Nanocrystals as performance-boosting materials for solar cells

Abstract

Nanocrystals (NCs) have been widely studied owing to their distinctive properties and promising application in new-generation photoelectric devices. In photovoltaic devices, semiconductor NCs can act as efficient light harvesters for high-performance solar cells. Besides light absorption, NCs have shown great significance as functional layers for charge (hole and electron) transport and interface modification to improve the power conversion efficiency and stability of solar cells. NC-based functional layers can boost hole/electron transport ability, adjust energy level alignment between a light absorbing layer and charge transport layer, broaden the absorption range of an active layer, enhance intrinsic stability, and reduce fabrication cost. In this review, recent advances in NCs as a hole transport layer, electron transport layer, and interfacial layer are discussed. Additionally, NC additives to improve the performance of solar cells are demonstrated. Finally, a summary and future prospects of NC-based functional materials in solar cells are presented, addressing their limitations and suggesting potential solutions.

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1. Introduction

Nanocrystals (NCs), sometimes termed as quantum dots (QDs) or nanoparticles (NPs), with a size less than 100 nm, are popular worldwide and have been paid extensive attention owing to their outstanding photoelectric properties. In the energy field, photovoltaic devices based on semiconductor NCs are among the most potential systems with high power conversion efficiency (PCE), good stability, low cost, facile fabrication, and other advantages.^1–16 In recent years, NCs have been widely used as light harvesting layers in high-performance solar cells. Semiconducting NCs such as PbX (X = S, Se, and Te), CdSe, and CuInS₂ have been suggested as excellent light harvesters in third-generation solar cells.^{11,14,15,17–22} Additionally, perovskite NCs are also good alternatives for light harvesting in solar cells.^{12–14,16,23–28} Besides active layers (light harvesting layers), functional layers based on NCs, mainly including hole transport layers (HTLs),^29–99 electron transport layers (ETLs)^100–173 and interfacial functional layers (IFLs),^174–221 are of great importance in solar cells. Notably, NC additives also boost the performance of solar cells through defect passivation, plasmonic effect, solar concentration, light up-conversion and down-conversion/shifting, light scattering and reflection, and heat sinks.^222–258

To efficiently transport charge in solar cells, HTLs and ETLs must have suitable energy levels matching the active layers such as dyes, Si, Pb-based light harvesting layers, and perovskite films. For example, to fabricate high-performance perovskite solar cells (PSCs), the work function of HTLs and ETLs should have hole and electron transport layers aligned with the valence band edge and conduction band edge of a perovskite. As is well known, the energy levels of NCs can be easily controlled and adjusted by changing their sizes, ligands, and dopants during their synthesis process, which is favourable to satisfy the energy alignment. Meanwhile, the easily-accomplished modifications in NCs mentioned above also favour high mobility to allow carrier transport and form a more effective current circulation path. Furthermore, the transmittance of HTL or ETL based on NCs can be improved during synthesis processes, which leads to less light loss and thus, higher performance of solar cells. Consequently, NCs are a good choice as HTLs and ETLs for efficient solar cells and have gained much attention.

Some researchers found that inserting different IFLs in solar cells could promote the power conversion efficiency (PCE) and stability of the devices. Good IFL can modulate the formation of adjacent layers (especially the perovskite layer in PSCs), optimize energy alignment, and impede charge recombination. As multi-functional IFL, NCs have been extensively investigated due to their compatible properties, such as appropriate morphology for compact, smooth films, gradual energy level to transfer carriers, and self-stability to protect the devices.

Besides acting as a separate functional layer in efficient solar cells, NCs have also been used as additives to improve active layer quality, accelerate carrier transfer, convert infrared or ultraviolet light to visible light, scatter and reflect light, sink heat, and other functions.^222–258 The addition of NCs could enlarge the grain size of perovskite film, reduce the recombination, enhance charge extraction, improve the charge mobility, broaden the spectral response range, and so on.

In this review, functional materials based on NCs for high-performance solar cells are summarized. NCs as HTL, ETL, IFL, and additives for NC-light-harvestor solar cells (NC-LHSCs), PSCs, organic solar cells (OSCs), Si solar cells, and dye-sensitized solar cells (DSSCs) are successively analyzed. In Section 2, NCs as charge transport layers are analyzed in detail. This can provide a deep understanding of the extensive application of NCs in solar cells. As an important functional layer, IFL based on NCs is summarized in the next section. Section 4 describes NCs as efficient additives in different functional layers due to their small sizes and excellent photoelectric properties. Finally, we discuss the existing challenges of NCs for boosting the performance of solar cells and provide some feasible suggestions on these issues, expecting to improve the performance of solar cells based on NCs.

2. Nanocrystals as charge transport layers

In this section, we focus on NCs-based charge transport layers for solar cells, mainly including top/bottom HTL and ETL in n-i-p and p-i-n solar cells.

2.1 Nanocrystals as HTL

As known to us, solar cells work as follows: absorption of sun light, generation and separation of hole–electron pairs, transport of holes through HTL and electrons through ETL, and current produced by the flow of electrons through external circuits. Solar cells using NCs as HTL are not exceptional, and the typical structures of solar cells based on NC HTL are shown in Fig. 1. According to the position of HTL, we call HTL between the ITO/FTO and active layer as bottom HTL while the HTL between metal electrode and active layer as top HTL.

Fig. 1 Typical structure of solar cells based on top (left) and bottom (right) NC HTLs.

2.1.1 Recent NC HTL. Semiconductor NCs have shown great potential in photoelectronic devices due to their excellent properties. The advances of NC HTLs for solar cells are listed in Table 1, including the size of NCs, device structure, PCE, and stability of solar cells.

Table 1 Recent advances in selected NC HTLs for solar cells (EDT = 1,2-ethanedithiol, relative humidity = RH)

