Comparison between conventional and inverted solar cells using open circuit voltage decay transients

In the development of new structures for solar cells, it has become important to extract the true ideality factor of a diode, disentangling it from other loss mechanisms in the device. We use the open circuit voltage decay (OCVD) transient to obtain ideality factors and isolate the internal loss mechanisms in P3HT: PCBM bulk heterojunction solar cells. We compare two different structures using OCVD transients over more than six orders in timescale. The equivalent circuit parameters of diodes can be reliably extracted from such decays. Specifically, the numerical solution of the transient allows the determination of the diode ideality factor, and the saturation leakage current. In addition, this technique makes it possible to determine the diode current in the presence of excess carriers under photo irradiance, and hence, the light induced recombination current and shunt resistance can be separately extracted. We compare the decay transients of an efficient device with a leaky device and demonstrate that the diode current changes in the same manner in both the cases. We study in detail the intensity power-law dependence of the recombination current that controls the decay transient and observe that it increases significantly faster in leaky devices with light intensity. Published by AIP Publishing.