Depth-Profiling Electronic and Structural Properties of Cu(In,Ga)(S,Se

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Depth-profiling Electronic and Structural Properties of Cu(In,Ga)(S,Se) Thin-film Solar Cell 2

Ching-Yu Chiang, Sheng-Wei Hsiao, Pin-Jiun Wu, Chu-Shou Yang, Chia-Hao Chen, and Wu-Ching Chou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03869 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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ACS Applied Materials & Interfaces

Depth-profiling Electronic and Structural Properties of Cu(In,Ga)(S,Se)2 Thin-film Solar Cell Ching-Yu Chiang,†,‡ Sheng-Wei Hsiao,† Pin-Jiun Wu,*,‡ Chu-Shou Yang,∥Chia-Hao Chen,‡ and Wu-Ching Chou† †

Institute and Department of Electrophysics, National Chiao Tung University, Hsinchu, 30010, Taiwan ‡

National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan

∥

Graduate Institute of Electro-Optical Engineering, Tatung University, Taipei, 10452, Taiwan

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Abstract

Utilizing a scanning photoelectron microscope (SPEM) and grazing-incidence X-ray powder diffraction (GIXRD), we studied the electronic band structure and the crystalline properties of the pentanary Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cell as a function of sample depth on measuring the thickness-gradient sample. A novel approach is proposed for studying the depth-dependent information of thin films, which can provide a gradient thickness and a wide cross section of the sample by polishing process. The results exhibit that the CIGSSe absorber layer possesses four distinct stoichiometries. The growth mechanism of this distinctive compositional distribution formed by a two-stage process is described according to the thermodynamic reaction and the manufacturing process. Based on the depth-profiling results, the gradient profiles of the conduction and valence bands were constructed to elucidate the performance of the electrical properties (in this case, Voc = 620 mV, Jsc = 34.6 mA/cm2 , and η = 14.04%); the valence-band maxima (VBM) measured with a SPEM in the spectroscopic mode coincide with this band-structure model, except for a lowering of the VBM observed in the surface region of the absorber layer due to the ordered defect compound (ODC). In addition, the depth-dependent texturing x-ray diffraction pattern presents the crystalline quality and the residual stress for each depth of a thin-film device. We find that the randomly oriented grains in the bottom region of the absorber layer and the different residual stress between the underlying Mo and the absorber interface, which can deteriorate the electrical performance due to peeling-off effect. An anion interstitial defect can be observed on comparing the anion concentration of the elemental distribution with crystalline

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composition; a few excess sulfur atoms insert in interstitial sites at the front side of the absorber layer whereas the interstitial selenium atoms at the back side.

KEYWORDS: CIGSSe, solar cell, scanning photoelectron microscope (SPEM), grazing-incidence X-ray powder diffraction (GIXRD), polishing, band diagram, residual stress, ordered defect compound (ODC)



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1. Introduction

The development of clean energy is an inevitable responsibility in human society. Renewable energy involves clean sources of energy that have a smaller environmental impact than conventional energy such as from oil, coal and natural gas. Among the various renewable energy resources, photovoltaic is currently the most improved as well as the most promising technology; it allows us to convert sunlight directly into electricity. To make the solar-cell technology become the most important power-supply system in the next generation, thin-film solar-cell systems have attracted much attention to decrease the cost of manufacturing and material utilization. Thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) become a promising candidate to compete with Si-based photovoltaics because of their specific properties including a direct band gap1 as well as a large optical absorption coefficient2, cost-effective manufacturing3, adjustable band gap4-6, excellent thermal stability7-9 and opportunity to be a flexible device10-12. A suitable band gap in an absorber layer is crucial for the efficiency of energy conversion of a solar cell. The theoretical maximum conversion efficiency, more than 33 %, can be achieved at band gaps of 1.18 and 1.36 eV13. In the CIGS-based thin-film solar cell, the band gap can be adjusted from 1 to 1.68 eV on controlling the ratio Ga/In. Moreover, the band gap can be enlarged to 2.43 eV by incorporating sulfur into the CIGS structure. In an industrial application, the two-stage process involving sputtering and subsequent selenization or sulfurization is convenient to achieve low-cost and large-scale properties of commercial CIGSSe solar-cell devices. The compositional grading profile is typically formed in the absorber layer via two-stage process and causes a gradient of the band gap.


