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 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.