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Physics Procedia 00 (2008) 000–000 Physics Procedia 2 (2009) 971–974 www.elsevier.com/locate/XXX www.elsevier.com/locate/procedia
Proceedings of the JMSM 2008 Conference
Optical and electrical characterization of In2S3 buffer layer for photovoltaics applications K. Benchouka*, J. Ouerfellib, M. Saadounb, A. Siyoucefa a
LPCM2E, université Es-Sénia, B.P 1524 Oran El Mnaouer UPDS, IPEIT, université de Tunis 1008 Montfleury-Tunis
b
Elsevier2009; use only: Received date here;form revised accepted date 31 hereAugust 2009 Received 1 January received in revised 31 date Julyhere; 2009; accepted
Abstract In2S3 thin films have been synthesized by annealing indium thin films under sulfur pressure. The indium thin films have been deposited by evaporation onto glass substrates. The sample is then introduced, with small grains of sulfur into a Pyrex tube. The latter is sealed under vacuum and then annealed for two hours at temperatures ranging from 280°C to 400°C in a step of 40°C.This simple and economic technique allows the elaboration of In2S3 thin films crystallized in the β structure. Optical density (O.D.) measurements were made for In2S3 thin films synthesized at different temperatures. The optical spectra show that the threshold value of optical absorption is approximately 610 nm. By increasing the synthesis temperature this threshold value moved towards higher wavelength values. The gap is around 3eV and the electrical resistivity measurements at room temperature, show that the films are highly resistive with an electric conductivity σ around 10-1 (ȍ cm)-1. © 2009 Elsevier B.V. Open access under CC BY-NC-ND license. PACS : 73.61.-r; 78.20.-e; 78.66.-w Keywords: thermal evaporation, thin film, In2S3, optical density, electrical resistivity.
1. Introduction The realization of a thin film solar cell implies several stages to deposit all the necessary films for its operation. The study described within this work is dedicated to the realization of a thin film used as buffer layer in the photovoltaic cells based on chalcopyrite structure For the purpose of an acceptable buffer layer having optimal optical and electrical properties, we chose a binary semiconductor of the (III-VI) compounds family, the indium trisulfure In2S3. It is an n-type semiconductor having all properties needed for elaborating a good buffer layer, namely a band gap which can be monitored between 2.1 and 2.9 eV [1] and small conductivity. In the present work we show that In2S3 thin films can be achieved using simple processes that consist of a thermal evaporation under secondary vacuum conditions of an indium layer, which is then annealed under sulfur vapour. We note that In2S3 is less toxic than CdS [2].
* Corresponding author. Tel.: 00-213-664953023; fax: +0-000-000-0000 . E-mail address:
[email protected].
doi:10.1016/j.phpro.2009.11.051
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The characterization of these thin films shows that they present n type semiconductor [3]. However, according to the elaboration process, the obtained layers are made up by various phases. Annealing temperature effects upon the thin films structure, morphology and optical properties are assessed in a purpose of trying to obtain a single phase thin films. 2. Techniques of preparation and characterization The soda lime glasses substrates were used after being cleaned firstly, by acetone in order to eliminate any greasy track and secondly, by ultrasounds in alcohol solution. They were then dried by a nitrogen flow. The vacuum inside the enclosure of evaporation is obtained and maintained by a pumping set, this latter is formed by a pump with palette, coupled with a zeolithe trap, insuring the primary vacuum of around 10-2 mbar. The primary pump was connected to an oil diffusion secondary pump, which is surmounted by a nitrogen liquid trap. This allows the achievement of a secondary vacuum pressure of around 10-5mbar without any contagions oil vapours issued from primary and secondary pumps. Used crucibles were in tungsten which contain indium (In) and some sulfur (S) of respectively 99,999% and 99,998% purity. These are then heated by using a rheostat connected to the transformer borders. The substrate carrier was fixed to the quartz balance support. This last one can revolve and so take place above every crucible. The indium films obtained by evaporation are placed in a 15 cm3 Pyrex tube, in the presence of some milligrams of sulfur grains. The tube is then sealed in secondary vacuum conditions of 10-6 mbar. The heat treatment of the sealed tube consists in an annealing process within a programmable oven at different temperatures of 280C °, 320C °, 360C ° and 400C° for a period of 2 hours for each annealing temperature. The obtained thin films, are fully analyzed using physico-chemical, optical and electrical characterizations techniques, such as thickness measurement, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron microprobe analysis (EMPA), X-ray photoelectron spectroscopy (XPS), optical absorption, electrical conductivity measurements and the determination of majority carriers type. 3. Results and discussions
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By comparing the depicted lines of the obtained X-rays diffraction diagrams with those of the reference JCPDS data [4,5,6], we notice that the elaborated thin films are crystallized either in the β-In2S3 cubic phase, or in the αIn2S3 and β-In2S3 tetragonal phases with effect of the annealing temperature (figure 1).
