Chemical properties of the Cu(In,Ga)Se2/Mo/glass interfaces in thin film solar cells

The Cu(In,Ga)Se₂/Mo and the Mo/glass interfaces in high efficiency thin film solar cells have been investigated by surface-sensitive photoelectron spectroscopy and bulk-sensitive X-ray emission spectroscopy. The interfaces were accessed by a suitable lift-off technique. Our experiments show a strong Se diffusion from the absorber into the Mo film, suggesting the formation of a MoSe₂ layer in the surface-near region of the back contact. In addition, we find a Ga diffusion into the Mo back contact, while no diffusion of In and Cu occurs. Furthermore, we derive a detailed picture of the Na distribution near the back and front side of the Cu(In,Ga)Se₂ absorber.     

Residual stress in CIGS thin film solar cells on polyimide: simulation and experiments

Sputtering technique is used to prepare Cu(In,Ga)Se₂ (CIGS) thin film solar cells on a UPILEX-S 50S polyimide substrate in order to investigate the residual stress in the Mo back contact and CIGS absorber layer after selenized annealing with various the thickness ratios of the Mo contact, R_Mo. A comparison between the results of numerical simulation and those obtained from X-ray diffractometry, indicate the existence of compressive residual stress in both the Mo contact and the CIGS absorber layer. The residual stress in the Mo contact was inversely proportional to the residual stress in the CIGS absorber layer. Residual stress decreased with an increase in the thickness ratio of the Mo contact. The empirical formulae for the residual stresses as a function of R_Mo in the Mo and CIGS films were deduced, from the results of this study.     

Selenization of CIS and CIGS layers deposited by chemical spray pyrolysis

Cu(In_1???xGa_x)Se₂ (CIGS) thin films with x?=?0 (CIS) and x?=?0.3 (CIGS) were prepared on Mo-coated glass substrate by using chemical spray pyrolysis at a substrate temperature of 350 °C, followed by selenization treatment at 550 °C in selenium environment under N₂ gas flow. X-ray diffraction patterns of as-deposited CIGS layers on Mo showed polycrystalline chalcopyrite phase with an intense (112) plane. Splitting of (204)/(220) and (116)/(312) planes for the film with x?=?0.3 reveals deviation of tetragonal nature. Field emission scanning electron microscopy cross-sectional images of selenized films showed clear re-crystallization of grains. During the selenization process of the CIGS absorber, a thin interface layer of MoSe₂ is formed. Line mapping of Mo/CIGS layer showed more gallium segregation at the interface of back contact resulting in band gap grading. Chemical composition and mapping of the as-deposited and selenized samples were determined by energy dispersive analysis of X-rays. This work leads to fabrication of low cost and large scale Mo/CIGS/CdS/ZnO/ZnO:Al device structure.     

Out‐of‐Plane Strain Induced in a Moiré Superstructure of Monolayer MoS2 and MoSe2 on Au(111)

Making contact of transition metal dichalcogenides (TMDCs) with a metal surface is essential for fabricating and designing electronic devices and catalytic systems. It also generates strain in the TMDCs that plays significant role in both electronic and phonon structures. Therefore, detailed understanding of mechanism of the strain generation is important to fully comprehend the modulation effect for the electronic and phonon properties. Here, MoS₂ and MoSe₂ monolayers are grown on Au surface by chemical vapor deposition and it is demonstrated that the contact with a crystalline Au(111) surface gives rise to only out‐of‐plane strain in both MoS₂ and MoSe₂ layers, whereas no strain generation is observed on polycrystalline Au or SiO₂/Si surfaces. Scanning tunneling microscopy analysis provides information regarding consequent specific adsorption sites between lower S (Se) atoms in the S? Mo? S (Se? Mo? Se) structure and Au atoms via unique moiré superstructure formation for MoS₂ and MoSe₂ layers on Au(111). This observation indicates that the specific adsorption sites give rise to out‐of‐plane strain in the TMDC layers. Furthermore, it also leads to effective modulation of the electronic structure of the MoS₂ or MoSe₂ layer.     

