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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.

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.

The copper/reduced graphene oxide (Cu@rGO) composites have been synthesized via a novel and facile chemical method and been characterized by a series of measuring techniques. Then the Cu@rGO as a primary back contact has been coated on the absorber in the CdTe solar cells. After optimizing the Cu@rGO back contact processing, significantly enhanced performance especially in efficiency, open-circuit voltage and fill factor has been achieved for CdTe solar cells. The Cu@rGO supplies an effective Cu diffusion source after annealing which achieves a considerable hole concentration, and the rGO is converted into p-type conducting with Au metallization, resulting in the decrease of the back barrier and improvement of the hole collection ability. The cells with Cu@rGO/Au exhibit more outstanding photovoltaic performance than those with Cu/Au or rGO/Au back contact.

A low-resistance and stable back contact to p-type CdTe semiconductor is crucial for the high-efficiency CdTe solar cells. In this work, ZnTe/Cu1.4Te bilayer alloys were fabricated by evaporation to form a new back-contact configuration for CdTe solar cells due to the well-matched interfacial property of ZnTe and the heavily p-doped property of Cu1.4Te. The devices with single layer or double layers have an effective doping level from ∼5 × 1013 to ∼3 × 1014 cm−3, resulting in the improvement of device performance and apparent increase in quantum response in the CdS and CdTe regions, compared to the devices with only Au. Device performance demonstrates lower Schottky barrier and less carrier recombination for CdTe solar cells with ZnTe/Cu1.4Te than that for the cells with Cu or Cu1.4Te due to a better modification of the band structure. Time-resolved photoluminescence and dark capacitance-voltage measurements indicate the introduction of Cu or single Cu1.4Te layer creates recombination centers to reduce the minority carrier lifetime from 100.9 to 78.7 ns and compensate net carrier concentration. The insertion of ZnTe buffer layers effectively prevents Cu diffusing to decrease recombination defects, resulting in longer minority carrier lifetime of 86.7 ns and higher carrier density, due to a well-matched interfacial property that is roughly 5% lattice mismatch at the CdTe/ZnTe interface. Finally, cell efficiency of >15%, open circuit voltage of >810 mV and fill factor of >73% for CdTe solar cells with an area of 0.24 cm2 have been achieved. Accelerated aging tests show the stability of the cells over time is also improved by the incorporation of the ZnTe buffer layer.

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 .

Developing a good back contact material is important for high efficiency CdTe solar cells. In this work, the graphite oxide (GO) was prepared by the Hummers method through liquid oxidization, and the Bi-doping reduced graphene oxides (Bi@RGO) with different concentrations were synthesized by a chemical reduction of GO and BiCl3 aqueous solution using hydrazine hydrate. The doping concentration of bismuth was varied by controlling the mass ratio of GO/BiCl3. The peaks of bismuth and RGO can be easily observed in the X-ray diffraction patterns. Both Raman spectra analysis and infrared spectra analysis also show that RGO is reduced successfully. Transmission electron microscopy shows Bi nanoparticles are entrapped inside the flake-like RGO. Furthermore, Bi-containing RGO/Au system was used as a back contact to CdTe thin film solar cells. The devices with Bi-containing RGO layers were fabricated and their photovoltaic characteristics such as short circuit current density, open circuit voltage, fill factor and efficiency were obtained. The results illustrate that Bi@RGO improves the hole collection ability, and supplies an active Bi diffusion source for the absorption layer, thereby enhancing CdTe solar cell performance.

The short-wavelength response for traditional CdS/CdTe thin film solar cells was dramatically restricted by the CdS window layer. In order to increase short-wavelength light collection, we tried to replace CdS with MgxZn1−xO (MZO). The short-wavelength quantum efficiency (QE) response was obviously increased to more than 80% at 400 nm, while fill factor (FF) and open circuit voltage (Voc) for CdTe solar cells were decreased. This decrease was mainly caused by the weak build-in potential formed by the MZO/CdTe heterojunction, which resulted in insufficient driving force to repel the photo-generated carriers. Hence, the thin CdS buffer layer was introduced between MZO and CdTe to improve the built-in potential. It efficiently increased the build-in potential from 0.369 to 0.579 V with an obvious improvement of FF and Voc. But this interlayer caused QE losses to some degree in the short-wavelength region. To avoid these losses, a composite CdS/CdSe buffer layer was incorporated in the CdTe solar cells. CdSe acted as a protective layer to avoid the excessive CdS consumption during the high temperature deposition processing of the CdTe absorption layer. Meanwhile the formation of CdSexTe1−x by inter-diffusion extended the long-wavelength response. Thus, cell performance was enhanced with a relatively high build-in potential 0.517 V and a substantial improvement of the QE response.

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.