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For the Cu(In1-xGax)(SySe1-y)2 (CIGSSe) material, the band gap can be adjusted from 1 to 2.43 eV on controlling the ratios Ga/In and S/Se. As the compositional gradient depends strongly on the conditions of selenization or sulfurization, the feature of a varied band gap in the depth of the absorber layer offers a possibility to engineer the band-gap profiles to improve the electronic performance11,14-17. Nevertheless, the varied concentration of five elements in the CIGSSe layer and elemental diffusion at the interface can further change the compositional distribution. Some authors reported that the elemental concentration varied with the depth of the absorber layer, using secondary-ion mass spectra (SIMS)11,18-20 or X-ray photoelectron spectra (XPS)14,21-23, but this information about the elemental distribution is inadequate for understanding the electronic properties of the materials, as the crystalline structure affects also the band-gap profile. Other authors have shown that the crystalline orientation24-29 and residual stress24,30-33 influence the electronic performance of a solar-cell device. It is difficult to explore depth-dependent structural variations because such measurements can provide solely average results. To observe the variations of the electronic properties and the crystalline structure related to the depth of a thin-film solar cell, therefore, a specific method should be implemented. In this work, we prepared the CIGSSe solar cell with a gradient thickness to study its properties in depth. The synchrotron experimental methods, including a scanning photoelectron microscope (SPEM) and grazing-incidence X-ray powder diffraction (GIXRD), were utilized to investigate the elemental distribution, the electronic properties and the crystalline structure of the solar cell. According to the quantitative results, the grading profile of the band gaps in the absorber layer has been modeled to comprehend



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the correlations between the band diagram and the electric properties.


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2. Experiments

The sample used in this study is a 5×10 mm2 piece cut from a 30×30 cm2 commercial CIGSSe solar cell, which has a stacking structure of ZnO:B/CdS/CIGSSe/Mo/soda-lime glass (SLG). The polycrystalline CIGSSe thin film of thickness ~1600 nm, formed on co-sputtering of a Cu-Ga-In alloy target with additional selenization at 415~440 oC and sulfurization at 515~540 oC, was deposited on a substrate of SLG coated with a SiOx layer (50 nm) and a Mo back contact layer (400 nm). The CdS buffer layers of thickness ~5 nm and boron-doped ZnO (ZnO:B) window layers of thickness ~2400 nm were subsequently deposited onto the CIGSSe absorber layer in a chemical bath at 60 oC and a metal-organic chemical vapor deposition (MOCVD) method at 150 oC, respectively.

To study the depth-dependent properties of a commercial solar cell, we polished the sample to have a gradient thickness by a wet grinding method. The polishing machine (ULTRA TEC) includes a rotating plate and a sample supporting arm. The sample supporting arm is mounted on a protractor and the level meter can be set for controlling the polished slop. The waterproof abrasive paper with 3 µm particle size was utilized to roughly grind the sample surface for removing the excess material with a small gradient (θ ~ 0.09o) along the sample plane. The waterproof abrasive paper with 0.1 µm particle size with a small gradient (θ ~ 0.09o) along the sample plane was utilized to further reduce the surface roughness of the sample. The slope of the polished sample, in this case, is about 1.7×10-3, which can enlarge the cross-section length ~580 times. A larger length scale of the cross section allows us to measure the elemental distribution and the structural characteristics related to the depth of each layer in the thin-film solar cell. After



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polishing, chemical etching with hydrochloric acid (0.1 %, 20 s) and low-energy Ar+ sputtering were undertaken to remove surface contamination and the surface stress induced by polishing mechanism from the sample.

The in-depth information about chemical compositions and band structure was obtained using a scanning photoelectron microscope (SPEM) at beamline BL-09A1 of Taiwan Light Source in National Synchrotron Radiation Research Center (NSRRC). A combination of a Fresnel zone plate and an order-sorting aperture was utilized to focus the monochromatic soft X-rays (380 eV); the beam size was about 200 nm at the focal plane. A two-dimensional mapping of a particular element on the material surface was performed on raster-scanning the sample relative to the focused soft X-ray beam and simultaneously collecting the emitted photoelectrons with a multiple-channel hemispherical

electron-energy

analyzer.

