Thin Solid Films 521 (2012) 45–49 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Low defect interface study of intrinsic layer for c-Si surface passivation in a-Si:H/c-Si heterojunction solar cells Sangho Kim a, Vinh Ai Dao b, c, Chonghoon Shin a, Jaehyun Cho b, Youngseok Lee a, Nagarajan Balaji a, Shihyun Ahn b, Youngkuk Kim b, Junsin Yi a, b,⁎ a b c Department of Energy Science, Sungkyunkwan University, Suwon, 440-746, Republic of Korea School of Information and Communication Engineering, Sungkyunkwan University, Suwon, 440-746, Republic of Korea Faculty of Materials Science, College of Science, Vietnam National University, Hochiminh, Viet Nam a r t i c l e i n f o Available online 27 March 2012 Keywords: Passivation Spectroscopy ellipsometry Heterojunction solar cells Fraction depletion VHF-PECVD a b s t r a c t High quality hydrogenated intrinsic amorphous silicon [a-Si:H(i)] layer with adequate hydrogen content and lesser void fraction is the key to obtaining good surface passivated crystalline silicon (c-Si), with high opencircuit voltage (Voc), which will ultimately make the heterojunction with intrinsic thin layer (HIT) solar cell highly efficient In this study, we performed good surface passivation of a HIT solar cell by depositing a-Si: H(i) layers at different working pressures from 26.7 to 107 Pa by using very high frequency of 60 MHz plasma-enhanced chemical vapor deposition Based on spectroscopic ellipsometry and gas depletion analysis, we discuss the influence of the working pressure on the deposition mechanism, interface passivation and ultimately cell efficiency Highest minority lifetime of about ms was achieved at the highest working pressure of 107 Pa The decrease in working pressure results in less denser and/or incorporation of epitaxy layer inside the a-Si:H(i) films, and leads to decrease in c-Si surface passivation The performance of heterojunction solar cell device was improved with the increase of working pressure and the best photo voltage parameters of the device were found to be Voc of 647 mV, short-circuit current density of 32.28 mA/cm and efficiency of 15.57% at working pressure of 107 Pa © 2012 Elsevier B.V All rights reserved Introduction Heterojunction with intrinsic thin-layer (HIT) solar cells, developed by Sanyo Ltd in 1994, offers low-cost fabrication for high-efficiency solar cells compared to crystalline silicon (c-Si) solar cell with diffused p–n junctions [1] The world record efficiency of 23% fabricated base on HIT structures, in which stacks of hydrogenated intrinsic amorphous silicon [a-Si:H(i)] and doped a-Si:H layers help to form both the emitter and the back surface field, is also held by Sanyo group [2] The a-Si:H(i) layers, which have high amorphicity, adequate hydrogen content and low void fraction, enables to suppress surface recombination at the aSi:H/c-Si heterointerface, to ensures high cell performance [3] Deposition condition, however, strongly affects the structural and physical properties of the thin a-Si:H(i) layer Completely amorphous silicon can be obtained at low temperatures (~100 °C), but, it is believed that an ultra-thin layer on c-Si surface is epitaxial Again, hydrogen dilution is reported to be a key deposition parameter that controls film quality and phase [1] During deposition, working pressure could also affect the properties of materials deposited by plasma-enhance vapor deposition via silane depletion [4] For good surface passivation, as well⁎ zCorresponding author at: Department of Energy Science, Sungkyunkwan University, Suwon, 440-746, Republic of Korea Tel.: +82 31 290 7139; fax: +82 31 290 7159 E-mail address: yi@yurim.skku.ac.kr (J Yi) 0040-6090/$ – see front matter © 2012 Elsevier B.V All rights reserved doi:10.1016/j.tsf.2012.03.074 known, a low order silane radical such as SiH3 is preferred, which can be obtained by depositing of a-Si:H(i) films at a high working pressure as well as silane depletion In spite of the effectiveness of the deposition parameter, fewer studies have focused on the optimization of the deposition condition, especially