Transparent Conductive Oxide Electrode Research Articles

Perovskite Solar Cells Go Bifacial-Mutual Benefits for Efficiency and Durability.

Bifacial solar cells hold the potential to achieve a higher power output per unit area than conventional monofacial devices without significantly increasing manufacturing costs. However, efficient bifacial designs are challenging to implement in inorganic thin-film solar cells because of their short carrier lifetimes and high rear surface recombination. The emergence of perovskite photovoltaic (PV) technology creates a golden opportunity to realize efficient bifacial thin-film solar cells, owing to their outstanding optoelectronic properties and unique features of device physics. More importantly, transparent conducting oxide electrodes can prevent electrode corrosion by halide ions, mitigating one major instability issue of the perovskite devices. Here, the theory of bifacial PV devices is summarized and the advantages of bifacial perovskite solar cells, such as high power output, enhanced device durability, and low economic and environmental costs, are reviewed. The limitations and challenges for bifacial perovskite solar cells are also discussed. Finally, the awareness of bifacial solar cells as a feasible commercialization pathway of perovskite PV for mainstream solar power generation and building-integrated PV is advocated and future research directions are suggested.

Boltzmann conductivity approach for charge transport in spray-deposited transparent Ta-doped SnO2 thin films

Transparent conducting oxide (TCO) thin films should possess good resistance stability with temperature, apart from high figure of merit, to be suitable as electrodes for several potential optoelectronic device applications. In most of the metal oxide systems, an increase in the electrical conductivity with temperature indicates semiconducting behavior up to a certain critical temperature; and above this critical temperature, metal-like behavior is witnessed. Theoretical and experimental evidence for such change in charge transport behavior is crucial for further development and applicability of these TCO systems. In the present work, temperature-dependent electrical properties of spray-pyrolyzed Ta-doped SnO2 (TTO) thin films are explored for their resistance behavior in the high temperature regime. In order to justify the experimentally determined electrical conductivity of the TTO films, the values are correlated with theoretically estimated electrical parameters by adopting Boltzmann conductivity approach. The structural, chemical, morphological, optical and electrical properties of the TTO films indicate the optimal Ta dopant concentration to be 4 wt%. The film deposited with 4 wt% Ta dopant exhibits sheet resistance of 17.96 Ω/□, resistivity of 4.36 × 10−4 Ω cm, transmittance ≅ 85% (at λ = 550 nm) and band gap of 4.01 eV, indicating its potential to serve as wide band gap TCO electrode in several optoelectronic applications.

Experimental study and theoretical modeling of coating-speed-dependent optical properties of TiO2-graphene-oxide thin films

Nanostructured titanium dioxide–graphene oxide (TiO2-GO) thin films have been successfully synthesized and deposited on glass substrates using spin coating technique. TiO2-GO nanocomposites were prepared using Titanium Tetra Isopropoxide (TIP) and Graphene Oxide (GO) nanosheets. The influences of spin coating deposition speed on the structural, morphological, and optical features of the resulting TiO2-GO samples have been investigated. X-ray diffraction (XRD) patterns showed an amorphous structure of all deposited films. Scanning electron microscopy (SEM) images demonstrated that these films are homogeneous and all free from cracking over the entire surface of the sample, regardless of the number of rotations used. Atomic force microscopy (AFM) topography images reveal that all samples are smooth with very small surface roughness. Optical data obtained from UV–Vis spectrophotometer measurement together with theoretical modeling using Tauc–Lorentz (TL) oscillator model showed that all films display high transmittance level around 85% from the visible to the near-infrared region. It is also shown that an increase in spin coating speed results in a decrease in the film thickness and the optical band gap. The observed optical features reveal that TiO2-GO thin films are promising candidate that can be used as transparent conducting oxide electrode in solar cells.

