A Proposal for Intermediate Band Solar Cells with Optimized ...

A Proposal for Intermediate Band Solar Cells with Optimized Transition Energy in Cr Doped 3C-SiC M. Esgandari, H. Heidarzadeh, A. Rostami, G. Rostami and M. Dolatyari

Abstract The Intermediate Band Solar Cell (IBSC) is a new concept proposed to better match the solar spectrum by absorbing sub-bandgap energy photons. One approach to implement this idea is to form an intermediate band (IB) with creating metallic intermediate band inside the host semiconductor. Excellent electronic properties of 3C-SiC such as high electron mobility and saturated electron drift velocity and its suitable band gap makes it an important alternative material for light harvesting technologies instead of conventional semiconductors like silicon. In this paper, the electronic band structure along with density of states calculated by the density functional theory (DFT). Main goal of this paper is proposing a new materials in the field of photovoltaic with intermediate band in the appropriate position. However our theoretical analysis show Cr is appropriate doping for 3C-SiC. In the other hand we demonstrated that our material choice is more advantageous in order to approach to efficiency of near 60 %.

1 Introduction Intermediate band solar cells have been considered as an interesting researches topic in recent years [1, 2]. They are expected to be a suitable solution for production of high efficiency solar cells. IBSCs are formed through the addition of a narrow band of states in the middle of the bandgap of a main semiconductor. So, they are characterized by three band states (valance band, conduction band and intermediate band). In most works, intermediate band is created by two basic M. Esgandari A. Rostami G. Rostami M. Dolatyari Photonics Research Group, School of Engineering Emerging Technology, University of Tabriz, Tabriz 5166614761, Iran H. Heidarzadeh A. Rostami (&) Photonic and Nanocrystal Research Lab. (PNRL), Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz 5166614761, Iran e-mail: [email protected] © Springer International Publishing Switzerland 2015 A.Y. Oral et al. (eds.), 2nd International Congress on Energy Efficiency and Energy Related Materials (ENEFM2014), Springer Proceedings in Energy, DOI 10.1007/978-3-319-16901-9_9

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strategies [3, 4]. The first is based on the use of quantum dots, the IB arising from the confined energy levels of the electrons in the dots. The second approach is based on the creation of intermediate bands by the doping of an appropriate impurity into a bulk semiconductor. 3d transition metals are considered as an appropriate impurity for intermediate band creation. In the intermediate band solar cell with metal doping an intermediate narrow metallic band is placed in the traditional forbidden bandgap which extends the absorption spectrum [5, 6]. 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 in 3C-SiC may give rise to a new type of high-efficiency photovoltaic materials based on silicon component. Numerous ab initio quantum calculations have been performed in the quest to realize IB materials [5, 7]. Wahnón and Tablero [5] published Ab initio electronic structure calculations for metallic intermediate band formation in photovoltaic materials in 2002. In 2006 Palacios et al. [8] introduced Energetics of formation of TiGa3As4 and TiGa3P4 intermediate band materials. The same authors [6] reported Transition metal-substituted indium thiospinels as novel intermediate-band materials: Prediction and understanding of their electronic properties in 2008. In 2012 Antonio Luque, Antonio Martí and Colin Stanley [3] reported a comprehensive article in nature photonics with the topic of understanding intermediate-band solar cells. Conventional materials that have been used in single junction solar cells fabrication have some problems. Low band gap single junction solar cells produce high current density but their voltage is low. This is in contrast for high band gap materials. Shockley and Queisser [9] showed that the maximum theoretical efficiency of a single junction solar cell is limit to 40.7 % for full concentration. However the maximum conversion limitation for these solar cells is low. In the intermediate band solar cells a high band gap material has been used that yield a high open circuit voltage. The IB is designed to be partially filled to permit the absorption of low-energy photons to pass the electrons from the VB to the partially filled IB and from the IB to the CB to support both high voltage and high photocurrent by absorbing sub-band gap photons. In order to best understand of the device performances the band diagram of intermediate band solar cells is depicted in Fig. 1. The limiting efficiency of an IBSC for full concentration and at room temperature is 63.2 %. Optimized absorption energy is placed in 0.7 eV up to valance band or below the conduction band with the main band gap of 1.9 eV, [1] significantly overcoming the Shockley-Queisser limit [9]. 3C-SiC is a wide-band gap semiconductor that is a compound of silicon and carbon. It is an intrinsic semiconductor with a band gap of about 2.3 eV at 25 °C [10, 11]. 3C-SiC is a promising material for photovoltaic application [12, 13], especially, intermediate band solar cell because of its band energy that is near to optimum value of IBSCs [14]. Also it has high mobility due to its lower density of interface states compared to other polytypes [15]. Here we describe a comprehensive study on electronic and optical properties of Cr doped 3C-SiC. They are studied with density functional theory to survey a new candidate of intermediate band (IB) material. The result shows that

A Proposal for Intermediate Band Solar Cells …

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Fig. 1 Band diagram of the intermediate band solar cells

the sub-band formed in the 3C-SiC and some bands are completely isolated from the CB and the VB. The absorption coefficient calculation for proposed materials were performed and it is greatly improved by the induction of the IB compared to the 3C-SiC host semiconductor.

