Electron Reflector Research Articles

Improving Photocurrent in Sb2Se3 Thin‐Film Solar Cells Through Sb2S3 Electron Reflector Layer: A Simulation Study

AbstractIn this paper, antimony sulfide (Sb2S3) material is utilized as an electron reflector layer (ERL) in antimony selenide (Sb2Se3) solar cells. The primary objective of this simulation study is to mitigate recombination at the back contact and improve cell performance. It is demonstrated that incorporating Sb2S3 ERL into a traditional Sb2Se3 cell yields enhancements in both the short‐circuit current density (Jsc) and carrier collection rate. This enhancement is assigned to the reduced interface recombination rate at the back electrical contact. The strong electric field at the p+‐Sb2S3/p‐Sb2Se3 electrical contact creates a barrier with a height of 0.34 eV in the conduction band that hinders the electron carriers' access to the back contact. This barrier effectively improves cell efficiency. The use of this method has demonstrated a significant improvement in the external quantum efficiency (EQE) at longer wavelengths, specifically ranging from ≈800 to 1100 nm. Using this method, the Sb2Se3 active layer thickness is reduced to 1 µm, which reduces the fabrication cost. After optimizing the device, the cell efficiency enhances from 9.2% to 13.1%, resulting in a significant improvement compared to traditional Sb2Se3 solar cells without an ERL, thanks to the incorporation of a suitable Sb2S3 ERL.

Thermally Evaporated Copper Iodide Hole-Transporter for Stable CdS/CdTe Thin-Film Solar Cells.

This study focuses on fabricating efficient CdS/CdTe thin-film solar cells with thermally evaporated cuprous iodide (CuI) as hole-transporting material (HTM) by replacing Cu back contact in conventional CdS/CdTe solar cells to avoid Cu diffusion. In this study, a simple thermal evaporation method was used for the CuI deposition. The current-voltage characteristic of devices with CuI films of thickness 5 nm to 30 nm was examined under illuminations of 100 mW/cm2 (1 sun) with an Air Mass (AM) of 1.5 filter. A CdS/CdTe solar cell device with thermally evaporated CuI/Au showed power conversion efficiency (PCE) of 6.92% with JSC, VOC, and FF of 21.98 mA/cm2, 0.64 V, and 0.49 under optimized fabrication conditions. Moreover, stability studies show that fabricated CdS/CdTe thin-film solar cells with CuI hole-transporters have better stability than CdS/CdTe thin-film solar cells with Cu/Au back contacts. The significant increase in FF and, hence, PCE, and the stability of CdS/CdTe solar cells with CuI, reveals that Cu diffusion could be avoided by replacing Cu with CuI, which provides good band alignment with CdTe, as confirmed by XPS. Such an electronic band structure alignment allows smooth hole transport from CdTe to CuI, which acts as an electron reflector. Hence, CuI is a promising alternative stable hole-transporter for CdS/CdTe thin-film solar cells that increases the PCE and stability.

Numerical investigation of the Cu2O solar cell with double electron transport layers and a hole transport layer

In this paper, a model of the Cu2O solar cell with double electron transport layers (ETLs) and a hole transport layer (HTL) has been investigated by SCAPS. Compared to the single ETL of ZnO, the problem of low open circuit voltage of the Cu2O solar cells can be improved by introducing the double ETLs of Zn(O,S)/ZnS. Our simulation results show that the sulfur concentration of Zn(O,S) and the donor density in double ETLs have a significant effect on device performance. The optimum S/S + O ratio and the best donor density are 0.82 and 8 × 1017cm−3, respectively. Due to the formation of an electron reflector and back surface field (BSF), an enhancement in both short circuit current density and open circuit voltage can be realized by introducing an appropriate HTL, and the best HTL is CuI, showing an efficiency of 12.15%. Then, the work function of metal electrode, the Cu2O thickness, the defect density and the acceptor density of Cu2O absorber layer, and the defect density at the Cu2O interfaces have been systematically investigated. After optimization, the best device efficiency of 16.49% can be achieved. Our results have demonstrated a new high-efficiency structure for cuprous oxide solar cells.

