Bulk Heterojunction Polymer Solar Cells Research Articles

Enhancing the photovoltaic performance of bulk heterojunction polymer solar cells by adding Rhodamine B laser dye as co-sensitizer

Ternary blend (TB) strategy has been considered as an effective method to enhance the photovoltaic performance of bulk heterojunction (BHJ) polymer solar cells (PSCs). Here, we report on TB-based PSCs containing two donor materials; poly-3-hexylthiophene (P3HT) and Rhodamine B (RhB) laser organic dye, and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) as an acceptor. The influence of RhB weight percentage and injection volume was extensively studied. To gain insight into the influences of RhB on the photovoltaic performance of PSCs, physicochemical and optical properties of TBs were compared with those of BHJ binary blend as a standard. RhB broadened the light absorption properties of the active layer and played a bridging role between P3HT and PC61BM. The PCE and short-circuit current density (Jsc) of the optimized TB-based PSCs comprising of 0.5 wt% RhB reached 5% and 12.12 mA/cm2, respectively. Compared to BHJ standard cell, the PCE and the generated current was improved by two orders of magnitude due to higher photon harvest of the active layer, cascade energy level structure of TB components and a considerable decrease in the charge carrier recombination. The results suggest that RhB can be considered as an effective material for application in PSCs to attain high photovoltaic performance.

High-performance solution-based CdS-conjugated hybrid polymer solar cells.

In this study, hybrid BHJ – bulk heterojunction polymer solar cells were fabricated by incorporating CdS quantum dots (QDs) in a blend of P3HT (donor) and PCBM (acceptor) using dichlorobenzene and chlorobenzene as solvents. CdS QDs at various ratios were mixed in a fixed amount of the P3HT and PCBM blend. The prepared samples have been characterized by a variety of techniques such as I–V and EQE measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM) and ultraviolet-visible (UV-vis) spectroscopy. The mixing of QDs in the polymer blends improved the PCE – power conversion efficiency of the solar cells under standard light conditions. The improved PCE from 2.95 to 4.41% is mostly due to the increase in the fill factor (FF) and short-circuit current (Jsc) of the devices with an optimum amount of CdS in the P3HT:PCBM blend. The increase in Jsc possibly originated from the formation of a percolation network of CdS. The conjugation of QDs has increased the absorption of the active layers in the visible region. These results well matched as reported, conjugation of CdS in the perovskite active layer increased the absorption and PCE of the devices relative to those of the perovskite films. This increment in parameters is attributed to the decrease in charge recombinations that improved the performance of the doped device.

Polymer solar cells: P3HT:PCBM and beyond

For the last two decades, polymer solar cells (PSCs) have been a cynosure of the photovoltaic community, as evidenced by the growing number of patent applications and scientific publications. Efforts to achieve high power conversion efficiency in PSC, propelled by advances in device architecture, material combination, and nanomorphology control, evolved into poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-Butyric-Acid-Methyl Ester (PCBM) bulk heterojunction PSCs, which had been the best seller in PSC research for a decade. Subsequently, PSC research was redirected towards the synthesis of low bandgap materials and optimization of tandem cells, which led to a power conversion efficiency of ∼13%. Even though this efficiency may not be sufficient enough to compete with that of inorganic solar cells, unique properties of PSCs, such as mass roll-to-roll production capability, as well as flexibility and lightness, suggest their niche market opportunities. In this review, an overview of developments in PSCs is presented during the last three decades encompassing pre- and post-P3HT:PCBM era. Emphasis is given on evolution in device architecture, coupled with material selection for pre-P3HT:PCBM era, and synthesis of low-bandgap materials, coupled with a tandem structure for post-P3HT:PCBM era. Last but not least, efforts toward the longer operational lifetime of PSCs by encapsulation are reviewed.

Effects of fluorination and thermal annealing on charge recombination processes in polymer bulk-heterojunction solar cells.

