Junction Partner Research Articles

Towards improved efficiency of SnS solar cells using back grooves and strained-SnO2 buffer layer: FDTD and DFT calculations

In this work, a novel SnS thin-film solar cell (TFSC) based on strained-SnO2 electron transport layer (ETL) combined with back grooves aspect is proposed. The density functional theory (DFT) is used to assess the influence of the unit cell volume engineering on the electronic and optical properties of SnO2 material. Simulated results demonstrate the potential of strained-SnO2 material as a junction partner of p-type SnS absorber, showing a favorable conduction band offset and thereby reduced recombination effects. Moreover, the role of back grooves texture in enhancing light-scattering effects is investigated using FDTD technique. It is revealed that the proposed SnS TFSC based on combined strained-SnO2/SnS heterostructure and optimized back grooves paradigm offers the possibility for bridging the gap between improved absorption capabilities and enhanced photo-induced carrier extraction characteristics. It is demonstrated that the optimized SnS TFSC exhibits superior performances compared to the conventional one in terms of photovoltaic parameters, offering a high short-circuit current of Isc = 28 mA/cm2, a superior open-circuit voltage of Voc = 0.47 V, a high fill factor of FF = 68.4% and over than 9% of PCE. Therefore, our work paves the way to use the strained-SnO2 buffer layer and back grooves to boost the poor performances of SnS-based TFSCs.

Performance analysis of SnS photodetector using strained SnO2 stacked layer: Numerical simulation and DFT calculations

In this work, a new broadband Photodetector (PD) based on strained-SnO2/SnS heterostructure combined with back grooves aspect is proposed. First-principles calculations are carried out to assess the influence of volume reduction using compressive stress on the electronic and optical properties of SnO2 wide band gap material. The obtained results demonstrate the potential of SnO2 under biaxial strain as a junction partner of SnS material, showing a lower band gap of 3.06 eV allowing a favorable band offset with SnS active layer. Moreover, numerical models based on Finite Difference Time Domain (FDTD) technique are developed to analyze the optical performances of strained-SnO2/SnS structure including periodic back grooves at the SnS/glass interface. It is revealed that the proposed device based on combined strained-SnO2/SnS heterostructure and back grooves approach offers the possibility for bridging the gap between extended absorption cutoff wavelength and improved photo-induced carrier extraction capability. The device shows a high ION/IOFF ratio of 78 dB and an enhanced responsivity under UV, visible and NIR lights (171mA/W at 365 nm, 67 mA/W at 550 nm and 93mA/W at 850nm). Therefore, the investigated PD based on strained SnO2/SnS heterostructure and optimized back grooves morphology can offer multiple sensing purposes with low power consumption and low elaboration cost, making it highly appropriate for the emerging optoelectronic applications.

The effect of ZnCl2 and CdCl2 treatment on ZnS/CdS junction partner on CdTe cell performance

In this study, the impacts of ZnCl2 and CdCl2 treatments on the structural and optical properties of ZnS/CdS bilayers and on the parameters of CdTe solar cells with ZnS/CdS junction partners were investigated. CdS and ZnS thin films were grown by chemical bath deposition. In the as deposited ZnS/CdS sample, hexagonal CdS and Zn(S,O) phases were formed. After the application of CdCl2 treatment to the sample, ZnO and CdZnS alloys also appeared. In the ZnCl2 treated bilayer, it was observed that the crystal structure of the CdZnS alloy changed from hexagonal to cubic phase. While similar grain structure was observed in the as deposited and the ZnCl2 treated samples; it was seen that CdCl2 treatment significantly affected the grain form and size. CdCl2 treatment resulted in a large increase of Cd-ratio and a more balanced increase Zn-ratio in ZnCl2 treatment. It was determined that ZnCl2 and CdCl2 treatments caused a decrease in the transmittance of the samples. PL spectroscopy revealed the presence of many structural defects such as interstitial zinc, sulfur vacancies, surface states, cadmium vacancies in all bilayer samples. Solar cells with ZnS/CdS, ZnS/CdS (CdCl2-treated) and ZnS/CdS (ZnCl2-treated) junction partners achieved efficiencies of 4.56%, 5.64% and 5.20%, respectively. Solar cell parameters showed that ZnCl2 treatment increased the FF value, while CdCl2 treatment improved the Voc value. An efficiency of 6.01% was obtained from the ZnS/CdS (CdCl2-treated)/CdTe/CdCl2 cell produced by obtaining the highest efficiency with the ZnS/CdS/CdCl2 junction partner. This cell revealed that applying CdCl2 treatment on CdTe significantly increased Voc and Jsc while deteriorating FF.