Evaluation of PbTe and SnTe as ohmic contact layers in CdTe solar cell devices

For solar cells based on CdTe, the choice of a suitable back contact material is limited by CdTe’s deep work function. Here, we explored p-type PbTe and SnTe as ohmic contacts to CdTe. These contact layers were grown on single crystal CdTe substrates by molecular beam epitaxy, and the valence band offset between film and substrate was measured using X-ray photoemission spectroscopy. Polycrystalline device structures were also grown by sublimation to assess performance improvements. Doping was achieved in PbTe by thallium incorporation. Only the highest Tl doped PbTe resulted in a desirable band alignment with the CdTe, forming an electron reflector and no hole barrier. Time-resolved photoluminescence measurements also revealed significant photocarrier lifetime improvements for only the highest doped PbTe. Consequentially, devices incorporating the highest doped PbTe layers showed increased power conversion efficiency, primarily from increased fill factor. Doping of the PbTe was measured via Hall effect with variable magnetic field, which was required due to the formation of an n-type parasitic interface layer. To properly interpret the variable field Hall measurements, we derived an expression for the magnetic field-dependent conductivity tensor of an L-valley semiconductor.

Sputter-deposited CdMgTe for rear contact to CdSeTe/CdTe solar cells

CdTe-based photovoltaic device performance improvement is predominantly limited by open-circuit voltage. This work approaches the voltage deficit challenge through the incorporation of a cadmium magnesium telluride (CdMgTe) electron reflector layer at the back of 1.5- μm CdSeTe/CdTe absorbers to reduce back-surface recombination through a conduction band offset. A sputtered CdMgTe electron reflector layer permitted low fabrication temperatures and circumvented the intolerance of CdMgTe to thermal processing present in traditional close-space sublimation (CSS)-deposited CdMgTe devices. We demonstrate that sputtered CdMgTe simultaneously promotes the retention of chlorine passivation and magnesium which is not feasible with CSS fabrication. We also show the importance of the Cu and Te back contact order in CdMgTe devices. Through the optimization of the sputtered CdMgTe substrate temperature, doping, and back structure configuration, 1.5 μm CdSeTe/CdTe devices with a sputtered CdMgTe layer achieved an efficiency of 16.0%, significantly higher than with CSS CdMgTe, and demonstrated partial enactment of electron-reflector behavior. • Sputtered CdMgTe electron reflector solar cells achieve higher efficiencies than close-space sublimation-deposited. • Low temperatures of sputtered CdMgTe reduce magnesium diffusion and chlorine loss. • CdTe devices with CdMgTe electron reflectors require back contact optimization. • CdTe voltage improvement with CdMgTe limited by a partial negative back valence band offset.

Understanding the Role of CdTe in Polycrystalline CdSexTe1–x/CdTe‐Graded Bilayer Photovoltaic Devices

Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te1–x /CdTe‐graded bilayer device has led to a device efficiency over 19%. A CdSe x Te1–x absorber would increase the short‐circuit current due to its lower bandgap but at the expense of open‐circuit voltage. It has been demonstrated that adding a CdTe layer at the back of such a CdSe x Te1–x film reduces the voltage deficit caused by the lower bandgap of absorber from selenium alloying while maintaining the higher short‐circuit current. This leads to a photovoltaic device that draws advantage from both materials with an efficiency greater than either of them. Herein, a detailed account using device data, ultraviolet photoelectron spectroscopy, electron microscopy, and first‐principles density functional theory modeling is provided, which shows that CdTe acts as an electron reflector for CdSe x Te1–x .

