Internal electric field and fill factor of amorphous silicon (a-Si:H) solar cells

The electric field E within the i-layer of hydrogenated amorphous silicon (a-Si:H) solar cells strongly affects the cell performances, and, specifically, the fill factor FF. It governs the drift length Ldrift = μτE which is the crucial parameter limiting charge collection. Ideally, a constant electric field is assumed across the i-layer, whereas in real devices, it is deformed by charged band tail states and dangling bonds. If the i-layer is too thick or has a high density of charged defects, E is deformed and reduced. To determine theoretically the charge states of band tails and dangling bonds, we must know the carrier density profiles within the i-layer. Here, the SunShine program is used to determine carrier generation profiles within i-layers of pincells on TCO-covered glass substrates. A classical model for transport and electron/hole capture is employed to determine charge conditions of band tail states and dangling bonds. Results are: (a) charged dangling bonds are predominant for the electric field deformation, affecting the output performance of the cell; (b) this effect is very pronounced especially in degraded cells; (c) it is independent of light intensity; (d) it accounts for performance breakdown of thick, degraded a-Si:H cells. Calculated results are confronted with experimental observations (measurements of FF, collection voltage Vcoll and external quantum efficiency EQE) on pin-type solar cells of 100, 200, 300, and 400 nm thickness produced at IMT Neuchâtel, in initial and degraded state. Ldrift is evaluated via Vcoll, determined here with the method of variable intensity measurements (VIM). Trends observed are explained to full satisfaction.