working pressure, for the excellent properties of a-Si:H(i), Thus, the present paper focuses on such an optimization Moreover, to obtain high performance devices, it is importance to monitor the properties of the a-Si:H(i) layers This monitoring is usually difficult for heterojunction solar cells, since the thickness of the a-Si:H(i) layer is of the order of nano-scale [3,5] Owing to the measurement of phase, spectroscopic ellipsometry (SE) is sensitive to subnanometer changes and the precision of measurements is sufficient to allow determination of dielectric function information at the shorter wavelengths from few nanolayers [6] The measured data of the dielectric function of a-Si:H(i) was first simulated by using the tetrahedron model in combination with the Bruggeman effective medium approximation The hydrogen content, crystalline fraction and void fraction of the film were determined This method also suggests a way to obtain good quality passivation a-Si:H(i) layer with information of interface property such as film density and included defects in the film In this paper, the fabrication of heterojunction solar cell with a different working pressure during the deposition of a-Si:H(i) layer is 46 S Kim et al / Thin Solid Films 521 (2012) 45–49 reported Spectroscopic ellipsometry studies and fractional depletion analysis have been utilized as a tool to optimize the a-Si:H(i) layer property Experimental details The solar cell devices fabricated for this experiment consist of a commercial Czochralski-grown wafer (1–10 Ωcm, 525 μm thick) with a specular surface on the front side and an unpolished surface on the rear side that is treated by a sequence of, acetone/methanol/ deionized water, RCA Immediately before a-Si:H deposition, native oxide was removed from the wafer by dipping the wafer into 1% hydrofluoric acid for Then, the a-Si:H(i) layer was deposited at various working pressures on n-type c-Si by 60 MHz very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) in order to investigate the effect of c-Si wafer passivation and on solar cell performance The details of the deposition condition are provided in Table The VHF-PECVD system had a load-lock system and three different chambers for the respective depositions of p, n and intrinsic i layers The deposition temperature of VHF-PECVD was 200 °C Radio frequency magnetron sputtering was used to deposit the Indium Tin Oxide (ITO) thin films at substrate temperature of 200 °C The ITO films thickness was fixed at 80 ± nm The deposition of aluminum(Al)/silver(Ag) finger as the emitter contacts has been performed using the evaporation system Al was evaporated on backside prior to area defining with mesa etching Our final HIT cell structure is constructed as following: (Al/Ag)/ ITO/a-Si:H(p)-7 nm/a-Si:H(i)-6.5 nm/ c-Si (n)/a-Si:H(i)-6.5 nm/a-Si:H(n)-10 nm/Al All the deposited films were measured by using SE (VASE®, J A Woollam, 240 nm b λ b 1700 nm) at room temperature The dielectric function, the film thickness and the band gap energy were obtained by using Tauc–Lorentz model In order to verify the SE analyses, cross-section of the transmission electron microscopy (TEM) micrographs was acquired by a JEM-ARM200F-JEOL system operated at an acceleration voltage of 200 kV The samples were prepared by mechanical polishing followed by keV Ar ion milling using a Gatan PIPS ion miller The silane fraction depletion (F) is defined as F = (Po − P)/Po, where Po and P are the partial pressures of the silane with the plasma turn off and on respectively To measure F during a film deposition, the plasma was first turned on with the required very high frequency input power, and the working pressure After the plasma stabilized the partial pressure of the silane (P) was measured The plasma was then turned off while the computer continued to monitor the Po [7,8] In order to determine the heterointerface quality, the effective lifetime (τeff) was measured by the quasi-steady-state Photoconductance (QSSPC) method, using a commercial Photo conductance set-up from Sinton Consulting (WCT-120) The quantum efficiency (QE) of the heterojunction solar cells was measured by using a xenon lamp, a monochromator, and optical filters, which filtered out the high orders with a light probe beam impinging normal on the samples Solar cell performance was characterized by current–voltage measurements under illuminated AM1.5, 100 mW/cm conditions Result and discussion Fig shows the effect of the a-Si:H(i) layer deposited at different working pressures on the real and imaginary parts of the pseudodielectric function, in which the dielectric function of the films is obtained multiplying the equation of the Lorentz oscillator by the equation of the Tauc join density of states [9]: 2 A:Eo :C: EEg imTL Eị ẳ : E > Eg ; 2 E2 Eg ỵ C :E2 E 1ị imTL Eị ẳ EEg ð2Þ where E0 is the peak transition energy, Eg is the band gap energy and C is a broadening parameter which can be related to the degree of disorder in the material A is another parameter which is proportional to the height of bεim,max> that is related to the film density At the lowest working pressure of 26.7 Pa, peaks started to appear near 3.4 eV and 4.2 eV, indicative of partial epitaxy growth With further increase in working pressure, a smooth, single-peaked dielectric function of amorphous silicon is observed at around 3.7 eV and the amplitude of A gradually increases, indicating the increase of density of the films [9] On the other hand, the minimum amplitude of bεreal,min>, which is related to the void fraction in the film, decreases with the increase of working pressure, particularly around 4.75 eV [5] Therefore, high quality a-Si:H(i) can be obtained for c-Si surface passivation layer by increasing the working pressure because the observed increase and decrease of bεim,max> and bεreal,min>, respectively, led to low percentage void fraction inside the material with adequate hydrogen content [9] In order to evaluate the validity of our SE analysis, the TEM image of the a-Si:H/c-Si heterointerface was performed for the samples with the lowest and the highest working pressure As shown in Fig 2, the film quality from the TEM image shows excellent agreement with the SE result From Fig 2a, it can be seen that the epitaxial growth occurs at lowest working pressure of 26.7 Pa, encompassing the intrinsic layer (~5 nm) and extending well into the p-layer For the higher working pressure of 107 Pa, abrupt amorphous-silicon growth is observed as clearly shown in Fig 2b Surface recombination velocity (Seff) was estimated from the τeff We assume that the device structure is symmetrical (Seff = Sfront = Sback) The Seff was estimated by the equation Seff ẳ w 1 ỵ eff τ b ! ð3Þ where τb is the recombination lifetime in the bulk and W is the wafer thickness The uncertainty of Seff depends on the value used for τb The Shockley–Read–Hall recombination was not considered to calculate the upper limit of the surface recombination velocity So τb has an unlimited value The τeff and Seff as a function of working pressure are depicted in Fig The minimum value of Seff which Table Deposition condition of hydrogenated intrinsic amorphous silicon layer in this study Sample no Pressure (Pa) Gas ratio (SiH4:H2) Temp (°C) Electrode distance (mm) Power (W) Deposition time (s) Thickness (nm) Deposition rate (nm/min) 26.7 53.3 80.0 107.0 1:5 200 60 37 15 26 30 40 6.5 ± 0.5 27.6 17.0 13 10.8 S Kim et al / Thin Solid Films 521 (2012) 45–49 47 εim 20 10 10 ε real 20 26.7 Pa 53.3 Pa Effective lietime,τeff (ms) 30 102 101 80.0 Pa 107.0 Pa -10 Photon Energy (eV) 0.1 20 40 60 80 100 Surface recombination,Seff (cm/s) 10 30 100 Working pressure (Pa) Fig Effect of the working pressure on the dielectric function of the a-Si:H(i) thin film deduced from the Tauc–Lorentz analysis of the experiment data Fig Measured effective lifetime (τeff) and calculated surface recombination velocity (Seff) of a-Si:H(i)/c-Si/a-Si:H(i) samples as a function of working pressure corresponds to the highest value of effective lifetime, was determined from QSSPC measurements, and is in agreement with the result of Olibet et al [10] Fig shows that the lowest effective lifetime is