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The effect of Cu concentration in the photovoltaic efficiency of CIGS solar cells prepared by co-evaporation technique
Accepted Manuscript The effect of Cu concentration in the photovoltaic efficiency of CIGS solar cells prepared by co-evaporation technique Hung-Hua Sheu, Yu-Tien Hsu, Shun-Yi Jian, Shih-Chang Liang PII:
S0042-207X(16)30224-X
DOI:
10.1016/j.vacuum.2016.07.008
Reference:
VAC 7070
To appear in:
Vacuum
Received Date: 16 June 2016 Revised Date:
6 July 2016
Accepted Date: 7 July 2016
Please cite this article as: Sheu H-H, Hsu Y-T, Jian S-Y, Liang S-C, The effect of Cu concentration in the photovoltaic efficiency of CIGS solar cells prepared by co-evaporation technique, Vaccum (2016), doi: 10.1016/j.vacuum.2016.07.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The effect of Cu concentration in the photovoltaic efficiency of CIGS solar cells prepared by co-evaporation technique Hung-Hua Sheu1*, Yu-Tien Hsu2, Shun-Yi Jian1, Shih-Chang Liang2
2
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Department of Chemical and Materials Engineering, Chung Cheng Institute Technology, National Defense University, Da-Xi, Tao-Yuan, Taiwan 335, ROC Materials & Electro-Optics Research Division, National Chung-Shan Institute of Science and Technology, Lung-Tan, Tao-yuan, 325, Taiwan, ROC
Corresponding author e-mail: [email protected]
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ABSTRACT
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1
Structural and electrical properties of polycrystalline CIGS thin films have been studied by changing the Cu/(In+Ga) ratio in the films. CIGS thin films with various Cu/(In+Ga) ratios are
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grown over Mo-coated soda-lime glass substrates using a co-evaporation technique. The Raman spectra provide information about the existence of the order defect compound (ODC) at Cu-poor
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compositions (Cu/(In+Ga) ratio < 0.8). The order defect compound (ODC) decreases with an
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increase of Cu concentration and disappears when the Cu/(In+Ga) ratio exceeded 0.8. The energy gap of the CIGS thin films reduce from 1.23 to 1.18 eV with an increase of Cu content. An increased carrier concentration and a decreased carrier mobility and resistivity of CIGS films occur at specimen CIGS-5 due to disappearance of ODC. The open circuit voltage (Voc) also enhances with an increment of Cu/(In+Ga) ratios from 410 to 540 mV. Finally, the experimental results reveal an optimum Cu/(In+Ga) ratio of 0.85 where the solar cell shows the highest efficiency of 10.3% with Jsc, Voc and FF of 28.0 mA cm-2, 520 mV and 0.706, respectively. 1
ACCEPTED MANUSCRIPT Keywords: preferred orientation, CIGS, co-evaporation, absorber layer
1. Introduction
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The performance of thin film CIS-based solar cells has been improved by the addition of Ga to form Cu(In,Ga)Se2 absorption layers with a higher band gap (between 1.02 to 1.68 eV) that
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more closely matches the solar spectrum [1]. Past researches have also pointed out that the increased band gap of CIGS absorption layer will lead to an increase of open circuit voltage (VOC)
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and voltage at maximum power (Vmax) which can reduce the power loss and enhance the photovoltaic efficiency than that of CIS absorption layer [2-4]. Recently, the laboratory efficiencies of CIGS solar cells have reached higher than 20% [5]. In general, the Ga content is usually
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expressed in terms of Ga/(In+Ga) atomic ratio in the CIGS thin film and the best CIGS solar cell is made with CIGS thin films whose Ga/(In+Ga) ratio is about 0.3; the optical property of CIGS solar
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cells will become worse when the Ga/(In+Ga) atomic ratio higher than 0.3 [6,7]. In addition, some
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researches indicate the Cu content within CIGS film also affects the P-V efficiency of CIGS solar cells. Zhang et al. [8] proposed that the formation of order defect compounds (ODC) in low Cu content (Cu/( In +Ga)<0.7) will reduce the electronic property of CIGS solar cells. Witte et al. confirmed the existence of order defect compounds (ODC) at Cu-poor (Cu/( In +Ga)<0.73) compositions using Raman spectra [9]. However, all the above literatures only propose the effect of ODC in the CIGS solar cells, and no investigations related to how the ODC at Cu-poor compositions to affect the efficiency of a real CIGS solar cell devices. Sung et al. [10] studied the 2
ACCEPTED MANUSCRIPT evolution of structure, composition distribution, and there action behavior in each of the three stages of CIGS growth at various Cu/( In +Ga) ratios from 0 to 1.25. Their solar cells presented the worst efficiency about 4.7 % in Cu-poor (Cu/( In +Ga)=0.49) condition due to the formation of impurity
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phases (Cu(In,Ga)3Se5) measured by XRD, better efficiency about 7.37 to 10.97 % occurred at Cu/( In +Ga) ratios from 0.86 to 0.99 were supposed without the effects of impurity phases and
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deep-level defects. The above-mentioned conclusion can be confirmed directly by other method such as Raman spectra. Therefore, in order to complete the study in the effect of ODC phase
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between electrical properties and efficiency of CIGS thin film solar cells, the Cu/( In +Ga) ratios are controlled at the range from 0.7 to 0.94 to avoid the formation of Cu2-xSe phase in this work and attempt to investigate the relationship between Cu content and ODC phase by Raman spectra and to
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2. Experimental
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CIGS thin films solar cells.
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directly confirm the effect of ODC phase on the electrical properties and the device performances of
The Mo back contact with a bi-layer structure and a thickness about 1-1.2 µm was deposited on a soda-lime glass substrate by dc magnetron sputtering. The CIGS absorber layer was grown by a two-stage process involving the co-evaporation of Cu, In, Ga and Se. In first stage, an (In0.7Ga0.3)2Se3 layer was grown by co-evaporation In, Ga and Se elements on Mo/glass substrates at 350 ℃. In second stage, a CIGS layer was formed by evaporating Cu and Se on the (In,Ga)2Se3 layer at 550 ℃ . The compositions of CIGS films were controlled at Cu-poor range, i.e., 3
ACCEPTED MANUSCRIPT Cu/(In+Ga)<1. To investigate the effects of Cu/(In+Ga) ratio, Cu and In fluxes were regulated by the temperature of effusion cells, while Ga/(In+Ga) ratio was kept 0.31±0.01 for all specimens. The fluxes were measured by the quartz crystal microbalances and kept constant during the deposition.
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The Cu/(In +Ga) ratio and Se/(Cu+In+Ga) was controlled in the range of 0.7-0.94 and 1.12-1.20, respectively (Table 1). The solar cells were finished by growing a chemical bath deposited CdS
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buffer layer and a radio-frequency sputtered i-ZnO/ZnO:Al top window layer. Finally, Al electrodes were deposited by thermal evaporation. No anti-reflection coating is deposited for these solar cells.
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The crystal structure of the CIGS layer was characterized by X-ray diffraction (XRD) using a Bruker D8 diffractometer. The wavelength of the incident radiation was λ= 1.5406 Å (Cu Kα). The ordering defects compound (ODC) within CIGS films was characterized using a Raman
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spectrometer. The optical transmittance and reflectance were measured by a HITACHI U-3310 spectrophotometer. External quantum efficiency (EQE) measurements were performed on the solar
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cells as well as current density-voltage (J-V) measurements under simulated AM 1.5 illumination at
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25℃ and with an intensity of 1000 W∙m-2. The J-V measurements were performed over the full area of the device and the EQE measurement area was a spot with a surface area of 4 mm2.
