Se2 Thin Film Solar Cells Research Articles

Impact of substrate temperature during NaF and KF post-deposition treatments on chemical and optoelectronic properties of alkali-free Cu(In,Ga)Se2 thin film solar cell absorbers

The substrate temperature TPDT during the alkali fluoride post-deposition treatment (PDT) of Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers is a critical process parameter. So far, TPDT is optimized empirically for a particular alkali distribution, and literature only reports on the effects of high TPDT (300–350 °C). To better understand the influence of TPDT and to close the gap for TPDT below 300 °C, a temperature series from 100 to 315 °C was studied. Furthermore, most studies on KF PDTs are performed with alkali-containing soda-lime glass substrates, in which the electrical effects of alkalis from the substrate and the PDT on the final device cannot be separated. To circumvent this obstacle, we use alkali-free substrates in our study and compare electrical and chemical effects of NaF and KF PDTs at different TPDT using current-density-voltage and capacitance-voltage measurements, as well as time-of-flight secondary-ion mass spectrometry and ultraviolet photoelectron spectroscopy. The alkali concentrations are quantified as a function of TPDT and discussed in relation to the respective cell parameters. Both, NaF and KF PDTs, improve the cell efficiency significantly, with the highest conversion efficiency of 16.3% obtained for a NaF PDT at TPDT = 315 °C. A given TPDT leads to a larger K concentration (after KF PDT) than Na concentration (after NaF PDT). For TPDT above 150 °C, similar conversion efficiencies are achieved for NaF and KF PDTs. In contrast, the KF PDT results in a significant decrease of the conversion efficiency at TPDT = 105 °C, likely caused by the formation of a Cu-Se secondary phase at the CIGSe surface, which is accompanied by a shift of the valence band maximum towards the Fermi energy.

Atomic Layer Deposition of Ultrathin ZnO Films for Hybrid Window Layers for Cu(Inx,Ga1-x)Se2 Solar Cells.

The efficiency of thin-film chalcogenide solar cells is dependent on their window layer thickness. However, the application of an ultrathin window layer is difficult because of the limited capability of the deposition process. This paper reports the use of atomic layer deposition (ALD) processes for fabrication of thin window layers for Cu(Inx,Ga1−x)Se2 (CIGS) thin-film solar cells, replacing conventional sputtering techniques. We fabricated a viable ultrathin 12 nm window layer on a CdS buffer layer from the uniform conformal coating provided by ALD. CIGS solar cells with an ALD ZnO window layer exhibited superior photovoltaic performances to those of cells with a sputtered intrinsic ZnO (i-ZnO) window layer. The short-circuit current of the former solar cells improved with the reduction in light loss caused by using a thinner ZnO window layer with a wider band gap. Ultrathin uniform A-ZnO window layers also proved more effective than sputtered i-ZnO layers at improving the open-circuit voltage of the CIGS solar cells, because of the additional buffering effect caused by their semiconducting nature. In addition, because of the precise control of the material structure provided by ALD, CIGS solar cells with A-ZnO window layers exhibited a narrow deviation of photovoltaic properties, advantageous for large-scale mass production purposes.

Effect of cesium fluoride treatment in bifacial Cu(In1-x,Gax)Se2 solar cell

Bifacial Cu(In1-x,Gax)Se2 (CIGS) thin film solar cells were fabricated on titanium-doped indium oxide (ITiO) coated soda-lime glass substrate, and the effect of cesium fluoride (CsF) treatment on solar cell performance was investigated. CsF treatment improved the photovoltaic properties of CIGS/ITiO-based solar cells not only under front-side illumination but also under rear-side illumination. The quantum efficiency for rear-side illumination improved significantly in CsF-treated CIGS solar cells. As a result, the bifaciality improved in the CsF-treated CIGS solar cell. ITiO exhibited good contact with the CIGS thin film, regardless of Cs incorporation. In addition, the effect of heat-light soaking treatment was also investigated in CIGS/ITiO-based solar cells.

