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The performance of CdS/CdSe/CdTe devices is related to the conduction band offset of CdSexS1−x/CdTe layers on CdTe solar cells.

This paper is the second part of a two-part series on evolutionary sequential transfer optimization (ESTO). The first part designs a problem generator to generate benchmark problems with diverse properties that can better resemble real-world problems. This part proposes five research questions (RQs) to empirically revisit a large body of knowledge transfer techniques in the context of solution-based ESTO (S-ESTO). Particularly, knowledge transfer techniques are organized from a perspective of transfer strategy, including solution selection, solution adaptation, and the integration of them. The results reveal that the performance of many algorithms is highly sensitive to similarity distribution of STOPs, which suggest the necessity of more flexible algorithm design in future studies. Specifically, when an STOP is full of source tasks that are highly similar to the target task in terms of optimum, one can randomly select one source task and reuse its elite solution(s) to accelerate the target search significantly. When an STOP contains source tasks with mixed similarities to the target task, an effective similarity metric that can represent the transferability is able to identify highly transferable solution(s). When an STOP is full of source tasks with low similarities to the target task, the transferability is expected to be improved by solution adaptation. In solution adaptation, a similarity metric in the form of a maximization (or minimization) objective function that is responsible for representing the transferability is optimized to obtain a mapping function for adapting source solution(s). The source code for reproducing our experiments is available at https://github.com/XmingHsueh/Revisiting-S-ESTOs-PartB.

Evolutionary sequential transfer optimization (ESTO), which attempts to enhance the evolutionary search of a target task using the knowledge captured from several previously-solved source tasks, has been receiving increasing research attention in recent years. Despite the tremendous approaches developed, it is worth noting that existing benchmark problems for ESTO are not well designed, as they are often simply extended from other benchmarks in which the relationships between the source and target tasks are not well analyzed. Consequently, the comparisons conducted on these problems are not systematic and can only provide numerical results without a deeper analysis of how an ESTO algorithm performs on problems with different properties. Taking this clue, this two-part paper revisits a large body of solution-based ESTO algorithms on a group of newly developed test problems, to help researchers and practitioners gain a deeper understanding of how to better exploit optimization experience towards enhanced optimization performance. Part A of the series designs a problem generator based on several newly defined concepts to generate benchmark problems with diverse properties, which are competitive in resembling real-world problems. Part B of the series empirically revisits various algorithms by answering five key research questions related to knowledge transfer. The results demonstrated that the performance of many ESTO algorithms is highly problem-dependent, which suggest the necessity of more research efforts on transferability measurement and enhancement in ESTO algorithm design. The source code of the benchmark suite developed in part A is available at https://github.com/XmingHsueh/Revisiting-S-ESTOs-PartA.

An evaporated CdSe layer was incorporated with a very thin CdS layer (40 nm) to optimize the performance of CdTe solar cells. Se alloys with narrower bandgaps were formed in the CdTe deposition and annealing process. With the incorporation of the CdSe layer at the optimal thickness, the highest short-circuit current density was obtained because of the simultaneous high carrier collection in the short-wavelength region and extended carrier collection in the long-wavelength region. Although narrow bandgap Se alloys were introduced, the open-circuit voltage was also significantly increased. As suggested by dark current–voltage measurements, the current leakage paths induced by the thin CdS layer can be significantly suppressed by the incorporation of the CdSe layer. The redshift of PL emission confirms the formation of the Se alloys. As revealed by transient photo-capacitance measurement, the Se alloys are less defective compared with that of CdTe, which is also in accordance with longer carrier lifetimes extracted from photoluminescence decays. The formation of Se alloys prior to the CdTe layer is favorable for the collection of photogenerated carriers. We attribute the performance enhancement to the formation of Se alloys, which are helpful to suppress the leakage current and enhance carrier collection. The incorporation of CdSe with thin CdS layers is validated to improve the diode quality and promising for further device performance improvement.

