Understanding graded a-SiGe solar cells using bifacial photocurrent collection

The authors have characterized p-i-n solar cell devices with front loaded or ungraded a-SiGe i-layers. The cells have transparent front and back contacts which allow bifacial photocurrent measurements to probe electron or hole limited collection using strongly absorbed blue light through the p or n-layer, respectively. Fill factor and quantum efficiency(V) data indicate that the primary effect of grading the i-layer in the front loaded device is to improve hole collection rather than electron collection. However, electron collection degrades more than hole collection after 200 hours light soaking. The authors conclude that device results do not agree with photoconductivity stability studies on a-SiGe films since stability studies of films show that the electron μτ is unaffected by light soaking while device results suggest that electron collection degrades significantly