Formation and functionalization of Ge-nanoparticles in ZnO

Semiconductor nanocrystals are often proposed as a viable route to improve solar energy conversion in photovoltaics and photoelectrochemical systems. Embedding the nanocrystals in, e.g. a transparent and conducting electrode of a solar cell will promote the photon absorption and subsequent transfer of the generated charge carriers from the nanocrystal, and thereby enhance the function of the electrode. This can be accomplished by embedding a semiconducting nanocrystal with a small bandgap in a transparent conducting oxide (TCO), which is commonly utilized as electrode in new generation solar cells. Here, we demonstrate the incorporation, formation, and functionalization of germanium (Ge) nanocrystals in zinc oxide utilizing ion implantation, where post implantation annealing at 800 °C results in diamond cubic Ge nanocrystals with sizes between 2 and 20 nm. Photoluminecence spectra show a distinct emission around 0.7 eV arising from the Ge nanocrystals, and with additional emission features up to 1.15 eV due to quantum confinement, demonstrating a novel functionalization and tunability of the TCO electrode.

Effects of Insertion of Ag Mid-Layers on Laser Direct Ablation of Transparent Conductive ITO/Ag/ITO Multilayers: Role of Effective Absorption and Focusing of Photothermal Energy.

From the viewpoint of the device performance, the fabrication and patterning of oxide–metal–oxide (OMO) multilayers (MLs) as transparent conductive oxide electrodes with a high figure of merit have been extensively investigated for diverse optoelectronic and energy device applications, although the issues of their general concerns about possible shortcomings, such as a more complicated fabrication process with increasing cost, still remain. However, the underlying mechanism by which a thin metal mid-layer affects the overall performance of prepatterned OMO ML electrodes has not been fully elucidated. In this study, indium tin oxide (ITO)/silver (Ag)/ITO MLs are fabricated using an in-line sputtering method for different Ag thicknesses on glass substrates. Subsequently, a Q-switched diode-pumped neodymium-doped yttrium vanadate (Nd:YVO4, λ = 1064 nm) laser is employed for the direct ablation of the ITO/Ag/ITO ML films to pattern ITO/Ag/ITO ML electrodes. Analysis of the laser-patterned results indicate that the ITO/Ag/ITO ML films exhibit wider ablation widths and lower ablation thresholds than ITO single layer (SL) films. However, the dependence of Ag thickness on the laser patterning results of the ITO/Ag/ITO MLs is not observed, despite the difference in their absorption coefficients. The results show that the laser direct patterning of ITO/Ag/ITO MLs is primarily affected by rapid thermal heating, melting, and vaporization of the inserted Ag mid-layer, which has considerably higher thermal conductivity and absorption coefficients than the ITO layers. Simulation reveals the importance of the Ag mid-layer in the effective absorption and focusing of photothermal energy, thereby supporting the experimental observations. The laser-patterned ITO/Ag/ITO ML electrodes indicate a comparable optical transmittance, a higher electrical current density, and a lower resistance compared with the ITO SL electrode.

Facile synthesis of binder free ZnO and its Indium, Tin doped materials for efficient dye sensitized solar cells

Research based on ZnO as a potential candidate in solar cells continues to draw greater attention as it provides an interlink between nanoparticles and improves the efficiency of electron transport towards the transparent conducting oxide electrode by reducing the recombination rate with holes. In the present investigation, ZnO paste is prepared using the solid-state synthesis technique. The preparative protocol was optimized by a screening of various precursor materials to get a smooth and viscous paste. The binder-free protocol is the key to successful paste preparation. The preliminary approach is the preparation of the ZnO paste mixture and its deposition on FTO substrate by doctor blade technique using a binder. The paste of ZnO with Indium, Tin doped paste was utilized without using the additional binder. Device tests of experimental DSSCs were performed with dye D149. The overall power conversion efficiency of the optimized ZnO-based DSSC reaches 1.16% and increases to 4.26% after the Sn doped ZnO. Indium doped ZnO DSSC shows 3.41% power conversion efficiency.

