Superlattices and Microstructures

Transcription

1 Superlattices and Microstructures 52 (2012) Contents lists available at SciVerse ScienceDirect Superlattices and Microstructures journal homepage: Improved efficiency of InGaN/GaN-based multiple quantum well solar cells by reducing contact resistance Jun-Hyuk Song a, Joon-Ho Oh a, Jae-Phil Shim b, Jung-Hong Min b, Dong-Seon Lee b, Tae-Yeon Seong a, a Department of Materials Science and Engineering, Korea University, Seoul , Republic of Korea b School of Information and Communications, Gwangju Institute of Science and Technology, Gwangju , Republic of Korea article info abstract Article history: Received 3 April 2012 Accepted 5 May 2012 Available online 22 May 2012 Keywords: InGaN Solar cell Cu-doped indium oxide Ohmic contact We report on the improvement in the performance of InGaN/GaN multi-quantum well-based solar cells by the introduction of a Cu-doped indium oxide (CIO) layer at the interface between indium tin oxide (ITO) p-electrode and p-gan. The solar cell fabricated with the 3 nm-sample exhibits an external quantum efficiency of 29.8% (at a peak wavelength of 376 nm) higher than those (25.2%) of the cell with the ITO-only sample. The use of the 3-nm-thick CIO layer gives higher short circuit current density (0.72 ma/cm 2 ) and fill factor (78.85%) as compared to those (0.65 ma/cm 2 and 74.08%) of the ITO only sample. Measurements show that the conversion efficiency of the solar cells with the ITO-only sample and the 3 nm-sample is 1.12% and 1.30%, respectively. Based on their electrical and optical properties, the dependence of the CIO interlayer thickness on the efficiency of solar cells is discussed. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction InGaN-based semiconductors have been extensively investigated for their applications in various optoelectronic devices, such as light emitting diodes (LEDs) and laser diodes (LDs) [1,2]. Depending on the content of InN, alloying GaN with InN (i.e., InGaN alloys) allows the band gaps to vary from 0.7 ev to 3.4 ev, covering a wide range of wavelengths from near infrared to ultraviolet solar spectrum. In addition, InGaN alloy layers have a high absorption coefficient, high mobility, high saturation Corresponding author. Tel.: ; fax: address: tyseong@korea.ac.kr (T.-Y. Seong) /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.

2 300 J.-H. Song et al. / Superlattices and Microstructures 52 (2012) velocity, and strong radiation tolerance [3 7]. Furthermore, the toxicity of arsenic in GaAs-based solar cells causes InGaN alloy layers to be more environmentally attractive in green energy fields. For these reasons, the InGaN alloys are of increasing interest for their application in solar cells. In order to enhance the external quantum efficiency of solar cells, the excellent electrical properties of electrodes must be realized. Series resistance plays an important role in achieving the overall conversion efficiency of solar cells. As in InGaN-based LEDs, InGaN-based solar cells also consist of p-type electrodes, such as semitransparent Ni/Au electrode [8]. However, such semitransparent electrodes absorb a significant amount of sunlight because of their transmittance lower than 60% in the nm wavelength region which corresponds to the bandgap of GaN [9,10]. Thus, the use of transparent electrodes is necessary so as to increase the performance of solar cells. Indium-doped tin oxide (ITO) is a well-known transparent conducting oxide (TCO) which has a low resistivity and a high transmittance at the visible wavelength region [11]. Shim et al. [12] investigating the effect of transparent contact layers on the performance of InGaN-based p i n solar cells, showed that the efficiency could be improved from 0.75% to 1% when Ni/Au layers were replaced by a transparent current spreading layer (ITO). On the one hand, Song et al. [13] showed that because ITO has smaller work function than does p-gan, the use of ITO resulted in a high Schottky barrier height (SBH) at the interface between p-type GaN and ITO, consequently limiting the charge transport across the interface of InGaN-based devices. In this work, we also employed Cu-doped indium oxide (CIO) layers (with a thickness of 1, 3 and 5 nm) at the interface between p-type GaN and ITO so as to improve the charge transport across the interface. In other words, we investigated the photovoltaic performance of InGaN-based solar cells as a function of the thickness of the CIO interlayers. Their open circuit voltage (V oc ), short circuit current density (J sc ), fill factor (FF), external quantum efficiency (EQE), and conversion efficiency (g) were characterized. 2. Experimental procedure InGaN/GaN multi-quantum well (MQW) epitaxial wafers were grown by using a metal organic chemical vapor deposition (MOCVD) system on c-plane patterned sapphire substrates. A 2-lm-thick undoped GaN buffer layer, a 2-lm-thick Si-doped n-type GaN layer, an intrinsic light absorption layers (i.e., 7 periods of In 0.15 Ga 0.85 N/GaN MQWs (3 nm and 8 nm thick, respectively), and a 100-nm-thick Mg-doped p-type GaN layer were sequentially grown on the sapphire substrate. The wafers were then mesa-etched by using a dry etcher to define each cell. Before deposition, the samples were dipped in buffered oxide etchant (BOE) for 1 min so as to remove native oxide on p-gan and were rinsed in deionized (DI) water. After that, CIO/ITO current spreading layers were deposited by an e-beam evaporator as follows: ITO (150 nm), CIO/ITO (1 nm/150 nm), CIO/ITO (3 nm/150 nm), and CIO/ITO (5 nm/ 150 nm), which were referred to herein as ITO-only sample, 1 nm-sample, 3 nm-sample, and 5 nm-sample, respectively. The CIO layers nominally contained 10 at% of Cu ([Cu]/[Cu] + [In]). Some of the samples were annealed at 500 C for 2 min in air by using a rapid thermal annealing system. (ITO films deposited on the glass substrates showed a low resistivity of Xcm when annealed at 500 C for 2 min). Then, Cr(30 nm)/au(300 nm) p-type contact pad (grid) and also same thickness of Cr/Au n-type electrode were deposited by means of an e-beam evaporator. Finally, each sample was electrically connected PCB through an Au wire bonder. Fig. 1a and b show plan-view optical microscopic chip image of a device (a chip size of 1 mm 1 mm) and schematic of the cross-section structure of a solar cell, respectively. The photovoltaic performance of devices was characterized by means of commercial solar simulator (Oriel Class AAA solar simulator) and an incident photocurrent efficiency measurement system. These measurements were carried out under air-mass (AM) 1.5 and 1 sun (100 mw/cm 2 ) condition. Transmittance of CIO/ITO structures were measured by using a Shimadzu UV-1800 spectrophotometer. 3. Results and discussion Fig. 2a shows typical light current density voltage (I V) characteristics obtained from the samples as a function of the thicknesses of CIO interlayers. It is shown that except the 5 nm-sample, the current

