DPP-based small molecule, non-fullerene acceptors for “channel II” charge generation in OPVs and their improved performance in ternary cells

Abstract

We synthesized three diketopyrrolopyrrole-thiophene-based small molecules (p-, m-, and o-DPP-PhCN) substituted with electron-withdrawing cyanide groups on both end phenyl rings in different positions. The physical properties of the oligomers varied based on the position of the CN groups. Compared to m- and o-DPP-PhCN, the p-DPP-PhCN film had a more red-shifted, strong UV absorption (λ_max = 670 nm). p-DPP-PhCN also exhibited a relatively well-aligned arrangement in the X-ray diffraction pattern, owing to a high degree of molecular packing in p-DPP-PhCN. Such an exceptionally strong aggregation of p-DPP-PhCN is expected to give rise to strong molecular orbital interactions and a subsequent decrease in the energy band gap (E_g). p-DPP-PhCN has a lower optical E_g (1.75 eV) than m- and o-DPP-PhCN (∼1.80 eV). Organic photovoltaic cells with the structure ITO/PEDOT:PSS/poly(3-hexylthiophene) (P3HT):DPP-PhCN/LiF/Al were fabricated. Two D/A-type binary cells using p- or o-DPP-PhCN showed similar power conversion efficiencies (PCEs) of 0.5% although the device parameters were different. A high open circuit voltage of 1.09 V in P3HT:o-DPP-PhCN comes from a high-lying lowest unoccupied molecular orbital energy level of o-DPP-PhCN. In contrast, the relatively high short circuit current density of P3HT:p-DPP-PhCN can be explained by the red-shifted UV absorption and superior molecular packing in p-DPP-PhCN. Furthermore, the maximum photocurrent response (13%) of P3HT:p-DPP-PhCN was observed at the λ_max of p-DPP-PhCN. In other words, the light absorption of p-DPP-PhCN contributes to the photocurrent along with the absorption of P3HT (e.g., “channel II” charge generation). Finally, a PCE of 1.00%, more than twice that of binary cells, was achieved in the D/A/A-type ternary cells composed of P3HT, p-, and o-DPP-PhCN. The contribution of the electron acceptor to the photocurrent of the devices was enhanced by adding a second acceptor. Improved film morphology and better charge separation were observed in the ternary cells.

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Introduction

Organic photovoltaic cells (OPVs) are useful devices that can produce infinite, renewable, and clean energy from sunlight.^1–3 While the polymeric and small molecule electron donors of bulk-heterojunction (BHJ) OPV cells have been actively developed,^4–6 the electron acceptors have been confined to fullerene derivatives such as [6,6]-phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) because of their good electron mobility.^7,8 Recently, the importance of developing non-fullerene acceptors has been emphasized to overcome the drawbacks of fullerene derivatives, such as weak absorption in the visible region and difficult synthesis and purification.^9–14 Together with conventional photocurrent generation derived from the absorption of an electron donor, light absorption by an electron acceptor followed by a photo-induced hole transfer (through the so-called “channel II” charge generation) was recently introduced as another important route for generating free charges in efficient D/A blends.^15–23 Such photocurrents were observed in devices using the electron acceptor, [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM)¹⁵ or non-fullerene acceptors^16,17 which exhibited better UV absorption compared to PC₆₁BM. Therefore, the development of new acceptors with absorption that is complementary to donor polymers with high absorption coefficients is very important.

Introduction of electron-withdrawing groups (EWGs) such as fluorine,^24–26 cyanide (CN),^27,28 and carbonyl groups^29,30 on the thiophene or phenyl rings of the molecular backbone can effectively lower the frontier orbital energy levels. The low-lying highest occupied molecular orbital (HOMO) energy levels induced by EWGs are known to be favorable for donor materials, resulting in a high open circuit voltage (V_OC). Traditional p-type materials can also be altered as acceptors by introducing EWGs due to the improved electron affinities (EAs).^31,32 For example, diketopyrrolopyrrole (DPP)-thiophene-based small molecules, known as good electron donor materials with a low band gap (E_g = 1.86 eV),^33,34 were successfully used as electron acceptors by substituting the carbonyl or fluorine groups,^35,36 and power conversion efficiencies (PCEs) of up to 1.00% were achieved by blending them with poly(3-hexylthiophene) (P3HT).

Along with the development of new donor/acceptor materials and device-manufacturing methods, the concept of ternary devices has been widely used to achieve high-performance OPVs.^37–39 For example, PCEs of low band gap polymers in binary cells (6.26% and 6.30%) could be further improved by using two low band gap polymers in ternary cells together with PCBM as an acceptor.⁴⁰ A 12% increase in PCE (7.02%) was obtained, demonstrating the positive role of the ternary cells. Various combinations of three components have been reported, i.e., two donors and one acceptor (D/D/A)^41–43 or one donor and two acceptors (D/A/A).⁴⁴ However, most research was based on the P3HT/PCBM system, and new polymeric^45,46 or small molecule^47,48 donors were introduced as a third component in the D/D/A systems, which led to positive changes such as complementary light absorption, better film morphology, and cascade energy levels. There are a few reports using fullerene derivatives into P3HT/PCBM system resulting D/A/A ternary cells.^49–51

Herein, we introduced new non-fullerene acceptors into D/A/A-type ternary cells, which exhibited improved performance over individual binary cells. To the best of our knowledge, this is the first report using two non-fullerene acceptors in a ternary cell. A series of small molecule acceptors composed of DPP, thiophene, and CN-substituted phenyl rings were synthesized and their physical properties including thermal, optical, and electrochemical properties were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), UV-visible spectroscopy, and cyclic voltammetry (CV) measurements. The OPV characteristics and physical properties varied according to the substitution pattern (p, m, and o) of the cyanide group. Furthermore, charge generation by light absorption of the acceptor (e.g., “channel II” charge generation) was observed, and a detailed study on the film morphology and charge transfer between P3HT and the acceptors were conducted using an atomic force microscope (AFM) and photoluminescence (PL) quenching experiments.

