Influence of surface properties on the performance of Cu(In,Ga)(Se,S)₂ thin-film solar cells using Kelvin probe force microscopy

Abstract

We have investigated the sulfurization process in a Cu(In,Ga)(Se,S)₂ (CIGSS) absorber layer fabricated by a two-step sputter and selenization/sulfurization method in order to make an ideal double-graded band-gap profile and increase the open circuit voltage (V_oc). The sulfurization process was controlled by temperature from 570 °C to 590 °C without changing H₂S gas concentration and reaction time. Although the energy band-gap of the CIGSS absorber layer was increased with increasing sulfurization temperature, the V_oc of the completed CIGSS device fabricated at 590 °C sulfurization temperature did not increase. In order to investigate this abnormal V_oc behavior, the CIGSS absorber layer was measured by local electrical characterization utilizing Kelvin probe force microscopy, especially in terms of grain boundary potential and surface work function. Consequently, the abnormal V_oc behavior was attributed to the degradation of grain boundary passivation by the strong sulfurization process. The optimum sulfurization temperature plays an important role in enhancement of grain boundary passivation. It was also verified that the V_oc degradation in the CIGSS solar cell fabricated by the two-step method is more influenced by the grain boundary passivation quality in comparison with the slight non-uniformity of material composition among grains.

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Thin-film chalcopyrite photovoltaic devices based on the compound Cu(In,Ga)Se₂ (CIGSe) recently have achieved power conversion efficiencies of over 20% in the laboratory.^1–3 Generally, the high-performance CIGSe absorbers have an intentionally double graded band-gap profile via Ga grading for a multi-stage co-evaporation method and sulfur diffusion for a two-step sputter and selenization/sulfurization method in order to improve the charge collection probability, the junction quality, etc.^4–6 The co-evaporation methods are well-known processes with many reports of research in this field and the present record efficiency of CIGSe solar cells.² However, there have not been many investigations about the two-step sputter and selenization/sulfurization method even though a successfully commercialized Cu(In,Ga)(Se,S)₂ (CIGSS) module is produced by Solar Frontier K.K in Japan. One of the most critical factors for a two-step sputter and selenization/sulfurization process for a CIGSS absorber layer is a sulfurization step for which it is hard not only to control the sulfur reaction because of very fast sulfur diffusion into an absorber layer at high temperature (generally above 500 °C), but also to easily influence device performance in accordance with sulfurization level. If the CIGSS absorber layer was fabricated by a strong sulfurization process (such as high temperature, long sulfurization time, and high partial pressure of H₂S gas), the CIGSS absorber quality capable of achieving a high open-circuit voltage (V_oc) may be obtained by the increase of S/(S + Se) (SSSe) and Ga/(Ga + In) (GGI) ratios. However, there can be problems particularly occurring in the strong sulfurization process in that the fill factor (FF) is decreased because of the deterioration of P/N junction quality by the sulfur surface etching effect.^7,8 Therefore, it is important to keep the absorber surface state in a good condition during the sulfurization step. However, there is a paucity of research reports of CIGSS surface state such as potential properties at a grain boundary, compositional uniformity, interface defect state, grain boundary passivation property, etc.

Kelvin probe force microscopy (KPFM) recently has been used to study electrical properties of photovoltaic devices which present surface states, such as work function, potentials of grain boundary/intra-grain, etc.^9–13 Furthermore, KPFM can directly measure the band bending on the grain boundary and also the electron–hole carrier transport near the grain boundaries in the absorber layer of CIGSS thin-film solar cells.^14,15 KPFM provides a non-destructive technique to measure the surface potential distribution in solar cell devices. Use of KPFM not only helps to optimize the charge separation and its contribution to the production photocurrents, but also provides a picture of the grain boundary potential profiles and surface state. This information is used to improve the efficiency of the solar cells.

In this paper, we have investigated a sulfurization process in a CIGSS absorber layer fabricated by a two-step sputter and selenization/sulfurization method. The local surface states of the CIGSS layer were also investigated by KPFM, especially the potential distributions at grain boundaries and the surface work-function distribution in order to resolve the harmful effect of a strong sulfurization process. Finally, the correlations of sulfurization degree with grain boundary passivation are discussed.

