Symposium EQ17—Emerging Materials for Contacts and Interfaces in Optoelectronics

Transparent conductive oxides (TCOs) are appealing materials for electronic and photonic applications as, amongst other advantages, their properties can be modulated by engineering their defects. Optimisation of this adjustment is, however, a complex design problem. In this work we address this problem following two ways: (i) establishing new high-throughput methods to selectively tune these properties towards the requirements of the application and (ii) developing better insights into the optoelectronic properties of TCOs. Specifically, we investigated the selective modification of the optoelectronic properties of sputtered tin-doped indium oxide (ITO), aluminium-doped zinc oxide (AZO) and gallium doped zinc oxide (GZO) thin films via laser annealing (LA) within an environmental cell containing a wide set of reducing and oxidising reactive gas mixtures.
In order to elucidate the precise mechanisms behind the ReLA process, the optical properties modifications were related to the structural, compositional, and electronic properties. In one case, where the sputtering conditions were optimised to produce an ITO thin film with low resistivity (4×10^-4 Ωcm), infrared-visible (0.034-3.4 eV) spectroscopic ellipsometry and Hall Effect measurements revealed a modulation of the optoelectronic properties that is dependant upon the ambient gas mixture. Explicitly, reducing environments (5% H₂ in N₂) promoted a greater enhancement of the carrier concentration. Examination of the X-ray diffractograms and cross-sectional Transmission Electron Microscopy images did not reveal any ReLA-induced structural modifications. However, X-ray photoelectron spectroscopy and energy-dispersive X-ray concluded that the modification of the optoelectronic properties of the ITO films arose from purely compositional changes (creation/filling of oxygen vacancies and activation of substitutional Sn dopants). The precise ReLA induced modifications were not identical for all TCO thin films. In the case of ITO thin films grown with “un-optimised” sputtering conditions (with a higher resistivity of 7×10^-3 Ωcm), the alteration of the optoelectronic properties was more significant and did not arise from changes to the composition alone. Instead, the depth-dependence of the laser annealing process was uncovered via a combination of advanced ellipsometric modelling and cross-sectional transmission electron microscopy. For the AZO and GZO films that form distinct grains even for room-temperature sputter depositions, the role of the grain boundary scattering on the optoelectronic properties is more significant. Therefore, we focussed our attention on the crystal structure in this case. By carefully considering the interplay between the compositional alterations and crystal structure changes for a wide range of TCO films, we revealed the influence of both the intra-grain and inter-grain carrier transport properties for doped ZnO and ITO thin films.
Altogether, we demonstrate that ReLA can be utilised to target specific defects within the TCO films and engineer them to achieve specific alterations to the optical and electronic properties. Hereby, ReLA is established as a novel and versatile tool to tailor the properties of TCO films towards the requirements of potential electronic and photonic applications.

Solar energy is by far the most abundant and widespread renewable energy resource, which is maintenance-free and envisioned to play a key role in the energy transition needed for the next decades. Organic photovoltaics (OPVs) technology, offering now power conversion efficiency of over 18%, is expected to provide alternative implementation technologies such as large area flexible panels for urban compatible exploitation with cost-effective printing techniques.
Organic solar cells architectures have become increasingly complex with stacking of both organic and inorganic materials optimized to provide largest efficiency in the final device. The rational design of electronic levels at the organic/organic interfaces or at the organic/inorganic interfaces is of fundamental importance in the transport of charges for improved efficiency. The use of electron spectroscopies carried out using synchrotron radiation allows an in-depth investigation of the electronic interplay at the interface evidencing charge transfer, band bending or chemical bonding together with a detailed picture of the band alignment.
In this context, we have used electron spectroscopies to investigate metal oxide interfacial layers acting as electron-transport layer (ETL) in organic solar cells, namely TiO2, in connection with acceptor molecules. During past twenty years, fullerenes have been dominant as electron accepting materials for designing high performance organic solar cells, however their drawbacks mainly in terms narrow absorption spectra have motivated their replacements by non-fullerene electron acceptor molecules. The comparison between C70 and non-fullerene acceptor, namely ITIC molecule, at the interface with low temperature sputtered TiO2 ETL will be presented. It will be shown that resonant photoemission is a powerful technique capable in revealing the role of defects states present at the TiO2 interface.

In the second example presented, we will focus our interest on organic/organic interfaces. It has been evidenced that the efficiency of organic solar cell suffers from the recombination of photo-induced electron-hole pair or excitonic state at the interface between the electron donor and the hole transport layer (HTL) or electrode. One of the strategies to limit this recombination consists in introducing an exciton blocking layer (EBL) at the interface between the electron donor and the HTL. Such strategy has been recently used successfully in an inverted solar cell design, in which a molecule commonly used in OLED, 4P-NDP has demonstrated its abilities as EBL in a planar heterojunction of 4P-NDP/DBP. Surprisingly, the optimal thickness of the EBL giving higher power conversion was found to be less than one nm. Our investigation of interfacial electronic properties in 4P-NDP/DBP heterojunction reveals the strong influence of DBP in term of band alignment on ultra-thin layer of 4P-NDP favoring the charge transport across the device.

The presented work is a join study by Sorbonne Université and University of Southern Denmark and supported by Danmarks Frie Forskningsfond, DFF FTP project ‘React-PV’, Danish Ministry of Higher Education and Science project ‘SMART’, Horizon2020 project CALIPSOplus.

Conventionally, thin-film synthesis of the high mobility, high figure of merit BaSnO₃ is achieved via cost-intensive, high temperature, high vacuum methods. Chimie-douce techniques for nanoparticle synthesis of this material are often accompanied by impurity phases such as BaCO₃, SnO₂ and Ba₂SnO₄, necessitating high annealing temperatures, consecutively rendering it impractical for numerous applications. Lowering the synthesis temperature to obtain phase pure BaSnO₃ can expand the horizons for its optoelectronic applications, with an obvious reduction in the thermal budget.

In this work, we have developed a novel solution combustion technique for the synthesis of BaSnO₃ nanoparticles. A peroxo/superoxo precursor to the nanoparticles is first synthesized by co-precipitation of the tin and barium salts via the H₂O₂ assisted route. We find that the crystallization temperature of BaSnO₃ is significantly reduced by adding an organic solvent to the precursor; temperatures as low as 130 °C lead to phase pure BaSnO₃ nanoparticles. The role of the organic solvent is established as the component responsible for an exothermic reaction occurring around 130 °C, that leads to crystalline BaSnO₃ formation. Importantly, this method allows for facile incorporation of dopants. Such low synthesis temperatures enable BaSnO₃ to be used in devices having temperature limitations during device processing, such as perovskite-based solar cells in an n-i-p architecture or heterojunction Si solar cells.

Semitransparent solar cells are used in diverse energy-related applications, such as Building Integrated Photovoltaics (BIPV), automobile windows, and wearable electronic devices. Perovskite is fairly attractive as an active layer in semitransparent solar cells. When this material is used, the thickness of the light absorption layer can be easily controlled based on a solution-based process. On the contrary, the fabrication of a transparent electrode layer in film solar cells with great performance remains a challenge so far. Since the layers of perovskite solar cells are substantially thin, the transparent electrode should be manufactured with the low surface roughness to prevent a detour current path between the different layers. In addition, the transparent electrode should be fabricated in low-temperature, moreover, the use of polar solvents is not beneficial because the active layer of solar cells can be damaged.
In this contribution, the Ag nanogrid electrode is fabricated by using directional electrospun polyethylene oxide (PEO) nanofibers as an etching mask in the wet etching process. Because polar solvents are not used, and high temperature is not required in this process, electrodes can be fabricated on Polydimethylsiloxane (PDMS) substrate; hence, electrodes can be transferable to various surfaces or shapes. In addition, when this method is applied, the high scalability can be achieved, such as for the roll-to-roll process for large-area substrates. The fabricated transferrable Ag nanogrid transparent electrodes has the transmittance of 92.7% (at 550 nm) and the sheet resistance of 18.0 Ω/sq. Furthermore, the MAPbI_2.5Br_0.5 based perovskite semitransparent solar cell with Ag nanogrid electrode attached as the top electrode was manufactured. And the performances, such as J-V characteristics, quantum efficiency, and transmittance, were investigated. Consequently, we report a semitransparent solar cell with an average visible wavelength transmittance (AVT) of 25.2%, and a power conversion efficiency (PCE) of 12.7%. Therefore, the light utilization efficiency rates (LUE) of 3.21%, which is the highest value in the conventional perovskite semitransparent solar cells, can be identified.

Several emerging technologies from the field of organic electronics have a great promise to disrupt our everyday lives. To achieve this goal, new materials need to be developed, that would meet the high expectations of the market.
Traditionally, self-assembled monolayers (SAMs) were widely used in material sciences for the modification of the interfaces of various semiconducting oxides. However, there were not many examples, where the SAMs would have an individual role, rather than that of the “assisting” or “improving”. The most significant demonstration of the power of the SAMs is dye-sensitized solar cells. There, in a combination with the mesoporous scaffold, a single layer of the dye molecules was sufficient to ensure efficient absorption of the light. Nevertheless, there were still only several examples of the other types of semiconducting SAMs demonstrated over the years.
Recently in our work, we have introduced carbazole-based phosphonic acids into the p-i-n perovskite solar cell architecture. It was used instead of the traditional hole-transporting materials, formed by a spin-coating method. Instead, the layers were formed on top of the indium tin oxide electrode by a simple dipping procedure.
Initially, phosphonic acid with dimethoxydiphenylamine-substituted carbazole chromophore was used for the construction of the proof-of-concept devices, showing a promising efficiency of over 17%. Eventually, the structure was simplified, and the devices with the [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) have demonstrated over 20% efficiencies, outperforming polytriarylamine (PTAA) standard. In further work 2PACz was used as a starting compound for the structural modifications, leading to some exceptional results.
The first example is a material called Me-4PACz ([4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid), which was used to fabricate record-breaking silicon/perovskite tandem solar cell, with 29.2% efficiency [1]. Another example is Br-2PACz ([2-(3,6-dibromo-9H-carbazol-9-yl)ethyl]phosphonic acid), which was used in bulk-heterojunction solar cells, to give 18.4% efficiency, which is one of the highest reported values in the literature [2].
The incorporation of the monolayers into devices as a functional layer is leading to a breakthrough in several aspects. First, due to the ultimately low thickness of the layer, the amount of the required material is significantly reduced. Next, the defined orientation of the molecule (“anchor” attached to the surface, while “head” staying on the surface), gives a promise for the more rational design of the materials. Finally, conventional synthetical procedures are allowing the creation of large libraries of the materials, making it possible to fine-tune the properties of the layer.

1. A. Al-Ashouri et al. Science, 370(6522) p. 1300-1309 (2020)
2. Y. Lin et al. ChemSusChem, Early View (2021)

The efficiency of perovskite solar cells (PSCs) almost reached that of silicon photovoltaics. Owing to the facile band-gap tunability of halide perovskites, partially see-through, i.e. semi-transparent, solar cell architectures with appealing colour impression become attractive. This opens up innovative areas of application, such as window integration, house facades, and in vehicles. Another essential challenge for PSCs is the susceptibility of the perovskite material to the effects of outside gases, such as moisture and oxygen.

