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Cu(In,Ga)Se2-based solar cells prepared from Se-containing precursors
Vacuum 102 (2014) 26e30
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Cu(In,Ga)Se2-based solar cells prepared from Se-containing precursors Jiang Liu, Da-ming Zhuang*, Ming-jie Cao, Xiao-long Li, Min Xie, Da-wei Xu School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2013 Received in revised form 2 October 2013 Accepted 4 October 2013
In this paper, we prepared the CIGS thin films with various [Cu]/[In þ Ga] ratios by selenization of Cu0.8Ga0.2 and In2Se3 precursor films. The properties of Cu(In,Ga)Se2 film and related solar cell were investigated. Raman spectra confirm that the secondary Cu2xSe phase tends to segregate at film surface. SEM results show that the grain-size improves noticeably with the increase of the [Cu]/[In þ Ga] ratios. For Cu-rich CIGS films, the performances of the related solar cells were damaged greatly and good photovoltaic characteristics cannot be obtained. For near-stoichiometric and Cu-poor films, the mean conversion efficiency close to 10% was achieved over a wide range of composition. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: CIGS Selenization Cu2xSe Solar cells
1. Introduction Cu(In1xGax)Se2 (CIGS) is a compound semiconductor consisting of group I element copper, group III element indium or gallium, and group VI element selenium. The excellent absorption characteristics of this material make it an attractive candidate for use as an absorber material in photovoltaic applications [1e3]. The worldrecord efficiency for small area of CIGS-based thin film solar cells has recently surpassed 20% [4], which is the highest conversion efficiency among all thin-film technologies [5]. By changing the ratio of gallium to indium, the band gap can be varied between 1.0 and 1.7 eV. Theoretically, the optimal band gap for a single-junction solar cell based on AM1.5G spectrum is 1.4 eV [6,7], which is higher than the present high-efficiency CIGS absorbers. The addition of small amounts of Ga could not only raise the band-gap to values more suitably matched to the AM1.5G solar spectrum, more importantly, but could also improve the electrical properties of CIGS films [8e10]. However, the CIGS films with higher gallium concentration show a significant loss of efficiency relative to lower gallium content films [11]. The optimal [Ga]/[In þ Ga] ratio of CIGS absorber for high efficiency CIGS solar cell is considered to be 0.2e 0.3 from a number of experimental results [12e14]. Many processing techniques for preparing CIGS thin films have been extensively studied. In terms of large-scale mass production,
* Corresponding author. School of Materials Science and Engineering, Tsinghua University, Room 411, West Main Building Sec. 3, Beijing 100084, PR China. Tel.: þ86 10 62773925; fax: þ86 10 62770190. E-mail address: [email protected] (D.-m. Zhuang). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.10.007
one of the most promising techniques is the so-called two-stage process consisting of the deposition of a precursor material with a subsequent reactive annealing in reactive Se vapor or H2Se ambient [15]. Typically, the precursor is CueIn or CueIneGa metal alloy film by sputtering. However, the presence of low-melting metal indium in CueIneGa metallic precursors could always result in rough morphologies of the CIGS films and loss of indium from films during the ramp-up and selenization stages [16,17]. Moreover, the CIGS films by selenization of metallic precursors tend to peel off from Mo-coated substrate, due to a three-fold volume expansion caused by the incorporation of Se into the metallic precursors during selenization [17]. Few studies have been reported using the selenium-containing precursors to prepare CIGS films. In this study, we introduce Cu0.8Ga0.2 and In2Se3 as precursor films and avoid the direct addition of elemental indium. Some structural and electrical properties of CIGS films from Se-containing precursors have been reported briefly [18]. In this paper, the characterization of CIGS films with various [Cu]/[In þ Ga] ratios and optoelectronic properties of the corresponding ZnO/CdS/CIGS heterojunction solar cells is further reported. 2. Experimental For this study we used a high purity In2Se3 ceramic target and a CuGa alloy target with Ga content of 20 at%. Bilayer CuGa/In2Se3 precursor films were sputter-deposited on Mo-coated and bare soda-lime glass substrate by mid-frequency (40 kHz) magnetron sputtering first CuGa target and then In2Se3 target. By adjusting the sputtering time of the two targets, the Se-containing precursor films with composition range of [Cu]/[In þ Ga] ratios ¼ 0.8e1.3
J. Liu et al. / Vacuum 102 (2014) 26e30
were obtained. Selenization of the precursor films then were carried out using Se vapor in a three-chamber furnace which can be easily scaled up to provide a large-area in-line sequential CIGS process. The selenization temperature and time were set at 550 C and 30 min. The compositions of the CIGS films with various [Cu]/[In þ Ga] ratios were measured using X-ray fluorescence analysis (LAB CENTER XRF-1800). Morphologies and structures of CIGS films were characterized using scanning electron microscope (thermal field emission type, TFE-SEM, LEO-1530), X-ray diffraction (D/maxRB diffractometer, Rigaku) and Raman spectrometry (Renishaw Raman microscope, model RM2000). The following sequential processes were carried out to complete solar cells with Mo/CIGS/CdS/i-ZnO/ZnO:Al/Al grid stack structure without antireflective film. About 60 nm thickness CdS film was deposited by chemical bath deposition using CdSO4, thiourea, and ammonia. The i-ZnO/ZnO:Al bilayer with total thickness of 500 nm was sputter-deposited by mid-frequency magnetron sputtering. Finally, an Al metal grid was deposited by evaporation through an aperture mask. An active cell area of 0.755 cm2 was defined by mechanical scribing along the edge of Al grid. The currentevoltage measurements of CIGS solar cells were performed under illumination from a solar simulator (Newport 91195A) set to produce an Air Mass 1.5 global (AM1.5G) spectrum. A standard silicon solar cell was used to calibrate the light intensity to 100 mW/cm2. The external quantum efficiency (EQE) of solar cell was measured using an IPCE test station (Qtest station 2000) equipped with a Digikrom monochromator and a DSP lock-in amplifier (Stanford Research Systems-SR830). 