CdS/CdSe composite window layer is a better candidate than CdS or CdSe single layer for CdTe solar cells. In order to find the underlying reason, the properties of co-sputtered CdSe1−xSx thin films with different compositions ( $$0\le x\le 1$$ ) were examined. XRD patterns show that all CdSe1−xSx films ( $$0\le x\le 1$$ ) are polycrystalline with hexagonal structures and the prominent peak position shows an increasing trend with the increase of x. This suggests that CdSe1−xSx window layer with a gradient composition, formed during the high-temperature growth and heat treatment of CdTe films, has the gradient lattice constant, meaning the less defect density. This is beneficial to the device performance. Hall measurements show that CdS1−xSex films (x ≥ 0.772) are n-type semiconductors with carrier concentrations ranging from 1015 cm−3 to 1016 cm−3. It reveals that CdS/CdSe bi-layers can form a strong built-in electric field with CdTe layer as CdS does. The electronic band structure of CdSe1−xSx window layer with a gradient composition suggests that the spikes formed in the conduction band are lower than 0.3 eV, which would not impede the electron transport. The valence band alignment is beneficial for the hole transport. Therefore, an appropriate band alignment can be formed between CdSe1−xSx and CdTe layers. These results reveal that CdSe1−xSx film is a good candidate as the window layer for high-efficiency CdTe solar cells considering its structural, optical and electrical properties as well as its electronic structure.

CdTe solar cell is a promising alternative to conventional silicon solar cells and it is quite potential to be utilized as a top sub-cell for tandem solar cells to improve the power conversion efficiency of the existing single-junction devices, whose efficiency has been approaching their practical efficiency limit. Thus, the research and development of semitransparent CdTe solar cells are urgently required. In this work, semitransparent CdTe solar cells were prepared by utilizing 900-nm-thick ultrathin CdTe absorber and CuCl/ITO transparent back contacts. The ultrathin CdTe in the present work was prepared by magnetron sputtering and the effects of substrate temperature and deposition ambient were extensively investigated. CdTe thin films with minimal dislocation density and lowest internal strain were fabricated at a substrate temperature of 235 °C with pure Ar atmosphere. The improved optical–electrical properties of CdTe absorber were then contributing on device performance, and as a result, we successfully fabricated a semitransparent CdTe solar cell with an optimal efficiency of 8.60%. It is worth noting that the transmittance around 1000 nm in the near-infrared region (NIR) is over 70%, which is the highest value among the as-reported semitransparent CdTe solar cells. The present work paves a way to optimize the performance and improve the optical transmittance of semitransparent CdTe solar cells so that it strongly supports their potential applications in bifacial and/or tandem configurations, building integrated photovoltaics, and so on.

We studied the impact of in situ post-growth annealing process on cadmium telluride (CdTe) polycrystalline thin film in this work. Samples of different annealing times have been characterized by x-ray diffraction (XRD) and a scanning electron microscope (SEM). The optoelectronic properties of CdTe film were deeply studied with light I–V testing, external quantum efficiency (EQE) and photoluminescence (PL)/time-resolved photoluminescence (TRPL) measurements. It is found that the in situ post-growth annealing treatment has a great effect on cadmium sulfide (CdS)/CdTe intermixing as well as interface pinholes. Elementary correlations between annealing time, film and junction morphology, carrier lifetime and device performance were investigated, and the annealing time turns out to be crucial to CdTe device performance. Solar cells with CdTe polycrystalline thin film annealed in situ at 550°C/550°C for 10 min show the best performance. With the help of precisely controlled annealing time, better CdTe film microstructure and morphology can be realized after in situ post-growth annealing.

The introduction of annealing treatment in CdCl2 atmosphere has been demonstrated to be an effective method in improving the efficiency of CdTe solar cells. Kelvin Probe Force Microscopy (KPFM) was used to study the effect of the CdCl2 treatment process by spatially resolved imaging of the surface contact potential difference (CPD). The results indicated that the grains were spatial depleted by grain boundaries (GBs). The quantitatively CPD distribution at GBs was analyzed by Gaussian fitting. During annealing process the carrier density was decreased and the electronic properties of the GBs were modified. The electronic structure changes resulted from the CdCl2 treatment at GBs were put forward to explain the efficiency enhancement in device performance. The results suggested that GB depletion regions were narrowed down and barrier height at the GBs was also significantly decreased. Therefore the carrier transport can be enhanced. A GB model including the electron traps was introduced. It was found that the barrier height at GBs can reach the maximum value at a specific hole density.