Simultaneous increases in electrical conductivity and work function of ionic liquid treated PEDOT:PSS: In-depth investigation and thermoelectric application

The electrical conductivity of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) should be at least comparable to that of transparent conductive oxide electrode for the practical implementation in the electronics industry. In this report, a clever post-treatment method for enhancing both electrical property and work function of the PEDOT:PSS film is studied for application in electronics requiring high work-function organic conducting materials. An ionic liquid that has strong electron-withdrawing group, bis(trifluoromethane)sulfonamide (TFSI), was applied to the PEDOT:PSS film during post-treatment. TFSI molecules were distributed throughout the resulting PEDOT:PSS film, which allowed the film to increase its electrical conductivity and work function, as confirmed by photoemission spectroscopy. The enhancement of the work function after TFSI treatment led to low hole injection barriers at various organic semiconductor/PEDOT:PSS interfaces. Moreover, the TFSI treatment was also found to be effective in changing the electronic properties of a composite of carbon nanotubes and PEDOT:PSS (denoted as MWPE). Finally, the effects of TFSI treatment on the thermoelectronic performance of the PEDOT:PSS and MWPE films were discussed, suggesting potential applicability of TFSI-treated PEDOT:PSS in organic thermoelectrics without inorganic fillers.

Thermal and Damp Heat Stability of High‐Mobility In2O3‐Based Transparent Conducting Films Fabricated at Low Process Temperatures

High electron mobility of transparent conductive oxide (TCO) facilitates high conductivity at a moderate carrier density, resulting in an extension of the degree of transparency from visible to near‐infrared due to less free carrier absorption. These broadband TCO electrodes provide opportunities to improve the performance of optoelectronic devices. Herein, the thermal and damp heat stability characteristics of TCO films are described. It is found that the difference in the crystal growth method has a significant influence on the initial electrical properties and stability characteristics. Polycrystalline In2O3:Me (Me: W, Ce) films deposited at 200 °C exhibit a relatively high mobility of 70–80 cm2 V−1 s−1, are stable after a damp heat test, and improve the mobility of 110–140 cm2 V−1 s−1 after postannealing at temperatures greater than the deposition temperature. Conversely, solid‐phase crystallized In2O3:Me,H films prepared by postannealing amorphous films exhibit an extremely high mobility of 100–160 cm2 V−1 s−1; however, they exhibit reduced stability characteristics after the damp heat test and high‐temperature annealing process compared to the polycrystalline films. The deterioration of the electrical properties increases as the hydrogen content in the films increases. The results suggest that polycrystalline and solid‐phase crystallized films should be used properly, according to the device manufacturing processes.

The characterization of various TCOs and metal oxide layers for dye sensitized solar cells

Dye sensitized solar cells have gained great interest for one decade because of easy fabrication and low cost. In this study, we synthesized various transparent conductive oxide (TCO) electrodes and metal oxide layers in laboratory for dye sensitized solar cells via spray pyrolysis technique. XRD results confirmed that the antimony doped tin oxide (ATO), flor doped tin oxide (FTO) and indium tin oxide (ITO) thin film TCO electrodes were fabricated successfully. UV–Vis spectrometer was employed to obtain transmittance and band gap energy values of the films. According to the UV–Vis spectrometer, all TCO films almost have 80% optical transmittance. The sheet resistance values were obtained by four-point probe technique for all TCO electrodes. SEM was performed for surface morphology of the TCO electrodes and SnO2, ZnO and Zn doped TiO2 metal oxides on the various TCOs. The TCO electrodes and metal oxide layers have homogeneous surfaces.

(Invited) Reliability Improvement and Remaining Issues on Photovoltaic Cells and Modules