3. Results and discussion The high quality of CIGS absorbers has much to do with the composition of films [12]. Four sets of Se-containing precursor films with four different [Cu]/[In þ Ga] ratios were prepared. After
27
selenization at 550 C for 30 min, XRF measurement indicated that the [Cu]/[In þ Ga] ratios of the CIGS films are 1.29, 1.02, 0.93 and 0.85, respectively. The [Ga]/[In þ Ga] ratios of the CIGS films (0.19, 0.22, 0.24 and 0.30, respectively) are nearly equal to these of their respective precursor films. The CIGS films with [Cu]/[In þ Ga] ratio ¼ 1.02 could be regarded as near-stoichiometric films ([Cu]/ [In þ Ga] ¼ 1) considering the error range of this XRF instrument. Others could be divided into Cu-rich films ([Cu]/[In þ Ga] > 1) and Cu-poor films ([Cu]/[In þ Ga] < 1). The SEM micrographs of the cross section of CIGS films with different [Cu]/[In þ Ga] ratios are shown in Fig. 1. From these micrographs, the increase in the crystallinity of the films with the increase of Cu contents is clearly observed, which is attributed to the aided growth of the lowtemperature liquid CueSe phase [19]. For the Cu-rich films [Fig. 1(a)] and near-stoichiometric [Fig. 1(b)], the presence of Cu2xSe enhances the mobility of the Se atoms on surface and the mass transport of the absorbed species [20], thus improving the grain size. Since the reaction proceeds from the surface towards the bulk during selenization, the crystalline quality of the bottom layer increases as the atomic diffusion ability increases. For the Cu-poor films [Fig. 1(c and d)], some small grains near the CIGS/Mo interface were observed, which also indicates a lower diffusion into the bottom layers, compared to Cu-rich films. Fig. 2 shows the XRD patterns of CIGS films with different [Cu]/ [In þ Ga] ratios. The peaks at 2q ¼ 40.5 and 73.5 are attributed to Mo (110) and (211), respectively. The others peaks can be attributed to CIGS. The strongest characteristic peaks of all the CIGS films is (112) peaks. The doublet peaks (204/220) and (311/116) are also observed. The peak ratios I(112)/I(220)/(204) are 10.62, 13.09, 3.19 and 2.00 for CIGS films with [Cu]/[In þ Ga] ratio ¼ 1.29, 1.02, 0.93 and 0.85, respectively, indicating that Cu-rich and near-stoichiometric films prefer the (112) texture in this process condition. No peaks related to CueSe phase could be clearly determined from the XRD pattern of Cu-rich film. There seems to be a contradiction between the above analysis of the presence of Cu2xSe phase and the lack of
Fig. 1. Cross sectional SEM images of the CIGS films on Mo-coated glass substrate with different [Cu]/[In þ Ga] ratios.
28
J. Liu et al. / Vacuum 102 (2014) 26e30
evidence from XRD patterns. The reason is that main diffraction peaks of Cu2xSe phase overlap that of CIGS. Raman spectroscopy is a powerful tool for the analysis of CIGS films and the Raman spectra may contain valuable information concerning secondary phases [21]. Therefore, in order to obtain further information about the phases present in the surface region, Raman spectroscopy was adopted. Fig. 3 shows Raman spectra from the front side of CIGS films with different [Cu]/[In þ Ga] ratios. The spectra were recorded using the 514.5 nm line of an Arþ laser which irradiated on the surface of CIGS films. A sharp peak at around 172e 174 cm1 is observed from all the four CIGS films. It is known that this peak is the A1 mode of CIGS, which corresponds to the vibration of the Se anions in the x-y plane with the cations at rest [22,23]. Mixed B2/E modes are also observed at around 213 cm1, which represent vibrations of anions and cations together [22]. For the CIGS films with [Cu]/[In þ Ga] ratio ¼ 1.29 and 1.02, an additional mode appears at 258e260 cm1, which is assigned to the A1 mode of CueSe compounds like CuSe or Cu2Se and is labeled with Cu2xSe in the following [24,25]. Furthermore, it has been reported that Cu2xSe phase tends to segregate on the surface of CIGS films
