Fine‐Tuning of Molecular Packing and Energy Level through Methyl Substitution Enabling Excellent Small Molecule Acceptors for Nonfullerene Polymer Solar Cells with Efficiency up to 12.54%

A novel small molecule acceptor MeIC with a methylated end‐capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520–740 nm. The MeIC‐based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short‐circuit current (J_SC) of 18.41 mA cm^?2, significantly higher than that of the device based on J71:ITCPTC (11.63% with a J_SC of 17.52 mA cm^?2). The higher J_SC of the PSC based on J71:MeIC can be attributed to more balanced μ_h/μ_e, higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X‐ray diffraction and resonant soft X‐ray scattering results. It is worth mentioning that the as‐cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as‐cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.     

High‐Performance Additive‐/Post‐Treatment‐Free Nonfullerene Polymer Solar Cells via Tuning Molecular Weight of Conjugated Polymers

In recent years, rapid advances are achieved in polymer solar cells (PSCs) using nonfullerene small molecular acceptors. However, no research disclosing the influence of molecular weight (M_n) of conjugated polymer on the nonfullerene device performance is reported. In this work, a series of polymers with different M_ns are synthesized to systematically investigate the connection between M_n and performance of nonfullerene devices for the first time. It is found that the device performance improves substantially as the M_n increases from 12 to 38 kDa and a power conversion efficiency (PCE) as high as 10.5% is realized. It has to be noted this PCE is achieved without using any additives and post‐treatments, which is among the top efficiencies of additive‐ and post‐treatment‐free PSCs. Most importantly, the variation trend of the optimal active layer thickness and morphology is significantly different from the device with fullerene as acceptor. The findings clarify the effect of M_n on the performance of nonfullerene PSCs, which would benefit further efficiency improvement of nonfullerene PSCs.     

Design of a New Small‐Molecule Electron Acceptor Enables Efficient Polymer Solar Cells with High Fill Factor

Improving the fill factor (FF) is known as a challenging issue in organic solar cells (OSCs). Herein, a strategy of extending the conjugated area of end‐group is proposed for the molecular design of acceptor–donor–acceptor (A–D–A)‐type small molecule acceptor (SMA), and an indaceno[1,2‐b:5,6‐b′]dithiophene‐based SMA, namely IDTN, by end‐capping with the naphthyl fused 2‐(3‐oxocyclopentylidene)malononitrile is synthesized. Benefiting from the π‐conjugation extension by fusing two phenyls, IDTN shows stronger molecular aggregation, more ordered packing structure, thus over one order of magnitude higher electron mobility relative to its counterpart. By utilizing the fluorinated polymer (PBDB‐TF) as the electron donor, the corresponding device exhibits a high efficiency of 12.2% with a record‐high FF of 0.78, which is approaching the theoretical limit of OSCs. Compared with the reference molecule, such a high FF in the IDTN system can be mainly attributed to the more ordered π–π packing of acceptor aggregates, higher domain purity and symmetric carrier transport in the blend. Hence, enlarging the conjugated area of the terminal‐group in these A–D–A‐type SMAs is a promising approach not only for enhancing the electron mobility, but also for improving the blend morphology, and both of them are conducive to the fill‐factor breakthrough.     

吩噻嗪基染料的结构改性及光伏性能

通过在吩噻嗪类基础染料PTZ-2的D单元上引入N-苯基吩噻嗪单元,构成D-D-π-A型染料分子PTZ-3。相比PTZ-2染料,PTZ-3染料的吸收光谱有所拓宽,其给电子性能也略有提升。通过DSSC器件光伏性能测试,PTZ-3的光电转换效率为5.5%,高于PTZ-2的4.9%。这一切都归因于N-苯基吩噻嗪单元的引入增强了光捕获性能与量子转换效率,同时末端的刚性结构在一定程度上起到了抑制暗电流的作用。     

Organic Solar Cells: A Review of Materials,Limitations, and Possibilities for Improvement

Significant attention has been given during the last few years to overcome technological and material barriers in order to develop organic photovoltaic devices (OPVs) with comparable cost efficiency similar to the inorganic photovoltaics (PVs) and to make them commercially viable. To take advantage of the low cost for such devices, major improvements are necessary which include: an efficiency of around 10%, high stability from degradation under real-world conditions, novel optically active materials, and development of novel fabrication approaches. In order to meet such stringent requirements, the research and development in OPVs need to improve upon the short diffusion length of excitons, which is one of the factors that are responsible for their low power conversion efficiency. This review discusses some of the most significant technological developments that were presented in the literature and helped improve photovoltaic performance, such as tandem architectures, plasmonics, and use of graphitic nanostructural materials, among others.

