Light intensity dependence of External Quantum Efficiency of fresh and degraded organic photovoltaics

The effect of light intensity on the External Quantum Efficiency (EQE) of encapsulated bulk heterojunction organic photovoltaics (OPV) is presented. The measurements were applied to devices based on poly(3-hexylthiophene) (P3HT) blended with the fullerene derivative phenylC61-butyric acid methyl ester (PCBM) in as-produced and various degradation states. The degradation of current collection in the OPV devices is shown to enhance the sub-linear dependence of the short-circuit current on light intensity, and the corresponding EQE decrease with increasing incident light intensity. On the other hand, fresh cells and cells exposed to a low photon dose demonstrated an increase in the fullerene-related part of the EQE with increasing light intensity, i.e. a super-linear dependence of the photocurrent in this spectral range. Generation of traps in PCBM was proposed as the underlying mechanism for this effect. Perusal of our results suggests that (1) EQE dependence on the incident light intensity should be always taken into account in measuring spectral response of fresh OPV and especially of degraded devices; (2) intensity-dependent characterization provides an insight to the degradation mechanisms of OPV and can help to separate degradation in absorption/generation from degradation of the charge collection in the cell.