Bifacial Solar Cells Based on Isotype Junctions

BIFACIAL SOLAR CELLS BASED ON ISOTYPE JUNCTIONSA. Simashkevich1, D. Sherban1, P. Morvillo2, E. Bobeico2, L. Bruk1, Iu. Usatii21Institute of Applied PhysicsAcademiei 5 street, MD 2028, Chisinau, Republic of Moldova, e-mail: Alexey.Simashkevich@phys.asm.md2ENEA Research Centre PorticiLocalita Granatello, 80055, Portici (Na), Italy, e-mail: morvillo@portici.enea.itABSTRACT: A novel type of bifacial solar cells, containing only homopolar junctions with a n+-n-n+ structure was elaborated. Single crystalline (100) oriented Cz-Silicon 350µm thick wafer having a resistivity 1Ohm·cm, concentration 5·1015cm-3, carriers mobility 1250cm2/V·s was used as substrate. The ITO layers are deposited on the n-Si wafer surface by spraying alcoholic solution of InCl4 and SnCl4 in different proportions. The obtained thin film thickness value of 0,35µm was chosen for the frontal semiconductor-insulator-semiconductor structure fabrication. The back n-n+ junction is an usual back surface field structure consisting in highly doped n-Si layer obtained by P diffusion. The spectral distribution of the quantum efficiency has been studied at frontal and back illumination. It is seen that in the region of wavelengths from 400nm to 870nm that in the case of frontal illumination their value changes in the limits 0,6–1,0. For back illumination quantum efficiency is equal to 0,2–0,3 in the same region of the spectrum. The photoelectric parameters of the elaborated bifacial solar cell have been determined in standard AM1,5conditions: for the frontal side V oc=0,425V, J sc=32,63mA/cm2, FF=68,29%, E ff=9,47%, R ser=2,08Ohm,R sh=6,7·103Ohm; for the back side V oc=0,392V, J sc=13,23mA/cm2, FF69,28%, E ff=3,6%, R ser=3,40Ohm, R sh=1,26·104Ohm.Keywords: Bifacial, ITO, Solar Cell.1 INTRODUCTIONBifacial solar cells (BSC) are promising devices because they are able to convert solar energy coming from both sides of the cell, increasing its efficiency. Different constructions of BSC have been proposed and investigated. In the framework of the classification suggested in [1] the BSC structures could be divided into groups according to the number of junctions: a) two p-n junctions, b) one p-n junction and one high-low junction and, c) just one p-n junction. In all these types of BSC are based on a heteropolar p-n junction. The best results have been obtained in the case of b)-type BSC [2, 3] which were fabricated on the base of n+-p-p+ or p+-n-n+ structures with back surface field (BSF) at rear contact.In this case it is necessary to obtain two junctions: a heteropolar p-n junction at the frontal side of the silicon wafer and a homopolar n-n+ or p-p+ junction at the rear side. Usually these junctions are fabricated by impurity diffusion in the silicon wafer. The diffusion process occurs at high temperatures higher then 8000C and requires special conditions and precise control. In the case of BSF fabrication these difficulties increases sinceit is necessary to realize the simultaneously diffusion of impurities which have a opposite influence on the silicon properties. Therefore appears the problem of the silicon surface protection from the undesirable impurities.The p-n junction in the solar cells could be replacedby semiconductor-insulator-semiconductor (SIS) structures. The formation of such structures by a simple and low-temperature spray method was demonstrated in [4-7].The main objective of this paper is to demonstrate the possibility of a novel type of BSC fabrication on the baseof nSi crystals and indium tin oxide mixture (ITO) layers obtained by spray method, containing only homopolar junctions with a n+-n-n+ structure. The utilization of such structures removes a considerable part of above-mentioned problems in BSC fabrication. In this case a single diffusion process is carried out for BSF obtaining.2 EXPERIMENTALThe main steps of the fabrication of our cell are schematized in Figure 1.Figure 1: Process sequenceIn our process we have chosen to use single crystalline (100) oriented Cz-Silicon 350µm thick wafer having a resistivity 1Ohm·cm, concentration 5·1015cm-3, carriers mobility 1250cm2/V·s.The first frontal junction is a SIS structure formed by a ITO layer deposited on the rough surface of n-type silicon wafer by spraying alcoholic solution of InCl4 and SnCl4 in different proportions using the installation described in [7].The back n-n+ junction is an usual BSF structure consisting in highly doped n-Si layer obtained by P diffusion.Grids obtained by cuprum (Cu) evaporation in vacuum were deposited on the frontal and back surfaces484of the cell. In such a way the bifacial Cu/n +ITO/SiO 2/nSi/n +Si/Cu structure have been obtained.3 RESULTSThe schematic view of the fabricated and investigated structures is presented in Figure 2.Figure 2: The schematic view of BSC structuresThe properties of the ITO films obtained in this way depend on the concentration of indium chloride and tin chloride in the solution, the temperature of the substrate, the time of spraying and the speed of deposition. ITO films with maximum conductivity (8,3·10-3Ohm -1·cm -1) and maximum transmission coefficient in the visible range of spectrum (~92%) were obtained from solutions containing 90% InCl 3 and 10% SnCl 4 under the following conditions: substrate temperature 450o C , deposition rate 50Å/s , spraying time 45s , solution viscosity 0,04P. X-ray analysis showed that these films are polycrystalline with a cubic structure and a lattice parameter of 10,14Å. They are n-type conductors with charge carriers of 1,9·1021cm -3, mobility of 27cm 2/(V·s) and band gap of 3,4eV . The obtained thin film thickness value of 0,35µm was chosen for the frontal SIS structure fabrication. The SiO 2 insulating layers at the ITO-nSi interface were obtained by thermal or chemical oxidation, their thickness varies from 30Å to 40Å.1E151E161E171E181E191E20C o n c e n t r a t i o n (A t o m s *c m -3)depth (µm)Figure 3: SRP characteristic on p-type c-Si test-wafer for the utilized diffusion processesThe rear n-n +junction was fabricated on the top of polished side of the wafer by a diffusion process starting from POCl 3 gas mixture. The junction formation ends with a wet chemical etching of POCl 3 residual in a 10% HF bath. A junction depth of 1µm has been chosen in order to minimize recombination. To reduce the surfacerecombination velocity the wafers have been thermally oxidized at a temperature of 850o C . During the diffusion process the test-wafers were introduced together with process wafers (1 Ωcm) on which PV device were realised. These p-type test-wafer were used for spreading resistance profile (SRP) measurements to measure the depth of realised junction (Figure 3).Current voltage (I-V) and capacity voltage characteristics of these SIS structures have been studied. The results of these investigations show that the structure obtained in such way represent an asymmetrical doped barrier structure in which the wide band gap oxide layer plays the role of a transparent metal. Therefore this structure may be considered as Schottky barrier with a thin SiO 2 layer at the interface.C u r r e n t (A )Voltage (V)Figure 4: I-V dark curve of BSC structuresI-V dark characteristic of BSC is presented in Figure 4. This characteristic can be approximated by the usual relation:⎥⎦⎤⎢⎣⎡−⎟⎠⎞⎜⎝⎛=1exp 0AkT eV J JThe electric parameters of this structure are: A=1,57,J 0=2·10-7A . The diffusion potential (V D ) is equal to 0,42V , the series resistance is 1,6Ohm .Q Yλ (nm)Figure 5: Spectral distribution of the quantum efficiencyThe spectral distribution of the quantum efficiency has been studied at frontal and back illumination (Figure 5). It is seen that in the region of wavelengths from 400nm to 870nm the value of QY changes in the limits4850,65–0,95 in the case of frontal illumination. For back illumination QY is equal to 0,2–0,3 in the same region of the spectrum.The I-V load characteristics at AM1,5 spectral distribution and 1000W/m 2 illumination are presented in Figure 6. The photoelectric parameters of the elaborated BSC have been determined in standard AM1,5 conditions: for the frontal side V oc =0,425V , J sc =32,63mA/cm 2, FF =68,29%, E ff =9,47%, R ser =2,08Ohm , R sh =6,7·103Ohm ; for the back side V oc =0,392V , J sc =13,23mA/cm 2, FF=69,28%, E ff =3,6%, R ser =3,40Ohm , R sh =1,26·104Ohm .J (A /c m 2)U (V)Figure 6: I-V load characteristics at AM1,5 illuminationOn the base of physical parameters of the siliconwafer, ITO layers and our experimental results wasproposed the band diagram of the n +Si/nSi/SiO 2/n +ITO structure which is shown in Figure 7. The space charge regions of the both junction, equal to 0,52µm and 0,36µm are situated in nSi. The SiO 2 insulator layer is tunnel transparent for the charge carriers.Figure 7: Band diagram of the bifacial Cu/n +ITO/SiO 2/nSi/n +Si/Cu structureAt the structure illumination through the frontal contact the solar radiation is absorbed in the silicon wafer. The light generated carriers are separated by the nSi/SiO 2/ITO junction. The back surface fields of the n+Si/nSi junction facilitate the transport of the carriers to the back contact. The same processes take place at the illumination through the rear contact.4 CONCLUSIONSIn such a way a novel type of bifacial cells containing only isotype junctions was elaborated. Their advantages consist in the following: the frontal junction is obtained with a simple spray technique at low temperatures; it is not necessary the double diffusion process; the frontal ITO layer is at the same time collecting electrode and antireflection coating. The overall efficiency of the elaborated bifacial solar cells reach 13%. The optimization of BSC parameters is possible.5 ACKNOWLEDGEMENTSThis work is accomplished under the joint project of the Academy of Sciences of the Republic of Moldova and the Russian Foundation of Fundamental Investigation (Project nr. 06.14 CRF).6 REFERENCES [1] A. Cuevas, Proceedings 20th European PV Solar Energy Conference, Vol. 3 (2005) 801.[2] A. Hubner, A. G. Aberle, R. Hezel, Applied Physics Letters, 70 (1997) 1008.[3] D. Munoz et al, Proceedings 20th European PV Solar Energy Conference, Vol. 3 (2005) 996.[4]. N. Adeeb, I. Kretsu, D. Sherban, et al, Solar Energy Materials, 15 (1987) 9.[5] V. Vasu, A. Subrahmanyam, Semicondactor Science and Technology, 7 (1992) 320. [6] L. Gagara, L. Gorcheac, D. Sherban, et al, Proceeding of the International Conference Euro-Sun 96, Vol. 2 (1996) 665.[7] A. Simashkevich, D. Sherban, L. Bruk. E. Bobeico, A. Coval, V. Fedorov, Iu. Usatii, Proceedings 20th European Photovoltaic Solar Energy Conference, Vol. 3 (2005) 980.486。