A key element of semi-transparent cells is the semi-transparent top electrode. Previously, we have shown that networks of silver nanowires ^[1] or thin metal layers, such as Ag ^[2], can be used to realize such electrodes. Both concepts are based on metals that are easily corroded by halide containing species potentially leaking from the perovskite (e.g. at elevated temperature). To mitigate this issue, an impermeable ALD grown SnO_x electron transport layer is typically inserted to prevent halide migration and to protect the perovskite against subsequent deposition processes.

While the SnO_x is highly transparent and an excellent permeation barrier, it lacks the intrinsic electrical conductivity (5 x 10^-3 S cm^-1) to qualify as a stand-alone semi-transparent top electrode without the requirement of additional metals.

In this work we propose ALD grown In₂O₃ from Cyclopentadienylindium(I) (CpIn) as semi-transparent top electrode with a high electrical conductivity of 2 x 10³ S cm^-1, which is six orders of magnitude higher than that of SnO_x and on par with that of sputtered indium tin oxide (ITO). Even at growth temperatures as low as 80°C and without any post deposition annealing, our In₂O₃ layers provide a high charge carrier density in the order of 10²¹ cm^-3 and an electron mobility up to 23 cm²/Vs. In stark contrast to sputter deposited ITO the ALD grown In₂O₃ provides outstanding properties as diffusion barrier, exemplified by a low water vapor transmission rate (WVTR) of 4.8 x 10^-4 g (m^-2d^-1) at 70°C/70%rH, which is orders of magnitude lower than number typically quoted for sputtered ITO ^[3].

Finally, a high optical transmittance of over 80% in the wavelength range from 400-1000 nm for layers with a thickness of 100 nm and a sheet resistance of 45 W/sq, qualifies the In₂O₃ as an excellent semi-transparent electrode to realize semi-transparent PSCs. Details of the growth conditions using oxygen and water as co-reactants in the ALD process as well as the requirement of a proper seed layer for the In₂O₃ on top of the perovskite will be discussed. In first experiments, we applied our In₂O₃ as semi-transparent top electrode in semi-transparent PSCs, which afford notable efficiencies up to 16.5%.

[1] Gahlmann et al., Adv. Energy Mater. 2020, 10, 1903897
[2] Zhao et al., Adv. Energy Mater. 2017, 7, 1602599
[3] Behrendt et al., Adv. Mater. 2015, 27, 5961-5967

Hybrid halide perovskites have overwhelmed the field of photovoltaics with unprecedented progress and skyrocketing efficiencies. Their facile bandgap tunability renders perovskite solar cells excellent building-blocks for multi-junction architectures, that provide the prospect to overcome fundamental efficiency limits of single-junctions. While perovskite/silicon or all-perovskite tandem cells have shown some remarkable progress, as of yet, perovskite/organic tandem cells show subpar efficiencies of ~20 per cent, limited by the low open circuit voltage of wide-gap perovskite cells, and serious optical/electrical losses introduced by the interconnect between the sub-cells.
Organic and perovskite semiconductors share similar processing technologies, which makes them attractive partners in multi-junction architectures. More importantly, the introduction of non-fullerene acceptors has boosted the efficiency of organic solar cells to levels beyond 18 per cent.
Here, we demonstrate perovskite/organic tandem cells with an efficiency up to 24 per cent, setting a new milestone for perovskite/organic tandem devices, which now for the first time outperform the most efficient single junction perovskite cells in p-i-n architecture. In an optimistic scenario, we envision that perovskite/organic tandem architectures bear a realistic prospect to reach efficiencies above 31 per cent.
This progress is not least based on significant improvements on the most critical interfaces in such a device:
Firstly, we managed to overcome interfacial losses, that are the predominant reasons limiting the performance of wide-gap perovskite cells. This allows us to access previously unreached territory of combined high open circuit voltage and fill factor. At the same time, our findings evidence that the proper choice of charge extraction layers allows to mitigate the detrimental halide segregation typically encountered in mixed-halide perovskites.
Secondly, we introduce a novel recombination interconnect for the two sub-cells, that is based on an ultra-thin (~1.5 nanometers) metal-like indium oxide layer. This interconnect offers unprecedented low optical and electrical losses, which unlocks the exploitation of the full potential of the two sub-cells without any discount. It can be foreseen that the applicability of this novel interconnect is not limited to perovskite/organic tandem cells but we expect that it will likewise revolutionize other tandem architectures.

This talk will focus on the combined use of photoelectron spectroscopy and electrochemical methods as a unique feedback loop to i) characterize the origin of defects in printable electronic materials; ii) quantify the energetics and effective density of states within the gap under applied voltage conditions; and iii) assess the validity of different passivation strategies using ionic liquid (IL) layers.

Defects and near-surface mid-gap states are a fundamental challenge in semiconductor materials. For example, in photovoltaic devices, controlling defects of charge selective layers and photoactive layers is critical to energy level alignment, functionality, and overall performance. For printable semiconductor materials and devices, a number of factors can increase the concentration of mid-gap defects, which can be coupled to complex Lewis and/or Bronsted acid/base chemistries, and redox reactions. We have recently shown that we can electrochemically quantify defect density and energetics in thin film opto-electronic materials using solid electrolytes, where the electrolytes act as transparent top contact in the dark and under illumination. Operando diffraction and spectroscopic tools can then be used under device-relevant conditions to probe changes in lattice spacing and band energetics.

Herein we expand on this to consider layer-by-layer vacuum deposition of IL films on prototype opto-electronic materials. Interfaces are characterized using photoemission spectroscopies (XPS/UPS) primarily to elucidate changes in band edge energetics at the electronic material/IL heterojunction. Striking differences are seen in the energetics and composition of these heterojunctions depending on the type of semiconductor substrate and type of IL deposited, which enhances our understanding of the electrostatic interactions between ILs and printed semiconductor materials which impact on both defect characterization and defect mitigation.

Metal halide perovskite semiconductors have captured significant interest in the thin film optoelectronics community. As one example, certified photovoltaic efficiencies of metal halide perovskite solar cells currently exceed 25%, making them competitive in the laboratory with well-established technologies like multicrystalline-Si, CdTe, and CIGS. Also recently, gains have been made in perovskite-based light emitting devices (LEDs), with external quantum efficiency (EQE) of approximately 20% realized for bromide (green) and iodide (red/near-infrared) based perovskite emission layers. But in order to better understand the physics of device operation as well as degradation, we need to understand what can occur at interfaces of halide perovskites with other materials. For example, we have determined that metal halide perovskites not only feature mixed ionic-electronic motion but are also considerably chemically reactive, for example via redox or acid-base reactions. In addition, it may be possible to exploit this reactivity to allow for charge transfer doping, as we have shown perovskite systems with the ability to illustrate charge transfer states as well as doping with important implications on perovskite optoelectronic devices.

Transparent amphiphobic surfaces (repelling water and other low surface tension liquids) are of great interest in applications such as self-cleaning, anti-icing, anti-fouling, condensation and water harvesting etc. Current state-of-the-art in highly liquid repellent surfaces highlights several limitations on their use in practical applications. For example, superhydrophobic surface (SHS) suffer from instabilities/collapse of air pockets/cushion responsible for high liquid contact angles. Similarly, the retention of slippery behaviour in lubricant infused surfaces (SLIPS) and other smooth surfaces (SS) are restricted by eventual lubricant depletion and mechanical susceptibility of short molecular chains, respectively. Herein, we present a way to overcome these challenges by combining a robust nanohierarchical morphology with long flexible alkyl chains and achieve high drop mobility (~10 times than SLIPS) and amphiphobicity (Singh et al., Nano Lett. 2021, 21, 8, 3480–3486). High drop mobility comparable to superhydrophobic surfaces is achieved due to nanohierarchy i.e. reduced solid fraction (= 0.4). Precisely controlled nanohierarchy is obtained from surface grown metal-organic frameworks (MOFs) via layer-by-layer method. Covalently attached zirconium-based MOFs (OH-UIO-66) on glass substrate are used for demonstration due to their highly stable framework structure and easy synthesis. We observed a free sliding of vegetable oil, alcohols or other polar liquids at <15°. In addition to the excellent optical transparency and robustness, our surface enables a unique combination of additional desirable properties, e.g., resistance to high-speed liquid impact due to high capillary pressure generated from MOF nanopores (V = 35 m/s, We_l = 42,500), thermal stability up to 200 °C, and mechanical stability of scratch resistance. Moreover, benefiting from inherent porous structure of MOFs, our surface is able to absorb carcinogenic pollutants such as dyes and volatile organic compounds (VOCs) from water. These features endow the usage of the presented nanohierarchical MOF coating in applications such as windows, hand-held displays, goggles, wind screens, optoelectronics and so on.

Mixtures of silica and polysiloxane form thixotropic inks useful for 3D printing. However, 3D printed structures of these inks have typically been opaque due to the refractive index mismatch between the base polysiloxane and silica. By tuning the composition of the base polysiloxane, we matched the refractive index to formulate transparent inks. The transparent inks transmit up to ca. 90% of 700 nm light through 1 cm and remain transparent when solidified. We observed and characterized a thermochromic effect, where the inks change in opacity depending on refractive index mismatch and temperature. We also observed that the rheological properties of the ink depend on the distribution of silica particles, which is dictated by silica functionality, weight content, and processing. We demonstrate utility of these inks by printing encapsulants for LEDs, scintillators, and microfluidic mixing devices.

Nematic liquid crystals (LC’s), ubiquitous in high information density displays, are typically composed of rod-shaped molecules that can exhibit large dielectric anisotropy, high birefringence and excellent transparency. These properties make them well suited for use in many optical components in high power laser systems, including polarization controllers and optoelectronic devices for beam shaping in 3D metal printing [“A new era In metal part production”, available at www.seuratech.com]. Recently, the single shot laser damage threshold of both saturated and un-saturated nematic LC’s revealed an intrinsic relationship between the degree of chemical bond saturation within LC molecules and laser damage [Kosc et al, Scientific Reports 2019, 9 (1), 16435]. Extrinsic factors such as alignment layer material selection and preparation, particulate contamination and LC chemical purification can also play an important role in determining the optical response and durability of the liquid crystals. In this study we investigate the efficacy of several absorption-based purification methods and their impact on the laser damage behavior of nematic LC’S. We focus on the lifetime of purified and as-prepared LC’S when exposed to multiple laser pulses at kW’s average power, over cm² areas, at near infrared wavelength and 20 to 40 Hz rep rates. Numerical simulations are used to estimate the degree of heat accumulation during exposure to rationalize the LC lifetime results.

