Commercial Production of the Group-V Doped Polycrystalline CdTe Source Material for the Next-Generation Photovoltaics

THE SunShot Initiative by the DOE has proposed a 2030 goal of achieving 3¢/kWhr for solar electricity [1] . The CdTe photovoltaic (PV) module performance still has room to improve and can help the DOE in achieving its SunShot initiative. Improving the p-type doping and carrier lifetime in polycrystalline CdTe absorber material is critical to achieving PV cell efficiency of 25%. CdTe modules are being manufactured now for approximately 40¢/W having efficiencies of >18%. As a result, multicrystalline Si (mc-Si) and CdTe PVs are now about even in cost and efficiency. CdTe PVs efficiency, however, can be pushed further where it can outperform mc-Si. The current industrial supply chain consists of CdTe feedstock that undergoes CdCl 2 and Cu treatments to help improve the material properties in effort to achieve a stable open circuit voltage (V oc ), typically in the 850-mV range. Group-V (Gp-V) doping has advantages over the current stateof-the-art where, unlike the typical Cu treatment, Gp-V doping has been shown to increase the hole density in CdTe and increase the carrier lifetime, which can improve the V oc to >850 mV, and thereby improve the PV efficiency. The CdTe feedstock is typically synthesized in vacuum-sealed quartz ampoules by conventional Bridgman and Traveler Heater Method (THM) systems. The very high heat of formation of CdTe, ∆H f = -100 kJ/mol, makes it difficult to compound in sealed quartz ampoules. Ampoules cannot be scaled beyond 3.0" – 4.0" diameter, because at greater ingot diameters the risk of ampoule failure becomes much greater. Furthermore, the addition of Gp-V elements often poses further challenges, as some have low sublimation temperatures and high vapor pressures. At these elevated vapor pressures, sealed quartz ampoules have a high probability of rupture during the synthesis process and constitute a safety hazard for synthesis. Considering these challenges, high-pressure synthesis in an open crucible is the most promising technique for producing these materials – both undoped CdTe and Gp-V doped CdTe. The high-pressure Bridgman (HPB) process uses thick-wall (~3 mm) high-purity graphite crucibles and high pressures (~80 – 100 atmospheres) of inert gas (typically argon) to perform CdTe synthesis. HPB can easily be scaled to produce 5 – 10 times larger crystal boules than obtainable using conventional Bridgman and THM techniques and thereby can radically reduce the CdTe feedstock cost. The HPB technique is a high-output and high-yield technique and is the only technique that can be used to produce group-V doped CdTe from elemental materials. The commercialization of the HPB process for production of CdTe feedstock materials will be significant to the thin film PV industry.