Measuring CdS/CdTe back contact barrier heights by internal photoemission

The back contact of the CdS/CdTe solar cell is one of the most critical aspects in optimizing efficiency and stability. A fundamental parameter is the barrier height Fbc and its measurement is important to fully understanding the device. Plotting the Internal photoemission (IPE) current response to sub-band-gap light according to the Fowler theory gives an intercept equal to the barrier height. IPE can be applied to completed cells, and to thick, thin, and n/i/p cells. Results are compared for a number of cells from various fabricators, including those with good and bad contacts and electronically thick and thin cells. Results suggest two barriers in parallel: a high one, (∼ 0.9 eV) which agrees well with the observed UPS values and a low one (∼ 0.3 eV) with a much smaller area fraction (1 – 4%) which dominates the contact transport and corresponds more closely to the values observed for efficient cells. For thick cells, IPE results agree well with thermal measurements.