Solar metal thin films

Dharmadasa IM, Samantileke AP, Chaure NB, Young J (2002) Semicond Sci Technol 17 1238 http //shura. shu.ac.uk/1270/ New ways of developing glass/ conductingglass/CdS/CdTe/metal thin-film solar cells based on a new model... 

Copper Sulfide—Cadmium Sulfide. This thin-film solar cell was used in early aerospace experiments dating back to 1955. The Cu S band gap is ca 1.2 eV. Various methods of fabricating thin-film solar cells from Cu S/CdS materials exist. The most common method is based on a simple process of serially overcoating a metal substrate, eg, copper (16). The substrate first is coated with zinc which serves as an ohmic contact between the copper and a 30-p.m thick, vapor-deposited layer of polycrystaUine CdS. A layer is then formed on the CdS base by dipping the unit into hot cuprous chloride, followed by heat-treating it in air. A heterojunction then exists between the CdS and Cu S layers. 

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

The deposition of CBD CdS as a junction layer for solar cell devices has proven to be a very successful industrially acceptable technique. Kessler et al.13 reported on copper indium gallium diselenide (CIGS) mini-modules (area = 16cm2) with a conversion efficiency of 16.6%, wherein CBD CdS was used as a junction layer. Basol et al.14 fabricated 9.3% active-area efficient thin-film flexible CuInSe2 (CIS) solar cells (specific power >1 kW/kg) on lightweight, flexible metallic, and polymeric (polymide-based) substrates using CBD CdS. 

The number of publications concerning utilization of the EISA process for fabrication of different structured materials is counted in the hundreds, which is far beyond the possibilities of this chapter to review in depth. Rather, we intend to provide a brief introduction into EISA and its application to the fabrication of functional thin films for electronic applications (e.g., electro-chromic layers and solar cells), with a special focus on fabrication of crystalline mesoporous films of metal oxides. Attention will also be given to techniques used to evaluate the pore structure of the thin films. For the other aspects of the EISA process, for example its mechanism,4 strategies for preparation of crystalline porous metal oxides,5 mesoporous nanohybrid materials,6 periodic organic silica materials,7,8 or postgrafting functionalization of mesoporous framework,9 we kindly recommend the reader to refer to the referenced comprehensive reviews. 

F.R. Zhu, T. Fuyuki, H. Matsunami, and J. Singh, Assessment of combined TCO/metal rare contact for thin film amorphous silicon solar cells, Sol. Energy Mater. Sol. Cells, 39 1-9, 1995. 

Electronic and Optoelectronic Applications of Tellurides. Most metal tellurides are semiconductors with a large range of energy gaps and can be used in a variety of electrical and optoelectronic devices. Alloys of the form HgCdTe and PbSnTe have been used as infrared detectors and CdTe has been employed as a gamma, ray detector and is also a promising candidate material for a thin-film solar cell. 

Another exciting thin-film company is FirstSolar (www.firstsolar.com). First Solar claimed to achieve a manufacturing cost of only 1.08 per watt during the third quarter of 2008. The company uses cadmium telluride as a semiconducting material, which is a byproduct of the mining and production of base metals such as zinc and copper. First Solar has entered into many excellent long-term sales agreements with major electric utility companies. 

The issue of Schottky barrier formation to ZnO is not treated in this chapter as such contacts are not of big importance in thin-film solar cells. This is related to the fact that in thin film solar cells metals are only used to contact highly-doped films. For degenerately doped semiconductors, the barrier heights become very small because of the large space charge associated with depletion layers in such materials. 

---