Non-Fullerene Polymer Solar Cells Based on a New Polythiophene Derivative as Donor

XU Qingqing, CHANG Chunmei, LI Wanbin, GUO Bing,GUO Xia , ZHANG Maojie

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu Province, P. R. China.

Abstract: With the development of non-fullerene small-molecule acceptors, non-fullerene polymer solar cells (PSCs) have garnered increased attention due to their high performance. While photons are absorbed and converted to free charge carriers in the active layer, the donor and acceptor materials both play a critical role in determining the performance of PSCs. Among the various conjugated-polymer donor materials, polythiophene (PT) derivatives such as poly(3-hexylthiophene),have attracted considerable interest due to their high hole mobility and simple synthesis. However, there are limited studies on the applications of PT derivatives in non-fullerene PSCs. Fabrication of highly efficient nonfullerene PSCs utilizing PT derivatives as the donor is a challenging topic. In this study, a new PT derivative, poly[5,5′-4,4′-bis(2-butyloctylsulphanyl)-2,2′-bithiophene-alt-5,5′-4,4′-difluoro-2,2′-bithiophene] (PBSBT-2F), with alkylthio groups and fluorination was synthesized for use as the donor in non-fullerene PSC applications. The absorption spectra,electrochemical properties, molecular packing, and photovoltaic properties of PBSBT-2F were investigated and compared with those of poly(3-hexylthiophene) (P3HT). The polymer exhibited a wide bandgap of 1.82 eV, a deep highest occupied molecular orbital (HOMO) of -5.02 eV, and an ordered molecular packing structure. Following this observation, PSCs based on a blend of PBSBT-2F as the donor and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno-[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) as the acceptor were fabricated.The absorption spectra were collected and the energy levels were found to be well matched. These devices exhibited a power conversion efficiency (PCE) of 6.7% with an open-circuit voltage (VOC) of 0.75 V, a short-circuit current density (JSC)of 13.5 mA·cm-2, and a fill factor (FF) of 66.6%. These properties were superior to those of P3HT (1.2%) under the optimal conditions. This result indicates that PBSBT-2F is a promising donor material for non-fullerene PSCs.

Key Words: Polymer solar cells; Non-fullerene acceptor; Polythiophene derivatives; Donor materials;Power conversion efficiency

1 Introduction

Solar cells are developing rapidly during these years. One of the most exciting developments is the emergence of perovskite solar cells with high efficiency1-3. At the same time, polymer solar cells (PSCs) have already attracted great attention due to their advantages of easy fabrication, low cost, light weight, and flexibility4-6. In the past few years, lots of conjugated polymers have emerged as donor materials in PSCs7-10. Among them,polythiophene (PT) derivatives are one of the most important donor material systems with advantages of simple structure and easy synthesis and so on11-13. Especially, the poly(3-hexylthiophene) (P3HT) has been the most outstanding photovoltaic donor material with high crystallinity, thermal stability, and good hole mobility14-19. Under optimizing device fabrication conditions, PSCs based on P3HT as an electrondonor material have achieved the power conversion efficiencies(PCEs) of ≈ 5%20and ≈ 7%11with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and indene-C70 bisadduct (IC70BA)as electron-acceptor materials, respectively.

Recently, many non-fullerene acceptors such as ITIC, IEIC,IDIC have emerged with advantages of easy synthesis, strong absorption in the visible region, easily tunable energy levels, and good stability21-23. The time of non-fullerene PSCs is coming24-32.For example, Li et al.33synthesized the polymer J61 with ITIC as acceptor and obtained a PCE of 9.5%. Our group developed the polymer donor PTZ6 with ITIC as acceptor and obtained a PCE of 10.3%34. In fact, very limited works have been reported to focus on the applications of P3HT or other PT derivatives in non-fullerene PSCs. To make highly efficient non-fullerene PSCs by using PT derivatives as donor is still a challenging topic.

