Incorporation of Ag into Cu(In,Ga)Se2 films in low-temperature process
1. Introduction
Nowadays, the fascinating thin films for photovoltaic generation are (CIGS) thin film[1,2], perovskite thin film[3,4], GaAs thin film[5], and so on. Compared with other films, polycrystalline CIGS thin film is more attractive because of its high-power conversion efficiency[1,2], high stability of heat and damp[6,7], and low cost of manufacturing process[8,9]. Typical efficient CIGS thin films are usually prepared at a substrate temperature of over 550°C[2,10], but the deformation of the soda-lime glass (SLG) substrate and high cost of the manufacturing process can restrict the process of industrial production of CIGS thin film solar cells[11]. A low-temperature deposition process, which is growing CIGS films at temperatures below 450°C, provides feasibility for the reduction of energy consumption and the application of flexible CIGS solar cells on a non-heat-resistance substrate, such as polyimide foil (PI)[9,12].
However, a low-temperature deposition process often produces an inferior device performance due to the poor crystallinity of CIGS films with the absence of the liquid phases during the deposition process[1315" target="_self" style="display: inline;">–
In this work, we investigate the effects of different Ag treatment processes (the Ag precursor and the Ag surface treatment process) on the material and the corresponding device properties during low-temperature processes. First, the material properties of different Ag treatment processes are investigated. Subsequently, the correlation with device performance, such as short-circuit current density () and open-circuit voltage (), is also addressed systematically. Finally, combining the analysis of the material properties and the device measurement data, the comprehension of the growth mechanism of different Ag treatment processes is obtained. Herein, we aim at providing a simple and effective way to deposit high-quality CIGS film at low temperature.
2. Experimental Methods
2.1. Preparation of CIGS films and solar cells
In this study, the solar cell has a Ni-Al/Al-ZnO/i-ZnO/CdS/CIGS/Mo/SLG structure with the active area of [2729" target="_self" style="display: inline;">–
Fig. 1. Sketch diagram of the absorber layer deposition process. The PRE-Ag process is referring to the Ag–Se precursor deposition process, which was prepared before the CIGS deposition process. In addition, the Ag surface treatment process carried out on the surface of CIGS films is denoted as the PDT-Ag process.
In this work, three group samples were prepared to study the characteristics of Ag-doped CIGS films in a low-temperature deposition process. The detailed deposition conditions of different samples are supplied in Table 1. Here, the CIGS sample without any treatment was used as the reference sample. Then, the influence of Ag on CIGS material properties was compared by the samples, A, B, and C. For all samples, the molar ratio of Ga and In + Ga (GGI) is in the range of 0.32–0.33. In addition, the thickness of all films ranges from 2.0 to 2.2 µm.
Table 1. Deposition Conditions of Different Samples with Aga
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Table 2. Statistics of Different CIGS Samples in the ΔE U and ΔV OCa
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2.2. Characterization
The integral compositions of CIGS films with Ag were determined by X-ray fluorescence spectroscopy (XRF), which was calibrated by inductively coupled plasma (ICP). A scanning electron microscope (SEM) was used to show the cross sections of Ag-treated films. The crystallinities of absorber layers were characterized by a Philips X-pert pro X-ray diffractometer (XRD) with a . A 2420 source meter under air mass (AM) 1.5 illumination () was employed to determine the current-voltage (J-V) curves of solar cells. An HP 4284A inductance, capacitance, resistance (LCR) meter was applied on ACIGS films to determine defect concentrations and its distributions (capacitance-voltage, CV; admittance spectroscopy characterizations, AS). Furthermore, the external quantum efficiency (EQE) of different samples was measured in the range of 350–1300 nm.
