Influence of Bulk Defect Density in CIGS on the Efficiency of Copper Indium Gallium Selenide Photocell

In CIGS-based thin films, bulk defects are believed to represent disturbances in a regular, periodic arrangement within the atoms or the crystalline media, which influence sheet resistivity. In this study, resistivity measurements were carried out on CIGS thin films using the Van Der Pauw technique. Then, a comparison between two Van Der Pauw four-point measurement configurations (aligned and square) was made, which showed that the square configuration was the most appropriate configuration that can be recommended to measure the thin film sheet resistance of CIGS films. Finally, a numerical simulation using SCAPS-1D software was used to study the influence of bulk defect density in CIGS films as an absorber layer in a model photocell. Using the simulated data, three operating zones for the model photocell were identified depending on its bulk defect density concentration. The influence of bulk defects on thickness, band gap, and doping were then analyzed. It was revealed that when the bulk defect density was less than 5 × 10¹³ cm⁻³ 5.10¹³ cm⁻³ for an absorber of thickness in the order of over 3 μm, a band gap between 1.3 eV–1.4 eV and acceptor density of NA = 10¹⁶ cm^-310¹⁶ cm⁻³ were the optimal operating conditions for the model photocell. It was concluded that the CIGS layer used as an absorber can be improved if its bulk defect density is tuned to optimal levels.

Introduction

Thin film technology can be used to fabricate copper indium gallium selenide (CIGS) photovoltaic cells. These are heterojunction photocells where the CIGS layer forms the absorber layer. In these cells, the CIGS is the p-type layer and occupies the largest surface area of the photocell. It is the cell’s most important layer [1], and an n-type buffer layQ1: In keywords as per the style we should follow 4 keywords. Please check.er usually covers it to provide a P-N junction interface [2]. Most of these heterojunction devices use a thin layer of cadmium sulphide (CdS) on top of the buffer layer with a transparent conductive oxide of higher resistivity, forming a third thin layer [3]. This complete assembly is then deposited on a recommended substrate to offer mechanical support. Most prototype CIGS photocells use the soda-lime glass substrate because of its compatibility with CIGS crystals, making them one of the most highly efficient CIGS-based photovoltaic devices. For testing purposes, two different ohmic contacts made out of either molybdenum or nickel-aluminum alloy (Ni/Al) [4] are used to collect any photo-generated electrons and holes upon illumination. Recent studies [5]–[7] have successfully investigated many optoelectrical parameters of CIGS photocells, but few have concentrated on the influence of bulk defect density on their performance.

A defect is considered to be shallow when its energy level is very close to the minima of a conduction band or very close to the maxima of a valence band [8]; otherwise, it is regarded as a deep defect. Deep defects are very common in many doped materials [8]–[10]. They are considered free carrier traps known to capture charge carriers introduced through doping [9]. The amount of bulk defects in an absorber layer may influence the optical and electrical capabilities of many thin film photocells. These bulk defects have been classified into four categories as follows: point defects believed to be represented by vacancies [11], [12], interstitial defects [13], anti-site defects [14], and linear defects that correspond to dislocations in crystalline materials. All these defects can be analyzed in two ways: two-dimensional defects (grain boundaries) or three-dimensional defects [5], [12]. Crystal imperfections indeed manifest themselves as bulk defects, and bulk defects in an active absorber can be regulated easily by regulating depositional conditions [15]. Bulk defects play a significant role. One way is by influencing charge carrier transport mechanisms in lattice crystalline structures by either influencing charge generation charge separation or electron-hole pair recombinations within the absorber [1]. That is why they act as periodic disturbances in crystalline materials, having regularly arranged atoms in their crystals. It is therefore necessary to evaluate the influence of bulk defect density to understand how to control or precisely create them [10] depending on their influence and application. The article highlights findings on the impact of bulk defect density on thickness, band gap, doping concentration, and performance of CIGS photovoltaic cells.