exhibited by the sample deposited at the lowest working pressure of 26.7 Pa As reported by Wang et al with incorporation of the epitaxy on c-Si surface, it extends through the a-Si:H(i) layer, so the defect interface will come in contact with the emitter layer, leading to lesser passivation than that of the pure a-Si:H(i) layer [11] The τeff linearly increased with increasing working pressure, because of the lower void fraction inside the film with adequate hydrogen content, at higher working pressures [5] The plasma diagnostics and growth mechanism were analyzed quantitatively to provide further insight into how the a-Si:H(i) properties, and subsequently, the surface passivation change with working pressure Fig shows the silane fractional depletion and deposition rate as a function of the working pressure We found that the fractional depletion is increased with the increase of the working pressure while the deposition rate is decreased This opposite trend can be attributed to the hydrogen effect with changing working pressure During the growth of films, atomic hydrogen can move in or out of the surface The hydrogen can react with the silicon network in the subsurface region after the silicon has attached to the surface The hydrogen, then, terminates dangling bonds and removes weak bonds while excess hydrogen is evolved from the film [12] With increasing working pressure resulting in increasing silane depletion, the atomic hydrogen, thus, dominates the film This phenomenon leads to the decomposition of H2 resulting in hydrogen coverage of the growing film surface, and then enhances the precursors absorbed at the growing surface to have enough time to find their energetically suitable sites that decreased the structural disorder in the films as well as the deposition rate [13,14] Therefore, with increasing working pressure, the silane depletion is more accomplished, leading to denser films with moderating hydrogen content, which is a merit property for c-Si surface passivation [5] The performance of the solar cell devices is indicated in Fig This figure shows an apparent increase in both open-circuit voltage (Voc) and short-circuits current density (Jsc) with the increase in working pressures The increase of Voc and Jsc, consequently, leads to an increase in solar cell efficiency from 11.95 to 15.57% with increasing working pressure from 26.7 Pa to 107 Pa As mentioned earlier, the lowest Voc of the sample deposited at the lowest working pressure could be attributed to the incorporation of epitaxy into the a-Si:H(i) layer, resulting in lesser surface passivation of the a-Si:H/c-S heterointerface With further increase in working pressure, however, the better a-Si:H(i) film properties with adequate hydrogen content and less void fraction lead to longer effective carrier lifetime of the Fig Cross-section TEM images of a-Si:H(i) deposition on a c-Si wafer (a) at working pressure of 26.7 Pa and (b) working pressure of 107 Pa 48 S Kim et al / Thin Solid Films 521 (2012) 45–49 10 30 25 20 15 Quantum efficiency (%) Deposition rate (nm/min) Fractional depletion (%) 80 70 60 Working pressure 50 26.7 Pa 40 53.3 Pa 80.0 Pa 30 107.0 Pa 10 20 40 60 80 100 20 400 120 600 Working pressure (Pa) Fig Fractional depletion and deposition rate as a function of working pressure 800 1000 Wavelength (nm) Fig The QE curves of HIT solar cells as function of working pressure Conclusions passivated wafers, which in turn leads to higher open-circuit voltage of the HIT solar cell To elucidate the reason for the increase in Jsc with increasing working pressure, the QE of the four different cells was characterized and shown in Fig The solar cells fabricated with lower working pressure indicate better spectral response at shorter wavelengths of about 400 nm, especially at 26.7 Pa In this case, the a-Si:H(i) layer may be thinner than the other due to the growth of the epitaxial on the c-Si surface This thinness leads to lower light absorption, resulting in better in QE at shorter wavelengths Solar cell devices, however, fabricated at higher working pressures indicate better