T=320C°. (411)
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(a) Fig.1. X-rays diffraction diagrams
(b) β -In2S3 cubic phase (a), and α -In2S3 and β -In2S3 tetragonal
phases (b) with effect the annealing temperature
Optical density (O.D.) measurements were made for the various synthesis temperatures. The transmission and reflection curves of our thin films are given in figure 2.
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T280 T320 T360 T400 R280 R320 R360 R400
Transmission,Réflection(%)
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Fig .2. Transmission and reflection spectra versus wavelength of thin films, annealed in various temperatures for In2S3compound .
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Fig.3. The evolution of direct and indirect gaps versus synthesis temperature of the In2S3 thin films.
The optical transmission spectra show that the optical absorption threshold is approximately 610 nm for films being synthesized at 400°C. By increasing the annealing temperature this threshold value moves towards the higher wavelengths values. Above the threshold value, the transmission of the In2S3 films is situated between 60% and 2 80%. We apply both direct and indirect transitions between the allowed parabolic bands [7]. The direct (αE ) and indirect (αE ) spectra versus the energy E = hυ , where E is the energy of the incidental photons allow to estimate the thin films band gap values. The optical gaps are determined from the extrapolation of the linear part of these curves. In our case we observe the direct and indirect gaps evolution with the synthesis temperature (figure.3). The evolution is the same as that found by Sandoval et al [8]. It can be seen that a critical value is observed in our case at 320°C The optical gap, according to the synthesis temperature, is seen to decrease until it reached a minimum value then its increases like is observed in the CdS films deposited by CVD [9,10]. Before annealing, the films were having cubic phases, whereas after annealing this compound changes phase for a stable hexagonal structure, this occurs in the temperature range 200°C-400°C . The minimum value of the gap of CdS is achieved at 300°C. This temperature is considered as being the temperature of transition between the cubic and hexagonal phases of CdS. The variation of optical gaps according to the temperature in our case is similar to that observed in the case of CdS. As for our sample and considering the information depicted in the XRD diagrams, the synthesized films at temperatures lower than 320°C (temperature of transition) have cubic structure, whereas for synthesis temperatures above this critical value, the In2S3 films have a tetragonal structure. From the representation of the gap evolution versus the annealing temperature, we can conclude that the structural transition occurs at T=320°C since at this temperature the optical gap takes a minimum value. Therefore, the gap diminution can be produced by the structural disorder result from the phase change during the increase of annealing temperature. For higher temperature, the increase of the gap shows that thin films crystallized in the second phase which is the tetragonal structure. When the annealing temperature increases, the phases changing can depend on the type of transition. In our case for the weak temperatures (below 320°C), thin films crystallized in cubic structures, a mix of α and β indirect transition, whereas for phase β-In2S3 tetragonal structure the transition is direct. Our In2S3 thin films were electrically characterized to determine their majority carriers type and electrical conductivity at room temperature. This was realised by means of the hot probe technique for which we noticed that our In2S3 thin films are of n type. Electrical resistivity measurements at room temperature show that thin films are quite resistive and their electric conductivity σ is equal to 10-1 (ȍ cm)-1. 12
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4. Conclusion The optical analysis show that the annealing temperature of In2S3 thin films has an effect on the value of optical direct and indirect gaps, according to the obtained phases, and that the transition temperature of the phases is T=320°C. However the obtained values of the optical band gaps are quite higher than those wanted, such high value can be attributed to contaminant existence, namely the oxygen which is introduced during the contact of thin films with air before the metal thin films sulfuration and which can replace the sulfur during the annealing [11]. Also some sodium issued from the glass substrate can replace the indium [12]. These contaminants allow the improvement of the crystalline structure of films. These thin films have interesting structural and optical properties and are n type. However they present a high electrical resistance that is equivalent to serial resistance added to the ones already present. In the future we need to minimize it by taking the other properties unchanged. A doping of our thin films by a metal (Cu, Zn) can increase their conductivity without affecting their majority type of carriers or their optical properties.
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