Several ZnO:Al thin films have been successfully deposited on glass substrates at different substrate temperatures by RF (radio frequency) magnetron sputtering method. Effects of the substrate temperatures on the optical and electrical properties of these ZnO:Al thin films were investigated. The UV-VIS-NIR spectra of the ZnO:Al thin films revealed that the average optical transmittances in the visible range are very high, up to 88%. X-ray diffraction results showed that crystallization of these films was improved at higher substrate temperature. The band gaps of ZnO:Al thin films deposited at 25°, 150°, 200°, and 250° are 3.59eV, 3.55eV, 3.53eV, and 3.48eV, respectively. The Hall-effect measurement demonstrated that the electrical resistivity of the films decreased with the increase of the substrate temperature and the electrical resistivity reached 1.990×10-3Ωcm at 250°. © 2017 Elsevier Ltd and Techna Group S.r.l.

The Schottky barrier between the CdTe layer and metal electrode has opposite polarity to the CdS/CdTe cell junction, which can greatly degrade cell performance. Adding a back contact (BC) layer can reduce the Schottky barrier at metal/p-CdTe interfaces. Paste including CuTe nanoparticles and carbon nanotubes (CuTe NPs/CNTs) was used as a BC in thin-film CdTe solar cells. The effect of the mass of carbon nanotubes (CNTs) in the paste and the BC annealing temperature on cell performance was explored. Cu film and paste including Cu nanoparticles and carbon nanotubes (Cu NPs/CNTs) were fabricated as the BC for CdTe solar cells. The performance of CdTe solar cells based on different kinds of Cu-containing BCs studied. The fill factor and open-circuit voltage (V OC) of devices with CuTe NPs/CNTs BC were greatly improved by optimizing the mass of CNTs in the paste and the annealing temperature. The carrier concentration in the CdTe layer was improved by one order of magnitude. The CuTe NPs/CNTs BC showed the best effect on cell efficiency for the Cu-containing BC.

In this work, a post annealing process in CdCl2 was applied to improve the quality of pulsed laser deposited CdS films, and the effect of annealing temperature on the quality of CdS films was studied. The research indicated that the grain size of CdS thin films obviously increased after the annealing process, and crystallinity and optical transmission performance of CdS films were enhanced as well. Moreover, the CdTe solar cell with the annealed CdS window layer obtained better performance. The annealing temperature is a significant factor in the annealing process. 420 °C may be an optimistic annealing temperature, since not only the CdS layer annealed at this temperature had good crystallinity and optical transmission performance with a relatively smooth surface, but also the photovoltaic device with this window layer obtained the best performance among all the samples, including higher short-circuit current, open-circuit voltage and efficiency, corresponding to 26.97 mA/cm2, 760.80 mV and 12.37% respectively.

Aluminum antimony (AlSb) is a potential absorber for solar cells. In present work, we fabricated AlSb thin films by using pulsed laser deposition method, and investigated the influence of substrate temperature on the structural, electrical and optical properties of AlSb thin films. The results suggested that the AlSb thin films were crystal when the substrate temperature was higher than 300 °C. The average grain size of AlSb films increased and the electrical resistivity decreased with the increase of substrate temperature. At the optimized temperature of 400 °C, the optical band gap of AlSb thin film was 1.82eV. The simulated device performance of ZnS/AlSb ultra-thin solar cells showed that the power conversion efficiency was 18.01% and the quantum efficiency values were higher than 55% in the visible range.

