Doping strategies for next generation CdTe solar cells
Over the last 6 years, CdTe photovoltaics (PV) have enjoyed a steady performance improvement after remaining relatively stagnant for the previous 10 years. These improvements have largely been pushed by one company, First Solar Inc, who have successfully commercialised the technology to become the largest thin film PV manufacturer in the world. This steady efficiency increase has come about from, in the first instance, focusing on increasing photon absorption to improve current collection. Whilst improving the current collection in these devices has yielded higher and higher efficiencies, it is important to address the voltage loss which CdTe devices currently suffer from. Only then will CdTe PV fully realise its potential. Currently, CdTe suffers from a significant voltage deficit, which prevents devices from exceeding 900mV open circuit voltage. This voltage deficit has been attributed to two phenomena: a) Low doping density of around 1014/cm3and b) low minority carrier lifetime (typically less than 5ns). Simultaneously improving both will lead to improved open circuit voltages. Strategies to improve doping in CdTe include using group V dopants, such as phosphorus, will be investigated in this project to improve doping levels in CdTe thin films from 1014 to 1016/cm3, which are typical in high efficiency CIGS solar cells. In addition to group V doping, selenium alloying will also be explored in this project. There is growing evidence that alloying CdTe with Se (to form CdTexSe1-x) can significantly improve the minority carrier lifetime of the film without resorting to exotic growth methods. In addition to this, alloying CdTe with Se also represents a unique opportunity to grade the band gap of the film, which has been successfully used in CIGS PV to promote a back surface field, which promotes collection of electrons generated near the back of the device. This is particularly important in materials which have high doping density. The investigation of the CdTexSe1-x material as well as group V doping represents a significant opportunity to improve CdTe device efficiency.