[20,26]. However, no clear proof has been demonstrated. It should be noted that the penetration depth for this laser (l ¼ 514.5 nm) from the surface of CIGS films is about 100 nm due to the high absorption coefficient of up to 105 cm1 of CIGS films [27]. Therefore, Fig. 3 could only give out the surface structural information of the CIGS films. In order to find out the possible structural variation in comparison to the front side, Raman spectra (Fig. 4) were obtained from the back side of CIGS films with different [Cu]/[In þ Ga] ratios. The incident laser irradiated on back side of CIGS films through glass substrate. To avoid the blocking of opaque back contact Mo layer, the CIGS films were prepared on bare glass and had the same process condition as that grown on Mo-coated substrate. It should be also noted that the Raman spectra recorded from the front side of CIGS films grown on glass substrate are almost the same as Fig. 3. From Fig. 4 it is observed that peaks at around 258e 260 cm1 related to Cu2xSe phase disappeared, which can confirm Cu2xSe phase prefers to segregate at film surface at least for the CIGS films grown on glass and is also consistent with the growth model presented in the literature [19]. Moreover, the position of the CIGS A1 mode in Fig. 4 is at around 177e178 cm1, which is higher than that in Fig. 3. For selenization of precursor films, there is a common problem that Ga tends to accumulate near Mo back contact or the back side of CIGS films [28]. A growth model consistent with the phenomenon of Ga accumulation was depicted, irrespectively of the substrate types [29]. A behavior that the CIGS A1 mode frequency increases linearly with the Ga content has also been reported earlier [22,30]. Therefore, the shift of the CIGS A1 mode in Fig. 4 can be explained that [Ga]/[In þ Ga] ratio at back side of CIGS films is higher than that at front side. Fig. 5 shows solar cell performance parameters with different [Cu]/[In þ Ga] ratios. The solar cells with [Cu]/[In þ Ga] ratios ¼ 0.85, 0.93, 1.02 and 1.29 are referred to as group A, B, C and D. The standard deviations (the error bars in Fig. 5) and the mean values (the square symbols in Fig. 5) of the parameters were also calculated to demonstrate the stabilization of each process. The best device efficiencies (the down triangle symbols in Fig. 5(d)) are 11.57% for cells A (open-circuit voltage, Voc ¼ 428.60 mV, shortcircuit current density, Jsc ¼ 37.70 mA/cm2, and fill factor, FF ¼ 71.59%), 12.80% for cells B (Voc ¼ 471.29 mV, Jsc ¼ 35.89 mA/ cm2, and FF ¼ 75.70%), 12.66% for cells C (Voc ¼ 455.87 mV, Jsc ¼ 36.69 mA/cm2, FF ¼ 75.71%), and 0.094% for cells D
Fig. 3. Raman spectra from the front side of CIGS films with different [Cu]/[In þ Ga] ratios. The spectra were recorded using the 514.5 nm line of an Arþ laser which irradiated on the surface of CIGS films.
Fig. 4. Raman spectra from the back side of CIGS films with different [Cu]/[In þ Ga] ratios. The incident laser irradiated on back side of CIGS films through glass substrate.
Fig. 2. XRD patterns of CIGS films with different [Cu]/[In þ Ga] ratios.
J. Liu et al. / Vacuum 102 (2014) 26e30
(a)
(b) 40
500 Jsc (mA/cm 2)
400 Voc (mV)
29
300 200 100
30 20 10
0 A
(c)
B C Group
D
0
(d)
70
B C Group
D
A
B C Group
D
Efficiency (%)
12
60 FF (%)
A
50 40
8 4
30 20
0 A
B C Group
D
Fig. 5. Solar cell performance parameters with different [Cu]/[In þ Ga] ratios. The solar cells with [Cu]/[In þ Ga] ratios ¼ 0.85, 0.93, 1.02 and 1.29 are referred to as group A, B, C and D. The squares show the average values of five cells with error bars showing the standard deviations, the down triangles denote the highest values attained from five cells.
(Voc ¼ 34.71 mV, Jsc ¼ 9.17 mA/cm2, and FF ¼ 29.59%). It is observed that the mean conversion efficiency close to 10% was achieved over a wide range of composition except for Cu-rich films. For the solar cells with [Cu]/[In þ Ga] ratios ¼ 1.29, all the parameters deteriorate greatly, which is attributed to the presence of too much Cu2xSe phase. The Cu2xSe phase is highly conductive and would produce the leakage current between the pen junctions of devices, thus deteriorating the photovoltaic performance. Fig. 6 shows the normalized external quantum efficiency (EQE) measured between 400 and 1300 nm for ZnO/CdS/CIGS heterojunction solar cells with three different [Cu]/[In þ Ga] ratios. All the three solar cells exhibit an absorption edge around 510 nm corresponding to the CdS band gap 2.4 eV. The thickness variation of CdS layer may lead to the variation of short-wavelength response of these devices. The absorption edge at near-infrared region is related to the absorption coefficient or band gap of CIGS absorbers. For a direct transition, the dependence of the external quantum efficiency on photon energy at long wavelength absorption edge could be given by applying the following expression [31].
hn lnð1 EQEÞ ¼ A hn Eg
1=2
to the decrease of [Ga]/[In þ Ga] of the CIGS absorber, thus leading to the decrease of the band gap of CIGS absorber. 4. Conclusions ZnO/CdS/CIGS heterojunction solar cells have been demonstrated using Se-containing precursors. Based on the SEM and XRD studies, it was found that the concentration of Cu has a strong influence on the preferred orientation and growth of grain size of CIGS films during selenization. The comparison of Raman spectra from the surface and back side of CIGS films confirmed that phase tends to segregate near the surface of CIGS films. The Cu2xSe phase is
(1)
where hn is the photon energy, Eg is the band gap of CIGS absorber, and A is a constant. By plotting [hn ln(1 EQE)]2 against hn (the inset of Fig. 6) and by extrapolating the linear portion of the absorption edge to find the intercept with energy axis, the band gap Eg have been calculated to be 1.04 eV, 1.02 eV and 1.00 eV for CIGS films with [Cu]/[In þ Ga] ratio ¼ 1.02, 0.93 and 0.85, respectively. The band gaps are close to that of CuInSe2. It is necessary to point out that even though the extrapolated band gap may not be completely accurate, the trend is still evident. During the preparation of precursor films, since Ga is incorporated in CuGa alloy target, the decrease of Cu concentration in precursor films may lead
Fig. 6. Normalized external quantum efficiency (EQE) curves of ZnO/CdS/CIGS heterojunction solar cells with three different [Cu]/[In þ Ga] ratios. The inset is plots of [hn ln(1 EQE)]2 against hn to determine band gap of the absorbers.