2 Simulation Methodology For the past 30 years density functional theory has been the dominant method for the quantum mechanical simulation of periodic systems [16]. In recent years it has also been adopted by quantum chemists. In our proposed material, for determining the electronic properties of Cr doped 3C-SiC, the electronic band structures along with the density of states (DOS) are calculated. Calculations are based on DFT using one of the three non-local gradient-corrected exchange-correlation functionals (generalized gradient approximation-Perdew-Burke-Ernzerhof parametrization, GGA-PBE). Despite its approximations, this theory is one of the few that allows the study of large systems such as crystalline solids and allows us to obtain realistic results for many electron systems. However, LDA and GGA approximations have the problem of underestimating the band gaps. Nevertheless, some methods to avoid this inconvenience have been proposed [17, 18]. Plane-wave DFT method has been developed, along with a plane-wave quantum mechanics. These methods are very powerful but they require thousands of plane waves to correctly compute the bands. In our case, the KS equations are solved using the CASTEP code [19–21]. CASTEP is a commercial (and academic) software package which uses density functional theory with a plane wave basis set to calculate the electronic properties of crystalline solids, surfaces, molecules, liquids and amorphous materials from first principles. We analyzed the electronic properties of a material derived from 3C-SiC hosed semiconductor where Cr atoms are substituted by some of 3C-SiC atoms that were shown in Fig. 2. 3C-SiC is semiconductor with the

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Fig. 2 Simulated supercell structure of Cr doped 3C-SiC

lattice parameters of a = 4.3596 Å, and a = 90° and the symmetry group of T2dF43 m. Theoretical calculations using the detailed balance model [22–24] show that the conversion efficiency of this photovoltaic device can overcome 60 and 70 % for one and two forbidden band, respectively. So in the end of this work we used detailed balance model to achieve maximum efficiency of silicon carbide based intermediate band solar cell as a function of sub-band energies.

3 Simulation Results and Discussion The electronic properties are obtained by using the above mentioned methodology for evaluation of Cr doped 3C-SiC to show intermediate band formation. The IB principally arise as a consequence of the interaction between the crystalline potential and spin interaction with transitions metal d orbitals. The main goal of this work is to show an IB formation within the forbidden band energy of 3C-SiC. Towards this end, we calculated band structure, density of states, and optical absorption coefficient of Cr doped and un-doped 3C-SiC.

3.1 Band Structure of Proposed Material Band structure calculation is an important topic in theoretical solid state physics. In this section we obtained the band structure of proposed materials. At first, we obtained the band structure of 3C-SiC with and without transition metal doping. The calculated energy-band structures for 3C-SiC without and with Cr are given in Fig. 3a, b, respectively. The obtained curves show it is possible to form intermediate band in 3C-SiC. Cr has an obvious effect on the band structure of 3C-SiC and


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Fig. 3 a The band structures of intrinsic 3C-SiC and b the band structures of Cr doped 3C-SiC

formed IB present sub-gap absorption for low energies. This absorption is responsible for the efficiency increase in solar cells based on these materials.

3.2 Density of State Calculation In this section we calculated the DOS of 3C-SiC with and without Cr substitution. Figure 4a, b shows DOSs for 3C-SiC without and with Cr, respectively. Where the creation of intermediate density of states by mentioned ions are clear. In the Cr doped system 3d electron orbital mainly form IB and this appears in DOS. In

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Fig. 4 a The density of state of intrinsic 3C-SiC and b the density of state of Cr doped 3C-SiC

Cr-doped 3C-SiC, the IB mainly originates from Cr 3d orbitals and the poor contribution of p orbitals. The conduction band is mainly formed from p electrons.

3.3 Optical Absorption Coefficient We present with first-principles calculations, optical properties of new Cr-substituted in 3C-SiC compounds presenting a narrow half-filled intermediate band isolated from the VB and the CB of the host semiconductor. Figure 5 depict the optical absorption coefficient for proposed materials. The computed optical absorption of Cr doped compound compared to the corresponding undoped semiconductor predicts a significant absorption below the band gap of the parent semiconductor and an enhancement of the optical absorption in the whole energy range of the visible region. Moreover, introducing these systems can be developed more efficient novel optoelectronic devices.

4 Efficiency Potential of Proposed Materials The energetic position of the intermediate band will strongly influence the power conversion efficiency of the solar cell. Calculations of the efficiency versus energetic position of intermediate band for 3C-SiC with band gap of 2.2 eV are shown in Fig. 6 as a dashed line. It has maximum value of 59 % at 0.8 eV up to valance band or 1.4 eV below the conduction band for 1000 sun concentration under AM1.5 spectrum irradiance.


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Fig. 5 The calculated optical absorption coefficient for undoped 3C-SiC and Cr doped 3C-SiC

Fig. 6 Efficiency versus intermediate band energy level, efficiencies for 3C-SiC is along the dashed line indicated in these figures

5 Conclusions The new intermediate band materials based on Cr doped 3C-SiC for high efficiency solar cell application were introduced in this article. Evaluation of band structure, DOS, and optical properties were performed for proposed materials. Development of new intermediate band materials based on 3C-SiC doped with the Cr opens new promising horizons in high efficiency solar cells. The simulation also shows 3C-SiC is an interesting candidate for intermediate band solar with efficiency near 60 %.

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