3. Results and discussion
3.1 XRD analysis of absorber layers In this study, CIGS absorber layer was grown by a two-stage process involving the co-evaporation of Cu, In, Ga and Se. In first stage, an (In0.7Ga0.3)2Se3 layer was grown by 4
ACCEPTED MANUSCRIPT co-evaporation In, Ga and Se elements on Mo/glass substrates at 350 ℃. In second stage, a CIGS layer was formed by evaporating Cu and Se on the (In,Ga)2Se3 layer at 550 ℃. Fig. 1 presents the XRD pattern of CIGS films, the preferred orientation (220/204) is strengthened than the orientation
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(112), the peak ratios of I(112)/I(220)/(204) is quite different than previous studies [11-15]. Contreras et al. [16] indicated that preferred orientation of CIS absorber layer will dominate by substrates
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temperature and Cu content, the substrates temperature lower than 500℃ the intensity of preferred orientation (220/204) will higher than the intensity of preferred orientation (112). Liu et al. [17]
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pointed out that the lower Cu/(Ga+In) ratio decreases the peak ratios of I(112)/I(220)/(204). Therefore, our experimental results present the intensity of preferred orientation (220/204) is higher than preferred orientation (112) due to lower substrates temperature (at 350 ℃) and Cu/(Ga+In)
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ratio. 3.2 Raman analysis of absorber layer
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Fig. 2 shows the Raman spectra of various Cu content within CIGS absorber layers, it reveals
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the frequency is around 175 to 177 cm-1 (Table 2) which corresponding the frequency of CuIn0.7Ga0.3Se2 phase occurred around 176 to 178 cm-1 [9]. The Raman spectra of CIGS-1, CIGS-2 and CIGS-3 all present a mode around 156 cm-1 to 158 cm-1 (Table 2) should be the ODC phase such as Cu(In,Ga)3Se5 or Cu2(In,Ga)4Se7 [18,19]. In general, the Cu2-xSe phase would be molten within a Cu-rich CIGS absorber layer and cause the surface of CIGS absorber layer denser and smoother when the substrate temperature higher than 523
[20-22]. Therefore, all specimens in this
study are controlled at a Cu-poor condition (Cu/(Ga+In)<0.95) to avoid the formation of Cu2-xSe 5
ACCEPTED MANUSCRIPT phase. The results of Raman spectra show that the peak corresponding Cu2-xSe phase occurred from 260 cm-1 to 270 cm-1 does not present in this work. The Raman spectra also present a broadening peak of order defects compound (ODC) occurred
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around 156-158 cm-1 and it became weak with an increase of Cu/(Ga+In) ratio (specimens No. CIGS-1 to CIGS-3). On the other hand, the binding energy of CIGS layer became more strength
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because of the Raman spectra shifted to shorter wavelength from 177 cm-1 to 175 cm-1. In generally, the diffused sodium atoms within CIGS layer will aggregate near grain boundaries, some sodium
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atoms substitute the position of Cu atoms and form a stable NaInSe2 compound [23]. The sodium atoms trend to form a selenium compound with selenium element within CIGS absorber layer [24-26]. Therefore, in specimens CIGS-4 and CIGS-5, the stable sodium-selenium compound
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decrease with an increase of Cu content, the Raman spectra would shift from 175 to 177 cm-1 again. The Raman spectra reveals that the crystalline structure of Cu(In,Ga)Se2 phase becomes better with
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an increase of Cu ratios, in specimens No. CIGS-1 to CIGS-3, the formation of ODC phase such as
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Cu(In,Ga)3Se5 and Cu2(In,Ga)4Se7 would control the binding energy of Cu(In,Ga)Se2 phase, an increase of ODC would reduce the binding energy of Cu(In,Ga)Se2 compound [27,28]. Fig. 3 presents that the FWHM (full width at half maximum) of the Raman spectra peak of Cu(In,Ga)Se2 films decreases with increasing Cu concentration from 20.6 to 11 cm-1. Previous studies have indicated the ODC structure, the formation of metastable domains with a different crystallographic order, will decompose the crystallinity of materials and lead a broad Raman spectra peak [9,16]. Therefore, the decreases of FWHM of Raman spectra peak with increasing Cu content 6
ACCEPTED MANUSCRIPT in this work can be attributed to the decrease of ODC and improve the crystallinity of CIGS layer. In order to distinguish the surface of CIGS films whether being contaminated by organics, the XPS depth profile measurement was carried out with a depth about 500Å from CIGS surface. In
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general, the binding energy peak of O1s and C1s which occurred at 531 and 285 eV will obviously appear in the full spectra when the surface of CIGS films is contaminated. The presence of C1s may
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be due to ambient contamination and the oxygen peak (O1s) is probably due to oxide formation on the surface [29]. In this study, the characteristic binding energy peak of O1s and C1s do not appear
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and indicate our CIGS films are not contaminated by organics during the co-evaporation process (Fig. 4). Fig. 5 and Table3 also present clearly the binding energy peaks of Cu2p3/2, In3d5/2, Ga2p3/2 and Se3d5/2 which binding energy occurred at about 931.6, 443.5, 1116.7 and 53 eV, respectively. In
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Cu-poor condition such as CIGS-1, all the binding energy peaks of Cu2p3/2, In3d5/2, Ga2p3/2 and Se3d5/2 shift to the lower binding energy side due to the formation of ODC.
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Fig. 5 presents the binding energy of Cu, In, Ga and Se increases with an increase of Cu ratios
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in the specimens No. CIGS-1 to CIGS-4, and then decreases in the specimen No. CIGS-5. The XPS results indicate the absorber layer has formed a Cu(In,Ga)Se2 compound and there are also some residual pure selenium within CIGS absorber layer (Fig. 6).