30
J. Liu et al. / Vacuum 102 (2014) 26e30
highly conductive and would produce the leakage current between the pen junctions of devices, thus deteriorating the photovoltaic performance. The mean conversion efficiency close to 10% was achieved over a wide range of composition except for Cu-rich films. Acknowledgments The authors are thankful to Song Jun for equipment maintenance, Li Jun for XRF measurement and Chen Feng-en for measurement of Raman spectra. This work was financially supported by Initiative Scientific Research Project of Tsinghua University. References [1] Chirila A, Buecheler S, Pianezzi F, Bloesch P, Gretener C, Uhl AR, et al. Highly efficient Cu(In, Ga)Se2 solar cells grown on flexible polymer films. Nat Mater 2011;10:857e61. [2] Contreras MA, Egaas B, Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, et al. Progress toward 20% efficiency in Cu(In, Ga)Se2 polycrystalline thin-film solar cells. Prog Photovolt Res Appl 1999;7:311e6. [3] Wallin E, Malm U, Jarmar T, Lundberg O, Edoff M, Stolt L. World-record Cu(In, Ga)Se2-based thin-film sub-module with 17.4% efficiency. Prog Photovolt Res Appl 2012;20:851e4. [4] Jackson P, Hariskos D, Lotter E, Paetel S, Wuerz R, Menner R, et al. New world record efficiency for Cu(In, Ga)Se2 thin-film solar cells beyond 20%. Prog Photovolt Res Appl 2011;19:894e7. [5] Green MA, Emery K, Hishikawa Y, Warta W, Dunlop ED. Solar cell efficiency tables (version 41). Prog Photovolt Res Appl 2013;21:1e11. [6] Green MA. Radiative efficiency of state-of-the-art photovoltaic cells. Prog Photovolt Res Appl 2012;20:472e6. [7] Shockley W, Queisser HJ. Detailed balance limit of efficiency of p-n junction solar cells. J Appl Phys 1961;32:510e9. [8] Jensen CL, Tarrant DE, Ermer JH, Pollock GA. In: Conference record of the Twenty Third IEEE photovoltaic specialists conference, Louisville, Kentucky 1993. p. 577e80. [9] Tuttle JR, Contreras M, Tennant A, Albin D, Noufi R. In: Photovoltaic specialists conference, 1993, Conference record of the Twenty Third IEEE, Louisville, Kentucky 1993. p. 415e21. [10] Rau U, Schock HW. Cu(In, Ga)Se2 thin-film solar cells. In: Markvart T, Castaner L, editors. Solar cells: materials, manufacture and operation. Oxford: Elsevier; 2005. p. 303e49. [11] Venkatachalam M, Kannan MD, Jayakumar S, Balasundaraprabhu R, Muthukumarasamy N. Effect of annealing on the structural properties of electron beam deposited CIGS thin films. Thin Solid Films 2008;516:6848e52. [12] Jackson P, Wurz R, Rau U, Mattheis J, Kurth M, Schlotzer T, et al. High quality baseline for high efficiency, Cu(In1x, Gax)Se2 solar cells. Prog Photovolt Res Appl 2007;15:507e19. [13] Ramanathan K, Teeter G, Keane JC, Noufi R. Properties of high-efficiency CuInGaSe2 thin film solar cells. Thin Solid Films 2005;480:499e502.
[14] Contreras MA, Ramanathan K, AbuShama J, Hasoon F, Young DL, Egaas B, et al. Diode characteristics in state-of-the-art ZnO/CdS/Cu(In1xGax)Se2 solar cells. Prog Photovolt Res Appl 2005;13:209e16. [15] Kaelin M, Rudmann D, Tiwari AN. Low cost processing of CIGS thin film solar cells. Sol Energy 2004;77:749e56. [16] Shi JH, Li ZQ, Zhang DW, Liu QQ, Sun Z, Huang SM. Fabrication of Cu(In, Ga)Se2 thin films by sputtering from a single quaternary chalcogenide target. Prog Photovolt Res Appli 2011;19:160e4. [17] Alberts V, Zweigart S, Schock HW. Preparation of device quality CuInSe2 by selenization of Se-containing precursors in H2Se atmosphere. Semicond Sci Technol 1997;12:217e23. [18] Liu J, Zhuang D, Cao M, Wang C, Xie M, Li X. Preparation and characterization of Cu(In, Ga)Se2 thin films by selenization of Cu0.8Ga0.2 and In2Se3 precursor films. Int J Photoenergy 2012;2012:149210. [19] Klenk R, Walter T, Schock HW, Cahen D. A model for the successful growth of polycrystalline films of CuInSe2 by multisource physical vacuum evaporation. Adv Mater 1993;5:114e9. [20] Wan L, Cao YS, Wang DL. P-type CuInSe2 thin films prepared by selenization of one-step electrodeposited precursors. J Mater Res 2009;24:2293e300. [21] Alvarez-Garcıa J, Rudigier E, Rega N, Barcones B, Scheer R, Pérez-Rodrıguez A, et al. Growth process monitoring and crystalline quality assessment of CuInS(Se)2 based solar cells by Raman spectroscopy. Thin Solid Films 2003;431e432:122e5. [22] Wolfram W, Robert K, Axel E, Michael P. In: Conference record of the IEEE 4th world conference on photovoltaic energy conversion, Waikoloa, Hawaii 2006. p. 553e6. [23] Delsol T, Samantilleke AP, Chaure NB, Gardiner PH, Simmonds M, Dharmadasa IM. Experimental study of graded bandgap Cu(InGa)(SeS)2 thin films grown on glass/molybdenum substrates by selenization and sulphidation. Sol Energy Mater Sol Cells 2004;82:587e99. [24] Witte W, Kniese R, Powalla M. Raman investigations of Cu(In, Ga)Se2 thin films with various copper contents. Thin Solid Films 2008;517:867e9. [25] Zaretskaya EP, Gremenok VF, Riede V, Schmitz W, Bente K, Zalesski VB, et al. Raman spectroscopy of CuInSe2 thin films prepared by selenization. J Phys Chem Solids 2003;64:1989e93. [26] Rau