20th European Photovoltaic Solar Energy Conference, 6-10 June 2005, Barcelona, Spain
SPRAY DEPOSITED ITO-nSI SOLAR CELLS WITH ENLARGED AREA A. Simaschevici1,2, D. Serban1, L. Bruc1, A. Coval1, V. Fedorov1, E. Bobeico2,3, Iu. Usatii1,2 1 Moldavian State University, str. Mateevici, 60, Chisinau, MD-2009, Republic of Moldova, e-mail:
[email protected] 2 Institute of Applied Physics of the Academy of Sciences of Moldova, str. Academiei 5, MD-2028, e-mail:
[email protected] 3 ENEA C.R. Portici, Loc. Granatello, 80055 Portici (NA), Italy, e-mail:
[email protected] ABSTRACT: ITO-nSi solar cells have been obtained by deposition of ITO layers using spray pyrolisis technique onto the surface of nSi wafers with the area up to 48.6cm2. The study of current-voltage and capacitance-voltage characteristics indicates that they are abrupt SIS heterostructures with a tunnel transparent SiO2 insulator layer at the ITO-nSi interface. The photosensitivity is observed in the wavelength range 350-1200nm and between 485nm and 920nm its value is more then 0.5A/W. At illumination conditions AM1.5 following parameters have been obtained: Uoc=0.54V; Jsc=25.1mA/cm2; ff=0.71; efficiency 9.6%. The viability of the developed technology was demonstrated by the fabrication of a number of PV modules composed from 36 solar cells with the output power of 15W. Keywords: Silicon, TCO Transparent Conducting Oxides, PV Module. 1
INTRODUCTION
The conventional energy production is based on no sustainable methods, exhausting our existing natural resources of oil, gas, coal, nuclear fuel. At the same tame the conventional energy systems causes also the majority of the environmental problems. Only renewable energy systems can meet the growing energy requirements in a sustainable way and without increasing the ecological damage. The photovoltaic conversion of the solar energy, which is a direct conversion of radiation energy into electricity, is one of the principal ways to resolve the above mentioned problem. New semiconductor materials are proposed for the fabrication of solar cells, nevertheless, the one of the latest World Conference on Photovoltaic Solar Energy Conversion (Osaka 2003) demonstrated that crystalline silicon continue to hold the dominant position in PV cell production. The efficiency of laboratory silicon cell based on p-n junctions reaches more than 24%. This value is near the upper theoretical limit of silicon based solar cells, but their cost is very high. Solar cells fabricated on the base of semiconductorinsulator-semiconductor (SIS) structures are promising for solar energy conversion due to its relative low cost [1]. These structures are obtained by deposition of transparent conductive oxide (TCO) films onto different crystal substrates [2-5]. The main purpose of this work is to demonstrate the possibility of fabrication of solar cells based on SIS structures using spray pyrolisis technique, which is the simplest method of obtaining TCO films: SnO2, In2O3 or their mixture (ITO). These thin films, deposited on the nSi crystals surface, are used for the formation of n+ITOSiO2-nSi structures with a shallow junction. At the same time the ITO thin films are frontal collecting electrodes and antireflection layers. 2
EXPERIMENTAL
The ITO layers are deposited on the nSi crystals surface by spraying of alcoholic solution of InCl3 and SnCl4 in different proportions using the special designed installation which contains four main units: the pulverization system, the system of displacement and 980
rotation of the support on which the substrate is fixed, the system of heating the substrate and the system of the evacuation of the residual products of the pyrolise. The heating system consists of an electric furnace and a device for automatic regulation of the substrate temperature (Fig.1).