3.3 Optical and electrical properties of CIGS absorber layers
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ACCEPTED MANUSCRIPT The optical band-gap (Eg) is studied using the optical data, including transmittance and reflectance spectra acquired from various Cu content absorption layers. The optical absorption coefficient (α) has been calculated by using the following equation [30]:
(1− R)
4
+ 4T
2
2T
R + (1− R) ] 2
2
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1 α= [ d
where d is the film thickness, R is the reflectance, and T is the transmittance. In order to calculate
ଵൗ ଶ
can be used, where, Aα is a constant that depends on the transition
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equation αhυ= ܣఈ ሺℎ߭ − ݃ܧሻ
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the optical band gap (Eg) of direct gap chalcopyrite compounds semiconductor, the following
nature, the effective mass and the reflective index and hυ is the incident photon energy. The band gap Eg is determined by extrapolating the slope of the (αhν)2 versus photon energy curve to the
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abscissa; for all absorption layers as shown in the Fig. 7. The band gaps for CIGS-1 to CIGS-5 are determined as 1.212, 1.189, 1.183, 1.165 and 1.163 eV, respectively. The band gap energies of CIGS absorption layers increase with an increase of Cu content. The past studies have indicated that
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the different band gap energies is attributed the different Ga/(Ga+In) ratios [31], but some
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literatures presented the band gap energies changed with the ratios of Cu/(Ga+In) [32-35]. In this study, the band gap energies decrease with an increase of Cu/(Ga+In) ratios. The formation of ODC phase such as CuIn3Se5 should play a dominate role in increasing band gap energy due to its band gap energy is about 1.3 eV [36], in general, the band gap energy of ODC phase is higher than Cu(In,Ga)Se2 phase about 0.2 to 0.3 eV [32]. On the other hand, the ratio of ODC phase in lower
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ACCEPTED MANUSCRIPT Cu content (specimen No. CIGS-1 to CIGS-3) is higher than that in Cu-richer condition and leads to the integrated band gap energy become larger (specimen No. CIGS-4 and CIGS-5). Fig. 8 shows the carrier concentration (n), resistivity (ρ) and Hall mobility (µ) of the CIGS thin
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films as a function of Cu/(Ga+In) ratios. As seen in Fig. 8 and Table 5, the carrier concentration increases from the order of 1014 to 1016 cm-3 and the Hall mobility (µ) decreases from 5.2 to 3.1 cm2
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V-1 s-1 as the Cu/(Ga+In) ratio increases from 0.7 to 0.94. In specimens CIGS-1 to CIGS-4, the carrier concentration are distributed from 3.4×1014 to 8.4×1014, the carrier concentration of CIGS-5
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significantly enhance to 2.1×1016 and the resistivity drop down to only 0.1 kΩ‧cm. The relationship between carrier concentration, resistivity and Cu content would be dominated by ordering defect compound (ODC) such as Cu(In,Ga)3Se5 and Cu2(In,Ga)4Se7 which leading to a lower carrier
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concentration and a resistivity in the CIGS solar cells at Cu-poor condition [37]. Therefore, the specimen CIGS-5 the highest Cu content without formation of ordering defect compound (ODC)
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(Fig. 2) presents a higher carrier concentration and a lower resistivity.
3.4 Photovoltaic properties of CIGS solar cells Fig. 9 presents the J-V curves of CIGS solar cells formed in various Cu/(Ga+In) ratios. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), conversion efficiency (η), series resistance (Rs), and shunt resistance (Rsh) are summarized in Table 6. Generally, the efficiency of solar cells is reduced by the dissipation of power through internal resistance. The parasitic resistance can be modeled as a shunt resistance (Rsh) and a series resistance (Rs), the shunt 9
ACCEPTED MANUSCRIPT resistance is due to fabrication defects, photo-generated current will find alternative paths rather than flowing through the cell’s junction. The shunt resistance will lower the open circuit voltage without changing the short circuit current. An increasing Rs will reduce the fill factor (FF) and
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photovoltaic conversion efficiency of solar cells such as the previous research studied CdS/CdTe thin film solar cells [38] and the device physics of CdS/CdTe ultrathin film solar cells is also
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modeled using “Sah–Noyce–Shockley” (SNS) theory [39]. In this study, the characterization parameters of CIGS solar cells are listed in Table 6, the lower Rs, the higher FF and the higher short
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circuit current density will lead to a better efficiency. Specimen No. CIGS-4 has the best conversion efficiency about 10.3 % with an approximate open-circuit voltage (Voc) of 520 mV, Jsc of 28.0 mA/cm2, fill factor (FF) of 70.6%, Rs of 12.0 Ω, and Rsh of 1141 Ω, respectively.
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Fig. 10 reveals the external quantum efficiency (EQE) of five CIGS solar cells, revealing a shift in the long-wavelength region from 400 to 1100 nm. The optoelectronic properties occur
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between 400 to 540 nm should be the characteristic of CdS buffer layer deposited by chemical bath
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deposition (CBD). The various CIGS solar cells present a slight change in EQE curves, indicating the quality of CdS buffer layer is quite stable in this study. Moreover, the optoelectronic properties occur between 540 to 1100 nm should be the characteristic of CIGS absorption layer, here we only discuss the wavelength region of visible spectrum from 540 to 700 nm. The EQE of CIGS solar cells (CIGS-1 to CIGS-5) prepared from different Cu/(In+Ga) ratios is about 71, 74, 77, 84 and 82 %, respectively. The CIGS-4 has the highest EQE about 84%, indicating reasonably good carrier generation and collection in the p-n junction. 10
ACCEPTED MANUSCRIPT Compared to the EQE values at 70% of five CIGS solar cells (CIGS-1 to CIGS-5) the CIGS-4 shows the widest absorption wavelength range from 530 to 910 nm; the CIGS-1 shows the narrowest absorption wavelength range from 550 to 590 nm and 630 to 740 nm. This also indicates
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the CIGS-4 has the best absorption efficiency from the visible and infrared light spectrum; the CIGS-1 has the worst absorption efficiency and only occurs at the range of visible light spectrum.
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All the five solar cells exhibit an absorption edge around 510 nm as same as the study presented by Liu et al. [17], and the absorption edge around 510 nm corresponding to the CdS band gap 2.4 eV.
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The EQE is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell from outside (incident photons); therefore it depends on both the absorption of light and the collection of charges. Once a photon has been
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absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. Generally, the EQE response of the solar cell can be attributed to the following three
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factors: absorption coefficient of the active layer, the surface recombination velocity and minority
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carrier lifetime [40]. In deeper depth within CIGS layer, the formations of electron hole pairs will far away the p-n junction with an increase of wavelength of incident light. The probability of the combination between electron hole will increase during the electron hole pairs diffuse to the p-n junction.
The CIGS-1 has the highest probability at the carrier recombination losses; the CIGS-4 and CIGS-5 have lower probability at the carrier recombination losses. The CIGS-4 has the best photovoltaic efficiency due to the larger grain size, and the less grain boundaries will reduce the 11
ACCEPTED MANUSCRIPT probability of carrier recombination.
4. Conclusions
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CIGS absorber layers with different Cu/(In+Ga) ratios were carried out using two-stage growth process. The ordering defects compound (ODC) within CIGS layers were characterized by Ramen
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spectra. Ramen spectra on Cu-poor (Cu/(In+Ga)<0.8) presented a broad peak at around 156-158 cm-1 in the Ramen signals due to the formation of the ODC phase. The ODC phase decreased with
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an increase of Cu/(In+Ga) ratio in specimens CIGS-1 to CIGS-3. In the electrical property, the formation of ODC phases leading to a higher resistivity, lower carrier concentration and higher energy band gap in CIGS thin films solar cells. Moreover, the ODC phase also reduced the
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conversion efficiency of CIGS solar cells. In higher Cu content specimens such as CIGS-4 and CIGS-5 have better conversion efficiency due to the disappearance of ODC. In different Cu/(Ga+In)
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ratios conditions, the best conversion efficiency of CIGS thin films solar cell was observed as
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10.3%, and the Voc, Jsc, and FF of CIGS solar cell were measured to be 520 mV, 28.0 mA/cm2, and 70.6%, respectively.
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ACCEPTED MANUSCRIPT Low-temperature growth of Na doped CIGS films on flexible polymer substrates by pulsed laser ablation from a Na containing target, Surface and Coating Technology, 231 (2013) 209-213. [31]T.F. Ciszek, R. Bacewicz, J.R. Durrant, Crystal growth and photoelectrical properties of
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(Cux,Ag1-x)InSe2 and Cu(Iny,Ga1-y)Se2 solid solutions, 19th IEEE PVSC, New York, USA (1987) 1448-1453.
