Search

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.

The performance impact of multiple parameters related to the use of MgZnO (MZO) as the emitter for bilayer CdSeTe/CdTe solar cells has been investigated in detail through numerical simulations. Such a comprehensive study is particularly important, because while cell fabrication using MZO has been highly successful in some cases, it has been less so in others, and it has not been clear which combinations of parameter values are most effective. The parameters considered here include the recombination velocity at the emitter/absorber interface, bulk recombination lifetime, and the carrier concentrations of the emitter and absorber. The ranges chosen for the simulation parameters are those most likely to be found experimentally. The primary finding is that independent of the interfacial recombination velocity and bulk recombination lifetime, the MZO carrier density should be $>$ 1017 cm–3 and in any case greater than that of the absorber to reduce interface recombination. At the same time, a shallow dopant the order of 50 meV or less should reduce Shockley-Read-Hall recombination in the bulk and enable $V_{\text{OC}} >$ 1 V.

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.

Cadmium telluride (CdTe) solar cells have achieved efficiencies of over 22%, despite having absorber layer grain sizes less than 10 μm and hence a very high density of grain boundaries. Recent research has shown that this is possible because of partial passivation of grain boundaries during the widely used cadmium chloride treatment, and passivation of grain interior defects by selenium alloying of the CdTe. Here, state-of-the art TEM-based cathodoluminescence imaging is used to show that, in addition to grain interiors, selenium also passivates grain boundaries in alloyed Cd(Sex,Te1-x) material (CST). Specifically, we find that recombination at CST grain boundaries is up to an order of magnitude lower than at CdTe grain boundaries. This further explains the superior performance of selenium graded CdTe devices and provides potential new routes for further efficiency improvement and solar electricity cost reduction.

Recent advances in design and processing technology have made possible commercialization of polycrystalline (px)-CdTe as a photovoltaic absorber. Grain boundaries (GBs) are the most prominent struc...

The realization of a transparent back buffer layer would be a significant boon for the CdTe community. Simulations presented here reveal a substantial performance increase in ultrathin cells through incorporation of a back buffer, as well as high bifaciality if transparent contacts are assumed. This project aims to deposit a p-type transparent conducting oxide (TCO) at the back of superstrate CdSeTe/CdTe cells to improve band bending and interface recombination at the rear of the cell, and ultimately to bring high performing bifacial cells to fruition. Four such oxides were sputter deposited to create a collection of functional bifacial cells. Back surface passivation was evaluated with rear-excitation time-resolved photoluminescence (TRPL). Several promising materials yielded rear-excitation lifetimes >2 ns, indicating effective passivation.

The passivating effects of MgO have been studied by integrating into an existing superstrate CdTe thin film solar cell device architecture. When implemented as an emitter to replace the typical MZO, the device performance was below par, but improved PL and TRPL was detected with significant carrier lifetimes in comparison to the MZO devices. However, when integrated at the back, the PL signal was detected upon the illumination of excitation laser from the back. This has opened the possibility of probing the back of the CdTe thin film solar cells with transparent back contact with improved back passivation and a bifacial device structure. Further, the TRPL measurements suggests that the MgO shows passivating effects to a CdTe surface irrespective of method of deposition implemented.

The doping effect of arsenic (As) concentration in the CdTe absorber has been studied by using the first principles method. Atomistic modeling based on density functional theory (DFT-1/2) and green’s function (GF) was utilized to simulate a first order approximated model of As doped CdTe surface. The band alignment results calculated using DFT-1/2 revealed that higher concentration of As doping at CdTe surface is required to obtain favorable band bending for hole charge carrier transport. Based on the results obtained from DFT models, CdTe solar cells were fabricated with two different As dopant concentrations. The experimental finding corroborates the theoretical result and provides a future pathway of using DFT simulations as a precursor in guiding the CdTe device fabrication experiments.

