Back Surface Field Research Articles

Bandgap engineered, quantum-confined ultrathin Te layer for highly effective back surface field in Si/organic hybrid solar cells

The suppression of back surface carrier recombination is primary importance for efficient Si-based solar cells. It is usually achieved with back surface field (BSF) by introducing a highly doped layer on the rear surface, which require a high temperature doping processes and involves toxic gasses. Here, we report that a high power conversion efficiency of 13.1% with device area of 1cm2 has been achieved for planar Si-PEDOT:PSS hybrid solar cells by depositing a ultrathin tellurium (Te) layer between silicon and rear contact, without any high temperature process or further treatment. Current-voltage, electrochemical impedance spectroscopy, quasi-steady-state-photoconductance measurements are used to investigate the effect of Te BSF layer on the device performance. The insertion of Te BSF layer have successfully reduced charge carrier recombination at rear interface. In addition, it could reduce the contact resistance thanks to the favorable band alignment originated from quantum confinement effect, resulting in improved current density and fill factor. This is the very first demonstration that a quantum-confined material can be used as an effective passivating back surface field layer in Si-organic hybrid solar cells.

Validation of ZnTe as back surface field layer for CdTe solar cells: A combined experimental and theoretical study

An excellent back contact layer is one of the key requirements for thin-film solar cells with a high energy conversion efficiency. For a highly resistive compound such as CdTe thin-film, fabrication of contact with low electrical contact resistance along with a high electron affinity is very difficult. Herein, we have thoroughly investigated the possibility of using ZnTe as a back contact layer for CdTe-based solar cells through combined experimental and first-principle studies. CdTe and ZnTe thin films were deposited on the glass substrate. Detailed structural, morphological, elemental, electrical, and optical properties are investigated through different experimental techniques. Then p-ZnTe/n-CdTe heterojunction was fabricated, and junction properties were studied. Precise electronic band-structures were obtained for CdTe, ZnTe, and CeTe/ZnTe heterojunctions. The interface properties, band edge position, and band alignments were estimated by using the HSE06 hybrid functional method. Detailed theoretical results substantiate our experimental findings.

Simulation aided design of a high efficient GaSb based single-junction solar cell

AbstractOwing to their elevated absorption coefficient, superior resistance to radiation and reduced effective electron mass, gallium antimonide (GaSb) semiconductors were documented to be suitable for photovoltaic systems applications. However, they were found to suffer from low efficiency. This work has been dedicated to enhance the design of GaSb based single-junction solar cells having window and back surface field made from AlGaAsSb material. The purpose is to maximize their electrical performance by considering doping and thickness of layers as design variables. A methodology of optimization was proposed. It is based on computer aided design through simulations performed under SILVACO-ATLAS software and a numerical procedure that was developed to achieve optimal design of this particular solar cell. The approach encompasses analysis of variance and derivation of response surface models to get explicit expression of the objective function corresponding to efficiency. The obtained results have shown an efficiency which is higher than all the previous known values established in literature.

Review of Laser Doping and its Applications in Silicon Solar Cells

Laser-doped selective emitter diffusion techniques have become mainstream in solar cell manufacture covering 60% of the market share in 2022 and are expected to continue to grow to above 90% within the next five years (ITRPV). This was a very rapid uptake of technology, coming from only ∼10% penetration in 2018, and has enabled over 20 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> front recombination current reductions on the dominant passivated emitter and rear cell concepts in the same short period. In this article, a broad overview of key concepts in relation to laser doping methods relevant to solar cell manufacturing is given. We first discuss the basic mechanisms behind laser doping along with the benefits over conventional doping methods. The main laser doping approaches reported in the literature are then discussed, along with implications for metallization strategy, particularly in relation to selective emitter and back surface field formation in the dominant passivated emitter and rear cell technology. Different cell concepts that have benefited from the application of laser doping are also discussed. In the last section, we discuss the main defects induced by laser processing of silicon which affect the finished devices, potential and debated causes, as well as some commonly applied treatments for their mitigation.

