CIGS Solar Cells Research Articles

Implementation of graphene as hole transport electrode in flexible CIGS solar cells fabricated on Cu foil

Graphene has great potential to be used as electrode in many opto-electronic devices, owing to its superior optical and electrical properties. Graphene films have been employed as window electrode in various thin film solar cells. However, to date graphene film has not been used as hole transport electrode, particularly in CIGS solar cell. In this work, we have demonstrated a novel structure for graphene-based flexible CIGS solar cell, in which graphene film on flexible Cu foil was implemented as hole transport electrode. CIGS solar cells were directly fabricated on the chemical vapor deposited graphene film on Cu foil, without any transfer process. Several techniques, including Raman spectroscopy, X-ray diffraction, scanning electron microscopy, external quantum efficiency and J-V characteristics under illumination, have been used to investigate the device performance. The graphene-based device displayed power conversion efficiency of 9.91 ± 0.89% with a fill factor of 64.75 ± 7.34%, which are substantially higher compared to reference cell fabricated using conventional Mo/stainless steel electrode. High open circuit voltage together with substantially large fill factor is primarily responsible for high cell efficiency of graphene/Cu foil based device. This study provides a plausible implication of graphene as hole transport electrode in flexible CIGS photovoltaic devices.

Evaluation of recombination losses in thin film solar cells using an LED sun simulator − the effect of RbF post-deposition on CIGS solar cells

Distinguishing among different electrical loss mechanisms − such as interface and bulk recombination − is a common problem in thin film solar cells. In this work, we report a J–V measurement technique using different illuminating spectra to distinguish between these two recombination losses. The basic idea is to change the relative contribution of bulk recombination to the total losses of photo-generated charge carriers by generating them in different depths within the absorber layer using different spectral regions of the illuminating light. The use of modern LED sun-simulators allows an almost free design of illumination spectra at intensities close to 1 sun. The comparison of two simple J–V measurements, one recorded with illumination near the absorber's band-gap energy and one with light of higher energy, in combination with supporting measurements of the absorber properties, as well as device modeling, enables the extraction of the diffusion length and the interface recombination velocity. Using this technique, we show that in CIGS solar cells, an RbF post-deposition treatment does not only reduce interface recombination losses, as often reported, but also reduces bulk recombination in the CIGS absorber. Furthermore, we find that both cells, with and without RbF treatment, are dominantly affected by interface recombination losses.

Improvement performance CIGS thin film solar cells by changing the thickness Cd_S layer

In this study the function of solar cells with the structure of CuIn1-xGaxSe2 is examined. CIGS solar cell consists of layers of ZnO (Layer TCO), Cd_S (buffer layer), CIGS (absorbent layer), and Layer MO (substrate), which Cd_S and CIGS layers form a PN Junction. Later using SILVACO software CIGS solar cell is simulated. Then, CIGS solar cell is simulated using SILVACO software, the thickness of the Cd_S layer is changed. Important parameters of a solar cell that will be discussed here, include open circuit voltage (VOC), short circuit current (ISC), maximum power (Pmax), filling factor (FF) and efficiency (η). After conducted simulations, it was that changing Cd_S layer thickness has impact on solar cells function.

Non-Isothermal Modeling of Dark Current-Voltage Measurements of a CIGS Solar Cell

Dark current density-voltage measurements were performed in a temperature range 110–290 K in both forward and reverse bias on a 0.5 cm2 solar cell sample to determine current and voltage losses in a CIGS solar cell. For the first time a JD − V model applicable in a wide temperature range was constructed by adding non-ideality terms to the Shockley diode equation and comparing the experimentally acquired data to the model simulation results. The shunt current was modeled assuming space charge limited current by a uniform distribution of trap levels. A total of only 9 adjustable parameters in the final JD − V model sufficed to simulate all experimental data simultaneously, i.e., no re-estimation had to be performed at different temperature levels. The estimates for the diode ideality factor n = 1.8 and the diode activation energy ΔE = 1.20 eV coincide with values typically obtained for CIGS solar cell dominated by recombination in the absorber. The quality of the CIGS solar cell used in this study was also confirmed by the low back contact activation barrier of 52 meV and the high shunt resistance. We demonstrate that this flexible model can be used to make an initial prediction of the resistive losses and non-idealities in thin film solar cells.

