Versatile third components for efficient and stable organic solar cells

Abstract

Organic solar cells (OSCs) with a third component, consisting of a donor material, an acceptor material and a third component (organic or inorganic, semiconductor or insulator), received increasing attention in the last five years and the power conversion efficiencies approached 10%. Compared with the traditional binary (two-component) blend, three-component OSCs presented some advantages: broader and stronger absorption, more efficient charge transfer, more efficient charge transport pathways, better charge extraction at the electrodes and improved stability. Various types of third components, such as light-absorbing small molecules or polymers, fullerene or non-fullerene acceptors, metal- or carbon-based nanomaterials, quantum dots, and polymer or small molecule nonvolatile additives were used in OSCs. In this contribution, we review the recent developments in three-component OSCs using various third components. In particular, the absorption complementation, phase separation and the nanostructure in three-component OSCs are addressed.

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Pei Cheng

Pei Cheng received his BS degree in polymer materials and engineering from Sichuan University in 2011. Now he is a PhD student at the Institute of Chemistry, Chinese Academy of Sciences. His research interests are focused on ternary blend organic solar cells, fullerene-free organic solar cells and new device structures of organic solar cells.

Xiaowei Zhan

Xiaowei Zhan obtained a PhD degree in chemistry from Zhejiang University in 1998. He was then a postdoctoral researcher at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) from 1998–2000, and in 2000 he was promoted to Associate Professor at ICCAS. Dr Zhan worked in the University of Arizona and Georgia Institute of Technology from 2002–2006 as a Research Associate and Research Scientist. He has been a full professor at ICCAS from 2006. In 2012 he moved to Peking University. His research interests are in the development of materials for organic electronics and photonics. Prof. Zhan is an associate editor of Journal of Materials Chemistry C and a Fellow of the Royal Society of Chemistry.

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1. Introduction

Organic solar cells (OSCs) are a promising cost-effective alternative for the utilization of solar energy, and present advantages, such as their simple preparation, low cost, light weight and large-area fabrication on flexible substrates. Much attention has been focused on the development of OSCs, which have seen a dramatic rise in efficiency over the last decade.^1–15

At the early stage of OSC development (1950s–1980s), the active layer typically consisted of a single component organic material, and yielded a very low power conversion efficiency (PCE).¹⁶ Since the bulk heterojunction (BHJ) concept was reported in 1995,¹⁷ a two-component (binary) blend had been predominant as the active layer. Relative to the single-component OSCs, the binary blend OSCs presented some advantages: efficient charge separation resulting from photoinduced electron transfer at the donor/acceptor (D/A) interfaces, and a high charge collection efficiency resulting from a bi-continuous network of the D/A blend. As a result, the binary blend OSCs exhibited PCEs of up to 10% for a single junction¹⁸ and 11%¹⁹ for a tandem junction.

Although the PCEs of binary blend OSCs were improved relative to 20 years ago, there are still some deficiencies in binary blend OSCs. For example, a poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C₆₁-butyric-acid-methyl-ester (PC₆₁BM) binary blend was most widely used in OSCs using spin coating^20–22 or a roll-to-roll (R2R) printing process.^23–26 This binary blend OSC still has some serious problems: (1) the absorption spectrum of this blend film is not broad (<650 nm) and not intense (a very thick film is required);²² (2) the offsets of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are considerably larger than 0.3 eV (the minimum requirement for charge transfer⁷), leading to energy loss and a reduced charge transfer efficiency;²⁷ (3) a simple blend of a donor and an acceptor cannot build efficient charge transport pathways; (4) the Schottky contact between the active layer and the electrodes¹⁰ and an unfavorable vertical phase separation^28–30 hinder efficient charge extraction at the electrodes; and (5) the morphology of the blend film is not stable during heat exposure or long term use.^31,32 In recent years, low-bandgap (LBG) polymers were developed to replace P3HT.^33–44 A LBG polymer/PCBM binary blend can solve some of the problems above, but a simple blend of them is still not good enough to achieve very high PCEs and good stability.

Three-component OSCs consisting of a donor material, an acceptor material and a third component received increasing attention in the last five years.^45–48 Compared with the binary blend, OSCs with a third component present some advantages: broader and stronger absorption, more efficient charge transfer, more efficient charge transport pathways, better charge extraction at the electrodes and improved stability. In the most recent two years, three-component OSCs have rapidly developed, and their PCEs have exceeded 8% for both an inorganic third component^49–54 and an organic third component.^55–65 It is worth mentioning that most of the reported OSCs with a third component were fabricated without an inverted structure, interlayer, tandem structure or optimized optical structure. The PCEs of three-component OSCs could be improved to >10% with further device work. OSCs with a third component are emerging as a new horizon in this field.

In this review, we focus our discussion on the recent developments in OSCs with a third component, especially on those accomplished in the last two years. Firstly, we introduce the fundamental design rules of OSCs with a third component. Then, we survey and analyze what is currently known concerning OSCs with a third component in terms of four types of structure: donor/organic light-absorbing molecule/acceptor, donor/acceptor/acceptor, donor/inorganic nanomaterials/acceptor and donor/organic nonvolatile additives/acceptor. The reasons for the better performance of OSCs with a third component relative to their control binary blend devices are also discussed. Finally, we address the possible research direction in the near future.

2. Fundamental design rules of three-component OSCs

In order to achieve highly efficient and stable OSCs, the third component is involved in every important physical process in OSCs. In this part, we discuss the fundamental design rules for OSCs with a third component in terms of light absorption, charge transfer, charge transport, charge extraction and morphology stability.

2.1. Broader and stronger absorption

Light absorption is the first step of the photovoltaic process. In most systems of OSCs, the vast majority of the photocurrent is attributed to the absorption of the donor materials. Lowering the bandgap (E_g) of the donor materials can extend the absorption spectrum, but will decrease the open circuit voltage (V_OC) simultaneously.^43,66 The absorption spectra of the most widely used donor materials, such as P3HT (E_g = 1.9 eV, absorption edge < 650 nm) and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno-[3,4-b]thiophenediyl]] (PTB7)³⁵ (E_g = 1.65 eV, absorption edge < 750 nm), do not match well with the solar spectrum.⁶⁷ Relative to broadening the absorption in the long wavelength region, broadening the absorption in the short wavelength region has received less attention, although it is also important for some systems. The molar absorption coefficient of organic materials is higher than that of inorganic materials, but is not high enough to absorb all light in their absorption spectrum when the films are thin (e.g. 100 nm). The thicker films can absorb more light, but the low mobility of organic materials does not allow the films to be very thick. To design a three-component OSC, the third component can play one or more roles (Fig. 1): it can show a strong light absorption which is complementary to the absorption of the host donor and acceptor materials; it can enhance the absorption of the active layer in its original absorption region by some optical effects; and it can enhance the crystallinity or the aggregation of the polymer donor to red-shift or improve its absorption.

Fig. 1 Broader and stronger absorption spectra of OSCs.

2.2. More efficient charge transfer at the D/A interfaces

The offsets of the HOMOs and LUMOs of the donor and the acceptor should be suitable. Too small offsets cannot provide a driving force large enough for charge transfer,^68,69 while too large offsets cannot suppress geminate charge recombination.²⁷ In a cascade energy heterojunction, the HOMO and LUMO levels of the cascade are located between the HOMOs and LUMOs of the donor and acceptor materials (Fig. 2). This is an efficient way to increase the charge transfer routes (donor–cascade, cascade–acceptor, donor–acceptor)⁵⁷ and suppress geminate charge recombination.²⁷ Typical cascade energy heterojunctions are ternary blends and tri-layers.⁷⁰ A ternary blend of a donor, an acceptor and a cascade is a simple and widely used way to fabricate a cascade energy heterojunction. On the other hand, the larger area of D/A interfaces can not only induce more efficient exciton dissociation and photoinduced charge transfer, but also cause more charge recombination. Thus, to design an OSC with a third component, the third component should have suitable energy levels to fabricate a cascade energy heterojunction and the D/A interface should be adjusted to a suitable area.

Fig. 2 The relative energy levels of the donor, cascade and acceptor.

2.3. More efficient charge transport pathways

Charge transport is very important for OSCs; efficient charge transport can endure a thicker film of the active layer which can absorb more light and is suitable for R2R fabrication.^71–73 The charge transport depends on the materials themselves, the molecular packing, the domain purity, the D/A interfaces, additional transport pathways and the network of the donor and the acceptor. OSCs with a third component can be designed to build more efficient charge transport pathways (Fig. 3). The third component can play one or more roles: it can induce more ordered molecular packing (maybe crystallization) of the donor or the acceptor; it can purify the domains of the donor or the acceptor; it can function as an interfacial compatibilizer; and it can act as additional hole or electron transport pathways.

Fig. 3 Schematic diagram of an active layer with efficient charge transport.

2.4. More efficient charge extraction at the electrodes

Charge extraction is the last step of the photovoltaic process, which is very important, but the efficiency is limited in simple binary blend systems.^74–76 For efficient charge extraction, the vertical phase separation of the active layer and the contact between the electrodes and the active layer play very important roles. In spin-coated polymer/fullerene blends, donor polymers are apt to aggregate on the top surface due to the relatively low surface energy compared with fullerene acceptors (Fig. 4a).^28,77 As a result, the holes transported by the polymer donor and the electrons transported by fullerene are extracted by the wrong electrode, the cathode and anode, respectively. Thus, the vertical phase separation in this simple binary blend structure is not desired for charge extraction.^78,79 The Schottky contact between the active layer and the electrodes is harmful for charge extraction,¹⁰ but the ideal ohmic contact between the active layer and the electrodes is hard to achieve without inserting some kinds of buffer layers.^80,81 OSCs with a third component can be designed to achieve more efficient charge extraction at the electrodes: (1) the third component can induce efficient vertical phase separation (like a p–i–n type²⁹); (2) the third component can self-organize as a hole or electron buffer layer to form a better contact between the active layer and the electrodes (Fig. 4b).

