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The open-circuit voltage (V $_\text{oc}$ ) of CdTe-based photovoltaics may be limited by carrier recombination at interfaces (front or back) or in the absorber layer. Reduction in recombination of a given dominant mechanisms can lead to improved device performance if the remaining mechanisms turn on in a narrow bias range just below the open circuit voltage. In this article, we demonstrate enhanced performance by incorporating solution-processed Cu $_\text{x}$ AlO $_\text{y}$ to form a back-buffer layer in CdSe/CdTe devices. Outstanding minority carrier lifetimes of 656 and 4.2 ns were measured with glass side and film side illumination for device stacks processed with Cu $_\text{x}$ AlO $_\text{y}$ . Devices demonstrated efficiencies of up to 17.4% with V $_\text{oc}$ of 859 mV, FF of 75.6% and J $_\text{sc}$ of 26.9 mAcm−2 while the efficiency of the reference device without the back-buffer layer was 16.5% with V $_\text{oc}$ of 839 mV, FF of 70.6%, and J $_\text{sc}$ of 27.9 mAcm−2.

Copper (Cu) incorporation is a key process for fabricating efficient CdTe-based thin-film solar cells and has been used in CdTe-based solar cell module manufacturing. Here, we investigate the effects of different Cu precursors on the performance of CdTe-based thin-film solar cells by incorporating Cu using a metallic Cu source (evaporated Cu) and ionic Cu sources (solution-processed cuprous chloride (CuCl) and copper chloride (CuCl2)). We find that ionic Cu precursors offer much better control in Cu diffusion than the metallic Cu precursor, producing better front junction quality, lower back-barrier heights, and better bulk defect property. Finally, outperforming power conversion efficiencies of 17.2 and 17.5% are obtained for devices with cadmium sulfide and zinc magnesium oxide as the front window layers, respectively, which are among the highest reported CdTe solar cells efficiencies. Our results suggest that an ionic Cu precursor is preferred as the dopant to fabricate efficient CdTe thin-film solar cells and modules.

CdTe solar cell technology is one of the lowest-cost methods of generating electricity in the solar industry, benefiting from fast CdTe absorber deposition, CdCl2 treatment and Cu doping. However, Cu doping has low photovoltage and issues with instability. Doping group V elements into CdTe is therefore a promising route to address these challenges. Although high-temperature in situ group V doped CdSeTe devices have demonstrated efficiencies exceeding 20%, they face obstacles including post-deposition doping activation processes, short carrier lifetimes and low activation ratios. Here, we demonstrate low-temperature and effective ex situ group V doping for CdSeTe solar cells using group V chlorides. For AsCl3 doped CdSeTe solar cells, the dopant activation ratio can be 5.88%, hole densities reach >2 × 1015 cm−3 and carrier lifetime is longer than 20 ns. Thus, ex situ As doped CdSeTe solar cells show open-circuit voltages ~863 mV, compared to the highest open-circuit voltage of 852 mV for Cu doped CdSeTe solar cells. Doping CdTe solar cells with group V elements could overcome the limitations in voltage output and device stability of copper doping, yet implementation remains challenging. Now, Li et al. have devised an ex situ doping approach that is based on group V chloride solutions and low-temperature annealing.

Antimony selenide (Sb2Se3) solar cells are an environmentally friendly and cost-effective photovoltaic technology. In this work, we fabricate Sb2Se3 solar cells using closed-space sublimation follo...

Although back-surface passivation plays an important role in high-efficiency photovoltaics, it has not yet been definitively demonstrated for CdTe. Here, we present a solution-based process, which achieves passivation and improved electrical performance when very small amounts of oxidized Al3+ species are deposited at the back surface of CdTe devices. The open circuit voltage (Voc) is increased and the fill factor (FF) and photoconversion efficiency (PCE) are optimized when the total amount added corresponds to ∼1 monolayer, suggesting that the passivation is surface specific. Addition of further Al3+ species, present in a sparse alumina-like layer, causes the FF and PCE to drop as the interface layer becomes blocking to current flow. The optimized deposit increases the average baseline PCE for both Cu-free devices and devices where Cu is present as a dopant. The greatest improvement is found when the Al3+ species are deposited prior to the CdCl2 activation step and Cu is employed. In this case, the best-cell efficiency was improved from 12.6 to 14.4%. Time-resolved photoluminescence measurements at the back surface and quantum efficiency measurements performed at the maximum power point indicate that the performance enhancement is due to a reduction in the interface recombination current at the back surface.

