Temperature dependence of open-circuit voltage and recombination processes in polymer-fullerene based solar cells

In this article, we have studied the temperature and illumination dependence of open-circuit voltage (V(oc)) in polymer-fullerene based solar cells. It has been observed that V(oc) at higher illumination intensities gets converged at 0 K which gives information about maximum achievable V(oc) in a particular donor-acceptor blend. Besides this, recombination processes have been studied by transient open-circuit voltage decay (TOCVD) and the transition between recombination regimes has been observed for the first time. At low V(oc) carrier lifetime exhibits a constant value around 500 mu s, which is interpreted in terms of a monomolecular recombination regime. At higher V(oc) carrier lifetime decreases as derived from a bimolecular relaxation law. The method allows estimating the recombination coefficient, which results in 2 x 10(-13) cm(3) s(-1). The results have been explained by considering Gaussian density-of-states (DOS) for highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO). (C) 2011 Elsevier B.V. All rights reserved.