Fig. 4 Schematic diagrams of active layers: (a) a simple binary blend; (b) a ternary blend with efficient charge extraction at the electrodes.

2.5. Better morphology stability

For industrial mass production of OSCs, the stability is as important as the efficiency.^31,82,83 However, in the most widely used binary blend systems, the electron acceptor PCBM tends to aggregate into big clusters after some time or after thermal treatment, which destroys the morphology of the active layer and reduces the PCE drastically.^84–86 OSCs with a third component can be designed to achieve better stability compared to the control devices; the third component can suppress the aggregation of PCBM or solidify the morphology of the active layer as a cross-linker or freeze the scale of phase separation via some special intermolecular interactions.

3. Main types of three-component OSCs

In this part, we focus our discussion on the recent development of OSCs with a third component, especially on the development accomplished in the last two years. The main four types of OSCs with a third component include donor/organic light-absorbing molecule/acceptor (third component: a light-absorbing polymer and a small molecule), donor/acceptor/acceptor (third component: an acceptor polymer and a small molecule), donor/inorganic nanomaterial/acceptor (third component: a metal nanomaterial, quantum dot and a carbon-based nanomaterial), and donor/organic nonvolatile additive/acceptor (third component: a polymer additive and a nonvolatile small molecule additive). The photovoltaic properties of devices without and with the addition of the third component are listed in Tables 1–6; the ratio of the third component is the weight ratio of the third component to the total weight of the donor, acceptor and third component in OSCs. The molecular structures of the donor and acceptor materials used in this review are shown in Chart 1. The molecular structures of the different third components are shown in Charts 2–6.

Table 1 The photovoltaic properties of devices without and with the addition of a third component (light-absorbing polymers)

Donor	Acceptor	Third component		V OC (V)	J SC (mA cm^−2)	FF	PCE (%)	Ref.
		Name	Weight ratio (%)
P3HT	PC61BM	PCPDTBT	0	0.57	7.1	0.63	2.5	87
			10	0.62	8.0	0.55	2.8
P3HT	PC61BM	PCPDTBT	0	0.59	8.1	0.66	3.17	88
			15	0.58	9.9	0.63	3.61
P3HT	PC61BM	Si-PCPDTBT	0	0.57	8.6	0.64	3.1	94
			20	0.59	11	0.62	4.0
P3HT	ICBA	Si-PCPDTBT	0	0.82	7.9	0.60	3.9	97
			10	0.79	10	0.65	5.1
DTBT	PC61BM	TAZ	0	0.87	10.2	0.50	4.39	99
			15	0.81	11.9	0.61	5.88
DTPyT	PC61BM	DTffBT	0	0.85	12.8	0.58	6.30	99
			25	0.87	13.7	0.59	7.02
PTB7	PC71BM	PID2	0	0.72	15.0	0.67	7.25	56
			4	0.72	16.8	0.69	8.22
P3HTT-DPP-10%	PC61BM	P3HT75-co-EHT25	0	0.57	14.4	0.62	5.07	100
			4.8	0.60	15.1	0.61	5.51
P3HT	PC61BM	THC8	0	0.60	10.6	0.48	3.10	107
			8.6	0.62	11.9	0.53	3.88
PCPDTBT	PC71BM	PTB7	0	0.63	11.1	0.42	2.93	108
			5	0.68	13.4	0.45	4.07
PTB7	PC71BM	PBDTT-SeDPP	0	0.72	15.1	0.66	7.2	55
			28.6	0.69	18.7	0.67	8.7

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Table 2 The photovoltaic properties of devices without and with the addition of a third component (light-absorbing small molecules)

Donor	Acceptor	Third component		V OC (V)	J SC (mA cm^−2)	FF	PCE (%)	Ref.
		Name	Weight ratio (%)
P3HT	PC71BM	TQTFA	0	0.60	9.7	0.67	3.90	121
			11	0.69	10.6	0.61	4.50
P3HT	PC61BM	BantHBT	0	0.62	8.6	0.56	3.0	122
			9.1	0.62	9.8	0.60	3.7
PBTADN	PC71BM	BantHBT	0	0.83	6.9	0.53	3.0	122
			4.3	0.91	11.0	0.56	5.6
P3HT	PC61BM	DMPA-DTDPP	0	0.60	8.3	0.61	3.02	123
			2.7	0.60	9.8	0.58	3.37
P3HT	PC71BM	DPP-CN	0	0.66	9.4	0.52	3.23	124
			3.8	0.70	10.9	0.58	4.37
P3HT	PC61BM	DTDCTB	0	0.58	8.7	0.64	3.25	125
			10	0.69	9.7	0.61	4.07
P3HT	PC61BM	SiPc	0	0.55	9.7	0.66	3.5	132
			4.8	0.58	11.1	0.65	4.2
P3HT	PC61BM	tBuSiNc	0	0.60	9.4	0.67	3.80	135
			8	0.62	11.4	0.63	4.46
P3HT	PC61BM	DIB-SQ	0	0.59	10.3	0.53	3.27	137
			0.5	0.60	11.6	0.65	4.51
P3HT	PC71BM	TBU-SQ	0	0.66	9.4	0.56	3.47	139
			2.5	0.70	11.2	0.58	4.55

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Table 3 The photovoltaic properties of devices without and with the addition of a third component (acceptor materials)

Donor	Acceptor	Third component		V OC (V)	J SC (mA cm^−2)	FF	PCE (%)	Ref.
		Name	Weight ratio (%)
a The device data after 25 h of heating at 150 °C is in brackets.
PTB7	PC71BM	ICBA	0	0.70	15.0	0.69	7.35	57
			9	0.72	16.3	0.69	8.24
P3HT	OXCMA	OXCTA	0	0.63	9.6	0.59	3.61	155
			11.3	0.67	10.3	0.57	3.96
PTBT	PC71BM	PC61BM	0	0.90	12.4	0.53	5.91	157
			26.7	0.90	13.2	0.59	7.00
P3HT	PC61BM	PCBPy	0	0.61	9.3	0.61	3.46	159
			10	0.64	11.4	0.66	4.82
P3HT	PC61BM	F-PCBM	0	0.55	8.72	0.65	3.09	162
			5.3	0.57	9.51	0.70	3.79
P3HT	PC61BM	PCBDAN	0	0.14	7.05	0.30	0.30	163
			4.5	0.58	9.87	0.65	3.70
PDTBCDTBT^a	PC71BM	PC61BP^F	0	0.80 (1.34)	10.3 (0.05)	0.65 (0.20)	5.34 (0.01)	84
			10	0.78 (0.78)	10.2 (8.6)	0.66 (0.63)	5.24 (4.24)
P3HT	PC61BM	DCF1	0	0.56	9.5	0.53	2.8	169
			33	0.61	11.1	0.55	3.7
P3HT	PC61BM	DCF2	0	0.56	9.5	0.53	2.8	169
			33	0.58	11.8	0.54	3.7

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Table 4 The photovoltaic properties of devices without and with the addition of a third component (inorganic nanomaterials)

Donor	Acceptor	Third component		V OC (V)	J SC (mA cm^−2)	FF	PCE (%)	Ref.
		Name	Weight ratio (%)
P3HT	PC61BM	Au NPs	0	0.60	8.27	0.61	3.26	179
			5	0.60	9.77	0.63	3.71
PCDTBT	PC71BM	Au NPs	0	—	—	0.64	5.77	180
			5	0.89	11.2	0.65	6.45
P3HT	PC61BM	Ag NPs	0	0.65	8.47	0.60	3.31	184
			20	0.65	9.28	0.59	3.56
P3HT	PC61BM	Ag NWs	0	0.65	8.47	0.60	3.31	184
			20	0.66	9.32	0.64	3.91
P3HT	PC61BM	Ag nanoprisms	0	0.64	8.99	0.62	3.60	185
			2	0.64	9.93	0.64	4.07
P3HT	PC61BM	Ag@SiO2–OT	0	0.62	10.3	0.54	3.44	186
			1	0.63	11.4	0.55	4.02
p-DTS(FBTTh2)2	PC71BM	Au@SiO2	0	0.77	12.2	0.70	6.52	49
			1	0.77	15.0	0.71	8.11
PBDTTT-C-T	PC71BM	MPs	0	0.77	15.0	0.62	7.38	52
			5	0.78	16.1	0.66	8.48
P3HT	PC61BM	CdSe	0	0.57	7.51	0.48	2.06	201
			4.8	0.60	8.15	0.62	3.05
PCDTBT	PC71BM	CdSe	0	0.83	11.69	0.53	5.22	202
			2.2	0.88	12.98	0.61	6.94
P3HT	PC61BM	CdS/TNTs	0	0.56	9.84	0.48	2.63	207
			2.7	0.59	13.3	0.49	3.52
P3HT	PC61BM	GQDs	0	0.59	14.3	0.31	2.61	208
			0.3	0.57	24.6	0.31	4.35
P3HT	PC61BM	CNT	0	0.63	8.7	0.63	3.4	213
			0.5	0.63	9.5	0.63	3.8
PTB7	PC71BM	NCNTs	0	0.71	15.0	0.68	7.2	50
			1.5	0.70	17.4	0.69	8.4
P3HT	ICBA	QD:NCNTs	0	0.75	10.7	0.58	4.68	215
			1.1	0.79	11.9	0.65	6.11
PTB7	PC71BM	Au:BCNT	0	0.73	16.7	0.68	8.31	53
			0.4	0.74	18.3	0.72	9.81
P3HT	PC61BM	N-RGO	0	0.59	10.4	0.51	3.2	220
			0.5	0.59	15.0	0.51	4.5