A novel design of high efficiency perovskite solar cell using electron reflector strategy and transport layers optimization

A novel design of high efficiency perovskite solar cell using electron reflector strategy and transport layers optimization

Improvement of CIGS solar cell efficiency with graded bandgap absorber layer

There are several ways to increase the efficiency of a solar cell. In addition to increasing efficiency, the important subject is to build the cell at a lower cost and try to reduce the heat losses of the cell. In this article, research was conducted on CIGS solar cell in which some methods were proposed that in addition to reducing the losses of a CIGS solar cell, we could achieve maximum efficiency with the minimum thickness of the absorber layer. To simulate and perform mathematical calculations, the Atlas software of Silvaco was used in this research. It should be noted that in all proposed structures, the thickness of the layer is minimized, and also the absorber layer is p-type. In the first stage, the structure of a ZnO:Al/Zn0.83Mg0.17O/CdS/Graded CIGS/MO was simulated in which the thickness of the graded CIGS (absorber layer) is 1 µm. Graded CIGS is a structure in which the bandgap of material CuIn1−xGaxSe2 changes linearly from x1 to x2. In this study, the x variation is from 0.7 to 0.1, so that the decrease in x corresponds to the decrease in band gap linearly from 1.45 to 1.07 eV. In this stage, the cell efficiency was 17.1%. In the second stage, the buffer layer was upgraded to increase the cell efficiency, that is, instead of CdS layer with the bandgap of 2.4 eV, ZnO0.5S0.5 with the bandgap of 2.8 eV was used that made the efficiency to become 19.0%. In the third stage, an electron reflector layer was added to the structure of the first stage; indeed at this stage, the effect of the electron reflector layer on the solar cell of the first stage was displayed. In the fourth stage (last stage), CGS (CGS is CuGaSe2) layer was used as the electron reflector layer, which caused the cell efficiency to reach 28.3%.

Optical modeling of periodic nanostructures in ultra-thin CdTe solar cells with an electron reflector layer

CdTe solar cells have become a competitive player in the commercial photovoltaic market competing with CIGS and Si wafers. Considerable research attention has been devoted to increasing the overall conversion efficiency of CdTe which reached 22%. Among the challenges facing this technology is the scarcity of Te. In this paper, we investigated the use of an ultra-thin CdTe layer, which would result in a substantial decrease in Te consumption by almost 90%. However, this substantial decrease in the absorber thickness must be accompanied by some embedded nano-light-trapping structures. The purpose of these nanostructures is to increase the light absorption of the UV-VIS and NIR wavelengths which leads to an increase in the generated photo-current. Also, an electron reflector layer must be added between the metallic contact and the thin absorber layer to mitigate the high recombination rate. Additionally, three different metallic contacts were explored: Au, Ag, and Al. The absorption in the CdTe layer which is responsible for the photo-generated current and the parasitic absorption in the metallic back contact, as well as the short circuit current densities, were determined and analyzed. The numerical Wave optics finite element method and the analytical transmission line theory were utilized. It was noted that these nanostructures showed a substantial increase in the absorption in the CdTe absorber layer in the visible and NIR ranges. A net increase in the short circuit current density of 36% has been realized with the top nano-structured CdTe layer comparing to the flat reference cell of the same absorber volume. Furthermore, the Ag metallic back-contact outperforms both Al and Au and achieved the highest short circuit current density and the lowest parasitic absorption. These findings would open a new route to integrate optimized light trapping nano-structures into the commercial CdTe solar cells to further enhance their material utilization and efficiency. • Ultrathin cadmium telluride CdTe solar cells with electron reflector layer. • Light-trapping-structures in the thin-film solar cells. • Optical modeling of the ultra-thin solar cells. • Materials utilization of ultra-thin cadmium telluride solar cells. • Transmission line theory model for the thin-film solar cells.

