Power Conversion Enhancement of CdS/CdTe Solar Cell Interconnected with Tunnel Diode

Wagah F. Mohammed, Omar Daoud, Munther Al-Tikriti

Communications and Electronics Department, Faculty of Engineering, Philadelphia University, Amman, Jordan.

DOI: 10.4236/cs.2012.33032                

Abstract

One of the most promising solar cell devices is cadmium telluride (CdTe) based. These cells however, have their own problems of stability and degradation in efficiency. Measurements show that CdS/CdTe solar cell has high series resistance which degrades the performance of solar cell energy conversion. Both active layers (CdS and CdTe) had been fabricated by thermal evaporation and tested individually. It was found that CdS window layer of 300 nm have the lowest series resistance with maximum light absorption. While 5 - 7 μm CdTe absorber layer absorbed more than 90% of the incident light with minimum series resistance. A complete CdS/CdTe solar cell was fabricated and tested. It was found that deposited cell without heat treatment shows that the short circuit current increment decreases as the light intensity increases. This type of deposited cell has low conversion efficiency. The energy conversion efficiency was improved by heat treatment, depositing heavily doped layer at the back of the cell and minimizing the contact resistivity by depositing material with resistivity less than 1 m??cm². All these modifications were not enough because the back contact is non-ohmic. Tunnel diode of CdTe (p++)/CdS (n++) was deposited in the back of the cell. The energy conversion efficiency was improved by more than 7%.

Keywords

CdS/CdTe; Solar Cells; Energy Conversion; Efficiency

1. Introduction

Silicon-based solar cells are currently the most successful commercial photovoltaic product. The PV market, dominated by crystalline silicon, has grown on average more than twenty percent per year but faces the problem of profitability as it must compete with traditional sources and methods of energy conversion. To become competitive, PV materials are needed as they are much less expensive than single crystal silicon and are compatible with large scale manufacturing. Thin film materials and manufacturing processes are an obvious choice for lowering the cost. Thin film solar cells based on polycrystalline Cadmium Telluride (CdTe) reached record efficiencies of 16.5% [1] for laboratory scale device and of 10.9% for terrestrial module [2]. Since the record efficiency of such type solar cells is considerably lower than the theoretical limit of 28% - 30% [3], the performance of the modules can be improved, through new advances in fundamental material science and engineering, and device processing. CdTe is one of the most suitable materials for photovoltaic applications. CdTe has a direct band gap material (E_g ≈ 1.5 eV at room temperature) with a high absorption coefficient (above 10⁵ cm^–1 at the wavelength of 700 nm). Few microns thick layer of CdTe absorbs more than 90% of the incident light with the photon energy higher than the band gap. The maximum theoretical efficiency corresponding to such band gap is about 27%. The small thickness required for an absorbing layer makes the cost of material for the solar cells relatively low. To date, CdTe has been deposited successfully by a variety of techniques [4].

CaCadmium sulfide (CdS) belonging to the II-VI group is one of the promising materials for optoelectronic devices. CdS has been the subject of intensive research because of its intermediate band gap (E_g ≈ 2.42 eV) making the material suitable as window material for a heterojunction solar cell [5], high absorption coefficient, reasonable conversion efficiency, stability and low cost [6]. Knowledge of the optical properties of CdS films is very important in the field of optoelectronic devices like photo-detectors and solar cells. A broad variety of deposition techniques can be used to fabricate CdS films with desirable optical properties [7]. Although CdTe can be doped both p-type and n-type CdTe: homojunction cells have not shown very high efficiency. Due to high absorption coefficient and small diffusion length, the junction must be formed close to the surface, which reduces the carrier lifetime through surface recombination. The In-doped CdTe (p) thin film is of high bulk resistivity which largely affects its photovoltaic properties particularly the short circuit current [8]. It was noted that, the deteriorative effect of the high bulk resistivity increases by increasing the light intensity, which in turn limits the benefit of using light concentrators that improve the short circuit current.

