15% Efficiency CdS/CdTe thin film solar cells using CdS layers doped with metal organic compounds

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1 Solar Energy Materials & Solar Cells 90 (2006) % Efficiency CdS/CdTe thin film solar cells using CdS layers doped with metal organic compounds Kengo Matsune a, Hiroyuki Oda a, Toshihiko Toyama a,, Hiroaki Okamoto a, Yuriy Kudriavysevand b, Rene Asomoza b a Department of Systems Innovation, Graduate School of Engineering Science, Osaka University Toyonaka, Osaka , Japan b Sección de Electrónica del Estado SólidoDepartamento de Ingeniería Eléctrica Cinvestav, Av. IPN 2508, México, D.F., México Available online 25 July 2006 Abstract For improving the photovoltaic performance of CdS/CdTe thin film solar cells, the CdS window layer is one of the most crucial factors. Here we demonstrate the photovoltaic performances of the lowenvironmental-load CdS/CdTe solar cell employing the CdS layer doped with various metal organic (MO) compounds, i.e., (CH 3 ) 2 SnCl 2,(C 6 H 5 ) 3 GeCl, (CH 3 CO 2 ) 3 In, [(C 2 H 5 ) 2 NCS 2 ] 2 Zn. Due to the MO doping, the degree of (1 1 1) preferential orientation of CdTe on the CdS layer is improved remarkably, influencing the increases in V oc and F.F. Being almost independent of the kind of the MO compounds, the short circuit current increases due to increasing optical transmittance of the MO-doped CdS layers. As a result, utilizing MO-doped CdS, we have achieved the conversion efficiency of 15.1%. r 2006 Elsevier B.V. All rights reserved. Keywords: Solar cell; CdTe; CdS; Metal organic compounds 1. Introduction Cadmium telluride is one of the most ideal materials for the photovoltaic application because of the following well-known reasons: the optimum band gap of 1.51 ev for solar spectrum and the direct band gap yielding high optical absorption coefficient. The energy conversion efficiency over 16% has been already realized in the R&D level [1,2]. However, the thickness of the photovoltaic layer, which is usually 5 10 mm, must be reduced from a Corresponding author. Tel.: ; fax: address: toyama@ee.es.osaka-u.ac.jp (T. Toyama) /$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi: /j.solmat

2 K. Matsune et al. / Solar Energy Materials & Solar Cells 90 (2006) viewpoint of the environmental load related to the Cd compounds. We have developed the low-environmental-load CdS/CdTe thin film solar cells with a photovoltaic layer thickness of o3 mm [3 6]. For the further improvement of the photovoltaic performance, the CdS window layer is one of the most crucial factors [4 6] because it acts as a window layer, as a seed layer for the CdTe growth, and as a sulfur source for the CdTe 1 x S x mixed crystal layer lying at CdS/CdTe metallurgical interface [7,8]. We have found that doping of the organic tin compound into CdS is effective for improving the photovoltaic performance, and already achieved a high conversion efficiency of 14.8% [6]. In this paper, we present experimental results the CdS layers doped with various metal organic (MO) compounds, i.e., (CH 3 ) 2 SnCl 2, (C 6 H 5 ) 3 GeCl, (CH 3 CO 2 ) 3 In, [(C 2 H 5 ) 2 NCS 2 ] 2 Zn. Structural changes due to MO doping found both in CdS and CdTe are described. Additionally, results on applying the CdS layers doped with the MO compounds the low-environmental-load CdS/CdTe solar cells are demonstrated. 2. Experimental The CdS layer was deposited by metal organic chemical vapor deposition (MOCVD) with a thickness of 80 nm on a glass substrate (Corning glass ]1737) coated by indium tin oxide (ITO). The MO compound, (CH 3 ) 2 SnCl 2, (C 6 H 5 ) 3 GeCl, (CH 3 CO 2 ) 3 In or [(C 2 H 5 ) 2 NCS 2 ] 2 Zn, was mixed into the MO CdS source. The doping concentration of the MO compound was changed up to 1 wt%. The CdTe layer was fabricated on the CdS layer by close-spaced sublimation (CSS) with a thickness of mm. The detailed other fabrication procedures of the CdS/CdTe solar cells are described in our previous reports [4]. Elemental analysis was performed by secondary ion microprobe mass spectrometry (SIMS) (CAMECA IMS-6f). The surface micrograph of the CdS layer was taken by fieldemission scanning electron microscope (SEM) (JEOL JSM 6340F). X-ray diffraction (XRD) measurements were carried out with a diffractometer (Rigaku RINT 2200) with Cu-Ka radiation. From the XRD peaks, the degree of (1 1 1) preferential orientation of CdTe, p(1 1 1), was derived in accordance with the method of Harris [9], while the (1 0 3) CdS grain size was estimated by the Scherrer s formula [10]. The optical transmittance of the CdS layers was measured by a double-monochromatic spectrometer (Shimadzu UV- 3100PC). Illuminated current voltage (J V) characteristics were measured under the standard condition (AM1.5, 100 mw/cm 2 ). The quantum efficiency (QE) spectrum was taken with a conventional lock-in system. Dark current voltage (J V) characteristics were measured with a pico-anmeter (HP 4140B). 3. Results and discussion Fig. 1 shows the SIMS depth profiles of Sn and Cl in the undoped CdS layer deposited on the ITO-coated glass substrate compared to those of the CdS layer doped with (CH 3 ) 2 SnCl 2. The doping concentration into the CdS source was 1 wt%. Even in the undoped CdS layer, Sn and Cl are presented with the atomic concentrations of and atoms/cm 3, respectively. This is likely to be caused by diffusion from the ITO/ glass substrate. Meanwhile, the atomic concentrations of Sn and Cl in the CdS layer doped with (CH 3 ) 2 SnCl 2 increases to and atoms/cm 3, respectively. Although the doping is not so efficient, the Sn concentration in the CdS layer actually increases.

