Simulation of CZTS(Se)4 Tandem Solar Cells By AFORS-HET Software

In this work, solar cells were simulated single, mechanically and monolithically stacked based on the CZTS (Se) 4 absorption layer, both layers of tandem solar cell were simulated using by AFORS-HET software (one-dimensional ). Electrical properties of the monolithical tandem cell, it's determined after the current matching to the top and bottom sub-cell. The short circuit current density Jsc is about 23.9 mA. cm -2 when the top cell thickness is 384 nm in conjunction with the 1000 nm bottom cell thickness. The maximum efficiency obtained is approximately 23.1% with an open circuit voltage Voc ~ 1146.9 mv. The efficiency of the mechanical tandem cell with series connection was 23.76 % at open circuit voltage Voc ~ 1166.9 mv and the efficiency about 26.62 % at open circuit voltage Voc ~ 475.2 mv for mechanical tandem cell with parallel connection when the both cells had the same thickness(1000nm).


Introduction:
Many specialists in recent years have been interested in tandem solar cells, because they have big efficiency and reduced production costs (very high efficiency can indirectly reduce the cost of solar ). The high efficiency of these photovoltaic PV cells semiconductor materials that absorb diverse parts of the spectrum of solar order to attain the technological benefit required for these cells, it is necessary to develop inexpensive materials that can be stacked in a certain way [1,2]. The tandem cell contains several different models, including a homogeneous stacked cell with two terminal (2T) and a mechanically stacked cell with two or four terminal (2T, 4T) as shown in Fig 1. The 2T design involves connecting the cells ( Top and Bottom sub cells) stacked in tandem electrically together through a tunnel junction or joint connection , resulting in a serial electrical connection, which means that only two external electrical contacts are required [3]. In mechanical design, cells are stacked (Top and Bottom) on separate substrates, working independently, where the two cells are connected separately to an outer circle in a seriese or parallel and must be the window layer semitransparent to allow light to pass [4] .

Simulation Approach By AFORS-HET:
AFORS-HET (Automat FOR Simulation of Heterostructures) is a one dimensional numerical computer program for simulation multi-layer homo-or Heterojuction solar cells as well as some usual solar cell characterization methods. like current-voltage (I-V), internal quantum efficiency (IQE), capacitance voltage, spectral response, etc, The software solves Poisson's equation and the continuity equations, based on Maxwell Boltzmann statistics, in the whole structure and simulate electrical properties of solar cell, by using different numeral

Theoretical Analysis:
J-V properties in solar cells are the standard method for calculating cell output and evaluating its performance under illumination. Using the diode equation, which is given as follows: [7,8] J SC is the short circuit current density due to the falling light, the current source in the circuit equivalent to a luminous solar cell, J 0 is the saturation current (or dark current) due to the recombination processes occurring inside the cell.
The saturation current density is given, taking into account the speed of surface recombination: where I and I 0 are the current flux and initial flux at a penetration depth x. According of this theory, creates electron-hole in active layer of a solar cell, wich that should be separated before recombination. The absorption coefficent α(λ) is calculated from the spectral absorption of each semiconductor layer within the stack [12,13]: Finally, the Fill factor (FF) and efficiency (η) of the solar cell is calculated by the following equations [7,8]: Where the η is described as the ratio between the maximum power generated by the cell and the power incident on it.

Mechanically Stacked Tandem Solar Cells:
The first concept of Tandem solar cells was already mentioned in 1955 and the concept was first piloted in 1978 [14]. The mechanically stacked solar cells are designed easily by integrating the bandgap subunits into any contact arrangement, and neglecting the splicing factor. The manufacturing of the cells was mechanically limited and limited due to: a-the cost of merging cells on multiple substrates; b-The assembly of parts of the devices off-site, which increases the difficulty of processing and cost, and lead to loss of performance due to joint delivery. c-Excessive weight also limits the practicality of these devices for space applications because of fixturing hardware used to stack the sub cells together and use the multiple substrates [15]. The CZTS(Se) 4 Direct bandgap compound semiconductor materials provide significant advantages, due better optical property and high electron mobility .It's having high absorption co-efficient thus with a very thin thickness (1 to 2μm), these material group can absorb appreciable amount of solar light due to their high absorption co-efficient (>10 -4 cm -1 ). Theoretically, Band gap of CZTSSe alloy varies in between 1.5 and 1.0 eV depending upon S/Se ratios. Suitable and tunable optical properties make kesterite as ideal solar absorber layers [16,17].
The structure of the studied cell, shown in Fig 2, contains ZnO as a window layer n-type, is transparent and conductive and which is responsible transmit the light to the absorber layer and extract photogenerated electrons, CdS as n-type a wide band gap thin buffer layer, allows more light to reach the junction and increases the EQE at the short wavelength region [18,19],  The properties of the materials are shown in Table 1 ,For CdS and ZnO, as specified in AFORS-HET program. As for the CZTS and CZTSe absorption classes taken from the indicated sources.

Density of states in VB
100 [23] 100 [21] 100 [23] 100 [23] µn (cm 2 .Vs -1 ) Electron mobility 25  In this research, the mechanical tandem cell is used, because a device has two advantages; First, the J-V characteristics of the individual bottom-and top cell as well as the tandem cell can be measured. Second, since the subcells are electrically and optically separated, it is easy to connect the two cells electrically, seriese or in parallel using external wires. This means allowing low-energy photons to generate photo current, increasing the shortcircuit current of the cell. Thus, efficiency CZTSe (Eff =16.91%) is higher than efficiency CZTS (Eff= 11.91%).

5-2 Parallel Connect:
When the two sub cells CZTS and CZTSe are electrically connected in parallel, the J-V characteristic are based on the following equations: V Tandem =V Top =V Bottom (14) J Tandem =J Top +J Bottom (15) The complete J-V property can be built into a parallel tandem as in the Fig .4a. refer to the equations (12,13), the fill factor (FF) and efficiency (η). For the parallel connection equat to FF = 79.92 % and η =26.62 % , respectively.
The voltage value of the tandem cell (Voc=475.2 mv ) is equal to cell's voltage that gives less voltage, CZTSe the Bottom cell because it have the lowest energy gap. Therefore the Jsc of the tandem cell current will be equal 70.09 mA/cm 2 .

Monolithically Stacked Tandem Solar Cells:
The fundamental concept of a monolithical tandem solar cell, is stacking different light absorber layers, connected electrically and optically in series, each utilizing a part of the solar spectrum and allowing the passage through of the other part, which the highest-energy photons are captured by the material with the largest bandgap, this material will be transparent for low energy light which can be passed on to the second absorber layer with the lower bandgap [26,27]. Because photons must cross the cell to gain access the appropriate layer to be absorbed, transparent conductors must be used to collect the electrons that are created in each layer, but producing a tandem cell is not an easy task, Because of the challenges facing the manufacture of 2T active devices: which are the current match between the top and bottom cell, minimizing the loss of recombination between cell layer and greatly reducing physical thinness [28]. The monolithic tandem solar cells are designed using the structure shown in   results obtained from simulating the cells studied for this study were included in Table 2.

Conclusion:
The different designs of the tandem cell were simulated by using the AFORS-HET software. The suggested tandem cell structures (mechanically and monolithically stacked) showed the ability to achieve improved efficiency of the solar cells based on the absorption (CZTS(Se) 4