Lee Tack Ho, Park Song Yi, Du Xiaoyan, Park Sujung, Zhang Kaicheng, Li Ning, Cho Shinuk, Brabec Christoph J, Kim Jin Young
Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea.
Department of Chemistry and Centre for Processable Electronics, Imperial College London, White City Campus, London W12 0BZ, U.K.
ACS Appl Mater Interfaces. 2020 Dec 16;12(50):55945-55953. doi: 10.1021/acsami.0c16854. Epub 2020 Dec 3.
We investigate the photovoltaic characteristics of organic solar cells (OSCs) for two distinctly different nanostructures, by comparing the charge carrier dynamics for bilayer- and bulk-heterojunction OSCs. Most interestingly, both architectures exhibit fairly similar power conversion efficiencies (PCEs), reflecting a comparable critical domain size for charge generation and charge recombination. Although this is, at first hand, surprising, a detailed analysis points out the similarity between these two concepts. A bulk-heterojunction architecture arranges the charge generating domains in a 3D ensemble across the whole bulk, while bilayer architectures arrange the specific domains on top of each other, rather than sharp bilayers. Specifically, for the polymer PBDB-T-2F, we find that the enhanced charge generation in a bulk composite is partially compensated by reduced recombination in the bilayer architecture, when nonfullerene acceptors (NFAs) are used instead of a fullerene acceptor. Overall, we demonstrate that bilayer-heterojunction OSCs with NFAs can reach competitive PCEs compared to the corresponding bulk-heterojunction OSCs because of reduced nonradiative open-circuit voltage losses, and suppressed trap-assisted recombination, as a result of a vertically separated donor-to-acceptor nanostructure. In contrast, the bilayer-heterojunction OSCs with the fullerene acceptor exhibited poor photovoltaic characteristics compared to the corresponding bulk devices because of highly aggregated acceptor molecules on top of the polymer donor. Although free carrier generation is reduced in a in a bilayer-heterojunction, because of reduced donor/acceptor interfaces and a limited exciton diffusion length, more favorable transport pathways for unipolar charge collection can partially compensate the aforementioned disadvantages. We propose that the unique properties of NFAs may open a technical venue for the bilayer-heterojunction as a great and easy alternative to the bulk heterojunction.
通过比较双层和体异质结有机太阳能电池（OSC）的电荷载流子动力学，我们研究了两种截然不同的纳米结构的有机太阳能电池的光伏特性。最有趣的是，这两种结构都表现出相当相似的功率转换效率（PCE），这反映出电荷产生和电荷复合的临界域尺寸相当。尽管乍一看这很令人惊讶，但详细分析指出了这两个概念之间的相似性。体异质结结构将电荷产生域以三维整体形式排列在整个体相中，而双层结构则将特定域相互堆叠，而非形成尖锐的双层。具体而言，对于聚合物PBDB-T-2F，我们发现当使用非富勒烯受体（NFA）而非富勒烯受体时，体复合材料中增强的电荷产生会被双层结构中减少的复合部分抵消。总体而言，我们证明，由于垂直分离的供体-受体纳米结构减少了非辐射开路电压损失并抑制了陷阱辅助复合，具有NFA的双层异质结OSC与相应的体异质结OSC相比可达到具有竞争力的PCE。相比之下，具有富勒烯受体的双层异质结OSC与相应的体器件相比，光伏特性较差，这是因为聚合物供体顶部的受体分子高度聚集。尽管在双层异质结中由于供体/受体界面减少和激子扩散长度有限，自由载流子产生减少，但单极电荷收集更有利的传输途径可以部分弥补上述缺点。我们提出，NFA的独特性质可能为双层异质结开辟一个技术途径，作为体异质结的一种出色且简便的替代方案。
