A Modeling Study on Utilizing In2S3 as the Buﬀer Layer of Cu(In,Ga)Se2 Based Solar Cell

doi:10.21203/rs.3.rs-650485/v1

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A Modeling Study on Utilizing In2S3 as the Buﬀer Layer of Cu(In,Ga)Se2 Based Solar Cell

https://doi.org/10.21203/rs.3.rs-650485/v1

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In Cu(In_1 − x,Ga_x)Se (CIGS)-based solar cells, the cadmium sulfide (CdS) layer is conventionally used as a buffer layer. In the current study, the CdS layer was replaced by the Indium sulfide (In₂S₃) layer, and the impact of various concentrations of Ga in the CIGS absorber, the band gap of the In₂S₃ buffer layer, and the band gap of the Na_yCu_1−yIn₅S₈ interfacial layer on the efficiency of these CIGS solar cells were investigated. The results indicated that in the absence of Na_yCu_1−yIn₅S₈, the optimal performance was obtained with an E_g−In2S3 value of 3.1 eV and the ratio of Ga/(Ga + In) (GGI) = 1, yielding an efficiency of 21.97%. The formation of the Na_yCu_1−yIn₅S₈ interfacial layer deteriorated the efficiency of the device, and the highest efficiency of the CIGS solar cells with the interfacial layer was 16.33 %.

Electrical Engineering

Electronic Materials and Devices

Optical Materials and Devices

CIGS solar cells

Buffer layer

Thin film

In2S3

CuIn_1 − xGa_xSe₂ (CIGS) has shown great promise as a solar cell absorber material with excellent properties, such as high absorption coefficient, tunable band gap, and good environmental stability [1–3]. In a study, Cu(In, Ga)(S, Se)₂ thin-film solar cells achieved a power conversion efficiency (η) of 23.35% on a laboratory scale [4]. The structure of CIGS solar cells typically comprises Glass/Mo/CIGS/Buffer layer/i-ZnO/n-ZnO:Al (transparent conductive oxide: TCO). It also includes cadmium sulfide (CdS) functioning as a buffer layer between the CIGS absorber and window layers.

However, because of the toxicity of cadmium sulfide and its incompatibility with in-line vacuum-based production methods, CdS is not a very reasonable choice as a buffer material in solar cells [5, 6]. Furthermore, owing to the low band gap energy (i.e., ~ 2.4 eV) of CdS, the short-wavelength photons of the sunlight spectrum cannot reach the CIGS absorber layer [5, 6]. As a result, replacing the CdS buffer layer with other Cd-free buffer layer materials has gained tremendous attention and has been considered one of the major goals in the area of CIGS thin-film solar cells [5, 6].

One of the most influential parameters in forming buffer layers is conduction band offset (CBO) at the buffer-CIGS absorber interface [7–9]. A good alignment of conduction bands reduces recombination at the buffer-CIGS absorber interface [7–9]. Thus, it is required to optimally align the conduction band edge of the p-type CIGS absorber layer with that of the n-type buffer layer so as to obtain a good p-n junction [7–9]. Extensive research has been carried out on the substitution of alternative buffer materials for CdS [5, 10–14].

Indium sulfide (In₂S₃) has emerged as a promising candidate for the buffer layer in CIGS thin-film solar cells [5, 15–20]. In₂S₃ is superior to CdS as it has a larger band gap (In₂S₃; 2.1–3.2 eV and CdS: 2.4eV)and greater stability [21]. In₂S₃ thin-film layers are also deposited via different in-line processing methods, such as atomic layer deposition (ALD) [22], physical vapor deposition (PVD) [15, 23], spray ion layer gas reaction (ILGAR) [24], and sputtering [25]. When In₂S₃ is deposited on the CIGS absorber layer, copper and sodium atoms from the CIGS absorber layer diffuse toward the In₂S₃ buffer layer, and an interfacial layer with a general formula of Na_yCu_1−yIn₅S₈ is generated [26, 27]. This interfacial layer substantially affects the efficiency of CIGS solar cells.

