Bulk Heterojunction Polymer Solar Cells Research Articles

Conducting Polymers for Optoelectronic Devices and Organic Solar Cells: A Review.

In this review paper, we present a comprehensive summary of the different organic solar cell (OSC) families. Pure and doped conjugated polymers are described. The band structure, electronic properties, and charge separation process in conjugated polymers are briefly described. Various techniques for the preparation of conjugated polymers are presented in detail. The applications of conductive polymers for organic light emitting diodes (OLEDs), organic field effect transistors (OFETs), and organic photovoltaics (OPVs) are explained thoroughly. The architecture of organic polymer solar cells including single layer, bilayer planar heterojunction, and bulk heterojunction (BHJ) are described. Moreover, designing conjugated polymers for photovoltaic applications and optimizations of highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy levels are discussed. Principles of bulk heterojunction polymer solar cells are addressed. Finally, strategies for band gap tuning and characteristics of solar cell are presented. In this article, several processing parameters such as the choice of solvent(s) for spin casting film, thermal and solvent annealing, solvent additive, and blend composition that affect the nano-morphology of the photoactive layer are reviewed.

Balanced Charge Transport Optimizes Industry‐Relevant Ternary Polymer Solar Cells

Bulk heterojunction polymer solar cells based on a novel combination of materials are fabricated using industry‐compliant conditions for large area manufacturing. The relatively low‐cost polymer PTQ10 is paired with the nonfullerene acceptor 4TIC‐4F. Devices are processed using a nonhalogenated solvent to comply with industrial usage in absence of any thermal treatment to minimize the energy footprint of the fabrication. No solvent additive is used. Adding the well‐known and low‐cost fullerene derivative PC61BM acceptor to this binary blend to form a ternary blend, the power conversion efficiency (PCE) is improved from 8.4% to 9.9% due to increased fill factor (FF) and open‐circuit voltage (VOC) while simultaneously improving the stability. The introduction of PC61BM is able to balance the hole–electron mobility in the ternary blends, which is favourable for high FF. This charge transport behavior is correlated with the bulk heterojunction (BHJ) morphology deduced from grazing‐incidence wide‐angle X‐ray scattering (GIWAXS), atomic force microscopy (AFM), and surface energy analysis. In addition, the industrial figure of merit (i‐FOM) of this ternary blend is found to increase drastically upon addition of PC61BM due to an increased performance–stability–cost balance.

Ladder-Type Heteroheptacenes with Different Heterocycles for Nonfullerene Acceptors.

The design, synthesis, and characterization of two novel nonfullerene acceptors (M8 and M34) based on ladder-type heteroheptacenes with different heterocycles are reported. Replacing the furan heterocycles with the thiophene heterocycles in the heteroheptacene backbone leads to a hypsochromically shifted absorption band and greatly improved carrier transport for the resulting nonfullerene acceptor (M34) although the π-π-stacking distances are barely affected. Bulk-heterojunction polymer solar cells fabricated from M34 and a wide band gap polymer (PM6) as the donor showed a best power conversion efficiency (PCE) of 15.24 % with an open circuit voltage (VOC ) of 0.91 V, much higher than a PCE of 4.21 % and a VOC of 0.83 V for the counterparts based on M8:PM6. These results highlight the importance of key atoms in the construction of high-performance nonfullerene acceptors.

Cationic Polyelectrolytes with Alkylsulfonate Counterions as a Cathode Interface Layer for High-Performance Polymer Solar Cells.

Three cationic polyelectrolytes polyethyleneimine ethoxylate (PEIE)-1,4-butanediol dimethylsulfonate (MSB), PEIE-1,4-butanediol diethylsulfonate (ESB), and PEIE-1,4-butanediol dibenzylsulfonate (BSB), containing methylsulfonate, ethylsulfonate, and benzylsulfonate, respectively, were prepared for cathode interface layers (CILs) via a one-step reaction with 1,4-butanediol dialkylsulfonate and PEIE as the reactants. The results indicate that PEIE-MSB and PEIE-ESB with smaller counterions possess more efficient electron extraction, higher electron mobilities, and better photovoltaic performance than PEIE-BSB with larger counterions. The PTB7-Th:PC71BM-based single junction bulk heterojunction polymer solar cells (PSCs) with PEIE-ESB as the CIL showed power conversion efficiencies (PCEs) of 10.44 and 9.23% under the thickness conditions of 8 and 30 nm, respectively. The PM6:Y6-based PSCs displayed a high PCE of 15.69%. The study provides not only new high-performance CILs but also a new strategy to construct light-soaking-free PSCs via tuning alkylsulfonate counterions.