Heterojunctions which consist of CdTe as one of the junction sides had been under investigation for many years [9]. The electrical properties of post-deposition annealed and as-deposited In-doped CdTe thin films were studied in details [10]. It was observed that the CdTe film was of modified Poole-Frenkel conduction mechanism and the resistivity of the film could be lowered by more than one order of magnitude due to indium doping. Also, considerable amount of work had been paid to develop the CdS/CdTe solar cells over the last twenty years [11]. Also the electrical, photoelectrical, and structural properties of CdS/CdTe heterostruture were studied [12]. Deposition of thin polycrystalline CdTe layers on the top of the CdS layer for solar cells has been successfully performed by using various methods. Considerably higher efficiencies were obtained by using n-CdS/p-CdTe heterojunctions. The CdS layer serves as a window layer and helps to reduce the interface recombination. Without special doping the CdS film has significantly higher carrier concentration (≈10¹⁶ - 10¹⁷ cm^–3) than the CdTe adjacent to the interface (≈10¹⁴ - 10¹⁵ cm^–3). As a result the built-in potential is applied mostly to the CdTe absorber layer, providing effective separation of the photo generated carriers. High efficiency solar cells of efficiencies up to 12.5% were developed with a CdTe low temperature (<450˚C) process [13]. Efficient solar cell performance requires minimizing the forward recombination current and maximizing the light generated current. Collection losses can be minimized in thin film of high absorption and short diffusion length. Voltage dependent photocurrent collection losses in CdTe films were observed [14]. The voltage dependence of photocurrent of CdTe/CdS solar cells was characterized by separating the forward current from the photocurrent. Recently, preparation and performance of CdS/CdTe tandem solar cells is introduced [15,16]. Thinner layers at the top and thicker ones at the bottom managed to increase the open circuit voltage and improve the spectral response.

2. Laboratory Preparations and Solar Cell Structure

Cadmium Sulphide/Cadmium telluride (CdS/CdTe) solar cell is composed of four main layers deposited on a glass substrate. A transparent conducting oxide deposited directly on top of the glass to form the front contact. The second layer is the window layer, which is usually n-type semiconductor. CdS, with band gap of 2.4 eV at room temperature, is the most suitable material for CdTe-based solar cells. The work of [17-19] showed that without special doping, the CdS films have significantly higher carrier concentration than the CdTe. The third layer is the absorber layer of CdTe, which is usually from 5 - 10 µm thick film. The deposition parameters, optical and electrical properties of active layers will be discussed deeply later. Finally, the fourth step in the solar cell fabrication is the application of the back electrical contact to the CdTe layer. 2 µm Aluminum is used as metal back contact. It was recognized that this step is critical for CdS/ CdTe solar cell performance due to low stability and resulted in a high contact resistance. In order to minimize this resistance (ρ_C < 1 mΩ·cm) tunnel diode is proposed to be connected in series with the solar cell [20]. These problems will be the main issues in this work and they will be addressed later.

Fabrication of CdS films of thickness up to 800 nm was carried out on a glass substrate using Balzer vacuum thermal evaporation system. The substrate temperature, vacuum pressure, deposition rate, film thickness and annealing temperature have been measured by the system. CdS film was evaporated at optimum evaporation parameters [18], under 10^–6 mbar vacuum. The substrate temperature was 300˚C and the deposition rate is 2˚ A/sec. Thickness of the layer and annealing temperature are varied to obtain maximum grain size at minimum thickness with very low resistivity. Few samples of CdTe thin film were prepared by thermal evaporation and deposited on glass substrate to be examined individually. A comprehensive study of CdTe layer in CdS/CdTe solar cell had been conducted [21], and the main parameters of CdTe material that affect the module efficiency had been discussed. Among these parameters are the lifetime, diffusion length, drift length of minority carriers and thickness of CdTe absorber layer. In this research; it is found that 7 µm of CdTe thickness deposited with 8˚ A/sec. rate of deposition on substrate with 100˚C temperature is optimum for maximum absorption of radiation and produces large enough grain size [10]. The annealing temperature is varied for optimum optical and electrical properties of the film.

3. Optical and Electrical Properties of the Solar Cell Layers

Transmission and absorption coefficient spectrum have been carried out for each individual layer at different annealing temperature. Figures 1 and 2 shows the transmission and absorption coefficients of CdS and CdTe layers respectively. CdS film exhibited high degree of transmittance in the infrared region and showed sharp falling of the absorption edge towards lower wavelength. The absorption edge is lowered as the annealing tem-

Figure 1. Transmittance and absorption coefficients of CdS layer at different annealing temperatures.

