Exploring Back Contact Technology to Increase CdS/CdTe Solar Cell Efficiency
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1012-Y07-05
Exploring Back Contact Technology to Increase CdS/CdTe Solar Cell Efficiency Alan L. Fahrenbruch Department of Physics, Colorado State University, Fort Collins, CO, 80523-1875 ABSTRACT The primary routes for increasing CdS/CdTe solar cell efficiency involve increasing free carrier density, reducing bulk and interface recombination, and/or reducing back contact barrier height Φbc. This paper focuses on the role of the back contact barrier in increasing cell efficiency. Measurement of Φbc and back surface recombination are outlined and three CdTe/MX/M back contact prototypes, each with particular strengths, are discussed. INTRODUCTION: EFFECT OF BACK CONTACT BARRIERS ON EFFICIENCY The Voc deficit (Eg/q ñ Voc) for CdS/CdTe cells is large in comparison with other cells (e.g., 0.65 V for CdTe vs. 0.44 V for CIS). Optimistically, one can view this as an opportunity for increasing efficiency. There is little room for improvement in short-circuit current, so the focus here is on increasing cell voltage and fill factor. Four basic ways to do this are: ï increase free carrier density (p) in the CdTe absorber, [1] ï decrease bulk recombination, [1] ï decrease back-contact barrier height (Φbc), [2] and ï reduce back surface recombination by surface treatment and/or an electron mirror. [3] All of these options are difficult and uncertain to do and to measure. There is increasing evidence that the density of deep acceptors is much greater than that of shallow acceptors [e.g., 4], suggesting that the hole density p is considerably smaller than the usual 1/C2 vs. V measurements indicate (≈ 3e14 cm-3). The experimental correlation between bulk recombination lifetime and real cell efficiency for CdS/CdTe has only recently become quantitative, when Metzger et al. [5] found a positive correlation between lifetime measured by Transient Photoluminescence and Voc. While the experimental connections between fabrication variables and efficiency are strong, a description in terms of specific defect species and their recombination properties is still under discussion and device modeling is still somewhat tentative [6]. This paper focuses on the role of the back contact barrier. This is perhaps the most uncertain choice for increasing Voc for thin cells, but arguably it has the highest potential payoff, especially if applied in combination with decreased bulk recombination. For electronically thick cells (XCdTe > ≈ 2 Wd, where XCdTe is the CdTe thickness and Wd the depletion layer width), the depletion layers of the main junction and the back contact donít overlap and the back contact can be treated as a series-opposing Schottky barrier. For values of Φbc ≤ 0.3 eV, ff and Voc arenít substantially affected. Increasing Φbc beyond 0.3 eV mainly decreases ff with a small loss of Voc. [7,8]. However, the cost-driven trend is toward thinner CdTe layers. Modeling suggests that efficiency gains due to decreased back-contact barrier height Φbc become greater for thinner CdTe and smaller space charge density SCD. Given the low SCD values ob
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