Performance Improvement of CIGS Solar Cells with Composition Graded Active Layer
Ali Abdolahzadeh Ziabari1
1Department of Physics, Lahijan Branch, Islamic Azad University, Lahijan, Iran, Postcode: 1616, E-mail:
Abstract- The objective of this study is to investigate a new way to create high efficiency Copper Indium Gallium Selenide (CIGS) solar cells. To accomplish that, composition grading of absorber layer was carried out using SILVACO’s technology aided computer design (TCAD) ATLAS program. Results showed a meaningful improvement of outputs including open circuit voltage (Voc), short circuit current (Isc), fill factor (FF) and conversion efficiency (ƞ).
Keywords: solar cell, CIGS, grading, TCAD, simulation
1. Introduction
Solar cells are semiconductor devices that rely on the absorption of light, particularly photons, from the solar cell spectrum to generate electron-hole pairs. The separation and subsequent collection of these electron-hole pairs is the method by which solar cells convert solar energy into useful electricity. CIGS is an alloy of copper indium diselenide (CInSe2) and copper gallium diselenide (CGaSe2) which result in a material with characteristics interpolated between both components. It has been shown that the trap density in CIGS solar cells is a function of Ga concentration [1]. It implies that higher Ga content may not cause of higher efficiency. Hence, an optimization problem is formed; how we can manage x value in CuInxGa1-xSe2 where x=Ga/(In+Ga) to obtain the best performance? In this paper we pursue the method that was applied in reference [2] where the absorber layer was deposited in a 3-step process.
2. Method Description
The initial configuration includes Mo layer as the back contact, a CIGS absorber layer, a CdS buffer layer and a ZnO window layer. The simulation was performed using the values that are stored in ATLAS library. The simulated cell was illuminated with AM1.5 spectrum.
Besides, the temperature was regarded 300K. To rise the accuracy of the modelling, finer meshing was used at junctions. Other parameters like electrons and holes lifetime and mobility, surface recombination velocities and doping levels were then defined.
Fig. 1 depicts the modelled configuration of the traditional CIGS solar cell schematically. The doping concentration of p type region and n type region is taken 1× 1016 cm-3 and 1× 1018 cm-3 respectively, and the doping concentration of ZnO is 5×1017cm-3. The finite element mesh grids are shown in Fig. 2.
Figure 1. The modeled conventional CIGS solar cell.
Figure 2. Mesh specification of solar cell structure.
3. Results and Discussion a. Verification of the modelling
Modelling was initiated with a basic cell (Fig. 1) to verify the appropriate implementation into ATLAS and the capability to produce outputs. The calculated solar cell parameters including the open-circuit voltage (Voc), fill factor (FF), short-circuit current (Isc) and efficiency (η) from simulation are shown in Fig. 3. The common value of x is 0.3 for the fabricated CuInxGa1-xSe2 compound. This value was taken in the first round simulation. Results are coincident with the reported values [3,4]. This confirms the validity and accuracy of the simulation operation.
Figure 3. IV curve along with output parameters for a traditional CIGS solar cell.
b. Constructing a Cell with Ultra-Thin Absorber Layer
Using expensive elements including In and Se in CIGS compound increases the production price. One way to overcome this challenge is to decrease the thickness of CIGS layer.
However, this will lead to shortening of the photon traveling path that in turn results in decrease of photocurrent. Ultra-thin CIGS absorber layers have been studied and efficiencies have been simulated to be in the 10–11% range but have not been widely produced [5]. Fig. 4 shows the IV curve for the ultra-thin CIGS solar cell with an absorber layer of 750 nm. As it was expected, all output parameters decreased with reduction of active layer.
Figure 4. IV curve along with output parameters for an ultra-thin CIGS solar cell.
