Effect of the cadmium chloride treatment on RF sputtered Cd0.6Zn0.4Te films for application in multijunction solar cells

Single phase Cd0.6 Zn 0.4Te (CdZnTe) films of 1 μm thickness were deposited by radio frequency planar magnetron sputter deposition on commercial soda lime glass samples coated with fluorine-doped tin oxide and cadmium sulphide (CdS). The stack was then treated with cadmium chloride (CdCl2) at different temperatures using a constant treatment time. The effect of the CdCl2 treatment was studied using optical, materials, and electrical characterization of the samples and compared with the as-deposited CdZnTe film with the same stack configuration. The band gap deduced from Tauc plots on the as-deposited CdZnTe thin film was 1.72 eV. The deposited film had good crystalline quality with a preferred orientation along the {111} plane. After the CdCl2 treatment, the absorption edge shifted toward longer wavelength region and new peaks corresponding to cadmium telluride (CdTe) emerged in the x-ray diffraction pattern. This suggested loss of zinc after the CdCl2 treatment. The cross sectional transmission electron microscope images of the sample treated at 400 °C and the energy dispersive elemental maps revealed the absence of chlorine along the grain boundaries of CdZnTe and residual CdTe. The presence of chlorine in the CdTe devices plays a vital role in drastically improving the device performance which was not observed in CdZnTe samples treated with CdCl2. The loss of zinc from the surface and incomplete recrystallization of the grains together with the presence of high densities of stacking faults were observed. The surface images using scanning electron microscopy showed that the morphology of the grains changed from small spherical shape to large grains formed due to the fusion of small grains with distinct grain boundaries visible at the higher CdCl2 treatment temperatures. The absence of chlorine along the grain boundaries, incomplete recrystallization and distinct grain boundaries is understood to cause the poor performance of the fabricated devices

Effect of CdCl2 passivation treatment on microstructure and performance of CdSeTe/CdTe thin-film photovoltaic devices

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

Polycrystalline CdSeTe/CdTe absorber cells with 28 mA/cm2 short-circuit 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 to 1.5 eV for CdTe. This lower band-gap allowed a current increase from approximately 26 to over 28 mA/cm2. Voltage same as earlier demonstrated high efficiency CdTe-only device was maintained. The fill-factor was not significantly affected. Improving the short-circuit current and maintaining the open-circuit voltage lead to device efficiency over 19%. QE implied that the approximately half the current was generated in the CdSeTe layer and half in the CdTe. Cross-section STEM and EDS showed good grain structure throughout and diffusion of Se into the CdTe layer was observed. To the best of authors’ knowledge this is the highest efficiency polycrystalline CdTe photovoltaic device demonstrated amongst universities and national labs

Advanced co-sublimation hardware for deposition of graded ternary alloys in thin-film applications

© 2018 IEEE. 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

Proceedings of the Thirteenth International Society of Sports Nutrition (ISSN) Conference and Expo

Meeting Abstracts: Proceedings of the Thirteenth International Society of Sports Nutrition (ISSN) Conference and Expo Clearwater Beach, FL, USA. 9-11 June 201

Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material: The Clear Role of Chalcogen Ratio on Light Absorption and Charge Transport for Cu_2–<i>x</i>Zn_1+<i>x</i>Sn(S_1–<i>y</i>Se_<i>y</i>)₄

Photovoltaic (PV) devices based on bulk polycrystalline Cu₂ZnSn­(S_1–<i>x</i>Se_<i>x</i>)₄ (CZTSSe) as the absorber material have historically shown the best efficiency with high Se compositions. The selenization process, which is employed in the formation of absorber layer, has been shown to result in maximum device efficiency at a lower than predicted optimal band gap (<i>E</i>_g= ∼1.1 eV as compared to the 1.34 eV predicted by the Shockley–Queisser detailed balance model). It is still not clear if this deviation is due to changes in the chalcogen composition, grain growth in the film, or increased order in the lattice. In contrast, CZTSSe nanocrystals (NCs) offer a unique opportunity to evaluate the effect of chalcogen ratio on light absorption, charge transport, and photovoltaic performance excluding the impact of the uncertain effects of the conventional selenization step and, importantly, offer a potential path to a dramatic reduction in PV manufacturing cost. Despite an abundance of literature reports on this compound, there is to date <i>no systematic study of the effects of controlled composition of the chalcogen on photocarrier generation and extraction at an optimal and constant cation ratio in a single system.</i> This is required to determine the interplay between light absorbance and transport without compositional convolution and, in turn, to identify the best chalcogen ratio for the unannealed NC PV devices. Here we show that the entire family of Cu_2–<i>z</i>Zn_1+<i>z</i>Sn­(S_1–<i>y</i>Se_<i>y</i>)₄ NCs can be made by a simple one-pot synthetic method with exquisite control over cation content and particle size across the entire range of chalcogen compositions. These NCs are then used to make solution-processed and electrically conductive CZTSSe NC films in the full range of S/(S + Se) ratios via ligand exchange without postdeposition annealing. The transport properties assessed by Hall-effect measurements revealed an intrinsic increase in film conductivity with selenium incorporation. These measurements are then correlated with the PV performance at the full range of band gaps (<i>E</i>_g = 1.0–1.5 eV), leading to an observed maximum in power conversion efficiency centered around <i>E</i>_g = 1.30 eV, which is much closer to the predicted Shockley–Queisser optimal band gap, an outcome predominantly dictated by the compromise between electrical conductivity and band gap

Effect of varying deposition and substrate temperature on sublimated CdTe thin film photovoltaics

A standardized process used for fabrication of CdTe solar cells was varied by increasing the substrate temperature during CdTe layer nucleation from approximately 460ºC to 610ºC 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 Mg1-xZnxO 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

Polycrystalline CdTe photovoltaics with efficiency over 18% through improved absorber passivation and current collection

© 2017 Elsevier B.V. Sublimated thin-film CdTe photovoltaic devices with conversion efficiencies over 18% and a fill-factor greater than 79% have been repeatedly obtained using high-rate fabrication processes on commercial soda-lime glass substrates used in CdTe modules. Four major improvements to the device have enabled an increase in efficiency from a baseline of approximately 12–18.7%: 1) A sputtered multilayer metal-oxide anti-reflection layer; 2) total replacement of the CdS window layer with a higher bandgap sputtered Mg x Zn 1−x O (MZO) window layer; 3) deposition of the CdTe layer at a higher thickness and substrate temperature; and 4) an evaporated tellurium back-contact. This work describes the effect of these changes on the device performance and film microstructural characteristics using various methods. Multiple devices with comparable high efficiency have been fabricated and demonstrated using methods described in this study, yielding very high efficiencies for CdTe polycrystalline thin-film photovoltaics using deposition processes and equipment in a university setting