Abstract

In thin-film solar cells based on CIGS (Copper Indium Gallium Selenide), bulk defects are disruptions in the regular, periodic arrangement of atoms within the crystalline structure. This research utilizes numerical simulations and the SCAPS-1D software to investigate how volume defects within the CIGS absorber impact the performance of the solar cell. The study reveals three operational zones for the cell, depending on the concentration of defects in the CIGS absorber. An examination of the influence of absorber thickness, bandgap, and doping on CIGS solar cell performance in these operational zones is conducted. The results indicate that optimal performance is achieved when the density of bulk defects in the absorber is below 5.10¹³ cm^-3 (Zone I). This optimization involves using an absorber with a thickness of approximately 2.5 μm, a bandgap ranging from 1.3 eV to 1.4 eV, and an acceptor density of N_A = 10¹⁶ cm^-3.

1. Introduction

In CIGS-based thin-film solar cells, crystal imperfections manifest themselves in a multitude of chemical or physical forms commonly referred to as defects. These defects can affect the electrical and optical performance of solar cells. The presence of defects within the absorber volume significantly impacts how charge carriers move within the solar cell. ¹ and their density depends on deposition conditions ². Defects in the crystal lattice are important material properties and are essential for their applications in solar cells. These defects directly influence the generation, separation and recombination of electron-hole pairs in the absorber ³. Defects are disturbances in the periodic and regular arrangement of atoms in the crystalline medium. Generally speaking, a defect state is said to be shallow if its energy level is close to the minimum of the conduction band or the maximum of the valence band. Otherwise, the defect state is said to be deep. Shallow levels doped the material, and deep levels are free-carrier traps, capturing charge carriers brought in by doping ⁴. It is therefore necessary to know and control the defects and impurities that create all these energy levels ⁵. Can increasing the concentration of defects in the CIGS absorber have a significant impact on solar cell performance? The aim of this article is to study the concentration of defects on solar cell performance as a function of electrical parameters. To do this, we studied the concentration of defects on solar cell performance. We then studied the impact of absorber thickness, bandgap and doping on CIGS solar cell performance as a function of defect concentration.

2. Materials and Methods

Numerical simulation is used to study the influence of electrical parameters on solar cell performance without carrying out experiments. It is used to characterize solar PV devices. In this work, we will use the SCAPS-1D software ⁶. to perform our numerical simulations. SCAPS-1D uses the finite-difference method with well-defined boundary conditions to solve the basic equations: the Poisson equation, the electron continuity equation and the hole continuity equation. The choice of this software is motivated by the fact that there is good agreement between the numerical simulation performed with SCAPS-1D and experimentation ^{7, 8}. CIGS-based thin-film solar cells are heterojunction cells where the p-type CIGS is the absorber of the cell. The CIGS absorber is the most important layer of the device and occupies most of the solar cell's surface ^{9, 5}. On this absorber, an n-type CdS buffer layer provides the P-N junction. A layer of high-resistivity OTC transparent conductive oxide is deposited on the buffer layer. The whole assembly is deposited on a substrate, which acts as the mechanical support. Generally speaking, soda-lime glass is the substrate most commonly used in high-efficiency CIGS cells, due to its properties compatible with the CIGS absorber ^{10, 11}. Ni/Al and Mo ohmic contacts are added to collect photo-generated electrons and holes. The structure of such a solar cell is shown in figure 1.

In solar cells, defects in the absorber volume affect the electrical and optical performance of solar cells. According to the theoretical work of Su.Huai Wei et al ¹², there are four classes of defects. Point defects which are represented by vacancies (e.g. V_Cu, V_Se), interstitial defects (e.g. Cu_i, In_i) and anti-site defects (e.g. In_Cu). Linear defects corresponding to a dislocation in the crystalline structure. Two-dimensional defects corresponding to grain boundaries and three-dimensional defects ¹³. These defects are shown in figure 2.

The information pertaining to the various layers and interfaces employed in the numerical simulation can be found in Table 1. These properties were obtained from theoretical and experimental results ^{14, 15, 16}. The solar cell temperature is maintained at 300K. It is illuminated under standard conditions by an AM 1.5 G spectrum that takes direct and diffuse radiation into account ¹⁷.

To validate our results, we compared the J-V characteristic curves of our numerical simulation with those obtained experimentally by Pettersson. Figure 2. shows that there is good agreement between the two results, which leads us to the results and discussion section.

3. Result and Discussion

CIGS absorber characteristics are strongly influenced by impurities or defects. In some cases, these are added intentionally to increase electrical conductivity and control charge carrier lifetime, but more often than not, these defects are closely linked to the deposition mechanisms ¹⁷. These impurities or imperfections in the lattice act as recombination sites. We utilized the SCAPS-1D simulation software to explore how the operational characteristics of the CIGS solar cell are affected by variations in defect density within the absorber layer, focusing on electrical parameters.