3 3110 ARTICLE IN PRESS K. Matsune et al. / Solar Energy Materials & Solar Cells 90 (2006) Depth (nm) CdS ITO Concentration (atoms/cm 3 ) Sn closed : 1-wt.%-doped open : Undoped 37 Cl Sputtering Time (sec) Fig. 1. SIMS depth profiles of Sn (squares) and Cl (circles) in the CdS layers (undoped; open symbols, (CH 3 ) 2 SnCl 2 -doped; closed symbols). Fig. 2. Surface SEM plan views of undoped (a) and (CH 3 ) 2 SnCl 2 -doped (b) CdS on ITO/glass substrates. The doping concentration was 1 wt%. Fig. 2 shows the surface SEM micrographs of the undoped (a) and 1-wt%-(CH 3 ) 2 SnCl 2 - doped CdS (b) layers deposited on the ITO/glass substrates. From the SEM images, the lateral grain size of the CdS layer doped with (CH 3 ) 2 SnCl 2 is estimated as 52 nm, which is

4 K. Matsune et al. / Solar Energy Materials & Solar Cells 90 (2006) Undooped Zn-compound-doped Sn-compound-doped P (111) Hexagonal (103) grain size of the CdS Fig. 3. The degrees of (1 1 1) preferential orientation of CdTe deposited on the various CdS layers, p(1 1 1), plotted as a function of the hexagonal (1 0 3) grain size of the CdS layer doped with (CH 3 ) 2 SnCl 2 (E), (C 2 H 5 ) 2 NCS 2 ] 2 Zn ( ). Also plotted here are those of undoped CdS layers deposited with different conditions (K). larger than that of undoped CdS layer of 33 nm. In Fig. 3, the degrees of (1 1 1) preferential orientation of CdTe deposited on the various CdS layers, p(1 1 1), are plotted as a function of the hexagonal (1 0 3) grain size of the CdS layer doped with (CH 3 ) 2 SnCl 2 together with those of undoped CdS layers deposited with different conditions [4,5]. With increasing the (1 0 3) CdS grain size, p(1 1 1) is gradually increased regardless of the doping. On the other hand, with increasing the doping concentration, the CdS (1 0 3) grain size tends to increase, which is consistent with the SEM results in Fig. 2. Furthermore, comparing at the identical CdS grain size, p(1 1 1) is increased drastically due to the MO doping. Fig. 4 summarizes the photovoltaic performance of the low-environmental-load CdS/ CdTe solar cells using the CdS layers doped with the different MO compounds, i.e., (C 6 H 5 ) 3 GeCl, (CH 3 CO 2 ) 3 In, [(C 2 H 5 ) 2 NCS 2 ] 2 Zn, and (CH 3 ) 2 SnCl 2. An optimized doping concentration into the CdS source differs for each MO compound as listed in Fig. 4. Compared to J sc of the solar cell with the undoped CdS layer, J sc of those with the MOcompound-doped CdS layers apparently increases, and the increase in J sc is obtained from samples employing the CdS layers doped with any kinds of MO compounds. Furthermore, with respect to the CdS/CdTe solar cell with the Zn- or Sn-compound-doped CdS layer, V oc and F.F. also increase. As shown in Fig. 1, the atomic concentration of Cl also increases in the CdS layer due to the MO doping, however, the photovoltaic performance is improved even when the In or Zn compounds are doped in which Cl is not incorporated. Therefore, the metal doping would be a dominant issue for improving the photovoltaic performance.

5 3112 ARTICLE IN PRESS K. Matsune et al. / Solar Energy Materials & Solar Cells 90 (2006) J sc (ma/cm 2 ) V oc (V) FF (%) Eff (%) Undoped Ge (0.05wt.%) In (0.1wt.%) Zn (0.5wt.%) Sn (1wt %) Fig. 4. Photovoltaic performance of CdS/CdTe solar cells CdS layers doped with different MO compounds. Also listed at the bottom label is the optimized doping concentration into the CdS source in this experimental series for each compound. Fig. 5 shows the spectral responses of the CdS/CdTe solar cells using the CdS layers doped with MO compounds. Being almost independent of the kind of the MO compounds, the quantum efficiencies of the solar cells with the MO-compound-doped CdS layers are higher than that of the solar cell with the undoped CdS layer, which is in good agreement