3 J.-H. Song et al. / Superlattices and Microstructures 52 (2012) Fig. 1. (top) A plan-view optical microscopic chip image of a device (a chip size of 1 mm 1 mm) and (bottom) schematic of the cross-section structure of a solar cell. density of the CIO interlayer samples increases with increasing interlayer thickness and is higher than that of the ITO-only sample. Fig. 2b reveals the external quantum efficiency (EQE)-wavelength relation of the samples as a function of the interlayer thickness. It is shown that major light absorption occurs in the nm wavelength region, because of the wide bandgap of the InGaN absorber layers used in this work. The solar cells fabricated with the ITO only-sample, the 1 nm-sample, the 3 nm-sample, and the 5 nm-sample give an EQE of 25.2%, 27.5%, 29.8%, and 22.2%, respectively, at a peak wavelength of 376 nm. It is noteworthy that the interlayer-thickness dependence of EQE is consistent with the variation of the current densities as shown in Fig. 2a. It is noted that the solar cells with the 3 nm-sample exhibits the best property possibly due to the enhanced short-wavelength response as will be discussed later. Fig. 3 shows the open circuit voltage (V oc ), short circuit current density (J sc ), fill factor (FF), and conversion efficiency (g) of the samples. These results are summarized in Table 1. The V oc of all the samples are in the range of approximately V. Such large value of V oc is the electrical characteristics of InGaN-based solar cells which have wide band gaps. The solar cell fabricated with the ITO-only sample shows a J sc of 0.65 ma/cm 2, which is comparable to the result observed previously by Shim et al. [12]. The solar cells fabricated with the 1 nm-sample, the 3 nm-sample, and the 5 nm-sample exhibit a J sc of 0.68 ma/cm 2, 0.72 ma/cm 2, and 0.54 ma/cm 2, respectively. Furthermore, the cell with the ITO-only sample gives a FF of 74.08%. On the other hand, the cells with the 1 nm-sample, the 3 nm-sample, and the 5 nm-sample yield a FF of 76.75%, 78.85%, and 74.90%, respectively. Measurements show that the conversion efficiency of the solar cells with the ITO-only sample, the 1 nm-sample, the 3 nm-sample, and the 5 nm-sample were 1.12%, 1.18%, 1.30%, and 0.93%, respectively. It is noted that apart from the 5 nm-sample, the use of the CIO interlayer improves the conversion efficiency. Since the transmittance of the CIO interlayer has an effect on the absorption of the incident light, the absorption of the incident light must be minimized at the top CIO/ITO layer, but maximized at the active layers [4]. Fig. 4 shows the transmittance of ITO-only sample, the 1 nm-sample, the 3 nm-sample,