Rear interface engineering of kesterite Cu2ZnSnSe4 solar cells by adding CuGaSe2 thin layers

AbstractKesterite Cu2ZnSn(S,Se)4 thin film technology has been thoroughly investigated during the last decade as a promising solution in the field of low‐cost, sustainable, and environmental‐friendly photovoltaic technologies. However, despite efforts to boost kesterite solar cells performance by numerous strategies, the efficiencies remain stagnant around 13%. Some commonly observed issues in this technology refer to recombination events due to the likely presence of defects and, largely in line with the latest, the presence of voids and poor morphologies at the rear interface. This work, partly inspired by the copper indium gallium selenide (CIGS) technology and the use of wide‐bandgap Ga‐rich region as back surface field (BSF), focuses on an innovative approach using ultrathin CuGa layers at the rear interface to promote the formation of wide‐bandgap CuGaSe2, acting as an efficient electron reflector or BSF, and to function as an effective interlayer improving the kesterite crystallinity at the back interface. Kesterite Cu2ZnSnSe4 devices fabricated with added CuGa layers show a general increase in photovoltaic parameters and a significantly enhanced collection efficiency compared with reference devices without CuGa. This strategy proves to be successful, for not only passivating but also for improving the Mo/kesterite interface morphology, preventing to a large extent the presence of voids at the back region of the absorber.

Successive Ionic Layer Adsorption and Reaction‐Deposited Transparent Cu–Zn–S Nanocomposites as Hole Transport Materials in CdTe Photovoltaics

Evolving material science and device architectures continue to drive improvements in photovoltaic solar cell performance. Herein, the synthesis and application of p‐type transparent copper–zinc–sulfide (Cu–Zn–S) nanocomposite thin films for application as a semi‐transparent back buffer layer for cadmium telluride (CdTe) photovoltaics is reported. Earth‐abundant and low‐toxicity Cu–Zn–S films are prepared at room temperature using successive ionic layer adsorption and reaction (SILAR). Transparency in the range of 500–800 nm, low resistivity, and composition‐controlled bandgap energy offer a compelling material system for high performance as an electron reflector enabling bifacial cell design. Implementing the Cu–Zn–S hole transport material (HTM) at the CdTe back contact, without intentional introduction of Cu doping, converts simulated AM1.5 sunlight to electricity at an efficiency up to 13.2%, with an average device performance of 13.0%. Intentional Cu doping yields a best efficiency of 14.3% with open‐circuit voltage (VOC) of 848 mV and fill factor (FF) of 77.3% (average 14.1%). Our study shows the clear promise of this material for highly efficient and semi‐transparent back contact to CdTe solar cells.

Enhancement of Ni-63 planar source efficiency for betavoltaic batteries

Source efficiency in planar betavoltaic batteries is drastically reduced by self-absorption and directional loss. In this paper, we present a technique for improving source efficiency by the use of beta particles reflectors based on metals. A metallic layer with a high atomic number is an effective electron reflector, capable of reflecting up to 50% of the escaping energy from the backside of a source and they are more efficient for thinner radioactive layers. The obtained results show that the design of separate planar one-sided sources with electron reflectors could be optimized to reach high efficiency before assembly with semiconductor converters. Using a Pb reflector for a 200 nm thick Ni-63 source, 38.5% of the directional lost energy could be recovered. They can lead to cost-effective technology compared to sandwiched, porous, or three-dimension microstructure designs.

Analysis and improvement of CIGS solar cell efficiency using multiple absorber substances simultaneously

In this paper, the efficiency of a CIGS solar cell was increased in several stages. The common structure and configuration of the CIGS solar cell are the ZnO:Al/ZnO/CdS/CIGS/MO combination whose efficiency is optimized approximately 20% for CIGS with thickness of 2 µm and 17% for CIGS with thickness of 1 µm. In this article, thickness of CIGS is 1 µm and the efficiency of this type of solar cell was calculated using Atlas software of Silvaco. In the first step, by applying Zn1−xMgxO material with x = 0.17 instead of ZnO material, the cell efficiency was 20.7% and then by adding GaAs as the electron reflector layer, we were able to achieve 27.1% efficiency and at the end, to increase the efficiency, one absorber layer is added under the CIGS absorber. This absorber layer is CIS (CIS is CuInSe2) that made the efficiency to become 27.9%. Indeed, CIGS absorber layer is not able to absorb all photons of the sun. So, this added absorber layer is able to absorb a part of low-energy photons, which lead to increasing the efficiency of the solar cell. It should be noted that in the whole process of this article, CIGS and CIS absorber layers are p-type.