Theory

Thin Film Resistivity

Two configurations are recommended in measuring thin film resistivity. One of them is the four-contact point configuration that can correct the insufficiencies of its two-point configuration counterpart. In the four-contact point configuration, two points inject current while the other two points measure potential difference using the LCR meter [16]. By knowing both the injected currents and voltages at specific film thickness, resistivity or impedance can be computed using the Van der Pauw technique [3]. Its complex impedance, Zx, is then computed using:

(1) $$\begin{array}{r}{Z}_x=-\left(\frac{V_1}{V_2}\right)\end{array}$$

The aligned configuration uses a rectangular shape of length, a, and width, d, with four arbitrary points aligned at equal tip spacing, s on a surface arranged as IHcuVHpot and VLpotILcu where IHcu and ILcu are dedicated to current injection points while VHpot and VLpot are dedicated to voltage measurement points [17]. Resistivity is then computed using (2) as: where ρ is resistivity, V is the measured potential difference, I is the low currents injected into the film and C is the coefficient of the correction factor, which depends on the width and distance between the configuration tip points. In a square configuration, only 4 points are arbitrarily arranged on the edges and numbered from 1 to 4 to form a square. Current is injected and voltage measured so that the R12, 34 and R14, 23 resistances are used to compute resistivity from using (3) and (4), respectively:

(2) $$\begin{array}{r}\rho =\frac{V}{I}(\frac{d}{s};\frac{a}{d})e\times C\end{array}$$

(3) $$\begin{array}{r}{e}^{\left(\frac{-\pi e}{\rho }{R}_{12,34}\right)}+e^{\left(\frac{-\pi e}{\rho }{R}_{14,23}\right)}=1\end{array}$$

(4) $$\begin{array}{r}\rho =\frac{\pi e}{ln2}R=\frac{\pi Ve}{Iln2}\end{array}$$

Thin Film Optical Properties

The CIGS band gap forms an important parameter to consider when improving the performance of its photocell [18]. It determines how photon absorption and charge carrier generation take place [22]. When gallium (Ga) is introduced into the CIS absorber to form CIGS by gallium levels between x = 0 and x = 1, its band gap widens from 1.021 eV to about 1.672 eV [3], [9]. It clearly indicates that the introduced bulk defects influence the band gap. Influencing the band gap implies that an increase in concentration influences the band gap by adjusting its conduction band minima [11]. Under a controlled deposition process, the band gap of CIGS has been found to vary linearly with gallium concentration [7], [19] by (5) as: forming a CuIn_1-xGa_xSe₂ composite. Parameters such as absorption coefficient [3], electron affinity [7] and bulk defect density [15] are adjusted by some correction factor that can also depend on bulk defect density. The density of bulk defects is increasing, becoming a key factor to consider when analysing the limits by which CIGS photocells can perform even at higher gallium concentration levels that may be above the theoretical values.

(5) $$\begin{array}{r}{E}_g=[1.021+0.611x-bx(1-x)]eV\end{array}$$

Photocell Performance

Numerical simulations can be used to study the influence of bulk defect density on the efficiency of many photocells without carrying out actual experiments. One can accurately characterize a photocell using simulations only. The SCAPS-1D is a software that uses the Finite-Difference Method to solve many basic equations with well-defined boundary conditions applicable to many optical devices and these equations include:

(i) the Poisson equation [20] given by:

(6) $$\begin{array}{r}{\mathrm{\nabla }}^2\varphi =-\left(\frac{\rho }{{\epsilon }_0}\right)\end{array}$$

The equation is commonly used in a region where there are no currents so that parameters ρ and J both vanish, resulting in Laplace’s equation expressed as:

(7) $$\begin{array}{r}{\mathrm{\nabla }}^2\varphi =0\end{array}$$

(ii) the electron continuity equation [21] or the law of conservation of charge expressed as:

(8) $$\begin{array}{r}\frac{\mathrm{\partial }p}{\mathrm{\partial }t}=\left(\frac{\mathrm{\partial }{P}_n}{\mathrm{\partial }n}\right)+gn(-\frac{p}{{\tau }_p})\end{array}$$

(iii) and the hole continuity equation [22] is expressed as: where all the symbols have their usual scientific meanings. Both continuity equations are used to evaluate carrier concentration rates when recombination, generation, drift and diffusion processes occur simultaneously. Under normal conditions, the rate of change of charge carrier concentration is equal to the sum of drift, diffusion and charge generation less its recombination rates [23].

