Introduction

The utilization of solar energy for the generation of electricity has become one of the most critical subjects in the renewable energy arena. Organic photovoltaics (OPVs) have attracted considerable attention in both academia and industry because of their advantages compared with solar cells based on inorganic materials, including their low weight, flexibility, low cost and ease of processing.1, 2 It has been well established that the active layer of OPVs is typically composed of a blend of a π-conjugated polymer as an electron donor (p-type) and a fullerene derivative as an electron acceptor (n-type), and these materials form an interpenetrating bulk-heterojunction (BHJ) structure to maximize the internal donor–acceptor (D–A) interfacial area for efficient photoinduced charge separation.3, 4, 5 To increase the power conversion efficiencies (PCEs), with regard to the electronic structure design, p-type materials must have the following properties: (1) a low optical band gap to broaden the absorption range of solar light, which contributes to an increase in the short-circuit current (JSC), and (2) a low-lying highest occupied molecular orbital (HOMO) energy level to increase the open-circuit voltage (VOC), which depends on the difference between the HOMO energy level of the p-type material and the energy level of the lowest unoccupied molecular orbital (LUMO) of the n-type material. Therefore, the development of D–A-conjugated copolymers based on alternating electron-rich and electron-deficient units has been recognized as a straightforward approach to realize an important class of semiconductor materials for OPVs.6, 7 In this context, a variety of π-conjugated electron-rich components represented by dithieno[3,2-b:2',3'-d]silole (DTS)8 and benzo[1,2-b:4,5-b']dithiophene (BDT)9 have been developed and utilized as donor units in D–A copolymers. On the other hand, the development of new acceptor units is still vital for synthesizing D–A copolymers because effective acceptor units are limited to 2,1,3-benzothiadiazole,8 diketopyrrolopyrrole,10 thieno[3,4-c]pyrole-4,6-dione,11 thieno[3,4-b]thiophene with electron-withdrawing substitutents12 and naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole.13 We have investigated the development of new electron-accepting materials for n-channel organic field-effect transistors (OFETs) and have applied the developed electron-deficient units to D–A copolymers for OPVs.

We previously reported that the hexafluorocyclopenta[c]thiophene (F)14 repeated oligothiophenes (F2, F4 and F6) (Figure 1a) clearly showed a positive shift of the electrochemical reduction potential and red shift of the absorption maximum as the number of the F units increased.15 These results clearly indicate that the annelation of hexafluorocyclopentene at the thiophene rings contributes to an increase in the electron-accepting nature of π-conjugated systems without disrupting the effective conjugation. Subsequently, we have systematically designed and synthesized a series of electron-deficient thiophene units annelated with a ring that contains the electron-withdrawing functional groups.16, 17, 18, 19 For example, the LUMO energy level of terthiophene BCB comprised of difluorodioxocyclopenta-[b]thiophene (B) and -[c]thiophene (C) units was estimated to be −4.17 eV; this value is low enough to exhibit an n-type semiconducting nature in OFETs.16 To improve the solubility in organic solvents, we synthesized quinquethiophene BHCHB, which consists of the difluorodioxocyclopentene-annelated thiophene and 3-hexylthiophene (H).17 Although the introduction of electron-donating H units elevated the LUMO energy level to −3.54 eV, bottom-gate bottom-contact OFETs (Figure 1b) fabricated by spin-coating showed n-channel characteristics with an electron mobility of the order of 10−4 cm2 V−1 s−1. Furthermore, to decrease the LUMO energy level while retaining good solution processability, we designed dialkyl-substituted electron-deficient naphtho[2,3-c]thiophene-4,9-dione units (C4 and C6) to develop BC4B and BC6B.18 These molecules showed a substantial improvement of two orders of magnitude in the performance of n-type OFETs (Figure 1c). On account of the effectiveness of our molecular design of electron-transporting OFET materials, we have designed a series of dioxocycloalkene-annelated thiophenes (C, Ch, Chy, C4, C6, CEH, C8, and T6) to be incorporated into D–A-type copolymers (Figure 2).20, 21, 22, 23 Several dioxocycloalkene-annelated compounds have also been developed by other groups and utilized as acceptor units in D–A-type copolymers.24, 25, 26, 27, 28, 29, 30, 31, 32 In this study, to systematically investigate the potential of these acceptor units in the copolymers, the donor units were fixed to DTS and BDT. All the copolymers listed in this review were synthesized by Stille-coupling reactions between dibrominated acceptor units and distannylated donor units.

