- Comment on “Analysis of the High Conversion Efficiencies
*β*-FeSi_{2}and BaSi_{2}n-i-p Thin Film Solar Cells”, Hyunki Kim and Joondong Kim

Journal of Nanomaterials

Letter to the Editor (2 pages), Article ID 896926, Volume 2015 (2015)

Published 15 September 2015

Journal of Nanomaterials

Volume 2014, Article ID 238291, 5 pages

http://dx.doi.org/10.1155/2014/238291

## Analysis of the High Conversion Efficiencies *β*-FeSi_{2} and BaSi_{2} n-i-p Thin Film Solar Cells

Department of Electronic Engineering, I-Shou University, Kaohsiung 840, Taiwan

Received 20 June 2014; Revised 20 September 2014; Accepted 22 September 2014; Published 29 December 2014

Academic Editor: Chien-Jung Huang

Copyright © 2014 Jung-Sheng Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Both *β*-FeSi_{2} and BaSi_{2} are silicides and have large absorption coefficients; thus they are very promising Si-based new materials for solar cell applications. In this paper, the dc - characteristics of n-Si/i-*β*FeSi_{2}/p-Si and n-Si/i-BaSi_{2}/p-Si thin film solar cells are investigated by solving the charge transport equations with optical generations. The diffusion current densities of free electron and hole are calculated first. Then the drift current density in the depletion regions is obtained. The total current density is the sum of diffusion and drift current densities. The conversion efficiencies are obtained from the calculated - curves. The optimum conversion efficiency of n-Si/i-*β*FeSi_{2}/p-Si thin film solar cell is 27.8% and that of n-Si/i-BaSi_{2}/p-Si thin film solar cell is 30.4%, both are larger than that of Si n-i-p solar cell ( is 20.6%). These results are consistent with their absorption spectrum. The calculated conversion efficiency of Si n-i-p solar cell is consistent with the reported researches. Therefore, these calculation results are valid in this work.

#### 1. Introduction

A silicide is a compound that has silicon with more electropositive elements. The metal silicides have been widely investigated for several years because of their potential applications in electronics [1]. Semiconducting beta-phase iron disilicide (*β*-FeSi_{2}) and orthorhombic barium silicide (BaSi_{2}) are two transition metal silicides and they are very promising Si-based new materials for solar cell applications. It is desirable for solar cell materials to have a large absorption coefficient to yield high conversion efficiencies. *β*-FeSi_{2} has a large optical absorption coefficient (>10^{5} cm^{−1} at 1.5 eV) and a direct band gap of ~0.87 eV [2–4]. Lin et al. [5] reported a conversion efficiency of 3.7% for p- (or n-) type *β*-FeSi_{2}/n- (or p-) type Si solar cell. Gao et al. [6] simulated a p-Si/i-*β*FeSi_{2}/n-Si solar cell structure by using the AMPS-1D software. The conversion efficiency is 24.7%. The orthorhombic barium silicide (BaSi_{2}) also has a large absorption coefficient of over 10^{5} cm^{−1} at 1.5 eV [7]. Recent reports on the photoresponse properties of BaSi_{2} have shown that BaSi_{2} is a new silicide material suitable for solar cell applications [8, 9].

Therefore, in this paper the conversion efficiencies of n-Si/i-*β*FeSi_{2}/p-Si and n-Si/i-BaSi_{2}/p-Si thin film solar cells are investigated by using self-developed analytical methods. For semiconductor solar cells, the n-i-p structure usually has superior - characteristics than the n-p structure. Since the built-in electric field exists in the intrinsic layer, the generated electron-hole pairs in the intrinsic layer are drifted by the electric field and produce larger short-circuit current and open-circuit voltage. In addition, the intrinsic silicide layer does not need doping and its manufacturing is compatible with the well-established Si solar cells. The distributions of minority carrier concentrations in the neutral n-Si and p-Si regions are calculated first. Then the total current density is the sum of diffusion current densities of free electron and hole and the drift current density in the depletion regions. The conversion efficiencies are calculated from the - curves of the solar cells with illumination of light. The calculated optimum conversion efficiency of *β*-FeSi_{2} n-i-p solar cell is 27.8% and that of BaSi_{2} p-i-n solar cell is 30.4% and that of Si n-i-p solar cell is 20.6%. Therefore, the conversion efficiencies of *β*-FeSi_{2} and BaSi_{2} n-i-p solar cells are significantly larger than that of the conventional Si n-i-p solar cells. The reported conversion efficiency of Si n-i-p solar cell is consistent with this calculation work [10]. Therefore, the calculation results are valid in this work.

#### 2. Analysis Methods

The n-i-p structure of solar cells under investigation is shown in Figure 1. The calculations are under global AM1.5 solar spectrum (Wm^{−2}) at 25°C. At the surface of the solar cell (i.e., at ), the generation rate of electron-hole pairs, (s^{−1}m^{−3}), is [11]
where = the wavelength of the incident light (m), = the intrinsic quantum efficiency to account for the average number (100% maximum) of electron-hole pairs generated per incident photon, = the optical reflectivity between the air and the semiconductor, = the incident optical power intensity (Wm^{−2}), = the energy of the incident photon (Joul), and = the absorption spectrum (m^{−1}).