Wide bandgap CIGS thin films via Ag-PDT to ameliorate the interface quality of CIGS/CdS heterojunction

Introduction

Wide bandgap Cu(In,Ga)Se₂ (CIGS) thin films play an important role in tandem solar cells. However, the wide bandgap CIGS obtained by high gallium content is usually in accompany with fine grains and serious interface problems, which leads to the degradation of device performance. In this work, we adopt silver post-deposition treatment (Ag-PDT) to reduce the interface recombination of high gallium CIGS/CdS heterojunction [CIGS_Eg≈1.3 eV, ((Ga/(Ga + In), GGI) = 0.5], resulting in improved device performance. When Ag is introduced into CIGS film, it preferentially reacts with In to produce AgInSe₂ that enhances the surface layer crystallization of the absorber. Simultaneously, the reaction of Ag and Se produces liquid phase Ag₂Se in the deposition process, which can smoothen the surface roughness of high-gallium CIGS absorber. Nevertheless, excessive Ag₂Se accumulation on the surface of absorber brings severe interface recombination in the heterojunction, generating higher current leakage. Finally, after optimization, the device with power conversion efficiency (PCE) of 17.8% (without anti-reflecting layer) was obtained in the process of Ag-PDT. For the further development of wide bandgap CIGS thin film solar cells with high performance, silver post-deposition treatment is validated as a simple optimization way of heterojunction interface.

Nowadays, polycrystalline Cu(In, Ga)Se2 (CIGS) thin film solar cell is competitive for photovoltaic generation, mostly because of its high power conversion efficiency (PCE) [[1]], good thermal as well as humid stability [[3]–[5]] and low cost of manufacture [[6]]. Simultaneously, the photovoltaic market has gradually paid its attention to tandem solar cells, in order to obtain higher power conversion efficiency. Then, CIGS thin film solar cells is one of the best candidates for tandem solar cells. Because the bandgap range of CIGS film is 1.04–1.67 eV, which can be used as both the top cell [[8]] and the bottom cell [[9]] of the tandem solar cells. National Renewable Energy Laboratory (NREL) [[10]] and Edoff research group of Uppsala University [[11]] both adopted the three-step co-evaporation process to deposit wide bandgap CIGS thin film solar cells [Eg > 1.2 eV, GGI (Ga/(Ga + In)) > 0.4], and illustrated the characteristics and advantages of wide bandgap solar cells. The bandgap of the absorber is closer to the optimal bandgap of 1.4 eV under the standard A.M. 1.5 illumination, resulting in higher open circuit voltage (Voc). At the same time, the significant characteristics of the wide bandgap solar cells are the high open voltage and low current, which can effectively reduce the energy loss (I2R), so as to decrease the thickness of transparent conductive oxide (TCO), and improve the stability of photovoltaic cells in actual working conditions. Therefore, the wide bandgap CIGS thin film solar cells have the potential application for industrialization.

However, as a top cell, wide bandgap CIGS films still have some problems, which restrict the enhancement of its efficiency. For example, with the increase of Ga content in the films, the CIGS/CdS interface will change from spike structure to cliff structure, and the open circuit voltage (Voc) saturation phenomenon will occur [[12]]. And, when the content of Ga in the film increases, fine grains will be yielded [[13]]. However, it leads to a degradation of device performance as more grain boundaries are arisen from the fine grains [[14]].

Currently, silver doping has become one of the research focuses to improve the performance of CIGS devices. This is mainly due to that the doping of Ag can induce the formation of (Ag,Cu)(In,Ga)Se2 (ACIGS), which contributes to lowering the melting point and enlarging the bandgap of absorber [[16]]. A number of research teams have used various silver treatments in the preparation of CIGS absorption layers to achieve efficiency gains of the corresponding devices [[1], [15], [18]–[22]]. William N. Shafarman et al. achieved a high PCE with a bandgap greater than 1.2 eV through Ag optimization for CIGS solar cells [[23]]. Robert W. Birkmire at el. adopted Ag modify of Ga depth profiles [[24]]. South Korean Kim et al. using Ag doping, improved the crystallization quality of the film and reduced internal defects [[15]]. AgGaSe2 prepared by Edoff et al. is the highest PCE so far [[25]]. In addition, our group [[7], [26]] has studied the mechanism of interaction between Ag and CIGS in different Ag doping ways. Even though some researches of Ag doping into Cu-based chalcopyrite thin films have been carried out, there are still few studies on Ag incorporation into wide bandgap CIGS film. In this work, we adopt the post-deposition treatment of Ag (Ag-PDT) approach to conduct the amelioration of the surface properties of wide bandgap CIGS absorber. Moreover, the mechanism of Ag-PDT on the morphology of surface and structure of the absorbers are investigated to clarify the influence of the Ag-PDT on the interface quality of CIGS/CdS heterojunction.

