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An atomic-scale perspective of energy band alignment in CdTe surfaces has not been spatially studied despite the major role surfaces play in forming interfaces within CdTe-based thin film photovoltaic devices. Atomistic modeling based on density functional theory coupled with surface Green’s function is used for calculating energy band alignment of CdTe surfaces. The CdTe(1 0 0) ((1 × 1) and c(2 × 2) reconstruction) and CdTe(1 1 1) ((1 × 1) and (2 × 2) reconstruction) facets without and with surface relaxation provide insightful band bending characteristics that influence charge carrier transport. Results show that unrelaxed (1 × 1) CdTe(1 0 0) and CdTe(1 1 1) surfaces bend the valence band downward with surface polarity dictating the surface potential magnitude. The reconstructed CdTe(1 0 0) c(2 × 2) and CdTe(1 1 1) (2 × 2) surfaces result in favorable surface electronic features in relation to their unreconstructed variants. In addition, the structurally relaxed CdTe(1 1 1) surfaces develop an internal energy cusp potential that may enhance hole charge transport toward the back of CdTe solar cell devices. Energy band alignments calculated within the study lead to a detailed understanding of how CdTe surfaces may affect CdTe-based thin film photovoltaic applications.

This paper investigates the suitability of CdTe photovoltaic cells to be used as power sources for wireless sensors located in buildings. We fabricate and test a CdTe photovoltaic cell with a transparent conducting oxide front contact that provides for high photocurrents and low series resistance at low light intensities and measures the photovoltaic response of this cell across five orders of magnitude of AM1.5G light intensity. Efficiencies of 10% and 17.1% are measured under ~1 W m-2 AM1.5G and LED irradiance respectively, the highest values for a CdTe device under ambient lighting measured to date. We use our results to assess the potential of CdTe for internet of things devices from an optoelectronic, as well as a techno-economic perspective, considering its established manufacturing know-how, potential for low-cost, proven long-term stability and issues around the use of cadmium.

Magnesium zinc oxide (MZO) is a promising front contact material for CdTe solar cells. Due to its higher band gap than traditional CdS, MZO can reduce parasitic absorption to significantly increase short-circuit current density while also providing a benefit of conduction band offset tuning through Mg:Zn ratio optimization. MZO has been successfully implemented into CdTe devices, however its stability has been of concern. The MZO stability issue has been attributed to the presence of oxygen in the CdTe device processing ambient, leading to double-diode behavior (S-kink) in the current density-voltage curves. Here we report on MZO thin films deposited by reactive co-sputtering. The reactively co-sputtered MZO thin films have encouraging stability, show no significant variation in work function of the surface over a period of 6 months, as measured by Kelvin probe. Energy conversion efficiencies of around 16% have been achieved both with and without presence of oxygen in device processing ambients across multiple research facilities. These efficiencies should be possible to increase further by tuning of the thin film deposition and device processing parameters, especially through optimization of the back contact.

Increasing doping density in CdTe absorbers remains one of the most promising avenues for increasing the open-circuit voltage $\mathrm{(V_{OC})}$ of photovoltaic devices. This work aimed to develop methods for quickly incorporating group V as a p-type dopant using Metal-Organic Chemical Vapor Deposition (MOCVD) precursor injection. CdTe:N was deposited by MOCVD injection during sublimation of CdTe. CdTe:As was deposited via MOVCD onto an existing sublimated CdSeTe layer to produce doped CdSeTe/CdTe devices. Nitrogen doping resulted in increased external quantum efficiency and an improved diode curve as compared to a device exposed to cadmium overpressure without the presence of MOVCD nitrogen.

The main obstacle to realizing the use of polycrystalline wide bandgap alloys of CdTe with Mg and Zn in PV is the CdCl 2 processing step. This step, essential to CdTe device performance, removes Zn and Mg from the alloy films while producing less of the benefits seen from the process with CdTe. In this study, the use of Al 2 O 3 CdS, and MgZnO films at the free surface of the alloys to prevent loss of Mg or Zn is investigated. It is found that Al 2 O 3 is the most effective at reducing loss of Mg and Zn, followed by CdS and then MgZnO. A new method of evaluating the transition to CdTe from the as deposited alloy is proposed to account for differences in transition behaviors.