Figure 1: Schematic imagines of the installation for ITO thin films deposition: 1-power unit; 2-electric furnace; 3-thermocouple; 4-rotating basis; 5-mechanism of basis rotation and moving; 6-basis position during layer reception; 7-system of spraying; 8-system of exhaust ventilation; 9-basis position during loading of silicon wafer; 10-cover; 11-wafer of silicon; 12-shielding plate. The silicon wafers are located on the support and with the aid of the displacement mechanism are moved in the deposition zone of the electric furnace. The construction of this mechanism provide the rotation of the support with the velocity of 60 rotation per minute, what is necessary for the obtaining of ITO thin films with uniform thickness on the all wafer surface. The alcoholic solution of InCl3 and SnCl4 is pulverized with the aid of compressed oxygen into the stove on the silicon wafer substrate, where is formed the ITO thin film due to thermal decomposition of the solution and the oxidation reaction. On the heated up substrate there are following chemical reactions:
20th European Photovoltaic Solar Energy Conference, 6-10 June 2005, Barcelona, Spain
SiO2 insulator layers were obtained on the silicon wafers surface by different methods: anodic, thermal or chemical oxidation. The best results have been obtained at the utilization of the two last methods. The chemical oxidation of the silicon surface was realized by immersing the silicon wafer into the concentrated nitric acid for 15 seconds. A tunnel transparent for minority carriers insulator layers at the ITO-Si interface have been obtained thermally, if the ITO layers deposition occurs in an oxygen containing atmosphere at the temperature which is determined by the expression:
⎡ ⎛ T T (t ) = Tm ⎢1 − ⎜⎜1 − 0 ⎣⎢ ⎝ Tm
⎞ ⎟⎟ exp ⎠
−
t
τ
⎤ ⎥ ⎦⎥
1.0
Reflection Transmission
0.8 0.6 0.4 0.2 0.0
500
1000
1500
2000
2500
Wavelength (nm)
(2)
where: T(t) – the temperature of the silicon surface at the moment t; T0 – the temperature of the oxidation start of the silicon surface; Tm – the temperature of the ITO layer deposition; τ - the time constant equal to 35 – 40sec. Ellipsometrical measurement showed that the thickness of the SiO2 insulator layer varies from 30Å to 60Å. In our experiments for the fabrication of SIS structures nSi wafers oriented in the (100) plane with different electron concentration from 1x1015cm-3 up to 8x1017cm-3 were used as substrates. The ohmic contacts have been obtained by evaporation in vacuum Cu+Sn for the back contact and Al grid for the frontal one. ITO-SiO2-nSi solar cells with active area of 8.1cm2 and 48.6cm2 have been fabricated. 3
where: n-refraction index equal to 1.8 for ITO [6]; λ1, λ2the wavelengths for two neighbouring maximum and minimum; d-the thickness of the ITO layer.
Transmission, Reflection (%)
(1)
RESULTS
Figure 2: Optical transmission and reflectance spectra of the ITO thin film deposited on the glass substrate at the substrate temperature 450°C by spray-pirolisys technique. Using this relation the thickness of ITO layers deposited on the nSi wafer surface in dependence on the quantity of the pulverized solution have been determined (insert in Fig.3). It is seen that this relation is linear and the layer thickness varies from 0.35µm up to 0.5µm. The ITO thickness value of 0.35µm was chosen for the solar cell fabrication. 0.8
Reflection (arb.unit.)
4InCl3+3O2=2In2O3+12Cl SnCl4+O2=SnO2+4Cl
0.7 0.6 0.5 600
0.4
d (nm)
550
0.3
500 450
0.2
400
The properties of the obtained in this way ITO films depend on the concentration of indium chloride and tin chloride in the solution, the temperature of the substrate, the time of spraying and the deposition speed. ITO films with maximum conductivity (4.7x103Ohm-1•cm-1) and maximum transmission coefficient in the visible range of the spectrum (87%) were obtained from solutions containing 90% InCl3 and 10% SnCl4 and under following condition: substrate temperature 450°C, deposition rate 100Å/s, spraying time 45s. The optical transmission and reflectance spectra of the deposited on the glass substrate ITO thin films are presented in Fig.2. The thickness of ITO layers in dependence of the quantity of pulverized solution has been evaluated from the spectral distribution of the light reflection coefficient from the surface of the deposited layers. In Fig.3 this dependence for an ITO layer obtained from 15ml of solution is presented. The variation of the solution quantity lead to the variation of maximum and minimum number which results the light interference reflected from the layer surface and layer-substrate interface. The location of the maximums and minimums is determined by the well known relation:
n=
λ1 * λ2 (λ 2 − λ1 ) * 2d
(3)
0.1 0.0
350
V (ml) 9
400
800
1200
10
1600
11
12
2000
13
14
15
2400
Wavelength (nm)
Figure 3: Spectral distribution of the reflection coefficient of the ITO layer deposited on the silicon wafer. Insert: the thickness of the ITO layer in dependence on the pulverized solution. The obtained in such a way structures represent asymmetrical doped barrier structures in which the wide band gap oxide semiconductor play the role of transparent metal. Therefore these structures may be considered as Schottky diodes with a thin insulating SiO2 layer at the interface. The study of dark current-voltage and capacitancevoltage characteristics of these structures indicates that they are abrupt SIS heterostructures. The spectral distribution of the quantum efficiency Q and the photosensitivity of the obtained PV cells have been studied. The monochromatic light from the spectrograph is falling on a semitransparent mirror and is divided in two equal fluxes (insert in Fig.4). One flux fall on the surface of a calibrated solar cell for the determination of the incident flux energy and the number (N) of incident photons.