In order to design PT derivatives to realize higher efficiency in non-fullerene PSCs, side chain engineering can be a wellknown and effective method to tune the absorption, electronic energy levels and charge carrier mobility of the conjugated polymers35. For example, in recent reports, by attaching electron-withdrawing carboxylate substituents, Hou et al.36demonstrated a highly efficient non-fullerene PSC using PDCBT:ITIC blends, which provided an impressive efficiency of 10.16% while PSCs based on P3HT as an electron-donor material with ITIC as an acceptor showed poor PCEs of ≈ 1%by various treatments. Besides, with electron-donating alkylthio side chains, the highest occupied molecular orbital (HOMO)energy level of the P3HST is decreased and absorption spectrum is broader, which is beneficial to the higher open-circuit voltage(Voc) of the PSCs based on the polymer as donor37. Introducing electron-donating alkylthio side chains is a promising way of molecular structure modification to achieve a high PCE38-41.Besides, fluorination of polymer donors is a facile and feasible strategy to reduce the HOMO and the lowest unoccupied molecular orbital (LUMO) energy level simultaneously without sacrificing the absorption spectrum, which made it pervasively adopted in improving the Vocof devices42-44.

Herein, we report a new PT derivative PBSBT-2F (Fig. 1a)with alkylthio side-chains and fluorine atoms. The optical and electrochemical properties as well as device characteristics of it are explored.

2 Results and discussion

The synthetic procedure of PBSBT-2F is shown in Scheme S1(Supporting Information) and PBSBT-2F was synthesized by Pd-catalyzed Stille coupling reaction with a yield of 58%. The polymer exhibits good solubility in chlorobenzene (CB) and odichlorobenzene (o-DCB). The number average molecular weight (Mn) and polydispersity index (PDI) of PBSBT-2F were measured using gel-permeation chromatography (GPC) with 1,2,4-tricholorobenzene as the solvent and polystyrene as a standard at a high temperature of 160 °C. The Mnof PBSBT-2F and PDI are 32.0 kDa and 2.59, respectively.

Fig. 1b shows the absorption spectra of PBSBT-2F and P3HT(for comparison) in thin solid film, and the absorption spectra of their solutions in o-DCB are shown in Fig. S1a (Supporting Information). In solution, PBSBT-2F shows a distinct absorption band in a long wavelength range of 400-680 nm, which can be ascribed to the intramolecular charge-transfer (ICT) of the polymer. The maximum absorption peak is located at 555 nm and a shoulder peak is at 612 nm. PBSBT-2F in a solid film exhibits a red-shifted absorption spectrum with a stronger shoulder peak at 618 nm due to the enhanced aggregation of the polymer chains in the solid state in comparison with that in solution. Compared with P3HT, the absorption spectrum of PBSBT-2F exhibits obvious red-shifts both in solution and in solid film. The absorption peak of PBSBT-2F film is at 562 nm in comparison with that (518 nm) of P3HT film, which can be ascribed to the introduced inter and/or intramolecular F···H hydrogen bond, S―F bond effect and the induced dipole along C―F bond to facilitate better π-π stacking and greater extent of backbone ordering45,46. Besides, the absorption edge (λonset) of PBSBT-2F is located at 680 nm corresponding to an optical bandgap of 1.82 eV which is conducive to form the complementary absorption with the ITIC from 400 to 800 nm.

Fig. 1 (a) Molecular structures of PBSBT-2F and ITIC; (b) UV-Vis absorption spectra of PBSBT-2F, ITIC and P3HT in thin films;(c) molecular energy level alignments of PBSBT-2F and ITIC.

Fig. 2 Optimized geometries and molecular orbitals for trimers of PBSBT-2F by DFT calculations (B3LYP/6-31G*).

The electronic energy levels of the polymers were measured by electrochemical cyclic voltammetry47. As shown in Fig. S2(Supporting Information), the onset oxidation potentials (φox) are 0.35 V and the reduction potentials (φred) are -1.92 V versus Ag/Ag+, respectively. The corresponding HOMO and LUMO levels are estimated to be -5.06 and -2.79 eV according to the equations48,49, EHOMO= -e(φox+ 4.71) (eV) and ELUMO= -e(φred+ 4.71) (eV).