In a study, the In₂S₃ buffer layer in CIGS-based thin-film solar cells was deposited through atomic layer deposition (ALD), and a conversion efficiency of 16.4% was reached for these solar cells [16]. The highest power-conversion efficiency reported for CIGS solar cells with the In₂S₃ buffer layer is 18.2% [26], which is notably lower than that reported for CIGS solar cells with the CdS buffer layer (22.9 %) [27]. Therefore, it is required to optimize CIGS solar cells with In₂S₃ buffer layers.

In a study, the CIGS solar cells were simulated with In₂S₃ layers and other Cd-free buffer layers [28]. Similarly, in another study, ZnO/In₂S₃/CIGS solar cells were modeled [29]. However, in both cases, the band gaps of CIGS and In₂S₃ were fixed. Two studies [30, 31] explored the effect of the properties of the In₂S₃ buffer layer on the performance of CIGS solar cells using numerical simulation, but variations in the CIGS absorber layer were not neglected in both studies. Sun et al. also examined the effect of the band gap (E_g) of In₂S₃ and the composition of interfacial layers between In₂S₃ and CIGS on the efficiency of CIGS solar cells [32] and found that the properties of these interfacial layers exerted a substantial influence on the performance of the CIGS device. However, they overlooked the possible variation of the band gap of the CIGS absorber layer.

In the present study, the structure of contact/CIGS/In₂S₃/i-ZnO/ZnO:Al/contact was analyzed using the Solar Cell Capacitance Simulator (SCAPS) developed by the University of Gent [33, 34]. Since the band gap is determined by the GGI ratio, the GGI was calculated for the CuIn_1 − xGa_xSe₂ absorber layer. Then, the impact of the In₂S₃ band gap and the GGI of the CuIn_1 − xGa_xSe₂ absorber layer on the efficiency of solar cells was examined. After that, the optimum values of the In₂S₃ band gap and Ga concentrations in the CIGS absorber layer were measured. The role of the band gap of the interfacial layer (Na_yCu_1−yIn₅S₈) and the CIGS layer in the performance of CIGS solar cells was also investigated.

In the current study, two structures, namely, ZnO:Al/i:ZnO/In₂S₃/CIGS/Mo and ZnO:Al/i:ZnO/In₂S₃/Na_yCu_1yIn₅S₈/CIGS/Mo, were investigated. The structures are shown in Fig. 1, and their base parameters are provided in Table 1. In Table 1, E_g and χ are band gap energy and electron affinity, respectively. N_A and N_D represent acceptor and donor densities, respectively. N_C and N_V denote the effective densities of conduction and valence bands, respectively. ε_r is relative permittivity, µ_p and µ_n are the mobility of hole and electron, respectively, and σ_h and σ_e are hole and electron capture cross sections, respectively. In both structures, the thickness of CIGS, i:ZnO, and ZnO:Al was fixed at 2 µm, 80 nm, and 300 nm, respectively.

Table 1

Base parameter set in simulation.
Parameter		CIGS [41]	In₂S₃ [32]	Na_yCu_1−yIn₅S₈ [32]	i:ZnO [41]	ZnO:Al [41]
Thickness (nm)		2000	40 (30)	0 (10)	80	300
E_g(eV)		variable	variable	variable	3.3	3.3
Χ (eV)		variable	variable	variable	4.4	4.4
ε_r		13.6	9	10	9	9
N_c (cm^− 3)		2×10¹⁸	2.22×10¹⁸	2.22×10¹⁸	3×10¹⁸	3×10¹⁸
N_v (cm^− 3)		1.5×10¹⁹	1.78×10¹⁹	1.78×10¹⁹	1.7×10¹⁹	1.7×10¹⁹
${\text{v}}_{\text{t}\text{h}}^{\text{e}}$ [cm/s]		3.9×10⁷	2.4×10⁷	2.4×10⁷	2.4×10⁷	2.4×10⁷
${\text{v}}_{\text{t}\text{h}}^{\text{p}}$‏‏[cm/s]		1.4×10⁷	1.3×10⁷	1.3×10⁷	1.3×10⁷	1.3×10⁷
µ_e [cm²/Vs]		100	100	100	100	100
µ_p[cm²/Vs]		12.5	25	25	31	31
Doping [cm^− 3]		7.95×10¹⁶	1.1×10¹⁸	1.1×10¹⁸	10¹⁹	10²⁰
	Bulk defect properties
N [cm^− 3]		5.5×10¹⁵	6×10¹⁷	8×10¹⁷	1×10¹⁶	1×10¹⁶
σ_e [cm²]		10^− 14	1×10^− 17	1×10^− 17	1×10^− 15	1×10^− 15
σ_h [cm²]		10^− 15	9.8×10^− 13	9.8×10^− 13	5×10^− 13	5×10^− 13
Interface defects		CIGS/CdS
N [cm^− 3]		6×10¹⁴
σ_e [cm²]		10^− 15
σ_h [cm²]		10^− 15