The effects of post-deposition CdCl2 annealing temperature on the electronic properties of CdTe solar cells were investigated. CdTe devices incorporate Mg doped ZnO as a buffer layer and selenization using a CdSe interlayer for reducing the buffer/absorber interface recombination and increasing solar energy absorption respectively. The post-deposition CdCl2 annealing treatments were done under separate, inert atmospheres of nitrogen and helium across the temperature range 380–430 °C. Electrical characterization of devices is carried out including temperature dependent current-voltage characteristics, admittance spectroscopy, and Shockley-Read-Hall recombination analysis. The best improvements in device efficiency are obtained upon annealing at temperature 410 °C. This anneal correlated with reduced back contact barrier in CdTe and reduced grain-boundary barrier height which is beneficial for enhanced charge transport.

The performance of CdTe solar cells has advanced impressively in recent years with the incorporation of Se. Instabilities associated with light soaking and copper reorganization have been extensively examined for the previous generation of CdS/CdTe solar cells, but instabilities in Cu-doped Se-alloyed CdTe devices remain relatively unexplored. In this work, we fabricated a range of CdSe/CdTe solar cells by sputtering CdSe layers with thicknesses of 100, 120, 150, 180, and 200 nm on transparent oxide-coated glass and then depositing CdTe by close-spaced sublimation. After CdCl

Electronic properties of a CdTe solar cell are reported using temperature-dependent capacitance spectroscopy and current-voltage characteristics, the latter in dark and illuminated conditions. The baseline solar cell material stack investigated is comprised of soda-lime-glass/SnO2:F/SnO2/CdS:O-buffer/CdTe-absorber/Cu/Au. Device properties are compared with CdTe solar cells in which the back surface was hydroiodic acid etched, before the back-contact formation, and a CdTe device in which Mg-doped ZnO (MZO) replaces buffer layers. Reduced back-contact barrier height and grain boundary barrier height are observed in the hydroiodic acid treated CdTe cell. Improved device performance in the MZO-based CdTe device is attributed to reduced emitter/absorber interface recombination when using the MZO window layer.

Band offsets at the back contact and front window layer in CdTe-based solar cells affect photovoltaic performance and challenge standard characterization methods. By analyzing the temperature and excitation dependence of both open circuit voltage and absolute photoluminescence intensity, we show that the effects band offsets can be separated from the effects of recombination and shunting. Solar cells were grown with MgZnO window layers and compared to cells with CdS window layers containing varying amounts of oxygen. It was demonstrated that band alignment rather than reduced recombination velocity is the reason for the success of MgZnO as a front interface contact. An assortment of thin back contact interlayers were also deposited, and a PbTe interlayer showed some promise as an Ohmic contact to the CdTe, though it appears to induce a photoconductive shunt. Finally, we show that the shunting resistance given by a standard current-voltage curve technique generally does not represent a physically meaningful quantity unless it is well below one kiloOhm square cm.

Thin film solar cells have, in general, three recombination locations that can limit the device performance – the bulk, front interface, and back interface. Unfortunately, it is difficult to determine which of these mechanisms limit any given device. Time resolved photoluminescence measurements provide an estimate for the carrier lifetime and, therefore, bulk recombination. Determining the role of the front and back interface in device performance, though, is more complicated. With this in mind, we investigated the use of current densityvoltage measurements with front and back illumination at varying intensities. Using numerical modeling, we show that recombination information about each interface can be gleaned, and the limiting interface can be determined. The response curve signatures learned from modeling are then applied to measurements of real devices.

Copper (Cu) is being widely used in CdTe solar cells to improve the p-type conductivity and reduce the back-barrier height. However, due to the defect compensate effect and the high spatial migration rate of Cu, the physical mechanism of Cu in CdTe is still a challenge. Here, three different Cu dopant source, metallic copper (Cu), and cuprous chloride (CuCl) and copper chloride (CuCl 2 ), are applied in ZMO/CdTe solar cells. Our results indicate that solution processed Cu treatment supplies lower Cu diffusion temperature and possibilities to reduce the Cu dosage and distribution profile in CdTe.