We report significant advances in understanding and reducing nonradiative Shockley-Read-Hall recombination in polycrystalline CdSe x Te 1-x , leading to microsecond charge carrier lifetimes. In undoped Al 2 O 3 -passivated heterostructures we find external radiative efficiency 0.2%, quasi-Fermi level splitting 950 mV, mobility 100 cm2/(Vs), and diffusion length 14 µm. In solar cells measured lifetimes can exceed 1 µs. We interpret this data to indicate MgZnO/CdSeTe interface recombination velocity

We report seminal advances in fabrication and understanding of group V (As) doped thin-film polycrystalline CdTe-based solar cells. The devices are fabricated using a novel approach, by sublimating layers of CdSeTe and CdSeTe:As. This new method allowed us to achieve minority carrier lifetime of over 1 µs, carrier concentration of more than 5×l015cc−1 and external radiative efficiency of over 2 % in a device configuration. We find an increase in open-circuit voltage when comparing As-doped, Cu-doped and undoped devices. The choice of CdSeTe instead of a CdTe-only absorber has been explained using first-principle density functional theory model. A SCAPS device model is used to analyze the potential causes for lower open-circuit voltage.

Since 2015, commercial sample holders have been available that enable cathodoluminescence imaging of semiconductors in the TEM. Despite this, issues with low signal have meant that high resolution TEM-CL imaging has so far not been achieved on a solar cell. Here, we use xenon ion milling and cryogenic sample cooling to boost signal from the TEM foil, enabling high resolution CL imaging of a bilayer CdSeTe/CdTe solar cell for the first time. The results show that selenium has a passivation effect on grain boundaries in alloyed CdSeTe material, helping to explain the superior performance of CdSeTe solar cells.

The first step to understand the origin of losses in any photovoltaic solar cell is to determine the fundamental thermodynamic efficiency and voltage limits of such a device. In this contribution, we detail techniques to calculate the voltage limit in the case of cadmium selenium telluride (CdSeTe) solar cells, and how approaches based on bandgap alone—i.e., the Shockley-Queisser approach with step-function absorptance—can overestimate the thermodynamic open-circuit voltage limit $V_{oc,ideal}$ . This is particularly true for arsenic-doped samples, which tend to exhibit below-bandgap absorptance.

Since excellent carrier lifetimes and front interface electronic quality are now achieved, rear interface recombination can limit V OC in $\text{CdSe}_{\mathrm{x}}\text{Te}_{1-\mathrm{x}}/\text{CdTe}$ solar cells. Several back-contact structures for devices were fabricated using combinations of tellurium, aluminum oxide, amorphous silicon, and indium tin oxide (ITO). Time-resolved photoluminescence was used to characterize such structures. We show increasingly improved interface passivation through the subsequent use of aluminum oxide, amorphous silicon, and ITO. Additionally, we show that arsenic-doped absorbers form a more passive interface with numerous back contact structures.

Graded absorbers devices with with CdSe 0.4 Te 0.6 (molar basis) and CdTe were fabricated. CdCl 2 treatment time, post-deposition CdCl 2 anneal time and thicknesses of CdSeTe and CdTe layers were varied. Photoluminescence and electrical measurements were performed on the fabricated devices. Results revealed that the individual thicknesses of CdSeTe and CdTe is critical to overall efficiency of the devices. Device fabricated on substrate with 0.5 µm CdSeTe, 3 µm CdTe with rest of process parameters kept unchanged, produced an efficiency of 20.14%. We report highest device efficiency among academia and research institutions.

The effects of the CdCl2passivation treatment on thin-film CdTe photovoltaic films and devices have been extensively studied. Recently, with an addition of CdSeTe layer at the front of the absorber layer, device conversion efficiencies in excess of 19% have been demonstrated. The effects of the CdCl2passivation treatment for devices using CdSeTe has not been studied previously. This is the first reported study of the effect of the treatment on the microstructure of the CdSeTe /CdTe absorber. The device efficiency is < 1% for the as-deposited device but this is dramatically increased by the CdCl2treatment. Using Scanning Transmission Electron Microscopy (STEM), we show that the CdCl2passivation of CdSeTe/CdTe films results in the removal of high densities of stacking faults, increase in grain size and reorientation of grains. The CdCl2treatment leads to grading of the absorber CdSeTe/CdTe films by diffusion of Se between the CdSeTe and CdTe regions. Chlorine decorates the CdSeTe and CdTe grain boundaries leading to their passivation. Direct evidence for these effects is presented using STEM and Energy Dispersive X-ray Analysis (EDX) on device cross-sections prepared using focused ion beam etching. The grading of the Se in the device is quantified using EDX line scans. The comparison of CdSeTe/CdTe device microstructure and composition before and after the CdCl2treatment provides insights into the important effects of the process and points the way to further improvements that can be made.