In the first structure, i.e., ZnO:Al/i:ZnO/In₂S₃/CIGS/Mo, the thickness of the In₂S₃ buffer layer was fixed at 40 nm. In the second structure, i.e., ZnO:Al/i:ZnO/In₂S₃/Na_yCu_1yIn₅S₈/CIGS/Mo, the sum of the thickness values of the Na_yCu_1−yIn₅S₈ and In₂S₃ layers was 40 nm, which indicates that the increase in the thickness of the Na_yCu_1−yIn₅S₈ layer is certainly accompanied by a decrease in that of the In₂S₃ buffer layer.

It is possible to alter the band gap of the CuIn_1 − xGa_xSe₂ absorber layer by varying the GGI ratio. With an increase in Ga content, the conduction band minimum was augmented, which boosted the band gap of CIGS [7] and declined the electron affinity by the same amount. Figure 2a depicts the band gap variations of the CIGS semiconductor [9]. Variation in Ga fraction yielded different band gaps of CuIn_1 − xGa_xSe₂. These bad gaps were calculated through the following equation [7]:

Eg (x) = 1.01 + 0.42x + 0.24x2

where x is Ga fraction. It was found that the bandgap ranged from 1.01 eV for CuInSe2 to 1.67 eV for CuGaSe₂.

Variation in the band gap of the In₂S₃ layer can be attributed to the grain sizes and composition of In₂S₃ [35, 36]. To be more precise, the sodium and oxygen doping of the In₂S₃ thin films enlarged the In₂S₃ band gap [37–39]. In this study, it was assumed that shifting the conduction band energy edge toward higher energies widened the band gap of the In₂S₃ buffer layer. It is worth mentioning that E_g−In2S3 varied from 2.1 to 3.2 eV. Figure 2b illustrates a schematic diagram of the band variations for the In₂S₃ buffer layer.

The main reason why the band gap of the Na_yCu_1−yIn₅S₈ interfacial layer extended with an increase in Na concentration was the shift in the valence band edge toward lower energies. A small shift in the conduction band edge toward higher energies also occurred [40]. Figure 2c shows a schematic diagram of the band variations for the Na_yCu_1−yIn₅S₈ interfacial layer.

By varying the band gap of In₂S₃ (Na_yCu_1−yIn₅S₈), a wide range of CBO between the absorber layer and the buffer (as an interfacial layer) was covered; thus, it allowed the offset to be tuned to an optimal value for any GGI ratio in the CIGS absorber layer. Figure 3a presents the CBO for the In₂S₃-CIGS interface for various In₂S₃ band gaps as a function of Ga content in the CIGS absorber layer. Figure 3b also displays the CBO for the Na_yCu_1−yIn₅S₈-CIGS interface for various Na_yCu_1−yIn₅S₈ band gaps as a function of Ga content in the CIGS absorber layer. The CBO ranged from + 0.69 to -0.97 eV for the CIGS-In₂S₃ interface and from + 0.19 to -0.47 eV for the CIGS-Na_yCu_1−yIn₅S₈ interface when the full range of E_g−In2S5, E_{g−NayCu1−yIn5S8}, and GGI ratio were considered. In all the cases, the CBO shifted toward more negative values with an increase in Ga fraction in the CIGS layer.