Simultaneous Passivation of Surface Vacancies and Enhancement in Charge Transfer Property of ZnO Electron Transport Layer for Inverted Organic Solar Cells

The design and development of a charge carrier transport material are decisive parameters for controlling the organic solar cell's power conversion efficiency (PCE). ZnO is one of the most suitable electron transport materials used in inverted bulk heterojunction polymer solar cells. However, the solution‐processed ZnO has surface defects, which hinders the power conversation efficiency of the device. Herein, it is designed and demonstrated that the 2D NO2 group functionalized reduced graphene oxide (rGN) sheet coated on top of the 1D ZnO nanoridges not only passivates the surface but also enhances the charge transport property of the electron transport layer (ETL), thereby improving the overall PCE by 31%. Atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), and optical measurements reveal that the highly transparent bilayer ZnO/rGN ETL has uniform film formation and, thereby, improved ohmic contact between the cathode and the photoactive layer. Due to the improved electron transport from the photoanode (PTB7‐Th:PC71BM) to the buffer layer, a photoinduced current density of 20.05 mA cm−2 is achieved. This interface modification by rGN can be an effective strategy to passivate the surface and retards the recombination rate to enhance the efficiency of organic photovoltaic cells.

A three-dimensional multi-chromophore naphthalene diimide acceptor for polymer bulk heterojunction solar cells

Non-fullerene acceptors are proving to be effective for high efficiency polymer bulk heterojunction solar cells. We report the synthesis and properties of a three-dimensional naphthalene diimide-based acceptor where four dithienylnaphthalene moieties are attached to a central tetraphenylethylene unit. In spite of the material being designed as an electron acceptor it was found to have similar and good hole and electron mobilities of 1.5 × 10−4 and 1.8 × 10−4 cm2/Vs, respectively. When blended with PTB7-Th, which had previously been used with other three-dimensional acceptors, at a weight ratio of 1:1 the hole mobility was found to increase slightly but the electron mobility decreased such that there was an imbalance of two orders of magnitude. The blend with a 1:1 donor:acceptor weight ratio was found to give devices with the best power conversion efficiency of 3.0 % in an inverted device architecture, which is amongst the best for non-polymeric naphthalene diimide acceptors. A combination of transient absorption and intensity dependent measurements, along with the charge mobility results indicated that while exciton separation was rapid, the performance of the devices was limited by recombination processes that decreased the extraction of the photogenerated charges.

Enhanced performance of polymer solar cells based on P3HT:PCBM via incorporating Au nanoparticles prepared by the micellar method

Surface plasmonic effect of metal nanoparticles is an effective method to improve the power conversion efficiency (PCE) of solar cells. In this work, the PCE of bulk heterojunction (BHJ) polymer solar cells was improved by Au nanoparticles (NPs). The Au NPs were embedded into PEDOT:PSS hole transport layer by spin coating on the ITO substrates. The Au NPs with a diameter of ~16 nm were prepared by the micellar method using polystyrene-block-poly (2-vinylpyridine) diblock polymer. The Au NPs prepared by this method are distributed uniformly in size and without agglomeration on the substrates. From both experimental and theoretical results, it can be seen that the light absorption of the active layer was increased because of the surface plasmonic effect of Au NPs. Meanwhile, the carrier transport performance of PEDOT:PSS was enhanced with introduced Au NPs. As a result, the PCE of BHJ solar cells was improved from 2.81 to 3.25% by incorporating Au NPs.

Synthesis of random copolymer using Zig-Zag Naphthodithiophene for bulk Heterojunction polymer solar cell applications

