Introducing High Efficiency Solar Cells Based on ...

SolarTR-2 Solar Electricity Conference and Exhibition November 7-9, 2012 Antalya, Turkey

Introducing High Efficiency Solar Cells Based on Crystalline Silicon Doped with Transition Metals A. Rostami1,2, H. Heidarzadeh2, H. Baghban1, M. Dolatyari 1, and H. Rasooli3 1

School of Engineering-Emerging Technologies, University of Tabriz, Tabriz, Iran Department of Electrical and computer Engineering, University of Tabriz, Tabriz, Iran 3 Islamic Azad University, Tabriz Branch, Tabriz, Iran Corresponding author: [email protected]

2

Abstract: Solar cell materials with more than one bandgap offer the possibility to increase the efficiency of the solar cell beyond that of a single bandgap cell. In the intermediate bandgap solar cell (IBSC) an intermediate narrow metallic band (IB) is placed in the traditional forbidden bandgap which extends the absorption spectrum. This generates extra electron–hole pairs and thus increases the current without decreasing the output voltage and therefore increases the quantum efficiency. Substitution of transition metal atoms (TM) in the crystalline silicon may give rise to a type of highefficiency photovoltaic materials with intermediate bands to absorb low energy photons. In the present study comprehensive analysis is carried out on this kind of materials. Theoretical studies confirm the formation of suitable mini-bands within silicon band gap by doping of transition metals in crystalline silicon. The mini bands mainly are created by nd orbitals of the transition metals. Absorption coefficient, density of states and band structure are three important features of the proposed materials. Here, we calculated these characteristics for the crystalline silicon doped with TM=Ti, Zr, Hf, Cr, Mo, W, Fe, Ru, Os as candidates for presenting an isolated partially-filled narrow bands between the valance band and the conduction band of silicon. The results show that a crystalline silicon solar cell with an intermediate band located at 0.35eV below the conduction band or above the valence band can reach a limiting efficiency of 44%, improving greatly than 31% of the Shockley–Queisser limit for the single junction Si solar cell. Ti doped silicon can be create this band between the valance band and conduction of silicon. Keywords: high efficiency solar cell, intermediate band, silicon, metal-doped silicon

1


1. INTRODUCTION Exploitation of new energy sources has gained considerable attention due to the world-wide energy shortage and environment pollution. It is possible to enhance solar cell efficiency by taking advantage of sub-gap absorption to increase the current density due to improvement the wavelength response [1-3].

The intermediate concept in the field of semiconductor solar cells is based on having a narrow band inside the main band gap, which allows the absorption of extra low energy photons that promotes a second electron-hole pair, and hence, increasing the solar cell photocurrent without affecting the cell voltage. The intermediate band (IB) solar cells present efficiencies higher than those established by the Shockley-Queisser limit [4], as has been shown in previous work of Luque and Marti [1]. IB solar cells can deliver a high photo voltage by absorbing two sub band gap photons to produce one high energy electron. As a result, the photocurrent of the solar cell can be increased without decreasing the voltage. The detailed balance method predicts the potential of the intermediate band solar cell (IBSC), which can improve the efficiency of the Si-based solar cell with a bandgap of 1.1eV. In this paper we show that a crystalline silicon solar cell with an intermediate band located 0.36eV below the conduction band or above the valence band can reach a limiting efficiency of 44%. Several direct band materials have been proposed as intermediate-band materials in order to achieve a highly-efficient solar cell [5-7]. In most cases, transition metals were introduced into host semiconductor, substituting group III atoms in such a way that nd electrons of the transition metal can form the IB. Theoretical studies have concluded that the synthesis of several of these candidates is not possible. Meantime, other possible intermediate-band materials have been recently proposed in order to demonstrate experimentally the working principles of the intermediate-band solar cells. The clear advantage of Si as host semiconductor is the thorough knowledge of it, which could lead more proficiently to the first experimental devices. For this reason in this paper we calculate electronic properties of crystalline silicon which transition metal atoms have been substituted with a number of Si atoms. In Fig. 1 silicon is the host semiconductor for the transition metal.