High-resolution

and

microscopic-area

photoemission spectra were also recorded on moving the focused beam to specific locations of the sample surface. The photon energies of soft X-rays were calibrated with the Au 4f core-level photoemission spectra of a gold foil. Information about crystal structures in a sample of varied thickness was obtained with grazing-incidence X-ray powder diffraction (GIXRD) at beamline BL-01C2. Photon energy 12 keV was selected for the measurements; a MAR345 image detector was used to collect the two-dimensional diffraction patterns. The grazing-incidence angle of the X-ray beams to the substrate was about 5°.


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3.

Results

3.1 Basics of a Cu(In,Ga)(S,Se)2 thin-film solar cell

Figure 1 Cu(In,Ga)(S,Se)2 chalcopyrite structure

Cu(In,Ga)(S,Se)2 (CIGSSe) is a group I-III-VI semiconductor, which has a crystal structure of chalcopyrite. As shown in Figure 1, this structure consists of a central cation atom (I, III) surrounded by four anion atoms (VI), forming a tetrahedral unit cell. Those unit cells are periodic, constituting a face-centered cubic structure and stacking two cubic unit cells to form a chalcopyrite structure.

3.2 Scanning photoemission microscope (SPEM) The elemental distribution and chemical properties are of interest in a CIGS-based solar cell because of a strong correlation with its electronic structure and performance. Figure 2(a) presents a schematic diagram of the SPEM measurements on a thickness-gradient CIGSSe-based solar cell, for which a photoelectron image of Mo 3d is taken as an example. The mapping area is about 2×0.6 mm2 and the intensity of the image



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corresponds to the concentration of the probed element. The SPEM images of all elements in the solar cell and the corresponding optical image are shown in Figure 2(b). For the S 2p1/2 image, sulfur is found in the entire device, even in SLG and the ZnO window layer, and is segregated in the bottom region ~250 nm near the Mo/CIGSSe interface. The Se 3d image indicates that Se atoms also diffuse into the back contact-layer region together with sulfur and form a Mo(SySe1-y)2 phase at the Mo/CIGSSe interface. For the Cu 3p3/2 image, few copper atoms diffuse into the back-contact layer and window layer, which could be a critical issue in the electronic performance of solar cells34. The intensity of each element, divided by the photoionization cross section, is shown in Figure 2(c). In the window layer, the Zn 3p signals remain almost a constant and abruptly drop at the interface of ZnO/CdS. The Cd signals contributed from the buffer layer CdS appear in a small region between ZnO and CIGSSe. In the absorber layer, it exhibits a Cu-poor composition with a Cu/(In+Ga) ratio of about 0.89. Ga and S atoms accumulated in the bottom region, whereas In and Se tended to distribute in the upper region. The Gibbs energy of reaction35 for the In2Se phase (-102.6 kJ/mol) is less than for the Ga2Se phase (-18.0 kJ/mol), whereas for the In2S phase (26.4 kJ/mol) is larger than for the Ga2S phase (-223.1 kJ/mol). The Gibbs energies of reaction for these compounds indicate that Ga prefers to form a GaS phase, whereas In prefers to form an InSe phase. A CuInxSey phase is hence formed first in the selenization, leading to a migration of Ga towards the back side of the absorber layer. In the subsequent sulfurization, the Se vacancies can be filled with sulfur, forming a CuGaxSy phase in the back side of the absorber layer. The core-level photoelectron intensities of each element correspond to its concentration in the material, but not all atoms form the chalcopyrite structure of the absorber layer, which is


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affected by defects of many kinds, such as interstitial atoms, intrinsic point defects and vacancies29,36. To identify the stoichiometry of a crystalline structure for the CIGSSe solar cell, the GIXRD has been employed.

Figure 2 (a) Photograph and schematic illustration of the thickness-gradient CIGSSe thin-film solar cell. (b) Mapping images of primary elements in the sample from a scanning photoelectron microscope (SPEM). (c) Photoelectron intensity of detected elements distributed from glass to the window layer region.



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3.3 Grazing-incidence X-ray powder diffraction

Figure 3 Schematic illustration of grazing-incidence X-ray diffraction on a thickness-gradient sample of ZnO/CdS/CIGSSe/Mo/SLG. The indices P0 to P9 represent the probing positions of the measurements.