Figure 1
figure 1

(a) Chemical structures of electron-accepting cycloalkene-annelated oligothiophenes with electron-withdrawing functional groups. (b) Schematic structure of bottom-gate bottom-contact OFET devices. (c) Output characteristics of BC4B-based OFET device. A full color version of this figure is available at the Polymer Journal online.

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Figure 2
figure 2

Our developed dioxocycloalkene-annelated thiophenes. Density functional theory calculations were performed at the B3LYP/6-31G(d, p) level. All the solubilizing alkyl groups were replaced with methyl groups to simplify the calculations.

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Difluorodioxocyclopenta[c]thiophene

First, we synthesized C-containing D–A copolymers DTS-C and DTS-D-C, where D denotes 3-dodecylthiophene. The target copolymers exhibited moderate solubility in chlorinated solvents, such as chloroform (CHCl3), chlorobenzene and o-dichlorobenzene (o-DCB), and their number-averaged molecular weights (Mns) were determined to be 6.1 kg mol−1 for DTS-C and 15.9 kg mol−1 for DTS-D-C, with molar-mass dispersities (DMs) of 1.80 and 1.38, respectively (Figure 3a, Table 1).20

Figure 3
figure 3

(a) Chemical structures of DTS-C and DTS-D-C. (b) UV-vis absorption spectra of DTS-C (black) and DTS-D-C (blue) in CHCl3 solution (solid line) and as films (dashed line). A full color version of this figure is available at the Polymer Journal online.

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Table 1 Characteristics of copolymers
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Ultraviolet–visible (UV-vis) absorption spectra of DTS-C and DTS-D-C were measured both in CHCl3 solution and as films on quartz plates (Figure 3b). The spectra in CHCl3 solution showed a broad absorption band that covers the region between 400 and 950 nm. Both the copolymers showed two absorption bands, which is a typical feature of the D–A configuration. The weak absorption band at a shorter wavelength corresponds to the local π–π* transition of the polymer backbone, whereas the strong main band at longer wavelength is attributed to the intramolecular charge-transfer (CT) transition between the donor and acceptor units. The red shift of the charge-transfer-transition maximum (λmax) for DTS-C (721 nm) compared with that of DTS-D-C (672 nm) implies that the absence of the D unit in the conjugated backbone contributes to an increase in the D–A nature. The absorption spectra of the polymer films were slightly red shifted compared with those obtained in solution, indicating an increase in planar conformations or a presence of π–π interchain interactions. The optical HOMO–LUMO energy gaps (ΔEgopts) for DTS-C and DTS-D-C were determined to be 1.38 and 1.44 eV, respectively, from the onset energy of absorption measured for the films. The HOMO energy levels (EHOMOs) of DTS-C and DTS-D-C were estimated to be −5.51 and −5.38 eV, respectively, from the onset potentials (vs Fc/Fc+) of oxidation observed in the cyclic voltammograms of the films; the energy level of Fc/Fc+ is assumed to be −4.8 eV below the vacuum level. Note that EHOMO for DTS-D-C determined by the ionization energy using photoelectron spectroscopy in air (PESA) was −5.30 eV, which is close to the EHOMO value determined from cyclic voltammetry. From the EHOMO and ΔEgopt values, the LUMO energy levels (ELUMOs) of DTS-C and DTS-D-C were estimated to be −4.13 and −3.94 eV, respectively. On the basis of the onset potentials of the first reduction waves, the ELUMOs of DTS-C and DTS-D-C were electrochemically estimated to be −3.96 and −3.73 eV, respectively.

The hole mobilities of DTS-C and DTS-D-C were measured using OFETs with a bottom-contact and bottom-gate configuration (Figure 1b). The DTS-D-C film exhibited typical p-type OFET characteristics with a hole mobility of 3.5 × 10−4 cm2 V−1 s−1. On the other hand, the DTS-C-based device did not show any OFET response. On the basis of this result and the fact that the ELUMO of DTS-C is too low lying compared with that (−3.80 eV) of typical n-type OPV materials with [6,6]-phenyl-Cx-butyric acid methyl ester (PCxBM, x=61 or 71), we only investigated the photovoltaic characteristics of DTS-D-C.