Probing hidden conduction mechanisms in diced silicon solar cells by low frequency noise analysis

Electrical low frequency noise (LFN) measurements are utilized in this work, in order to identify parasitic low voltage transport mechanisms in diced aluminum back surface field (Al-BSF) photovoltaic (PV) cells, which cannot be identified by I–V characterization. A detailed method of LFN analysis is proposed, taking into account the co-existence of various transport mechanisms, that typically occur in parallel in such devices. A precise examination of the 1/f noise level dependency with both current and voltage, combined with static I–V characterizations at various temperatures, enabled the identification of two flicker noise sources appearing at different current regions: one attributed to local linear shunt mechanisms and a second one to a fluctuation in a nonlinear parasitic conductivity localized at the samples’ edges. The latter source exhibits noise signatures resembling either tunneling or Poole-Frenkel transport, while excluding Shockley-Read-Hall recombination processes. This hypothesis was further verified by measurements of thermal activation energies typical of tunnel and Poole-Frenkel conduction mechanisms.

Vertically Aligned One-Dimensional Crystal-Structured Sb2Se3 for High-Efficiency Flexible Solar Cells via Regulating Selenization Kinetics

Recently, antimony selenide (Sb2Se3) has exhibited an exciting potential for flexible photoelectric applications due to its unique one-dimensional (1D) chain-type crystal structure, low-cost constituents, and superior optoelectronic properties. The 1D structure endows Sb2Se3 with a strong anisotropy in carrier transport and a lasting mechanical deformation tolerance. The control of the crystalline orientation of the Sb2Se3 film is an essential requirement for its device performance optimization. However, the current state-of-the-art Sb2Se3 devices suffer from unsatisfactory orientation control, especially for the (001) orientation, in which the chains stand vertically. Herein, we achieved an unprecedented control of the (001) orientation for the growth of the Sb2Se3 film on a flexible Mo-coated mica substrate by balancing the collision rate and kinetic energy of Se vapor particles with the surface of Sb film by regulating the selenization kinetics. Based on this (001)-oriented Sb2Se3 film, a high efficiency of 8.42% with a record open-circuit voltage (VOC) of 0.47 V is obtained for flexible Sb2Se3 solar cells. The vertical van der Waals gaps in the (001) orientation provide favorable diffusion paths for Se atoms, which results in a Se-rich state at the bottom of the Sb2Se3 film and promotes the in situ formation of the MoSe2 interlayer between Mo and Sb2Se3. These phenomena contribute to a back-surface field enhanced absorber layer and a quasi-Ohmic back contact, improving the device's VOC and the collection of carriers. This method provides an effective strategy for the orientation control of 1D materials for efficient photoelectric devices.

High efficiency Cu2MnSnS4 thin film solar cells with SnS BSF and CdS ETL layers: A numerical simulation

The quaternary compound copper manganese tin sulfide Cu2MnSnS4 is a potential absorber semiconductor material for fabricating thin film solar cells (TFSC) thanks to their promising optoelectronic parameters. This article numerically investigated the performance of Cu2MnSnS4 (CMTS)-based TFSC without and with tin sulphide (SnS) back surface field (BSF) thin-film layer. First, the impact of several major influential parameters such as the active material's thickness, doping concentration of photoactive materials, density of bulk and interface defect, working temperature, and metal contact, were studied systematically without a BSF layer. Thereafter, the photovoltaic performance of the optimized pristine cell was further investigated with an SnS as BSF inserted between the absorber (CMTS) with a Platinum back metal of an optimized heterostructure of Cu/ZnO:Al/i-ZnO/n-CdS/p-Cu2MnSnS4/Pt. Thus, the photoconversion efficiency (PCE) of 25.43% with a JSC of 34.41 mA/cm2 and VOC of 0.883 V was achieved under AM1.5G solar spectrum without SnS BSF layer. Furthermore, an improved PCE of 31.4% with a JSC of 36.21 mA/cm2 and VOC of 1.07 V was achieved with a quantum efficiency of over 85% in the wavelengths of 450–1000 nm by the addition of SnS BSF layer. Thus, this obtained systematic and consistent outcomes reveal immense potential of CMTS with SnS as absorber and BSF, respectively and provide imperious guidance for fabricating highly a massive potential efficient solar cell.