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Table 5 The photovoltaic properties of devices without and with the addition of a third component (polymer additives)

Donor	Acceptor	Third component		V OC (V)	J SC (mA cm^−2)	FF	PCE (%)	Ref.
		Name	Weight ratio (%)
P3HT	PC61BM	P3HT-b-PTMSM	0	0.63	8.1	0.51	2.6	226
			5	0.66	9.2	0.61	3.7
P3HT	PC61BM	P3HT-b-P4VP	0	0.54	9.0	0.57	2.7	227
			3.8	0.57	12.2	0.61	4.3
P3HT	PC61BM	P3HT-b-PS	0	0.64	8.70	0.56	3.25	228
			5	0.63	9.86	0.62	4.11
P3HT	PC61BM	P3HT-b-PTP	0	0.60	9.81	0.55	3.24	229
			5	0.60	10.4	0.65	4.03
P3HT	C60	C60-BCP	0	0.50	1.9	0.51	0.48	230
			16.7	0.49	9.0	0.58	2.56
P3HT	PC61BM	PEG-C60	0	0.61	10.2	0.58	3.60	242
			2.4	0.66	10.3	0.65	4.41
P3HT	PC61BM	P3HT-b-P3TEGT:Li	0	0.53	6.07	0.50	1.6	243
			1.9	0.59	9.91	0.62	3.6
PCDTBT	PC71BM	PTE	0	0.89	12.1	0.46	5.0	251
			6.7	0.90	13.8	0.49	6.1
p-DTS(FBTTh2)2	PC71BM	PS	0	0.80	13.0	0.67	7.1	58
			2.5	0.80	14.5	0.69	8.2
DR3TBDTT	PC71BM	PDMS	0	0.91	13.2	0.63	7.51	59
			1.4	0.93	13.2	0.66	8.12

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Table 6 The photovoltaic properties of devices without and with the addition of a third component (nonvolatile small molecule additives)

Donor	Acceptor	Third component		V OC (V)	J SC (mA cm^−2)	FF	PCE (%)	Ref.
		Name	Weight ratio (%)
a The device data after 16 h of heating at 150 °C are in brackets.
p-DTS(FBTTh2)2	PC71BM	DMDBS	0	0.76	3.0	0.32	0.72	260
			1	0.78	9.4	0.56	4.15
P3HT	PC61BM	a	0	0.55	4.51	0.39	1.01	262
			5	0.54	6.37	0.53	1.81
SMDPPEH	PC61BM	CP3MS	0	0.68	10.5	0.39	2.75	264
			0.1	0.72	12.4	0.51	4.55
P3HT	PC61BM	TT-TTPA	0	0.62	8.91	0.68	3.85	265
			3.8	0.62	10.3	0.67	4.41
P3HT	PC61BM	CBP	0	0.59	4.93	0.50	1.43	266
			1.6	0.59	5.74	0.55	1.85
PBDTTPD-HT	PC71BM	BDT-3T-CNCOO	0	0.99	11.8	0.58	6.85	60
			20	0.97	12.2	0.71	8.40
PBDTTT-C-T	PPDIDTT	PDI-2DTT	0	0.75	7.92	0.49	3.04	267
			1	0.75	8.55	0.52	3.45
SiIDT-BT	PC71BM	DAZH	0	0.88	12.71	0.54	6.04	281
			2.2	0.89	12.74	0.62	7.03
PTB7^a	PC61BM	BABP	0	0.71 (0.62)	11.2 (4.51)	0.68 (0.32)	5.45 (0.89)	86
			2	0.71 (0.73)	12.3 (10.8)	0.66 (0.59)	5.78 (4.60)
PCDTBT	PC71BM	F4-TCNQ	0	0.87	11.0	0.67	6.41	282
			0.4	0.90	14.0	0.63	7.94

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Chart 1 The molecular structures of the donor and acceptor materials used in this review.

Chart 2 The molecular structures of the third components (light-absorbing polymers) used in OSCs.

Chart 3 The molecular structures of the third components (light-absorbing small molecules) used in OSCs.

Chart 4 The molecular structures of the third components (acceptor materials) used in OSCs.

Chart 5 The molecular structures of the third components (polymer additives) used in OSCs.

Chart 6 The molecular structures of the third components (nonvolatile small molecule additives) used in OSCs.

3.1. Donor/organic light-absorbing molecule/acceptor

3.1.1. Donor/light-absorbing polymer/acceptor. To broaden the absorption spectra of OSCs, various light-absorbing polymers (Chart 2) were employed as the third components in OSCs.^87–103 The photovoltaic properties of devices with and without the addition of the third component are listed in Table 1. One of the earliest OSCs with a third component based on a donor/light-absorbing polymer/acceptor system was reported by Koppe et al. in 2010.⁸⁷ Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT)¹⁰⁴ exhibited near infrared (NIR) absorption (Fig. 5a) and suitable energy levels for a cascade energy heterojunction (Fig. 5b), and was used as the third component in P3HT/PCPDTBT/PC₆₁BM OSCs.^{88,89,91–93} After the addition of 10% PCPDTBT into the P3HT/PC₆₁BM blend, the V_OC, the short circuit current (J_SC) and the PCE increased from 0.57 V, 7.1 mA cm⁻² and 2.5% to 0.62 V, 8.0 mA cm⁻² and 2.8%, respectively. Inverted OSCs based on the P3HT/PCPDTBT/PC₆₁BM blend allowed a higher PCPDTBT ratio (15%) and exhibited better performance: the PCEs of the OSCs without and with the addition of the third component were 3.17% and 3.61%, respectively.⁸⁸ The J_SC had a large enhancement of 20% due to the higher ratio of PCPDTBT. To explore the reason for the higher efficiency and the restriction of the low ratio of the third component, some researchers studied the morphology of P3HT/PCPDTBT/PC₆₁BM blend films. Li et al.⁹² reported that the crystallinity of blend films decreased with addition of the dominantly amorphous polymer PCPDTBT, but a small amount of PCPDTBT (0–20%) only slightly affected the eutectic point of the ternary blends. The addition of PCPDTBT reduced the crystallinity of PC₆₁BM, but had no influence on the crystalline part of P3HT.⁹³ The addition of a small amount of PCPDTBT (0–20%) slightly reduced the crystallinity of PC₆₁BM, but the crystalline part of PC₆₁BM would be destroyed after the addition of a large amount of PCPDTBT (>20%).

Fig. 5 (a) Absorption spectra of thermally annealed layers of P3HT/PC₆₁BM, PCPDTBT/PC₆₁BM, and P3HT/PCPDTBT/PC₆₁BM. (b) Energy levels of the electrodes and semiconductors used in the P3HT/PCPDTBT/PC₆₁BM blends. The values for the organic semiconductors were determined by cyclic voltammetry.⁸⁷ Reprinted with permission from ref. 87. Copyright 2010, Wiley.

Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (Si-PCPDTBT)¹⁰⁵ exhibited much higher crystallinity and a lower bandgap than its counterpart PCPDTBT and accordingly was used to replace PCPDTBT as the third component in the OSCs.^94–98 Incorporating 20% Si-PCPDTBT in P3HT/PC₆₁BM led to a higher efficiency (4.0%) relative to the binary counterpart (3.1%). The J_SC of the OSCs with a third component presented an approximate 30% increase relative to that of the binary blend OSCs, which was attributed to the higher Si-PCPDTBT ratio (20%).⁹⁴ Ameri et al.^97,98 used an indene-C₆₀ bisadduct (ICBA)¹⁰⁶ to replace PC₆₁BM and fabricated P3HT/Si-PCPDTBT/ICBA OSCs. The efficiency increased to 5.1% due to the higher LUMO energy level of ICBA which produced a higher V_OC of 0.79 V.

The structure of PCPDTBT and Si-PCPDTBT was completely different from that of P3HT. The ternary blend tended to separate into three phases, and the third component must have its HOMO and LUMO levels between the corresponding energy levels of P3HT and PCBM. This imposed a stringent requirement on the selection of the third component. Chemists introduced a new ternary blend approach of a “parallel-like BHJ” (PBHJ), in which two donor polymers with similar skeleton structures but different alkyl chains or substituent groups were used. These two polymer donors had good compatibility and different absorption spectra. In 2012, Yang et al.⁹⁹ fabricated DTBT/TAZ/PC₆₁BM OSCs. The polymers DTBT and TAZ had similar structures but different alkyl chains and substituent groups. After addition of 15% TAZ into a DTBT/PC₆₁BM binary blend, the PCE of the OSCs increased from 4.39% to 5.88%. The absorption and external quantum efficiency (EQE) spectra of the PBHJ devices were essentially a linear combination of the spectra of their two “subcells” (Fig. 6a and c). Similarly, as shown in Fig. 6b and d, OSCs based on a DTPyT/DTffBT/PC₆₁BM blend (25% DTffBT) exhibited an improved PCE (7.02%) relative to their binary counterpart (6.3%). In 2014, Lu et al.⁵⁶ reported OSCs based on a PTB7/PID2/PC₇₁BM blend. The third component, PID2, had a similar chemical structure to PTB7, while PID2 had a downshifted HOMO and LUMO relative to PTB7, leading to the formation of a cascade energy heterojunction. The larger bandgap of PID2 broadened the absorption of the blend films in the short wavelength region. In comparison with the binary blend OSCs based on PTB7/PC₇₁BM, the OSCs based on PTB7/PID2/PC₇₁BM demonstrated a higher PCE (8.22%). Recently, the same group changed the polymer donor and obtained a higher PCE of 9.20%.⁶⁵

Fig. 6 (a and b) Absorption spectra of the ternary blend devices and their “subcells” based on (a) TAZ/DTBT and (b) DTffBT/DTPyT. (c and d) EQE spectra of the ternary blend devices and their “subcells” based on (c) TAZ/DTBT and (d) DTffBT/DTPyT.⁹⁹ Reprinted with permission from ref. 99. Copyright 2012, American Chemical Society.

