Spectroscopic analysis of CIGS2/CdS thin film solar cell heterojunctions on stainless steel foil

This paper presents a spectroscopic analysis of the interface between a CuIn_1−xGa_xS₂ (CIGS2) absorber and a CdS buffer layer on stainless steel foil by Auger electron spectroscopy (AES), inverse photoemission spectroscopy (IPES) and photoelectron spectroscopy (PES) such as X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). By combining these spectroscopic techniques, detailed information about the electronic and chemical properties of the CIGS2 surface and the CdS/CIGS2 interface can be obtained. The gallium concentration in CIGS2 films was found to increase continuously towards the Mo back contact. XPS analysis showed the presence of KCO₃ on the surface of CdS, deposited on etched and un-oxidized samples indicating diffusion of potassium. No potassium was observed on oxidized as well as samples having thicker CdS (50 nm) indicating the effectiveness of oxidation and chemical bath deposition (CBD) process in cleaning the sample surface effectively. In addition, investigation of the electronic level alignment at the interface has been carried out by combining PES and IPES. Conduction band offset of −0.45 (±0.15) eV and a valence band offset of −1.06 (±0.15) eV were measured. These unfavorable conditions limit efficiency of CIGS2 thin film solar cells.     

Extending absorption of near-infrared wavelength range for high efficiency CIGS solar cell via adjusting energy band

The efficient photon harvesting in near infrared wavelength range is still a challenging problem for high performance Cu(In_1-x, Ga_x)Se₂ (CIGS) solar cell. Herein, adjusting the energy band distribution of CIGS solar cell could provide significant academic guidance for devices with superior output electric power. To understand the role of each functional layer, the optimal 3000 nm CIGS absorber layer with 1.3 eV bandgap and 30 nm CdS buffer layer were firstly obtained via simulating the uniform band-gap structures. By introducing CIGS absorber layer with a double grading Ga/(Ga+In) profile, the power conversion efficiency of the double gradient band gap cell is superior to that of uniform band-gap cell through extending absorption of near-infrared wavelength range. Upon optimization, the best power conversion efficiency of CIGS with a double gradient band gap solar cell is improved significantly to 24.90%, among the best values reported in literatures, which is an 8.17% relative increase compared with that of the uniform band-gap cell. Our findings provide a theoretical guide toward the design of high performance solar cells and enrich the understandings of the energy band engineering for developing of novel semiconductor devices.     

Cu(In,Ga)Se2 thin film solar cells from nanoparticle precursors

A non-vacuum process for Cu(In,Ga)Se₂ (CIGS) thin film solar cells from nanoparticle precursors was described in this work. CIGS nanoparticle precursors was prepared by a low temperature colloidal route by reacting the starting materials (CuI, InI₃, GaI₃ and Na₂Se) in organic solvents, by which fine CIGS nanoparticles of about 15 nm in diameter were obtained. The nanoparticle precursors were then deposited onto Mo/glass substrate by the doctor blade technique. After heat treating the CIGS/Mo/glass layers in Se gas atmosphere, a complete solar cell structure was fabricated by depositing the other layers including CdS buffer layer, ZnO window layer and Al electrodes by conventional methods. The resultant solar cell showed a conversion efficiency of 0.5%.     

Growth of Sn(O,S)2 buffer layers and its application to Cu(In,Ga)Se2 solar cells

Sn-based thin films as new buffer layer for Cd-free Cu(In,Ga)Se₂ (CIGS) solar cells were developed. The Sn(O,S)₂ films were formed on CIGS substrates by chemical bath deposition from an alkaline ammonia solution by reacting tin(IV) chloride with thiourea. Optimization of the growth process allowed the smooth and conformal coverage of the films on the CIGS substrates with a thickness of 20 nm that was a self-limited thickness in the chemical bath deposition process. XPS analysis revealed that the as-deposited films contained Sn–O, Sn–OH, and Sn–S bondings and the ratio of Sn–S bonding to Sn–O bonding was 0.3. The CIGS solar cell fabricated with a 20-nm thick Sn(O,S)₂ buffer layer had the best efficiency of 11.5% without AR coating. The open circuit voltage, short circuit current, and fill factor were 0.55 V, 34.4 mA/cm², and FF = 0.61, respectively. The open circuit voltage and fill factor were low compared to the conventional CIGS solar cell with a 50-nm thick CdS buffer due to too thin Sn(O,S)₂ buffer layer.     