Design of n-CdS/p-CuInTe2/p + -MoS2 thin film solar cell with a power conversion efficiency of 34.32%

Copper indium telluride (CuInTe2)-based n-CdS/p-CuInTe2/p + -MoS2 double-heterostructure solar cell has been investigated numerically by solar cell capacitance simulator (SCAPS-1D). Initially, an adjusted condition among the most influencing parameters e.g. thickness, carrier doping level, and bulk defects of active materials such as CdS window, CuInTe2 absorber, and p + -MoS2 back surface field (BSF) layers has been obtained by a systematic computation. The proposed solar cell exhibits an improved power conversion efficiency (PCE) of 34.32% with VOC =0.927 V, JSC = 42.50 mA/cm2, and FF = 87.14% under the optimized condition. The PCE can be further enhanced to 38.87% introducing sub-bandgap absorption in the MoS2 (300 nm) BSF with Urbach energy, E0 of 0.4 eV. These detailed simulation results reveal a huge potential of CuInTe2 absorber with MoS2 BSF layer for the manufacture of a cost-effective, high-efficiency double-heterojunction thin film solar cell.

Design and numerical analysis of CIGS-based solar cell with V2O5 as the BSF layer to enhance photovoltaic performance

Copper indium gallium selenide (CIGS)-based solar cells have exhibited greater performance than the ones utilizing cadmium telluride (CdTe) or hydrogenated amorphous silicon (a-Si: H) as the absorber. CIGS-based devices are more efficient, considering their device performance, environmentally benign nature, and reduced cost. In this article, we proposed a potential CIGS-absorber-based solar cell with an FTO/ZnSe/CIGS/V2O5/Cu heterostructure, with a V2O5 back-surface field (BSF) layer, SnO2:F (FTO) window layer, and ZnSe buffer layer. Using the solar cell capacitance simulator one-dimensional simulation software, the effects of the presence of the BSF layer, the thickness, bulk defect density, and acceptor density of the absorber layer, buffer layer thickness, interfacial defect density, device resistance, and operating temperature on the open-circuit voltage, short-circuit current, fill factor, and efficiency, as well as on the quantum efficiency and recombination and generation rate, of the device have been explored in detail. The simulation results revealed that only a 1 μm-thick-CIGS absorber layer with V2O5 BSF and ZnSe buffer layers in this structure offers an outstanding efficiency of 31.86% with a VOC of ∼0.9 V. Thus, these outcomes of the CIGS-based proposed heterostructure provide an insightful pathway for fabricating high-efficiency solar cells with performance more promising than the previously reported conventional designs.

Numerical prediction and optimization of the performance of CCTS-based thin film solar cell

Despite its direct band gap of about 1.5 eV, fabricated CCTS-based solar cells suffer from low efficiency. This paper presents a numerical simulation, using SCAPS software, for improving the efficiency of these solar cells. We optimized the work function of the back contact, the choice of the buffer layer, its thickness, its doping density, and finally, the absorber layer thickness. To minimize the reflection on the surface, we coted the solar cell with an anti-reflection layer of AlF3. We found that the optimal structure is: Ni/CCTS/ SnS2/ ZnMgO:Al/ AlF3 whose efficiency is 14.8 %. To avoid the problem of the expensive price of Ni, we proposed to use Mo as back contact and insert a p + -CCTS back surface field. With this solution, the output parameters are very close to that of the optimal structure.

Performance of Sb2Se3‐Based Solar Cell: With and Without a Back Surface Field Layer

Optimizing thin‐film‐based solar cells’ efficiency is essential for expanding renewable energy deployment. Fundamental device properties such as life‐span and optical properties must be enhanced simultaneously to reduce recombination and losses. Herein, the design and numerical study of an efficient Sb2Se3‐based dual heterojunction (n‐ZnSe/p‐Sb2Se3/p+‐MoSe2) photovoltaic cell with the help of SCAPS‐1D simulation software by using the physical parameters of various layers are presented. The thickness and doping of each layer are considered during the optimization of the device. The work functions of back and front contact also affect the overall performance. Defects in each layer and interface defects are highly responsible for the increment in series resistance. Thus, these defects are also studied. The efficiency improved significantly from 22.52% to 24.57% when MoSe2 is used as a back surface field (BSF) layer. The results demonstrate that the addition of BSF significantly increased the open‐circuit voltage, short‐circuit current density, and fill factor, all of which are within the Shockley–Queisser limits of a dual heterojunction solar cell. This study indicates the possibility of producing highly efficient Sb2Se3 heterojunction‐based solar cells.