Results and discussion

Synthesis and thermal properties

Three small molecules (p-, m-, and o-DPP-PhCNs) were synthesized in high yield (86–88%) by a Suzuki coupling reaction between the diboronic ester of a DPP-thiophene-based core (3,6-bis-(5-(4,4,5,5-tetramethyl-1,3,2-dioxabrolan-2-yl)thiophen-2-yl))-N,N′-bis((2-ethylhexy)-1,4-dioxo-pyrrolo[3,4-c]pyrrole) (DPP-B) and three corresponding bromides (4-, 3-, and 2-bromobenzonitrile, respectively) using a tris(dibenzylideneacetone)dipalladium(0) catalyst (Pd₂(dba)₃) (Scheme 1).⁵² The synthesized oligomers were successfully characterized by ¹H and ¹³C NMR spectroscopy (Fig. S1–S6, ESI†) and elemental analysis. It was observed that three DPP-PhCNs had excellent thermal stability, losing less than 5% of their weight upon heating upto 420 °C in TGA thermograms (Fig. 1a). Despite having the same conjugated backbone, the thermal behaviors of the oligomers were different based on the substitution pattern. In DSC (Fig. 1b), p-DPP-PhCN had a relatively high endothermic melting peak (T_m) at 250 °C with a sharp exothermic recrystallization peak (T_cryst) at 215 °C compared to o-DPP-PhCN (T_m = 187 °C and T_cryst = 100 °C, broad). The higher degree of crystallization of p-DPP-PhCN can be explained by its superior intermolecular interaction and packing property compared to o-DPP-PhCN, which matches the relatively low solubility of p-DPP-PhCN in common organic solvents such as chloroform.

Scheme 1 Synthesis of DPP-PhCNs by Suzuki coupling reaction (i) Pd₂(dba)₃, P(t-Bu)₃ × HBF₄, K₃PO₄, THF, H₂O, 80 °C, N₂ for 3 hours.

Fig. 1 (a) TGA and (b) DSC curves of DPP-PhCNs.

Optical and electrochemical properties

Fig. 2a shows the UV-visible absorption spectra of the oligomers in chloroform solution and thin solid films. The oligomer solutions showed similar UV-vis absorption with two absorption bands at around 350 and 600 nm, and these peaks were assigned as the π–π* transition of the conjugated backbone and intramolecular charge transfer interactions between the thiophene donor and DPP acceptor units, respectively.^33,45,53 While the absorption spectra of the o- and m-DPP-PhCN films were slightly broader than that of the solution, the p-DPP-PhCN film had a more red-shifted absorption (λ_max = 670 nm), probably due to the high degree of molecular aggregation of p-DPP-PhCN. Furthermore, the absorption coefficient of p-DPP-PhCN was 1.26 × 10⁵ cm⁻¹ at 670 nm, which is almost two times higher than the maximum absorption coefficient of o-DPP-PhCN (0.76 × 10⁵ cm⁻¹ at 577 nm). It is well known that materials with high absorption coefficients can be good candidates for OPV devices as they result in high short circuit current densities (J_SC) by absorbing more sunlight. The optical E_g of the oligomers were estimated from the absorption onset wavelength of the oligomer films (E_g = 1240/λ_onset eV). p-DPP-PhCN (1.75 eV) has a slightly lower E_g than o-DPP-PhCN (1.80 eV) and m-DPP-PhCN (1.81 eV). In addition, the distinguishing absorption peak of p-DPP-PhCN in 670 nm is maintained in the UV absorption spectrum of P3HT:p-DPP-PhCN blended film (Fig. 2b).

Fig. 2 UV-vis absorption spectra of (a) p-, m-, and o-DPP-PhCN in solution (solid line) and films (circle) and (b) two binary blend films (P3HT : p-DPP-PhCN = 1 : 1 and P3HT : o-DPP-PhCN = 1 : 1) with pristine films (p-, o-DPP-PhCN, and P3HT), (c) CV graph, and (d) energy diagram of DPP-PhCNs with P3HT (E_g are shown in parenthesis).

The electrochemical properties of the oligomers were studied by CV (Fig. 2c, Table 1). The films were prepared by dip-coating the oligomer solution on a Pt wire, and the measurements were calibrated using the ferrocene value of −4.8 eV as the standard. The anodic scans showed that the onsets of oxidation for p-, m-, and o-DPP-PhCN occurred at 0.99 V, 1.13 V, and 0.98 V (vs. SCE), respectively. The HOMO levels of p-, m-, and o-DPP-PhCN were calculated to be −5.38 eV, −5.52 eV and −5.37 eV, respectively, using the empirical equation proposed by Leeuw et al. (I_p(HOMO)) = −(E_onset,ox + 4.39) (eV), where E_onset,ox is the onset potentials of oxidation.⁵⁴ The lowest unoccupied molecular orbital (LUMO) energy levels of p-, m-, and o-DPP-PhCN were estimated to be −3.63 eV, −3.71 eV and −3.57 eV, respectively, using the HOMO levels and E_g of oligomers. o-DPP-PhCN had a higher LUMO energy level than p-DPP-PhCN. The relatively high LUMO level of o-DPP-PhCN could result in a better V_OC.^38,55 The band diagram for the oligomer is shown in Fig. 2d, together with the energy levels of P3HT for comparison.⁵⁰ The substitution of the electron-withdrawing CN group on both the phenyl rings could decrease the molecular energy levels of the DPP-PhCNs and could provide sufficiently large offset between LUMO energy levels of P3HT and acceptors for effective electron transfer.

