The effect of grain boundaries on the performance of poly-Si thin film solar cells was studied theoretically using a 2-D simulation assuming the presence of either rectangular-shaped or graded width grain boundaries in the i-layer of p/i/n structure of solar cells. The grain boundary had an adverse effect mainly on V-oc. J(sc) gradually increased and saturated with increasing solar cell thickness in cells without grain boundaries, whereas it reached a maximum for an i-layer thickness of 5 mum in polycrystalline silicon cells. The calculation using the graded width model showed that the efficiency of the p(+)/p(-)/n(+) structure was better than that of the p(+)/n(-)/n(+) structure. A slight p-type doping of the i-layer was found to be effective in improving cell performance. (C) 2002 Elsevier Science B.V. All rights reserved.