Investigation of Absorber Layer Thickness Effect on CIGS Solar Cell in Different Cases of Buffer Layers

Investigation of Absorber Layer Thickness Effect on CIGS Solar Cell in Different Cases of Buffer Layers

Rapid and Accurate Measurement of Ideality Factor and Parasitic Resistances of Thin Film Solar Cells

The sunshade method for the measurement of the ideality factor and resistances of solar cells is commonly used in silicon solar cells. However, the method is rarely employed for thin-film solar cells. In this paper, the sunshade method is applied to CIGS solar cells, and it is shown that the three major assumptions of the method remain valid for CIGS solar cells. The extracted parameters are compared to those calculated using other common methods, and the ideality factors and Rs values are quickly determined and found to be nearly the same as those extracted through illuminated I–V fitting and dark I–V fitting.

How Does CIGS Performance Depend on Temperature at the Microscale?

Unveiling the correlation among electrical performance, elemental distribution, and defects at the microscale is crucial for the understanding and improvement of the overall solar cell performance. While this is true in general for solar cells with polycrystalline absorber layers, it is particularly critical for defect engineering of the complex quaternary CuIn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-∞</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) material system. Studying these relationships under standard ambient conditions can provide important insights but does not provide input on the behavior of the cell under real operating conditions. In this contribution, we take a close look at the complex temperature dependence of defects and voltage in CIGS at the microscale. We have developed correlative X-ray microscopy methods and adapted them for temperature-dependent measurements of the locally generated voltage and elemental compositions at the microscale. We have applied these techniques to industrial CIGS solar cells covering temperatures from room temperature up to 100 °C. We find underperforming areas spanning multiple grains that do not correlate with the elemental distribution of major absorber constituents. However, we demonstrate that low-performing areas perform better at higher temperatures relative to the high-performing areas.

High-Efficiency Nanoparticle Solution-Processed Cu(In,Ga)(S,Se)2Solar Cells

Thin film Cu(In,Ga)(S,Se) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based (generally referred to as CIGS) solar cells represent a promising alternative to conventional crystalline silicon solar cells due to their high efficiencies, reduced cost, and better material utilization. In recent years, it has been demonstrated that it is possible to form thin films by annealing nanoparticulate material such that the nanoparticles coalesce to form large grained thin films. In this paper, we present a 13.8% efficient CIGS solar cell derived from printed nanoparticle inks. The approach was successfully extended to fabricate monolithic devices on larger substrates. These results demonstrate that low-cost, nonvacuum printing of CIGS nanoparticles has great potential to achieve high efficiencies and reduce the performance gap with the more traditional vacuum co-evaporation and sputtering techniques.

Increase in conversion efficiency of above 14% in Cu(In,Ga)3Se5 (β-CIGS) solar cells by Na2S incorporation through the surface of β –CIGS film

Little work has been reported on the performance improvement of the β-CIGS solar cell itself even though the β-CIGS phase can have an ideal band gap for high-conversion efficiency solar cells. We incorporated Na2S to β-CIGS film by supplying Na2S to three different stages: on the (In,Ga)2Se3 layer, on the α-CIGS layer, and on the β -CIGS layer in the three-stage co-evaporation process. The purpose of Na2S incorporation was to control the carrier concentration and passivate grain boundaries in β–CIGS film. With Na2S incorporation on the β–CIGS surface, both the Cu and Se concentrations at the β–CIGS surface were greatly reduced and the Na-depleted subsurface area that existed in the referenced β–CIGS film without Na2S was eliminated. The carrier concentration determined at 100kHz was lowest with Na2S incorporation on the β–CIGS surface, while that determined at 1MHz was similar with various Na2S supply stages. The open-circuit voltage and fill factor greatly increased in the β–CIGS solar cell with the Na2S incorporation. The cell conversion efficiency with Na2S incorporation on the β-CIGS layer improved from 10.3% to 14.2% without AR coating, which is a record efficiency in β-CIGS solar cells at this time.

