Impact of the bilayer electron transport layer in the donor acceptor bulk heterojunctions for improved inverted organic photovoltaic performance

Functionalization at interface of the electron transport layer and the organic polymeric donor and non-fullerene acceptor bulk heterojunction layer in photovoltaic devices that have an inverted structure, is a unique strategy for improving performance. Here we demonstrate improved interfacial nanomorphology of the well-known PTB7-Th donor and IEICO-4F acceptor bulk heterojunction (BHJ) on an atomic layer deposited (ALD) ultrathin titanium oxide (TiO2) passivated sol-gel grown zinc oxide (ZnO) electron transport bilayer and highlight how this is correlated to the overall organic photovoltaic device performance. Synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS) has been used to demonstrate an enhanced face-on conformation within the BHJ with highly crystalline ZnO/TiO2/BHJ layers and this is greatest for the very thinnest of TiO2 layers on ZnO (ZnT1), from just two deposition cycles, relative to both four (ZnT2) and eight (ZnT3) cycles. Photovoltaic devices with BHJs grown over ZnT1, showed an overall average increase of 30.3 +/- 0.5 % in the power conversion efficiency (PCE 10.3 +/- 0.5 %), a 14.3 +/- 1 % increase in the fill-factor, FF (60.1 +/- 1 %), 2.9 +/- 0.05 % improvement in Voc (0.70 +/- 0.05 mV) and 13.6 +/- 0.5 % higher JSc (24.2 +/- 0.5 mA/cm2), compared to the control device,with a best efficiency of 11.1 +/- 0.5 % PCE, 61.9 +/- 1 % FF, 0.70 +/- 0.5 mV Voc and 25.0 +/- 0.5 mA/cm2 Jsc. We conclude that the molecular crystal conformation of the organic donor acceptor BHJ layer can be partly controlled through the ETL along with an enhanced organic inorganic interface which strongly influences the OPV performance.