The recent report of a breakthrough in light emission in silicon-germanium alloys has renewed interest in light emission in silicon related materials and structures. The ability to tailor solid-solid interfaces, for a desired functionality, is of enormous interest in a vast variety of applications in materials science and engineering. In this work, engineered doped coatings catalyzed by several acids are deposited on silicon to introduce random interfacial stresses that can modulate the silicon bandgap. This modulation has been demonstrated to be beneficial by adding two functionalities to ordinary Czochralski-Si. It has appreciably improved bandgap radiative recombination of carriers by over two folds which leads to the possibility of engineering silicon-enabled absorber by circumventing Auger recombination processes, i.e. better light emitter and photovoltaic candidate at the same time. This talk will brief properties of the films that lead to light emission enhancement and will present the room-temperature photoluminescence results as well as the nature of the interfacial strains and will quantify bandgap modulations. Theoretical interpretation of these interesting results will be presented along with some comparative studies of stressed and alloyed silicon.

Silver nanowire networks show great promise as transparent conductors, but many nanowires in these materials participate only meagerly in conduction. Using graph theoretic measures to rank the importance of different nanowires, we show that simulated networks can be strategically de-densified to below the percolation threshold while maintaining low resistance. We achieve performance reaching 63% of the theoretical maximum, a roughly 91% gain over the performance predicted by percolation theory. Furthermore, the mathematical methods applied here are sufficiently general to apply to any networked material in which there is a non-monotonic relationship between density and performance.

Silver nanowire networks (AgNWs) are a promising candidate for transparent electrodes due to their high transparency, low sheet resistance, flexibility, and potential for low-cost implementation, owing to their high DC-to-optical conductivity ratio and low metal loading. These properties make it suitable for applications such as transparent heaters and solar cell electrodes. Unfortunately, AgNWs are susceptible to electrical, thermal and chemical degradation, which have largely stifled their implementation; effective encapsulation strategies are needed. To ensure the encapsulation approaches are scalable, industrial scale, high throughput deposition methods should be targeted. Moreover, encapsulation methods should use low cost, high-abundance elements with low-temperature processing. A variety of encapsulation approaches have been considered in the past, but physical vapor deposition approaches are the most promising for these criteria.

In this work, reactive pulsed DC and RF magnetron sputtering at room temperature are demonstrated as a promising approach to encapsulate AgNWs with thin films. Since using O₂ as a reactive gas can significantly oxidize and damage the AgNWs and active layer surfaces, N₂ is used as a reactive gas under moderate base chamber pressures (0.5×10^-4 Torr). Low- to moderate- cost metal sputtering targets (Al, Ti, Zr) are then used to deposit as little as 20 nm of oxynitrides using mixed Ar and N₂ gas flow with high deposition rates. Oxynitride-encapsulated AgNW films show exceptional electrothermal stability, surviving at temperatures of 100°C more than unencapsulated AgNWs (>250°C). They also show exceptional chemical stability, where the electrical properties of Al oxynitride encapsulated wires stay nearly unchanged after >7 days under modified Damp Heat testing conditions (80°C, 80%RH). This work demonstrates a versatile approach to encapsulating AgNWs with sputtering, which can be extended to encapsulate lithographically patterned electrodes and other solar cell device architectures.

Oxide electronic materials is one of the most promising technologies for electronic devices, as distinct from the traditional silicon technology. The fact that circuits based on conventional semiconductors such as silicon and conductors such as copper can be made transparent by using different materials, the so-called transparent semiconducting and conducting oxides (TSOs and TCOs, respectively), is of great importance and allows for the definition of innovative fields of application with high added value.
Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge.
In this paper we will present the most important landmarks achieved by these stimulating scientific area as well as some insights to emerging applications.

During the last decade, the constant growth of interest in the field of optoelectronic devices has been observed in almost every commercial electronic device. Advances in interdisciplinary researches and industrial approaches have practically improved the optical characteristics of optoelectronic devices comparable with conventional silicon-based photodiodes. Among these extensive ranges of studies, transparent metal oxide semiconductors such as indium–gallium–zinc oxide or indium-zinc oxide have emerged as an important candidate owing to the wide variety of their electronic and optical properties, for instance, high field-effect mobility, low off-state current, optical transparency, high uniformity in large area deposition and processing versatility. However, the wide bandgap above 3 eV of metal oxide layer fundamentally restricts its light absorption range under about 420 nm which is the spectrum of ultraviolet and blue light. In this regard, more detailed researches about metal oxide semiconductors-based optoelectronic devices are required, which focus on how to engineer them for excellent responsivity upon the visible light and stability concurrently. Studies focusing on expanding the absorption range of metal oxide semiconductors-based phototransistors have been conducted by inserting a visible light absorption layer such as nanomaterials, quantum dots, perovskite, or two-dimensional materials. Although these results manifested the feasibility of metal oxide semiconductors-based optoelectronic devices, they still have some inherent barriers for practical large-scale devices due to the high cost and complicate fabrication process.
In this work, we introduce a facile method to control the photoresponse characteristic of oxide semiconductor under visible light using a reducing agent, sodium dithionite (Na₂S₂O₄), that is able to compensate for the aforementioned limitations. Na₂S₂O₄ exhibits strong reducing properties due to its weak S-S bond. Photoresponse characteristic of hafnium–indium–gallium–zinc oxide (HIGZO) thin-film was enlarged to absorb visible light by modulating sub-gap states and oxygen vacancy of HIGZO through reduction treatment, immersing in Na₂S₂O₄ solution. Moreover, we revealed that photoresponsivity of HIGZO thin-film is easy to control substantially depending upon the molarity of Na₂S₂O₄. This controllability plays a key role in widening its potential applications and fine adjustment of performance from phototransistor to photo memory such as neuromorphic devices using persistent photoconductivity (PPC). Specifically, we achieved HIGZO phototransistors with photoresponsivity of 626.32 A/W, photosensitivity of 3.32 × 10⁷, and detectivity of 7.18 × 10¹¹ Jones under 10 mW/mm² of red light using 0.1 M of Na₂S₂O₄. In addition, HIGZO photomemory-based neuromorphic devices with a paired-pulse facilitation index of 142.6% under the input pulse of red light with 1 mW/mm² intensity were fabricated by increasing the molarity of Na₂S₂O₄ to 0.5 M. As a result, we successfully demonstrated the facile method for improving the performance of HIGZO optoelectronic devices by controlling the photoresponse characteristic which makes it possible to be applied in various optoelectronic applications.

The last decade has witnessed a remarkable development in thin film technologies, the application of which has marked a new era in electronics research and industry. To this end, thin film-based optoelectronic devices have created the foundation of outstanding technology applications, from flexible displays to smart window coatings, to name a few. Whilst the demand for improvement in device performance is increasing, Metal Oxides (MOs) have lately emerged as a prominent class of optoelectronic materials, whilst gathering a plethora of unique features such as increased carrier mobility, high optical transparency and mechanical stability. Besides their advantages, MOs present an excellent compatibility with solution processes, thus providing the opportunity of a novel, high-throughput manufacturing scheme of MO-based high-performance optoelectronic devices.
“Sol-gel” is established as one of the currently favourable solution processes, as it uniquely links MOs to solution processing. This deposition of organometallic precursors via wet techniques, followed by a subsequent thermal treatment, enables the formation of high-quality MOs, while accommodating essential advantages such as ambient processing conditions and composition tuning. The growing implementation of sol-gel ultra-thin MO layers (<10 nm) in optoelectronic devices renders the assessment and control of their optoelectronic properties essential. However, the cost and time-consuming features of current characterization techniques such as XPS and TEM hinders their implementation in a high-throughput production line. In addition, their “destructive” character (Pt deposition in TEM, Argon etching in depth-profile XPS) allows only for final product analysis, thus restricting the possibility of in situ monitoring during fabrication. Therefore, the presence of non-invasive, highly-sensitive characterization techniques could introduce a remarkable upgrade of MO thin film manufacturing.
To address this challenge, we propose Infra-Red Spectroscopic Ellipsometry (IRSE) as an alternative and novel approach towards the clarification of the sol gel precursor conversion stages, the control of which leads to high quality MO ultra-thin films. Particular focus is being paid in the role of substrates, as their optoelectronic properties may tune the presence of spectral features, thus allowing the detection of vibrational modes in ultra-thin films (∼ 1 nm).
In this study, In(NO₃)₃ dissolved in 2-Methoxyethanol (C₃H₈O₂) is converted into an ultra-thin In₂O₃ film (<10 nm), a popular MO semiconductor, widely used as a Transparent Conductive Oxide (TCO) host. The individual stages are captured by IRSE, aiming to shed light into the sol gel reaction conversion. For this, various substrates are being utilized (Al, TiN, Au and Si (including highly doped)), aiming to enhance the IRSE sensitivity, thus provide a clear monitoring of the conversion process through vibrational mode detection. We thus identify the intermediate products during transition towards In₂O₃. This introduces IRSE as a new route towards the detection of sol-gel stages in ultra-thin films, with its implementation in solution processing technologies being potentially essential. The suggested methodology can also be employed in the non-destructive characterization of an expanded material palette, such as 2-D materials. This approach could constitute a novel tool in thin film industry, thus boosting the new generation of optoelectronic devices.

SrVO₃ thin films with a high figure of merit for applications as transparent conductors were crystallized from amorphous layers using solid phase epitaxy (SPE). Epitaxial metal oxides incorporating multivalent ions can be formed with thermodynamics and kinetics derived from crystallization processes that are initiated at buried amorphous/crystalline interfaces. We report crystallization methods allowing control of the ionic valence during crystallization and the synthesis of epitaxial SrVO₃ on SrTiO₃. The SrVO₃ layers exhibited room temperature resistivity as low as 2.5 × 10^-5 Ω cm and visible light transmission ranging from 0.89 to 0.52 for thicknesses from 16 to 60 nm. Amorphous SrVO₃ was deposited at room temperature using radio-frequency sputtering and crystallized by heating in a gas environment with selected oxygen activity. The lattice parameters and mosaic angular width of x-ray reflections from the crystallized films are consistent with partial relaxation of the strain resulting from the epitaxial mismatch between SrVO₃ and SrTiO₃ in the 60 nm layer while the 16 nm layers were found to be fully strained. A reflection high-energy electron diffraction study of the kinetics of SPE indicates that crystallization occurs via the thermally activated propagation of the crystalline/amorphous interface, similar to SPE in other perovskite oxides. Thermodynamic calculations based on density functional theory predict the temperature and oxygen partial pressure conditions required to produce the SrVO₃ phase and are consistent with the experiments. The separation of deposition and crystallization processes used in this work are scalable to large areas and can form the basis of technologies incorporating complex oxide electronic materials in emerging devices.

In modern day life, transparent conducting materials (TCMs) are ubiquitous, playing an integral role in touch and display screens, solar cells and smart windows. P-type TCMs have often been overshadowed by n-type TCMs, due to the increased difficulty in synthesis, poorer stability and inferior opto-electronic properties. With technologies such as pop up windshield displays and fully transparent touch-display screens on the horizon, there is an impending need for the development of a high performance p-type TCM, in turn allowing the fabrication of a transparent p-n junction.

