The open circuit voltage in amorphous silicon p-i-n solar cells and its relationship to material, device and dark diode parameters

We review the sensitivity of the open-circuit voltage (V-oc) to material, device and dark diode parameters, and try to assess the possibilities of improving this quantity in hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells, having a wide band-gap emitter layer. We find that parameters that affect V-oc can broadly be classified into two categories: those that alter the built-in potential (V-bi) appreciably, and those that produce small or no change in V-bi, but can still alter V-oc by changing the dark recombination current in the intrinsic absorber or the interfaces. The study is carried out using an electrical-optical model based on the solution of the Poisson's and the continuity equations. In agreement with existing work, we find that V-oc is very sensitive to all parameters that appreciably alter V-bi, viz., the transparent conducting oxide/P contact barrier height, the P layer thickness and its activation energy, although V-oc is found to be more sensitive to these parameters than V-bi itself. However, we cannot correlate such changes in V-oc to the dark diode parameters. On the other hand, parameters that have little effect on V-bi, but alter the dark recombination current, viz., the density of states, capture cross-sections and the Urbach edge, have only a minor influence on V-oc, although these changes can qualitatively be linked to changes in the dark current. Also we find that the common belief that V-oc decreases as the reverse bias generation current increases is not always true. Boron profiling of the P/I interface and its effect on V-oc has also been studied. All in all our study indicates that in a-Si:H p-i-n cells under AM1.5 light, there is little scope of further improving V-oc, while retaining the cell efficiency. (C) 2004 American Institute of Physics.