	NC	Active layer	PCE (%)	Long-term stability	Ref.
Top HTL	PbS-EDT	CH3NH3PbI3	7.88	8% decay after 2 days	29
		PbS	11.6	Null	30
			10.4	100%, after 103 days in the dark under ambient conditions	32
			9.44	∼100%, after 4 months in ambient air	33
			13.2	Null	37
			10.4	①96%, after baking on a hotplate in air for 120 min; ②97%, after 60 min oxygen plasma treatment	38
			13.3	Null	39
			10.5	Null	40
			13.0	Null	41
		PbS–PbI2	11.2 ± 0.22	Nearly no drop of PCE after 180 h under continuous heating at 85 °C in ambient air	44
			11.29	>90, after 600 h under ambient condition	45
		PbS:F	12.7	Null	46
		PbX2–PbS	8.16	Null	47
			10.35	Null	48
			10.6	Null	49
		PbS–PbX2–KI3	12.1	94%, after 20 h continuous operation in air	50
		PbSe–PbS	1.24 (infrared PCE)	∼95%, after 25 days in air	51
		PbSe–PbI2	10.4	90%, after 30 days in ambient condition	51
		CsMAFA–PbS	11.3	96%, after 1200 h shelf storage	53
		MAPbI3–PbS	9.5	95%, after 2 months in ambient environment	54
		Sb2Se3	6.5 (certified)	Null	55
		KPbS	12.6	83%, after 300 h under continuous operation at MPP in ambient air	56
	CuInS2/ZnS core/shell	MAPbI3	8.38	Null	57
	CdZnSe@ZnSe	CdZnSe@ZnSe	8.65	Null	58
	Cu2ZnSnSe4	MAPbI3	9.72	Null	59
	Cu2ZnSnS4		10.72
	SnS	(CsPbI3)0.05(FAPbI3)0.79(MAPbI3)0.16(PbI3)0.03	13.7	①99%, after 1000 h storage in air; ②75%, after 500 h under continuous 1 sun illumination in a N2 atmosphere at 25 °C	60
	CuInSe2	MAFAPbClBrI	12.8	78%, after 96 h in air	61
	Ag–In–Ga–S	CsPbBr3	8.46	96.1%, after 240 h in air	62
	Cu12Sb4S13	CsPbI3	10.02	94%, after storage in ambient air for 360 h	63
	Cu2SnS3	Cs0.05(MA0.17–FA0.83)0.95Pb(I0.83Br0.17)3	13.01	90%, after 1200 h in an ambient atmosphere	64
	CuGaS	(FAPBI3)1−x(MAPbBr3)x	17.56	81%, aging for 30 days	65
	CuInS2	(FAPBI3)1−x(MAPbBr3)x	18.81	91%, aging for 30 days	66
	NiO	MAPbI3	6.2	Null	67
		MAPbI3NCs	10.89	Null	68
	Ti-doped MoO2	MAPbI3	15.8	∼95%, after 15 days with 50–70% RH	70
	Co3O4	MAPbI3	13.27	Up to 2500 hours under ambient conditions	71
	Cu2O	Cs0.05FA0.81MA0.14PbI2.55Br0.45	18.9	80%, after 30 days in air with ∼30% RH	72
	CuCrO2	Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3	16.68	88%, after 500 h at the MPP under one sun and a N2 atmosphere	73
	CuGaO2	MAPbI3	18.51	>85%, after 30 days at 25 °C with 30–55% humidity without encapsulation	74
	CsPbI3	MAPbI3	17.0	Null	75
Bottom HTL	PbS	MAPbI3	7.5	Null	76
	Cu2ZnSnS4	MAPbI3	15.4	Null	77
			6.02	87%, after 43 days in N2 atmosphere	78
	Ni–NiO core–shell	P3HT	0.86	Null	79
	NiOx	Cs0.08(MA0.17FA0.83)0.92Pb(I0.83Br0.17)3	12.8	Null	81
		MAPbI3	16.1	90%, after 60 days in air at RT	84
		Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br0.1)3	18.6	>80%, after 1000 h	85
		MAPbI3	17.60	93%, after 30 days	86
			1.02 cm^2: 18.49 (rigid) and 15.89 (flexible)	90%, after 500 h in the thermal aging test (85 °C, 85% RH)	87
	Cu:NiOx		15.01 (flexible, >1 cm^2)	86%, after 1 month in an ambient environment at 25 °C with about 40% humidity	88
			18.3	Null	89
	(Li,Cu):NiOx	MAPbI3−xClx	20.80	95%, after 60 days of storage	90
	NiOx	CsPbIxBr3−x	16.1	85%, after 350 h light soaking	91
	CuO	MAPbI3	15.3	Null	92
	CuCrO2		19.0	∼90, after 30 days in an Ar-filled dry glove box and continuously irradiated by a UV optical fiber with 5 mW cm^−2	94
	CuGaO2		15.6	Null	95
	NiCo2O4	MAPbI3−xClx	18.23	∼90%, after 500 h illumination at AM 1.5G	96
	ZnCo2O4	PTB7-Th:PC71BM	9.37	>60%, after 60 h in ambient environment with 50% RH without capsulation	98
		MAPbI3−xClx	18.14	>60%, after 110 h in ambient environment with capsulation and under continuous 1 sun illumination soaking
	In:CuCrO2	Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3	20.54	∼90%, after 800 h of continuous radiation in glovebox	99

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Lead chalcogenide NCs, especially PbS, have been massively used as efficient top HTL in NC-LHSCs.^29–56 Additionally, other metal chalcogenide NCs are also excellent choices as top HTL for high-performance perovskite solar cells (PSCs), mainly containing CuInS₂/ZnS (core/shell),⁵⁷ CdZnSe@ZnSe,⁵⁸ Cu₂ZnSnS₄/Se₄,⁵⁹ SnS,⁶⁰ CuInSe₂,⁶¹ Ag–In–Ga–S,⁶² Cu₁₂Sb₄S₁₃,⁶³ Cu₂SnS₃,⁶⁴ CuGaS,⁶⁵ and CuInS₂.⁶⁶ Metal oxide NCs of NiO have shown potential for hole transport on top of the light harvesting layer in PSCs and Si solar cells.^67–69 Besides NiO, some other metal oxide NCs such as MoO₂,⁷⁰ Co₃O₄,⁷¹ Cu₂O,⁷² CuCrO₂,⁷³ CuGaO₂,⁷⁴ and CsPbI₃ (ref. 75) have been used as efficient top HTL for PSCs due to their excellent properties. It can be noticed that when PbS NCs are used as top HTL in solar cells, the light harvesting layers are mostly based on the PbS/Se family. There are not many perovskite used as light harvest materials when PbS/Se is used as top HTL. Only MAPbI₃ was applied in 2015, and 7.88% PCE was gained.²⁹ We suppose this is because the PbS NCs with long carbon chains or 1,2-ethanedithiol are not good at charge transport. If high-temperature annealing is adopted to enhance the conductivity of PbS, the perovskite underneath will be destroyed. This encourages us to regulate the synthesis or modification of NCs with less or even no charge transfer inhibition capture and improve their electronic properties.

Similar to top HTL, NCs can also be applied as bottom HTL for high-performance solar cells. NCs of PbS,⁷⁶ Cu₂ZnSnS₄,^77,78 NiO_x and its doped derivatives,^79–91 CuO,⁹² and ternary oxides of CuCrO₂,^93,94 CuGaO₂,⁹⁵ NiCo₂O₄,^96,97 ZnCoO₄,⁹⁸ and doped ternary oxide In:CuCrO₂ (ref. 99) have offered excellent hole transport ability in PSCs and PTB7-Th-based organic solar cells (OSCs).

Ever since demonstrated as efficient HTL by Luther et al. in 2008,²⁴⁸ PbS NCs with a 1,2-ethanethiol (EDT) ligand (named as PbS-EDT or EDT-PbS) have been massively applied for hole transport in solar cells. From the established NC HTLs listed in Table 1, we can find some regular patterns: (i) as top HTL, EDT-linked sulphides were mainly used in NC-LHSCs but were not popular in PSCs. This is because EDT will strongly attack the perovskite materials in the n-i-p device and decrease the device performance.⁶¹ (ii) As bottom HTL, oxides are widely used and sulphides are rare. We speculate that it is (iii) NiO_x with high hole transport quality mostly used to transport holes in PSCs.

The previous light harvesting NC layers are mainly based on the Pb-based chalcogenide family or their mixture. NC-LHSCs using NCs as HTL have gained high PCE above 13%,^45,50 but this is significantly lower than that of the theoretical value.¹¹ Some strategies were also adopted to modify the light-harvesting NCs, which is not the keynote in this review. The current relatively low PCE of NC-LHSCs may be on account of non-radiative recombination resulting from the high density of surface traps due to some intrinsic properties of NCs, such as high surface-to-volume ratios. Using NC top HTL, the PCE value can be improved. Some oxides, for instance, NiO, Ti-doped MoO₂, Co₃O₄, Cu₂O, CuCrO₂, and CuGaO₂, have acted as excellent top HTL in high-performance PSCs. As shown in Fig. 2a, SnS NCs prepared by the one-pot hot-injection method were utilized as top HTL in efficient and stable (CsPbI₃)_0.05(FAPbI₃)_0.79(MAPbI₃)_0.16 PSCs. The high PCE mainly resulted from good surface coverage and an excellent hole extraction ability demonstrated by Nyquist plots. Additionally, SnS-based PSC presented better air stability than the 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD)-based device. Surface-modified Cu₂O NCs boosted the efficiency of PSC to 18.9% with distinctly better stability than the reference device based on spiro-OMeTAD.⁶¹ One important reason is the difference in hydrophobicities illustrated by water contact angles in Fig. 2b. Ternary oxide CuGaO₂NCs with promising photoelectronic properties boosted the n-i-p PSCs with higher PCE and stability than spiro-OMeTAD (Fig. 2c).⁷⁵

Fig. 2 (a) Evolution of PCE over time of the unencapsulated SnS-based PSCs in the ambient air of ∼30–50% humidity, Nyquist plots at 0.9 V forward bias measured in the dark, forward, and reverse J–V curves of champion PSCs based on SnS. (b) Schematic view of PSC configuration, device performance durability of PSCs based on different HTLs in ambient air for 30 days, and water contact angles of unmodified and modified Cu₂O. (c) Schematic illustration of the crystal structure of CuGaO₂, device architecture of a regular PSC based on CuGaO₂, standard deviations of PCEs to evaluate reproducibility by statistics of 50 devices based on CuGaO₂ and spiro-OMeTAD. (a) Adapted with permission.⁶¹ Copyright 2019, American Chemical Society. (b) Adapted with permission.⁷³ Copyright 2019, Wiley-VCH. (c) Adapted with permission.⁷⁵ Copyright 2017, Wiley-VCH.

