Improvement of efficiency in CZTSSe solar cell by using back surface field

A kesterite CZTS based solar cell is studied with back surface field. In order to achieve this, a p+ layer of SnS is added to the standard cell configuration. The resulting p+-p-n configuration of SnS/CZTS/CdS is studied using Solar Cell Capacitance Simulator (SCAPS). Low Voc is responsible for lower efficiencies of kesterite solar cells. The proposed p+-p-n structure increases the Voc and thus the efficiency. Simulation of CZTSSe and CZTS with back surface p+ layer has shown considerably higher efficiency than the highest reported value for kesterite cell of CZTSSe from 12.3% to 15.7%. A comparative summary is presented with literature reported performance and our simulated performance for kesterite cell.


Introduction
Solar cells based on Cu 2 ZnSnS 4-x Se x (CZTSSe) can have Shockley Quisser limit of 32% as per their bandgap, and is a potential low cost photovoltaic (PV) material. The reported highest efficiency of 12.6% for kesterite CZTSSe based solar cells [1] is much less than SQ limit and chalcopyrite CuIn x Ga 1x S 2 (CIGS) counterpart (21.6% for) [2]. This inferiority in performance of kesterite suggests complex defect physics resulting in Lower V oc and overall performance [3,4]. Secondary phase and point defect in absorber layer, non optimized interfaces [3] and possible flaws in cell structure are issues which require careful attention for kesterite solar cells. For optimization of cell structure simulation tools can be utilized where cell structures/configuration, interface and their performance can be optimized. A simulation, studying the effect of various parameters on photovoltaic performance is an easy step to predict optimized conditions without actually fabricating a solar cell. AMPS, PC1D, SILVACO, SCAPS [5,6] are some extensively used solar cell simulation software. SCAPS-1D simulation software is used in this study, which solve Poisson equation, relating the charge to the electrostatic potential (φ) and Continuity equation for electrons (n i ) and holes (p i ) for simulation of devices. In one dimension, the cell is divided into N intervals and solved for (φ, n i , p i ) at each interval. The SCAPS can simulate CuInSe 2 , CdTe, c-Si a-Si [7], CZTSSe [6,8] and perovskite [9]. The present work focuses on the role of back surface field. The back surface field has been reported for enhanced PV performance in a variety of solar cells [10,11]. A high-low junction p + -p at back cause conducive band banding, an electric field in direction of p-n junction is setup decreasing electrons current at back contact. H. Cui et al [12] reported the increase in efficiency by adding a back surface layer in CZTSSe. In this regard we propose an SnS back surface layer to modify the overall cell structure. Cell configuration of p + (SnS)/p(CZTSSe)/CdS/ZnO is simulated with SCAPS-1D software to study and optimize overall performance. A comparative study of p-n junction and p + -p-n junction is presented in following section. 2. Solar cell description with back surface field A p-n junction based photovoltaic device has built in voltage at the junction is given by

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When a p + layer (layer with higher hole density) is added at back, a p + -p high low junction is formed giving rise to a built-in voltage Where Na, Na + , is the hole density in the p, p + , N d is electron density in n layer respectively. For a p + -p junction, by virtue of acceptor concentration variations, band bending occurs at junction and an electric field in direction of p-n junction is set up. The generated voltage at the p + -p junction produces a barrier for electron current at rear contact. Such high-low junction can be employed in solar cell structure at appropriate locations to induce directional flow of charge carriers in solar cell for selective contacts. This arrangement can provide higher V oc and less recombination at junction and bulk. A higher V oc is observed in case of an applied p + -p in solar cell as discussed in the next section.

Simulation Details
A p-type SnS is used as a back surface layer. The configuration of standard cell is Mo/CZTSe/CdS/ZnO and of that for the modified cell is Mo/SnS/CZTSe/CdS/ZnO as shown in Fig. 1.
A comparative study of the two cell configurations is performed using SCAPS-1D [7]. SnS is used since it has similar material composition as CZTS, suitable p-type nature and easily synthesized by various methods [13]. The material parameters used are listed in the Table I [5,8]. The device is illuminated from window layer (ZnO) side with AM1.5 spectrum using relevant SCAPS option at temperature 300 K. For the series resistance (R s ) and shunt resistance (R sh ), we have used typical values of 0.72 ohm-cm 2 and 620 ohm-cm 2 , respectively which are conservative estimates considering the experimentally reported values of 0.72 ohm-cm 2 and 620 ohm-cm 2 , respectively [1].The Schematic of the two configuration is shown in the Fig.1.

Absorber layer: CZTSSe
The quality of interface between the CZTSSe absorber layer and the CdS buffer layer is crucial in the separation of generated electron-hole pairs. The recombination in bulk leading to losses is primarily caused by radiative processes, non-radiative (Shockley-Read-Hall mechanism) processes, and the interface recombination which increases saturation current and thus decreases V oc . In case of polycrystaline absorber, inhomogeneities in band gap variations and electrostatic fluctuations increases the saturation current. Here, CZTSSe passivated with SnS is observed to have a better performance as shown in Fig. 2. The gain in performance is seen along with higher V oc , fill factor (FF) and J sc in cell with SnS. Typical thicknesses for SnS and CdS were chosen as 100 nm and 80 nm, respectively. The p + -p and the p-n junctions at either side of absorber layer creates an asymmetry for charge flow. The efficiency with the thickness of the absorber layer is shown in Fig.2. The V oc for the SnS/CZTSSe/CdS/ZnO configuration is observed to be higher than the unpassivated cell due to the generation of the built-in voltage at both the ends of the absorber layer. Short circuit current, I sc of the passivated cell is higher than the unpassivated counterpart due to the presence of a directed field for charge flow in absorber layer.

Band-diagram for CZTS solar cell.
Simulated band diagram shown in Fig. 3 convey band bending at junction/interfaces and energy levels in bulk. The Fermi level splitting at p-n junction gives rise to a light induced voltage. The addition of SnS back surface layer leads to the formation of a high-low junction by virtue of difference in acceptor concentration in SnS and CZTSSe. The higher energy level of SnS than CZTSe in band diagram in Fig. 3 is a favorable condition as an electric field is set up at both ends of the absorber layer. Generated charge carrier in absorber will feel a drift field thereby increasing its probability of collection at junction. A high low junction at the rear also acts as the near ohmic contact.

Quantum efficiency.
Quantum efficiency shows the charge pair generated by absorption of different wavelength. Different wavelengths are absorbed at different thickness. Higher wavelengths are absorbed in deeper in bulk of absorber and low wavelength light is absorbed at surface. The band gap of CZTSSe (1.3eV) corresponds to 954nm wavelength. The QE is observed to improve at higher wavelength i.e. in depth at rear side in the modified cell configuration. This is due to SnS back surface field which efficiently created at field thus efficient collection of charge carriers.

Conclusions
The back surface field in CZTSSe can improve V oc in cell by creating a built in voltage at rear, It also improve the contact of metal semiconductor making a near ohmic metal-p + -p. In our work we simulated back surface using an SnS layer and showed that a structure of p + -p-n is useful for solar cell performance improvement. A higher V oc is obtained in CZTSSe cell along with improved FF and QE. We have increased the efficiency of cell from 12.3 to 15.7% by incorporation of back surface field. The comparative performance of all the simulated and experimentally reported cell of CZTSSe cell is shown in Fig. 5 and tabulated in Table 2.