Polymer Solar Cell Devices Research Articles

Polythiophenes with alkylthiophene side chains for efficient polymer solar cells

We synthesized two polythiophene derivatives with alkylthiophene side chains, named PT4T-0F and PT4T-2F. The introduction of alkylthiophene side chains endowed PT4T-0F with more ordered edge-on stacking in film, resulting in higher hole mobility of 0.45 cm2V−1s−1 in organic field-effect transistors in comparison to P3HT. Further introducing fluorine atoms, a mainly face-on orientation was achieved for PT4T-2F neat film, which was facilitated to charge transport and collection in polymer solar cells (PSCs). Using F8IC as acceptor, the PSC devices based on PT4T-2F achieved the champion efficiency of 10.48%, owing to the appropriate film morphology with moderate phase separation and favorable face-on molecular orientation. Such efficiency greatly outperformed those of PT4T-0F (2.85%) and P3HT (0.35%) based devices.

A New Dibenzoquinoxalineimide-Based Wide-Bandgap Polymer Donor for Polymer Solar Cells.

The molecular design of a wide-bandgap polymer donor is critical to achieve high-performance organic photovoltaic devices. Herein, a new dibenzo-fused quinoxalineimide (BPQI) is successfully synthesized as an electron-deficient building block to construct donor–acceptor (D–A)-type polymers, namely P(BPQI-BDT) and P(BPQI-BDTT), using benzodithiophene and its derivative, which bears different side chains, as the copolymerization units. These two polymers are used as a donor, and the narrow bandgap (2,20-((2Z,20Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo [3,4-e]thieno[2,″30′:4′,50]thieno[20,30:4,5]pyrrolo[3,2g]thieno[20,30:4,5]thieno[3,2-b]indole-2,10 diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) Y6 is used as an acceptor to fabricate bulk heterojunction polymer solar cell devices. Y6, as a non-fullerene receptor (NFA), has excellent electrochemical and optical properties, as well as a high efficiency of over 18%. The device, based on P(BPQI-BDTT):Y6, showed power conversion efficiencies (PCEs) of 6.31% with a JSC of 17.09 mA cm−2, an open-circuit voltage (VOC) of 0.82 V, and an FF of 44.78%. This study demonstrates that dibenzo-fused quinoxalineimide is a promising building block for developing wide-bandgap polymer donors.

The effect of boron-doped carbon nanotubes blended with active layers in achieving high-efficiency polymer solar cells and X-ray detectors

Polymer solar cells (PSCs) usually offer inferior power conversion efficiency (PCE), lower open-circuit voltage (Voc), and inefficient charge extraction abilities. We studied boron-doped carbon nanotubes (B-CNTs) as the third active layer (AL) component to improve the PCE and Voc, due to their tunable work function, superior conductivity, and transport behavior. A novel arc discharge (AD) method produced B-CNTs without any residual catalysts. This work utilized PBDB-T: ITIC as the initial AL to prepare PSCs and X-ray detectors with glass/ITO/PEDOT: PSS/AL/LiF/Al structure. The excitation effects of PSC and X-ray detector devices based on ALs embedded with different amounts of B-CNTs were also studied. The derived outcomes revealed a PCE of 10.21 % for the constructed PSC with 3 wt% of B-CNTs blended AL. Furthermore, the fabricated X-ray photodetector with 3 wt% of B-CNTs obtained an excellent sensitivity of 2.49 mA/Gy·cm2. These results show that B-CNTs can act as both an exciton dissociation center and a charge transfer channel, promoting charge separation and transport in the AL.

Heteroheptacene-based acceptors with thieno[3,2-b]pyrrole yield high-performance polymer solar cells.

Rationally utilizing and developing synthetic units is of particular significance for the design of high-performance non-fullerene small-molecule acceptors (SMAs). Here, a thieno[3,2-b]pyrrole synthetic unit was employed to develop a set of SMAs (ThPy1, ThPy2, ThPy3 and ThPy4) by changing the number or the position of the pyrrole ring in the central core based on a standard SMA of IT-4Cl, compared to which the four thieno[3,2-b]pyrrole-based acceptors exhibit bathochromic absorption and upshifted frontier orbital energy level due to the strong electron-donating ability of pyrrole. As a result, the polymer solar cells (PSCs) of the four thieno[3,2-b]pyrrole-based acceptors yield higher open-circuit voltage and lower energy loss relative to those of the IT-4Cl-based device. What is more, the ThPy3-based device achieves a power conversion efficiency (PCE) (15.3%) and an outstanding fill factor (FF) (0.771) that are superior to the IT-4Cl-based device (PCE = 12.6%, FF = 0.758). The ThPy4-based device realizes the lowest energy loss and the smallest optical band gap, and the ternary PSC device based on PM6:BTP-eC9:ThPy4 exhibits a PCE of 18.43% and a FF of 0.802. Overall, this work sheds light on the great potential of thieno[3,2-b]pyrrole-based SMAs in realizing low energy loss and high PCE.