(9) $$\begin{array}{r}\frac{\mathrm{\partial }p}{\mathrm{\partial }t}=\left(\frac{\mathrm{\partial }{P}_p}{\mathrm{\partial }n}\right)+gn(-\frac{p}{{\tau }_p})\end{array}$$

Method

Materials

An argon gas source, molybdenum foil substrates, a selenium-rich gaseous source, hexanethiol and sodium fluoride solutions were among the reagents that were purchased from Sigma and Aldrich Co.

Film Deposition

A magnetron sputtering machine was used to sputter CIG on a molybdenum foil substrate with a target power density of 10 mW cm⁻² in the presence of an argon gas flowing at a pressure of 10 mTorr at about 27 °C. The thickness of the CIGS absorber film was varied by precisely controlling the repeated spin coating process and heating in a selenium-rich environment for 15 minutes at 590 °C. About 1.96 mg of sodium fluoride was mixed with 1.55 mL of hexanethiol precursor solution to form a sodium ink doping solution. This solution was then deposited on the molybdenum-coated substrate directly by spin coating. All other film processing and procedures were carried out following the work by Li et al. [1].

Characterization

Measurement of Resistivity

Four-point measurements were repeatedly carried out using an H.P. 4284A LCR meter with its tester in serial R.L. mode at short factor of correction to eliminate any residual impedance. For measurements in the aligned four-point configuration, a rectangular structure with a length of “a” and “d” wide at equal distance “s” were taken to determine I_Hcu V_Hpot V_Lpot I_Lcu where I_Hcu and I_Lcu were current injection at the tips while V_Hpot and V_Lpot were voltage measured at the dedicated tips. The four points were then arranged in the form of a square and current was injected and corresponding voltages were obtained with their R_12,34 and R_14,23 respective resistances computed. The spacing between tip points was each varied at fixed values of 1 mm to 5 mm and resistivity was determined as a function of frequency, thickness, and type of configuration. Five points were tested for frequency ranges from 5 × 10³ Hz to 4 × 10⁴ Hz. Two different films with thicknesses of 3.3 μm and 5.2 μm were chosen and tested both in aligned and square configurations.

Numerical Simulation

Numerical simulations were carried out using the SCAPS-1D software on the model photocell shown in Fig. 1.

Fig. 1. CIGS model photocell.

During simulation, most parameters of the different layers were kept constant to determine the qualitative and quantitative relationship between bulk defect density and its electrical parameters. bulk defect density was varied from 10¹² cm⁻³ to 10¹⁸ cm⁻³ to investigate the photocell’s quantum efficiency. Some extra external parameters were added to the SCAPS-1D software to help extract electrical and photocell parameters of interest from each layer. Interface and photocell parameters are summarized in Table I.

Table I. Selected Parameters Used in the SCAPS-1D Software to Simulate the CIGS Photocell Performance

Layers properties	Units	i-ZnO	CdS	CIGS
Photocell temperature	K	300	300	300
Standard illumination spectrum	G	1.5	1.5	1.5
Doping concentration	cm−3	1017 (D)	1017 (D)	1016 (A)
Approximate layer thickness	nm	350	60	2550
Working density of bulk defects	cm−3	1016	1016	1014
Theoretical band gap energy	eV	3.321	2.413	1.245
Effective electron affinity	eV	4.445	Variable	Variable
Effective electron mobility	cm2/Vs	100.0	70.0	100.0
Effective electron thermal velocity	cm/s	2.41 × 107	2.43 × 107	4.19 × 107
Electron cross-section capture area	cm2	10−15	10−15	Variable
External ambient conditions	°C	15–35	15–35	15–35
Relative dielectric permittivity		9.03	9.81	13.56
Effective BC density of state	cm−3	3.11 × 1018	3.11 × 1018	2.0 × 1018
Effective B.V. density of state	cm−3	1.8 × 1019	3.1 × 1018	1.5 × 1019
Photo constant	W/m2	103	103	103
Effective thermal velocity of holes	cm/s	1.3 × 107	1.6 × 107	1.4 × 107
Effective mobility of holes	cm2/Vs	31.3	19.8	12.5
Hole capture cross-section area	cm2	2.0 × 10−13	2.0 × 10−13	2.0 × 10−13