Theoretical calculation was performed by density functional theory (DFT) at the B3LYP/6-31G(d, p) level to understand the molecular coplanarity and the HOMO and LUMO energy levels of PBSBT-2F50,51. The alkyl groups in the structures were replaced with methyl groups to simplify the structure. As shown in Fig. 2, three repeating units were calculated. Dihedral angles between thiophenes in the second repeating unit are 7.96°, 5.78°and 1.05°, respectively, indicating that the polymer has a good planarity. Furthermore, the molecular geometry shows a linear backbone conformation from the side view. Besides, the HOMO/LUMO levels from the calculations are -2.62 eV/-4.85 eV, which is in good agreement with the electrochemical characterization data.

X-ray diffraction (XRD) measurement was used to investigate the crystalline properties of PBSBT-2F, as shown in Fig. 3.PBSBT-2F displays two slightly obvious diffraction peaks at 2θ =5.02° (100) and 10.01° with a d-spacing value of 1.76 nm, which corresponds to the interchain distance separated by the side chains of PBSBT-2F. The high crystallinity of PBSBT-2F film may be attributed to the strong self-assembly into an ordered lamellar structure52, which is conductive to the charge transportation and the better photovoltaic performance of PBSBT-2F.

To investigate the photovoltaic performance of the polymer,single junction PSCs based on the blend of PBSBT-2F and ITIC were fabricated with an inverted structure of ITO/ZnO7/PBSBT-2F : ITIC/MoO3/Al. where the ZnO and MoO3were inserted as the cathode and anode interlayers, respectively. The blend solution was prepared in CB with a polymer concentration of 8 mg·mL-1. Table 1 listed the parameters of devices under optimized conditions. To find the optimized PBSBT-2F : ITIC blend ratio, devices with different Donor/Acceptor (D/A) ratios(1.5 : 1, 1 : 1 and 1 : 1.5, mass ratio) were fabricated. As a result,the optimized PBSBT-2F : ITIC blend ratio is clearly 1 : 1 (mass ratio) as is shown in Table 1 while the current density-voltage(J-V) characteristics and external quantum efficiency (EQE)curves are shown in Fig. 4a,b. The results showed that the PCE is only 4.5% without any post treatment, which is much higher than that of the P3HT : ITIC system, so actions should be taken to obtain an optimized PCE.

Fig. 3 The XRD pattern of PBSBT-2F film.

First, as we all know, additives can influence the morphology of active layer, in consideration of which, the widely used solvent additive N-methyl-2-pyrrolidone (NMP) was used to further improve photovoltaic performance of the devices53. The PSCs based on PBSBT-2F : ITIC (1 : 1, mass ratio) with a different content of NMP (0.5%, 1%, 2%, 3%) as the solvent additive under AM 1.5G illumination (100 mW·cm-2) were fabricated. Fig. S3 (Supporting Information) shows the J-V characteristics and EQE curves of devices with different additive content ratios. The corresponding photovoltaic parameters of the devices are summarized in Table S1. When 1% NMP was used as a solvent additive, the PCE of the PBSBT-2F based device was improved to 5.0%, which is slightly higher than the deviceprocessed using pure CB.

Table 1 Summary of photovoltaic properties under optimized conditions (AM 1.5 G illumination, 100 mW·cm-2).

Furthermore, to optimize the morphology of active layer,thermal annealing (TA) treatment was conducted. Table S2 summarizes the photovoltaic characteristics for the devices with thermal annealing at different temperature for 10 min with 1%NMP with the optimal D/A mass ratio (1 : 1) under AM 1.5G standard solar spectrum (100 mW·cm-2) illumination. The PBSBT-2F : ITIC device showed a higher PCE of 6.7% when annealing at 140 °C for 10 min with a Jscof 13.5 mA·cm-2, a Vocof 0.75 V and a FF of 66.6% and the EQE spectrum covered from 300 to 800 nm with a maximum EQE of 59% was observed at ca. 570 nm.

Fig. 4 (a) J-V characteristics; (b) EQE curves of solar cellsbased on based on PBSBT-2F : ITIC with different conditions;(c) Jph versus Veff characteristics, and dependence of Jph (d) on light intensity for the PSCs based on based on PBSBT-2F : ITIC(1 : 1, mass ratio) processed with nothing or 1% NMP or thermal annealing at 140 °C for 10 min and 1% NMP.