Novel Pyrrolo [3,4-b] Dithieno [3, 2-f:2″,3″-h] Quinoxaline-8,10 (9H)-Dione Based Wide Bandgap Conjugated Copolymers for Bulk Heterojunction Polymer Solar Cells.

Two D-A copolymers consisting of fused ring pyrrolo-dithieno-quinoxaline acceptors are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2-ethylhexyloxy (P117) side chains. These copolymers are used as donors and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron-deficient fused ring pyrrolo-bithieno-quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells (PSCs) based on P134:Y6 attain the power conversion efficiency (PCE) of 15.42%, which is higher than the P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 can be associated with more well-adjusted charge transport, weak charge recombination, proficient exciton generation, and dissociation into free charge carriers and their subsequent charge collection owing to the dense π-π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor-weak donor tactic for developing efficient D-A copolymers consists of quinoxaline acceptor for PSCs.

Light harvesting coating design accelerated by deep learning for semi-transparent polymer solar cells

The reduction in optical loss in polymer solar cells (PSCs) plays a crucial role in the development of high-performance PSCs devices. Especially for the semi-transparent PSCs, high reflective transparent electrodes lead to low energy utilization. Optical multi-layer coating is proven to be an effective approach to reduce the reflection and transmission loss. In this work, a double-sided PSCs device coating strategy was used to reduce the device optical loss. Optical coating design on a multi-layer PSCs device is far more complex. The dispersion and thickness of each layer both have an impact on the optical property. Meanwhile, the illuminance spectrum is based on the solar AM1.5 spectrum rather than a common-used standard illuminance CIE-E spectrum. It brings many difficulties to the optical design, and the global optimization is generally time-consuming. To fast solve the optimization problem in optical design of the multi-layer coating for PSCs, we combine deep learning (DL) method with hybrid optimization algorithms. By designing a multi-layer device structure to achieve the highest light harvesting with tandem simplex simulated annealing and assisted simplex simulated annealing, we show unambiguously that DL is a powerful tool to minimize the computation cost and maximize the design efficiency for optical multi-layer design. The optical loss of the semi-transparent device is reduced from 52.71% to 27.95%, and the simulation time is reduced by a factor of 276 compared with standard simplex simulated annealing. This provides an efficient optical design strategy in multi-layer coating design for PSCs to achieve desired optical performance.

Bis(thieno[3,2-b]thieno)cyclopentafluorene-Based Acceptor with Efficient and Comparable Photovoltaic Performance under Various Processing Conditions.

The morphology of a bulk heterojunction (BHJ) blend within a polymer solar cell (PSC) device plays a crucial role in its performance. The ideal morphology is generally achieved through molecular engineering and optimization under film processing conditions. Under different processing conditions, the deviation of the resulted morphology characteristics from the ideal one leads to the dispersion of device performance. For a specific donor/acceptor BHJ blend, it is of great challenge to maintain an efficient and comparable photovoltaic performance under various processing conditions. The solution to this challenge would be of great value in offering more choices for a suitable processing technology in practical applications. Based on the acceptor BTTFIC with the core of bis(thieno[3,2-b]thieno)cyclopentafluorene (BTTF) in our previous work, we chemically modified BTTFIC by fluorination of the end groups of 1,1-dicyanomethylene-3-indanones (IC) and the switching part of octyls in BTTF with 4-hexylphenyls to offer a novel acceptor (BTTFIC4F-Ar). The inverted PBDB-T-2Cl:BTTFIC4F-Ar blend device provided an average power conversion efficiency (PCE) of 10.61, 11.08, and 11.55% when processed under solvent annealing (SA), thermal annealing (TA), and additive treatment with 1,8-diodooctane (DIO), respectively. Different from the reported discrete performance under various processing conditions for a specific donor/acceptor BHJ blend, a low mean absolute performance deviation of 3% was attained. This slight enhancement trend was unexceptionally reflected on charge generation, transportation, and recombination within the blend films from SA, TA, and DIO conditions. A slightly improved ordering of BTTFIC4F-Ar within the DIO blend was observed. Meanwhile, very similar molecular packings as well as a close amorphous domain size of the mixture of PBDB-T-2Cl and BTTFIC4F-Ar within the three blends were observed. These morphological characteristics are in good agreement with the photoelectrical conversion performance of the blends under the three processing conditions. Furthermore, similar attenuation behaviors in performance were also observed. This investigation may provide new guidance on the molecular engineering of nonfullerene acceptors to achieve an efficient BHJ blend with more options for a suitable and cost-effective processing method in practical applications.