The OPV performance was investigated using a BHJ solar cell with a standard configuration of glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)/DTS-D-C:PC71BM (1:2 w/w)/Al. The active BHJ layer was prepared by spin coating an o-DCB solution of a DTS-D-C/PC71BM blend. This device showed typical OPV characteristics under AM 1.5G simulated solar illumination (100 mW cm−2) with a PCE of 0.77%, a VOC of 0.88 V, a JSC of 1.9 mA cm−2 and a fill factor (FF) of 0.46 (Table 2). The value of JSC is relatively low, which results in the poor PCE value, although the VOC and FF values are reasonably high. We rationalized that the low-lying LUMO energy level of DTS-D-C prevented efficient charge separation between the copolymers and PC71BM, which resulted in the low JSC.

Table 2 OPV characteristics of copolymers
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Dihexyldioxocyclopenta[c]thiophene and (dihexylmethylidene)dioxocyclopenta[c]thiophene

As described in the preceding section, owing to the insufficiency of solubilizing alkyl groups in the conjugated backbone, the Mn of DTS-C was limited to 6.1 kg mol−1.20 More importantly, the strong electron-withdrawing nature of the C unit led to an excessive decrease in the LUMO energy level of the resulting D–A copolymers. To overcome this, we decided to introduce solubilizing groups and to concomitantly fine-tune the orbital energy levels by replacing the fluorine substituents in the C unit. Based on this concept, we designed dihexyldioxocyclopenta[c]thiophene (Ch) and (dihexylmethylidene)dioxocyclopenta[c]thiophene (Chy).21 In the Ch unit, we substituted the fluorine atoms of the C unit with hexyl groups while retaining a tetrahedral carbon atom; the Chy unit is a planar structure with a dihexylmethylidene group (Figure 2). Density functional theory calculations indicated that the replacement of fluorine atoms with alkyl groups resulted in an increase of both the HOMO and LUMO energy levels. To investigate the effectiveness of these modified structures as acceptor units, we developed new D–A copolymers with the DTS donor unit (DTS-Ch and DTS-Chy) and donor-part-extended copolymers (DTS-T-Ch and DTS-T-Chy) (Figure 4a). Owing to the solubilizing alkyl groups in the Ch and Chy units, we can adopt pristine thiophene (T) as the π-bridge instead of 3-dodecylthiophene (D) in DTS-D-C, which might induce better π–π interactions in BHJ active layers. The Mn and DM values were measured using high-temperature gel permeation chromatography at 140 °C with o-DCB as an eluent; DTS-Ch, DTS-T-Ch, DTS-Chy and DTS-T-Chy had Mns of 8.3, 11.7, 6.3 and 7.9 kg mol−1, respectively, and DMs of 1.6, 2.0, 1.7 and 2.5, respectively. The dialkylated tetrahedral structure increased the solubility and thus the Mns of Ch-based copolymers were higher than those of Chy-based copolymers.

Figure 4
figure 4

(a) Chemical structures, (b) UV-vis absorption spectra and (c) energy diagram of DTS-Ch, DTS-Chy, DTS-T-Ch and DTS-T-Chy. (d) Current density–voltage curves of the OPV devices based on blend of DTS-T-Ch and DTS-T-Chy/PC71BM. (e) Atomic force microscopic height images of DTS-T-Ch/PC71BM without DIO and with 3% DIO.

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The red shift observed in the absorption maxima of DTS-Ch and DTS-Chy in solution compared with those of DTS-T-Ch and DTS-T-Chy indicated that the absence of T between the donor and acceptor units contributes to an increase in the D–A characteristics (Figure 4b). This trend is similar to what is observed in the DTS-D-C and DTS-C systems, as described above. The shoulder at approximately 750 nm observed for DTS-T-Chy indicated the appearance of interchain π–π interactions even when dilute solutions were used. Although the absorption spectra of DTS-Ch and DTS-T-Ch in the films showed no noticeable difference from their corresponding solution spectra, the CT transition bands of DTS-Chy and DTS-T-Chy in the films clearly broadened and were red shifted compared with those observed in solution. This result indicates that the coplanar conformation between the dihexylmethylidene group and the cyclopenta[c]thiophene skeleton may facilitate cofacial interchain aggregations. As a result, the ΔEgopts of DTS-Chy (1.57 eV) and DTS-T-Chy (1.56 eV) are smaller than those of DTS-Ch (1.68 eV) and DTS-T-Ch (1.73 eV). On the other hand, as summarized in Table 1, both the low-lying ELUMOs and EHOMOs of DTS-Ch and DTS-Chy compared with those of DTS-T-Ch and DTS-T-Chy indicate that the absence of thiophene between the donor and acceptor units contributes to an increase in the electron-accepting characteristics of the conjugated backbones. It should also be mentioned that, as we expected, the ELUMOs of all these copolymers are higher than those of DTS-C and DTS-D-C,20 which can be attributed to the relatively weak electron-accepting ability of the Ch and Chy units compared with the C unit.

