New technology yielding flexible solar cells was developed by scientists at Empa, the Swiss Federal Laboratories for Materials Science and Technology. Key to the breakthrough is the control of the energy band gap grading in the copper indium gallium (di)selenide semiconductor, or CIGS, the layer that absorbs light and converts it into electricity.
High-performance flexible and lightweight solar cells on, for example, plastic foils have excellent potential to lower the manufacturing costs through roll-to-roll processing, thus enabling affordable solar electricity in the near future. However, flexible solar cells on polymer films have been lagging behind in performance compared to rigid cells, primarily because polymer films require much lower temperatures during deposition of the absorber layer, generally resulting in much lower efficiencies.
The research team at Empa's Laboratory for Thin Film and Photovoltaics, led by Ayodhya N. Tiwari, has been involved in the development of high-efficiency CIGS solar cells on both glass and flexible substrates with a special focus on reducing the deposition temperature of the CIGS layer. The group has repeatedly increased efficiency of flexible CIGS solar cells over the past years. With their current record value of 18.7%, Tiwari and his team nearly closed the efficiency gap to cells based on multi-crystalline silicon (Si) wafers or CIGS cells on glass.
“To achieve such high efficiency values, we had to reduce the recombination losses of photo-generated charge carriers”, said Tiwari. CIGS layers grown by co-evaporation at temperature of around 450°C have a strong composition grading because of inadequate inter-diffusion of intermediate phases and preferential diffusion of gallium (Ga) towards the electrical back contact.
To overcome this problem doctoral students Adrian Chirilã and Patrick Bloesch developed novel processes for optimizing the solar cell performance. To achieve an appropriate composition profile in the CIGS layer – for enabling more efficient charge carrier collection and reduced interface recombination – Chirilã and colleagues developed a growth process by carefully controlling the Ga and indium (In) evaporation flux during different stages of the evaporation process.
Such high-efficiency CIGS solar cells up to now were developed only on glass substrates with processes where CIGS layers are grown at temperatures of 600°C or above. In contrast, polymer foils cannot withstand such high temperatures. The new low-temperature process yielded another record efficiency of 17.7% on steel foil without any diffusion oxide or nitride barrier layer commonly used in high-temperature processes.