Doped Emitters to Unlock Lowest Cost Solar Electricity

Solar PV is on the cusp of becoming the lowest cost source of electricity for many regions of the world, displacing fossil fuels, with the prospect of dramatically reducing carbon emissions. The second generation thin film PV based on CdTe has lower manufacturing cost and lower carbon footprint than silicon PV. This proposal will enable the solar energy conversion efficiency of thin film CdTe PV modules to equal or exceed that of silicon and enabling more rapid and wider adoption of solar PV electricity.

This proposal brings fresh thinking to the front emitter layer that is widely recognised in the CdTe PV community as being the limiting factor in realising the potential of the arsenic doped CdTe and CdSeTe absorber layers. This is predicted to achieve over 25% cell efficiency and over 22% module efficiency. To achieve this goal we have put together a world leading team to work on a new n-type emitter layer. The teams at Swansea-CSER and Loughborough-CREST have combined expertise on As doping of the CdTe absorber layer along with sputter deposition of oxide layers. The world leading team includes project partners - Colorado State University (leading academic team in the USA), First Solar (leading thin film PV manufacturer) and NSG Pilkington (leading coated glass products for thin film PV).

The challenge for realising the potential for arsenic doped CdTe (pioneered by the Swansea team) is to combine the acceptor doped CdTe layer with a transparent emitter layer where the n-type doping concentration exceeds the acceptor doping concentration of the CdTe layer. For an acceptor doping of >1x1016 cm-3, the emitter donor doping needs to be >1x1017 cm-3. In addition the conduction band alignment must give a small positive step for electron collection which will reduce non-radiative recombination. To achieve this exacting specification we will explore a wide range of potential oxides and their alloys with different dopants using combinatorial techniques. This will be matched to the optimised alloy composition and doping of the CdSeTe absorber layer using MOCVD. Stability of candidate doped emitters will be tested from an early stage with regard to air exposure and exposure to process steps in fabricating the complete thin film PV device. Extensive materials and device characterisation will be used to understand the relationship between the novel doped emitters and improved PV cell efficiency.