Thin-Film Photovoltaic Devices with Asymmetric Heterocontact Geometries

Abstract

Silicon (Si) is the material of choice for the photovoltaic (PV) industry since the beginning (1954). Precisely, in most of the PV industries, Si solar cells (Si-SCs) are dominating. Si-SCs require nearly 10–15 kg of Si to produce per kilowatt-peak, while the traditional SCs use 200–300 µm thick Si layer for efficient photon absorption. So, highly purified raw materials are required for manufacturing Si-SC. Therefore, the cost of electricity produced by Si-PV solar cells cannot be much lower than that from fossil fuels. Comparing the active layer thickness of conventional Si-SCs with thin-film solar cells (TF-SCs), TF-SCs require a few µm, while the conventional Si-SCs require 200–300 µm. Hence, the electricity produced by TF-SCs might be cost-effective due to less material consumption. Additionally, TF-SCs are benefited by simplified processing steps 216and monolithic integration. Among TF-SC materials, (i) copper zinc tin selenide (CZTS) and copper indium gallium selenide (CIGS) have complex structures with high defect densities that limit their cost-competitiveness, efficiencies, and industrial uptake for decades; (ii) Perovskite suffers from various intrinsic instabilities (i.e., iodine diffusion) and degrade quickly in the presence of heat, UV light, and moisture; (iii) GaAs-based III-V single and multi-junction cells are too expensive to be deployed in large areas. On the other hand, cadmium telluride (CdTe) TF-SC panel technology has outshined multicrystalline Si solar panels in terms of cost-efficiency. Although CdTe has a large and almost ideal bandgap (1.5 eV) that can utilize its energy at shorter wavelengths more efficiently compared to Si (1.1 eV). But the performance of CdTe solar cells is limited due to traditional use of cadmium sulfide (CdS) as a window layer that causes the parasitic absorption and recombination loss at CdS/CdTe junction along with the lack of an electron blocking layer at the traditional CdTe/metal back contact (e.g., Au) interface. While the parasitic absorption in CdS reduces the short-circuit current (JSC ) and the poor band alignment of CdS and Au with CdTe reduces the open-circuit voltage (VOC ), the fill factor (FF), and ultimately the efficiency. For such reasons, top priority is given to the solid research for having a cost-effective, easy-to-fabricate, robust, and scalable methods to improve the performance of CdTe PV technology. The large bandgap (i.e., low parasitic absorptive) transition metal oxide (TMO) dopant-free carrier transport layers have emerged as an alternative contact material that can be deposited at low temperatures to improve the overall performance and efficiency of CdTe solar cells. As it allows only one kind of carrier and blocking the another, thus, the probability of recombination decreases. As a result, there is a significant increase in VOC, thus, increasing the overall efficiency of the solar cell as well. Therefore, this chapter will be focusing mainly on exploring the paths for obtaining high VOC , which will perhaps yield high efficiency for TF-PV technology.