CdTe photovoltaic devices with efficiency over 22% have been demonstrated. Sublimated CdTe photovoltaics with efficiency over 19% have been reported using graded alloying of Se in CdTe absorber films. Grading of alloy films has been identified as an important characteristic to achieve higher device performance using more complex device structures. An advanced co-sublimation source has been designed and developed to deposit highly controlled CdTe based ternary alloys. An advanced shutter mechanism enables changing the composition of the deposited films during sublimation. The hardware used for advanced co-sublimation and initial materials characterization is presented in this study.

A computational study on band alignment of the cadmium telluride (CdTe) absorber layer and tellurium (Te) backcontact interface was performed using a first-principles approach. Inclusion of a Te back-contact on the CdTe layer establishes beneficial diode characteristics in polycrystalline CdTe-based photovoltaic devices. Density Functional Theory coupled with Nonequilibrium Green’s Function was used to identify the mechanisms in Te responsible for enhancing CdTe device performance. The computational model reveals several factors that explain why Te significantly improves CdTe thin-film solar cells that include increased hole carrier concentration at the back of CdTe and mitigation of recombination at the interface.

A recent detailed analysis was performed by DOE and NREL [1] that identified avenues needed to lower solar LCOE. Two key areas are identified: reduce module costs and improved module reliably and lifetime. The NREL/DOE study found that for both crystalline silicon (c-Si) and thin film PV, the encapsulation and module assembly costs were the largest single cost category for manufacturing. The capital equipment (cap-ex) costs for the module assembly are also shown to be high. Current module designs have opportunities for reliably improvements. Studies of fielded modules by NREL have shown degradation of over 1.8%/yr. Reducing degradation to 0.2%/yr. while increasing module lifetime will facilitate further cost reductions. Supported by the US Dept. of Energy’s PVRD program, Colorado State University’s Next Generation PV Center is developing a new module architecture and associated manufacturing processes to reduce costs and improve reliably.

This study focuses on the microstructural and crystallographic characteristics of cadmium telluride thin film photovoltaics using the novel characterization technique of transmission electron back-scatter diffraction (T-EBSD). Taking advantage of the increase in resolution of transmission electron back-scatter diffraction capabilities, identification of localized changes of phase within the cadmium telluride grains have been detected. T-EBSD of the cadmium telluridegrains show areas containing very high defect densities indexed to the hexagonal phase whereas the rest of the grain is indexed to the cubic phase, showing that the high densities of defects alters the stacking formation enough to causes a localized change of phase, forming two different phases within the same grain.

The activation of CdTe with a cadmium chloride annealing treatment is a vital step in the fabrication of high efficiency solar cells. Thin film MZO/CdTe cells have been activated using CdBr 2 instead of CdCl 2 with a lower activation process temperature. Using this method, CdBr 2 does activate the cell as revealed by J-V and EQE measurements. TEM and EDX elemental maps from device cross-sections confirm that bromine is present in the grain boundaries. TEM shows that the treatment removes stacking faults at 425 °C. CdBr 2 treatment resulted in a relatively modest conversion efficiency of 5.49% when treated at 375 °C. Nevertheless, the experiments shed further light on the mechanisms involved in the activation.

There are two major hurdles in the fabrication of top cell using polycrystalline films of CdTe alloys. First, the deposition of the films is difficult due to the difference in the vapor pressure of the individual elements present in the alloy or longer process time making it not economical. The retention of the original stoichiometry of the CdTe alloy after the CdCl 2 treatment is the second hurdle. In this work, polycrystalline films of Cd (1-x) Zn x Te (Cd-Zn-Te) were fabricated with different band gaps by co-sublimation of CdTe and Zn. A strategy to retain the composition of the polycrystalline Cd-Zn-Te films is investigated and demonstrated. Quantum efficiency of greater than 0.7 was observed in the fabricated devices and band gap was maintained.

