Sn02 Implementation in Solar Cells
Sn02 has been widely used in the solar cell field, being employed as a transparent conductive layer usually doped with Fluor as fluorine doped tin oxide or alloyed with indium oxide as ITO. Even when the solar cells industry is still well dominated by silicon-based technologies, hybrid solar cells, combining organic and inorganic materials, seem promising. In this field. Sn02 has been used as a passivation element of the silicon surface embedded in an organic host as the polymer PEDOT:PSS, which by itself possesses high conductivity and transparency in the visible range (Garcia-Tecedor, Maestre, Cremades, & Piqueras, 2017). Furthermore, Sn02 also has an important role in the manufacture of transparent electrodes in photovoltaic devices of amorphous silicon (a-Si), CdTe and Graztel cells (dye sensitized solar cells, DSSC) and as an electron selective layer in perovskite solar cells where it improves the carrier extraction (Bolzan, Fong, Kennedy, & Howard 1997; KiIiq, & Zunger, 2002; Gopel, 1985; Hagen. Lambrich. & Lagois, 1983; Zhu, Yang, et ah. 2016; Fortunato, Ginley, Hosono, & Paine, 2007; Madou, & Morrison, 2012; Klein et ah, 2010). Some of these promising Sn02-based solar cells devices consist in hybrid materials, as the patent from the Schenzhen Xianjin Clean Power Tech. Research Co (CN109904331(A)) in which Sn02 and rGO composites in form of thin films, with a 30-50 nm thickness, are produced and evaluated as electron transmission layer perovskites. Some other approaches are focused on an improvement of the passivation behavior of Si substrates by using hybrid organic—inorganic films formed by PEDOT:PSS and Sn02 or Ti02 nanoparticles, as described in the patent (patent published as ES2650213B2; EP3482426A1; W02018010935A1; N020161150A1; US2019319161A1). In that case charge carrier lifetimes of about 275 gs were achieved by adding a low concentration of Sn02 nanoparticles to the conductive polymer, involving low- cost processing and the potential implementation in flexible substrates.
Nowadays, Sn02 has been also employed as a hole mirror layer in heterojunction architectures, mainly with BiV04, for solar water splitting, improving notably the performance of the photoanodes (Garcia-Tecedor, Cardenas-Morcoso, Fernandez-Climent, & Gimenez, 2019; Liang, Tsubota, Mooij, & van de Krol. 2011).
5.3.3 Gas Sensors with Sn02 as Active Material
Detection of gases can be mediated by tin oxide, where the surface reaction between the oxide surface and adsorbed oxygen species involves charge transfer, which modifies the surface resistance depleting the oxide surface (Das, & Jayaraman, 2014), as described in section 5.2.2. In the last decades, Sn02 has played an important role in the functionalization of applications based on gas sensing where the charge transfer of the Sn02 is modified in the presence of different atmospheres (Gopel, 1985), presenting high sensitivity to the processes of chemisorption. Both, its thermal stability at low temperature (Hagen. Lambrich, Lagois, 1983) and easy ceramic fabrication have made Sn02 a very cost- and performance-effective material in the sensing of gases such as H2. H20, 02, CO, and C02 (Choi et al., 2008; Sber- veglieri et ah, 1993; Zhang et ah, 2014). Actually, Sn02 is one of the most employed materials for gas sensing based on the resistance or conductance change due to gas adsorption (Das, & Jayaraman. 2014). The use of Sn02 nanostructures allows achieving better sur- face-to-volume ratio and enhanced performance; hence the use of gas sensor based on Sn02 nanomaterials has been greatly increased. The control of the size, morphology, doping, and defect distribution can tailor the electrical and chemical properties of the Sn02 leading to variable selectivity and sensitivity to different gases. Gas sensors based on Sn02 have been developed in the form of thin sheets, which exhibit good sensitivity but low response time (Choi et al., 2008). However, by reducing the size of the active materials by using morphologies such as nanoparticles, nanowires, nanotubes, and nanobelts, it has been possible to significantly increase their response in resistive sensors (Leite, Weber, Longo, & Varela, 2000; Ying, Wan, Song, & Feng, 2004).Many different approaches have been developed to increase Sn02 sensing abilities, based on improving the following two main aspects: sensing sensibility (detection limit) and recovery time. One is the addition of noble metals (Au, Pt), which are known to enhance the rate of response (Lin, Li, Chen, & Fu, 2017; Moseley, 2017), as catalytic properties of metals lower the temperature range of gas response due to the formation of a Schottky barrier on the oxide surface. Also, «-type/ «-type junctions have been used as gas sensors, as the combination of Sn02 with Ti02 or ZnO. On the other hand, the addition of dopants could inhibit the growth of nanoparticles while controlling conveniently the carrier concentrations, thus enhancing sensing abilities, as for Ni-doped Sn02 (Lin et al., 2017). On the other hand, composites of tin oxide had been also used in gas sensing (Joshi et al., 2018) mainly because of its possibility to work under room temperature, lowering the operation temperature with improvements in sensibility and recent focus has been aiming in this direction (Joshi et al., 2018). As an example, Sn02 quantum wire/rGO nanocomposites have been reported as selective gas sensors for H2S, with remarkable sensitivity (Song et al., 2016), although still exhibiting long recovery times and high cost.