To investigate the OPV performances of the copolymers, BHJ solar cells were fabricated with a typical layered configuration of glass/ITO/PEDOT:PSS/active layer/Ca/Al. As summarized in Table 2, all the devices composed of the copolymer and PC61BM showed photovoltaic behavior. The OPV device based on DTS-Ch showed a markedly high VOC of 0.95 V. This result can be attributed to the relatively low-lying HOMO energy level of DTS-Ch because the VOC of OPVs is proportional to the difference between the HOMO energy level of the donor and the LUMO energy level of the acceptor.33 In fact, the trend of the observed VOC values is in qualitative agreement with the relative HOMO energy levels of the donor polymers (Figure 4c). The devices based on DTS-T-Ch and DTS-T-Chy showed higher Jsc and FF values than the devices based on DTS-Ch and DTS-Chy. This phenomenon is consistent with the fact that the balanced ambipolar (hole and electron) mobilities of DTS-T-Ch/PC61BM and DTS-T-Chy/PC61BM blend films were observed in space-charge-limited current (SCLC) measurements, whereas DTS-Ch/PC61BM and DTS-Chy/PC61BM blend films exhibited only unipolar (hole) mobilities. Overall, the DTS-T-Ch- and DTS-T-Chy-based devices exhibited moderate PCEs of 2.68% and 2.65%, respectively.

The active layers fabricated from a blend of DTS-T-Ch and DTS-T-Chy with PC71BM showed improved PCEs of 2.84 and 3.43%, respectively (Figure 4d and Table 2). This improvement of PCEs is generally attributed to the increased absorption of PC71BM in the visible region compared with PC61BM, which resulted in improved JSC values. Furthermore, although the addition of 3% 1,8-diiodooctane (DIO)34 to the processing solvent for the DTS-T-Chy-based device did not considerably improve the OPV performance, the addition of 3% DIO to o-DCB for DTS-T-Ch/PC71BM led to a drastic increase of Jsc (10.1 mA cm−2) and FF (0.64) while the VOC remained at a high level (0.80 V); this resulted in a substantial improvement of the overall device performance to a PCE of 5.17%. The X-ray diffraction patterns of DTS-T-Ch/PC71BM films showed a broad peak at approximately 2θ=20°, which is ascribed to π–π stacking between the backbones of the copolymers, irrespective of whether DIO was added during processing. This phenomenon is rationalized by possible cofacial interchain aggregations derived from the coplanar backbone conformation of the copolymer, and the presence of these aggregations does not depend on the addition of DIO. On the other hand, the atomic force microscopic images of the DTS-T-Ch-containing blend films clearly indicated that the addition of DIO caused a substantial increase in the domain size with an average roughness (Ra) value of 2.25 nm, which differs from the value of 1.90 nm for the blend film without DIO (Figure 4e). This result indicates that the addition of DIO contributed to the formation of appropriate phase separation in the blend films.

Dihexyl-substituted naphtho[2,3-c]thiophene-4,9-dione and 4H,8H-benzo[1,2-c:4,5-c]dithiophene-4,8-dione

As an alternative way of tuning the energy level of DTS-C by replacing the strongly electron-deficient C unit, we focused on the C6 unit because the replacement of C with C6 in a BCB system resulted in an increase of approximately 0.3 eV in the LUMO energy level for BC6B (the structures are shown in Figure 1a).18 This result indicates that the electron-accepting ability of C6 is weaker than that of C. To investigate the influence of the annelated dihexyl-substituted benzene unit on the electronic properties and OPV performance of D–A copolymers, we also designed 5,7-dihexyl-4H,8H-benzo[1,2-c:4,5-c]dithiophene-4,8-dione (T6) as a new electron-deficient unit (Figure 2). Density functional theory calculations suggest that the frontier orbital energy levels of both C6 and T6 are comparable to those of the Ch and Chy units (Figure 2). Moreover, their extended π-systems compared with Ch and Chy are expected to facilitate cofacial interchain aggregations of resulting D–A copolymers and hence to enhance carrier transport in blend OPV films. We therefore developed copolymers DTS-C6, DTS-T6, BDT-C6 and BDT-T6 (Figure 5a).22 Using analytical gel permeation chromatography with CHCl3 as an eluent, DTS-C6, DTS-T6, BDT-C6 and BDT-T6 have Mns of 19.5, 19.1, 18.6 and 18.8 kg mol−1, respectively, and DMs of 1.51, 1.46, 2.38, and 3.53, respectively (Table 1). Note that DTS-C6 with an Mn of 19.5 kg mol−1 is hereafter designated DTS-C6-L.

