III-V Nanowire Solar Cells: Growth and Characterization

Abstract: To mitigate dangerous climate change, a transition to a new and sustainable energy system is needed. In this system, solar energy will need to be a key player. Prices of electricity made from solar cells have declined rapidly over the recent decades, making solar energy competitive in more markets. However, further price reductions and new innovations are needed for solar cell technology to fulfil its potential.In this thesis, we look at a class of materials that have gained increasing interest in the recent decade; III-V semiconductor nanowires. For solar cells, the III V semiconductors hold excellent optical and electrical properties, but high material and manufacturing costs have so far prevented competitiveness with the dominating Si based technology. However, III-V material in the nanowire geometry has a number of interesting advantages when it comes to reducing cost, as well as for adding III-Vs to conventional Si in tandem solar cell architectures. This has motivated substantial research efforts during recent years, both at universities and private companies. In this thesis, we have mainly studied InP and InxGa1-xP nanowire arrays as a solar cell material. The nanowires were grown by metal organic vapor phase epitaxy (MOVPE) via gold seeded vapor-liquid-solid growth. The gold seed particles were placed in a pattern on the growth substrate by help of nanoimprint lithography. Developing strategies to preserve this pattern through the stages of nanowire growth was an important foundation for the thesis work. These strategies allowed us to reproducibly grow dense and ordered arrays of nanowires, optimized for sunlight absorption. Controllably changing the electrical properties of the semiconductor through impurity doping is important to make a good solar cell. Doping nanowires is challenging since the growth mechanism is different from established layer growth by MOVPE, and nanowire characterization is demanding. We have studied doping in our nanowires in various ways. Most importantly, we have performed some of the first systematic doping studies in ternary III-V materials, with bandgaps needed to create tandem nanowire solar cells. Knowledge from these studies allowed us to realize and study the first nanowire tunnel junction connecting two materials of appropriate bandgap to match the solar spectrum.Finally, we have developed a characterization procedure to optimize nanowire solar cell characteristics. This helped us create a better understanding of performance limiting factors in our InP nanowire solar cells. As a result, we achieved more than a sevenfold performance improvement in these cells, with the best device having a certified power conversion efficiency of 15.0%. This is the highest reported efficiency value for a bottom-up synthesized InP nanowire solar cell.