Enhancement in Efficiency of CIGS Solar Cell by Using a p-Si BSF Layer

Copper–indium–gallium–diselenide Cu(In,Ga)Se2 (CIGS) is a semiconductor compound belonging to group I-III-VI, with a chalcopyrite crystal structure. CIGS is promising for the development of high-performance photovoltaic applications in terms of stability and conversion efficiency. It is one of the main candidates to rival the efficiency and stability of conventional crystalline silicon cells, due to its high light absorption coefficient, lower material cost, and high stability. The limitation of its use is that CIGS integrates indium (In) and gallium (Ga), which are rare and expensive materials. The amount of these materials in the CIGS cell can be reduced by optimizing the thickness of the absorber. We show that the introduction of a layer of highly doped silicon in the structure of the solar cell between the absorber layer and the back surface field layer effectively allows for decreasing the thickness of the absorber. Within the same objective, we focus on the danger of cadmium in the CdS buffer layer. In the first optimizations, we replaced the n-type CdS buffer layer with a n-type Zn(O,S) buffer layer. For this work, we used a one-dimensional simulation program, named Solar Cell Capacitance Simulator in one Dimension (SCAPS-1D), to investigate this new CIGS solar cell structure. After optimization, a maximum conversion efficiency of 24.43% was achieved with a 0.2 μm CIGS absorber layer and a 1 µm Si BSF layer.

Bandgap engineered 1.48 eV GaAs0.95P0.05 solar cell with enhanced efficiency using double BSF layer

One of the most important criteria to design more than 30% efficient III–V compound/Si based dual junction solar cell is that we must design atleast 20% efficient III–V compound material top cell. In this regard, we designed a bandgap engineered GaAs0.95P0.05 single junction solar cell with reduced bandgap of (E g ) = 1.48 eV. Reducing the bandgap from 1.72 eV to 1.48 eV for GaAs0.95P0.05 cell leads to generate higher short circuit current, while having the tradeoff with the open circuit voltage. Due to small change in lattice constant of GaAs0.95P0.05 cell, some recombination is observed near the junction area. Although the minimal degradation is observed in open circuit voltage, the higher short circuit current drives the overall efficiency of the GaAs0.95P0.05 single junction solar cell. The designed solar cell provides an extended internal absorption for longer wavelength of spectrum. The high electron mobility of 8500 cm2 V–S−1 was observed with very high electron to hole mobility ratio of 21.25. The optimization of the cell is done using two back surface field layers (AlInP and AlGaInP) of higher bandgap material. The high short circuit current density of J SC = 25.93 mA cm−2 with V OC = 1.1635 V achieved by the designed cell with the highest efficiency of η = 25%. The solar cell is irradiated under 1-Sun solar irradiation in the AM1.5 G environment providing 1000 W m−2 of power spectral density. The External and Internal Quantum efficiency of more than 95% is achieved by the designed solar cell.

Degradation of Monocrystalline Silicon Photovoltaic Modules From a 10-Year-Old Rooftop System in Florida

A system of 180 monocrystalline aluminum back-surface field modules were installed in Cocoa, Florida, for 10 years. In total, 156 modules are characterized and compared to 3 controls. Power degradation rates vary between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 0.14% to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 3.22% per year, with median and average rates of −0.92% and −1.05% per year, respectively. The losses are primarily resistive with minor optical and recombination loss contributions. Electroluminescence imaging shows a characteristic pattern, which is shown to be resistive in nature when compared to photoluminescence. Resistive losses are due to corrosion of the rear contact Ag/solder interface and, to a much lesser degree, gridline Ag oxidation. Moisture ingress through the backsheet is likely responsible for mediating corrosion. Optical losses are due mostly to a combination of antireflection coating degradation, minor encapsulant browning, and delamination. Minor front contact corrosion may contribute to recombination. This study expands upon previous work on this vintage of the module by examining a large sample set, comprehensive characterization including techniques not previously used on these modules, and a comparison between two other systems of different climates.