In order to investigate the relationship between the device efficiencies and the miscibility of the polymers (donor polymer and the third component) in ternary blend devices, Thompson and co-workers carried out some theoretical work.^100–103 They studied two OSCs with a third component: P3HTT-DPP-10%/P3HT₇₅-co-EHT₂₅/PC₆₁BM (with good miscibility of the two polymers) and P3HTT-DPP-10%/PCDTBT/PC₆₁BM (with bad miscibility of the two polymers). The OSCs with good polymer miscibility showed tunability of V_OC, which was explained by invoking an alloy model for the two polymer components. In this alloy model, optical excitation within the alloy was at the molecular level due to the highly localized nature of the exciton. The donor–acceptor interface and the corresponding interface bandgap (CT state), which determined V_OC, displayed a material-averaged electronic structure due to the more delocalized nature of the one-electron states, and thus the V_OC reflected the average composition of the interface.¹⁰¹ On the other hand, the OSCs with bad polymer miscibility did not show tunability of V_OC. Unlike P3HTT-DPP-10% and P3HT₇₅-co-EHT₂₅, the structures of P3HTT-DPP-10% and PCDTBT were completely different and their surface energies were also highly different. The V_OC was pinned to the smallest V_OC of the corresponding binary blend, even at a P3HTT-DPP-10% content as low as 5%. The lack of miscibility and different surface energies between the polymers facilitated the preferential location of P3HTT-DPP-10% away from the PC₆₁BM interface at high PCDTBT content and prevented the formation of an organic alloy, leading to a significant decrease in the device performance.¹⁰² The same group studied the electronic states of mobile carriers and excitons as well as the miscibility of the blend. Excitons originating from a dilute low-bandgap polymer in a polymer mixture diffused easily in the miscible system but not in the nonmiscible system, which was a consequence of their morphological and/or aggregation properties.¹⁰³

Some other light-absorbing polymers were used as the third component to enhance the performance of OSCs.^107–118 For instance, OSCs based on P3HT/THC8/PC₆₁BM exhibited a better PCE (3.88%) compared with that of the binary blend (3.10%).¹⁰⁷ The addition of 5% PTB7 enhanced the PCE from 2.93% to 4.07% in PCPDTBT/PC₇₁BM-based OSCs.¹⁰⁸ Very recently, Yang et al. improved the PCE of the PTB7/PC₇₁BM blend from 7.2% to 8.7% by incorporating a NIR light-absorbing polymer PBDTT–SeDPP.⁵⁵

3.1.2. Donor/light-absorbing small molecule/acceptor. Compared with polymeric semiconductors, small molecules have several advantages, such as their easy synthesis and purification, definite molecular weight, well-defined molecular structure and high purity without batch-to-batch variation.^5,119,120 Therefore, light-absorbing small molecules used as the third component (Chart 3) have attracted considerable attention in recent years. The photovoltaic properties of devices with and without the addition of light-absorbing small molecules are listed in Table 2.

In 2010, Huang et al.¹²¹ used a broad-bandgap (BBG) small molecule TQTFA to broaden the absorption of a P3HT/PC₇₁BM blended film in the short wavelength region. After the addition of 11% TQTFA, the PCE of the devices increased from 3.90% to 4.50%. In 2013, Cha et al.¹²² employed a BBG small molecule BantHBT as the third component in OSCs. The strong absorption of BantHBT ranging from 300–600 nm, especially at 450 nm, was complementary with that of P3HT and PBTADN (Fig. 7a and b). The PCE of the P3HT/PC₆₁BM-based devices increased from 3.0% to 3.7% with 9.1% of BantHBT incorporated. For the PBTADN/PC₇₁BM system, the V_OC, J_SC, fill factor (FF) and PCE increased from 0.83 V, 6.9 mA cm⁻², 0.53 and 3.0% to 0.91 V, 11.0 mA cm⁻², 0.56 and 5.6%, respectively, after the addition of 4.8% BantHBT. The third component BantHBT not only increased the absorption of the active layer, but also displayed good miscibility with the polymer donors, and balanced the charge transport.

Fig. 7 UV-visible absorption spectra of (a) P3HT, PC₆₁BM, HBantHBT and BantHBT films,¹²² (b) PBTADN, PC₇₁BM, HBantHBT, and BantHBT films,¹²² (c) DMPA-DTDPP, P3HT and PC₆₁BM films,¹²³ and (d) P3HT, DTDCTB and P3HT/DTDCTB films,¹²⁵ respectively. Reprinted with permission from ref. 122. Copyright 2013, Wiley. Reprinted with permission from ref. 123. Copyright 2012, Wiley. Reprinted with permission from ref. 125. Copyright 2014, American Chemical Society.

Compared with BBG small molecules, the use of LBG small molecules as the third component in OSCs was more popular.^123–130 Lee et al.¹²³ fabricated OSCs based on P3HT/DMPA-DTDPP/PC₆₁BM. DMPA-DTDPP was a LBG small molecule based on the diketopyrrolopyrrole (DPP) unit and broadened the absorption of P3HT/PC₆₁BM (Fig. 7c). The PCE of devices increased from 3.02% to 3.37% with the use of DMPA-DTDPP. Sharma et al.¹²⁴ added DPP-CN as the third component in a P3HT/PC₇₁BM blended film to fabricate OSCs. The photosensitivity in the wavelength region from 620 to 750 nm was improved, and J_SC was enhanced from 9.40 mA cm⁻² to 10.9 mA cm⁻². As a result, the PCE was improved from 3.23% to 4.37%. Ye et al.¹²⁵ fabricated OSCs based on P3HT/DTDCTB/PC₆₁BM, in which the absorption edge of the third component DTDCTB was >800 nm (Fig. 7d). After the addition of 10% DTDCTB, the PCE of the OSCs increased to 4.07%, better than that of the binary blend OSCs (3.25%).

Förster resonance energy transfer (FRET) was a promising strategy to improve exciton migration over long distances and reduce energy loss. Employing dye molecules which have a large spectral overlap between their absorption and the emission of the donor as the third component was a new way to utilize FRET. These dye molecules had a high absorbance in the red and NIR spectral regions. Some LBG small molecule dyes, which had a narrow and strong absorption that overlapped with the emission band of donor materials, were used as the third component in OSCs. In 2009, Honda et al.¹³¹ used silicon phthalocyanine bis(trihexylsilyl oxide) (SiPc) as the third component in P3HT/PC₆₁BM; their energy levels were suitable to form a cascade energy heterojunction (Fig. 8a). The absorption peak of SiPc was around 680 nm, and overlapped with the emission peak of P3HT (Fig. 8b). After the addition of SiPc to the P3HT/PC₆₁BM blend, the PCE increased from 2.2% to 2.7%. Later, the same group investigated the morphology of this OSC with the third component.^132–134 After the optimization of the device conditions and the ratio of SiPc in OSCs, the V_OC, J_SC, FF and PCE increased to 0.58 V, 11.1 mA cm⁻², 0.65 and 4.2% respectively, which were higher than those of the binary blend.¹³² As shown in Fig. 8c, the light harvesting based on the dye sensitization and the polymer exciton harvesting were highly efficient because the SiPc molecules were selectively localized at the polymer/fullerene interface (interfacial coverage around 40%). In particular, the SiPc molecules were closely in contact with both P3HT and PC₆₁BM at the interface instead of forming aggregations (bulky groups suppress the formation of dye aggregations), which was an ideal interfacial structure for the efficient cascade energy heterojunction.¹³³

Fig. 8 (a) Energy diagrams of P3HT, PC₆₁BM, SiPc, and SiNc, (b) absorption spectra of the P3HT/PC₆₁BM film (dotted line), SiPc (dashed line), and SiNc (solid line) in solution.¹³⁴ (c) Schematic diagram of the morphology, energy transfer and charge transfer of P3HT/SiPc/PC₆₁BM films.¹³³ Reprinted with permission from ref. 134. Copyright 2010, Royal Society of Chemistry. Reprinted with permission from ref. 133. Copyright 2011, American Chemical Society.

Silicon naphthalocyanine bis(trihexylsilyloxide) (SiNc)¹³⁴ was also introduced as the third component in OSCs because of its narrower bandgap and red-shifted absorption relative to SiPc (Fig. 8a and b). However, due to the limited solubility of the SiNc dye, the PCE did not exhibit an obvious enhancement after the addition of the third component. The PCEs of the OSCs with 1.5% SiNc and the binary blend OSCs were 3.7% and 3.5%, respectively. In 2014, Lim et al.¹³⁵ employed a modified dye molecule, tBuSiNc, as the third component in OSCs. The t-butyl groups improved the solubility, which allowed more dye incorporation. The P3HT/PC₆₁BM device incorporating 8% tBuSiNc exhibited a higher efficiency of 4.46% compared to the binary blend OSCs (3.80%).