An approximately 1.6 μm thick CIGSS absorber was processed as follows, the method consisting of two fabrication processes. In the first fabrication process, Cu_0.72Ga_0.28 and In precursors were sequentially deposited by DC sputtering on a 300 nm thick Mo back electrode which was also deposited by a sputtering system on a cleaned high strain point glass at room temperature. The Cu/(Ga + In) composition was controlled by film thickness and was fixed at about 0.87. The second fabrication process forms the completed CIGSS structure by selenization and sulfurization which is a chemical reaction process with H₂Se and H₂S gases, respectively. In this experiment, the sulfurization process was carried out by changing the temperature from 570 °C to 590 °C in order to control the diffusion of sulfur into the CIGSe absorber by sulfurization. The detailed temperature profile of the selenization and sulfurization process is shown in Fig. 1. At the selenization stage, a graded band-gap CIGSe absorber was fabricated intentionally by the selenization of metal precursors in H₂Se gas atmosphere. Then in the sulfurization stage, a thin CIGSS surface layer was formed on the CIGSe absorber surface by sulfurization with H₂S gas in order to prepare a double graded band-gap profile. Heterojunctions were formed by a chemical bath deposition (CBD) process of an about 5 nm Zn(O,OH,S) buffer layer in order to reduce the shunt path and increase interface quality. The advantages of Zn-based buffer layer are that it is an eco-friendly non-toxic material and has improved absorption at short wavelength. One laser (P1) and two mechanical (P2 and P3) scribing techniques were applied to form a monolithic interconnection. The gap between P1 and P3 should be minimized to reduce the dead area of the total module area. A transparent conducting oxide top contact layer was deposited on the CBD buffer layer using low-pressure chemical vapor deposition. Morphologies and microstructure of the absorber layer were measured by field emission scanning emission microscopy (SEM). X-ray fluorescence and glow discharge optical emission spectrometry (GD-OES) were used to determine the composition ratio and the depth profile in the CIGSS films, respectively. The solar cell performance was measured under AM 1.5, 100 mW cm⁻² illumination at 25 °C. Optical response characteristic of the solar cell was measured as external quantum efficiency (EQE) which was used to calculate energy band-gap (E_g). The bulk and surface crystal structures were studied by X-ray diffraction (XRD) and grazing-incidence X-ray diffraction (GIXRD), respectively. Local electrical properties of the CIGSS absorber layer were measured by KPFM, which utilized a commercial atomic force microscope. A Pt/Ir coated tip was used for measuring the CIGSS layer. The surface potential and topography were investigated using a non-contact mode. The KPFM measurements are described in detail elsewhere.^13,16 For a comparison between the surface properties and the device performance, it is essential to understand the polycrystalline CIGSS solar cell characteristics.

Fig. 1 The temperature profiles of two-step selenization/sulfurization process.

The GD-OES depth profile of the CIGSS absorber layer is shown in Fig. 2, presenting GGI, SSSe, and E_g profiles. The CIGSS absorber layers were made with different sulfurization temperatures from 570 °C to 590 °C when the other conditions were fixed. GD-OES was applied on samples with removed window and buffer layers in order to evaluate the content and distribution of sulfur in the absorber layer. It is well known that the E_g can be engineered by changing GGI and SSSe ratios in chalcopyrite compound semiconductors.^17,18 For example, the E_g of the CIGSS absorber can be increased by substituting In and Se by Ga and S, respectively. It was verified by the GD-OES measurements that more sulfur element was incorporated into the CIGSe absorber when the sulfurization was performed at higher temperature as shown in Fig. 2(a). In addition, Fig. 2(b) shows that the Ga concentration at the absorber surface was increased by increasing the sulfurization temperature. It was confirmed from GD-OES depth profile results that the E_g of CIGSS absorber was increased by increasing the sulfurization temperature as shown in Fig. 2(c). The E_g profile was calculated using the following equation:¹⁹