Considering the above mentioned characteristics, several material parameters were incorporated into the SCAPS to delve into the effect of the E_g−In2S3 and composition of the CIGS layer on the efficiency of the ZnO:Al/i:ZnO/In₂S₃/CIGS/Mo structure. In what follows, the results obtained for the role of the Eg of the interfacial layer and the composition of the CIGS layer in the performance of the ZnO:Al/i:ZnO/In₂S₃/Na_yCu_1−yIn₅S₈/CIGS/Mo structure are presented and discussed.

3.1. Optimization of ZnO:Al/i:ZnO/In₂S₃/CIGS/Mo structure

Initially, based on the structure described in the previous section, the impact of the band gap of the In₂S₃ buffer layer and Ga concentrations in the CuIn_1 − xGa_xSe₂ absorber layer on the performance of the ZnO:Al/i:ZnO/In₂S₃/CIGS/Mo structure was scrutinized.

Various short-circuit currents (J_SC) were obtained for different Ga fractions in CIGS as a function of the band gap of the In₂S₃ buffer layer, as indicated in Fig. 4a. With increasing the band gap of the In₂S₃ buffer layer, the current was boosted slightly since the wide band gap of In₂S₃ allowed for greater absorption of short-wavelength photons [8, 41]. J_SC was not notably changed by varying the GGI ratio. One possible explanation is that the recombination of the photogenerated carriers does not occur at the interface due to the electric field generated by the space charge region (SCR) [minemoto 2013]. At low concentrations of gallium, the spike of the TCO-CIGS interface also impedes the photo-generated carrier, which decreases J_SC dramatically [8, 9, 41, 42]. With increasing the Ga concentration, the current (J_SC) remained relatively constant in most of the ranges of the In₂S₃ band gap. J_SC substantially declined when low Ga fractions were used in wider In₂S₃ bang gaps. This can be attributed to the formation of a large electron barrier at the absorber-buffer interface, which limits the current flow of the circuit [9, 41, 42].

Figure 4b shows the variations in the open-circuit voltage (V_OC) as a function of the band gap of the In₂S₃ buffer layer for different Ga fractions in CIGS. The results revealed that for almost all the band gaps of the buffer layer, the V_OC was augmented as the Ga fraction in the absorber layer increased from 0 to 1. One can argue that the incorporation of high Ga content into the SCR increases the barrier height, which facilities band-gap widening in the SCR and reduces the recombination rate [8, 43].

As seen in Fig. 3a and Fig. 4b, for a constant GGI ratio, the value of V_OC decreased with a reduction in the band gap of the buffer layer so long as the CBO was negative. When the band gap of the buffer layer lessened, its electron affinity (χ) increased and as a result, a larger negative value of CBO was obtained at the absorber/buffer interface. The negative CBO at the absorber-buffer interface acted as a barrier against the injected electrons. This barrier facilitated the recombination of the majority carriers via defects at the absorber-buffer interface. Therefore, the total recombination rate was promoted, and the V_OC decreased with increasing the absolute negative value of CBO at the absorber-buffer interface [7]. In a nutshell, the negative CBO at the interface enhanced the interfacial recombination rate, which reduced the V_OC [7, 8, 44]. As also illustrated in Fig. 3a and Fig. 4b, when the CBO at the interface of the absorber and the buffer was positive, the voltage reached its saturation point and became independent of the band gap of In₂S₃. When the CBO of the buffer layer was greater than that of the absorber layer, the barrier that was required for the recombination of the majority carriers was not formed; therefore, the V_OC remained approximately constant [7]. As mentioned before, these results were observed in all the In₂S₃ bang gaps except for the higher In₂S₃ bang gaps.

Figure 4c depicts the impact of various Ga fractions in the CIGS absorber layer and different band gap ranges of the buffer layer on the fill factor (FF). For all the GGI ratios, the FF was initially promoted and then reduced with increasing the band gap of the In₂S₃ buffer layer. Lower GGI ratios triggered more profound changes.

When the CBO value was negative, the FF decreased because of the recombination of the majority carriers including the electrons in the In₂S₃ buffer and the holes in the CIGS absorber layer at the In₂S₃-CIGS interface [7].

When the CBO value was positive, the FF was nearly constant. This value remained the same until the CBO value became so large that the spike of the In₂S₃-CIGS interface impeded the photo-generated carrier [7].