Except for the bulk defect density in the absorber, the properties of the different layers are kept constant in order to obtain qualitative information about the effect of defect density on the electrical parameters of the solar cell. Varying the defect density from 10¹² cm^-3 to 10¹⁸ cm^-3 clearly affects the J-V characteristic, as can be seen in Figure 4. This current-voltage characteristic highlights three operating zones of the cell as a function of bulk defect concentration. To clearly illustrate these three operating zones, we have extracted the electrical parameters from the current-voltage (J-V) characteristic, as shown in Figure 5.

In the first zone (zone I), where the bulk defect density is less than 5.10¹³ cm^-3, all the electrical parameters of the solar cell are almost constant. In this zone, bulk defects have less impact on solar cell operation and very good performance. This performance can be explained by a reduction in Shockley-Read-Hall (SRH) recombinations via defects. In the second zone (zone II), where the defect density is between 5.10¹³ cm^-3 and 5.10¹⁵ cm^-3, all the cell's electrical parameters decrease slightly with increasing defect concentration. In third zone, where defect density is greater than 5.10¹⁵ cm^-3, all electrical parameters are highly dependent on defects and decrease abruptly with their concentration. Indeed, a very large amount of defects in the absorber decreases the lifetime of charge carriers as shown by equation (1). This affects their diffusion lengths, as shown in equation (2), and favours recombination via defects.

(1)

(2)

In these equations, is lifetime, is Capture cross-section,is thermal velocity,diffusion constant and is diffusion lenght of the electrons. is bulk defect density in absorber

The reduction in electrical performance with increasing defect concentration can also be explained by a reduction in photon absorption in the solar cell. To verify this hypothesis, we studied the influence of defect concentration on quantum efficiency (Figure 6). A very good quantum yield is observed in the first zone, reflecting almost complete absorption of incident photons. This may explain the good electrical performance previously observed in this zone. In the second zone (II), the decrease in cell performance can be explained by a reduction in the absorption of incident photons, favoring recombination phenomena in the CIGS absorber volume. The same applies to the third zone (III). This drastic decrease can be explained by a very low absorption of incident photons.

The study of defects in the CIGS absorber has a significant impact on solar cell performance, and has highlighted three zones of solar cell operation depending on defect concentration. These three operating zones of the solar cell as a function of defect concentration are linked to the quality of the CIGS absorber. For further work in this article, we will study the impact of other absorber parameters such as thickness, bandgap and doping on the CIGS solar cell in these three zones.

The studies carried out in the previous section have shown that the improvement in solar cell performance depends on the density of volume defects in the absorber. In this section, we will investigate the impact of absorber defects on electrical parameters by varying the absorber thickness from 3.0 μm to 0.5 μm as shown in Figure 7.

Generally speaking, whatever the density of the bulk defects (Zone I, II and III), all the electrical parameters are affected by variation in the thickness of the CIGS absorber. With a thickness absorber (W_CIGS > 2,5 µm), the electrical parameters are almost insensitive to thickness variation, but are highly dependent on the defect zone. Nevertheless, very good performance is achieved. For absorbers with thicknesses between 0.5 μm and 2.5 μm, electrical parameters decrease slightly. With an ultra-thin absorber (W_CIGS ≤ 0,5 µm), a drastic decrease in all electrical parameters of the solar cell is observed. There are several possible reasons for this observation: (i) incomplete absorption of incident photons due to the thinner absorber thickness ¹⁸, (ii) increased recombination due to the closer proximity between the front surface of the absorber and the rear contact ¹⁸, (iii) absorption of short-wavelength photons near the rear contact, which is an area of high recombination ¹⁹. Electrical performance as a function of thickness is closely linked to absorber quality. In first zone, electrical parameters are significantly better than in the other two zones (Zone II and III). This may be due to a reduction in recombinations due to the low density of volume defects in the absorber, leading to long lifetimes for the photo-generated charge carriers in the absorber, thus improving cell performance. Optimum performance is achieved with a high-quality solar cell (Zone I) and an absorber of the order of 2.5 μm.

Absorber bandgap is an important factor in improving the performance of CIGS solar cells. Its importance is no longer in question because of its impact on absorption and, consequently, on charge carrier generation. The material bandgap energy equation CuIn_1-xGa_xSe₂ is given by equation (3).

