Three-stage Co-evaporation Process Research Articles

Optoelectronic Effects of Copper-Indium-Gallium-Sulfur (CIGS2)-Solar Cells Prepared by Three-Stage Co-Evaporation Process Technology.

In this paper, the performance of Cu-(In,Ga)-S2 (CIGS2) solar cells with adjusting composite [Cu]/([Ga] + [In]) (CGI)-ratio absorber was explored and compared through an improved three-stage co-evaporation technique. For co-evaporating CIGS2 absorber as a less toxic alternative to Cd-containing film, we analyzed the effect of the CGI-ratio stoichiometry and crystallinity, and explored its opto-electric sensing characteristic of individual solar cell. The results of this research signified the potential of high-performance CIGS2-absorption solar cells for photovoltaic (PV)-module industrial applications. For the optimal CIGS2-absorption film (CGI = 0.95), the Raman main-phase signal (A1) falls at 291 cm-1, which was excited by the 532 nm line of Ar+-laser. Using photo-luminescence (PL) spectroscopy, the corresponding main-peak bandgaps measured was 1.59 eV at the same CGI-ratio film. Meanwhile, the best conversion efficiency (η = 3.212%) and the average external quantum efficiency (EQE = 51.1% in the visible-wavelength region) of photo-electric properties were achieved for the developed CIGS2-solar cells (CGI = 0.95). The discoveries of this CIGS2-absorption PV research provided a new scientific understanding of solar cells. Moreover, this research undeniably contributes to a major advancement towards practical PV-module applications and can help more to build an eco-friendly community.

Impact of tellurium as an anion dopant on the photovoltaic performance of wide-bandgap Cu(In,Ga)Se2 thin-film solar cells with rubidium fluoride post-deposition treatment

The development of wide-bandgap Cu(In,Ga)Se2 thin films is crucial in order to reach the theoretical Shockley–Queisser limit values in single-crystal solar cells. However, the performance of solar cells based on wide-bandgap thin film absorbers has lagged significantly compared to that of their narrow-bandgap counterparts. Herein, we develop a feasible strategy to improve the photovoltaic performance of wide-bandgap Cu(In,Ga)Se2 chalcopyrite thin-film solar cells by simultaneously doping with both RbF PDT and Te2− anions as dopants in the absorber layer during the three-stage co-evaporation process. Besides inducing significant change in the GGI gradient, the synergistic effect of the Te2− anion dopant is rather beneficial in terms of controlling grain size, defects in grain boundaries, and charge carrier lifetime for encouraging charge separation and extraction, which contributes to simultaneously boosting short-circuit current density and fill factor. Te-poor devices afford an impressive efficiency of 9.58%, compared to 6.43% for control devices. More importantly, the efficiency and Voc values obtained for wide-bandgap-based thin-film solar cells containing Te anions were the highest compared to their counterparts as reported in the literature. These results demonstrate the role of Te2− anions in wide-bandgap absorber thin films on the photovoltaic performance of thin-film solar cells and the potential of this approach for use in reasonable and effective design of highly efficient wide-bandgap thin-film solar cells.

Improved carrier transport in CIGS solar cells induced by Ag treatment

In this work, to improve the performance of three-stage co-evaporated Cu(In,Ga)Se2 (CIGS) solar cells, a small amount of Ag was incorporated during the third stage, and the materials/device characteristics were investigated. Although the amount of added Ag was limited to less than 1 at%, both the film microstructure and the resulting device performance were affected. The Ag-alloyed CIGS thin films exhibited enhanced recrystallization and homogenous Ag distribution throughout the CIGS film bulk, even though the Ag incorporation was performed near the end of the three-stage co-evaporation process. Well-controlled Ag alloying into CIGS solar cells (approximately 0.6 at%) led to increased fill factors, accompanied by increased charge carrier densities as well as reduced detrimental state densities. The approach performed in the present work suggests that the device quality of CIGS solar cells can be tuned by small Ag additions in a similar manner to extrinsic alkali incorporation.

Enabling Low-Temperature Deposition of High-Efficiency CIGS Solar Cells with a Modified Three-Stage Co-Evaporation Process

CuIn1–xGaxSe2 (CIGS) absorber layers with good crystallinity and large grain sizes deposited at low substrate temperatures are essential to the development of CIGS solar cells. The main challenges ...

