Understanding energy level alignment in PCDTBT:PC70BM solar cells

Blended heterojunctions continue to improve in overall efficiency, with current power conversion efficiencies (PCE) at >;7%, with interfaces being a dominating factor in improving PCE. Charge separation is dictated by energetic alignment between the donor and acceptor. Selective interlayers are used to preferentially harvest one charge via either thermodynamic or kinetic pathways. Energetic level alignment between the blend and the charge collection interfaces can impact the open circuit voltage (V_OC) through both loss mechanisms and poor choice of interlayers can result in less than ideal charge collection efficiencies. Here we report on the surface work function characterization of interlayers and phosphonic acid modifiers using ultraviolet photoelectron spectroscopy and the measured energetic level alignment between the interlayers and PCDTBT:PC₇₀BM BHJs. We will address the overall charge extraction from the bulk heterojunction of PCDTBT:PC₇₀BM and subsequent impact on V_OC, J_SC, and PCE.