In n-i-p type solar cells, NiO_X and its doped family oxide NCs are the most popular NC HTLs because of their facile synthesis and outstanding photoelectronic properties.^86,101 Meanwhile, sulphides of PbS and Cu₂ZnSnS₄ together with multi basic oxides of CuCrO₂, CuGaO₂, NiCo₂O₄, ZnCo₂O₄, and In:CuCrO₂ are valuable substitutes for NiO_X family. Ligand-free NiO_x NCs in ethanol (E-NiO_x) are spin-coated onto a substrate to form a smooth and compact NiO_x film that has good hole extraction capability. As seen in Fig. 3a, this E-NiO_x bottom HTL can be used both in rigid and flexible PSC, producing high PCE and stability. Similar to top HTL, ternary oxide NCs like CuCrO₂ were also utilized as bottom HTL for high-performance solar cells. The low-temperature solution-processed CuCrO₂ NCs provide suitable electronic structure, charge carrier transport properties, and greater UV light-harvesting, demonstrating its potential as an efficient HTL for highly efficient and photostable n-i-p PSCs (Fig. 3b).

Fig. 3 (a) Energy level diagram of the materials used in the device, J–V curves of rigid and flexible PSCs, normalized PCE. (b) UV-vis spectra of the CuCrO₂ and NiO_x layer with optimum thickness with energy level in the inset. Adapted with permission.^87,94 Copyright 2018, Wiley-VCH.

2.1.2 Advantages of NC HTL. One important inherent advantage of semiconductor NCs is their bandgap variation along with modification. The conduction band, valence band, and Fermi level of PbS NCs can be modified by changing surface ligands (Fig. 4), thus enhancing the performance of solar cells.²⁵²Fig. 4 shows that the energy level of PbS NCs is easily changed in a large range by different ligands, facilitating energy matching with the light-harvesting layer and other functional layers. Compared with mostly used organic HTLs of spiro-OMeTAD, poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), inorganic NCs show higher stability with good hole transport ability, suggesting great potential for boosting solar cell performance. Equally important, facile synthesis and low cost would be helpful for the commercialization of new-generation solar cells.

Fig. 4 Energy band position of PbS NC films for different surface ligands. Adapted with permission.²⁴⁹ Copyright 2014, American Chemical Society.

2.1.3 Potential NC HTL. To further develop more strategies for fabricating high-performance solar cells and boost the device PCE and stability, we should find more potential NCs for efficient hole transport in solar cells. According to the previous reports and our understanding, excellent NC HTLs should satisfy the following requirements: (i) matched energy level alignments with other functional layers; (ii) high hole mobility and conductivity; (iii) enhancing the quality of adjacent layers; (iv) intrinsic resistance to heat, light and water; (v) convenient fabrication with low cost; and (vi) high transmittance for bottom HTL and reflectivity for top HTL. NCs have many good properties such as easily-controlled energy level, excellent spreading and filling ability due to their small sizes, stable intrinsic structure, skilled synthesis process, and variant surface ligand. Considering the above rules and the advantages of NCs, besides the existing NC HTLs, some other p-type sulphide and oxide NCs have huge potential for efficient and stable solar cells. Additionally, semiconductor NCs with enhanced hole-transporting ability by p-type doping are also good alternatives.

2.2 Nanocrystals as ETL

2.2.1 Recent advances of NC ETL. As important as HTL, ETL is also of great significance for high-performance solar cells to transport electrons and block holes, and it acts as well as a trap passivating layer and water/oxygen preventing layer. The typical structure of solar cells based on NC ETL is shown in Fig. 5.

Fig. 5 Structure of solar cells based on top (left) and bottom (right) NC ETLs.

ZnO is good at electron transport in photoelectrical devices due to excellent properties such as bandgap (3.3 eV), low cost, high electron mobility (∼10⁻⁵–10² cm² V⁻¹ s⁻¹), and matching energy levels.^100,101 ZnO NCs with different sizes have been used as efficient top ETL in NC-LHSCs, OSCs, and PSCs.^{43,84,85,91,101–108} TiO₂ and SnO₂NCs are as popular as ZnO for electron transport due to their outstanding photoelectronic properties of high mobility and conductivity.²⁵⁰ Certainly, these oxide NCs can be modified by doping, ligand changing, and other strategies. Tetrabutylammonium hydroxide (TBAOH)-capped metal oxide NCs for SnO₂ also extend to TiO₂, ITO and CeO₂NCs as top ETL for PSCs.¹⁰⁸ TiO₂NCs have always been as the top ETL for PSCs.^109–111 CeO_x,¹¹² In₂O₃ and its Sn doped derivative formed bilayer ETL,¹¹³ and CdSe¹¹⁴ were also used for high-performance n-i-p PSCs. As efficient bottom ETL, modified TiO₂ by Sn, Al, Co, Cu, and N doping and N, F and S co-doped graphene NCs were widely used in dye-sensitized solar cells (DSSCs) based on N719 and N3 ({cis-Ru(H₂dcbpy)₂(NCS)₂, H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl }).^115–120 Additionally, TiO₂, CdS NCs-modified TiO₂, and Nb-doped TiO₂NCs were applied for electron transport in PSCs with high PCE and stability.^121–123 Meanwhile, TiO₂NCs have been widely applied in n-i-p solar cells using chalcogenide NC as light harvesters, such as CdSe, CdS, and PbS.^124–130 Similarly, ZnO NCs were used as ETL for n-i-p PbS-based NC-LHSCs,^{30,32,35,39,40,44,45,58,131–145} PSCs,¹⁴⁶ and OSCs.^147–149 SnO NCs as ETL for DSSCs,^150–156 for OSCs,^157–159 and for PSCs,^27,160–168 have been demonstrated. Additionally, a bilayer of SnO₂/TiO₂ (ref. 169) and SnO₂/InP–ZnS¹⁷⁰ were used for PSCs and DSSCs. Ni-doped Co₃S₄ and Co₄S₃ and ternary sulphide NCs of NiCo₂S₄ and Zn₂SnO₄ have been used for DSSCs,^171,172 while doped SrSnO₃ NCs for PSCs.¹⁷³ NCs have been used as IFL for NC-LHSCs,^{49,141,174–179} OSCs,^180–185 DSSCs,^186–195 PSCs,^196–212 and Si solar cells.^213–220 FAPbBr₃ perovskite NCs have been used as a multifunctional luminescent-downshifting passivation layer for GaAs solar cells.²²¹

From the reported literature listed in Table 2, selenide NCs of CdSe, and doped sulfide NCs of Ni:Co₂S₄, Ni:Co₄S₃, and Ni:Co₂S₄ have shown their potential, suggesting more substitutes for traditional NC ETL. The advantages of NC ETL and some potential NCs suitable as ETLs are discussed in the following sections.

