Match the Interfacial Energy Levels between Hole Transport Layer and Donor Polymer To Achieve High Solar Cell Performance

The interfacial energy level alignment is shown to play an important role in determining solar cell performance. Replacing hole transport layer poly­(3,4-ethylene dioxythiophene)–(polystyrene sulfonic acid) (PEDOT:PSS) with vanadium pentoxide (V₂O₅) leads to a simultaneous improvement in short-circuit current density (<i>J</i>_sc), open-circuit voltage (<i>V</i>_oc) and fill factor (FF) for two donor polymers with deep HOMO energy levels, resulting in a power conversion efficiency (PCE) of 7.03% and 4.14%. This is 18% and 106% increase in PCE over the 5.97% and 2.01% achieved with PEDOT:PSS. V₂O₅ is shown to increase <i>J</i>_sc and FF by enhancing hole mobility, reducing bimolecular recombination, and facilitating charge collection and to maximize <i>V</i>_oc by providing a better ohmic contact. We also demonstrate that PEDOT:PSS still works better for donor polymers with a HOMO energy level around 5.1 eV, such as PTB7