Result and Discussion

Influence of Bulk Defect Density on Resistivity

Influence on Frequency and Tip Spacing

Figs. 2–6 represents graphical representations of the variations in frequency against resistivity. Fig. 2, in particular, shows the variation of resistivity with frequency when a distance of 1 mm separates the tip points.

Fig. 2. Influence of frequency of 1 mm tip-spacing on resistivity.

Fig. 3. Influence of frequency of 2 mm tip-spacing on resistivity.

Fig. 4. Influence of frequency of 3 mm tip-spacing on resistivity.

Fig. 5. Influence of frequency of 4 mm tip-spacing on resistivity.

Fig. 6. Influence of frequency of 5 mm tip-spacing on resistivity.

As shown in Fig. 2, whenever there was a constant tip spacing in the aligned configuration, the resistivity was always higher in the 5.2 μm (thick film) as compared to the 3.3 μm (thinner film). A similar observation was seen in the square configuration. However, at the same frequency in both types of configuration, a notable difference in resistivity was noticed at different magnitudes for each CIGS film thickness. This difference was attributed to the influence of bulk defect density [24]. Fig. 3 shows the resistivity variation when tip spacing was increased to 2 mm.

Similarly, in the aligned configuration in Fig. 3, a thicker film (5.2 μm) exhibited higher resistivity as compared to a thinner film (3.3 μm). However, resistivity increased with frequency in both cases, suggesting that frequency contributed to resistivity. In the square configuration, the thicker film had higher resistivity as compared to a thinner film, which was frequency-independent. This observation suggested that the influence of bulk defect density on resistivity depends on the type of configuration [25]; there was higher restriction to charge flow in an aligned configuration more as opposed to in the square configuration. Fig. 4 shows the relation between resistivity and frequency when tip spacing was 3 mm.

Resistivity in the square configuration remained constant with increased frequency, though the 5.2 μm film exhbited a relatively higher resistivity than the 3.3 μm film. In the aligned configuration, resistivity increased as frequency increased. At 2 × 10³ Hz, the resistivity of both films exhibited almost similar values attributed to bulk defects at resonance. These bulk defects exhibited a perfect multi-oscillator mechanism [26] that transfers charge carriers from one defect to another, like conductors, irrespective of their thickness. Beyond the 2 × 10³ Hz, the 5.2 μm film increased its resistivity sharply, suggesting higher charge collisions between charges and defects behaving as charge traps or higher degree restriction centers. As a result, the resistivity for the 3 mm tip spacing was higher as compared to both the 2 mm and 1 mm spacings.

Fig. 5 shows a comparison in the variation in resistivity with frequency between the aligned configuration and square configuration when the tip spacing distance was increased to 4 mm. Resistivity was not influenced by either tip spacing or frequency as opposed to when the tip spacing was 1 mm, 2 mm and 3 mm in the square configuration while in the aligned configuration, the frequency had a notable influence in resistivity of the thinner film. Resistivity in both aligned and square configurations at 2 × 10³ Hz was notably very close to each other, but thereafter, the thicker film exhibited a greater increase than the thinner film. Fig. 6 shows how resistivity varied with frequency when the tip spacing increased to 5 mm.

