Efficiency enhancement of silicon solar cells with silicon nanocrystals embedded in PECVD silicon nitride matrix

c ri . A a a a b 031 119 PECVD Silicon nanostructures RC stal bric N fi dep con ls- to a & 2012 Elsevier B.V. All rights reserved. eading of the silicon ll, lattic s for 47 nconv wncon range of 254–365 nm with ultrathin films of silicon nanoparticles cells Siltronix . Three types of solar cells were fabricated with labels Contents lists available at SciVerse ScienceDirect .els Solar Energy Mater Solar Energy Materials & Solar Cells 101 (2012) 32–35 optimized KOH based etching process which contained 3 v% ofE-mail address: [email protected] (W.R. Taube). dispersed over a commercial polycrystalline silicon solar cell. A, B and C using standard semiconductor processing steps with three levels of photolithography for selective emitter, contact window and metal patterning. In the first step, the wafers were selectively textured with 0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2012.02.010 n Corresponding author. Tel.: þ1596 246021; fax: þ1596 242294. than 425 nm has been observed in solar cells covered by a spin- on-glass with silicon nanocrystals [15]. Stupca et al [16] has reported a power performance enhancement of �60% in the fabricated and studied are shown in Fig. 1. Solar cells were fabricated using 200, /100S, 300 mm thick, 1–10O-cm resistivity, p-type Czochralski single side polished silicon wafers from TM processes is easier to achieve [4,5,8]. Various studies have reported efficient photoluminescence in silicon based nanomaterials [9–14]. An increase in the internal quantum efficiency (IQE) for photons with wavelengths shorter 2. Experimental procedure The cross section schematics of the monocrystalline solar of a conventional solar cell, high energy photons will be absorbed to emit low energy photons. These photons are then absorbed efficiently by the solar cell below, resulting in increased conversion efficiency. While there are other materials and nanomaterials which can exhibit such downconversion behavior [6,7], silicon based nanostructured materials are preferable as its integration with existingmanufacturing refractive indices (RI) of these films are crucial to ensure antire- flection behavior along with their downconverting property. This motivated us to take-up this study on developing nanostructured PECVD silicon-rich-nitride films and their integration in fabri- cated monocrystalline silicon solar cells. Photoluminescence 1. Introduction Silicon solar cell technology is the l Silicon based nanostructures are one dates for enhancing the efficiency of different loss mechanisms in a solar ce energy photons (Ephoton4Eg) account be reduced to some extent using dow silicon nanostructures [4,5]. With a do photovoltaic technology. most promising candi- solar cells. Among the e thermalization of high % loss [1]. This loss can ersion [2,3] property of verting layer on the top Recently, Yuan et. al. [17] has demonstrated a relative increase of 14% in IQE, by silicon-rich-oxide layer containing silicon nano- crystals, on the top of a conventional silicon solar cell. But this downconverting layer was not optimized for antireflection and hence there was a decrease in power conversion efficiency from 15% on the conventional cell to 9.9% on the cell with SRO layer. As per the authors’ knowledge, significant improvement in conver- sion efficiency using downconverting silicon nanostructures is lacking. This suggests that choice of optimal thicknesses and Silicon solar cell Letter Efficiency enhancement of silicon solar embedded in PECVD silicon nitride mat William R. Taube a,n, A. Kumar a, R. Saravanan a, P.B a CSIR-Central Electronics Engineering Research Institute (CEERI), Pilani, Rajasthan 333 b Electronics Science Department, Kurukhshetra University, Kurukhshetra, Haryana 136 a r t i c l e i n f o Article history: Received 24 August 2011 Received in revised form 7 February 2012 Accepted 9 February 2012 Available online 9 March 2012 Keywords: Downconversion Silicon-rich-nitride a b s t r a c t Anti reflection coatings (A converting silicon nanocry standard semiconductor fa one with single layer SR enhanced chemical vapor annealed to precipitate sili for single layer nanocrysta were observed compared journal homepage: www garwal , P. Kothari , B.C. Joshi , D. Kumar , India , India ), comprising of silicon-rich-nitride (SRN) films embedded with down- s were integrated into monocrystalline silicon solar cells fabricated using ation techniques, and their effects were studied. Two types of ARC layers, lm and other with double layer SiOx/SRN were deposited by plasma osition (PECVD) during fabrication of silicon solar cells and thermally nanocrystals. A relative increase in power conversion efficiency of 15.6% embedded-ARC and 22.8% for