Electrodeposited CdTe Solar Cells

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. 

Aqueous cathodic electrodeposition has been shown to offer a low-cost route for the fabrication of large surface n-CdS/p-CdTe solar cells. In a typical procedure, CdTe films, 1-2 xm thick, are electrodeposited from common acidic tellurite bath over a thin window layer of a CdS-coated substrate under potential-controlled conditions. The as-deposited CdTe films are stoichiometric, exhibit strong preferential (111) orientation, and have n-type conductivity (doping density typically 

Peter and Wang [266] invented a channel flow cell for rapid growth of CdTe films they showed that 2 p,m Aims can be deposited in less than 20 min, as opposed to the 2-3 h normally required in the conventional stirred single batch cells. The as-deposited films were structurally more disordered than the conventional ones, but after annealing and type conversion they became suitable for fabrication of efficient solar cells. A test cell with an AMI.5 efficiency approaching 6% was fabricated using a film prepared in the channel cell. 

Electrolyte contacts have been used to characterize as-deposited and annealed CdS/CdTe solar cell structures by photocurrent spectroscopy and electrolyte elec-troabsorbance/electroreflectance measurements (EEA/EER) [267-269]. 

Hodes G (1995) Electrodeposition of 11-Vl semiconductors, in Rubinstein I (ed.) Physical Electrochemistry Principles, Methods, and Applications. Marcel Dekker, New York 

---

McGregor SM, Dharmadasa IM, Wadsworth 1, Care CM (1996) Growth of CdS and CdTe by electrochemical technique for utilisation in thin film solar cells. Opt Mater 6 75-81 Morris GC, Das SK (1992) Some fabrication procedures for electrodeposited CdTe solar cells. Int J Sol Energy 12 95-108... 

Kampmann A, Cowache P, MokiU B, Lincot D, Vedel J (1995) Characterization of (111) cadmium telluiide electrodeposited on cadmium sulphide. J Cryst Growth 146 256-261 Britt J, Ferekides C (1993) Thin-film CdS/CdTe solar cell with 15.8% efficiency. Appl Phys Lett 62 2851-2852... 

Bhattacharya RN, Rajeshwar K (1985) Heterojunction CdS/CdTe solar cells based on electrodeposited p-CdTe thin films Fabrication and characterization J Appl Phys 58 3590... 

From Acidic Aqueous Solutions The electrodeposition of CdTe from acidic aqueous solutions has been carried out on various substrates, including the single crystals of Au(l 1 1) or Au(l 0 0) [67], n-Si(l 0 0) [68], and Si(l 1 1) [69]. CdTe has also been electrodeposited on CdS to form thin-film CdS/ CdTe solar cells [70]. 

Basol BB (1988) Electrodeposited CdTe and HgCdTe solar Cells. Sol Cells 23 69-88 Bhattacharya RN, Rajeshwar K, Noufi RN (1985) In situ preparation of p-Type CdTe thin films by cathodic electrodeposition. J Electrochem Soc 132 732-734 Llabres J (1984) In situ preparation of undoped p-Type CdTe by cathodic electrochemical deposition. J Electrochem Soc 131 464 65... 

Barker J, Binns SP, Johnson DR, Marshall RJ, Oktik S, Ozsan ME, Patterson MN, Ransome SJ, Roberts S, Sadeghi M, Sherborne J, Turner AK, Woodcock JM (1992) Electrodeposited CdTe for thin film solar cells. Int J Sol Energy 12 79-94... 

Peter EM, Wang RL (1999) Channel flow ceU electrodeposition of CdTe for solar cells. Electrochem Commun 1 554-558... 

Preliminary results were recently reported [145] on the use of CdTe electrodeposition on diamond in the fabrication of a solid-state solar cell based on the boron-doped p-type diamond/n-type CdTe junction. In this cell, the wide-bandgap diamond is an optical window that generates photovoltage, whereas the narrow-bandgap CdTe generates photocurrent. We note that no appropriate p-type material for the fabrication of optical windows has existed so far therefore, one would use an n-type CdS window coupled with narrow-bandgap p-type CdTe. However, the pro-... 

This determination is of key importance for the optical and electronic characterisation of solar cells, since a thickness determination is otherwise quite challenging. The as-grown electrodeposited CdTe is nanocrystalline and has rather poor electronic properties. Marked improvement is obtained in a subsequent annealing process involving CdCb vapour and temperatures around 430 C, as described by Birkmire et al. (1991). If this process is performed in air, the crystallisation is accompanied by a type-conversion towards p-type material. 

There is considerably less work on very thin absorber layers, prepared with the methods that are typically used for the preparation of highly structured cells. We will present results from work on our CdTe fdms electrodeposited on TiOi. These results appear to indicate that the transport parameters in these nanocrystalline films are much inferior to those typically observed in polycrystaUine films of micrometre thickness. Nonetheless, they may be sufficient for solar cell operation in non-planar arrangements. 

The methods available for preparation of the different layers in thin-film solar cells include physical methods such as vacuum sputtering, vapor-phase deposition, and molecular beam epitaxy as well as chemical methods such as chemical vapor-phase deposition, metal organic vapor-phase epitaxy, chemical bath deposition (CBD), and electrochemical deposition (ED). This chapter explores the potential of electrodeposition as a route to the fabrication of absorber layers such as CdTe, CIGS, and CZTS for thin-film solar cells. Electrochemistry may also be usefiil for the preparation of transparent layers such as ZnO this topic has been reviewed by Pauporte and lincot [13]. 

Another interesting example of the application of EQE spectroscopy is its use to follow type conversion in CdS [CdTe heterojunction solar cells fabricated by electrodeposition (see Section 1.2). The method was used originally by Basol on completed solid-state CdSjCdTe cells [143] and later extended by others to characterize partially completed structures using electrolyte contacts [144—147]. The location of the photoactive regions is probed by comparing the EQE spectra for substrate side (SS) and electrolyte side (ES) illumination of the samples as illustrated schematically in Figure 1.32. 

Duffy, N.W., Peter, LM., Wang, R.L, Lane, D.W., and Rogers, K.D. (2000) Electrodeposition and characterization of CdTe films for solar cell applications. Electrochim. Acta, 45, 3355-3365. 

There is one report of a PCE of 1% using vertically aligned CdTe nanorods obtained by electrodeposition into an AAO template. After the AAO template was removed, poly(3-octylthiophene) (P30T) was filled in by spin coating. This indicates that CdTe NCs may be useful for hybrid solar cells when their energy levels match the energy levels of the polymer [103]. 

Morris GC, Das SK(1993) 0-7803-1220-l/93 3.00 (1993) Influence of CdC12 treatment of CdS on the properties of electrodeposited CdS/CdTe thin film solar cells. IEEE... 

---