Table 2 Recent advances in NC ETLs (bathocuproine = BCP)

	NC	Active layer	PCE (%)	Long-term stability	Ref.
Top ETL	ZnO	PbS–PbX2	10.35	Null	43
		PCDTBT:PC61BM	2.66	Null	100
		CsPbI3NCs	13.1	80%, after 40 h under 25 °C with 30–40% RH	103
		MAPbI3	17.2	66%, after 120 days stored in air	105
		MAPbI3	16.1	90% after 60 days in air at RT	84
		Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br0.1)3	18.6	>80% after 1000 h	85
		CsPbIxBr3−x	16.1	85%, after 350 h light soaking	91
	Ag modified BCP:ZnO	MAPbI3	15.5	Null	106
	In:ZnO	MAPbI3	16.2	85% after 460 h of light soaking	107
	TBAOH-SnO2	MAPbI3	18.77	90% after aging at 45 °C for 240 h, followed by 65 °C for 240 h and 85 °C for 240 h	108
	TiO2	Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3	20.5	∼90% after 350 h MPP tracking test	109
	Carbide-TiO2	CsPbI2Br	14.8	①>94%, after 1000 h at 85 °C in dark under N2; ②>90%, after 1000 h at 60 °C under continuous illumination	110
	CeOx	MAPbI3	16.7	100%, after 200 h in air with 30% RH	112
	Sn:In2O3/In2O3 (bilayer)	Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3	20.65	①91.9% after 69 days under 85 °C;②91.8% after 2000 h, 12 h continuous 1 sun illumination and then 12 h interval in the dark	113
	CdSe	MAPbI3	15.1	Null	114
Bottom ETL	Sn:TiO2	N719	6.24	Null	115
	Al:TiO2		4.27	Null	116
	Co:TiO2		4.85	Null	117
	Ti0.94Cu0.06O2	N3/electrolyte	6.51	Null	118
	TiO2 modified by N, F and S, co-doped graphene NCs	N719	11.7	∼85.5, after one month	120
	TiO2	Cs–FA–MA mixed cation perovskite	19.03	84%, after 30 days	121
	CdS NCs-modified TiO2	MAPbI3	8.16	Null	122
	Nb-doped TiO2	Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3	18.97	Null	123
	TiO2	CdS	4.59	Null	125
	Cu2O doped TiO2	CdSe/CdS/ZnS/electrolyte	3.01	Null	126
	TiO2	Mn–CdSe	3.55	Null	128
	TiO2	CdSe/ZnS/SiO2/Cu2S	4.00	Null	129
	Cl@ZnO	PbS	11.6	Null	30
	ZnO	PbX2–PbS	9.5	95%, after 2 months in ambient environment	33
	MgZnO	PbS	10.4	100%, after 103 days in the dark under ambient conditions	32
	ZnO	Lead halide-PbS	12.7	Null	46
	ZnO	PbS	13.1 ± 0.1	Null	37
	ZnO	PbS NC	12.44	Null	140
	Cs-ZnO	PbS	10.43	97%, after 3 months under 20 °C and 30% RH without any encapsulation	144
	ZnO	CsPbBr3–CsPb2Br5	6.81	No detectable decay after 100 days under 25 °C and 45% RH	146
	ZnO	PTB7-Th:PC71BM	12.02	Null	147
	Na–ZnO	p-DTS(FBTTh2)2:PC70BM	9.2	90%, after 28 h	149
	SnO2	N719	3.2	Null	151
	Ni:SnO2		3.6
	Zn:SnO2		4.2
	SnO2	Eosin-Y	3.89	Null	155
	Sn0.92O2:Sb0.08		4.15	Null	156
	SnO2	PBDR-T & ITIC-4	12.023	①>90%, up to 75 h after exposure to an ambient atmosphere with continuous illumination; ②>95%, after storage in N2 for 200 h with continuous illumination	157
		PM6:Y6	14.9	81%, after 15 days stored in the air at RM without encapsulation	158
		PTB7-Th:PC71BM	10.30	Null	159
		PM7:ITC6-4F	13.93	Null
		PM6:Y6	15.38	94.3%, after 100 h light aging
	Cl@SnO2	CsPbI3NCs	14.5	80%, without encapsulation under 1-sun light soaking and 50% relative humidity for 8 h	27
	Ga:SnO2	Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3	18.18	Null	161
	SnO2	CsPbI3/FAPbI3	16.07	96%, after 1000 h in ambient storage	162
		Csx(MA0.17FA0.83)(100−x)Pb(I0.83Br0.17)3	17.92	89%, after 2500 h stored under 20 ± 5% RH	163
		MAPbI3	13.90 (flexible device)	Null	164
		(CsPbI3)0.04(FAPbI3)0.82(MAPbBr3)0.14	20.34 ± 0.5	90%, after 720 h storage	165
		Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3	20.79	Null	167
		FAPbI3	25.39 (0.0803 cm^2)	①80% after 1000 h in ambient air at 25% RH and 25 °C without sealing; ②70.5% after 700 h light soaking; ③95% (encapsulated cell) after 100 h MPP tracking and 2 h dark recovery under ambient conditions	168
			23.3 (1 cm^2)	Null
			21.7 (20 cm^2)	Null
			20.6 (64 cm^2)	Null
	SnO2/TiO2 (bilayer)	CsPbI2Br	15.86	①∼95%, after 1 month storage in N2 glovebox without encapsulation; ②>80%, after 1 month stored under RT and 20–30% RH without encapsulation	169
	SnO2/InP–ZnS (bilayer)	PM6:Y6	15.22	>80%, after 500 h in N2 without encapsulation	170
	Ni:Co3S4	N719	6.01	Null	171
	Ni:Co4S3		6.82
	NiCo2S4		7.43
	Zn2SnO4		4.9 ± 0.2		172
	Y:SrSnO3	FA0.85MA0.15Pb(I0.85Br0.15)3	19.0	91%, after 1000 h stored in N2	173

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2.2.2 Advantages of NC ETL. Compared with common organic ETLs such as fullerenes (C₆₀/C₇₀) and phenyl-C61-butyric acid methyl ester (PCBM), semiconductor NCs can easily overcome the shortcomings of poor stability, high cost, and unsuitable energy levels. In detail, PCBM film degrades at 85 °C, indicating its thermal instability.¹¹⁰ The cost of synthesis and purification for organic electron transport materials is higher than most semiconductor NCs.²⁵² Moreover, the semiconductor NCs can offer adjustable energy levels by easily controlling the size. The above advantages strongly demonstrate that semiconductor NCs are a very good choice as efficient ETL in high-performance solar cells.

2.2.3 Potential NC ETL. Thanks to their unique application advantages in solar cells, NC ETLs have been widely used, boosting the devices to higher PCE and stability. More oxide and chalcogenide NCs as well as their doped congeners are anticipated alternatives. As we know, TiO₂NCs always act as splendid ETL in solar cells. The properties of TiO₂NCs can also be improved by doping, enhancing the device performance. As shown in Fig. 6, many elements were successfully doped in TiO₂ as ETL for PSCs. Meanwhile, more elements are potential dopants for high-quality TiO₂ and this suggests that there is much room for doped-TiO₂NC ETL. Additionally, we reasonably think this strategy is also applicable to other oxide NCs like SnO₂ due to its comparable nature with TiO₂.

Fig. 6 Summary of the explored and potential elements as dopants in TiO₂-based electron transporters for PSCs in the periodic table, as in 2017.²⁵³ Copyright 2017, Wiley-VCH.

3. Nanocrystals as IFL

Besides HTL and ETL, another essential layer is the interfacial functional layer (IFL). Interfaces play a non-ignorable role in improving both PCE and stability through many paths. The position of a typical IFL is shown in Fig. 7. The IFL can be located at different positions in the solar cells to make different contributions. NCs IFL can enhance light absorption, decrease carrier recombination, improve charge transport ability, reduce impedance, convert ultraviolet (UV) or near-infrared (NIR) light to visible light, enhance optical transmission, decrease trap states of the active layer, and so on. Table 3 shows the recent advances of NC IFLs, and the detailed discussion is as follows.

Fig. 7 Typical position sketch of interfacial layers in solar cells.

