CdTe PV Research Articles

Performance analysis of tungsten disulfide (WS2) as an alternative buffer layer for CdTe solar cell through numerical modeling

The cadmium telluride (CdTe) solar cells are the most widely used high efficiency thin film solar cells that have shown stable performance over a long time. Although being a frontier in thin film technology, the performance of CdTe solar cells is setback by pinhole effects when thin CdS buffer layers are used in devices for compensating the spectral loss. To overcome this, generally, a highly resistive ZnO is used as window layer between the transparent conducting oxide (TCO) and the CdS layer. In this work, a single tungsten disulfide (WS2) is proposed to replace the bilayer window in conventional CdTe solar cells. The band discontinuities at two interfaces of the WS2 in the proposed structure (p-CdTe/n-WS2/n-ITO) are studied with varying defect state density. The performance parameters such as open circuit voltage, short circuit current, fill factor, and efficiency are observed respectively for different defect state densities of interfaces and the bulk layer of WS2. Comparison between the conventional CdTe structure and proposed structure shows that the CdTe solar cells with a single WS2 buffer layer can perform as efficiently as the one with CdS/ZnO bilayer.

Study of the second-generation of CdTe and CIGS thin film PV modules under natural sunlight conditions

There is a significant amount of research in the literature concerning the performance of solar panels operating during outdoor exposure, but the full topic is not yet exhausted. One of the reasons is that the photovoltaic cell technology is constantly evolving. In this paper, a comparison of two types of CdTe and CIGS modules operated with a nominal power of 80 W and 140 W, respectively is studied. The module tests were performed under external conditions during autumn, winter, spring, and summer from October 2019 to July 2020 in the temperate climate of Miękinia, South Poland. The photovoltaic panels were connected to the electric grid via microinverters. During the tests, the temperature of the panels was monitored. To determine the influence of solar radiation on the energy conversion efficiency of photovoltaic panels, a pyranometer installed in the plane of the panels was used. Based on the monitoring of the atmospheric conditions and the measurement of instantaneous power, the efficiency of the modules is determined.

Understanding the Copassivation Effect of Cl and Se for CdTe Grain Boundaries.

Chlorine passivation treatment of cadmium telluride (CdTe) solar cells improves device performance by assisting electron-hole carrier separation at CdTe grain boundaries. Further improvement in device efficiency is observed after alloying the CdTe absorber layer with selenium. High-resolution secondary ion mass spectroscopy (NanoSIMS) imaging has been used to determine the distribution of selenium and chlorine at the CdTe grain boundaries in a selenium-graded CdTe device. Atomistic modeling based on density functional theory (DFT-1/2) further reveals that the presence of selenium and chlorine at an exemplar (110)/(100) CdTe grain boundary passivates critical acceptor defects and leads to n-type inversion at the grain boundary. The defect state analysis provides an explanation for the band-bending effects observed in the energy band alignment results, thereby elucidating mechanisms for high efficiencies observed in Se-alloyed and Cl-passivated CdTe solar cells.

Numerical Investigation of Graphene as a Back Surface Field Layer on the Performance of Cadmium Telluride Solar Cell.

This paper numerically explores the possibility of ultrathin layering and high efficiency of graphene as a back surface field (BSF) based on a CdTe solar cell by Personal computer one-dimensional (PC1D) simulation. CdTe solar cells have been characterized and studied by varying the carrier lifetime, doping concentration, thickness, and bandgap of the graphene layer. With simulation results, the highest short-circuit current (Isc = 2.09 A), power conversion efficiency (η = 15%), and quantum efficiency (QE~85%) were achieved at a carrier lifetime of 1 × 103 μs and a doping concentration of 1 × 1017 cm−3 of graphene as a BSF layer-based CdTe solar cell. The thickness of the graphene BSF layer (1 μm) was proven the ultrathin, optimal, and obtainable for the fabrication of high-performance CdTe solar cells, confirming the suitability of graphene material as a BSF. This simulation confirmed that a CdTe solar cell with the proposed graphene as the BSF layer might be highly efficient with optimized parameters for fabrication.

Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time‐Resolved Photoluminescence

Thin-Film Photovoltaics In article number 2000775, scientists at NREL and First Solar used charge carrier lifetime spectroscopy and microscopy to quantify grain boundary and back-contact recombination in cadmium telluride solar cells. About 7.5 GWpp CdTe PV will be produced in 2021, which will constitute about 50% of all PV installed in the U.S. in 2021. Results indicate pathways to further improve this leading PV technology.

Chlorine passivation of grain boundaries in cadmium telluride solar cells

Cadmium telluride is the most commercially important second generation thin-film photovoltaic, with a record solar cell conversion efficiency of 22.1%. However as-deposited cells are 5% efficient and require a cell activation treatment with ${\mathrm{CdCl}}_{2}$ at about 400 ${}^{\ensuremath{\circ}}\mathrm{C}$ to reach commercially viable efficiencies. Such a treatment is a routine process during CdTe module manufacturing. However, the precise mechanisms at work for this remarkable efficiency enhancement are not well understood. In this paper, atomistic modeling techniques are used to improve the fundamental understanding of the structural and electronic properties of CdTe by modeling the effects of chlorine and other elements with their interaction with extended defects and grain boundaries (GBs). Studies at high spatial resolution with nanoscale secondary ion mass spectrometry, transmission electron microscopy (TEM), and energy dispersive x-ray analysis show that chlorine atoms are concentrated at grain boundaries in CdTe after the ${\mathrm{CdCl}}_{2}$ treatment. Density functional theory calculations show that both ${\mathrm{Cl}}_{\mathrm{Te}}$ and ${\mathrm{Cl}}_{i}$ are stabilized at the grain boundaries compared to bulk CdTe. Similar defect formation energies of these defects suggest both will be present at the grain boundaries. As expected, four single-particle levels are present in the $\mathrm{\ensuremath{\Sigma}}3$ (112) GB band gap, which explains the low efficiencies prior to treatment. ${\mathrm{Cl}}_{\mathrm{Te}}$ substitutions passivate one of these levels and partially passivate another two. Remarkably, further addition of ${\mathrm{Cl}}_{i}$ fully passivates the remaining single-particle levels. This passivation of single-particle levels is most likely to be the primary cause of the efficiency enhancement on chlorine treatment. Further to this, alternative halogens were then trialed as activation treatments. All halogens show similar electronic effects and their defect formation energies follow ionic radii trends.

Comparison of different simplistic prediction models for forecasting PV power output: Assessment with experimental measurements

This paper tested the energy outputs of different types of PV modules and evaluated the accuracies of different simplistic PV module power prediction models. A test rig was developed in Hong Kong to assess the PV power outputs of ten PV modules. The solar radiation, ambient temperature and power generation are recorded. The evaluated models include simple efficiency model, temperature correction model and one-diode model. The results show that the mono-Si PV module is the highest in terms of the annual energy outputs per unit area, and a-Si PV modules is the lowest. The simple efficiency model overestimates the power output of all types of PV modules for more than 10% except for a CdTe PV module. The one-diode model demonstrated the highest accuracy for mono-Si and poly-Si PV modules. The accuracies of the evaluated models are low for thin-film PV technologies. Although the mean bias error of the one-diode model is larger than 10% for the thin-film PV modules (expect for one CdTe PV module), the one-diode model has the highest accuracy among the three models. Further studies should be conducted to investigate the energy performance of thin-film PV modules and then improve their prediction accuracy.

A simulation approach for investigating the performances of cadmium telluride solar cells using doping concentrations, carrier lifetimes, thickness of layers, and band gaps

This paper describes the simulation study for the optimization of high-performance cadmium telluride (CdTe) solar cells using different doping concentrations, carrier lifetimes, temperature, and thickness of layers of CdTe absorber and CdS window layers. In this simulation, the highest efficiencies of ~18% and ~18.29% achieved when the doping concentrations were 1.5 × 1017 cm−3 for absorber layer and 1 × 1015 cm−3 for window layer, respectively. The efficiency of the solar cell increases with increase in carrier lifetime and the highest efficiency of 18.26% achieved at carrier lifetime 100 μs with doping concentration of 1 × 1017 cm−3. Solar cell with the thickness of absorber layer 8 μm at carrier lifetime 100 μs attained the maximum efficiency of 19.18% whereas the efficiency of 18.33% was noticed in thickness of window layer 70 nm at 100 μs carrier lifetime. The optimum efficiency of 18.3% with short-circuit current 2.66 A and open-circuit voltage 0.79 V of solar cell has been achieved at operating temperature 25 °C. The optimized energy band gap of absorber (1.7 eV) accomplished the highest efficiency of 18.31%. The photogeneration rate increases logarithmically as distance from front increases, while the recombination rate increases linearly, which could be suitable for fabrication of efficient solar cell.