Organic–inorganic hybrid perovskite solar cells have attracted significant research attention in terms of perovskite materials, fabrication, device architecture, and interfacial engineering to increase their power conversion efficiency (PCE). However, the state-of-the-art front electrode of perovskite solar cells is mainly focused on indium tin oxide (ITO) and fluorine-doped tin oxide (FTO). To further improve the optical characteristics of front electrodes for perovskite devices, it is necessary to explore a new and suitable transparent conductive oxide material. Herein, we introduce a Cd2SnO4 film for constructing a perovskite device with a novel structure. The as-prepared Cd2SnO4 film shows higher optical transmission in the visible region compared to the FTO substrate. The matching energy band alignment can ensure efficient carrier transport and collection between the TiO2 layer and Cd2SnO4 electrode. The higher PCEs with an average of 15.58% under AM 1.5 irradiation for Cd2SnO4-based perovskite solar cells were obtained compared to those of the FTO-based devices. This renders the Cd2SnO4 film a promising transparent conductive oxide candidate for highly efficient perovskite solar cells. However, the toxicity of lead and cadmium components still remain a major concern for its commercial applications.

Grain boundaries and interfacial impurities are the main factors that limit the further development of polycrystalline perovskite solar cells because their existence severely deteriorates the device performance. In order to optimize the efficiency of perovskite solar cells, it is essential to eliminate these defects. In the present work, potassium iodide (KI) is incorporated into the perovskite absorber. KI incorporation improves the crystallinity of the perovskite, increases the grain size, and decreases the contact potential distribution at the grain boundary, which are verified by X-ray diffraction, scanning electronic microscopy and Kelvin probe force microscopy. Besides, the activation energy of the recombination, estimated from the temperature dependent current–voltage of perovskite solar cells, is larger than the bandgap calculated from the temperature coefficient. These suggest that KI incorporation effectively passivates the grain boundaries and interfacial defects. As a result, charge trapping in the absorber as well as the bimolecular and trap-assisted recombination of the device are significantly suppressed. Consequently, the open circuit voltage and fill factor of the incorporated devices are greatly improved, enabling an optimized power conversion efficiency of 19.5%, in comparison with that of 17.3% for the control one. Our work provides an effective strategy of defect passivation in perovskite solar cells by KI incorporation and clarifies the mechanism of the performance optimization of KI incorporated devices.

Wider band-gap window layers can enhance the transmission of sunlight in the short-wavelength region and improve the performance of CdTe solar cells. In this work, we investigated the band structure of In-doped Zn1&minus, xMgxO (ZMO:In) by using first-principles calculations with the GGA + U method and simulated the performance of ZMO:In/CdTe devices using the SCAPS program. The calculation results show that with the increased Mg doping concentration, the band gap of ZMO increases. However, the band gap of ZMO was decreased after In incorporation due to the downwards shifted conduction band. Owing to the improved short circuit current and fill factor, the conversion efficiency of the ZMO:In-based solar cells show better performance as compared with the CdS-based ones. A highest efficiency of 19.63% could be achieved owing to the wider band gap of ZMO:In and the appropriate conduction band offset (CBO) of ~0.23 eV at ZMO:In/CdTe interface when the Mg concentration x approaches 0.0625. Further investigations on thickness suggest an appropriate thickness of ZMO:In (x = 0.0625) in order to obtain better device performance would be 70&ndash, 100 nm. This work provides a theoretical guidance for designing and fabricating highly efficient CdTe solar cells.

We report use of lithium chloride (LiCl) as a non-Cd dopant to deal with the environmental issues associated with use of traditional CdCl2 dopant in CdTe solar cells. It has been found that, after LiCl treatment, device performance parameters including external quantum efficiency and conversion efficiency were improved considerably, being comparable to those of a counterpart treated with CdCl2. The optimal efficiency of 9.58% was obtained at 405°C, and V oc as high as ∼737.3 mV was obtained at 385°C. Thorough study of the properties of the CdTe film treated by LiCl by x-ray diffraction analysis, scanning electron microscopy, x-ray photoelectron spectroscopy, and secondary-ion mass spectroscopy further verified the feasibility of posttreatment with nontoxic LiCl for fabrication of CdTe photovoltaic devices. The doping level of p-type CdTe thin film was improved by lithium. This represents a nontoxic approach for fabrication of commercial CdS/CdTe thin-film solar cells with better performance.