Highly efficient inverted polymer solar cells by using solution processed MgO/ZnO composite interfacial layers

A highly efficient inverted polymer solar cell (PSC) has been successfully demonstrated by utilizing a wide bandgap magnesium oxide (MgO) film and ZnO stacked structure as an effective cathode interfacial layer. The MgO/ZnO bilayer structure is designed to combine the superiorities of both ZnO ETL and MgO film, based on the efficiency comparison of the PSCs without and with MgO interlayer. The ZnO film can serve as an efficient electron transport layer (ETL), while the MgO layer can reduce the surface defects of FTO and block the holes effectively, leading to an elevated electron collection and suppressed carrier recombination at the interface. With the excellent dual functions interface characteristics induced by the MgO/ZnO stacked films, the corresponding inverted PSC device based on the PTB7-Th:PC71BM photoactive layer system presents a superior power conversion efficiency (PCE) of 11.02%, which is higher than that of the PSC without MgO (8.79%). We believe that the MgO/ZnO bilayer structure is a superior interfacial contender for the organic photovoltaics and other photoelectric devices requiring cathode interfacial layers.

Synthesis and characterization of a wide‐bandgap polymer based on perfluorinated and alkylthiolated benzodithiophene with a deep highest occupied molecular orbital level for organic photovoltaics

AbstractA perfluorinated and alkylthiolated benzodithiophene (BDT)‐ttTPD‐based donor polymer (P2FS‐ttTPD) was synthesized via a Stille polymerization, and found to have a number average molecular weight (Mn) of 13,000 g/mol (Đ = 2.3). P2FS‐ttTPD has a wide bandgap (1.96 eV) and a deep highest occupied molecular orbital (HOMO) level (−5.70 eV). The perfluorination and alkylthiolation of the polymer backbone lower the polymer's HOMO level significantly. The hole and electron mobilities of P2FS‐ttTPD were determined to be 1.12 × 10−4 and 9.38 × 10−7 cm2/V s, respectively. Polymer solar cell devices prepared with a P2FS‐ttTPD:IT‐4F (1:1) blend as the active layer were found to exhibit power conversion efficiencies of 4.15%, a short‐circuit current density (JSC) of 10.29 mA/cm2, an open‐circuit voltage (VOC) of 0.97 V, and a fill factor of 41.6%. The (1:1) blend devices were found to exhibit high VOC and low Eloss values.

Comparison of TiO2 and ZnO electron selective layers on the inverted-type polymer solar cells

In this study, P3HT:PCBM polymer-based solar cell devices with TiO2 and ZnO electron selective layers were separately investigated to compare effects of metal oxide layer in the inverted-type polymer solar cell structures. R.f. magnetron sputtering technique was used to fabricate highly oriented polycrystalline TiO2 and ZnO thin film layers onto FTO-coated glass substrates. The surface morphology, structural and optical properties of TiO2 and ZnO electron selective layers were studied by performing X-ray diffraction (XRD), confocal Raman spectroscopy, transmittance spectra and Atomic Force Microscope (AFM). Photovoltaic performance of inverted-type polymer solar cell based on Poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) with different electron selective layers was executed to analyze the effects of TiO2 and ZnO thin film layers on the power conversion efficiency (PCE) value of inverted polymer solar cell (IPSC) devices. Produced devices with TiO2 and ZnO thin film indicated the best device performance with maximum PCE values of 2.88 and 3.49%, respectively.