Mechanisms of extrinsic alkali incorporation in CIGS solar cells on flexible polyimide elucidated by nanoscale and quantitative analyses

In this work, Cu(In,Ga)Se2 (CIGS) solar cells on polyimide (PI) substrates were fabricated using a low-temperature three-stage co-evaporation process. To enhance device performance, the CIGS films were extrinsically doped with alkali (Na and K) using an in-situ post deposition treatment (PDT). To account for mechanisms of extrinsic alkali incorporation in CIGS solar cells on flexible polyimide, the alkali dopant concentrations in the film bulk (intragrain and grain boundary) and the surface chemistries/band structures were quantitatively investigated with various advanced characterization methods. In addition, the effects of the PDT sequences on the resulting device performance were studied with a particular emphasis on the characteristics of CIGS surfaces. By controlling the alkali incorporation into the CIGS absorber films, flexible lightweight CIGS thin-film solar cells with an efficiency of approximately 19% were obtained.

A simple and robust route toward flexible CIGS photovoltaic devices on polymer substrates: Atomic level microstructural analysis and local opto-electronic investigation

In this work, copper indium gallium selenide (Cu(In,Ga)Se2; CIGS) absorbers were grown on polyimide (PI)/molybdenum substrates by a three-stage co-evaporation process at various temperatures, film formation was systemically studied using various advanced characterization methods such as transmission electron microscopy, micro-Raman spectroscopy, Kelvin probe force microscopy, and atom probe tomography. The CIGS films on PI were found to exhibit considerable physical and electrical variations with respect to the process temperature of three-stage co-evaporation. In particular, when the process temperature reached 400 °C, the CIGS absorber on PI began to exhibit controlled microstructure and intergrain properties. By adjusting the microstructure and intergrain properties of the absorber films by means of the process temperature of three-stage co-evaporation, flexible CIGS solar cells on PI with an efficiency of 16.7% (with anti-refection coating) were achieved.

CIGS thin film solar cells with graded-band gap fabricated by CIS/CGS bilayer and CGS/CIS/CGS trilayer systems

High efficiency CuIn1-xGaxSe2 (CIGS) thin film solar cells are usually deposited on Mo-coated soda-lime glass (SLG) substrates by the three-stage co-evaporation process which is intended to create double-graded band gap. It requires short total deposition time and rapid changing of substrate temperature in order to obtain the double-graded band gap which is quite difficult for high source-temperatures. When the total deposition time cannot be made short enough, uniform band gap is rather achieved. In this work, the depositions of CIS/CGS bilayer are employed in order to create back grading while CGS/CIS/CGS trilayers are for the double-grading, owing to non-uniformity and different diffusivity of In and Ga constituents in the films. The bilayers with back grading show the increasing short-circuit current density (Jsc) due to the assisting back surface field, but the open-circuit voltage (Voc) is relatively low due to the reduction of Ga content at the front surface. For the CGS/CIS/CGS trilayers, double grading and the increasing of Voc are observed when compared with the bilayers due to the increasing Ga content near the junction. The efficiencies of the devices fabricated from the CIS/CGS bilayer and CGS/CIS/CGS trilayer absorbers show the maximum value of 12.5% and 15.5%, respectively. The external quantum efficiency (EQE) of the bilayer and trilayer absorbers exhibit the enhancement in the long wavelengths compared to that of the three-stage process absorbers.

In vacuo XPS investigation of Cu(In,Ga)Se2 surface after RbF post-deposition treatment

Latest record efficiencies of Cu(In,Ga)Se2 (CIGSe) solar cells were achieved by means of a rubidium fluoride (RbF) post-deposition treatment (PDT). To understand the effect of the RbF-PDT on the surface chemistry of CIGSe and its interaction with sodium that is generally present in the CIGSe absorber, we performed an X-ray photoelectron spectroscopy (XPS) study on CIGSe thin films as-deposited by a three-stage co-evaporation process and after the consecutive RbF-PDT. The sample transfer from the deposition to the XPS analysis chamber was performed via an ultra-high vacuum transfer system. This allows to minimize air exposure, avoiding oxide formation on the CIGSe surface, especially for alkali-treated absorbers. Beside an expected reduction of Cu- and Ga-content at the surface of RbF-treated CIGSe films, we find that Rb penetrates the CIGSe and, contrary to fluorine, it is not completely removed by subsequent ammonia etching. The remaining Rb contribution at 110.0 eV binding energy, which appears after the RbF-PDT is similar to the one detected on a co-evaporated RbInSe2 reference sample and together with a new Se 3d contribution may hence belong to an Rb-In-Se secondary phase on the CIGSe surface. In addition, Na is driven towards the surface of the CIGSe absorber as a direct result of the RbF-PDT. This proves the ion-exchange mechanism in the absence of moisture and air/oxygen between heavy Rb atoms incorporated via PDT and lighter Na atoms supplied by the glass substrate. A remaining XPS signal of Na 1 s is observed after etching the vacuum transferred RbF-CIGSe sample, indicating that Rb and/or F are not as much a driving force for Na as oxygen usually is.

