The Effects of ZnO and Al2O3 Layers on Semiconductor-Insulator-Metal (MIS) Solar Cell Performance

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1 Applied mathematics in Engineering, Management and Technology 2 (6) 2014: The Effects of ZnO and Al2O3 Layers on Semiconductor-Insulator-Metal (MIS) Solar Cell Performance Arezo Abdolhay 1,*, Sahar Royaniyan 2, Zahra Taheri Nezhad 3 1 Department of electrical engineering, Naragh Branch, Islamic Azad University, Naragh, Iran. ( * Corresponding author) 2 Department of electrical engineering, Fras branch, Islamic Azad University, Fasa, Iran 3 Department of electrical engineering, Ashtiyan branch, Islamic Azad University, Ashtiyan, Iran Abstract We first analyzed the solar cell (Metal-SiO 2 -Si(N)) under illumination AM 1.5; maximum conversion efficiency of these cells is 17.26%. Then, we suggested various structures of MIS solar cells on a single-crystal silicon substrate to enhance efficiency in the form (Metal-Al 2 O 3 -Si(N)) with 18.5% conversion efficiency and a solar cell Metal-SiO 2 -Al 2 O 3 -SiO 2 -Si with 19.14% conversion efficiency and a solar cells (Metal-SiO 2 -ZnO-SiO 2 -Si(N)) with 19.88% conversion efficiency and 0.566V open circuit voltage under illumination AM 1.5. Given that short circuit current of solar cells is calculated based on absorption of photons, aligned layer of zinc oxide and aluminum oxide between silicon dioxide plays an important role in increasing light absorption which reduces dark current and improves the open-circuit voltage and conversion efficiency of solar cells. The results obtained by numerical simulation were described and discussed using Silvaco software. 1.Introduction In Schottky barrier cells, open voltage drop is associated with increased short-circuit current due to reduction of the cell dark current and low height of Schottky barrier, which reduces efficiency. To establish a balance between increase and decrease in the open circuit voltage and short circuit current as well as efficiency of Schottky barrier, MIS solar cell have been introduced. In structure of an MIS solar cell, growth of a thin oxide layer leads to increased penetration depth of the carriers and, according to (1) and (2) and (3) [1], leads to increased optic flow and decreased dark current and reduced reverse saturation current density and thus increased efficiency and open circuit voltage of MIS solar cells compared to cells of Schottky barrier by tunneling phenomena of the majority carriers from semiconductor to metal [2]. (1) (2) (3) Using a semiconductor with a high absorption coefficient and anti-reflective coating for maximum absorption of incident photons, finger-shaped perforated metal fittings cause transmission of minority carriers at the front and in neutral semiconductor zones and accumulation of optical carriers into the thinner barrier metal layer which all reduce I S (reverse saturation current) and reduce carrier remix in the surface field and increase J sc and V oc and efficiency of the MIS cells [3]. Insulating layer in MIS cell acts as a chemical barrier to prevent improper surface reactions between the metal and semiconductor leading to the formation of silicide [4]. At first, we review types of MIS solar cell from 1978 to 2012 and summarize it in Table

2 Table 1: different MIS solar cells Cell η )%( V oc (mv) J sc ) Am/mc 2 ( Reference MIS (with a stacking structure) [5] MIS on polycrystalline silicon [6] Ti-SiO 2 -Si (p) [7] Cr-SiO 2 -Si (p) [7] MIS/IL (Si(p),1Ωcm)) Metal:Al [8] SnO 2 -SiO 2 -Si(n) [9] Mg-SiO 2 -Si(p) [9] Au- SiO 2 - GaAs(n) [6] MS'S (Al-Ga 2 Se 3 - Si(n)) [10] In structure of the solar cell Meta-SiO 2 -Si, insulating layer on n-type single crystal silicon substrate so formed that a thin metal layer transparent to light with higher work function than silicon (~5.4 ev) settles on it to accumulate the n-type semiconductor surface as the substrate due to higher work function of the upper layers ( m > s ). Accumulation phenomenon in the substrate acts as a source of majority carriers (electrons) leading to tunneling into the top layers by majority carriers [11]. Then, we could obtain maximum efficiency in an optimal condition (N A =10 16 mc -3 and δ SiO2 =10 A ), as Figure 1 shows, by changing layer thickness of SiO 2 from 8 to 17 Å and changing the amount of impurities in semiconductor silicon from cm -3 to cm -3. Generally speaking, increase in thickness of oxide layer reduces the electron transfer across the insulating layer, increases voltage in the insulating layer and increases effective barrier height [12]. Figure 1: Effect of changes in insulation thickness and the amount of impurities in semiconductors on efficiency Photovoltaic properties of the solar cell Metal-SiO 2 -Si under AM1.5 illumination using simulation by Silvaco are presented in Table 2. Table 2: Photovoltaic properties of the solar cell Metal-SiO2-Si (%) FF(%) V OC (mv) J SC (ma/cm 2 ) By further studies on the absorption and better coupling of light illuminated into a cell, and by reducing the dark current and subsequent increase in short-circuit current, we could improve conversion efficiency of the solar cell (Metal SiO 2 Si(n)). Therefore, we suggest different structures of MIS solar cells made on single-crystal silicon substrate by various layers of insulation to improve the conversion efficiency. 2. New Structures of MIS Solar Cells to Improve Conversion Efficiency 2.1. The Cell Metal-Al 2 O 3 -Si(n) For the reasons described below, the current structure of (Metal-Al 2 O 3 -Si(n)) increased significantly compared to (Metal-SiO 2 -Si) cell, which contributes to increase the conversion efficiency up to 18.5%. According to (4) the relationship between absorption coefficient of refractive index (n r ) and wavelength of the absorbed light is inverse. Given that n r (SiO 2 )=2.326 and n r (Al 2 O 3 )=1.77. Substituting SiO 2 layer by Al 2 O 3, the light absorption coefficient increases. By increasing α(λ), the optical current generated in the bulk area (I dl (λ)) [1] increases 349