[32]B.J. Stanbery, Copper Indium Selenides and related materials for photovoltaic devices, Critical
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Reviews in Solid State and Material Sciences, 27 (2002) 73-117.
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[33]H.C. Lai, K.W. Cheng, The preparation of copper indium gallium diselenide (CuInxGa1-xSe2) photo absorber layers using co-evaporation for photovoltaic application, Master Dissertation (2011) 44-46.
[34]Y.D. Chung, Incorporation of Cu in Cu(In,Ga)Se2-based thin-film solar cells, Journal of the Korean Physical Society, 57 (2010) 1826-1830.
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[35]C. Guillen, J. Herrero, CuInS2 and CuGaS2 thin films grown by modulated flux deposition with various Cu contents, Physica Status Solidi (A) Applications and Materials, 203 (2006)
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2438-2443.
[36]D. Schmid, M. Ruckh, F. Grunwald, H.W. Schock, Chalcopyrite/defect chalcopyrite
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heterojunctions on the basis of CuInSe2, Journal of Applied Physics, 73 (1993) 2902. [37]T. Negami, N. Kohara, M. Nishitani, and T. Wada, Preparation of ordered vacancy chalcopyrite-type CuIn3Se5 thin films, Japanese Journal of Applied Physics, 33 (1994) L1251– L1253. [38] N.E. Gorji, Deposition and doping of CdS/CdTe thin film solar cells, Journal of Semiconductors, 36 (2015) 054001. 16
ACCEPTED MANUSCRIPT [39] L. Kuhn, U. Reggiani, L. Sandrolini, N.E. Gorji, Physical device modeling of CdTe ultrathin film solar cells, Solar Energy, 132 (2016) 165-172. [40] M.C. Tseng, R.H. Horng, F.L. Wu, S.N. Lin, H.H. Yu, D.S. Wuu, Crystalline quality and
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photovoltaic performance of InGaAs solar cells grown on GaAs substrate with
20
30
40
SC (211) (316/332)
(312/116)
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Intensity(a.u.)
∆
(220/204)
(110)
(112)
large-misoriented angle, Vacuum 86 (2012) 843-847.
50
∆
C IG S ∆
Mo
C IG S-1 C IG S-2 C IG S-3 C IG S-4 C IG S-5
60
70
80
90
2 T heta (D eg.)
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Fig. 1. The XRD pattern of different Cu content of CIGS absorber layers using co-evaporation process.
17
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temperature.
25
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Fig. 2 Raman spectra of CIGS absorbers with different Cu/(In+Ga) ratios recorded at room
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20
17.7 15.7
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-1
FWHM (cm )
20.6
15
13 11
10
5 CIGS-1
CIGS-2
CIGS-3
CIGS-4
CIGS-5
Fig. 3. FWHM of Ramen spectra measured at various CIGS absorption layers.
18
ACCEPTED MANUSCRIPT In 3d 5/2 Cu 2p 3/2
O 1s
C 1s
CIGS-1
Intensity (a.u.)
Ga 2p 3/2
Se 3d 5/2
CIGS-2 CIGS-3
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CIGS-4 CIGS-5
1000
800
600
400
200
0
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1200
Binding energy (eV)
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Fig. 4. The full XPS spectrum analyzed from different Cu content CIGS solar cells with a depth at 500Å in CIGS absorption layers.
CIGS-1 CIGS-2 CIGS-3 CIGS-4 CIGS-5
933
932
931
CIGS-1 CIGS-2 CIGS-3 CIGS-4 CIGS-5
930
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934
Intensity (a.u.)
In 3d5/2
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Intensity (a.u.)
Cu 2p3/2
Binding energy (eV)
445
Ga 2p3/2
443
442
Se 3d5/2
Intensity (a.u.)
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444
Binding energy (eV)
CIGS-1
CIGS-2
CIGS-3 CIGS-4
CIGS-1 CIGS-2 CIGS-3 CIGS-4
CIGS-5 CIGS-5
1119
1118
1117
1116
1115
Binding energy (eV)
Fig. 5.
56
55
54
53
52
51
Binding energy (eV)
The Cu2p3/2, In3d5/2, Ga2p3/2 and Se3d5/2 spectrum analyzed from different Cu content CIGS solar cells with a depth about 500Å in CIGS absorption layers. 19
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C IG S-1 C IG S-2
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Intensity (a.u.)
Cu(In,G a)Se 2
C IG S -1 C IG S -2 C IG S -3 C IG S -4 C IG S -5
Se
C IG S-3 C IG S-4
55.0
54.5
54.0
53.5
53.0
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C IG S-5
52.5
52.0
51.5
B inding E nergy (eV )
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Fig. 6. The Se3d core level analyzed from different Cu content CIGS solar cells.
Fig. 7. Optical band-gap of different Cu/(In+Ga) ratios of CIGS absorption layers.
20
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10 16
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n 15
µ 14
CIGS-1
CIGS-2
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6
10
10
2 Hall mobirity (cm /V.s)
8
CIGS-3
CIGS-4
4
k
ρ
Resistivity ( Ω•cm)
10
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-3
Carrier concentration (cm )
4
2
0
CIGS-5
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Fig. 8. The curves of measured carrier concentration, Hall mobility and resistivity from different Cu content CIGS solar cells
21
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Fig. 9. Illuminated J-V curves of CIGS solar cells formed with various Cu contents.
Fig. 10 External quantum efficiency curves of CIGS thin films solar cells fabricated with different Cu/(Ga+In) ratios of CIGS-1, CIGS-2, CIGS-3, CIGS-4 and CIGS-5. Table 1 The operated parameters of CIGS absorption layer 22
ACCEPTED MANUSCRIPT Sample Cu/ Ga/ Se/ Thickness No. (In+Ga) (In+Ga) (Cu+In+Ga) (µm) 0.7
0.32
1.20
1.34
CIGS-2
0.75
0.31
1.19
1.41
CIGS-3
0.8
0.31
1.16
1.40
CIGS-4
0.85
0.30
1.16
1.47
CIGS-5
0.94
0.31
1.12
1.43
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Table 2 The Raman shift values of Cu(In,Ga)Se2 and ODC phase
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CIGS-1
CIGS-1
CIGS-2
CIGS-3
CIGS-4
Cu(In,Ga)Se2
177 cm-1
177 cm-1
175 cm-1
175 cm-1
ODC
156 cm-1
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Sample No.
158 cm-1
158 cm-1
Table 3. The shifted binding energy peaks of Cu2p3/2, In3d5/2, Ga2p3/2 and Se3d5/2
CIGS-1 CIGS-2
In3d5/2
Ga2p3/2
Se3d5/2
931.5 931.65
443.4 443.65
1116.65 1116.7
52.9 53
931.7 931.7
443.7 443.7
1116.9 1116.9
53.2 53
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CIGS-3 CIGS-4
Cu2p3/2
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Sample No.