6 K. Matsune et al. / Solar Energy Materials & Solar Cells 90 (2006) Quntum Efficiency(%) Undoped Ge(0.05wt.%) In(0.1wt.%) Zn(0.5wt.%) Sn(1wt.%) Wavelength (nm) Fig. 5. Spectral responses of CdS/CdTe solar cells as a function of doped impurities of CdS undoped Zn(0.5wt.%) Sn(1wt.%) Current (A/cm 2 ) Voltage (V) Fig. 6. J V characteristics of solar cells with different CdS layers (undoped, solid line; Sn-compound-doped, dotted line; Sn-compound-doped, dashed line). with the increase in J sc in Fig. 4. Furthermore, the increased QE is obtained in the almost entire spectral region. From the optical transmittance spectra of the CdS layers deposited on the ITO-coated glass substrate, which are also increased in any wavelengths due to the MO doping apart from a decrease due to the optical interference, the increase in QE is mainly attributed to the increase in optical transmittance of CdS layers doped with MO compounds. Fig. 6 shows dark J V characteristics of the CdS/CdTe solar cells using undoped CdS layer as well as the CdS layers doped with Sn and Zn compounds. At reverse bias voltages

7 3114 ARTICLE IN PRESS K. Matsune et al. / Solar Energy Materials & Solar Cells 90 (2006) and at forward bias voltageso0.3 V, the magnitude of the current of the solar cells employing the CdS doped with the Sn and Zn compounds is suppressed apparently. The suppressions in the dark current for which the carrier recombination is a dominant mechanism [11] must be caused by the increase in the (1 0 3) CdS grain size as well as in p(1 1 1) as found in Figs. 2 and 3, leading the increase in V oc and F.F. as shown in Fig. 4. Influence of the structural changes also appear in the forward bias current at 40.7 V where the series resistance of the diode dominates the J V characteristic; due to the Zn or Sn compound doping, the series resistance of the diode decreases. Finally, we have achieved the conversion efficiency of 15.1% (J sc ¼ 25.5 ma/cm 2, V oc ¼ V, F.F. ¼ 72.0%, CdTe thickness ¼ 2.7 mm, cell area ¼ 0.11 cm 2 ) using the CdS layer doped with (CH 3 ) 2 SnCl 2 in 1 wt%. 4. Conclusions We have investigated the CdS/CdTe solar cells with the CdTe thickness of o3 mm employing the CdS layers doped with various MO compounds, i.e., (CH 3 ) 2 SnCl 2, (C 6 H 5 ) 3 GeCl, (CH 3 CO 2 ) 3 In, and (C 2 H 5 ) 2 NCS 2 ] 2 Zn. The MO doping is effective for enhancing the optical transmittance of the CdS layer, resulting in the increase in QE and J sc. In addition, V oc and F.F. are also improved in the case of the solar cells with the organic Zn- or Sn-doped CdS layer. An increase in the (1 0 3) CdS grain size as well as in the degree of the (1 1 1) preferential orientation of CdTe is induced by the organic Sn doping, leading to the increase in V oc and F.F. as evidenced by dark J V characteristics. As a result, we have achieved the conversion efficiency of 15.1% for the solar cell with the 2.7-mm-thick CdTe layer. References [1] X. Wu, R.G. Dhere, D.S. Albin, T.A. Gessert, C. DeHart, J.C. Keane, A. Duda, T.J. Coutts, S. Asher, D.H. Levi, H.R. Moutinho, Y. Yan, T. Moriarty, S. Johnston, K. Emery, P. Sheldon, Proceedings of the NCPV Program Review Meeting, Lakewood, Colorado, [2] T. Aramoto, S. Kumazawa, H. Higuchi, T. Arita, S. Shibutani, T. Nishino, J. Nakajima, M. Tsuji, A. Hanafusa, T. Hibino, K. Omura, H. Ohyama, M. Murozono, Jpn. J. Appl. Phys. 36 (1997) [3] T. Toyama, T. Suzuki, M. Gotoh, K. Nakamura, H. Okamoto, Sol. Energy Mater. Sol. Cells 67 (2001) 41. [4] K. Nakamura, M. Gotoh, T. Fujihara, T. Toyama, H. Okamoto, Sol. Energy Mater. Sol. Cells 75 (2003) 185. [5] T. Toyama, H. Oda, K. Nakamura, T. Fujihara, K. Shimizu, H. Okamoto, Mat. Res. Soc. Proc. 763 (2003) 155. [6] T. Toyama H. Oda, K. Matsune, H. Okamoto, presented at Third World Conference on Photovoltaic Energy Conversion, Osaka, Japan, May 11 18, 2P-D3-57, [7] T. Toyama, T. Yamamoto, H. Okamoto, Sol. Energy Mater. Sol. Cells 49 (1997) 213. [8] T. Yamamoto, T. Toyama, H. Okamoto, Jpn. J. Appl. Phys. 37 (1998) L916. [9] G.B. Harris, Philos. Mag. 43 (1952) 113. [10] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, [11] S.M. Sze, Physics in Semiconductor Devices, second ed., Wiley, New York, 1981, pp