High Aspect Ratio Electrodeposited Ni/Au Contacts for GaAs-Based III-V Concentrator Solar Cells

We report on a photolithographic and electro-deposition process that results in an optimized front grid structure for high efficiency multi-junction III-V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6-μm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10^-4 to mid 10^-5 Ohm cm² range along with excellent adhesion without sintering.