Second Generation Thin-Film Technologies

Thin-film technologies are becoming a competitive class of PV, doubling production from 2006 to 2007. The majority is produced in the USA where investment in alternative thin-film technologies is highest, but China should soon overtake the USA in terms of production capacity. The market share of thin-film technologies increased from only a few per 

1% yr match c-Si. Stable laboratory efficiencies of 13.5% and module efficiencies around 5.3-6.4% have been achieved corresponding to a cost of 3.5-8.6 Wp . The expanding industry now accounts for around 6% of the PV market with a number of 10-30 MWp yr capacity plants based in the USA, China and Japan, with Sharp being the market leader with a 1 GWp yr capacity facility. 

Heterojunction solar cells are constructed from p-type absorber layers and n-type, wide-band gap layers (windows) to form a p-n junction. Compound semiconductors such as CdTe are used which, unlike Si and Ge, have direct band gaps and, therefore, high extinction coefficients, making them suitable for thin-film applications. 

CdTe is a crystalline compound with a cubic zinc blende (sphalerite) crystal structure (lattice constant of 6.481 A), a direct band gap of 1.5 eV, an ideal match to the solar spectrum, and an extinction coefficient around 5 x 10 cm . ° The intrinsic defects include cadmium interstitials and cadmium vacancies, and extrinsic doping can be achieved using In (donor) substitution or Cu, Ag, Au (acceptor) substitution for Cd. The mobilities have been measured to be up to 1100 cm s for electrons and up to 8 cm s for holes. Dopant densities up to 10 cm  

Back contact (Graphite, Cu) p-type absorber (2 pm, CdTe, CSS) n-type window (0.07 pm, CdS, CBD] 

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Solar cell technologies can be divided into three generations. The first is the established technology such as crystalline silicon. The second includes the emerging thin-film technologies that have just entered the market, while the third generation covers future technologies, which are not yet commercialised. 

Ferroelectric crystals (especially oxides in the form of ceramics) are important basic materials for technological applications in capacitors and in piezoelectric, pyroelectric, and optical devices. In many cases their nonlinear characteristics turn out to be very useful, for example in optical second-harmonic generators and other nonlinear optical devices. In recent decades, ceramic thin-film ferroelectrics have been utilized intensively as parts of memory devices. Liquid crystal and polymer ferroelectrics are utilized in the broad field of fast displays in electronic equipment. 

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