  We present a "one-step" approach to modify ZnO electron transport layers
(ETLs) used in organic solar cells. This approach involves adding benzoic acid
(BZA) derivatives directly to the ZnO precursor solution, which are then
present at the surface of the resulting ZnO film. We demonstrate this approach
for three different BZA derivatives, namely benzoic acid, chlorobenzoic acid,
and 4-hydrazinobenzoic acid. For all molecules, improved device performance and
stability is demonstrated in solar cells using an active layer blend of PTQ10
(donor) and ITIC-Br (non-fullerene acceptor) compared to such cells prepared
using untreated ZnO. Furthermore, similar or improved device performance and
stability is demonstrated compared to conventional PEIE treatment of ZnO. The
presence of the BZA derivatives at the surface after processing is established
using X-ray photoelectron spectroscopy and near-edge X-ray absorption
fine-structure spectroscopy. From atomic force microscopy analysis and X-ray
diffraction studies, the addition of BZA derivatives appears to restrict ZnO
grain growth; however, this does not negatively impact device performance. ZnO
layers treated with BZA derivatives also exhibit higher water contact angle and
lower work function compared to untreated ZnO. This approach enables
simplification of device manufacture while still allowing optimization of the
surface properties of metal oxide ETLs. Keywords: electron transport layers,
zinc oxide, organic solar cells, surface modification
