Suppressing Interfacial Dipoles to Minimize Open-Circuit Voltage Loss in Quantum Dot Photovoltaics

Abstract

Quantum-dot (QD) photovoltaics (PVs) offer promise as energy-conversion devices; however, their open-circuit-voltage (V-OC) deficit is excessively large. Previous work has identified factors related to the QD active layer that contribute to V-OC loss, including sub-bandgap trap states and polydispersity in QD films. This work focuses instead on layer interfaces, and reveals a critical source of V-OC loss: electron leakage at the QD/hole-transport layer (HTL) interface. Although large-bandgap organic materials in HTL are potentially suited to minimizing leakage current, dipoles that form at an organic/metal interface impede control over optimal band alignments. To overcome the challenge, a bilayer HTL configuration, which consists of semiconducting alpha-sexithiophene (alpha-6T) and metallic poly(3,4-ethylenedioxythiphene) polystyrene sulfonate (PEDOT:PSS), is introduced. The introduction of the PEDOT:PSS layer between alpha-6T and Au electrode suppresses the formation of undesired interfacial dipoles and a Schottky barrier for holes, and the bilayer HTL provides a high electron barrier of 1.35 eV. Using bilayer HTLs enhances the V-OC by 74 mV without compromising the J(SC) compared to conventional MoO3 control devices, leading to a best power conversion efficiency of 9.2% (>40% improvement relative to relevant controls). Wider applicability of the bilayer strategy is demonstrated by a similar structure based on shallow lowest-unoccupied-molecular-orbital (LUMO) levels.