Table 3 Recent advances in selected NCIFLs

	IFL and its adjacent layers	PCE (%)	Functions	Ref.
PbCdS	TiO2/PbCdS/CdS	3.35	Enhance light absorption; suppress interfacial recombination	174
CdS + amine/ZnS	TiO2:Al^3+/CdS + amine/ZnS/electrolyte/CuS	2.4	Decrease electron–hole recombination	175
CuxS	CIZS + mp-TiO2/electrolyte/CuxS/electrode	1.13	Null	176
CdSe	ZnO/CdSe/PbS	7.9	Suppress interface recombination; contribute additional photogenerated carriers	141
NiO	PbS-EDT/NiO/Au	10.4	Improve hole extraction efficiency; suppress the penetration of moisture and oxygen; good band alignment	49
PbS-EDT	PbS-TBAI/PbS-EDT/P3HT	8.7	Adjust the valence band; improve charge transfer	177
CsPbBr3	PbSe-MPA/CsPbBr3/Au	7.22	Suppress carrier recombination	178
ZnO	Mg-doped ZnO/ZnO/PbS	7.06	Decrease the interface recombination	179
CdSe	ZnO/CdSe/P3HT:PCBM	2.25	Increase conductivity; electron transport and hole block	180
CuInS2–ZnS	TiO2/CuInS2–ZnS/PCDTBT:PC71BM	7.01	Increase the electron extraction; reduce impedance	181
CdSe	P3HT:PCBM/CdSe/Al	3.08	Down-conversion, reduce charge recombination	182
ZnCdS	TiO2/ZnCdS/PTB7:PC71BM	7.75	Suppress the recombination; reduce the series resistance	183
SnO2	ITO/SnO2/ZnO	7.16	Enhance optical transmission; reduce energy barrier; suppress carrier recombination	184
Ag2Se	TiO2/Ag2Se/N719	5.89	Null	186
CdS	TiO2/CdS/N719	7.54	Suppress the charge recombination; increase the optical absorption	187
CeO2:Eu^3+	N719/CeO2:Eu^3+/electrolyte	8.36	Down-convert UV light to visible light, light scattering	190
NaGdF4:Eu^3+	FTO/TiO2/N719/electrolyte/Pt/FTO/NaGdF4:Eu^3+	9.34	Act as luminescent down-conversion centers and light scatterers in the ultraviolet and visible domains	191
CaCe2(MoO4)4:Er^3+/Yb^3+	Electrolyte/CaCe2(MoO4)4:Er^3+/Yb^3+/Pt	7.78	Convert the NIR and UV radiation to visible emissions	192
NaYF4:20%Yb, 2%Er@NaYF4:7%Eu	N719/NaYF4:20%Yb,2%Er@NaYF4:7%Eu/Pt	7.664	Convert NIR and UV lights to visible lights	193
SrF2:Pr^3+–Yb^3+	FTO/SrF2:Pr^3+–Yb^3+/TiO2-N719	9.07	Absorb blue light and emit green and red light	194
BaWO4:Pr^3+	TiO2/BaWO4:Pr^3+/N719	8.08	Absorb UV light and emit blue, green, and red light	195
MAPbBr0.9I2.1	TiO2/MAPbI3/MAPbBr0.9I2.1	13.32	Facilitate hole transfer from MAPbI3 to HTL	196
CuInS	MAPbI3/CuInS/spiro-MeOTAD	13.8	Enhance charge transfer; suppress charge recombination pathways	198
MgO	FTO/MgO/SnO2	18.23	Smoother surface; less FTO surface defects; suppressed electron–hole recombination	199
CdS	TiO2/CdS/MAPbI3	10.52	Longer electron lifetime; lower charge carrier recombination rate	200
SnO2	PC61BM/SnO2/Al	19.7	Block holes; enhance the conductivity; reduce the recombination	201
Cl–SnO2	FTO/Cl–SnO2/CsMAFAPbI3Br3−x	17.3	Fill the pinholes; passivate the trapping defects	202
FAPbX3	MAPbI3/FAPbX3/C60	7.59	Enhance absorption	203
PbS	MAPbI3/PbS/spiro-OMeTAD	19.24	Enhance hole extraction; retard interfacial recombination; improve perovskite film morphology	204
CuOx	NiOx/CuOx/MAPbI3	19.91	Lead higher transfer efficiency and lower carrier recombination	205
MAPbBr0.9I2.1	SnO2/MAPbBr3/MAPbBr0.9I2.1	20.21	Optimize the energy level; improve hole extraction	206
Co-CuGaO2	Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3/Co-CuGaO2/spiro-OMeTAD	20.39	Reduce the energy gap; prevent direct contact between PVK and oxygen and moisture	207
MoS2:RGO	MAPbI3/MoS2:RGO/spiro-OMeTAD	20.12	Extract holes and block electrons	208
CsCu5S3	MAFAPbI3/CsCu5S3/spiro-OMeTAD	22.29	Favor for energy levels; reduce carrier recombination	209
Cu2−xS@SiO2@Er2O3	TiO2/Cu2−xS@SiO2@Er2O3/MAPbI3	17.8	Upconvert infrared light to visible light	210
NaYF4:Yb^3+,Er^3+/@NaYF4:Yb^3+,Nd^3+	IR-783 + NaYF4:Yb^3+,Er^3+/@NaYF4:Yb^3+,Nd^3+ + Au/CsMAFAPbBrI	20.5	Convert IR to visible light, scatter light	212
MoS2	SiOx/MoS2/MoOx	22.8	Provide electron-blocking and hole-extraction properties	213
PbS	PbS/c-Si solar cell	12.6	Luminescent solar concentrator	214
CdS	Si solar cell/CdS	9.37	Reduce the reflectance spectral ranging from 250 to 1100 nm, passivation, and down-conversion	216
ZnxCd1−xS–ZnS:Mn	Si solar cell/ZnxCd1−xS–ZnS:Mn	14.271 ± 0.268	Down-conversion	217
Cd0.5Zn0.5S–ZnS:Mn	Si solar cell/Cd0.5Zn0.5S–ZnS:Mn	17.90	Down-convert UV light of 250–450 nm to yellow-orange light at 583 nm	218
NaGdF4:Ce@NaGdF4:Nd/Yb@NaYF4	c-Si solar cell/NaGdF4:Ce@NaGdF4:Nd/Yb@NaYF4	0.8 (under 254 nm illumination)	Expand the spectrum of quantum cutting in the NIR	219
CsPbCl1.5Br1.5:Yb^3+,Ce^3+	c-Si solar cell/CsPbCl1.5Br1.5:Yb^3+,Ce^3+	21.5	Larger absorption cross-section, weaker electron-phonon coupling and higher inner luminescent quantum yield	220

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3.1 Suppressing recombination

As shown in Fig. 7, IFL is a separate layer between the charge transport layer/active layer or charge transport layer/electrode. So, it can convincingly act as a functional layer to prevent hole–electron recombination. Ternary semiconductor NCs of PbCdS were deposited on TiO₂ as an IFL to impede the direct contact between ETL and the active layer of CdS, thus suppressing recombination efficiently and boosting the device PCE significantly (Fig. 8a). As the same, oxide NCs of MgO and SnO₂ film inserted between ETLs and electrodes blocked holes and reduced recombination (Fig. 8b and c).^199,201

Fig. 8 (a) Schematic illustration of the possible electron transfer process in bare CdS and PbCdS/CdS based solar cells and J–V curves of CdS and PbCdS/CdS based solar cells. (b) The perovskite can directly contact the FTO surface along a shunt pathway in the absence of SnO₂ ETLs; the MgO₂ layer can inhibit the penetration of perovskite reaching the FTO surface, and the best performance of the PSCs with and without MgO. (c) Corresponding energy level diagram of PSCs, steady-state PL spectra of the perovskite film deposited on the PC₆₁BM layer and PC₆₁BM:SnO₂ bilayer, J–V characteristics in the illumination for the devices based on the PC₆₁BM layer and PC₆₁BM:SnO₂ bilayer. (a) Adapted with permission.¹⁷⁴ Copyright 2017, Elsevier. (b) Adapted with permission.¹⁹⁹ Copyright 2017, Wiley-VCH. (c) Adapted with permission.²⁰¹ Copyright 2018, American Chemical Society.