Two‐step selenization using nozzle free Se shower for Cu(In,Ga)Se2 thin film solar cell

AbstractThe production of commercialized Cu(In,Ga)(S,Se)2 (CIGS) photovoltaic absorber layers uses expensive H2Se gas with a high utility cost. To reduce the manufacturing cost of CIGS photovoltaic modules, a process technology capable of supplying Se vapor uniformly over a large area is required to replace H2Se. In this study, a nozzle‐free Se shower was implemented using a porous material to pass Se vapor while confining liquid Se, and the highly effective selenization of the CuInGa precursor was performed. The nozzle‐free Se‐shower vehicle could be mounted in a commercial rapid thermal process chamber. The chamber pressure and the temperatures of the shower module and substrate, which were controlled independently by the upper and lower heaters, respectively, were varied to control the amount of Se supplied during the entire selenization reaction in real time. In particular, the precursor should be soaked with a sufficient amount of Se at a relatively low substrate temperature of 300°C or less to obtain a good quality absorber. In addition, at a chamber pressure of 100 Torr during the soaking stage, the Ga content in the surface region of the absorber increased considerably with a concomitant improvement in the open‐circuit voltage. The highest performance obtained using this method was an open‐circuit voltage of 0.638 V, short‐circuit current density of 34 mA/cm2, fill factor of 67.2%, and an active area efficiency of 14.57%. This performance is very high compared with other CIGS solar cells manufactured by a 2‐step process using Se vapor.

Quenching Mo optical losses in CIGS solar cells by a point contacted dual-layer dielectric spacer: a 3-D optical study.

A 3-D optical modelling was calibrated to calculate the light absorption and the total reflection of fabricated CIGS solar cells. Absorption losses at molybdenum (Mo) / CIGS interface were explained in terms of plasmonic waves. To quench these losses, we assumed the insertion of a lossless dielectric spacer between Mo and CIGS, whose optical properties were varied. We show that such a spacer with low refractive index and proper thickness can significantly reduce absorption in Mo in the long wavelength regime and improve the device's rear reflectance, thus leading to enhanced light absorption in the CIGS layer. Therefore, we optimized a realistic two-layer MgF2 / Al2O3 dielectric spacer to exploit (i) the passivation properties of ultra-thin Al2O3 on the CIGS side for potential high open-circuit voltage and (ii) the low refractive index of MgF2 on the Mo side to reduce its optical losses. Combining our realistic spacer with optically-optimized point contacts increases the implied photocurrent density of a 750 nm-thick CIGS layer by 10% for the wavelengths between 700 and 1150 nm with respect to the reference cell. The elimination of plasmonic resonances in the new structure leads to a higher electric field magnitude at the bottom of CIGS layer and justifies the improved optical performance.

Impact of heat‐light soaking on potassium fluoride treated CIGS solar cells with CdS buffer layer

AbstractIn this study, the effects of light‐soaking (LS), heat‐soaking (HS), and combined LS and HS, that is, heat‐light soaking (HLS) on potassium fluoride (KF)‐treated and KF‐free copper indium gallium selenide (CIGS) solar cells with CBD‐CdS buffer layer were investigated. LS and HS did not change the basic solar cell parameters of CIGS solar cells when they were performed separately. In contrast, HLS improved cell efficiency with increased open‐circuit‐voltage for KF‐treated CIGS solar cells, whereas it reduced cell performance for KF‐free CIGS cells. Capacitance‐voltage measurements confirmed a significantly increased carrier concentration in KF‐treated CIGS solar cells, as compared to KF‐free cells by HLS. X‐ray photoelectron spectroscopy measurement revealed that the HLS did not change the atomic concentration of Cd, S, and O in CBD‐CdS buffer layer. However, the concentration of Na atoms slightly increased at the CIGS surface region, as confirmed from SIMS measurement. It implies a possible reason for increased carrier concentration in KF‐treated CIGS solar cells after HLS. Temperature‐dependent current‐voltage measurements suggests that HLS modify a K‐containing new layer and affects cell performance.