We investigate the effect of fluorination on the photovoltaic properties of an alternating conjugated polymer composed of 4,8-di-2-thienylbenzo[1,2-b:4,5-b']dithiophene (BDT) and 4,7-bis([2,2'-bithiophen]-5-yl)-benzo-2-1-3-thiadiazole (4TBT) units in bulk-heterojunction solar cells. The unsubstituted and fluorinated polymers afford very similar open-circuit voltages and fill factor values, but the fluorinated polymer performed better due to enhanced aggregation which provides a higher photocurrent. The photovoltaic performance of both materials improved upon thermal annealing at 150-200 °C as a result of a significantly increased fill factor and open-circuit voltage, counteracted by a slight loss in photocurrent. Detailed studies of the morphology, light intensity dependence, external quantum efficiency and electroluminescence allowed the exploration of the effects of fluorination and thermal annealing on the charge recombination and the nature of the donor-acceptor interfacial charge transfer states in these films.

Non-halogenated diphenyl-chalcogenide solvent processing additives for high-performance polymer bulk-heterojunction solar cells.

The ability to control the morphologies of active layers is a critical factor in the successful development of polymer solar cells (PSCs), and solvent processing additives offer a simple and effective way to accomplish this. In particular, diphenyl ether (DPE) is one of the most effective solvent additives but analogous additives based on this structure have not yet been extensively investigated. In this work, we have fabricated PSCs and investigated photovoltaic device characteristics using the series of non-halogenated, diphenyl-chalcogen solvent additives; DPE, diphenyl sulfide (DPS) and diphenyl selenide (DPSe). DPS devices showed optimal power conversion efficiencies (PCEs) of up to 9.08%, and DPE devices also showed similarly high PCEs of up to 8.85%. In contrast, DPSe devices showed relatively low PCEs (5.45% at best) which we attribute to significant surface recombination and high series resistance, which led to limited open-circuit voltage (VOC). In the case of DPS, fast, field-independent photocurrent saturation with negligible bimolecular recombination led to efficient charge separation and collection, which resulted in the highest PCEs. Additionally, using 1,2,4-trimethylbenzene and DPS as an entirely non-halogenated solvent/additive system, we successfully demonstrated device fabrication with comparably high PCEs of up to 8.4%. This work elucidates the effects of diphenyl-based solvent additives in PSCs and suggests a great potential of DPS as an effective non-halogenated solvent additive.

Facile embedding of gold nanostructures in the hole transporting layer for efficient polymer solar cells

Abstract Photon harvesting by the active layers of bulk heterojunction (BHJ) polymer solar cells (PSCs) greatly influences the power conversion efficiency (PCE) of the devices. Inclusion of novel metal nanostructures in photoactive layers is one of the effective ways to enhance light trapping without increasing the device thickness. Here, we demonstrate a dramatic enhancement (up to 43%) of the power conversion efficiency (PCE) by exploiting gold nanostructures (AuNSs)-embedded graphene oxide (GO) as the hole transporting layer (HTL) in BHJ PSCs. The enhancement of the device performance could be attributed to the increase in both short circuit current density (Jsc) and fill factor (FF). The increased FF is most likely the result of the enhanced charge collection through modified HTL and anode contact. Furthermore, the optical properties of the photovoltaic devices suggest that AuNSs cause light trapping via plasmonic effects resulting enhanced Jsc.

Porphyrin Antenna-Enriched BODIPY-Thiophene Copolymer for Efficient Solar Cells.

Low bandgap A-π-D copolymer, P(BdP-DEHT), consisting of alternating BOronDIPYrromethene (BODIPY) and thiophene units bridged by ethynyl linkers, and its porphyrin-enriched analogue, P(BdP/Por-DEHT), were prepared, and their optical and electrochemical properties were studied. P(BdP-DEHT) exhibits strong absorption in the 500-800 nm range with an optical bandgap of 1.74 eV. On the basis of cyclic voltammetry, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are evaluated to be -5.40 and -3.66 eV, respectively. After the anchoring of zinc(II) porphyrin on the BODIPY unit, P(BdP/Por-DEHT) displays broadened absorption, thanks to porphyrins, and the optical bandgap decreases to 1.59 eV because of extension of BODIPY conjugation. The resulting estimated HOMO and LUMO energy levels, respectively, move to -5.32 and -3.73 eV. After optimization of the P(BdP-DEHT) or P(BdP/Por-DEHT) to PC71BM weight ratio to 1:2 in dichlorobenzene solution, the bulk heterojunction polymer solar cells show overall power conversion efficiencies (PCEs) of 3.03 and 3.86%, respectively. After solvent vapor annealing (SVA) treatment in CH2Cl2 for 40 s, the PCEs increased to 7.40% [Voc of 0.95 V, Jsc of 12.77 mA/cm2, and fill factor (FF) of 0.61 with energy loss of 0.79 eV] and 8.79% (Voc of 0.92 V, Jsc of 14.48 mA/cm2, and FF of 0.66 with energy loss of 0.67 eV). The increase in the PCE for P(BdP/Por-DEHT)-based devices is mainly attributed to the enhancement in Jsc and FF, which may be related to the broader absorption spectra, lower band gap, and better charge transport of P(BdP/Por-DEHT) compared to P(BdP-DEHT). This could also be related to the optimized nanoscale morphology of the active layer for both efficient exciton dissociation and charge transport toward the electrodes and a balanced charge transport in the device, induced by the SVA treatment of the active layer.