At 5 mm tip spacing, the square configuration was not influenced by bulk defects. In the aligned configuration, a linear increase in both films was noticed at equal ratios, and the influence of bulk defect density was negligible. They could not influence resistivity irrespective, suggesting a limit below the 5 mm tip spacing where bulk defect density can influence resistivity [27]. It can be noted that it does not matter how thick or how far the tip spacing is located in the square configuration; defect density does not influence resistivity. In the aligned configuration, frequency and tip spacing influence resistivity if the system is exposed to frequencies below 1000 Hz due to skin effects and proximity effects on charge flow. The abnormal resistivity levels observed above 1000 Hz should be avoided and only frequencies below 1000 Hz should be considered when studying the influence of tip spacing in CIGS films. It was also observed that, for a tip spacing that lies below or less than 3 mm, resistivity remained constant irrespective of the type of configuration and thickness as long as the frequency was below 1000 Hz. Similarly, for tip spacing above 3 mm, resistivity inversion as a function of thickness only in the aligned configuration (such that resistivity values for the thickest films become lower whiles those of the thinner films become higher). This observation was not the same in the square configuration, where resistivity remained constant for all cases when tip spacing was less than 3 mm. The inversion can only be justified by considering the limitations of the aligned configuration, where injected current must be adjusted to control the measured voltage across the film to obtain the same sheet resistance. Therefore, resistivity is a function of the tip spacing, which can be considered as a restriction when using the aligned configuration. It is necessary to limit resistivity measurements to below 1000 Hz when tip spacing is less than 3 mm so that both between aligned and square configurations can agree in their measurements.

Influence on Tip Disposition

Figs. 7–9 show the variation between resistivity as function tip spacing for frequencies of 50 Hz, 500 Hz and 1000 Hz in both the aligned and square configurations on 3.3 μm and 5.2 μm CIGS film thicknesses.

Fig. 7. Influence of tip-spacing on resistivity at 50 Hz.

Fig. 8. Influence of tip-spacing on resistivity at 400 Hz.

Fig. 9. Influence of tip-spacing on resistivity at 1000 Hz.

Figs. 7–9 show a constant resistivity for both thickness and tip spacing in the square configuration with variance that followed (3). In aligned configuration, resistivity changed with changes in tip spacing and thickness by notable variations. The 3.3 μm film exhibited less resistivity than the nominal resistivity of a metallic copper solid (a component in CIGS film) but whose resistivity increased as soon as tip spacing increased above 3 mm. In the 5.2 μm film, resistivity was higher than solid copper’s but decreased whenever the tip spacing was above 3 mm. This was a sign of an influence by an intrinsic phenomenon [28] that conforms to an aligned configuration which was suspected to be bulk defect density. To obtain a resistivity that is comparable in both configurations, the tip spacing must be limited to below 3 mm. When tip spacing was 1 mm, both configurations had an almost perfect concordance in measurements. In aligned configuration, the resistivity depended on spacing between tips unlike in the square configuration. A temperature rise caused an increase in thermal vibrations [29] of atomic particles generating lattice irregularities that acted as electron scattering centres [30], increasing resistivity and grain boundaries and hence becoming effective electron scattering sites. As the number of grain boundaries increases [16], resistivity also increases, implying an increase in effective electron scattering sites. Bulk defects or grain boundaries provide barriers for charge dislocations [31] and transmission through thin films but do not interfere with electrons trespassing. Table II illustrates the percentage variance for three selected frequencies of 50 Hz, 500 Hz and 1000 Hz for tip spacing of 2.0 mm, 3.0 mm, 4.0 mm and 5.0 mm as compared from tip spacing of 1.0 mm.

Table II. Percentage Variance in Resistivity in Aligned Configuration for Three Selected Frequencies and Four Tip Spacing Distances

Tip spacing (mm)	3.3 µm	5.2 µm	3.3 µm	5.2 µm	3.3 µm	5. 2 µm
Frequency (Hz)	50		500		1000
2.0	−5.13	−5.22	−5.12	−5.20	−5.21	−5.13
3.0	−9.21	−9.18	−20.2	−9.14	−20.5	−9.17
4.0	61.11	−20.00	61.11	−20.11	61.23	−19.97
5.0	46.52	−27.13	46.45	−27.5	46.43	−27.5

Table II shows that the percentage resistivity variance values are below 10% for all tip spacing below 3.0 mm. This is the most acceptable variance; therefore, resistivity measurement in CIGS thin films in aligned configurations can be recommended to be done when the tip spacing is less than 3.0 mm and the frequency is below 1000 Hz. It is recommended that when designing a photocell that will use CIGS as an absorber layer, one has to consider the influence of bulk defect density as a factor influencing charge flow.