The effect of post annealing in CdCl2 was investigated to elucidate the correlation between the evolution of the microstructure of CdTe thin films with the annealing time and the performance of CdS/CdTe thin-film solar cells made by in situ pulsed laser deposition (PLD). It has been found that the crystallinity of the annealed film is improved considerably as the CdTe polycrystalline grains evolved into grain clusters with a dendrite structure. While longer annealing time allows larger grain size, the increased film surface roughness outweighs the benefit of the improved crystallinity, resulting in increased structural disorders, such as pinholes. In fact, the concentration of the pinholes was found to increases monotonically with annealing time when it exceeds the optimal value, resulting in degradation of device performance and yield especially when pinholes appeared near the CdS/CdTe interface. This result illustrates the importance of controlling the microstructure of CdTe film and its interface with CdS to high-performance CdTe/CdS thin-film solar cells.

Over the past few years, the studies of CdS/CdTe solar cells have become the mainstream, which have achieved good performance of exceeding 20 %. Here, we prepare CdSe thin films by pulsed laser deposition (PLD) which is one of promising techniques to synthesize semiconductor thin films. In this paper, the optical and electrical properties as well as the crystal structure of CdSe thin films deposited under different temperatures are investigated. We find that the deposition rate adds firstly, and then decreases with the increase of substrate temperature. Transmittance spectrum suggests the absorption edges of CdSe thin films under the condition of high temperature move towards short wavelength. The band gaps of CdSe thin films are 1.948, 1.976, 2.013 and 1.978 eV at the deposition temperature of 150, 250, 350, 450 °C, respectively. X-ray diffraction analysis indicates the formation of cubic phase with a strong (111) preferential orientation. The surface morphology of CdSe thin film and the cross-sectional structure of CdSe/CdTe layer are also analyzed. Based on these studies, we further fabricate the CdSe (PLD)/CdTe solar cells that present excellent performance of response in the range of long wavelength over 900 nm and a good efficiency of near 10 % has been realized.

To meet the large-scale fabrication technology of perovskite solar cells in the future, a novel method is proposed herein to achieve an excellent and highly reproducible CH3NH3PbI3−xClx film based on a high concentration spinning process. Pinholes in the CH3NH3PbI3−xClx film were substantially reduced, leading to better uniformity and film coverage. The precursor ions and molecules promoted nucleation and growth while releasing by-products gradually, which was beneficial to continuous growth and crystallization of the perovskite film in high concentration conditions and then resulted in significantly enhanced optoelectronic characteristics and device performance. Up to 13.8% of power conversion efficiency had been achieved by employing the FTO/bl-TiO2/mp-TiO2/CH3NH3PbI3−xClx/Spiro-MeOTAD/Au device structure under AM1.5 global spectral irradiation (100 mW cm−2). This method offered an efficacious and simple strategy for a highly reproducible large-area fabrication process of low-cost perovskite solar cells.

Pulsed laser deposition (PLD) technique is suitable for the deposition of high-quality compound semiconductor thin films, and has been widely developed in recent years. However, pulsed laser deposition of CdS films has rarely been reported. In this work, we prepared CdS thin films using PLD. The effects of growth temperature on the PLD-CdS thin films were studied towards high-performance CdS/CdTe thin film solar cells. Results showed that the CdS film prepared at 400 °C has the best crystallinity and optical transmittance, while 200 °C is more suitable for the window layer of CdTe solar cells due to the highest energy conversion efficiency and the best short-wavelength response. CdCl2 annealing treatment was also employed on the 200 °C-deposited and 400 °C-deposited PLD-CdS layer. Annealing treatment further enhanced crystallinity, and obviously enlarged the grain size. Optical transmittance spectra showed that the band gap of the CdS films increased after annealing. Fermi level of CdS films shifted closer to the conduction band from XPS analysis. CdTe solar cells with annealed windows obtained further improved performance, including higher short-circuit current, open-circuit voltage and energy conversion efficiency.