Tandem organic solar cells with embedded plasmonics are a promising approach to further increase the power conversion efficiency of organic solar cells, by harvesting complementary spectral regions with high quantum efficiencies. Polymeric nanocomposites incorporating graphitic nanostructures were extensively investigated for the next generation of efficient and low-cost solar cells, since such nanomaterials show excellent electrical and mechanical properties, excellent carrier transport capabilities, and provide an efficient pathway to the dissociated charge carriers.     

13.9%‐Efficiency and Eco‐Friendly Nonfullerene Polymer Solar Cells Obtained by Balancing Molecular Weight and Solubility in Chlorinated Thiophene‐Based Polymer Backbones

To industrialize nonfullerene polymer solar cells (NFPSCs), the molecular design of the donor polymers must feature low‐cost materials and a high overall yield. Two chlorinated thiophene‐based polymers, P(F–Cl) and P(Cl–Cl), are synthesized by introducing halogen effects like fluorine (F) and chlorine (Cl) to the previously reported P(Cl), which exhibits low complexity. However, the molecular weights of these polymers are insufficient owing to their low solubility, which in turn is caused by introducing rigid halogen atoms during the polymerization. Thus, they show relatively low power conversion efficiencies (PCEs) of 11.8% and 10.3%, respectively. To overcome these shortcomings, two new terpolymers are designed and synthesized by introducing a small amount of 1,3‐bis(5‐bromothiophen‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) unit into each backbone, namely, P(F–Cl)(BDD = 0.2) and P(Cl–Cl)(BDD = 0.2). As a result, both polymers remain inexpensive and show a better molecular weight–solubility balance, achieving high PCEs of 12.7% and 13.9%, respectively, in NFPSCs processed using eco‐friendly solvents.     

基于噻吩-苯非对称单元的DPP类聚合物给体材料的合成及光伏性能

为了优化聚合物太阳能电池的光伏性能,设计合成了一种基于噻吩-苯非对称单元的二酮吡咯并[3,4-c]吡咯(DPP)类聚合物给体材料(PDPP-PT)。非对称结构的设计使得该聚合物具有较好的分子堆积,有利于器件的制备。该聚合物具有范围在300~900nm的宽吸收光谱、1.5eV的窄光学带隙。在器件性能方面,活性层厚度达260nm时,测得开路电压(Voc)为0.68V,光电转换效率(PCE)为1.51%。因此,PDPP-PT给体材料在制备厚活性层太阳能电池时具有一定的优势并为聚合物给体材料的分子设计提供了一种新的思路。     

Conformation‐Tuning Effect of Asymmetric Small Molecule Acceptors on Molecular Packing,Interaction, and Photovoltaic Performance

Understanding the conformation effect on molecular packing, miscibility, and photovoltaic performance is important to open a new avenue for small‐molecule acceptor (SMA) design. Herein, two novel acceptor–(donor‐acceptor1‐donor)–acceptor (A‐DA1D‐A)‐type asymmetric SMAs are developed, namely C‐shaped BDTP‐4F and S‐shaped BTDTP‐4F . The BDTP‐4F ‐based polymer solar cells (PSCs) with PM6 as donor, yields a power conversion efficiency (PCE) of 15.24%, significantly higher than that of the BTDTP‐4F ‐based device (13.12%). The better PCE for BDTP‐4F ‐based device is mainly attributed to more balanced charge transport, weaker bimolecular recombination, and more favorable morphology. Additionally, two traditional A‐D‐A‐type SMAs ( IDTP‐4F and IDTTP‐4F ) are also synthesized to investigate the conformation effect on morphology and device performance. Different from the device result above, here, IDTP‐4F with S‐shape conformation outperforms than IDTTP‐4F with C‐shape conformation. Importantly, it is found that for these two different types of SMA, the better performing binary blend has similar morphological characteristics. Specifically, both PM6:BDTP‐4F and PM6:IDTP‐4F blend exhibit perfect nanofibril network structure with proper domain size, obvious face‐on orientation and enhance donor‐acceptor interactions, thereby better device performance. This work indicates tuning molecular conformation plays pivotal role in morphology and device effciciency, shining a light on the molecular design of the SMAs.