Unraveling the effects of a GeSe BSF layer on the performance of a CuInSe2 thin film solar cell: a computational analysis

In this work, a dual-heterojunction (DH) thin film solar cell of notable efficiency has been designed and simulated where p-type CuInSe2 (CIS) has been employed as the base layer in combination with an n-type CdS window and a p + -type GeSe back surface field (BSF) layer. The influences of each layer have been revealed using the SCAPS-1D simulator. While the n-CdS/p-CIS single heterojunction (SH) structure acting alone has been found to be resulted with 24.86% of photoconversion efficiency (PCE) with the JSC = 42.80 mA/cm2, VOC = 0.70 V, and FF = 83.44%, an enhancement to PCE of 30.52% is observed with the corresponding JSC of 44.10 mA/cm2, VOC of 0.86 V, and FF of 80.30% owing to the addition of GeSe as BSF layer in the proposed structure with optimized parameters. Because of the enormous built-in potential of the CIS/GeSe interface, increased VOC mostly contributes to the efficiency enhancement. These findings suggest that the CIS absorber layer with GeSe BSF layer is a promising choice for solar energy harvesting in the near future.

Numerical modeling of CuSbSe2-based dual-heterojunction thin film solar cell with CGS back surface layer

Ternary chalcostibite copper antimony selenide (CuSbSe2) can be a potential absorber for succeeding thin film solar cells due to its non-toxic nature, earth-abundance, low-cost fabrication technique, optimum bandgap, and high optical absorption coefficient. The power conversion efficiencies (PCEs) in conventional single heterojunction CuSbSe2 solar cells suffer from higher recombination rate at the interfaces and the presence of a Schottky barrier at the back contact. In this study, we propose a dual-heterojunction n-ZnSe/p-CuSbSe2/p+-copper gallium selenide (CGS) solar device, having CGS as the back surface field (BSF) layer. The BSF layer absorbs low energy (sub-bandgap) light through a tail-states-assisted upconversion technique, leading to enhanced conversion efficiency. Numerical simulations were run in Solar Cell Capacitance Simulator-1 dimensional software to examine how the performance of the proposed solar cell would respond under different conditions of absorber layer thickness, doping levels, and defect densities. The simulation results exhibit a PCE as high as 43.77% for the dual-heterojunction solar cell as compared to 27.74% for the single heterojunction n-ZnSe/p-CuSbSe2 counterpart, demonstrating the capability of approaching the detailed balance efficiency limit calculated by Shockley–Queisser.

Improved performance of Cd-free CZTS thin-film solar cells by using CZTS0.4Se0.6 BSF layer

Cu2ZnSnS4 (CZTS) thin film solar cells (TFSCs) have received great attention from the solar cell industry for their environment, price, high absorption coefficient and great electronic properties. This work provides a strategy to prompt the photoelectric conversion efficiency (η) of CdS/CZTS-based TFSCs via introducing the back surface field (BSF) layer and wxAMPS to simulate the results. Meanwhile, the optimum ratio of sulfur to selenium in the BSF layer material CZTSSe has been also studied, and a new device structure has been constructed. Beneficial from the introduction of CZTSSe as a BSF layer, the interface recombination is suppressed and leads to an enhanced Voc. Subsequently, MoO3, ZnS, WO3, TiO2 and In2S3 as candidates to substitute for the toxic Cd buffer layer are investigated. The results demonstrate that In2S3/CZTSSe exhibits the best performance with 28.59 % and 0.99 V, ascribed to a superior spike-like value. It was found that the presence of a spike-like conduction band (CB) at the In2S3/CZTSSe interface and a more suitable conduction band offset (CBO) value could suppress the interfacial complexation. Therefore, In2S3 performs best as a buffer layer material. However, due to the cost of indium, this paper recommends the utilization of wide bandgap semiconductor materials MoO3 and ZnS, which also have good stability and conversion efficiencies that can reach 28.38% and 28.41%, respectively. The work of this paper provides important guidance for researchers to manufacture CZTS TFSCs.

