MoO x as an Efficient and Stable Back Contact Buffer for Thin Film CdTe Solar Cells
- PDF / 654,886 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 92 Downloads / 215 Views
MoOx as an Efficient and Stable Back Contact Buffer for Thin Film CdTe Solar Cells Hao Lin, Wei Xia, Hsiang N. Wu, Ching W. Tang Department of Chemical Engineering, University of Rochester, Rochester, NY, 14627 Irfan Irfan, Yongli Gao Department of Physics and Astronomy, University of Rochester, Rochester, NY, 14627 ABSTRACT A low-resistance back contact for n-CdS/p-CdTe solar cells has been developed, which utilizes a thermally evaporated MoOx thin film as the buffer layer between the p-CdTe and the back electrode. The low-resistance behavior of back contact is attributed to the high work function of MoOx, which reportedly is as high as 6.8 eV, and thus adequately matches that of pCdTe. With MoOx as the buffer, a variety of common metals, even those with a low work function such as Al, have been found to be useful as the electrode in forming the back contact. Other advantages of the MoOx buffer include dry application by vacuum deposition, and thus it is particularly suitable for the fabrication of ultra-thin CdTe solar cells without introducing additional shorting defects. Surface cleaning of CdTe films prior to MoOx deposition has also been studied. The cell stability has been evaluated through thermal annealing tests. Thermal degradation has been explained in terms of oxidation of the metal electrodes. CdTe cells with high efficiency and good stability have been demonstrated with MoOx as the back contact buffer and Ni as the electrode. INTRODUCTION Back contact improvement is a critical issue for thin film n-CdS/p-CdTe solar cells. The difficulties to achieve barrier-free ohmic contact to p-CdTe are because of two technical issues. The first issue is the high work function of p-CdTe, which is reportedly as high as 5.7 eV [1]. As a result, no common metals can be directly applied to p-CdTe to make an ohmic contact. The second issue is hard to highly dope CdTe (an intrinsic semiconductor) to achieve a shallow acceptor concentration of 1017 cm-3 or higher in order to realize a tunneling contact [2, 3]. So far various back contacting methods for p-CdTe have been employed, which are summarized in Table 1. Among these methods, a well-known way is to apply a solution etching process (typically nitric-phosphoric acid or NP etching) to generate a Te-rich surface, and then deposit a layer of Cu-containing buffer followed by a thermal activation process. Through this method, highly efficient CdTe solar cells have been reported [4]. However, the devices comprising Cu buffer usually suffer from instability problems because Cu tends to diffuse along the grain boundaries of polycrystalline CdTe and accumulate in CdS layer. One strategy to address the cell degradation issue is to investigate other Cu-free buffers in place of Cu-containing buffers. In our CdTe research, we have utilized transition metal oxides (TMOs) especially molybdenum oxide (MoOx) as a contact buffer to realize low resistance contact in CdTe solar cells [5-7]. The key is to achieve high work function with MoOx and other TMOs, i.e. 6~7eV, which is high enough
Data Loading...