In 2017, CaCuP (and MCuP where M = Mg, Sr, Ba) was identified as a potential p-type TCM by Williamson et al, owing to its large valence band dispersion and optical absorption onset at around 2.7 eV.[1] Powders were synthesised and pressed into pellets, which were shown to be phase pure from XRD analysis, to possess a strong absorption onset of around 2.7 eV, and to display p-type conductivity of around 2 x 10^-3 S cm^-1, with the origin of this conductivity suggested to be Cu vacancies. For a nominally undoped pressed powder sample, this was an impressive level of conductivity, and served as the motivation for this work.

Formation energies for the intrinsic defects in CaCuP have been calculated using hybrid density functional theory within VASP. The Cu vacancy possesses extremely low formation energies under cation poor growth conditions, and is stable into the valence band giving rise to degenerate semiconducting behaviour. The resulting hole charge density is delocalised through the Cu-P layers of the material, indicating high charge carrier mobility and conductivity.

Phase pure thin films have been successfully grown across a Cu gradient via the pulsed laser deposition technique. The measured room temperature conductivity is 1040 S cm^-1, a vast improvement over the pressed powder samples. Hall measurements indicate a mobility of 36 cm² V^-1 s^-1 and a carrier concentration of around 1.8 x 10²⁰ cm^-3, supporting the defect calculation results and indicative of a degenerate semiconductor. Temperature dependent charge transport calculations are being performed using the AMSET code[2] to assess if this is the intrinsic material limit, or if improved electronic properties could be achieved with a better optimised deposition process. The optical behaviour of these films is not straightforward, with a larger absorption coefficient than expected seen at the indirect band gap value of around 1.2 eV – optical defect transition calculations are being performed in order to understand this further.

[1] B. A. D. Williamson et al, Chem. Mater., 29, 2402-2413 (2017).
[2] A. M. Ganose et al, Nat. Comm., 12, 2222 (2021).

Organic Photovoltaics (OPVs) have now attained high power conversion efficiencies (PCEs) over 15%, and research on OPV fabrication via solution processes on flexible substrates, and replacement of toxic halogenated solvents by environmentally benign solvents, is underway, as are extensive efforts to improve the PCEs. In this work, we synthesized solution processable cathode interfacial materials and used in OPVs. Two small-molecule interfacial materials with phosphine oxide and benzoimidazole groups were designed to induce strong interface dipoles and chelate with the metal electrodes. We used isopropanol, an environmentally benign solvent, to dissolve the small-molecule materials and spin-coated solutions onto photoactive layers. The interfacial materials formed ohmic contacts between the electron acceptor materials and the OPV cathodes due to the strong interface dipoles. As the highest occupied molecular orbital (HOMO) energy level of the interfacial materials is -7.05 eV, hole carriers cannot travel toward the cathode. The appropriate energy levels of the interfacial materials and the efficient energy level alignment facilitated charge extraction at the cathodes and increased the PCEs of OPVs. Of the two, BIPO (with two benzoimidazole groups) more efficiently formed smooth interfacial layers on active layers that allowed of better energy level alignment between the electron acceptor materials and the cathodes.

Circularly polarized luminescence (CPL) refers to the differential photoemission between left- and right-handedly polarized lights. The polarity of CPL provides a unique principle for photonic applications, including three-dimensional displays and quantum encryption. However, previous CPL-active materials have suffered from a trade-off between a photoluminescence quantum yield (PLQY) and a luminescence dissymmetry factor (g_lum). We recently discovered that helical co-assemblies of square-planar cycloplatinated complexes could overcome the intrinsic limitation. This strategy exploited an asymmetric exciton coupling and the magnetically allowed metal-metal-to-ligand charge-transfer (MMLCT) transition within the Pt(II) complex co-assemblies. However, utility of this strategy is limited because it inevitably requires chiral guest molecules to form helical co-assemblies.
In this research, we have developed a combined top-down and bottom-up strategy which enables strong CPL emissions without chiral guest molecules. Our strategy takes advantage of the chiral induction effect exerted by counter anions of charge-cationic Pt(II) complexes. The Pt(II) complexes stack in a face-to-face fashion to form helical self-assemblies, where the helical sense is dictated by the point chirality of counter anions. We tailored the chemical structures of the counter anions, including (R)-/(S)-tetrahydrofuran-2-carboxylates ((R)-/(S)-TC), L-/D-dipivaloyl tartarate (L-/D-PT), and L-/D-dibenzoyl tartarate (L-/D-BT).
The Pt(II) complex self-assemblies could be grown by in-situ anion metathesis between [(2-phenylpyridinato)(4,4′-bis(nonyl)-2,2′-bipyridine)platinum]ClO₄ ([PtN]ClO₄) and tetrabutylammonium (TBA) salts of the chiral anions in 1,2-dichloroethane (1,2-DCE):CH₃CN (8:2, wt/wt). The formation of PtN self-assemblies were supported by an emergence of the MMLCT absorption at 500 nm and the strong MMLCT phosphorescence at 613 nm (PLQY = 0.19 for the [PtN]₂BT self-assemblies). The MMLCT transitions of the self-assemblies were chiroptical, as seen from strong couplets at 500 nm in electric circular dichroism. Among the tested anions, BT having the -2 charge and the elongated π-conjugation structure resulted in the most tight and helical stacking of PtN. As expected, helical supramolecular structures were absent for the self-assemblies of [PtN]ClO₄.
To further enhance the CPL activities, we allowed the self-assembly formation within microchannels with a 28 μm width which were formed by a conformal contact between a glass substrate and a poly(dimethylsiloxane) mold. The self-assemblies of [PtN]₂BT formed within the microchannels exhibited a two-fold improvement in PLQY (0.32) over that (0.19) of the self-assemblies formed onto the planar glass substrate. In addition, an order of magnitude enhancement in |g_lum| was also found; g_lum (572 nm) were -0.12 and 0.13 for the self-assemblies of [PtN]₂L-BT and [PtN]₂D-BT, respectively, grown within the microchannels, whereas g_lum (572 nm) were -0.027 and 0.037 for the dropcasted self-assemblies of [PtN]₂L-BT and [PtN]₂D-BT, respectively. The notable improvement in |g_lum| was attributed to the tight and ordered supramolecular packing structures. Our approach is valuable as it does not rely on chiral guest molecules. The successful combination with soft-lithographic techniques will stimulate the future research to construct CPL-active materials.

Highly efficient 10-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10H-dispiro[acridine-9,9′-anthracene-10′,9″-fluorene] (OBOtSAc) and 10-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10H-dispiro[acridine-9,9′-anthracene-10′,9″-fluorene] (tBuOBOtSAc) emitters, comprising almost perpendicularly linked rigid 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (OBO) electron acceptors and a rigid and linear tri-spiral acridine electron donor, are reported here. OBOtSAc and tBuOBOtSAc show deep-blue emission (λ_max = 452 and 446 nm) with narrow full width at half maximum (FWHM) values of 50 and 48 nm, respectively. Due to the rigid and twisted structures, and the appropriate singlet and triplet energy levels of both emitters, 10 wt% doped films of OBOtSAc and tBuOBOtSAc in a bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) host show efficient thermally activated delayed fluorescence (TADF) emission and photoluminescence quantum yields (PLQYs) of 97% and 90%, respectively. Moreover, linear-shaped OBOtSAc and tBuOBOtSAc lead to excellent horizontal emitting dipole orientations (88% and 90%, respectively). Consequently, organic light-emitting diode (OLED) devices using OBOtSAc and tBuOBOtSAc exhibit maximum external quantum efficiencies (EQEs) of 31.2% and 28.2%, respectively, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.147, 0.092) and (0.149, 0.061), respectively.

Transparent amphiphobic surfaces (repelling water and other low surface tension liquids) are of great interest in applications such as self-cleaning, anti-icing, anti-fouling, condensation and water harvesting etc. Current state-of-the-art in highly liquid repellent surfaces highlights several limitations on their use in practical applications. For example, superhydrophobic surface (SHS) suffer from instabilities/collapse of air pockets/cushion responsible for high liquid contact angles. Similarly, the retention of slippery behaviour in lubricant infused surfaces (SLIPS) and other smooth surfaces (SS) are restricted by eventual lubricant depletion and mechanical susceptibility of short molecular chains, respectively. Herein, we present a way to overcome these challenges by combining a robust nanohierarchical morphology with long flexible alkyl chains and achieve high drop mobility (~10 times than SLIPS) and amphiphobicity (Singh et al., Nano Lett. 2021, 21, 8, 3480–3486). High drop mobility comparable to superhydrophobic surfaces is achieved due to nanohierarchy i.e. reduced solid fraction (= 0.4). Precisely controlled nanohierarchy is obtained from surface grown metal-organic frameworks (MOFs) via layer-by-layer method. Covalently attached zirconium-based MOFs (OH-UIO-66) on glass substrate are used for demonstration due to their highly stable framework structure and easy synthesis. We observed a free sliding of vegetable oil, alcohols or other polar liquids at <15°. In addition to the excellent optical transparency and robustness, our surface enables a unique combination of additional desirable properties, e.g., resistance to high-speed liquid impact due to high capillary pressure generated from MOF nanopores (V = 35 m/s, We_l = 42,500), thermal stability up to 200 °C, and mechanical stability of scratch resistance. Moreover, benefiting from inherent porous structure of MOFs, our surface is able to absorb carcinogenic pollutants such as dyes and volatile organic compounds (VOCs) from water. These features endow the usage of the presented nanohierarchical MOF coating in applications such as windows, hand-held displays, goggles, wind screens, optoelectronics and so on.

The fabrication of polymer-based organic light-emitting diodes (OLEDs) using the inverted method, where the device is built on top of a metal back electrode has advantages over more conventional OLED fabrication methods. For example, the inverted method provides a way to control the structure and morphology of the metal electrode. This, in turn, enables improved light management by control of the photonic and plasmonic behavior within the OLED devices. Ag is a good option as a back metal electrode in OLEDs because of its low optical loss, high reflectivity and high conductivity. However, Ag has a work function (~4.4 eV) which is not compatible with the band gap of many polymeric organic semiconductors.
Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) is a conjugated polymer commonly used in OLED prototypes that has high electron affinity (∼3.3 eV), large ionization potential (∼5.9 eV), and high fluorescence quantum yield. Combining F8BT with an Ag back electrode results in a high barrier height for charge injection, which results in poor device performance and a high turn on voltage. A method to overcome this, as some studies on organic field-effect transistors (OFETs) have, used self-assembled monolayers (SAMs) of sulphur compounds like thiols to reduce the work function of Ag and the barrier height for charge injection into the polymer.
Here, we have extended this methodology in using the more commercially available lipoic acid (LA) and its derivatives to form SAMs in between Ag back electrodes and F8BT thin films. By selecting LA, we can tailor the functional groups at one end of the compound by converting the acid to an ester, allowing the influence of difference alkyl chains as well as chain lengths to be investigated. We have studied how having two sulphur linkages instead of one in the case of conventional thiols like 1-decanethiol(1-DT), as well the dipole orientation of the molecules can change the work function of Ag and improve the contact at the metal-polymer interface. X-ray photoelectron spectroscopy and Raman spectra reveal the binding properties of the SAMs, while current density-voltage and ultraviolet photoelectron spectroscopy measurements show that (±)-α-lipoic acid and (±)-4-phenylbutyl 5-(1,2-dithiolan-3-yl)pentanoate improve the charge injection greatly by changing the work function of the Ag and by altering the morphology of the semiconducting polymer. The results obtained gave us valuable insight into the effect of chain length and functional groups of the SAMs in engineering morphological and electronic properties of the metal-polymer interface.

