CIGSSe Films Research Articles

Interfacial modification mechanism by aging effect for high-performance Cd-free and all-dry process Cu(In,Ga)(S,Se)2 solar cells

Modification of the hetero-junction interface in Cd-free and all-dry process Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells by introducing the aging process for the bare CIGSSe films is demonstrated, ultimately achieving the highest conversion efficiency of 22.0%. In this article, two positive effects of the aging process are discussed. First, additional phases, such as Inx(O,S)y and Cu-deficient phases, are naturally formed on the CIGSSe surface. Second, unintentional segregation of Zn-rich particles at the (Zn,Mg)O/CIGSSe interface during the (Zn,Mg)O deposition is suppressed. These effects lead to the reduction of recombination at the hetero-junction interface and dramatic enhancement of performance for the Cd-free and all-dry process CIGSSe solar cells.

Spontaneous and Stimulated Emission in Thin Films of Cu(In1 –xGax)(SySe1 –y)2 Solid Solutions in the Сomposition of Solar Cells

The emission spectra of thin nanocrystalline films of Cu(In1 – xGax)(SySe1 – y)2 (CIGSSe) direct-gap solid solutions in the structure of solar cells, recorded upon continuous-wave laser excitation (~0.5 W/cm2) and nanosecond pulsed laser excitation with a power density in the range of 0.1–53 kW/cm2 at temperatures ranging from 10 to 300 K, are analyzed. It is found that stimulated emission (SE) occurs in thin CIGSSe films at temperatures from 10 to 90 K in the spectral range hν = 1.062–1.081 eV, with a minimum threshold-pump level of ~1 kW/cm2. It is shown that the spontaneous emission band is blue-shifted with an increase in the excitation intensity. It is also established that, with an increase in temperature, the photoluminescence (PL) band (upon weak excitation) and the stimulated-emission band are blue-shifted, whereas the PL band is red-shifted upon strong excitation. The possible reasons and mechanisms for the influence of temperature and excitation intensity on the spectral positions of the spontaneous and stimulated emission bands for the solid-solution films are discussed.

Controlling the morphology of solution-processed CuIn(SSe)2 absorber layers by film thickness and annealing temperature

We demonstrated the solution-processed CuIn(S,Se)2 solar cells using a diamine/dithiol cosolvent. Compared to the traditional CIGS materials made with the Selenization process, we fabricated CISSe films without the Selenization process. The crystallinity, optical properties, and morphology of the CIGSSe films were changed by annealing temperature and film thickness. The band gap energy were found approximately 1.5 eV with the different film thickness.

Effect of cesium for Cu(In,Ga)Se2 and Cu(In,Ga)(S,Se)2 films studied by depth-resolved XAFS

Alkali metal treatment is an indispensable technology for obtaining highly efficient CuInSe2 (CIS) solar cells. This study evaluated the structural change of Cu(In,Ga)Se2 (CIGSe) and Cu(In,Ga)(S,Se)2 (CIGSSe) films with and without the CsF post-deposition treatment (CsF-PDT) (for CIGSe) and Cs treatment (for CIGSSe) by the Se K-edge depth-resolved X-ray absorption fine structure. The CIGSe films were deposited by a three-stage process, and the CIGSSe films were deposited by the sulfurization after the selenization method. Although CsF-PDT can re-form the surface of CIGSe film, the Cs treatment cannot modify the surface of the CIGSSe film. Our result suggests that the improvement is due to not only the surface reforming but also an effect on the inside of the CIGSe and CIGSSe films. This study provides useful information for the preparation of highly efficient CIS solar cells.

