Investigation of Mo films deposited on high temperature polyimide substrate by magnetron sputtering for flexible CIGS thin film solar cells application

CIGS solar cell is one of the most efficient photovoltaic technologies to utilize solar energy, which has the record efficiency of 22.9% reported by Solar Frontier on glass substrate.^1,2 CIGS solar cells on flexible polyimide (PI) substrate provide significant advantages over conventional, rigid solar cells such as lightness, flexibility, high power density and power to weight ratio, etc.^3–6 However, the record cell and module efficiencies of CIGS solar cells on flexible PI substratesare much lower than those of the CIGS solar cells on rigid glass substrate.⁷ The challenge existing in fabricating higher efficiency CIGS on PI substrate is that the PI does have thermal stability good enough to withstand the high temperature CIGS fabrication process.^8–10 The high efficiency CIGS solar cells are fabricated at substrate temperatures around 550 °C. However, the glass transition temperature (T_g) of the PI films is below 500 °C.^11,12 In addition, the coefficient of thermal expansion (CTE) of the PI films is much higher than that of the Mo back contact of the CIGS solar cells, leading to a poor adhesion of the CIGS solar cells on the PI substrate.^6,9,13–15

In this study, high temperature PI films with a high glass decomposition temperature of 595 °C were obtained by two-step solution process. The high temperature PI films coated glass were employed as substrate to deposited Mo films by magnetron sputtering. The effects of the deposition pressure on the properties of the Mo films are investigated to obtained suitable back contact for fabricating high efficiency CIGS solar cell on flexible PI substrate.

The preparation of polyimide was carried out through a two-step procedure, involving the polycondensation using self-made 2,2′-di(4-aminophenyl)-6,6′-bi-1,3-benzoxazole (BAPBBOA, purity>99.8%) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) monomers and then imidization process with the procedure as shown in Scheme 1. In a typical experiment, BAPBBOA (4.1816 g, 0.01mol) and 40 mL of DMAc were added into a 100 mL three-neck round-bottom flask with a mechanical stirrer and a nitrogen inlet. After the diamine completely dissolved, BPDA (2.9422 g, 0.01mol) and additional 28.5 mL of DMAc were added into the solution with a stirring under nitrogen atmosphere. The mixture was stirred at room temperature for 24 h under a nitrogen atmosphere to give a highly viscous polymeric solution with a concentration of 10% poly (amic acid) (PAA) in DAMc. After filtration of above solution with a funnel of 30–50 μm pore size, the solution was cast onto a glass substrate and followed by air-drying for 2 h at 60°C. The resultant gel film was cured by heating at a rate of 2.5 °C/min from room temperature to 200 °C and holding at 200 °C for 0.5 h, then heating at a rate of 5 °C/min to 490 °C and holding for 20 minutes to obtain the completely imidized poly(BPDA-BAPBBOA) polyimide film (BBAPBBOA PI, Scheme 1). Fourier transform infrared (FTIR) spectra of BAPBBOA and BBAPBBOA PI were captured on a Nicolet 8700 FTIR spectrometer at the range of 4,000–400 cm^-1 by averaging 32 scans. Thermo-gravimetric analyses (TGA) of the film samples were performed on a thermal analyzer (TA Q5000 IR) from 50 to 900°Cwith a heating rate of 10 °C/min under N₂ atmosphere.

Mo films were deposited on PI coated glass by DC magnetron sputtering without intentionally heating substrate. The background vacuum of the sputtering chamber was below 2 × 10^-4 Pa. The size of Mo target used in this study was 254 mm × 102 mm × 4 mm with a purity of 99.97%. The flow of working gas Ar was kept at a constant of 20 cm³/min (20 sccm) during the deposition process controlled by a mass flow controller. The pressure of the chamber was tuned by adjusting the valve of molecular pump. The thickness of the Mo films was measured by X-ray fluorescence (XRF) and cross-sectional SEM. The sheet resistance of the Mo films was measured by four point probe. The resistivity of the Mo film was obtained by multiplying its sheet resistance by its thickness. The crystallinity of the deposited Mo films was analyzed by X-ray diffraction (XRD, Rigaku MiniFlex 600). The surface morphology of the Mo films was measured by scanning electron microscopy (SEM, TESCAN) and atomic force microscopy (AFM). The deposition parameters of the Mo on PI coated glass by magnetron sputtering are summarized in Table I.

The FT-IR spectra of BAPBBOA and BBAPBBOA PI are shown in Fig. 1. The characteristic peaks of symmetric C=O stretching and asymmetric C=O stretching of the imide group are visible at 1776 and 1723 cm^-1. The bending vibration of C=O appears at 725 cm^-1, and the assignment of the stretching of the imide ring is at 1362 cm^-1. The NH₂-featured peak in the diamine BAPBBOA appears at 3321cm^-1 and the NH₂-featured peak in the BBAPBBOA PI films disappeared completely, which reveal the PAA intermediate was cyclized completely.

