Improvement to thin film CdTe solar cells with controlled back surface oxidation

• Low temperature thermal treatment in air improves a CdTe solar cell PV performances by enhancing the Voc and FF.
• X-ray photoelectron spectroscopy revealed an increase in oxygen at the back surface associated with Te–O binding energies.
• The CdTe solar cells remained stable over a broad exposure period of oxidation treatments until FF eventually deteriorated.

Thin film CdTe solar cells were produced by MOCVD, at atmospheric pressure, under a hydrogen atmosphere (i.e. oxygen-free). Window layer alloying with zinc (forming Cd1−xZnxS) and extrinsic p-type doping with arsenic (giving CdTe:As) have been used to improve photovoltaic solar cell performances, but as-grown MOCVD-CdTe PV cells are still typically characterised by low Voc (~620–690 mV). Post-deposition annealing in air for 30 min at low temperature (170 °C) prior to evaporation of the back contacts led to significant increases in Voc and FF. XPS measurements revealed back surface oxidation, resulting in formation of Te–O species. This was also the case for a device aged under ambient laboratory conditions. Extended annealing in air of a fresh device, for up to 180 min, continued to improve both Voc and FF. At longer annealing times the Voc remained relatively stable whilst the FF started to deteriorate. External quantum efficiency showed loss of photocurrent generation after excessive oxidation prior to back contact metallisation. Controlled back surface oxidation resulted in Voc values exceeding 800 mV and a best cell efficiency of 15.3%.