The research of post-treatment on Cd1-xMgxTe thin films and CdTe solar cells

As for it higher band gap, Cd1-xMgxTe (CMT) material has been taken into consideration to improve the open-circuit voltage of CdTe solar cells as an electron reflector layer. An essential step in the development of this new interlayer is the passivation process. X-ray Diffraction (XRD) and scanning electron microscope (SEM) results give us the suitable annealing temperature range of CMT thin films from 350°C to 400°C, and suggest that the annealing process is beneficial to grain growth and recrystallization. The concentration of CdCl2 solution has an influence on the crystallinity of the thin films and coating method would contribute to electrical performance of the devices. The CMT/CdTe cells which underwent the two-step annealing procedure at high purity nitrogen atmosphere showed the significant improvements in device performance. Furthermore, the CdTe solar cells with the CdTe cap layer and Te/Cu back contacts also demonstrated higher performance.

Coevaporated Cd1-xMgxTe thin films for CdTe solar cells

Substantial improvement of the open-circuit voltage of thin film solar cells has been investigated by applying an electron reflector strategy. Cd1-xMgxTe has a strong potential as an electron reflector to keep minority carriers away from the CdTe back surface to reduce the back surface recombination. In this paper, SCAPS simulations were first used to investigate device performance for CdTe solar cells with the electron reflector layers. The theoretical results indicate that the ∼0.4 eV electron barrier height for CdTe solar cells with Cd1-xMgxTe (Eg∼1.85 eV) is sufficient to decrease the back surface recombination and improve device performance, especially the open-circuit voltage. Thus the variation of energy gap of the Cd1-xMgxTe thin films prepared by coevaporation as a function of x was investigated from the transmittance spectra. Then Cd1-xMgxTe (x∼0.3) thin films were used as the electron reflectors for the CdTe thin film solar cells. It is found that CdTe solar cells with Cd1-xMgxTe yielded open-circuit voltage of 804 mV and fill factor more than 70% after an 425 °C anneal, which is higher than those without Cd1-xMgxTe. The electron reflector in CdTe solar cells can effectively reduce the carrier surface recombination, thereby resulting in the increase of the fill factor and the open-circuit voltage.

CdTe thin film solar cells with copper iodide as a back contact buffer layer

In this study, CuI was employed as a back contact buffer layer which not only assisted hole transport, but also acted as an electron reflector at back contact. Quantitative analysis of the band alignment at the CdTe/CuI interface was carried out by using X-ray photoelectron spectroscopy (XPS). The results showed that the valence band offset and conduction band offset between CuI and CdTe were 0.08 and 1.63 eV, respectively. By optimizing the thickness of the CuI layer, a CdS/CdTe solar cell with a maximum efficiency of 14.5% has been fabricated. Capacitance–voltage (C–V) and impedance spectroscopy measurements were carried out and the results demonstrated that the CuI buffer layer between CdTe and metal electrode eliminated contact barrier and reduced carrier recombination at the back contact. The current–voltage (J–V) characteristics measured under different spectrum illumination conditions provided convincing evidences that CuI buffer layer acted as an electron reflector and increased the open-circuit voltage (VOC) of the CdTe solar cells. Long time device stressing test results demonstrated that CdTe solar cells with a CuI buffer layer have much better device stability compared to the cells with a Cu/Au bi-layer metal contact.

