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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.

In this study, bromide-iodide lead perovskite (CH3NH3Pb(I1−xBrx)3) thin films were fabricated and applied as copper-free back contact layers for CdTe solar cells. The results reveal that the open-circuit voltage (Voc) and fill factor (FF) of the CdTe solar cells are greatly enhanced by the utilization of perovskite back contact layers. This is mainly due to the evidently reduced charge transportation barrier at the back contact, which is verified by temperature-dependent current–voltage (J-V-T) and apparent quantum efficiency (AQE) measurements. Specifically, after the application of perovskite back contact layer, the best-performing device shows 15.80% improvement in efficiency (from 10.82 to 12.53%), with Voc and FF increasing by 3.94% and 6.91%, respectively. Besides, the overall uniformity of the solar cells is also improved by the perovskite back contact, suggested by laser beam induced current (LBIC) results. Further simulation results confirm the experimental results and clarify the optimized cell performance by perovskite back contact layer in terms of both energy band alignment and charge carriers’ recombination. In addition, in the simulation section, we have also investigated the effect of the bandgap, thickness and doping level of perovskite and the doping level of CdTe on the cell performances. The experiments and the numerical simulation reported in this work suggest perovskite a potential copper-free back contact layer for the fabrication of efficient 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.

The optical and electrical properties of the front transparent conductive film of a heterojunction with intrinsic thin-layer solar cell are the key factors that determine the photovoltaic conversion efficiency of the device. First, the relationship between the structure as well as optoelectronic properties and sputtering process of ITO-97/3 (In2O3:SnO2 = 97 wt.%:3 wt.%) thin film with low SnO2 content and conventional ITO-90/10 (In2O3:SnO2 = 90 wt.%:10 wt.%) thin film was compared and studied in this article. Next, the application of ITO-97/3 thin film for heterojunction with intrinsic thin-layer solar cells was verified. It was found that, although the spectral response was improved, the series resistance of the device deteriorated. Finally, the bilayer structure of ITO-90/10 and ITO-97/3 was introduced into a heterojunction with intrinsic thin-layer solar cell. The results showed that the overall performance of the device was improved because of the combination of electrical and optical characteristics of the bilayer.

Semitransparent CdTe solar cell is a promising technique for the development of ultrathin CdTe photovoltaics, which can not only be used as architectural components of window, fence or roof, but also allows a bifacial structure or tandem configuration to improve sunlight utilization. In this work, the ultrathin polycrystalline CdTe is prepared by sputtering and semitransparent solar cells with a structure of glass/FTO/SnO2/CdS/CdTe (~920 nm)/CuCl/ITO are fabricated. The CdCl2 post treatment was conducted on sputtered ultrathin semitransparent CdTe by varying the CdCl2 treatment amount. The effects of the CdCl2 treatment amount on the microstructure, optical property of ultrathin CdTe films, and the performance of semitransparent CdTe solar cells were extensively studied. By optimizing treatment amount, a front illuminated efficiency of up to 9.74% was achieved. Furthermore, the completed solar cells exhibited excellent near-infrared transmittance of ~70% in the wavelength range of 850–1100 nm, which strongly supports the future application of semitransparent CdTe solar cells in building integrated photovoltaics or tandem configurations.

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.

Cd2SnO4-based CdTe solar cell is a potential technology to fabricate low cost and highly efficient thin film photovoltaic modules due to the excellent optical and electrical properties of Cd2SnO4 conductive transparent layer. However, the detailed research on patterning process is still deficient. In the present work, we developed laser scribing techniques to pattern the Cd2SnO4 thin film and the functional layers on top of the transparent conductive layer. By modifying the process and precisely adjusting the average energy density, the technique was further optimized to remove the upper layers completely without inducing any damage to the Cd2SnO4. The laser scribing process eliminates the exaggerated current density of CdTe solar cells contributed by the light absorption of the residual upper layers and as a result, device performance of the laser scribed cells could be precisely characterized. Besides, the isolative laser scribing of Cd2SnO4 layer and layers on top of Cd2SnO4 (window layer, absorption layer, back contact and metal electrode) is equal to the so-called P1, P2 and P3 steps in thin-film PV module production, which suggests that the whole set of laser scribing technique is applicable to the mass production of the Cd2SnO4-based CdTe solar cell modules.

In this paper, MgxZn1-xO (MZO) films with x from 0 to 0.25 were prepared by radio frequency (RF) magnetron co-sputtering, and the structure, surface morphology, optical and electronic properties of MZO films were investigated. With the increase of Mg content, the crystallinity of MZO films became worse with the appearance of grain agglomeration, and the optical energy band gap (Eg) linearly changed from 3.228 eV to 3.824 eV. The MZO films with different Mg content were used as the window layer for CdTe solar cells with or without CdS layer. The efficiency of MZO/CdTe devices is much lower than MZO/CdS/CdTe devices. Incorporating a buffer layer CdS into CdTe solar cells can significantly improve the devices efficiency, and the devices still maintain a high spectral response in the short wavelength region.