The efficiency of cadmium telluride (CdTe) solar cells is limited primarily by voltage, which is known to depend on the carrier concentration and carrier lifetimes within the absorber layer of the cell. Here, cathodoluminescence measurements are made on an as-deposited CdSeTe/CdTe solar cell that show that selenium alloyed CdTe material luminesces much more strongly than non-alloyed CdTe. This reduction in non-radiative recombination in the CdSeTe suggests that the selenium gives it a certain defect tolerance. This has implications for carrier lifetimes and voltages in cadmium telluride solar cells.\ud

With research scale CdTe devices reaching efficiencies over 22%, thin-film CdTe solar cells are part of a growing industry. ZnTe with copper doping can improve overall device efficiency through enhancements in open-circuit voltage and fill-factor. ZnTe has been credited with good device performance and improved device stability. ZnTe displays a promising route for further device improvement. Exploration of ZnTe as a back contact was done with and without copper doping. Devices were characterized to determine how to further improve the incorporation of ZnTe into CdSeTe/ CdTe devices.

Over 19% device efficiency with over 28 mA/cm$^{2}$ short-circuit current density has been achieved with thin-film using CdSeTe/CdTe graded absorber. A deep pocket sublimation source was used to deposit CdSeTe alloy. However, cross-section line scan using energy dispersive X-ray spectroscope showed that actual Se incorporation in the absorber films was much lower than the feed stock composition. Further lowering the band-gap of deposited CdSeTe films will further improve absorption of higher wavelengths leading to higher short-circuit current density. To overcome the limitation preferential sublimation of CdSeTe, advanced co-sublimation of Se and CdTe to achieve higher Se incorporation and lower bandgap is presented.

Adding a thin layer of Al2O3 to the back of CdSeTe/CdTe devices has previously been shown to passivate the back interface and drastically improve surface recombination lifetimes. Using such a structure, lifetimes of over 400 ns have been recorded. Despite this, such devices do not currently show an improvement in open-circuit voltage (VOC) that is commensurate with this effect. Devices were fabricated using a range of Al2O3 thicknesses at the back interface. High efficiency devices exceeding 16.4% were fabricated with 0.5 nm Al2O3.

Arsenic dopant was incorporated in CdTe absorber layers in high-efficiency CdTe cells using feedstock doped with 1018cc-1for sublimation of films. The goal of the work was to develop a feasible method for creating a hole density equal to, or greater than that currently achievable using copper doping using a CuCl treatment. Doping with arsenic resulted in a modest increase in open-circuit voltage (V OC ) and a large improvement in fill-factor and conversion efficiency when compared with copper-doped devices with similar structure. All experiments were performed in the presence of cadmium overpressure to encourage dopant activation in tellurium vacancy sites. Arsenic incorporation was measured using Secondary Ion Mass Spectrometry (SIMS) at 4E +16 atoms/cc-1,about four times greater incorporation than previously seen by the authors. Using a CdSeTe/CdTe graded absorber and arsenic doping, a conversion efficiency of 16.79% was achieved.

Postdeposition CdCl

Purpose – The purpose of this paper is to present a real world application of an innovative hybrid system reliability optimization algorithm combining Tabu search with an evolutionary algorithm (TSEA). This algorithm combines Tabu search and Genetic algorithm to provide a more efficient search method. Design/methodology/approach – The new algorithm is applied to an aircraft structure to optimize its reliability and maintain its structural integrity. For retrofitting the horizontal stabilizer under severe stall buffet conditions, a decision support system (DSS) is developed using the TSEA algorithm. This system solves a reliability optimization problem under cost and configuration constraints. The DSS contains three components: a graphical user interface, a database and several modules to provide the optimized retrofitting solutions. Findings – The authors found that the proposed algorithm performs much better than state-of-the-art methods such as Strength Pareto Evolutionary Algorithms on bench mark problems. In addition, the proposed TSEA method can be easily applied to complex real world optimization problem with superior performance. When the full combination of all input variables increases exponentially, the DSS become very efficient. Practical implications – This paper presents an application of the TSEA algorithm for solving nonlinear multi-objective reliability optimization problems embedded in a DSS. The solutions include where to install doublers and stiffeners. Compromise programming is used to rank all non-dominant solutions. Originality/value – The proposed hybrid algorithm (TSEA) assigns fitness based upon global dominance which ensures its convergence to the non-dominant front. The high efficiency of this algorithm came from using Tabu list to guidance the search to the Pareto-optimal solutions.