Table 1 Electrochemical properties of DPP-PhCNs

Oligomers	Eonset,ox^a (V vs. SCE)	HOMO^b (eV)	LUMO^c (eV)	Eg^d (eV, V nm^−1)
a Eonset,ox stands for onset potential of oxidation.b Calculated using the empirical equation: Ip(HOMO) = −(Eonset,ox + 4.39).c Calculated from HOMO and Eg.d The optical Eg taken as the absorption onset (value in parentheses) of the UV-vis spectra of the oligomer films.
p-DPP-PhCN	0.99	−5.38	−3.63	1.75 (709)
m-DPP-PhCN	1.13	−5.52	−3.71	1.81 (685)
o-DPP-PhCN	0.98	−5.37	−3.57	1.80 (689)

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Film morphology

The molecular orientation and charge transport properties of the oligomer films were investigated by X-ray diffraction (XRD) analysis (Fig. 3). The oligomer films for XRD analysis were prepared on quartz glass substrates by drop-casting oligomers dissolved in chloroform. The p-DPP-PhCN film exhibited a series of multiple (h00) reflections including a strong first-order diffraction peak (100) at 2θ = 7.87°, corresponding to a d spacing value of 1.12 nm, whereas the o- and m-DPP-PhCN films had more complex patterns apart from a (100) reflection. Similar interlayer spacings of 1.1–1.4 nm were observed in other ethylhexyl-substituted DPP-based small molecules and polymers^56–58 with high degrees of crystallization from interdigitated alkyl side chains. Although the films were made by solution-casting the oligomers dissolved in chloroform, the superior packing ability of p-DPP-PhCN resulted in a relatively well-aligned arrangement, as suggested by the red-shifted UV-vis spectrum above. The characteristic diffraction peaks from the acceptors were maintained in the P3HT:acceptor blend films along with the diffraction peaks of P3HT at around 2θ = 5°. The relative high intensity of p-DPP-PhCN to P3HT than those of m- and o-DPP-PhCN also showed the better molecular aggregation of p-DPP-PhCN. It is expected that p-DPP-PhCN films will have better charge transport between the molecules than o- and m-DPP-PhCN films.

Fig. 3 XRD pattern of (a) DPP-PhCN and (b) P3HT:DPP-PhCN blend films.

Photovoltaic performances

All BHJ cells were fabricated with the configuration ITO/PEDOT:PSS/active layer/LiF/Al and the well-known polymer, P3HT, was used as the donor material in the active layer to combine with the synthesized DPP-PhCN acceptors. We used various blending ratios, annealing temperatures and solvents for optimization and the representative photovoltaic properties are summarized in Table 2 (see also Table S1, ESI†). Fig. 4 shows the plots of current-density versus voltage (J–V) and external quantum efficiency (EQE) curves of the devices annealed at 120 °C using chloroform as solvent. The devices using m-DPP-PhCN as acceptor showed very low performances of upto 0.23%. Two binary cells using p-DPP-PhCN and o-DPP-PhCN as acceptors showed similar PCEs of 0.47% and 0.46%, respectively, but the device parameters V_OC and J_SC varied based on the nature of the used acceptor. While P3HT:p-DPP-PhCN device showed a better J_SC of 1.64 mA cm⁻² than P3HT:o-DPP-PhCN device (1.19 mA cm⁻²), P3HT:o-DPP-PhCN device had a higher V_OC of 1.09 V than P3HT:p-DPP-PhCN device (0.56 V). The high V_OC of P3HT:o-DPP-PhCN device comes from the high-lying LUMO energy level of o-DPP-PhCN compared to p-DPP-PhCN.^38,55 The high J_SC from P3HT:p-DPP-PhCN device can be explained by the red-shifted UV absorption and superior molecular packing of p-DPP-PhCN, which can facilitate charge transport through acceptors in BHJ cells. Furthermore, the maximum photocurrent response (13%) of P3HT:p-DPP-PhCN device in the EQE spectrum (black circle of Fig. 4b) occurred at 670 nm, coinciding with the maximum absorption peak of p-DPP-PhCN (black circle of Fig. 2a) In other words, the light absorption of the acceptor contributes to the photocurrent of the P3HT:acceptor device along with the absorption of P3HT. Similar phenomena (e.g., “channel II” charge generation) were also observed in the devices using PC₇₁BM or non-fullerene acceptors, resulting in an enhanced broad EQE response and improved efficiencies.^16,17

Table 2 Photovoltaic performances of binary cells with various solvent and annealing temperatures^a

