New Technology to increase the efficiency of Photovoltaic cells,

Introduction

The photovoltaic (PV) industry was based on the niche applications of powering satellites and remote locations. However, the tide has changed dramatically, with growing recognition of the environmental impact of non-renewable energy sources and the economic volatility that comes from reliance on oil and gas. Subsidy-based market strategies, particularly in Japan and Germany in the late 1990s, pump-primed a PV-industry that is increasing in economic importance and is now a billion dollar industry undergoing staggering growth (Fig. 1).

Fig. 1. Yearly worldwide production in photovoltaics in MWp (EurObserv'ER, 2008).

As well as providing an energy source that is acceptable to environmentalists, photovoltaics has the interest of a financial sector that now sees a business case for investment. It seems possible that it will be economic arguments rather than environmental arguments that will push PV energy into the mainstream. Cost, in terms of $/W, remains the greatest barrier to further expansion of PV-generated power and cost reduction is the prime goal of the PV sector.

Current status of photovoltaic technology

The current PV market consists of a range of technologies including wafer-based silicon and a variety of thin-film technologies. The range of current technologies and possible future options have been grouped from current first-generation to future third-generation technologies (Green, 2006).

First-generation PV

Current PV production is dominated by single-junction solar cells based on silicon wafers including single crystal (c-Si) and multi-crystalline silicon (mc-Si). These types of single-junction, silicon-wafer devices are now commonly referred to as the first-generation (1G) technology, the majority of which is based on a screen printing-based device similar to that shown in Fig. 2.

Fig. 2. Schematic of a single-crystal solar cell.

Originally built using single-crystal wafer silicon (c-Si) and processing technology from the integrated circuit (IC) industry, it is clear that 1G silicon PV benefited greatly from its symbiosis with the IC industry which provided the materials, processing know-how, and manufacturing tools necessary to allow a rapid move to large-scale production.

New device technologies including BP Solar ('Saturn cells'; see, for example, Bruton et al., 2003), Sanyo ('HIT cells') and Sunpower are pushing commercial single-crystal wafer silicon efficiencies to the 18-21% range offering the potential for lower $/W as a result of increased efficiencies (see company websites, e.g. Sharp; Q-Cells; Kyocera; Sanyo; Sunpower) for the most up-to-date information). However, multi-crystalline silicon (mc-Si) currently accounts for 63% of the world market, including manufacturers with cell efficiencies around 13-14% but at overall lower $/W cost.

Second-generation PV

The obvious next step in the evolution of PV and reduced $/W is to remove the unnecessary material from the cost equation by using thin-film devices. Second-generation (2G) technologies are single-junction devices that aim to use less material while maintaining the efficiencies of 1G PV. 2G solar cells use amorphous-Si (a-Si), CuIn(Ga)Se2 (CIGS), CdTe/CdS (CdTe) or polycrystalline-Si (p-Si) deposited on low-cost substrates such as glass (Fig. 3). These technologies work because CdTe, CIGS and a-Si absorb the solar spectrum much more efficiently than c-Si or mc-Si and use only 1-10 µm of active material. Meanwhile, in ...