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A 7.3% efficiency non-fullerene polymer solar cell is realized by combining a large-bandgap polymer PffT2-FTAZ-2DT with a small-bandgap acceptor IEIC. The complementary absorption of donor polymer and small-molecule acceptor is responsible for the high-performance of the solar-cell device. This work provides important guidance to improve the performance of non-fullerene polymer solar cells., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Electron transporting materials (ETMs) play vital roles in determining the efficiency and stability of inverted perovskite solar cells. The widely used PCBM is prone to undesirable aggregation and migration in a cell, thus impairing device stability. In this work, we develop a new type of ETMs by polymerizing C60 fullerene with an aromantic linker unit. The resultant polyfullerene (PFBS‐C12) not only maintains the good optoelectronic properties of fullerenes, but also can address the aforementioned aggregation problem of PCBM. The polyfullerene‐based blade‐coated cells exhibit a high efficiency of 23.2 % and good device stability that maintain 96 % of initial efficiency after >1300‐hour light soaking. An aperture efficiency of 18.9 % is also achieved on a 53.6‐cm2 perovskite mini‐module. This work provides a new strategy for designing ETMs that retain the key figure‐of‐merits of conventional fullerene molecules and enable more stable perovskite solar devices simultaneously. [ABSTRACT FROM AUTHOR]

All‐polymer solar cells (all‐PSCs) are one of the most promising application‐oriented organic photovoltaic technologies due to their excellent operational and mechanical stability. However, the power conversion efficiencies (PCEs) are mostly lower than 16%, restricting their core competitiveness. Furthermore, the improvement of mechanical durability is rarely paid attention to cutting‐edge all‐PSCs. This work deploys a low‐cost "technical grade" PCBM (incompletely separated but pure mixtures containing ≥90% [70]PCBM or [60]PCBM), into the efficient PM6:PY‐IT all‐polymer blend, successfully yielding a high‐performance ternary device with 16.16% PCE, among the highest PCE values for all‐PSCs. Meanwhile, an excellent mechanical property (i.e., crack onset strain = 11.1%) promoted from 9.5% for the ternary system is also demonstrated. The "technical grade" PCBM slightly disrupts the crystallization of polymers, and disperses well into the amorphous polymer regions of the all‐PSC blends, thus facilitating charge transport and improving film ductility simultaneously. All these results confirm introducing low‐cost "technical grade" PCBM with high electron mobility into all‐polymer blends can improve carrier mobility, reduce charge recombination, and optimize morphology of the amorphous polymer regions, thus yielding more efficient and mechanically durable all‐PSCs. [ABSTRACT FROM AUTHOR]

Central π‐core engineering of non‐fullerene small molecule acceptors (NF‐SMAs) is effective in boosting the performance of organic solar cells (OSCs). Especially, selenium (Se) functionalization of NF‐SMAs is considered a promising strategy but the structure‐performance relationship remains unclear. Here, we synthesize two isomeric alkylphenyl‐substituted selenopheno[3,2‐b]thiophene‐based NF‐SMAs named mPh4F‐TS and mPh4F‐ST with different substitution positions, and contrast them with the thieno[3,2‐b]thiophene‐based analogue, mPh4F‐TT. When placing Se atoms at the outer positions of the π‐core, mPh4F‐TS shows the most red‐shifted absorption and compact molecular stacking. The PM6 : mPh4F‐TS devices exhibit excellent absorption, high charge carrier mobility, and reduced energy loss. Consequently, PM6 : mPh4F‐TS achieves more balanced photovoltaic parameters and yields an efficiency of 18.05 %, which highlights that precisely manipulating selenium functionalization is a practicable way toward high‐efficiency OSCs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Herein, a pair of tetrahalogenated nonfullerene small molecular acceptors (NF‐SMAs) (TSeIC4Cl and TSeIC4Br) are designed and synthesized, with the same indacenodithieno[3,2‐b]selenophene central unit and two different dihalogenation terminal groups, respectively. The systematic investigation is achieved to reveal the impact of two different nondifluorinated terminal groups on the device performance of the resultant ITIC series NF‐SMAs. TSeIC4Br shows red‐shifted absorption range and higher frontier energy levels compared with that of TSeIC4Cl. Moreover, PM6:TSeIC4Br blend film exhibits more suitable phase‐separated morphology with more ordered molecular packing and improved miscibility compared with that of the tetrachlorinated counterpart. Importantly, PM6:TSeIC4Br‐based device exhibits a better power conversion efficiency (PCE) of 11.92%, with a higher open‐circuit voltage (Voc) and an enhanced short‐circuit current density (Jsc) when compared with that of PM6:TSeIC4Cl‐based device (11.13%). Furthermore, the energy loss can be reduced by replacing the disubstituents of end group from chlorine to bromine atoms. The results demonstrate that incorporation of indacenodithieno[3,2‐b]selenophene and replacement of tetrachlorination by tetrabromination on the end group contributes to elevating the Jsc, reduce energy loss, and enhancing the PCE for its relevant ITIC series device simultaneously, which may give a new avenue for achieving high‐performance multihalogenated ITIC series NF‐SMAs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Herein, a novel small molecule donor is first developed, FSM6, which is a cognate molecule to BTR possessing similar molecular structure with comparable optical absorption but different crystallinity. The efficient fullerene‐type ternary small molecular solar cells (SMSCs) based on an alloy donor of BTR and FSM6 in a thick film of 250 nm reveal the improved hierarchical phase separation morphology and molecular structural order of ternary active layers with improved crystallinity of the key donor component BTR. Furthermore, FSM6 as the key third component also plays a role of charge transfer accelerator in ternary SMSCs. As a result, the optimal ternary SMSCs based on BTR:FSM6:PC71BM achieve a high power conversion efficiency (PCE) up to 10.21% with the synergistically improved open‐circuit voltage of 0.950 V, short‐circuit current density of 13.85 mA cm−2, and fill factor of 77.6%, in comparison with either the binary SMSCs of BTR:PC71BM (PCE = 9.37%) or FSM6:PC71BM (PCE = 8.00%). This work provides a promising methodology to optimize device morphology for high‐performance ternary SMSCs by combining two cognate small molecules with similar absorption spectra but different crystallinity as an alloy donor. [ABSTRACT FROM AUTHOR]