Semiconductors industry necessitate p-type materials in a wide range of applications like photovoltaics, light-emitting diodes, detectors, transistors and thermoelectrics. Copper iodide (CuI) has received significant interest as an alternative beyond silicon. Intriguing optoelectronic properties of CuI include but not limited to, direct and wide bandgap (3.1 eV measured as thin film), high hole mobility (in excess of 40 cm²/V. s) and large exciton binding energy (62 meV). Direct iodination of copper thin films allows production of CuI films even at room temperature, which makes CuI a suitable candidate for flexible and transparent electronics. In this study, γ-CuI nanowire networks were fabricated through direct vapor iodination of copper nanowire (Cu NW) networks. Cu NWs were synthesized and purified by solution means and deposited onto glass substrates by spray coating [1,2]. A house-made setup was used for the vapor iodination of Cu NW networks. The transformation of Cu to γ-CuI was monitored through investigating morphological, structural, chemical and optoelectronic properties of the fabricated γ-CuI NW networks. Upon 120 seconds exposure to iodine vapor, initially smooth Cu NW surface became rough, indicating transformation to γ-CuI with a zinc-blende structure. γ-CuI NWs were found to consist of only Cu⁺ and I^-1. Hall-effect measurements revealed a mobility and resistivity of 12 cm²/V.s and 19.9 ohm-cm, respectively. Optical transmittance of γ-CuI NW networks was over 68% in the wavelength range of 300-1200 nm, while that of Cu NWs was 85%. The direct optical band gap value was determined as 3.2 eV with the help of Tauc’s plot. Following materials characterization, UV-photodetectors in metal-semiconductor-metal structure were fabricated using γ-CuI NW networks. A three times increase in current was registered in response to UV light illumination (with a wavelength 365 nm) under 5 V.

References
[1] Tigan, D., Genlik, S. P., Imer, B., & Ünalan, H. E. Transparent Thin Film Heaters Based on Copper Nanowire Networks
[2] Sevim Polat Genlik, Dogancan Tigan, Yusuf Kocak, Kerem Emre Ercan, Melih Ogeday Cicek, Sensu Tunca, Serkan Koylan, Sahin Coskun, Emrah Ozensoy, and Husnu Emrah Unalan
ACS Applied Materials & Interfaces 2020 12 (40), 45136-45144

Nowadays, displays are evolving as a way of interacting with users rather than simply conveying information. Interactive display is one of the new high-value-added display technologies and means a display that enables interaction between users and display devices through interface technology with transparent display panels that incorporate various sensors. These interactive displays can perform various roles, such as providing new experiences to users through interaction with users and responding to emergency situations through real-time environment awareness. For the implementation of these interactive displays, research on transparent and flexible phototransistors for embedding in transparent display panels is essential. The phototransistor is an essential element to perform various functions such as user's touch, motion, and gaze recognition. To apply it to transparent displays, it is also essential to ensure transparency, to facilitate processing for large-area substrates, and to have low off current for high photosensitivity. Therefore, oxide semiconductors with high transparency, large area processability, and reasonable mobility (~ 10cm²/Vs) are suitable candidates for the channel layer of phototransistors. However, oxide semiconductors cannot absorb visible light with large bandgap energy of ~ 3.0 eV. For fabricate an oxide semiconductor-based phototransistor that can absorb the full range of visible light, one of the most widely used methods is to add absorption layer such as chalcogenides, organics, perovskites, and nanodots. However, in the case of the materials that can absorb visible light, it is not transparent due to a small bandgap. Thus, it is difficult to fabrication transparent phototransistors.
In this work, we propose a method for fabricating polyimide (PI) doped single layer (PSL) phototransistor that is transparent and can absorb visible light. Amorphous indium-gallium-zinc oxide (IGZO) and PI can be deposited using co-sputtering process for the channel layer itself to absorb visible light. The PI is well known in the display industry as a flexible substrate and can be made into a sputter target, so it can be used in sputter equipment. We used a SiO₂/p⁺-Si substrate in which SiO₂ is thermally oxidized on a p⁺-Si wafer; the p⁺-Si and SiO₂ were used as gate and gate insulator, respectively. On the SiO₂/p⁺-Si substrate, the PSL phototransistor was deposited as a channel layer using a co-sputtering process; a shadow mask was used for patterning. The powers of IGZO and PI were set to 150 W and 0 to 30 W, respectively. After that, thermal annealing was performed at 300 ^oC for 1 hr. Finally, aluminum was deposited as a source/drain using a shadow mask for patterning. When fabricating the PSL phototransistor, it was optimized by controlling the sputter power of the PI target. We confirmed that the PSL phototransistor fabricated with sputter power of PI 10 W featured excellent optoelectronic characteristics compared with other PSL phototransistors. In addition, when compared with the IGZO phototransistor, the optoelectronic characteristics of the PSL phototransistor also improved in red lights at 1mW/mm² intensity. Specifically, photoresponsivity improved from 14.43 to 623.79 A/W, photosensitivity improved from 2.30 × 10⁰ to 9.89 × 10⁶, and detectivity improved from 8.92 × 10⁶ to 5.83 × 10¹¹ Jones. Furthermore, as the bending test was conducted 10,000 times, the flexible PSL phototransistor exhibited stable optoelectronic characteristics.
As the phototransistors can be fabricated with an organic material, this manufacturing method shows mechanical flexibility and reliability, which enables its usage in the display industry in the future.

We demonstrate, for the first time, the synthesis of highly ordered titanium oxynitride nanotube arrays sensitized with Ag nanoparticles (Ag/TiON) as an attractive class of materials for visible-light-driven water splitting. The nanostructure topology of TiO2, TiON and Ag/TiON was investigated using FESEM and TEM. The X-ray photoelectron spectroscopy (XPS) and the energy dispersive X-ray spectroscopy (EDS) analyses confirm the formation of the oxynitride structure. Upon their use to split water photoelectrochemically under AM 1.5 G illumination (100mW/cm2, 0.1M KOH), the titanium oxynitride nanotube array films showed significant increase in the photocurrent (6mA/cm2) compared to the TiO2 nanotubes counterpart (0.15mA/cm2). Moreover, decorating the TiON nanotubes with Ag nanoparticles (13±2nm in size) resulted in exceptionally high photocurrent reaching 14mA/cm2 at 1.0VSCE. This enhancement in the photocurrent is related to the synergistic effects of Ag decoration, nitrogen doping, and the unique structural properties of the fabricated nanotube arrays.

X-ray diffraction (XRD) and Raman spectroscopy measurements of relatively new energetic crystal DTO [N-(1,7-dinitro-1,2,6,7-tetrahydro-[1,3,5]triazino[1,2-c][1,3,5]oxadiazin- 8(4H)-ylidene)nitramide] are accompanied by DFT calculations. Calculated XRD and Raman spectra are in reasonable agreement with experiments. Calculated vibrational spectra identified in terms of atomic movements help us determine the `weakest link` in the molecular crystal, whereas DFT partial charge density calculations related with N, C, and O atoms are used to find most chemically active atoms in crystal decomposition. In addition, the calculations show that the cage-containing N atoms provide the main contribution near the bottom of the DTO valence band. The DTO band gap calculated with hybrid function is compared to the band gap of several other energetic materials with regards to impact sensitivity.

Infrared (IR) emitting phosphorescent materials have unique photophysical properties which make them useful for applications in optoelectronic devices. Ir(azp)₂(acac) (Bis(1-monoazaperylene)iridium(acetylacetonate)) is a promising phosphor for IR emitting applications; however, little information is available about the frontier orbitals and triplet states of this molecule. Here, density-functional theory (DFT) is used to calculate the singlet and triplet electronic structure of Ir(azp)₂(acac), and the triplet emission energy. Typically, DFT calculation results of organometallic phosphors, especially of iridium-based emitters, are reported with varying accuracy compared to experimental results. Therefore, first a calculation method is established by carrying out DFT calculations of two well-known organometallic phosphorescent materials: FIrpic (Bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium) and Ir(ppy)₃ (tris(2-phenylpyridine)iridium). A set of different hybrid functionals that are a mixture of Hartree-Fock exchange with DFT exchange-correlation (e.g., B3LYP, M05, M06), or a long-range correlation functional (e.g., CAM-B3LYP), are used with different basis sets including double-zeta (such as LANL2DZ) and polarized triple-zeta basis sets (such as DEF2TZVP) to optimize the ground state and triplet state geometries of the molecules in solvents (using polarizable continuum model) and in the gas phase. It is determined that the B3LYP functional performs better as compared to other functionals in terms of performance as well as accuracy; most likely because it uses non-local correlation, which works better with large molecules. From the DFT calculations of FIrpic and Ir(ppy)₃, and comparing the calculation results with relevant literature, it is determined that the LANL2D basis set gives results within less than 0.2 eV of error. The DEF2TZVP, being a bigger basis set used with the B3LYP functional, gives the most accurate energy levels; however, it requires much longer computational time. Later, the B3LYP functional and DEF2TZVP basis set are used to carry out geometry optimization of Ir(azp)₂(acac) in the ground state and in the first triplet state in dichloromethane and in the gas phase. Energy levels of the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital and first triplet level (T₁) are determined from the optimized geometry. Finally, the triplet emission energy is calculated from the difference in energy level of T₁ and HOMO. This study not only provides useful information about the frontier orbitals and triplet states of Ir(azp)₂(acac) but also will help researchers working in the field in selecting appropriate functionals and basis sets for DFT calculations of the similar molecules.

Transparent conducting oxide (TCO) thin films play an important role in the development of optoelectronic devices. To fulfill their function as contacts and interfaces in optoelectronics, these materials need to perform well in terms of electronic conductivity, while at the same time maintaining their optical transparency. The most widely-used TCO is indium tin oxide. However, a remaining long-standing challenge in the synthesis of such indium-oxide-based thin films is achieving a sufficiently high crystalline quality and smooth surface morphology, which has complicated their integration into next-generation nanoscale devices. Here, we use oxide molecular beam epitaxy to explore the optimal growth parameters for both undoped and doped indium oxide on various technologically relevant substrates. By combining in-situ electron diffraction during thin-film growth with post-deposition X-ray diffraction and atomic-force microscopy, we achieve improved epitaxy and smoothness of our films as an important step to facilitate their integration in nanoscale optoelectronic devices.