Worldwide rapid growth of photovoltaics requires not only high-efficiency but also high-reliability and long-lifetime photovoltaic cells and modules. Reliability of photovoltaic modules is determined by module materials such as encapsulant, backsheet, etc. and, of course, also by photovoltaic cells themselves. There are some complicated phenomena concerning degradation of photovoltaic properties. Those physical and chemical degradation phenomena should be microscopically analyzed in detail in order to clarify degradation mechanism and also realize highly reliable photovoltaic cells and modules. Although such degradation phenomena are complicated and often related each other, degradation mechanism is classified into only three categories; 1) less incident light into solar cells, 2) less collection of photogenerated carriers, and 3) less photovoltaic ability itself. Less incident light often originates from soiling of cover glass; however, in some cases originates from browning of encapsulant or reduction of transparent conductive oxide electrode. Less correction of carriers originates from damage of electrodes on the cell or damage of interconnector ribbons between the cells. Less photovoltaic ability originates from surface or bulk recombination or shunting. These three categories are schematically shown in the attached figure. There are various degradation factors, for example, thermal or mechanical stress, high temperature, high humidity, UV irradiation, and potential difference. In particular, hygrothermal stress, UV irradiation, potential difference, and also their combination are considerable factors. For example, UV irradiation accelerates degradation by hygrothermal stress [1], on the other hand, delays degradation by potential difference [2].Both development of indoor acceleration test method and estimation of acceleration factor are also important for predicting the lifetime of photovoltaic modules. As mentioned above, although UV light irradiation much influences degradation phenomena, test method with UV light irradiation is not popular. The reason is that it is quite difficult to keep the module temperature under irradiation during test duration and also to realize uniform irradiation for large-size photovoltaic modules over 1.5 m. However, using acetic acid concentration in the encapsulant as the mediator between outdoor exposure and indoor acceleration test, it was elucidated that damp heat test at 85°C and 85% relative humidity for 4000 h corresponds to outdoor exposure at Japan for 30 years [3].Based on degradation mechanism clarified, highly reliable photovoltaic cells and modules should be developed. It is clarified that the most important key material is electrode of solar cells. Even when photocarriers are generated, if damage or degradation occurs in the electrodes, such carriers are not collected and photovoltaic performance is lowered. Problems in photocarrier collection occupy majority of origins for degradation observed outdoors. Degradation by potential difference has also attracted much attention in the last 10 years especially in mega-watt scale photovoltaic plant with high system voltage. Encapsulant materials and anti-reflection coating of solar cells are key materials for suppressing such kinds of degradation.There are some remaining issues on photovoltaic cells and modules for improving reliability and increasing lifetime. First, interaction between module materials and cells should be completely clarified. Photovoltaic modules are composed of various materials including polymers, metals, ceramics, and, of course, semiconductors. Complicated reactions should occur both at the interface and in the bulk of those materials by many stressors such as high temperature, high humidity, thermal cycle, UV irradiation, and potential difference. Many observations and knowledges have been obtained so far, however, we do not reach the complete understanding, and essential point is that materials with less change and less interaction with other materials should be employed. Second, acceleration test results should be always analyzed considering outdoor exposure results. Especially, climate conditions such as desert, tropical zone, etc. for outdoor exposure also should be employed for acceleration test conditions. Photovoltaic cells and modules appropriate to specific climate conditions should be developed. Third, high efficiency photovoltaic cells and modules should be developed always considering high reliability and long lifetime. Those are inseparable issues for development of photovoltaic cells and modules. Especially, reliability issues on high-efficiency perovskite cells and modules should be intensively studied and urgently solved for realizing high-efficiency and high-stable perovskite/silicon tandem solar cells and modules.The author is grateful to Professors Keisuke Ohdaira, Sachiko Jonai, Yasuaki Ishikawa, Yasushi Sobajima, Fumitaka Ohashi, Kentaro Iwami, Dr. Seira Yamaguchi, Mr. Yasushi Tachibana, and Ms. Yukiko Hara for their collaboration and fruitful discussion. This study was in part supported by the New Energy and Industrial Technology Development Organization.

Influence of Oxygen on Sputtered Titanium‐Doped Indium Oxide Thin Films and Their Application in Silicon Heterojunction Solar Cells

One of the challenges in fabricating high‐performance n‐type silicon heterojunction (SHJ) solar cells is developing a high‐quality transparent conductive oxide (TCO) electrode. Herein, the development and application of low‐temperature sputtered titanium‐doped indium oxide (ITiO) in n‐type, rear junction SHJ solar cells as a function of the oxygen flow ratio is presented. The microstructure, morphology, and optoelectronic properties are analyzed. The grain size of ITiO thin films decreases rapidly as the oxygen flow ratio is increased. Compared with an indium tin oxide (ITO) thin film, ITiO shows a superior balance in achieving excellent optoelectronic properties by reducing film resistivity but maintaining weak absorption. Higher fill factor is obtained by substituting ITiO for ITO as the front electrode in SHJ solar cells, which is mainly due to the improved carrier transport. Resistivity contributions of front‐side vertical and lateral carrier transport are disclosed by Quokka3 simulation. A champion cell efficiency of 23.81% with ITiO is achieved, which is so far the highest efficiency among the application of ITiO in SHJ solar cells to the best of our knowledge. The study demonstrates that ITiO is a promising TCO candidate for SHJ solar cells.

