Properties of Arsenic–Doped ZnTe Thin Films as a Back Contact for CdTe Solar Cells

Abstract

As-doped polycrystalline ZnTe layers grown by metalorganic chemical vapor deposition (MOCVD) have been investigated as a back contact for CdTe solar cells. While undoped ZnTe films were essentially insulating, the doped layers showed significant rise in conductivity with increasing As concentration. High p-type carrier densities up 4.5 × 1018 cm−3 was measured by the Hall-effect in heavily doped ZnTe:As films, displaying electrical properties comparable to epitaxial ZnTe single crystalline thin films in the literature. Device incorporation with as-deposited ZnTe:As yielded lower photovoltaic (PV) performance compared to reference devices, due to losses in the open-circuit potential (VOC) and fill factor (FF) related to reducing p-type doping density (NA) in the absorber layer. Some minor recovery observed in absorber doping following a Cl-free post–ZnTe:As deposition anneal in hydrogen at 420 °C contributed to a slight improvement in VOC and NA, highlighting the significance of back contact activation. A mild CdCl2 activation process on the ZnTe:As back contact layer via a sacrificial CdS cap layer has been assessed to suppress Zn losses, which occur in the case of standard CdCl2 anneal treatments (CHT) via formation of volatile ZnCl2. The CdS sacrificial cap was effective in minimising the Zn loss. Compared to untreated and non-capped, mild CHT processed ZnTe:As back contacted devices, mild CHT with a CdS barrier showed the highest recovery in absorber doping and an ~10 mV gain in VOC, with the best cell efficiency approaching the baseline devices.

An overview on the role of ZnTe as an efficient interface in CdTe thin film solar cells: a review

“…216 Arsenic-doped ZnTe layers demonstrated higher conductivity compared to the undoped ZnTe, where on applying a heavily doped p-type 100 nm ZnTe:As back contact with the device architecture of glass/ITO/Cd 1Àx Zn x S/CdTe-CdCl 2 /ZnTe:As/CdS/ CdCl 2 /Au and annealed stack up to ZnTe:As at 420 1C in a hydrogen atmosphere, a significantly enhanced V oc and PCE 11.9% were achieved. 217 To reduce the back contact barrier, ZnTe:Cu was applied with the device architecture of glass/FTO/ SnO 2 /n-CdS/p-CdTe/ZnTe:Cu/Au, and it was revealed that the reaction at the CdTe/ZnTe:Cu interface during the heat treatment process could modify the interfacial band alignment, which reduced the defect-associated recombination and supported the transport of holes, resulting in a PCE of 15.8%. 218 The deposition time and temperature to the ZnTe back contact alter the performance, and accordingly a device was developed with the configuration of glass/FTO/CdS/CdTe/ZnTe/Cu/Au, as depicted in Fig.…”

Back contacts materials used in thin film CdTe solar cells—A review

“…The high bandgap means that ZnTe is not suitable for use as a single absorber in a high PCE solar cell. ZnTe can be doped p-type by pnictides (N, 69 P, 70 As, 71 Sb, 72 and Bi) and by copper. The electron affinity of ZnTe is such that the valence band offset to CdTe is very low.…”

“…It is clear from Table 3 that in order to obtain high PCE either the back contact process must contain Cu or the CdTe is already heavily p-type doped (Oklobia et al (2019) 71 used arsenic doping).…”

Optical, Electrical Properties and Surface Morphology of Thermal Evaporated Zinc Telluride (ZnTe) Thin Films for Photovoltaic Applications

“…It is a p-type semiconductor with a direct bandgap energy of 2.21 to 2.26 eV at room temperature with a low electron affinity of 3.53 eV. [13] Different methods of deposition have been employed by different researchers to synthesize ZnTe thin films: metal-organic chemical vapour deposition (MOCVD) [14], spray pyrolysis [15], electrodeposition [16], thermal [17]. Due to the peculiarity of the thermal evaporation technique capable of forming multiple layers on a single substrate, making it easy to monitor the deposition rate and thickness of the film with a quartz crystal thickness monitor equipped with a vacuum coating unit, there was no wastage of materials and the time-saving technique was necessitated for this study.…”

“…An investigation into the structural and optical properties of nanocolumnar ZnTe thin films grown using the glancing angle technique was conducted [12] and determined that the band gap energy of the samples was modified at higher deposition angles, which is promising to be applicable in optoelectronic devices. Also, the properties of arsenic-doped ZnTe thin films as a back contact for CdTe solar cells were studied by [14],…”