Sensitive Intensity Dependent Photocurrent Measurement (IPC)
This method provides useful information about recombination dynamics in solar cells. The linear dynamic range (LDR) of photodetectors can be determined by using this method as well. Our home-built system uses CW lasers with different wavelengths as an excitation source linked to an integrating sphere acting as a beam homogenizer. The detection system operates at extremely low frequency (~ 0.1 Hz) in order to be able to probe the contribution of trap states with long lifetime. Photocurrent and photovoltage (and EQE) can be determined over several decades of light intensity. From these measurements trap density and energy can be estimated.
Figure 1: Experimental setup for intensity dependent photocurrent (IPC) measurement with multi-wavelength CW laser light source and integrating sphere.
Figure 2: External quantum efficiency (EQE) plotted as a function of light irradiance for a 100 nm thick PCDTBT:PCBM (1:4) solar cell. Different photocurrent loss mechanisms, which are dominant at particular light irradiance, cause drops in photocurrent, respectively EQE, namely generation and absorption loss and first-order trap-assisted Shockley-Read-Hall (SRH) recombination at low and moderate light irradiance as well as second-order bimolecular recombination and device series resistance limitation at high light irradiance. The EQE is calculated from the raw IPC data by relating the photocurrent to the light power and the photon energy.