Great efforts have been devoted to patent continuous improvements in the sensitivity, selectivity, and miniaturization of Sn02-based sensor devices with improved versatility. As an example, researches from the University of Korea recently patented an N02 sensing device usable at room temperature that consists of a multi-array chip based on 3D laminated structures which comprised Sn02 nanoparticles and carbon nanotubes (Patent KR20180072980A).
5.3.4 Optical Applications of Low-Dimensional Sn02
In the present section, a less exploited role of Sn02 in waveguides, optical resonators, and white luminescence devices will also be presented. Microstructures of different TCOs as ZnO, I n203, or Ga203 have been reported as optical resonators, with promising applicability in sensing, and lab-on-a-chip devices (Bartolome, et al., 2013; Huang et al., 2010; Lopez, Nogales, Mendez, & Piqueras, 2012). However, it is only recently that Sn02 microstructures have been considered as optical resonators, which can be further exploited in future devices. The optical resonances observed in microcavities with regular sections are frequently attributed to FP or WGM (Born, & Wolf, 1980). FP resonances involve reflections between opposite flat ends of the cavity, while WG modes involved total internal reflections around the inner faces of the cavity. Each mode exhibits different properties such as the small mode volume for WGM presenting higher light confinement or the easy tuning of the resonance frequency in FP resonators; hence, mode selection could be a determining factor for specific applications. Microstructures with flat and smooth surfaces are crucial not only for the generation of optical resonances, but also, as it will be explained further, for achieving waveguiding behavior. Obtaining optical resonances and waveguiding behavior in the same material could bring up the possibility of achieving optical microsystems with a monolithic architecture, using the same material as light input element and optical microcavity. Therefore, a high control of the synthesis method used for the growth of the microstructures is required, as well as the ability of engineering the resonant modes through size, morphology, and doping. In addition, the quality of the cavities as optical resonators can be studied through the estimation of the finesse (F) and quality (Q) factors, which provide information about the resonator losses and stored photon energy, respectively. The behavior as optical resonators and waveguides of different undoped and Cr-doped Sn02 microstructures as tubes, rods, and plates was recently reported (Garcia-Tecedor et al., 2017).
Figure 5.10 shows an Sn02 microtube in which different optical resonances coexist. As explained before, the optical resonances depend on the morphology of the optical cavity. In this specific case, simultaneous FP and WGM resonances were detected due to the morphology of the tube, which is schematized in Figure 5.11. In that work, different morphologies were analyzed in order to tune the optical resonances.
In addition, the possible application of undoped and Cr-doped Sn02 elongated microstructures with different morphologies were also reported as waveguides by Garcia-Tecedor et al. (2017). These microstructures were illuminated with a laser by one end and a bright light signal could be observed at the opposite end. Also the light can be guided toward both ends of the tube when illuminating at its center. This phenomenon confirms the wave- guiding behavior with few losses along these structures. In the case of the microtubes, the light was guided along the lateral faces, therefore a red frame-feature was observed at the opposite end, as shown in Figure 5.12a. However, in the microrods, an intense red spot was found at the apex, marked with an arrow in Figure 5.12b, at hundreds of microns from the incidence of the laser. This light-guide behavior was observed even for the longest rods, with lengths in the range of millimeters. For the microplate, not only the final ending of the structure, but also the surrounding area was illuminated (Figure 5.12c), as a difference with the microrod. No scattered light can be appreciated along the structures, which is an indirect indication of low optical losses. Similar waveguiding behavior has been reported for microstructures of other semiconducting oxides as In203 and Ga203 (Bartolome et al., 2013; Lopez et al., 2012). It is important to point out that the guided light along the different microstructures is the laser itself, not the laser-induced luminescence of the material.
