Concept of epitaxial silicon structures for edge illuminated solar cells J. SARNECKI *1 , G. GAWLIK 1 , M. TEODORCZYK 1 , O. JEREMIASZ 2 , R. KOZŁOWSKI 1 , D. LIPIŃSKI 1 , K. KRZYŻAK 1 , and A. BRZOZOWSKI 1 1 Institute of Electronic Materials Technology, 133 Wólczyńska Str., 01−919 Warsaw, Poland 2 ABRAXAS, 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 cm 2 has been il− luminated with 2500 suns of AM1.5. Under such illumina− 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 (10 15 –10 17 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 486 Opto−Electron. Rev., 19, no. 4, 2011 OPTO−ELECTRONICS REVIEW 19(4), 486–490 DOI: 10.2478/s11772−011−0047−x * e−mail: jerzy.sarnecki@itme.edu.pl Unauthenticated Download Date | 7/25/18 3:07 PM