Towards High-Efficiency CZTS Solar Cell Through Buffer Layer Optimization
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Materials for Renewable and Sustainable Energy (2019) 8:6 https://doi.org/10.1007/s40243-019-0144-1 ORIGINAL PAPER Towards high‑efciency CZTS solar cell through bufer layer optimization Farjana Akter Jhuma1 · Marshia Zaman Shaily1 · Mohammad Junaebur Rashid1 Received: 18 April 2018 / Accepted: 8 February 2019 / Published online: 13 February 2019 © The Author(s) 2019 Abstract Cu2ZnSnS4 (CZTS)-based solar cells show a promising performance in the feld of sunlight-based energy production system. To increase the performance of CZTS-based solar cell, bufer layer optimization is still an obstacle. In this work, numerical simulations were performed on structures based on CZTS absorber layer, ZnO window layer, and transparent conducting layer n-ITO with diferent bufer layers using SCAPS-1D software to fnd a suitable bufer layer. Cadmium sulfde (CdS), zinc sulfde (ZnS) and their alloy cadmium zinc sulfde (Cd 0.4Zn0.6S) were used as potential bufer layers to investigate the efect of bufer thickness, absorber thickness and temperature on open-circuit voltage (Voc), short-circuit current (Jsc), fll factor (FF) and efciency (η) of the solar cell. The optimum efciencies using these three bufer layers are around 11.20%. Among these three bufers, Cd0.4Zn0.6S is more preferable as CdS sufers from toxicity problem and ZnS shows drastic change in performance parameters. The simulation results can give important guideline for the fabrication of high-efciency CZTS solar cell. Keywords CZTS · SCAPS-1D · CdS · ZnS · Cd0.4Zn0.6S Introduction market [4]. On the other hand, CIGS and CdTe ofer high efciencies (around 21% and 21%, respectively) for which The green, clean, renewable and sustainable energy source, they attracted the researchers for the last few years [5]. The sun has a very high potential to meet up the electric- main problem with these two materials is the toxicity of the ity demand of the world and the sunlight can directly be constituent materials cadmium and selenium and also the converted to electricity through solar cell using the princi- less availability of tellurium and indium on earth [6]. To ple known as photovoltaic (PV) efect. To obtain low cost meet up the problems, an alternative material, CZTS has and high-efciency solar cell, researchers are working on drawn the attraction with good advantages over CIGS and many diferent solar materials such as Si, CdTe, Cu(In,Ga) CdTe. During last few years, a good amount of researches Se2 (CIGS), Cu 2ZnSnS4 (CZTS), CZTSSe and organic had been done to improve the solar cell efciency using resources [1, 2]. CZTS absorber layer [7]. Wei Wang et al. presented a record The most widely used Si-based solar cell exhibits high cell efciency of around 12.6% using a hydrazine pure solu- conversion efciency (up to 24.5% at the University of New tion approach for Cu2ZnSnS4-based solar cell [8]. South Wales) [3]. However, it sufers from low throughput The material CZTS is a quaternary semiconductor with and high cost, therefore, could not afect the world’s energy two diferent crystal structures stannite and kesterite. Kester- ite structure shows more stability than stannite structure [9]. * Farjana Akter Jhuma One important advantage of CZTS is that it consists of earth- [email protected] abundant materials which are non-toxic. Thus, CZTS ofers Marshia Zaman Shaily lower cost in comparison with other thin-flm solar cell that [email protected] makes it more promising in PV technology. CZTS shows Mohammad Junaebur Rashid excellent photovoltaic properties such as a direct bandgap [email protected] with bandgap energy of 1.45–1.6 eV and absorption coef- fcient over 1 × 104 cm−1 [10]. CZTS can be fabricated using 1 Department of Electrical and Electronic Engineering, University of Dhaka, Dhaka 1000, Bangladesh Vol.:(0123456789)1 3 6 Page 2 of 7 Materials for Renewable and Sustainable Energy (2019) 8:6 diferent processes such as sputtering, evaporation, spray Methodology and device structure pyrolysis, electrodeposition, sol–gel technique, etc. [11]. Though CZTS has become a good choice as solar cell SCAPS-1D, a one-dimensional solar cell simulation soft- material, bufer layer optimization is still an issue for its pro- ware developed at the Department of Electronics and Infor- gress. The bufer layer provides band alignment between the mation Systems (EIS), University of Gent, Belgium, was CZTS and window layer and also reduces defects and inter- used for numerically analyzing the solar cell. Up to seven facial strain due to the window layer [12]. Cadmium sulfde diferent layers can be added in the cell defnition panel of (CdS) can be used as a prominent bufer as it improves inter- the software which makes it more