Organic–inorganic doped nickel oxide nanocrystals for hole transport layers in inverted polymer solar cells with color tuning

Abstract

Polymer solar cells using non-fullerene acceptors are nowadays amongst the most promising approaches for next generation photovoltaic applications. However, there are still remaining challenges related to large-scale fully solution-processing of high efficiency solar cells as high efficiencies are obtained only for very small areas using hole transport layers based on evaporated molybdenum oxide. Solution-processable hole transport materials compatible with non-fullerene acceptor materials are still scarce and thus considered as one of the major challenges nowadays. In this work, we present copper-doped nickel oxide nanocrystals that form highly stable inks in alcohol-based solutions. This allows processing of efficient hole transport layers in both regular and inverted device structures of polymer solar cells. As the initial work function of these ionic doped materials is too low for efficient hole extraction, doping the nanocrystals with an organic electron acceptor, namely 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino dimethane (F4-TCNQ), was additionally applied to make the work function more suitable for hole extraction. The resulting hybrid hole transport layers were first studied in polymer solar cells based on fullerene acceptors using regular device structures yielding 7.4% efficiency identical to that of reference cells based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). For inverted device structures, the hybrid hole transport layers were processed on top of blends based on the non-fullerene acceptor IT-4F and PBDB-T-2F donor. The corresponding solar cells showed promising efficiencies up to 7.9% while the reference devices using PEDOT:PSS showed inferior performances. We further show that the hybrid hole transport layer can be used to tune the color of the polymer solar cells using optical spacer effects.