E_g = 0.98 + 0.167x² + 0.17y² + 0.023x²y − 0.17xy² + 0.397xy + 0.31y + 0.523x

where x is the GGI ratio and y is the SSSe ratio. Fig. 3 gives the E_g determined by differentiation of the EQE curve²⁰ in order to obtain the E_g values more exactly. The raw data of EQE are depicted in the inset of Fig. 3. The E_g of the CIGSS absorber was increased by increasing the sulfurization temperature, similar to the GD-OES results. The E_g of the CIGSS absorber layer fabricated with 570 °C, 580 °C, and 590 °C sulfurization temperatures are 1.15 eV, 1.17 eV, and 1.24 eV, respectively.

Fig. 2 The (a) GGI and (b) SSSe GD-OES profiles and (c) calculated E_g profiles of CIGSS absorber layer fabricated by various sulfurization temperatures.

Fig. 3 d(QE)/dλ curve as a function of wavelength in order to determine the E_g. Inset shows EQE raw data.

Fig. 4 shows cross-sectional SEM images of the CIGSS solar cells fabricated with 570 °C, 580 °C, and 590 °C sulfurization temperatures. The CIGSS grain sizes are important for device performance, as smaller grains may lead to both unfavorable hopping-based electron transport and stronger recombination at grain boundaries,^21–23 and also the grain size is related to Ga gradient profile.^23–25 In our samples, overall grain sizes of the CIGSS were slightly increased with increasing sulfurization temperature in addition to a small grain area and void region adjacent to the Mo bottom electrode significantly decreasing as the sulfurization temperature increased because higher Ga concentration generally tended to form a smaller grain region.²⁶ This change in small grain area and void region of CIGSS films did not substantially affect power conversion efficiency, but these have an effect on the process window because of the correlation of adhesion strength between CIGSS and Mo layer.

Fig. 4 Cross-sectional SEM images of the CIGSS cells fabricated with different sulfurization temperatures: (a) 570 °C, (b) 580 °C, and (c) 590 °C.

The photovoltaic performances of the devices obtained with three different sulfurization temperatures (570 °C, 580 °C, and 590 °C) were extracted from one-sun current–voltage measurements. The results are shown in Fig. 5 and the characteristic device parameters are summarized in Table 1. Generally, the J_sc decreases with increasing E_g and the V_oc increases with increasing E_g in photovoltaic devices. In our devices, the J_sc and V_oc depend on the absorber E_g apart from the V_oc of the device made at 590 °C sulfurization temperature as shown in Table 1. In addition, for the highest sulfurization temperature, the lowest efficiency was observed due to the large decrease of the FF. This result means that too much sulfur incorporated into the CIGSS absorber layer may lead to detrimental impacts on the FF because of deteriorated P/N heterojunction quality as mentioned above. Then, “Why was the V_oc decreased with 590 °C sulfurization temperature?” was a major question in this experiment.

Fig. 5 One-sun I–V curves of 900 cm² sized 64 series connected CIGSS device fabricated by different sulfurization temperatures. Black, red, and green curves are 570 °C, 580 °C, and 590 °C sulfurization temperatures, respectively.

Table 1 CIGSS device characteristics of a device fabricated by different sulfurization temperatures (570–590 °C). Listed are the device parameters: the efficiency (Eff.), open circuit voltage (V_oc), short circuit current (J_sc), series resistance (R_s), shunt resistance (R_sh), fill factor (FF), diode ideality factor (N-factor), and E_g

Temperature (°C)	Eff. (%)	Voc (V)	Jsc (mA cm^−2)	FF (%)	Rs (Ω cm^2)	Rsh (Ω cm^2)	N-factor	Band-gap by QE (eV)
570	15.10	0.646	33.15	70.53	1.35	1363	1.51	1.15
580	15.15	0.669	31.63	71.62	1.26	1225	1.50	1.17
590	12.84	0.662	28.78	67.43	1.58	850	1.83	1.24