Lateral Photocurrent Method for Directly Measuring the Sheet Resistance of a Junction Partner for a Chalcogenide Light Absorber in a Thin-Film Solar Cell

The purpose of this study is to develop a method for directly measuring the sheet resistance of a junction partner for a chalcogenide light absorber in a thin-film solar cell. The developed method is applied to $\mathrm{Cd}\mathrm{S}$ in a $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ (CIGS) thin-film solar cell as a proof of concept. The suggested method utilizes a combination of the lateral photocurrent flowing along $\mathrm{Cd}\mathrm{S}$ in a custom-designed cell and the current density--voltage curve of a lab-scale CIGS solar cell. By combining an equivalent circuit analysis and a derived approximate equation, a semiempirical formula is proposed for directly measuring the sheet resistance of $\mathrm{Cd}\mathrm{S}$ in the CIGS thin-film solar cell. The semiempirical formula is a function of the basic characteristics (fill factor, short-circuit current density, open-circuit voltage) of the CIGS solar cell, and the linear lateral photocurrent density measured in the custom-designed cell. The proposed semiempirical formula is validated by measuring the sheet resistance of a $\mathrm{Zn}\mathrm{O}:\mathrm{Al}$ thin film in a custom-modified cell. The sheet resistance value agrees well with the sheet resistance value measured by a four-point probe. This method can be easily extended to ${\mathrm{Cu}}_{2}{\mathrm{Zn}\mathrm{Sn}\mathrm{S}}_{4}$, $\mathrm{Sn}\mathrm{S}$, and ${\mathrm{Sb}}_{2}{\mathrm{Se}}_{3}$ thin-film solar cells, which have very similar device structures to a CIGS thin-film solar cell.

Effect of ultra-thin CdSexTe1−x interface layer on parameters of CdTe solar cells

Effects of an ultra-thin CdSexTe1-x junction interface layer on CdTe solar cell parameters were investigated employing a CdSexTe1-x/CdTe absorber structure. CdSexTe1-x thin films with varying composition were grown by vacuum evaporation and CdTe films were produced by the close spaced sublimation (CSS) method. XRD analysis showed that while the CdSexTe1-x layers with x values less than 0.39 crystallized in cubic structure, films that were richer in Se displayed a (cubic + hexagonal) mixed phase. SEM analysis demonstrated a morphology with compact grains for all films. However, the grain size decreased appreciably with increasing Se content. Optical measurements showed that the band gaps of the alloys reached the minimum value of 1.40 eV at x ∼ 0.32. CdS/CdSexTe1-x/CdTe solar cells were fabricated employing 100 nm thick CdSexTe1-x interlayers. The Grazing Incidence (GI)-XRD spectra of CdSexTe1-x used in the device structure showed that these inter-layers had graded alloy composition. The average Se-concentration within the graded alloy films were found to agree with the values obtained by EDS. Conversion efficiencies of 9.59% 11.69% and 10.13%, were obtained for x values of 0.24, 0.32 and 0.39, respectively. Spectral response showed enhanced long wavelength response for all devices due to the presence of the CdSexTe1-x interlayer. It was concluded that using an ultra-thin CdSexTe1-x inter-layer with optimum properties between CdS (junction partner) and CdTe improves the cell performance by increasing the current density of the device.