Active layer (D : A ratio = 1 : 1)	Solvent	Temp (°C)	VOC (V)	JSC (mA cm^−2)	FF (%)	PCE^best/PCE^avg (%)
a The standard deviation and average PCE (PCE^avg) based on more than 5 devices, CF: chloroform, CB: chlorobenzene.b The optimized temperatures.
P3HT : p-DPP-PhCN	CF	120	0.56 (±0.02)	1.64 (±0.02)	50 (±3)	0.47/0.43 (±0.03)
	CF	90	0.78 (±0.07)	1.24 (±0.04)	37 (±1)	0.36/0.27 (±0.05)
	CF	150	0.28 (±0.04)	0.73 (±0.04)	43 (±2)	0.09/0.08 (±0.02)
	CB	90^b	0.50 (±0.01)	0.79 (±0.03)	43 (±0)	0.17/0.16 (±0.01)
P3HT : m-DPP-PhCN	CF	120	0.45 (±0.03)	0.56 (±0.05)	31 (±1)	0.08/0.05 (±0.01)
	CF	90	0.43 (±0.04)	0.47 (±0.03)	30 (±4)	0.06/0.05 (±0.01)
	CF	150	0.28 (±0.02)	0.29 (±0.02)	36 (±1)	0.03/0.02 (±0.02)
	CB	150^b	0.73 (±0.04)	0.80 (±0.08)	39 (±2)	0.23/0.17 (±0.08)
P3HT : o-DPP-PhCN	CF	120	1.09 (±0.01)	1.19 (±0.04)	35 (±2)	0.46/0.44 (±0.03)
	CF	90	1.08 (±0.06)	0.81 (±0.02)	46 (±3)	0.40/0.39 (±0.02)
	CF	150	0.25 (±0.05)	0.39 (±0.03)	36 (±1)	0.04/0.01 (±0.01)
	CB	150^b	0.94 (±0.01)	1.18 (±0.03)	48 (±1)	0.54/0.47 (±0.02)

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Fig. 4 (a) J–V curves and (b) EQE responses of P3HT:DPP-PhCN devices.

Theoretical results on the frontier molecular orbitals (FMOs) of DPP-Ph derivatives

FMO energies and ionization potentials (IPs)/EAs of DPP-Ph derivatives were calculated and collected in Table 3. Upon the substitution of CN group, due to its electron-pulling nature, both HOMOs and LUMOs are stabilized. However, what is more interesting would be the evolution of FMO energies as a function of the substitution pattern. When going from ortho- via meta- to para-substitution, both HOMOs and LUMOs are consistently more stabilized. We note that the inductive effect of CN groups would not depend on the substitution position.⁵⁹ Furthermore, molecular orbital interactions are expected not to be effective due to the large energy difference between FMOs of CN group and those of DPP-Ph backbone; Fig. 5 shows, indeed, non-bonding interactions between DPP-Ph and CN groups. On the other hand, the FMO energies can be affected by the size and the direction of the dipole moment of the other subunits.^60,61 To investigate this electrostatic effect, we carried out calculations of DPP-Ph molecule with point charges being in place of the atoms of the CN substituents. The results are depicted in Fig. 6a and demonstrate that HOMO/LUMO energies of DPP-Ph with CN substituents evolve in the parallel way to those with point charges. This suggests the definite role of electrostatic interactions between main backbone and substituents in modulating FMO energies.

Table 3 Calculated HOMO/LUMO energies and IPs/EAs of DPP-Ph derivatives^a

	HOMO	LUMO	ΔEHL^b	IP	EA^c
a All the values are in eV.b ΔEHL denotes the energy gap between HOMO and LUMO levels.c EA is defined as E(anion) − E(neutral molecule).
DPP-Ph	−4.80	−2.56	2.24	6.10	−1.87
o-DPP-PhCN	−5.44	−3.16	2.28	6.44	−2.24
m-DPP-PhCN	−5.54	−3.26	2.28	6.52	−2.31
p-DPP-PhCN	−5.59	−3.37	2.22	6.57	−2.46

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Fig. 5 FMOs of DPP-Ph derivatives.

Fig. 6 (a) Pictorial depiction of the evolution of FMO energies of DPP-Ph derivatives (closed symbols correspond to FMO energies of CN substituted DPP-Ph and open symbols represent those of unsubstituted DPP-Ph under the field generated by dipole of CN.) and (b) conceptual diagram for molecular orbital interactions of FMOs of p-DPP-PhCN dimer.

Although theory suggests that HOMO/LUMO energies of p-DPP-PhCN should be lower than those of its o- and m-counterparts, experiments state otherwise; according to CV experiments, for example, in going from p- via m- to o-DPP-PhCN, HOMO energies change from −5.38 eV to −5.52 eV and −5.37 eV, respectively (Fig. 2d). However, intrinsic properties of organic semiconductors can be modulated by intermolecular interactions as well and p-DPP-PhCN may serve as the representative example for this. On the basis of the marked red-shift in the absorption spectra, the strong aggregation of p-DPP-PhCN is expected such that the HOMO [LUMO] level of the aggregate is destabilized [stabilized] via the enhanced molecular orbital interactions (Fig. 6b). In addition, the LUMO levels are experimentally determined using optical E_g which is subject to vary depending upon the exciton binding energy. Since the strong molecular orbital interactions between molecules would render the FMO wavefunctions of the aggregate significantly delocalized, its excitons are considered to be more like Wannier excitons rather than Frenkel ones and therefore their exciton binding energies are very likely smaller than those of monomer. As a result, LUMO energy of the aggregate tends to be estimated higher than the one that should be. This picture is supported by the fact that the V_OC of the device with p-DPP-PhCN is significantly lower than that with its o-counterpart despite the marginal difference in their LUMO energies (−3.63 eV vs. −3.57 eV).