Synthesized a random copolymer here, namely P(zNDT-TPDBT), consisting of an electron-rich unit ‘zig-zag’ 4,9-bis-(2-ethylhexyloxy)naphtho[1,2-b:5,6-b’]dithiophene (zNDT) and different ratio of two-electron acceptor units of thieno[3,4-c]pyrrole-4,6-dione (TPD) and benzodiathiazole (BT) via Stille coupling polymerization. The photophysical, electrochemical, and photovoltaic properties of P(zNDT-TPDBT) were investigated. The differences in the photophysical property of P(zNDT-TPDBT) indicate (i) red-shifted and broadening absorption spectrum located at 509 nm in thin-film as compared to solution (λmax = 492 nm) (ii) the fluorescence spectrum shows dual emission bands at 545 nm and 608 nm in the longer wavelength in solution, however, relatively complete quenching fluorescence property was observed in blended film with PC71BM. The copolymer exhibited an optical bandgap of 2.03 eV, with a highest occupied molecular orbital (HOMO) level of −5.87 eV. The optimized structure for the copolymer was also determined through DFT calculation. The bulk-heterojunction (BHJ) polymer solar cell (PSC) with a device structure of ITO/PEDOT:PSS/P(zNDT-TPDBT):PC71BM/Al exhibited a promising efficiency of 2.4% with the short circuit current density (Jsc) of 5.7 mA cm−2, open-circuit voltage (Voc) of 0.91 V, and fill factor (FF) of 47%, without processing addictive. The clear correlations between film morphology and device efficiency were observed through atomic force microscopy technique.

Wide Band-gap Two-dimension Conjugated Polymer Donors with Different Amounts of Chlorine Substitution on Alkoxyphenyl Conjugated Side Chains for Non-fullerene Polymer Solar Cells

In this study, wide bandgap (WBG) two-dimensional (2D) copolymer donors (DZ1, DZ2, and DZ3) based on benzodithiophene (BDT) on alkoxyphenyl conjugated side chains without and with different amounts of chlorine atoms and difluorobenzotriazole (FBTZ) are designed and synthesized successfully for efficient non-fullerene polymer solar cells (PSCs). Three polymer donors DZ1, DZ2, and DZ3 display similar absorption spectra at 300–700 nm range with optional band-gap (Egopt) of 1.84, 1.92, and 1.97 eV, respectively. Compared with reported DZ1 without chlorine substitution, it is found that introducing chlorine atoms into the meta-position of the alkoxyphenyl group affords polymer possessing a deeper the highest occupied molecular orbital (HOMO) energy level, which can increase open circuit voltage (VOC) of PSCs, as well as improve hole mobility. Non-fullerene bulk heterojunction PSCs based on DZ2:MeIC demonstrate a relatively high power conversion efficiency (PCE) of 10.22% with a VOC of 0.88 V, a short-circuit current density (JSC) of 17.62 mA/cm2, and a fill factor (FF) of 68%, compared with PSCs based on DZ1:MeIC (a PCE of 8.26%) and DZ3:MeIC (a PCE of 6.28%). The results imply that adjusting chlorine atom amount on alkoxyphenyl side chains based on BDT polymer donors is a promising approach of synthesizing electron-rich building block for high performance of PSCs.

2-D Modeling of Bulk Heterojunction Solar Cells With a Gaussian Shape in Donor-Acceptor Interface as Morphology

In this work, we presented numerical modeling for the bulk heterojunction polymer solar cells (BHJ) based on the finite difference method to obtaining the BHJ characteristics. Our model for the BHJ, where the electron donor and acceptor materials are mixed in a solution, is two separate domains with gaussian shape interface. In this model we consider a two-dimensional geometry of thickness L on y-axis and periodic along x-axis. We assume that the rate of exciton generation to be homogenous throughout device. Some of obtained characteristics are the density of electrons and the density of the holes, current density, open circuit voltage and short circuit current density.

A Non‐Fullerene Acceptor with Chlorinated Thienyl Conjugated Side Chains for High‐Performance Polymer Solar Cells via Toluene Processing

Summary of main observation and conclusionSmall molecular acceptors (SMAs) BTC‐2F and BTH‐2F, based on heptacyclic benzodi(cyclopentadithiophene) electron‐donating core (CBT) with chlorinated‐thienyl conjugated and thienyl conjugated side chains, respectively, are designed and synthesized. Compared with non‐chlorine acceptor BTH‐2F, BTC‐2F exhibits slightly blue‐shifted absorption spectra, similar the lowest unoccupied molecular orbital (LUMO) (–3.91 eV), deeper highest occupied molecular orbital (HOMO) energy level and higher electron mobility than that of BTH‐2F. PM6, a wide bandgap polymer, is selected as the donor material to construct bulk heterojunction polymer solar cells processed with nonhalogenated solvent toluene. The optimized PM6:BTC‐2F‐based device presents a 12.9% power conversion efficiency (PCE), while the PCE of PM6:BTH‐2F‐based device is only 11.3%. The results suggest that it is an effective strategy to optimize the photoelectric properties of SMAs by incorporating chlorine atom into the conjugated side chains.