Varied deposition time of third stage process to fabricate Cu(In,Ga)Se2 absorber layer and device through a three-stage process

A Cu-deficient Cu(In,Ga)Se2 (CIGS) absorber layer with different deposition time of third stage was fabricated using three-stage co-evaporation process. An appropriate deposition time not only eliminated excess Cu2−xSe but also formed the order defect compound (ODC) phase near the CIGS surface. This ODC surface phase can minimize the recombination rate between absorber and buffer layer. The highest efficiency of 15.82% was achieved when the deposition time was 7 min and bulk [Cu]/([In] + [Ga]) ratio at the end of the three-stage process was 0.98. It indicated that the suitable deposition time of third stage and [Cu]/([In] + [Ga]) ratio could improve the defect concentration, surface properties and recombination of interface between CIGS/CdS.

A new simple route to grow Cu(In,Ga)Se2 thin films with large grains in the co-evaporation process

In the conventional three-stage co-evaporation process to grow Cu(In,Ga)Se2 (CIGS) film, a large grain is achieved by the co-evaporation of Cu and Se on (In,Ga)2Se3 layer at 550 °C in the second stage and then a p-type is achieved by the co-evaporation of In, Ga, and Se in the third-stage. We reported a new process where a CIGS film with a large gain and p-type is achieved by evaporation of Cu only in the second stage at 400 °C and by the Se annealing in the third stage. In the new process, thermal budget was lowered and the third-stage co-evaporation process was eliminated. It was found that the CIGS gain size increased when the Cu/(In + Ga) ratio was above 0.7 and an addition thin CIGS layer appeared on the CIGS surface. The reaction path with Cu was described in the Cu-In-Se ternary phase diagram. The cell conversion efficiency increased from 9.6 to 15.4% as the Se annealing temperature increased from 400 to 550 °C in the third stage, mainly due to the increase of open-circuit voltage and fill factor. Our process demonstrated a new route to grow a CIGS film with a less thermal budget and simpler process in the co-evaporation process.

Reversible correlation between subnanoscale structure and Cu content in co-evaporated Cu(In,Ga)Se2 thin films

Thin film solar cells based on Cu-poor Cu(In,Ga)Se2 absorbers grown by a three-stage co-evaporation process have been shown to strongly benefit from a Cu-rich intermediate growth stage and the presence of alkali elements such as Na, K, and Rb, pushing the current record efficiency to 22.6%. However, some details of the mechanisms underlying these improvements are not yet fully understood, particularly on a very local scale. We therefore used extended X-ray absorption fine structure spectroscopy to study element-specific local structural parameters of Cu(In,Ga)Se2 thin films with varying final Cu content, varying history of the Cu content, and varying alkali treatment. We find that the bulk structure on a subnanometer scale is unaffected by the presence of alkali elements, confirming that their beneficial effect mostly stems from improving the electronic properties of the material. In contrast, local structural disorder clearly increases with decreasing Cu content but does not depend on whether or not the material has passed through a Cu-rich intermediate stage. While the latter is known to improve film morphology and microstructure, it obviously has no effect on the subnanoscale structure, which exhibits a fully reversible correlation with the final Cu content of the Cu(In,Ga)Se2 absorber.

Real Time Spectroscopic Ellipsometry Analysis of First Stage CuIn1-xGaxSe₂ Growth: Indium-Gallium Selenide Co-Evaporation.