A schematic diagram, describing the grazing-incidence X-ray powder diffraction method

to

study

the

structural

information

about

the

thickness-gradient

ZnO/CdS/CIGSSe/Mo/SLG solar cell, appears in Figure 3. The incident X-ray beam enters the sample with a grazing angle of 5° relative to its surface; the beam width is about 0.5 mm. The 2-D image detector enables us to collect simultaneously the crystalline information along the out-of-plane and the off-normal-plane directions of ZnO, CIGSSe and Mo layers. The diffraction patterns at the various positions integrated along the surface normal direction (out-of-plane) of the 2-D diffraction patterns with a sectorial integration 3° and the deconvolution of CIGSSe (112) diffraction patterns are shown in Figure 4. The diffraction patterns of the thickness-gradient sample varying with the positions from P0 to P9 exhibit structural features from the increasing contribution of the upper material in the solar-cell device. According to the diffraction patterns that we obtained, the positions P1 and P5 are the CIGSSe/Mo and ZnO/CdS/CIGSSe interfaces,


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respectively. At P0, all diffraction features correspond to the cubic phase of Mo, which is indicating a polycrystalline structure as the Mo (200) reflection reveals a circular ring of uniform intensity. For the pattern taken at P1, the Mo(Se,S)2 has a preferred orientation alone the (100) planes (marked #) at q = 2.24 A-1 was found at the CIGSSe/Mo interface, and phases of CIGSSe-I at q = 1.987 A-1 and CIGSSe-II at q = 1.941 A-1 are found at the back side of the absorber layer. The diffraction signal at larger q for the chalcopyrite structure presumably contributed to the increased ratios Ga/(Ga+In) and S/(S+Se). At P2, the phases of CIGSSe-III at q = 1.916 A-1 and CIGSSe-IV at q = 1.896 A-1 are formed on top of CIGSSe-I and CIGSSe-II. For the depth-dependent XRD results, the CIGSSe absorber layer has at least four distinct stoichiometries via co-sputtering of Cu/In/Ga precursors and selenization-sulfurization process. In addition, the CIGSSe-based phases are polycrystalline; the grains grow preferentially along direction (112). The window layer ZnO with a wurtzite structure has also been found at positions from P5 to P10. To determine the structural stoichiometry of CIGSSe from the GIXRD data is difficult because the lattice parameter can be compensated by ratios Ga/Se and In/S. Compared with In and Ga, smaller atoms, such as sulfur, more easily occupy the interstitial sites. The ratio In/Ga extracted from the SPEM data is used to determine the S/Se ratios on fitting the GIXRD signals, whereas the Cu deficiency in the absorber layer (in this case, Cu/(In+Ga)~0.89) is not taken into consideration due to the negligible influence on the diffraction peak position (JCPDS card # 89-5648 for CuInSe2 and JCPDS card # 86-1504 for Cu0.8InSe2). In addition, the references of JCPDS card # 87-2265 (CuInSe2), JCPDS card # 75-0104 (CuGaSe2), JCPDS card # 65-1572 (CuInS2), and JCPDS card # 85-1574 (CuGaS2) are introduced in the calculation of structural stoichiometry. With the ratio



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Ga/(In+Ga) = x extracted from the SPEM data and the peak positions of the ternary compounds from the JCPDS references, qCIGSe = [qCGSe x + qCISe (1-x)] A-1 and qCIGS = [qCGS x + qCIS (1-x)] A-1 for the quaternary compounds CuIn1-xGaxSe2 and CuIn1-xGaxS2, respectively, are calculated according to the Vegard’s law. Each deconvoluted peak of the (112) reflection with the peak position between qCIGSe and qCIGS is involved in evaluating the ratio S/(S+Se) = {qCIGSSe - [qCGSe x + qCISe (1-x)]} / {[ qCGS x + qCIS (1-x)] - [qCGSe x + qCISe (1-x)]}. The diffraction signal at higher q can be representing the higher ratio S/(S+Se) for the chalcopyrite structure. The results show the stoichiometries of the CIGSSe absorber layer (from bottom to surface) for each deconvoluted phase to be Cu(In0.42Ga0.58)(S0.91Se0.09)2 of thickness about 160 nm for the CIGSSe-I phase, Cu(In0.5Ga0.5)(S0.51Se0.49)2