Figure 5
figure 5

(a) Chemical structures, (b) UV-vis absorption spectra and (c) energy diagram of DTS-C6, DTS-T6, BDT-C6 and BDT-T6.

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As shown in Figure 5b and Table 1, the absorption maxima of DTS-containing copolymers in CHCl3 solution (600 nm for DTS-C6 and 581 nm for DTS-T6) are red shifted compared with those of BDT-containing copolymers (543 nm for BDT-C6 and 539 nm for BDT-T6). A similar trend has also been observed for other BDT- and DTS-based copolymers.35 The absorption spectra of the copolymer films were red shifted compared with those in solution, which indicates the increase in planar conformations and/or the presence of π–π interchain interactions in the solid state. From the onset of the UV-vis absorption spectra of the films, the ΔEgopts of DTS-C6, DTS-T6, BDT-C6 and BDT-T6 were estimated to be 1.63, 1.72, 1.86 and 1.92 eV, respectively. These values indicate that the C6 unit contributes to the narrowed ΔEgopt of D–A copolymers, compared with the T6 unit.

The ionization potentials evaluated by PESA for DTS-C6, DTS-T6, BDT-C6 and BDT-T6 correspond to EHOMOs of −5.19, −5.21, −5.23 and −5.32 eV, respectively. Considering the differences in these energy levels, the overall behavior of the EHOMOs primarily depends on the electron-donating ability of the BDT and DTS units. The ELUMOs of these copolymers (estimated from cyclic voltammetry (Table 1)) are approximately 0.2–0.3 eV higher than the ELUMO of DTS-D-C, so an improvement in the charge separation between these copolymers and fullerene derivatives is expected (Figure 5c).

The OPV performance of the copolymers was investigated in BHJ solar cells with a conventional device configuration of ITO/PEDOT:PSS/copolymer:PC61BM/Al. All the copolymers showed typical photovoltaic responses, and the representative photovoltaic parameters are summarized in Table 2. All the devices exhibited relatively high VOC values (0.83–0.98 V), which reflects the low-lying EHOMOs of the copolymers. Interestingly, depending on the chemical structures of the D–A copolymers, the devices exhibited significant differences in their PCEs, which is primarily a reflection of the different JSC values: the devices with DTS-containing polymers (DTS-C6-L and DTS-T6) exhibited good PCEs of 4.12% and 2.70%, respectively, whereas those with BDT-containing polymers (BDT-C6 and BDT-T6) exhibited poor PCEs of <1%. This disparity between these polymers can be attributed to a difference in surface morphology; surface atomic force microscopic images showed that the blends of DTS-containing polymers formed aggregated nanometer-scale domains without apparent phase segregation, whereas the blends of BDT-containing polymers formed large segregated domains. These results indicate that the morphologies of the blends of DTS-containing polymers are beneficial for efficient charge separation and charge transport, which resulted in the observed difference in the PCEs. Furthermore, the device with DTS-C6-L had a higher PCE value compared with the device with DTS-T6, which was mainly attributed to the JSC value. This high JSC value is attributed to the higher field-effect hole mobility of DTS-C6-L (1.4 × 10−4 cm2 V−1 s−1) than of DTS-T6 (3.2 × 10−5 cm2 V−1 s−1).