Embedded plasmonic nanoprisms in polymer solar cells: Band-edge resonance for photocurrent enhancement

Introduction of metallic nanoparticles that can generate the surface plasmon resonance (SPR) has been considered as a prominent option for enhancing the performance of polymer solar cells (PSCs), as the radiative scattering and field confinement by the SPR can extend the effective photon traveling length and manipulate the spatial absorption profile. Despite many successful efforts to favorably exploit metallic nanoparticles, further studies of their effects on the PSC performance have been demanded to achieve the full benefit from them. Here, we systematically investigate the optical and photovoltaic performances of PSCs with disorderly distributed silver nanoprisms embedded in the photoactive material. Due to the superior properties of the plasmonic scattering of this class of nanoparticles, a significant improvement of photon absorption is gained from the devices with silver nanoprisms, particularly in the wavelength range of substandard absorption property including the band-edge wavelengths. While such absorption improvement can be obviously reinforced as an increase in the particle density, its level becomes saturated and decayed eventually because of the concurrently promoted photon loss by plasmonic absorption. At the optimal configurations of silver nanoprisms for the productive light trapping effect, the incorporated PSC devices present a photocurrent of ∼17.76 mA/cm2 and a power conversion efficiency of ∼9.68%, where their net increase ratios are ∼10% and ∼13% compared to the reference PSC devices, respectively. Details of numerical modeling and experiments for both metal nanoprisms and PSC devices offer an optimum route to tailoring metallic nanoparticles for high-performance PSC systems.

Enhanced Charge Transport in Conventional Polymer Solar Cells with a Perovskite-Type LaNiO3 Layer.

In this study, a novel metal oxide, lanthanum nickelate (LNO) with a perovskite structure, was introduced into a polymer solar cell (PSC) device, replacing the PEDOT:PSS hole transport layer (HTL). The results show that the LNO-based PTB7-Th:PC71BM solar cell exhibits a higher circuit current density, power conversion efficiency, and stability compared with a device with PEDOT:PSS HTL. To understand the effect of LNO HTL on the performance of devices, the active layer morphology and charge transport characteristics in PSCs were systematically analyzed. The morphology of active layer was affected by the HTL, which further regulated the generation and transport processes of charge carrier in the PSC device. For the LNO HTL, an appropriate thickness (8 nm) and a small surface roughness (Sq = 0.7 nm) can coordinate the energy-level structure of device and improve the interface contact between the FTO electrode and PTB7-Th:PC71BM active layer, promoting the charge transport performance of device. Therefore, this work provides a new consideration for the preparation of efficient, stable, and low-cost polymer solar cells.

Difluorobenzoxadiazole-based conjugated polymers for efficient non-fullerene polymer solar cells with low voltage loss

Two donor-acceptor (D-A) conjugated copolymers based on difluorobenzoxadiazole (ffBX) and oligothiophenes, i.e. , PffBX-2T and PffBX-TT, were designed and synthesized for polymer solar cells (PSCs). Compared to the polymers based on difluorobenzothiadiazole (ffBT) units, the two ffBX-based polymers presented identical optical bandgaps (~1.62 eV), but lower highest occupied molecular orbital (HOMO) energy levels. Owing to the down-shifted HOMO levels, the PSCs based on PffBX-2T and PffBX-TT showed lower voltage loss, and the open-circuit voltage ( V oc ) was ~0.1 V higher than that of the devices with the ffBT-based polymer. As a result, higher photovoltaic performance was achieved for the devices based on the ffBX-based polymers. The power conversion efficiencies (PCEs) of the non-fullerene PSCs with PffBX-2T and PffBX-TT as the donor were 8.72% and 10.12%, respectively. The superior device performance of PffBX-TT resulted from the efficient exciton dissociation and charge transport as well as weak charge recombination, which could be ascribed to the favorable face-on packing of the conjugated backbones and the desired morphology in the blend film. Our study demonstrates that difluorobenzoxadiazole is a promising building block for constructing conjugated polymers for high-performance non-fullerene PSCs. • Difluorobenzoxadiazole-based polymers with alkylthiophene side chains were synthesized. • PffBX-2T and PffBX-TT based PSC devices showed low voltage loss of 0.54 and 0.52 V, respectively. • A high PCE up to 10.12% was achieved for PSC devices based on PffBX-TT.

