Internal electric field and fill factor of amorphous silicon 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 L_drift = μTE 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 pin-cells 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 V_coll 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. L_drift is evaluated via V_coll, determined here with the method of variable intensity measurements (VIM). Trends observed are explained to full satisfaction.