Fluorine-doped Mg0.21Zn0.79O (MZO:F) thin films were successfully deposited on glass substrates at room temperature by radio frequency (RF) magnetron co-sputtering method with Mg0.21Zn0.79O, MgF2, and ZnF2 targets. Low resistivity (1.26 × 10-1 Ω⋅cm), high Hall mobility (3.55 cm2/Vs) and high carrier concentration (1.39 × 1019 cm−3) were simultaneously realized in the annealed MZO:F film. The effects of doping concentration and annealing temperature on the structural, morphological, electronic, electrical and optical properties of MZO:F films were systematically investigated in this study. XRD and XPS analysis confirmed that F atoms have entered into crystal lattices of MZO and acted as donors when they occupied O sites in the hexagonal lattice. Moreover, it was observed that both doping concentration and annealing temperature greatly affected the crystallinity, grain size, growth orientation and surface morphology of the MZO:F thin films. In addition, the optical gap changed dramatically with the F content and annealing temperature, which could be attribute to the Burstein-Moss effect accompanied by the effects of fluctuating potential arising from the randomly situated impurities.

In this work, a composite window layer of the sandwich structure containing CdS/ZnS multilayer for CdTe thin film solar cell was designed. Two composite windows of this kind with 5 cycles and 8 cycles CdS/ZnS respectively were prepared by pulsed laser deposition, and finally were fabricated into CdTe-based thin film solar cell. SEM images show that the pulsed laser deposited composite window layers have good crystallinity. CdS and ZnS are layer-by-layer stacking together as the design. In comparison with CdS single-layer window, the composite windows have a blue-shift transmittance edge due to introduction of wide-bandgap ZnS, and show higher transmittance in the short-wavelength region. CdTe solar cells with the composite window obtain further improved performance, including higher short-circuit current (JSC) and energy conversion efficiency. Also, enhanced QE of CdTe solar cells with the composite window is observed in both short-wavelength and visible region. Meanwhile, some differences exist in the J–V and QE curves between the two composite window solar cells, indicating the volume proportion of ZnS may be an important factor that affects the performance of CdTe solar cell with the CdS/ZnS composite window.

CdTe films have been prepared by close-spaced sublimation under different conditions. When CdTe is deposited at a low temperature, grains with a single predominant growth plane (111) adhere to the substrate and gather into scattered particles. With increasing deposition temperature, the number of CdTe grains on the substrate increases very quickly, the grains begin to form a quasi-continuous film, and (220), (311), (400), and (331) growth planes of CdTe begin to appear. When the deposition temperature and time are increased further, the CdTe grains begin to accumulate, the pin-holes formed initially begin to disappear, and grain boundaries with traces of layer growth can be observed. Different transmittance of samples of different thickness is clearly apparent but changes of the band gap, E g, of the CdTe films is negligible. XPS results suggest CdTeO3 is generated on the CdTe film surface as the deposition time is increased.

The light and dark output performances of CdS/CdTe solar cells made by close-spaced sublimation (CSS) were investigated to elucidate the transportation properties of carriers at CdS/CdTe heterojunction interface. It has been found that the interfacial transportation properties were relatively sensitive to variations of the characteristics of heterojunction due to the series resistance and shunting effects. For the high quality cell with 12.1% efficiency, narrow depletion region of ~1.1 microns and large electric field intensity of ~1.3 V/μm allow the sufficient energy-band bending close to CdS layer at CdS/CdTe heterojunction, which changes the carrier transportation mechanism from emission to diffusion and leads to the optimal rectifying characteristics with small dark saturation current density ~6.4 × 10−10 A/cm2. As a result, the schematic diagram of heterojunction band structure corresponding to various performances of solar cells has also been presented.

Cadmium telluride (CdTe) thin films deposited by pulsed laser deposition (PLD) on fluorine–tin–oxide substrates under different pressures of argon (Ar) + oxygen (O2) at high substrate temperature (Ts = 500 °C) was reported in this paper. In our work, the CdTe thin films were prepared successfully at high Ts by inputting Ar + O2. As reported, PLD-CdTe thin films were almost prepared at low substrate temperatures (

It is a reasonable way to increase power generation and reduce the expensive solar cell materials by combining low-cost concentrator and solar cells. The characterization of low concentration CdTe solar cell under standard test condition (STC) and monochromatic light were carried out. The application of concentrator significantly increases the power output of CdTe solar cell. The monochromatic I-V results suggest that the FF of CdTe solar cell was critically affected in the wave length range of 525 to 780 nm. Also the effect of the anti-reflective film on the low concentration CdTe solar cells were revealed, and the results imply that the anti-reflective effect is magnified mainly at the wave length range of 525 to 780 nm.