about

Cu(In0.66Ga0.34)(S0.12Se0.88)2 about

120 300

nm nm

for for

the

the

CIGSSe-II

CIGSSe-III

phase,

phase, and

Cu(In0.88Ga0.12)(S0.35Se0.65)2 about 920 nm for the CIGSSe-IV phase. Liao et al.21 and Hanket et al.25 reported a similar depth profile of crystalline structure in the CIGS quaternary compounds to our results. They also observed the In-rich and the Ga-rich CIGSe phases coexisted in the front- and back-side CIGSe layer, respectively, after selenization or sulfurization process. However, most CIG(S)Se materials are composed of inhomogeneous microstructures and the average result of XRD is difficult to quantitatively evaluate the structural grading profile of the absorber layer. The thicknesses of the CIGSSe-based phases are estimated from the SPEM data, which also corresponds to the intensities of the deconvoluted XRD signal areas.


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Figure 4 X-ray powder diffraction data integrated along the surface normal direction (out-of-plane) in the 2-D diffraction patterns from P0 to P9 with 3∘sectorial integration and deconvolution of CIGSSe (112) diffraction patterns. The P1 and P5 are interfaces of Mo/CIGSSe and CIGSSe/CdS/ZnO, respectively.

Figure 5 Ratio S/(S+Se) of the CIGSSe absorber layer calculated from SPEM (black line) and GIXRD (red line) results.



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In Figure 5, the ratio S/(S+Se) in the absorber layer calculated from SPEM and GIXRD results has a similar depth profile, but the absolute values are different. This difference is attributed to the existence of intrinsic defects, such as interstitial anions, in the material. After the selenization, the CuInSe-based phase is formed in the top region of the absorber layer; it causes the migration of Ga to the bottom region. During the subsequent sulfurization, a small proportion of sulfur fills the Se vacancies in the CuInSe-based phase and most excess sulfur atoms occupy the interstitial sites in the top region, leading to a larger ratio S/(S+Se) estimated from SPEM results than that from GIXRD data for the top and middle regions. In the bottom region of the absorber layer, Ga preferably forms CuGaS with sulfur, leading to Se being expelled into the interstitial site. The ratio S/(S+Se) obtained from the SPEM results is consequently smaller than that from the GIXRD results in the bottom region.

3.4 Analysis of out-of-plane and in-plane crystalline orientation A 2-D image detector is widely used to collect diffraction patterns of polycrystalline samples. By collocating with the solar cell with a thickness gradient, the several specific lattice planes depending on the sample depth can be obtained simultaneously from the surface normal direction to near the in-plane direction. Figure 6 shows the lattice parameters and the intensity of the diffraction signals fitted with a Voigt function from out-of-plane and off-normal45∘ directions for ZnO (100), CIGSSe-I~IV (112), and Mo (110) planes. The differences of the lattice parameters between out-of-plane and off-normal directions reveal a varied lateral stress in the thin films24. In the materials science, polycrystalline films are composed of many grains of varying size and


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non-preferred orientation. If the crystalline grains have a preferred orientation, the texture profile of diffraction produces a discontinuous Debye cone37. From the texture profile, the degree of intensity ratio out-of-plane/off-normal-plane is related to the degree of preferred orientations of crystalline grains and has a great influence on the materials properties. For example, the crystalline orientation is randomly distributed in the thin films if the intensity ratio out-of-plane/in-plane is identical to 1. In the region of the Mo back contact, a uniform compressive stress induced by the SLG substrate is observed from the Mo (110) reflections; the intensity becomes decreased on probing the thicker CIGSSe layer. The degree of grain orientation of Mo (110) increased from 1.7 to 2.36 with increasing thickness of Mo but decreased to 2.15 at the CIGSSe/Mo interface because of elemental inter-diffusion. Some reports revealed38-39 that the Mo film with a compressive stress has a poor adhesion to a glass substrate but possesses a smaller resistivity. In the absorber layer region, CIGSSe-I to III experiences a tensile stress, constrained by the compressive stress of the Mo film and released in the middle region (CIGSSe-IV) of the absorber layer. The degree of grain orientation is greatly increased from 1.67 to 2.03 in the upper region and decreased at the ZnO/CdS/CIGSSe interface because of elemental inter-diffusion. The stress in CIGSSe-based thin films is generally constrained by the stress of the Mo thin film and likely affects the conversion efficiency. Hultqvist et al.30 reported that the increased compressive stress in Mo thin films on increasing coefficients of thermal expansion of glass enhances the efficiency of energy conversion, but the stress in a CIGS absorber layer cannot be determined because of other effects on the structural variations, such as a Ga gradient in the CIGS thin film. In our work, the elements graded in the CIGSSe