Optical modeling of periodic nanostructures in ultra-thin CdTe solar cells with an electron reflector layer

CdTe solar cells have become a competitive player in the commercial photovoltaic market competing with CIGS and Si wafers. Considerable research attention has been devoted to increasing the overall conversion efficiency of CdTe which reached 22%. Among the challenges facing this technology is the scarcity of Te. In this paper, we investigated the use of an ultra-thin CdTe layer, which would result in a substantial decrease in Te consumption by almost 90%. However, this substantial decrease in the absorber thickness must be accompanied by some embedded nano-light-trapping structures. The purpose of these nanostructures is to increase the light absorption of the UV-VIS and NIR wavelengths which leads to an increase in the generated photo-current. Also, an electron reflector layer must be added between the metallic contact and the thin absorber layer to mitigate the high recombination rate. Additionally, three different metallic contacts were explored: Au, Ag, and Al. The absorption in the CdTe layer which is responsible for the photo-generated current and the parasitic absorption in the metallic back contact, as well as the short circuit current densities, were determined and analyzed. The numerical Wave optics finite element method and the analytical transmission line theory were utilized. It was noted that these nanostructures showed a substantial increase in the absorption in the CdTe absorber layer in the visible and NIR ranges. A net increase in the short circuit current density of 36% has been realized with the top nano-structured CdTe layer comparing to the flat reference cell of the same absorber volume. Furthermore, the Ag metallic back-contact outperforms both Al and Au and achieved the highest short circuit current density and the lowest parasitic absorption. These findings would open a new route to integrate optimized light trapping nano-structures into the commercial CdTe solar cells to further enhance their material utilization and efficiency. • Ultrathin cadmium telluride CdTe solar cells with electron reflector layer. • Light-trapping-structures in the thin-film solar cells. • Optical modeling of the ultra-thin solar cells. • Materials utilization of ultra-thin cadmium telluride solar cells. • Transmission line theory model for the thin-film solar cells.

Thermal, electrical and purification performance of a novel thermal-catalytic CdTe double-layer breathing window in winter

CdTe PV cell is one perfect candidate for window application because of its good daylighting performance, low-temperature coefficient and excellent electrical performance under weak light. In addition, thermal catalytic oxidation is an thermal driven air purification technology and the thermal energy can be easily obtained from PV window systems. Therefore, a novel thermal-catalytic CdTe double-layer breathing window was proposed. Firstly, the electrical, thermal and daylighting performances of proposed window were investigated experimentally. Secondly, the removal of formaldehyde was investigated using the verified coupled formaldehyde mass transfer and thermal-catalytic reaction kinetic model. Finally, the evolution of indoor formaldehyde concentration considering formaldehyde release source was calculated. Main results were: (1) The thermal and electrical efficiencies were respectively 0.297 and 0.0715, which were closed to the reported results. Also, the proposed window had better daylighting uniformity compared with conventional window. (2) The total generated clean air was 25.7 m3/(m2•day), with the equivalent formaldehyde degradation efficiency of 0.010 based on purification performances of various air purifiers in market. (3) The comprehensive thermal efficiency was 0.518, with increase of 9.28% considering the contribution from formaldehyde removal. (4) The proposed window could make the indoor concentration approach to standard concentration under most working conditions.

Direct Microscopy Imaging of Nonuniform Carrier Transport in Polycrystalline Cadmium Telluride

Summary Inhomogeneous microscopic carrier transport is difficult to study, but important in many condensed-matter applications. For example, the role of grain boundaries (GBs) in polycrystalline semiconductors has been controversial for 20 years. In cadmium telluride (CdTe) solar cells, electron-beam-induced current (EBIC) measurements consistently demonstrate enhanced current collection along GBs, which is argued as evidence for interpenetrating CdTe p-n current-collection networks critical to high efficiency. Conversely, cathodoluminescence (CL) measurements consistently indicate that GBs are deleterious low-lifetime regions. Here, we apply transport imaging (TI) in conjunction with spatially correlated EBIC, CL, and scanning Kelvin probe force microscopy measurements to understand carrier drift, diffusion, and recombination in polycrystalline CdTe. We simultaneously observe GB potential wells, reduced carrier lifetime at GBs, and seemingly contradictory enhanced GB current collection, and then describe their coexistence with microscopic TI and physical arguments. The results provide visualization of inhomogeneous transport that is critical to understanding and engineering polycrystalline solar technology.