Novel benzodithiophene type low band gap polymer solar cell application and device stability study with atomic layer deposition encapsulation technique

Novel benzodithiophene type copolymer was synthesized through a solvent evaporation technique. Poly(5-(5-(4, 8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4, 5-b’]dithiophen-2-yl)thiophen-2-yl)-2, 3-bis(3, 4-bis(decyloxy)phenyl)-8-(thiophen-2-yl) quinoxaline) (P-TQTBDT) solar cells were encapsulated with conventional and Atomic Layer Deposition (ALD) techniques. Characterizations of the samples were carried out via nuclear magnetic resonance (NMR), gel permeation chromotagraphy (GPC) and cyclic voltammetry (CV) techniques. Stability studies were carried out both P3HT and P-TQTBDT solar cells for comparison of commercial P3HT and our novel polymer solar cell. Our results confirm that ALD is a promising technique for encapsulation of polymer solar cell since stability of P-TQTBDT improved to 72% durability from 38% durability with ALD encapsulation technique during 300 h under AM1.5 G solar irradiation. P-TQTBDT solar cell showed higher stability (about 10%) than P3HT solar cell for both encapsulation methods. We focused on the stability of polymer solar cell can be improved with ALD encapsulation technique. Our aim was to compare P-TQTBDT polymer solar cell with P3HT solar cell. The stability results of P-TQTBDT showed that P-TQTBDT copolymer could be promising polymer to obtain very high stability of polymer solar cells.

Tetrahydroxy‐Perylene Bisimide Embedded in a Zinc Oxide Thin Film as an Electron‐Transporting Layer for High‐Performance Non‐Fullerene Organic Solar Cells

AbstractBy introduction of four hydroxy (HO) groups into the two perylene bisimide (PBI) bay areas, new HO‐PBI ligands were obtained which upon deprotonation can complex ZnII ions and photosensitize semiconductive zinc oxide thin films. Such coordination is beneficial for dispersing PBI photosensitizer molecules evenly into metal oxide films to fabricate organic–inorganic hybrid interlayers for organic solar cells. Supported by the photoconductive effect of the ZnO:HO‐PBI hybrid interlayers, improved electron collection and transportation is achieved in fullerene and non‐fullerene polymer solar cell devices, leading to remarkable power conversion efficiencies of up to 15.95 % for a non‐fullerene based organic solar cell.

Tetrahydroxy-Perylene Bisimide Embedded in a Zinc Oxide Thin Film as an Electron-Transporting Layer for High-Performance Non-Fullerene Organic Solar Cells.

By introduction of four hydroxy (HO) groups into the two perylene bisimide (PBI) bay areas, new HO‐PBI ligands were obtained which upon deprotonation can complex ZnII ions and photosensitize semiconductive zinc oxide thin films. Such coordination is beneficial for dispersing PBI photosensitizer molecules evenly into metal oxide films to fabricate organic–inorganic hybrid interlayers for organic solar cells. Supported by the photoconductive effect of the ZnO:HO‐PBI hybrid interlayers, improved electron collection and transportation is achieved in fullerene and non‐fullerene polymer solar cell devices, leading to remarkable power conversion efficiencies of up to 15.95 % for a non‐fullerene based organic solar cell.

Carboxylate-Substituted Polythiophenes for Efficient Fullerene-Free Polymer Solar Cells: The Effect of Chlorination on Their Properties

Two new wide-bandgap polythiophenes, i.e., poly[5,5′-bis(2-hexyldecyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-3-chloro-2,2′-bithiophene] (PDCBT-Cl) and poly[5,5′-bis(2-hexyldecyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-3,3′-dichloro-2,2′-bithiophene] (PDCBT-2Cl) comprising 3-chloro-2,2′-bithiophene and 3,3′-dichloro-2,2′-bithiophene moieties, respectively, were synthesized for fullerene-free polymer solar cells (PSCs). For comparison, three other polymers based on [2,2′-bithiophene]-4,4′-dicarboxylate (DCBT), i.e., PDCBT, PDCBT-F, and PDCBT-2F with 2,2′-bithiophene, 3-fluoro-2,2′-bithiophene, and 3,3′-difluoro-2,2′-bithiophene as comonomers, respectively, were also prepared. PSC devices were fabricated with these polymers as donor materials and ITIC-Th1 as acceptor. The incorporation of chlorine (Cl) or fluorine (F) atoms into polymers both efficiently downshifted the highest occupied molecular orbital (HOMO) energy levels, leading to higher open-circuit voltage (Voc) in the PSCs. Owing to...