Performance improvement of polymer solar cells with binary additives induced morphology optimization and interface modification simultaneously

Active layer morphology optimization and electrode buffer layer interface modification are commonly used strategies in improving the performance of polymer solar cells (PSCs). In this study, we prepared PTB7: PC71BM bulk heterojunction PSCs with 1,8-diiodooctane (DIO) and polyethylene glycol (PEG) additives, and studied the influence of binary additives on exciton dissociation, charge transport and charge extraction. DIO facilitates donor/acceptor phase separation for efficient exciton dissociation and charge transport. The migration of PEG from active layer to the PEDOT:PSS layer improves the crystallinity of PTB7, optimizes charge transport pathway, and enhances the conductivity of PEDOT:PSS layer. With the combined advantages of binary additives in active layer morphology optimization and anode buffer layer modification, the device exhibits a high short-circuit current density of 20.03 mA/cm2 and an improved power conversion efficiency. Binary additive provides a promising method to optimize active layer morphology and improve interfacial buffer layer of PSCs simultaneously.

Toward efficient polymer solar cells with thick bulk heterojunction by introducing iridium complex as an aggregation reshaping auxiliary

The morphology control of thick bulk heterojunction (BHJ) polymer solar cells (PSCs) is an important factor to determine their power conversion efficiency (PCE). Particularly, during the building of phase separation, aggregation morphology plays a prominent role in the control of both horizonal and vertical gradient distribution of donor/acceptor (D/A) in thick BHJ. In this work, we introduced a novel iridium complex of (tfmppy)2Ir(tpip) into the active layer of PffBT4T-2OD: PC71BM as an “aggregation reshaping auxiliary” to form a long and narrow aggregation shape in both horizontal and vertical directions. Through characterizing the morphology of active layer in details, it was found that the combination of 5% (tfmppy)2Ir(tpip) assists PffBT4T-2OD aggregation shape with a 1: 2.5 aspect ratio while maintaining high crystallinity. In addition, the results showed that the (tfmppy)2Ir(tpip) facilitates efficient exciton dissociation and charge transport because of increased contacting area of D/A interface. As a result, the short circuit current (JSC) and fill factor (FF) performances were both improved contemporaneously, leading to a 20.3% enhancement in PCE.

Synergistic Effects of Polymer Donor Backbone Fluorination and Nitrogenation Translate into Efficient Non-Fullerene Bulk-Heterojunction Polymer Solar Cells.

State-of-the-art non-fullerene bulk-heterojunction (BHJ) polymer solar cells outperform the more extensively studied polymer-fullerene BHJ solar cells in terms of efficiency, thermal-, and photostability. Considering the strong light absorption in the near-infrared region (600-1000 nm) for most of the efficient acceptors, the exploration of high-performing large band gap (LBG) polymer donors with complementary optical absorption ranging from 400 to 700 nm remains critical. In this work, the strategy of concurrently incorporating fluorine (-F) and unsaturated nitrogen (-N) substituents along the polymer backbones is used to develop the LBG polymer donor PB[N][F]. Results show that the F- and N-substituted polymer donor PB[N][F] realizes up to 14.4% efficiency in BHJ photovoltaic devices when paired with a benchmark molecule acceptor Y6, which largely outperforms the analogues PB with an efficiency of only 3.6% and PB[N] with an efficiency of 11.8%. Systematic examinations show that synergistic effects of polymer backbone fluorination and nitrogenation can significantly increase ionization potential values, improve charge transport, and reduce bimolecular recombination and trap-assisted recombination in the PB[N][F]:Y6 BHJ system. Importantly, our study shows that the F- and N-substituted conjugated polymers are promising electron-donor materials for solution-processed non-fullerene BHJ solar cells.

A Benzo[1,2-b:4,5-c']Dithiophene-4,8-Dione-Based Polymer Donor Achieving an Efficiency Over 16.