Photovoltaic performances of ternary cells

To gain the synergistic effect from both the high light absorption of p-DPP-PhCN and high LUMO energy level of o-DPP-PhCN, we used both DPP-PhCNs as the two acceptors in the D/A/A-type ternary cells together with a P3HT donor. For comparison, the total blending ratio of donor to acceptor was adjusted to 1 : 1, and the blending ratios of the two DPP-PhCNs were varied (25 : 75, 50 : 50, and 75 : 25). Fig. 7 and Table 4 show the photovoltaic properties of ternary cells (Device II–IV) with binary cells (Device I and V) for comparison (see also Fig. S8 in ESI†). Interestingly, all ternary cells had enhanced performances compared to the binary cells. Among the devices, Device II had the best PCE of 1.00%, which was more than two times the PCEs of binary cells, and a J_SC of 2.04 mA cm⁻², V_OC of 0.99 V, and FF of 49%. The values of V_OC (0.96–0.99 V) for all three ternary cells (Devices II–IV) are higher than the V_OC (0.56 V) of Device I and similar to the V_OC (1.09 V) of Device V, which has only o-DPP-PhCN as an acceptor. A similar increase in the V_OC of ternary cells was previously reported for other P3HT–PCBM-based D/D/A systems, and it was generally explained by the preferred energy cascade build up in ternary cells.^62,63 As shown in the energy diagram (Fig. 2d), the energy difference between the LUMO energy level of the acceptor and HOMO energy level of the P3HT donor at the D/A surface can be increased by introducing o-DPP-PhCN into P3HT:p-DPP-PhCN, leading to an enhanced V_OC. In addition, the introduction of o-DPP-PhCN into P3HT:p-DPP-PhCN could increase the V_OC of the resulting ternary cells, regardless of the amount (25–75%) of o-DPP-PhCN added. This phenomenon is presumably related to the strong tendency of p-DPP-PhCN to aggregate. In other words, para-substituted acceptor would not mingle with other molecules but drive others to blend with themselves. Consequently, o-DPP-PhCN is expected to dominate the interface with P3HT and thus excitons of donor are to dissociate more likely at the interface with o-DPP-PhCN. More study, however, is warranted to draw a more conclusive picture. The introduction of the third component (in other words, the second acceptor) in ternary cells also improved the J_SC values. Adding 25% of o-DPP-PhCN (Device II) resulted in a 24% increase in J_SC, 2.04 mA cm⁻² from 1.64 mA cm⁻² in Device I. Device IV (1.62 mA cm⁻²) also showed a 36% increase in J_SC compared to Device V (1.19 mA cm⁻²) upon the introduction of p-DPP-PhCN. The increased J_SC values are in accordance with the improved EQE responses, as shown in Fig. 7. More importantly, the EQE intensities were further enhanced in the wavelength regions where the acceptors have light absorption.

Fig. 7 (a) J–V curves, (b) EQE of Devices I–V, and (c) EQE of Device I and II overlaid with the absorption spectra of P3HT, p-DPP-PhCN and o-DPP-PhCN films. The devices were annealed at 120 °C.

Table 4 Photovoltaic performances using chloroform as a solvent with various blend ratios and annealing temperatures^a

Active layer (D : A : A ratio) P3HT : o-DPP-PhCN : p-DPP-PhCN	Temp (°C)	VOC (V)	JSC (mA cm^−2)	FF (%)	PCE^best/PCE^avg (%)
a The standard deviation and average PCE (PCE^avg) based on more than 5 devices.b Binary cells.c Ternary cells.
1.00 : 0.00 : 1.00 (Device I)^b	120	0.56 (±0.02)	1.64 (±0.02)	50 (±3)	0.47/0.43 (±0.03)
1.00 : 0.25 : 0.75 (Device II)^c	120	0.99 (±0.01)	2.04 (±0.06)	49 (±1)	1.00/0.97 (±0.05)
	90	0.98 (±0.00)	1.61 (±0.02)	54 (±2)	0.85/0.82 (±0.02)
	150	0.75 (±0.02)	0.96 (±0.06)	35 (±1)	0.25/0.24 (±0.01)
1.00 : 0.50 : 0.50 (Device III)^c	120	0.96 (±0.01)	1.19 (±0.02)	44 (±2)	0.50/0.47 (±0.03)
	90	0.94 (±0.01)	0.80 (±0.02)	43 (±2)	0.32/0.30 (±0.01)
1.00 : 0.75 : 0.25 (Device IV)^c	120	0.98 (±0.01)	1.62 (±0.03)	45 (±2)	0.72/0.68 (±0.04)
	90	0.96 (±0.02)	1.11 (±0.01)	46 (±1)	0.49/0.48 (±0.02)
	150	0.71 (±0.01)	0.76 (±0.07)	34 (±1)	0.19/0.16 (±0.02)
1.00 : 1.00 : 0.00 (Device V)^b	120	1.09 (±0.01)	1.19 (±0.04)	35 (±2)	0.46/0.44 (± 0.03)

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For example, Device II exhibited a maximum EQE response of 22% at 670 nm, which is the UV absorption maximum for p-DPP-PhCN (Fig. 7c), and this EQE is nearly double that of Device I at the same wavelength. Therefore, the contribution of the electron acceptor to the photocurrent of the devices (through channel II pathway) can be enhanced by adding a second acceptor, and better conduction pathways for excitons and holes can be made in ternary cells. In addition, an AFM image of Device II also showed lower surface root-mean-square (rms) of 3.85 nm than that of Device I (5.22 nm) (Fig. 8), implying that adding small amount of a second acceptor makes smooth surface and the better charge transfer. The similar phenomenon was observed from Device V (rms = 4.10 nm) and IV (rms = 2.98 nm), whereas the rms value of Device III (8.11 nm) was higher than those of Devices II and IV, meaning the importance of adequate blending ratios. In addition, Device II had large features which are approximately 1.2 μm long, 0.8 μm wide and protrude 20 nm from surface (Fig. S9, ESI†). We expect that these features might be resulted from good packing ability of p-DPP-PhCN and a distinct aggregation already started to be appeared in the film annealed at 90 °C (Fig. S9f†).