Efficiency refers to the ratio of the product of J_SC, the V_OC, and the FF to incident power. The obtained efficiency in this study is plotted in Fig. 4d. As evident in Fig. 4d, the efficiency was promoted as absorber layer was transferred from the pure CIS phase to the pure CGS pure phase. Almost in all the GGI ratios, there was an increase in the efficiency as the E_g−In2S3 value was boosted up to 2.9 eV, and then, the efficiency decreased for the E_g−In2S3 values greater than 2.9 eV. At a E_g−In2S3 value of 2.9 eV, the efficiency values for x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 were 10.55, 12.25, 13.78, 15.34, 16.98, 1.73, 20.34, 21.43, 21.78, 21.88, and 21.92%, respectively. The optimum performance was obtained at E_g−In2S3=3.1 eV and GGI = 1, leading to an efficiency value of 21.97%.

3.2. Optimization of ZnO:Al/i:ZnO/In₂S₃/Na_yCu_1−yIn₅S₈/CIGS/Mo structure

The formation of Na_yCu_1−yIn₅S₈ at the CIGS-In₂S₃ interface strongly affects the performance of CIGS solar cells [32]. This section presents the results obtained for the impact of various E_g ranges of the interfacial layer and different GGI ratios in the CIGS layer on the efficiency of the ZnO:Al/i:ZnO/In₂S₃/Na_yCu_1−yIn₅S₈/CIGS/Mo structure.

The variation in J_SC as a function of the band gap of the Na_yCu_1−yIn₅S₈ interfacial layer for different Ga fractions in CIGS is displayed in Fig. 5a. With increasing the band gap of the interfacial layer, the current was augmented from about 32.1 to 33.5 mA/cm². It can be interpreted that the larger band gap of the interfacial layer allowed more photons to reach the CIGS absorber and generate more carriers [8, 41]. Over all the interfacial band gap ranges, as the GGI ratio increased in the absorber layer the current, J_SC, remained nearly constant. One can construe that the recombination of the photogenerated carriers does not occur at the interface due to the electric field generated by the SCR [7]; consequently, the current remained unchanged.

A boost in the V_OC value was observed for all the band gap ranges of the buffer layer as the Ga fraction in the absorber layer increased from 0 to 1 (Fig. 5b). Owing to an increase in the band gap within the SCR, the barrier height was augmented, which diminished the recombination rate, and ultimately promoted the V_OC [8, 43, 45–47].

As demonstrated in Fig. 3b and Fig. 5b, the obtained V_OC value was constant when the GGI ratio was fixed and the CBO at the Na_yCu_1−yIn₅S₈-CIGS interface was positive. It was found that for a constant GGI ratio, the V_OC value lessened slightly when the band gap of the interfacial layer decreased and the CBO was negative. ‏ Based on these results, it can be concluded that using a constant GGI ratio, with increasing the band gap of the interfacial layer a slightly larger negative CBO value is achieved at the absorber-buffer interface. The negative CBO at the interface decreased the energy barrier between the conduction band of the buffer layer and the valence band of the absorber layer across the interface, which induced the carrier recombination via interface defects and eventually lowered the V_OC [8]. When the CBO was positive, the barrier triggering the recombination of the majority carriers was not generated and the V_OC was nearly constant [7, 48]. These results are consistent with those of other studies undertaken on the effect of the CBO of buffer/CIGS layers on solar cell performance [7, 48]. The maximum value of V_OC was about 0.66 V at E_{g−NayCu1−yIn5S8}=2.4eV and GGI ≥ 0.5.

The achieved values for the fill factor (FF) over the total range of gallium content in the absorber layer and the band gap of the interfacial layer are shown in Fig. 5c.

The variations of the FF followed the trends in V_OC as expected based on the analytical relationship between FF and V_OC [49]. For all the E_{g− NayCu1−yIn5S8} values, the FF increased as the Ga fraction in the absorber layer was boosted, and with an increase in the GGI ratio, this effect lessened. For all GGI ratios, the FF was not under the influence of the variations in the interfacial band gap.

