Measuring the current density - voltage characteristics of individual subcells in two-terminal polymer tandem solar cells

Measuring the current density to voltage (J-V) characteristics of the separate subcells in two-terminal polymer tandem solar cells enables assessing the individual contributions of the two subcells to the total performance. We present two different methods to determine the J-V characteristics of the subcells in a two-terminal tandem cell without interfering with the light incoupling of the cell. The first method employs an extra proximity metal electrode that acts as a voltage probe. The proximity electrode is in contact with the recombination layer that separates the two subcells, but is positioned outside the illuminated area of the tandem cell. The second method uses bias-dependent external quantum efficiency measurements of two-terminal tandem solar cells and subsequent integration with the solar spectrum to determine the J-V curves of the subcells. The methods show good mutual agreement. For a 4.7% efficient solution processed two-terminal polymer tandem cell, based on PFTBT:PCBM as wide band gap and pBBTDPP2:PCBM as small band gap layer, we find that the subcells contribute with efficiencies of 2.9% and 1.8%. (C) 2011 Elsevier B.V. All rights reserved.