Shockley-Read-Hall (Trap Assisted) recombination is a form of efficiency loss in a solar cell.
This process occurs when there is an impurity which introduces an energy level within the band gap of the semiconductor. Usually, an electron or hole will first get trapped in the impurity, and the other carrier will find it. This type of recombination is usually the biggest problem in silicon solar cells. This efficiency loss can be avoided by purifying the silicon more – however this adds more cost to the solar cell.