Applicability of the suns-Voc method on organic solar cells
The Suns-VOC technique is used to acquire pseudocurrent-voltage (JV) characteristics of solar cells which are unaffected by their series resistance and, therefore, only reflect generation and recombination. It is well established for inorganic solar cells. In this study, the applicability of the Suns-VOC method on organic solar cells is investigated. In the first part, the necessary assumptions on which the method is based are reviewed. It is discussed to what extent transient effects are caused by long charge carrier lifetimes, and capacitive effects have to be considered. Experiments show that this measurement technique can be successfully applied on organic solar cells based on blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). By comparing pseudo-JV curves and JV curves measured under standard conditions, the series resistance can be extracted. It is composed of an ohmic contribution of the circuitry Rcirc and an injection dependent part caused by the transport of the charge carriers through the photoactive layer Rtransp. The results clearly indicate that the relative contribution of Rtransp is significantly larger than in inorganic solar cells due to the low charge carrier mobilities. By comparing different organic solar cell designs, qualitative information about the recombination processes is acquired.