Characterizing the Back-Contact Interface for CdTe PV through HRSTEM, EELS, and XEDS

Characterizing the Back-Contact Interface for CdTe PV through HRSTEM, EELS, and XEDS

Inert gas bubble formation in magnetron sputtered thin-film CdTe solar cells.

Cadmium telluride (CdTe) solar cells are deposited in current production using evaporation-based tech- niques. Fabricating CdTe solar cells using magnetron sputtering would have the advantage of being more cost-efficient. Here, we show that such deposition results in the incorporation of the magnetron working gas Ar, within the films. Post deposition processing with CdCl2 improves cell efficiency and during which stacking faults are removed. The Ar then accumulates into clusters leading to the creation of voids and blisters on the surface. Using molecular dynamics, the penetration threshold energies are determined for both Ar and Xe, with CdTe in both zinc-blende and wurtzite phases. These calculations show that more Ar than Xe can penetrate into the growing film with most penetration across the (111) surface. The mechanisms and energy barriers for interstitial Ar and Xe diffusion in zinc-blende are determined. Barriers are reduced near existing clusters, increasing the probability of capture-based cluster growth. Barriers in wurtzite are higher with non-Arrhenius behaviour observed. This provides an explanation for the increase in the size of voids observed after stacking fault removal. Blister exfoliation was also modelled, showing the formation of shallow craters with a raised rim.

Application of multi-stage vacuum distillation for secondary resource recovery: potential recovery method of cadmium telluride photovoltaic waste

With the widespread use of cadmium telluride solar cells, environmental protection, and effective recycling approaches for solar cell component waste have become topics of concern. As an emerging metal smelting technology, vacuum metallurgy technology has broad application prospects in primary metal refining and the recycling of secondary non-ferrous scraps. Thus, this study proposes a new method for the separation and recovery of rare metals from cadmium telluride waste through sulfur smelting and vacuum distillation. In this process, sulfur is smelted with cadmium telluride to produce tellurium with a high vapor pressure. The tellurium was then separated and purified by multi-stage vacuum distillation. The results show that the tellurium purity can reach up to 99.97% and the tellurium recovery rate was above 99%. The cadmium and sulfur contents in the tellurium were 0.0061% and 0.0194%, respectively. Moreover, the cadmium sulfide produced during the smelting process can be enriched by vacuum distillation, resulting in 99.6% purity. This technology strongly aligns with the “reduce, reuse, and recycle” principle for solid waste, and provides a basis for the sustainable development of the photovoltaic industry.

Sputtered p-Type CuxZn1–xS Back Contact to CdTe Solar Cells

As thin film cadmium telluride (CdTe) solar cells gain prominence, one particular challenge is optimizing contacts and their interfaces to transfer charge without losses in efficiency. Back contact...

CuSCN as the Back Contact for Efficient ZMO/CdTe Solar Cells.

The replacement of traditional CdS with zinc magnesium oxide (ZMO) has been demonstrated as being helpful to boost power conversion efficiency of cadmium telluride (CdTe) solar cells to over 18%, due to the reduced interface recombination and parasitic light absorption by the buffer layer. However, due to the atmosphere sensitivity of ZMO film, the post treatments of ZMO/CdTe stacks, including CdCl2 treatment, back contact deposition, etc., which are critical for high-performance CdTe solar cells became crucial challenges. To realize the full potential of the ZMO buffer layer, plenty of investigations need to be accomplished. Here, copper thiocyanate (CuSCN) is demonstrated to be a suitable back-contact material with multi-advantages for ZMO/CdTe solar cells. Particularly, ammonium hydroxide as the solvent for CuSCN deposition shows no detrimental impact on the ZMO layer during the post heat treatment. The post annealing temperature as well as the thickness of CuSCN films are investigated. Finally, a champion power conversion efficiency of 16.7% is achieved with an open-circuit voltage of 0.857 V, a short-circuit current density of 26.2 mA/cm2, and a fill factor of 74.0%.