U, Schock HW. Electronic properties of Cu(In, Ga)Se2 heterojunction solar cells-recent achievements, current understanding, and future challenges. Appl Phys A 1999;69:131e47. [27] Alonso MI, Wakita K, Pascual J, Garriga M, Yamamoto N. Optical functions and electronic structure of CuInSe2, CuGaSe2, CuInS2, and CuGaS2. Phys Rev B 2001;63:75203. [28] Kim K, Hanket GM, Huynh T, Shafarman WN. Three-step H2Se/Ar/H2S reaction of CueIneGa precursors for controlled composition and adhesion of Cu(In, Ga)(Se, S)2 thin films. J Appl Phys 2012;111:83710. [29] Gabor AM, Tuttle JR, Bode MH, Franz A, Tennant AL, Contreras MA, et al. Band́ gap engineering in Cu(In, Ga)Se2 thin films grown from (In, Ga)2Se ́ 3 precursors. Sol Energy Mater Sol Cells 1996;41e2:247e60. [30] Tanino H, Deai H, Nakanishi H. Raman spectra of CuGaxIn1xSe2. Jpn J Appl Phys Suppl 1993;32:436e8. [31] Zoppi G, Forbes I, Miles RW, Dale PJ, Scragg JJ, Peter LM. Cu2ZnSnSe4 thin film solar cells produced by selenisation of magnetron sputtered precursors. Prog Photovolt Res Appl 2009;17:315e9.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Cu(In,Ga)Se2-based solar cells prepared from Se-containing precursors Jiang Liu, Da-ming Zhuang*, Ming-jie Cao, Xiao-long Li, Min Xie, Da-wei Xu School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2013 Received in revised form 2 October 2013 Accepted 4 October 2013
In this paper, we prepared the CIGS thin films with various [Cu]/[In þ Ga] ratios by selenization of Cu0.8Ga0.2 and In2Se3 precursor films. The properties of Cu(In,Ga)Se2 film and related solar cell were investigated. Raman spectra confirm that the secondary Cu2xSe phase tends to segregate at film surface. SEM results show that the grain-size improves noticeably with the increase of the [Cu]/[In þ Ga] ratios. For Cu-rich CIGS films, the performances of the related solar cells were damaged greatly and good photovoltaic characteristics cannot be obtained. For near-stoichiometric and Cu-poor films, the mean conversion efficiency close to 10% was achieved over a wide range of composition. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: CIGS Selenization Cu2xSe Solar cells
1. Introduction Cu(In1xGax)Se2 (CIGS) is a compound semiconductor consisting of group I element copper, group III element indium or gallium, and group VI element selenium. The excellent absorption characteristics of this material make it an attractive candidate for use as an absorber material in photovoltaic applications [1e3]. The worldrecord efficiency for small area of CIGS-based thin film solar cells has recently surpassed 20% [4], which is the highest conversion efficiency among all thin-film technologies [5]. By changing the ratio of gallium to indium, the band gap can be varied between 1.0 and 1.7 eV. Theoretically, the optimal band gap for a single-junction solar cell based on AM1.5G spectrum is 1.4 eV [6,7], which is higher than the present high-efficiency CIGS absorbers. The addition of small amounts of Ga could not only raise the band-gap to values more suitably matched to the AM1.5G solar spectrum, more importantly, but could also improve the electrical properties of CIGS films [8e10]. However, the CIGS films with higher gallium concentration show a significant loss of efficiency relative to lower gallium content films [11]. The optimal [Ga]/[In þ Ga] ratio of CIGS absorber for high efficiency CIGS solar cell is considered to be 0.2e 0.3 from a number of experimental results [12e14]. Many processing techniques for preparing CIGS thin films have been extensively studied. In terms of large-scale mass production,
* Corresponding author. School of Materials Science and Engineering, Tsinghua University, Room 411, West Main Building Sec. 3, Beijing 100084, PR China. Tel.: þ86 10 62773925; fax: þ86 10 62770190. E-mail address: [email protected] (D.-m. Zhuang). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.10.007
one of the most promising techniques is the so-called two-stage process consisting of the deposition of a precursor material with a subsequent reactive annealing in reactive Se vapor or H2Se ambient [15]. Typically, the precursor is CueIn or CueIneGa metal alloy film by sputtering. However, the presence of low-melting metal indium in CueIneGa metallic precursors could always result in rough morphologies of the CIGS films and loss of indium from films during the ramp-up and selenization stages [16,17]. Moreover, the CIGS films by selenization of metallic precursors tend to peel off from Mo-coated substrate, due to a three-fold volume expansion caused by the incorporation of Se into the metallic precursors during selenization [17]. Few studies have been reported using the selenium-containing precursors to prepare CIGS films. In this study, we introduce Cu0.8Ga0.2 and In2Se3 as precursor films and avoid the direct addition of elemental indium. Some structural and electrical properties of CIGS films from Se-containing precursors have been reported briefly [18]. In this paper, the characterization of CIGS films with various [Cu]/[In þ Ga] ratios and optoelectronic properties of the corresponding ZnO/CdS/CIGS heterojunction solar cells is further reported. 2. Experimental For this study we used a high purity In2Se3 ceramic target and a CuGa alloy target with Ga content of 20 at%. Bilayer CuGa/In2Se3 precursor films were sputter-deposited on Mo-coated and bare soda-lime glass substrate by mid-frequency (40 kHz) magnetron sputtering first CuGa target and then In2Se3 target. By adjusting the sputtering time of the two targets, the Se-containing precursor films with composition range of [Cu]/[In þ Ga] ratios ¼ 0.8e1.3