Discovering Low-Electron-Affinity Semiconductors for Junction Partners of Photovoltaic BaSi2 by Database Screening and First Principles Calculations

Discovering Low-Electron-Affinity Semiconductors for Junction Partners of Photovoltaic BaSi₂ by Database Screening and First Principles Calculations

Screening and optimization of processing temperature for Sb2Se3 thin film growth protocol: Interrelation between grain structure, interface intermixing and solar cell performance

Deliverability of controllable deposition processes represents the current state-of-the-art strategy for the development of emerging 1D crystal-structure photovoltaic (PV) materials. For Sb2Se3, a compound with tremendous potential for next-generation cost-efficient thin-film PV, the current reported findings proposed that columnar grain growth protocol is required to promote efficient carrier transport and to achieve highly efficient solar cells. It is still a matter of debate, however, whether the growth of the thin-film absorber should follow the weak contacted nanorod grain structure or the columnar-sintered large grain protocol, in relation with the processing temperature, the under-layer junction partner, and interface intermixing. In this work, close-spaced sublimation processing temperature in conjunction with CdS and TiO2 junction partner layers is systematically investigated towards optimization of Sb2Se3 thin film growth conditions. The desideratum of columnar-sintered large grains protocol is validated and insights into the mechanism of absorber-buffer interface intermixing are provided. Following a systematical technological approach, a solar cell efficiency of 5.3% is demonstrated. Combining classical and advanced measurement techniques, insights into physicochemical processes and device functionality in connection with grain structure, defect density, carrier concentration, and recombination mechanism in Sb2Se3 absorber are provided.

Processing CdS- and CdSe-containing window layers for CdTe solar cells

The influence of heat treatment steps on the characteristics of (CdS, CdSe) junction partners and on solar cell performance was studied. CdS films were obtained by chemical bath deposition, and CdSe layers were evaporated. Structural and compositional properties of CdS/CdSe bilayer stacks did not change upon heat treatment at 400 °C up to 10 min, whereas heat treatment in the presence of CdCl2 for 10 min caused formation of a CdSSe alloy with a bandgap value of about 2.05 eV. Originally, the cubic structure of the stack was also transformed into a hexagonal structure during this treatment. CdSe–CdTe interdiffusion was also studied using CdS/CdSe/CdTe triple layer stacks. CdTe films were deposited using a close-spaced sublimation method. Limited CdSe–CdTe interdiffusion was seen when CdTe was deposited over the as-deposited CdSe layer at 580 °C. However, such interdiffusion was not detected for samples where CdTe deposition was carried out on CdS/CdSe stacks pre-annealed in the presence of CdCl2. This suggests that partial crystallization of the CdS/CdSe bilayer stack by CdCl2 reduced such an interaction. Solar cells with CdSe/CdTe, CdS/CdTe and CdS/CdSe/CdTe structures with efficiencies of 8.39%, 10.12% and 11.47% were fabricated using 4.5–5 µm thick CdTe layers and a final CdCl2 treatment. Quantum efficiency measurements demonstrated the benefit of CdSe–CdTe alloying during the final CdCl2 treatment in improving the short circuit current values.

Optimal CdS Buffer Thickness to Form High-Quality CdS/Cu(In,Ga)Se2 Junctions in Solar Cells without Plasma Damage and Shunt Paths.

CdS has been known to be one of the best junction partners for Cu(In,Ga)Se2 (CIGS) in CIGS solar cells. However, the use of thick CdS buffer decreases the short-circuit current density of CIGS solar cells. There are two obstacles that limit the use of ultrathin CdS. The first is plasma damage to CIGS during the preparation of transparent conducting windows and the second is a low shunt resistance due to the direct contact between the window and CIGS via pinholes in the thin CdS buffer. In other words, to avoid plasma damage and shunt paths, we may have to use a CdS buffer that is thicker than necessary to form a high-quality CdS/CIGS junction. This work aims to determine how thin the CdS buffer can be employed without sacrificing device performance while also eliminating the above two obstacles. We investigate the effect of CdS thickness on the performance of CIGS solar cells with silver nanowire-based window layers, which can eliminate both obstacles. An approximately 13 nm thick CdS buffer allows us to achieve high short-circuit current density and fill factor values. To attain an even high open-circuit voltage, an additional CdS buffer with a thickness of 13 nm is needed. The data from this study imply that an approximately 26 nm thick CdS buffer is sufficient to form a high-quality CdS/CIGS junction.