4 302 J.-H. Song et al. / Superlattices and Microstructures 52 (2012) Fig. 2. (a) Typical light current density voltage (I V) characteristics obtained from samples as a function of the thicknesses of CIO interlayers. (b) External quantum efficiency (EQE)-wavelength relation of samples as a function of interlayer thickness. The solid line denotes light power intensity simulated by means of an incident photocurrent efficiency measurement system. and the 5 nm-sample deposited on glass. The CIO/ITO samples show a transmittance in the range of %, which is only slightly lower than those (73.2%) of the ITO-only sample across the nm wavelength region. It is noted that the transmittance of the CIO/ITO samples decreases slightly with an increase in the thickness of the CIO layer because the CIO layer has somewhat higher extinction coefficient than does the ITO layer, as shown in the inset in Fig. 4. It is well known that the efficiency of solar cells is determined by four different operation steps, such as absorption, generation, separation, and collection [14]. The precise mechanisms for the CIO interlayer-induced improvement are not clearly understood at the moment. However, a comparison of their transmittance (Fig. 4) indicates that the thickness effects may be explained in terms of the improvement in the collection efficiencies of the solar cells fabricated with the interlayers. Schottky barriers at the TCO electrode/p-gan interface could have an influence on J sc and FF of the cells. This indicates that the contact resistance at the p-gan/p-electrode interface plays an important role in improving the efficiency. It should be noted that p-gan has a fairly large work function of 6.7 ev [15], which is larger than those of any metals and conventional transparent conducting oxides. Thus, large Schottky barrier heights (SBHs) are formed at the p-gan/electrode interfaces. Consequently, such large SBHs limit the collection of carriers across the interface. For this reason, efforts

5 J.-H. Song et al. / Superlattices and Microstructures 52 (2012) Fig. 3. Open circuit voltage (V oc ), short circuit current density (J sc ), fill factor (FF), and conversion efficiency (g) of samples as a function of CIO interlayer thickness. Table 1 Summary of open circuit voltage (V oc ), short circuit current density (J sc ), fill factor (FF), and conversion efficiency (g) of samples. CIO thickness (nm) V oc (V) J sc (ma/cm 2 ) FF (%) Eff. (%) were made in order to reduce the SBHs at the interfaces. For example, Song et al. [16] reported that for InGaN/GaN near-uv LEDs, the use of the CIO interlayers at the p-gan/ito interface effectively lowered the contact resistivity. As shown in Figs. 2 and 3, the InGaN-based solar cells fabricated with the 1 nmsample and the 3 nm-sample produced higher J sc and FF than did the cells with ITO only-sample. However, the cells with the 5 nm-sample exhibited lower J sc and FF than the cells with the ITO only-sample. The dependence of the interlayer thickness on the efficiency could be explained in terms of the electrical and optical properties of the CIO interlayer at the interface. For the 1 nm-sample and the 3 nm-sample, annealing may cause thin CIO layers to be broken into nano-dots (8 25 nm in size) having Ga In Cu multi-components at the GaN/ITO interface, as previously described by Song et al. [16]. This results in the electrode/p-gan interface with inhomogeneous SBHs. The electronic transport theory at the electrode/semiconductor interface with inhomogeneous SBHs [17] shows that the presence of such nano-dots causes the generation of electric field at the interface. It was shown that the electric field increased with decreasing the size of nano-dots [13] and so the increase in the electric field brought about a lowering of barrier height [16]. Consequently, this causes carriers to be effectively transported to the electrode, namely, effective carrier collection. On the other hand, the lower efficiency of the solar cells with the 5 nm-sample may be explained as follows. Fang et al., investigating the electrical characteristics of a-si solar cells fabricated with (2 nm and 5 nm-thick) amorphous WO x