Annealing temperature dependent on structural,optical and electrical properties of indium oxide thin films deposited by electron beam evaporation method

In the present work the influence of annealing temperature on the structural and optical properties of the In₂O₃ films deposited by electron beam evaporation technique in the presence of oxygen was studied. The deposited films were annealed from 350 to 550 °C in air. The chemical compositions of In₂O₃ films were carried out by X-ray photoelectron spectroscopy (XPS). The film structure and surface morphologies were investigated as a function of annealing temperature by X-ray diffraction (XRD) and atomic force microscopy (AFM). The structural studies by XRD reveal that films exhibit preferential orientation along (2 2 2) plane. The refractive index (n), packing density and porosity (%) of films were arrived from transmittance spectral data obtained in the range 250–1000 nm by UV–vis-spectrometer. The optical band gap of In₂O₃ film was observed and found to be varying from 3.67 to 3.85 eV with the annealing temperature.     

Investigation of Cu2ZnSnS4 solar cell buffer layer fabricated via spray pyrolysis

Copper zinc tin sulfide solar cells were fabricated by using spray pyrolysis from a window layer to an absorber layer. ZnS and In₂S₃ buffer layers were deposited on the TiO₂ layer, and the photovoltaic characteristics were investigated. The ZnS buffer demonstrated a poor photovoltaic performance because of its poor surface coverage and micro-cracks at fluorine-doped tin oxide/TiO₂ layers. The In₂S₃ buffer layer sprayed at low temperature (<360 °C) showed a large difference between photo and dark currents beyond the open-circuit voltage (V_OC). When the spraying temperature exceeded 390 °C, the devices showed high dark leakage currents at reverse biases because of the high conductivity of the buffer layer, resulting in decreased V_OC and short-circuit current density (J_SC). The optimum temperature for spraying In₂S₃ is 360 °C, and the best performing device showed 410 mV, 30.4 mA/cm², 35.3%, and 4.4% of V_OC, J_SC, fill factor, and efficiency, respectively.     

Cu(In,Ga)Se2 superstrate-type solar cells with Zn1−xMgxO buffer layers

Superstrate-type Cu(In,Ga)Se₂ (CIGS) thin film solar cells were fabricated using Zn_1−xMg_xO buffer layers. Due to the diffusion of Cd into CIGS during the growth of the CIGS layer, the conventional buffer material of CdS is not suitable. ZnO is a good candidate because of higher thermal tolerance but the conduction band offset (CBO) of ZnO/CIGS is not appropriate. In this study, the Zn_1−xMg_xO buffer layers were used to fulfill both the requirements. The superstrate-type solar cells with a soda-lime glass/In₂O₃:Sn/Zn_1−xMg_xO/CIGS/Au structure were fabricated with different band gap energies of the Zn_1−xMg_xO layer. The CIGS layers [Ga/(In + Ga)∼0.25] were deposited by co-evaporation method. The substrate temperature during the CIGS deposition of 450 °C did not cause the intermixing of the Zn_1−xMg_xO and CIGS layers. The conversion efficiency of the cell with Zn_1−xMg_xO was higher than that with ZnO due to the improvement of open-circuit voltage and shunt resistance. The results well corresponded to the behavior of the adjustment of CBO, demonstrating that the usefulness of the Zn_1−xMg_xO layer for the CBO control in the superstrate-type CIGS solar cells.     

Laser and electrical current induced phase transformation of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> semiconductor thin film on Si(111)

Phase transformation of thin film (∼30 nm)In₂Se₃/Si(111) (amorphous→crystalline) was performed by resistive annealing and the reverse transformation (crystalline→amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In₂Se₃ is of interest for non-volatile phase-change memory. Amorphous In _x Se _y was deposited at room temperature on Si(111) after pre-deposition of a crystalline In₂Se₃ buffer layer (0.64 nm). Upon resistive annealing to 380°C, the film was transformed into a γ-In₂Se₃ single crystal with its {0001} planes parallel to the Si(111) substrate and parallel to Si , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and X-ray diffraction. Laser annealing with 20-ns pulses (0.1 millijoules/pulse, fluence≤50 mJ/cm²) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous In _x Se _y film.     

