The Consequences of Interface Mixing on Organic Photovoltaic Device Characteristics

• Published in: Advanced functional materials 2011-05, Vol.21 (9), p.1657-1665
• Main Authors: Huang, David M., Mauger, Scott A., Friedrich, Stephan, George, Simon J., Dumitriu-LaGrange, Daniela, Yoon, Sook, Moulé, Adam J.
• Format: Article
• Language: English
• Subjects: charge transport; conducting; organic electronics; photovoltaic devices; polymers; solar cells
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201002014

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. Upon heating an organic solar cell, poly(3‐hexylthiophene) (P3HT) reacts with poly(3‐hexylthiophene) (HPSS) to form a mixed interface layer in which charge transport is controlled by p‐doped P3HT. A byproduct of the reaction is hydrogen gas.