Photovoltaic Properties of a Porphyrin‐Containing Polymer as Donor in Bulk Heterojunction Solar Cells With Low Energy Loss

A low bandgap D‐π‐A polymer (PPDPP; D = Zn‐porphyrin, A = diketopyrrolopyrrole, π = ethynyle linker) has been synthesized and used as an electron donor for the fabrication of solution processed bulk heterojunction polymer solar cells (PSCs). PC71BM is used as the electron acceptor. After the optimization of the PPDPP: PC71BM weight ratio in 2‐methyltetrahydrofuran (2‐MeTHF), the PSC based on PPDPP: PC71BM (1:2) showed an overall power conversion efficiency (PCE) of 4.18% with Jsc = 9.75 mA cm−2, Voc = 0.78 V, and FF = 0.54. After the thermal annealing (TA) and subsequent solvent vapor annealing (SVA) of the active layer, the resulting device showed an overall PCE of 6.44% (Jsc = 13.18 mA cm−2, Voc = 0.74 V, and FF = 0.66) with a small voltage loss of 0.51 eV. The improvement in PCE after the TA and SVA treatment of the active layer is attributed to an enhancement of Jsc and FF related to a more favorable nanoscale morphology for the exciton dissociation and charge transport as evidenced by the increased hole mobility.

Incorporating an Electrode Modification Layer with a Vertical Phase Separated Photoactive Layer for Efficient and Stable Inverted Nonfullerene Polymer Solar Cells.

For bulk heterojunction polymer solar cells (PSCs), the donors and acceptors featuring specific phase separation and concentration distribution within the electron donor/acceptor blends crucially affect the exciton dissociation and charge transportation. Herein, efficient and stable nonfullerene inverted PSCs incorporating a phase separated photoactive layer and a titanium chelate electrode modification layer are demonstrated. Water contact angle (WCA), scanning kelvin probe microscopy (SKPM), and atomic force microscopy (AFM) techniques are implemented to characterize the morphology of photoactive layers. Compared with the control conventional device, the short-circuit current density (Jsc) is enhanced from 14.74 to 17.45 mAcm-2. The power conversion efficiency (PCE) for the inverted PSCs with a titanium (diisopropoxide)-bis-(2,4-pentanedionate) (TIPD) layer increases from 9.67% to 11.69% benefiting from the declined exciton recombination and fairly enhanced charge transportation. Furthermore, the nonencapsulated inverted device with a TIPD layer demonstrates the best long-term stability, 85% of initial PCE remaining and an almost undecayed open-circuit voltage (Voc) after 1440 h. Our results reveal that the titanium chelate is an excellent electrode modification layer to incorporate with a vertical phase separated photoactive layer for producing high-efficiency and high-stability inverted nonfullerene PSCs.

Enhancing Indacenodithiophene Acceptor Crystallinity via Substituent Manipulation Increases Organic Solar Cell Efficiency

The post-fullerene indacenodithiophene acceptor ITIC is a highly effective n-type component of high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs) for reasons that are not well-understood. Here, the impact of the ITIC alkyl sub-stituent architecture on PSC active layer film morphology, charge transport, and photovoltaic (PV) performance is investigated with the donor polymers PBDB-T and PBDB-TF. On progressing from n-propyl to n-hexyl to n-nonyl ITIC substituents, PSC power conversion efficiency (PCE) increases from <0.1 % to 9.31% to 10.24%, respectively. BHJ blend morphology, carrier recombina-tion dynamics, and PV performance with both donor polymers as probed by AFM, XRD, GIWAXS, and light intensity dependence correlate with marked differences in ITIC acceptor crystallinity. The DSC cold crystallization temperatures of the n-hexyl and n-nonyl-functionalized acceptors are found to closely track the anneal-ing temperatures for optimum PSC performance. These results identify a promising...