Cadmium Selenide (CdSe) is a II-VI group compound semiconducting material with a direct band gap about 1.74 eV, which is widely applied in thin film transistors, light emitting diodes and photoelectrochemical (PEC) cells. In recent years, the application of CdSe thin films to CdTe thin film solar cells has attracted the attention of people. CdSe thin films were synthesized using radio frequency magnetron sputtering method on Corning glass substrates at room temperature. The as-deposited films were annealed at various temperatures (200°C, 300°C, 400°C and 500°C) in N2 atmosphere for 1h and the effect of annealing temperature on structural, morphological, compositional, optical and electrical properties was studied. X-ray diffraction (XRD) indicates that the structure of CdSe thin films transforms from cubic to hexagonal structure when annealing temperature changes from room temperature to 500°C. Scanning electron microscope (SEM) images show that the crystallite size of CdSe thin films has no obvious increase with annealing temperature increasing from 200°C to 400°C, which is all about 33 nm. However, grains with size of 200 nm were precipitated on the surface of CdSe thin film annealed at 500°, and the Se content of CdSe thin film decreased. The band gaps of CdSe thin films were found to firstly decrease, and then gradually increase with the increase of annealing temperature. The minimal band gap of CdSe thin film is 1.63eV when annealed at 300°C. The conductivity of CdSe thin film annealed at 500° increased by two orders of magnitude compared with as-deposited CdSe thin film.

Se alloying CdTe absorber has attracted much attention for enhancing the short current density (J sc ) of CdTe device with high fill factor (FF) and open circuit voltage (V oc ). The structural, morphological, optical properties and electronic structure of co-sputtered CdTe1−x Se x films (0 ≤ x ≤ 1) are determined in this study. All CdTe1−x Se x films are polycrystalline in nature. Phase transition occurs as x increases. The band gap (E g ) of CdTe1−x Se x films can be tuned from 1.35 eV to 1.70 eV. CdTe and CdTe0.838Se0.162 films are p-type but the others are n-type, judging from the relative position of Fermi energy (EF) and the intrinsic Fermi energy (EI). Appropriate electric field can form in Se alloying CdTe absorber with a gradient composition, which enhances carrier collection. The barrier height at the emitter/absorber interface can be designed appropriately by changing the composition of CdTe1−x Se x films. These results give reasons why CdTe1−x Se x absorber is better than CdTe and device performance can be further improved by optimizing the composition distribution of CdTe1−x Se x alloys.

The application of thinner cadmium sulfide (CdS) window layer is a feasible approach to improve the performance of cadmium telluride (CdTe) thin film solar cells. However, the reduction of compactness and continuity of thinner CdS always deteriorates the device performance. In this work, transparent Al2O3 films with different thicknesses, deposited by using atomic layer deposition (ALD), were utilized as buffer layers between the front electrode transparent conductive oxide (TCO) and CdS layers to solve this problem, and then, thin-film solar cells with a structure of TCO/Al2O3/CdS/CdTe/BC/Ni were fabricated. The characteristics of the ALD-Al2O3 films were studied by UV&ndash, visible transmittance spectrum, Raman spectroscopy, and atomic force microscopy (AFM). The light and dark J&ndash, V performances of solar cells were also measured by specific instrumentations. The transmittance measurement conducted on the TCO/Al2O3 films verified that the transmittance of TCO/Al2O3 were comparable to that of single TCO layer, meaning that no extra absorption loss occurred when Al2O3 buffer layers were introduced into cells. Furthermore, due to the advantages of the ALD method, the ALD-Al2O3 buffer layers formed an extremely continuous and uniform coverage on the substrates to effectively fill and block the tiny leakage channels in CdS/CdTe polycrystalline films and improve the characteristics of the interface between TCO and CdS. However, as the thickness of alumina increased, the negative effects of cells were gradually exposed, especially the increase of the series resistance (Rs) and the more serious &ldquo, roll-over&rdquo, phenomenon. Finally, the cell conversion efficiency (&eta, ) of more than 13.0% accompanied by optimized uniformity performances was successfully achieved corresponding to the 10 nm thick ALD-Al2O3 thin film.

