80% Fill Factor in Organic Solar Cells with a Modified Nickel Oxide Interlayer

Abstract

The efficiency of organic solar cells has raised drastically in the past years. However, there is an undeniable lack of hole transport layers that can provide high carrier selectivity, low defect density, and high processing robustness, simultaneously. In this work, this issue is addressed by studying defect generation and surface passivation of nickel oxide (NiO_x). It is revealed that the generation of high oxidation state species on NiO_x surface lowers contact resistance but hinders charge extraction when employed as transport layer in organic solar cells. By using them as coordination centers, a straightforward surface modification strategy is implemented using (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz) that enhances charge extraction and increases the solar cell efficiency from 11.46% to 17.12%. Additionally, the robustness of this modification across different deposition methods of the carbazole molecule is demonstrated. Finally, by fine-tuning the Fermi level using various carbazole-based molecules, and in particular with ((4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (4PADCB), a power conversion efficiency of 17.29% is achieved, with an outstanding combination of a V_OC of 0.888 V and a fill factor of 80%.