Mixtures of silica and polysiloxane form thixotropic inks useful for 3D printing. However, 3D printed structures of these inks have typically been opaque due to the refractive index mismatch between the base polysiloxane and silica. By tuning the composition of the base polysiloxane, we matched the refractive index to formulate transparent inks. The transparent inks transmit up to ca. 90% of 700 nm light through 1 cm and remain transparent when solidified. We observed and characterized a thermochromic effect, where the inks change in opacity depending on refractive index mismatch and temperature. We also observed that the rheological properties of the ink depend on the distribution of silica particles, which is dictated by silica functionality, weight content, and processing. We demonstrate utility of these inks by printing encapsulants for LEDs, scintillators, and microfluidic mixing devices.

It is well known that increasing the surface area and reducing the electrical resistance of the photoanode electrode are important factors for realizing a highly efficient dye-sensitized solar cell. Long and dense nanorods are a suitable structure for obtaining a large surface area. However, in long nanorods, the photoexcited carriers tend to recombine before reaching the electrode, which causes a decrease in photoelectric conversion efficiency. Therefore, a low resistivity material is desired as the nanorod material. ZnO possess higher electron transport properties than traditional anode material, TiO₂, and thus improve the conversion efficiency of the solar cell. However, bare ZnO nanorods are prone to chemical etching. In this point, the ZnO/TiO₂ core-shell heterojunction is candidate for the nanodot, because TiO₂ shell can prevent both the chemical eiching and material corrosion caused by the external environment such as acid and/or alkali solutions. Nevertheless, the conversion efficiency of present ZnO/TiO₂ nanorod dye-sensitized solar cells is not sufficiently high. In this study, we adopted Al-doped ZnO (AZO) as the core nanorod material. This is because the resistivity of AZO is lower than that of ZnO, so higher conversion efficiency can be expected.

AZO/TiO₂ core-shell heterojunction nanorods are prepared by a combination of hydrothermal method and successive ionic layer absorption and reaction (SILAR) method. Fluorine-doped tin oxide (FTO) sheet glass was used as substrate. AZO seed layer was deposited on the FTO glass substrate by sputtering. AZO nanorods were prepared by immersing the substrate in a mixtured solution of zinc nitrate hexahydrate, aluminum nitrate hexahydrate, hexamethylenetetramine and polyethyleneimine at 90°C for 3 hours. TiO₂ shell layer was then prepared by SILAR method, repeatedly dipping three kinds of solution, “titanium tetraisopropoxide + diethanolamine + IPA”, “IPA” and “deionized water”, in this order. From SEM observations, the typical length and diameter of the nanorods were 1.6µm and 60 nm, respectively.

Comparing the performance of our dye-sensitized solar cells using AZO nanorods and that of ZnO nanorods, the short-circuit current increased by 12% and the photoelectric conversion efficiency improved by 43% by using AZO nanorods.

Nematic liquid crystals (LC’s), ubiquitous in high information density displays, are typically composed of rod-shaped molecules that can exhibit large dielectric anisotropy, high birefringence and excellent transparency. These properties make them well suited for use in many optical components in high power laser systems, including polarization controllers and optoelectronic devices for beam shaping in 3D metal printing [“A new era In metal part production”, available at www.seuratech.com]. Recently, the single shot laser damage threshold of both saturated and un-saturated nematic LC’s revealed an intrinsic relationship between the degree of chemical bond saturation within LC molecules and laser damage [Kosc et al, Scientific Reports 2019, 9 (1), 16435]. Extrinsic factors such as alignment layer material selection and preparation, particulate contamination and LC chemical purification can also play an important role in determining the optical response and durability of the liquid crystals. In this study we investigate the efficacy of several absorption-based purification methods and their impact on the laser damage behavior of nematic LC’S. We focus on the lifetime of purified and as-prepared LC’S when exposed to multiple laser pulses at kW’s average power, over cm² areas, at near infrared wavelength and 20 to 40 Hz rep rates. Numerical simulations are used to estimate the degree of heat accumulation during exposure to rationalize the LC lifetime results.

Interfaces govern the behaviour of all electronic devices. Herbert Kroemer coined the famous phrase “the interface is the device” in his 2000 Nobel Prize lecture, and we are still applying tremendous effort to understand interfaces in new material generations, with transparent and wide band gap materials being no exception. Understanding gained for the bulk behaviour of materials can often not be extended to their behaviour in structured film stacks, where interfaces play a vital role.

This talk will give an overview of analytical strategies to probe buried interfaces between metallisation, dielectrics and semiconductors using both soft and hard X-ray photoelectron spectroscopy (XPS and HAXPES). It will contrast opportunities and limitations of these techniques to qualitatively and quantitatively explore heterostructures both in the laboratory and at synchrotron sources. It will explore how the different types of spectral features, such as shallow and deep core levels, Auger lines, and valence spectra, can be used to understand aspects of device critical interfaces such as elemental distributions, local chemical environments, intermixing and interdiffusion, and interface defects.

To illustrate the capabilities of PES techniques to address device interfaces and their structure and electronic structure, application examples including bulk Ga₂O₃, transparent conducting oxide (TCO) based transistors and diodes and copper-based industrial metallisation schemes will be used.

Tin dioxide (SnO₂) is one of the most studied and used metal oxide semiconductors and is often used in gas sensors. Despite the extensive research into SnO₂ and its wide range of uses, its surface chemistry is still not fully understood. A detailed literature search of previously proposed surface structures and gas adsorption mechanisms for SnO₂ reveals many conflicting models with zero consensus. This disparity between different mechanistic theories demonstrates the need for a more thorough investigation into SnO₂ if we are to design new gas sensing materials in a methodical way in the future.

In this work, we use levels of accuracy within a Density Functional Theory (DFT) framework which have hitherto been inaccessible due to their computational expense. We initially calculate the optoelectronic properties of bulk SnO₂ and systematically investigate the effect of slab and vacuum thickness on surface model validity. We go on to calculate the energetics and electronic structures for a range of proposed surface defects, as well as for various proposed gas adsorption sites.

Our initial results show that larger slab models than those previously used are required to accurately represent the SnO₂ crystal structure. Overall, this work highlights how modern first-principles methods can elucidate surface phenomena in well-studied materials and paves the way for the rational design of new inorganic compounds for gas sensing applications.

The ability to accurately model, understand and predict the behaviour of crystalline defects would constitute a significant step towards improving photovoltaic device efficiencies and semiconductor doping control, accelerating materials discovery and design.¹ In this work, we apply hybrid Density Functional Theory (DFT) including spin-orbit coupling to accurately model the atomistic behaviour of the cadmium vacancy (V_Cd) in cadmium telluride (CdTe).¹ In doing so, we resolve several longstanding discrepancies in the extensive literature on this species.

CdTe is a champion thin-film absorber for which defects, through facilitation of non-radiative recombination, significantly impact photovoltaic (PV) performance, contributing to a reduction in efficiency from an ideal detailed-balance limit of 32% to a current record of 22.1%. Despite over 70 years of experimental and theoretical research, many of the relevant defects in CdTe are still not well understood, with the definitive identification of the atomistic origins of experimentally observed defect levels remaining elusive.^2–4

In this work, through identification of a tellurium dimer ground-state structure for the neutral Cd vacancy, we obtain a single negative-U defect level for V_Cd at 0.35 eV above the VBM, finally reconciling theoretical predictions with experimental observations. Moreover, we reproduce the polaronic, optical and magnetic behaviour of V_Cd^-1 in excellent agreement with previous Electron Paramagnetic Resonance (EPR) characterisation.^5,6 We find the cadmium vacancy facilitates rapid charge-carrier recombination, reducing maximum power-conversion efficiency by over 5% for untreated CdTe—a consequence of tellurium dimerisation, metastable structural arrangements, and anharmonic potential energy surfaces for carrier capture.

The origins of previous discrepancies between theory and experiment, namely incomplete mapping of the defect potential energy surface (PES) and approximation models, are highlighted and discussed. Importantly, these results demonstrate the necessity to include the effects of both metastability and anharmonicity for the accurate calculation of both charge-carrier recombination rates in emerging photovoltaic materials and the efficiency limits imposed by both native and extrinsic defect species.

¹ Y.-T. Huang, S. R. Kavanagh, D. O. Scanlon, A. Walsh and R. L. Z. Hoye, Nanotechnology, 2021, 32, 132004.
² S. R. Kavanagh, A. Walsh and D. O. Scanlon, ACS Energy Lett., 2021, 6, 1392–1398.
³ A. Lindström, S. Mirbt, B. Sanyal and M. Klintenberg, J. Phys. D: Appl. Phys., 2015, 49, 035101.
⁴ J.-H. Yang, W.-J. Yin, J.-S. Park, J. Ma and S.-H. Wei, Semicond. Sci. Technol., 2016, 31, 083002.
⁵ A. Shepidchenko, B. Sanyal, M. Klintenberg and S. Mirbt, Scientific Reports, 2015, 5, 1–6.
⁶ P. Emanuelsson, P. Omling, B. K. Meyer, M. Wienecke and M. Schenk, Phys. Rev. B, 1993, 47, 15578–15580.

Fabrication of low-resistivity, robust front contacts for solar cell electrodes typically requires temperatures exceeding 200 ^oC to evaporate high-resistivity organic additives and sinter the suspended silver particles that form screen printing pastes. Novel technologies such as passivating contact, perovskite, and silicon heterojunction solar cells, along with their tandems, stand to benefit greatly from high performance metallization formed at temperatures that are compatible with each solar cell architecture [1,2,3].
Passivating contact technologies like silicon heterojunction (SHJ) cells have shown record power conversion efficiencies of 25.11% on full-size M2 monocrystalline wafers and continue to show promise as the platform for ever higher efficiencies (e.g. tandem) [4,5]. One of the main drivers of the SHJ solar cell’s high efficiency is the passivating layers of a-Si that offer extremely low surface recombination velocity, <5 cm/s at room temperature [6]. The efficiencies of these cells are typically limited by their series resistance, which arises from the low conductivity of the low temperature silver paste used for front-grid metallization. While higher processing temperatures would enable lower metallization-related losses it would inevitably degrade the surface passivation quality of the SHJ cells. Therefore, metallization technologies that can offer silver-like resistivity and low contact resistance at temperatures below 150 ^oC have the potential to further propel passivating contact technologies.
In this presentation we will present the use of reactive silver ink (RSI) and dispense printing to fabricate solar cell front-grids that result in a low-resistivity, low-temperature metallization. When used to metallize a front-grid, the RSI results in media resistivities of 2.4-6 μΩ-cm and a cell series resistance of 0.76 Ω-cm², which is lower than commercially available screen-printed low-temperature silver paste with a media resistivity of 10 μΩ-cm and series resistance of 0.79 Ω-cm². This RSI metallization can be deposited at temperatures between 60-115 ^oC depending on the ink composition with a much lower silver loading than low temperature screen-printing pastes. These inks composition offer also the added benefit of being more corrosion resistant that the screen-printing paste counterparts. Using RSI allows a 55% decrease in silver consumption with improved cell efficiency when compared to the standard low-temperature silver paste metallized cell. This results in a substantial decrease in the cell metallization cost, which accounts for 30% of cell production costs [7]. Herein we will show the electrical performance of cells and modules prepared with RSI metallization, optical modeling of the dispensed RSI fingers as well as aging results under damp heat conditions.