As illustrated in Fig. 5d, the efficiency first increased with a boost in the GGI ratio up to 0.4, and then remained approximately constant. The efficiency was nearly unchanged for the GGI ratio less than 0.4 in the full range of E_{g−NayCu1−yIn5S8} while it promoted for higher GGI ratios than 0.4 with the augmentation of E_{g−NayCu1−yIn5S8}. The optimal performance was obtained at E_{g−NayCu1−yIn5S8}=3 eV and GGI = 1, yielding an efficiency of 16.33% (J_SC= 33.5 mA/cm², V_OC= 655 mV, & FF = 74.4%).

The comparison between Fig. 4d and Fig. 5d revealed that the formation of the Na_yCu_1−yIn₅S₈ interfacial layer deteriorated the efficiency of the device. It seems that the major reason for the decline in the device efficiency is the decrease of the V_OC. The highest V_OC and efficiency of the CIGS solar cell with no interfacial layer were 0.98 V and 21.97 %, respectively, while those of the cell with an interfacial layer were 0.66 V and 16.33 %, respectively.

The Na_yCu_1−yIn₅S₈ interfacial layer is generated owing to the diffusion of copper from the CIGS layer during In₂S₃ deposition at substrate temperatures above 250. Thus, it is of crucial importance to deposit In₂S₃ at temperatures below 250 ̊C and minimize copper diffusion from the CIGS layer.

In the current study, the impact of gallium content in Cu(In,Ga)Se₂ and the variation in the band gap of buffer and interfacial In₂S₃ and Na_yCu_1−yIn₅S₈ layers on the performance of CIGS solar cells was investigated. The cell parameters were measured for all the ranges of the band gap of In₂S₃ and Na_yCu_1−yIn₅S₈ layers and the GGI ratios in the CIGS layer were computed.

Based on the results, it can be concluded that in the absence of the Na_yCu_1−yIn₅S₈ interfacial layer, the efficiency of CIGS solar cells enhances for all the GGI ratios when E_g−In2S3 increases up to 2.9 eV, and there is a decrease in their efficiency with an E_g−In2S3 value above 2.9 eV. The best performance is obtained with an E_g−In2S3 value of 3.1 eV and a GGI ratio of 1, which yields an efficiency of 21.97%. The results also reveal that the formation of Na_yCu_1−yIn₅S₈ interfacial layers weakens the performance of the device since it decreases V_OC. The highest V_OC value obtained for the CIGS solar cell with no interfacial layer was 0.98 V while this value dropped to 0.66 V for the cell with an interfacial layer. The optimum efficiency achieved for the CIGS solar cell with a Na_yCu_1−yIn₅S₈ interfacial layer was 16.33 %.

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You are reading this latest preprint version

A Modeling Study on Utilizing In2S3 as the Buﬀer Layer of Cu(In,Ga)Se2 Based Solar Cell

Status:

Version 1

Abstract

Figures

1. Introduction

2. Modeling And Simulation Of Cigs Solar Cells

3. Results And Discussion

3.1. Optimization of ZnO:Al/i:ZnO/In₂S₃/CIGS/Mo structure

3.2. Optimization of ZnO:Al/i:ZnO/In₂S₃/Na_yCu_1−yIn₅S₈/CIGS/Mo structure

4. Conclusion

5. References

Status:

Version 1

A Modeling Study on Utilizing In2S3 as the Buﬀer Layer of Cu(In,Ga)Se2 Based Solar Cell

Status:

Version 1

Abstract

Figures

1. Introduction

2. Modeling And Simulation Of Cigs Solar Cells

3. Results And Discussion

3.1. Optimization of ZnO:Al/i:ZnO/In2S3/CIGS/Mo structure

3.2. Optimization of ZnO:Al/i:ZnO/In2S3/NayCu1−yIn5S8/CIGS/Mo structure

4. Conclusion

5. References

Status:

Version 1

3.1. Optimization of ZnO:Al/i:ZnO/In₂S₃/CIGS/Mo structure

3.2. Optimization of ZnO:Al/i:ZnO/In₂S₃/Na_yCu_1−yIn₅S₈/CIGS/Mo structure