Influence of vacuum annealing on the structural and photoelectrochemical properties of nanocrystalline MoBi2S5 thin films

In the present paper we report structural, optical, morphological and electrical properties of thin films of MoBi₂S₅ prepared by facile self organized arrested precipitation technique (APT) from aqueous alkaline bath. X-ray diffraction study on thin films suggests orthorhombic and rhombohedral mixed phase structure. The samples are further annealed under vacuum at 373 and 473 K. The EDS pattern shows minor loss of sulphur upto 473 K. The optical absorption in visible region shows direct allowed transition with band gap variation over 1.2–1.1 eV. Post-heat treated samples exhibit n-type electrical conductivity. SEM images show uniform distribution of spherical grains with diameter ∼200 nm for as-synthesized MoBi₂S₅ thin film. The grain size increases with annealing temperature and morphology becomes more compact due to crystallization of thin film. The surface roughness deduced from AFM, was in the range of 1.29–1.92 nm. The MoBi₂S₅ thin films are employed for the fabrication of photoelectrochemical solar cells as all the samples exhibit strong absorption in visible to near IR region. Due to vacuum annealing it gives a significant enhancement of power conversion efficiency (η) upto 0.14% as compared to as-synthesized MoBi₂S₅ thin film.     

Electroreflectance study of CuIn1−xGaxSe2 thin film solar cells

We have investigated the optical properties of CuIn_1−xGa_xSe₂ (CIGS) thin film solar cells using their electroreflectance (ER) at room temperature. The ER spectra exhibited one broad and two narrow signal regions. Using the photoluminescence (PL) and photocurrent (PC) spectra, the peaks in the low-energy region (1.02–1.35 eV) can be assigned to the CIGS thin film. The PC results implied that the peaks in the high-energy region (2.10–2.52 eV) can be assigned to the CdS band-gap energy. Using the applied bias voltage, the broad signals in the 1.35–2.09 eV region can be assigned to the Franz–Keldysh oscillation (FKO) due to the internal electric field. The ER spectra exhibited a distorted CdS signal for the CIGS thin film solar cell with low shunt resistance and efficiency.     

Direct growth of CdSe nanorods on ITO substrates by co-anchoring of ZnO nanoparticles and ethylenediamine

This work presents results from a study carried out on the Al/Cu₃BiS₃/Buffer/ZnO stacked layer, using high-resolution transmission electron microscopy (HRTEM). This system is used to fabricate solar cells with Al/Cu₃BiS₃/In₂S₃/ZnO and Al/Cu₃BiS₃/ZnS/ZnO structures. The conforming layers function as electrical contact, absorber layer, buffer layer, and optical window, respectively. The detailed results of Cu₃BiS₃ thin film investigation by HRTEM are presented. The Cu₃BiS₃ thin films are non-homogeneous and are strongly dependent on deposition conditions with grain size between 6.5 and 20?nm showing a nano-crystalline character. We found that the buffer layer of In₂S₃ grows in a polycrystalline structure, whereas the layer of ZnS reveals an amorphous structure. The performed study of these solar cells gives us significant information about their crystalline structure and allows us to visualize each of the constituting layers as well as of the Al/Cu₃BiS₃, Cu₃BiS₃/buffer, and buffer/ZnO interfaces. This study was correlated with electrical properties.     

Improving the performance of pure sulfide Cu(InGa)S2 solar cells via injection annealing system

In this study, we present an effective method of improving the performance of pure sulfide Cu(InGa)S₂ (CIGS) solar cells via injection annealing system. The injection annealing system can perform annealing at desired temperatures, and therefore, the CIGS thin film passed over the temperature range in which secondary phases occurs. Via the injection annealing system, secondary phase InS_x was effectively removed from the surface of the CIGS thin films at the temperatures over 550°C. This resulted in the formation of good-quality P–N junction CIGS devices, thereby improving significantly the performance of the CIGS solar cell. In addition, the open-circuit-voltage (V_OC) and fill factor (FF) of the CIGS devices increased gradually with increasing annealing temperature in the range of 550–640°C. It is speculated that the bulk defects were decreased as the annealing temperature increased. Finally, via injection annealing system, a pure sulfide CIGS solar cell with an efficiency of 12.16% was achieved.     