Optimization of Cu(In, Ga)Se2 (CIGSe) thin film solar cells parameters through numerical simulation and experimental study

This study utilized the 2-D Silvaco ATLAS simulator to design and optimize the CIGSe thin-film solar cell (TFSC), considering experimental results from the hybrid-deposited CIGSe thin-film layer. The enhanced hybrid fabrication technique promises to yield higher power conversion efficiencies (PCE) for CIGSe-based TFSC and attain the Shockley–Queisser limit for single-junction devices. Another consideration is material utilization to avoid wastages without compromising on efficiency. The novel hybrid technique incorporates vacuum and non-vacuum deposition methods to fabricate the high-efficiency CIGSe absorber material for solar cell applications. The experimental studies involved optimizing the deposition methods and characterizing the deposited thin films for morphological, structural, optical, and electrical properties. Employing simulations in a study allows optimizing parameters, especially defects that may not be observed simply by experiments. During simulations for the CIGSe TFSC structure materials, the defined tail defect parameters studied the performance's deep defects, optimizing parameters favorable in experimental conditions for the CdS/CIGSe heterojunction in the based solar cell structure. The optimized simulated study achieved a record 21.92% of conversion efficiency. The simulated results can pave the way to understanding some carrier and device mechanisms, aiding fabricating low-coat and high-efficient CIGSe TFSC. The substitution of some results with experimental data from the hybrid deposited CIGSe absorber layer and reported optimized experimental values yielded an efficiency of 15.01%.

Effects of Sputtered InxSy Buffer on CIGS with RbF Post-Deposition Treatment

Indium sulfide (InxSy) is an attractive candidate to be used as a buffer layer for Cu(In,Ga)Se2 (CIGS) thin-film solar cells and modules. InxSy sputtering would be the preferred deposition method of choice as it allows for much higher growth rates compared to all other current deposition methods. A key feature of CIGS absorber processing, established during the last years, is the use of a post-deposition treatment (PDT) with alkali compounds. In this work, we apply rf-magnetron sputtered InxSy buffers from ceramic targets, optionally doped with NaF, on industrially relevant CIGS absorbers with or without RbF-PDT prepared in an in-line coater. We analyze the influence of wet chemical treatments of the CIGS surface after RbF-PDT and alkali accumulation at the InxSy/CIGS interface on the solar cell performance. Good efficiencies could be achieved with sputtered InxSy on CIGS with RbF in an all-dry process. An InxSy deposition temperature around 200 °C and/or a post-annealing step of the complete cell in air emerged as a key feature for decent efficiencies. This is also due for sputtering from a NaF-doped In2S3 target. Our best cell with sputtered InxSy on CIGS with RbF-PDT shows an efficiency of 18.0% with a high open-circuit voltage of 701 mV.

Application of Quantum Dot Down-Conversion Layer in Thin-Film Solar Cells to Increase Short-Wavelength Spectral Response

The short-wavelength optical loss in the Cu(In,Ga)Se2 (CIGS) thin-film solar cells is inevitable owing to the substantial light absorption in the front layers such as the buffer layer and transparent conducting oxide (TCO) layer. Quantum dots (QDs) with CdSe/ZnS core–shell structure is utilized to increase the short-wavelength spectral response of the CIGS thin-film solar cells. The QDs absorbs photons in the short-wavelength region (<540 nm) and re-emits the photons at approximately 540 nm; these photons penetrate the front layers and reach the CIGS absorber layer. The thickness of the QD layer was varied via drop coating with different QD concentrations, thereby facilitating the application of the optimized QD layer as a down-conversion layer in the CIGS thin-film solar cells. The photoelectric parameters of the CIGS thin-film solar cells were dependent on the QD thickness, and they were characterized using quantum efficiency measurements, spectrophotometric analysis, and current–voltage measurements. The CIGS thin-film solar cells with a 0.7 μm-thick QD layer exhibited the highest increase of 1.86 mA cm−2 and 0.75% in the short-circuit current density and efficiency, respectively.

Effect of combined treatment of cesium fluoride as precursor and post-treatment on Cu(In,Ga)Se2 thin film solar cell

In this study, we investigate the combined effect of cesium fluoride (CsF) as a precursor and post-treatment on a Cu(Inx,Ga1-x)Se2 (CIGS) thin film solar cell fabricated onto sodalime glass substrates. The combined treatment improves the open-circuit voltage, fill factor, and cell efficiency of the solar cell compared with its single counter treatment. Secondary ion mass spectroscopy measurements show a higher concentration of Cs diffusion in the surface regions in the combined treatment, which change the electronic properties of the buffer/CIGS interface by reducing interfacial recombination. Solar cells fabricated using CsF as a precursor indicate a deterioration in device performance owing to the formation of bulk defects, as confirmed from admittance spectroscopy. This study unveils device performance improvement by combining a heavy alkali metal as a precursor and post-treatment on CIGS thin film solar cells.

