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The emergence of the latest generation of small-molecule acceptor (SMA) materials, with Y6 as a typical example, accounts for the surge in device performance for organic solar cells (OSCs). This study proposes two new acceptors named Y6-C2 and Y6-C3, from judicious alteration of alkyl-chains branching positions away from the Y6 backbone. Compared to the Y6, the Y6-C2 exhibits similar optical and electrochemical properties, but better molecular packing and enhanced crystallinity. In contrast, the Y6-C3 shows a significant blue-shift absorption in the solid state relative to the Y6 and Y6-C2. The as-cast PM6:Y6-C2-based OSC yields a higher power conversion efficiency (PCE) of 15.89% than those based on the Y6 (15.24%) and Y6-C3 (13.76%), representing the highest known value for as-cast nonfullerene OSCs. Prominently, the Y6-C2 displays a good compatibility with the PC71BM. Therefore, a ternary OSC device based on PM6:Y6-C2:PC71BM (1.0:1.0:0.2) was produced, and it exhibits an outstanding PCE of 17.06% and an impressive fill factor (FF) of 0.772. Our results improve understanding of the structure-property relationship for state-of-the-art SMAs and demonstrate that modulating the structure of SMAs via fine-tuning of alkyl-chains branching positions is an effective method to enhance their performance.

End group engineering is critical for developing A-D-A-type non-fullerene acceptors with suitable absorption properties and high crystallinity for organic solar cells. Here, we report the synthesis of a novel chlorinated thiophene end group and its application in A-D-A-type acceptors. When combined with a tetraoctyl-substituted indacenodithieno[3,2-b]thiophene core, the resulting C8-ITCC-Cl acceptor exhibited red-shifted absorption and downshifted energy levels relative to those of the nonchlorinated analogue, C8-ITCC, due to the strong electron-withdrawing ability of the chlorine atoms as revealed by theoretical calculations. Moreover, the noncovalent interactions induced by the chlorine atoms enabled improved lamellar and π–π stacking of C8-ITCC-Cl, leading to a higher electron mobility of 6.6 × 10–4 cm2 V–1 s–1 and a fill factor of 73% of the PBDB-TF:C8-ITCC-Cl devices. Consequently, a high power conversion efficiency of 12.7% was achieved by the C8-ITCC-Cl-based devices, which outperformed the C8-ITCC...

Thiophene and its derivatives have been extensively used in organic electronics, particularly in the field of polymer solar cells (PSCs). Significant research efforts have been dedicated to modifying thiophene-based units by attaching electron-donating or withdrawing groups to tune the energy levels of conjugated materials. Herein, we report the design and synthesis of a novel thiophene derivative, FE-T, featuring a monothiophene functionalized with both an electron-withdrawing fluorine atom (F) and an ester group (E). The FE-T unit possesses distinctive advantages of both F and E groups, the synergistic effects of which enable significant downshifting of the energy levels and enhanced aggregation/crystallinity of the resulting organic materials. Shown in this work are a series of polymers obtained by incorporating the FE-T unit into a PM6 polymer to fine-tune the energetics and morphology of this high-performance PSC material. The optimal polymer in the series shows a downshifted HOMO and an improved morphology, leading to a high PCE of 16.4% with a small energy loss (0.53 eV) enabled by the reduced non-radiative energy loss (0.23 eV), which are among the best values reported for non-fullerene PSCs to date. This work shows that the FE-T unit is a promising building block to construct donor polymers for high-performance organic photovoltaic cells.

Reducing the voltage loss is considered a promising route to improve the performance of organic solar cells (OSCs). In this work, two donor polymers PT4FB and PTT4FB based on a tetra-fluorinated benzene building block are synthesized and applied to non-fullerene OSC devices, which achieve high open-circuit voltage (VOC). When PT4FB is combined with IDIC, the blend exhibits large fiber-like domains (size ∼39 nm) due to the excessive aggregation of PT4FB, which results in a relatively low short-circuit current (JSC) of 14.06 mA cm−2 and a moderate power conversion efficiency (PCE) of 9.81%. To weaken the aggregation of PT4FB, we replace the alkyl chains on the polymer backbone with alkylthiophene side chains to afford a novel donor polymer PTT4FB with reduced packing and crystallinity. As a result, the PTT4FB:IDIC-based blend shows significantly smaller domain sizes (24 nm) than the PT4FB-based device, contributing to a remarkably increased JSC and an overall higher PCE of 10.60%. The synergistic effects of abundant fluorination and alkylthiophene side chains can be utilized as a general method to tune the morphological and energetic properties when designing novel donor polymers to achieve both high VOC and favorable morphology for efficient non-fullerene OSCs.