Chemical passivation of the perovskite layer and its real-time effect on the device performance in back-contact perovskite solar cells

The back-contact architecture for perovskite solar cells (PSCs) offers the possibility of positioning both electrodes on one side of the absorber layer and shining light directly on the perovskite photoactive layer. This helps us to avoid the occurrence of transmission losses caused by the charge collecting transparent conductive oxide electrode in the conventional sandwich structure for PSCs. The back-contact device architecture is also useful for conducting fundamental studies as it has an exposed photoactive area, permitting in situ measurements on the effects of chemical treatment, passivation, and annealing. A successful application of back-contact PSCs in studying the effect of chemical passivation of the perovskite photo-absorber layer trap states with pyridine and its influence on the device performance have been studied. The real-time effect of pyridine vapor treatment on the device performance is visualized by monitoring the maximum power output of the devices under operation conditions. The device performance enhancement by ∼12% owing to the surface passivating effect of pyridine is demonstrated.

High-Mobility In2O3:H Electrodes for Four-Terminal Perovskite/CuInSe2 Tandem Solar Cells.

Four-terminal (4-T) tandem solar cells (e.g., perovskite/CuInSe2 (CIS)) rely on three transparent conductive oxide electrodes with high mobility and low free carrier absorption in the near-infrared (NIR) region. In this work, a reproducible In2O3:H (IO:H) film deposition process is developed by independently controlling H2 and O2 gas flows during magnetron sputtering, yielding a high mobility value up to 129 cm2 V–1 s–1 in highly crystallized IO:H films annealed at 230 °C. Optimization of H2 and O2 partial pressures further decreases the crystallization temperature to 130 °C. By using a highly crystallized IO:H film as the front electrode in NIR-transparent perovskite solar cell (PSC), a 17.3% steady-state power conversion efficiency and an 82% average transmittance between 820 and 1300 nm are achieved. In combination with an 18.1% CIS solar cell, a 24.6% perovskite/CIS tandem device in 4-T configuration is demonstrated. Optical analysis suggests that an amorphous IO:H film (without postannealing) and a partially crystallized IO:H film (postannealed at 150 °C), when used as a rear electrode in a NIR-transparent PSC and a front electrode in a CIS solar cell, respectively, can outperform the widely used indium-doped zinc oxide (IZO) electrodes, leading to a 1.38 mA/cm2 short-circuit current (Jsc) gain in the bottom CIS cell of 4-T tandems.

Ultrafast 27\u2009GHz cutoff frequency in vertical WSe2 Schottky diodes with extremely low contact resistance

Ultra-thin two-dimensional semiconducting crystals in their monolayer and few-layer forms show promising aspects in nanoelectronic applications. However, the ultra-thin nature of two-dimensional crystals inevitably results in high contact resistance from limited channel/contact volume as well as device-to-device variability, which seriously limit reliable applications using two-dimensional semiconductors. Here, we incorporate rather thick two-dimensional layered semiconducting crystals for reliable vertical diodes showing excellent Ohmic and Schottky contacts. Using the vertical transport of WSe2, we demonstrate devices which are functional at various frequency ranges from megahertz AM demodulation of audio signals, to gigahertz rectification for fifth-generation wireless electronics, to ultraviolet–visible photodetection. The WSe2 exhibits an excellent Ohmic contact to bottom platinum electrode with record-low contact resistance (~50 Ω) and an exemplary Schottky junction to top transparent conducting oxide electrode. Our semitransparent vertical WSe2 Schottky diodes could be a key component of future high frequency electronics in the era of fifth-generation wireless communication.

