OPTO−ELECTRONICS REVIEW 19(4), 486–490 DOI: 10.2478/s11772−011−0047−x Concept of epitaxial silicon structures for edge illuminated solar cells J. SARNECKI*1, G. GAWLIK1, M. TEODORCZYK1, O. JEREMIASZ2, R. KOZŁOWSKI1, D. LIPIŃSKI1, K. KRZYŻAK1, and A. BRZOZOWSKI1 1Institute of Electronic Materials Technology, 133 Wólczyńska Str., 01−919 Warsaw, Poland 2ABRAXAS, 27 Piaskowa Str., 44−300 Wodzisław Śląski, Poland A new concept of edge illuminated solar cells (EISC) based on silicon epitaxial technique has been proposed. In this kind of photovoltaic (PV) devices, sun−light illuminates directly a p−n junction through the edge of the structure which is perpendicu− lar to junction surface. The main motivation of the presented work is preparation of a working model of an edge−illuminated silicon epitaxial solar cell sufficient to cooperation with a luminescent solar concentrator (LSC) consisted of a polymer foil doped with a luminescent material. The technological processes affecting the cell I–V characteristic and PV parameters are considered. Keywords: silicon epitaxy, solar cell, edge illuminated silicon solar cell. 1. Introduction Edge illuminated solar cells (EISC) are an alternative to conventional planar solar cells. The vertical geometry of these cells causes the illuminated surface is not obstructed by contact stripes. Moreover, in edge illuminated cells, the area of p−n junction is much larger than the area of illumi− nated surface. This makes the junction current density in these cells relatively smaller comparing to the conventional planar cells under the same illumination. Also the contact serial resistance can be small comparing to the regular pla− nar cells due to the large contact area. The high voltage solar batteries can be created by serial connection of many edge− −illuminated solar cells in the form of compact stack of many edge illuminated cells soldered side by side creating vertical−multijunction module (VMJ) [1]. Taking into ac− count properties of the edge illuminated PV cells it seems that cells of such geometry are especially useful for applica− tions with light concentrators. The application potential of such design caused that the edge illuminated solar cells have been investigated for about ten years starting from the early 70’s [2–7]. Recently, the similar edge illuminated solar cells have been developed and investigated by Sater at the beginning of the 21st century [8]. Photovoltaic characteristics of the VMJ under high inten− sity illumination was previously examined using a structure of serial connected 16 edge illuminated unit cells under 100 suns AM0 light intensity [3,4]. The VMJ ability to work un− der high light intensity was then checked by Sater using a module constructed from 40 serial connected vertical cells [8]. The working surface of the area of 0.78 cm2 has been il− luminated with 2500 suns of AM1.5. Under such illumina− *e−mail: 486 jerzy.sarnecki@itme.edu.pl tion, the VMJ module produced 31.8 W at 25.5 V, i.e., very good conversion efficiency of about 20% was reached. This corresponds to a very high ratio of output power to silicon weight of PV module of about 300 W/g. For a regular PV c−sili− con planar cell under AM1.5 illumination, the power to silicon weight ratio is about 0.2 W/g. The detailed review of theoreti− cal and experimental works on edge illuminated solar cells published up to the year 2006 has been presented by Rafat [1]. The concept of edge illuminated silicon PV cells, made using silicon epitaxial p and n layers grown on p+ or n+ sili− con substrates, has been proposed in this work. The p−n junc− tion has been created directly during sequential epitaxial growth of silicon p and n type layers. The relatively low dop− ing level (1015–1017 cm–3) in both n and p type epi−layers pro− motes long diffusion length of minority carriers. The thick− ness of the cell has been settled at about 0.1 mm with p−n junction located at about middle of the cell thickness. Dis− tance from a junction to the contact plane is essentially smal− ler than minority carrier diffusion length in the proposed structure. The l−h junctions of p−p+ and n−n+ type have been applied under both contacts creating back surface field re− gions directing charge carriers to the p−n junction and de− creasing the structure serial resistance. Taking into account that minority carrier diffusion length is essentially longer than a distance between a contact and p−n junction, the l−h junction