Squaraine (SQ)-based small molecules were also widely used dyes as the third component in OSCs.^136–140 In particular, DIB-SQ was the most representative SQ molecule used as the third component in OSCs.^136–138 In 2013, Huang et al.¹³⁷ used DIB-SQ in a P3HT/PC₆₁BM system to fabricate OSCs. DIB-SQ had suitable energy levels which could form a cascade energy heterojunction with P3HT/PC₆₁BM (Fig. 9a) and its absorption overlapped with the emission spectrum of P3HT (Fig. 9b). In comparison with the control devices, the device with 0.5% DIB-SQ exhibited a significant increase of 38% in PCE due to the improved J_SC and FF. Compared with DIB-SQ, TBU-SQ exhibited a red-shifted absorption peak from 650 to 710 nm, which had a better overlap with the emission spectrum of P3HT.¹³⁹ The PCE of devices increased from 3.47% to 4.55% after the addition of 2.5% of the third component TBU-SQ. Thermal annealing of the P3HT/TBU-SQ/PC₇₁BM OSCs further increased the PCE to 5.15%. In addition to the widely used SiPc, SiNc and SQ-based dye molecules, other dyes such as BODIPY were also introduced as the third component in OSCs to achieve a higher PCE.^141–145

Fig. 9 (a) Energy level diagram of the components in an OSC highlighting pathways for charge generation, (b) extinction coefficient of SQ in 1,2-dichlorobenzene, and the absorption and emission spectra of a P3HT film.¹³⁷ Reprinted with permission from ref. 137. Copyright 2013, Nature Publishing Group.

3.2. Donor/acceptor/acceptor

Electron acceptor materials were also used as the third component in OSCs (Chart 4). In the last decade, a mass of acceptor materials were developed including fullerene derivatives^146–148 and non-fullerene molecules.^149–153 Some of these molecules had up-shifted LUMO and HOMO levels compared to PCBM, resulting in a cascade energy heterojunction with a donor polymer and PCBM; some of them were chemically modified to form a buffer layer or enhance the device stability (Table 3).

ICBA¹⁰⁶ was the most representative third component in donor/acceptor/acceptor ternary blends and used for the formation of a cascade energy heterojunction. In 2011, Khlyabich et al.¹⁵⁴ fabricated OSCs using P3HT as a donor and two soluble fullerenes, PC₆₁BM and ICBA, as acceptors. The V_OC of the ternary blend OSCs could be tuned between the limiting V_OC values of the corresponding binary blend (P3HT/PC₆₁BM, P3HT/ICBA) solar cells. Recently, Cheng et al.⁵⁷ used ICBA as an electron-cascade acceptor material (the third component) in PTB7/PC₇₁BM blends to fabricate OSCs. Due to the higher LUMO energy level of ICBA relative to PC₇₁BM, the V_OC increased with the addition of ICBA. ICBA played a bridging role between PTB7 and PC₇₁BM (cascade energy heterojunction), thus providing more routes for charge transfer at the D/A interface. As shown in Fig. 10, the morphology of the blend films with a third component was similar to that of PTB7/PC₇₁BM when the ICBA content was much lower than the PC₇₁BM content which guaranteed suitable phase separation and efficient charge transport. However, a higher ICBA content would destroy the morphology. Compared with the binary blend of PTB7/PC₇₁BM, the devices with 9% ICBA content exhibited enhanced V_OC, J_SC, FF and PCE values. The highest PCE of this OSC with a third component was 8.24%, which was the highest reported for ternary blend OSCs and ICBA-related OSCs. Recently, the same group promoted the PCE to 8.88% by use of the method of diluting a concentrated solution.⁶³

Fig. 10 Atomic force microscopy (AFM) height images (a, c and e) and phase images (b, d and f) of PTB7/ICBA/PC₇₁BM films spin-coated on ITO/PEDOT:PSS substrates: (a, b) 0% ICBA, (c, d) 9% ICBA, and (e, f) 18% ICBA.⁵⁷ Reprinted with permission from ref. 57. Copyright 2014, Royal Society of Chemistry.

In addition to ICBA, some other fullerene-based acceptors were also used as the third components in OSCs to achieve better efficiency.^155–161 For instance, Kang et al.¹⁵⁵ fabricated OSCs based on P3HT/OXCTA/OXCMA; the device incorporating 11.3% OXCTA exhibited an efficiency of 3.96%, higher than that of its binary counterpart (3.61%). Santo et al.¹⁵⁶ and Ko et al.¹⁵⁷ improved the PCEs of P3HT and PTBT-based OSCs by using a mix of PC₆₁BM and PC₇₁BM as acceptors. Chen et al.¹⁵⁹ used PCBPy as the third component in OSCs. PCBPy could self-organize into nanostructures to control the morphology; the PCE of P3HT/PC₆₁BM-based OSCs was significantly improved to 4.8%.

In addition to their use in the formation of a cascade energy heterojunction, fullerene acceptor molecules were used to form a buffer layer or a desired vertical phase separation. Wei et al.¹⁶² introduced a fluorocarbon chain-modified fullerene acceptor, F-PCBM, in a P3HT/PC₆₁BM blend. F-PCBM had a lower surface energy compared with P3HT and PC₆₁BM. The addition of a small amount (5.3%) of F-PCBM improved the photovoltaic performance due to the spontaneous formation of the thin buffer layer at the top surface of the active layer. On the other hand, some fullerene acceptor materials with a high surface energy can also be used as the third components in OSCs. Ma et al.¹⁶³ employed an amino-containing fullerene derivative PCBDAN as the third component in a P3HT/PC₆₁BM blend to fabricate inverted OSCs. Due to its higher surface energy relative to P3HT and PC₆₁BM, PCBDAN served as the cathode interfacial material at the bottom and decreased the work function (WF) of ITO for its use as the cathode.

Fullerene derivatives with a spherical geometry tended to diffuse out of the blend and aggregate into larger clusters. In order to suppress the aggregation and improve the morphology stability, some fullerene derivatives with chemical modification were used as the third component in OSCs. In 2011, Cheng et al.¹⁶⁴ employed the fullerene derivatives PCBSD and PCBS in a P3HT/PC₆₁BM blend to improve the morphology stability, since PCBSD and PCBS suppressed the severe migration of PC₆₁BM. After adding a small amount (8.3%) of PCBSD into the P3HT/PC₆₁BM blend, the morphology stability of the blend was improved. In 2014, the same group⁸⁴ synthesized a fluorine-modified fullerene derivative PC₆₁BP^F, and used it as the third component in OSCs. The merit of PC₆₁BP^F was to impart a complementary attraction between the pentafluorophenyl group of PC₆₁BP^F and the C₆₀ cores of PC₆₁BP^F/PC₆₁BM. Such a supramolecular interaction could effectively suppress the aggregation of PC₆₁BM and improve the morphology stability. By adding 10% PC₆₁BP^F, the PDTBCDTBT/PC₆₁BP^F/PC₇₁BM device yielded a PCE of 4.24% after heating at 150 °C for 25 h, preserving 81% of its original value. In sharp contrast, the PCE of the device using a traditional PDTBCDTBT/PC₇₁BM blend decayed drastically to 0.19% over 25 h of heating. Similarly to this strategy, Schroeder et al.¹⁶⁵ improved the morphology stability of polymer/fullerene OSCs by incorporating a PC₆₁BM dimer ((PCB)₂C₂) as the third component. After the use of 10% (PCB)₂C₂ in OSCs, the PCE was higher and the morphology stability was much better. Some groups also improved the thermal stability of OSCs by using crosslinkable fullerene derivatives as the third component.^166–168

In addition to fullerene acceptors, non-fullerene acceptors were also used as the third component in donor/acceptor/acceptor OSCs. For example, Andrew et al.¹⁶⁹ added DCF1 or DCF2 to a P3HT/PC₆₁BM blend as the third component. These two compounds had suitable energy levels and could form a cascade energy heterojunction with P3HT/PC₆₁BM. After the addition of 33% DCF1 or DCF2 in the P3HT/PC₆₁BM blend, the PCEs of both P3HT/DCF1/PC₆₁BM and P3HT/DCF2/PC₆₁BM OSCs increased to 3.7%, which was higher than that of the P3HT/PC₆₁BM control device. Some ambipolar molecules were also used as the third component in OSCs.^170–174

3.3. Donor/inorganic nanomaterial/acceptor

3.3.1. Donor/metal nanomaterial/acceptor. In order to enhance the light-absorbing ability of the active layer, metal nanomaterials were employed in OSCs as the third component since they can enhance the local absorption within the adjacent plasmonic structure due to the effect of localized surface plasmon resonance (LSPR) (Table 4).^175–178

Gold (Au)-based^179–183 nanomaterials were widely used in OSCs as the third components to achieve stronger light absorption and a higher efficiency. Paci et al.¹⁷⁹ reported OSCs based on P3HT/Au nanoparticles (Au NPs)/PC₆₁BM. The device incorporating 5% Au NPs exhibited a higher efficiency of 3.71% compared to that of the control device (3.26%). The Au NPs were also used in an amorphous polymer donor/PCBM blend; for example, Wang et al.¹⁸⁰ employed Au NPs in a PCDTBT/PC₇₁BM blend as the third component and the PCE was improved from 5.77% to 6.45%. This improvement resulted from a combination of enhanced light absorption caused by the light scattering of Au NPs in the active layer and the plasmon-induced light concentration at specific wavelengths.