Cd1-xZnxSthin films have been prepared by the vacuum coevaporation method. The structural, compositional, and optical properties ofCd1-xZnxSthin films have been investigated using X-ray diffraction, X-ray fluorescence, and optical transmittance spectra. As-depositedCd1-xZnxSthin films are polycrystalline and show the cubic structure forx=1and hexagonal one forx<1with the highly preferential orientation. The composition ofCd1-xZnxSthin films determined from Vegard's law and quartz thickness monitors agrees with that determined from the X-ray fluorescence spectra. Optical absorption edge of optical transmittance forCd1-xZnxSthin films shows a blue shift with the increase of the zinc content. The band gap forCd1-xZnxSthin films can be tuned nonlinearly withxfrom about 2.38 eV for CdS to 3.74 eV for ZnS. A novel structure for CuInS2-based solar cells with aCd0.4Zn0.6Slayer is proposed in this paper.

The apparent quantum efficiency measurement under forward bias can be used to analyze the window layer inter-diffusion, the photocurrent loss mechanism and the battery back-contact barrier height. In this article, quantum efficiency measurement and apparent quantum efficiency measurement under forward bias are performed on the MgxZn1−xO/CdS/CdTe solar cells. The results show that the introduction of MZO thin film is benefit to reduce CdS thickness which improves the short-wavelength response of quantum efficiency. The inter-diffusion occurs at the interface between CdS and CdTe, where CdS0.844Te0.156 polycrystalline thin film possibly forms. However, there might be an obvious electron barrier at the conduction band interface between CdS0.844Te0.156 and MZO, which reduces the carrier collection efficiency. In addition, more photo-generated carriers are collected with increasing forward bias in the space charge region of the back contact barrier to generate a current opposite to the primary photocurrent. So the AQE response for the cell without the back contact decreases monotonously in the wavelength range of 300–600 nm and has a lower negative peak in the range of 800–900 nm when the forward bias exceeds 0.6 V.

Using a wider band-gap transparent conducting oxide (TCO) instead of CdS as the window layer of CdTe solar cells can increase the transmission of sunlight in the ultra-violet portion of the solar spectrum. In this paper, we tried to use Zn1-x Mg x O (ZMO) which is a ternary alloy with a controllable wide band gap as an alternative to CdS layer. The band structures of ZMO with different Mg compositions were calculated by using first-principles. Based on the theoretical calculation results, ZMO/CdTe solar cells were modeled and the device performances were simulated by using SCAPS software. The results show that the high conversion efficiency was obtained when the ZMO conduction band minimum exceeded that of CdTe layer by more than about 0.13 eV (x = 0.125). Furthermore, we focused on the effect of the thickness, donor density and interface states density of ZMO film (x = 0.125) on the performances of CdTe solar cells, and compared it with the simulation results of CdS/CdTe solar cell. For ZMO/CdTe solar cell, the primary contribution of the efficiency improvement was from the enhancement of the short-circuit current density, which resulted from the increase of the EQE at the wavelength less than 510 nm. The carrier recombination was increased for the high donor density and interface state density of ZMO film, which resulted in the low EQE at the short wavelength. Therefore, the efficiency of CdTe solar cell could be further improved up to 18.69% for the ZMO film with the appropriate donor density and the low interface state density of 1014 cm−2 eV−1.

Deposition ofSb2Te3thin films on soda-lime glass substrates by coevaporation of Sb and Te is described in this paper.Sb2Te3thin films were characterized by x-ray diffraction (XRD), x-ray fluorescence (XRF), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), electrical conductivity measurements, and Hall measurements. The abnormal electrical transport behavior occurred fromin situelectrical conductivity measurements. The results indicate that as-grownSb2Te3thin films are amorphous and undergo an amorphous-crystalline transition after annealing, and the posttreatment can effectively promote the formation of Sb-Te bond and prevent oxidation of thin film surface.