Impact of an electron withdrawing group on the thiophene-fused benzotriazole unit on the photovoltaic performance of the derived polymer solar cells

In this work, two new donor-acceptor (D-A) copolymers (P32 and P33) for non-fullerene polymer solar cells (PSCs) have been synthesized based on a thiophene-fused benzotriazole (BTAZT) unit and thiophene substituted benzodithiophene (BDTT) unit, in which two different electron-withdrawing groups (i.e., fluorine and ester groups) have been introduced at the side of BTAZT unit, respectively. It is found that P32 with a long alkyl chain ester group shows a more redshifted intramolecular charge transfer absorption, and also exhibits a better complementary absorption spectrum with a small molecular acceptor based on the dithienocyclopentathieno[3,2-b]thiophene core end-capped with 1,1-dicyanomethylene-3-indanone unit (IHIC) than P33. Moreover, P32:IHIC blended film shows higher and more balanced hole/electron charge mobilities as well as better morphologies with more suitable phase separation scale for efficient charge separation and transport than P33:IHIC blended film. Thus, the P32:IHIC based PSC device exhibits the higher power conversion efficiency of 8.35% with higher short-circuit current density (JSC = 16.77 mA cm−2) and fill factor (FF = 64.89%) compared with the P32:IHIC based PSC device, indicating that the different electron-withdrawing side groups at BTAZT unit exerts a significant influence on the properties of the polymers as well as the photovoltaic performances of the PSCs.

Fine-tuning head-to-head bithiophene-difluorobenzothiadiazole polymers for photovoltaics via side-chain engineering

Abstract High-performance polymer semiconductors generally should have wide absorption, good solution process-ability, and fine-tuned physicochemical properties in order to achieve satisfying photovoltaic performance. Herein, two new alkynyl-functionalized electron donor units, 3-alkyl-3′-alkynyl-2,2′-bithiophene (TRTRy) and 3-alkoxy-3′-alkynyl-2,2′-bithiophene (TORTRy), were invented through the combination of the alkynyl side chain with the alkyl or alkoxy side chain, respectively. Copolymerization of these bithiophenes with difluorobenzothiadiazole (ffBT) afforded polymers ffBT-TRTRy and ffBT-TORTRy, respectively. When applied into polymer solar cells (PSCs), devices using ffBT-TRTRy donor polymer display a maximum power conversion efficiency (PCE) of 2.34% with a short-circuit current (Jsc) of 7.46 mA cm−2, an open circuit voltage (Voc) of 0.93 V, and a fill factor (FF) of 34.0%. The TORTRy containing polymer ffBT-TORTRy affords PSCs with a much improved PCE of 6.60% with a higher Jsc of 15.09 mA cm−2, a larger FF of 58.3%, and a Voc of 0.75 V. The performance improvement of PSC devices using ffBT-TORTRy donor is mainly attributed to the simultaneous realization of higher Jsc and FF, while maintaining a good Voc. The results demonstrate that alkynyl-functionalized H-H bithiophenes are promising building blocks for efficient PSCs.

The effect of alkylthio substituents on the photovoltaic properties of conjugated polymers

Abstract Alkylthio groups are effectively utilized in molecule design to improve the performance of polymer solar cells (PSCs). In this study, two conjugated polymers, P1 and P2, were designed and synthesized with alkyl and alkylthio side chains substituted on thiophene as π bridges, respectively. Owing to the twist of the backbone induced by the steric hindrance of the hexyl side chains, polymer P1 shows a low power conversion efficiency (PCE) of 2.83% in PSC devices with PC71BM as acceptor and a low hole mobility of 1.85 × 10−5 cm2 V−1 s−1 in the blend film. In contrast to the steric hindrance of the alkyl side chain, the alkylthio side chain can form S⋯S non-covalent interaction with an adjacent thiophene to maintain the molecular planarity and strengthen intermolecular interaction, which is designed in polymer P2 to improve the photovoltaic performance. As a result, the P2-based devices exhibit a higher PCE of 6.15% with an open-circuit voltage (VOC) of 0.71 V, a short-circuit current (JSC) of 14.55 mA cm−2 and a fill factor (FF) of 59.5%. The hole mobility is also increased to 2.20 × 10−4 cm2 V−1 s−1. Our results demonstrate that introducing S⋯S non-covalent interaction into conjugated polymers could be a useful strategy for building high performance photovoltaic materials.