2,1,3-Benzothiadiazole-5,6-dicarboxylicimide-Based Polymer Semiconductors for Organic Thin-Film Transistors and Polymer Solar Cells.

A series of polymer semiconductors incorporating 2,1,3-benzothiadiazole-5,6-dicarboxylicimide (BTZI) as strong electron-withdrawing unit and an alkoxy-functionalized head-to-head linkage containing bithiophene or bithiazole as highly electron-rich co-unit are designed and synthesized. Because of the strong intramolecular charge transfer characteristics, all three polymers BTZI-TRTOR (P1), BTZI-BTOR (P2), and BTZI-BTzOR (P3) exhibit narrow bandgaps of 1.13, 1.05, and 0.92 eV, respectively, resulting in a very broad absorption ranging from 350 to 1400 nm. The highly electron-deficient 2,1,3-benzothiadiazole-5,6-dicarboxylicimide and alkoxy-functionalized bithiophene (or thiazole) lead to polymers with low-lying lowest unoccupied molecular orbitals (-3.96 to -4.28 eV) and high-lying highest occupied molecular orbitals (-5.01 to -5.20 eV). Hence, P1 and P3 show substantial and balanced ambipolar transport with electron mobilities/hole mobilities of up to 0.86/0.51 and 0.95/0.50 cm2 V-1 s-1, respectively, and polymer P2 containing the strongest donor unit exhibited unipolar p-type performance with an average hole mobility of 0.40 cm2 V-1 s-1 in top-gate/bottom-contact thin-film transistors with gold as the source and drain electrodes. When incorporated into bulk heterojunction polymer solar cells, the narrow bandgap (1.13 eV) polymer P1 shows an encouraging power conversion efficiency of 4.15% with a relatively large open-circuit voltage of 0.69 V, which corresponds to a remarkably small energy loss of 0.44 eV. The power conversion efficiency of P1 is among the highest reported to date with such a small energy loss in polymer:fullerene solar cells.

One-Step Synthesis of New Electron Acceptor for High Efficiency Solution Processable Organic Solar Cells

Here, we demonstrate a new fulleropyrrolidine electron acceptor coded as DIF-ful-C60, following a one-step synthetic protocol. The absorption spectrum of DIF-ful-C60 suggests broader absorption over PC61BM while the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the molecule are −5.32 and −3.60 eV, respectively. The solution processed bulk-heterojunction polymer solar cells fabricated using a blend of DIF-ful-C60 with PTB7 and PTB7-Th as electron donors show power conversion efficiencies (PCEs) of 6.8 and 8.6%, respectively, which is highest reported so far for fulleropyrrolidine type electron acceptors. In comparison, the control devices made from PTB7:PC61BM and PTB7-Th:PC61BM blends showed PCEs of 6.2 and 7.9%, respectively. The increase in efficiency of the devices based on DIF-ful-C60 is due to increase in the short circuit current as well as open circuit voltage of ca. 0.8 V.

Dithienosilole–phenylquinoxaline‐based copolymers with A‐D‐A‐D and A‐D structures for polymer solar cells

ABSTRACTTwo copolymers having D‐A‐D‐A (P1) and D‐A (P2) structures with quinoxaline acceptor unit and dithienosilole donor unit were synthesized and their optical and electrochemical (both experimental and theoretical) properties were investigated. The optical properties showed that these copolymers P1 and P2 exhibit optical bandgaps of 1.54 and 1.62 eV, respectively, with broader absorption profiles extending up to 800 nm and 770 nm, respectively. The electrochemical investigation of these two copolymers indicates that they exhibit suitable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels for efficient exciton dissociation and high open circuit voltage in the resultant polymer solar cells (PSCs). These copolymers were used as donors along with the PC71BM as acceptor for the fabrication of solution processed bulk heterojunction PSCs. The optimized P1:PC71BM and P2:PC71BM active layers treated with solvent vapor treatment showed overall power conversion efficiency (PCE) of 7.16% and 6.57%, respectively. The higher PCE of P1‐based device as compared to P2 might be attributed to higher crystallinity of P1 and good hole mobility resulting more balanced charge transport. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 376–386