Influences of donor defect passivation on the performance of Cu(In,Ga)Se2 thin-film solar cell

In this work, the study of donor defect passivation in CIGS solar cell was simulated by wxAMPS software. The effects of near-interface shallow donor defect and bulk donor defect on the performances of solar cell were investigated mainly through changing the state densities and their position distributions. The results show that the high density of the near-interface shallow donor defect state decreases the open circuit voltage. By contrast, the low density of the near-interface shallow donor defect state has the higher open circuit voltage and short-circuit current density. But it easily leads to the CdS conduction band offset increased, then raising carrier recombination rate which decreases the fill factor. For the passivation of the bulk donor defect in the absorption layer, the performances of the CIGS solar cell are much dependent on the position distribution of donor defect state density. Among them, the decrease of the state density in the space charge region can improve the open-circuit voltage. Meanwhile, the reduction of the state density in the quasi-neutral region can increase the short-circuit current density. With the bulk donor defect state density decreased, the conversion efficiency will saturate due to the limitation of the acceptor concentration. When the donor defect state density is low enough, the influence of capture cross section on the device performance is reduced. The conversion efficiencies varying with passivation region were compared from two directions (CdS to Mo and Mo to CdS). It was found the passivation of the donor defect in the transition region where the position from the quasi-neutral region to the space charge region in the absorption was critical to the improvement in the conversion efficiency of the solar cell under a suitable alkali element concentration.

Measurement of Relaxation Time of Excess Carriers in Si and CIGS Solar Cells by Modulated Electroluminescence Technique

Excess carrier lifetime plays a crucial role in determining the efficiency of solar cells. In this paper, we use the frequency dependence of inphase and quadrature components of modulated electroluminescence (MEL) to measure the relaxation time (decay) of excess carriers. The advantage of the MEL technique is that the relaxation time is obtained directly from the angular frequency at which the quadrature component peaks. It does not need knowledge of the material parameters like mobility, etc., and can be used for any finished solar cells which have detectable light emission. The experiment is easy to perform with standard electrical equipment. For silicon solar cells, the relaxation time is dominated by recombination and hence, the relaxation time is indeed the excess carrier lifetime. In contrast, for the CIGS solar cells investigated here, the relaxation time is dominated by trapping and emission from shallow minority carrier traps.

Proposed suitable electron reflector layer materials for thin-film CuIn1−xGaxSe2 solar cells

This paper investigates the electrical properties of electron reflector layer to survey materials as an electron reflector (ER) for chalcopyrite CuInGaSe solar cells. The purpose is optimizing the conduction-band and valence-band offsets at ER layer/CIGS junction that can effectively reduce the electron recombination near the back contact. In this work, an initial device model based on an experimental solar cell is established, then the properties of a solar cell with electron reflector layer are physically analyzed. The electron reflector layer numerically applied to baseline model of thin-film CIGS cell fabricated by ZSW (efficiency = 20.3%). The improvement of efficiency is achievable by electron reflector layer materials with Eg > 1.3 eV and −0.3 < Δχ < 0.7, depends on bandgap. Our simulations examine various electron reflector layer materials and conclude the most suitable electron reflector layer for this real CIGS solar cells. ZnSnP2, CdSiAs2, GaAs, CdTe, Cu2ZnSnS4, InP, CuO, Pb10Ag3Sb11S28, CuIn5S8, SnS, PbCuSbS3, Cu3AsS4 as well as CuIn1-xGaxSe (x > 0.5) are efficient electron reflector layer materials, so the potential improvement in efficiency obtained relative gain of 5%.

