Search

Solar cells are essentially minority carrier devices, and it is therefore of central importance to understand the pertinent carrier transport processes. Here, we advanced a transport imaging technique to directly visualize the charge motion and collection in the direction of relevant carrier transport and to understand the cell operation and degradation in state-of-the-art cadmium telluride solar cells. We revealed complex carrier transport profiles in the inhomogeneous polycrystalline thin-film solar cell, with the influence of electric junction, interface, recombination, and material composition. The pristine cell showed a unique dual peak in the carrier transport light intensity decay profile, and the dual peak feature disappeared on a degraded cell after light and heat stressing in the lab. The experiments, together with device modeling, suggested that selenium diffusion plays an important role in carrier transport. The work opens a new forum by which to understand the carrier transport and bridge the gap between atomic/nanometer-scale chemical/structural and submicrometer optoelectronic knowledge.

Mg x Zn 1-x O (MZO) has recently gained popularity as a transparent electron contact in CdTe-based solar cells. The value of the conduction band offset (CBO) and, hence, of the MZO bandgap are often considered essential parameters of this interface, as—in theory—the CBO has a strong impact on both the conduction of electrons and the recombination of holes. However, in this contribution, we report MZO/CdSeTe interfaces where recombination appears to be independent of the CBO. The implications of such results are discussed.

In this contribution, we investigate why different dopant species and back-contact architectures lead to different sub-bandgap behaviors in CdSeTe solar cells. Through extraction of the absorptance from photoluminescence spectra, we parse the contributions from material properties and from cell optics. We show that, as expected, arsenic doping leads to an increase in sub-bandgap features over traditional copper doping, and that this is a material property of arsenic-doped CdSeTe. Conversely, the increase in sub-bandgap absorption and emission using alternative back contact architectures can be attributed to the cell optics, and more specifically to the increased reflectance of the back interface, leading to at least a doubling of the pathlength for sub-bandgap photons.

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.

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.

Magnesium zinc oxide (MZO, MgxZn1-xO) is a leading emitter for CdTe-based solar cells due to its transparency and the ability to tune its conduction band offset with the absorber. Devices employing alloyed cadmium selenide telluride (CST, CdSeyTe1-y) absorbers achieved high efficiency (>19%) using MZO deposited by reactive sputtering over a broad composition range (3.68–3.92 eV, x: 0.20–0.35). Minimal differences in implied and measured open circuit voltage indicate that the contacts are well passivated and highly selective across the spectrum of MZO employed. Device performance insensitivity to MZO composition, which is not observed in CdTe devices, is attributed to the formation of an oxygenated interface layer. Se volatility creates a group VI deficiency at the interface that drives O migration from the MZO into the absorber. This introduces conductivity in the emitter not present in its as-deposited state, contributing to the exceptional performance observed. It is shown that the quality of device passivation depends on the oxidation state of the as-deposited MZO such that intelligent control and management of the reactive sputtering process is required.

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.

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.

OBJECTIVE. The purpose of this article is to review the tools and opportunities available for patient-centered care in radiology and to create a quality patient-centered care process map to organize them for radiology practices. CONCLUSION. This article provides a review of the many opportunities to increase and improve patient-centered care in radiology. A process map that organizes and highlights key elements of patient-centered care throughout the radiology care process is provided that can be implemented to enhance the patient experience of our services and improve the quality of care provided.

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.

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.

Oncologic 18F-FDG PET/CT is rapidly gaining acceptance in clinical practice. However, the referring physician’s attitude toward the usefulness of this diagnostic modality is unknown. This survey was undertaken to collect information regarding the current perspective of referring physicians on oncologic PET/CT. Methods: We conducted a prospective worldwide, Web-based survey of physicians who manage cancer patients. A total of 963 referring physicians completed a 20-question survey focused on their experience with oncologic 18F-FDG PET/CT. Attention was directed toward their confidence about indications, their satisfaction with related educational resources, the quality of interaction with interpreting physicians, and practical problems encountered. The respondents included oncologists (38.5%, n = 371), hematologists (16.4%, n = 158), radiation oncologists (9.0%, n = 87), surgeons (30.3%, n = 292), and other physicians (5.7%, n = 55). Results: Only 25.2% of respondents considered the oncologic 18F-FDG PET/CT indications to be well established and defined. Frequent uncertainty about the need for a PET scan was indicated by 62.3% of the respondents. High cost and overinterpretation of findings were the most commonly reported concerns (47.0% and 40.9%, respectively). The experience and skill level of the interpreting physician was considered very important by 96.8% of the surveyed physicians. Conclusion: Referring physicians expressed considerable uncertainty about the appropriate use of oncologic PET/CT. Additional major concerns are procedure costs and quality of interpretation. The responses suggest a strong need for efforts to educate referring and interpreting physicians about the appropriate use of 18F-FDG PET/CT in oncology.