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 SnO2-based transparent conducting oxide and high resistance transparent coated substrates using in-line compatible processes. Copyright © 2015 John Wiley & Sons, Ltd

The addition of a CdMgTe (CMT) layer at the back of a CdTe solar cell should improve its performance by reflecting both photoelectrons and forward-current electrons away from the rear surface. Higher collection of photoelectrons will increase the cell’s current, and reduction of forward current will increase its voltage. To achieve electron reflection, conformal CMT layers were deposited at the back of CdTe cells, and a variety of measurements including performance curves, transmission electron microscopy, x-ray photoelectron spectroscopy, and energy-dispersive x-ray spectroscopy were performed. Oxidation of magnesium in the CMT layer was addressed by adding a CdTe capping layer. MgCl2 passivation was substituted for CdCl2 in some cases, but little difference was seen.

To investigate the effects of CdCl$_2$ treatment on the local electronic properties of polycrystalline CdTe films, we conducted a photoemission electron microscopy (PEEM) study of polished surfaces of CdTe films in superstrate configuration, with and without CdCl$_2$ treatment. From photoemission intensity images, we observed the tendency for individual exposed grain interiors to vary in photoemission intensity, regardless of whether or not films received CdCl$_2$ treatment. Additionally, grain boundaries develop contrast in photoemission intensity images different from grain interiors after an air exposure step, similar to observations of activated grain boundaries using scanning Kelvin probe force microscopy studies. These results suggest that work function varies locally, from one grain interior to another, as well as between grain boundaries and grain interiors., 5 pages, 3 figures, Photovoltaics Specialist Conference (PVSC), 2017

CdTe solar cells currently use a hetero-junction design allowing for ideal band alignment engineering. The back contact has often been a source of CdTe performance limitations due to the formation of a barrier at the back interface. Tellurium has been studied as a possible high quality back contact to CdTe. Temperature dependent current density-voltage measurements and x-ray and ultraviolet photoelectron spectroscopy were carried out in order to approximate offsets at the CdTe/Te interface.

Thin-film polycrystalline CdTe photovoltaic devices were studied using electrical and material characterization methods to understand the effects of hydrogen plasma passivation treatment. Devices were fabricated using sublimation and were exposed to hydrogen plasma for 10, 20 and 30 minutes. Current density vs voltage measurements were performed to measure the performance of the devices. Capacitance vs voltage graphs showed that dopants are active and the device behaved like a CdCl2 passivated device. Microscopic characterization was performed using SEM and (S)TEM that showed larger grains and more homogenous film coverage as compared to films without passivation suggesting grain growth during H2 passivation.

In thin-film CdTe solar cells, a high quality back contact is essential for high voltage and fill factor. Properties such as a high work function, stability to air and moisture, and an ohmic contact to the CdTe interface are important considerations. MoO x and MoN x thin films are promising alternative back contacts. Films were deposited with varying composition via DC reactive sputter deposition. Thin film properties were measured through characterization techniques such as four-point probe measurements, profilometry measurements, Hall measurements, XPS, and GAXRD. Devices were compared between current back contact types and alternatives such as molybdenum. Experimental results for MoO x and MoN x as back contact buffer layers are discussed in detail.

Pre-synthesis of virtual screening of newly designed four small molecules was approached using Density Functional Theory method. ‘Medium donor-vinylene-strong acceptor’ concept was used to design four small molecules. Malononitrile is flanked on the donor moiety of benzo[1,2-b:6,5-b, dithiophene-4,5-dione(BDTD) which were blended with vinyl linked benzothiadiazole (BTD) acceptor moiety studied with strong electron withdrawing groups (EWG) such as F, CN, and NO2onto the benzothiadiazole unit. DFT and TD-DFT (Time Dependent Density Functional Theory) results of studied small molecules with 1.79eV- 2.06eV band-gap with simulated high open-circuit voltage of 1.38V - 1.49V and predicted power conversion efficiency of designed small molecules is shown using schraber diagram. Detailed investigation of this study is described in this manuscript.

