Introduction to Solar Cells

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Edmond Becquerel discovered the photovoltaic effect in 1838, when he observed a small voltage and current when two silver halide coated platinum plates immersed in an acidic solution were exposed to light. Only far into the next century a major improvement was achieved: over 50 years ago, the first silicon solar cell was developed by Chapin, Fuller, and Pearson of Bell Telephone Laboratories [1].

Molecular Materials and Nanosystems, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands e-mail: [email protected]

L. Merhari (ed.), HybridNanocomposites for Nanotechnology, 321

DOI: 10.1007/978-0-387-30428-1_8, © Springer Science + Business Media, LLC 2009

Silicon solar cells consist of p- and n-type silicon (Fig. 8.1). In-between these two layers there is a transition layer, a p-n junction. The internal field over this junction separates the photogenerated charges, the holes are swept into the p-layer and the electrons are swept into the n-layer. Although these opposite charges attract each other, most of them can only recombine by passing through an external circuit outside the material because of the internal potential energy barrier.

Since the first silicon solar cell, many other types of cells, often based on new materials, new deposition methods, or novel device architectures have been described. The operation principle of each photovoltaic device, however, involves three basic steps, i.e., absorption of photons, creation and separation of free and mobile charges, and transport of these charges towards electrodes for collection. When each of these steps is efficient and occurs with minimal energy losses, a photovoltaic device can convert solar light into electrical free energy, with a maximum thermodynamic efficiency of about 31% in the radiative limit assuming detailed balance and a single band gap absorber [2].

Many different designs of this general p-n junction type silicon solar cell have been developed. Nowadays, single crystalline and multi-crystalline silicon solar cells have reached solar energy conversion efficiencies of 24.7 and 20.3%, respectively [3-5]. In essence silicon is not the optimal material for solar cells, its band gap of 1.1 eV (crystalline Si) is at the lower limit for optimal solar light conversion. The ideal solar cell would have a band gap between 1.1 and 1.7 eV for good photovoltaic conversion efficiency, a direct band gap for a high absorption coefficient, and consist of non-toxic, readily available materials that can be easily and repro-ducibly deposited with a technique that is suitable for large areas [6] . Of course such ideal solar cell would also have excellent long term stability.

Silicon suffers from its disadvantage of being an indirect semiconductor; as a result it is only a weakly absorbing material. For a silicon layer to absorb 90% of the light, at least a 100 |im thick film is needed. Using direct band gap semiconductors, top contact

Fig. 8.1 A simple scheme of a silicon solar cell, showing the basic elements: p- and n-type silicon and the junction in between

like GaAs, a 1 |im thin film is sufficient. Because the photocarriers have to reach the p-n junction for separation, the silicon used in solar cells has to be of very high purity and crystalline perfection. For this reason the production of crystalline silicon is far from easy and cheap, and much attention is paid to the development of micro-crystalline and amorphous silicon solar cells, promising high efficiencies at lower costs. Nonetheless single crystalline silicon solar cells still have a market share of 31.0, vs. 59.6% for multi-crystalline silicon, 3.1% for ribbon silicon and 5.3% for amorphous silicon, leaving ~1% for non-silicon based solar devices [7] . One of the major challenges for future photovoltaic devices will be to overcome the high material- and production-cost of silicon and other semiconductor materials. Relatively new materials like semiconducting polymers and hybrid composites with metal oxides promise to become an attractive alternative.

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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