Fine tuning of dialkyl-substituted naphtho[2,3-c]thiophene-4,9-dione

To further investigate the photovoltaic performance of DTS-C6-L, OPV devices were fabricated using PC71BM as the n-type material with the device structure of glass/ITO/PEDOT:PSS/ DTS-C6-L:PC71BM (1:2 weight ratio)/Ca/Al. This device retained a high VOC of 0.90 V. Nevertheless, the PCE was not considerably enhanced and was limited to 4.87% mainly because of moderate JSC and FF values. This was partially considered as a result of the low Mn of DTS-C6-L because Mn is known to influence the photophysical properties of polymers and the morphology of BHJ films.36 The copolymer side chains are also known to have an important role in controlling the film morphology, which determines OPV performance.37 Therefore, to obtain a high PCE, we fine-tuned the following: (1) the synthesis of copolymers to obtain a higher Mn, (2) the alkyl chain components in the naphtho[2,3-c]thiophene-4,9-dione unit (C4, C6, CEH and C8) (Figure 6a) and (3) the thin-film fabrication process for BHJ blends with PC71BM.23

Figure 6
figure 6

(a) Chemical structures and (b) UV-vis absorption spectra of DTS-C4, DTS-C6, DTS-CEH and DTS-C8 in CHCl3 solution (dashed line) and as films (solid line). (c) Current density–voltage curves and (d) external quantum efficiency spectra of the DTS-C6/PC71BM OPV devices fabricated using o-DCB (blue), o-DCB/DIO (black) and o-DCB/CHCl3/DIO (red). (e) In-plane (left) and out-of-plane (right) two-dimensional grazing incidence X-ray diffraction profiles for the DTS-C6/PC71BM blend films fabricated using o-DCB (blue), o-DCB/DIO (black) and o-DCB/CF/DIO (red).

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The distannylated DTS unit of the Stille-coupling copolymerization component, 4,4-bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4H-dithieno[3,2-b:2',3'-d]silole (DTS-Sn), was carefully purified by reverse-phase preparative liquid chromatography to guarantee the stoichiometry of DTS-Sn relative to its dibromo counterpart during the copolymerization. As a result, the improved purity of DTS-Sn leads to a drastic increase in the molecular weight of the resulting copolymers. The Mns for DTS-C4, DTS-C6, DTS-CEH and DTS-C8were determined to be 41.5, 42.8, 53.4 and 46.3 kg mol−1, respectively, from analytical gel permeation chromatography with CHCl3 as an eluent, and the DMs were measured to be 2.0, 1.7, 1.7 and 2.0, respectively (Table 1). The differential scanning calorimetry profiles and X-ray diffraction patterns indicated that these copolymers intrinsically adopt an amorphous morphology, regardless of the Mns and alkyl chain substituents.

The physical properties of DTS-C4, DTS-C6, DTS-CEH and DTS-C8 are summarized in Figure 6b and Table 1. From the UV-vis spectra, these copolymers exhibited similar spectroscopic shapes and peak wavelengths even as films, which indicates that the alkyl groups in the acceptor units have little influence on the interchain interactions in the solid state. The ΔEgopts of these copolymers, extracted from the film absorption onset values, are ca. 1.62–1.64 eV. The ionization potential of DTS-C6, obtained from PESA, was nearly at the same level as DTS-C6-L, and the EHOMOs for all the copolymers were determined to be in a narrow range between −5.25 and −5.33 eV. Based on these EHOMO and Egopt values, their ELUMOs were also calculated in a narrow range between −3.63 and −3.70 eV.

To investigate the influence of molecular weights, the photovoltaic properties of DTS-C6 and DTS-C6-L were evaluated using PC71BM as the acceptor. For an 80-nm-thick BHJ film, DTS-C6-L delivered a moderate PCE of 4.55%. In contrast, the DTS-C6-based device exhibited an improved PCE of 5.21% (Table 2). This PCE improvement is mainly ascribed to the increased JSC and FF, which are derived from the morphological change and increased SCLC hole mobility of the BHJ layer (1.0 × 10−4 cm2 V−1 s−1 for DTS-C6 vs 3.4 × 10−5 cm2 V−1 s−1 for DTS-C6-L). Under the same fabrication conditions, DTS-CEH exhibited a comparable PCE of 5.15%. In contrast, although the VOC values were similarly high, the devices with DTS-C4 and DTS-C8 resulted in lower overall efficiencies. This deterioration is due to the decreased FF for DTS-C4 and the reduced JSC for DTS-C8. We attributed the low FF for DTS-C4 to its low hole mobility (7.8 × 10−5 cm2 V−1 s−1). On the other hand, from the atomic force microscopic measurements, which showed the largest estimated Ra value of 3.2 nm among these five blend films, we concluded that the relatively high alkyl chain density for DTS-C8 may cause poor intermixing with PC71BM, which would hamper efficient exciton dissociation and result in low JSC.

