Identification of process induced defects affecting the performance of nanowire based solar cells by spatially resolved photocurrent measurements

Abstract: Solar power clearly has one of the greatest potential among renewable energy but a big setback is that it is still relatively expensive. Engineering of materials on a nanoscale, e.g. the growth of nanowires makes it possible to make semiconductor-materials with better optical properties while using less material than for traditional bulk semiconductors. This means that nanowire based solar cells has the potential to be cheap to produce while they also could reach high efficiencies possibly making solar cells on a large scale a reality. Lund University is one of the best in the world in growing rows of high quality nanowires and has produced a record solar cell based on InP nanowires with an efficiency of 13.8% in 2012. During this project, a photocurrent measurement technique that can resolve the photocurrent on a solar cell has been further developed and evaluated. There has also been several measurements and investigations on a sample called SCIL 51-84 that had 7 III-V InP nanowire solar cells. The work has led to identification of local defects in material and processing, and an evaluation technique that in this project is called inverted images and may lead to a safer and faster way of detecting abnormalities on these nanowire-based solar cells. Four kinds of defects were investigated and two of these could be removed using reactive ion etching with oxygen plasma. Removal of the optical defects led to an increased performance with about 40% compared to before. The limitations and suggestions on further development of the setup are also discussed.