Influence of Ga back grading on voltage loss in low‐temperature co‐evaporated Cu(In,Ga)Se2 thin film solar cells

AbstractThe performance of Cu(In,Ga)Se2 (CIGS) solar cells is limited by the presence of the highly recombinative CIGS/Mo interface. The recombination at the CIGS/Mo interface is influential for the open circuit voltage (VOC) in high quality CIGS absorbers with increased charge carriers diffusion length. A quantitative understanding of the role of the Ga back grading height (ΔGGI) in suppressing back interface recombination is needed. In this work, we take advantage of a low temperature process to modify the ΔGGI while keeping the composition in the notch and front regions almost unchanged. Improvement in both VOC deficit and time‐resolved photoluminescence lifetime are observed with increasing ΔGGI. With a combination of back surface modification experiments and numerical simulations, we quantify a voltage loss in ungraded devices of approximately 100 mV solely from the back interface recombination. Nice agreement between simulation and experimental data is reached while constraining the values of possible diffusion lengths. Our results suggest that a ΔGGI of about 0.50 is required to effectively suppress the back interface recombination, highlighting the importance of grading control in high‐performance CIGS solar cells and devices.

Challenges in the deposition of (Ag,Cu)(In,Ga)Se2 absorber layers for thin-film solar cells

The partial replacement of Cu by Ag in Cu(In,Ga)Se2 thin-film solar cells is strategically interesting to achieve smooth devices with high conversion efficiencies. Yet, the industrial exploitation requires further understanding of the deposition process and control of the absorber layer properties. In this study, three-stage co-evaporation of (Ag,Cu)(Ga,In)Se2 films with [Ag]/([Ag] + [Cu]) contents up to 0.2 was investigated. Deep crevices and voids, sometimes extending down to the rear contact, were found. They mainly occur for high Ag contents and excessive group-I richness during the second stage of the deposition. The formation of cavities is attributed to the segregation of Ag–Se phases and slow Ag diffusion into the chalcopyrite during the deposition. Another identified challenge is the flattening of the desired bandgap grading which is correlated with the Ag content. Optimized process conditions allow fabrication of smooth (Ag,Cu)(Ga,In)Se2 films in a manufacturing-like inline deposition with cell efficiencies up to 20.5%.

12.95% Efficient Cu(In,Ga)Se2 Solar Cells by Single-Step Atmospheric Selenization, Scaled to Monolithically Integrated Modules

High-quality Cu(In,Ga)Se2 (CIGS) thin-film solar cells are often prepared by a two-step process, sputtering of Cu-Ga-In precursors followed by multistep selenization, including toxic additional pos...

Cross-sectional Kelvin probe force microscopy on Cu(In,Ga)Se2 solar cells: Influence of RbF and KF post-deposition treatment on the surface potential of the absorber layer

Post-deposition treatment (PDT) of the absorber layer with alkali fluorides has led to a significant increase in the efficiency of Cu(In,Ga)Se2 (CIGS) thin-film solar cells. In this contribution, we investigate the influence of alkali PDTs on the absorber's surface potential by means of Kelvin probe force microscopy (KPFM). To this end, we perform KPFM on cross sections of complete CIGS solar cells. To improve the reliability of the measurement procedure, we deposit a gold layer on top of the solar cell as a reference layer. Using this approach, we study the influence of RbF and KF PDT on the absorber's surface potential for CIGS solar cells with different absorber and buffer compositions. In all cases, an increased surface potential of the cross section of the absorber layer is measured for the cells with PDT.

Photon-induced defects and dynamics of photogenerated carriers in Cu(In,Ga)Se2 thin film solar cells

A detailed understanding of charge-carrier behavior could provide new avenues to improve the performance of the solar cell by enhancing the active materials and electronic properties. In the present work, the transport mechanism of photogenerated carriers in Cu(In,Ga)Se2 or CIGS)-based solar cells is investigated by analysis of the temperature-dependent current density-voltage (J-V) curves obtained under various illumination (i.e., dark, AM 1.5G, 650 nm, and 405 nm). The results demonstrate that J-V curve distortion, termed the “roll-over” effect, is significantly correlated with the illumination wavelength and, more importantly, is strongly associated with the persistent characteristics of the CIGS solar cell. In addition, the nature of the photo-induced defects and their influence on the transport mechanism of the photogenerated carriers were investigated via deep level capacitance profiling (DLCP) and admittance spectroscopy (AS) under various illumination conditions. The photo-capacitance results indicate distinct spatial distributions and persistent behaviors depending upon the illumination wavelength. Based on the findings in this work, the origin of the non-ideal J-V behavior in the CIGS solar cell is explained, and the spatial defect distribution and illumination wavelength sensitivity are discussed.

Characteristics of TAA-ZnS Buffer Layer by Addition of Sodium Citrate for CIGS Thin Film Solar Cell.

Zinc Sulfide (ZnS) is an environmentally friendly material with a wide bandgap (Eg = 3.7 eV) comparable to that of cadmium sulfide (CdS) (2.4 eV), which is conventionally used as buffer layer in Cu(In,Ga)Se2 (CIGS) thin film solar cells. Conventional ZnS buffer layers are manufactured using thiourea, and, these layers possess a disadvantage in that their deposition rate is lower than that of CdS buffer layers. In this paper, thioacetamide (TAA) was used as a sulfur precursor instead of thiourea to increase the deposition rate. However, the ZnS thin films deposited with TAA exhibited a higher roughness than the ZnS thin films deposited with thiourea. Sodium citrate was therefore added to increase the uniformity and decrease the roughness of the former ZnS thin films. When sodium citrate was used, the thin films demonstrated a high transmittance via the controlled generation of particles. In the case of TAA-ZnS thin films doped with a sodium citrate concentration of 0.04 M, the granules on the surface disappeared and these thin films were denser than the TAA-ZnS thin films deposited with a lower sodium citrate concentration. It is considered that the rate of the ion-by-ion reaction increased due to the addition of sodium citrate, thereby resulting in a uniform thin film. Consequently, TAA-ZnS thin films with thicknesses of approximately 40 nm and high transmittances of 83% were obtained when a sodium citrate concentration of 0.04 M was used.

A comparative study of the effects of light and heavy alkali-halide postdeposition treatment on CuGaSe2 and Cu(In,Ga)Se2 thin-film solar cells

Enhancement of photovoltaic performance due to alkali-metal doping in various thin-film compound solar cells such as chalcogenide Cu(In,Ga)Se2 (CIGS), Cu2ZnSnS4 (CZTS), and organic–inorganic hybrid perovskite, is significant in the development of photovoltaic technologies. In this work, the effects of light Li- and heavy Cs-doping in ternary CuGaSe2 (CGS) and quaternary CIGS thin-films and devices are compared. The new findings are: (i) heavy alkali-metal halide CsF postdeposition treatment (PDT) modifies the CGS film surface morphology and increases the carrier density; however, the beneficial effects of Cs on the CGS photovoltaic performance are low when compared to the results observed from quaternary CIGS devices; (ii) LiF-PDT leads to no significant CGS and CIGS film surface (hetero p-n junction interface) modifications; thus, the use of a thin (approximately 30 nm) CdS buffer layer is not effective in enhancing photovoltaic performance; and (iii) the beneficial effects of either light Li or heavy Cs on ternary CGS devices are modest; nonetheless, Li may be more effective than Cs, unlike quaternary CIGS devices.

Superstrate-type flexible and bifacial Cu(In,Ga)Se2 thin-film solar cells with In2O3:SnO2 back contact

Bifacial and flexible Cu(In,Ga)Se2 (CIGS) thin-film solar cells with superstrate-type structure were fabricated. For this fabrication, the substrate-type solar cells were first transferred from Mo/Glass substrates to 50-µm-thick flexible fluorinated ethylene propylene (FEP) copolymer films as superstrates using a lift-off process. The 300-nm-thick In2O3:SnO2 (ITO) thin-films were subsequently deposited on the rear sides of the CIGS absorber layers as back contacts (BC) of the finished bifacial devices with the structure of FEP/Epoxy/Al/Ni/Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CdS/CIGS/ITO/Ni/Al. Particular attention was paid to maintain the high optical and electrical transport properties of ITO-BC as well as to improve the Ohmic-like contact behavior at the CIGS/ITO interface to improve cell performances. Under the optimized ITO preparation, the resulting ITO thin film possesses the average optical transparency of ITO (>77%) in the wavelength range of 400–1100 nm, and the resistivity of 6.1 × 10−4 Ωcm with corresponding high carrier mobility of 42.8 cm2/Vs, as well as Ohmic-like characteristics at the CIGS/ITO interface can be realized with shunt resistance as low as 5.5 Ωcm2. Consequently, the flexible and bifacial CIGS thin-film solar cell was fabricated with efficiencies of 9.1% and 2.4% for frontside and backside illuminations, respectively. In this study, highly transparent and conductive ITO thin films have been successfully used as BC in flexible and bifacial CIGS solar cells.

Cu(In,Ga)Se2 Solar Cells Integrated with Subwavelength Structured Cover Glass Fabricated by One-Step Self-Masked Etching.

We report an anti-reflective cover glass for Cu(In,Ga)Se2 (CIGS) thin film solar cells. Subwavelength structures (SWSs) were fabricated on top of a cover glass using one-step self-masked etching. The etching method resulted in dense whiskers with high aspect ratio. The produced structure exhibited excellent anti-reflective properties over a broad wavelength range, from the ultraviolet to the near infrared. Compared to a flat-surface glass, the average transmittance of the glass integrated with the SWSs improved from 92.4% to 95.2%. When the cover glass integrated with the SWSs was mounted onto the top of a CIGS device, the short-circuit current and the efficiency of the solar cell were enhanced by 4.38 and 6%, respectively, compared with a CIGS solar cell without cover glass.

Phototransistor Behavior in CIGS Solar Cells and the Effect of the Back Contact Barrier

In this paper, the impact of the back contact barrier on the performance of Cu (In, Ga) Se2 solar cells is addressed. This effect is clearly visible at lower temperatures, but it also influences the fundamental parameters of a solar cell, such as open-circuit voltage, fill factor and the efficiency at normal operation conditions. A phototransistor model was proposed in previous works and could satisfactorily explain specific effects associated with the back contact barrier, such as the dependence of the saturated current in the forward bias on the illumination level. The effect of this contribution is also studied in this research in the context of metastable parameter drift, typical for Cu (In, Ga) Se2 thin-film solar cells, as a consequence of different bias or light soaking treatments under high-temperature conditions. The impact of the back contact barrier on Cu (In, Ga) Se2 thin-film solar cells is analyzed based on experimental measurements as well as numerical simulations with Technology Computer-Aided Design (TCAD). A barrier-lowering model for the molybdenum/Cu (In, Ga) Se2 Schottky interface was proposed to reach a better agreement between the simulations and the experimental results. Thus, in this work, the phototransistor behavior is discussed further in the context of metastabilities supported by numerical simulations.

Cu(In,Ga)Se2 thin film solar cells produced by atmospheric selenization of spray casted nanocrystalline layers

Cu(In,Ga)Se2 (CIGS) is suitable absorber material for thin film photovoltaic owing to its excellent thermo-chemical stability and demonstration of high power conversion efficiency of 23.35% for lab-scale devices. The manufacturing of CIGS thin film solar cells need to be economical for commercially viability. Non-vacuum approach for the synthesis of the CIGS thin film absorber is desired for its cost-effective benefits over vacuum based processes. We report development of solution-processed CIGS absorber layer involving spraying of sonochemically prepared nanocrystal ink to obtain pristine CIGS nanocrystal layer, followed by single-step atmospheric pressure selenization utilizing Se pallets as a Se vapor source. Influence of selenization conditions and NaF doping on grain growth of the CIGS thin film absorber is studied in detail by structural, morphological, compositional and electrical characterization. Solar cells prepared under optimized conditions yielded a maximum efficiency of 6.7% without employing any thermal pre-treatment and toxic Potassium Cyanide etching.

Microscopic origins of performance losses in highly efficient Cu(In,Ga)Se2 thin-film solar cells

Thin-film solar cells based on polycrystalline absorbers have reached very high conversion efficiencies of up to 23-25%. In order to elucidate the limiting factors that need to be overcome for even higher efficiency levels, it is essential to investigate microscopic origins of loss mechanisms in these devices. In the present work, a high efficiency (21% without anti-reflection coating) copper indium gallium diselenide (CIGSe) solar cell is characterized by means of a correlative microscopy approach and corroborated by means of photoluminescence spectroscopy. The values obtained by the experimental characterization are used as input parameters for two-dimensional device simulations, for which a real microstructure was used. It can be shown that electrostatic potential and lifetime fluctuations exhibit no substantial impact on the device performance. In contrast, nonradiative recombination at random grain boundaries can be identified as a significant loss mechanism for CIGSe solar cells, even for devices at a very high performance level.

Cover Image

The cover image is based on the Color tuning in Cu(In,Ga)Se2 thin-film solar cells by controlling optical interference in transparent front layers Dae-Hyung Cho et al., https://doi.org/10.1002/pip.3272.