3 according to (5) and (6). According to (7), increase in J SC(TOTAL) increases conversion efficiency of the suggested solar cell. where, I pw (λ) is a part of holes penetrated in the depletion layer boundary, that is, the same current generated in the transition area. In addition, λ min is the minimum wavelength of the solar spectrum. λg represents the wavelength consistent with gap band energy of silicon. ζ = 2n r 2.x.w.ε 0 is semiconductor conductivity and ε 0 =8.854*10 represents vacuum permittivity coefficient; c is the velocity of light in vacuum, w is the frequency of light and the refractive index of semiconductors The Solar Cell Metal-SiO 2 -Al 2 O 3 -SiO 2 -Si In this structure, silicon dioxide and aluminum oxide layers were placed between the two insulating layers to increase the light absorption and increase the conversion efficiency of (Metal-SiO 2 -Si(n)) solar cell. Table 3 shows photovoltaic properties of the solar cell Metal-SiO 2 -Al 2 O 3 -SiO 2 -Si under AM1.5 illumination. Table 3: input and output parameters of the cell Metal- SiO 2 - Al 2 O 3 - SiO 2 -Si (%) FF(%) V OC (mv) J SC (ma/cm 2 ) (4) (5) (6) (7) 2.3. The Cell Metal-SiO 2 -ZnO-SiO 2 -Si Due to light absorption in structure of a MIS solar cell and absorption spectra of zinc oxide, shown in Figure 2 and 3, respectively, light absorption parts of the solar cell Metal-SiO 2 -ZnO-SiO 2 -Si can be expressed as follows: 1. The light with energy q BN hv (barrier height) can be absorbed in the Schottky barrier junction; 2. Zinc oxide layer can absorb energies (~3.3ev) and wavelengths ( nm); ( hv Eg ) 3. The light with a small wavelength enters a semiconductor and mainly absorbs energies (~1.1ev) in the empty silicon semiconductor area [14, 13]. 4. Figure 2: (a) Schottky barrier solar cells (MS) b) MIS solar cells [14, 13] Figure 3: Absorption spectra of ZnO thin films at different temperatures [1] 350

4 Thus, increase in the light absorption by zinc oxide layer adds a component of current, corresponding to (8), to (5); as (9) shows, therefore, conversion efficiency can be improved by 19.88% by an increase in short-circuit current. (8) where, inc is the optical spectrum reached the cells and QE(λ) is the quantum coefficient of the cell [15] Changes in Energy Band Films of Solar Cell Metal-SiO 2 -ZnO-SiO 2 -Si For a thin layer of zinc oxide between two insulation layers of SiO 2 with larger band gap, there are discontinuities in the energy band. In this case, zinc oxide layer is so thin that causes quantum modes in the conduction and capacity band. This causes changes in the energy emitted by photons by formation of discrete states in zinc oxide layer. That is, one electron in one discrete state of conduction band (E 1 ) goes to one discrete state of capacity band in zinc-oxide-like quantum well (E h ) instead of normal states of the conduction band, and returns a photon with the energy E h +E g +E 1 greater than band gap of zinc oxide; while holes occupy similar discrete states (E h ) in the quantum well [16]. In these cells, amount of light absorption is determined by the depth and length of the quantum well. The deeper well, the more light current generated. Thus, the voltage again reduces. Besides, the wider well, the less light absorbed. The energy of absorbable photons is between the effective band gap energy of the well and the cell level. Suppose a particle trapped in the potential well so that value of V(X) is infinitely large in boundary X= 0, L and zero in the rest. For any integer quantum (n) in the potential well and potential-free zone, in accordance with (10), the quantized energy (E n ) is [16]: (11) where, m and L denote electron effective mass and the width of quantum well, respectively. Therefore, ΔE n is equal to the difference between electron affinity of zinc oxide and silicon after connecting ZnO and Si. According to (10) and (11), the width of quantum well is calculated as thickness of zinc oxide layer: 0.445nm. As thickness of zinc oxide layer changes from 0.3 to 0.6 nm during the simulation, maximum efficiency was obtained in 0.5nm for zinc oxide layer. (9) (10) Figure 4: Energy band diagram of zinc oxide and silicon before connecting to each other [16] Figure 5: Energy band film of cell Metal-SiO2-ZnO-SiO2-Si Table 4 presents photovoltaic properties of the solar cell Metal-SiO 2 -ZnO-SiO 2 -Si under AM1.5 illumination. 351

