The effect of temperature on resistive ZnO layers and the performance of thin film CdTe solar cells

The use of a highly resistive transparent (HRT) layer has been shown to increase the efficiency of thin film CdTe heterostructure solar cells incorporating a thin CdS layer. In this study ZnO HRT layers were deposited at different substrate temperatures on soda lime glass and on fluorine-doped tin oxide-coated glass to enable structural, optical and electrical characterization. The performance of equivalent films was tested within CdS/CdTe solar cells. The ZnO thickness was limited to 150 nm, whilst the substrate temperature was varied from 20 °C to 400 °C during deposition. X-ray diffraction patterns and transmission electron microscopy of the cross-sectional microstructure of completed devices showed that the growth of the ZnO is improved when the films are deposited at higher temperatures. Film resistivity was lowest at 100 °C and highest at 400 °C, ranging from 10− 2 Ω·cm to 0.33 Ω·cm. The high temperature deposited ZnO exhibits improved micro-structural growth and an improvement in device efficiency

Intragranular defects in As-deposited and Cadmium Chloride treated Polycrystalline Cadmium Telluride solar cells

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

Effect of varying process parameters on CdTe thin film device performance and its relationship to film microstructure

The performance of CdTe thin film photovoltaic devices are sensitive to process parameters. In this study, efforts are made to further understand the effects of process parameters like process temperature and variation in cadmium chloride passivation treatment on CdTe films deposited using a sublimation based deposition system. The effects on film microstructure are studied using advanced microstructural characterization methods like TEM, SEM, EDS and SIMS while electrical performance is studied using various electrical measurements such as current density vs. voltage and electroluminescence. The aim of this study is to provide new insight into the understanding of relationship between fabrication process, device performance and thin film microstructure

High resolution 3D chemical characterisation of a cadmium telluride solar cell by dynamic SIMS

Impurity elements such as chlorine and sulphur can have significant effects on the electrical performance of cadmium telluride (CdTe) solar cells. Here, the 3D distribution of such elements in a cadmium chloride treated CdTe device has been determined by high resolution dynamic SIMS, a novel technique that has not been applied to thin-film PV cells. It is found that as well as segregating to grain boundaries following treatment, chlorine also segregates to the CdS/CdTe interface. Conversely, sulphur shows a U-shaped diffusion profile. These results have potential implications for the processing thin-film CdTe devices

Testing the durability of anti-soiling coatings for solar cover glass by outdoor exposure in Denmark

The presence of soiling on photovoltaic modules reduces light transmission through the front cover glass to the active absorber, thereby reducing efficiency and performance. Current soiling mitigation techniques are expensive and/or ineffective. However, anti-soiling coatings applied to the solar cover glass have the potential to reduce soiling for long periods of time without continuous maintenance. This paper reports the performance of two transparent hydrophobic coatings (A and B) exposed to the outdoor environment of coastal Denmark for 24 weeks. A comparison was made between the performance of coated and uncoated glass coupons, periodically cleaned coupons, and accelerated laboratory tests. Although initial results were promising, water contact angle and transmittance values were found to decline continuously for all coated and uncoated coupons. Surface blisters, film thickness reduction, changes in surface chemistry (fluorine loss), and abrasion damage following cleaning were observed. Coupons cleaned every 4 weeks showed a restoration in transmittance. Cycles of light rainfall and evaporation combined with a humid and salty environment led to cementation occurring on all coupons. The development of an abrasion-resistant, super-hydrophobic coating with a low roll-off angle and high water contact angle is more likely to provide an anti-soiling solution by reducing the build-up of cementation

Cadmium chloride assisted re-crystallisation of CdTe: The effect on the CdS window layer

© 2015 Materials Research Society. The cadmium chloride annealing treatment is an essential step in the manufacture of efficient thin film CdTe solar cells. In previous work we have shown that the primary effect of the treatment is to remove high densities of stacking faults from the as-deposited material. Use of density functional theory has shown that some of the higher energy stacking faults are hole traps. Removal of these defects dramatically improves cell efficiency. In this study we focus on the effect of the activation treatment on the underlying n-type cadmium sulphide layer. A range of techniques has been used to observe the changes to the microstructure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include High Resolution Transmission Electron Microscopy (HRTEM) for subgrain analysis, EDX for chemical analysis and XPS and SIMS for composition-depth profiling. By studying the effect of increasing the treatment time and temperature, we will show that the cadmium sulphide layer depletes to the point of complete dissolution into the absorber layer. We will also show that chlorine penetrates and decorates the grain boundaries in the cadmium sulphide. In addition we will show that chlorine builds up at the heterojunction and concentrates in voids at the cadmium telluride/cadmium sulphide interface. A combination of these effects damages the electrical performance of the solar cell

Cadmium chloride-assisted re-crystallization of CdTe: the effect of annealing over-treatment

Although the cadmium chloride treatment is an essential process for high efficiency thin film cadmium telluride photovoltaic devices, the precise mechanisms involved that improve the cadmium telluride layer are not fully understood. The treatment parameters have a narrow window, deviating from these even slightly can be detrimental to cell performance. In this investigation we apply advanced microstructural characterization techniques to study the effects of varying two parameters: the temperature of the substrate during the cadmium chloride treatment and the length of time of the treatment. In both cases, the devices have been deliberately over-treated. The effect of the over-treatment on the microstructure of cadmium telluride solar cells, deposited by close spaced sublimation is investigated and related to cell performance. A range of techniques has been used to observe the changes to the microstructure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the microstructural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis, EDX for chemical analysis and XPS for composition-depth profiling

Activation of thin film CdTe solar cells using a cadmium bromide treatment

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 CdBr2 instead of CdCl2 with a lower activation process temperature. Using this method, CdBr2 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. CdBr2 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 effect of a post-activation annealing treatment on thin film CdTe device performance

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 400ºC to 480 ºC 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

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