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CIGS) thin film deposition by the three-stage co-evaporation process used for fabrication of high efficiency thin film photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1−xGax)2Se3 (IGS) on a substrate of Mo-coated soda lime glass maintained at a temperature of 400 °C. This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result precise compositional control is desired in order to achieve the optimum performance of the resulting CIGS solar cell. RTSE is sensitive to monolayer level film growth processes and can provide accurate measurements of bulk and surface roughness layer thicknesses. These in turn enable accurate measurements of the bulk layer optical response in the form of the complex dielectric function ε = ε1 − iε2, spectra. Here, RTSE has been used to obtain the (ε1, ε2) spectra at the measurement temperature of 400 °C for IGS thin films of different Ga contents (x) deduced from different ranges of accumulated bulk layer thickness during the deposition process. Applying an analytical expression in common for each of the (ε1, ε2) spectra of these IGS films, oscillator parameters have been obtained in the best fits and these parameters in turn have been fitted with polynomials in x. From the resulting database of polynomial coefficients, the (ε1, ε2) spectra can be generated for any composition of IGS from the single parameter, x. The results have served as an RTSE fingerprint for IGS composition and have provided further structural information beyond simply thicknesses, for example information related to film density and grain size. The deduced IGS structural evolution and the (ε1, ε2) spectra have been interpreted as well in relation to observations from scanning electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy profiling analyses. Overall the structural, optical and compositional analysis possible by RTSE has assisted in understanding the growth and properties of three stage CIGS absorbers for solar cells and shows future promise for enhancing cell performance through monitoring and control.

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.

Evolution of opto-electronic properties during film formation of complex semiconductors

Optical and electrical properties of complex semiconducting alloys like Cu(In,Ga)Se2 (CIGS) are strongly influenced by the reaction pathways occurring during their deposition process. This makes it desirable to observe and control these properties in real-time during the deposition. Here we show for the first time the evolution of the band gap and the sub-band-gap defect absorption of CIGS thin film as well as surface roughness during a three-stage co-evaporation process by means of an optical analysis technique, based on white light reflectometry (WLR). By simultaneously recording structural information with in-situ energy dispersive X-ray diffraction and X-ray fluorescence we can directly correlate the evolution of opto-electronic material parameters with the structural properties of the film during growth. We find that the surface roughness and the sub-gap light absorption can be correlated with the phase evolution during the transformation from (In,Ga)2Se3 to Cu(In,Ga)Se2 by the incorporation of Cu into the film. Sub-bandgap light absorption is found to be influenced by the Cu-saturated growth phase and is lowered close to the points of stoichiometry, allowing for an advanced process design.

Secondary phase formation in (Ag,Cu)(In,Ga)Se2 thin films grown by three-stage co-evaporation

(Ag,Cu)(In,Ga)Se2 thin films deposited by a three-stage co-evaporation process have shown potential as mid- and wide-bandgap absorber materials for solar cell devices. Further growth optimization requires a thorough understanding of secondary phases formed in addition to the primary chalcopyrite phase, during the group-I rich growth near the end of the second stage. In this study, we investigate how the composition ratio Ag/(Ag+Cu) controls the formation of the secondary phases containing Ag, Cu and Se. Group-I rich [composition ratio (Ag+Cu)/(In+Ga)>1] ACIGS films with varying silver content—Ag/(Ag+Cu)=0, 0.1 and 0.5—are grown until the end of the second stage of three-stage processes. Secondary phases are found to locate near film surfaces. However, they intermix with the chalcopyrite phase in the surface layer when Ag/(Ag+Cu)>0, while they form a separate layer if no Ag is alloyed. The crystal structures of the secondary phases shifts along the Cu2Se - Ag2Se pseudo-binary tie-line as Ag/(Ag+Cu) increases. Concurrently, the film surfaces become smoother with reduced facets and voids, the accumulation of sodium is suppressed, and both the degree of In and Ga gradient and the film emissivity are reduced, all of which affect the subsequent deposition process and final solar device performance.

Effect of Se flux on CuGaSe2 absorbers deposited on ITO-coated SLG substrates by using a three-stage co-evaporation process

As the key factor for top-cell application in a tandem structure, wide-bandgap chalcopyrite CuGaSe2 (CGS) absorbers were deposited at a thickness of 2 μm on soda-lime glass (SLG) which was coated with radio-frequency sputtered indium-tin-oxide (ITO) films. The semi-transparent CGS absorbers with a bandgap energy of 1.65 eV were grown by using a three-stage co-evaporation process with Cu, Ga, and Se elemental sources. During CGS absorber growth, the composition ratio [Cu]/[Ga] was fixed at about 0.85 and the Se-to-Ga flux ratio P[Se]/[Ga] was varied from 22 to 61 by increasing the temperature of the Se source. In this study, the compositional, structural, optical and electrical properties of top-cell CGS absorbers, which absorbed the short wavelength range of the solar spectrums for tandem solar cell application, were investigated as a function of the Se flux. On the basis of our experimental results, the highest CGS solar cell efficiency of 4.7 % in the cell structure described as Al/ZnO:Al/i-ZnO/CdS/CGS/ITO/SLG was demonstrated using a P[Se]/[Ga] value of 22.