It is of great significance to develop efficient donor polymers during the rapid development of acceptor materials for nonfullerene bulk-heterojunction (BHJ) polymer solar cells. Herein, a new donor polymer, named PBTT-F, based on a strongly electron-deficient core (5,7-dibromo-2,3-bis(2-ethylhexyl)benzo[1,2-b:4,5-c']dithiophene-4,8-dione, TTDO), is developed through the design of cyclohexane-1,4-dione embedded into a thieno[3,4-b]thiophene (TT) unit. When blended with the acceptor Y6, the PBTT-F-based photovoltaic device exhibits an outstanding power conversion efficiency (PCE) of 16.1% with a very high fill factor (FF) of 77.1%. This polymer also shows high efficiency for a thick-film device, with a PCE of ≈14.2% being realized for an active layer thickness of 190 nm. In addition, the PBTT-F-based polymer solar cells also show good stability after storage for ≈700 h in a glove box, with a high PCE of ≈14.8%, which obviously shows that this kind of polymer is very promising for future commercial applications. This work provides a unique strategy for the molecular synthesis of donor polymers, and these results demonstrate that PBTT-F is a very promising donor polymer for use in polymer solar cells, providing an alternative choice for a variety of fullerene-free acceptor materials for the research community.

Investigation of Optical and Electrochemical Properties of Benzene Based Solution Processable 2,1,3-Benzooxadiazole Comprising Polymers

Four novel solution processable benzooxadiazole containing alternating copolymers namely poly (5,6-bis(octyloxy)-4-(5-phenylthiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]oxadiazole)PBOTPh, poly (5,6-bis(octyloxy)-4-(5-phenylselenophen-2-yl)-7-(selenophen-2-yl)benzo[c][1,2,5]oxadiazole)PBOSPh, poly (4-(furan-2-yl)-5,6-bis(octyloxy)−7-(5-phenylfuran-2-yl)benzo[c][1,2,5]oxadiazole)PBOFPh and poly (5,6-bis(octyloxy)-4-(5-phenylthieno[3,2-b]thiophen-2-yl)-7-(thieno[3,2-b]thiophen-2-yl)benzo[c][1,2,5]oxadiazole) PBOTTPh incorporating 5,6-bis(octyloxy)benzo[c][1,2,5]oxadiazole as the acceptor moiety and benzene as the donor moiety were synthesized via Suzuki polycondensation reaction. Various electron donor moieties were used as π bridges in the polymer backbone to tailor the optoelectronic properties of the polymers. To explore the electrochemical and electrochromic properties and the photovoltaic properties of the copolymers, cyclic voltammetry, spectroelectrochemistry and kinetic studies were conducted. Thiophene, selenophene, furan and thienothiophene π bridges were incorporated into the polymer backbones. Thiophene and selenophene based alternating copolymers showed purple color in its neutral state whereas furan and thienothiophene bearing copolymers have pinkish purple and dark purple in their neutral states respectively. The optical band gaps of the polymers were calculated as 1.85, 1.76, 1.58, 1.79 eV for thiophene, selenophene, thienothiophene and furan based copolymers, respectively. Among all the alternating copolymers, the lowest optical band gap corresponds to thienothiophene comprising alternating copolymer which is 1.58 eV and red shifted in absorption due to the stronger donor character compared to other analogs. HOMO and LUMO levels were calculated from cyclic voltammetry studies using onsets of oxidation and reduction, respectively. HOMO levels were calculated as −5.96 eV for PBOTPh, −5.85 eV for PBOSPh, −5.63 eV for PBOTTPh and −5.74 eV for PBOFPh. LUMO levels were determined as −4.02 eV for PBOSPh, −4.03 eV for PBOTTPh and −3.87 eV for PBOFPh. Bulk heterojunction (BHJ) polymer solar cells based on selenophene containing polymer were fabricated. The device architecture was: ITO/PEDOT:PSS/PBOSPh:PC71BM/Ca/Al. After optimization studies, best results for 130 nm thickness, Polymer:PC71BM with 1:2 ratio, Voc, Jsc, FF, PCE (%) were found as 0.62 V, 4.53 mA cm−2, 0.58, 1.63% under illumination of AM 1.5 G 100 mW cm−2.

Bithieno[3,4-c]pyrrole-4,6-dione-Mediated Crystallinity in Large-Bandgap Polymer Donors Directs Charge Transportation and Recombination in Efficient Nonfullerene Polymer Solar Cells

Solution-processed nonfullerene bulk-heterojunction (BHJ) polymer solar cells (PSCs), which are composed of polymer donors and organic acceptors, are proven to manifest promising performance and lo...

