The option of using conjugated π-linkers is critical for rational molecular design toward an energy-level strategy for organic sensitizers. To further optimize photovoltaic performance, methyl- and octyl-substituted 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) are introduced into D–A−π–A featured sensitizers. Along with CPDT, instead of thiophene as conjugated bridge, WS-39 and WS-43 exhibit an extended spectral response due to the excellent conjugation and coplanarity of CPDT. Specifically, we focused on the critical effect of length of the alkyl group linked to the bridging carbon atoms of CPDT on the photovoltaic performances. Octyl-substituted WS-39 shows a broader IPCE onset with an enhanced photovoltage relative to the analogue WS-5. In contrast, WS-43, with methyl substituted on the CPDT moiety, presents a relatively low quantum conversion efficiency within the whole spectral response region, along with low photocurrent density. WS-43 displays a distinctly low IPCE platform, predominately arising from the short electron diffusion length with significant electron loss during the electron transport. The relative movement of the conduction band edge (E_CB) and charge transfer resistance as well as lifetime of injected electrons are studied in detail. Under standard AM 1.5 conditions, WS-39-based solar cells show a promising photovoltaic efficiency of 9.07% (J_SC = 16.61 mA cm^–2, V_OC = 770 mV, FF = 0.71). The octyl chains attached on CPDT can provide dual protection and exhibit a high propensity to prevent binding of the iodide–triiodide redox couple, producing an efficient shielding effect to retard the charge recombination and resulting in improvement of V_OC. Our research paves the way to explore more efficient sensitizers through ingenious molecular engineering.

中文翻译：

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长烷基对环戊二噻吩作为D–A–π–A有机增敏剂的共轭桥的影响：IPCE，电子扩散长度和电荷复合

对于有机敏化剂的能级策略而言，使用共轭π-连接剂的选择对于合理的分子设计至关重要。为了进一步优化光伏性能，将甲基和辛基取代的4 H-环戊[2,1- b：3,4- b ']二噻吩（CPDT）引入D-A-π-A特色敏化剂中。WS-39和WS-43与CPDT一起代替噻吩作为共轭桥，由于CPDT具有出色的共轭性和共面性，因此具有扩展的光谱响应。具体而言，我们重点研究了与CPDT桥碳原子相连的烷基长度对光伏性能的关键影响。辛基取代的WS-39相对于模拟WS-5，它显示了更宽的IPCE起始和更高的光电压。相反，在CPDT部分上被甲基取代的WS-43在整个光谱响应区域内具有相对较低的量子转换效率，以及较低的光电流密度。WS-43显示出明显较低的IPCE平台，主要是由于电子传输过程中短的电子扩散长度和明显的电子损失而引起的。详细研究了导带边缘（E _CB）的相对运动和电荷转移电阻以及注入电子的寿命。在标准AM 1.5条件下，WS-39的太阳能电池显示出有希望的9.07％的光伏效率（J _SC = 16.61 mA cm ^–2，V _OC = 770 mV，FF = 0.71）。连接在CPDT上的辛基链可以提供双重保护，并具有防止碘化物-三碘化物氧化还原对结合的高倾向性，从而产生有效的屏蔽作用，从而延迟了电荷复合并改善了V _OC。我们的研究为通过巧妙的分子工程探索更有效的敏化剂铺平了道路。