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Table 4. The binding energy of selenium compounds measured at different Cu content absorption layers Sample No. CIGS-1 CIGS-2 CIGS-3 CIGS-4 Cu(In,Ga)Se2 53.5 53.7 53.8 53.8 Se 52.8 53.0 53.0 53.0
23
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Table 5. The measured carrier concentration, Hall mobility and resistivity from different Cu content CIGS solar cells Hall mobility Resistivity Sample Carrier concentration 2 -3 (cm /V‧s) (kΩ‧cm) No. (cm ) 3.6×1014
5.2
3.4
CIGS-2
5.0×1014
4.9
2.8
CIGS-3
7.1×1014
4.6
2.1
CIGS-4
8.4×1014
4.3
1.9
CIGS-5
2.1×1016
3.1
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CIGS-1
0.1
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Table 6 Characterization parameters retrieved from the measurements Voc (mV)
Jsc (mA/cm2)
FF (%)
η (%)
Rs(Ω)
Rsh (Ω)
CIGS-1
410
23.2
55.8
5.3
21.8
608
CIGS-2
510
23.6
50.3
6.1
34.0
833
CIGS-3
520
25.3
54.6
7.2
28.6
1039
CIGS-4
520
28.0
70.6
10.3
12.0
1141
CIGS-5
540
27.6
65.1
9.7
16.4
2177
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Sample No.
24
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Highlights 1. ODC phase within CIGS layers was studied by Raman spectra.
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2. Band gap of CIGS layers are affected by Cu content. 3. Effect of ODC phase on electrical property of CIGS layers is studied.
S0042-207X(16)30224-X
DOI:
10.1016/j.vacuum.2016.07.008
Reference:
VAC 7070
To appear in:
Vacuum
Received Date: 16 June 2016 Revised Date:
6 July 2016
Accepted Date: 7 July 2016
Please cite this article as: Sheu H-H, Hsu Y-T, Jian S-Y, Liang S-C, The effect of Cu concentration in the photovoltaic efficiency of CIGS solar cells prepared by co-evaporation technique, Vaccum (2016), doi: 10.1016/j.vacuum.2016.07.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The effect of Cu concentration in the photovoltaic efficiency of CIGS solar cells prepared by co-evaporation technique Hung-Hua Sheu1*, Yu-Tien Hsu2, Shun-Yi Jian1, Shih-Chang Liang2
2
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Department of Chemical and Materials Engineering, Chung Cheng Institute Technology, National Defense University, Da-Xi, Tao-Yuan, Taiwan 335, ROC Materials & Electro-Optics Research Division, National Chung-Shan Institute of Science and Technology, Lung-Tan, Tao-yuan, 325, Taiwan, ROC
Corresponding author e-mail: [email protected]
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ABSTRACT
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1
Structural and electrical properties of polycrystalline CIGS thin films have been studied by changing the Cu/(In+Ga) ratio in the films. CIGS thin films with various Cu/(In+Ga) ratios are
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grown over Mo-coated soda-lime glass substrates using a co-evaporation technique. The Raman spectra provide information about the existence of the order defect compound (ODC) at Cu-poor
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compositions (Cu/(In+Ga) ratio < 0.8). The order defect compound (ODC) decreases with an
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increase of Cu concentration and disappears when the Cu/(In+Ga) ratio exceeded 0.8. The energy gap of the CIGS thin films reduce from 1.23 to 1.18 eV with an increase of Cu content. An increased carrier concentration and a decreased carrier mobility and resistivity of CIGS films occur at specimen CIGS-5 due to disappearance of ODC. The open circuit voltage (Voc) also enhances with an increment of Cu/(In+Ga) ratios from 410 to 540 mV. Finally, the experimental results reveal an optimum Cu/(In+Ga) ratio of 0.85 where the solar cell shows the highest efficiency of 10.3% with Jsc, Voc and FF of 28.0 mA cm-2, 520 mV and 0.706, respectively. 1
ACCEPTED MANUSCRIPT Keywords: preferred orientation, CIGS, co-evaporation, absorber layer
1. Introduction
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The performance of thin film CIS-based solar cells has been improved by the addition of Ga to form Cu(In,Ga)Se2 absorption layers with a higher band gap (between 1.02 to 1.68 eV) that
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more closely matches the solar spectrum [1]. Past researches have also pointed out that the increased band gap of CIGS absorption layer will lead to an increase of open circuit voltage (VOC)
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and voltage at maximum power (Vmax) which can reduce the power loss and enhance the photovoltaic efficiency than that of CIS absorption layer [2-4]. Recently, the laboratory efficiencies of CIGS solar cells have reached higher than 20% [5]. In general, the Ga content is usually
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expressed in terms of Ga/(In+Ga) atomic ratio in the CIGS thin film and the best CIGS solar cell is made with CIGS thin films whose Ga/(In+Ga) ratio is about 0.3; the optical property of CIGS solar
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cells will become worse when the Ga/(In+Ga) atomic ratio higher than 0.3 [6,7]. In addition, some
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researches indicate the Cu content within CIGS film also affects the P-V efficiency of CIGS solar cells. Zhang et al. [8] proposed that the formation of order defect compounds (ODC) in low Cu content (Cu/( In +Ga)<0.7) will reduce the electronic property of CIGS solar cells. Witte et al. confirmed the existence of order defect compounds (ODC) at Cu-poor (Cu/( In +Ga)<0.73) compositions using Raman spectra [9]. However, all the above literatures only propose the effect of ODC in the CIGS solar cells, and no investigations related to how the ODC at Cu-poor compositions to affect the efficiency of a real CIGS solar cell devices. Sung et al. [10] studied the 2
ACCEPTED MANUSCRIPT evolution of structure, composition distribution, and there action behavior in each of the three stages of CIGS growth at various Cu/( In +Ga) ratios from 0 to 1.25. Their solar cells presented the worst efficiency about 4.7 % in Cu-poor (Cu/( In +Ga)=0.49) condition due to the formation of impurity
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phases (Cu(In,Ga)3Se5) measured by XRD, better efficiency about 7.37 to 10.97 % occurred at Cu/( In +Ga) ratios from 0.86 to 0.99 were supposed without the effects of impurity phases and
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deep-level defects. The above-mentioned conclusion can be confirmed directly by other method such as Raman spectra. Therefore, in order to complete the study in the effect of ODC phase
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between electrical properties and efficiency of CIGS thin film solar cells, the Cu/( In +Ga) ratios are controlled at the range from 0.7 to 0.94 to avoid the formation of Cu2-xSe phase in this work and attempt to investigate the relationship between Cu content and ODC phase by Raman spectra and to
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2. Experimental
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CIGS thin films solar cells.
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directly confirm the effect of ODC phase on the electrical properties and the device performances of
The Mo back contact with a bi-layer structure and a thickness about 1-1.2 µm was deposited on a soda-lime glass substrate by dc magnetron sputtering. The CIGS absorber layer was grown by a two-stage process involving the co-evaporation of Cu, In, Ga and Se. In first stage, an (In0.7Ga0.3)2Se3 layer was grown by co-evaporation In, Ga and Se elements on Mo/glass substrates at 350 ℃. In second stage, a CIGS layer was formed by evaporating Cu and Se on the (In,Ga)2Se3 layer at 550 ℃ . The compositions of CIGS films were controlled at Cu-poor range, i.e., 3
ACCEPTED MANUSCRIPT Cu/(In+Ga)<1. To investigate the effects of Cu/(In+Ga) ratio, Cu and In fluxes were regulated by the temperature of effusion cells, while Ga/(In+Ga) ratio was kept 0.31±0.01 for all specimens. The fluxes were measured by the quartz crystal microbalances and kept constant during the deposition.