Fig. 8 AFM images (5 × 5 μm) of the (a) Device I (rms = 5.22 nm), (b) Device II (rms = 3.85 nm), (c) Device III (rms = 8.11 nm), (d) Device IV (rms = 2.98 nm), and (e) Device V (rms = 4.10 nm). The films were annealed at 120 °C.

PL quenching

Charge transfer between P3HT and the acceptors was demonstrated by the PL quenching experiment (Fig. 9).^16,47,64,65 The PL emission spectra were obtained using pristine and blend films. The pristine films of P3HT, p- and o-DPP-PhCN showed PL maxima at 660 nm, 730 nm, and 735 nm, respectively, with excitation wavelengths (λ_ex) of 517 nm (λ_max of P3HT), 610 nm (absorption shoulder peak of p-DPP-PhCN), and 578 nm (λ_max of o-DPP-PhCN), respectively (see Fig. 9a–c, respectively). The PL intensity and the calculated PL quenching efficiency (PLQE) are summarized in Table 5. All blend films, including the binary and ternary cells, showed PL quenching of P3HT at wavelengths between 650 nm and 720 nm (Fig. 9a). The highest PLQE of 93% (at 660 nm) was achieved in Device II and matched well with its highest J_SC value. Fig. 9b and c showed the PL quenching of p- and o-DPP-PhCN, respectively. The λ_ex of Fig. 9b and c are 610 nm (absorption shoulder peak of p-DPP-PhCN) and 578 nm (λ_max of o-DPP-PhCN), respectively. As expected from the EQE responses originating from the absorption of acceptors, the PL of the acceptors was also quenched in two binary cells of Device I and V with PLQEs of 27% and 19%, respectively. The PL of p- and o-DPP-PhCN in Devices II and IV, respectively, was more efficiently quenched by the addition of a second acceptor compared to the corresponding binary cells of Devices I and V, respectively, supporting the idea that the light absorption of acceptors in ternary cells enhances charge generation. For example, the PL of o-DPP-PhCN in Device V (PLQE = 19%) was more effectively quenched by the addition of 25% of p-DPP-PhCN in Device IV (PLQE = 56%).

Fig. 9 PL spectra of P3HT, p-, o-DPP-PhCN, and blend films corresponding to Device I–V. The excitation wavelengths (λ_ex) are (a) 517 nm, (b) 610 nm, and (c) 578 nm.

Table 5 PL intensity and the calculated PLQE of the films

	PL intensity, a.u. (PLQE, %)
	At 660 nm (Fig. 9a)	At 730 nm (Fig. 9b)	At 735 nm (Fig. 9c)
a The pristine films of P3HT, p-, and o-DPP-PhCN showed PL maxima at 660 nm, 730 nm, and 735 nm, respectively, with λex of 517 nm (λmax of P3HT), 610 nm (absorption shoulder peak of p-DPP-PhCN), and 578 nm (λmax of o-DPP-PhCN), respectively.b The blend films were excited at 517 nm, 610 nm, and 578 nm to see the PL quenching of P3HT, p-, and o-DPP-PhCN, respectively.
Pristine films^a
P3HT	0.90 (00)	—	—
p-DPP-PhCN	—	0.60 (00)	—
o-DPP-PhCN	—	—	0.73 (00)
Blend films^b
(P3HT : o-PhCN : p-PhCN)
1.00 : 0.00 : 1.00 (Device I)	0.08 (91)	0.44 (27)	—
1.00 : 0.25 : 0.75 (Device II)	0.06 (93)	0.40 (33)	—
1.00 : 0.50 : 0.50 (Device III)	0.10 (89)	0.50 (16)	0.50 (32)
1.00 : 0.75 : 0.25 (Device IV)	0.07 (92)	—	0.32 (56)
1.00 : 1.00 : 0.00 (Device V)	0.09 (90)	—	0.59 (19)

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Conclusions

We synthesized small molecules of p-, m-, and o-DPP-PhCNs and studied their physical properties by varying the substitution pattern. The superior packing ability of p-DPP-PhCN resulted in a red-shifted, strong UV absorption at 670 nm, where a high EQE response was achieved, indicating charge generation by light absorption of the acceptor. The ternary cells of P3HT:p-DPP-PhCN:o-DPP-PhCN had a higher PCE, up to 1.00%, than binary cells owing to a synergistic effect of the two acceptors. The improved V_OC and J_SC values were explained by the preferred energy cascade and improved light absorption.