3. Result and Discussion
3.1. Independently controlling material properties of CIGS films with Ag
Figure 2(a) shows the XRD patterns of CIGS films to explore the phase transition of Ag-treated CIGS films. Except for the Mo peak (about 40°), all XRD patterns of the three samples (A, B, and C) are consistent with chalcopyrite CIGS phases[21], which means that Ag was well synthesized into the CIGS films. The reference CIGS sample (A) exhibits an obvious (220/204) preferred orientation, while Ag-treated samples (B and C) have a strong (112) preferred orientation. In fact, the (112) peak in chalcopyrite film has the lowest surface energy[18]. When Ag is incorporated into CIGS film, the melting temperature of the absorber layer is reduced obviously, leading to the enhanced element diffusion. As a result, the film has a lower surface energy. Therefore, ACIGS films show different preferred orientation (112) in contrast with the reference CIGS sample. The corresponding grazing incident X-ray diffraction (GIXRD) patterns of CIGS films with the incident angle of 0.3° are presented in Fig. 2(b). The significant shift to a lower degree in ACIGS films indicates the successful substitution of Ag for Cu. In addition, the peak of sample C [post deposition treatment (PDT)-Ag process] is slightly lower than that of the sample B [precursor deposition treatment (PRE)-Ag process], which suggests that the Ag content of the sample C is a little higher on the CIGS surface. It is known that the diffusion of the Ag element in CIGS films deposited at the high substrate temperature is uniform, which is not affected by the different Ag treatment processes. Therefore, the Ag content on the surface of CIGS thin films with different Ag treatment processes should be equivalent at the high substrate temperature. In this low-temperature deposition process, the Ag content of sample C (PDT-Ag process) is a little higher on the CIGS surface than that of sample B (PRE-Ag process), although they possesses almost the same Ag content as sample C. Therefore, it is likely that the low substrate temperature limits the diffusion of the Ag element in CIGS films. According to the standard equation of the tetragonal lattice ()[26], the lattice constants of CIGS films can be calculated: sample A, (1 Å = 10−10 m) for and for ; sample B, for and for ; and sample C, for and for . These results indicate that substituting Ag for Cu can increase the lattice constants of CIGS films. Besides, the corresponding values of of different samples (A, B, and C) are 0.9919, 0.9951, and 1.0006, respectively. Considering that the CIGS film is chalcopyrite structure, the ideal lattice constant is one[30]. It is found that the incorporation of Ag into CIGS film could address lattice distortions, which also verifies our previous work[26].
Fig. 2. (a) XRD patterns of the CIGS films with Ag. (b) The corresponding grazing incident X-ray diffraction (GIXRD) patterns of absorber layers.
Cross-sectional SEM images of CIGS films are given in Fig. 3. Clearly, the crystallinity of the reference CIGS film, grown in a low-temperature process, is poor in Fig. 3(a). The grain sizes in the upper part of the film are larger than that near the Mo layer owing to the Ga distribution in the absorber layer. For the Ag precursor-treated sample B, its crystallinity is improved obviously, and the fine grain layer near the Mo layer disappears. In addition, the grains in the upper part of sample B also become better. Currently, two possible explanations can be considered for this phenomenon. First, the melting temperature of the whole films is decreased after the incorporation of Ag, which can enhance the recrystallization. In addition, it has been demonstrated that the Cu–Se phases are also observed in the Cu-poor condition during the second stage[31]. Given the fact that the bond dissociation energy of the Ag–Se phases is lower than that of the Cu–Se phases[25,32,33], we can deduce that more Ag atoms of sample C with the PDT-Ag process can be separated to the lattice of Cu to improve the quality of CIGS films. Compared with the reference CIGS sample A, sample C possesses similar crystal quality (the poor microstructure near the Mo layer). The increased grain size in the upper part of sample C means that the Ag surface process in a low-temperature process mainly improves the upper CIGS crystallization, which agrees with the XRD results in Fig. 2(b). In combination with the result of sample C, it is obvious that both the Ag precursor and the Ag surface process can heighten the crystallinity of CIGS film, and the former is more obvious than the latter. Therefore, incorporating Ag into CIGS films can contribute to improving the crystallization of the absorber layers.
