Photovoltaic research and education at the Department of Applied Physics and Informatics of Moldova State University

Transcription

1 Photovoltaic research and education at the Department of Applied Physics and Informatics of Moldova State University TAMARA POTLOG Physics Department Moldova State University 60, A.Mateevici str., Chisinau MOLDOVA Abstract: - The paper reviews the activities in photovoltaic at the Department of Applied Physics and Informatics of Moldova State University. Most activities emphasize research and education. We focused our paper on the characterization of a solar cells based on A B 6 p-n heterojunctions, prepared in laboratory conditions, with the tools available at the department of the university. Key-Words: - renewable energy sources, education, solar cells, A B 6 p-n heterojunctions Introduction Potential resources of solar radiation in Moldova range from 0.99 kw/m in the clear sky conditions to 0.76 kw/m at the average nebulosity. The maximum amount of solar energy is during summer months, the solar radiation can be used with enough efficiency in autumn and even in winter. In the north of the country, the number of effective work days of the solar installation reaches 60, but in the south 30 per year. The average density of solar energy is 800 W/m. The energetic coefficient E, calculated as average solar energy that drops on the m area during 7 months (March-September) E=5.0 kw h/m day []. So the energetic parameters of solar radiation in Moldova are high enough. There is a real possibility to develop the production of solar cells in Moldova being aware technique and scientific potential of our country. Subject using the new energy sources versus the classic one is on increased interest for humanity, therefore it can be proposed to the young students to prepare them as the main beneficiaries of use such renewable energy sources and to develop a positive attitude about the environmental protection [ 4]. For this purpose at Moldova State University (MSU), Physics Department was opened specialty Renewable energy which required organization of activities of students in scientific research, aiming for: - development the creative skills with high interest for a photovoltaic, through scientific research; - acquisition of additional knowledge in this area, familiarity with research methods, with specific literature and improvement of skills on intellectual work. The fundamental photovoltaic research at Moldova State University (MSU) is carried out on thin-film A B 6 materials. The wide gap materials ZnSe, ZnTe, CdS, and CdTe have the band gap corresponding to the photon energies in the visible region of the solar spectrum and are characterized by relatively large mobility and high photosensitivity. Cadmium telluride has long been identified as an absorber in A B 6 thin film photovoltaic application because of ability to be doped n-and p- type permitting the formation of a variety of junction structures. All CdTe thin film solar cells (CdS/CdTe and ZnSe/CdTe) reported in this study were fabricated in the superstrate configuration by close space sublimation. In order to develop scientific research capacity of students, they initially got acquainted with the general aspects on physical processes in the p-n junction theory and then with the methods of manufacture, investigation of materials and devices. Method of investigation is a part of the training process and represents the link of connection between theory and practice, ensuring on a hand the understanding of physical processes which take place in a photovoltaic cell and on the other hand the gain of the experimental skills needed to develop photovoltaic module industry. The work of group concentrates on optimizing substrate and source temperatures, distances between sources of evaporation and substrates. The group s activity also covers characterization by various optical and photoelectric methods. The optical transmission of the photovoltaic windows was investigated using JASCO V-670 ISBN:

2 spectrophotometer. A briefly characterization of a solar cells was made investigating current-voltage characteristic at illumination and quantum efficiency.. Education Because the technique and technology progress, and finally progress of human society can only be achieved through research and study, and conventional energy sources are exhaustible, we consider that the study of renewable energy sources is a subject of really interest for physics curricula in high school. A number of courses concerning photovoltaic conversion are carried out at the Department of Applied Physics and Informatics of Moldova State University during the last 5 years. At speciality Renewable Energy a group of 0 students was enrolled. The Department of Applied Physics and Informatics has offered some courses Technology of semiconductor materials, Optical and electrical properties of semiconductors, Semiconductors and optoelectronic devices, The non-traditional methods of energy conversion, Solar energy heat converters, Solar Energy Photovoltaic converters and others. Structural changes on recent years in our country highlights the need for the Moldavian education reconstruction on new bases, according to current social and economic needs. The economic needs require improvement of human resources, qualification and labour flexibility. University wants to be the promoter of such an attitude, and this requires a diversification of supply department in order to give to each generation the individualized courses to University. Method of fabrication of the photovoltaic devices at MSU CdTe Moldova State University photovoltaic devices were fabricated on the commercial Solaronix glass/sno substrates. Because the highest laboratory record efficiency of 6.5% has been achieved for solar cells fabricated on glass substrate by close space sublimation (CSS) at NREL, USA [6], for the fabrication Moldova State University also used the same method. The CSS home-made growth system used in the present study consists of two heated graphite blocks. It is well known [7] that the main parameters that have an effect on the growth rate are the source substrate separation, source substrate temperatures, and the pressure and composition of gases in the deposition chamber. The temperature and growth rate are maintained using BPT-3 temperature controllers that allow keeping a constant temperature with a precision of ~0.5 o C. The pressure during deposition was below Torr. Our aim was to reduce both source and substrate temperatures, because this growth system will be used in the future for the fabrication of CdS/CdTe solar cells on polyimide substrates. Thus, the 0-mm substrate source distance of our evaporation system was employed to reduce the substrate temperature. For the fabrication of CdS and CdTe thin films the optimal substrate temperature 30 o C and a source temperature of 590 o C were found. The ZnSe layers were deposited at T s = 450 o C and T ev = 70 o C. The polycrystalline films were obtained in a short time (about 40min) without an additional transport agent gas. It is well known that the CdCl treatment is an important step in the formation of a solar cell containing CdTe. The obtained structures were dipped into a CdCl saturated solution for min and then annealed in the atmosphere at 40 o C.. Characterization of the photovoltaic windows The window layer's role is to absorb light energy from only the high-energy end of the spectrum. It must be transparent and have a wide enough bandgap to let all available light through the interface (heterojunction) to the absorbing layer and have a good conductivity. As window layers were used CdS and ZnSe. The SEM images show that the CdS and ZnSe thin films deposited in the optimal regime have a granular structure and the cross-sectional images indicate that both grow in a columnar morphology... Optical properties of CdS and ZnSe thin films Fig. and Fig. present the transmittance spectra at usual incidence of the light on the surface of CdS and ZnSe layers depending on the time of deposition. As one can see, the transmittance spectra possess a structure with values characteristic of interference phenomenon. The highest transmittance of CdS layers is observed for deposition time of min. The highest value of transmittance in the visible spectral range is (85-90) % for CdS with deposition time min. ISBN:

3 λλ d = (), ( λ n λ n ) Fig.. Transmittance of the CdS thin films, deposition time, min: - 5; - 4; 3-3; 4 - ; 5 -. It can be observed as well that with the growth of CdS layer deposition time, the minimal and maximal interference values become more prominent, deeper, narrower and lesser in number, and this fact indicates on the unreasonableness of big thickness of CdS layers for solar cells. It can be easily noticed that the wavelengths, which correspond to maximum and minimum values, depend on the layers thickness. The transmission spectra of as deposited ZnSe thin films on glass substrates were studied in a wavelength range of 00 to 00 nm. T,% Wavelength, nm Fig.. Effect of film thickness on the transmittance of ZnSe thin films, µm: -.4; -.7; Knowing the position of two neighbouring maximum or minimum values from the transparency spectra, the thickness of the ZnSe or CdS buffer layers was determined using the following relation [8]: 3 where n and n are the refractive indexes at two adjacent maxima (or minima); λ and λ the respective wavelength values. The thickness of CdS thin films changes from 0.56 µm (deposition time min) to.88 µm (deposition time 5 min). The transmittance of the ZnSe films shifts in the transmission threshold to lower wavelengths with decreasing thickness (see Fig.), which indicates that, in this case, the energy band gap of the ZnSe films increases. This particular behavior of the ZnSe films seems to be related with changes in the band structure induced by changes in their thickness. The conductivity of CdS varies from Ω - cm to Ω - cm -, respectively. Undoped ZnSe films are high-resistive: ρ= ( ) Ω cm. The deposition of Zn thin layer by thermal evaporation on ZnSe and annealing at 470 o C reduced the resistivity till 0 3 Ω cm. It was found that an increase in the annealing temperature from 40 o C to 450 o C results in an increase in the donor concentration by an order of magnitude to a value of 0 7 cm -3. The study of electrical properties showed the contribution of noncontrolled donor-type native defects to increasing conductivity of the ZnSe samples... Optical properties of CdTe thin films The absorbing layer under the window, usually doped p-type must have a high absorptivity (ability to absorb photons) for high current and a suitable band gap to provide a good voltage. The relation between absorption coefficients α and the incident photon energy hν can be written as [9] (αhν) = A (hν-e g ) ½ (), where E g is the band gap of the CdTe films, A is the constant. The (α) versus (hν) plots (Fig. 3) for all CdTe films show straight line portions that cut the hν axis (x-axis) upon extrapolation giving the band gap values. The optical energy gap decreases with the increasing film thickness. For the (-3) thin films a larger variation of E g from.496 ev to.487 ev is observed, which is probably determined by a strong strain of the zincblende lattice. Hence, we consider that the observed decrease in E g with increasing thickness is due to the decrease in lattice strain. The values are found to be comparable with earlier reports [0]. ISBN:

4 3,00E+009,50E+009,00E+009,50E+009,00E+009 5,00E+008 α, [cm - ] 0,00E+000,450,475,500,55,550 hν, ev 3 Current-voltage characteristics of ZnSe/CdTe and CdS/CdTe thin film heterojunction solar cells The study of the dark current-voltage characteristics of several ZnSe/CdTe solar cells with different thicknesses of ZnSe shows that the direct curves of the J-U characteristic shift in the direction of the current abscissa, which corresponds to an increase in the build-in voltage from 0.46 to 0.5 V. A further increase in the thickness of the ZnSe layer shows that the build-in voltage remains constant. Thus, this thickness of ZnSe was taken as optimal. The rectification coefficient of the heterostructure with d ZnSe =.4 µm calculated at U d =U ind = V is about and increases to 60 with increasing thickness of the ZnSe film. A further increase in the thickness of the ZnSe layer results in an increase in the series resistance of the heterojunction, which limits the direct current that leads to a decrease in the rectification coefficient. The conclusion of the students was that the solar cell in the dark is acting like a diode. The achievement of high efficiency for solar cells requires the understanding of the junction transport. In order to obtain information on the transport current mechanisms for the best ZnSe/CdTe and CdS/CdTe devices, we measured current-voltage characteristics in the dark in the temperature interval of (90-393) K. In the case of our structures, the logarithm of forward current versus voltage at different temperatures was analyzed. Intercepts of the plot with the current (y-) axis gives the value of 3 Fig. 3. Dependence of (α) as a function of the photon energy, hν, for as-deposited CdTe thin films with different thicknesses, µm: -8.9; -7.9; the saturation current J o at any given temperature. At room temperature for ZnSe/CdTe J o =.8E- ma/cm, for CdS/CdTe reaches.07e-4 ma/cm. Dependence n=f(t) exhibits two linear regions. The value of the ideality factor in this temperature range varies with temperature from 6 (93K) to.85 (400 K) while for the region where is