Figure 2. Transmittance and absorption coefficients of CdTe layer at different annealing temperatures.

perature of the film increased. It was found that small changes in the thickness of CdS had a greater influence on transmission. It must, however, be emphasized that more reduction of CdS will increase the resistivity of the layer which will deteriorate the electrical properties of the layer. Thicker CdTe layer is used in order that all light is absorbed in this layer. CdTe film exhibits transmittance at short wavelength (λ ≈ 500 nm). The transmittance becomes more pronounced at wavelength higher than 800 nm. The absorption edge shifted toward lower wavelength at high annealing temperature (250˚C).

Figures 3 and 4 show that the variation of resistivity and photo generated current of CdS and CdTe layers with wavelength at different annealing temperatures. It is clear that CdS sample annealed at 250˚C gave minimum resistivity and of course maximum photo generated current. This is because that the absorption coefficient for this sample is very high which is inversely proportional to resistivity. The material becomes more n-type due to excessive Cadmium under layer and enhanced diffusion at grain boundaries or impurities incorporated in the deposit [7].

4. The Effect of Series Resistance

Figure 5(a) represents a schematic representation of the CdS/CdTe solar cell heterostructure. The layers succession and thicknesses are the one used in the present work. An electronic solar cell model can be considered, as shown in Figure 5(b), taking into account the effect of the series resistance (R_s) and shunt resistance (R_sh). The solar cell current source generates a light current (I_PH) which is directly proportional to the solar illumination. The two resistors (R_sh) and (R_s) represent the losses incurred in the solar cell. The series resistor (R_s) caused by the ohmic losses in the surface of the solar cell. The par-