suitable for solar cell face with the absorber CZTS and has higher transmission in simulation. Physical properties such as bandgap, electron the blue wavelength region. It has a bandgap of 2.42 eV and afnity, dielectric permittivity, etc., for diferent layers can absorbs photons with wavelength lower than 590 nm cover- be modifed in the layer properties panel which helps to ing 24% of the solar spectrum [13]. However, CdS contains achieve the desired structure. The necessary working point large amount of Cd which is toxic and also produces a large specifcation can be indicated in the action panel. The soft- amount of wastage during the deposition processes. To avoid ware supports grading of all physical parameters and also this problem, an environment friendly material zinc sulfde specifcation of properties of front and back contact. A large (ZnS) can also be used as an alternative bufer layer. ZnS has number of AC and DC electrical measurements including a higher bandgap of 3.5 eV compared to CdS which results short-circuit current density (Jsc), fll factor (FF), open-cir- in less absorption of low-wavelength photons. It also pro- cuit voltage (Voc), conversion efciency (η), quantum ef- duces better interface with CZTS creating a potential barrier ciency (QE), spectral response, generation and recombina- to separate the electron–hole pair [14]. tion profle can be calculated as well as displayed using the Cd1−xZnxS is another bufer layer which is promising as software SCAPS-1D, compared to other simulation software its bandgap can be varied between 2.42 eV (CdS) and 3.5 eV [18]. (ZnS). The bandgap of Cd 1−xZnxS for diferent Zn composi- Figure 1 shows the solar cell structure used in this study. tion (x) can be calculated using the following equation which The structure started with soda lime glass substrate. A thin is a modifed Vegard’s law [15]: MoS2 layer of thickness of 100 nm to avoid high series resistance was used over the absorber layer, as this layer can Cd Zn S 2.566 0.041 1.086 2 eV . Eg 1−x x = + x + x ( ) (1) be experimentally formed by the reaction of Mo back con- tact with sulfur contained in the absorber precursor (ZnS or With increasing Zn concentration, bandgap increases SnS) in the case of CZTS solar cell [19]. The next layer was letting lower wavelength photon through the device. Thus, CZTS absorber layer in which most of the incident photons instead of CdS, Cd 1−xZnxS can be used to obtain a more were absorbed to produce electron–hole pairs. CdS or ZnS transparent window in the short-wavelength region, already or Cd0.4Zn0.6S bufer layer was used after that to provide the demonstrated by Oladeji et al. [16]. On the other hand, band alignment between CZTS and following window layer. the grain size decreases as Zn concentration increases in Afterwards, less costly and available zinc oxide (ZnO) of Cd1−xZnxS [17]. It turns out that for a Zn content of x = 0.6 in Cd1−xZnxS, highest current density can be achieved which leads to a signifcant enhancement in the solar cell efciency [15]. Therefore, in this simulation study, 60% Zn is used in Sunlight Cd1−xZnxS and the bandgap of 2.98 eV is calculated using Eq. (1). The main objective of this work is to ofer a CZTS solar n-ITO (60 nm) cell with high efciency using a suitable bufer layer. For this reason, three CZTS solar cell device models using three dif- ZnO (80 nm) ferent bufer layers such as CdS, ZnS, and Cd0.4Zn0.6S were CdS or ZnS or Cd0.4Zn0.6S (50-150 nm) numerically analyzed using SCAPS-1D to obtain the solar cell performance parameters. Diferent solar cell parameters CZTS (100-2000 nm) Voc, Jsc, fll factor (FF), and efciency (η) were observed and performance analysis was conducted between the three MoS2 (100 nm) structures CdS/CZTS, ZnS/CZTS and Cd0.4Zn0.6S/CZTS. Substrate (glass) Fig. 1 Structure of CZTS thin-flm solar cell 1 3 Materials for Renewable and Sustainable Energy (2019) 8:6 Page 3 of 7 6 thickness of 80 nm was used for the window layer over the Results and discussion bufer layer which enhances light scattering to enable the efcient use of sun light to maximize the number of incident Efect of bufer layer’s thickness photons to the bufer and absorber layers [20]. Finally, a transparent conducting flm n-type indium tin oxide (n-ITO) To investigate the efect of bufer layer’s thickness, simula- of thickness of 60 nm was used to provide a high mobility tions were done on the structure shown in Fig. 1 with dif- leading to an increase in visible absorption to obtain a lower ferent bufer layers. As already mentioned in Sect. 2, the sheet resistance [21]. bufer layer thickness was varied from 50 to 150 nm with The illumination of light was through the side of n-ITO a fxed CZTS layer of 2000 nm for the frst steps of simu- layer with “Air mass 1.5 global” spectrum with 1000 W/ lation.