Abstract: The field of nonfullerene organic solar cells (OSCs) has seen an impressive progress, largely due to advances in high‐performance small molecule acceptors (SMAs). As a large portion of the solar energy is located in the near‐infrared region, it is important to develop ultralow‐bandgap SMAs that have extended absorption in the spectral range of 800–1000 nm to maximize light absorption and efficiencies. In this work, three low‐bandgap SMAs, namely, IXIC, IXIC‐2Cl, and IXIC‐4Cl, are designed and synthesized with same fused terthieno[3,2‐b]thiophene donor unit and different end groups (EGs). The three SMAs all have low optical bandgap (Eg) of 1.35, 1.30, and 1.25 eV, respectively. The chlorination on EGs can lower the energy level and broaden absorption range of the SMAs. As a result, the Voc of the devices is reduced but the Jsc is significantly increased. In addition, the addition of chlorine atoms can enhance π–π stacking and crystallinity of the SMAs, which result in high fill factors. Overall, the optimum EGs are monochlorine‐substituted IC and OSCs based on PBDB‐T:IXIC‐2Cl that can achieve remarkable power conversion efficiencies (PCEs) of 12.2%, which is one of the highest PCEs for nonfullerene organic solar cells based on low‐bandgap SMAs. [ABSTRACT FROM AUTHOR]

Abstract: Compared to the rapid development of nonfullerene organic solar cells (OSCs) based on the state‐of‐the‐art indacenodithiophene (IDT)‐based small molecule acceptors (SMAs), the progress for perylene diimide (PDI)‐based electron acceptors has lagged behind owing to the lack of understanding on the structure–morphology–performance relationship of PDI SMAs. Given the ease of synthesis for PDIs and their high intrinsic electron mobility, it is crucial to identify key material parameters that influence the polymer:PDI blend morphology and to develop rational approaches for molecular design toward high‐performance PDI‐based SMAs. In this study, three pairs of PDI‐based SMAs with and without ring‐fusion are investigated and it is found that ring‐fusion and domain purity are the key structural and morphological factors determining the fill factors (FFs) and efficiencies of PDI‐based nonfullerene OSCs. This data shows that nonfullerene OSCs based on the ring‐fused PDI‐based SMAs exhibit much higher average domain purity and thus increased charge mobilities, which lead to enhanced FFs compared to those solar cells based on nonfused PDIs. This is explained by higher Florry Huggins interaction parameters as observed by melting point depression measurements. This study suggests that increasing repulsive molecular interactions to lower the miscibility between the polymer donor and PDI acceptor is the key to improve the FF and performance of PDI‐based devices. [ABSTRACT FROM AUTHOR]