Fig. 5 AFM (5 µm × 5 µm) height images of PBSBT-2F : ITIC (1 : 1, mass ratio) blend films processed with: (a) nothing, (b) 1% NMP, (c) thermal annealing at 140 °C for 10 min and 1% NMP. AFM (5 µm × 5 µm) phase images of PBSBT-2F : ITIC (1 : 1, mass ratio) blend films processed with:(d) nothing, (e) 1% NMP, (f) thermal annealing at 140 °C for 10 min and 1% NMP.

Fig. 6 TEM images of PBSBT-2F : ITIC (1 : 1, w/w) blend films processed with: (a) nothing, (b) 1% NMP, (c) thermal annealing at 140 °C for 10 min and 1% NMP.

To further study the influence of the NMP additive and the TA treatment on the charge generation and extraction behavior of the PSC devices, the dependence of the photocurrent (Jph) on the effective voltage (Veff) was measured, as shown in Fig. 4c. Jphis defined as JL-JD, where JLand JDare the current densities under illumination and in the dark, respectively. Veffis defined as V0-V, where V0is the voltage at which the photocurrent is zero and V is the applied voltage54. For the PBSBT-2F : ITIC solar cells,Jphreaches saturation (Jsat) at large Veff(Veff≥ 2 V), suggesting that all of the photogenerated excitons are dissociated into free carriers and collected by the electrodes. Thus, the charge dissociation probability (P(E, T)) can be estimated using the Jph/Jsatratio. Under short-circuit and maximal power output conditions, the ratios were 83%, 62% for the as-cast device,86%, 65% for the device with 1% NMP, and 88%, 66% for the device with 1% NMP and TA treatment, respectively. The improved P(E, T) implied that the NMP additive and TA treatment of the PSC device leads to more efficient exciton dissociation and charge collection efficiency, which is directly related to the high PCE of the corresponding device. The relationship between Jphand Plightcan be described by Jph∝(Plight)S, where S implies the extent of the bimolecular recombination. It is a linear dependence between log(Jph) and log(Plight) and the slope S value close to 1 indicates weak bimolecular recombination in the device55. As shown in Fig. 4d,the S value was 0.99 for the as-cast device, 1.0 for the device with 1% NMP and 1.0 for the device with TA and NMP. The higher S value for all devices means less bimolecular recombination in PSCs.

Further effort was required to reveal the surface and bulk morphology of the blend films, so atom force microscope (AFM)and transmission electron microscope (TEM) were used. As illustrated in Fig. 5, the film processed with 1% NMP showed a significantly increased root-mean-square (RMS) roughness value of 2.88 nm in compared to those of the blends without NMP treatment with a RMS of 1.86 nm, which may be due to the enhanced crystallinity of PBSBT-2F and ITIC in the blends by processing with 1% NMP. When annealing at 140 °C for 10 min with 1% NMP, the morphology of the blend film became more homogeneous and the RMS value further decreased to 2.79 nm. Fig. 6 shows TEM images of PBSBT-2F : ITIC processed without (a) and with (b) 1% NMP, (c) 1% NMP and thermal annealing at 140 °C for 10 min. When the blend film is treated with NMP and thermal annealing, the suitable phase separation and better interpenetrating network morphology are obtained for effective charge separation and transport, resulting in the enhanced Jscand FF for the higher PCE.

3 Conclusions

In summary, we have presented the synthesis and photovoltaic properties of a new polythiophene derivative PBSBT-2F with alkylthio side chains and fluorine atoms. The polymer has a strong absorption at 400-680 nm, which is complementary with that of the small molecule acceptor ITIC.The PSCs based on PBSBT-2F:ITIC achieved a PCE of 6.7%with a Vocof 0.75 V, a Jscof 13.5 mA·cm-2and a FF of 66.6%.Meanwhile, the polymer has higher Vocand Jscthan that of P3HT. Our result indicates that PBSBT-2F is a promising donor material for non-fullerene PSCs.

Supporting Information:available free of charge via the internet at http://www.whxb.pku.edu.cn.