Experimental methods

Preparation of device

In the experiment, we use the mainstream process preparation process [[8], [12], [28]]. First, the absorber is evaporated on an electrode of Mo thin film using a three-step evaporation process under high vacuum, where the Mo layer is deposited on the soda-lime glass by sputtering [[13], [29]]. Whereafter, CdS (chemical bath deposition) is deposited on the Ag-doped absorber, the thickness of which is 50 nm. Then, the high resistance (i-ZnO 60 nm) and low resistance (Al-ZnO 360 nm) layers are sputtered. Finally, Ni–Al (electron beam vapor deposition method) grids lines is deposited on the top of the device as the top electrode. The preparation details of the absorber are shown in Fig. 1.

Graph: Fig. 1 Preparation process of high-gallium CIGS absorber with Ag-doped (Ag-PDT refers to Ag treatment on the surface, that is, deposition of Ag after three-step CIGS thin film preparation)

There are the four groups of samples in this study. In order to find the variation trend of device efficiency with Ag-PDT more quickly, we set the Ag-PDT thickness with the common ratio of 2 (the first item was 25nm). That is, the Ag-PDT thickness of the four samples is 0, 25, 50 and 100nm, respectively. The four groups of samples are kept at GGI = 0.50 and Eg ≈ 1.3 eV. In addition, the [(Ag + Cu)/(In + Ga), ACGI] of ACIGS films with Ag-PDT thickness of 0, 25, 50, 100 nm were 0.883, 0.917, 0.946, 1.034, respectively. The total thickness of the absorber layer is about 2 μm.

Characterization

The chemical element composition of the absorber is calibrated by inductively coupled plasma (ICP) and X-ray fluorescent spectrometer (XRF). The surface-sections and cross-sections of the samples are shown by scanning electron microscope (SEM). The chemical element distribution of the ACIGS films is analyzed by energy dispersive spectrometer (EDS). The X-ray diffraction (XRD) is used to analyze the crystal structure of the thin films. The surface morphology of the absorber is determined by Atomic Force Microscopy (AFM). The reflectance and transmittance of the sample are measured by ultraviolet-visible-near-infrared spectrophotometer. The 2420 Source Meter is used to test the current–voltage (I–V) of the solar cells under AM 1.5 illumination (100 mW/cm2). The external quantum efficiency [EQE, range (350–1350nm)] of diverse samples is tested. In addition, to characterize the carrier concentration and judge the recombination mechanism of different solar cells, capacitance-voltage (CV) and temperature-dependent (current density)–voltage (J–V) tests are carried out by multifunctional digital bridge system 4284L.

Results and analysis

Material properties

Figure 2 shows the SEM images of high-gallium CIGS surface with Ag-PDT of variant thicknesses. It is obvious that the surface crystallization is improved with the successively increase of Ag-PDT thickness.

Graph: Fig. 2 SEM images of CIGS film surface morphology with different Ag doping thicknesses: a 0 nm, b 25 nm, c 50 nm, d 100 nm

Figure 3 shows surface-sectional EDS image of Ag in CIGS films preparative under diverse conditions. It can be clearly seen that the silver content in the surface of the absorber grows with the enhancement of Ag thickness.

Graph: Fig. 3 Surface-sectional EDS image of Ag in CIGS films preparative under diverse conditions: Ag-PDT (a) 0 nm, (b) 25 nm, (c) 50 nm, (d) 100 nm

Figure 4 shows the AFM diagram of high-gallium CIGS film with different amount of Ag deposition. The average surficial roughness (Ra) of CIGS with different Ag deposition thicknesses (0 nm, 25 nm, 50 nm and 100 nm) is decreased, 102 nm, 95.2 nm and 93.8 nm and 87.6 nm, respectively, which are calculated by Nano Scope Analysis software. Thus, Ag-PDT can smoothen the surface of CIGS film, which is conductive to the subsequent deposition of CdS.