Improved CuGaSe2 absorber properties through a modified co-evaporation process

The present study deals with CuGaSe2-thin-films for solar cell applications. With the aim of achieving chemically and structurally homogeneous CuGaSe2 layers, it is proposed a modified co-evaporation process, which implies two so-called relaxation sequences during films growth. The resulting layers are characterized by scanning electron microscopy and Raman spectroscopy. By comparing their characteristics with those of CuGaSe2 grown without relaxation sequence, it is demonstrated that the modified process yields improved distribution of elements throughout the whole layer. The performance of the resulting solar cells is improved exclusively through increased quantum efficiency in the large wavelengths; further output voltage increase would require alternative junction partners.

Finding junction partners for CsPbI3 in a two-terminal tandem solar cell: A theoretical prospect

Abstract Screening ideal partner materials for the CsPbI3 perovskite is crucial for developing all-inorganic perovskite-based tandem solar cells. Here, we quantize the performance of all-inorganic tandem solar cells combining the CsPbI3 perovskite with c-Si, GaAs, CdTe, CIGS, and CsSnI3, by analyzing the measured spectral efficiencies and developing macroscopic continuum device (MCD) model. Firstly, we integrate the MCD model and the principles of detailed balance (Shockley-Queisser limit model, SQM) to investigate the top CsPbI3 solar cell with varying thickness and the rear sub-cell with different bandgaps. Without considering the nonradiative recombination and absorptivity in the rear sub-cell, an efficiency of over 32% can be reached by selecting the low bandgap materials (Egap ~ 1.1 eV) as the rear sub-cell. Furthermore, after the simulation by MCD model that takes the charge recombination, absorptivity and optimal thicknesses for both sub-cells in tandem architectures into account, CIGS is found to be one of the best junction partners for CsPbI3. During the simulations, a unique bisection search algorithm is used to guarantee that the continuity of electric potential near the junction layer and the current matching of sub-cells in the tandem solar cells. As a result, a CsPbI3(470 nm)/CIGS(2000 nm) tandem solar cell with the best-modeled efficiency of 24.40% can be achieved, which is affirmed by a recent experimental result. Finally, we simulate the corresponding bifacial tandem photovoltaic device, and the optimal modeled efficiency can be further enhanced to 25.58%.

Design and Simulation of FeSi2‐Based Novel Heterojunction Solar Cells for Harnessing Visible and Near‐Infrared Light

Herein, three novel third‐generation (3G) solar cells: n‐Si/p‐FeSi2/p+‐Si, n‐Si/p‐FeSi2/p+‐BaSi2, and n‐CdS/p‐FeSi2/p+‐BaSi2 based on the orthorhombic iron disilicide (β‐FeSi2) absorber are demonstrated theoretically for multikilowatt photovoltaic (PV) systems and space applications. These cells overcome the complication of producing low voltages (≤450 mV) of FeSi2‐based solar cells due to the narrow bandgap (≈0.87 eV) of the absorber. Using crystalline silicon (c‐Si), cadmium sulfide (CdS), and orthorhombic barium disilicide (β‐BaSi2) as junction partners, effects of parameters such as the thickness, doping and defect densities, band offsets, and temperature are studied systematically by a solar cell capacitance simulator (SCAPS‐1D). The highest open‐circuit voltage of 958 mV is attained materially with a 300 nm thin absorber. This article renders the optimization of the PV parameters to improve the device performance with power conversion efficiencies (PCEs) of 28.18%, 31.61%, and 38.93% by the three novel npp+ approaches compared to the PCEs of 15.78% and 24.96% for the solar cells n‐Si/p‐FeSi2 and p‐Si/i‐FeSi2/n‐Si, respectively.

Impact of RbF-PDT on Cu(In,Ga)Se2 solar cells with CdS and Zn(O,S) buffer layers

Alkali-fluoride post-deposition treatments (PDTs) of Cu(In,Ga)Se2 (CIGS) absorbers have repeatedly resulted in device efficiency improvements, observed mainly due to an open-circuit voltage (Voc) enhancement. Replacement of the CdS buffer layer with a higher band gap alternative can increase the short-circuit current density (Jsc) and also eliminate the use of Cd. In many alternative-buffer attempts, however, the Jsc gain was accompanied by a Voc loss, resulting in some degree of performance loss. In order to better understand the impact of RbF-PDT, we analyze a combination of experimental devices produced in the same in-line CIGS run with and without RbF-PDT in combination with chemical-bath-deposited CdS and Zn(O,S) buffers. Low-temperature current–voltage curves indicate a difference in Rb impact on the CIGS/CdS and CIGS/Zn(O,S) p-n junctions. For example, the diode-current barrier which creates a rollover often observed in RbF-treated CIGS/CdS current–voltage curves is significantly reduced for the CIGS/Zn(O,S) junction. Although the RbF-PDT had a positive impact on both junction partner combinations, the CIGS/Zn(O,S) devices' Voc and fill factor (FF) benefited stronger from the RbF treatment. As a result, in our samples, the Jsc and FF gain balanced the Voc loss, thus reducing the efficiency difference between cells with CdS and Zn(O,S) buffers.