Back-surface recombination, electron reflectors, and paths to 28% efficiency for thin-film photovoltaics: A CdTe case study

As thin-film and silicon solar technologies mature, questions emerge about the upper bounds of thin-film solar performance and realistic experimental paths to reach them. Directions include increasing absorber hole density and bulk lifetime, improving the junction interface, reducing back-surface recombination, and implementing a back-surface electron reflector. Textbook solutions of idealized p-n junctions create a powerful conceptualization of solar cells as predominantly minority-carrier-driven devices. We demonstrate that thin films are distinct, and models often fail to capture the important role of majority-carrier lifetime, leading to contradictions with lifetime measurements and overestimates of potential device improvement from back-surface passivation and/or reflectors. Furthermore, we identify methods to probe majority-carrier lifetime and re-examine the degree to which back-surface passivation and electron reflectors can increase efficiency for a range of common thin-film interface and absorber properties, using current and emerging CdTe technology as an example. Results indicate that a practical approach is to focus first on improving front-interface recombination velocity and the absorber properties, and then on implementing the back-surface passivation or reflector, which can ultimately allow thin-film solar technology to reach 28% efficiency.

Numerical simulation of ultra-thin CdTe solar cells with a buffer layer of MoOx in the backwall configuration

The software of Solar Cell Capacitance Simulator (SCAPS) is used to investigate the performance of ultra-thin CdTe solar cells in the backwall configuration (glass/ITO/MoOx/CdTe/CdS/SnO2/Ag). The backwall structure utilizes ultra-thin CdTe absorber layer instead of CdS film facing light illumination, which eliminates the absorption of CdS in short-wavelength region and improves the blue response of CdTe. A buffer layer of MoOx is added to modify the contact between CdTe and ITO, reducing the valence band barrier height and simultaneously forming an electron reflector, which can reduce electron-hole recombination at this contact. When the thickness of MoOx is 2 nm, the simulation results show that an efficiency can reach up to 25.5% with high ITO work function and ideal interface recombination velocity.

Proposed suitable electron reflector layer materials for thin-film CuIn1−xGaxSe2 solar cells

This paper investigates the electrical properties of electron reflector layer to survey materials as an electron reflector (ER) for chalcopyrite CuInGaSe solar cells. The purpose is optimizing the conduction-band and valence-band offsets at ER layer/CIGS junction that can effectively reduce the electron recombination near the back contact. In this work, an initial device model based on an experimental solar cell is established, then the properties of a solar cell with electron reflector layer are physically analyzed. The electron reflector layer numerically applied to baseline model of thin-film CIGS cell fabricated by ZSW (efficiency = 20.3%). The improvement of efficiency is achievable by electron reflector layer materials with Eg > 1.3 eV and −0.3 < Δχ < 0.7, depends on bandgap. Our simulations examine various electron reflector layer materials and conclude the most suitable electron reflector layer for this real CIGS solar cells. ZnSnP2, CdSiAs2, GaAs, CdTe, Cu2ZnSnS4, InP, CuO, Pb10Ag3Sb11S28, CuIn5S8, SnS, PbCuSbS3, Cu3AsS4 as well as CuIn1-xGaxSe (x > 0.5) are efficient electron reflector layer materials, so the potential improvement in efficiency obtained relative gain of 5%.

Modeling Back Electronic Reflectors in CdTe Solar cells.

Abstract A photovoltaic system (or PV system) is a system which uses one or more solar panels to convert sunlight into electricity. The CdTe thin-film photovoltaic has a large absorption coefficient and is theoretically highly efficient. Large-area CdTe photovoltaic panels can be economically fabricated. These features potentially make the CdTe thin-film solar cell the leading alternative energy source. However, the record CdTe efficiency of (16.5%), is much less than its theoretical maximum efficiency of (29%) [1]. “The open-circuit voltage 0.845 V is well below what is expected for its band gap of 1.5 eV. The incorporation of an electron reflector is a strategy to improve the open-circuit voltage of solar cells, and thus a strong possibility for improving the efficiency of CdTe thin-film” [1]. An electron reflector is a conduction-band energy barrier at the back surface of the solar cell, which can reduce the recombination due to the electron flow to the back surface. The different methods of creating an electron reflector are explained in the electron-reflector strategy paper