The photovoltaic industry is in a phase of rapid expansion, growing at over 30 % per annum over recent years. Although technologies based on thin-film compound and alloy solar cells are under active development, most commercial solar cells presently use self-supporting bulk crystalline or multicrystalline silicon wafers, similar to those used in microelectronics. The laboratory performance of these cells, at 25 % solar energy conversion efficiency, is now approaching thermodynamic limits, with the challenge being to incorporate these improvements into low-cost commercial products. Improvements in cell optical design, particularly in their ability to "trap" weakly absorbed light, has also led to a growing interest in thin-film cells based on polycrystalline silicon, having advantages over other thin film photovoltaic candidates.