You may know solar cells on satellites and street lights, but you may not know the reasons why these traditional inorganic solar cells are not very commonly found in our daily life. The complex device structure, difficult fabrication, small area, high cost, and heavy weight have limited their wide application. Comparatively, the polymer solar cells (PSCs) are emerging and promising alternatives. If these flexible PSCs are available, our office lights can consume the electricity generated from the window curtains containing PSCs. 

An organic molecule, namely benzodithiophene (BDT), has been regarded as one of the most promising electron donor units to obtain high-performance PSCs. The BDTs were usually employed as the building units in a symmetric mode. However, up to now, these symmetric BDTs are still unable to simultaneously attain decent short-circuit current density and high open-circuit voltage, both of which are the key parameters responsible for higher efficiency of PSCs. 

For the first time, asymmetric BDTs have been designed to build up PSCs in a research group led by Prof. YANG Renqiang from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of Chinese Academy of Sciences. Surprisingly, the photovoltaic properties of PSCs were improved greatly in comparison with corresponding symmetric BDT based polymers. More remarkably, the best PSC showed the highest power conversion efficiencies (9.44%) among the reported photovoltaic polymers with similar structures. 

The novel asymmetric idea was somewhat inspired by an interesting experimental incident. Before they got the idea, the research group had done a lot of tests to improve the performance of PSCs by using the symmetric BDTs that the other groups also used, however, the enhancements were quite tiny. One day, the PhD student who handled the experiment added the wrong raw materials into the reaction system, and it turned out that the final PSC showed a much higher property than before. The researchers found that, after careful analyses, it is the asymmetric structure of BDT that improved the photovoltaic performance a lot.