Abstract
A new approach to back contacting CdTe solar cells that uses an organic poly(3-hexythiophene-2,5-diyl) (P3HT) back contact layer is reported. The most striking benefit of P3HT was demonstrated to be through a “pinhole blocking” effect, significantly improving performance uniformity. This was demonstrated through comparison of open circuit voltage values for a large sample set (600 cells) and through measurement of a device with a graded absorber layer thickness (0.7–1.9µm). The conversion efficiency achievable and the electrical barrier height of the contacts to the CdTe were also investigated for P3HT/Au and Au control contacts – both being tested with and without additional Cu. Temperature dependent JV measurement showed the use of P3HT reduced the barrier to (0.29–0.33eV) from the value achievable with Au (0.39–0.42eV), but inclusion of Cu into either of the structures gave the lowest barriers (0.21–0.22eV). For the data sets recorded, P3HT/Au yielded higher peak efficiencies than the Au control contact. However, when Cu was included the peak performance of devices having P3HT/Cu/Au and Cu/Au contacts were comparable at 14.7% respectively but the P3HT/Cu/Au contact displayed a significantly higher average performance through increased uniformity of the device response.
Highlights
Establishing an Ohmic back contact for CdTe thin film solar cells is a fundamental issue of the material and is one of the primary process challenges of cell fabrication
This paper reports a different approach wherein P3HT is directly deposited from solution via spin coating and unlike the electrochemical techniques is able to act as a dual back contact and pinhole blocking layer
It was found that the usual variables in spin coating affected the quality of the deposit, including the smoothness of the CdTe – sputtered CdTe is considerably smoother than CSSgrown CdTe
Summary
Establishing an Ohmic back contact for CdTe thin film solar cells is a fundamental issue of the material and is one of the primary process challenges of cell fabrication. Cell performance becomes compromised by a reduced fill factor (FF) [3] To account for this issue the typical solution is to incorporate Cu into the CdTe back surface via the formation a CuTe layer [4], that acts to dope the surface highly ptype. Due to variations in the grain structure, arising from factors such as the choice of deposition conditions, contamination on the surface prior to deposition or scratches during post deposition processing, either partial or full physical shunting pathways may be formed in the CdTe layer which degrade device performance [16] These may take the form of a complete pinhole in which case the metallic back contact completely penetrates the CdTe layer and the cell short circuits. They have improved performance uniformity over that of Cu/Au contacted devices while maintaining comparable peak efficiencies
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