Among other applications, an increasing interest in the luminescent properties of GO in combination with TCOs such as ZnO (Williams, & Kamat, 2009) or transition metals (Cu) (Udayabhaskar et al., 2017) has recently emerged. Nevertheless, the luminescent properties of GO-Sn02 nanoparticles have been scarcely studied and even lessen the possibility of synergetic effects between the Sn02 and the GO matrix (Del Prado et al., 2019). These properties can be studied by means of luminescence techniques to get a deeper insight into the different processes involved as the charge transfer between the GO matrix and the nanoparticles. Even more, the luminescence of GO with metal oxides has been a motivation of study in the last decade (Chien et al., 2012; Cuong et al., 2010; Mei et al., 2010) in different fields with biomedical purposes or to develop optoelectronic devices (Eda et al. 2008; Mei et al., 2010). Nevertheless, the studies reported until the moment, related with GO and inorganic materials (Shen et al., 2011; Zhou, Zhu, Yang, & Li, 2011; Zou, Zhu, Sun. & Wang, 2011) are still scarce (Williams, & Kamat, 2009). In the majority of the cases, the luminescence has been explained applying different model or theories attending to the active emission as originating in the GO matrix. Usually, as a function of the fabrication processes and final components of the samples, isolated emissions in the near infrared range (Chien et al., 2012; Jiang et al., 2015), or alternatively in the yellow-green region (Jiang et al., 2015; Stengl et al., 2013) or in the blue or near-UV region (Eda et al., 2010; Jiang et al., 2015; Stengl et al., 2013) have been reported. The origin of these bands has been commonly associated with the aperture of a bandgap due to the functionalization of GO by different functional groups (Luo, Vora, Mele, Johnson. & Kikkawa, 2009; Mathkar et al., 2012), to the disorder of the GO containing the sp2 and sp3
FIGURE 5.10 (a) SEM image of an undoped Sn02 microtube, (b) PL spectrum showing optical resonances. (c) and (d) Amplified regions of the PL spectrum in (b) showing different resonant modes, where (d) corresponds to the marked region with a red rectangle in (c). (e) Linear fit ДЛ versus X2 corresponding to an FP mode, (f) Resonance wavelength X as a function of the mode number N for the ТЕ (triangular symbols) and TM (square symbols) polarizations corresponding to a WG mode.
Figures reprinted from Garcia-Tcccdor ct al. (2017). Reproduced with permission from IOP Publishing.
FIGURE 5.11 (a) Confocal optical image of an undoped SnC)2 microtube, (b) Scheme of the FP (red
lines) and WGM (blue lines) optical resonances taking place in the tube.
domains (Chien et al., 2012), or alternatively to some other defects that can generate new energy bands in the bandgap in graphene like related material.
Lately, Del Prado et al. (2019) reported the wide band emission in the visible range of different composites with a matrix of GO and nanoparticles of Sn02 as fillers. The composites were synthe- tized by different chemical routes or by blending the counterparts including Sn02 nanoparticles obtained by Liquid-Mix, Hydrolysis, and Hydrothermal methods. In that work, the authors show a contribution of four bands to the total emission of the PL spectra of the samples, with the main peak centered around 2.25 eV, whereas a difference in the total contribution of the bands to the PL spectra could be observed as a function of the fabrication route of the composites. Moreover, the authors reported a near-white luminescence after the integration of the PL spectra and their transformation to chromaticity coordinates (see Figure 5.13), opening the opportunity to these materials in the field of science and engineering for optoelectronic devices. Recently, the fabrication of Sn02-Li2Sn03 branched micro- and nanostructures with white luminescence was also reported (Garcia-Tecedor et al., 2019). In that work, the authors reported the bright white luminescence of the grown micro- and nanostructures without the use of any phosphors or complex multi-quantum wells, which are the two traditional approaches to obtain white light emission.