Fig. 1 Silicon as host material for transition metal

In this work the electronic and optical properties of Si(TM) materials, where TM is the transition metals Sc, Ti, V, Cr, Mn and Fe, are studied with density functional theory to survey the candidates of intermediate band (IB) material. The absorption coefficient in sub-band-gap energy is greatly improved by the introduction of the IB compared to the Silicon host. 2


2. CALCULATION METHOD Quantum calculations based on density-functional theory are carried out with the aim of discovering the origin of the electronic properties of transition-metal-implanted Si. The simulation is based on Ab-initio calculation, using DFT method [8, 9]. The Kohn-Sham equation was solved selfconsistently to calculate the properties of the proposed intermediate band materials. Silicon has the space group of Fd3m with lattice parameters a0=5.433 angstrom at 300 oK. The size of the K-point set to 6×6×6 in all this work simulation. 3. SIMULATION RESULTS AND DISCUSSION The electronic structure features of Si(TM) obtained in this work shows the formation of new band inside the main band gap. This new band, formed mainly from TM nd orbitals. The three important features are absorption coefficient, density of state, and band structure. We have calculated these parameters by the mentioned method. The presence of metal ions in silicon introduces new energy levels into the band gap of Si. Fig. 2 (a)-(c) depict the band formation between the valance band and conduction band of Si for TM=Ti, Cr, and Fe. The electronic transitions from the valence band to dopant level or from the dopant level to the conduction band can effectively red shift the band edge absorption threshold. The response to the visible light was improved by the intermediate band formed by the d orbital of doping metal ions.

Fig. 2 Energy bands of Si(TM) in several directions of the Brillouin zone: (a) TM=Ti, (b) TM=Cr, and (c) TM=Fe.

Then, we focus on the absorption coefficients calculation for three group of transition metals and compare them by the absorption coefficient of pure silicon. Results of the absorption coefficient have been shown in fig. 3(a) for TM=Fe, Ru and Os, in fig. 3(b) for TM=Cr, Mo and W and in fig. 3(c) for TM=Ti, Zr and Hf as 3


candidates for presenting an isolated partially-filled narrow band between the valance band and the conduction band of silicon.

Fig. 3 Optical Absorption Coefficient of Si(TM): (a) TM=Ti, Zr, Hf (b) TM=Cr, Mo, W and (c) TM=Fe, Ru, Os. Fig. 4 depicts the limiting efficiency of the crystalline silicon IBSC as a function of position of the IB. It shows that a crystalline silicon solar cell with an intermediate band located at 0.36eV below the conduction band or above the valence band can reach a limiting efficiency of 44%, improving greatly than 31% of the Shockley–Queisser limit for the single junction Si solar cell.

Fig. 4 Efficiency of crystalline silicon IBSC as a function of position of the IB level.

4.

CONCLUSIONS

The band structure and absorption coefficients of Si(TM) structures, where TM is the transition metal substituting with Si atoms in the silicon semiconductor host, were simulated based on Ab-initio calculations. 4


The results show that these materials exhibit the IB properties, which can enhance the absorption of the solar radiation compared to crystalline bulk Silicon. Application of these IB materials will increase the photovoltaic efficiency via sub-band-gap absorption, thus, improving the efficiency of photovoltaic devices such as solar cells.

References [1] Luque A., Marti A., Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels, Phys. Rev. Lett, vol. 78, 1997, pp. 5014 [2] Luque, A., operation of the intermediate band solar cell under nonideal space charge region conditions and half filling of the intermediate band," J. Appl. Phys, vol. 99, 2006, pp. 094503 [3] Martí, A., Cuadra, L., Luque, A., Quasi-Drift Diffusion Model for the Quantum Dot Intermediate Band Solar Cell, IEEE Tran. Electron Dev., vol. 49, 2002, pp. 1632 [4] Shockley, W., Queisser, H.J., Detailed balance limit of efficiency of p–n junction solar cells, Journal of Applied Physics, Vol. 32, 1961, pp. 510–519 [5] Luque, A., Marti, A., A Metallic Intermediate Band High Efficiency Solar Cell, Prog. Photovolt. Res. Appl, vol. 9, 2001, pp. 73 [6] Tablero, C., Electronic and magnetic properties of ZnS doped with Cr, Phyc. Rev. B, vol. 74, 2006, pp.195203-1-195203-9 [7] Palacios, P., Aguilera, I., Sanchez, K., Conesa, J. C., Wahnon, P., Transition-MetalSubstituted Indium Thiospinels as Novel Intermediate-Band Materials: Prediction and Understanding of Their Electronic Properties, Phys. Rev. Lett, vol. 101, 2008, pp. 046403 [8] Karch, K., Pavone, P., Mindi, W., Schutt, O., Strauch, D., Ab initio calculation of structural and lattice-dynamical properties of silicon carbide, Phys. Rev B. vol. 50, 1994, pp. 1705417063 [9] Callaway J., March, N. H., Density Functional Methods: Theory and Applications, Solid State Physics, vol. 38, 1984, pp.135

5