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compound are deconvoluted into four single-phase components; the stress in each CIGSSe-based single phase can be extracted from each diffraction signal of the in- and out-of-plane signals. With increasing thickness of ZnO, the tensile stress in this film is decreased, accompanied by the decreased intensity degree of grain orientation.

Figure 6 Lattice parameters and intensities of x-ray diffraction patterns for ZnO(100), CIGSSe-based (112), and Mo (110) planes in the thickness-gradient sample. The grain orientation intensity can be extracted from the ratio of out-of-plane/in-plane intensities; out-of-plane/in-plane = 1 indicates a crystalline orientation randomly distributed in thin films.

3.5 Modeling the band diagram for the CIGSSe solar cell According to the earlier report40, on increasing the content of Ga in the CuIn1-xGaxSe2 solar cell, the band gap is increased because of an increased conduction-band minimum


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(CBM) and an almost unaffected valence-band maximum (VBM). In addition, the S content in CIGSSe can affect both the CBM and the VBM; being poor in Cu would lower the VBM. The compositional distribution of Ga/In and S/Se in the CIGSSe absorber layer hence leads to the profiles of the conduction-band and valence-band gradients. In this work, the stoichiometries of each CIGSSe-based compound are determined from the SPEM and GIXRD results. The band gaps of each Cu(In1-xGax)(SySe1-y)2-based compound were calculated using the following expression41, which is derived from an analysis of elastic recoil detection.

EgCIGSSe ( x, y ) = (1.00 + 0.13x 2 + 0.08 x 2 y + 0.13xy + 0.55 x + 0.54 y )eV

(1)

The ratio S/(S+Se) in the CIGSSe absorber layer affects varied levels of the VBM and the CBM; we can calculate ∆Ec and ∆Ev on maintaining their ratio 60/40 from the literature42-43. The offsets of the Fermi levels for p- and n-type semiconductors are adopted from Cu(In0.7Ga0.3)Se2 and CdS compounds in the literature44. The space-charge region, Xp ~ 960 nm, is also calculated from basic functions of the p-n junction for a semiconductor45-46 (with mn*/mo = 0.09, mh*/mo = 0.71, Vbi = 0.86 eV); a n-type window layer is considered a heavily doped ZnO:B. The calculated band diagram for the CIGSSe-based solar cell is shown in Figure 7. The black squares denote values of the VBM, measured from the microscopic-area photoelectron spectra of the SPEM system. At the back side of the absorber layer, the VBM curve matches our band model: it increases with increasing S/(S+Se) in the CIGSSe-I compound and decreases with decreasing S/(S+Se) in the CIGSSe-III compound. At the front side of the absorber layer, the observed data points of VBM deviate from our band model, perhaps because of the ordered-defect-compound (ODC)47-49 in the surface Cu-poor absorber layer. The band



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gap of the ZnO compound was estimated from the plasmon energy loss of O 1s photoelectrons50. The band diagram extracted from the SPEM and the GIXRD results of the solar cell device can serve to predict the corresponding performance and provides manufacturing information to improve the efficiency of solar cells, which is discussed in the following section.

Figure 7 Band diagram of the CIGSSe-based solar cell. The black squares are the data points of VBM measured by SPEM.