Combinatorial Tuning of Structural and Optoelectronic Properties in CuxZn1−xS

Summary P-type transparent conductors (TCs) are important enabling materials for optoelectronics and photovoltaics, but their performance still lags behind n-type counterparts. Recently, semiconductor CuxZn1−xS has demonstrated potential as a p-type TC, but it remains unclear how properties vary with composition. Here, we investigate CuxZn1−xS across the entire alloy space (0 ≤ x ≤ 1) using combinatorial sputtering and high-throughput characterization. First, we find a metastable wurtzite alloy at an intermediate composition between cubic endpoint compounds, contrasting with solid solutions or cubic composites (ZnS:CuyS) from the literature. Second, structural transformations correlate with shifts in hole conductivity and absorption; specifically, conductivity increases at the wurtzite phase transformation (x ≈ 0.19). Third, conductivity and optical transparency are optimized within a "TC regime" of 0.10

Finding a junction partner for candidate solar cell absorbers enargite and bournonite from electronic band and lattice matching

An essential step in the development of a new photovoltaic (PV) technology is choosing appropriate electron and hole extraction layers to make an efficient device. We recently proposed the minerals enargite (Cu3AsS4) and bournonite (CuPbSbS3) as materials that are chemically stable with desirable optoelectronic properties for use as the absorber layer in a thin-film PV device. For these compounds, spontaneous lattice polarization with internal electric fields—and potential ferroelectricity—may allow for enhanced carrier separation and novel photophysical effects. In this work, we calculate the ionization potentials for non-polar surface terminations and propose suitable partners for forming solar cell heterojunctions by matching the electronic band edges to a set of candidate electrical materials. We then further screen these candidates by matching the lattice constants and identify those that are likely to minimise strain and achieve epitaxy. This two-step screening procedure identified a range of unconventional candidate junction partners including SnS2, ZnTe, WO3, and Bi2O3.

Suitable junction partners for promising solar absorbers

Screening for optimal electronic band offsets and low strain identifies four partner materials that merit future study.

Band Edge Positions and Their Impact on the Simulated Device Performance of ZnSnN2-Based Solar Cells

ZnSnN2 (ZTN) has been proposed as a new earth abundant absorber material for photovoltaic (PV) applications. While carrier concentration has been reduced to values suitable for device implementation, other properties such as ionization potential, electron affinity, and work function are not known. Here, we experimentally determine the value of ionization potential (5.6 eV), electron affinity (4.1 eV), and work function (4.4 eV) for ZTN thin film samples with Zn cation composition Zn/(Zn+Sn) = 0.56 and carrier concentration $n$ = 2 × 1019 cm−3 . Using both experimental and theoretical results, we build a model to simulate the device performance of a ZTN/Mg:CuCrO2 solar cell, showing a potential efficiency of 23% in the limit of no defects present. We also investigate the role of band tails and recombination centers on the cell performance. In particular, device simulations show that band tails are highly detrimental to the cell efficiency, and recombination centers are a major limitation if present in concentration comparable to the net carrier density. The effect of the position of the band edges of the p- type junction partner was assessed too. Through this study, we determine the major bottlenecks for the development of ZTN-based solar cell and identify avenues to mitigate them.