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The Cu/(In +Ga) ratio and Se/(Cu+In+Ga) was controlled in the range of 0.7-0.94 and 1.12-1.20, respectively (Table 1). The solar cells were finished by growing a chemical bath deposited CdS
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buffer layer and a radio-frequency sputtered i-ZnO/ZnO:Al top window layer. Finally, Al electrodes were deposited by thermal evaporation. No anti-reflection coating is deposited for these solar cells.
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The crystal structure of the CIGS layer was characterized by X-ray diffraction (XRD) using a Bruker D8 diffractometer. The wavelength of the incident radiation was λ= 1.5406 Å (Cu Kα). The ordering defects compound (ODC) within CIGS films was characterized using a Raman
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spectrometer. The optical transmittance and reflectance were measured by a HITACHI U-3310 spectrophotometer. External quantum efficiency (EQE) measurements were performed on the solar
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cells as well as current density-voltage (J-V) measurements under simulated AM 1.5 illumination at
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25℃ and with an intensity of 1000 W∙m-2. The J-V measurements were performed over the full area of the device and the EQE measurement area was a spot with a surface area of 4 mm2.
3. Results and discussion
3.1 XRD analysis of absorber layers In this study, CIGS absorber layer was grown by a two-stage process involving the co-evaporation of Cu, In, Ga and Se. In first stage, an (In0.7Ga0.3)2Se3 layer was grown by 4
ACCEPTED MANUSCRIPT co-evaporation In, Ga and Se elements on Mo/glass substrates at 350 ℃. In second stage, a CIGS layer was formed by evaporating Cu and Se on the (In,Ga)2Se3 layer at 550 ℃. Fig. 1 presents the XRD pattern of CIGS films, the preferred orientation (220/204) is strengthened than the orientation
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(112), the peak ratios of I(112)/I(220)/(204) is quite different than previous studies [11-15]. Contreras et al. [16] indicated that preferred orientation of CIS absorber layer will dominate by substrates
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temperature and Cu content, the substrates temperature lower than 500℃ the intensity of preferred orientation (220/204) will higher than the intensity of preferred orientation (112). Liu et al. [17]
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pointed out that the lower Cu/(Ga+In) ratio decreases the peak ratios of I(112)/I(220)/(204). Therefore, our experimental results present the intensity of preferred orientation (220/204) is higher than preferred orientation (112) due to lower substrates temperature (at 350 ℃) and Cu/(Ga+In)
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ratio. 3.2 Raman analysis of absorber layer
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Fig. 2 shows the Raman spectra of various Cu content within CIGS absorber layers, it reveals
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the frequency is around 175 to 177 cm-1 (Table 2) which corresponding the frequency of CuIn0.7Ga0.3Se2 phase occurred around 176 to 178 cm-1 [9]. The Raman spectra of CIGS-1, CIGS-2 and CIGS-3 all present a mode around 156 cm-1 to 158 cm-1 (Table 2) should be the ODC phase such as Cu(In,Ga)3Se5 or Cu2(In,Ga)4Se7 [18,19]. In general, the Cu2-xSe phase would be molten within a Cu-rich CIGS absorber layer and cause the surface of CIGS absorber layer denser and smoother when the substrate temperature higher than 523
[20-22]. Therefore, all specimens in this
study are controlled at a Cu-poor condition (Cu/(Ga+In)<0.95) to avoid the formation of Cu2-xSe 5
ACCEPTED MANUSCRIPT phase. The results of Raman spectra show that the peak corresponding Cu2-xSe phase occurred from 260 cm-1 to 270 cm-1 does not present in this work. The Raman spectra also present a broadening peak of order defects compound (ODC) occurred
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around 156-158 cm-1 and it became weak with an increase of Cu/(Ga+In) ratio (specimens No. CIGS-1 to CIGS-3). On the other hand, the binding energy of CIGS layer became more strength
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because of the Raman spectra shifted to shorter wavelength from 177 cm-1 to 175 cm-1. In generally, the diffused sodium atoms within CIGS layer will aggregate near grain boundaries, some sodium
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atoms substitute the position of Cu atoms and form a stable NaInSe2 compound [23]. The sodium atoms trend to form a selenium compound with selenium element within CIGS absorber layer [24-26]. Therefore, in specimens CIGS-4 and CIGS-5, the stable sodium-selenium compound
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decrease with an increase of Cu content, the Raman spectra would shift from 175 to 177 cm-1 again. The Raman spectra reveals that the crystalline structure of Cu(In,Ga)Se2 phase becomes better with
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an increase of Cu ratios, in specimens No. CIGS-1 to CIGS-3, the formation of ODC phase such as
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Cu(In,Ga)3Se5 and Cu2(In,Ga)4Se7 would control the binding energy of Cu(In,Ga)Se2 phase, an increase of ODC would reduce the binding energy of Cu(In,Ga)Se2 compound [27,28]. Fig. 3 presents that the FWHM (full width at half maximum) of the Raman spectra peak of Cu(In,Ga)Se2 films decreases with increasing Cu concentration from 20.6 to 11 cm-1. Previous studies have indicated the ODC structure, the formation of metastable domains with a different crystallographic order, will decompose the crystallinity of materials and lead a broad Raman spectra peak [9,16]. Therefore, the decreases of FWHM of Raman spectra peak with increasing Cu content 6
ACCEPTED MANUSCRIPT in this work can be attributed to the decrease of ODC and improve the crystallinity of CIGS layer. In order to distinguish the surface of CIGS films whether being contaminated by organics, the XPS depth profile measurement was carried out with a depth about 500Å from CIGS surface. In
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general, the binding energy peak of O1s and C1s which occurred at 531 and 285 eV will obviously appear in the full spectra when the surface of CIGS films is contaminated. The presence of C1s may
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be due to ambient contamination and the oxygen peak (O1s) is probably due to oxide formation on the surface [29]. In this study, the characteristic binding energy peak of O1s and C1s do not appear
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and indicate our CIGS films are not contaminated by organics during the co-evaporation process (Fig. 4). Fig. 5 and Table3 also present clearly the binding energy peaks of Cu2p3/2, In3d5/2, Ga2p3/2 and Se3d5/2 which binding energy occurred at about 931.6, 443.5, 1116.7 and 53 eV, respectively. In
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Cu-poor condition such as CIGS-1, all the binding energy peaks of Cu2p3/2, In3d5/2, Ga2p3/2 and Se3d5/2 shift to the lower binding energy side due to the formation of ODC.
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Fig. 5 presents the binding energy of Cu, In, Ga and Se increases with an increase of Cu ratios
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in the specimens No. CIGS-1 to CIGS-4, and then decreases in the specimen No. CIGS-5. The XPS results indicate the absorber layer has formed a Cu(In,Ga)Se2 compound and there are also some residual pure selenium within CIGS absorber layer (Fig. 6).