Influence of Bulk Defect Density on I–V Curve

Fig. 10 shows the I–V characteristics profile curves for different bulk defect densities in CIGS absorber layers as generated by the SCADS-1D simulation software.

Fig. 10. I–V profile characteristics for different bulk defect densities.

By varying the bulk defect density from 10¹² cm⁻³ to 10¹⁸ cm^−3, the fill factor, I_sc, efficiency, ɳ and V_oc revealed three distinct zones. There was a notable reduction in photon absorption in the photocell. The profile I–V curves highlight three distinctive operational zones shown in Fig. 11 and labeled as I, II and III, respectively.

Fig. 11. Influence of bulk defect density on short circuit current, fill factor, efficiency and open circuit voltage.

The bulk defect density in zone I, was less than 5 × 10¹³ cm⁻³ and had a less influence on the photocell operation and as a result, the photocell had very good and almost constant performance due to the Shockley-Read-Hall recombinations [32]. In zone II, the bulk defect density was between 5 × 10¹³ cm⁻³ and 5 × 10¹⁵ cm^−3, decreasing the photocell’s performance. In zone III, the defect density was slightly above 5 × 10¹⁵ cm⁻³ where most of the electrical parameters highly depend on the concentration of defects. There was an abrupt decrease in the photocell’s performance. A large defect concentration decreases charge carrier lifetime [33] proportionally as: where ${\tau }_n$ is lifetime, $D_n$ is diffusion constant, $v_n$ is thermal velocity, $N_t$ is bulk defect density in absorber, ${\sigma }_n$ is capture cross-section of the film and $L_n$ is diffusion length of the electrons. The decrease in lifetime favour recombination rates but proportionally affects their diffusion lengths by [34]:

(10) $$\begin{array}{r}{\tau }_n=\frac{1}{{\sigma }_n{v}_n{N}_t}\end{array}$$

(11) $$\begin{array}{r}{L}_n=\sqrt{D_n{\tau }_n}\end{array}$$

Literature confirms that impurities [32], [33], [35] and defects [36], [37] strongly influence the properties of CIGS absorbers. Impurities or defects can be added to either increase conductivity or control charge carrier lifetime and recent studies reveal that defects are closely linked to film growth mechanisms in CIGS that include but are not limited to gravitational sedimentation [37], inertial impaction [38] and Brownian diffusion [39], mixed induction by turbulent flow [40], electrostatic precipitation [41], interception [42] and in part by elongation of particles [43]. Some impurities cause imperfections in a lattice crystal and end up acting as recombination sites, which explains the reduction in photon absorption by the photocell [44].

Influence of Bulk Defect Density on Quantum Efficiency

Fig. 12 shows the variation of quantum efficiency against wavelength for different bulk defect densities.

Fig. 12. Quantum efficiency for different bulk defects.

From Fig. 12, there is a very high quantum yield in zone I, suggesting an almost 100% photon absorption, which explains the quality of electrical features in zone I. In zone II, photon absorption decreased, reducing the photocell performance due to recombinations per unit volume space as compared to zone III, where there is a drastic decrease in photon absorption. The three operating zones establish a quality link to the CIGS absorber layer, suggesting that thickness, bandgap and doping concentration are influenced by bulk defect density. Therefore, defects in CIGS have a significant impact on the performance of the CIGS photocell.

Influence Bulk Defect Density on Thickness

Fig. 13 illustrates the influence of bulk defect density in CIGS on fill factor, short circuit current, open circuit voltage and photocell efficiency at different thicknesses.

Fig. 13. Influence of CIGS layer on thickness and short circuit current, fill factor, efficiency and open circuit voltage.