Graph: Fig. 4 AFM images of absorber films surface with different Ag doping thicknesses: a 0 nm, b 25 nm, c 50 nm, d 100 nm

Figure 5 shows grazing incident x-ray diffraction (GIXRD) patterns of the absorber films with the different Ag-PDT. When the Ag deposition thickness grows, all peaks shift gradually to the left, because the atomic mass of silver is heavier than that of copper. Besides, the X-ray diffraction results reveal that the primary peaks for Ag2Se appear at 2θ values of 35.7 for (121). This means that the Ag–Se compounds are formed in the absorber surface. Ag2Se is liquid phase at high temperature (555 °C), which can reduce the surface roughness (Fig. 4) of the film.

Graph: Fig. 5 GIXRD diffraction patterns of a all peaks, b the primary peaks and c enlarged Ag2Se peak with different Ag-PDT conditions

To further explore what form Ag exists on the heterojunction interface of the absorber, the CIGS absorber films with high gallium content of different Ag-PDT processes are characterized by SEM images of cross section (Fig. 6), EDS line scan of cross section (Fig. 7) and X-ray diffraction peaks (Fig. 8) respectively. As shown in Fig. 6, with the increase of Ag doping thickness, the interior crystallinity of the CIGS absorber becomes progressively better. From Fig. 6a, we can clearly see that the Mo layer and CIGS absorber layer. The absorber layer (Fig. 6a) appears the worst crystal quality compared to other samples. When the thickness of Ag doping is 25 nm, the crystallinity in the upper part (Fig. 6b) of CIGS thin film are basically massive, while there are still fine grains in the bottom (inside the white rectangle of Fig. 6b) of CIGS thin film. When the thickness of Ag doping is 50 nm (Fig. 6c), the crystallinity of the overall CIGS thin film becomes better, the fine grains in the bottom of CIGS thin film are almost gone. However, when the thickness of Ag doping is 100 nm, the stratification of grain crystals (Inside the white rectangle of Fig. 6d) on the top of absorber occurs and the irregularity of grain crystallization on the interior of absorber appears (Inside the white circular of Fig. 6d).

Graph: Fig. 6 SEM images of absorber films cross-sectional with Ag doping thicknesses: a 0 nm, b 25 nm, c 50 nm, d 100 nm. The SEM images include the Mo layer(bottom) and CIGS absorber layer. The white frame is to emphasize the crystalline part that is different from other positions

Graph: Fig. 7 Cross-sectional EDS images of line scan in CIGS films fabricated under difference conditions: Ag-PDT a 0 nm, b 25 nm, c 50 nm, d 100 nm

Graph: Fig. 8 XRD diffraction patterns of a all peaks, b the primary peaks and c enlarged Ag2Se peak with different Ag-PDT conditions. d Normalized transmission spectrum T/(1-R) of CIGS films with distinct Ag-doped thickness, where T is transmission and R is reflection. And (αhν)2 and hν curves of the different sample. The dashed line shows the optical bandgap Eg obtained by fitting the linear region

Figure 7 shows that EDS line scan of cross section of the samples with different Ag doping thicknesses. Obviously, the fluctuation of In is most affected by Ag. In fact, Ag and Cu belong to the same group of elements and have similar chemical properties. When Ag diffuses into CIGS thin films, the Cu vacancy in the lattice is preferentially replaced by Ag, which reduces the concentration of holes in the absorber. In addition, the formation energy of Ag and In is lower than that of Ag and Ga. When Ag is introduced into CIGS film, it reacts preferentially with In to generate AgInSe2, whose chemical formula is shown as follows [[24]]:

$$2{\text{Ag + Se + In}}_2{\text{Se}}_3\to 2{\text{AgInSe}}_2(\text{priority)}$$

Graph

2 $$2{\text{Ag + Se + In}}_2{\text{Se}}_3\to 2{\text{AgGaSe}}_2(\text{hysteresis})$$

Graph

AgInSe2 has a lower melting point than CuInSe2, which promotes the recrystallization of thin films and forms larger grains. Sample of 25 nm (Fig. 7b) shows In content on the surface layer of the film is more than sample of 0 nm (Fig. 7a), while their content at the back interface is smaller than that of the whole film. This is consistent with the enhanced crystallization on the upper surface of the 25 nm sample (Fig. 2b) and the presence of fine grains on the back of the 25 nm sample (Fig. 6b). For the 50 nm sample (Fig. 7c), In is more evenly distributed. For the 100 nm sample (Fig. 7c), The distribution of In is irregular compared to other samples. In addition, the distribution of Ag in 100 nm sample is disordered relative to other samples, which may be the cause of the crystallization irregularity (Fig. 6d).