Photovoltaics is an important energy technology for large scale energy generation. In the past few years cost of photovoltaic module manufacturing and installation as well as electricity generation has substantially decreased while the production volume has seen a steep increase. These changes can be attributed to improvement in solar cell efficiencies as well as better manufacturing practices. There are several photovoltaic technologies available in the market but the two primary technologies commercially manufactured for large scale installations are polycrystalline thin-film CdTe and crystalline silicon. Crystalline Si is the oldest and the most widely installed technology while thin-film CdTe is the technology that has demonstrated the largest growth and lowest LCOE (levelized cost of energy). In this study, commercial modules from both these technologies are installed side by side for an accurate comparison of their performance. The modules for comparison are installed with the same approximate nameplate capacity in three different configurations viz. Roof-top, floating on water and ground. Their performance is monitored and analyzed over a 3 month period. Thin-film CdTe demonstrated substantial advantage under all three conditions over crystalline Si in Thailand's tropical climate which is characterized by high temperatures and humidity throughout the year. Advantage demonstrated by thin-film CdTe is further supported by greater economic, environmental, reliability and life-cycle advantages that are summarized in the later part of the study.

Commercial CdTe PV modules have polycrystalline thin films deposited on glass, and devices made in this format have exceeded 22% efficiency. Devices made by the authors with a magnesium zinc oxide window layer and tellurium back contact have achieved efficiency over 18%, but these cells still suffer from an open-circuit voltage far below ideal values. Oxide passivation layers made by sputter deposition have the potential to increase voltage by reducing interface recombination. CdTe devices with these passivation layers were studied with photoluminescence (PL) emission spectroscopy and time-resolved photoluminescence (TRPL) to detect an increase in minority carrier lifetime. Because these oxide materials exhibit barriers to carrier collection, micropatterning was used to expose small point contacts while still allowing interface passivation. TRPL decay lifetimes have been greatly enhanced for thin polycrystalline absorber films with interface passivation. Device performance was measured and current collection was mapped spatially by light-beam-induced current.

An 800 nm CdSeTe layer was added to the CdTe absorber used in high-efficiency CdTe cells to increase the current and produce an increase in efficiency. The CdSeTe layer employed had a band-gap near 1.41 eV, compared with 1.5 eV for CdTe. This lower band-gap enabled a current density increase from approximately 26 to over 28 mA/cm2. The open-circuit voltage obtained in the high-efficiency CdTe-only device was maintained and the fill-factor remained close to 80%. Improving the short-circuit current density and maintaining the open-circuit voltage lead to device efficiency over 19%. External quantum efficiency implied that about half the current was generated in the CdSeTe layer and half in the CdTe. Cross-sectional STEM and EDS showed good grain structure throughout. Diffusion of Se into the CdTe layer was observed. This is the highest efficiency polycrystalline CdTe photovoltaic device demonstrated by a university or national laboratory.

Distortions in the current–voltage curves of CdTe-based thin-film solar cells with MgZnO emitter layers are not uncommon. Even when such distortions are very subtle with standard sunlight, they can be greatly accentuated by the removal of short-wavelength photons with optical filters, and the dark-curve turn-on voltage can be noticeably increased. The strong suggestion is that good J–V curves with full-spectrum light often rely on photogeneration of electrons from trap states in the MgZnO emitter. When the carrier concentration of the CdTe absorber is increased with external doping, there is a corresponding need for a larger MgZnO carrier concentration. Based on the assumption that the emitter carrier concentration needs to be comparable to that of the absorber, Ga-doping of the MgZnO was shown to very nearly eliminate the large filtered-light distortions and is arguably superior to MgZnO that relies on photogeneration. In addition, it was shown that photogeneration and recovery in MgZnO is relatively slow, so that without Ga-doping, the cell efficiency improves gradually for most of an hour in the light, and the turn-on voltage of the dark curve increases on a similar time scale when the light is removed.