Cadmium Sulphide is the conventional n-type buffer layer used in thin film Cadmium Telluride solar cells. It is well known that Cadmium Sulphide causes optical losses and sulphur diffuses into the absorber during high temperature activation. Sputter-deposited Mg-doped ZnO (MZO) has been shown to be an attractive buffer layer for Cadmium Telluride solar cells due to its transparency and tuneable band gap. It is also stable to high temperature processing and avoids diffusion of elements into the cadmium telluride absorber during the cadmium chloride activation treatment. However, degradation is observed in solar cells incorporating MZO buffer layers. Analysis of the MZO film surface potential has revealed significant fluctuations in the thin film work function once the layer is exposed to the atmosphere following deposition. These fluctuations are attributed to the high reactivity to water vapour of the MgO contained in the MZO films. This has been analysed using X-ray Photoelectron Spectroscopy to determine corresponding changes in the surface chemistry. The Zinc Oxide component is relatively stable, but the analysis shows that MgO forms a Mg(OH)2 layer on the MZO surface which forms a secondary barrier at the MZO/CdTe interface and/or at the interface between MZO and the Fluorine-doped SnO2. This affects the Fill Factor and as a consequence it degrades the conversion efficiency.

CdTe-based solar cell efficiency has rapidly improved over the last few years. Some of the reasons have been a change to the absorber composition including the incorporation of selenium, and better front contact and emitter materials in CdTe photovoltaic devices. In addition to the increase in short-circuit current by reducing the bandgap, Se plays other important roles in passivation of defects thus improving the conversion efficiency of CdSeTe/CdTe graded absorber photovoltaic devices. Here, we combine structural and optical characterizations with first principles calculations to investigate the role of Se and Cl segregation in CdSeTe devices. We find that in the presence of Se and Cl, the minority carrier lifetime improves due to a reduction of midgap defect states. We also correlate this effect with defect passivation in CdSeTe devices and suggest innovative ways to further improve CdTe-based photovoltaic efficiency.

It is well known that the cadmium chloride annealing treatment is an essential step in the manufacture of efficient thin film cadmium telluride solar cells. It has been recognized that the combination of annealing at ∼4000C together with the addition of cadmium chloride at the surface induces re-crystallisation of the cadmium telluride layer and also affects the n-type cadmium sulfide. We have applied advanced micro-structural characterization techniques to distinguish the effect of the annealing and the cadmium chloride treatments on the properties of the cadmium telluride deposited via close space sublimation (CSS) and relate these observations to device performance. Transmission electron microscopy (TEM) has shown a variation in stacking fault density with annealing temperature and annealing time. Stacking faults observed within the cadmium telluride grains in TEM were partially removed post annealing; these findings show that temperature alone has a role in the reduction of stacking faults. However, since we have previously observed almost complete removal of stacking faults with annealing in combination with cadmium chloride, the cadmium chloride is essential to defect removal and high efficiency cells.

In our work on laser scribing CdTe solar cells we have found what appears to be an unpublished laser material interaction that allows precise laser etching of SnO2 films to an arbitrary thickness with high uniformity. This precise and efficient laser etching mechanism allows arbitrary reduction of the film thickness in a controlled manner on the scale of tens of nm. In addition to the fine depth selection, we find that there develops a pulse duration dependent microstructure on the surface. This micro microstructure results in a strong diffraction effect in the visible portion of the spectrum. In this work we propose a physical mechanism behind this novel depth selective laser interaction as well as the resultant micro-structure. Finally we demonstrate and propose some possible applications for this process.

A standardized process used for fabrication of CdTe solar cells was varied by increasing the substrate temperature during CdTe layer nucleation from approximately 460°C to 610°C and by increasing the CdTe sublimation vapor source temperature. Higher substrate temperatures increase device efficiency, but cause significant CdS re-sublimation. This effect was eliminated by using a Mg 1−x Zn x O window layer that also has higher transparency. Elevated CdTe source temperatures were found to increase contamination in the deposition system but did not further improve device efficiency. The improvement using high substrate temperatures is attributed to larger CdTe grains and better crystalline quality. TEM cross section analysis, X-ray diffraction measurements and device results are presented.

In this study Cd 1−x Mg x Te thin films are investigated for use as an absorber layer for future use in top cells of a tandem device. An essential step in the development of this new absorber layer is the passivation process. An experimental passivation process using a mixture of MgCl 2 and CdCl 2 was conducted. Localized magnesium loss during the passivation process is shown to be along grain boundaries as well as in the formation of an oxide layer at the back surface. Devices which underwent the experimental passivation process showed significant improvement in device performance. A next generation n-type window layer is used in junction with a Cd( 1−x )Mg (x) Te absorber, resulting in higher performing cells than CdS.

