Abstract
One of the most promising solar cell devices is cadmium telluride (CdTe) based. These cells however, have their own problems of stability and degradation in efficiency. Measurements show that CdS/CdTe solar cell has high series resistance which degrades the performance of solar cell energy conversion. Both active layers (CdS and CdTe) had been fabricated by thermal evaporation and tested individually. It was found that CdS window layer of 300 nm have the lowest series resistance with maximum light absorption. While 5 - 7 μm CdTe absorber layer absorbed more than 90% of the incident light with minimum series resistance. A complete CdS/CdTe solar cell was fabricated and tested. It was found that deposited cell without heat treatment shows that the short circuit current increment decreases as the light intensity increases. This type of deposited cell has low conversion efficiency. The energy conversion efficiency was improved by heat treatment, depositing heavily doped layer at the back of the cell and minimizing the contact resistivity by depositing material with resistivity less than 1 m??cm2. All these modifications were not enough because the back contact is non-ohmic. Tunnel diode of CdTe (p++)/CdS (n++) was deposited in the back of the cell. The energy conversion efficiency was improved by more than 7%.
Highlights
Silicon-based solar cells are currently the most successful commercial photovoltaic product
Measurements show that CaCadmium sulfide (CdS)/cadmium telluride (CdTe) solar cell has high series resistance which degrades the performance of solar cell energy conversion
While 5 - 7 μm CdTe absorber layer absorbed more than 90% of the incident light with minimum series resistance
Summary
Silicon-based solar cells are currently the most successful commercial photovoltaic product. Since the record efficiency of such type solar cells is considerably lower than the theoretical limit of 28% - 30% [3], the performance of the modules can be improved, through new advances in fundamental material science and engineering, and device processing. The small thickness required for an absorbing layer makes the cost of material for the solar cells relatively low. CdS has been the subject of intensive research because of its intermediate band gap (Eg ≈ 2.42 eV) making the material suitable as window material for a heterojunction solar cell [5], high absorption coefficient, reasonable conversion efficiency, stability and low cost [6]. Knowledge of the optical properties of CdS films is very important in the field of optoelectronic devices like photo-detectors and solar cells. Thinner layers at the top and thicker ones at the bottom managed to increase the open circuit voltage and improve the spectral response
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