As shown in Fig. 8, all peaks except Mo peak (110) shift to the left (Fig. 8b), and the peak (112) was the dominant peak. This result (Fig. 8b) is consistent with Fig. 5b. When the XRD of CIGS film exhibits a preferred orientation of (112) (Fig. 8a, b), its surface energy is lower [[30]]. This is related to the recrystallization of the absorber film by Ag-PDT. The incorporation of Ag promotes the interaction diffusion of elements, which makes the film have a lower melting point. At the same time, the existence of Ag2Se is also found, which proves that superfluous Ag exists at the interior of the absorber (Fig. 6d) in the form of Ag2Se (Fig. 8c). Hence, a mass of Ag2Se accumulation may lead to the deterioration of device performance. Figure 8d shows normalized transmission spectrum T/(1-R) of CIGS films with distinct Ag doping thickness, where T is transmission and R is reflection. The embedded chart of Fig. 8d shows (αhν)2 and hν curves of the different sample. The dashed line in the embedded chart (Fig. 8d) shows the optical bandgap Eg obtained by fitting the linear region. This illustrates that with the increase of silver content in the absorber, the bandgap is widening in turn.

Device performances

To clarify the effect of Ag-PDT on the recombination paths of the high-gallium CIGS solar cells, a temperature-dependent current–voltage measurement is performed as shown in Fig. 9. The relationship between Voc and Jsc is given by [[15], [31]–[34]]

3 $$V_{\mathit{OC}}=\frac{E_a}{q}-\frac{\mathit{AkT}}{q}ln(\frac{J_{00}}{J_{\mathit{SC}}})$$

Graph

Graph: Fig. 9 The activation energy (Ea) of different absorbers with Ag deposition thickness of a 0 nm, b 25 nm, c 50 nm, and d 100 nm

The value of Ea in Eq. 3 is compared with the band-gap value. If the value is significantly lower than that of the bandgap, the recombination position of the absorber is mainly on the surface of its heterojunction; otherwise, it will mainly bulk recombination inside the absorption layer. Where Ea is the intercept between Voc extension line and ordinate when T = 0 K. The Ea is called activation energy of the recombination.

Figure 8d shows that the bandgaps of all samples are about 1.3 eV. The bandgaps of these samples are obviously larger than Ea values (Fig. 9). This means that the recombination paths in CIGS thin film solar cells are mainly located at the interface of heterojunction. The sample (Ag-PDT 0 nm) has a smaller Ea (Fig. 9a), this is caused by the roughest surface (Fig. 4a) and the worst crystallinity (Fig. 2a). the value of Ea (Fig. 9a–c) increases with the Ag-PDT content (from 0 to 50 nm), which proves that the interface recombination can be reduced by Ag-PDT. However, for the 100 nm sample, the grain boundary has the extra Ag exists in the sample [Ag-PDT 100 nm (ACGI = 1.034)] [[22]]. Thus, the value of Ea (Fig. 9d) is the lowest, which is because excrescent Ag2Se can give rise to severer interface recombination and higher current leakage.

As shown in Fig. 10a, the carrier concentration decreases of the samples with silver deposition thickness increase from 0 to 100 nm. When the doping thickness of Ag increases to 100 nm, the carrier concentration (NA) value decreases to the minimum, because excessive doping of Ag results in the conversion of absorber from P type to N type and the occupation of Cu vacancy, which reduces the hole concentration. As can be seen from the EQE (Fig. 10b) of different samples, the medium-long wave (750–950 nm) response becomes better with Ag-PDT, which is due to the optimization of absorber film crystallization [[15], [35]], thus facilitating carrier collection.