Seed layer effect on morphological, structural, and optical properties of electrochemically grown ZnO nanowires over different SnO2:F/glass substrates

Transparent conductive oxide electrodes and specifically SnO2:F/glass are widely employed substrates in the preparation of optoelectronic devices. This study deals with the surface effect of commercially SnO2:F/glass electrodes on the properties of ZnO nanowires grown onto them by using an electrochemical method. Four types of commercially SnO2:F/glass with sheet resistances in the range of 7–15 Ω cm−2 and roughness from 9 to 39 nm were used. The deposition process of ZnO nanowires was performed with zero, one, and two ZnO seed layers, respectively. The seed layer was grown from an ethanolic solution of zinc acetate as a Zn2+ precursor, and further, a thermal treatment was performed. The formation of the nanomaterial was carried out by using the potentiostatic method at − 1 V and until reaching a total charge of − 1.5 C at a temperature of 75 °C. Different densities of nanowires were obtained for each case, with diameters ranging from 30 to 400 nm. The SnO2:F/glass roughness affects the diameter and alignment of the ZnO nanowires. Measurements of diffusive spectra in the optical range, as well as the determination of polarization properties, show that the seed layer deposition is indispensable to obtain optimal optical quality. The findings here discussed constitute another critical factor to be considered in the synthesis of well-aligned nanowires.

Transparent Conducting Oxide Electrodes: Layer‐by‐Layer Assembled Oxide Nanoparticle Electrodes with High Transparency, Electrical Conductivity, and Electrochemical Activity by Reducing Organic Linker‐Induced Oxygen Vacancies (Small 8/2020)

Transparent Conducting Oxide Electrodes: Layer‐by‐Layer Assembled Oxide Nanoparticle Electrodes with High Transparency, Electrical Conductivity, and Electrochemical Activity by Reducing Organic Linker‐Induced Oxygen Vacancies (Small 8/2020)

Photonic-structured TCO front contacts yielding optical and electrically enhanced thin-film solar cells

Wavelength-structured transparent conductive oxide (TCO) electrodes are highly promising to improve both the optical and electrical performance of photovoltaic (PV) devices, due to wave-optical light-trapping (LT) effects and higher TCO volume without increasing optical losses.Herein we present a complete study of the benefits of microstructured IZO contacts applied on amorphous-silicon (a-Si) thin film solar cells. The IZO LT structures were integrated by an innovative colloidal lithography process on the front contact of the cells, resulting in enhancements of 26.7% in photocurrent, with respect to planar reference cells, when using an ultra-thin (30 nm) flat IZO layer between the LT structures and the a-Si absorber. However, the best efficiency enhancement (23.1%) was attained with an optimized thickness of 190 nm for this layer, due to a more favorable combination of optical and electrical gains.In view of the application of this LT strategy in flexible PV devices operating under bending, the angular response of the cells was studied for 0-90° incidence angles. This showed that the LT enhancements are generally higher at oblique incidence, reaching 53.2% and 52%, respectively in photocurrent and efficiency, at ± 70° angles with the optimized flat IZO thickness of 190 nm; and 52.2% in efficiency at ± 40° with the ultra-thin thickness of 30 nm. These results are among the highest gains reported thus far for LT-enhanced thin film solar cells.

Synthesis of PtSe catalysts using atmospheric-pressure plasma and their application as counter electrodes for liquid-junction photovoltaic devices

This study reports on the synthesis of PtxSe1-x(0 ≤ x ≤ 1) nanoparticles (NPs) with different volume ratios of Pt and Se in mixed precursor solutions on a transparent conducting oxide electrode using dry plasma reduction (DPR). DPR can work under atmospheric pressure without the use of any toxic chemicals and at a low temperature. The developed PtxSe1-x(0 ≤ x ≤ 1) NPs immobilized on FTO glass substrates were first employed to the counter electrodes (CEs) for efficient dye-sensitized solar cells (DSCs). The morphology and trends of PtxSe1-x(0 ≤ x ≤ 1) NPs were observed by scanning electron microscope (HRSEM) and transmission electron microscope (TEM), while varying the ratio of platinum to selenium precursor. In addition, physical and chemical characteristics of PtxSe1-x(0 ≤ x ≤ 1) NPs were analyzed by photoelectron spectroscopy (XPS). Electrochemical properties for the catalytic activity of CEs were also measured and analyzed by Tafel, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV). As a result, the CE prepared at the volume ratio of Pt0.9Se0.1 exhibits the lowest charge transfer resistance of 0.85Ω. Therefore, the device fabricated with Pt0.9Se0.1 alloy electrode shows the highest efficiency of 5.91% among all cells. This is also higher than the efficiency rates of 5.54% and 0.13% for cells fabricated with Pt NP and Se NP CEs, respectively. The obtained results indicate the potential use of PtSe alloy CEs in robust and efficient DSCs.

