Development of High Efficiency Flexible CdTe Solar Cells

Polycrystalline thin film solar cells of II-VI compound semiconductors are important because of their low cost, high efficiency and stable performance. Flexible and lightweight solar cells are interesting for a variety of terrestrial and space applications that require a very high specific power (ratio of output electrical power to the solar module weight). Moreover, light modules are advantageous in terms of transport and mounting. We have previously described the development of flexible CdTe/CdS solar cells on polyimide substrates with a novel method; efficiencies of 8.6 % were reported. Further improvements in the processing have now increased the solar cell efficiency to 11%. The CdTe solar cell is fabricated in a \u201csuperstrate\u201d configuration where the light passes through the polyimide substrate. Absorption of the incident light and possible degradation of polyimide under UV and particle irradiation are the major limitations of this configuration. To overcome these a new approach is introduced here. The CdTe/CdS/TCO stacks are grown on a NaCl/glass substrate. After the solar cell has been processed, a polymer layer is spin coated on top and the flexible solar cells are detached from the glass by dissolving the NaCl buffer layer. Solar cells of 7.3% efficiency have been obtained

Lock-in EBIC studies of CdS/CdTe solar cells grown by high vacuum evaporation

The use of an electostatically blanked electron beam has allowed the application of phase sensitive detection to the electron beam-induced current technique. This has allowed the recovery of smaller induced signals, even from films of low mechanical integrity following substrate removal. This technique has been applied to CdTdCdS solar cells deposited by the high vacuum evaporation technique. EBlC images of untreated and chloride treated devices suggest that this treatment produces higher efficiency regions near CdTe grain boundaries than In the grain interiors. This is explained by the collecting junction being closer to the CdStCdTe interface in the near boundary regions. compared with a more deeply buried junction within the grains. This conclusion is supported by beam voltage dependent EBIC measurements, which show unexpectedly efficient collection deep in the absorber layer. Comparison with previous work implies that this variation in junction position is due to higher doping near the grain boundaries

Effect of back contact metallization on the material stability of CdTe/CdS solar cells

The electrical contact on CdTe is important for the high efficiency and long term stability of CdTe/CdS solar cells. As most of the metals form a Schottky barrier on p-type CdTe, a buffer layer is needed to reduce the effective barrier height and enhance the acceptor concentration at the back contact. In order to obtain stable but highly efficient cells we investigated different back contact materials and etching procedures. Accelerated life time tests revealed the stability of the cells. CdTe/CdS cells with Al or Au metallization showed degradation, whereas Mo metallization produced very stable cells with Sb2Te3 and Sb buffer layers. These cells have up to 10 % efficiency after 9 months accelerated life time tests which corresponds to 75 years under normal conditions. The barrier height and the doping profile have been measured with C-V characteristics

CdTe/CdS Solar Cell Performance under Low Irradiance

Solar cells are usually characterised under standard test conditions (STC), which do not resemble realistic operation conditions, especially for indoor applications where the cells operate at much lower irradiances than for outdoor applications. The potential of CdTe/CdS solar cells developed in our laboratory for low irradiance applications typical of indoor use is investigated and compared to conventional cell materials like c-Si and GaAs. Parameters of the IV-characteristics are normalized to the STC values for comparative analysis. The CdTe/CdS cells show a superior relative efficiency and voltage at low intensities than the c-Si and GaAs cells. The CdTe/CdS solar cells with STC efficiencies of around 11 % retain around 8 % efficiency at 1W/m2 and the Voc remains as high as 600 mV under the same conditions. Together with a low cost production the suitability of these CdTe/CdS cells for indoor application is preeminent, especially for applications requiring high voltages. This study gives an insight on the irradiance dependent performance of different solar cell materials, as well as explanations of the observed dependencies by analysing the underlying transport phenomena. Therefore irradiance was varied over almost 5 orders of magnitude

Radiation Hardness of CdTe/CdS solar cells

The performance stability of CdTe/CdS thin film solar cells against radiation damage caused by protons and electrons has been investigated. High vacuum evaporated superstrate configuration cells of 10 - 12.5 % were developed on soda-lime glass.The irradiation experiments were performed under extremely damaging conditions. The subsequent characterisation confirms that CdTe/CdS solar cells are highly suitable for space application. A minor degradation is measured at high fluences for low energy protons of 650 keV and fluences of some 1016cm-2 for 3 MeV electrons

High Efficiency Flexible CdTe Solar Cells on Polymer Substrates

Development of flexible and lightweight solar cells is interesting for terrestrial and space applications that require a very high specific power (kW/kg) and flexibility for curved shaping or rolling. Flexible CdTe/CdS solar cells of 11% efficiency in superstrate and 7.3% efficiency in substrate configurations have been developed with a \u201clift-off\u201d approach. However, roll-to-roll manufacturing is desired in future. Therefore, flexible superstrate solar cells were directly grown on commercially available 3c10 \u3bcm thin polyimide (Upilex\u2122) foils. A process for the deposition of ITO (front contact) has been developed to have a stable front contact on the Upilex\u2122 foil. Post deposition annealing treatments of the ITO/polyimide stacks bring a significant stability to the front contact, having almost the same sheet resistance at the beginning and at the end of the cell fabrication process. Solar cells with AM1.5 efficiency of 11.4% on Upilex\u2122 foils (highest efficiency recorded for flexible CdTe cell) have been developed. A comparison of the cells prepared on different polyimides is presented

Stability Aspects in CdTe/CdS Solar Cells

The stability of CdTe/CdS solar cells depends on spatial changes of defects and impurities throughout the cell. Degradation effects are often associated with metal diffusion from the back contact of the cell, which is Cu in most cases. However, cells with stable back contact can also exhibit instability, as also all the other cell layers are potential sources of impurities causing instability. Cell degradation due to generation of defects from external influences like particle irradiation, e.g. in space, is another reason for instability. The development of stable back contacts as well as sources of instability in the cell performance are discussed with a special focus on the US layer and CdTe/CdS interface, which are very sensitive to the accumulation of impurities and defects. A non-destructive method to assess the stability issue is described. An analysis of performance stability with respect to defect generation caused by high-energy protons and electrons is presented. Additionally, effects of meta-stability and the capability to recover from degradation by defect relaxation are shown