Electrodeposition of semiconductors for applications in thin film solar cells

Electrodeposition was used to deposit thin film semiconductor materials for use in solar cell devices. Copper indium diselenide (CuInSe₂) was deposited from ethylene glycol at 150°C with the aim of improved material properties due to the elevated temperature. The broad nature of the X-ray diffraction (XRD) peaks before and after annealing indicated the layers were comprised of multiple phases identified as CuInSe₂ and Cu-Se binaries. Insufficient indium inclusion for device quality materials was incorporated into the layers over the explored growth range of -0.800 to -1.000 V vs Se reference electrode. The layers deposited at more positive deposition voltages were metallic and contained mainly Cu-Se binary phases. At more negative deposition voltages the formation of CuInSe₂ was confirmed although above -1.000 V vs Se the layers were often powdery and disintegrated on removal from the electrolyte. There were no noticeable improvements in the CuInSe₂ layers deposited from ethylene glycol compared to reports from aqueous media, which is less toxic and lower cost, therefore electrodeposition from aqueous solution is preferable.</p> <p>Undoped zinc oxide (ZnO) and aluminium doped ZnO (ZnO: Al) were deposited from zinc nitrate solutions with the aim of using electrodeposition for the ZnO bilayer in CuInSe₂ devices to unify the production process. ZnO was deposited at a range of deposition voltages from -0.900 to -1.050 V vs silver/silver chloride reference electrode as identified using XRD. Various morphologies were observed using scanning electron microscopy (SEM) and the electrical resistivity was determined at 6.9x 10⁶ Ω cm and decreased to 3.4x10⁵ Ω cm after Al doping. To make this method suitable for commercialisation more work would need to be carried out to address consistency issues mainly regarding the electrolyte conditions, including pH and oxygen concentration as a function of growth time.</p> <p>A comparison was made between electrodeposited and sputtered ZnO and ZnO: AI. Some differences in the material properties were found; all layers were identified as hexagonal wurtzite ZnO. A considerable change in morphology was observed by SEM between the electrodeposited and sputtered materials. Little change in the electrical resistivity was observed between electrodeposited and sputtered undoped ZnO, having 6.9x 10⁶ and 6.2x 10⁷ Ω cm. The electrical resistivity of ZnO: AI was 3.4 x l 0⁵ and 2.3 x 10⁵ Ω cm for electrodeposited and sputtered materials respectively. Further work would need to be carried out to quantify the concentration of Al dopant in the electrodeposition solution as a function of growth time if this method were to be used for commercialisation. Cadmium telluride (CdTe) was electrodeposited from aqueous solution onto glass/fluorine doped tin oxide/cadmium sulphide substrates. Little improvement in XRD spectra was observed for annealed layers compared to the as-deposited material and the CdTe was identified exhibited cubic phase having (111) preferential orientation. Working solar cell devices were fabricated over a range of growth voltages with superior performance being observed for materials deposited between -0.620 to -0.650 V vs saturated calomel electrode (SCE). Furthermore high uniformity over a2 cm² area completed with an array of 2 mm diameter contacts was observed for devices deposited in this growth voltage range. All devices fabricated using CdTe grown at -0.610 to -0.690 V vs SCE indicated photovoltaic activity although layers deposited between -0.630 and -0.650 V vs SCE indicated the highest performance, with device parameters of open circuit voltage = 420-540 mV, short circuit current density = 3.2-19.1 mA cm^-2 and fill factor = 0.48.