may essentially increase the number of minority carriers which pass through p−n junction. The l−h junctions have been pro− duced during the same epitaxial process as p and n epi−layers. The main advantage of the CVD epitaxial process is the unique quality of the silicon crystalline structure. Moreover, both p and n layers are created by doping of pure silicon without compensation with the opposite dopant. Conse− quently, the diffusion length of minority carriers in both p and n type layers is high and may be close to a theoretical Opto−Electron. Rev., 19, no. 4, 2011 Unauthenticated Download Date | 10/2/16 4:20 PM value. So, the layers of both types may contribute to PV transformation efficiently. The unit edge illuminated PV cells have been fabricated in the form of 5–8 cm long, 0.2–0.3 cm wide and about 0.01 cm thin stripes. The thickness of an individual stripe corre− sponds to the total thickness of an epitaxial layer, so in the cell manufacturing process almost the whole substrate has been removed using mechanical process. Contact metalliza− tion was evaporated on both sides of the thinned epitaxial wafers. After thermal processing of the contact, the epi− −wafers ware then cut on 2–3 mm wide stripes. Finally, the illuminating edge surface of the unit cell with an area of about (5–8 cm)×0.01 cm = 0.05 cm2 – 0.08 cm2 has been finished by chemical and plasma processing. The geometry of the edge illuminated solar cells (EISC) is well suited for cooperation with foil luminescent solar collectors (LSC) [9]. However, taking into account a thick− ness of the unit EISC, the modules of more serial connected EISCs with the total thickness matched to the thickness of LSC will be used for the best cooperation with luminescent collector foil. 2. Fabrication of EISC Fig. 1. Majority carriers concentration profile in n+/n/p/p+ epitaxial structure. surface and metal contact, the additional n+ sub−micron layer was deposited on n type layer (emitter). The acceptors and donors concentration profile vs. depth in epitaxial multilayers structure p++/p/n deposited on n+ substrate is presented on Fig. 2. Additionally, for same epitaxy run per− formed on highly doped (~5×1018 cm–3) p type substrate was deposited a back surface field (BSF) layer between the substrate and a p layer as shown in Fig. 3. 2.1. Deposition of silicon epitaxial layers The epitaxial layers of EISC structure were grown by means of chemical vapour deposition technique (CVD) in the industrial Gemini 1 two−chamber inductive heated vertical reactors equipped with rotated susceptor. The used equip− ment ensures high uniformity of silicon epitaxial layer thic− kness (±2.5%) and resistivity (±5%). The silicon tetrachlo− ride (SiCl4) was used as a silicon source. The diborane (B2H6) and the phosphine (PH3) were used as dopant sour− ces. The epitaxial layers were grown on the substrates of different conductivity type, resistivity and orientation. We have used p (1016–1017 cm–3), p+ (~5×1018 cm–3) and n+ (~2×1018 cm–3) type, Czochralski grown (Cz Si) <111> or <100> oriented, 4” diameter Si substrates with the thickness in the range of 300–350 mm. During epitaxial growth, the temperature of susceptor was in the range 1080–1110°C. Such conditions and a proper SiCl4 concentration ensure a growth rate close to 1 μm/min. To select a most suitable multilayer epitaxial structures for EISC application, the three types of epitaxial structures such as n+/n/p/p+, n+/n/p/ p++/p+ and p+/p/n/n+ with different layer thickness and resistivity have been performed in several epitaxy runs. Fig. 2. Majority carriers concentration profile in p++/p/n/n+ epitaxial structure. 2.2. Characterization of epitaxial structures The majority carrier concentrations, the thickness of depo− sited epitaxial layers and the p−n junction width were mea− sured by spreading resistance (SR) method. The doping pro− file of p type layer (base) deposited on p+ type Si Cz <111> oriented substrate followed by n type layer has been pre− sented in Fig. 1. To ensure high phosphorus surface concen− tration, which enables low contact resistance between Si Opto−Electron. Rev., 19, no. 4, 2011 Fig. 3. Majority carriers concentration profile in n+/n/p/p++/p+ epitaxial structure with BSF layer between the substrate and p base layer. 487 J. Sarnecki Unauthenticated Download Date | 10/2/16 4:20 PM Concept of epitaxial silicon structures for edge illuminated solar cells The quality of epitaxial structures was determined with deep level transient spectroscopy (DLTS) method. Concen− tration of metal contaminants in the epitaxial layer was below DLTS sensitivity < 1×1012 cm–3. 