3.2. Device performance of Ag-treated CIGS solar cells
The photovoltaic parameters of Ag-treated CIGS films are shown in Fig. 4. It can be observed that the conversion efficiency of Ag-treated CIGS samples increases from 14.0% to 15.1%, mostly because of the enhanced short-circuit current density (). In order to give an explanation for the changed , EQE results are exhibited in Fig. 5(a). Compared with the reference CIGS sample, the Ag-treated samples have a blue shift of the band-edge wavelength, leading to the reduction of . This is related to the enlarged band gap caused by Ag doping. The band gap values of different samples were obtained by EQE curve fitting. As shown in Fig. 5(b), after adding Ag into CIGS films, values of the band gap increase slightly. Besides, the improved spectral response of the near-infrared wavelengths (600–900 nm) of Ag-treated CIGS films indicates that Ag can improve the minority carrier collection of the bulk of CIGS films due to the enhanced recrystallization[18]. Therefore, the improved mainly benefits from the increased minority carrier collection.
Fig. 4. (a) J-V curves of the best CIGS solar cells fabricated from the different absorber samples. Statistic boxes for the (b) VOC, (c) JSC, and (d) FF and each box contains over 10 solar cells.
Fig. 5. (a) External quantum efficiency (EQE) spectrum. (b) The band gap value obtained by EQE fitting curve. (c) Doping concentration (Na) of different samples. (d) ln(EQE) as a function of photon energy to determine the Urbach energy (EU) values.
In addition, adding Ag into CIGS films also increases the slightly, as shown in Figure 4(b). This result can be attributed to the enlarged band gap, the reduced defect concentration, and the enhanced CdS/CIGS heterojunction, which have been discussed in detail in our previous work[27,34]. It is well known that the Urbach energy () of CIGS films, which is calculated by the EQE results in the long-wavelength edge, can be used to measure the change in (). Here, a decreased corresponds to an improved . In order to further explore the reason for the changed , the of different samples should be calculated by the following equation:
From previous work[35], the decrease in the value of the CIGS films by 1 meV corresponds to the reduction of by 8.6 mV. This result indicates that when the value reduces 1 meV, the of solar cells should increase about 8.6 mV. As shown in Table 2, to get clearer comparison, we calculated the (the reduced Urbach energy) and
Exploring the reason for the slightly improved , CV results of different samples are provided in Fig. 5(c). The doping concentrations () are , , and for samples A, B, and C, respectively. From the previous studies[30,36,37], the relationship between the and can be expressed as follows:
4. Conclusion
In this paper, different Ag treatment processes were utilized to improve the CIGS material properties and the corresponding device performance in a low-temperature process. The incorporation of Ag into CIGS films can not only reduce the melting temperature of the absorber layer but also enhance the element diffusion. Simultaneously, it leads to a lower surface energy and generates a strong (112) preferred orientation. Furthermore, according to the XRD results, we can observe the larger lattice constant of the CIGS films compared with the reference sample, which suggests that the addition of Ag can increase the lattice constant of CIGS films. In contrast to the reference film, the values of of Ag-doping CIGS films are closer to one (the ideal lattice constant of CIGS film). It is demonstrated that both the Ag precursor and the Ag surface process can heighten the crystallinity of CIGS film, and the former is more obvious than the latter. The enhanced crystallinity of CIGS films is beneficial to the collection of minority carriers and the improvement of . Furthermore, is also reduced with Ag doping: the Ag precursor process decreases the value from 36.2 to 22.7 meV because of the enhanced crystallinity, and, for sample C, the value decreases from 36.2 to 21.5 meV, which attributes to the passivation of the defect at the CIGS/CdS heterojunction. However, the reduced does not make sufficient contributions to the increased for the CIGS thin films with Ag doping, which is ascribed to the decreased carrier concentration. Besides, compared with the reference CIGS films, the values of charge barrier height from the absorber layers to Mo back electrodes of Ag-treated CIGS films decrease slightly, which may be attributed to the carrier density and affect the transport of the carrier. Therefore, additional Na, not in the SLG substrate, can be introduced into CIGS thin films to improve the carrier concentration. Herein, we expect to provide a simple and effective Ag treatment process for the CIGS film in a low-temperature process.
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Article Outline
Zhaojing Hu, Yunxiang Zhang, Shuping Lin, Shiqing Cheng, Zhichao He, Chaojie Wang, Zhiqiang Zhou, Fangfang Liu, Yun Sun, Wei Liu. Incorporation of Ag into Cu(In,Ga)Se2 films in low-temperature process[J]. Chinese Optics Letters, 2021, 19(11): 114001.