spectral response above 500 nm wavelength Fig shows the energy band gap of the a-Si:H layer as a function of working pressure The a-Si:H(i) layer deposited at higher pressures show the higher energy band gap, which leads to more light absorption by the wafer and the creation of more electron hole pairs Hence, the higher spectral response at wavelengths around 500 to 600 nm may be expected On the back surface, in addition, the higher energy band gap of the a-Si: H(i) layer deposited at higher working pressure leads to the larger valence band offset, and this provides a back surface “mirror” for holes, however does not much hinder electron transport due to the smallness of the offset in the conduction band edges [15] Thus, both higher energy band gap and merit properties for c-Si surface passivation of the a-Si:H(i) layer deposited at higher working pressure conduct to a better back-surface passivation, thus, in turn improved QE in the longer wavelength; which is in accord with Yamanaka et al [16] In summary, the effect of working pressure on a-Si:H/c-Si interface passivation during deposition of the a-Si:H(i) layer on a-Si: H(p)/c-Si(n) interface passivation and hence on device performance was investigated by spectroscopic ellipsometry as well as by fraction depletion analysis From SE, we found the formation of mixing phase, epitaxy and hydrogenated amorphous silicon, on the c-Si surface at the low working pressure of 26.7 Pa This incorporation of the epitaxial layer in a-Si:H(i) led to the suppression of the surface passivation of the c-Si surface, and hence to the reduction of Voc, and also cell performance For working pressures above 26.7 Pa, the silane depletion was increased, which led to denser films with adequate hydrogen content, which resulted in the longer lifetime of the passivation of the c-Si surface Highest minority lifetime of about ms was achieved at the highest working pressure of 107 Pa At the optimum condition, we obtained a conversion efficiency of 15.57% Thus, the high quality of the a-Si:H(i) layer with low defects and low void fraction in the film is very important for the a-Si:H/c-Si heterointerface, one of the major factors to improve the junction properties and enable the high Voc and also enhance cell efficiency Acknowledgments This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No 20113010010100) η (%) 16 1.95 14 12 Jsc (mA/cm2) Eg (eV) Voc (mV) 1.90 650 585 1.85 33 1.80 30 27 20 40 60 80 100 Working pressure (Pa) Fig Parameters of short-circuits current, Jsc, open-circuit voltage, Voc, and cell efficiency, η, measured on heterojunction solar cells as function of working pressure 20 40 60 80 100 120 Working pressure (Pa) Fig Optical band gap, Eg, of a-Si:H(i) films deposited on c-Si substrate as a function of working pressure S Kim et al / Thin Solid Films 521 (2012) 45–49 References [1] V.A Dao, J Heo, H Choi, Y Kim, S Park, S Jung, N Lakshminarayan, J Yi, Solar Energy 84 (2010) 777 [2] T Mishima, M Taguchi, H Sakata, E Maruyama, Sol Energy Mater Sol Cells 95 (2011) 18 [3] H Fujiwara, M Kondo, Appl Phys Lett 90 (2007) 013503 [4] A Descoeudres, L Barraud, R Barlome, C Ballif, Appl Phys Lett 97 (2010) 183505 [5] V.A Dao, Y.S Lee, S.H Kim, J Yi, J Electrochem Soc 158 (2011) H312 [6] C Pickering, Surf 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USA, 2004, p [16] M Yamanaka, I Sakata, R Shimokawa, H Takato, Conf Record of the 2006 IEEE 4th World Conf on Photovoltaic Energy Conversion, Hawaii, USA, 2006, p 1421 ... the lowest Voc of the sample deposited at the lowest working pressure could be attributed to the incorporation of epitaxy into the a-Si:H( i) layer, resulting in lesser surface passivation of the... found the formation of mixing phase, epitaxy and hydrogenated amorphous silicon, on the c-Si surface at the low working pressure of 26.7 Pa This incorporation of the epitaxial layer in a-Si:H( i)... come in contact with the emitter layer, leading to lesser passivation than that of the pure a-Si:H( i) layer [11] The τeff linearly increased with increasing working pressure, because of the lower