2.3. EISC technological cycles Six groups of epitaxial structures have been fabricated. The to− tal thickness of deposited layers in each epitaxial structure was about 100 μm. The wafers p, p+, and n+ type with deposited epitaxial layers n+/n, n+/n/p, n+/n/p/p++ and p+/p/n were thinned up to about 105–110 μm. For parts of thinned structures, the chemical etching of grinded surface in HF:HNO3: H2O mix− ture follows the thinning operation. The etching in Ar plasma preceded the deposition of 0.4 μm thick Ni/Al layers on both surfaces of thinned wafers. The metal contacts were deposited in sputtering processes. After deposition, the Ni/Al contacts were annealed in Ar atmosphere in about 500°C for a few minutes to form an ohmic contact. Table 1. Technological cycles employed for fabrication of epitaxial stripe cells. 3. Experimental results The influence of technological processes on stripe cells PV performance was evaluated by using computerized sun simu− lator at the Photovoltaic Laboratory of the Institute of Meta− llurgy and Material Science Polish Academy of Sciences in Cracow. The set of four halogen lamp was used as a light source which ensures the standardized light power density of 100 mW/cm2 at the illuminated edge surface of the mea− sured stripe. The cell temperature during measurements was kept constant at 25°C. It should be noted that the light spec− trum of simulator differs from the real sunlight spectrum as illustrated in Fig. 4. The maximum intensity of simulator spectrum is shifted towards the red and simultaneously the intensity of the NIR part is slightly lower than in the solar spectrum. Here, total energy of the light from the simulator is supplied by photons of mean energy lower compared to the sun light. Table 2 summarises the PV parameters of different stripe cells performed by means of technological operations designed as I–III. All stripes were of the same shape, 2−mm width and about 0.1−mm thick. Technological cycles I II III IV V VI Grinding + + + + + + Chemical etching of grinded surface + + + + Deposition and annealing of contacts + + + + + + Cutting into stripes + + + + + + Chemical etching of stripe edges + + + Ar plasma etching of stripe edges + + + + Deposition SiO2 layers on stripe edges Technological operation Finally, the wafers were cut by diamond saw into 40–100 mm long and 2–4 mm width stripes which form unit cells. Edges of some samples after cutting process were sub− jected to further chemical and plasma etching processes. In selected samples, edges were then covered with 180−nm− −thick SiO2 layer. Table 1 lists technological cycles used for fabrication of edge illuminated stripe cells. Fig. 4. Comparison of sun light spectrum and simulator spectrum. Table 2. An example of PV parameters of individual stripe obtained from different epitaxial structures after processing sequences. L (cm) Pmax (mW) FF h (%) I–1 n /n/p/p <111> 4 0.134 0.42 3.4 I–3 n++/n/p/p+<111> 5 0.21 0.41 4.0 5 0.23 0.45 4.6 5 0.21 0.47 4.1 6 0.424 0.56 7.7 6 0.33 0.53 5.4 6 0.42 0.52 7.7 Technological cycle – No. of epitaxial structure ++ + + I–5 n /n/p d<100> ++/n/p/p+<111> II–1 n ++/n/p/p++/p+ II–2 n <111> II–6 p++/p/n+<111> ++/p/n/n+ <111> II–7 p p++ = BSF III–1 n ++/n/p/p+ <111> 6 0.22 0.50 3.7 III–3 n ++/n/p/p+ <111> 4.5 0.21 0.47 4.7 7.5 0.43 0.53 5.6 5 0.26 0.44 5.0 III–5 n+/n/p <111> III–7 p 488 ++/p/n/n+ <111> Opto−Electron. Rev., 19, no. 4, 2011 © 2011 SEP, Warsaw Unauthenticated Download Date | 10/2/16 4:20 PM A series resistance of single stripe structure was esti− mated from the dark I–V characteristics of the PV structure. The measured resistance was in the range 0.1–0.2 Wcm2. The values of main PV parameters of EISC fabricated in the above mentioned technological cycles from the epitaxial structures n+/n/p/p++/p+ and n+/n/p/p+ are listed in Table 3. Table 3. PV parameters of individual stripe obtained from epitaxial structures No. 3 (n+/n/p/p+ – see Fig. 1) and No. 4 (n+/n/p/p++/p+ – see Fig. 3) after different processing sequences. Technological L (cm) cycle No. 4 No. 3 I 5 3.5 II 6 6 III 8 5 IV 8 5 V 8 6 VI 6 5 Pmax (mW) No. 4 0.16 0.37 0.36 0.95 0.93 0.89 No. 3 0.22 0.43 0.29 0.69 0.92 0.85 FF No. 4 0.43 0.54 0.45 0.61 0.64 0.61 No. 3 0.46 0.56 0.50 0.64 0.65 0.62 h (%) No. 4 No. 3 3.3 4.2 6.1 7.7 4.2 5.8 11.7 