Thermal stability of sputtered Mo/polyimide films and formation of MoSe2 and MoS2 layers for application in flexible Cu(In,Ga)(Se,S)2 based solar cells

Molybdenum (Mo) films with a thickness of about 800 nm were room temperature sputtered onto flexible polymeric substrates. Upilex® films were chosen as substrates on the basis of their high thermal endurance and reduced coefficient of thermal expansion. Thermal stability of Mo films has been proved by heat treatment of the Mo/Upilex® structures at a temperature comparable to that used in the preparation of the Cu(In,Ga)(Se,S)₂ absorber layer. A combination of high optical reflectance (maximum values of 75-80%), low electrical resistivity (about 30 μΩ cm) and a smooth surface free of cracks for heated films highlights their good thermal stability. The formation of MoSe₂ and MoS₂ layers, after selenization/sulfurization of the Mo/Upilex® structures, has been further investigated in view of their application as back contact layers in flexible CIGS based solar cells.     

Chemical synthesis and compositional analysis of mixed [Mo(S1 − x Se x )2] semiconductor thin films

The synthesis of binary MoS₂, MoSe₂ and mixed [Mo(S_{1 – x} Sex)₂] thin films onto a glass substrates using arrested precipitation technique (APT) is presented in this investigation. Growth kinetics and mechanism of film formation were studied for these films and are explained in brief. The stoichiometry of the film is confirmed by analyzing films using Extractive spectrophotometric (ESP), atomic absorption spectroscopic (AAS) and electron difftraction X-ray microanalysis (EDAX) techniques. The semiconductor solution containing Mo(VI) and Se(IV) is extracted with N-n-octylaniline in xylene and determined by ESP, AAS and EDAX techniques. Further these films are characterized for its semiconducting behavior to test the suitability of molybdenum chalcogenides as a photoelectrode to convert radiant energy into electricity. It is found that stoichiometry of the film formed by our recently developed arrested precipitation technique (APT) has strong influence on photoconduction in molybdenum chalcogenide photoelectrodes.     

Pulsed electrodeposition and characterization of molybdenum diselenide thin film

Molybdenum dichalcogenides are semiconductors with layered type structure, which can act as efficient electrodes in the realization of photoelectrochemical solar cells. The main advantage of this molybdenum diselenide (MoSe₂) semiconductor is the prevention of electrolyte corrosion because of the phototransitions involving non-bonding d-d orbital of the Mo atoms. Polycrystalline molybdenum diselenide thin films are prepared by pulsed electrodeposition on conducting glass and titanium substrates in galvanostatic mode from an ammoniacal solution of H₂MoO₄ and SeO₂. The growth kinetics of the film was studied and the deposition parameters such as electrolyte bath concentration, bath temperature, time of deposition, deposition current, pH of the electrolyte and duty cycle of the current are optimized. X-ray diffraction analysis of the as deposited and annealed films showed the presence of highly textured MoSe₂ films with polycrystalline nature. EDAX spectrum of the surface composition confirms the nearly stoichiometric MoSe₂ nature of the film. Surface morphology studies by scanning electron microscope (SEM) shows that the films are pinhole free and of device quality nature. The optical absorption spectra show an indirect band gap value of 1.16 eV. Conductivity measurements were carried out at different temperatures and electrical constants such as activation energy, trapped energy state and barrier height were calculated.     

Preparation of layered semiconductor (MoSe2) by electrosynthesis

Molybdenum dichalcogenides are semiconductors, which can act as efficient electrodes in the realization of photoelectrochemical solar cells. Among molybdenum dichalcogenides (MoS₂, MoSe₂ and MoTe₂), MoSe₂ has led to the best solid state cells with efficiencies exceeding 6%. The main advantage of these MoSe₂ semiconductor is the prevention of electrolyte corrosion, because of the phototransitions involving non-bonding d–d orbital of the Mo atoms. MoSe₂ thin films have been electrodeposited cathodically on tin oxide (SnO₂)-coated conducting glass substrates from an ammonaical solution of H₂MoO₄ and SeO₂ under potentiostatic condition. The electrode potential was fixed at −0.9 V_SCE and the pH was maintained at 9.3±0.1. The bath temperature was maintained at 40°C. X-ray diffraction analysis showed the presence of highly textured MoSe₂ films with polycrystalline nature. The optical absorption spectra show that the material has an indirect band-gap value of 1.17 eV. Atomic force microscope (AFM) study was used to find the surface roughness of the film and surface morphology studies by scanning electron microscope (SEM) show that the films are smooth, uniform and pin-hole-free useful for device fabrication.     