5 Table 4: input and output parameters of the cell Metal- SiO 2 - ZnO - SiO 2 -Si (%) FF(%) V OC (mv) J SC (ma/cm 2 N ) A(si) W Si W 3SiO2 W 2ZnO (cm -3 ) (μm) (nm) (nm) W 1SiO2 (nm) 3. Conclusions Solar cell (Metal-SiO 2 -Si(n)) has the maximum efficiency of energy conversion in ~17.26%. By changing the structure of above solar cell to Metal-SiO 2 -Al 2 O 3 -SiO 2 -Si and placement of aluminum dioxide layer and zinc oxide between insulating layer of silicon dioxide in the form of Metal-SiO 2 -Al 2 O 3 -SiO 2 -Si and Metal-SiO 2 - ZnO-SiO 2 -Si(n) under AM 1.5 illumination, we could improve conversion efficiency of the suggested cells to 18.5%, 19.14% and 19.88% respectively. Placing layers of SiO 2 /ZnO/SiO 2 or SiO 2 /Al 2 O 3 /SiO 2 on single-crystal silicon substrate increases absorption of incident photons whereby increases the optical current and decreases slightly the open circuit voltage and thus improves conversion efficiency of the solar cell. In addition, the absorption spectra of the suggested cells range from 300 to 1800 nm. References 1. F.Bouzid et S.Ben Machich,"The Effect of solar spectral irradiance and temperature on the electrical characteristics of ZnO SiO2/Si(N) photovoltaic structure", Vol.13, No.2, p , Juin O.M. Nielsen, "Current mechanism of tunnel m.i.s, solar cells ". IEEE PROC, Vol. 127, No 6, December, Pt. I. 3. Davidl. Pulfrey," MIS Solar Cells: A Review", IEEE Transactions On Electron Devices, VOL. ED-25, NO. 11, November B. L. Sharma," Metal-Semiconductor Schottky Barrier Junctions andtheirapplications ", Plenum: New York, Tzu.Yueh Chang,Chung.Lung Chang, Hsin.Yu Lee, and Po.Tsung Lee, 2010".A Metal- insulator- semiconductor Solar Cell With High Open- Circuit Voltage Using a Stacking Structure. "IEEE Electronic Device letters,vol.13, NO.12, December. 6. David. Pulfrey, "MIS Solar Cells: A Review ". IEEE Transactions On Electron Devices, VOL. ED-25, NO. 11, November. 7. Bhattacharya.Pallab, 2002.Semiconductor Optoelectronic Devices. translated by Dr.shahram mohamadnejad, p LIU Xinming,JIA Quanxi and LIU Enke, "High Efficiency MIS/IL Silicon Solar Cells with Silicon Oxynitride as Ultra-thin Tunneling Films. "Solar Energy Materials,vol.17, p A.K. Ghosh, C. Fishman, and T. Feng, "SnO 2 /Si Solar Cells- Heterostructure or Schottky-barrier or MIS- Type Device. " J.Appl. Phys, Vol. 49, N.6, June. 10. M. Bhatnagar, P. K. Bhatnagar, " High level illumination effect on MS'S solar cell characteristics with a new material Ga 2 Se 3,as an intermediate layer ". Journal Of Materials Science, vol.33,pp Rajondra singh," phd Thesise on Theory Of Metal Insulator _Semiconductor (MIS) and Semiconductor- Insulator semiconductor (SIS) Solar cells ", submitted as partial fulfillmen of the requirement for the degree of doctor of philosophy, April D.Hocine, "Effect of interfacial oxide layer thickness and interface states on conversion efficiency of SnO 2 /SiO 2 /Si(N) Solar cells", Juin S.M.SZE, Semicinductor devices physics & technology. translated by G.H.Sodair Abedi. 14. Michael.Bertolli, "Solar Cell Materials".University of Tennessee. 15. Christopher E. Valdivia, Eric Desfonds, Denis Masson, Simon Fafard,Andrew Carlson, John Cook, Trevor J. Hall, Karin Hinzer,"Optimization of antireflection coating designfor multi-junction solar cells and concentrator systems ", RooeenTan, Gh, Samadi S., Electronic physics, IUST publication, Tehran. (2008). 352