CdSe thickness and Cu doping play significant roles in achieving a highly efficiency in CdTe solar cells. Using an evaporated CdSe/CdTe device stack to avoid vacuum breaks between layers, we investigate the role of the CdSe thickness on the device performance and show that when the CdSe thickness is above a critical value the device performance, specifically the short circuit current (J SC ), suffers. From these measurements, we determine the critical thickness of CdSe to be 120 nm. However, in some cases the CdSe thicknesses can be increased above the critical value and increasing the Cu doping process temperature can lead to an increase in J SC and an overall improvement in device efficiency. Specifically, for a device with ~270 nm of CdSe, the J SC increases from 7.9 mAcm-2 with CuCl 2 processing temperature of 200 °C to ~29 mAcm-2 when the processing temperature is increased to 250 °C.

Comparing to the traditional CdS buffer layer, zinc magnesium oxide (ZMO) offers the following advantages for CdTe-based thin-film solar cells: it introduces a spike to conduction band offset, whic...

Cu doping of CdTe has enabled reasonable device performance, but has some significant drawbacks, including limited achievable carrier concentration and high mobility in the CdTe films. Group V elements, on the other hand, are expected to facilitate higher carrier concentration and expected to have low mobility. Consequently, incorporate of As in CdTe is of great interest to the CdTe community. Here, we deposited CdTe from the CdTe:As feedstock to complete devices. We demonstrate that the As diffuses through the CdTe absorber layer and accumulates at the front and back interfaces with Cl activation. Current density-voltage and capacitance-voltage measurements indicate that the devices have low carrier concentration, even though the concentration of As is high. These results indicate that the process employed here was insufficient to activate the As doping.

Copper (Cu) doping is a crucial process to improve the hole concentration in CdTe absorber layer and reduce the back-barrier height for efficient CdTe solar cells. However, excess Cu creates detrimental defects and deteriorates device stability due to its high diffusivity. Here, we introduce a wet chemical method to incorporate Cu in CdTe using a CuCl solution. This approach allows a better control of Cu doping than the conventional method using evaporated Cu. Our investigation shows that the CuCl treatment can significantly improve the overall device performances due to an increased carrier concentration in the CuCl treated CdTe absorber layer. Furthermore, the CuCl treated devices show a longer minority carrier lifetime than the conventional Cu treated devices.

As the efficiency of single junction devices improve, bifacial thin film (BFT) devices are being increasingly investigated as a way to increase energy yield per unit area. At the same time, increasing doping density of thin film devices are also being developed to increase the energy yield per unit area. Here, we investigate how critical parameters to improving backside illuminated devices and increasing the absorber doping density affect backside illuminated device performance. We show that reducing recombination at the back of the device through increasing initial Fermi level offset (IFLO) or decreasing back surface recombination velocity (BSRV) is far more important than increasing absorber doping density. We also show that there is a wide range of the IFLO-BSRV parameter space that can lead to high backside illumination device efficiency for highly doped thin film devices.

Sb2Se3 is a promising low-cost and low-toxicity photovoltaic material. Recent research revealed that recombination losses in the absorber limited the efficiency of Sb2Se3 solar cells. Herein we demonstrated a strategy of grain boundary (GB) inversion to alleviate such recombination loss. Owning to its one-dimensional crystal structure, we successfully inverted the GBs of Sb2Se3 films by introducing n-type Cu interstitial doping at GBs via low-temperature CuCl2 treatment. A built-in electric field is established between p-type grain interiors and n-type GBs, which spatially separates phototogerated carriers, suppresses recombination, and enhances carrier collection. Finally, we obtained an efficiency of 7.04%, which is the highest efficiency of Sb2Se3 solar cells based on rapid thermal evaporation technology. We envision that this GB inversion strategy is generally applicable to Sb2Se3 solar cells with different device configuration or produced from other methods and is extendable to other emerging low-dim...