Influence of an additional carbon layer at the back contact-absorber interface in Cu(In,Ga)Se2 thin film solar cells

For paste-coated Cu(In,Ga)Se₂ (CIGS) absorber layers used for thin film solar cells one often gets a residual carbon layer between back contact and absorber layer. We investigate the influence of this layer on the solar cells’ performance with co-evaporated CIGS absorbers and find a beneficial effect. The power conversion efficiencies of thin chalcopyrite absorber layers are often limited by the influence of back contact recombination. It is assumed that the carbon layer between the back contact and the absorber layer helps lower this recombination and allows higher open circuit voltages and thus higher conversion efficiencies.     

GaSe Formation at the Cu(In,Ga)Se2/Mo Interface–A Novel Approach for Flexible Solar Cells by Easy Mechanical Lift‐Off

Implementing photovoltaic devices based on high efficiency thin‐film technologies on cheap, light‐weight and flexible polymeric substrates is highly appealing to cut down costs in industrial production and to accelerate very large scale deployment of photovoltaics in the upcoming years. Lift‐off processes, which allow separating active layers from primary substrates and subsequent transfer onto an alternative substrate without modifying the upstream production process and without performance losses, are an emerging alternative to direct growth on polymeric substrates. This study concerns the feasibily of direct mechanical lift‐off process for high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells grown by coevaporation on glass/molybdenum substrates without performance losses. The study presents an in depth characterization (SEM,AFM,GIXRD,XPS) of samples leading to excellent lift‐off properties. They are explained by a specific gallium rich CIGS graded interface structure according to the interfacial sequence glass/Mo/MoSe₂/Ga_xSe_y/Ga‐rich‐CIGS. The interfacial layer, attributed to GaSe, has a layered structure and out performs the molybdenum diselenide layered layer which forms spontaneously at the interface Mo/CIGS. It allows a very easy lift‐off process at the interface GaSe/CIGS thanks to Van‐der‐Waals adhesion mechanism in GaSe. Key physical‐chemical parameters are identified and analyzed. After lift‐off, an efficiency of 14.3%, higher than the initial reference CIGS solar cell efficiency (13.8%) is measured.     

Edge‐Riched MoSe2/MoO2 Hybrid Electrocatalyst for Efficient Hydrogen Evolution Reaction

Molybdenum diselenide (MoSe₂) is widely considered as one of the most promising catalysts for the hydrogen evolution reaction (HER). However, the absence of active sites and poor conductivity of MoSe₂ severely restrict its HER performance. By introducing a layer of MoO₂ on Mo foil, MoSe₂/MoO₂ hybrid nanosheets with an abundant edge and high electrical conductivity can be synthesized on the surface of Mo foil. Metallic MoO₂ can improve the charge transport efficiency of MoSe₂/MoO₂, thereby enhancing the overall HER performance. MoSe₂/MoO₂ exhibits fast hydrogen evolution kinetics with a small overpotential of 142 mV versus RHE at a current density of 10 mA cm^?2 and Tafel slope of 48.9 mV dec^?1.     

The potential of textured front ZnO and flat TCO/metal back contact to improve optical absorption in thin Cu(In,Ga)Se2 solar cells

The role of additionally textured front transparent conductive oxide − TCO (ZnO:Al) and flat TCO/metal contact on optical improvements in thin Cu(In,Ga)Se₂ (CIGS) solar cells are investigated by means of numerical simulations. A de-coupled analysis of two effects related to additional texturing of front surface of ZnO:Al TCO − (i) enhancement of light scattering and (ii) decreased total reflectance (antireflective effect) − reveals that the improvements in quantum efficiency, QE, and short-circuit current, J_SC, of the solar cell originate from an antireflective effect only. In order to improve optical properties of the back contact the introduction of a TCO layer (undoped ZnO) between CIGS and back metal contact is investigated from the optical point of view. In addition to ZnO/Mo, a highly reflective ZnO/Ag contact (ZnO is also assumed to work as a protection layer for Ag) is also included in simulations. Results show significant increase in reflectance related to introduced ZnO in front of Mo. Drastically increased reflectance is obtained if ZnO/Mo is substituted with ZnO/Ag. The improvements in QE and J_SC of a thin CIGS solar cell, related to ZnO/metal contacts are presented.     