Boosting Solar Cell Performance via Centrally Localized Ag in Solution-Processed Cu(In,Ga)(S,Se)2 Thin Film Solar Cells

Fabrication of Cu(In,Ga)(S,Se)2 (CIGSSe) absorber films from environmentally friendly solutions under ambient air conditions for use in solar cells has shown promise for the low-cost mass production of CIGSSe solar cells. However, the limited power conversion efficiency (PCE) of these solar cells compared with their vacuum-processed counterparts has been a critical setback to their practical applications. This study aims to fabricate solution-processed CIGSSe solar cells with high PCEs by incorporation of Ag into the precursor layer of the CIGSSe absorber films. The results showed that Ag doping promoted grain growth by accelerating Se uptake irrespective of location within the CIGSSe film. Nevertheless, uniform Ag doping formed crevices that lowered the PCE of the cells, while centrally localizing the doped Ag prevented the formation of crevices, resulting in high PCEs up to 15.3%. Our results demonstrate that carefully doping Ag into a selected area of the precursor layer of CIGSSe films can realize solution-processed chalcopyrite solar cells with high PCE.

Спонтанное и стимулированное излучение в тонких пленках твердых растворов Cu(In-=SUB=-1-x-=/SUB=-Ga-=SUB=-x-=/SUB=-)(S-=SUB=-y-=/SUB=-Se-=SUB=-1-y-=/SUB=-)-=SUB=-2-=/SUB=- в составе солнечных элементов

The results of a study of the emission spectra of thin nanocrystalline films of Cu(In1-xGax)(SySe1-y)2 (CIGSSe) direct-band-gap solid solutions in the structure of solar cells at ~ 0.5 W/cm2 continuous wave and nanosecond pulsed laser excitation in the range of excitation power density 0.1 - 53 kW/cm2 and temperatures of 10-300 K are presented. It was found that stimulated emission (SE) occurs in thin CIGSSe films in the temperature range from 10 K to 90 K in the spectral region h = 1.062 - 1.081 eV with a minimum threshold pump level of about 1 kW/cm2. It was shown, that, with increasing intensity of the exciting emission, the spontaneous emission bands shift toward higher energies. It was found that the photoluminescence bands at low excitation levels and the SE bands shift with increasing temperature toward higher energies, and the PL bands at high excitation levels shift toward low energies. Possible causes and mechanisms of the influence of temperature and excitation intensity on the spectral positions of spontaneous and SE of the films of solid solutions are discussed.

Sulfur-Alloying Effects on Cu(In,Ga)(S,Se)2 Solar Cell Fabricated Using Aqueous Spray Pyrolysis.

We fabricated Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells using aqueous spray based deposition, which is inexpensive and covers a large area. To apply the sprayed film to a photoabsorber of a solar cell, post-sulfo-selenization was carried out. Through the sulfo-selenization process, we were able to fabricate various S-alloyed CIGSSe films from S/(S + Se) = 0 (S-0.0) to S/(S + Se) = 0.4 (S-0.4). CIGSSe solar cells were made with the S-alloyed CIGSSe absorbers. Power conversion efficiency of CIGSSe solar cell was found to be increased with S-alloying up to S-0.3, and the best efficiency of 10.89% was obtained with the S-0.3 CIGSSe absorber. Comparison study of S-alloyed CIGSSe solar cells showed that enhanced efficiency in S-0.3 solar cell is due to the increased open-circuit voltage and an improved fill factor, which is induced by S-alloying. In addition, admittance spectroscopy revealed that the defect density of the deep level was developed in the S-alloyed S-0.3 CIGSSe absorber. However, the defect density was observed to be rather reduced. Details of characterization and analysis results are discussed in this paper.

A simple non-toxic simultaneous selenization/sulfurization process for Cu(In, Ga)(S, Se)2 solar cells

