CuIn1-xGaxSe2 Research Articles

Influence of Na and Ga on the electrical properties of perfect 60° dislocations in Cu(In, Ga)Se2 thin-film photovoltaic absorbers

The segregation of GaIn and NaCu to perfect 60° dislocations in CuIn1–xGaxSe2 is investigated by means of density functional theory calculations. We find that the segregation process is mainly driven by the elastic interaction of both defect types with the strain field of the dislocation. GaIn moves into the negatively strained region, while NaCu is found in the positively strained region. We show that both defects affect the electronic defect levels induced by the dislocation core and GaIn is able to passivate the β-core in CuInSe2. This result indicates that β-cores are inactive in CuIn1–xGaxSe2. NaCu; however, they do not have a significant effect on the electrical properties of the studied dislocation cores. Therefore, the experimentally observed sodium segregation to dislocation cores in CuIn1–xGaxSe2 cannot be considered as the passivation mechanism of the electrically active cores in that material.

Selenization of CIS and CIGS layers deposited by chemical spray pyrolysis

Cu(In1 − xGax)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.

Light trapping in ultrathin CuIn1-xGaxSe2 solar cells by dielectric nanoparticles

In this work, light trapping effects based on dielectric nanoparticles (NPs) are numerically evaluated in ultrathin CuIn1-xGaxSe2 (CIGSe) solar cells for different locations of NPs, cell architectures and illumination directions, with relevant implications and optimized NP parameters being specified. The severe absorption in Mo is the main constraining factor for the effective implementation of light trapping NPs in ultrathin CIGSe cells. For a significant light absorption enhancement, it is favoured to integrate dielectric NPs at the rear interface of CIGSe/back contact and employ transparent conductive oxide (TCO) back contacts rather than the conventional Mo. It is demonstrated, that under front illumination, the low-index (n = 1.5) hemispherical NPs at the CIGSe/TCO interface cause significant light trapping effects. The NP-patterned ultrathin cells achieve a maximum short circuit current density (Jsc) of 36.4 mA/cm2 at an upper limit of 500 nm CIGSe thickness, which is as high as 94% Jsc of their thick flat counterparts with a CIGSe thickness of 2000 nm. In contrast, under back illumination, the patterned ultrathin cells realize comparable absorption to the corresponding thick counterpart at a CIGSe thickness of only 300 nm and the maximum Jsc (35.2 mA/cm2) saturates at a CIGSe thickness of 425 nm. Further, Jsc is less attenuated by the parasitic absorption in Al:ZnO (AZO) under back illumination than under front illumination in module configuration, where a much thicker AZO is required. This suggests that patterned ultrathin CIGSe solar cells under back illumination will be a promising cell architecture simultaneously for high efficiencies and less material usage in industrial module production.

Green and low-cost synthesis of CIGSe nanoparticles using ethanol as a solvent by a sonochemical method - A new approach

A sonochemical method was utilized to synthesize Cu(InxGa1-x)Se2 (CIGSe) nanoparticles using NaBH4 with ethanol as a solvent. The effect of the ultrasonification time intervals on the synthesis was investigated in the range of 2–5 h, 5 h was found to be the most suitable time to obtain the expected CuIn0.7Ga0.3Se2 compounds. The as-synthesized CIGSe nanoparticles possessed a tetragonal structure with quasi-spherical shape, band gap of ∼1.25 eV, and targeted Cu(InxGa1-x)Se2 composition. A new reaction scheme is proposed to explain the role of NaBH4 in the plausible reaction paths. The non-toxic solvent used in the synthesis with no additional heating makes the developed method cheaper and “greener” than the previously reported methods.

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(In1-x, Gax)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.

Development of highly efficient CuIn0.5Ga0.5Se2-based photocathode and application to overall solar driven water splitting

Optimization of CBM offset boosted hydrogen evolution from water on CuIn1−xGaxSe2(CIGS) photocathode surface modified with Pt and CdS under simulated sunlight.