Microstructure and electrical conductivity in shape and size controlled molybdenum particle thick film

We study the effects of particle morphology on the microstructure and electrical conductivity of Mo particle thick films. In our study, the shape and size of molybdenum (Mo) particles are modified by mechanical ball-milling and atomic layer deposition (ALD). As the total number of collisions between Mo particles and ball-milling media increases, Mo particles are deformed, and the shape of Mo particles changed from irregular polyhedrons to thin flakes. In the ball-milling process, stress frequency is an important processing parameter governing the deformation and breakage of Mo particles. In addition, ALD-grown TiO₂ layer is found to significantly suppress the growth of Mo particles at high temperature. After 1000 °C annealing, the particle size of the TiO₂ layer-coated film is only half of that of bare Mo particle films. The shape of the particles changes electrical conductivity of the Mo thick films. Large contact area between flake shape particles can increase the carrier mobility of the film and the 5-nm thick TiO₂ layer can provide the inter-particle carrier transport path via a tunneling mechanism. Our results show that the combined use of the ball-milling and the ALD coating leads to Mo thick films with high electric conductivity and large surface area.     

Opto-structural and electrical properties of chemically grown Ga doped MoBi2Se5 thin films

An arrested precipitation route was developed to obtain gallium doped MoBi₂Se₅ thin films on substrate support. High purity organometallic complexes of Mo–triethanolamine (Mo–TEA), Bi–triethanolamine (Bi–TEA), Ga–triethanolamine (Ga–TEA) allow to react with sodium selenosulphite (Na₂SeSO₃) in the presence of sodium dithionite (Na₂S₂O₄) as a reducing agent in an aqueous alkaline reaction bath. As deposited thin films were characterized by X-ray diffraction, which reveals that material is nanocrystalline with mixed phases of rhombohedral (Bi₂Se₃)-hexagonal (MoSe₂)-hexagonal (GaSe) structures. Scanning electron micrographs show the grain of granular morphology decreases with increase in Ga concentration. Energy dispersive x-ray analysis shows presence of Mo, Bi, Ga and Se elements in stoichiometric ratio confirms the chemical formula MoBiGaSe₅. Optical absorbance of the films show direct allowed transition in visible region having band gap energy in the range of 1.30–1.47 eV. Thermoelectric power and electrical conductivity measurements have been carried out for thin film samples in the temperature range 300–500 K and results revealed that n-type semiconducting behavior. It is interesting to note that Ga doping in MoBi₂Se₅ changes n to p type conductivity.     

Interstitial Mo‐Assisted Photovoltaic Effect in Multilayer MoSe2 Phototransistors

Thin‐film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe₂) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W^?1), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe₂ TFTs is sought. A unique structural characteristic of the APCVD‐grown MoSe₂ is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe₂ phototransistors is identified, suggesting a novel route to high‐performance, multifunctional 2D material devices for future wearable sensor applications.     

Optimization of Ti/TiN/Mo back contact properties for Cu(In,Ga)Se2 solar cells on polyimide foils

Molybdenum is conventionally used as electrical back contact for Cu(In,Ga)Se₂ (CIGS) solar cells. In this work, a multifunctional stack of Ti/TiN/Mo is introduced as back contact for flexible CIGS solar cells. The multilayer back contact was deposited on 25 μm thick polyimide foil by means of DC reactive sputtering.To optimize electrical conductivity and film stress of the alternative back contact sputter parameters such as total gas pressure, sputtering power, substrate temperature and RF substrate bias have been varied. XRD measurements and quantitative analysis of foil curvature revealed that the film stress is significantly influenced by the argon gas pressure and sputtering power. The electrical conductivity was improved by applying higher sputtering power or RF substrate bias. Analysis of the film microstructure with SEM shows that applied substrate bias influences the density of the sputtered film. The solar cells processed on Ti/TiN/Mo as well as on a conventional Mo bilayer back contact have been compared using standard current density to voltage (J-V) measurements and external quantum efficiency measurements. Conversion efficiencies of 13.4% for the alternative and 14.9% for the conventional design have been obtained.     