Life cycle assessment of photovoltaic manufacturing consortium (PVMC) copper indium gallium (di)selenide (CIGS) modules

Thin film copper indium gallium (di)selenide (CIGS) photovoltaic (PV) modules show promise for significant growth. The Photovoltaics Manufacturing Consortium (PVMC) is leading research and development of CIGS in New York State. This study presents the results of a life cycle assessment (LCA) study of CIGS technology, currently being advanced by PVMC, and compares the environmental performance of several emerging alternative materials and processes. The results provide manufacturers with an understanding of how to produce CIGS sustainably. The cradle-to-gate LCA study followed the International Standards Organization (ISO) 14040 series. In addition to analyzing CIGS cells, the study evaluated zinc oxysulfide (Zn(O,S)) as an alternative for cadmium sulfide (CdS) for the junction partner, and an integrated cell interconnect (ICI) system as an alternative for the screen printing and stringer. Life cycle inventory data for the CIGS cell and alternatives were obtained from PVMC for the product manufacturing stage. Global Solar Energy, Inc. (GSE), a PVMC member and CIGS manufacturer, provided data for the ICI alternative. This data was supplemented by secondary data, from thinkstep, and modeled in GaBi6. The end-of-life stage was not modeled due to limited inventory data. The results identified the following key drivers of environmental and toxicity impacts from the manufacture of CIGS photovoltaic cells: (i) silver used in the stringer and screen printing processes, (ii) metals comprising the CIGS layer, (iii) surface washing of the stainless steel substrate, and (iv) copper in the cable for the balance of system. In addition, the study found that the zinc oxysulfide alternative had lower overall impacts compared to cadmium sulfide. Substitution of the ICI system also reduced impacts of the CIGS system, by decreasing the silver needed. Finally, comparison of the overall life cycle impact results to similar systems in literature, which focused primarily on greenhouse gas emissions, found that the CIGS system fell in the lower end of the range of estimated global warming impacts. PVMC’s CIGS module impacts are likely lower than those made by other manufacturers due to their use of a stainless steel versus glass substrate and reliance on a relatively clean energy mix in New York State. To further reduce impacts, PVMC may wish to substitute the ICI component for the stringer and screen printing processes, recycle additional waste materials, substitute recycled metals for virgin materials, and substitute the cadmium sulfide with the zinc oxysulfide alternative.

Design prospects of cadmium telluride/silicon (CdTe/Si) tandem solar cells from numerical simulation

Tandem or multi-junction structures have been utilized to enhance the performance of existing solar cell to achieve higher efficiency in thin film solar cells with the least material usage. In this paper, silicon (Si) has been used as the multi junction partner with the standard CdTe solar cell (n-SnO2/n-CdS/p-CdTe/Al) structure to investigate the efficiency and stability. Photovoltaic properties of CdTe/Si tandem solar cell with the configuration of n-SnO2/n-CdS/p-CdTe/p+-CdTe/n+-Si/n-Si/p-Si/p+-Si/Al have been studied by Analysis of Microelectronic and Photonic Structure (AMPS-1D) simulation software. This modified structure has been investigated by adding tunnel junction, which consists of highly doped p-CdTe and n-Si with the variation of simulation parameters such as layer thickness, carrier concentration and operating temperature. Enhanced photovoltaic effects have been observed by optimizing the layer thickness of p-CdTe, n-Si, p-Si and carrier concentration of p-CdTe. From the simulation result, the best efficiency of 28.457% has been achieved for CdTe/Si tandem structure with Voc=1.15V,Jsc=27.612mA/cm2, FF=0.894 as compared with the baseline CdTe solar cell efficiency of 19.701%.

Effect of CdS deposition temperature for thermally grown Cu(In,Ga)Se2 and two-step chalcogenized Cu(In,Ga)(S,Se)2 absorbers on solar cell performances

CdS buffer layer of varying thickness ranging from 23 to 58 nm deposited at different substrate temperature were prepared as n-type junction partner for thermally grown Cu(In,Ga)Se2 and two-step chalcogenized Cu(In,Ga)(S,Se)2 photovoltaic absorber films and the effect of deposition temperature and time on the CdS growth behavior and solar cell performance were evaluated. High deposition temperature resulted in a thicker CdS layer and more importantly lower density and shallower depth of open voids, which attributed to the improved open-circuit voltage and fill factor due to reduced interface recombination. The solar cell efficiency of thermally grown absorber saturated at about 30 nm thickness of CdS, while that of chalcogenized absorber gradually increased with CdS thickness up to 60 nm without significant loss of short-circuit current density.