Efficient Method of Fabricating Polymeric Solar Cells in Multilayered Configuration Using Electrospray

Electrospray deposition (ESD) for the fabrication of multilayered (ML) bulk heterojunction polymeric solar cells via direct/inverted configurations was investigated. The active layer heterojunction (P3HT/PCBM) is deposited both in single and multilayered architectures. In ML configuration, the P3HT/PCBM blend film is sandwiched between thin donor (P3HT) and acceptor (PCBM) layers. In ZnO-based inverted solar cells, ZnO film synthesized by sol–gel process was deposited on ITO substrate using spin coatinng. Solar cells were fabricated via ESD and spin coating and the device’s performance parameters were compared. Higher efficiency was obtained in the case of (ML) ESD device (direct structure). Post thermal treatment showed that ESD devices exhibit a power conversion efficiency (PCE) of 1.85% ML and 1.64% SL in direct structure while in the case of inverted structure PCEs of 1.30% ML and 0.82% SL were obtained as compared to 1.64% from the spin coated device at 120°C. The ML ESD devices have shown an overall efficiency enhancement of 13% (direct) and 58% (inverted) over the single layer (SL) ESD devices. The enhanced performance in ML devices is because there is a spatially uninterrupted pathway to the charge carrier transport towards their respective electrodes.

Rational Design of 2D p–π Conjugated Polysquaraines for Both Fullerene and Nonfullerene Polymer Solar Cells

AbstractSo far, squaraine‐based polymer donors have been less explored for the bulk heterojunction (BHJ) polymer solar cells. In this work, two new p–π conjugated polysquaraines (PASQ‐BDT1 and PASQ‐BDT2) with different electron‐rich subunits on the squaraine skeleton are rationally developed as new polymer donors based on the 2D structure design concept. PASQ‐BDT2 with N,N‐diisobutylaniline subunits shows superior device performances in both fullerene and nonfullerene PSCs compared to PASQ‐BDT1 containing triphenylamine subunits, with power conversion efficiencies (PCEs) of 4.34% and 3.72%, respectively, owing to increased light‐harvesting ability and more favorable nanoscale morphology in the BHJ films. Moreover, its demonstrated that solvent effects can play an effective role in elevating the device performance. For the PASQ‐BDT2/PC71BM blend, the PCE is improved from 3.19% to 4.34% after solvent vapor annealing treatment, mainly attributed to the optimized film morphology and increased hole mobility. More interestingly, when the processing solvent for nonfullerene devices is changed from chlorobenzene to chloroform, a significant enhancement on PCE from 1.96% to 3.72% is yielded for the PASQ‐BDT2/ITIC blend, due to suppressed charge recombination and enhanced crystallinity in the chloroform‐processed BHJ films.

Impact of alkyl side chain on the photostability and optoelectronic properties of indacenodithieno[3,2‐b]thiophene‐alt‐naphtho[1,2‐c:5,6‐ c′]bis[1,2,5]thiadiazole medium bandgap copolymers

AbstractThree donor‐π‐acceptor (D‐π‐A) type alternating conjugated polymers, namely PIDTT‐DTNT‐C16, PIDTT‐DTNT‐HD and PIDTT‐DTNT‐OD bearing the same backbone of indacenodithieno[3,2‐b]thiophene (IDTT) as the D unit and naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole (NT) as the A moiety but with different flexible side chain (n‐hexadecyl (C16), 2‐hexyldecyl (HD) and 2‐octyldodecyl (OD)) substituted thiophene employed as π‐bridges, were synthesized and characterized. The effects of the side chain on absorption, photostability, energy levels, aggregation, backbone conformation, morphology and photovoltaic properties were systematically investigated. Because moderate D and strong A units were selected to construct the polymer backbone, a medium optical bandgap (ca. 1.66 eV) and low‐lying highest occupied molecular orbital energy level (EHOMO ≈ −5.36 V), thus resulting in a relatively higher open‐circuit voltage (VOC) of 0.80–0.83 V, were achieved. It was found that the side chain gave rise to an insignificant impact on absorption, aggregation and photostability in chlorobenzene solution and energy levels but a non‐negligible influence on absorption, photostability and aggregation behavior in the film state. It was found that PIDTT‐DTNT‐C16 with the densest and most ordered packing structure exhibited the best photostability. Inverted bulk heterojunction polymer solar cells based on PIDTT‐DTNT‐HD:PC61BM ([6,6]‐phenyl‐C61‐butyric acid methyl ester) showed at least a 1.5‐fold increase in power conversion efficiency, chiefly originating from its slightly improved absorption, more balanced μh/μe ratio and favorable morphology of the active layer as a result of incorporating branched HD side chains into the IDTT‐alt‐DTNT backbone. © 2019 Society of Chemical Industry