Microstructural analysis of high efficiency thin film CdTe solar cells has been obtained over large areas. Analysis regions are device cross-sections approximately 0.325 mm in length. The samples have been prepared using a xenon-plasma focused ion beam (Xe- PFIB). The detailed images of the microstructure were obtained using backscattered electron imaging and electron backscatter diffraction (EBSD). As deposited devices and those with a low level of cadmium chloride treatment both show strong (111) growth texture. A high density of twins is seen in the columnar grains. Three As doped FTO/CdZnS/CdTe with varying process conditions we devices with 13.1 %, 16.3% and 17% conversion efficiency were investigated. Lowest efficiency device was CdCl 2 treated at 420°C for 10 minutes while the 16.3 and 17% devices were both treated at 440°C for 10 minutes. The large area analysis revealed a partial recrystallisation state in the 16.3% efficient device which was induced by an incomplete chloride activation process. The analysis confirms that the efficiency of the devices tends to correlate with grain size. It also showed that a strong correlation exists between device efficiency and the randomization of the texture away from the (111) grain orientation. EBSD can be used to survey large areas and to mark out features for more detailed analysis using transmission electron microscopy (TEM). As an example, we show how using an EBSD scanned cross-sectional area can identify a partially recrystallized region which is then extracted and analyzed in detail using TEM.
Abstract The efficiency of as-deposited CdTe solar cells is typically <5 %. An activation process involving the post-treatment of the CdTe surface with cadmium chloride at approximately 400 deg C improves the absorber microstructure by removing stacking faults, fills the grain boundaries with chlorine and leads to efficiencies of up to ~22 %. Stacking fault removal and improvement in device efficiency are thus correlated but the question of whether this correlation is direct or indirect has to date not been established. Although some types of stacking fault could be hole traps, those most commonly observed are tetrahedral in nature which recent work has shown to be electrically benign. For the first time, in this paper, we not only explain the passivation responsible for this efficiency increase but crucially elucidate the associated stacking fault removal mechanism. Experimental work shows that as chlorine is added to the system, the stacking faults gradually disappear from the absorber layer upwards to the surface with some grains being fault-free in the intermediate state before full saturation. At saturation chlorine decorates all the grain boundaries and so the effect of chlorine on a model system of a tetrahedral stacking fault bounded by two grain boundaries is investigated using density functional theory (DFT). We find that if the stacking faults between the grain boundaries were to be removed without Cl, this would result in an energy increase for the system. Increasing the chlorine concentration in the grain boundaries decreases the energy difference between the faulted and unfaulted system until a cross-over occurs close to the point at which chlorine saturates the grain boundaries. The atomic mechanisms and energy profile for the stacking fault removal is presented. The removal process is via a cascade effect whereby the system energy gradually increases as the fault is sequentially removed until the layers snap into place. The energy barrier for this to occur is easily overcome with the 400 deg C temperature treatment. Density of States (DOS) plots of the chlorine saturated structures show that defect passivation occurs in the highly reconstructed grain boundaries due to both interstitial and substitutional chorine. Chlorine saturation is shown to disconnect the two sides of the grain boundary avoiding electronic defects which arise from the interaction of dangling bonds across this region. It is thus concluded that the cell efficiency increase observed is due to the electronic effects of chlorine in the grain boundaries and that the observed stacking fault removal is a bi-product of the chlorine grain boundary saturation.
Recent developments in CdTe solar cell technology have included the incorporation of ternary alloy Cd(Se,Te) in the devices. CdTe absorber band gap grading due to Se alloying contributes to current density enhancement and can result in device performance improvement. Here we report Cd(Se,Te) polycrystalline thin films grown by a chamberless inline atmospheric pressure metal organic chemical vapour deposition technique, with subsequent incorporation in CdTe solar cells. The compositional dependence of the crystal structure and optical properties of Cd(Se,Te) are examined. Selenium graded Cd(Se,Te)/CdTe absorber structure in devices are demonstrated using either a single CdSe layer or CdSe/Cd(Se,Te) bilayer (with or without As doping in the Cd(Se,Te) layer). Cross-sectional TEM/EDS, photoluminescence spectra and secondary ion mass spectroscopy analysis confirmed the formation of a graded Se profile toward the back contact with a diffusion length of ~1.5 μm and revealed back-diffusion of Group V (As) dopants from the CdTe layer into Cd(Se,Te) grains. Due to the strong Se/Te interdiffusion, CdSe in the Se bilayer configuration was unable to form an n-type emitter layer in processed devices. In situ As doping of the Cd(Se,Te) layer benefited the device junction quality with current density reaching 28.3 mA/cm2. The results provide useful insights for the optimisation of Cd(Se,Te)/CdTe solar cells.