Metal halide perovskite quantum dots (QDs) recently have attracted great research attentions. However, blue-emitting perovskite QDs generally suffer from low photoluminescence quantum yield (PLQY) because of easily formed defects and insufficient surface passivation. Replacement of lead with low toxicity elements is also preferred toward potential commercial applications. Here, we apply Cl-passivation to boost the PLQY of MA3Bi2Br9 QDs to 54.1% at the wavelength of 422 nm, a new PLQY record for blue emissive, lead-free perovskite QDs. Because of the incompatible crystal structures between MA3Bi2Br9 and MA3Bi2Cl9 and the careful kinetic control during the synthesis, Cl– anions are engineered to mainly locate on the surface of QDs acting as passivating ligands, which effectively suppress surface defects and enhance the PLQY. Our results highlight the potential of MA3Bi2Br9 QDs for applications of phosphors, scintillators, and light-emitting diodes.

Sb2Se3 is a promising candidate for low toxicity, low-cost and high efficiency solar cells. Thin film solar cells employing Sb2Se3 as the absorber have achieved power conversion efficiencies of up to 6.5%. However, these solar cells are typically fabricated using rapid thermal evaporation, a fast deposition technique but suffer from entangled substrate and source temperatures. Here, we report on the fabrication of stable and efficient CdS/Sb2Se3 thin film solar cells using close-space sublimation process, which allows separate control of the source and substrate temperatures. This enables better control of Sb2Se3 film deposition and reduction of interfacial diffusion, leading to higher quality absorber films and improved device heterojunctions. Additionally, we introduced a novel low-cost material 4,4',4'',4'''-(9-octylcarbazole-1,3,6,8-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline) (CZ-TA) as the hole transport layer (HTL) to create n-i-p structured devices. This CZ-TA HTL reduced the back contact resistance and promoted photogenerated carrier collection, boosting device photo conversion efficiency to 6.84%.

Antimony selenide (Sb2Se3) is a new thin film photovoltaic material and has received great research attention these days. ZnO, a nontoxic and wide bandgap material, has been successfully applied as the buffer layer in Sb2Se3 solar cells. Device with high efficiency and exceptional stability has been obtained, and ZnO is thus regarded as the most-promising buffer layer for Sb2Se3 photovoltaics. Herein we reported a simple sol–gel method to deposit ZnO film and construct ZnO/Sb2Se3 heterojunction solar cells. We applied one-step annealing process to obtain smooth and compact ZnO and ZnO/Zn1−xMgxO thin films. We revealed that 10% Mg content was optimal for the sandwiched Zn1−xMgxO layer and finally obtained 4.45% device performance. Through careful characterization, we found out that the insertion of Zn0.9Mg0.1O upshifted the Fermi-level, leading to higher built-in potential (VBi) and consequently larger open circuit voltage (VOC); it also promoted [2 2 1] orientation in Sb2Se3 films and reduced interfacial defects of PN junction, enhancing short-circuit current density (JSC).

PbS quantum dot solar cells are promising candidates for low-cost and highly efficient light harvesting devices owing to their solution processability and bandgap tunability. The p-type ethanedithiol (EDT) treated PbS quantum dot film plays an important role in PbS quantum dot solar cells with an n-i-p junction device structure. However, despite their sulphur-rich surface the EDT-treated PbS quantum dot film still have a relatively low carrier concentration. Higher carrier concentrations in this layer are desirable to extend depletion regions and improve hole extraction. Also imbalances in the charge mobility between the intrinsic layer and the p-type layer may lead to charge build-up at this interface. These obstacles limit further improvement of the device performance. Herein, we utilize EDT-treated Ag-doped PbS quantum dots as a p-type layer to fabricate PbS quantum dot photovoltaic cells. The carrier carrier concentration, mobility and band extrema as well as Fermi energy levels of Ag doped PbS quantum dot film can be tailored by tuning the Ag/Pb mole ratio from 0.0% to 2.0% during fabrication. The device performance has been significantly improved from 9.1% to 10.6% power conversion efficiency largely due to improvements in carrier concentration in the PbS-EDT layer through the incorporation of silver impurities.