The annealing effect on structural,optical and photoelectrical properties of CuInS2/In2S3 films

Successive Ionic Layer Adsorption and Reaction (SILAR) technique was used to deposit the CuInS₂/In₂S₃ multilayer thin film structure at room temperature. The as-deposited film was annealed at 100, 200, 300, 400 and 500 °C for 30 min in nitrogen atmosphere and the annealing effect on structural, optical and photoelectrical properties of the film was investigated. X-ray diffraction (XRD) and optical absorption spectroscopy were used for structural and optical studies. Current–Voltage (I–V) measurements were performed in dark environment and under 15, 30 and 50 mW/cm² light intensity to investigate the photosensitivity of the structure. Also, the electrical resistivity of the film was determined in the temperature range of 300–470 K. It was found that annealing temperature drastically affects the structural, optical and photoelectrical properties of the CuInS₂/In₂S₃ films.     

On the structure, morphology, and optical properties of chemical bath deposited Sb2S3 thin films

In the present paper, we have reported the room temperature growth of antimony sulphide (Sb₂S₃) thin films by chemical bath deposition and detailed characterization of these films. The films were deposited from a chemical bath containing SbCl₃ and Na₂S₂O₃ at 27 °C. We have analysed the structure, morphology, composition and optical properties of as deposited Sb₂S₃ films as well as those subjected to annealing in nitrogen atmosphere or in air. As-deposited films are amorphous to X-ray diffraction (XRD). However, the diffused rings in the electron diffraction pattern revealed the existence of nanocrystalline grains in these films. XRD analysis showed that upon annealing in nitrogen atmosphere these films transformed into polycrystalline with orthorhombic structure. Also, we have observed that during heating in air, Sb₂S₃ first converts into orthorhombic form and then further heating results in the formation of Sb₂O₃ crystallites. Optical bandgap energy of as deposited and annealed films was evaluated from UV-vis absorption spectra. The values obtained were 2.57 and 1.73 eV for the as-deposited and the annealed films respectively.     

Thickness study of Al:ZnO film for application as a window layer in Cu(In1−xGax)Se2 thin film solar cell

Structural, electrical and optical properties of Al doped ZnO (Al:ZnO) thin film of various thicknesses, grown by radio-frequency magnetron sputtering system were studied in relation to the application as a window layer in Cu(In_1−xGa_x)Se₂ (CIGS) thin film solar cell. It was found that the electrical and structural properties of Al:ZnO film improved with increasing its thickness, however, the optical properties degraded. The short circuit current density, J_sc of the fabricated CIGS based solar cells was significantly influenced by the variation of the Al:ZnO window layer thickness. Best efficiency was obtained when CIGS solar cell was fabricated with electrically and optically optimized Al:ZnO window layer.     

Modification in cell structure for better performance of spray pyrolysed CuInS2/In2S3 thin film solar cell

We report a new structure for CuInS₂/In₂S₃ solar cell, in which both absorber and buffer layers were deposited using chemical spray pyrolysis (CSP) technique. The usual superstrate structure, having buffer layer just above ITO, was not functioning mainly due to diffusion of Cu into In₂S₃ layer as seen from X-ray photoelectron spectroscopy (XPS) results. However, when the configuration of the cell was ITO/CuInS₂/In₂S₃/Ag, cell parameters obtained were V_oc=0.45 V, J_sc=44.03 mA/cm², fill factor (FF) = 29.5% and η=5.87%. Good results could be obtained by using indium sulfide thin films having maximum photosensitivity. The cell was characterized using X-ray diffraction, optical absorption, current–voltage and spectral response measurements. PACS 81.15.Rs; 82.45.Mp; 84.60.Jt     

Optimization of annealing conditions of In2S3 thin films deposited by vacuum thermal evaporation

In₂S₃ thin films were grown on glass substrates by means of the vacuum thermal evaporation technique and subsequently thermally annealed in nitrogen and free air atmosphere from 250 to 350 °C for different durations. Experimental parameters have been adjusted in order to optimize the annealing conditions, and to obtain high band gap energy at low deposition temperature, as required for photovoltaic applications. In order to improve our understanding of the influence of the deposition and annealing parameters on device performance, we have investigated our indium sulfide material by X-ray diffraction, energy dispersive X-ray analysis (EDAX), atomic force microscopy (AFM) and spectrophotometry. The optical and structural properties of the films were studied as a function of the annealing temperature and durations. X-ray diffraction analysis shows the initial amorphous nature of deposited In-S thin films and the phase transition into crystalline In₂S₃ upon thermal annealing. Films show a good homogeneity and optical direct band gap energy about 2.2 eV. An annealing temperature of 350 °C during 60 min in air atmosphere were the optimal conditions.     