Study on the thickness effect of wide-bandgap CuGaSe2 thin films for applications with tandem solar cells

For application on the top cell of a tandem structure, chalcopyrite CuGaSe2 (CGS) thin films were prepared on as-deposited 200-nm-thick indium-tin-oxide (ITO) thin films grown by using radiofrequency (RF) magnetron sputtering. CGS thin films with a wide bandgap of 1.63 eV for use in high open-circuit-voltage solar cells were deposited by using a three-stage co-evaporation process with Cu, Ga, and Se elemental sources with compositional ratios of Cu/Ga = 0.88 and Se/(Cu + Ga) = 0.98. In this study, we examined the effect of the thickness of the top-cell CGS thin films on the tandem cell performance, which is a key factor for improving the cell’s efficiency for optimum light absorption. The film thickness was varied from 0.5 μm to 2 μm in intervals of 0.5 μm by controlling the process time to confirm the optical and the electrical properties of solar cells. Based on our experimental results as a function of the CGS film’s thickness, we achieved a solar cell efficiency of 5.77% with a 1.5-μm-thick CGS thin film in the cell structure fabricated as Al/ZnO: Al/i-ZnO/CdS/CGS/ITO/SLG.

Spectroscopic ellipsometry studies of 3-stage deposition of CuIn1 − xGaxSe2 on Mo-coated glass and stainless steel substrates

In order to achieve proper process monitoring and control for the deposition of thin film photovoltaic absorbers, for example CuIn1−xGaxSe2 (CIGS), optical techniques such as spectroscopic reflectometry and polarimetry are advantageous because they can be set up in an unobtrusive manner in the manufacturing line, and collect data in-line and in-situ. The use of these techniques requires accurate optical models that correctly represent the properties of the layers being deposited. In this study, spectroscopic ellipsometry (SE) has been applied for the characterization of each individual stage of CIGS layers deposited using the three-stage co-evaporation process. Dielectric functions have been determined for the energy range between 0.7eV and 5.1eV. As the layers' thicknesses are in the order of 2.5μm, thickness non-uniformity along with surface roughness has been included in the analysis. A high similarity between the optical models developed for CIGS on glass and on stainless steel substrate was observed. Critical-point line-shape analysis was used in this study to determine the critical point energies of the CIGS based layers. These results can assist in the development of optical process-control tools for the manufacturing of photovoltaic absorbers, increasing the uptime and yield of the production line.

Vacancy behavior in Cu(In1 − xGax)Se2 layers grown by a three-stage coevaporation process probed by monoenergetic positron beams

Vacancy-type defects in Cu(In1−xGax)Se2 (x≅0.45 and 1) grown by a three-stage coevaporation process were probed using monoenergetic positron beams. Measurements of Doppler broadening spectra of the annihilation radiation and positron lifetime spectra showed that two different defect species coexist in the Cu(In1−xGax)Se2 layers, and these were identified as mono/divacancy-type defects and vacancy clusters, respectively. The vacancy clusters were mainly introduced during the third growth stage, and were located in the subsurface region. The concentration of the defects affected the short-circuit current density and the conversion efficiency of the solar cells. The defect concentration and their depth distributions varied depending on Se beam equivalent pressure, growth time, and post-growth annealing time. The behavior of the vacancy-type defects is discussed also with respect to results obtained using an electron probe micro-analyzer.

Remarkable enhancement of the efficiency of Cu(In,Ga)Se2 solar cells by annealing the (In,Ga)2Se3 precursor layer

Thin film solar cells based on chalcopyrite Cu(In, Ga)Se2 (CIGSe) absorber layers were fabricated successfully using a three-stage co-evaporation process. The effects of annealing the (In,Ga)2Se3 (IGSe) precursor layer on the preferred orientation and morphology of the CIGSe films as well as the performance of the Ag/Ni/(Ga,Al):ZnO/i-ZnO/CdS/CIGSe/Mo/Glass cell were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and illuminated J–V and quantum efficiency measurements. Layer studies indicated that the CIGSe film texture changed from (112) to (220) with increasing annealing time. Cell studies showed that the performance of the solar cell was improved with an abrupt change in the open circuit voltage and a remarkable increase in efficiency from 7.35% to 14.35% according to the annealing time in the range, 0–30 min.