A cadmium chloride activation treatment is essential for the production of high efficiency cadmium telluride (CdTe) solar cells. However, the effects of the treatment on the distributions of chlorine and sulphur within the device are not fully understood. Here, the detailed locations of chlorine and sulphur in a treated CdTe cell are determined in three dimensions by high resolution dynamic SIMS measurements. Chlorine is found to be present in grain boundaries, grain interiors, extended defects within the grain interiors, at the front interface, and in the cadmium sulphide layer. In each of these regions, the chlorine is likely to have significant effects on local electronic properties of the material, and hence overall device performance. Sulphur is found to have a U-shaped diffusion profile within CdTe grains, indicating a mixed grain boundary and lattice diffusion regime.

Chlorine is known to have numerous effects on the electronic performance of cadmium telluride (CdTe) solar cells, such as doping the CdTe absorber material and pacifying crystal defects. However the mechanisms by which the element improves device efficiency following the cadmium chloride treatment are still not fully understood. In this work the distributions of chlorine in a high efficiency CdTe device are tracked over large areas and in three dimensions by high resolution dynamic SIMS measurements. The results give new insights into the role of chlorine and defects on the performance of CdTe solar cells, particularly when combined with correlative backscatter diffraction measurements.

A thin layer of Al 2 O 3 at the back of CdSe x T e1-x /CdTe devices is shown to passivate the back interface and drastically improve surface recombination lifetimes and photoluminescent response. Despite this, such devices do not show an improvement in open-circuit voltage (VOC.) Adding a p+ amorphous silicon layer behind the Al 2 O 3 bends the conduction band upward, reducing the barrier to hole extraction and improving collection. Further optimization of the Al 2 O 3 , amorphous silicon (a-Si), and indium-doped tin oxide (ITO) layers, as well as their interaction with the CdCl 2 passivation process, are necessary to translate these electro-optical improvements into gains in voltage.

CdTe based thin film solar cells have shown to be competitive in terms of efficiency and low cost, but the polycrystalline structure and low minority carrier lifetime constrain CdTe based devices from reaching the theoretical efficiency limit. In this work, the effect of Se and Cl segregation in polycrystalline CdSeTe photovoltaic devices was studied. We demonstrated using two-photon time-resolved photoluminescence (TRPL) that the Se alloyed CdSeTe absorber layer shows high minority carrier lifetime, and used density functional theory (DFT) calculations to explain the origin of such high lifetime.

The treatment of CdTe with a CdBr 2 activation annealing step has been performed and the device characterized for its electrical, compositional and structural properties with Capacitance-Voltage, Current-Voltage, Transmission Electron Microscopy, Energy Dispersive X-ray analysis (EDX), Electron Backscattered Diffraction Analysis, photoluminescence and high spatial resolution- Secondary Ion Mass Spectrometry (nanoSIMS). NanoSIMS and EDX show the bromine segregating in the grain boundaries and also in through grain defects. This CdBr 2 treated sample has a net doping density of 1.05x1014 cm-3, an efficiency of 8.6 % with a J sc of 23.7 mAcm-2 and V oc of 762 mV. The bandgap has been calculated to be 1.49 eV.

CdTe thin-film photovoltaics have demonstrated some of the lowest costs of electricity generation owing to its low material cost and ease of manufacturing. However, the full potential of polycrystalline CdTe photovoltaics can only be realized if the open-circuit voltage can be increased beyond 1 V Open-circuit voltage ~850-900 mV has been consistently observed for state-of-the-art polycrystalline CdTe solar cells. Open-circuit voltage of over 1V has been demonstrated for single crystal CdTe devices by doping with Group V elements. Therefore, this study is aimed at understanding behavior of polycrystalline CdTe devices with arsenic doping, its activation and process and performance optimization in order to overcome current voltage limitations in CdTe solar cells.