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	X. Wu, J. C. Keane, R. G. Dhere, C. DeHart, A. Duda, T. A. Gessert, S. Asher, D. H. Levi and Sheldon, “16.5% Efficient CdS/CdTe Polycrystalline Thin-Film Solar Cell,” Proceedings of the 17th E-PVSEC, München, October 2001, pp. 995-1000.
[2]	D. Cunningham, K. Davies, L. Grammond, E. Mopas, M. O’Connor, M. Rubcich, M. Sadeghi, D. Skinner and T. Trumbly, “Large Area Apollo Module Performance and Reliability,” Proceedings of the 28th IEEE Photovoltaic Specialists Conference, Fairfield, 2000, pp. 13-18. doi:10.1109/PVSC.2000.915743
[3]	S. Sze, “Physics of semiconductor Devices,” 2nd Edition, Wiley, New York, 1981.
[4]	R. Birkmire, “CdTe1–xSx Absorber Layers for Thin-Film CdTe/CdS Solar Cells,” 26th IEEE Photovoltaic’s Specialists Conference (PVSC), Anaheim, Vol. 295, 1997, pp. 307-312.
[5]	M. V. Garcia-Cuenc, J. L. Morenza, E. Bertran and A. Lousa, “Electrical Conduction in Polycrystalline CdS Films: Comparison of Theory and Experiment,” Journal of Physics D: Applied Physics, Vol. 20, No. 7, 1987, pp. 958-962. doi:10.1088/0022-3727/20/7/020
[6]	A. Sanchez, P. J. Sebastian and O. Gomez-Daza, “LowResistivity CdS Thin Films Formed by a New Chemical Vapor Transport Method,” Semiconductor Science and Technology, Vol. 10, No. 1, 1995, pp. 87-90. doi:10.1088/0268-1242/10/1/014
[7]	W. F. Mohammed, “The Effect of Temperatures and Doped Level on CdS Thin Films,” Engineering & Technology Journal, Vol. 14, No. 7, 1995, p. 34.
[8]	W. F. Mohammed and M. A. Shehathah, “Effect of Series Resistance on Photovoltaic Properties of In-Doped CdTe (p) Thin Film Homojunction Structure,” Renewable Energy, Vol. 21, No. 3, 2000, pp. 141-152. doi:10.1016/S0960-1481(00)00008-2
[9]	F. Zhou, X. C. Wang, H. C. Wu and C. Z. Zhao, “Achievements and Challenges of CdS/CdTe Solar Cells,” International Journal of Photo Energy, Vol. 2011, 2011, pp. 1-8.
[10]	W. F. Mohammed and M. A. Shehathah, “The Electrical Properties of Post-Deposition Annealed and As-Deposited In-Doped CdTe Thin Films,” Renewable Energy, Vol. 26, 2002, pp. 285-249. doi:10.1016/S0960-1481(00)00195-6
[11]	M. M. Al-Jassim, R. G. Dhere, K. M. Jons, F. S. Hasoon and Sheldon, “The Morphology, Microstructure, and Luminescent Properties of CdS/CdTe Films,” 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, 6-10 July 1998.
[12]	D. H. Levi, L. M. Woods, D. S. Albin and T. A. Gessert, “The Influence of Grain Boundary Diffusion on the Electro-Optical Properties of CdTe/CdS Solar Cells,” 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, 6-10 July 1998.
[13]	G. Khrypunov, A. Romeo, F. Kurdesau, D. L. Batnzer, H. Zogg and D. L. Tiwari, “Recent Development in Evaporated CdTe Solar Cells,” Solar Energy Material and Solar Cells, Vol. 90, No. 6, 2006, p. 664. doi:10.1016/j.solmat.2005.04.003
[14]	S. Hegedus, D. Desai and C. Thompson, “Voltage Dependent Photocurrent Collection in CdTe/CdS Solar Cells,” Progress Photovoltaic: Research and Applications, Vol. 15, No. 7, 2007, p. 587. doi:10.1002/pip.767
[15]	Y. J. Li, et al., “Preparation and Performance of CdS/ CdTe Tandem Solar Cells,” Chinese Journal of Semiconductors, Vol. 5, 2007, pp. 722-725.
[16]	X. Mathew, J. Drayton, V. Parikh, N. R. Mathews, X. X. Liu and A. D. Compaan, “Development of a Semitransparent CdMgTe/CdS Top Cell for Applications in Tandem Solar Cells,” Semiconductor Science Technology, Vol. 24, No. 1, 2009, Article ID 015012. doi:10.1088/0268-1242/24/1/015012
[17]	W. F. Mohammed and A. Nori, “The Photo-Electric and Thermal Properties of Vacuum Deposited CdS Thin Films,” Renewable Energy Journal, Vol. 14, No. 1-4, 1998, pp. 129-134. doi:10.1016/S0960-1481(98)00058-5
[18]	W. F. Mohammed and A. Nori, “The Effect of Deposition Parameters on Hall Mobility and Carrier Concentration of CdS Thin Films,” Abhath Al-Yarmook Journal, Vol. 11, No. 1B, 2002, pp. 402-412.
[19]	H. A. Ahmed and L. S. Ali, “Characterization of In-Doped CdTe Thin Film,” Muutah Journal for Research and Studies, Vol. 11, No. 5, 1996, pp. 207-218.
[20]	W. Guter, F. Dimroth, M. Meusel and A. W. Bett, “Tunnel Diodes for III-V Multi-Junction Solar Cells,” 20th European Photovoltaic Solar Energy Conference, Barcelona, June 2005, pp. 515-518.
[21]	L. Kosyachenko, “Efficiency of Thin-Film CdS/CdTe Solar Cells,” In: R. D. Rugescu, Ed., Solar Energy, 2010, pp. 105-130. doi:10.5772/8065
[22]	M. Wolf and H. Rauschenbusch, “Series resistance Effects on Solar Cell Measurements,” Advanced Energy Conversion, Vol. 3, 1963, pp. 455-479. doi:10.1016/0365-1789(63)90063-8
[23]	A. W. Brinkman and S. M. Al-Amri, “Thin Film CdTe Based Solar Cell,” Proceedings of the 6th Arab International Solar Energy Conference, Muscat, 1998.
[24]	J. Sites and J. Pan, “Strategies to Increase CdTe SolarCell Voltage,” Thin Solid Films, Vol. 515, No. 15, 2007, pp. 6099-6102. doi:10.1016/j.tsf.2006.12.147
[25]	N. Amin, K. Sopian and M. Konagai, “Numerical Modeling of CdS/CdTe and CdS/CdTe/ZnTe Solar Cells as a Function of CdTe Thickness,” Solar Energy Materials and Solar Cells, Vol. 91, No. 13, 2007, pp. 1202-1208. doi:10.1016/j.solmat.2007.04.006
[26]	W. Guter and A. W. Bett, “I-V Characterization of Tunnel Diodes and Multijunction Solar Cells,” IEEE Transactions on Electron Devices, Vol. 53, No. 9, 2006, pp. 2216-2222. doi:10.1109/TED.2006.881051