This paper proposes a non-toxic simultaneous selenization/sulfurization process to produce Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells without the need for the conventional two-stage process using toxic H2Se/H2S gases. The absorber layer is applied via sputtering with Cu-In-Ga ternary targets, whereupon non-toxic selenium and sulfur vapor are introduced simultaneously to produce chalcopyrite CIGSSe film. Experiment results demonstrate that the total sulfur content of the absorber layer increases with an increase in the S/(S + Se) ratio. When the S/(S + Se) ratio ≧ 0.12, the sulfur content of the surface absorber layer is higher than inside the sample. This contributes to an increase in the surface energy gap and Voc of the solar cell. However, an excessively high S/(S + Se) ratio can undermine the crystallinity of the CIGSSe while introducing phase segregation at the surface of the absorber layer as well as ordered vacancy compounds (OVCs) and secondary phases, which can decrease cell efficiency. A suitable proportion of sulfur in the absorber layer can inhibit excessive growth of the Mo(S,Se)2 layer and thereby enhance cell efficiency. A small number of CuGaSe2 grains were observed at the bottom of the CIGSSe absorber layer in specimens with various S/(S + Se) ratios. A maximum cell efficiency of 12.8% was achieved when the S/(S + Se) ratio was 0.12. This cell efficiency is close to the reference specimen fabricated using a two-stage selenization/sulfurization process based on toxic H2Se/H2S gases.

Growth and characterization of high quality CIGS films using novel precursors stacked and surface sulfurization process

This study produces high quality Cu(In,Ga)Se2 (CIGS) solar cells using a two-step process. Stacked In (200 nm)/CuGa (150 ~ 300 nm)/In (500 nm) layers are deposited onto Mo bilayer soda-lime glass by sputtering, using CuGa and In targets, followed by vapor stacking of the elemental Se layers. To produce the CIGS film, the CuGa film is coated in thicknesses of 150, 200, 250 and 300 nm. An appropriate atomic ratio [Cu/(In + Ga), CGI and Ga/(In + Ga), GGI] for the precursors and the CIGS absorption layers composition is easily obtained. Compared to those for the as-deposited precursors, after selenization, the CGI and GGI ratios for CIGS films are almost constant. All CIGS thin films exhibit a peak in the Raman curves at around 173–174 cm−1, which is identified as the CIGS phase. Following sulfurization, the main peaks for CIGS thin films at (112), (220)/(204) and (312)/(116) indicated that the crystalline quaility were improved. The main peaks for the CIGSS films are slightly greater for (112), (220)/(204) and (312)/(116). The band gap energy is increased from 1.19 eV (for the as-grown) to 1.34 eV (for the absorber layer that is sulfurized at 500 °C for 10 min). The performance of the CIGS absorber films is improved by using a proper holding time for sulfurization.

A highly efficient Cu(In,Ga)(S,Se)2 photocathode without a hetero-materials overlayer for solar-hydrogen production

Surface modification of a Cu(In,Ga)(S,Se)2 (CIGSSe) absorber layer is commonly required to obtain high performance CIGSSe photocathodes. However, surface modifications can cause disadvantages such as optical loss, low stability, the use of toxic substances and an increase in complexity. In this work, we demonstrate that a double-graded bandgap structure (top-high, middle-low and bottom-high bandgaps) can achieve high performance in bare CIGSSe photocathodes without any surface modifications via a hetero-materials overlayer that have been fabricated in a cost-effective solution process. We used two kinds of CIGSSe film produced by different precursor solutions consisting of different solvents and binder materials, and both revealed a double-graded bandgap structure composed of an S-rich top layer, Ga- and S-poor middle layer and S- and Ga-rich bottom layer. The bare CIGSSe photocathode without surface modification exhibited a high photoelectrochemical activity of ~6 mA·cm−2 at 0 V vs. RHE and ~22 mA·cm−2 at −0.27 V vs. RHE, depending on the solution properties used in the CIGSSe film preparation. The incorporation of a Pt catalyst was found to further increase their PEC activity to ~26 mA·cm−2 at −0.16 V vs. RHE.

Improvement of Ga distribution in Cu(In, Ga)(S, Se)2 film by pretreated Mo back contact

In the two-step selenization process, Ga accumulation near back contact is a common problem, resulting in a lower band gap near front surface and then lower open circuit voltage (VOC) of solar cell. In this work, Ga accumulation in Cu(In, Ga)(S, Se)2 (CIGSSe) film was weakened by pre-sulfurized and selenized (PSS) Mo back contacts. The sputtered dense Mo back contacts were sulfurized and selenized at different temperatures before Cu-In-Ga precursor deposition. Raman results show that MoS2, MoSe2 and pure Se phases were formed on the PSS Mo surface. These selenide and sulfide phases will become the bottom Se and S sources in the structure of PSS Mo/metal precursor during selenization process. The thermodynamic calculation confirms the bottom S and Se sources are effective. SIMS measurement shows the Ga back grading in CIGSSe films decreases when the PSS Mo back contacts are used. For higher substrate temperature (Tsub) we find reduced steepness of the Ga back gradient. The S atomic percentage in CIGSSe is so small that the band gaps of CIGSSe are mainly determined by the Ga/(Ga + In) distribution. Therefore, the band gap in the space charge region (SCR) of solar cell increased when PSS Mo was used. As expected, VOC increased about 10 mV when TSub increased to 600 °C. Due to the enhancement of VOC and fill factor (FF), the conversion efficiencies (η) of solar cells was improved.

Rapid supersonic spraying of Cu(In,Ga)(S,Se)2 nanoparticles to fabricate a solar cell with 5.49% conversion efficiency

We demonstrate production of high-quality Cu(InGa)(SSe)2 (CIGSSe) films by high-rate supersonic spray deposition. This technique is unique in creating particle-based films without introducing impurities, because no additives or binders are used. The thin film deposition process was investigated computationally, to understand the pulverization of the incoming particles. These simulations were consistent with experimental observations. Grain growth was improved by adding a 300-nm copper layer atop the CIGSSe film; selenization of the resulting bilayer produced a CuSe liquid flux that assisted the sintering process. The final CIGSSe film-based solar cell had a conversion efficiency of 5.49% with Jsc = 18.73 mA/cm2, Voc = 0.488 V, and FF = 59.99% in an active area of 0.44 cm2.

A Comparative Study of Nanoparticle‐Ink‐Based CIGSSe Thin Film Solar Cells on Different Back Contact Substrates

The effect of the back contact electrode in nanoparticle‐ink‐based CIGSSe solar cells was investigated using different glass substrates with either a fluorine‐doped SnO2 (FTO) conducting layer or a molybdenum (Mo) conducting layer. The morphologies, crystal structures, and compositional distributions of the Cu(In x Ga1– x )(S y Se1– y )2(CIGSSe) films are very similar in spite of the different back contact materials. The observed performances of the solar cell device, however, are somewhat different. The device that was fabricated on the FTO back contact substrate revealed a lower open‐circuit voltage ( V oc) and fill factor (FF) than the Mo back contact substrate, resulting in lower solar cell efficiencies (6.5% for FTO and 7.4% for Mo). The differences between the behaviors are attributed to the interfacial properties between the CIGSSe film and the back contact electrode.

Chalcogenization-Derived Band Gap Grading in Solution-Processed CuIn(x)Ga(1-x)(Se,S)₂ Thin-Film Solar Cells.

Significant enhancement of solution-processed CuIn(x)Ga(1-x)(Se,S)2 (CIGSSe) thin-film solar cell performance was achieved by inducing a band gap gradient in the film thickness, which was triggered by the chalcogenization process. Specifically, after the preparation of an amorphous mixed oxide film of Cu, In, and Ga by a simple paste coating method chalcogenization under Se vapor, along with the flow of dilute H2S gas, resulted in the formation of CIGSSe films with graded composition distribution: S-rich top, In- and Se-rich middle, and Ga- and S-rich bottom. This uneven compositional distribution was confirmed to lead to a band gap gradient in the film, which may also be responsible for enhancement in the open circuit voltage and reduction in photocurrent loss, thus increasing the overall efficiency. The highest power conversion efficiency of 11.7% was achieved with J(sc) of 28.3 mA/cm(2), V(oc) of 601 mV, and FF of 68.6%.

Ecofriendly and Nonvacuum Electrostatic Spray-Assisted Vapor Deposition of Cu(In,Ga)(S,Se)2 Thin Film Solar Cells

Chalcopyrite Cu(In,Ga)(S,Se)2 (CIGSSe) thin films have been deposited by a novel, nonvacuum, and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) method. The generation of a fine aerosol of precursor solution, and their controlled deposition onto a molybdenum substrate, results in adherent, dense, and uniform Cu(In,Ga)S2 (CIGS) films. This is an essential tool to keep the interfacial area of thin film solar cells to a minimum value for efficient charge separation as it helps to achieve the desired surface smoothness uniformity for subsequent cadmium sulfide and window layer deposition. This nonvacuum aerosol based approach for making the CIGSSe film uses environmentally benign precursor solution, and it is cheaper for producing solar cells than that of the vacuum-based thin film solar technology. An optimized CIGSSe thin film solar cell with a device configuration of molybdenum-coated soda-lime glass substrate/CIGSSe/CdS/i-ZnO/AZO shows the photovoltaic (j-V) characteristics of Voc=0.518 V, jsc=28.79 mA cm(-2), fill factor=64.02%, and a promising power conversion efficiency of η=9.55% under simulated AM 1.5 100 mW cm(-2) illuminations, without the use of an antireflection layer. This demonstrates the potential of ESAVD deposition as a promising alternative approach for making thin film CIGSSe solar cells at a lower cost.

Na Incorporation in Cu(In,Ga)(Se,S)<sub>2</sub> Films Grown on Insulator-Coated Stainless Steel Foil Using a Metal Precursor Reaction

Cu(In,Ga)(Se,S)2 (CIGSS) films are formed on flexible 430 stainless steel foils through reacting Cu–In–Ga metal precursors in H2Se and H2S. The foils are coated using an electrically insulating silicone-based resin that can sustain temperatures as high as 600 °C. NaF films with various thicknesses are deposited between the metal precursor and Mo back contact in order to incorporate Na into the CIGSS films. The metal precursor reaction with the extrinsic Na incorporation results in greater Ga homogenization and S incorporation. The higher Na incorporation enhances $V_{{\rm OC}}$ and lowers $J_{{\rm SC}}$ . The temperature-dependent $V_{{\rm OC}}$ and drive-level capacitance profiling measurements reveal that the Na incorporation results in higher activation energy of the recombination and increased carrier density.

Cu(In,Ga)(S,Se)₂ thin film solar cell with 10.7% conversion efficiency obtained by selenization of the Na-doped spray-pyrolyzed sulfide precursor film.

Selenium-rich Cu(In,Ga)(S,Se)2 (CIGSSe) thin films on an Mo-coated soda-lime glass substrate were fabricated by spray pyrolysis of an aqueous precursor solution containing Cu(NO3)2, In(NO3)3, Ga(NO3)3, and thiourea followed by selenization at 560 °C for 10 min. We studied the effects of intentional sodium addition on the structural and morphological properties of the fabricated CIGSSe films by dissolving NaNO3 in the aqueous precursor solution. The addition of sodium was found to affect the morphology of the final CIGSSe film: the film had denser morphology than that of the CIGSSe film obtained without addition of NaNO3. Photoelectrochemical measurements also revealed that the acceptor density of the nondoped CIGSSe film was relatively high (N(a) = 7.2 × 10(17) cm(-3)) and the addition of sodium led to a more favorable value for solar cell application (N(a) = 1.8 × 10(17) cm(-3)). As a result, a solar cell based on the sodium-modified CIGSSe film exhibited maximum conversion efficiency of 8.8%, which was significantly higher than that of the cell based on nondoped CIGSSe (4.4%). In addition, by applying MgF2 antireflection coating to the device, the maximum efficiency was further improved to 10.7%.