Synthesis of new 2,6-bis(6-fluoro-2-hexyl-2H-benzotriazol-4-yl)-4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene based D-A conjugated terpolymers for photovoltaic application

Two D1-A-D2-A conjugated terpolymers with same A (fluorinated benzotriazole, fBTz), D2 (dithienosilole, DTS) and different D1 (bis(undecyl)benzo[2,1-b:3,4-b']dithiophene for P1 and didodecyloxy-benzo[1,2-b:4,5-b']dithiophene for P2) were synthesized and their optical and electrochemical properties were investigated. Compared to P1, the P2 with didodecyloxy side chains in benzodithiophene showed strong interchain π-π interaction, more redshifted absorption spectrum and higher molar extinction coefficient. The X-ray diffraction results indicated their excellent crystallinity and molecular stacking features, especially P2 containing didodecyloxy side chains in BDT. These two terpolymers were used as donor along with PC71BM for the fabrication of bulk heterojunction polymer solar cells. After the optimization of active layers, i.e. weight ratios of terpolymer to PC71BM and concentration of solvent additive (DIO) in the host solvent, the resultant polymer solar cells showed overall power conversion efficiency of 6.95% (Jsc = 12.23 mA/cm2, Voc = 0.93 V and FF = 0.61) and 8.14% (Jsc = 14.08 mA/cm2, Voc = 0.85 V and FF = 0.68) for P1 and P2 based bulk heterojunction active layers, respectively. The higher power conversion efficiency of P2 based polymer solar cell than P1 based counterpart, may be attributed to the more exciton dissociation, better nanoscale morphology and higher hole mobility in P2:PC71BM active layer than P1:PC71BM.

Bromine-Terminated Additives for Phase-Separated Morphology Control of PTB7:PC71BM-Based Polymer Solar Cells

Trace amounts of solvent additive can effectively regulate the phase-separated morphology of the active layer composed of donor and acceptor materials for improved power conversion efficiency (PCE) of polymer solar cells (PSCs). However, applicable solvent additives for PSCs are still limited, and it is difficult to rationally design or select appropriate solvent additives for optimal morphology control of the active layer, mainly due to the lack of sufficient understanding of the morphological regulation mechanism. Here, on the basis of a series of bromine-terminated additives with different chain lengths, we systematically investigated the relations between properties of solvent additives, active layer morphology, and photovoltaic performance of PTB7:PC71BM bulk heterojunction PSCs. In addition to the widely acknowledged requirements of solvent additives with selective solubility toward one of the components in the active layer and remarkably higher boiling point than that of the host solvent, it was fo...

Water-soluble polythiophenes as efficient charge-transport layers for the improvement of photovoltaic performance in bulk heterojunction polymeric solar cells

Water-soluble regioregular poly{3-[(6-sodium sulfonate)hexyl]thiophene} (PT6S) and poly{3-[(6-trimethylammoniumbromide)hexyl]thiophene} (PT6N) have been synthesized and employed both as photoactive layers for the assembling of “green” bulk-heterojunction organic solar cells and as charge-collection layers in a cell with “classic” architecture. While the photovoltaic performances obtained with the two aforementioned polymers were lower than the reference cell, their latter use allowed to notably increase the inherent J-V properties, leading to a considerable enhancement in the overall photovoltaic output. The power conversion efficiency of the optimized multilayer BHJ solar cell reached 4.78%, revealing a higher efficiency than the reference cell (3.63%).

In Situ GIWAXS Analysis of Solvent and Additive Effects on PTB7 Thin Film Microstructure Evolution during Spin Coating.

