The consequences of interface mixing on organic photovoltaic device characteristics

Organic bulk-heterojunction solar cells are being developed as a low-cost alternative to inorganic photovoltaics. A key step to producing high-efficiency bulk-heterojunction devices is film 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 filling factor, short-circuit current density, and efficiency 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. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.David M. Huang, Scott A. Mauger, Stephan Friedrich, Simon J. George, Daniela Dumitriu-LaGrange, Sook Yoon, and Adam J. Moul