Interfacial microstructure and defect analysis in Cu(In,Ga)Se([sub]2)-based multilayered film by analytical transmission electron microscopy and focused ion beam

Interfacial microstructures of Cu(In,Ga)Se₂(CIGS)-based multilayered film are closely characterized by TEM (transmission electron microscopy), SEM (scanning electron microscopy) and FIB (focused ion beam). A cross-sectional TEM, energy dispersive X-ray spectroscopy and energy-filtered TEM reveal that a pronounced Cu diffusion occurs across the interface of the CdS/CIGS, which leads to a large amount of Cu rich in the CdS layer and a Cu-deficient sub-surface in the CIGS layer as well as a rough interfacial structure. TEM studies further reveal that the interface microstructures in the multilayered film are dissimilar, both ZnO/CdS and CdS/CIGS interfaces are strongly bonded whereas the CIGS/Mo interface is weakly bonded and interface separation occasionally occurs. Mo back contact layer shows a well adhesion to glass substrate.Detailed observation on defects in the CIGS-based multilayered film is carried out by 3D (3-dimensional) FIB and SEM techniques. Sequential 2D (2-demensional) cross-sectioning shows that dominant growth-defects in the CIGS and top SiO₂ layers are micro-scale crack, appearing as diversified morphologies. The micro-scale crack in the CIGS layer is possibly released by propagating into the adjacent layer while the crack in the SiO₂ layer is relieved usually by forming a small particle behind. It is noted that in the multilayered film the interface frequently acts as crack initiation sites due to distinct thermal expansion coefficients.     

Na-doped Mo target sputtering for Cu(In,Ga)Se2 thin film solar cells

This study employed a Mo–5 % Na thin film on a soda-lime glass substrate as the bottom layers of a Mo back contact using a sputtering process to achieve large area Cu(In,Ga)Se₂ (CIGS) cells application and uniform distribution. Our results demonstrate that increasing the ratio of Mo–5 % Na to Mo film thickness (R %) from 0 to 11 % enhanced the crystallinity of the deposited bi-layer Mo film, thereby increasing surface roughness and slightly reducing resistivity. Following selenization, optimal CIGS crystalline characteristics appeared when R % = 8 % (sodium content = 1.57 at.%), such that secondary phases were not generated, and the surface and depth distribution of sodium were uniform.     

Investigation into the thermal annealing effects on CuIn(S,Se)2 thin films prepared by solution processes

CuIn(S,Se)₂ polycrystalline films were grown on soda-lime glass substrates by a solution process. Different annealing temperatures led to the variations of structural and electrical properties of the thin films. The pre-annealing temperatures changed from room temperature (RT) to 250°C, and all the post-annealing temperatures were set at 550°C. High quality film was obtained after post-annealing when the pre-annealing temperature increased to 250°C. The (112) X-ray diffraction peak’s position shifted at different pre-annealing temperatures after post-annealing, and the intensity of the (112) orientation increased with the pre-annealing temperature rising. Raman spectra exhibited A₁ mode was stable and the mixed B₂-E modes disappeared gradually with pre-annealing temperature increasing. The band gap energy of the film pre-annealed at 250°C is about 1.29 eV. The resistivity of the films without pre-annealing was 1230 Ω·cm, and 1.5 Ω·cm was achieved when the pre-annealing temperature was 250°C.     

One-step electrodeposition of CuGaSe2 thin films

CuGaSe₂ thin films have been prepared by one-step electrodeposition and rapid thermal annealing process. According to composition and morphology analysis, deposition potential of − 0.6 V vs. SCE is considered to be optimum for electrodeposition. From the X-ray diffraction and Raman studies, the as-deposited film exhibits poor crystallinity without the evidence of CuGaSe₂ or other Ga-containing phases, while the rapid thermal annealing-treated film shows chalcopyrite structure CuGaSe₂ phase containing MoSe₂ phase between the Mo substrate and the absorber and minor second phase Cu_2 − xSe. The obtained CuGaSe₂ thin film has a band gap of about 1.68 eV and p-type conductivity.