Achieving high open-circuit voltage ( V o c ) for CdTe thin film solar cells is challenging. We report that a ZnMgO (ZMO) buffer can improve V o c and the performance of CdTe solar cells when deposited on the tin oxide (SnO2 or TO) coated fluorine-doped tin oxide (FTO) substrates. The TO/ZMO bilayer buffer brings a better energy band matching at the front contact of CdTe thin film solar cells, which can reduce interface carrier recombination and facilitate electron extraction. Our best-performing CdTe solar cell using the TO/ZMO bilayer buffer has achieved a power conversion efficiency of 16.76%, with an open-circuit voltage of 838.75 mV, a short-circuit current density of 27.32 mA cm−2, and a fill factor of 73.14%, much higher than those of devices based on TO buffer. Our results suggest that a careful interface modification of front contact layers provides a simple and effective route to further improving the performance of CdTe solar cells with a higher V o c .

In this work, CdS thin films were deposited on glass/fluorine-doping tin oxide (FTO) substrates by low-cost chemical bath deposition (CBD), followed by annealing with CdCl2-doping (Cl-doping) to fabricate resistive-type photodetectors (PDs) with a vertical structure of glass/FTO/CBD-CdS:Cl/electrode. The effect of annealing with Cl-doping (AwCl) on the various properties of films and the photodetection performance of PDs were systematically characterized by special testing methods. The results revealed that the surface morphology and crystal structure of CBD-CdS:Cl films were improved, accompanied by an enhancement of hexagonal phase (002) crystal plane (wurtzite structure), and the optical bandgap of films can be adjusted between 2.43 and 2.26 eV while the electron activation energy of the films was reduced from 75.9 to 10.3 meV after AwCl. Furthermore, the light responsivity of CBD-CdS:Cl PDs were 0.57, 0.59 and 1.67 A/W under 365, 400 and 530 nm illumination at a bias of −2.0 V, respectively, and the detectivity of photons at corresponding wavelengths were 3.7 × 108, 3.9 × 108 and 1.08 × 109 Jones. Meanwhile, the PDs showed optimized repeatability and improved sensitivity under periodic illumination. The above results demonstrated that the CBD-CdS:Cl PDs with a vertical structure developed in this work had good detection performance at certain characteristic wavelengths.

CdTe devices with CdTe and CdTeSe/CdTe as absorber layer were fabricated. The highest short circuit current density reaches 28.73mA/cm2 of CdTeSe/CdTe device. Besides, defects of devices with different absorber layers were investigated using admittance. A shallow defect with activation energy of 0.09eV was only detected in CdSeTe/CdTe device, which may assign to the hydrogenic defects in CdSeTe alloy. A defect with activation energy of 0.34eV was detected in both devices which might relate to the native impurities. And the defect with activation energy of 0.55eV was only observed in CdTe only devices. Overall, Se is significant to reduce defects in CdTe.

Poor qualities of ZnMgO thin films are likely to be obtained at higher Mg content, which can decrease the performance of ZnMgO-based devices. This study focuses on improving the qualities of ZnMgO thin films to increase the performance of CdTe solar cell. It is found that the rapid thermal annealing (RTA) treatment increases the crystallinity, eliminates defects and improves the conductivity of ZnMgO thin films. These are helpful to suppress the recombination caused by bulk and interface defects as well as to reduce the cell's series resistance. The Fermi level moves close to conduction band and both the conduction and valence band energy shift lower relative to band energy of CdSSeTe film after RTA, which could not only increase the built-in voltage but also enhance the electron transport and block the hole recombination. As a result, the efficiency of CdTe solar cells is enhanced when using the RTA-treated ZnMgO, with maxium power conversion efficiency reaching values of 15.7%, which are much higher than those of the cells with the as-grown ZnMgO (11.6%). RTA is proved to be one of the excellent methods to control the properties of ZnMgO for improving the performance of CdTe solar cell.

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.