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4. Discussion

Precisely controlling the depth profile of elemental composition in the CIGS solar cell is an important method to improve its conversion efficiency. Some techniques, such as, SIMS11,18-20, energy dispersive spectroscopy (EDS)19-20, and XPS14,21-23 are widely used to analyze the in-depth properties of CIGS absorber layers, but there are several drawbacks of applying these methods to a thin-film system. SIMS is sensitive to very low elemental concentration but it is difficult to quantitatively analyze the results due to the dependence of the signal intensity on the elemental mass, the smaller yield of secondary ions and the substrate effect, and the application limit in a rough sample surface. EDS has a poor energy resolution and a poor reliability of elemental concentration. In general, a bombardment with energetic ions should be applied in traditional XPS for the purpose of depth profiling study, but sputtering a thick film would take a long time and might alter the chemical composition via bombardment with energetic ions. Different other techniques and methods are also used to characterize thin-film solar cells. For example, Bär et al.22 used a suitable lift-off technique and XPS to investigate the chemical properties of the CIG(S)Se/Mo interface. However, the lift-off technique could not prevent the possibility of mutual contamination between the CIG(S)Se and Mo side. Powalla et al.51 studied the compositional depth-profiling and grain orientation on a double-graded composition and a Cu-depleted surface CIGSe cells and modules by GDOES and XRD techniques, respectively. SPEM has never been used to explore the depth-dependent electronic and chemical properties of CIGS-based solar cell, which possesses advantages such as great elemental sensitivity and quantitative reliability, compared to other techniques. Contrarily, SPEM can achieve an energy resolution 0.7 eV,



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capability for small (nm) to large (mm) analyzed area, and provide a reliability of 0.1% in quantitative analysis and a spatial resolution about 200 nm due to the sub-micrometre beam size. Utilizing a slow slop of thickness grading sample, SPEM has an opportunity to obtain the high-resolution depth profile of elemental concentration and chemical state of elements. The gradient-etched CIGSe absorber layer by chemical method has been fabricated to study the depth profiling of crystalline orientation and residual stress of the CIGSe and Mo layers by utilizing synchrotron x-ray powder diffraction24. The thickness-gradient sample can provide an opportunity to study the depth-dependent properties of materials with different experimental technologies, such as GIXRD, micro-photoluminescence, micro-Raman spectrum, and Fourier transform infrared spectroscopy (FTIR). However, the realistic depth profile of structural variation in the CIGS cells is still lacking, especially in the pentanary CIGSSe compounds. With SPEM and GIXRD, our results show

four

main

stoichiometries

of

CIGSSe-based

compounds

Cu(In0.42Ga0.58)(S0.91Se0.09)2, Cu(In0.5Ga0.5)(S0.51Se0.49)2, Cu(In0.66Ga0.34)(S0.12Se0.88)2, and Cu(In0.88Ga0.12)(S0.35Se0.65)2 distributed from the bottom to the top of the absorber layer. The Cu(In0.88Ga0.12)(S0.35Se0.65)2 phase appears at the n-/p-type interface; the Ga/(In+Ga) ratio 0.12 can lower the CBM but the ratio S/(S+Se) = 0.35 can raise the CBM and decrease ∆Ec for a n-/p-type to approach the small spike (∆Ec > 0) for the conduction-band offset. A large spike obstructs the generating photoelectrons sweeping from the p-type absorber layer to the n-type buffer layer, whereas the cliff (∆Ec < 0) increases the interface recombination and lowers the open-circuit voltage Voc4,52. The conduction band offsets varied from a flat conduction band alignment to the small spike


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are hence the preferable band structure for the solar-cell performance. Table 1. Photovoltaic parameters of 30×30 cm2 CIGSSe solar cell.

Eff (%)

FF

VOC (mV)

JSC (mA/cm2)

14.04

0.654

620

34.6

RS (ohm-cm2) RSH (ohm-cm2) 2.64

203

The distribution of Ga in the CIGS-based absorber layer is highly related to the conduction-band gradient, thus attracts much attention to optimize the solar-cell performance. On tuning both S and Cu contents in the CIGSSe solar cell, in addition, the profile of the valence-band gradient can also be varied15. Basically, the band gradient has three profiles: normal gradient, reverse gradient, and double gradient11,14-17. The influence of the band gap grading on the performance of CIGS-based solar cell is pronounced. Jackson et al.53 reported that the CIGSe thin-film solar cell of a new record efficiency (21.7%) has a double-graded band gap studied by sputtered neutral mass spectrometry (SNMS): a higher Ga/(Ga+In) ratio at the back side of absorber layer and a lower Ga/(Ga+In) ratio (Eg = 1.13 eV) in the SCR. In our sample, the conduction band exhibits a normal-gradient profile with a linear increase of the band level towards the back contact and the valence band shows a double-gradient profile with an increase of the band level in both front and back sides of absorber (see Figure 7). At the front side of the absorber layer, a wider band gap in the SCR and a lower recombination rate at the p-n junction increase the Voc 16. At the back side, an additional quasi-electric field for the conduction band enlarges the diffusion length of the electrons and decreases the interface recombination rate at the CIGSSe/Mo interface. Moreover, a small gradient of the valence band at the back side of the absorber layer leads to an increased Voc because the