Experimental

Materials

Pd₂(dba)₃, tri-tert-butylphosphonium tetrafluoroborate (P(t-Bu)₃ × HBF₄), and potassium phosphate tribasic (K₃PO₄) were purchased from Aldrich. 2-, 3-, and 4-bromobenzonitrile were purchased from TCI. All chemicals were used without further purification, and all reactions were performed under nitrogen atmosphere with anhydrous solvents.

Physical measurements

NMR spectra were recorded on a Bruker AVANCE II 400 spectrometer and Varian Unity Inova 500 MHz spectrometer. Elemental analyses were performed with a Flash EA 1112 series from Thermo Electron Corporation. DSC was performed on a TA instrument Q100 at the heating and cooling rates of 10 °C min⁻¹ under a nitrogen atmosphere. TGA was performed under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹ using a Perkin Elmer TGA7 thermogravimetric analyzer. UV-vis spectra were obtained using a Shimadzu UV/vis spectrometer, and PL spectra were obtained using a Perkin Elmer spectrofluorometer. The oligomer and blend films used in the UV-vis and PL measurements were prepared by spin coating from chloroform solution. The optical E_g were estimated from the absorption onset wavelengths (E_g = 1240/λ_onset (eV)) of the oligomer film. The electrochemical properties of the oligomers were studied by CV with a BAS 100B electrochemical analyzer. A three-electrode system was used and consisted of a non-aqueous reference electrode (0.1 M Ag/Ag⁺ acetonitrile solution), a platinum working electrode, and a platinum wire as a counter electrode. The oxidation potential of the oligomer was measured in acetonitrile with 0.1 M (n-C₄H₉)₄N-PF₆. The films were prepared by dip-coating the oligomer solution onto the platinum working electrode, and the measurements were calibrated using the ferrocene value −4.8 eV as the standard. XRD patterns were recorded in the reflection mode at 30 kV and 60 mA with a scanning rate of 0.03° per 60 s and Cu Kα radiation (with wavelength λ = 1.5406 nm). The oligomer films for XRD analysis were prepared on quartz glass substrates by drop-casting oligomers dissolved in chloroform, followed by annealing at 80° for 10 min. The AFM images were obtained with a Scanning Probe Microscope (XE-100) from Park Systems. The AFM measurements were performed on the same samples as were used in the OPV devices. Geometry optimizations were conducted by means of Density Functional Theory (DFT) method employing B3LYP hybrid functional and 6-31G(d) basis sets. Subsequent vibrational frequency calculations were carried out to confirm all the structures to be local minima. Neutral ground-state structures were derived via spin-restricted calculations while radical ionic structures were optimized via spin-unrestricted calculations. Then, further single point calculations were performed at B3LYP/6-31+G(d,p) level to obtain the energetic results, such as the adiabatic IP and EA values which were obtained via the ΔSCF method. All the calculations were carried out with Gaussian 09 program package.⁶⁶

Fabrication of OPVs devices

PEDOT:PSS (Clevios P VP AI 4083) was purchased from EM Index and Heraeus. In this study, we fabricated the devices including binary and ternary cells with a structure of ITO/PEDOT:PSS/active layer/LiF/Al using P3HT as a donor and oligomers as acceptors in the active layer. The ITO glass was cleaned with sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropyl alcohol for 15 min each and subsequently dried in an oven for 5 h. The ITO-coated glass substrates were pre-treated in a UV–ozone oven for 15 min. A layer of PEDOT:PSS (∼30 nm) was spin-coated on top of the ITO-coated glass substrates. The BHJ active layers were spin-cast at 3000 rpm from a D/A solution in chloroform and chlorobenzene with a total solids concentration of 15 and 20 mg mL⁻¹, respectively. The average thickness of the active layers (∼100 nm) was measured with an Alpha-Step IQ surface profiler. A LiF (∼0.5 nm) and Al (∼100 nm) layer were directly deposited on the active layer under a vacuum of ∼10⁻⁶ Torr. The effective area of all devices was measured to be 9 mm². The J–V curves of the devices were measured using a computer-controlled Keithley 236 source measure unit. The characterization of un-encapsulated solar cells was carried out in air under white light AM 1.5G illumination, 100 mW cm⁻², using a xenon lamp-based solar simulator. The simulator irradiance was characterized using a calibrated spectrometer, and the illumination intensity was set using an NREL-certified silicon diode with an integrated KG1 optical filter. The EQE was measured using a reflective microscope objective to focus the light output from a 100 W halogen lamp outfitted with a monochromator and optical chopper (PV Measurements, Inc.). The photocurrent was measured using a lock-in amplifier, and the absolute photon flux was determined using a calibrated silicon photodiode.

Synthesis of small molecules

DPP-B was synthesized according to the literature.^34,52 p-, m-, and o-DPP-PhCN were synthesized using a palladium-catalyzed Suzuki coupling reaction between DPP-B and the corresponding bromides, 4-, 3-, and 2-bromobenzonitrile, respectively.⁵²