Optical simulation of external quantum efficiency spectra of CuIn1−xGaxSe2 solar cells from spectroscopic ellipsometry inputs

Applications of in-situ and ex-situ spectroscopic ellipsometry (SE) are presented for the development of parametric expressions that define the real and imaginary parts (ε1, ε2) of the complex dielectric function spectra of thin film solar cell components. These spectra can then be utilized to analyze the structure of complete thin film solar cells. Optical and structural/compositional models of complete solar cells developed through least squares regression analysis of the SE data acquired for the complete cells enable simulations of external quantum efficiency (EQE) without the need for variable parameters. Such simulations can be compared directly with EQE measurements. From these comparisons, it becomes possible to understand in detail the origins of optical and electronic gains and losses in thin film photovoltaics (PV) technologies and, as a result, the underlying performance limitations. In fact, optical losses that occur when above-bandgap photons are not absorbed in the active layers can be distinguished from electronic losses when electron-hole pairs generated in the active layers are not collected. This overall methodology has been applied to copper indium-gallium diselenide (CuIn1−xGaxSe2; CIGS) solar cells, a key commercialized thin film PV technology. CIGS solar cells with both standard thickness (>2 μm) and thin (<1 μm) absorber layers are studied by applying SE to obtain inputs for EQE simulations and enabling comparisons of simulated and measured EQE spectra. SE data analysis is challenging for CIGS material components and solar cells because of the need to develop an appropriate (ε1, ε2) database for the CIGS alloys and to extract absorber layer Ga profiles for accurate structural/compositional models. For cells with standard thickness absorbers, excellent agreement is found between the simulated and measured EQE, the latter under the assumption of 100% collection from the active layers, which include the CIGS bulk and CIGS/CdS heterojunction interface layers. For cells with thin absorbers, however, an observed difference between the simulated and measured EQE can be attributed to losses via carrier recombination within a ∼0.15 μm thickness of CIGS adjacent to the Mo back contact. By introducing a carrier collection probability profile into the simulation, much closer agreement is obtained between the simulated and measured EQE. In addition to the single spot capability demonstrated in this study, ex-situ SE can be applied as well to generate high resolution maps of thin film multilayer structure, component layer properties and their profiles, as well as short-circuit current density predictions. Such mapping is possible due to the high measurement speed of <1 s per (ψ, Δ) spectra achievable by the multichannel ellipsometer.

EuIII‐Based Nanolayers as Highly Efficient Downshifters for CIGS Solar Cells

We report herein on the use of 13 ternary europium(III) complexes as downshifting (DS) material for copper indium gallium selenide (CIGS) solar cells. Their general formulae are [Eu(β‐Dik)3(NL)x] and [Eu(L)3], in which β‐Dik = 2‐thenoyltrifluoroacetonate, 4,4,4‐trifluoro‐1‐phenyl‐1,3‐butanedione and 4,4,4‐trifluoro‐1‐(2‐naphthyl)‐1,3‐butanedione and NL = diphenyl sulfoxide (x = 2), triphenylphosphine oxide (x = 2), bis[2‐(diphenylphosphino)phenyl] ether (x = 1), 5,6‐epoxy‐5,6‐dihydro‐1,10‐phenanthroline (x = 1) and 2‐(N,N‐diethylanilin‐4‐yl)‐4,6‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,3,5‐triazine (x = 1) and L = 4‐(4′‐tert‐butylbiphenyl‐4‐yl)‐2,2′‐bipyridine‐6‐carboxylate. Tris(β‐diketonate) ternary EuIII complexes are very good candidates for DS applications, and some of them have shown very promising results when embedded in a host organic polymer matrix typically used for solar cell encapsulations. Herein we report a new method for the direct deposition of luminescent nanolayer films on CIGS solar cells. Highly luminescent nano‐ and microlayer films with a thickness in the range of 150 nm to 10 µm have been obtained and characterized by optical microscopy, profilometry, spectrofluorimetry and external quantum efficiency (EQE) measurements. The efficient conversion of incident photons into red light was observed for films with a thickness of 400–600 nm, leading to a significant improvement in the EQE and short‐circuit current density (Jsc) in the UV region up to 0.81 mA/cm2. The absolute conversion efficiency increased by up to 0.8 %. The nanofilms, with the exception of those containing naphthyl derivatives, were found to be photostable and therefore represent a competitive alternative to doped polymer encapsulants.