Atomic-scale defects limit the open circuit Voltage, and the conversion efficiency of thin film polycrystalline cadmium telluride solar cells. Using state of the art aberration-corrected high resolution transmission electron microscopy, the type, density and atomic structure of intragranular defects present in cadmium chloride treated and untreated CdTe has been established. The cadmium chloride activation process dramatically reduces defect density but faults do remain. Characterizing the defects in both materials is an essential first step to determining their potential electrical effects, and to understanding how the cadmium chloride treatment reduces their density. Improving our knowledge of the mechanisms involved can lead to further process improvements.

Different organic compounds are evaluated to form back contact for CdTe photovoltaics (PV) and to study the effect of band alignment. CdTe devices with CdS n-type window layer were fabricated using closed space sublimation (CSS) and organic back contact layers were deposited using dip coating. In order to prepare photovoltaic devices, the appropriate deposition parameters and thicknesses of organic compounds were selected experimentally. The best results were obtained with dip coated PEDOT-PSS aqueous dispersion with high electrical conductivity. The performance of devices with organic back contacts are compared to those with baseline CdTe devices with reduced copper doping.

CdTe solar-cell efficiency at Colorado State has now been independently verified above 18% on commercial glass. The parameters for the highest-efficiency cell are 863 mV for V OC , 26.8 mA/cm2 for J SC , and 79.2% for fill-factor, combining for 18.3% efficiency. The cell features that allowed the increases include higher-temperature pre-heating before CdTe deposition, a Te layer to facilitate the back contact, MgZnO for the buffer layer, and an anti-reflective coating. The current and fill-factor have achieved large fractions of their ideal values for the CdTe band gap, but as is typical for CdTe cells, the voltage has not. Two general strategies to address the voltage deficit are described. One of these, electron reflection with fully-depleted CdTe, has achieved voltage increases of 60 mV with 1-μ m CdTe and higher with sub-micron absorbers.

Reducing recombination in polycrystalline solar cells by orders of magnitude is currently one of the greatest challenges for increasing thin-film solar cell efficiency to theoretical limits. The question of how to do this has been a challenge for the thin-film community for decades. This work indicates that effective interface passivation is critical. Here, polycrystalline Al2O3/CdSeTe/Al2O3/glass heterostructures are grown, and a combination of spectroscopic, microscopic, and time-resolved electro-optical measurements demonstrates that the interface recombination velocity at alumina/thin-film interfaces can be less than 100 cm/s. This is three orders of magnitude less than typical CdTe interfaces without passivation, commensurate with single-crystal epitaxial CdMgSeTe/CdSeTe/CdMgSeTe double heterostructures, and enables minority-carrier lifetimes in polycrystalline CdSeTe well above 100 ns. Microscopic interfacial electric-field measurements identify the field effect as a potential mechanism for polycrystalline Al2O3/CdSeTe interface passivation. The results provide guidance for modeling and interface passivation in devices and indicate future paths to realize highly efficient thin-film solar cells.Reducing recombination in polycrystalline solar cells by orders of magnitude is currently one of the greatest challenges for increasing thin-film solar cell efficiency to theoretical limits. The question of how to do this has been a challenge for the thin-film community for decades. This work indicates that effective interface passivation is critical. Here, polycrystalline Al2O3/CdSeTe/Al2O3/glass heterostructures are grown, and a combination of spectroscopic, microscopic, and time-resolved electro-optical measurements demonstrates that the interface recombination velocity at alumina/thin-film interfaces can be less than 100 cm/s. This is three orders of magnitude less than typical CdTe interfaces without passivation, commensurate with single-crystal epitaxial CdMgSeTe/CdSeTe/CdMgSeTe double heterostructures, and enables minority-carrier lifetimes in polycrystalline CdSeTe well above 100 ns. Microscopic interfacial electric-field measurements identify the field effect as a potential mechanism for polycrys...