Synthesis of 3,6-bis(5-(4-cyanophenyl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (p-DPP-PhCN). Degassed 1.7 mL of H₂O solution of K₃PO₄ (0.50 g, 2.36 mmol) was added to degassed 15 mL of THF solution of DPP-B (0.60 g, 0.77 mmol), Pd₂(dba)₃ (0.019 g, 0.021 mmol), P(t-Bu)₃ × HBF₄ (0.012 g, 0.041 mmol), and 4-bromobenzonitrile (0.56 g, 3.08 mmol) under nitrogen. After the mixture was heated to 80 °C for 3 h, the reaction was cooled to room temperature. The mixture was extracted with dichloromethane and water. The collected organic layer was dried over MgSO₄. After removing the solvent under reduced pressure, the crude product was purified by column chromatography on silica using dichloromethane as the eluent, to afford p-DPP-PhCN (0.48 g, yield 86%) as purple crystals. ¹H NMR (CDCl₃, 400 MHz): δ (ppm) 8.96 (d, J = 4.2 Hz, 2H), 7.79 (d, J = 8.5 Hz, 4H), 7.74 (d, J = 8.6 Hz, 4H), 7.59 (d, J = 4.2 Hz, 2H), 4.10 (m, 4H), 1.94 (m, 2H), 1.28–1.44 (m, 16H), 0.94 (t, J = 7.5 Hz, 6H), 0.89 (t, J = 7.0 Hz, 6H). ¹³C NMR (1,1,2,2-tetrachloroethane-d₂, 125 MHz): δ (ppm) 161.91, 147.09, 140.14, 137.70, 136.62, 133.26, 131.20, 126.82, 126.66, 118.77, 116.63, 116.54, 112.32, 109.64, 46.59, 39.74, 30.97, 29.03, 24.39, 23.32, 14.24, 11.06. Anal. calcd for C₄₄H₄₆N₄O₂S₂: C, 72.69; H, 6.38; N, 7.71; O, 4.40; S, 8.82. Found: C, 72.86; H, 6.42; N, 7.25; S, 8.34%.

Synthesis of 3,6-bis(5-(3-cyanophenyl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (m-DPP-PhCN). m-DPP-PhCN was prepared in the same manner as p-DPP-PhCN using degassed 1.23 mL of H₂O solution of K₃PO₄ (0.38 g, 1.77 mmol), degassed 10.7 mL of THF solution of DPP-B (0.45 g, 0.58 mmol), Pd₂(dba)₃ (0.015 g, 0.016 mmol), P(t-Bu)₃ × HBF₄ (0.009 g, 0.03 mmol), and 3-bromobenzonitrile (0.42 g, 2.32 mmol), to afford m-DPP-PhCN (0.37 g, yield 88%) as a brown solid. ¹H NMR (CDCl₃, 400 MHz): δ (ppm) 8.95 (d, J = 4.1 Hz, 2H), 7.96 (s, 2H), 7.92 (d, J = 7.9 Hz, 2H), 7.67 (d, J = 7.7 Hz, 2H), 7.58 (dd, J = 7.9, 7.7 Hz, 2H), 7.54 (d, J = 4.1 Hz, 2H), 4.10 (m, 4H), 1.93 (m, 2H), 1.31–1.45 (m, 16H), 0.94 (t, J = 7.4 Hz, 6H), 0.92 (t, J = 7.0 Hz, 6H). ¹³C NMR (CDCl₃, 100 MHz): δ (ppm) 161.63, 146.40, 139.80, 136.58, 134.46, 131.79, 130.17, 130.13, 130.09, 129.37, 125.72, 118.17, 113.66, 108.83, 46.06, 39.28, 30.35, 28.58, 23.72, 23.09, 14.07, 10.58. Anal. calcd for C₄₄H₄₆N₄O₂S₂: C, 72.69; H, 6.38; N, 7.71; O, 4.40; S, 8.82. Found: C, 72.88; H, 6.39; N, 7.21; S, 8.38%.

Synthesis of 3,6-bis(5-(2-cyanophenyl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (o-DPP-PhCN). o-DPP-PhCN was prepared in the same manner as p-DPP-PhCN using degassed 1.23 mL of H₂O solution of K₃PO₄ (0.38 g, 1.77 mmol), degassed 10.7 mL of THF solution of DPP-B (0.45 g, 0.58 mmol), Pd₂(dba)₃ (0.015 g, 0.016 mmol), P(t-Bu)₃ × HBF₄ (0.009 g, 0.03 mmol), and 2-bromobenzonitrile (0.42 g, 2.32 mmol), to afford o-DPP-PhCN (0.37 g, yield 88%) as a purple solid. ¹H NMR (CDCl₃, 400 MHz): δ (ppm) 8.93 (d, J = 4.2 Hz, 2H), 7.81 (dd, J = 7.8, 0.8 Hz, 2H), 7.78 (d, J = 4.2 Hz, 2H), 7.71 (dd, J = 7.9, 0.9 Hz, 2H), 7.66 (td, J = 7.7, 1.3 Hz, 2H), 7.48 (td, J = 7.6, 1.4 Hz, 2H), 4.07 (m, 4H), 1.94 (m, 2H), 1.26–1.44 (m, 16H), 0.91 (t, J = 7.4 Hz, 6H), 0.86 (t, J = 7.0 Hz, 6H). ¹³C NMR (CDCl₃, 100 MHz): δ (ppm) 161.55, 144.26, 139.81, 136.12, 136.09, 134.67, 133.18, 131.21, 129.69, 128.73, 128.63, 118.37, 110.26, 108.88, 46.10, 39.25, 30.25, 28.45, 23.62, 23.09, 14.05, 10.51. Anal. calcd for C₄₄H₄₆N₄O₂S₂: C, 72.69; H, 6.38; N, 7.71; O, 4.40; S, 8.82. Found: C, 72.65; H, 6.27; N, 7.17; S, 8.23%.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A3005083).

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra, additional normalized UV absorption spectra, J–V characteristics under various conditions, and AFM images (10 × 10 μm) based on three acceptors. See DOI: 10.1039/c4ra12184h

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