Improved CdTe Solar-Cell Performance with An Evaporated Te Layer before The Back Contact
- PDF / 1,089,345 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 57 Downloads / 201 Views
Improved CdTe Solar-Cell Performance with An Evaporated Te Layer before The Back Contact Andrew Moore1, Tao Song1, James Sites1 1 Colorado State University, Fort Collins, CO, USA ABSTRACT CdTe solar cells with a Te-buffer layer adjacent to the back contact were fabricated. The effects of the Te layer on cell performance were evaluated in detail. The carrier density of the Te layer (1018 cm-3) was measured. The valence band offset of the CdTe/Te interface (~0.3-0.5 eV) was determined from current-voltage-temperature measurements and published reports. These values were incorporated into a simulation model and compared to the measured experimental performance with good agreement. Most notably, it was found that the Te layer allowed improved cell performance with less Cu required to form the back contact. INTRODUCTION CdTe solar cells have demonstrated efficiencies of 22.1% [1]. However there is much to be improved in order to more closely approach their theoretical maximum of ~30% [2]. One area of potential enhancement is the back contact of the device. CdTe is a difficult material to make true ohmic contact due to its high electron affinity (4.3 eV) in addition to its wide band gap (1.5 eV) [3,4]. Most metals do not have a sufficiently high work function, and have a tendency to form a Schottky diode at the CdTe/metal interface, which both creates a barrier to photogenerated holes and increases forward electron current [5]. Several strategies have been employed to mitigate the detrimental impact of the back contact by creating a buffer layer between the CdTe and metal contact. One technique is back surface modification by heavily doping the CdTe, usually with Cu, and/or by wet-etch in order to create a Te-rich layer [6-8]. Even though Cu substitution for a Cd vacancy (CuCd) does create a non-shallow acceptor, Cu sitting interstitially (Cui) is a shallow donor [9]. Cui is known to be highly mobile in CdTe, and can self-compensate the doping or form unfavorable complexes with Cl, which causes limits to stability and performance [8-11]. Using a wet-etch also has drawbacks such as not being easily integrated into a manufacturing line and preferentially etching grain boundaries causing shunting [12]. Occasionally, in place of or in conjunction with the surface modification, the physical deposition of a buffer layer has been used. In this work an evaporated Te layer is explored. Using the Te buffer, we have seen performance improvement and are able to reduce the amount of Cu surface treatment compared to our traditional C buffer contact. While the use of a Te buffer layer in CdTe has been studied previously, (Niles [12, 13], Uda [14], Kraft and Fritsche [15-16], and recently Xia [17]) there have been conflicting conclusions concerning the physical mechanisms. This work gives a more complete view of the Te buffer layer for CdTe solar cells by incorporating measured material properties into a simulation model. Simulated device characteristics are compared to measured electrical parameters with good agreement. EXPERIMENT The d
Data Loading...