As single-junction silicon solar cells approach their theoretical limits, tandems provide the primary path to higher efficiencies. CdTe alloys can be tuned with magnesium (CdMgTe) or zinc (CdZnTe) for ideal tandem pairing with silicon. A II-VI/Si tandem holds the greatest promise for inexpensive, high-efficiency top cells that can be quickly deployed in the market using existing polycrystalline CdTe manufacturing lines combined with mature silicon production lines. Currently, all high efficiency polycrystalline CdTe cells require a chloride-based passivation process to passivate grain boundaries and bulk defects. This research examines the rich chemistry and physics that has historically limited performance when extending Cl treatments to polycrystalline 1.7-eV CdMgTe and CdZnTe absorbers. A combination of transmittance, quantum efficiency, photoluminescence, transmission electron microscopy, and energy-dispersive X-ray spectroscopy clearly reveals that during passivation, Mg segregates and out-diffuses, initially at the grain boundaries but eventually throughout the bulk. CdZnTe exhibits similar Zn segregation behavior; however, the onset and progression is localized to the back of the device. After passivation, CdMgTe and CdZnTe can render a layer that is reduced to predominantly CdTe electro-optical behavior. Contact instabilities caused by inter-diffusion between the layers create additional complications. The results outline critical issues and paths for these materials to be successfully implemented in Si-based tandems and other applications.

In order to study the microstructure of cadmium telluride (CdTe) photovoltaic thin film solar cells, manufactured by an in-line manufacturing process, Scanning Electron Microscopy characterization (SEM) and X-ray diffraction (XRD) characterization were performed. SEM measurement showed that no substantial changes in the grain structure of CdTe layers occurred during the Cadmium Chloride (CdCl2) treatment. No change in the cubic CdTe lattice parameter “a” was observed for the CdCl2 treated sample. It is inferred that the primary effect of the CdCl2 treatment in the devices studied is the passivation of grain boundaries and bulk defects. XRD studies show a loss of preferred orientation (as determined from the peak ratios) of planes during the copper compound treatment indicating recrystallization of the grains due to the Cu treatment. Also the Cu treated sample showed decrease in value of the lattice parameter “a”.

Steady state photocapacitance (PHCAP) was used to survey the deep levels in the energy range 0.73 eV to 1.38 eV with respect to the valence band in CdS/CdTe PV devices. The effect of the cadmium chloride treatment process on deep level densities is shown qualitatively. Estimates of the optical transition thresholds E p o for three deep minority type levels E 1, E 2, and E 3 are given and a discussion of possible assignment to specific defects is presented. Preliminary results using deep level optical spectroscopy (DLOS) to measure the optical cross sections for holes σ p o of the E 2 and E 3 levels are given. The thermal emission rates e n t for electrons at 79 K are estimated for the E 2 and E 3 levels.

In-line manufactured CdTe thin film photovoltaic devices were evaluated for their performance with respect to power conversion efficiency and reliability (stability). A National Renewable Energy Laboratory (NREL) certified power conversion efficiency (η) of 12.44% was measured for one CdTe device. A mean efficiency of 11.4% for 53 devices is shown. Results of reliability studies give a conservative estimate of −1% relative degradation per year in efficiency over a 20‐year lifetime for these CdTe devices.

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.

Sublimation based processing has proven to be an efficient method for manufacturing thin film CdS/CdTe PV devices. Recent advancements in computational simulation have opened a door for comprehensive modeling and analysis of the many processes involved in creating a fully functional solar cell. Computational models have been developed for CdS, CdTe, CdCl2, and CuCl that include fluid mechanics, heat transfer, and chemical reactions. This modeling will allow manufacturers to evaluate source geometries, process temperatures, operating pressures, and film properties before committing money to developing hardware.

The cadmium chloride activation treatment of cadmium telluride solar cells is essential for producing high efficiency devices. The treatment has many effects but the most significant is the complete removal of stacking faults in the cadmium telluride grains and the diffusion of Chlorine along the grain boundaries of the device. Chlorine decorates all cadmium telluride and cadmium sulphide grain boundaries and also builds up along the CdTe/CdS junction.. This paper reveals that by annealing devices to temperatures of 400°C to 480 °C for times ranging from 30 to 600 seconds in moderate vacuum results in the re-appearance of stacking faults and the removal of Choline from the grain boundaries. STEM analysis confirms the re-appearance of the stacking faults and SIMS and EDX confirm the removal of chlorine from the grain boundaries. This directly corresponds to a lowering in cell efficiency. The study provides further evidence that CdCl2 diffusion and certain microstructural defects directly affect the performance of cadmium telluride photovoltaic devices.