AgBiS2 nanocrystals are good candidates in optoelectronic applications due to their solution-processability, earth abundance and non-toxic properties. AgBiS2 thin film solar cells have demonstrated promising performance due to their high absorption coefficients and suitable bandgaps for light harvesting. However, their physical properties such as mobility, carrier concentration and photosensitivity, which are very important for photovoltaics, remain unreported. Herein, we develop an improved amine-based synthesis route and systematically investigate these properties by characterizing field effect transistors and photodetectors fabricated from AgBiS2 nanocrystals. We found that the amine-based synthesis improved their measurable semiconducting properties and their solar cell performance. The optimal thickness of a champion solar cell was found to increase to 65 nm, which achieved a power conversion efficiency of 4.3% based on a ZnO/AgBiS2/P3HT/Au device structure, compared with 35 nm of a champion solar cell with a power conversion efficiency of 3.9% reported previously. This modified approach could be applied in the preparation of other high quality nanocrystals that uses silver, where single valence Ag cations are of increasing importance in stable solution processed nanomaterials.

Sb2Se3 is a promising candidate for thin-film photovoltaics, with a suitable band gap, benign grain boundaries, Earth-abundant and nontoxic constituents, and excellent stability. However, the low doping density (1013 cm–3) of Sb2Se3 absorber and back contact barrier limit its efficiency. Here we introduced a PbS colloidal quantum dot (CQD) film as the hole-transporting layer (HTL) to construct a n-i-p configured device and overcame these problems. Through simulation-guided optimization, we have significantly improved the efficiency of a Sb2Se3 thin-film solar cell to a new certified record of 6.5%. The PbS CQD HTL not only minimized carrier recombination loss at the back contact and boosted carrier collection efficiency but also contributed photocurrent by its own near-infrared absorption. Furthermore, these n-i-p devices also demonstrated improved device uniformity, achieving 6.39% in a 1.02 cm2 device.

Defects present in the absorber layer largely dictate photovoltaic device performance. Recently, a binary photovoltaic material, Sb2Se3, has drawn much attention due to its low-cost and nontoxic constituents and rapid performance promotion. So far, however, the intrinsic defects of Sb2Se3 remain elusive. Here, through a combined theoretical and experimental investigation, we revealed that shallow acceptors, SeSb antisites, are the dominant defects in Sb2Se3 produced in an Se-rich environment, where deep donors, SbSe and VSe, dominate in Sb2Se3 produced in an Se-poor environment. We further constructed a superstrate CdS/Sb2Se3 thin-film solar cell achieving 5.76% efficiency through in situ Se compensation during Sb2Se3 evaporation and through careful optimization of absorber layer thickness. The understanding of intrinsic defects in Sb2Se3 film and the demonstrated success of in situ Se compensation strategy pave the way for further efficiency improvement of this very promising photovoltaic technology. Copyright © 2017 John Wiley & Sons, Ltd.

The high electron affinity (4.4eV) of CdTe presents a major technological challenge for forming a good back contact to CdTe, limiting the power conversion efficiency of CdTe solar cells. Commonly used metals do not possess an adequately deep work function to create an ohmic contact with CdTe. Instead, approaches such as doping the back surface of CdTe or incorporating a buffer layer between CdTe and the back-metal contact are used to lower the back-barrier height. Here, we investigate the impact of sputtered ZnTe back buffer layer on the performance of CdS:O/CdTe solar cells. We demonstrated CdTe solar cells with an efficiency of around 15% with a 35 nm ZnTe buffer layer.

Magnesium Zinc oxide (MZO), as a promising alternative buffer layer to replace CdS, is predicted to improve both the open circuit voltage and short circuit current, and, therefore, efficiency of CdTe solar cells due to its larger band gap and higher conduction position. However, few published works have achieved these benefits due to the S-kink in J-V curve. Here, the S-kink in MZO/CdTe films is eliminated through performing CdCl2 treatment in oxygen-free atmosphere and as a result, a significant improvement on efficiency (from 9.2% to 16.1%) is achieved.