Effect Of Fluorine Doping On The Structural, Optical And Electrical Properties Of Spin Coated Tin Oxide Thin Films For Solar Cells Application

Transparent conducting oxide (TCO) thin films are materials of significance for their applications in optoelectronics and sun powered cells. Fluorine-doped tin oxide (FTO) is an elective material in the advancement of TCO films. This paper reports the impact of fluorine doping on structural, optical and electrical properties of tin oxide thin films for solar cells application. The sol-gel was prepared from anhydrous stannous chloride, SnCl2 as an originator, 2-methoxyethanol as a solvent, di-ethanolamine as a preservative and ammonium fluoride as the dopant source. FTO precursor solution was formulated to obtain 0, 5, 10, 15 and 20 % doping concentration and deposited on glass substrates by means of spin coater at the rate of 2000 rpm for 40 seconds. After pre-heated at 200 oC, the samples were annealed at 600 oC for 2 h. The structural, optical and electrical characteristics of prepared films were characterized using X-ray diffraction (XRD) analysis, UV-visible spectroscopy and electrical measurement. X-ray diffraction (XRD) investigation of the films demonstrated that the films were polycrystalline in nature with tetragonal-cassiterite structure with most extraordinary pinnacle having a grain size of 17.01 nm. Doping with fluorine decreases the crystallite size. There was increment in the absorbance of the film with increasing wavelength and the transmittance was basically reduced with increasing fluorine doping in the visible region. The energy band gaps were in the range of 4.106-4.121 eV. The sheet resistance were observed to decrease as the doping percentage of fluorine increased with exception at higher doping of 15 and 20 %. In view of these outcomes, FTO thin films prepared could have useful application in transparent conducting oxide electrode in solar cell.

Zinc Phthalocyanine–Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics

Efficient charge harvesting at electrodes is critical for the effective performance of organic photovoltaics and is strongly influenced by the first molecular monolayer at the transparent conducting oxide electrode. Herein, we present a study of the relationship between molecular orientation and tunneling distance on charge-transfer rates between a tethered sub-monolayer/monolayer of phosphonic acid (PA)-functionalized Zn phthalocyanines (ZnPcs) and an indium tin oxide (ITO) surface by a combination of electrochemical techniques and waveguide tools such as attenuated total reflectance (ATR) UV–vis spectroscopy and potential-modulated ATR (PM-ATR) spectroelectrochemistry. The distance between the main chromophore and PA anchoring group was modulated by an aliphatic carbon of various lengths (n = 3, 9, 10, or 11) that resulted in an extended distance of 7–18 Å between the ZnPcs and the ITO surface. Modified ITO surfaces were composed of monomeric and aggregated subpopulations of ZnPcs with molecular orientations predominantly in-plane (36–39°) and out-of-plane (72–75°), respectively. Charge-transfer rate constants (ks,opt) were measured using PM-ATR. For a given tether length, the aggregated subpopulations exhibit higher ks,opt values compared to the monomeric subpopulations. The observed ks,opt values had an exponential dependence on the effective tunneling distance with a decay constant (β) that ranged from 0.32 to 0.47 Å–1, depending on the chromophore orientation and aggregation state. The fastest charge-transfer rate constants were found for the chromophores with the smallest tunneling distance (n = 3). A ks,opt of 3.9 × 104 s–1 represents the fastest rate constant measured by PM-ATR for a PA-functionalized ZnPc chromophore tethered to an ITO waveguide electrode.