Organic bulk-heterojunction solar cells are being developed as a low-cost alternative to inorganic photovoltaics. A key step to producing high-effi ciency bulk-heterojunction devices is fi lm curing using either heat or a solvent atmosphere. All of the literature examining the curing process have assumed that improvement of the bulk-heterojunction morphology is the reason for the increased fi lling factor, short-circuit current density, and effi ciency following heat or solvent treatment. We show in this article that heat treatment causes the donor polymer (P3HT) and polymer electrode (PEDOT: PSS) to mix physically to form an interface layer. This interface layer is composed of a mixture of P3HT and PSS in which the P3HT is oxidized to P3HT +. This mixed layer affects the open-circuit voltage and compensation voltage by limiting the dark current. This result implies that a simplistic description of the P3HT/PEDOT: PSS contact as a sharp interface between bulk P3HT and bulk PEDOT: PSS cannot adequately capture its electrical characteristics.