MoOx: Cu thin films were deposited by thermal co-evaporation. The copper atoms prevented MoOx thin films growing along or perpendicular to the substrates, resulting in the decrease of the crystallinity. The energy band gaps for the films decreased from 3.22 to 2.29 eV with the increase of Cu concentration from 0 to 20 at%. Owing to the formation of the oxygen vacancies, MoOx thin films were n-type semiconductors. As MoOx thin films were doped with Cu, the films also showed the characteristics of electronic conduction. In contrast to the difference between the Fermi level and the bottom of conduction band was 0.27 eV for MoOx thin films, a strong hybrization among the O 2p, Mo 4d, and Cu 3d states occurred in the Cu-doped MoOx thin films, thus the electrons occupied the anti-bonding d-d* band and the Fermi level was shifted within the conduction band. With the use of MoOx as a back contact buffer layer, performance of the CdTe solar cells was mainly improved by the reduced Schottky barrier, which is ascribed to the presence of the defect states. CdTe solar cells with Cu-doped MoOx thin films demonstrated higher efficiency (~14.63%) than those with undoped MoOx or without a back contact buffer in our work, which results from the enhancements in the open circuit voltage (~830 mV) and fill factor (~72.7%) due to the zero barrier in the MoOx: Cu thin films. Therefore, MoOx: Cu buffer layer is a promising contact material for CdTe solar cells.

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.

A low electric resistive and stable ohmic back contact are crucial to the commercial application of CdTe solar cells. In this study the Sb2Te3 thin film is employed as a buffer layer in the back contact of CdTe solar cells. The Sb2Te3 thin films are deposited at different substrate temperature by pulsed laser deposition and then characterized to investigate the thin film properties, firstly. Hall measurements shows the Sb2Te3 thin film presents strong p-type semiconductor features with high Hall mobility and carrier concentration. CdTe solar cells with Sb2Te3 buffer layer in different layer thickness and deposition temperature are fabricated to obtain the optimal experiment conditions for device performance. The dark J-V curve of CdTe solar cell with Sb2Te3 buffer layer exhibits a typical diode curve without roll-over phenomenon. An insertion of Sb2Te3 buffer layer eliminates the Schottky barrier resulting in remarkable enhancement of open circuit voltage, short-circuit current and fill factor. Besides, the external quantum efficiency gets improvement in almost all of the absorber wavelength compared to the cells with Au-only contact.

CdSe film is noteworthy for its application in CdTe solar cells. In this paper, the crystal structure, surface profile and optical properties of CdSe films deposited at 25 °C, 200 °C, 250 °C, 300 °C and 350 °C by magnetron sputtering are investigated by XRD, SEM, AFM and an UV–VIS spectrometer. XRD patterns show that CdSe films deposited at 200 °C and 250 °C have cubic structure, which hasn’t been reported so far for sputtered CdSe films. CdSe films deposited at 300 °C and 350 °C are hexagonal. Then CdSe films with different substrate temperatures (25 °C, 200 °C, 250 °C and 300 °C) were used as the window layer for CdTe solar cells. CdSe/CdTe device with CdSe film deposited at 250 °C reaches the highest efficiency. Quantum efficiency (QE) was enhanced in the range of 300–510 nm and 830–900 nm for CdSe/CdTe device (CdSe film deposited at 250 °C) compared to CdS/CdTe device while in the range of 510–830 nm, QE of CdSe/CdTe device was lower than that of CdS/CdTe device indicating an inferior junction between CdSe and CdTe. The results also suggest that CdSe deposition temperature influences the film structure and further to the interfusion between CdSe and CdTe which has a significant effect on device performance.

AlSb is a new type of optoelectronic material with wide potential application prospects, but the applications of AlSb thin films for solar cells were few reported before, owing to the intrinsic drawbacks of deliquescence for AlSb. In this work, four types of AlSb/Sb stacks were fabricated by using pulsed laser deposition: (1) AlSb/Sb (5 nm), (2) AlSb/Sb (10 nm), (3) AlSb/Sb (15 nm) and (4) the in situ annealed AlSb film covered with Sb film which thickness is 15 nm. The experimental results indicate that the thicker the Sb layer deposited on the surface of the AlSb film is, the lower the degree of deliquescence will be. XRD measurements showed that the AlSb/Sb stacks were not deliquesced completely after exposed to air for 48 h. The EDS spectrum suggested that the percentage of oxygen atoms in the annealed AlSb film covered with Sb (15 nm) was smaller than the unannealed. The SEM morphology analysis revealed the deliquescence of AlSb film: the local film deliquesced to form sand inclusions first, after then the gradual increased of the sand inclusions lead to all deliquescence of the crystalline AlSb.