J. Liu et al. / Vacuum 102 (2014) 26e30
were obtained. Selenization of the precursor films then were carried out using Se vapor in a three-chamber furnace which can be easily scaled up to provide a large-area in-line sequential CIGS process. The selenization temperature and time were set at 550 C and 30 min. The compositions of the CIGS films with various [Cu]/[In þ Ga] ratios were measured using X-ray fluorescence analysis (LAB CENTER XRF-1800). Morphologies and structures of CIGS films were characterized using scanning electron microscope (thermal field emission type, TFE-SEM, LEO-1530), X-ray diffraction (D/maxRB diffractometer, Rigaku) and Raman spectrometry (Renishaw Raman microscope, model RM2000). The following sequential processes were carried out to complete solar cells with Mo/CIGS/CdS/i-ZnO/ZnO:Al/Al grid stack structure without antireflective film. About 60 nm thickness CdS film was deposited by chemical bath deposition using CdSO4, thiourea, and ammonia. The i-ZnO/ZnO:Al bilayer with total thickness of 500 nm was sputter-deposited by mid-frequency magnetron sputtering. Finally, an Al metal grid was deposited by evaporation through an aperture mask. An active cell area of 0.755 cm2 was defined by mechanical scribing along the edge of Al grid. The currentevoltage measurements of CIGS solar cells were performed under illumination from a solar simulator (Newport 91195A) set to produce an Air Mass 1.5 global (AM1.5G) spectrum. A standard silicon solar cell was used to calibrate the light intensity to 100 mW/cm2. The external quantum efficiency (EQE) of solar cell was measured using an IPCE test station (Qtest station 2000) equipped with a Digikrom monochromator and a DSP lock-in amplifier (Stanford Research Systems-SR830). 3. Results and discussion The high quality of CIGS absorbers has much to do with the composition of films [12]. Four sets of Se-containing precursor films with four different [Cu]/[In þ Ga] ratios were prepared. After
27
selenization at 550 C for 30 min, XRF measurement indicated that the [Cu]/[In þ Ga] ratios of the CIGS films are 1.29, 1.02, 0.93 and 0.85, respectively. The [Ga]/[In þ Ga] ratios of the CIGS films (0.19, 0.22, 0.24 and 0.30, respectively) are nearly equal to these of their respective precursor films. The CIGS films with [Cu]/[In þ Ga] ratio ¼ 1.02 could be regarded as near-stoichiometric films ([Cu]/ [In þ Ga] ¼ 1) considering the error range of this XRF instrument. Others could be divided into Cu-rich films ([Cu]/[In þ Ga] > 1) and Cu-poor films ([Cu]/[In þ Ga] < 1). The SEM micrographs of the cross section of CIGS films with different [Cu]/[In þ Ga] ratios are shown in Fig. 1. From these micrographs, the increase in the crystallinity of the films with the increase of Cu contents is clearly observed, which is attributed to the aided growth of the lowtemperature liquid CueSe phase [19]. For the Cu-rich films [Fig. 1(a)] and near-stoichiometric [Fig. 1(b)], the presence of Cu2xSe enhances the mobility of the Se atoms on surface and the mass transport of the absorbed species [20], thus improving the grain size. Since the reaction proceeds from the surface towards the bulk during selenization, the crystalline quality of the bottom layer increases as the atomic diffusion ability increases. For the Cu-poor films [Fig. 1(c and d)], some small grains near the CIGS/Mo interface were observed, which also indicates a lower diffusion into the bottom layers, compared to Cu-rich films. Fig. 2 shows the XRD patterns of CIGS films with different [Cu]/ [In þ Ga] ratios. The peaks at 2q ¼ 40.5 and 73.5 are attributed to Mo (110) and (211), respectively. The others peaks can be attributed to CIGS. The strongest characteristic peaks of all the CIGS films is (112) peaks. The doublet peaks (204/220) and (311/116) are also observed. The peak ratios I(112)/I(220)/(204) are 10.62, 13.09, 3.19 and 2.00 for CIGS films with [Cu]/[In þ Ga] ratio ¼ 1.29, 1.02, 0.93 and 0.85, respectively, indicating that Cu-rich and near-stoichiometric films prefer the (112) texture in this process condition. No peaks related to CueSe phase could be clearly determined from the XRD pattern of Cu-rich film. There seems to be a contradiction between the above analysis of the presence of Cu2xSe phase and the lack of
Fig. 1. Cross sectional SEM images of the CIGS films on Mo-coated glass substrate with different [Cu]/[In þ Ga] ratios.
