Improving Performance of Perovskite Solar Cells Using [7]Helicenes with Stable Partial Biradical Characters as the Hole‐Extraction Layers

• Published in: Advanced functional materials Vol. 29; no. 13
• Main Authors: Lee, Chia‐Chen, Chen, Chih‐I, Fang, Chia‐Te, Huang, Pei‐Yu, Wu, Yao‐Ting, Chueh, Chu‐Chen
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 28.03.2019
• Subjects: Charge transfer; Crystal defects; Energy conversion efficiency; helicene; hole‐extraction layer; Materials science; Motion pictures; perovskite; Perovskites; Photovoltaic cells; solar cell; Solar cells; stable biradical
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201808625

Organic–inorganic hybrid perovskites have realized a high power conversion efficiency (PCE) in both n–i–p and p–i–n device configurations. However, since the p–i–n structure exempts the sophisticated processing of charge‐transporting layers, it seems to possess better potential for practical applications than the n–i–p one. Currently, the inorganic NiOx is the most prevailing hole‐transporting layer (HTL) used in p–i–n perovskite solar cells. Nevertheless, defects might exist on its surface to influence the charge transfer/extraction across the interface with perovskite and to affect the quality of the perovskite film grown on it. Herein, two novel [7]helicenes with stable open‐shell singlet biradical ground states at room temperature are demonstrated as an effective surface modifier of the NiOx HTL. Their nonpolar feature effectively promotes the crystallinity of the perovskite film grown on them; meanwhile, their unique partial biradical character seems to provide a certain degree of defect passivation function at the perovskite interface to facilitate interfacial charge transfer/extraction. As a result, both 1ab‐ and 1bb‐modifed devices yield a PCE of >18%, exceeding the value (15.6%) of the control device using a sole NiOx HTL, and the maximum PCE can reach 19%. Detailed characterizations are carefully conducted to understand the underlying reasons behind such enhancement. The [7]helicenes with stable partial open‐shell biradical ground states are demonstrated as effective surface modifiers of the inorganic NiOx hole‐transporting layer in p–i–n perovskite solar cells. Their nonpolar feature improves the crystallinity of the perovskite films grown on them. Meanwhile, their biradical character provides a certain defect passivation function to facilitate charge transfer/extraction across the perovskite interface.