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electrical junction is shifted away from the high-recombination interface between the CdS and CIGS layer17. The additional shifts of VBM, related to the calculated band diagram, in the surface region of the absorber (about 300 nm) are observed, which can be correlated to the ordered defect compound (ODC). The ODC is formed due to the disordered point defect pairs, 2VCu+InCu 48. The point defect pairs can usually be observed in the Cu-poor CIGS absorber materials, forming α phase of Cu(InGa)Se2, β phase of Cu(InGa)3Se5, and γ phase of Cu(InGa)5Se8. The CIGSSe-IV phase with the Cu/(Ga+In) ratio of ~0.89 is slightly Cu-poor and possibly forms α and β phase instead of γ phase. In the surface layers of absorber, a slightly Cu-poor phase has to reconstruct by changing the Cu/(Ga+In) ratio to reduce the surface free energy49, forming the ODC phase. This ODC keeps positive carriers away from the p-n heterojunction and thus prevents the interface recombination to improve the Jsc. Here we utilized the depth profile of the elemental concentration, combined with the structural information to more accurately determine the band diagram, which is highly correlated with the cell performance, of the pentanary Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cells. With the information mentioned above, the optimized profile of the band-gap gradient for improving electrical performance is achievable on modifying the conditions of selenization and sulfurization in the two-stage process. The electrical characteristics of this sample are shown in Table 1. It is noted that the shunt resistance (RSH) has a small value (203 ohm cm2), which is detrimental to the efficiency due to the peeling off effect33,54-55. The peeling off effect would occur while a high residual stress is existed between CIGS and Mo layers33 or the MoSe2 layer is oriented parallel to the Mo layer54. MoSe2 at the CIGSe/Mo interface can ameliorate the


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contact ohmically and create an additional back surface field (BSF) in the conduction band56 that can decrease the recombination current at the CIGS/Mo interface. Hence, it is important to characterize the crystalline qualities of the CIGS/Mo interface. In our case, the peeling off effect is existed at the interface of the absorber layer and the Mo film due to a randomly oriented crystalline structure of the CIGSSe-I phase grown on the Mo layer and a large residual stress at the interface of Mo and CIGSSe thin films (see Figure 6). To diminish the peeling-off effect, decreasing the degree of grain orientation of the Mo layer and increasing the thickness of Mo(S,Se)2-x might help.

5. Conclusion

In this study, we investigated the in-depth electronic and structural properties of CIGSSe-based solar cell on a thickness-gradient sample by using SPEM and GIXRD. The elemental distribution of the absorber layer can be elucidated by the mechanism of the thermodynamic reaction for a two-stage manufacturing process. The structural research exhibits that the CIGSSe layer has four stoichiometries and experiences a tensile stress. A large residual stress has been found at the interface of CIGSSe/Mo, which can deteriorate the electric efficiency due to the peeling-off effect. The calculated band diagram of the ZnO/CdS/CIGSSe/Mo solar cell, corresponding to the measured VBM by SPEM, has been correlated with the electric characteristics of the sample. A lowering of the VBM is observed at the top of the absorber layer, which can be attributed to the existence of ODC in the surface Cu-deficient layers. Author information Corresponding Author:



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Tel: 886-3-5780281 ext. 6416 Fax: 886-3-5783805. E-mail:[email protected]

Acknowledgments The authors would like to thank Dr. Lo-Yueh Chang, Dr. Hung-Wei Shiu, and Mr. Chung-Kai Chang for experimental assistance. This work is supported by the project No. 103CTSMC-S of National Synchrotron Radiation Research Center.


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Table of Contents Graphic


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Depth-Profiling Electronic and Structural Properties of Cu(In,Ga)(S,Se

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