Silver (Ag)-based^184–189 nanomaterials were also widely used in OSCs as the third component. These nanomaterials had a lot of geometric constructions. For instance, Kim et al.¹⁸⁴ used Ag NPs or Ag nanowires (NWs) (Fig. 11a) in a P3HT/PC₆₁BM blend. Both the Ag NPs and Ag NWs enhanced the absorption of the active layer in its original absorption region. Compared with P3HT/PC₆₁BM solar cells, the P3HT/Ag NPs/PC₆₁BM and P3HT/Ag NWs/PC₆₁BM OSCs had nearly the same V_OC, and a reasonably higher J_SC, FF, and PCE. Li et al.¹⁸⁵ fabricated OSCs based on Ag NPs (Fig. 11b) or Ag nanoprisms (Fig. 11c). After the addition of Ag nanoprisms to a P3HT/PC₆₁BM blend, the PCE increased from 3.60% to 4.07% (for Ag NPs, the PCE was 3.99%). Due to the improved overlap between the absorption spectra of the active layer material and the Ag nanoprism, the high-order resonances contributed a lot to the absorption enhancement of the OSCs. Lee et al.¹⁸⁶ employed Ag-based core–shell nanoparticles (Fig. 11d) in OSCs as the third component. As the surface of the Ag NPs was modified by introducing a SiO₂ layer and an oligothiophene (OT) derivative, the resulting Ag@SiO₂–OT NPs became miscible with the P3HT/PC₆₁BM blend without aggregation, leading to a large enhancement (18%) in the PCE value compared to the pristine P3HT/PC₆₁BM device. The same concept of a core–shell nanostructure was also used in Au-based nanoparticles. Xu et al.^49,54 used Au@SiO₂ as the third component in a 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]-dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithio-phen]-5yl)benzo[c][1,2,5]-thiadiazole) (p-DTS(FBTTh₂)₂)/PC₇₁BM blend. Due to the plasmonic and scattering effects caused by the Au NPs, the PCE of OSCs with a third component (8.11%) was much higher than that of the control device (6.52%).

Fig. 11 Transmission electron microscope (TEM) micrographs of (a) Ag NWs,¹⁸⁴ (b) Ag NPs, (c) Ag nanoprisms,¹⁸⁵ and (d) Ag@SiO₂–OT NPs.¹⁸⁶ Reprinted with permission from ref. 184. Copyright 2011, American Chemical Society. Reprinted with permission from ref. 185. Copyright 2013, Wiley. Reprinted with permission from ref. 186. Copyright 2014, Royal Society of Chemistry.

In addition to Au or Ag-based nanomaterials, a platinum–nickel (Pt–Ni) nanomaterial⁵² was also used as the third component. Recently, Chou et al.⁵² employed Pt–Ni multipods (MPs) in PBDTTT-C-T/PC₇₁BM-based solar cells and the PCE was improved from 7.38% to 8.48%. The MPs were an excellent scattering center to improve light absorption, conductivity and charge transport.

3.3.2. Donor/quantum dot/acceptor. Quantum dots (QDs) were used as electron acceptor materials in OSCs due to their size dependent optical response, efficient multiple carrier generation and low cost.^190–192 In recent years, QDs were also used as the third component in OSCs. PbS,^193–198 CdSe^199–202 and CdTe^203,204 were the most widely used third components, but unfortunately, only a few works achieved higher PCE values. For example, OSCs that incorporated CdSe QDs showed an enhanced performance as compared with the binary system of P3HT/PC₆₁BM.²⁰¹ An efficiency of 3.05% was achieved, which was comparable to the record efficiency of shape-tailored CdSe-based solar cells. This promising performance of OSCs with a third component was attributed to the increased sunlight harvesting and better electron transport. Recently, Guo et al. incorporated CdSe QDs into the PCDTBT/PC₇₁BM active layer; they achieved a PCE of up to 6.94% (control device: 5.22%).²⁰²

Some special QDs were also used in OSCs and showed good performance.^205–209 For instance, Li et al.²⁰⁷ employed CdS QD-sensitized TiO₂ nanotube arrays (CdS/TNTs) as the third component in a P3HT/PC₆₁BM blend. The CdS/TNTs were synthesized using the chemical bath deposition (CBD) method and dispersed in the P3HT/PC₆₁BM layer, which not only increased the light absorption but also reduced the charge recombination. As a result, a high PCE of 3.52% was achieved for the OSCs (incorporating 2.7% CdS/TNTs), which was a 34% increase compared with the P3HT/PC₆₁BM devices. In 2013, Li et al.²⁰⁸ fabricated three-component OSCs using graphene quantum dots (GQDs) as the third component. The GQDs were synthesized from double walled carbon nanotubes (DCNTs) with a uniform size distribution (3–5 nm). The J_SC (24.6 mA cm⁻²) and PCE (4.35%) of the OSCs based on the P3HT/GQDs/PC₆₁BM blend were significantly improved compared with those of the P3HT/PC₆₁BM device (14.3 mA cm⁻² and 2.61%). This improvement was attributed to the enhanced absorption of the blended film and the suitable energy level structure of the GQDs (cascade energy heterojunction).

3.3.3. Donor/carbon-based nanomaterial/acceptor. Carbon-based nanomaterials such as carbon nanotubes (CNTs) and graphene were used as the third component in OSCs. They can provide conductive pathways in the active layer due to their extremely high charge carrier mobility.^210–212 CNTs were the most widely used carbon-based nanomaterials in OSCs as the third component. Nam et al.²¹³ dispersed CNTs in a P3HT/PC₆₁BM blend. The CNT networks provided hole or electron transport pathways and extra exciton dissociation centres in P3HT/PC₆₁BM blend devices and resulted in a 10% increase in the device PCE (3.8% vs. 3.4%). Some modified CNTs were also used in OSCs as the third component.^214–219 Lu et al.⁵⁰ employed N-doped multiwall carbon nanotubes (NCNTs) in a PTB7/PC₇₁BM blend. Compared with non-doped CNTs, NCNTs had selective charge transport properties and could help to more efficiently extract electrons hopping between different PC₇₁BM domains (much better electron transport).²¹⁴ Incorporated with 1.5% NCNTs, the OSCs had a higher J_SC and PCE compared with the control device. Lee et al.²¹⁵ combined indium phosphite (InP) QDs and NCNTs as the third component in a P3HT/ICBA blend (Fig. 12a). The QDs encouraged exciton dissociation by promoting electron transfer, while the NCNTs enhanced the transport of the separated electrons and eventual charge collection. Such a synergistic effect successfully improved the V_OC, J_SC, FF and PCE from 0.75 V, 10.7 mA cm⁻², 0.58 and 4.68% to 0.79 V, 11.9 mA cm⁻², 0.65 and 6.11%, respectively. Very recently, the same group⁵³ combined Au NPs and boron doped CNTs (Au:BCNT) as the third component in a PTB7/PC₇₁BM blend (Fig. 12b). The use of a small amount of Au:BCNT enhanced the photoexcitation, exciton dissociation into charge carriers, and charge transport to the electrodes. The multiple synergistic effects led to a very high PCE of 9.81%.

Fig. 12 (a) Chemical interaction for the QD:NCNT nanohybrid formation.²¹⁵ (b) Schematic chemical interactions for Au:NCNT (left) and Au:BCNT (right) hybrids.⁵³ Reprinted with permission from ref. 215. Copyright 2013, Wiley. Reprinted with permission from ref. 53. Copyright 2015, Wiley.

Graphene can promote more efficient charge-carrier transport compared with CNTs, and was also used in OSCs as the third component.^220–222 For instance, Jun et al.²²⁰ fabricated OSCs based on P3HT/N-doped reduced graphene oxide (N-RGO)/PC₆₁BM. The N-RGO was well dispersed in the polymer matrix and enhanced the electron transport properties due to its high carrier mobility and charge selectivity. Compared with the control device, the J_SC and PCE of the OSCs using N-RGO (0.5%) increased significantly from 10.4 mA cm⁻² and 3.2% to 15.0 mA cm⁻² and 4.5%, respectively.

3.4. Donor/organic nonvolatile additive/acceptor

3.4.1. Donor/polymer additive/acceptor. In addition to their use as a third light-absorbing component in OSCs, polymers can also be used as an additive to control the horizontal and vertical phase separation of the active layer, achieve better film-forming properties, stabilize the morphology and lower the cost (Chart 5, Table 5).^223–257

Block copolymers were used to improve the morphological stability of immiscible material blends, and were described as a compatibilizer in OSCs.^223,224 The interfacial energy between the immiscible components decreased, as the block copolymer compatibilizers were preferentially located at the interface of the immiscible materials to span the heterojunction itself, consequently ensuring good miscibility in the systems.²²⁵ In the last 5 years, various block copolymer compatibilizers were synthesized and used as the third component in OSCs.^226–241 Block copolymers with a rod–coil structure were the most widely used polymer additives. Han et al.²²⁶ used an asymmetric rod–coil block copolymer P3HT-b-PTMSM in a P3HT/PC₆₁BM blend to form a passivation layer between the microscopically phase-separated D/A domains. As shown in Fig. 13, the P3HT (rod) crystallized and phase-separated from the PC₆₁BM domains, while the PTMSM blocks (coil) were localized in the interfacial region between the P3HT and PC₆₁BM domains. Thermal condensation of the trimethoxysilyl groups of the PTMSM blocks yielded a SiO_x-like interfacial passivation layer to depress the local shunts and efficiently blocked the injection of holes into the leakage pathway, thus the recombination of charge carriers was suppressed. The incorporation of 5% P3HT-b-PTMSM in the active layer effectively enhanced the charge transport and increased the PCE from 2.6% to 3.7%. Renaud et al.²²⁷ employed P3HT-b-P4VP in a P3HT/PC₆₁BM blend to control the morphology of the active layer. After the addition of 3.8% P3HT-b-P4VP, the J_SC and PCE increased from 9.0 mA cm⁻² and 2.7% to 12.2 mA cm⁻² and 4.3%, respectively. The better performance of OSCs with compatibilizers was related to the formation of a well-optimized nanoscale structure that allowed for more efficient exciton dissociation and charge transport, by lowering the charge recombination and/or trapping. Sun et al.²²⁸ used PS-b-P3HT as a polymer additive in P3HT/PC₆₁BM based OSCs. The interaction between the polystyrene (PS) segments and PC₆₁BM was a major driving force to control phase separation in the P3HT/PC₆₁BM blend. The PS-b-P3HT induced a favourable active layer morphology with interpenetrating nanoscale domains, and the enhanced P3HT crystallinity and orientation facilitated hole transport within the active layer. Moreover, the addition of the PS-b-P3HT compatibilizer affected the PC₆₁BM segregation in the vertical direction, in which the PC₆₁BM accumulation near the substrate interface was reduced, with a corresponding increase in the middle region of the active layer. The PCE of the OSCs with compatibilizers was 4.11%, which was higher than that of the control device (3.25%).