applied bias greater than 0.5 V n changes slightly from.9 until. with temperature. Based on these results, students assume that the current flow mechanism is generation-recombination. For the best CdS/CdTe photovoltaic device the forward I-U branches in the entire temperature range (93-400) K at voltages 0. V<U<U D show the temperature dependence of ideality factor which varies from.9 to.. These data indicate the same mechanism transport current flow as in the case of ZnSe/CdTe device. This is in agreement with many other studies [0-] because CdS/CdTe interface is heavily dislocated due to the lattice and structural mismatch. The J-V characteristics of the ZnSe/CdTe and CdS/CdTe thin film heterojunction solar cells were investigated under 00 mw/cm illumination through the wide band-gap ZnSe and CdS layers and are shown in Fig. 4. The J-U characteristic for a PV cell based on an ideally p-n junction in the light is given by the Shockley equation []: qu J = Jo[exp( ) ] J L (3), nkt where: J 0, q, k, T and I L, are the reverse saturation current density, electronic charge, Boltzmann constant, absolute temperature and photocurrent, respectively. In Fig. 4, the students recognize the open circuit voltage, U oc =0.785 V for CdS/CdTe and U oc =0.698 V for ZnSe/CdTe and the short circuit current density Jsc =5. ma/cm and J sc =8.7 ma/cm, respectively. The fill factor FF, is the ratio of the maximum power (P max ) point divided by the open circuit voltage U oc and the short circuit current density J sc [3]: Pmax FF = U (4) oc J sc The calculated fill factor FF, of our solar cells is ~ 0.54 for CdS/CdTe and ~ 0.37 for ZnSe/CdTe. A new challenge for students is to find the conversion efficiency of light in electricity. A PV cell energy ISBN:

5 conversion efficiency η is calculated using the ratio of the maximum power point, P max, divided by the input light irradiance E [W/m ] under standard test conditions (STC means a temperature of 5 C and an irradiance of.000 W/m and the surface area of the solar cell S [m ] [3]: P max η = (5) E S The efficiency of ZnSe/CdTe is about 5%. For CdS/CdTe device efficiency is higher and reaches ~0%. materials make an equal contribution to photocurrent if and only if the thickness of the epitaxial layer is on the order of the penetration depth of the contact field. If the thickness is less than the penetration depth, the major contribution to the EQE is obtained in the sensitivity region of the narrow-bandgap material; for the layer thickness higher than the penetration depth of the contact field, it is in the sensitivity region of the widebandgap material.,0 0,8 5 d e m o d e m o 0 d e m o ZnSe/CdTe CdS/CdTe 5 d e m o d e m o d e m o 0 d e m o d e m o 5 0 -, -0,8-0,4 o -50,0 m o 0,4 o 0,8 m o, d e m o d e m d e d e m d e -0 U,V d e m o d e m o d e m o -5 d e m o d e m o -0 d e m o d e m o d e m o -5 d e m o d e m o -30 J, ma/cm QE,[a.u] 0,6 0,4 0, 0,0 3,,4,6,8,0,,4,6,8 3,0 3, hν (ev) Fig. 5. Quantum efficiency of ZnSe/CdTe thin film heterojunction solar cells with different thicknesses of ZnSe. Fig. 4. Current-voltage characteristics for the best ZnSe/CdTe and CdS/CdTe thin film heterojunction solar cells, 00 mw/cm and room temperature. 4 Effect of the thickness on quantum efficiency of ZnSe/CdTe and CdS/CdTe thin film heterojunction solar cells Students should also see the effects of the different light wavelengths on the PV cell. The quantum efficiency (QE) is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell from outside (incident photons). The quantum efficiency therefore may be given either as a function of wavelength or as energy. The Fig. 5 shows the effect of the thickness on the quantum efficiency (QE). In the case of samples, almost all space charge is situated in CdTe, and (QE) is determined by the electron-hole generation in it. The redistribution of electron-hole pair generation occurs with increasing thickness of ZnSe (samples: without Zn layer at the interface and 3 with Zn layer at the interface). It is obvious that the contacting Fig.6. QE of CdS/CdTe solar cells with different thickness of CdS (CdS deposition time, min: - ; ; 3 3; 4 4; 5 5). The quantum efficiency illustrated in Fig.6 shows the response for a set of solar cells made with different thicknesses of CdS window layer. Decreasing the window layer thickness increased response in the 400 nm nm wavelength range (curves, ). In the cell with thick CdS layer (curves 4, 5) some interdiffusion of S and Te across the metallurgical junction probably occurs during film growth or during the post-deposition treatment near 400 o C in the presence of CdCl. ISBN:

6 The shape of the spectral response curve 5 is consistent of a mixture layer having a band gap narrower than CdS. In the blue region from 400 nm to 500 nm, absorption in CdS attenuates the light reaching the CdTe layer, and thus reduces the quantum efficiency of the cell. No carrier collection appears to occur in the CdS, either because the minority carrier (hole) lifetime is too low or because other factors present barriers to hole collection. As the CdS thickness is more reduced the quantum efficiency in the region from 400 nm to 500 nm rises. The quantum efficiency (QE) for four of the cells is reasonably good for wavelengths between the band gap of the CdS window layer and that of the CdTe. The three-layer tandem structure ZnSe/ZnTe/CdTe was prepared by sequential deposition of ZnTe and CdTe onto SnO /ZnSe substrates. The structure was supplied with contacts which allowed switching on ZnSe/ZnTe and ZnTe/CdTe heterojunctions into separate electrical circuits. At 300 K in AM.5 conditions the summary V oc was. V and efficiency about 3. %. These results were received with non optimized structures without antireflection coating and they could be improved on the expense of diminishing of losses both for current and voltage by selecting the component thickness and doping level. For all data analysis was used an Origin Software. The Origin Lab is professional software specializing in data analysis and plotting of graphs. It contains several sheets, data import capabilities, query databases, making professional graphics. The software is used in colleges and universities worldwide, with friendly interface that is a very good tool in experimental data processing laboratory [5]. 5 Conclusion The students understand the mechanism of photovoltaic effect generally and gain the experimental skills to characterize a solar cell. Measuring the transmission and reflectance spectra the students determined the band gap of wide gap materials observing the good agreement between their value and the values obtained by other measurements. Plotting the current-voltage characteristics in both forward and reverse conditions, in the dark, the students observe that the p-n heterojunction cells are a nonlinear element of circuit, with high asymmetry. The measured current-voltage characteristics in fourth quadrant under the light, gave the possibility to the students to find with accuracy, the typical parameters of the cell working in regime of photoelement (U oc, J sc, FF, and η). The students were fascinated by their good results obtained using the experimental setups available in their laboratories, acquiring the really skills for research in this field. Acknowledgements This research was supported by the EU 7 th Framework Program PEOPLE International Research Staff Exchange Scheme project Development of Flexible Single and Tandem II-VI Based High Efficiency Thin Film Solar Cells GA-008, No References:. Ambros T., Arion V., Gutu A, Sobor I, et al. Surse regenerabile de energie. Editura Tehnica-Info, Chisinau, Eds.: Mary D. Archer and Robert Hill, Clean electricity from Photovoltaics, Imperial College Press, E. Lorenzo, G. L. Araújo, P. Davies, A. Cuevas, M. Egido, J. Minano, R. Zilles, Solar electricity: engineering of photovoltaic systems, Progensa, G.P. Smestad, Education and solar conversion: Demonstrating electron transfer, Solar Energy Materials and Solar Cells, 55, 998, pp X. Wu, High-efficiency polycrystalline CdTe thin-film solar cells, Solar Energy 77, 004, pp T. Potlog, N. Spalatu, N. Maticiuc, J. Hiie, V. Valdna V. Mikli, A. Mere. Structural Reproducibility of CdTe Thin Films Deposited on Different Substrates by Close Space Sublimation Method. Physica status solidi (a), Vol. 09, Issue, 0, pp Huanyong Li, Wanqi Jie Growth and characterizations of bulk ZnSe single crystal by chemical vapor transport, Journal of Crystal Growth, 57, 003, pp T. S. Moss, G. J. Burrell, B. Ellis, Semiconductors Opto-Electronics, Butterworth & CoLtd., London, S. Saha, U. Pal, A. K. Chaudhuri, V. V. Rao, H. D. Banerjee. Optical Properties of CdTe Thin Films. Physica status solidi (a), Vol. 4, Issue, 989, pp ISBN:

7 . W. Shockley and W. T. Read, Statistics of the Recombinations of Holes and Electrons, Phys. Rev., 87, 95, pp El-Nahass, M.M., Zeyada, H.M., Aziz, M.S. and El-Ghamaz, N.A. Carrier Transport Mechanisms and Photovoltaic Properties of Au/p-ZnPc/p-Si Solar Cells. Solid State Electronics, V. 49, 005, pp ISBN: