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

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 NiO _x 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 NiO _x 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 NiO _x HTL, and the maximum PCE can reach 19%. Detailed characterizations are carefully conducted to understand the underlying reasons behind such enhancement.