3.2 Charge transport improvement

As we know, charge transport ability inevitably affects the performance of solar cells. As shown in Fig. 9a, MAPbBr_0.9I_2.1NCs with good energy alignment enhance the hole transport ability and the PCE of the PSCs. Co-CuGaO₂ NCs with ∼20 nm size were synthesized by hydrothermal method and used for surface passivation at the interface of perovskite and spiro-OMeTAD. Furthermore, the larger bandgap and lower valence band energy of Co-CuGaO₂ reduced the energy gap between Co-CuGaO₂ and perovskite. Considering that the reduced energy gap improved hole conduction and electron blocking, the PCE of PSCs was enhanced from 18.60% to 20.39%.²⁰⁷ MoS₂NCs were also used to improve hole transport and the device PCE and stability (Fig. 9b).

Fig. 9 (a) TRPL spectra of the perovskite film with and without MAPbBrI₂ NCs, schematic diagram of band bending in the heterojunction structure formed by the perovskite and MAPbBrI₂ NCs. (b) Steady-state PL measurements of MAPbI₃ after the deposition of spiro-OMeTAD and different IFL/spiro-OMeTAD, I–V characteristics of tested PSCs using different IFLs, normalized PCE trends vs. time extracted by I–V characteristics under 1 sun illumination, periodically acquired during the shelf life test for the PSCs. (a) Adapted with permission.²⁰⁶ Copyright 2020, American Chemical Society. (b) Adapted with permission.²⁰⁸ Copyright 2018, American Chemical Society.

3.3 Light conversion and harvest enhancement

It is known that the active layer of recent solar cells cannot respond to the whole solar spectrum, resulting in energy loss and relatively low efficiency. Broadening the absorption of the active layer is an efficacious strategy to reduce energy loss and enhance the performance of solar cells. CdS NCs with general Stocks shifts were used in Si solar cells for spectral range enhancement.²¹⁶ In Fig. 10a, we can see that Mn-doped NCs expand the spectral range response of solar cells by absorbing short-wave lights and emitting the characteristic light around 580 nm, suggesting a larger range of light harvesting by active layer and PCE improvement. Zn_xCd_1−xS/ZnS:Mn²⁺ NCs were also used to broaden the light response range of Si solar cells.²¹⁷ Furthermore, Mn-doped semiconductor NCs have a large Stocks shift, avoiding self-absorption and thus reducing energy loss. Sr₂CeO₄ NCs with down-shifting properties could improve the stability of organic P3HT:PCBM solar cells without significant loss of short-circuit current.¹⁸⁵ Certainly, by using NCs with a Stocks shift for expanded light harvesting, the photoluminescence yield and other photoelectronic properties should keep the rules of high-performance solar cells.

Fig. 10 (a) Scheme of the down-shifting mechanism of the Zn_0.5Cd_0.5S:Mn (5%)/ZnS NC converter material and the design of proof-of-concept solar cell; J–V curves of the corresponding solar cells. (b) Schematic and proposed energy transfer for Ce³⁺-sensitized quantum cutting in Nd³⁺ ions; schematic design for boosting the energy-harvesting efficiency of c-Si solar cells with quantum cutting nanocrystals; comparison of current density–voltage characteristic for c-Si solar cells with and without a nanocrystal coating layer (the solar cells were illuminated with a 254 nm UV lamp at a power density of 7 mW cm⁻²). (c) Schematic diagram of energy transfer mechanism in the Yb³⁺, Ce³⁺ codoped CsPbCl_1.5Br_1.5NCs, absorption; visible and near-infrared emission spectra of CsPbCl_1.5Br_1.5 perovskite NCs codoping with different rare earth ions; PCE of Si solar cells with different perovskite NCs. (a) Adapted with permission.²¹⁷ Copyright 2016, the Royal Society of Chemistry. (b) Adapted with permission.²¹⁹ Copyright 2017, American Chemical Society. (c) Adapted with permission.²²⁰ Copyright 2017, Wiley-VCH.

Besides the above NCs, rare elements are good at light conversion due to their special energy levels. Undoped and doped NCs based on rare elements were massively applied to broaden the light response in solar cells and efficiently boost the device performance. As seen in Fig. 10b and c, Yb³⁺, Ce³⁺, and Nd³⁺ based NC layers efficiently convert light and enhance the device performance. Additionally, Eu³⁺, Er³⁺, and Pr³⁺ were also used for efficient solar cells.

3.4 Energy level optimization

To ensure propitious charge transfer among the functional layers in solar cells, un-matched energy levels are a tricky issue that we need to address. One origin of the open-circuit voltage (V_oc) loss of NC-LHSC, Si solar cells, PSCs, and OSCs is mainly analyzed quantitatively via the energy difference between bandgap and the Schokley–Queisser limit voltage.¹⁵ For n-i-p solar cells, band alignment between the active layer and ETL directly limits the splitting of the quasi-Fermi level. For p-i-n solar cells, the band alignment between HTL and active layer has a great impact on the V_oc. The conduct band of recent HTL is relatively deep, and the barrier for electron transport is not sufficient, resulting in electron leakage and reducing the device performance. Therefore, energy level is of great importance to improve the performance of solar cells. Shown in Fig. 11a and b, Co-doped CuGaO₂ and SnO₂NCs film were used for energy level optimization for high-performance solar cells, illuminating the large potential of NCs as ITL for reducing V_oc loss of solar cells.

Fig. 11 (a) Energy level diagram of PSCs with the ITO/TiO2/PVK/Co-CuGaO₂/spiro-OMeTAD/Au structure, dark I–V curves of devices based on spiro-OMeTAD and Co-CuGaO₂/spiro-OMeTAD, forward and reverse scan J–V curves based on spiro-OMeTAD and Co-CuGaO₂/spiro-OMeTAD. (b) Band edge alignment and photocarrier dynamics in the resultant device. (c) Schematic illustration of an NC-luminescent solar concentrator, PL spectra of PbS NC-luminescent solar concentrator with different concentrations of PbS, and J–V curves for solar cells with and without PbS NC-luminescent solar concentrator. (a) Adapted with permission.²⁰⁷ Copyright 2022, Elsevier. (b) Adapted with permission.²⁰² Copyright 2019, Elsevier. (c) Adapted with permission.²¹⁵ Copyright 2015, Springer.

3.5 Concentrating luminescent solar radiation

Solar radiation is geographically extensive, but the energy density is not high. So, concentrating solar light for higher density is a feasible strategy to improve solar cell performance. PbS NC luminescent solar concentrator (LSC) was found to show potential advantages over silicon solar cell panels (Fig. 11c). They can reduce the size of solar cells and offer great flexibility in design, which results in cost reduction with any desired shape. One of the attractive LSCs is based on NCS.

3.6 Prevention

The main reason for unstable solar cells is the invasion of oxygen and water. IFL between the active layer and HTL, ETL, or electrode can arrest this invasion to a great degree and increase the stability of solar cells. The above-mentioned IFL of Co-CuGaO₂ NCs in Fig. 11a not only acts as a hole transport accelerator but also prevents the direct contact of perovskite with oxygen and moisture, boosting the stability of the PSCs.²⁰⁷

4. Nanocrystals as efficient additives

Besides an independent layer to improve the performance of solar cells, NCs have also been used as additives for boosting active layer quality, carrier transfer acceleration, spectral response broadening due to plasmonic effect, light conversion, light scattering/reflection, heat sinking, and some other functions. Table 4 lists the recent progress of NCs as efficient additives for high-performance solar cells.