double layer nanocrystals-embedded-ARC reference cell. ells with silicon nanocrystals x evier.com/locate/solmat ials & Solar Cells W.R. Taube et al. / Solar Energy Materials & Solar Cells 101 (2012) 32–35 33 45% KOH and 10 v% IPA. The top emitter was realized by phosphorus diffusion at 850 1C resulting in emitter sheet resis- tance of 75O/square. Boron diffusion was done for back surface field. Second phosphorus diffusion step was carried out at 1000 1C for selective emitters. PECVD SRN, SiNx, SiOx based antireflection coatings were deposited on these cells. The films’ thicknesses and refractive indices were optimized using transfer matrix method (TMM) as discussed in Section 3. Solar cell C is the reference cell with normal SiNx ARC of refractive index 1.83 and thickness 83 nm. Solar cell A has a single Layer ARC of SRN with refractive index 2.00 and thickness 76 nm. Solar cell B has a double layer ARC with bottom layer comprising of SRN of refractive index 2.2 and thickness 62 nm and top layer comprising of normal PECVD SiOx of refractive index 1.46 and thickness 93 nm. Film thicknesses were measured using TalystepTM surface profiler and refractive indices were measured using Metricon2010TM Prism Coupler and single wavelength laser ellipsometer both at wavelength 632.8 nm. The increased refrac- tive indices of SiNx in samples A and B are achieved by increasing the [SiH4]/[NH3] ratio during PECVD deposition process thus making it silicon-rich. The deposited SRN films (cells A and B) were annealed at 700 1C for 30 minutes in N2 ambient, leading to precipitation of silicon nanocrystals [9,18]. Aluminum was depos- ited on both sides for contact formation by RF sputtering. The top side aluminum films were patterned by photolithography fol- lowed by chemical etching to define contact fingers. The backside Fig. 1. Schematics of the solar cells (a) Cell A is with SRN ARC with embedded silicon na layer ARC (c) Reference cell C is with normal PECVD SiNx ARC. of the cell is completely covered with aluminum. The size of the each cell is 3.00 cm�3.00 cm with active area of 2.85 cm�2.85 cm. The fabricated solar cells are shown in Fig. 2. Efficiency measurements were performed in a solar simulator under 100 mW/cm2, corresponding to AM1.5 G insolation. Before measurement, the intensity of the source was calibrated using a standard calibrated solar cell. Current density-voltage (J-V) char- acteristics were measured with a source measure unit using a 4 point probe technique. The temperature of the solar cells was maintained at 2571 1C during the measurements. Spectral response and external quantum efficiency (EQE) measurements were done on solar cell spectral response measurement system (Model: Bunkoh-Keiki Co. CEP-45 HS-40 SR) owing to IEC60904 standard. The short circuit current densities obtained from J-V measurements and spectral response are matching well within 73%. Fig. 3 shows the J-V characteristics of the three solar cells. Short circuit current density Jsc, open circuit voltage Voc, fill factor FF and efficiency Z extracted from the J-V characteristics are given in Table 1. 3. Downconversion mechanisms and optimization of ARC Two mechanisms of downconversion are possible with silicon nanocrystals [4]: (1) Photo- luminescence (PL) and (2) Photon splitting via MEG (Multiple Exciton Generation) also called as nocrystals (b) Cell B is with SiOx/SRN (with embedded silicon nanocrystals) double W.R. Taube et al. / Solar Energy Materials & Solar Cells 101 (2012) 32–3534 interband Auger process. In PL downconversion process the quantum efficiency cannot exceed one. But if the passivation is poor for example for industrial solar cells with a limited blue response, the influence of photoluminescence down-conversion is expected to be double layer ARC were used for fabricating solar cells. Fig. 2. Actual photograph of fabricated silicon solar cells before dicing. Fig. 3. Measured current-voltage characteristics of fabricated solar cells under AM1.5 G illumination. C is the reference cell with normal PECVD SiNx ARC, cell A is with single layer silicon nanocrystals embedded SRN ARC and cell B is with normal PECVD SiOx and silicon nanocrystals embedded SRN double layer ARC. 4. Results and discussions In solar cell A with single layer SRN ARC there was an increase in short circuit current density, JSC of 11.5% (from 31.4 to 35.00 mA/cm2) and an increase in open circuit voltage, VOC of 6.1% (from 0.529 to 0.561 V) compared to the reference cell. This change in JSC corresponds to additional 2.25�1016 photons/cm2 contributing to the short circuit current. In solar cell B, JSC increased by 12.8% (from 31.4 to 35.41 mA/cm2) and VOC increased by 6.2% (from 0.529 to 0.562 V) compared to the reference cell. Here, additional 2.51�1016 photons/cm2 are con- verted into photo generated short