The thermal stability of the PI films was studied by thermogravimetric analysis (TGA). The PI film coated on 10 cm × 10 cm size glass substrate is shown in Fig. 2(a). The TGA measurement was conducted as a heating rate of 20 °C/min in N₂ atmosphere. As we can see in Fig. 2(b), the decomposition temperature (T_d) and 5% weight loss temperature (T_d,5) of the BBAPBBOA PI were 551 and 596 °C, respectively, which were 57 °C and 19 °C higher than that of the Upilex-S, respectively. The fantastic thermal stability of the BBAPBBOA PI films was attributed to the introduction of the rigid and stable biphenylbenzoxazolyl groups from the BBAPBBOA monomer and biphenyl groups from the dianhydride monomer into the main chain of BBAPBBOA PI. So obtained BBAPBBOA PI may meet the requirements for flexible solar cell substrate materials.

The growth rate and the resistivity of the Mo films deposited at different working pressure are shown in Fig. 3. We can see clearly from this figure that the growth rate of the Mo films reduced gradually from 35.9 to 26.3 nm/min and the resistivity of the Mo films increasing significantly from 67.0 to 727.9 µΩcm with deposition pressure increasing from 0.3 to 2.5 Pa. These findings can be explained by the fact that the collision between the ionized Ar⁺ ions and the Ar atoms in the chamber increased with increasing pressure, thus the energy of the Ar⁺ reaching to the surface of the target reduced gradually, which means that less target materials mwas knockedout from the target, resulting in slower growth rate. In addition, the energy transferring from the Ar⁺ to the Mo particles deposited on the PI substrate reduced, resulting in reduced diffusion length of the Mo atoms and poorer crystallinity of the Mo film, and thus higher resistivity. The resistivity of the Mo films deposited on flexible PI substrate in this study is comparable to that of Mo films deposited on glass substrate reported in literature.^16,17

XRD was employed to investigate the effects of the deposition pressure on crystallinity of the Mo films. The XRD patterns of the Mo films are shown in Fig. 4. As we can see from this picture, all the Mo films showed preferred (110) orientation, in addition the intensity of the Mo (112) peak was much weaker that of the Mo (110) peak, indicating the good crystallinity of Mo films deposited on PI films. The grain size of the Mo films was estimated by Scherer Equation:

where K is a constant of 0.89, λ is the wavelength of the x-ray of 0.15406 nm, B is full width at half maximum of Mo (110) peak. The FWHM of the Mo (110) peak and the calculated grain size of the Mo films are shown in Fig. 4(b). From this figure, we can see that the FWHM of the Mo (110) peak increased gradually while the grain size of the Mo films reduced gradually with increasing deposition pressure. This is attributed to the larger energy loss of the Mo particles upon collisions with the Ar atom during the deposition process and thus shorter diffusion length of the Mo particles deposited on PI substrate at high deposited pressure, resulting in small grains and poorer crystallinity.

The morphology of the Mo films deposited on PI films was investigated by AFM and SEM. The AFM images and the surface roughness (RMS, root mean square) of the Mo films are shown in Fig. 5. As we can see from Fig. 5(g), the RMS of the PI film was 2.27 nm showing a smooth surface. However, the surface roughness of the Mo increased gradually from 2.09 to 7.70 nm with increasing deposition pressure. The surface roughness of the Mo films in this study was much smaller than that of Mo films reported in literatures,^18,19 which has the potential to obtain high efficiency CIGS solar cells on the Mo coated PI substrate. The smoother surface of the Mo films deposited at lower pressure can be explained by the fact that the Mo particle deposited at lower pressure on the PI substrate had higher energy and diffusion length leading to a more uniform and flatter surface; while the Mo films deposited at higher pressure had lower energy and shorter diffusion length, leading to a rougher surface. The SEM images of the Mo films deposited on PI substrate at different pressures are shown in Fig. 6. As illustrated in Fig. 6, all the Mo films showed a relatively smooth surface and there were no cracks and pinholes observed at the surface. The thickness of the Mo films measured by SEM vary from 670 to 502 nm, which were nearly same as them were measured by XRF.

In this study, BBAPBBOA PI films were prepared by a two-step procedure. The decomposition temperature (T_d) of the BBAPBBOA PI is 551 °C and 57 °C higher than that of the Upilex-S, which can meet the requirements of flexible substrate in CIGS solar cell. The fantastic thermal stability of the BBAPBBOA PI films is the result of the introduction of the rigid and stable biphenylbenzoxazolyl group into the main chain of BBAPBBOA PI structure. In addition, Mo films were deposited on the PI films by magnetron sputtering at different pressure varying from 0.3 to 2.5 Pa. We find that the growth rate of the Mo films reduces gradually and the sheet resistance of the Mo films increases significantly with increasing deposition pressure. All the Mo films show a preferred (110) orientation, that the surface morphologies of the Mo films were investigated by AFM and SEM, all the Mo films showed a relatively smooth surface and there were no cracks and pinholes observed at the surface. In conclusion, the Mo coated high temperature PI films have the potential for substrate application in high efficiency flexible CIGS solar cell fabrication.

The work was supported by the Science and Technology Innovation Major Project of Guangdong Provincial Education Department no. 2017GKTSCX080, the Major Social Welfare Project of Zhongshan Municipality No. 2018B1020, the National Key R&D Program of China Grant nos. 2018YFB1500201 and 2018YFB1500205, the National Natural Science Foundation of China under Grant no. 61804159, 61574157 and 61774164, the Shenzhen Basic Research Grant no. JCYJ20150529143500956, JCYJ20150925163313898 and JCYJ20160331193134437.