[1] Y. Wang et al., Solar RRL, vol. 2(1), 1700180, 2017.
[2] S. Zhu et al., Nano Energy, vol. 45, 280-286, 2017.
[3] A. Moldovan., IEEE 42nd PVSC, 2015.
[4] X. Ru et al., J. Solmat, vol. 215, 110643, 2020.
[5] Hermle, M., … & Glunz, S. W. Appl Phys Rev 7, 021305 (2020).
[6] Bernardini, S. & Bertoni, M. physica status solidi (a) 1800705–6 (2018)
[7] Louwen et al., J. Solmat, vol. 147, 295-314, 2016.

While crystalline silicon solar cells (cSi) continue their steady march towards ever higher efficiencies, the practical physical efficiency limit for these devices looms on the horizon. In order to continue to increase module efficiency and lower overall system costs, new technologies which can surpass this limit must be realized. A tandem solar cell approach which pairs cSi with a Perovskite solar cell is the most promising current possibility to achieving this. Such an approach promises the potential of minimal additional production costs over existing cSi with a significantly higher efficiency limit. Indeed, with efficiencies of over 29% having been reported by several laboratories it is clear that this technology has great potential. However, significant work still must be done in order to assess the compatibility of this technology with industrialization and to challenge the assumption that the processing steps will incur a minimal cost increase. One central question in this vein is can we adapt existing industrial solutions for the large-scale production of Silicon/Perovskite tandems? Towards answering this question, three critical areas are examined: 1) Metallization, 2) Large-area perovskite deposition, and 3) Encapsulation.
Screen-printed Ag metallization is a standard process for commercial solar cells. However, the heat sensitive nature of perovskite solar cells makes this step challenging to adapt in tandem solar cells. Specifically, the requirement to have high conductivity and low contact resistance between the Ag fingers and the TCO appears difficult to achieve with most commercial pastes. Using optimized Ag pastes and standard screen-printing processes we demonstrate 4 cm² devices with a flat or textured front surface can be produced achieving efficiencies of 27.7% and 26.2%, respectively.
While these results are very promising, they were accomplished using spin coated layers on non-standard PV wafers. Moving towards a normal PV wafer requires a process with a very high throughput, good process control, and ultimately result in good semiconductor properties. However, the design parameters of this process are convoluted with the surface topology of the silicon wafer. As mechanically polished wafers are not realistic for commercial production, the question of surface preparation on commercial wafers becomes very important. To this end, we demonstrate a strategy of using meniscus deposited perovskite layers on top of chemically polished silicon M2 CZ bottom cells to achieve efficiencies over 22% on an area of 100cm². Such an approach should be compatible with existing industry standard wet-chemical preparation of silicon wafers. Additionally, meniscus coating of perovskite layers promises extremely high throughput potential using a technique that is already widespread commercially (albeit not in the PV world).
Finally, encapsulation of these devices is a critical step to ensuring good stability and reliability in the field. To enable adoption of this technology into existing production lines, demonstration of glass-glass encapsulation using a membrane laminator appears to be essential. It is demonstrated that by optimizing the encapsulant materials and lamination process for lower temperature conditions allows for good encapsulation of tandem solar cells. However, a significant performance drop related to degradation of the cell during the process is observed. Through process optimization and optical refinement of the tandem stack we can minimize this performance loss and demonstrate a 4cm² device with printed metallization and glass-glass encapsulation with a stabilized efficiency of 24.8% (representing a 0.3% drop in efficiency over the unencapsulated cell).

The photovoltaic conversion efficiency of two-terminal (2T) perovskite/Si tandem solar cells is strongly dependent on the degree of current matching that can be achieved between the top and bottom cell. In conventional cell designs, the distribution of light that is absorbed in the top and bottom cells is simply determined by the single or double-pass absorption in the planar sub-cells. For high-bandgap perovskites this will always lead to a current mismatch, as too small an amount of light is absorbed in the top cell. Here we show how a spectrum-splitting metasurface, placed at the perovskite/Si interface can help optimize the absorption distribution between the subcells for certain tandem cell geometries.

As a starting point we use detailed-balance calculations of the tandem cell efficiency for ideal subcells. For a 500 nm thick perovskite top cell the optimum perovskite bandgap is 1.57 eV for single-pass absorption (1.66 eV for double-pass absorption), in order to achieve optimal current matching. We then add a Lambertian-scattering metasurface at the perovskite/Si interface that reflects light in a narrow spectral band above the perovskite bandgap. This shifts the bandgap for which current matching is achieved to 1.70 eV and yields a higher efficiency, as the bandgap is closer to the ideal bandgap for perovskite/Si tandems (1.73 eV).

For wider perovskite bandgaps >1.73 eV, a spectrum splitter shows a major improvement as it strongly enhances the current in the (current-limiting) top-cell. For an idealized geometry we predict an absolute efficiency enhancement of more than 5% absolute for a top cell bandgap of 1.77 eV. Using experimental external quantum efficiency data for perovskite and Si cells this would translate to a practical efficiency gain of 2.7% (absolute) for this perovskite bandgap.

In practice, spectrum splitter metasurfaces may have small parasitic absorption. We include this in our model and find that for a top cell bandgap of 1.70 eV, a Lambertian spectral splitter with 15% parasitic loss and a reflectivity of 0.5 would still yield an efficiency gain of 2% absolute. Hydrogenated amorphous Si scatterers are ideal candidates to achieve this experimentally, as we have experimentally shown in four-terminal perovskite/Si tandem solar cells.

A subtlety in our analysis is the fact that the optimum energy at which the spectral splitter reflects light back into the top cell does not necessarily coincide with the top cell bandgap. This energy depends on which cell is current limiting, and for bandgaps around the ideal bandgap, like 1.70 eV, an onset of 20 meV above the bandgap yields the highest efficiency.

Our calculations show that a planar or Lambertian spectral splitter could be of significant benefit for two-terminal perovskite/Si tandems, especially around and above the ideal perovskite bandgap of 1.73 eV. Furthermore, we find that for non-ideal reflectivities and considering parasitic loss, significant efficiency gains can be achieved in these tandem solar cells.

The multitude of layers, interfaces, elements, impurities, defects, etc. contributing to the structure, function, and performance of optoelectronic devices means that characterization and fundamental understanding of the chemical and electronic structures of each component, as well as their interactions at interfaces, are crucial to support technical progress. The deliberate combination of complementary electron and x-ray spectroscopies can reveal crucial information that open routes to insight-driven strategies to improve device performance.
To showcase the power of this approach, several examples from the field of photovoltaics will be presented. In particular, the chemical and electronic structures (including energy level alignment) of the emitter/absorber and absorber/back contact interfaces for Cu(In,Ga)Se₂ (CIGSe) thin-film solar cell absorbers and how they are modified by different alkali post-deposition treatments will be presented and discussed against the background of the respective device performances.

The performance of halide perovskite-based electronic and optoelectronic devices is often related to interfacial charge transport. To shed light on the underlying physical and chemical properties of CH₃NH₃PbI₃ (MAPbI₃) in direct contact with common electrodes Al, Ti, Cr, Ag, and Au, the evolution of interfacial properties and Fermi level pinning is systematically studied. Given a unique experimental facility, pristine interfaces without any exposure to ambient air were prepared. We observe aggregation of substantial amounts of metallic lead (Pb⁰) at the metal/MAPbI₃ interface, resulting from the interfacial reaction between the deposited metal and iodine ions from MAPbI₃. It is found that the Schottky barrier height at the metal/ MAPbI₃ interface is independent of the metal work function due to strong Fermi level pinning, possibly due to the metallic Pb⁰ aggregates, which act as interfacial trap sites. The charge neutrality level of MAPbI₃ is consistent with the energy level of Pb⁰-related defects, indicating that Pb⁰ interfacial trap states can be nonradiative recombination sites. This work underlines that control of chemical bonding at interfaces is a key factor for designing future halide perovskite-based devices.

Device architectures for high-performance solar cells have evolved over decades to maximise current collection and to minimise voltage losses. The two main challenges caused by moving to new systems is the change in crystal structure and the variation in electron energies (ionisation potential and electron affinity). These issues are magnified for non-cubic crystals where anisotropic materials properties can result in a mistmatch between the ideal orientations for absorption and transport verses the preferred orientation for crystal growth. I will discuss our development and application of computational tools to accelerate the design process including the electron-lattice-strain (ELS) descriptor[1] and its extension to systems including Sb2Se3, SnS, and CuBiS2 [2,3].

1. Screening procedure for structurally and electronically matched contact layers for high-performance solar cells: hybrid perovskites, J. Mater. Chem A 4, 1149 (2016); https://doi.org/10.1039/c5tc04091d

2. Finding a junction partner for candidate solar cell absorbers enargite and bournonite from electronic band and lattice matching, J. App. Phys. 125, 055703 (2019); https://doi.org/10.1063/1.5079485

3. Upper limit to the photovoltaic efficiency of imperfect crystals from first principles, Energy Environ. Sci. 13, 1481 (2020); https://doi.org/10.1039/D0EE00291G

Organic light emitting diodes (OLEDs) have been of great interest for display and lighting applications during decades and they have recently been utilized in smartphones and flat-panel displays. After the development of fluorescent materials, the phosphorescent materials have been introduced and have achieved high efficiency. However, phosphorescent materials require expensive heavy metal such as Ir and Pt. Recently, thermally activated delayed fluorescence (TADF) materials have been developed as a new class of light emitting material where triplet excitons are converted into singlet excitons without heavy metals in phosphorescent materials.[1] Theoretically, an internal quantum efficiency of 100%, the same as for phosphorescent materials, can be expected.
In order to study the optical properties of materials, we have employed the time dependent density functional theory (TDDFT), which is one of the most prominent and widely used methods for calculating excited states of various molecules, and it is recognized as a powerful tool for studying their electronic transition. In our calculations, the real-time and real-space (RSRT) techniques are employed in solving time dependent Kohn-Sham equation by the finite difference approach [2,3] without using explicit bases such as plane waves and Gaussian basis. Within the frame work of this approach, we can solve for the wave functions on the grid with a fixed domain, which encompasses the physical system of interests.[4]
In this study, we have focused on the spectrum of OLED materials. For material design, it is highly desirable to simulate the spectral profile, not only the peak wavelength of absorption and/or emission, but also its spectrum shape. RSRT-TDDFT is applied to analyze some typical organic materials to study their spectrum profiles. In adition to the convensional fluorescent materials, phosphorescent and TADF materilas are studied. Although the research is in an early stage, current results show that this approach may be able to simulate the spectrum profile reasonably well and to apply the material design.