28
J. Liu et al. / Vacuum 102 (2014) 26e30
evidence from XRD patterns. The reason is that main diffraction peaks of Cu2xSe phase overlap that of CIGS. Raman spectroscopy is a powerful tool for the analysis of CIGS films and the Raman spectra may contain valuable information concerning secondary phases [21]. Therefore, in order to obtain further information about the phases present in the surface region, Raman spectroscopy was adopted. Fig. 3 shows Raman spectra from the front side of CIGS films with different [Cu]/[In þ Ga] ratios. The spectra were recorded using the 514.5 nm line of an Arþ laser which irradiated on the surface of CIGS films. A sharp peak at around 172e 174 cm1 is observed from all the four CIGS films. It is known that this peak is the A1 mode of CIGS, which corresponds to the vibration of the Se anions in the x-y plane with the cations at rest [22,23]. Mixed B2/E modes are also observed at around 213 cm1, which represent vibrations of anions and cations together [22]. For the CIGS films with [Cu]/[In þ Ga] ratio ¼ 1.29 and 1.02, an additional mode appears at 258e260 cm1, which is assigned to the A1 mode of CueSe compounds like CuSe or Cu2Se and is labeled with Cu2xSe in the following [24,25]. Furthermore, it has been reported that Cu2xSe phase tends to segregate on the surface of CIGS films
[20,26]. However, no clear proof has been demonstrated. It should be noted that the penetration depth for this laser (l ¼ 514.5 nm) from the surface of CIGS films is about 100 nm due to the high absorption coefficient of up to 105 cm1 of CIGS films [27]. Therefore, Fig. 3 could only give out the surface structural information of the CIGS films. In order to find out the possible structural variation in comparison to the front side, Raman spectra (Fig. 4) were obtained from the back side of CIGS films with different [Cu]/[In þ Ga] ratios. The incident laser irradiated on back side of CIGS films through glass substrate. To avoid the blocking of opaque back contact Mo layer, the CIGS films were prepared on bare glass and had the same process condition as that grown on Mo-coated substrate. It should be also noted that the Raman spectra recorded from the front side of CIGS films grown on glass substrate are almost the same as Fig. 3. From Fig. 4 it is observed that peaks at around 258e 260 cm1 related to Cu2xSe phase disappeared, which can confirm Cu2xSe phase prefers to segregate at film surface at least for the CIGS films grown on glass and is also consistent with the growth model presented in the literature [19]. Moreover, the position of the CIGS A1 mode in Fig. 4 is at around 177e178 cm1, which is higher than that in Fig. 3. For selenization of precursor films, there is a common problem that Ga tends to accumulate near Mo back contact or the back side of CIGS films [28]. A growth model consistent with the phenomenon of Ga accumulation was depicted, irrespectively of the substrate types [29]. A behavior that the CIGS A1 mode frequency increases linearly with the Ga content has also been reported earlier [22,30]. Therefore, the shift of the CIGS A1 mode in Fig. 4 can be explained that [Ga]/[In þ Ga] ratio at back side of CIGS films is higher than that at front side. Fig. 5 shows solar cell performance parameters with different [Cu]/[In þ Ga] ratios. The solar cells with [Cu]/[In þ Ga] ratios ¼ 0.85, 0.93, 1.02 and 1.29 are referred to as group A, B, C and D. The standard deviations (the error bars in Fig. 5) and the mean values (the square symbols in Fig. 5) of the parameters were also calculated to demonstrate the stabilization of each process. The best device efficiencies (the down triangle symbols in Fig. 5(d)) are 11.57% for cells A (open-circuit voltage, Voc ¼ 428.60 mV, shortcircuit current density, Jsc ¼ 37.70 mA/cm2, and fill factor, FF ¼ 71.59%), 12.80% for cells B (Voc ¼ 471.29 mV, Jsc ¼ 35.89 mA/ cm2, and FF ¼ 75.70%), 12.66% for cells C (Voc ¼ 455.87 mV, Jsc ¼ 36.69 mA/cm2, FF ¼ 75.71%), and 0.094% for cells D
Fig. 3. Raman spectra from the front side of CIGS films with different [Cu]/[In þ Ga] ratios. The spectra were recorded using the 514.5 nm line of an Arþ laser which irradiated on the surface of CIGS films.
Fig. 4. Raman spectra from the back side of CIGS films with different [Cu]/[In þ Ga] ratios. The incident laser irradiated on back side of CIGS films through glass substrate.
Fig. 2. XRD patterns of CIGS films with different [Cu]/[In þ Ga] ratios.
J. Liu et al. / Vacuum 102 (2014) 26e30
(a)
(b) 40
500 Jsc (mA/cm 2)
400 Voc (mV)
29
300 200 100
30 20 10
0 A
(c)
B C Group
D
0
(d)
70
B C Group
D
A
B C Group
D
Efficiency (%)
12
60 FF (%)
A
50 40
8 4
30 20
0 A
B C Group
D
Fig. 5. Solar cell performance parameters with different [Cu]/[In þ Ga] ratios. The solar cells with [Cu]/[In þ Ga] ratios ¼ 0.85, 0.93, 1.02 and 1.29 are referred to as group A, B, C and D. The squares show the average values of five cells with error bars showing the standard deviations, the down triangles denote the highest values attained from five cells.