CdTe photovoltaic devices with a ZnTe back contact have the potential to improve device performance and stability. After performing a sweep of ZnTe deposition and annealing temperatures, device performances were evaluated. Copper doping was performed after the ZnTe depositions by sublimating CuCl. Initial results indicate that ZnTe deposited and annealed for 20 minutes at 250°C improved device performance in terms of fill factor, J SC , and V OC as compared to other deposition temperatures. Copper doping also impacted device performance and a longer copper treatment on ZnTe led to a 17.6% device.

Electricity produced by cadmium telluride (CdTe) photovoltaic modules is the lowest-cost electricity in the solar industry, and now undercuts fossil fuel-based sources in many regions of the world. This is due to recent efficiency gains brought about by alloying selenium into the CdTe absorber, which has taken cell efficiency from 19.5% to its current record of 22.1%. Although the addition of selenium is known to reduce the bandgap of the absorber material, and hence increase the cell short-circuit current, this effect alone does not explain the performance improvement. Here, by means of cathodoluminescence and secondary ion mass spectrometry, we show that selenium enables higher luminescence efficiency and longer diffusion lengths in the alloyed material, indicating that selenium passivates critical defects in the bulk of the absorber layer. This passivation effect explains the record-breaking performance of selenium-alloyed CdTe devices, and provides a route for further efficiency improvement that can result in even lower costs for solar-generated electricity. Selenium in cadmium telluride solar cells is known to allow bandgap engineering, thus enabling highly efficient devices. Here, Fiducia et al. show that selenium also plays a role in passivating defects in the absorber layer.

Atomistic modeling based on Desnity Functional Theory (DFT)- 1/2 method coupled with surface Green’s function has been employed to investigate the energy band alignment results in cadmium selenium telluride (CdSeTe) surfaces. The structural and electronic properties of the bulk ternary alloy were established before cleaving the CdSeTe low index surface facets. The dependency of atomic-scale electronic features on different plane orientations was explored by studying the energy band alignment diagrams in the unreconstructed and reconstructed surfaces. While the low index CdSeTe surface geometry reconstructions replicate the CdTe surface geometries, the energy band alignment features of the unreconstructed and reconstructed CdSeTe surfaces differ from those observed in CdTe low index surfaces. The structurally relaxed reconstructed CdSeTe surface results in purely flat bands as opposed to an internal cusp feature observed in CdTe surfaces. The presence of an internal energy cusp in CdTe(111) surfaces is expected to play a key role in enhancing the hole charge transport towards the back of CdTe-only solar cell device and the absence of such feature in CdSeTe surface may explain one of the reason for the lower performance of CdSeTe-only solar cell seen experimentally.

Thin film CdTe (cadmium telluride) is an important technology in the development of sustainable and affordable electricity generation. More than 10 GW of installations have been carried out using this technology around the globe. It has been demonstrated as a sustainable, green, renewable, affordable and abundant source of electricity. An advanced sublimation tool has been developed that allows highly controlled deposition of CdTe films onto commercial soda lime glass substrates. All deposition and treatment steps can be performed without breaking the vacuum within a single chamber in an inline process that can be conveniently scaled to a commercial process. In addition, an advanced cosublimation source has been developed to allow the deposition of ternary alloys such as CdxMg1−xTe to form an electron reflector layer which is expected to address the voltage deficits in current CdTe devices and to achieve very high efficiency. Extensive materials characterization, including but not limited to scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, high resolution transmission electron microscopy and electron back-scatter diffraction, has been performed to get a better understanding of the effects of processing conditions on CdTe thin film photovoltaics. This combined with computer modeling such as density function theory modeling gives a new insight into the mechanism of CdTe photovoltaic function. With all these efforts, CdTe photovoltaics has seen great progress in the last few years. Currently, it has been recorded as the cheapest source of electricity in the USA on a commercial scale, and further improvements are predicted to further reduce the cost while increasing its utilization. Here, we give an overview of the advantages of thin film CdTe photovoltaics as well as a brief review of the challenges that need to be addressed. Some fundamental studies of processing conditions for thin film CdTe are also presented along with fabrication conditions using the closed-space sublimation method.

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.

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.

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.

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.