Crystalline silicon thin films: A promising approach for photovoltaics?
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Crystalline silicon thin films: A promising approach for photovoltaics? A. Slaoui and R. Monna Laboratoire PHASE (UPR 292 CNRS), 23 rue du Loess, F-67037 Strasbourg cedex, France
J. Poortmans, T. Vermeulen, O. Evrard, K. Said, and J. Nijs IMEC, Kapeldreef 75, B-3001 Leuven, Belgium (Received 2 March 1998; accepted 2 March 1998)
In this paper we review the achievements in the field of silicon crystalline thin film solar cells and correlate these with the different types of growth techniques and substrates. As a starting point we discuss the characteristics of photovoltaic devices based on the use of highly doped monocrystalline substrates as mechanical carriers for the thin films. These films are epitaxially deposited from the gas (CVD) or liquid phase (LPE). The comparison of both techniques is extended to growth on defective silicon substrates, i.e., multicrystalline wafers or silicon ribbons. The intrinsic grain boundary recombination activity in the thin films is assessed as a function of the deposition technique. Bulk passivation by hydrogenation considerably improves the recombination properties. The optimization of the hydrogen passivation conditions is looked at in conjunction with the used surface passivation process. This review is completed with the approaches to realize thin film cells on nonsilicon substrates, including recrystallization in solid and liquid phases.
I. INTRODUCTION
Silicon in its three basic forms, “monocrystalline, sc-Si”, “multicrystalline, mc-Si,” or “amorphous, a-Si” is nowadays the main material to produce solar cells. It has a number of advantages compared to other materials, such as high abundance, availability in high purity, a mature technology, and environmental safety. Crystalline silicon solar cells made from relatively thick (200 to 450 mm) substrates/sheets are characterized by fairly high efficiencies. However, their potential for cost reduction seems to be of wafers. On the other hand, thin film silicon solar cells have the potential of performance improvement and offer the perspective of low material and energy consumption and relative ease of large scale production. First, improved efficiency can be reached, in spite of reduced material quality, by the use of appropriate electrical structures (electrical confinement, e.g., by back surface field, BSF) and also of effective light trapping (optical confinement), which increases the effective absorption length in the material. Figure 1 demonstrates the effect of thickness reduction and furthermore shows that thin active layers tolerate shorter minority carrier diffusion lengths and therefore allow the use of a less pure material. Second, reduced cost can be achieved by the use of low-cost supporting, large area substrates. Although the theoretical basis of this type of thin film solar cell has been unfolded about 15 years ago,1,2 there are still severe technical problems to be solved J. Mater. Res., Vol. 13, No. 10, Oct 1998
until a cost-effective manufacturing of these cell
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