(Voc ¼ 34.71 mV, Jsc ¼ 9.17 mA/cm2, and FF ¼ 29.59%). It is observed that the mean conversion efficiency close to 10% was achieved over a wide range of composition except for Cu-rich films. For the solar cells with [Cu]/[In þ Ga] ratios ¼ 1.29, all the parameters deteriorate greatly, which is attributed to the presence of too much Cu2xSe phase. The Cu2xSe phase is highly conductive and would produce the leakage current between the pen junctions of devices, thus deteriorating the photovoltaic performance. Fig. 6 shows the normalized external quantum efficiency (EQE) measured between 400 and 1300 nm for ZnO/CdS/CIGS heterojunction solar cells with three different [Cu]/[In þ Ga] ratios. All the three solar cells exhibit an absorption edge around 510 nm corresponding to the CdS band gap 2.4 eV. The thickness variation of CdS layer may lead to the variation of short-wavelength response of these devices. The absorption edge at near-infrared region is related to the absorption coefficient or band gap of CIGS absorbers. For a direct transition, the dependence of the external quantum efficiency on photon energy at long wavelength absorption edge could be given by applying the following expression [31].
hn lnð1 EQEÞ ¼ A hn Eg
1=2
to the decrease of [Ga]/[In þ Ga] of the CIGS absorber, thus leading to the decrease of the band gap of CIGS absorber. 4. Conclusions ZnO/CdS/CIGS heterojunction solar cells have been demonstrated using Se-containing precursors. Based on the SEM and XRD studies, it was found that the concentration of Cu has a strong influence on the preferred orientation and growth of grain size of CIGS films during selenization. The comparison of Raman spectra from the surface and back side of CIGS films confirmed that phase tends to segregate near the surface of CIGS films. The Cu2xSe phase is
(1)
where hn is the photon energy, Eg is the band gap of CIGS absorber, and A is a constant. By plotting [hn ln(1 EQE)]2 against hn (the inset of Fig. 6) and by extrapolating the linear portion of the absorption edge to find the intercept with energy axis, the band gap Eg have been calculated to be 1.04 eV, 1.02 eV and 1.00 eV for CIGS films with [Cu]/[In þ Ga] ratio ¼ 1.02, 0.93 and 0.85, respectively. The band gaps are close to that of CuInSe2. It is necessary to point out that even though the extrapolated band gap may not be completely accurate, the trend is still evident. During the preparation of precursor films, since Ga is incorporated in CuGa alloy target, the decrease of Cu concentration in precursor films may lead
Fig. 6. Normalized external quantum efficiency (EQE) curves of ZnO/CdS/CIGS heterojunction solar cells with three different [Cu]/[In þ Ga] ratios. The inset is plots of [hn ln(1 EQE)]2 against hn to determine band gap of the absorbers.
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J. Liu et al. / Vacuum 102 (2014) 26e30
highly conductive and would produce the leakage current between the pen junctions of devices, thus deteriorating the photovoltaic performance. The mean conversion efficiency close to 10% was achieved over a wide range of composition except for Cu-rich films. Acknowledgments The authors are thankful to Song Jun for equipment maintenance, Li Jun for XRF measurement and Chen Feng-en for measurement of Raman spectra. This work was financially supported by Initiative Scientific Research Project of Tsinghua University. References [1] Chirila A, Buecheler S, Pianezzi F, Bloesch P, Gretener C, Uhl AR, et al. Highly efficient Cu(In, Ga)Se2 solar cells grown on flexible polymer films. Nat Mater 2011;10:857e61. [2] Contreras MA, Egaas B, Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, et al. Progress toward 20% efficiency in Cu(In, Ga)Se2 polycrystalline thin-film solar cells. Prog Photovolt Res Appl 1999;7:311e6. [3] Wallin E, Malm U, Jarmar T, Lundberg O, Edoff M, Stolt L. World-record Cu(In, Ga)Se2-based thin-film sub-module with 17.4% efficiency. Prog Photovolt Res Appl 2012;20:851e4. [4] Jackson P, Hariskos D, Lotter E, Paetel S, Wuerz R, Menner R, et al. New world record efficiency for Cu(In, Ga)Se2 thin-film solar cells beyond 20%. Prog Photovolt Res Appl 2011;19:894e7. [5] Green MA, Emery K, Hishikawa Y, Warta W, Dunlop ED. Solar cell efficiency tables (version 41). Prog Photovolt Res Appl 2013;21:1e11. [6] Green MA. Radiative efficiency of state-of-the-art photovoltaic cells. Prog Photovolt Res Appl 2012;20:472e6. [7] Shockley W, Queisser HJ. Detailed balance limit of efficiency of p-n junction solar cells. J Appl Phys 1961;32:510e9. [8] Jensen CL, Tarrant DE, Ermer JH, Pollock GA. In: Conference record of the Twenty Third IEEE photovoltaic specialists conference, Louisville, Kentucky 1993. p. 577e80. [9] Tuttle JR, Contreras M, Tennant A, Albin D, Noufi R. In: Photovoltaic specialists conference, 1993, Conference record of the Twenty Third IEEE, Louisville, Kentucky 1993. p. 415e21. [10] Rau U, Schock HW. Cu(In, Ga)Se2 thin-film solar cells. In: Markvart T, Castaner L, editors. Solar cells: materials, manufacture and operation. Oxford: Elsevier; 2005. p. 303e49. [11] Venkatachalam M, Kannan MD, Jayakumar S, Balasundaraprabhu R, Muthukumarasamy N. Effect of annealing on the structural properties of electron beam deposited CIGS thin films. Thin Solid Films 2008;516:6848e52. [12] Jackson P, Wurz R, Rau U, Mattheis J, Kurth M, Schlotzer T, et al. High quality baseline for high efficiency, Cu(In1x, Gax)Se2 solar cells. Prog Photovolt Res Appl 2007;15:507e19. [13] Ramanathan K, Teeter G, Keane JC, Noufi R. Properties of high-efficiency CuInGaSe2 thin film solar cells. Thin Solid Films 2005;480:499e502.
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