(3)

The introduction of Ga into the CIS absorber to form the CIGS, results in a widening of the band gap from 1.02 eV to 1.67 eV ²⁰. These values are the CIS and CGS gaps corresponding to Gallium levels x = 0 and x =1 respectively ²¹. Work by Hanna et al. has shown that the density of defects in the absorber depends not only on the band gap, but also on the absorber deposition technique ²². These studies showed that the bandgap of the CIGS absorber varies almost linearly with the gallium content. The increase in bandgap mainly affects the conduction band minimum of the absorber ²³. It also affects various CIGS material parameters such as absorption coefficient, electron affinity and bulk defect density ^{22, 23, 24}. To date, the best performances have been obtained with a gallium content of between 0.25 and 0.35 ^{2, 22}. Indeed, the density of defects in the absorber is shown to be a factor that limits solar cell performance at high gallium levels ²². Given the correlation between defect density and absorber bandgap, we investigated the influence of absorber volume defects and bandgap on electrical parameters, as shown in figure 8. It appears that all electrical parameters depend on absorber quality and bandgap. The poorer the quality of the absorber, the poorer the electrical performance. The open-circuit voltage () increases linearly with the bandgap, whatever the quality of the absorber as shown figure 8.a.

Indeed, increasing the Ga concentration in the absorber reduces the recombination rate at the CIGS/Mo back interface. The short-circuit current density J_SC evolves linearly when <1,4 eV. Above this value, a decrease in J_SC is observed in Figure 6.b. This decrease in J_SC can be explained by the decrease in the optical absorption coefficient as the bandgap increases ²¹, in accordance with equation (4).

(4)

is planck constant and is wave frequency.

It can also be attributed to the increase in Ga, which reduces the generation rate at the p-n junction and at the CIGS volume. Form factor FF and conversion efficiency η (Figure 6.c,d) show almost the same evolution. They grow with the bandgap and the best performance is obtained for 1,3 eV < Eg < 1,4 eV corresponding to 0,45 < x <0,6. Outside this zone, conversion efficiency and FF form factor decrease.

The doping of CIGS-based thin-film solar cells is linked to the presence of intrinsic defects in the material structure, some of which are electrically charged. An excess of selenium leads to N-type doping, while a deficiency of selenium leads to P-type doping. In our work, the absorber is P-doped and the buffer layer is N-doped. Acceptor density is one of the parameters that can significantly affect solar cell operation ²⁵. Figure 7 shows the influence of hole density on electrical parameters as a function of absorber quality.

The electrical characteristics of the solar cell remain unaffected by doping when the acceptor densities are below 10¹³ cm^-3. They depend only on the quality of the absorber. When N_A > 10¹³ cm^-3, V_OC increases regardless of absorber quality. For excessive doping (N_A > 10¹⁶ cm^-3), a decrease in V_OC is observed. In contrast to V_OC, short-circuit current density J_SC decreases for acceptor concentration exceeding 10¹⁴ cm^-3. When acceptor density is below 10¹⁴ cm^-3, J_SC is almost insensitive to doping (Figure 8.b). This implies that for moderate doping, absorber quality plays a decisive role on cell performance. Conversion efficiency η and form factor FF show virtually the same trend (Figure 8.c.d). As doping levels rise, these characteristics experience an uptick, however, their reliance on the absorber's quality remains crucial. Both parameters decrease for very high doping (N_A > 10¹⁶ cm^-3), as shown Figure 5.c.d especially for (Zone II) and (Zone I) quality absorbers. In view of the above, it can be said that doping control is very important for optimizing solar cell performance. This phenomenon can be elucidated by the observation that a high concentration of acceptors has the potential to diminish both charge carrier mobility and lifetimes ^{6, 7, 8, 9, 10, 11, 12, 13, 14 14, 15, 16, 17, 18} thus reducing performance. Very good performance is achieved for a very good quality absorber (Zone I) and acceptor density N_A > 10¹⁶ cm^-3.

4. Conclusion

In this research paper, we employed numerical simulations and utilized the SCAPS-1D software to investigate how variations in bulk density within the CIGS absorber impact the performance of CIGS solar cells. The study highlighted three zones of influence of bulk defects on electrical and optical parameters, depending on their density. It can be seen from this study that optimum performance is obtained when the bulk defects density in the absorber is less than 5.10¹³ cm^-3, corresponding to an absorber of very good quality. These optimum performances are obtained with an absorber of the order of 2.5 μm in thickness and a bandgap of between 1.3 and 1.4 eV corresponding to a gallium (x) content of between 0.45 and 0.6 and an acceptor density (N_A = 10¹⁶ cm^-3). The findings presented in this paper offer promising pathways for enhancing the efficiency of CIGS-based solar cells by improving the quality of the absorber material

References