SILICON HETEROJUNCTION CELLS: A BREAKTHROUGH IN PHOTOVOLTAICS

Julio Carabe, CIEMAT. Avda. Complutense, 22. E-28040 Madrid, Spain Francesco Roca, ENEA. Localita Granatello. 80055 Portici (Naples) Italy.

ABSTRACT

Photovoltaics are facing the challenge to find new approaches to make solar cells competitive with respect to more conventional electricity sources. Crystalline-silicon (c-Si) wafer-based technology must evolve towards lower costs by implementing new material — fabrication processes and making wafers thinner. On the other hand, silicon grown directly on low cost substrates is forced to progress in the direction of improving the optoelectronic properties of the material and its growth rate and consistently of making the cell active — layer thicker. The key silicon material in next-generation PV is characterised by a medium crystallinity, a medium active-layer thickness and a high fabrication rate. The efforts towards this material are generating new approaches involving the combination of wafer and thin-film technologies.

The characteristics of new materials impose new limitations, thus requiring innovative solutions. Silicon-heterojunction (SHJ) cells, basically made of a crystalline-silicon wafer or ribbon absorber and one or two thin-film-silicon emitter(s), represent a promising option. Its features are: a simple low temperature fabrication process, an important cost-reduction capability, high efficiencies and a high potential for improvements. Furthermore SHJ technology has proven to provide excellent surface-passivation approaches. Particularly remarkable is the work done by Sanyo, who have reported 21% efficiency on a cell of this kind (so-called HIT®) and have attracted much attention. This new product has allowed to the Japanese company to reach a market share of 6% of all PV sales in the world. A number of research groups and companies are working hard on SHJ cells in Europe. Their individual results are promising and reveal an excellent scientific level. The need to address the fragmentation of European R&D in this field must however be recognised by creating a permanent structure ensuring the harmonisation of the whole R&D on SHJ cells.

INTRODUCTION

Photovoltaics are dominated by silicon. About 84% of the world PV market share corresponds to mono — and multicrystalline-silicon wafer technology. Only 0.4% is covered by thin-film chalcogenides, basically CdTe. The remaining 15.6% is shared by various silicon-based technologies, such as ribbon silicon, amorphous silicon and silicon heterojunction cells1. This primacy is very likely to remain for at least the next ten years. The cause of this predominant position is the combination of a number of factors, such as the maturity of silicon PV technologies, the good and well-known optoelectronic properties of the material, its availability, lack of toxicity, cost, chemical stability, etc.

Europe is a leader in the production of crystalline ingots and wafers for PV applications. At present, the cost of materials and processes used in c-Si PV applications is high. Every effort done to reducing the costs of the materials involved in the process without detrimental effects on conversion efficiency is considered of vital importance. In the search for breakthroughs, the most promising initiatives for the reduction of costs in the production of c-Si solar cells are addressed towards replacing the present raw material by thinner, less-crystalline silicon (EFG, string ribbon silicon, Silicon Film®, dendritic-web silicon, etc.).

If these new materials are to be used, a number of issues become essential, such as low — temperature approaches for junction formation and cost-effective processes for passivation.

On the other hand, thin-film silicon research is evolving towards the preparation of thicker, more crystalline films (nano — and microcrystalline silicon) where the stability and high deposition rate of the material and the quality of the interfaces are key factors.

It seems that the two main silicon technologies are mutually approaching. This is particularly apparent in the Sanyo development of hybrid wafer — and thin-film approaches, such as the silicon heterojunction (sHj) solar cell. Here the advantages of both technologies mutually interact for the development of low-cost, stable high-efficiency solar cells one of whose types is the well-known 21%-efficiency HIT® cell developed by Sanyo2. The fundamental approach is the replacement of the more conventional emitter thermal diffusion by a low-temperature deposition process. Key features of SHJ technology are: a very simple fabrication process, an important cost-reduction capability, relatively high efficiencies, and a high potential for significant improvements. Of particular relevance is the suitability of the approach to process low-cost wafers or ribbons without degradation of their transport properties (given the low temperatures involved, around 250°C), and relatively thin substrates, since the control of emitter thickness is extremely good.