The influence of solvent and processing additives on the pathways and rates of crystalline morphology formation for spin-coated semiconducting PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno[3,4-b]thiophenediyl]]) thin films is investigated by in situ grazing incidence wide-angle X-ray scattering (GIWAXS) and optical reflectance, to better understand polymer solar cell (PSC) optimization approaches. In situ characterization of PTB7 film formation from chloroform (CF), chlorobenzene (CB), and 1,2-dichlorobenzene (DCB) solutions, as well as CB solutions with 1% and 3% v/v of the processing additives 1-chloronapthalene (CN), diphenylether (DPE), and 1,8-diiodooctane (DIO), reveals multiple crystallization pathways with: (i) single-solvent systems exhibiting rapid (<3 s) crystallization after a solvent boiling point-dependent film thinning transition, (ii) solvent + additive systems exhibiting different crystallization pathways and crystallite formation times from minutes (CN, DPE) to 1.5 h (DIO). Identifying crystalline intermediates has implications for bulk-heterojunction PSC morphology optimization via optimized spin-casting processes.

A Highly Crystalline Wide-Band-Gap Conjugated Polymer toward High-Performance As-Cast Nonfullerene Polymer Solar Cells.

A new wide-band-gap conjugated polymer PBODT was successfully synthesized that showed high crystallinity and was utilized as the active material in nonfullerene bulk-heterojunction polymer solar cells (PSCs). The photovoltaic devices based on the as-cast blend films of PBODT with ITIC and IDIC acceptors showed notable power conversion efficiencies (PCEs) of 7.06% and 9.09%, with high open-circuit voltages of 1.00 and 0.93 V that correspond to low energy losses of 0.59 and 0.69 eV, respectively. In the case of PBODT:ITIC, lower exciton quenching efficiency and monomolecular recombination are found for devices with small driving force. On the other hand, the relatively higher driving force and suppressed monomolecular recombination for PBODT:IDIC devices are identified to be the reason for their higher short-circuit current density (Jsc) and higher PCEs. In addition, when processed with the nonchlorinated solvent 1,2,4-trimethylbenzene, a good PCE of 8.19% was still achieved for the IDIC-based device. Our work shows that such wide-band-gap polymers have great potential for the environmentally friendly fabrication of highly efficient PSCs.

Non-halogenated solvent-processed single-junction polymer solar cells with 9.91% efficiency and improved photostability

High-efficiency bulk heterojunction (BHJ) polymer solar cells (PSCs) with power conversion efficiencies of 9.91% are achieved by using non-halogenated and environment-friendly solvent. To the best of our knowledge, it is the champion efficiency achieved by the random polymer based PSCs which are processed with all non-halogenated solvents. The photostability of the PSCs processed from halogenated solvent and non-halogenated solvent is intensively investigated by using grazing incidence wide-angle X-ray scattering (GIWAXS) combined with resonant soft X-ray scattering (R-SoXS) measurement, which is found to be efficient in monitoring the changes of the blend film morphology during irradiation. The results suggest that the crystallinity of the polymer as well as the domain purity of the blends with 1,8-diiodooctane (DIO) residual decrease dramatically in the illumination condition, due to sensitivity of DIO based device to the illumination. In contrast, the PffBT-2TPF4-9/1:PC61BM based PSCs processed with non-halogenated solvents show much better photostability and high efficiency. Therefore, our study offer an effective method to investigate the photostability of PSCs. It suggests that non-halogenated solvents can be excellent alternatives for large area printing of high-performance PSCs.

Acceptor-rich bulk heterojunction polymer solar cells with balanced charge mobilities

Abstract In this work, we reported efficient polymer solar cells with balanced hole/electron mobilities tuned by the acceptor content in bulk heterojunction blend films. The photovoltaic cells were fabricated with two new wide band-gap D-A polymers PBDDIDT and PBDDIDTT as the donor material. The molecular conformations of new polymers are carefully evaluated by theoretical calculations. The results of photovoltaic studies show that two devices reach their optimal conditions with rich PC 71 BM content up to 80% in blend films, which is uncommon with most of reported PSCs. The as-cast devices based on PBDDIDT and PBDDIDTT reveal good photovoltaic performance with PCE of 7.04% and 6.40%, respectively. The influence of PC 71 BM content on photovoltaic properties is further detailed studied by photoluminescence emission spectra, charge mobilities and heterojunction morphology. The results exhibit that more efficient charge transport between donor and acceptor occurs in rich PC 71 BM blend films. Meanwhile, the hole and electron mobilities are simultaneously enhanced and afford a good balance in rich PC 71 BM blend films (D/A, 1:4) which is critical for the improvement of current density and fill factors.