13.6 14.8 18.5 11.7 14.1 where L is the length of stripe, Pm is the maximal generated power, FF is the fill factor, and h is the energy conversion efficiency. The examples of illuminated I–V characteristics of the stripes originated from the same epitaxial structure No. 7 and fabricated in IV and V technological cycles are shown in Fig. 5. The doping profile of outgoing p+/p/n/n+ structure No. 7 is presented in Fig. 2. The results presented in Fig. 5 indicate that the open cir− cuit voltage (VOC) reaches only about 0.4 V. This low VOC value was attributed to relatively low concentration of major− ity carriers in base and emitter. The weak effect of outgoing epitaxial structures on VOC can be explained by low p and n dopant concentration in active part of all investigated struc− tures. Moreover, the deep part of the structure (from illumi− nated surface) is very low illuminated and may cause dimi− nishing of resulting value of the VOC. The data given in Table 3 show that the proper technological operations im− proved the conversion efficiency up to several percent. Fig. 5. Illuminated I–V curves of stripes processed from epitaxial structures No. 7 by means of technological IV and V cycles. The doping profile of outgoing p++/p/n/n+ structure is presented in Fig. 2. A and B denote different grinding conditions. Opto−Electron. Rev., 19, no. 4, 2011 The energy conversion efficiency of about 19.0% and the FF value equal to 0.70 were observed for samples series V – 7A. Generally, for all stripes fabricated according to the cycle denoted as V, the FF values were estimated above 0.6. The satisfactory values of h and FF in connection with insufficient open circuit voltage result probably from high quality of epitaxial structures and relatively low concentra− tion of majority carriers in n and p type epitaxial layers. The chemical and plasma etching of stripe edges after cutting operation improved PV cell performance. The etch− ing processes caused significant reduction of surface recom− bination effects which result on increasing PV parameters. 4. Discussion In the EISC, the p−n junction plane is perpendicular to the illuminated surface. So, the area of the p−n junction may be many times higher than the area of illuminated surface. In the proposed design, the p−n junction area is about 20–30 times higher than illuminated area. Incident light is guided parallel to the junction plane inside a cell and generates electron – hole pairs in the whole volume of the silicon material in close vicinity to the junction defined by the thickness of the epi−layers. So, generated minority carriers can reach the p−n junction with the same probability inde− pendently of the energy of incident photon if its energy is higher than silicon energy gap Eg. Consequently, photons with the energy only slightly higher than Eg may effectively contribute to the PV effect, if the depth (from illuminated surface) of the cell is high enough. In the proposed cell design, this distance has been settled at 2–3 mm, taking into account optimization of the measured cell external efficiency. The p−n junction area is many times higher than illumi− nated area. So, under regular 1 sun illumination, the EISC junction is working under low illumination level and den− sity of current passing through the p−n junction is relatively low. It creates a potential for working with concentrated light. On the other hand, under 1 sun illumination, the parameters of EISC like FF and open circuit voltage VOC may be smaller than the same parameters for conventional planar PV silicon cells under the same illumination. The conversion efficiency of a PV cell defined as the proportion of sunlight energy that the cell converts to elec− trical energy may be confusing in case of EISC because the area of illuminated p−n junction of EISC is many times larger than illuminated cell surface. So, the conversion effi− ciency per unit area of illuminated EISC surface may be very high even when conversion efficiency per unit area of the junction is small. Consequently, the conversion effi− ciency per unit illuminated area of EISC may be higher than for planar PV cells under illumination of the same light power density, but the conversion efficiency per unit vol− ume of EISC is usually lower. However, ability of EISC to work with concentrated light may increase the conversion efficiency per unit volume of the EISC cell material by using effective light concentrator. 