One advantage of nonfullerene polymer solar cells (PSCs) is that they can yield high open-circuit voltage ( VOC) despite their relatively low optical bandgaps. To maximize the VOC of PSCs, it is important to fine-tune the energy level offset between the donor and acceptor materials, but in a way not negatively affecting the morphology of the donor:acceptor (D:A) blends. Here, an effective material design rationale based on a family of D-A1-D-A2 terthiophene (T3) donor polymers is reported, which allows for the effective tuning of energy levels but without any negative impacts on the morphology of the blend. Based on a T3 donor unit combined with difluorobenzothiadiazole (ffBT) and difluorobenzoxadiazole (ffBX) acceptor units, three donor polymers are developed with highly similar morphological properties. This is particularly surprising considering that the corresponding quaterthiophene polymers based on ffBT and ffBX exhibit dramatic differences in their solubility and morphological properties. With the fine-tuning of energy levels, the T3 polymers yield nonfullerene PSCs with a high efficiency of 9.0% for one case and with a remarkably low energy loss (0.53 V) for another polymer. This work will facilitate the development of efficient nonfullerene PSCs with optimal energy levels and favorable morphology properties. [ABSTRACT FROM AUTHOR]

Benzene units are inserted into the backbone of a quaterthiophene‐based polymer named PffBT4T, and the resulting polymer, PffBT4T‐B, exhibits remarkably tight alkyl chain interdigitation, which can expel the ITIC‐Th molecules from the polymer domains thus forming more pure and crystalline ITIC‐Th domains. As a result, PffBT4T‐B‐based non‐fullerene organic solar cells achieve a high power conversion efficiency of 9.4%. [ABSTRACT FROM AUTHOR]

Methanofullerenes such as the well-known [70]PCBM are commonly synthesized under harsh conditions to obtain the product as a mixture of site-isomers (namely α, β and minor γ) due to the D5h symmetry of the C70 cage. We report the first site-selective synthesis of [70]methanofullerenes under light irradiation and low temperatures, thus avoiding time-consuming and highly expensive HPLC separations. Pure major site-isomers α-[70]PCBM and α-[70]DPM have been thus efficiently prepared including the crystal structure of 5b. Photovoltaic preliminary results revealed a slightly beneficial performance for α-pure [70]PCBM site-isomer devices. [ABSTRACT FROM AUTHOR]

Developing a high-performance donor polymer is critical for achieving efficient non-fullerene organic solar cells (OSCs). Currently, most high-efficiency OSCs are based on a donor polymer named PM6, unfortunately, whose performance is highly sensitive to its molecular weight and thus has significant batch-to-batch variations. Here we report a donor polymer (named PM1) based on a random ternary polymerization strategy that enables highly efficient non-fullerene OSCs with efficiencies reaching 17.6%. Importantly, the PM1 polymer exhibits excellent batch-to-batch reproducibility. By including 20% of a weak electron-withdrawing thiophene-thiazolothiazole (TTz) into the PM6 polymer backbone, the resulting polymer (PM1) can maintain the positive effects (such as downshifted energy level and reduced miscibility) while minimize the negative ones (including reduced temperature-dependent aggregation property). With higher performance and greater synthesis reproducibility, the PM1 polymer has the promise to become the work-horse material for the non-fullerene OSC community. The batch reproducibility of polymer donor materials limits the performance of polymer solar cells. Here Wu et al. develop a polymer donor PM1 by random terpolymerization strategy with a high efficiency of 17.6% in the device and excellent batch-to-batch reproducibility. [ABSTRACT FROM AUTHOR]

Although ternary solar cells (TSCs) offer a cost‐effective prospect to expand the absorption bandwidth of organic solar cells, only few TSCs have succeeded in surpassing the performance of binary solar cells (BSCs) primarily due to the complicated morphology of the ternary blends. Here, the key factors that create and limit the morphology and performance of the TSCs are elucidated. The origin of morphology formation is explored and the role of kinetic factors is investigated. The results reveal that the morphology of TSC blends considered in this study are characterized with either a single length‐scale or two length‐scale features depending on the composition of the photoactive polymers in the blend. This asymmetric morphology development reveals that TSC blend morphology critically depends on material compatibility and polymer solubility. Most interestingly, the fill factor (FF) of TSCs is found to linearly correlate with the relative standard deviation of the fullerene distribution at small lengths. This is the first time that such a correlation has been shown for ternary systems. The criteria that uniform sized and highly pure amorphous domains are accomplished through the correct kinetic path to obtain a high FF for TSCs are specifically elucidated. The findings provide a critical insight for the precise design and processing of TSCs. Linear correlation of fill factor and relative standard deviation of fullerene distribution reveals that a ternary blend morphology with a uniform and pure mixed amorphous domain is required to achieve efficient ternary solar cells. This is achieved by the right kinetic path, controlled by the material and process compatibility. [ABSTRACT FROM AUTHOR]