Investigation of CdSe and ZnSe as Potential Back Surface Field Layers for CdTe‐Based Solar Cells: A Study from First Principles Calculations

AbstractA class of II–VI semiconductors, especially CdTe, is a highly photo‐reactive compound that would be suitable for photovoltaic applications. However, being a highly resistive material, CdTe produces considerable contact resistance and drastically reduces the efficiency of photovoltaic devices. Introducing a back surface field layer at the contact region may significantly improve the device's performance. This work investigates the suitability of using ZnSe and CdSe layer as a back‐surface‐field layer in CdTe‐based solar cells through accurate electronic structure calculations using the hybrid‐density functional theory method. The calculations show that both ZnSe/CdTe and CdSe/CdTe behave as type‐II heterojunctions with band gaps of 2.0 and 1.1 eV, respectively. The Mulliken electronegativity method is used to determine the correct band edge positions concerning the vacuum level for all the pristine semiconductors and their interfaces. Calculation shows that a significant charge redistribution in the interface leads to the formation of an effective local field near the contact region for both ZnSe/CdTe and CdSe/CdTe heterostructures. This local field may help to separate the photogenerated electron–hole pairs in the active layer by pushing the opposite charges into the two different sections of the heterojunction. Additionally, the heterojunctions also exhibit better light‐absorption characteristics in the visible light range.

Defect engineering enabling p-type Mo(S,Se)2:TM (TM = V, Nb, Ta) towards high-efficiency kesterite solar cells

One important issue limiting the development of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is the severe recombination loss at CZTSSe/Mo(S,Se)2/Mo back contact, primarily arising from the non-matched potential barrier and inferior electric contact between absorber and weak n-type Mo(S,Se)2 interfacial layer. It is expected that a p-type Mo(S,Se)2 can effectively improve the hole extraction and enhance the device performance. In this work, we propose a plausible direction to build up an electrically benign back surface field by in-situ doping Mo(S,Se)2 with group VB elements (TM = V, Nb and Ta). It is turned out that the generated VMo, NbMo, or TaMo shallow acceptors could convert Mo(S,Se)2 from weak n-type to p-type conductivity and increase its work function (WF) and hole concentration. With the contact of p-type absorber (WF,absorber < WF,Mo(S,Se)₂), a benign ohmic contact with favorable downward band bending is formed, thereby accelerating the holes extraction and minimizing interface recombination loss. With significant gains in open-circuit voltage (Voc) and fill factor (FF), the Mo(S,Se)2:Ta device finally increases the efficiency from 10.82 % to 12.72 %. This convenient in-situ Mo(S,Se)2 doping strategy is different from previous complicated or unstable cases and should serve as a basis for high-quality back contact engineering in kesterite photovoltaics.

The use of back surface field and passivation layer to enhance the performance of silicon heterojunction solar cells

c-Si/a-Si:H heterojunction solar cells (Si-HJSCs) are simulated by AFORS-HET to study their performance with the back surface field (BSF) and passivation layer. Si-HJSCs(ninip) with the best open circuit voltage (Voc)(760.8 V), short circuit current density (Jsc)(36.97 mA/cm2), fill factor FF (85.22%), and efficiency (ɳ) (24.17%) were achieved. Whereas, estimated values of 675.2 mV, 33.5 mA/cm2, 83.34%, and 18.98% for a basic Si-HJSC (np) SC, which corresponds to the Voc,Jsc, FF and ɳ. The intrinsic -a-Si:H layer on both sides of the c-Si wafer has passivated the dangling bond density on the silicon, and the defect density at the interface between the c-Si and a-Si:H layers and improved the performance of Si-HJSCs. These simulation results were compared with experimental data for accuracy and validation. It was observed that the simulation results are highly promising and almost match experimental data.