Post-annealing effect on the reactively sputter-grown CIGS thin films and its influence to solar cell performance

Using a reactive co-sputtering from Cu_0.6Ga_0.4 and Cu_0.4In_0.6 alloy targets, we prepared CuIn_1−xGa_xSe₂ (CIGS) thin films on Mo/soda-lime glass (SLG) in association with a thermal cracker for elemental atomic Se radicals. The film growth was performed at 500 °C for 90 min. To achieve the composition ratio of CIGS absorber layer, Cu_0.6Ga_0.4 target was set at RF power of 50 W, 60 W, 70 W, and 80 W while keeping at 100 W for Cu_0.4In_0.6 alloy target. Post-annealing was done for all the CIGS films at 550 °C for 30 min. The composition ratio of [Cu]/[In + Ga] and [Ga]/[In + Ga] was increased with RF power but showed no change after post-annealing. X-ray diffraction analysis revealed all the samples has grown dominantly in the [112] crystal orientation. We found the Cu_2−xSe and (InGa)_2−xSe₃ defect phase both at the surface and in the bulk, and developed with post-annealing. From the devices fabricated in the structure of grid/ITO/i-ZnO/CdS/CIGS/Mo/soda-lime glass (SLG), the external quantum efficiency (EQE) was observed to improve in the wavelength, λ ≥ 550 nm in the samples treated with annealing. In the current–voltage (J–V) measurements, the solar cell showed the best performance of FF = 54.1%, V_oc = 0.48 V, J_sc = 33.1 mA/cm² and η = 8.5% in the sample with [Cu]/[In + Ga] = 0.84 that improved largely from η = 4.6% for the solar cell with an as-grown CIGS films.     

一种印刷型薄膜太阳能电池p-n结调制技术

能带值为0.5~0.85 eV材料的稀缺是多结太阳能电池面临的一个主要挑战,本文使用非真空的机械化学法合成了能带值为0.83 eV的Cu₂SnS₃化合物,使用印刷技术将其制备成吸收层薄膜,并采用superstrate太阳能电池结构(Mo/Cu₂SnS₃/In₂S₃/TiO₂/FTO glass)对其光伏特性进行了研究.实验表明所制备的太阳能电池短路电流密度、开路电压、填充因子和转换效率分别为12.38 mA/cm²、320 mV、0.28和1.10%.此外,为更好地满足多结太阳能电池对电流匹配的需求,本文对所制备太阳能电池的Cu₂SnS₃/In₂S₃ p-n结进行了分析.通过在p-n结界面植入一层薄的疏松缓冲层,使调制后的太阳能电池短路电流密度从最初的12.38 mA/cm²增加到了23.15 mA/cm²,相应太阳能电池转换效率从1.1%增加到了1.92%.该p-n调制技术对印刷型薄膜太阳能电池具有重要借鉴意义.     

Improved growth of solution‐deposited thin films on polycrystalline Cu(In,Ga)Se2

CdS and Zn(O,S) grown by chemical bath deposition (CBD) are well established buffer materials for Cu(In,Ga)Se₂ (CIGS) solar cells. As recently reported, a non‐contiguous coverage of CBD buffers on CIGS grains with {112} surfaces can be detected, which was explained in terms of low surface energies of the {112} facets, leading to deteriorated wetting of the chemical solution on the CIGS surface. In the present contribution, we report on the effect of air annealing of CIGS thin films prior to the CBD of CdS and Zn(O,S) layers. In contrast to the growth on the as‐grown CIGS layers, these buffer lay‐ ers grow densely on the annealed CIGS layer, even on grains with {112} surfaces. We explain the different growth behavior by increased surface energies of CIGS grains due to the annealing step, i.e., due to oxidation of the CIGS surface. Reference solar cells were processed and completed by i‐ZnO/ZnO:Al layers for CdS and by (Zn,Mg)O/ZnO:Al for Zn(O,S) buffers. For solar cells with both, CdS and Zn(O,S) buffers, air‐annealed CIGS films with improved buffer coverage resulted in higher power‐conversion efficiencies, as compared with the devices containing as‐grown CIGS layers. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)