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Resistive oxide materials play an important role in the front contact of CdTe solar cells. The highresistance transparent (HRT) or "buffer" layer has been used extensively in CdTe thin-film photovoltaics to enable a reduction in CdS thickness while maintaining near-maximum device voltage and fill factor. SnO 2and ZnO-based alloys were tested as HRT layers on a fluorine-doped tin oxide transparent conducting oxide. SnO 2-based alloy HRT layers were deposited via atmospheric pressure chemical vapor deposition (APCVD). Alloying ZnO with MgO to create Mg x Zn 1−x O (MZO) via radio-frequency sputter deposition was explored as a way to reduce the electron affinity of ZnO HRT layers. To fully understand the behavior of these materials, many devices were fabricated with either no CdS layer, a sublimated CdS layer, or a sputtered, oxygenated CdS layer. MZO layers resulted in high open-circuit voltage and device efficiency even with the complete elimination of the CdS layer. In both HRT systems, controlling electron affinity to optimize front contact band alignment is an important consideration. Band measurements using photoelectron spectroscopy and synchrotron techniques correlate band alignment measurements with efficiency parameters in the design of HRT and CdS layers.

A major source of loss in cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells results from light absorbed in the CdS window layer, which is not converted to electrical current. This film can be made more transparent by oxygen incorporation during sputter deposition at ambient temperature. Prior to this work, this material has not produced high-efficiency devices on tin oxide-coated soda-lime-glass substrates used industrially. Numerous devices were fabricated over a variety of process conditions to produce an optimized device. Although the material does not show a consistent increase in band gap with oxygenation, absorption in this layer can be virtually eliminated over the relevant spectrum, leading to an increase in short-circuit current. Meanwhile, fill factor is maintained, and open-circuit voltage increases relative to baseline devices with sublimated CdS. The trend of device parameters with oxygenation and thickness is consistent with an increasing conduction band offset at the window/CdTe interface. Optimization considering both initial efficiency and stability resulted in a National Renewable Energy Laboratory verified 15.2%-efficient cell on 3.2-mm soda-lime glass. This window material was shown to be compatible with SnO 2 -based transparent conducting oxide and high resistance transparent coated substrates using in-line compatible processes.

Sublimated thin film CdTe photovoltaic devices with conversion efficiencies over 18% and a fillfactor greater than 79% have been repeatedly obtained using high-rate fabrication processes on commercial soda-lime glass substrates used in CdTe modules. Four major improvements to the device have enabled an increase in efficiency from a baseline of approximately 12% to 18.7%: 1) A sputtered multilayer metal-oxide anti-reflection layer; 2) total replacement of the CdS window layer with a higher bandgap sputtered MgxZn1-xO (MZO) window layer; 3) deposition of the CdTe layer at a higher thickness and substrate temperature; and 4) an evaporated tellurium back-contact. This work describes the effect of these changes on the device performance and film microstructural characteristics using various methods. Multiple devices with comparable high efficiency have been fabricated and demonstrated using methods described in this study, yielding some of the highest efficiencies for CdTe polycrystalline thin-film photovoltaics. Thin film CdTe photovoltaics have consistently demonstrated the lowest cost solar electricity generation, particularly for utility scale applications. CdTe is a p-type absorber that has a bandgap of 1.5 eV which is nearly optimal for photovoltaic conversion. Approximately 2 µm is sufficient to absorb most of the visible solar spectrum. 1,2 CdTe films are typically deposited on glass substrates using low-cost hardware and high-rate deposition processes 3,4,5 reducing production costs. Typical crystalline silicon photovoltaics require wafers that are 150-200 microns thick and use a more complex and capital-intensive fabrication process. 3 The low-cost manufacturing of thin-film CdTe PV has enabled agreement for a record low cost power purchase agreement of ¢3.8/kWh for a 100 MW field, 6 which is significantly lower than the average cost of electricity in the U.S. of ¢11/kWh. 7 With recent improvements, research-scale small devices have record efficiencies of 22.1%, 8 while modules with up to 18.6% 9 efficiency have been produced. The leading CdTe PV manufacturer, First Solar Inc., has increased average production module efficiency from 13.5% in the first quarter of 2014 10 to 16.7% in the first quarter of 2017. 11 Further improving the efficiency without substantial increase in production cost will reduce the levelized cost of energy for CdTe photovoltaics. 12,13 Maintaining the dual requirement of high efficiency and low cost requires the use of film deposition techniques suitable for mass production of millions of solar modules per year. The vapor deposition methods used for this study, including sublimation, evaporation, and sputter deposition, have been used in large scale manufacturing for solar and other industries. Commercially available 3.2-mm soda-lime glass with a fluorine-doped tin-oxide (FTO) transparent conducting layer is a standard substrate for thin-film PV manufacturing, including for CdTe, due to its sufficient strength, reliability, and low cost. Using processes suitable for large scale manuf...

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Nanocrystal (NC) Cu 2 ZnSnS 4 (CZTS) solar cells, composed of a nontoxic and earth abundant absorber material, have great potential in low-cost solar energy harvesting. However, CZTS NC films typically must be thermally annealed at elevated temperatures and under harsh environments to produce high-efficiency devices. The efficiencies of unannealed CZTS NC solar cells have been hampered by low open circuit potentials (V oc , <325 mV) and low short circuit current densities (J sc , <2 mA), primarily because of the incomplete passivation of the crystal surface. Although great progress has been made in understanding the surface chemistry of II−VI and IV−VI semiconductor NCs, the surface chemistry of complex quaternary CZTS NCs is largely unexplored. Here, for the first time, we report a comprehensive study of the surface chemistry of CZTS NCs focusing on depositing ligand-passivated, uniform NC thin films to address the issue of large V oc deficit and low current collection efficiency typically observed for CZTS NC solar cells. The ligand exchange reactions were rationally designed to target each metal ion on the surface [using both organic L-type ligands such as ethylenediamine and inorganic X-type ligands (I − and S 2− )] and to passivate anionic chalcogen sites with inorganic Z-type ligands (ZnCl 2 ). Herein, we show that CZTS/CdS heterojunction NC solar cells made of uniformly passivated CZTS NCs demonstrate a >180 mV improvement in V oc . Furthermore, the influence of device configuration on the collection efficiency of photogenerated carriers in the CZTS NC absorber layer is presented, and the implications of both surface and internal defects in CZTS NCs for photovoltaic performance are discussed.

On the basis of a combination of X-ray photoelectron spectroscopy and synchrotron-based X-ray emission spectroscopy, we present a detailed characterization of the chemical structure of CdS:O thin films that can be employed as a substitute for CdS layers in thin-film solar cells. It is possible to analyze the local chemical environment of the probed elements, in particular sulfur, hence allowing insights into the species-specific composition of the films and their surfaces. A detailed quantification of the observed sulfur environments (i.e., sulfide, sulfate, and an intermediate oxide) as a function of oxygen content is presented, allowing a deliberate optimization of CdS:O thin films for their use as alternative buffer layers in thin-film photovoltaic devices.