Accurate mobility determination is essential to model and improve the efficiency of organic solar cells. A frequently used method to determine charge carrier mobilities is called CELIV(charge extraction by a linearly increasing voltage). In this technique a voltage ramp is applied to the device in order to extract the free charge carriers inside the cell. With an extended method called photo-CELIV the free charge carriers are first generated by a short laser pulse and are then extracted after an adjustable time. To analyse the experiment analytical formulas are used. We simulate the CELIV and photo-CELIV method with a fully coupled electro-optical model. Our numerical model allows us to reveal the limitations of analytical expressions used to analyse CELIV transients. The influence of the mobility, the series resistance, the voltage slope and the illumination intensity on the CELIV transients are studied. We show that using the analytical formulas only the order of magnitude of the mobility can be determined. We also perform CELIV measurements on organic bulk heterojunction solar cells based on a PT5DPP:PCBMC70 blend. By fitting the numerical simulation to the measured transients we extract charge carrier mobilities, the recombination efficiency and the series resistance.

Date Published: 18 May 2010
PDF: 12 pages
Proc. SPIE 7722, Organic Photonics IV, 77220V (18 May 2010); doi: 10.1117/12.854668

Published in SPIE Proceedings Vol. 7722:
Organic Photonics IV
Paul L. Heremans; Reinder Coehoorn; Chihaya Adachi, Editor(s)