Fig. 13 Generation of silica-containing passivation layers embedded in the interfacial region of P3HT/PC₆₁BM domains using a block copolymer: (a) the formation of a ternary blend with the addition of the P3HT-b-PTMSM block copolymer. (b) A schematic representation of the phase-separated morphology of a P3HT/P3HT-b-PTMSM/PC₆₁BM blend. (c) Thermal cross-linking of PTMSM domains to form a silica-containing layer in the P3HT/PC₆₁BM interface. Note that one among several P3HT chains contains a PTMSM tail.²²⁶ Reprinted with permission from ref. 226. Copyright 2012, Wiley.

Some liquid crystalline segments can also be used as one block in P3HT-based compatibilizers. Yuan et al.²²⁹ employed a self-assembled liquid crystalline rod–coil block copolymer P3HT-b-PTP in P3HT/PC₆₁BM-based OSCs; the PCE was increased from 3.24% to 4.03%. The self-assembly of liquid crystalline blocks (much stronger after thermal annealing to form the liquid crystalline states), the interaction between the P3HT block in the compatibilizer and the chains of the P3HT donor, and the liquid crystalline copolymer located at the interface of the donor and the acceptor induced a long range crystallinity of the P3HT domains and the formation of interpenetrating networks, which resulted in improved charge transport. For polymer/fullerene OSCs, to improve the miscibility between the donor and acceptor, block copolymers with fullerene as a pendant group were used as the compatibilizer in ternary blends. Chan et al.²³⁰ synthesized a block copolymer C₆₀-BCP and used it as the third component in OSCs. After the addition of 16.7% C₆₀-BCP, the PCE of the P3HT/C₆₀-based OSCs had a large enhancement from 0.48% to 2.56%. Mixing P3HT with C₆₀-BCP induced the formation of a self-organized nanostructure and enhanced the interchain interactions in the P3HT domain. C₆₀-BCP induced a moderate phase-separated morphology with crystalline P3HT and C₆₀ domains in the active layer, which was the key to a higher J_SC value (increased from 1.9 mA cm⁻² to 9.0 mA cm⁻²) because it benefited both charge separation and charge transport.

In addition to controlling the horizontal phase separation of the active layer, the polymer additives can also induce more efficient vertical phase separation; they can self-organize to form hole or electron buffer layers. Jung et al.²⁴² added PEG-C₆₀ to the P3HT/PC₆₁BM blend. The PEG-C₆₀ molecules migrated to the top surface of the P3HT/PC₆₁BM active layer and induced the segregation of PC₆₁BM near the top surface, which formed an ideal vertical distribution of PC₆₁BM in the active layer (Fig. 14). When 2.4 wt% of PEG-C₆₀ was used, the performance of OSCs was largely enhanced. The increase of V_OC upon the addition of PEG-C₆₀ was due to the generation of the interfacial dipole moment and the vacuum level shift of the metal cathode; the enhanced FF suggested that an ideal vertical morphology in the active layer was well developed. Furthermore, since the PEG-C₆₀ layer protected the active layer from oxygen penetration, as well as from the invasion of aluminium (Al) into the active layer during metal deposition, the stability of the OSCs was improved. Shi et al.²⁴³ used a block copolymer with a lithium ion (Li⁺) (P3HT-b-P3TEGT:Li⁺) in a P3HT/PC₆₁BM blend as the third component. Due to the spontaneous vertical migration of P3HT-b-P3TEGT:Li⁺ to the surface of the active layer during the spin-coating process and the formation of a self-assembled anode buffer layer (inverted structure), a vertical distribution of components in the active layer and an ordered arrangement of donor polymer chains were obtained at the same time. In addition, the self-assembled anode buffer layer could provide the interfacial modification between the active layer and the anode electrode, increase the WF of the anode electrode, increase the electrical conduction of the device, and consequently remarkably improve the device performance and stability. After the addition of 1.9% P3HT-b-P3TEGT:Li⁺, the PCE of the OSCs increased from 1.6% to 3.6%.

Fig. 14 Schematic illustration of a bulk heterojunction solar cell with PEG-C₆₀ as the buffer layer.²⁴² Reprinted with permission from ref. 242. Copyright 2011, Wiley.

Insulating polymers were another series of polymer additives and can improve the film-forming properties and control the film morphology.^251–257 Huh et al.²⁵¹ used poly(oxyethylene tridecyl ether) (PTE) surfactant as the third component in a PCDTBT/PC₇₁BM blend. A high PCE (6.1%) was obtained for OSCs by the use of a 6.7% PTE additive. This improvement in the PCE may be attributed to the increased selective flow of dissociated charge carriers; not only at the interfaces between the active layer and the electrodes, but also at the D/A interface. Huang et al.⁵⁸ added a small amount of high molecular weight PS to a p-DTS(FBTTh₂)₂/PC₇₁BM blend as the third component. The addition of PS increased the solution viscosity at low concentrations and led to an increase in the film thickness of the small molecule blend, control over the dewetting and uniformity of the film, and a desirable adjustment of the active layer morphology. Because of the favourable interaction between PS and the solvent, the film may retain the solvent for longer, which enhanced the crystallization of p-DTS(FBTTh₂)₂ and the evolution of individual phases of p-DTS(FBTTh₂)₂ and PC₇₁BM. The PCE of the small molecule OSCs increased from 7.1% to 8.2% after the use of 2.5% PS. Also for the optimization of small molecule OSCs, Zhou et al.⁵⁹ employed poly(dimethylsiloxane) (PDMS) as the third component in OSCs. The PDMS could control the morphology of the active layer and enhanced the PCE of DR3TBDTT/PC₇₁BM small molecule OSCs from 7.51% to 8.12%. The addition of an insulating polymer in a non-negligible amount showed benefits for OSCs, with the prospect of cost-effective module manufacture and mechanical stability.²²³ Ferenczi et al.²⁵⁵ used high density polyethylene (HDPE) in a P3HT/PC₆₁BM blend as the third component. After the addition of HDPE (up to ≈50%), the photovoltaic properties of P3HT/PC₆₁BM OSCs were not seriously decreased, which suggested the feasibility of cost-effective module manufacture. HDPE could assist the deposition process and reduce the occurrence of defects in the films, thus offering a high manufacturing yield over a large area.

3.4.2. Donor/nonvolatile small molecule additive/acceptor. Unlike the widely used volatile solvent additives, such as diiodooctane (DIO) and chloronaphthalene (CN), nonvolatile small molecule additives can stay in the active layer as the third component after the solvent evaporation (Chart 6). Nonvolatile small molecule additives can function as nucleating agents, adjust phase separation and improve device stability (Table 6). Some small molecule additives were used as a nucleating agent to control the crystallinity of donor materials.^258–261 Sharenko et al.²⁶⁰ used the nucleating agent DMDBS as the third component to control the crystallization of small molecule donors. The use of DMDBS could reduce the crystallite sizes of p-DTS(FBTTh₂)₂, but increased the quantity of crystallites. After the addition of 1% DMDBS, the J_SC, FF and PCE were significantly increased from 3.0 mA cm⁻², 0.32 and 0.72% to 9.4 mA cm⁻², 0.56 and 4.15%, respectively.

Some of the nonvolatile small molecule additives were used to control the horizontal and vertical phase separation.^262–276 As some representative examples, Kim et al.²⁶² employed a fractional amount of nonvolatile additives (a) in a P3HT/PC₆₁BM blend and the PCE was improved. This additive, selectively partitioned into P3HT due to its special hydrophobicity, effectively enhanced the phase separation between the electron donor and acceptor phases. Farahat et al.²⁶⁴ used a silicon based small molecule additive CP3MS in the SMDPPEH/PC₆₁BM blend. The PCE increased from 2.75% to 4.55% after the addition of 0.1% CP3MS. This nonvolatile additive was a morphological controller and also could form an ultrathin buffer interlayer between the Al cathode and the active layer by self migration (Fig. 15). Cheng et al.²⁶⁵ fabricated OSCs based on P3HT/TT-TTPA/PC₆₁BM, in which TT-TTPA had good miscibility with PC₆₁BM. During solvent annealing and thermal annealing, the small amount of TT-TTPA in the P3HT domains could move from the P3HT domains to the PC₆₁BM domains, thus increasing the phase purity of the P3HT domains that can undergo crystallization. After the addition of 3.8% TT-TTPA in the P3HT/PC₆₁BM blend, a better PCE (4.41%) was achieved relative to the binary blend (3.85%). Qin et al.²⁶⁶ employed CBP as the third component in a P3HT/PC₆₁BM blend. The phase separation of P3HT and PC₆₁BM was altered by adding a small amount of crystalline CBP (1.6%) due to the difference in the surface energy of P3HT and CBP. After the use of CBP, the PCE of the OSCs was increased from 1.43% to 1.85%. Zhang et al.⁶⁰ fabricated OSCs based on PBDTTPD-HT/BDT-3T-CNCOO/PC₇₁BM. The optimal content of BDT-3T-CNCOO (20%) resulted in the formation of favourable nanostructures for charge generation and collection. The OSCs with a third component yielded a high PCE of 8.40%, which was higher than the binary systems based on small molecule/fullerene (7.48%) and polymer/fullerene (6.85%). Recently, the same group increased the PCE of this type of OSCs to 10.5%.⁶⁴

Fig. 15 Schematic representation of the dual function of the additive CP3MS when combined with a post-annealing treatment.²⁶⁴ Reprinted with permission from ref. 264. Copyright 2014, Royal Society of Chemistry.

