Organic solar cells characterized by dark lock-in thermography

This article presents the lock-in thermography measurement technique applied to encapsulated organic heterojunction solar cells, built from poly(3-hexylthiophene) (P3HT) and 1-(3-methaoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM). The realizable temperature resolution grants the possibility to visualize even very weak thermal losses in solar cells. The thermal behavior of the cells is demonstrated for different applied voltages. Especially, parallel and serial resistances can be spatially resolved. In order to explain the nonuniform behavior of the serial resistance a simplified replacement circuit of the organic cell is created, according to the Kirchhoff laws. The matrix of series and parallel resistivities allows the simulation of the currents flowing vertically through the semiconducting layer. Bias dependent simulations reveal that the considerable difference of the sheet resistances between the indium-tin-oxide (ITO) and aluminum contacts is responsible for the experimentally observed inhomogeneous temperature profiles of the organic solar cells.