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In order to analyze the abnormal behavior of the V_oc, the surface properties of the CIGSS absorber were investigated by KPFM, since the sulfurization temperature influences surface quality. Fig. 6(a)–(c) and (d)–(f) show the topography and surface potential images, respectively, of the CIGSS absorber measured by KPFM. In Fig. 6(d)–(f), the yellow and blue regions represent the positive potential value and negative potential, respectively. From the KPFM measurements, we can obtain the surface work function of the CIGSS absorber as follows. KPFM measures contact potential difference (V_CPD) between a conducting atomic force microscope tip and the sample. We measured the tip work function by using highly ordered pyrolytic graphite. The V_CPD between the tip and sample is defined as V_CPD = (ϕ_tip − ϕ_sample)/−e where ϕ_sample and ϕ_tip are the work functions of the sample and tip, respectively, and e is the electronic charge. Fig. 7 shows the surface work-function distributions in the CIGSS absorber layer processed at different sulfurization temperatures as calculated from the above equation. The CIGSS work function value is around 4.5–5.5 eV. As shown in Fig. 7, the work-function distributions become narrower with increasing sulfurization temperature. This means that the surface state in the CIGSS absorber treated at higher sulfurization temperature is more uniform than that of the CIGSS absorber treated at lower sulfurization temperature. Many work-function peaks mean that diverse phases having different compositional ratio might exist at the surface.

Fig. 6 (a)–(c) Topography images of the CIGSS absorber layer surface and (d)–(f) surface potential mapping images of the corresponding topographies.

Fig. 7 Surface work-function distributions in CIGSS absorber layer processed at different sulfurization temperatures: (a) 570 °C, (b) 580 °C, and (c) 590 °C.

In order to analyze the KPFM results more in detail, we have investigated bulk and surface crystallization by XRD and GIXRD, respectively. Fig. 8 presents the CuKα two-theta XRD patterns of the CIGSS absorber layer sulfurized at different temperatures. The overall-range XRD spectra of the CIGSS absorber are shown in Fig. 8(a). The preferred orientation for CIGSS samples was in (112) direction. The XRD intensity was higher than background at about 27° and a small peak can be seen at about 28°. This small peak indicates the existence of a separated Ga-rich CIGSS phase (dotted arrows in Fig. 8(a) and (b)). The CIGSS absorber film sulfurized at high temperature showed a decrease of Ga-rich CIGSS peak intensity, indicating that the CIGSS absorber layer showed improved depth compositional homogeneity and the formation of a high-quality chalcopyrite phase. However, a shoulder peak occurs at the high-angle side of the (112) and (220) reflections for the CIGSS sample, as indicated by solid arrows in Fig. 8(b) and (c); in addition, these shoulder peaks decreased with increasing sulfurization temperature as shown in Fig. 8(b) and (c). The shoulder peak may be caused by inhomogeneous sulfur composition between grains. Therefore, an increase of sulfurization temperature makes a more uniform sulfur composition between grains. The high sulfurization temperature also gives a more uniform work-function distribution as shown in Fig. 7(c). The uniform surface states such as narrow work-function distribution correspond with uniform sulfur composition and better crystallinity of the CIGSS thin films. Thus, the XRD results have something in common with the KPFM results.

Fig. 8 Conventional XRD spectra for CIGSS absorber layer on Mo-coated glass substrates obtained with different sulfurization temperatures. (a) The overall range of XRD pattern, and enlarged views around (b) (112) peak and (c) (220) peak of CIGSS absorber layer.

Fig. 9 presents GIXRD patterns of the CIGSS absorber on glass substrate fabricated by a basic sulfurization process. Using the GIXRD mode the X-rays penetrate the absorber only close to the surface. Therefore, the GIXRD data provide information from the CIGSS near-surface layers, while the conventional XRD data reveal little information about the film surface. The GIXRD scan was carried out with a grazing incidence angle of 0.2°. The difference in overall-range patterns between Fig. 8(a) and 9(a) is that the intensity of the Mo peak (about 40°) was greatly reduced due to the almost reflecting surface properties by using grazing incidence angle. Fig. 9(b) shows an enlarged part of the GIXRD pattern of the (220) peak of the CIGSS. As shown in Fig. 9(b), the shoulder peak was slightly decreased with increasing sulfurization temperature for a similar reason as mentioned for the conventional XRD result. Therefore, the surface state in the CIGSS absorber obtained with higher sulfurization temperature is also more uniform than that in the CIGSS absorber sulfurized at low temperature.

