Handbook of Modern Coating Technologies

Spectroscopic ellipsometry application examples in coatings

In this part, the applications of SE to characterize various coatings, such as photovoltaic films, display coatings, protective coatings, films of biological molecules as well as films of two-dimensional (2D) materials are illustrated. The huge potential of SE in characterizing coatings and thus yielding insight into the coating's nature and formation mechanism is demonstrated (Fig. 2—12).

• Photovoltaic films

Photovoltaic cells are developed rapidly in the past decades because of their advantages of environmental-friendly and economical harvesting of solar energy. Most photovoltaic researches are focused on the effects of the compositions, microstructure as well as optical properties of photovoltaic thin films on the energy conversion efficiency. In these investiga­tions, the SE measurements were commonly carried out to obtain the fundamental charac­teristics of photovoltaic thin films fabricated by various technique, such as the optical properties, thickness and roughness, and even to understand the growth mechanism of the films. Cu₂ZnSn(S, Se)₄ (CZTSSe) is a prospective material to prepare the absorber layer in thin-film solar cells. Caballero et al. [84] prepared CZTSSe thin film by chemical vapor trans­port technique and used SE to research its bandgap engineering. They found that the funda­mental band gap E₀ was within the range of 1.59—1.94 eV when the Ge content was within the range of 0.1—0.5. SE measurements were also carried out by Hermann et al. [85,86] to extract the surface roughness and optical properties of large area Cu(In, Ga)Se2 (CIGS) thin films grown with sequential sputtering method. To deconvolute SE data, the Gaussian broad­ened polynomial superposition parametric relationship was introduced to describe the

Display coatings                                      Protective coatings

FIGURE 2-12 Typical application fields of SE for coatings characterization. SE, Spectroscopic ellipsometry.

optical properties of CIGS films. Based on the SE results, the film preparation was optimized and the photovoltaic efficiency was higher than 10%. McLaughlin and Pearce [87] used SE to investigate the influence of medium indium content on the properties of wurtzite indium
gallium nitride (In_xGa!__xN) thin films (0.38 < x < 0.68). They developed a Kramers—Kronig consistent parametric model to fit SE data and obtained the film thicknesses, absorption coefficients, and dielectric functions. For the tantalum oxide (TaO_x or Ta₂O₅) tunnel barrier with the thickness of 3 nm, Tyagi [88] revealed by using SE that the extinction coefficient of TaO_x was evidently larger than that of Ta₂O₅ and TaO_x is a promising photoelectric material. This was confirmed by the fact that TaO_x ultrathin film with 3 nm thickness absorbed about 12% of the energy of the incident light within the range of 400—1000 nm. Woo et al. [89] obtained the complex dielectric function of a lead halide perovskite (CH₃NH₃PbBr₃) quan­tum dots, which is helpful to fabricate effective optoelectronic devices. SE was also used to optimize the parameters (thickness, porosity, dielectric constants, and distribution of the ele­ments) of inhomogeneous layers of single-crystal silicon nanowires and Ag porous nanolayer for the better photoelectrical performance by Zharova et al. [90].

SE has been applied to study the influence of doping on the photovoltaic performance. For example, it was found that Mg doping into CdO could improve optical properties and stability of CdO films [91]. SE combined with Cauchy—Urbach model, which describes the N—X relationship very well, were used to fit the thicknesses and refractive indices of the films. This Mg doped CdO films had enhanced photovoltaic properties and were promising photovoltaic optoelectronic materials. Undoped and Cu doped CdS films, which were pre­pared by Kost et al. [92] using ultrasonic spray pyrolysis method, were investigated with SE and UV/vis spectrophotometer. Cauchy model was used for fitting the SE data and then the optical constants (n and k) and thicknesses of the films were determined. The structure and values about the band gap of the films were also obtained. An important conclusion of this work was that increase in Cu dosage would probably change the conductivity type of the films from n-type to p-type, which would be applied in optoelectronic devices in the future. Leem and Yu [93] studied the influence of thermal annealing on the properties of (Sb)-doped tin oxide (SnO₂) films with SE, X-ray diffraction (XRD) and SEM techniques. It was found that upon the increase in the relative void fraction, the refractive index and extinction coeffi­cient gradually decreased for the annealed films.

Organic photovoltaic thin films have been developed rapidly and SE is used to charac­terize the films. Liang et al. [94] prepared thin films of conjugated polymers and CdSe nanoparticles with layer-by-layer method and measured the thickness of films by using SE. It was found that the film thickness was linearly related to the number of layers. Tse et al. [95] reported the similar linear relationship existed for multilayer thin films of rhe­nium containing hyperbranched polymer and poly [2-(3-thienyl)ethoxy-4-butylsulfonate] (PTEBS). These reports showed that SE has been a powerful tool for organic photovoltaic thin studies.

Real-time SE has been a powerful tool to track and measure the dynamic evolution of photo­voltaic thin films. For example, Li et al. [96] studied the preparation, the changes of the struc­ture, and dielectric functions of sputtered CdTe, CdS, and CdTe_12XS_X thin films. They first found that the deposition temperature (T) was the most important preparation condition for CdTe and CdS, and the rf power level was the most important condition for the cosputtered CdTe_12XS_X alloys. Then they analyzed the interdiffusion at CdS/CdTe or CdTe/CdS heterojunctions according to the obtained dielectric functions є of CdTe₁__xS_x. Moreover, they also extracted the dynamic information on the nucleation, coalescence, and structural evolution of CdS and CdTe films base on the RTSE results [97]. In situ SE and GIXS were used by Want et al. [98] to provide insights into kinetics of poly(3-hexylthiophene) and [6,6]-phenyl C61- butyric acid methyl ester blend film formation process. Three stages in film drying were identi­fied: (1) rapid solvent-evaporation, (2) moderate solvent-evaporation and rapid crystallization, and (3) slow solvent-evaporation and slow crystallization. The results would be helpful for pre­paring efficient photovoltaic devices and RTSE exhibits its ability as a process-control technique.