Thin film CdTe-based photovoltaic devices have achieved high efficiency above 22 %. The recent improvement in efficiency is due to Se alloying in the CdTe absorbers to form a CdSeTe/CdTe structure. The subsequent band gap grading increases the short circuit current density. The Se can be introduced by depositing a precursor thin film of either CdSe or a CdSeTe alloy and then diffusing the Se into the CdTe during the high temperature cadmium chloride activation process. Using CdSe is preferred because it is easier to control the Se concentration. However, during fabrication of the CdSeTe/CdTe devices, the CdSe thickness needs to be precisely controlled to prevent the retention of a CdSe remnant layer after the activation treatment. Retention of a remnant CdSe layer causes a dramatic reduction in device efficiency. In this work, we show that the reduction in efficiency is caused by a number of factors. The remnant CdSe layer is n-type which moves the position of the p-n junction. Also, it is widely thought that the CdSe remnants are photo-inactive. In this work, we clarify that the individual CdSe grains are actually highly photo-active. However, the grain sizes in the CdSe remnant and the adjacent CdSeTe layer are very small resulting in a high grain boundary area. Although the grain boundaries are passivated with chlorine, cathodoluminescence imaging and electrical measurements show that this is only partially effective. Also, EQE measurements show that the remnant CdSe causes parasitic absorption. Overall, the remnant CdSe layer causes a reduction in short circuit current density and device efficiency. The thickness of the CdSe precursor layer and the cadmium chloride activation process conditions must be precisely optimised to ensure that all the CdSe is consumed and inter-diffused to form the CdSeTe alloy for highest efficiency devices.
We report on the effect of a new laser annealing treatment for thin film CdTe solar cells using a 808 nm diode laser. As-deposited, laser annealed and MgCl 2 treated/laser annealed CdTe thin films have been analysed. One part of the work has been focused on understanding the efficacy of the activation treatment by laser annealing. The results show partial chlorine diffusion and associated partial re-crystallisation of the absorber. The second part of this work has been focused on the effect of the treatment on the chemical composition of the CdTe surface. It has been found that the process also contributes to the formation of a Te-rich layer on the surface of the CdTe absorber, which may provide a useful process to produce a back contact. This paper reveals the effect of the laser treatment on the microstructural properties of the CdTe absorber material. The microstructure has been analysed using STEM/EDX, HRTEM and XRD. Further work is required to optimise the process but it has the potential to provide much greater control than current activation methods and also to provide a Te back contact suitable for CdTe solar cells.
Cadmium telluride (CdTe)-based cells have emerged as the leading commercialized thin film photovoltaic technology and has intrinsically better temperature coefficients, energy yield, and degradation rates than Si technologies. More than 30 GW peak (GWp) of CdTe-based modules are installed worldwide, multiple companies are in production, modules are shipping at up to 18.6% efficiency, and lab cell efficiency is above 22%. We review developments in the science and technology that have occurred over approximately the past decade. These achievements were enabled by manufacturing innovations and scaling module production, as well as maximizing photocurrent through window layer optimization and alloyed CdSexTe1-x (CST) absorbers. Improved chlorine passivation processes, film microstructure, and serendipitous Se defect passivation significantly increased minority carrier lifetime. Efficiencies >22% have been realized for both Cu and As doped CST-based cells. The path to further efficiency gains hinges primarily on increasing open circuit voltage (Voc) and fill factor (FF) through innovations in materials, fabrication methods, and device stacks. Replacing the longstanding Cu doping with As doping is resulting in better module stability and is being translated to large-scale production. To realize 25% efficiency and >1 V Voc, research and development is needed to increase the minority carrier lifetime beyond 100 ns, reduce grain boundary and interface recombination, and tailor band diagrams at the front and back interfaces. Many of these goals have been realized separately however combining them together using scalable manufacturing approaches has been elusive to date. We review these achievements and outstanding opportunities for this remarkable photovoltaic technology.
Cadmium sulphide (CdS) thin films were deposited by two different processes, chemical bath deposition (CBD), and pulsed DC magnetron sputtering (PDCMS) on fluorine doped-tin oxide coated glass to assess the potential advantages of the pulsed DC magnetron sputtering process. The structural, optical and morphological properties of films obtained by CBD and PDCMS were investigated using X-ray photoelectron spectroscopy, X-ray diffraction, scanning and transmission electron microscopy, spectroscopic ellipsometry and UV–Vis spectrophotometry. The as-grown films were studied and comparisons were drawn between their morphology, uniformity, crystallinity, and the deposition rate of the process. The highest crystallinity is observed for sputtered CdS thin films. The absorption in the visible wavelength increased for PDCMS CdS thin films, due to the higher density of the films. The band gap measured for the as-grown CBD-CdS is 2.38 eV compared to 2.34 eV for PDCMS-CdS, confirming the higher density of the sputtered thin film. The higher deposition rate for PDCMS is a significant advantage of this technique which has potential use for high rate and low cost manufacturing.
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