In Fig. 13, the short circuit current, fill factor, photocell efficiency and open circuit voltage are affected by thickness in Zone I, II and Zone III. When the thicknesses were between 0.6 μm and 2.7 μm, these parameters decreased slightly. They were almost insensitive to variation in thickness but highly depended on defect density. Below 0.5 µm, there was a drastic decrease due to incomplete photon absorption by the thinner absorber layer [10] attributed to the closer proximity towards the front surface of the absorber and the rear contact [10] that increased recombinations. There was also a decreased photon absorption near the rear contact at short-wavelengths. The rear contact is a region where we have the highest recombination rates [20]. In Zone I, short circuit current, fill factor, photocell efficiency and open circuit voltage are significantly better as compared to Zone II and Zone III, respectively, due to reduced recombinations that can be attributed to low bulk defect density [45] in CIGS layer which leads to a longer lifetimes of photo-generated charge carriers. Zone I is the only zone with optimal performance.

Influence of Bulk Defect Density on Band Gap Energy

Fig. 14 illustrates the influence of bulk defect density on band gap for different electrical parameters, showing a correlation between defect density and bandgap.

Fig. 14. Influence of bulk defect on band gap on the short circuit current density, fill factor, open circuit voltage, and photocell efficiency.

In Fig. 14, J_sc, FF, V_oc and η depend on bulk defect density, which influences band gap. Open-circuit voltage increased linearly whenever the band gap was less than 1.412 eV in Fig. 14a and vice versa in agreement with the absorption coefficient $\alpha$ relationship given as [9]:

where $h$ is the Planck constant while $\gamma$ is wave frequency. Increasing the concentration of Ga in CIGS films reduced the regeneration rate at the P-N junction [45], and thus, fill factor and efficiency exhibited almost the same trend, increasing the band gap to have its optimal performance when it lies between 1.31 eV and 1.42 eV to produce a corresponding efficiency of between 0.45 and 0.61 respectively.

(12) $$\begin{array}{r}\alpha =\frac{B}{h\gamma }{(h\gamma -Eg)}^{\frac{1}{2}}\end{array}$$

Influence of Bulk Defect Density on Acceptor Density

Fig. 15 illustrates how bulk defect density influences short circuit current density, fill factor, open circuit voltage and efficiency in CIGS photocells.

Fig. 15. Influence of bulk defects on short circuit current density, fill factor, open circuit voltage and efficiency.

From Fig. 15, its panels (a)–(d), all the cell parameters were independent of acceptor densities below 10¹³ cm⁻³, while when it was greater than 10¹³ cm⁻³, open circuit voltage increased. For every excessive acceptor density above 10¹⁶ cm⁻³, open circuit voltage decreased significantly. Similarly, short-circuit current density became almost insensitive for acceptor densities below 10¹⁴ cm⁻³ but decreased whenever acceptor concentration exceeded 10¹⁴ cm⁻³. A similar trend was observed for conversion efficiency and fill factor in line with Fig. 5 in Zone I and Zone II, respectively. Bulk defect density determined the quality (defect-free) of the CIGS absorber layer. Therefore, by controlling doping, you control the density of bulk defects, or you control the quantity of acceptors, charge carrier mobility and lifetimes [12] available, which affect photocell performance. Doping has been linked to the presence of intrinsic defects [6], [9], [17], [46] in thin films and one study revealed that an excess of selenium introduces a n-type [46] conductivity, while a deficiency of selenium leads to p-type [41]. Therefore, acceptor density is a key parameter to consider in addition to bulk defect density.

Conclusion

CIGS thin films are commonly used to manufacture photocells and solar cells. A study was carried out on the influence of bulk defect density in CIGS thin films. Resistivity measurements were experimentally performed using the aligned and square configurations. The findings revealed that a square configuration is more accurate than an aligned configuration. It enables quality measurements up to and over 40 kHz in frequency without being affected by tip spacing distances. However, well-defined conditions must be established first for more accurate results when using an aligned configuration. The second section of the study involved using the SCAPS-1D software to simulate the influence of bulk defect density on electrical parameters and photocell performance. Results identified three photocell operating zones influenced by bulk defect densities, revealing that CIGS films had optimal performance when their bulk defect density was lower than 5 × 10¹³ cm⁻³ at 1.31 eV–1.42 eV band gap energy. It was concluded that bulk defect density plays a significant role in CIGS thin films when used as an absorber layer for photon sensing applications.