489 J. Sarnecki Unauthenticated Download Date | 10/2/16 4:20 PM Concept of epitaxial silicon structures for edge illuminated solar cells Stripe geometry of the edge illuminated silicon epitaxial solar cells is well fitted to the foil luminescent solar collec− tors (LSC). The LSC consisted of polymer transparent sheet with embedded organic dyes, quantum dots (QD) and rare earth doped oxide nanocrystals has been investigated simul− taneously at our Institute [10]. 5. Conclusions The presented results concerning the edge illuminated stripe solar cell show that a CVD epitaxy is a proper technique to fabricate them. The used technological operations (contact deposition, etching, cutting and passivation of illuminated surface of stripe edges) affecting PV parameters need at this time optimization. To explain our observations and the obtained results it should be stated that: in epitaxial structures, the thickness of p and n type layer is much smaller than diffusion length of majority carrier holes and electrons, respectively, distribution (distance to the junction) of carrier pairs generated by incident photons is independent of energy (wavelength). These properties are responsible for high amount of light generated charge carriers passing through the p−n junction. The relatively high value of power conversion efficiency for the same best stripes was reached despite low open circuit voltage. Technological processes presented in this paper have been used to create, in laboratory scale, the working models of epitaxial edge illuminated PV cells. It should be noted that the used thinning procedure by substrate grinding is a silicon consuming process and such operation can only be approved by the model developed in the laboratory conditions. The very promising energy conversion efficiency of the investigated EISC structures justifies the developing thin− ning methods which allow at least on partial preservation of the substrate for the next epitaxial runs. An exfoliation from porous silicon substrate is considered as an alternative method for production of epitaxial PV cells. Such approach may limit consumption of silicon substrate in EISC manu− facturing process. l l 490 Acknowledgements The authors would like to express their thanks to Dr P. Panek, D.M. Lipinski, and Dr K. Drabczyk from the Photo− voltaic Laboratory (the Institute of Metallurgy and Material Science Polish Academy of Sciences in Cracow) for help with I–V characteristics and photovoltaic conversion mea− surements and valuable discussions as well. This work has been supported by the National Centre for Research and Development within the research project NR 15−0030−04. References 1. N.H. Rafat, “A simple analytical treatment of edge−illumi− nated VMJ silicon solar cells”, Sol. Energy 80, 1588–1599 (2006). 2. P. Stella and A. Gover, “Vertical multi−junctions solar cell”, in Proc. the 9th IEEE Photovoltaic Specialists Conf., 85–86 (1972). 3. B.L. Sater, H.W. Brandhorst, T.J. Riley, and R.E. Hart, “The multiple junction edge illuminated solar cell”, in Proc. the 10th IEEE Photovoltaic Specialists Conf., 188–193 (1973). 4. B.L. Sater and C. Goradia, “The high intensity solar cell−key to low cost photovoltaic power”, in Proc. the 11th IEEE Pho− tovoltaic Specialists Conf., 356–363 (1975). 5. C. Hu, J.K. Carney, and R.I. Frank, “New analysis of a high− −voltage vertical multijunction solar cell”, J. Appl. Phys. 48, 442–444 (1977). 6. C. Goradia and B.L. Sater, “A first order theory of the p+−n−n+ edge−illuminated silicon solar cell at very high injec− tion levels”, IEEE T. Electron Dev. ED−24, 342–351 (1977). 7. R.J. Soukup, “The lensed high−voltage vertical multijunction solar cell”, J. Appl. Phys. 48, 440–441 (1977). 8. B.L. Sater and N.D. Sater, “High voltage silicon VMJ solar cells for up to 1000 suns intensities”, in Proc. the 29th IEEE Photovoltaic Specialists Conf., 1019–1022 (2002). 9. B.C. Rowan, L.R. Wilson, and B.S. Richards, “Advanced material concepts for luminescent solar concentrators”, IEEE J. Sel. Top. Quant. 14, 1312 (2008). 10. O. Jeremiasz, M. Socha, W. Nikiel, L. Lipińska, J. Sarnecki, M. Nakielska, G. Gawlik, M. Teodorczyk, and A. Wnuk, “Nanocrystals doped polymer material for luminescent solar concentrator for absorption of visible and NIR part of solar spectrum and emission within Si absorption range”, Proc. the 25th EU PVSEC, 587–590 (2010). Opto−Electron. Rev., 19, no. 4, 2011 © 2011 SEP, Warsaw Unauthenticated Download Date | 10/2/16 4:20 PM