The performance of Cadmium telluride (CdTe) solar cell devices is limited by the properties of front and back interfaces as well as the absorber layer, which can be influenced by the fabrication atmosphere. In this work, we report a detailed study of electrical properties of ZnMgO (ZMO)/CdTe and cadmium sulfide (CdS)/CdTe solar cells with the cadmium chloride (CdCl2) treatment performed in different atmospheres using temperature-dependent impedance spectroscopy and capacitance – voltage measurements. An equivalent circuit model consisting of two serial combinations of the front and back junctions is employed to fit the complex impedance spectra of devices measured in dark. Fitted data from equivalent circuit provides the value of each element, from which the bulk conductivity, back contact barrier height, as well as spatial inhomogeneities within the cell are extracted. Impedance spectroscopy analysis shows that there are negative and positive effects of back surface treatment in oxygen free ambient on device performance. For oxygen-free treatment, an obvious increase in the bulk conductivity is observed, suggesting an increased copper doping in the device. Additionally, ZMO devices show less junction inhomogeneity. All these improvements lead to better device performance of ZMO/CdTe solar cells.

Bifacial solar cells have the potential to increase the energy yield per unit area over traditional monofacial devices without significant added cost, driving $/kWh costs lower and accelerating the adoption of solar photovoltaics. However, the performance of bifacial thin film solar cells significantly lags that achieved by crystalline silicon cells. Here we incorporate wide bandgap CuxAlOy as a back buffer layer for CdTe devices and achieve a backside illuminated device with high current density and high fill factor. Moreover, these values remain nearly constant even as the absorber layer thickness changes, indicating that a fully-depleted device is not required for efficient charge collection. We show that this response is indicative of a back surface field, albeit with a persistent high back surface recombination velocity. By managing electron reflection, we achieved a backside illumination conversion efficiency of 7.1% and bifaciality of 0.55 for a 3.3 µm CdTe device and 8.0% and 0.62 for a 2 µm device. Future improvements can be made by identifying and incorporating a passivation material that reduces the back surface recombination velocity.

Colloidal quantum dots (CQDs) exhibit extraordinary features due to their bandgap tunability and solution processing. Instead of the ZnO layer usually used as the electron transport layer (ETL) in CQD heterojunction devices, we developed, for the first time, tin dioxide (SnO2) as the ETL in colloidal quantum dot solar cells (QDSCs). Its wider bandgap and higher electron mobility, as well as appropriate band alignment with PbS QDs, could favor light absorption and photocarrier extraction. Our low-temperature processed SnO2 film could retain chlorine atoms (SnO2–Cl) to achieve interface passivation in QDSCs. Utilizing 1-ethyl-3-methylimidazolium iodide (EMII) as the absorber ligand, our superior device obtained a power conversion efficiency of 9.37%, which was 44% higher than that of a control device. Physical characterizations revealed that this remarkable improvement could be ascribed to the chlorine passivation of the SnO2/QD interface contact and to the EMII ligand passivation effect on the QD surface. Our newly developed ETL, along with an efficient interface passivation technique, is expected to enhance the performance of full solution-processed colloidal QDSCs.

Organic–inorganic hybrid perovskite solar cells have attracted great attention due to their unique properties and rapid increased power conversion efficiency. Currently, PC61BM is widely used as the electron transport layer (ETL) for inverted hybrid perovksite solar cells. Here we propose and demonstrate that Bi2S3, a ribboned compound with intrinsic high mobility and stability, could be used as the ETL for perovksite solar cells. Through a simple thermal evaporation with the substrate kept at room temperature, we successfully produced a compact and smooth amorphous Bi2S3 ETL with high conductivity. Our NiO/CH3NH3PbI3/Bi2S3 solar cell achieved a device efficiency of 13%, which is comparable with our counterpart device using PC61BM as the ETL. Moreover, our device showed much improved ambient storage stability due to the hydrophobic and hermetic encapsulation of the perovskite layer by the Bi2S3 ETL. We believe thermally evaporated Bi2S3 is a promising ETL for inverted hybrid perovskite solar cells and wor...

The incorporation of copper (Cu) is one of the critical processes for fabricating high-efficiency CdTe thin-film solar cells. However, due to its high mobility in CdTe, the distribution and concentration of Cu must be carefully engineered to reduce the compensative donor-like interstitial defects in CdTe bulk and the recombination centers at the buffer layer/CdTe interface to maximize device performances. Here, a cuprous chloride (CuCl) solution treatment and a rapid thermal annealing (RTA) process are used to control the concentration and distribution of Cu in CdTe absorbers, enabling a champion CdTe thin-film solar cell with a power conversion efficiency of 17.5% without selenium incorporation. The results demonstrate that the use of a CuCl solution can substantially reduce the amount of Cu needed in CdTe and the RTA process is a viable approach to engineer the Cu distribution in CdTe solar cells.