We attempted to optimize spin-coating solvents and additives for the DTS-C6/PC71BM blend films to further improve the OPV performance by controlling the film morphology. Upon adding 3% (v/v) DIO to the spin-coating solvent (o-DCB), the PCE further increased to 5.56%, which was attributed to the increased FF. The SCLC measurements indicated that the addition of DIO led to an increase in both the hole and electron mobilities without sacrificing the carrier mobility balance (Table 3). When CHCl3 with 3% DIO was used as the solvent instead of o-DCB with 3% DIO, the device exhibited a slightly decreased PCE of 5.42%. However, the thicker film (190 nm) formed as a result of these fabrication conditions led to a markedly high JSC of 15.54 mA cm−1, which was derived from improved light absorption, whereas FF was considerably reduced owing to the decreased overall conductance. As summarized in Table 3, the SCLC electron mobility of the CHCl3:DIO-processed film was 7.5 × 10−6 cm2 V−1 s−1, which is almost an order of magnitude lower than that of the o-DCB:DIO-processed film (3.6 × 10−5 cm2 V−1 s−1). To maintain both the high FF (o-DCB/DIO) and high JSC (CHCl3/DIO), a ternary solvent composition of o-DCB/ CHCl3 with 3% DIO was used in a 4:1 volume ratio. The OPV device based on DTS-C6/PC71BM (with a 1:2 weight ratio) exhibited the best PCE of 7.85% (Figure 6c). The external quantum efficiency spectrum of this device showed a high photoresponse between 300 and 800 nm with a maximum value of 70% at 500 nm (Figure 6d). In the o-DCB/CHCl3/DIO-processed films, the SCLC measurements showed an increase in their hole- (2.0 × 10−4 cm2 V−1 s−1) and electron-transporting (7.0 × 10−5 cm2 V−1 s−1) characteristics.

Table 3 SCLC mobilities of DTS-C6/PC71BM blend films
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Two-dimensional grazing incidence X-ray diffraction was used to further investigate the microstructures of the o-DCB, o-DCB/DIO and o-DCB/CF/DIO films. The two-dimensional grazing incidence X-ray diffraction (2D GIXD) profiles are shown in Figure 6e. In all the films, no significant differences were observed in the degree of crystallinity for PC71BM. In contrast, visible differences can be detected for DTS-C6. Thin films fabricated with o-DCB do not show the diffraction peaks derived from DTS-C6, which indicates an amorphous nature of the copolymer in this BHJ film. When the thin films were fabricated using o-DCB/DIO, a weak diffraction peak is observed at 1.6 Å−1 in the qz axis. This peak corresponds to the π–π stacking in DTS-C6. In addition, this peak becomes slightly more intense after o-DCB/CF/DIO processing was performed, which indicates an improved ordering of DTS-C6. Thus we concluded that the use of the ternary solvent in the amorphous DTS-C6/PC71BM system generates larger domains accompanied by subtly ordered π–π stacking of DTS-C6 backbones, which may contribute to enhanced carrier mobility and charge-separation efficiency, and lead to improved photovoltaic performance.

Summary and outlook

In this focused study, we investigated the use of new low band-gap D–A copolymers based on dioxocycloalkene-annelated thiophenes as electron-acceptor units. Our systematic structural modifications of the acceptor units can tune their energy levels and solubility. The development of a series of D–A copolymers based on the combination of these acceptor units with fixed donor DTS or BDT units enabled us to determine the relationship among molecular structures, physical properties and OPV characteristics. These results demonstrate that dioxocycloalkene-annelated thiophene can be a promising electron-acceptor unit for D–A-type OPV materials. On the other hand, a noticeable connection between structural variations and the photovoltaic performance was observed, which correlates with the influence of the film morphology and/or carrier mobility on the performance of BHJ blend films. We expect that OPV performance can be further improved by the precise optimization of both molecular design and thin-film fabrication processes. The key strategy for molecular design involves fine-tuning the electronic structures of dioxocycloalkene-annelated thiophene units and finding appropriate combinations with donor units. For thin-film fabrication, the establishment of novel fabrication techniques that promote the ordering of interchain π–π interactions in amorphous D–A copolymers based on dioxocycloalkene-annelated thiophenes in BHJ films is essential. Further studies based on these concepts will give us important insight into the structure–property–OPV performance relationships and will accelerate advances in BHJ solar cells.