Characterization of sputtered MoOx thin films with different oxygen content and their application as back contact in CdTe solar cells

Transparent MoOx thin films with varied composition were successfully prepared by sputtering and the effects of oxygen contents on the composition stoichiometry, defect states, work functions, electrical and optical properties of MoOx films were systematically studied. These thin films were then utilized in cadmium telluride solar cells to act as back contact buffers. As compared with the CdTe solar cells with Au back contact, deteriorated performance was witnessed in those with MoOx buffers. The deterioration was demonstrated to be caused by the oxidation of CdTe surface, the large valence band offset and the insufficient work function. Finally, by optimizing the back contacts to form a CuCl/MoOx/ITO composite back contact, the efficiency was improved to 11.8%. This work demonstrates feasible approaches to prepare MoOx thin films with varied oxygen concentrations which could be easily applied to many other thin film solar cells and provide important indications in the preparation and optimization of the performance for bifacial devices.

Three-Dimensional Imaging of Selenium and Chlorine Distributions in Highly Efficient Selenium-Graded Cadmium Telluride Solar Cells

Thin-film solar modules based on cadmium telluride (CdTe) technology currently produce the world's lowest cost solar electricity. However, the best CdTe modules now contain a cadmium selenium telluride (CST) alloy at the front of the absorber layer. Despite this, research characterizing the behavior of selenium in alloyed CdTe devices is currently very limited. Here we employ advanced secondary ion mass spectrometry measurements to map the three-dimensional distribution of selenium in a graded CST/CdTe device for the first time. We find significant interdiffusion of selenium between the CST and CdTe layers in the cell, primarily out of the CST grain boundaries and up into the CdTe grain boundaries and grain fringes above. This results in significant lateral variations in selenium concentrations across grains and hence also lateral fields, which we estimate using the measured selenium concentrations.

Efficient materials for thin-film CdTe solar cell based on back surface field and distributed Bragg reflector

Modifications in the physical properties of photovoltaic materials lead to enhanced conversion efficiency of a solar cell. This paper focuses on the suitable material selection for the cadmium telluride (CdTe) solar cell with special emphasis on improving the electrical parameters such as open-circuit voltage Voc, short-circuit current density Jsc, fill factor FF, and efficiency ƞ. Simulation results showed that materials having wide bandgap are more appropriate for each layer in the proposed cell compared to small-bandgap materials. Moreover, the back metal contact, which acts as electrode and also used as light reflector, usually suffers from high intrinsic absorption losses. To reduce such losses, we replaced the metal contact by a highly reflective one-dimensional distributed Bragg reflector, which increases the optical path length in greater amount over a broad span of incident angles and frequencies, and subsequently enhances the performance of the cell. The measured electrical parameters of the proposed cell under global AM 1.5G conditions are JSC = 25.036 mA/cm2, Voc = 1.065 V, FF = 87.56%, and ƞ = 23.94%, respectively.

Impact of Back Surface Field (BSF) Layers in Cadmium Telluride (CdTe) Solar Cells from Numerical Calculation

In this study, numerical simulation has been executed using Solar Cell Capacitance Simulator (SCAPS-1D) to study the prospect of favourable efficiency and stable CdS/CdTe cell in various cell configurations. A basic structure of CdS/CdTe cell is studied with 4 μm CdTe as absorber layer, 100 nm tin oxide (SnO2) as front contact and 25 nm cadmium sulfides (CdS) as buffer layer. Four back surface fields (BSF) layers namely ZnTe, ZnTe: Cu, Cu2Te and MoTe2 are investigated to reduce the minority carrier recombination at back contact. The cell structure of glass/SnO2/CdS/CdTe/MoTe2 has shown a nearly ohmic contact with CdTe with the highest efficiency of 17.02% (Voc=0.91 V, Jsc=24.79 mA/cm2, FF=75.41). Simulation results have verified that MoTe2 as BSF layer is appropriate for an efficient CdS/CdTe cell. Moreover, it is found that a few nanometres (about 40 nm) of BSF layer are enough to achieve high efficiency. For MoTe2 layer, more than 17% efficiency has been achieved compared to other BSF layers.