Zinc oxide-based TCO films have high mobility and transmission, but are limited by a bandgap of approximately 3.5 eV. High-efficiency CdTe thin-film solar cells often use window layers which are transparent over the entire spectrum. In these cells, TCO absorption can be a significant source of current loss. Relative to ZnO, MgxZn1−xO (MZO) has a higher optical bandgap and and a lower electron affinity, both of which are desirable for the front contact. MZO doped with gallium showed high conductivity and transmission over a broad range, indicating that it is suitable for high-performance CdTe thin-film solar cells. Window layer band alignment and device results are presented.

The production of stable thin-film photovoltaic cells requires tight control of temperature uniformity within the glass substrates during the vacuum deposition process. Though traditional approaches such as radiation shielding and channeling more power to outer lamps result in substantial improvements in temperature uniformity they fail in meeting the stringent requirement of less than 1 °C variation across the substrate required to guarantee the long-term stability of the devices. The problem becomes especially acute while scaling up to larger commercially-viable panel sizes. To this end, a finite element thermal model of a commercial-scale deposition station has been developed and optimized with the target of achieving the desired temperature uniformity of 1 °C. The effects of improvements such as radiation shielding, addition of radiation spreader, contouring of radiation spreader and optimizing power distribution among the radiation lamps have been studied. A new lamp configuration has been proposed for attaining the desired uniformity levels.

The problem of predicting deposition rates and film thickness variation is relevant to many high-vacuum physical vapor deposition (PVD) processes. Analytical methods for modeling the molecular flow fail when the geometry is more complicated than simple tubular or planar sources. Monte Carlo methods, which have traditionally been used for modeling PVD processes in more complicated geometries, being probabilistic in nature, entail long computation times, and thus render geometry optimization for deposition uniformity a difficult task. Free molecular flow is governed by the same line-of-sight considerations as thermal radiation. Though the existence of an analogy between the two was recognized by Knudsen (1909, Ann. Phys., 4(28), pp. 75–130) during his early experiments, it has not been exploited toward mainstream analysis of deposition processes. With the availability of commercial finite element software having advanced geometry modelers and built-in cavity radiation solvers, the analysis of diffuse thermal radiation problems has become considerably simplified. Hence, it is proposed to use the geometry modeling and radiation analysis capabilities of commercial finite element software toward analyzing and optimizing high-vacuum deposition processes by applying the radiation-molecular flow analogy. In this paper, we lay down this analogy and use the commercial finite element software ABAQUS for predicting radiation flux profiles from planar as well as tube sources. These profiles are compared to corresponding deposition profiles presented in thin-film literature. In order to test the ability of the analogy in predicting absolute values of molecular flow rates, ABAQUS was also employed for calculating the radiative flux through a long tube. The predictions are compared to Knudsen’s analytical formula for free molecular flow through long tubes. Finally, in order to see the efficacy of using the analogy in modeling the film thickness variation in a complex source-substrate configuration, an experiment was conducted where chromium films were deposited on an asymmetric arrangement of glass slides in a high-vacuum PVD chamber. The thickness of the deposited films was measured and the source-substrate configuration was simulated in ABAQUS. The variation of radiation fluxes from the simulation was compared to variation of the measured film thicknesses across the slides. The close agreement between the predictions and experimental data establishes the feasibility of using commercial finite element software for analyzing high vacuum deposition processes.

A correlation between the CdCl 2 treatment and the change in conversion efficiency with light and heat stress indoors (stability) has been shown previously by our group for CdS/CdTe:Cu PV devices. In the present work CdTe devices were fabricated with various CdCl 2 treatments and with and without a Cu containing back contact. The electrical characteristics of the defects acting as traps in these devices were studied using thermal admittance spectroscopy (TAS). The activation energy E t − E V , the apparent capture cross section and the densities of state functions (using Walter's method) of the traps in the devices were estimated.