Detection of trap charge in small molecular organic bulk heterojunction solar cells

We detect and measure the trap charges in a small molecular bulk heterojunction solar cell under operating conditions. The trap-charge density is estimated from capacitance measurements with light incident on the sample. At high intensities ($\ensuremath{\sim}1\text{ }\text{sun}$, $100\text{ }\text{mW}/{\text{cm}}^{2}$), the trapped charge concentration leads to a spatial distortion of the electric field in the device. The lower limit of the trap-charge density is estimated to be $6\ifmmode\times\else\texttimes\fi{}{10}^{16}\text{ }{\text{cm}}^{\ensuremath{-}3}$. The frequency dependence of the capacitance suggests that the charges are trapped in a manifold of deep states present in the energy gap of the semiconductors. The distortion of the electric field by this trap charge affects the charge-carrier collection efficiency.