Low-cost solution-processed lead chalcogenide colloidal quantum dots (CQDs) have garnered great attention in photovoltaic (PV) applications. In particular, lead selenide (PbSe) CQDs are regarded as attractive active absorbers in solar cells due to their high multiple-exciton generation and large exciton Bohr radius. However, their low air stability and occurrence of traps/defects during film formation restrict their further development. Air-stable PbSe CQDs are first synthesized through a cation exchange technique, followed by a solution-phase ligand exchange approach, and finally absorber films are prepared using a one-step spin-coating method. The best PV device fabricated using PbSe CQD inks exhibits a reproducible power conversion efficiency of 10.68%, 16% higher than the previous efficiency record (9.2%). Moreover, the device displays remarkably 40-day storage and 8 h illuminating stability. This novel strategy could provide an alternative route toward the use of PbSe CQDs in low-cost and high-performance infrared optoelectronic devices, such as infrared photodetectors and multijunction solar cells.

The replacement of traditional CdS with zinc magnesium oxide (ZMO) has been demonstrated as being helpful to boost power conversion efficiency of cadmium telluride (CdTe) solar cells to over 18%, due to the reduced interface recombination and parasitic light absorption by the buffer layer. However, due to the atmosphere sensitivity of ZMO film, the post treatments of ZMO/CdTe stacks, including CdCl2 treatment, back contact deposition, etc., which are critical for high-performance CdTe solar cells became crucial challenges. To realize the full potential of the ZMO buffer layer, plenty of investigations need to be accomplished. Here, copper thiocyanate (CuSCN) is demonstrated to be a suitable back-contact material with multi-advantages for ZMO/CdTe solar cells. Particularly, ammonium hydroxide as the solvent for CuSCN deposition shows no detrimental impact on the ZMO layer during the post heat treatment. The post annealing temperature as well as the thickness of CuSCN films are investigated. Finally, a champion power conversion efficiency of 16.7% is achieved with an open-circuit voltage of 0.857 V, a short-circuit current density of 26.2 mA/cm2, and a fill factor of 74.0%.

Thin film CdTe solar cells were prepared using close-space sublimation followed by a standard cadmium chloride activation annealing treatment. After the activation process, some of the samples were subjected to a brief etching process using a 10% hydrogen iodide solution in methanol while the others were only rinsed with methanol. Completed solar cells from both sample types were prepared and characterized using current and capacitance-voltage scanning methods. The results show that the etching process increases the open circuit voltage by 10-20mV and the fill factor by 1-5% compared to the devices without the etching, thus increasing the efficiency from $\sim$ 13% to $\sim$ 15%. Capacitance-voltage measurements indicate that the bulk carrier (hole) density of the CdTe films increase by approximately a factor of three, which could partially explain the increase in device performance. The cause of the change in carrier density is still under investigation.

The linear permanent magnet vernier machine (LPMVM) is the combination of the linear permanent magnet machine and vernier machine, which yields high thrust density at low speed and thus is a preferable solution in the direct-drive linear-motional applications [1]. In order to further improve the thrust density of the LPMVM, many new configurations have been proposed in recent years, such as the double-stator spoke-type LPMVM [2], the LPMVM with Halbach permanent magnet (PM) array [3], the LPMVM with consequent-pole and Halbach PM array [4], the linear stator PM vernier machine [5], and so on. It is worth noted that the flux density of the tooth of the LPMVM is even smaller that of the conventional linear PM (CLPM) machines due to the multi-polar and fewerteeth structure. In order to improve the flux density and the thrust of the LPMVM, the linear interior permanent magnet vernier machine (LIPMVM) is proposed in this paper, in which the PMs are inserted into the stator core. Due to the improvement of the flux density and additional reluctance thrust, the average thrust of the proposed LIPMVM is significantly higher that of the conventional linear surface PM vernier machines (LSPMVM).