3.3 Optical and electrical properties of CIGS absorber layers
7
ACCEPTED MANUSCRIPT The optical band-gap (Eg) is studied using the optical data, including transmittance and reflectance spectra acquired from various Cu content absorption layers. The optical absorption coefficient (α) has been calculated by using the following equation [30]:
(1− R)
4
+ 4T
2
2T
R + (1− R) ] 2
2
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1 α= [ d
where d is the film thickness, R is the reflectance, and T is the transmittance. In order to calculate
ଵൗ ଶ
can be used, where, Aα is a constant that depends on the transition
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equation αhυ= ܣఈ ሺℎ߭ − ݃ܧሻ
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the optical band gap (Eg) of direct gap chalcopyrite compounds semiconductor, the following
nature, the effective mass and the reflective index and hυ is the incident photon energy. The band gap Eg is determined by extrapolating the slope of the (αhν)2 versus photon energy curve to the
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abscissa; for all absorption layers as shown in the Fig. 7. The band gaps for CIGS-1 to CIGS-5 are determined as 1.212, 1.189, 1.183, 1.165 and 1.163 eV, respectively. The band gap energies of CIGS absorption layers increase with an increase of Cu content. The past studies have indicated that
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the different band gap energies is attributed the different Ga/(Ga+In) ratios [31], but some
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literatures presented the band gap energies changed with the ratios of Cu/(Ga+In) [32-35]. In this study, the band gap energies decrease with an increase of Cu/(Ga+In) ratios. The formation of ODC phase such as CuIn3Se5 should play a dominate role in increasing band gap energy due to its band gap energy is about 1.3 eV [36], in general, the band gap energy of ODC phase is higher than Cu(In,Ga)Se2 phase about 0.2 to 0.3 eV [32]. On the other hand, the ratio of ODC phase in lower
8
ACCEPTED MANUSCRIPT Cu content (specimen No. CIGS-1 to CIGS-3) is higher than that in Cu-richer condition and leads to the integrated band gap energy become larger (specimen No. CIGS-4 and CIGS-5). Fig. 8 shows the carrier concentration (n), resistivity (ρ) and Hall mobility (µ) of the CIGS thin
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films as a function of Cu/(Ga+In) ratios. As seen in Fig. 8 and Table 5, the carrier concentration increases from the order of 1014 to 1016 cm-3 and the Hall mobility (µ) decreases from 5.2 to 3.1 cm2
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V-1 s-1 as the Cu/(Ga+In) ratio increases from 0.7 to 0.94. In specimens CIGS-1 to CIGS-4, the carrier concentration are distributed from 3.4×1014 to 8.4×1014, the carrier concentration of CIGS-5
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significantly enhance to 2.1×1016 and the resistivity drop down to only 0.1 kΩ‧cm. The relationship between carrier concentration, resistivity and Cu content would be dominated by ordering defect compound (ODC) such as Cu(In,Ga)3Se5 and Cu2(In,Ga)4Se7 which leading to a lower carrier
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concentration and a resistivity in the CIGS solar cells at Cu-poor condition [37]. Therefore, the specimen CIGS-5 the highest Cu content without formation of ordering defect compound (ODC)
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(Fig. 2) presents a higher carrier concentration and a lower resistivity.
3.4 Photovoltaic properties of CIGS solar cells Fig. 9 presents the J-V curves of CIGS solar cells formed in various Cu/(Ga+In) ratios. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), conversion efficiency (η), series resistance (Rs), and shunt resistance (Rsh) are summarized in Table 6. Generally, the efficiency of solar cells is reduced by the dissipation of power through internal resistance. The parasitic resistance can be modeled as a shunt resistance (Rsh) and a series resistance (Rs), the shunt 9
ACCEPTED MANUSCRIPT resistance is due to fabrication defects, photo-generated current will find alternative paths rather than flowing through the cell’s junction. The shunt resistance will lower the open circuit voltage without changing the short circuit current. An increasing Rs will reduce the fill factor (FF) and
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photovoltaic conversion efficiency of solar cells such as the previous research studied CdS/CdTe thin film solar cells [38] and the device physics of CdS/CdTe ultrathin film solar cells is also
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modeled using “Sah–Noyce–Shockley” (SNS) theory [39]. In this study, the characterization parameters of CIGS solar cells are listed in Table 6, the lower Rs, the higher FF and the higher short
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circuit current density will lead to a better efficiency. Specimen No. CIGS-4 has the best conversion efficiency about 10.3 % with an approximate open-circuit voltage (Voc) of 520 mV, Jsc of 28.0 mA/cm2, fill factor (FF) of 70.6%, Rs of 12.0 Ω, and Rsh of 1141 Ω, respectively.
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Fig. 10 reveals the external quantum efficiency (EQE) of five CIGS solar cells, revealing a shift in the long-wavelength region from 400 to 1100 nm. The optoelectronic properties occur
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between 400 to 540 nm should be the characteristic of CdS buffer layer deposited by chemical bath
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deposition (CBD). The various CIGS solar cells present a slight change in EQE curves, indicating the quality of CdS buffer layer is quite stable in this study. Moreover, the optoelectronic properties occur between 540 to 1100 nm should be the characteristic of CIGS absorption layer, here we only discuss the wavelength region of visible spectrum from 540 to 700 nm. The EQE of CIGS solar cells (CIGS-1 to CIGS-5) prepared from different Cu/(In+Ga) ratios is about 71, 74, 77, 84 and 82 %, respectively. The CIGS-4 has the highest EQE about 84%, indicating reasonably good carrier generation and collection in the p-n junction. 10
ACCEPTED MANUSCRIPT Compared to the EQE values at 70% of five CIGS solar cells (CIGS-1 to CIGS-5) the CIGS-4 shows the widest absorption wavelength range from 530 to 910 nm; the CIGS-1 shows the narrowest absorption wavelength range from 550 to 590 nm and 630 to 740 nm. This also indicates
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the CIGS-4 has the best absorption efficiency from the visible and infrared light spectrum; the CIGS-1 has the worst absorption efficiency and only occurs at the range of visible light spectrum.
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All the five solar cells exhibit an absorption edge around 510 nm as same as the study presented by Liu et al. [17], and the absorption edge around 510 nm corresponding to the CdS band gap 2.4 eV.
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The EQE is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell from outside (incident photons); therefore it depends on both the absorption of light and the collection of charges. Once a photon has been
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absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. Generally, the EQE response of the solar cell can be attributed to the following three
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factors: absorption coefficient of the active layer, the surface recombination velocity and minority
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carrier lifetime [40]. In deeper depth within CIGS layer, the formations of electron hole pairs will far away the p-n junction with an increase of wavelength of incident light. The probability of the combination between electron hole will increase during the electron hole pairs diffuse to the p-n junction.
The CIGS-1 has the highest probability at the carrier recombination losses; the CIGS-4 and CIGS-5 have lower probability at the carrier recombination losses. The CIGS-4 has the best photovoltaic efficiency due to the larger grain size, and the less grain boundaries will reduce the 11
ACCEPTED MANUSCRIPT probability of carrier recombination.
4. Conclusions
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CIGS absorber layers with different Cu/(In+Ga) ratios were carried out using two-stage growth process. The ordering defects compound (ODC) within CIGS layers were characterized by Ramen
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spectra. Ramen spectra on Cu-poor (Cu/(In+Ga)<0.8) presented a broad peak at around 156-158 cm-1 in the Ramen signals due to the formation of the ODC phase. The ODC phase decreased with
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an increase of Cu/(In+Ga) ratio in specimens CIGS-1 to CIGS-3. In the electrical property, the formation of ODC phases leading to a higher resistivity, lower carrier concentration and higher energy band gap in CIGS thin films solar cells. Moreover, the ODC phase also reduced the
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conversion efficiency of CIGS solar cells. In higher Cu content specimens such as CIGS-4 and CIGS-5 have better conversion efficiency due to the disappearance of ODC. In different Cu/(Ga+In)
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ratios conditions, the best conversion efficiency of CIGS thin films solar cell was observed as
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10.3%, and the Voc, Jsc, and FF of CIGS solar cell were measured to be 520 mV, 28.0 mA/cm2, and 70.6%, respectively.