6 304 J.-H. Song et al. / Superlattices and Microstructures 52 (2012) Fig. 4. Transmittance of ITO-only sample, 1 nm-sample, 3 nm-sample, and 5 nm-sample deposited on glass. Extinction coefficient of ITO and CIO layers are shown in the inset. interlayers (at the F-doped SnO 2 /p-type a-sic interface) [18], showed that the solar cells with the 5 nm-thick interlayer produced lower J sc and FF. This poorer performance was attributed to the amorphous nature of WO x. In other words, they argued that the increase in the oxide interlayer thickness might increase the density of trap states within the energy gap, serving as non-radiative recombination centers. Similar mechanisms might also account for the lower performance of our cells with the 5 nm-sample. In addition, our previous results showed that the 5-nm-thick CIO layer was broken into relatively large nano-dots (30 60 nm in size) when annealed at 500 C [19]. Such large nano-dots generate relatively lower electric fields, so making an insignificant contribution to the reduction in the SBHs [16]. Combined effects of the two mechanisms may be responsible for the performance of cells with the 5 nm-thick sample. However, the exact mechanisms remains to be further clarified. 4. Summary We investigated the effect of the CIO interlayer on the performance of InGaN/GaN MQW-based solar cells as a function of the thickness of the interlayer. The transmittance of the CIO/ITO samples was slightly lower than those of the ITO-only sample. The introduction of the 3-nm-thick CIO layer produced higher short circuit current density and fill factor than did the ITO-only sample. The solar cell fabricated with the 3 nm-sample showed higher conversion efficiency than did those with the ITOonly sample. This result implies that the use of the 3-nm-thick CIO interlayer may serve as a potentially important buffer layer for enhancing the performance of InGaN/GaN MQW-based solar cells. Acknowledgments This work was supported by Energy Resource R&D Program (No ) under the Ministry of Knowledge Economy and the World Class University Program through the National Research Foundation of Korea funded by MEST (R ). References [1] E. Fred Schubert, 2nd Light-Emitting Diodes, Cambridge University Press, Cambridge, [2] M.K. Kelly, O. Ambacher, B. Dahlheimer, G. Groos, R. Dimitrov, H. Angerer, M. Stutzmann, Appl. Phys. Lett. 69 (1996) [3] O. Jani, I. Ferguson, C. Honsberg, S. Kurtz, Appl. Phys. Lett. 91 (2007)

7 J.-H. Song et al. / Superlattices and Microstructures 52 (2012) [4] C.J. Neufeld, N.G. Toledo, S.C. Cruz, M. Iza, S.P. DenBaars, U.K. Mishra, Appl. Phys. Lett. 93 (2008) [5] J.Q. Wu, J. Appl. Phys. 106 (2009) [6] J. Wu, W. Walukiewicz, K.M. Yu, M. Shan, J.W. Ager, E.E. Haller, H. Lu, W.J. Schaff, W.K. Metzger, S. Kurtz, J. Appl. Phys. 94 (2003) [7] J.F. Muth, J.H. Lee, I.K. Shmagin, R.M. Kolbas, H.C. Casey, B.P. Keller, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 71 (1997) [8] J.K. Ho, C.S. Jong, C.C. Chiu, C.N. Huang, C.Y. Chen, K.K. Shih, Appl. Phys. Lett. 74 (1999) [9] K.M. Chang, J.Y. Chu, C.C. Cheng, Solid-state Electron 49 (2005) [10] C.H. Kuo, S.J. Chang, Y. Su, R.W. Chuang, C.S. Chang, L.W. Wu, W.C. Lai, J.F. Chen, J. Sheu, H.M. Lo, J.M. Tsai, Mater. Sci. Eng. B- Solid State Mater. Adv. Technol. 106 (2004) [11] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 83 (1998) [12] J.P. Shim, S.R. Jeon, Y.K. Jeong, D.S. Lee, IEEE Electron Device Lett. 31 (2010) [13] J.O. Song, J.S. Ha, T.Y. Seong, IEEE Trans. Electron Devices 57 (2010) [14] H. Tsumomura, H. Kobayashi, Crit. Rev. Solid State Mater. Sci. 18 (1993) [15] R.J. Nemanich, M.C. Benjamin, S.W. King, M.D. Bresmer, R.F. Davis, B. Chen, Z. Zhang, J. Bernhole, Gallium Nitride and Realated Materials, in: R.D. Dupuis, J.A. Edmond, F.A. Ponce, S. Nakamura (Eds.), Mater. Res. Soc. Symp. Proc. 395, Pittsburgh, PA, [16] J.O. Song, J.S. Kwak, Y. Park, T.Y. Seong, Appl. Phys. Lett. 86 (2005) [17] R.T. Tung, Phys. Rev. B 45 (1992) [18] L. Fang, S.J. Baik, K.S. Lim, S.H. Yoo, M.S. Seo, S.J. Kang, J.W. Seo, Appl. Phys. Lett. 96 (2010) [19] In Ref. [16], we investigated only the 3-nm-thick CIO interlayer, because the thicker interlayer (e.g., 5 and 10 nm in thickness) insignificantly improved the electrical properties of the CIO/ITO contacts.