Table 4 Recent advances in NC additives for high-performance solar cells

NC	Added layer	NC additives and the mixture	PCE (%)	Functions	Ref.
SnO2–Sb2O3	Bottom ETL	SnO2–Sb2O3 + TiO2	7.7	①Improve the electron mobility and light harvesting; ②avoid the recombination	222
PbS		PbS + m-TiO2	5.04	①Enlarge the grain size of CsPbBr3 perovskite film; ②suppress the activation of intrinsic trap sites of m-TiO2	223
		PbS + TiO2	14.95	Downshift the conduction band of TiO2, promote the driving force of an electron injection	227
Cu–Zn–In–S–Se (CZISSe)		CZISSe + mp-TiO2 + CsPbBr3	5.37	①Enhance charge extraction; ②reduce charge recombination	225
Ho^3+–Yb^3+–F^− tri-doped TiO2		Ho^3+–Yb^3+–F^− tri-doped TiO2 + TiO2	9.91 ± 0.3	Convert NIR light to green light	232
Gd1.54Er0.46(MoO4)3 (GMO:Er)		GMO:Er + TiO2	3.41	Convert NIR light to the visible region (near 550 nm)	233
β-NaYF4:Yb^3+/Er^3+/Sc^3+@NaYF4		β-NaYF4:Yb^3+/Er^3+/Sc^3+@NaYF4 + TiO2	20.19	Convert NIR to red and green light	243
SrAl2O4:Eu^3+		SrAl2O4:Eu^3+ + TiO2	4.64	Elevation of the Fermi energy level of TiO2, improves light harvesting	245
ZnSTe	Bottom HTL	ZnSTe + PEDOT:PSS	2.31	Reduce series resistance, increase shunt resistance, improve mobility	230
NaCsWO3@NaYF4@NaYF4:Yb,Er	Top HTL	NaCsWO3@NaYF4@NaYF4:Yb,Er + spiro	18.28 ± 0.34	①Widen the perovskite spectral response range; ②increase the light reflection; ③ prolong the light path, and light absorption	231
NaLuF4:Yb,Er@NaLuF4		NaLuF4:Yb,Er@NaLuF4 + PTAA	15.86	Convert NIR light to visible light, scatter light	237
Li(Gd,Y)F4:Yb,Er		Li(Gd,Y)F4:Yb,Er + spiro-MeOTAD	18.34	Convert NIR to visible light	242
SnO2	Active layer	SnO2 + P3HT-PCBM	3.39	Reduce the recombination	224
Fe-doped SnO2		Fe-doped SnO2 + P3HT	3.04	Extension of photogenerated exciton lifetime, overcome the burn-in regime faster	247
NaYF4:Yb,Er/NaYF4		NaYF4:Yb,Er/NaYF4 + N719	9.15	Convert NIR light to visible light (450–700 nm)	235
TiO2:Sm^3+		TiO2:Sm^3+ + P25 + N719	5.31	Convert UV to visible light	244
SnS		SnS + MAPbI3	14.26	①Provide more nucleation sites for the growth of perovskite grains; ② accelerate carrier transfer and reduce the recombination	226
NaYF4:Yb/Er		NaYF4:Yb,Er + MAPbI3	17.8	Broaden the solar spectral use to NIR light, minimize the non-absorption energy loss	236
IR-806-β-NaYF4:Yb,Er		IR-806-β-NaYF4:Yb,Er + MAPbI3Er^3+–Yb^3+ doped Zn	17.49	Convert NIR light (800–1000 nm) to visible emissions	238
Ho^3+–Yb^3+–Li^+-doped TiO2		Ho^3+–Yb^3+–Li^+-doped TiO2 + FAMAPbBrI3	16.88 ± 0.5	Convert NIR to visible light, improve electron injection efficiency, and decrease recombination	240
β-NaYF4:Yb^3+,Tm^3+@TiO2		β-NaYF4:Yb^3+,Tm^3+@TiO2 + MAPbI3	16.27	Convert NIR to visible light, serve as the light scatter centers	241

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4.1 Active layer quality improvement and carrier transfer accelerator

The performance of solar cells largely depends on the quality of the active layer, such as the purity of Si for Si solar cells and the composition of perovskite for PSCs. For example, defects unavoidably exist at the surface of perovskite thin film during the low-temperature fabrication process, and reducing defects is a very useful way to improve the performance of PSC.²⁵⁴ This subsection discusses NCs as outstanding additives in the active layer for boosting its quality and carrier transfer acceleration.

SnO₂–Sb₂O₃ NCs were used to modify TiO₂ nanorod arrays for electron mobility improvement and electron transport resistance reduction. This NCs modified ETL enhanced the PCE of CH₃NH₃PbI_3−xCl_x PSC from 6.5% to 7.7%.²²² PbS NCs suppress the activation of the intrinsic trap sites, provide nucleation sites to enlarge the grain size, and suppress the charge combination in CsPbBr₃ PSCs (Fig. 12).²²³ Adding SnO₂ NCs of size around 5 nm into the active layer of P3HT:PC₆₁BM made electrons more easily pass through the active layer and accelerate the electron transfer, improving the PCE of OSC from 2.67% to 3.39%.²²⁴ S₂O₃²⁻-capped Cu–Zn–In–S–Se NCs with ∼5 nm size was introduced in the perovskite precursor of PbBr₂ solution to boost 22.6% enhancement of the PCE of inorganic Cs-based PSCs, which was due to promoted crystallization of CsPbBr₃ and hole extraction.²²⁵ SnS NCs with an average size of 6.9 nm were implanted into the active layer of carbon-based HTL-free mesoporous PSCs, and the device gained a high PCE of 14.26% with a 12.42% improvement. This improvement was demonstrated by more nucleation sites for the growth of perovskite grains and the accelerated carrier transfer.²²⁶ PbS NCs doped TiO₂ nanotubes (TNTs) modified the electronic and optical properties by downshifting the conduction band of TiO₂ ETL from −4.22 to −4.58 eV and promoting the driving force of an electron injection to the conductive electrode.²²⁷

Fig. 12 Top-view and cross-section images of CsPbBr₃ film decorated with PbS NCs, and J–V curves of all-inorganic PSCs based on different TiO/PbS photoanodes. Adapted with permission.²⁴³ Copyright 2019, American Chemical Society.

4.2 Plasmonic Effect

Semiconductor NCs have exhibited localized surface plasmon resonances (LSPR), and this plasmonic effect has been used in many fields, such as solar photovoltaics, in-door energy comfort, water splitting, and so on (Fig. 13). Compared with traditional LSPR materials (noble metals), the semiconductor LSPR NCs allow a wide range of wavelength tunability from visible towards near-infrared (NIR) and further to mid-IR, leading to larger absorption of solar light. Higher absorption of the active layer in solar cells increases the current intensity and thus, the device performance. DSSCs based on plasmonic effect by ZnO or SnO₂ and TiO₂/SnO₂ have been investigated.²⁵⁴ From Fig. 13, it can be seen that different oxides, sulfides, and selenides such as MoO_3−x, Cs_xWO₃, TiO_2−x, CdO_1−x, doped In₂O₃, doped ZnO, Cu_2−xS, Cu_xIn_yS₂, Cu_2−xSe, etc. broaden the response spectra from 500 nm to nearly 3800 nm. Effective light harvesting due to the plasmonic effect shows great potential in solar cell application. Given the limited self-absorption bands of solar cells, the above oxide and sulfide NCs can be applied as additives to widen the light harvesting range. Clearly, this will reduce energy loss and raise the device performance in a considerable way.

Fig. 13 Schematic of the applications of LSPR semiconductor NCs; the solar spectrum and its relevancy for energy-targeted applications of LSPR semiconductor NCs via their tunable plasmon absorbance. Adapted with permission.²⁵⁵ Copyright 2021, Elsevier.

4.3 Light conversion

As discussed in Section 3, light conversion is an efficient way to boost the PCE and stability of solar cells. Based on this point, NC light-converting layers were investigated in the previous part. It is also true that light conversion can be realized by NC additives, which will be summarized in this part.