One-dimensional simulation of sequentially processed [formula omitted] heterojunction solar cells with vertically graded absorber composition

The successful definition and verification of a one-dimensional device simulation baseline for sequentially processed Mo/Cu(In1−xGax)(Se1−ySy)2/CdS/i-ZnO/ZnO:Al thin film solar cells is presented. The appropriate modeling of the layer sequence requires experimental assessment of the relevant material properties of each film within the layer sequence of the device. The properties of the CdS/i-ZnO/ZnO:Al window layer stack and of the molybdenum back electrode have been approximated by their bulk properties obtained from experiment and from the literature, respectively. This approach is, however, not meaningful for the sequentially processed Cu(In1−xGax)(Se1−ySy)2 (CIGSSe) thin films due to characteristic segregation/grading of the chemical composition and the presence of structural inhomogeneities. This work shows that a suitable modeling of the CIGSSe film can be achieved by definition of effective medium properties. With this approach a 1D device simulation baseline for the studied system was established, which accurately reproduces the experimental characteristics of the studied CdS/CIGSSe heterojunction solar cells.

Structural and electrical properties of radio frequency magnetron sputtered Cu(InxGa1 − x)Se2 thin films with additional post-heat treatment

Cu(In1−xGax)(Se,S)2 (CIGSS) thin films were deposited using a single quaternary target of Cu(In0.75Ga0.25)Se2(CIGS) by rf magnetron sputtering, followed by sulfurization. The effects of substrate temperature and post-sulfurization on the properties of CIGSS films were investigated. As the substrate temperature increased, the crystallinity of the films increased significantly and the grain size also increased. Energy dispersive X-ray spectroscopy of CIGS films showed that Cu, Ga and Se contents approached the stoichiometry of CIGS films with increasing substrate temperature.Post-sulfurization of as-deposited CIGS films was carried out to improve their properties. The resultant CIGSS thin films revealed a noticeable increase in (112) peak for films deposited under 200°C, while the intensities of the (220)/(204) and (312)/(116) peaks increased as well. Grains with a size of approximately 100nm were grown after sulfurization. The results of EDX of CIGSS thin films after sulfurization indicated that the Cu content increased slightly and the Ga and (Se+S) contents decreased significantly. The band gaps of the films were also noticeably reduced after sulfurization. The carrier concentrations of the films after sulfurization decreased greatly at substrate temperatures below 400°C, while they showed little change at temperatures above 400°C. The resistivity of the films after sulfurization increased with increasing substrate temperature and showed little change at temperatures above 400°C. As a result, CIGS films were transformed to the chalcopyrite phase with well crystallized grains through sulfurization.

Three-step H2Se/Ar/H2S reaction of Cu-In-Ga precursors for controlled composition and adhesion of Cu(In,Ga)(Se,S)2 thin films

Control of the through-film composition and adhesion are critical issues for Cu(In,Ga)Se2 (CIGS) and/or Cu(In,Ga)(Se,S)2 (CIGSS) films formed by the reaction of Cu–In–Ga metal precursor films in H2Se or H2S. In this work, CIGSS films with homogenous Ga distribution and good adhesion were formed using a three-step reaction involving: (1) selenization in H2Se at 400 °C for 60 min, (2) temperature ramp-up to 550 °C and annealing in Ar for 20 min, and (3) sulfization in H2S at 550 °C for 10 min. The 1st selenization step led to fine grain microstructure with Ga accumulation near the Mo back contact, primarily in a Cu9(In1−xGax)4 phase. The 2nd Ar anneal step produces significant grain growth with homogenous through-film Ga distribution and the formation of an InSe binary compound near the Mo back contact. The 3rd sulfization step did not result in any additional change in Ga distribution or film microstructure but a small S incorporation near the CIGSS film surface and complete reaction of InSe to form CIGSS were observed. The three-step process facilitates good control of the film properties by separating different effects of the reaction process and a film growth model is proposed. Finally, CIGSS solar cells with the three-step reaction were fabricated and devices with efficiency = 14.2% and VOC = 599 mV were obtained.