c. Graded CIGS Layer
Another advancement in the design of Cu(InGa)Se2 cells is the inclusion of graded Ga concentration within the absorber layer in order to increase the overall efficiency of the solar cell. The first method is known as back grading, where the concentration of Ga is increased moving towards the back contact. The second method is known as front grading, where the Ga concentration is decreased moving towards the back contact. The third is essentially a blend of the first two concepts, in which higher Ga concentration at both the top and bottom contacts is achieved with a minimum Ga concentration in the middle of the absorber layer. The bandgap of CuInxGa1-xSe2 varies from 1.01 eV to 1.67 eV as shown in Fig. 5 when the x composition is changed from 0 to 1. The band gap of CIGS is related to the x composition according to the following equation [6]:
Eg (x) = 1.01 + 0.42x +0.24x2 (1)
After verification of the basic cell, the first change was from an ultra-thin absorber layer (750nm) with constant Ga concentration to one with a graded concentration. The first attempts only had a single gradient in the absorber layer, which was varied for a variety of slopes, all with a midpoint Ga mole fraction of 0.3. Two different cases of decreasing and increasing Ga mole fraction going from top to bottom were tested and the maximum efficiency of 15.76% was recorded for the cell with the smallest forward graded absorber layer. This is linked to the increased internal electric field stemming from the concentration gradient in the absorber. In the second attempt the absorber layer was divided into two equal sections that allow for the specification of two Ga concentration gradients; a decreasing gradient from top to bottom in the top absorber layer creates a front field that sweeps photo- generated carriers towards the p-n heterojunction and an increasing gradient from top to bottom in the bottom absorber layer creates a back field that sweeps photo-generated carriers towards the back contact. The combination of these two fields should increase the overall efficiency of the cell by sweeping carriers towards the contacts before they can recombine.
Figure 5. Bandgap versus x composition for CIGS.
The largest efficiency of 16.12% was observed for the cell with the smallest negative slope. This slope corresponds to the top absorber layer having concentration decreasing from a Ga mole fraction of 0.31 to 0.3 going from the top of the layer to the bottom. For the bottom layer, the Ga mole fraction was increased from 0.3 to 0.31 going from the top to bottom. Much like the single-absorber layer simulations, the higher magnitude negative slopes resulted in increases in the open circuit voltage. The short circuit currents decreased significantly for these higher slopes, resulting in an overall decrease in efficiency. Despite promising results came out of the mentioned single and dual-graded absorber layer, the best output was achieved by a trapezoidal profile of grading accomplished on a three-stage processed CIGS layer with a constant content of Ga in the middle layer (250 nm) and a decreasing Ga concentration from the top layer (250 nm) plus an increasing Ga concentration profile from the mid layer to the bottom layer (250 nm). The resulting structure of this file is illustrated in Fig. 6. The results of these simulations led to a cell with the highest efficiency at 16.80%, as illustrated in Fig. 7. All other output parameters improved after performing the mentioned trapezoidal grading procedure (shown inset of Fig. 7). Compare to the common thick CIGS solar cells, the designed modelled ultra-thin solar cell shows a quite high performance.
Figure 6. The three-stage processed CIGS layer for dual-grading of the ultra-thin cell.
4. Conclusion
In summary, a series of simulations was performed on CIGS solar cells to survey the impact of decreasing the expensive absorber layer. Results showed a significant degradation of output parameters with downsizing the absorber layer. The bandgap grading of the active layer showed promising results and caused output values quite analogous to the thick solar cell.
Figure 7. IV curve along with output parameters for an ultra-thin trapezoidal-graded CIGS solar cell.
REFERENCES
[1] G. Hanna, A. Jasenk, U. Rau, and H. Schock, “Influence of Ga-content on the bulk defect densities of Cu(In,Ga)Se2” Thin Solid Films, 387 (1) (2001)71-73.
[2] T. Dullweber, O. Lundberg, J. Malmström, M. Bodegård, L. Stolt, U. Rau, H. Schock, and J. H.
Werner, “Back surface band gap gradings in Cu (In, Ga) Se2 solar cells”, Thin Solid Films
,
387(1) (2001)11–13.
[3] S. K. Lee, H.-J. Jeong, Y.-C. Kim, and J.-H. Jang, “Improvement in CIGS solar cell efficiency using a micro-prism array integrated with sub-wavelength structures”, Sol.
Energy Mate. Sol. Cells 186 (2018) 254–258.
[4] J. Li, B. Deng, H. Zhu, F. Guo, X. You, K. Shen, M. Wan, and Y. Mai, “Rear interface modification for efficient Cu(In,Ga)Se2 solar cells processed with metallic precursors and low-cost Se vapour”, Sol. Energy Mater. Sol. Cells 186 (2018) 243–253.
[5] M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, “Solar cell efficiency tables (Version 45),”
Progress in Photovoltaics Research Applications,
23(1) (2015) 1–9.[6]S. Sharbati and R. Sites James, “Impact of the Band Offset forn-Zn (O, S)/p-Cu (In, GA) Se2 Solar Cells”, IEEE Journal of photovoltaics, 4 (2014) 2.