Fig. 9 GIXRD spectra of CIGSS sample prepared by the different sulfurization temperatures showing (a) the overall range and (b) enlarged view of the range around (220) peak of CIGSS absorber layer surface.

Even though the CIGSS absorber layers become more uniform with increasing sulfurization temperature in composition between grains as evident from the KPFM, XRD, and GIXRD results, we still cannot solve the abnormal V_oc behavior for the CIGSS device processed by 590 °C sulfurization. Therefore, we have investigated in detail the potential distributions at grain boundaries using KPFM. Generally, the device performances in chalcopyrite thin-film solar cells are closely connected to potential properties of a grain boundary.²⁷ Fig. 10 shows the statistical potential distribution of the CIGSS absorber surface focusing on grain boundaries. Histograms of potential distribution are indicated at the grain boundaries based on KPFM data for the grain boundary of each sample. According to the previous result of the local electrical properties, positive potential distributions at grain boundaries can improve the device performance because the surface energy band at grain boundaries is bent by the positive potential at grain boundaries. The bent energy band plays an important role in passivation at grain boundaries.^13,26,28 From the results, we compared the distribution ratio of the potential of each CIGSS thin film depending on the device performance, especially the V_oc. In Fig. 10, the CIGSS absorber layer fabricated at 580 °C sulfurization temperature shows 80% of positive surface potential ratio at grain boundaries, but the CIGSS processed at 590 °C sulfurization temperature shows only 65%. The lower positive potential barrier distribution reflects that for the CIGSS sulfurized at 590 °C recombination becomes increased at the grain boundaries, and, consequently, V_oc is decreased compared with the energy band-gap. In other words, it was verified to be possible to add more sulfur into the CIGSe absorber layer by adjusting the temperature of the sulfurization process but it is difficult to improve the device performance with high-sulfur-content CIGSS absorber. Although the uniformity of material composition among grains is better when the sulfurization temperature is increased and over the optimum point, the passivation characteristics at a grain boundary are degraded for too much sulfurization. Hence, the V_oc loss is more significant due to the recombination at a grain boundary than to composition uniformity. This also means that the existence of an upper limit of incorporated sulfur is a good indication to improve the passivation property at grain boundaries.

Fig. 10 Statistical characterization of surface potential focusing on grain boundaries based on the line profile results of many regions of grain boundaries of CIGSS samples fabricated by different sulfurization temperatures: (a) 570 °C, (b) 580 °C, and (c) 590 °C.

In conclusion, we have investigated the surface states of a CIGSS absorber layer measured by KPFM focusing on the potential distributions at a grain boundary and surface work-function distributions in order to optimize the sulfurization process in the CIGSS absorber layer fabricated by a two-step sputter and selenization/sulfurization method. An efficiency of 15.15% is achieved for a 900 cm² sized module obtained at 580 °C sulfurization temperature that is the optimized condition in our experiment. The sulfurization process at 580 °C improved the grain boundary passivation in the surface of the CIGSS absorber, and resulting in a decreased V_oc loss. The experimental results also show that the V_oc loss by recombination at a grain boundary is more dominant in comparison with the uniformity of material composition among grains. In addition, there is an upper limit of sulfur content in the CIGSS absorber layer due to the degradation of passivation quality at the grain boundaries. Consequently, the passivation quality of surface grain boundaries may depend on the sulfurization process, and this plays a crucial role in the final device performance.

Acknowledgements

This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) (grant no. 20119010100010) in 2011. This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government, Ministry of Trade, Industry and Energy (no. 20123010010130).

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