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ACCEPTED MANUSCRIPT [39] L. Kuhn, U. Reggiani, L. Sandrolini, N.E. Gorji, Physical device modeling of CdTe ultrathin film solar cells, Solar Energy, 132 (2016) 165-172. [40] M.C. Tseng, R.H. Horng, F.L. Wu, S.N. Lin, H.H. Yu, D.S. Wuu, Crystalline quality and
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photovoltaic performance of InGaAs solar cells grown on GaAs substrate with
20
30
40
SC (211) (316/332)
(312/116)
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Intensity(a.u.)
∆
(220/204)
(110)
(112)
large-misoriented angle, Vacuum 86 (2012) 843-847.
50
∆
C IG S ∆
Mo
C IG S-1 C IG S-2 C IG S-3 C IG S-4 C IG S-5
60
70
80
90
2 T heta (D eg.)
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Fig. 1. The XRD pattern of different Cu content of CIGS absorber layers using co-evaporation process.
17
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temperature.
25
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Fig. 2 Raman spectra of CIGS absorbers with different Cu/(In+Ga) ratios recorded at room
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20
17.7 15.7
AC C
-1
FWHM (cm )
20.6
15
13 11
10
5 CIGS-1
CIGS-2
CIGS-3
CIGS-4
CIGS-5
Fig. 3. FWHM of Ramen spectra measured at various CIGS absorption layers.
18
ACCEPTED MANUSCRIPT In 3d 5/2 Cu 2p 3/2
O 1s
C 1s
CIGS-1
Intensity (a.u.)
Ga 2p 3/2
Se 3d 5/2
CIGS-2 CIGS-3
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CIGS-4 CIGS-5
1000
800
600
400
200
0
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1200
Binding energy (eV)
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Fig. 4. The full XPS spectrum analyzed from different Cu content CIGS solar cells with a depth at 500Å in CIGS absorption layers.
CIGS-1 CIGS-2 CIGS-3 CIGS-4 CIGS-5
933
932
931
CIGS-1 CIGS-2 CIGS-3 CIGS-4 CIGS-5
930
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934
Intensity (a.u.)
In 3d5/2
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Intensity (a.u.)
Cu 2p3/2
Binding energy (eV)
445
Ga 2p3/2
443
442
Se 3d5/2
Intensity (a.u.)
AC C Intensity (a.u.)
444
Binding energy (eV)
CIGS-1
CIGS-2
CIGS-3 CIGS-4
CIGS-1 CIGS-2 CIGS-3 CIGS-4
CIGS-5 CIGS-5
1119
1118
1117
1116
1115
Binding energy (eV)
Fig. 5.
56
55
54
53
52
51
Binding energy (eV)
The Cu2p3/2, In3d5/2, Ga2p3/2 and Se3d5/2 spectrum analyzed from different Cu content CIGS solar cells with a depth about 500Å in CIGS absorption layers. 19
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C IG S-1 C IG S-2
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Intensity (a.u.)
Cu(In,G a)Se 2
C IG S -1 C IG S -2 C IG S -3 C IG S -4 C IG S -5
Se
C IG S-3 C IG S-4
55.0
54.5
54.0
53.5
53.0
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C IG S-5
52.5
52.0
51.5
B inding E nergy (eV )
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Fig. 6. The Se3d core level analyzed from different Cu content CIGS solar cells.
Fig. 7. Optical band-gap of different Cu/(In+Ga) ratios of CIGS absorption layers.
20
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10 16
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n 15
µ 14
CIGS-1
CIGS-2
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6
10
10
2 Hall mobirity (cm /V.s)
8
CIGS-3
CIGS-4
4
k
ρ
Resistivity ( Ω•cm)
10
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Carrier concentration (cm )
4
2
0
CIGS-5
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Fig. 8. The curves of measured carrier concentration, Hall mobility and resistivity from different Cu content CIGS solar cells
21
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Fig. 9. Illuminated J-V curves of CIGS solar cells formed with various Cu contents.
Fig. 10 External quantum efficiency curves of CIGS thin films solar cells fabricated with different Cu/(Ga+In) ratios of CIGS-1, CIGS-2, CIGS-3, CIGS-4 and CIGS-5. Table 1 The operated parameters of CIGS absorption layer 22
ACCEPTED MANUSCRIPT Sample Cu/ Ga/ Se/ Thickness No. (In+Ga) (In+Ga) (Cu+In+Ga) (µm) 0.7
0.32
1.20
1.34
CIGS-2
0.75
0.31
1.19
1.41
CIGS-3
0.8
0.31
1.16
1.40
CIGS-4
0.85
0.30
1.16
1.47
CIGS-5
0.94
0.31
1.12
1.43
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Table 2 The Raman shift values of Cu(In,Ga)Se2 and ODC phase
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CIGS-1
CIGS-1
CIGS-2
CIGS-3
CIGS-4
Cu(In,Ga)Se2
177 cm-1
177 cm-1
175 cm-1
175 cm-1
ODC
156 cm-1
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Sample No.
158 cm-1
158 cm-1
Table 3. The shifted binding energy peaks of Cu2p3/2, In3d5/2, Ga2p3/2 and Se3d5/2
CIGS-1 CIGS-2
In3d5/2
Ga2p3/2
Se3d5/2
931.5 931.65
443.4 443.65
1116.65 1116.7
52.9 53
931.7 931.7
443.7 443.7
1116.9 1116.9
53.2 53
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CIGS-3 CIGS-4
Cu2p3/2
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Sample No.
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Table 4. The binding energy of selenium compounds measured at different Cu content absorption layers Sample No. CIGS-1 CIGS-2 CIGS-3 CIGS-4 Cu(In,Ga)Se2 53.5 53.7 53.8 53.8 Se 52.8 53.0 53.0 53.0
23
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Table 5. The measured carrier concentration, Hall mobility and resistivity from different Cu content CIGS solar cells Hall mobility Resistivity Sample Carrier concentration 2 -3 (cm /V‧s) (kΩ‧cm) No. (cm ) 3.6×1014
5.2
3.4
CIGS-2
5.0×1014
4.9
2.8
CIGS-3
7.1×1014
4.6
2.1
CIGS-4
8.4×1014
4.3
1.9
CIGS-5
2.1×1016
3.1
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CIGS-1
0.1
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Table 6 Characterization parameters retrieved from the measurements Voc (mV)
Jsc (mA/cm2)
FF (%)
η (%)
Rs(Ω)
Rsh (Ω)
CIGS-1
410
23.2
55.8
5.3
21.8
608
CIGS-2
510
23.6
50.3
6.1
34.0
833
CIGS-3
520
25.3
54.6
7.2
28.6
1039
CIGS-4
520
28.0
70.6
10.3
12.0
1141
CIGS-5
540
27.6
65.1
9.7
16.4
2177
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Highlights 1. ODC phase within CIGS layers was studied by Raman spectra.
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2. Band gap of CIGS layers are affected by Cu content. 3. Effect of ODC phase on electrical property of CIGS layers is studied.