4.3.1 Up-conversion. Rare-earth (RE) elements are famous for light up-conversion and further application in solar cells. RE element-doped semiconductor NCs have been added as additives in active layers and charge transport layers for high-performance DSSCs and PSCs. NIR lights were up-converted to visible lights and thus elevated the device PCE due to wider solar radiation absorption (Fig. 14). The RE element-based NCs, which can be used in up-converting materials as additives in solar cells, mainly contain Yb, Er, Ho, and Sc doped materials like Ho–Yb–F doped TiO₂, Er–Yb doped ZnO₂, Ho–Yb doped Gd₂O₃, Yb–Er doped NaYF₄ and Yb–Er doped Li(Gd,Y)F₄. As shown in Fig. 14, the schematic energy diagram for Yb³⁺ and Er³⁺ are suitable for up-conversion and the related solar cells gain high efficiency. The up-conversion NCs can be added in the active layer, ETL, and HTL in different solar cells like OSC, DSSC, and PSC. The original active layers cannot absorb the whole range of sunlight and thus result in energy loss. After assembling NCs with up-conversion ability, the non-responsive long-wavelength range of sunlight will be converted to shorter wavelengths and absorbed by active layers to re-generate hole–electron pairs. This strategy can enhance the utilization of the infrared range of sunlight and the performance of solar cells.

Fig. 14 Solar spectrum, absorption and emission spectrum of up-conversion nanocrystals and EQE curve of γ-CsPbI₃ PSC, J–V curves of PSCs based on different nanocrystals, and statistical PCE distribution histograms of 30 devices, schematic energy diagram for Yb³⁺ and Er³⁺. Adapted with permission.²³⁷ Copyright 2019, American Chemical Society.

4.3.2 Down-conversion and down-shifting. Down-conversion and down-shifting NCs are advantageous for high-performance solar cells due to the efficient utilization of UV lights. Typical Sm³⁺-based TiO₂NCs were used in DSSCs and obvious improvement of PCE was gained (Fig. 15) through converting ultraviolet to visible light. Better performance of solar cells was obtained by means of down-converting NCs such as ZnS:Er in Si-based devices and CeO₂:Gd in OSC.^249,250 The shortest wavelength response by perovskite is about 400 nm.²⁵⁶ The energy of sunlight with wavelengths shorter than 400 nm will be wasted. Furthermore, the UV lights can damage the perovskite or organic active layer and reduce the device stability. So, the PCE and light stability of solar cells can be increased by using down-conversion or down-shifting NCs.

Fig. 15 Excitation spectrum (λ_em = 567 nm) and emission spectrum (λ_ex = 395 nm) of TiO₂:Sm³⁺ NCs with different Sm³⁺ doping concentrations, and schematic energy-level diagrams to show the details of down-conversion mechanisms via excitation using 395 nm radiation for TiO₂:Sm³⁺ NCs. Adapted with permission.²⁴⁴ Copyright 2016, Elsevier.

4.4 Light scattering and reflection

Light scattering and reflection are well-known for boosting the optical absorption of different solar cells. TiO₂:Zn NCs can scatter light and promote the performance of conventional DSSCs.²⁵⁷ Due to the ultralow (<1%) photoluminescence quantum yield, NaLuF₄:Yb,Er@NaLuF₄NCs acted as scattering centers and extended the sunlight optical path by combining scattering and reflecting sunlight.²³⁷ NaYF4:Yb³⁺,Tm³⁺ NCs serve as scatter centers to enhance light harvesting for PSCs.²⁴¹ We can conclude that NCs are good at light absorption enhancement due to light scattering and reflecting, and this is an available approach for improving the device performance.

4.5 Heat sinks

Except for optical management, heat control is also important for solar cell operation because elevated temperatures may increase energy loss and destroy the devices. In the traditional photovoltaic/thermal (PV/T) system, the temperature of thermal energy is always limited by the operation temperature of PV cells. The oleylamine solution of Cu₉S₅NCs was adopted in the spectral splitting filter to harvest the moderate-temperature heat. After successful thermal energy collection, the maximum overall efficiency of the present PV/T collector is 34.2%, with a 17.9% improvement.²⁵⁸ In a concentrator photovoltaic nanocrystal-phase change material (PCM) hybrid system, Al₂O₃, CuO, and SiO₂ were used to save energy and offer safe operating conditions. Compared with pure PCM (0 wt%), Al₂O₃-PCM at 5 wt% increased the thermal conductivity, and the melting rate reduced the solar cell temperature. The electrical efficiency was improved from 6.36% to 8% and gained temperature uniformity from 20 °C to 12 °C. This strategy would be recommended for residential and industrial applications in solar cells.²⁵⁹

4.6 Other functions

ZnSTe NCs with an average of 2.96 nm in the active layer demonstrated increased photo-generation and improved efficiency by reduced series resistance and improved mobility.²³⁰ Fe-doped SnO₂ NCs incorporated into the active layer of P3HT:PCBM improved the J_sc of OSC due to the extension of photogenerated exciton lifetime as a result of the magnetic field. Meanwhile, these NC-reinforced devices showed the tendency to overcome the burn-in regime faster and indicated the diluted magnetic semiconductor NCs had the potential to increase the stability of the devices.²⁴⁷

5. Summary and outlook

In recent years, NCs as functional layers and additives have been widely used in solar cells, significantly enhancing their performance. Here, we summarize NCs-based HTL, ETL, IFL, and additives for solar cells. NCs can boost the device performance in many ways, such as increasing the charge transport ability, suppressing charge recombination, broadening light harvest, and so on.

Based on previous investigations, we propose some promising strategies to enhance the performance of solar cells by using NCs.

(I) Optical management. Full spectrum absorption under low-cost conditions: both up-converting and down-shifting materials. For down-shifting, doped NCs with large Stokes shift, such as Mn or Cu doped NCs, have great potential due to no self-absorption, facile synthesis, and low cost. Cu⁺, Ag⁺ doped n-type metal oxide NCs,²⁶⁰ Fe_1−xS₂ NCs,²⁶¹ and In doped CuxS NCs,²⁶² with great potential for spectra broadening are also suggested for high-performance solar cells. CaMoO₄:Er³⁺,Yb³⁺ NCs would offer great potential for conserving energy in Si solar cells.²⁶³ Certainly, the photoluminescence efficiency of NCs is very important when they are used to convert light in solar cells. Excellent optical management of solar cells can utilize more sunlight and improve the device performance.

(II) Electronic optimization. The charge transfer ability is mainly determined by the electronic properties of charge transport materials. The performance of solar cells can be improved by electronic optimization. One approach is a component change of materials such as n doping for n-type NCs and p doping for p-type NCs. Finding more suitable dopants for NCs will further boost the PCE and stability of solar cells. Meanwhile, the size and ligand control of NCs are also considered to optimize their electronic properties and fabricate better-performance solar cells.

(III) Interface engineering. Interfacial layers between different functional layers show different functions in solar cells. In further developments, more NCs IFL will be used to improve device performance by preventing direct contact between the active layer and charge transport layer, impeding the entry of water and oxygen, and protecting and destroying the active layer with UV lights.²⁶⁴ So, NC IFL, with good photoelectronic properties, can adjust energy alignment, accelerate charge transport, enhance light harvest, and protect the active layer.

(IV) Cross utilization. The metal–organic framework (MOF) materials can improve the efficiency and stability of solar cells due to their unique properties.²⁶⁵ NCs with small sizes can be considered to mix with MOF and enhance the performance of devices. In addition, NCs can be utilized as light harvesters, HTL, ETL, and IFL, so we suggest their application in all-NC solar cells.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the Natural Science Foundation of the Guizhou Province (No. 2071). The authors thank Prof. Ma Wanli, Liu Zeke, and Dr Lu Huiyuan at Soochow University for their helpful discussion and suggestions for improving the manuscript.

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