Effects of non-uniformity on rollover phenomena in CdS/CdTe solar cells

The behavior of CdS/CdTe devices with very little Cu in the back-contact was investigated. Though the devices maintain decent voltage (Voc ∼760–780 mV), the fill factor suffers (FF ∼ 45–50%) due to a secondary curvature in the power quadrant of the J-V plane. Capacitance results indicate full depletion, and laser-beam-induced-photocurrent mapping indicates spatial QE variations of several percent at voltage-bias near that of the initial diode turn-on. Guided by the LBIC results, we have developed an expanded equivalent circuit model to approximate effects of non-uniform back-contact barrier not explained by the traditional two-diode model [1]. PSpice modeling of the new circuit model successfully accounts for the non-standard rollover in the experimental J-V behavior.