Nonvolatile small molecule additives were also used in all-polymer solar cells. All-polymer solar cells, which consist of a polymer donor and a polymer acceptor, have attracted considerable attention in the last decade due to their advantages over polymer/fullerene solar cells.^153,277,278 In 2014, Cheng et al.²⁶⁷ used a nonvolatile small molecule additive, PDI-2DTT, as the third component in a PBDTTT-C-T/PPDIDTT blend. PDI-2DTT had a similar chemical structure to that of the polymer acceptor PPDIDTT, which could suppress the aggregation of PPDIDTT and facilitate its mixing with the polymer donor PBDTTT-C-T. After the incorporation of 1% PDI-2DTT, the PCE of all-polymer solar cells was increased from 3.04% to 3.45%. Recently, the same group employed the inverted structure in these all-polymer solar cells with PDI-2DTT additives and achieved PCEs of 3.505% and 0.727% with a spin-coated process and a roll-coated process, respectively.²⁷⁹

In addition to adjusting the phase separation, nonvolatile small molecule additives had some other functions.^280–285 For example, Rumer et al.²⁸¹ used a dual function additive (the small molecule crosslinker DAZH) to simultaneously enhance the PCE and stability of SiIDT-BT/PC₇₁BM solar cells. A 2.2% bis-azide DAZH additive increased the device efficiency from 6% to 7%. Moreover, the OSCs with a third component preserved 85% of their initial PCE after 130 h heating at 85 °C (control device: 60% preserved). As shown in Fig. 16, the addition of DAZH greatly suppressed fullerene crystallization. Furthermore, the BHJ microstructure had a fibrillar character, as observed by AFM, as a result of frustrated fullerene aggregation and suppressed polymer skin formation at the cathode. Derue et al.⁸⁶ employed a bis-azide cross-linking reagent (BABP) as the third component in OSCs. This nonvolatile small molecule additive could stabilize the morphology of the active layer by preventing the fullerenes from diffusing and forming large crystallites. As shown in Fig. 17, long time thermal annealing induced PC₆₁BM aggregation and a large size phase separation of the PTB7/PC₆₁BM film, but the morphology of the PTB7/BABP/PC₆₁BM film was not changed after a long thermal annealing time. By adding 2% BABP, the PTB7/BABP/PC₆₁BM device yielded a PCE of 4.60% after 16 h of heating at 150 °C, preserving 80% of its original value. In sharp contrast, the PCE of the device using a PTB7/PC₆₁BM blend decayed drastically to 16% over 16 h heating at 150 °C. Zhang et al.²⁸² used a small molecule F4-TCNQ as the third component in PCDTBT/PC₇₁BM solar cells to increase the photoconductivity of the active layer. Under low doping concentrations (0.4%), the photocurrent in the PCDTBT/PC₇₁BM solar cells exhibited a considerable increase from 11.0 mA cm⁻² to 14.0 mA cm⁻² due to the improved hole mobility, leading to an ultimate enhancement of the PCE from 6.41% to 7.94%.

Fig. 16 Crosslinking and thermal ageing morphology: small molecule additives confer macrostructure thermal stability, through suppressed fullerene crystallization (central optical microscopy images), with the bis-azide DAZH additionally enhancing the microstructure through fibril formation (flanking AFM images). Such a “double” picture of morphology highlights the various donor–acceptor phases in bulk heterojunctions and the microstructural basis of stability.²⁸¹ Reprinted with permission from ref. 281. Copyright 2015, Wiley.

Fig. 17 Optical microscopy images (scale bar: 50 μm) of a PTB7/PC₆₁BM BHJ solar cell: (a) before thermal annealing and (b) after thermal annealing at 150 °C for 16 h without cross-linker. Numerous micrometer-sized PC₆₁BM crystals were observed. (c) Image of the same blend with the addition of BABP after thermal annealing at 150 °C for 16 h, indicating the absence of microcrystals.⁸⁶ Reprinted with permission from ref. 86. Copyright 2014, Wiley.

4. The reasons for the better performance of three-component OSCs

OSCs with a third component generally had better performance compared with their control devices. Some of them had higher V_OC, J_SC or FF values which enhanced the PCE; some of them had better morphology stability which increased the durability. Reasons for the improvements are summarized below.

A higher V_OC of the OSCs can be achieved in two ways using the third component. The first one was to introduce a second donor with a lower HOMO energy level (D2) or a second acceptor with a higher LUMO energy level (A2) as the third component in the OSCs, and to form some kind of ternary phase separation. As in type 1 in Fig. 18, two donors (or two acceptors) blended with each other (good miscibility and similar surface energies) and contacted with the anode (or cathode). In this type, some holes (or electrons) were extracted by the anode (or cathode) from the HOMO of D2 (or the LUMO of A2), which increased the V_OC of the OSCs. The other way was to introduce the third component which can self-organize to form the hole (or electron) transport interlayer between the anode (or cathode) and the active layer. The self-organized interlayer can significantly increase the V_OC because it can generate the interfacial dipole and shift the vacuum level of the anode or cathode (i.e. increase the WF of the anode electrode or decrease the WF of the cathode electrode).

Fig. 18 Schematic diagrams of ternary blend OSCs with two representative phase separation types.

A higher J_SC of OSCs with a third component was attributed to a stronger and broader absorption and efficient charge transfer, charge transport and charge extraction. Some kinds of third components (organic light-absorbing polymers and molecules) can broaden the absorption of the binary blend at the short wavelength or long wavelength region, since they have a strong absorption which is complementary with the absorption of the host donor and acceptor material. Some kinds of third components (metal nanomaterials and quantum dots) can enhance the absorption of the binary blend by LSPR effects or quantum effects. The broader or stronger absorption caused by these third components can increase J_SC. Charge transfer was also very important for J_SC. As shown in Fig. 18 (type 2), by the introduction of D2 (or A2) as a cascade energy material in OSCs and the formation of some kind of ternary phase separation, the third component was located at the D/A interfaces and did not contact with the anode (or cathode). Some holes were transferred by the A–D2–D1 (or A1–A2–D) route, while electrons were transferred by the D1–D2–A (or D–A2–A1) route, both of which induced efficient charge transfer and thus increased the J_SC of the OSCs. Efficient charge transport was another important factor in achieving a higher J_SC. Third components with a high hole or electron mobility (e.g. carbon-based nanomaterials) can obviously increase the charge transport properties as they provided extra charge transport pathways. Some organic third components can optimize the molecular arrangement (e.g. enhance the crystallinity of the donor or acceptor), purify the donor or acceptor domains so that the charge recombination traps were reduced, and optimize the D/A interfaces. The charge transport properties and J_SC can be significantly improved after the use of these third components. The efficient vertical phase separation induced by some third components can also enhance J_SC due to the improved charge extraction at the right electrodes (holes extracted by the anode and electrons extracted by the cathode).

Some third components (e.g. block polymer compatibilizers) can improve the FF of OSCs as they can increase the hole or electron mobility due to the formation of more efficient charge transport pathways, balance the hole and electron transport, and/or induce efficient charge extraction at the right electrodes. After the use of these third components, the energy loss in the charge transport and extraction of the OSCs was significantly reduced, leading to an increase in the FF. Given the fact that the typical exciton diffusion length in a disordered blend layer is about 10 nm, the third component can control the phase separation of the donor and acceptor to ca. 10 nm to reduce the energy loss during exciton diffusion, which can increase the FF.

Better morphology stability was achieved in OSCs with a third component because some third components can suppress the aggregation of the fullerene acceptor and freeze the morphology of the active layer. These third components had a supramolecular interaction with the fullerene acceptor, or crosslinked the fullerene acceptors before their excessive aggregation. Thus, these OSCs with a third component had much better morphology stability and showed great potential for future industrial manufacture.

5. Summary and outlook

Adding a third component is an emerging and promising strategy towards high-performance OSCs and presents some advantages: broader and stronger absorption, more efficient charge transfer, more efficient charge transport pathways, better charge extraction at the electrodes and improved stability. Various types of third components such as light-absorbing small molecules or polymers, fullerene or non-fullerene acceptors, metal or carbon-based nanomaterials, quantum dots, and polymer or small molecule nonvolatile additives were added to OSCs. In order to promote this research field and achieve a higher performance of OSCs, some fundamental research may deserve further attention in the future: (a) the synthesis of new light-absorbing materials with a strong absorption, which is complementary with the absorption of the host donor and acceptor materials; (b) the control of the ternary phase separation to enhance both V_OC and J_SC; (c) the introduction of some molecular interactions to build ideal nanostructures in the active layer (e.g. a self-organized interlayer, unrestricted charge transport pathways, frozen morphology); (d) characterization of the ternary phase separation; and (e) a theoretical study on the device physics of OSCs with a third component. So far, OSCs with a third component exhibited PCEs approaching 10%, and have the potential for even higher efficiency.

Acknowledgements

We thank the NSFC (91433114, 51261130582, 21025418), the 973 Program (2011CB808401) and the Chinese Academy of Sciences for financial support.

Notes and references

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