circuit current. This increase in photon generated short circuit current density may be attributed to; downconversion of higher energy photons into lower energy photons by photoluminescence resulting in efficient absorption in the solar cell, also observed by Yuan et. al in SRO films [18], or photon splitting followed by absorption of these lower energy photons into the bottom cell, but this needs further investiga- tions. This increase in JSC led to increase in VOC for cell A and B compared to reference cell C. ARC in cell A is lesser silicon-rich than that of cell B, but SRN layer in cell A is 16 nm thicker than cell B. Solar cell B has an incremental JSC of 410 mA/cm2 compared to solar cell A. This incremental current corresponds to substantial. Photon splitting downconversion, where the quantum efficiency may exceed one, can enhance the efficiency for both industrial and high efficiency passivated cells. For effective down- conversion, the reflectance of the cell for higher energy photons should beminimal, at the same time the overall weighted reflectance should also be minimal. Achieving this with a single layer ARC is not possible, as it does not have a broad band anti-reflection response. In view of this, optimization for refractive indices RI and thicknesses were performed using Transfer Matrix Method (TMM) [19] implementation in MATLABTM as given in the block diagram of Fig. 4(a). Refractive indices and thicknesses of layers were given as input to the TMM implementation. This computes reflectance corresponding to each wavelength. For this calcula- tion, RI¼1 was taken for the ambient, and wavelength dependent refractive indices of silicon were used for the substrate [20]. The reflectance, corresponding to individual wavelengths, were weighed by photon flux in AM1.5 G [21], integrated and normal- ized to find the weighted reflectance. Fig. 4(b) shows the surface plot of weighted reflectance for top and bottom layer thicknesses for fixed refractive indices (SRN RI¼2.2 and SiOx RI¼1.46). The optimization resulted in a minimal weighted reflectance of 5%, for top SiOx thickness¼90 nm and bottom SRN thickness¼60 nm. These optimized parameters for Table 1 Solar cell parameters extracted from J-V measurements. Solar cell VOC (V) JSC (mA/cm 2) FF Efficiency (%) C Ref. 0.529 31.40 0.77 12.7 A 0.561 35.00 0.75 14.7 B 0.562 35.41 0.78 15.6 2.56�1015 additional photons/cm2 converted into current den- sity. Reference Cell C has power conversion efficiency (PCE) of 12.7%. Single layer SRN ARC cell A has a PCE of 14.7%, thus 15.8% relative increase in efficiency compared to the reference cell C. Similarly, double layer ARC (SiOx/SRN) cell B has a PCE of 15.6%, thus showed relative increase in efficiency by 22.8%. This increase may be attributed to downconversion by silicon nanocrystals embedded in silicon rich nitride ARC. Based on these observa- tions, it can be said that, the optimized ARC with transfer matrix 5. Conclusion Acknowledgments [16] M. Stupca, M. Alsalhi, T. Al Saud, A. Almuhanna, M.H. Nayfeh, Enhancement gram W.R. Taube et al. / Solar Energy Materials & Solar Cells 101 (2012) 32–35 35 The authors would like to thank Director, CEERI, Pilani for his motivation and facility members of SNTG for their support. This work has been carried out under CSIR network program on nanostructured advanced materials. Financial support from CSIR-India is acknowledged. Efficiency enhancements have been observed in monocrystal- line silicon solar cells with an ARC of SRN with embedded Si nanocrystals. Increase in power conversion efficiency of 15.8% in single layer SRN ARC and 22.8% in double layer SiOx/SRN ARC solar cells have been observed. This demonstrates that nanopar- ticles embedded SRN downconversion films contribute signifi- cantly to efficiency enhancement of solar cells without adding much to the manufacturing cost. method and downconversion have helped to achieve better efficiency in cell B compared to other cells. Results from cells A and B indicate that embedded silicon nanocrystals are effective in increasing the power conversion efficiency of silicon solar cells and easy to integrate in conventional manufacturing technology. Fig. 4. Optimization of DLARC (Double Layer Anti Reflection Coating): (a) Block dia and bottom silicon-rich-nitride (RI¼2.2, thickness¼60 nm) with WR 5%. References [1] M.C. Beard, K.P. Knutsen, P. Yu, J.M. 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[21] ASTM G-173-03, /http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ ASTMG173.htmlS, (accessed 17.08.2011). Efficiency enhancement of silicon solar cells with silicon nanocrystals embedded in PECVD silicon nitride matrix Introduction Experimental procedure Downconversion mechanisms and optimization of ARC Results and discussions Conclusion Acknowledgments References