Performance enhancement in Sb doped Cu(InGa)Se 2 thin film solar cell by e-beam evaporation

Abstract To investigate the effects of Sb doping on the structural and electrical properties of Cu(InGa)Se 2 (CIGS) thin films and solar cells, CIGS thin films, prepared by e-beam evaporation on soda-lime glass, were doped with lower and upper Sb layers in the precursor stacks respectively. Change of structure and introduction of stress were observed in the CIGS thin films with upper Sb layer in stack through XRD and Raman measurement. Both crystalline quality and compactness of CIGS thin films were improved by the doping of upper Sb layer in stack and the CIGS thin film showed an optimal structural property with 20 nm Sb layer. Movement of Fermi level of the surface of CIGS thin film after doping of upper Sb layer in stack and electrons transfer between Cu/Cu + redox couple and CIGS thin films, which provided probability for the substitution of Sb for Cu sites at the surface of CIGS thin films, were proposed to explain the migration of Cu from the surface to the bulk of CIGS thin films. The larger barrier at the CIGS/CdS interface after doping of upper Sb layer in stack made contribution to the increase of V OC of CIGS solar cells. The efficiency of CIGS solar cell was improved from 3.3% to 7.2% after doping with 20 nm upper Sb. Compared to the CIGS solar cell with lower Sb layer in stack, in which an additional Cu 2-x Se phase was found, the CIGS solar cell with upper Sb layer in stack possessed a higher efficiency.

Optimization of the properties of the molybdenum back contact deposited by radiofrequency sputtering for Cu([formula omitted]) [formula omitted] solar cells

In this work, we report on using RF sputtering technique for the deposition of the molybdenum back contact for CIGS solar cells. This allows to realize the back and front contact (Aluminum doped ZnO is used for the front contact) with the same deposition system. We demonstrate that deposition pressure controls the morphology and resistivity of the Mo back contact. This latter point has been the subject of apparently contradicting reports in the literature, but an analysis of the deposition parameters versus sputtering mechanisms allow to understand this spread of results. Full cells have been grown onto the various sets of Mo films, and their electrical properties (efficiencies, Voc, Jcc and fill factors) are analyzed and modeled. From this, we obtain the ideality factors (reflecting the amount of carrier recombination) and with the shunt resistances we find these to be the main parameters which explains the different cells performances versus Mo back contact growth conditions. Low temperature photoluminescence is used as a tool to get a deeper insight on the recombination paths in the samples and support the conclusions drawn from electrical measurements: the molybdenum films grown at 1Pa yields the best cells results, owing to minimized non radiative recombination. Finally, using the optimum Mo films and improving the quality of the top contact allow us to obtain CIGS solar cells with efficiencies of 17%, although the CIGS, CdS and ZnO layers are not still fully optimized.

Full space device optimization for solar cells

Advances in computational materials have paved a way to design efficient solar cells by identifying the optimal properties of the device layers. Conventionally, the device optimization has been governed by single or double descriptors for an individual layer; mostly the absorbing layer. However, the performance of the device depends collectively on all the properties of the material and the geometry of each layer in the cell. To address this issue of multi-property optimization and to avoid the paradigm of reoccurring materials in the solar cell field, a full space material-independent optimization approach is developed and presented in this paper. The method is employed to obtain an optimized material data set for maximum efficiency and for targeted functionality for each layer. To ensure the robustness of the method, two cases are studied; namely perovskite solar cells device optimization and cadmium-free CIGS solar cell. The implementation determines the desirable optoelectronic properties of transport mediums and contacts that can maximize the efficiency for both cases. The resulted data sets of material properties can be matched with those in materials databases or by further microscopic material design. Moreover, the presented multi-property optimization framework can be extended to design any solid-state device.