Graph: Fig. 10 a Carrier distribution of CIGS thin film tested by multifunctional digital bridge system 4284L. The black, red, blue and purple represent samples with Ag deposition thickness of 0 nm, 25 nm, 50 nm and 100 nm, respectively. b EQE (External Quantum Efficiency) test curves of CIGS film solar cell with different Ag-PDT

Figure 11 shows the device performance of the different Ag-doped films, and Table 1 shows the detailed parameters of the best device prepared by each condition. As shown in Fig. 11, the Voc, Jsc and fill factor (FF) of the solar cell all increase at first, until the thickness was increased to 100 nm, the device performance deteriorates greatly. The Voc (open-circuit voltage) of Ag-PDT samples (0 ~ 50 nm) increases successively. For Ag-PDT samples (0 ~ 50 nm), the interfacial quality (Fig. 9) of CIGS/CdS heterojunction ameliorated gradually, although the carrier concentration (Fig. 10) decreased in turn. Therefore, for open circuit voltage rise, the main heterojunction interface recombination with CIGS/CdS is reduced. At the same time, the optimization of crystallization is another important factor for the increase of FF and Jsc. However, the device performance of 100 nm Ag-doped samples degrade significantly. This bad outcome may be caused by the excessive Ag, which induces the formation Ag2Se on the surface of CIGS. Excessive Ag2Se accumulation on the surface layer of the absorber leads to numerous recombination centers on the interface of the heterojunction, generates higher current leakage. Therefore, excessive Ag doping is harmful to the interface of the heterojunction. Finally, the device parameters are all improved through Ag doping 50 nm into Cu(In,Ga)Se2 absorber of high gallium content, which makes the power conversion efficiency of 17.8% (without anti-reflecting layer).

Graph: Fig. 11 Statistical boxplot of a PCE, bVoc, cJsc, and d FF of CIGS thin film solar cells under Ag-PDT conditions (0, 25, 50 and 100 nm). Each box contains more than 10 small cells

Table 1 Highest PCE of CIGS film solar cell devices with different Ag deposition thickness

Sample	Voc	Jsc	FF	PCE
	(mV)	(mA/cm2)	(%)	(%)
0 nm	658	32.3	71.3	15.2
25 nm	670	33.4	73.1	16.3
50 nm	682	34.6	75.4	17.8
100 nm	634	32.8	66.3	13.7

Conclusion

In this work, we focus on studying the effect of surficial Ag-doped on both the material properties and device performance of high-gallium CIGS films. In term of material properties, it found that Ag-PDT can smoothen the surface roughness of high-gallium CIGS absorber and increase the crystallization on the surface and inside of the absorber. The decrease of roughness is due to the Ag–Se liquid phase generated at high temperature in the Ag-PDT process. The crystallization enhancement of the absorber surface-layer is due to the fact that when silver is doped into the film, it will preferentially react with In to form AgInSe2 with lower melting point. Simultaneously, the incorporation of Ag can reduce the melting point of the film and promote the interaction diffusion of elements, which is conducive to the recrystallization of the film, so that the crystallinity of the whole film becomes better. Accordingly, Ag-PDT can reduce the interface recombination of CIGS/CdS heterojunction and enhance collection of carriers, resulting in improved Voc and Jsc. However, when silver doping is excessive, a large amount of Ag2Se will be produced. Unnecessary Ag2Se will accumulate on the interface of the CIGS/CdS heterojunction and at the grain boundaries of the CIGS absorber. This inevitably leads to the formation of leakage channels and a large number of recombination centers in the CIGS films, which will cause the deterioration of device performance. Appropriate Ag-PDT is favorable for the growth of CIGS thin films. Finally, after optimization, the device with PCE of 17.8% (without anti-reflecting layer) was obtained by the process of Ag-PDT in the surface layer of absorber. Herein, our work could be regard as a handy mode for the interface engineering of high-gallium CIGS solar cells.

Acknowledgements

The work was supported by the National Key R&D Program of China (Grant No. 2018YFB1500200), the National Natural Science Foundation of China (Grant No.61774089, 61974076), National Natural Science Foundation of China (U1902218).

Author contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by CW. The first draft of manuscript was written by CW. The manuscript was reviewed and edited by CW and WL. All authors commented on previous versions of manuscript. All authors read and approved the final manuscript.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

Declarations

Conflict of interest

The authors declare no conflict of interest.

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By Chaojie Wang; Zhaojing Hu; Yunfeng Liu; Shiqing Cheng; Yifeng Yao; Yunxiang Zhang; Xudong Yang; Zhiqiang Zhou; Fangfang Liu; Yi Zhang; Yun Sun and Wei Liu

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