981
20th European Photovoltaic Solar Energy Conference, 6-10 June 2005, Barcelona, Spain
1.0
1.0
1
Quantum Efficiency
0.6
0.6
2
0.4
0.4
0.2
0.2
0.0
0.0 1400
400
600
800
1000
1200
Sensibility (A/W)
0.8
0.8
Wavelength (nm)
Figure 4: Spectral distribution of the quantum efficiency (1) and sensibility (2) of the n+ITO-SiO2-nSi solar cells. The second flux fall on the surface of the analysed sample and the short circuit current Isc is measured, what permit to calculate the number “q” of charge carriers, generated by the light and separated by the junction, and than the quantum efficiency η=q/N for each wavelength. The spectral distribution of the quantum efficiency of the n+ITO-SiO2-nSi structure is presented in Fig.4 (curve 1). It is seen that in the region of wavelengths from 400 to 870nm the value of η changes on the limits 0.6-0.97. The photosensitivity measured in A/W for a fixed wavelength is determined from the ratio of Isc and the incident light energy. The curve 2, Fig.4 represents the spectral distribution of the photosensitivity for the same sample. The photosensitivity is observed in the wavelength range of 350-1200nm and in the wavelength range of 485920nm its value is more than 0.45A/W. 0.010 0.005 0.000
2
J (A/cm )
-0.005 -0.010 -0.015 -0.020
Voc=0.536 V 2 Jsc=2.511E-2 A/cm FF=71.14% Eff=9.567% 2 Rser=2.573 Ohm∗cm Rsh=0.124E+4 Ohm
-0.025 -0.030 -0.3 -0.2 -0.1 0.0
0.1
0.2
0.3
0.4
0.5
0.6
U (V)
Figure 5: I-V characteristic of the n+ITO–SiO2– nSi cells with active area 8.1cm2. The obtained n+ITO-SiO2-nSi structures were used as photovoltaic converters. For solar cells with active area of 8.1cm2 at the AM1.5 illumination conditions the following photoelectric parameters were obtained: the short circuit current 25.11mА/cm2, the open circuit voltage 536mV, the fill factor 71.14%, the efficiency 9.567% (Fig.5). The reproducibility of the process and the performances of the devices during
982
samples realization was checked in each batch of samples as well as batch-to-batch. The viability of the developed technology for the solar cells obtaining in laboratory conditions was demonstrated by fabrication of the PV modules each of them formed by 36 cells and possessing the active area of 1752cm2 and the output power of 15W. CONCLUSIONS ITO-nSi semiconductor – insulator – semiconductor structures have been produced with a simple spraying technique. The obtained in such a way structures may be considered as Schottky diodes with a thin insulating SiO2 layer at the interface. Solar cells on the base of ITO-nSi structures with active area of 8.1cm2 and 48.6cm2 have been fabricated and studied. Their quantum efficiency reaches 0.97 at λ=550nm. At the AM1.5 illumination conditions the efficiency is 9.567% for cells with area of 8.1cm2 and 6.979% for cells with area 48.6cm2. PV modules with the output power of 15W have been fabricated on the base of ITO-nSi solar cells. ACKNOWLEDGEMENTS This work was has been funded by the Supreme Council for the Science and Technological Development of the Academy of Sciences of the Republic of Moldova (Project nr. 45.005P). REFERENCES [1] Proc. of 2nd World Conf. on PV solar energy conversion, 1998, Vienna. [2] J.Shewchun, G.Dubow, A.Myszkowsky, R.Singh “The operation of the semiconductor-insulatorsemiconductor (SIS) solar cells: Theory” J. Appl. Phys. 1978, v.49, N9, p.855-864. [3] N.Adub, I.Kretsu, D.Sherban, V.Sushkevich, A.Simaschkevici „Spray deposited ITO-CdTe solar cells“ Sol. Energy Mater., 1987, v.15, N1, p.9-19. [4] Andronic, A.Simashkevich, L.Gagara, L.Gorceac T.Potlog. D.Sherban. InP based radiation stable solar cells. Proc of the 2nd World Conf. on PV Solar Energy Conversion, Vienna, 1998, v.1, p.249-252. [5] V.Vasu, A.Subrahmanyam “Photovoltaic properties of ITO/Si junctions prepared by spray pyrolisis. dependence on oxidation time” Semicond. Sci. and Tech., v.7, p.320, 1992. [6] K.L.Chopra, S.Major, and D.K.Pandya. Transparent Conductors - A Status Review // Thin Solid Films. – 1983. – V.102, N.1. - p.1–46.