Semi-transparent photovoltaic glazing based on electrodeposited CIGS solar cells on patterned molybdenum/glass substrates

In this paper, a new way of preparing semi-transparent solar cells using Cu(In1−xGax)Se2 (CIGS) chalcopyrite semiconductors as absorbers for BIPV applications is presented. The key to the elaboration process consists in the co-electrodeposition of Cu-In-Ga mixed oxides on submillimetric hole-patterned molybdenum substrate, followed by thermal reduction to metallic alloys and selenisation. This method has the advantage of being a selective deposition technique where the thin film growth is carried out only on Mo covered areas. Thus, after annealing, the transparency of the sample is always preserved, allowing light to pass through the device. A complete device (5 × 5 cm2) with 535 μm diameter holes and total glass aperture of around 35% shows an open circuit voltage (VOC) of 400 mV. Locally, the I-V curves reveal a maximum efficiency of 7.7%, VOC of 460 mV, JSC of 24 mA.cm−2 in an area of 0.1 cm2 with 35% aperture. This efficiency on the semi-transparent area is equivalent to a record efficiency of 11.9% by taking into account only the effective area.

Identification and analysis of partial shading breakdown sites in CuInxGa(1-x)Se2 modules

In this study, CuInxGa(1-x)Se2 (CIGS) mini-modules are stressed under reverse bias, resembling partial shading conditions, to predict and characterize where failures occur. Partial shading can cause permanent damage in the form of ‘wormlike’ defects on thin-film modules due to thermal runaway. This results in module-scale power losses. We have used dark lock-in thermography (DLIT) to spatially observe localized heating when reverse-bias breakdown occurs on various CIGS mini-modules. For better understanding of how and where these defects originated and propagated, we have developed techniques where the current is limited during reverse-bias stressing. This allows for DLIT-based detection and detailed studying of the region where breakdown is initiated before thermal runaway leads to permanent damage. Statistics of breakdown sites using current-limited conditions has allowed for reasonable identification of the as-grown defects where permanent breakdown will likely originate. Scanning electron microscope results and wormlike defect analysis show that breakdown originates in defects such as small pits, craters, or cracks in the CIGS layer, and the wormlike defects propagate near the top CIGS interface.

Experimental scheme for a stable molybdenum bilayer back contacts for photovoltaic applications

Molybdenum (Mo) back contacts have been widely used for thin film based solar cells, especially for CuIn1-xGaxSe2 (CIGS) and Cu2ZnSnS4 (CZTS) based devices. Traditionally, a bilayer approach at two different sputtering pressures is used to produce highly conducting and adhesive films on glass substrates. Mo bilayers undergo a harsh heat treatment during the absorber layer (CIGS/CZTS) deposition or formation process, which in turn changes the residual stress and the morphological properties of the back contact layers and can cause poor adhesion of the absorber layers. This is a key layer for the photovoltaic industry because the entire device is deposited on top of this layer. Adhesion issues can degrade device performance and can affect the long-term stability of the solar cell. This work reports in-situ study of Mo stress variation during anneals ranging from 20 °C to 500 °C and compares the film’s morphological and electrical properties before and after heating to develop a thermally stable Mo bilayer. For bilayer films with a total thickness of 1 μm, this study revealed that the optimum pressure and thickness are 5 mTorr and 150 nm, and 1 mTorr and 850 nm for the bottom and top layers, respectively.

Core Levels, Band Alignments, and Valence-Band States in CuSbS2 for Solar Cell Applications

The earth-abundant material CuSbS2 (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuInxGa(1-x)Se2 (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.

Deposition and characterization of Cu(In,Ga)Se2 thin films from the ink of sonochemically prepared CIGSe nanoparticles

CuIn1-xGaxSe2 (CIGSe) nanoparticles were synthesized via a sonochemical method at different Ga content (x = 0.15, 0.30 and 0.45) and dispersed in a low toxic isobutanol to form the nanoparticle-ink. Thin films of CIGSe were grown by a facile, non-vacuum and inexpensive “nanoparticle-ink based air-spray coating” method. Effects of Ga-content and annealing time on the physical properties of CIGSe thin films were investigated. The elemental composition, crystalline structure, surface morphology and optical properties of the films were explored by various characterization methods. XRD studies of as-synthesized films showed a tetragonal structure with (112) orientation. After annealing, the CIGSe films showed an improvement in the intensity ratio of I(220)/I(112) for the annealing time of 60 min. The morphological studies of annealed CIGSe films showed plank-like larger grains of size ∼1–2 µm. The films deposited at different gallium content, x = 0.15, 0.30 and 0.45 showed PL peak maxima at 954, 1049 and 1168 nm, respectively. The present method is capable of producing CIGSe absorbers by a greener route in large scales at lower cost.

Modeling and optimization of a superstrate solar cell based on Cu2ZnSn(SxSe1-x)4/ZnS structure

The Kestrite semiconductor material Cu2ZnSnSe4 (CZTSe) is believed to be a suitable candidate for replacing the CuIn1-xGaxSe2 (CIGS) absorber for the abundance and the non-toxicity of its components. However, the record efficiency of solar cells based on this material reaches 11% which is lower than the conversion efficiency of the CIGS based solar cell for which the efficiency has reached 25%. The aim of this study is to model and optimize the electrical performances of a superstrate type solar cell based on the kestrite material Cu2ZnSn(SxSe1-x)4 (CZTSSe). The goal is to investigate the effect of mixing the sulfide (S) component with selenide (Se) on the conversion efficiency η, band gap Eg open circuit voltage Voc, short circuit current density Jsc, fill factor FF and maximum power density P of the device, through the evaluation of their behavior as a function of the ratio S/(S+Se), which represents the concentration of sulfur in the absorber material CZTSSe. It is also shown in this work, through the calculation of the mismatch strain ε at the interface between the absorber and the buffer layers, that the zinc sulfide (ZnS) is a more appropriate buffer than cadmium sulfide (CdS) for the CZTSSe absorber. The effect of strain at the interface buffer/absorber on the bandgap energy of CZTSSe and then on the cell performances is evaluated. This evaluation is based on the strain theory in order to obtain more realistic results close to experimental results. It is noted that adding 72% of Sulfur in the absorber material, meaning that x=0.72, increases the efficiency to 13.1% therefore an improvement of 21.3% is obtained compared to the efficiency of the CZTSe solar cell with a strain equal to 0 meaning no deformation, Jsc= 15.35mA/cm², Voc= 0.800 V, FF = 74.1% and Pmax=9.45mW/cm².

Synthesizing three-dimensional ordered macroporous CuInxGa1−xSe2 thin films by template-assisted electrodeposition from modified ionic liquid

Three-dimensional ordered macroporous (3DOM) CuInxGa1−xSe2 (CIGS) film is synthesized by one-step galvanostatic electrodeposition based on polystyrene templates from a special mixture of ionic liquid 1-butyl-3-methylimidazolium tetra-fluoroborate and ethanol. X-ray diffraction pattern, Raman spectroscopy and transmission electron microscopy present the CIGS film has ideal chalcopyrite crystallization. 3DOM CIGS film with high Ga content has photochemical activity and good photo-stability, its band gap is 1.34eV. This is a simple, feasible and additive free method without selenization process to synthesize 3DOM CIGS film, the film is qualified and available for absorber-layer fabrication or other fields.

A resistance ratio change phenomenon observed in Al doped ZnO (AZO)/Cu(In 1-x Ga x )Se 2 /Mo resistive switching memory device

In this work, the Cu(In1-xGax)Se2 (CIGS), Al doped ZnO (AZO) and Mo has been used for constructing a resistive switching device with AZO/CIGS/Mo sandwich structure grown on a transparent glass substrate. The device represents a high-performance memory characteristics under ambient temperature. In particularly, a resistance ratio change phenomenon have been observed in our device for the first time.

Electrochemical synthesis of CuIn(1−x)GaxSe2 nanowires with controlled stoichiometry

We present a solution-based approach for fabrication of CuIn(1−x)GaxSe2 (CIGS) nanowires by electrodeposition method with stoichiometry close to the ideal one for photovoltaic applications. Fabrication of CIGS nanowires is based on a one-step electrodeposition to deposit CIGS nanowires into the nanometer-scale cylindrical pores of the anodized aluminum oxide (AAO) template with molybdenum (Mo) thin-film deposited on the backside of the AAO as the counter electrode. A key to the commercialization of CIGS nanowires as active absorber layer for photovoltaic applications is to improve the composition control and quality of the CIGS nanowires. In this study, CIGS nanowires with close to the ideal stoichiometry and chalcopyrite nanocrystalline structure for photovoltaic applications were obtained by varying the deposition parameters such as deposition potential and electrolyte’s composition during electrodeposition and the annealing process after electrodeposition.

Self-anti-reflective density-modulated thin films by HIPS technique

A critical factor for an efficient light harvesting device is reduced reflectance in order to achieve high optical absorptance. In this regard, refractive index engineering becomes important to minimize reflectance. In this study, a new fabrication approach to obtain density-modulated CuInxGa(1−x)Se2 (CIGS) thin films with self-anti-reflective properties has been demonstrated. Density-modulated CIGS samples were fabricated by utilizing high pressure sputtering (HIPS) at Ar gas pressure of 2.75 × 10−2 mbar along with conventional low pressure sputtering (LPS) at Ar gas pressure of 3.0 × 10−3 mbar. LPS produces conventional high density thin films while HIPS produces low density thin films with approximate porosities of ∼15% due to a shadowing effect originating from the wide-spread angular atomic of HIPS. Higher pressure conditions lower the film density, which also leads to lower refractive index values. Density-modulated films that incorporate a HIPS layer at the side from which light enters demonstrate lower reflectance thus higher absorptance compared to conventional LPS films, although there is not any significant morphological difference between them. This result can be attributed to the self-anti-reflective property of the density-modulated samples, which was confirmed by the reduced refractive index calculated for HIPS layer via an envelope method. Therefore, HIPS, a simple and scalable approach, can provide enhanced optical absorptance in thin film materials and eliminate the need for conventional light trapping methods such as anti-reflective coatings of different materials or surface texturing.

An improved method for determining carrier densities via drive level capacitance profiling

We demonstrate that an analytic relationship between coefficients in the Taylor expansion of the junction capacitance can be exploited to yield more precise determinations of carrier densities in drive level capacitance profiling (DLCP). Improvements are demonstrated on data generated according to the DLCP theory and in measurements performed on a CuInxGa1–xSe2 device. We argue that the improved DLCP method is especially important for non-uniform devices, which are more susceptible to noise in the capacitance data used in DLCP because they require that the amplitude of the drive level be restricted. Importantly, the analysis does not require the collection of any data other than what is typically collected during a DLCP measurement while employing fewer independent parameters than the model that is typically used in DLCP. Thus, we expect that it will be readily adoptable by those who perform DLCP measurements.

PV Absorber Layered Materials Formation Using Potential Pulse Atomic Layer Deposition (PP-ALD),

Thin film photovoltaic solar cells are a promising avenue for solar technology. However, they currently are not as cost effective as fossil fuels. Several absorber layer materials have been formed using various methods in the past. CuIn(1-x)GaxSe2(CIGS) is a promising material due to its tunable direct bandgap, allowing thin films to be used in photovoltaic devices. Previous studies of CIGS done by the author’s group investigated thin film deposition using electrochemical atomic layer deposition (E-ALD). Potential Pulse Atomic Layer Deposition (PP-ALD) is an electrochemical method of atomic layer deposition that relies on alternating cathodic and anodic potential pulses. Cathodic potential pulses long enough to form fractions of a monolayer are used. This is followed by an anodic potential which makes use of the underpotential deposition (UPD) phenomenon to strip away excess bulk, allowing stoichiometric deposits. Because short pulses are used, there is not enough time for excess elemental layers to be formed. PP-ALD is also a one solution co-electrodeposition technique. This differs from the previous E-ALD, which required multiple solution switching, allowing for PP-ALD to have shorter deposition cycles, saving solution and time. Initial results for the formation of In2Se3 and other components of the CIGS compound will be discussed.

Synthesis and Crystallization of CuIn1–xGaxSe2 Compounds Formed via Co-Sputtering with Se Vapor

Synthesis and Crystallization of CuIn₁_<i>–x</i>Ga_<i>x</i>Se₂ Compounds Formed via Co-Sputtering with Se Vapor

Influence of process parameters on the gallium composition of a CuIn1−xGaxSe2 solar cell on the efficiency of non-vacuum blade coating stacking

The distribution of an energy gap gradient can influence photovoltaic conversion, which in turn influences the performance of solar cells. On the other hand, controlling the energy gap gradient can influence the range of the absorption spectrum, and thus the overall solar photovoltaic conversion efficiency is affected. To control the energy gap of a CuIn1−xGaxSe2 (CIGS) solar cell, this study employs a non-vacuum stack coating technique. The chemical composition of the ink is regulated and a multilayer stack is coated to investigate (1) the distribution range of the energy gap and (2) the variability of conversion efficiency in multilayer stack and single-layer composition conditions, in order to determine the feasibility of using a coating stack design to change and control an energy gap. The effect of film composition on crystallinity is tested through high-temperature selenylation. Finally, a miniature solar module is prepared and the conversion efficiency is found to improve from 5.02% to 7.08%, and the feasibility of energy gap control is demonstrated.