• Display coatings

Flat panel display apparatus are much lighter and smaller than the cathode ray tube appara­tus, so that they have been widely used in computers, cell phones, and other electric devices. Organic light-emitting diode (OLED) technique is one of the strong competitors for the future flat panel display technique due to their outstanding nature such as flexibility, low power consumption, high luminance, and full color capability. To optimize the OLED devices, the optical characterization of organic thin films in OLEDs is required. As a nonde­structive, rapid, and efficient characterization technique, SE has been widely employed to extract optical constants of organic emitters, such as 1-methyl-1,2,3,4,5-pentaphenylsilole [99], tris(2-phenylpyridine) iridium (Ir(ppy)₃) [100], platinum octaethyl porphine (PtOEP) [101], N,N'-diphenyl-N,N-foi's(3-methyl-phenyl)-1,1^,biphenyl-4,4diamine                    [102], tris (8-

hydroxy) quinolato aluminum (Alq₃) [102,103], doped grapheme [104], Si(2,6-bis(benzimida- zol-2'-yl)pyridine)₂ (Si(bzimpy)₂) [105], 4,4'bis(9-carbazolyl)-1,1'biphenyl (CBP) and 2,2',2"- (1,3,5-benzinetriyl)-tris(1-phenly-1-H-benzimidazole) (TPBi) [106], and so on. Tsuboi et al. used a phase-modulated SE to study the optical constants of fac Ir(ppy)₃ layers [100] and PtOEP layers [101] in single-layer OLED devices and on quartz plates, respectively. The results showed that the optical properties of the Ir(ppy)₃ and PtOEP films as emitting layer in single-layer OLED devices were different from those of the thin films evaporated on quartz plates. Doping a few optical active molecules in emission layer is an alternative way to improve the optical device efficiency of OLED. Hartmann et al. [103] studied the doping effects of dicyanomethylene-4H-pyran (DCM) molecule on the optical indices of the Alq₃ emitting layer using SE. For the Alq₃ emitting layers which were doped with DCM molecules at different concentrations, their extinction coefficient (k) values corresponding to the absorption peak at around 0.55 pm increased with increasing in dopant concentration. As the comparison, the values of k corresponding to the peak located at 0.39 pm were almost not affected by the dopant concentration. Pfeiffer et al. [106] measured the refractive indices and extinction coefficients of three organic semiconductors [CBP, Ir(ppy)₃, and TPBi] in a transparent OLED device [including glass, indium tin oxide (ITO), organic semiconductors, and cathode] with SE, optical transmittance, and reflectance techniques. Based on the results, they optimized the structure of the diode stack and obtained the OLEDs with an opti­cal transmittance which was enhanced from 47% to 65%.

Thin film transistor liquid crystal display (TFT-LCD) is another kind of dominant flat panel display technology. In the TFT-LCD panels, the red, green, and blue (RGB) color filter coating is one of the main components for generating the color images. Lee et al. [107] applied SE to extract the effective thickness and refractive index of the RGB color filter coat­ing. Considering the scattering and absorption properties of the layers as well as the rough­ness of the surface, they built an effective layer—included model which could be used to determine the (n, k, and d) values of the RGB color filter coatings simultaneously. SE also could be used to characterize the conductors in display devices. For example, ITO thin film is widely used as the transparent conductor in display devices. Bartella et al. [108] introduced SE to provide insight into the relationship between the optical constants n(A) and k(A) of ITO coating, which were obtained by SE, and the stoichiometry, film density, surface rough­ness, and crystalline structure. It was demonstrated that SE could serve as an effective quality-control tool for display manufacturing.

Dielectric protection layer is also an important component for display devices because it can improve the firing voltage, delay time, discharge efficiency and lifetime of display devices. MgO film is the most common protection layer. To improve the discharge perfor­mance, Lee et al. [109] used an ion plating technique to prepare hydrogen—doped MgO thin films. They then monitored the changes of optical constants of the doped MgO films with the hydrogen flow rates and found that the most optimal hydrogen flow rate was 50 sccm, which corresponded to the lowest value of refractive index of the films.

Lyum et al. [110] used transmission ellipsometry to characterize the ultrasmall optical anisotropy of a rubbed polyimide film for liquid crystal alignment. The ordinary refractive index (n_o), the extraordinary refractive index (n_e), the thickness of rubbed polyimide film (d) were found to be 1.732, 1.743, and 38.8 nm, respectively. These optical properties of the film had influence on the color gamut and response time of the LCD directly, so the precise values of the optical constants were important for the LCD manufacture. Similarly Son et al. [111] compared the optical anisotropy of the ion-beam—treated polyimide layers with that of rubbed layers by using SE. The results indicated that the anisotropy of the former was much larger than that of the later because the former modified the whole polyimide layer while the latter only changed the polyimide surfaces along the rubbing direction.