SnO2:F (FTO), as a kind of transparent conductive oxide (TCO), exhibits excellent transmittance and conductivity and is widely used as transparency electrodes in solar cells. It’s very important to modifying the surface of FTO for it plays a critical role in CdTe solar cells. In this study, modifying effects of oxygen plasma on FTO was investigated systematically. Oxygen plasma treatment on FTO surface with ion accelerating voltage ranged from 0.4 kV to 1.6 kV has been processed. The O proportion of surface was increased after ion implantation. The Fermi level of surface measurement by XPS valance band spectra was lowered as the ion accelerating voltage increased to 1.2 kV and then raised as accelerating voltage was elevated to 1.6 kV. The work function measured by Kelvin probe force microscopy increased after ion implanting, and it was consistent with the variation of Fermi level. The change of energy band structure of FTO surface mainly originated from the surface composition variation. As FTO conduction was primarily due to oxyanion hole, the carrier was electron and its concentration was reduced while O proportion was elevated at the surface of FTO, as a result, the Fermi level lowered and the work function was enlarged. It was proved that oxygen plasma treatment is an effective method to modulate the energy band structure of the surface as well as other properties of FTO, which provides much more space for interface and surface modification and then photoelectric device performance promotion.

Cd2SnO4 (CTO) thin films were prepared by RF sputtering technique from a CTO target and then annealed under Ar or Ar/CdS atmosphere. The detailed characterizations on the surface properties of as-deposited, Ar-annealed, and Ar/CdS-annealed CTO thin films were carried out by AFM, XPS, UPS, etc. The surface properties of the films are greatly dependent on the annealing atmosphere. Ar/CdS atmosphere promotes the grains to grow but slightly increases the surface roughness. Cadmium atoms sublimated off the CTO thin films during the annealing process and this sublimation can be suppressed under Ar/CdS atmosphere, which causes the surface of CTO thin films not homogeneous as inside. The fundamental band gap is determined to be 2.83 eV. The Fermi level position is 1.7 eV above the valence band maximum for the as-deposited films, while it shifts to 2.98 eV and 3.21 eV for the films annealed in Ar and Ar/CdS atmospheres, respectively. Work function for the Ar/CdS-annealed films is about 0.16 eV higher than the Ar-annealed ones. These results could be used to design high efficiency thin film solar cells and water photolysis devices, in which the efficiency is sensitive to the surface properties of TCO thin films.

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.

The study of CdS/CdTe solar cells has become popular in the photovoltaic field, in which the highest efficiency achieved is more than 20%. However, the CdTe thin film as the absorbing layer doesn’t perform very well in the long wavelength photon absorption regime. Here, a new single offset superlattice was designed to be the composite absorbing layer, which is based on a super lattice-structure of CdTe/ZnTe using cycle growth. Different cycles are to applied to the solar cells to characterize properties with respect to the super lattice. The crystallinity and transmission spectrum as well as the cross sectional morphology of the ZnTe/CdTe composite layers with different cycles are also characterized in this work. X-ray diffraction analysis showed that there is a strong (111) preferred orientation of CdTe growth. The solar cells based on the structure of FTO/CdS/CdTe/ZnTe:Cu/(ZnTe|CdTe) n /ZnTe:Cu/Au are fabricated which exhibited a desired spectral response in the range of 550–800 nm and still possess slight spectral response in the range of 850–900 nm. The photoelectric conversion efficiency of the cell of this structure is 12.8%, with a short-circuit current density of 25.59 mA/cm 2 and fill factor of 66.8%, significantly improving these properties compared with a cell without the composite layer. This structure can improve the spectral response of the solar cell when integrated with other leading technologies.

In recent years, the power conversion efficiency (PCE) up to 20% was obtained based on CdTe solar cells consisting of Glass/SnO2: F/SnO2/CdS/CdTe/ZnTe: Cu/Au. The underlying carrier transport and collection mechanisms of this architecture become complicated while quantum efficiency with external direct current bias is tremendously informative. Quantum efficiency measurement under forward voltage changes dramatically and it is defined as apparent quantum efficiency (AQE). It is accessible to give insight to the impact of the high resistant transparent buffer layer and CdS window layer on the carrier collection mechanism and photocurrent loss. Herein, five simpler structures were fabricated and tested together with the structure mentioned above for both direct current (DC) and alternating current (AC) modes of quantum efficiency under forward bias and four types (470 nm, 525 nm, 780 nm, 850 nm) of monochromatic light to achieve a pair of I-V curves under different light intensities. Comparing the AQE acquired via applying various forward bias with different device structures along with their collection efficiency of different wavelength, it is available to establish the connection between AQE effects and device structures. The consistent tendency of the AQE response under DC mode at forward bias for different structures of CdTe devices indicates a same current direction without phase shift. While the introduction of SnO2 buffer layer is prone to generate a large phase angle in the shortwave AQE response, resulting in AQE > 1. When there is no photoconductive CdS layer, the device photocurrent under forward bias changes slightly and uniformly in the measured spectra, avoiding intricate band diagram in the CdS region.

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.