Ternary blends have shown great potential to increase the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we studied a ternary OSC system with a donor polymer (PM6) and two structurally similar non-fullerene acceptors (named ITCPTC and MeIC). Although these two small molecular acceptors (SMAs) exhibit similar absorption spectra, they introduce a surprising synergistic effect on tuning the domain size and crystallinity of the OSC blend. More specifically, MeIC is a SMA with strong crystallinity, which results in excessive phase segregation and large domain size for the PM6:MeIC binary blend. By adding a structurally similar and less crystalline SMA (ITCPTC) into the binary blend, the domain size and morphology of the blend are much improved without sacrificing the electron mobility of the blend. As a result, the optimal blend ratio of PM6 : ITCPTC : MeIC (1 : 0.4 : 0.6) led to an impressive FF of 78.2% and PCE of 14.13%, which are the highest values reported for ternary non-fullerene OSCs reported to date.

The side-chain structures of conjugated molecules are well recognized to sensitively influence the crystallinity, morphology and thus carrier transport properties of organic semiconductors. Here, by varying the alkyl side-chain length in the polymer acceptors, the effect of side-chain engineering on the photovoltaic performance is systematically studied in all-polymer solar cells. Clear trends of first an increase and then a decrease in the Jsc and FF values are observed as the branched alkyl groups are extended from 4 to 8 carbons. Correspondingly, the maximum average PCE (ca. 7.40%) is attained with an acceptor bearing a branched side-chain length of seven carbon atoms.

Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted intensive research attention over the past two decades owing to their unique advantages including mechanical flexibility, light weight, large area, and low-cost fabrications. To date, OSC devices have achieved power conversion efficiencies (PCEs) exceeding 12%. Much of the progress was enabled by the development of high-performance donor polymers with favorable morphological, electronic, and optical properties. A key problem in morphology control of OSCs is the trade-off between achieving small domain size and high polymer crystallinity, which is especially important for the realization of efficient thick-film devices with high fill factors. For example, the thickness of OSC blends containing state-of-the-art PTB7 family donor polymers are restricted to ∼100 nm due to their relatively low hole mobility and impure polymer domains. To further improve the device performance and promote commercialization of OSCs, there is a strong demand for the design of new donor polymers that can achieve an optimal blend morphology containing highly crystalline yet reasonably small domains. In this Account, we highlight recent progress on a new family of conjugated polymers with strong temperature-dependent aggregation (TDA) property. These polymers are mostly disaggregated and can be easily dissolved in solution at high temperatures, yet they can strongly aggregate when the solution is cooled to room temperature. This unique aggregation property allows us to control the disorder-order transition of the polymer during solution processing. By preheating the solution to high temperature (∼100 °C), the polymer chains are mostly disaggregated before spin coating; as the temperature of the solution drops during the spin coating process, the polymer can strongly aggregate and form crystalline domains yet that are not excessivelylarge. The overall blend morphology can be optimized by various processing conditions (e.g., temperature, spin-rates, concentration, etc.). This well-controlled and near-optimal BHJ morphology produced over a dozen cases of efficient OSCs with an active layer nearly 300 nm thick that can still achieve high FFs (70-77%) and efficiencies (10-11.7%). By studying the structure-property relationships of the donor polymers, we show that the second position branched alkyl chains and the fluorination on the polymer backbone are two key structural features that enable the strong TDA property. Our comparative studies also show that the TDA polymer family can be used to match with non-fullerene acceptors yielding OSCs with low voltage losses. The key difference between the empirical matching rules for fullerene and non-fullerene OSCs is that TDA polymers with slightly reduced crystallinity appear to match better with small molecular acceptors and yield higher OSC performances.

Although most polymer/fullerene-based solar cells are cast from a blend of the components in solution, it is also possible to sequentially process the polymer and fullerene layers from quasi-orthogonal solvents. Sequential processing (SqP) not only produces photovoltaic devices with efficiencies comparable to the more traditional bulk heterojunction (BHJ) solar cells produced by blend casting (BC) but also offers the advantage that the polymer and fullerene layers can be optimized separately. In this paper, we explore the morphology produced when sequentially processing polymer/fullerene solar cells and compare it to the BC morphology. We find that increasing polymer regioregularity leads to the opposite effect in SqP and BC BHJ solar cells. We start by constructing a series of SqP and BC solar cells using different types of poly(3-hexylthiophene) (P3HT) that vary in regioregulary and polydispersity combined with [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM). We use grazing incidence wide-angle X-ray scattering to demonstrate how strongly changes in the P3HT and PCBM crystallinity upon thermal annealing of SqP and BC BHJ films depend on polymer regioregularity. For SqP devices, low regioregularity P3HT films that possess more amorphous regions allow for more PCBM crystallite growth and thus show better photovoltaic device efficiency. On the other hand, highly regioregular P3HT leads to a more favorable morphology and better device efficiency for BC BHJ films. Comparing the photovoltaic performance and structural characterization indicates that the mechanisms controlling morphology in the active layers are fundamentally different for BHJs formed via SqP and BC. Most importantly, we find that nanoscale morphology in both SqP and BC BHJs can be systematically controlled by tuning the amorphous fraction of polymer in the active layer. © 2014 American Chemical Society.