REFERENCES
[1] K. Goushi, K. Yoshida, K. Sato, C. Adachi, Nat. Photonics, 6(2012), 253
[2] E.Runge, E. K. U. Gross, Phys. Rev. Lett. 52, 997(1984)
[3] K. Yabana and G.F. Bertsch, Phys. Rev. B54, 4484(1996)
[4] N. Akino and Y. Zempo, MRS Advances, 1(2016), 1773

Despite long-standing research efforts to develop p-type transparent conductive materials (TCMs), the current generation of optoelectronic devices still relies exclusively on n-type TCM contacts due to their much better trade-off between conductivity and transparency. An important issue in oxide-based p-type TCMs is the deep energy and localized nature of the 2p oxygen states in the valence band, which has negative consequences on both hole dopability and hole mobility in most oxides.

Recent computational work [1,2] has identified several earth-abundant phosphides (particularly BP and CaCuP) that could potentially outperform the existing p-type TCMs. These phosphides have shallow, disperse valence bands that are ideal for high hole dopability and mobility. Although their indirect band gaps are rather low (around 2 eV), their direct band gaps are much wider and could ensure transparency in thin film samples. However, thin films of CaCuP have not been made yet, and previously reported BP thin films have not been thoroughly evaluated as transparent contacts [3].

Using a unique combinatorial sputter chamber with reactive PH₃ gas, we have synthesized BP and CaCuP films as a function of various process parameters. Last year we gave a preliminary report of their properties at this conference. This year, we will discuss their suitability as p-type TCMs in much greater detail.

In particular, we have found that the electrical conductivity of CaCuP under optimized growth conditions is almost on par with the conductivity of state-of-the-art n-type TCMs such as ITO and FTO, even in the absence of any extrinsic dopant. However, CaCuP films are only moderately transparent in the visible region due to unexpectedly high absorption strength above the indirect band gap.

We have also found that BP can be doped p-type up to remarkably high hole concentrations by using extrinsic dopants combined with off-stoichiometry. However, it is challenging to obtain BP films of high crystal quality by sputter deposition. This issue has so far prevented us from reaching high hole mobilities.

Despite the experimental challenges, we confirm that both CaCuP and BP could be outstanding transparent conductors if their growth processes could be further improved. More generally, the field of metal phosphides appears to be a fertile search space for potential p-type transparent conductors.

[1] B. Williamson et al., Chem. Mater., 29, 2402–2413 (2017).
[2] J. Varley et al., Chem. Mater., 29, 2568–2573 (2017).
[3] A. Fioretti, M. Morales-Masis, J. Photonics Energy, 10, 042002 (2020)

Effective sensing of optical signals by a solid-state detector that convert photons to electricity are widely used in digital imaging, optical communications, remote-sensing, spectroscopy, and astronomy. These photodetectors should possess high sensitivity, i.e., the ability to differentiate signal from noise, especially at low light intensities down to single-photon level. In recent years, colloidal quantum dots (CQDs) have gained significant attention in optoelectronics due to their wide spectral tunability, high absorption coefficients, and large oscillator strengths. Colloidal quantum dots also allow facile solution-processing that enables an easy room-temperature integration onto variety of substrates, rigid or flexible, including read-out integrated circuits with near-unity fill factor. In semiconductor optoelectronics, fast and sensitive photodetection is achieved when photocarrier recombination is limited by physically separating holes and electrons through electron/hole-transport layer. In our work, we developed vertical-stack photodetectors, which uses a solution-processed nanocrystal photoconversion layer coupled to an amorphous selenium wide-bandgap avalanche charge transport layer, in the desired p-i-n fabrication sequence that enables compatibility with active-matrix readout circuitry. Amorphous-selenium detectors have gained significant attention due to their single-carrier hole impact ionization phenomenon achieving high gains (~1000) with a very low excess noise factor and are suitable for a large-area deposition at room-temperature without damaging the underlying read-out circuits. Here, in our recent work on colloidal quantum dot/glass avalanche transport layer heterostructures, optical absorption and electrical transport are physically separated which enables increased flexibility for spectral tuning and increased charge extraction reaching high specific detectivity (5x10^12 Jones), fast photoresponse with megahertz 3-dB electrical bandwidth (~ 25 Mhz), ultra-low dark current density (~ 10 pA/cm2), low noise current (~ 20 fW/Hz1/2), and high linear dynamic range (~ 150 dB).

X-ray photoelectron spectroscopy has played an enormous role in developing our understanding of a range of interesting materials over the last five decades, primarily arising from its ability to energy-resolve electrons ejected from any sample due to its interaction with sufficiently high energy photons, thereby defining one of the most powerful spectroscopic techniques. However, when combined with space-resolution to achieve a microscopy based on this spectroscopy, it can provide us with unique information, not available otherwise. In my presentation, I shall describe two qualitatively different ways^1,2 to achieve such a space-resolution to provide complementary insights into diverse systems.

In the first part of the presentation, I shall focus on an elusive metastable phase, existing only as small patches in chemically exfoliated few-layer, thermodynamically stable 2H phase of MoS₂ that is believed to critically influence properties of MoS₂ based devices. Its electronic structure is little understood in absence of any direct experimental data and conflicting claims from theoretical investigations. I shall present data^3,4 to conclusively resolve this issue based on probing the electronic structure of chemically exfoliated few layer systems using lateral-space-resolution in photoemission spectroscopy.

In the second approach, I shall explore the rapidly expanding field that deal with emergent properties at a variety of interfaces formed in heterostructured materials, including optically active nanomaterials. It is in general difficult to probe directly the nature of such interface states since these are typically buried under a depth and represent a very small volume fraction of the entire sample. We have shown over the years^2,5-10 that x-ray photoelectron spectroscopy with a widely tunable photon energy from synchrotron sources can be used very effectively to obtain vertical-space resolution leading to layer-resolved electronic structure information; This makes this technique an ideal probe to investigate interface properties in diverse heterostructured systems. I shall illustrate this by discussing electronic properties of a few interesting systems, such as LaAlO₃-SrTiO₃, magnetic tunnel junctions with CoFeB and MgO, and optical properties of highly luminescent semiconductor nanoparticles with integrated internal interfaces.

References:
1. D. D. Sarma et al., Phys. Rev. Lett. 93, 097202 (2004).
2. D. D. Sarma et al., Chem. Mater. 25, 1222 (2013).
3. Banabir Pal et al., Phys. Rev. B 96, 195426 (2017).
4. Debasmita Pariari et al., Applied Materials Today 19, 100544 (2020); and APL Mater. 8, 040909 (2020).
5. S. Sapra et al., J. Phys. Chem. B 110, 15244 (2006).
6. Pralay K. Santra et al., J. Am. Chem. Soc. 131, 470 (2009).
7. D. D. Sarma et al., J. Phys. Chem. Lett. 1, 2149 (2010).
8. Banabir Pal et al., J. Electron Spectrosc. Relat. Phenom. 200, 332 (2015).
9. Sumanta Mukherjee et al., Phys. Rev. B 91, 085311 (2015).
10. Sumanta Mukherjee et al., EPL (Europhys. Lett.) 123, 47003 (2018).

Electron transport layers (ETLs) have been instrumental in breaking the efficiency boundaries of optoelectronic devices. A key ETL for organic electronics is ZnO, however, depending on its preparation it can be a sub-optimal ETL suffering from surface defects. The application of a thin interlayer between ZnO and the active layer of optoelectronic devices has proven to be a successful method to achieve performance enhancements. Ultrathin MgO in combination with metal oxides such as SnO₂ and TiO₂ to form effective ETLs has afforded tremendous success in various solution-processed systems, such as perovskite photovoltaics. However, its application to improve ZnO based ETLs for organic electronics is limited.¹

In our work, we investigate MgO as a modifying interlayer on top of a ZnO ETL formed by simple solution-processing and low temperature annealing and apply it to organic photovoltaics (OPVs). We show that the modified ETL enhances the performance of OPV devices with respect to a single ZnO ETL without significantly affecting the photon capture rate and blend morphology. The ZnO/MgO ETL is shown to have a more uniform top surface with a root mean square roughness decreasing from 4.5 ± 0.2 nm to 3.2 ± 0.4 nm and a lower work function (by 0.11 ± 0.05 eV) compared to the single ZnO ETL, which is expected to be beneficial to electron extraction. We demonstrate that insertion of the thin MgO interlayer in devices leads to a reduced leakage current and an increase in the shunt resistance. The power conversion efficiency of OPVs employing the ZnO/MgO ETL is enhanced by ≈ 10% (relative increase) as a result of a boosted short circuit current density and fill factor.²

1. J. Dagar et al., “Highly efficient perovskite solar cells for light harvesting under indoor illumination via solution processed SnO₂/MgO composite electron transport layers”, Nano Energy, 49, (2018) 290–299.
2. I. Ierides et al., “Inverted organic photovoltaics with a solution-processed ZnO/MgO electron transport bilayer”, J. Mater. Chem. C, 9, (2021) 3901-3910

Transparent conductive oxides (TCOs) are essential for optoelectronic devices. Among TCOs, ITO is widely utilized due to its high electrical conductivity (~10000 S cm^−1[1]) and easy synthesis. However, the bandgap of ITO (~3.5 eV) is insufficient compared to the requirements of electrodes for deep-ultraviolet light-emitting diodes (DUV-LEDs) (hν > 4.1 eV). As the electrode, La-doped SrSnO₃ (LSSO) is promising due to its both wide bandgap (~4.6 eV) and high electrical conductivity (~3000 S cm^−1[2]). Nevertheless, there is a problem that single-crystal LSSO epitaxial film requires high deposition temperature, which is beyond the heat resistant temperature of the DUV-LEDs. In order to solve this problem, we proposed a lift-off and transfer method, which enables preparation of LSSO sheet on many kinds of substrates at room temperature.
The LSSO thin films were heteroepitaxially grown on the water-soluble Sr₃Al₂O₆ (SAO)^[3,4]-buffered SrTiO₃ (STO) substrate by pulsed laser deposition method. Then, we removed the SAO layer with water. The LSSO sheet was further transferred on glass or PET substrates.
The out-of-plane X-ray diffraction (XRD) patterns of the LSSO/SAO bilayer film and transferred LSSO sheet clearly exhibited the LSSO peaks without any impurity peaks. In addition, the sheet exhibited coexistence of high electrical conductivity (~1600 S cm⁻¹) and wide bandgap (~4.6 eV) at room temperature. The single-crystal LSSO sheet with lateral size of 5 mm × 5 mm was successfully obtained.


[1] H. Ohta et al., Appl. Phys. Lett. 76, 2740 (2000).
[2] M. Wei et al., Appl. Phys. Lett. 116, 022103 (2020).
[3] D. Lu et al., Nat. Mater. 15, 1255 (2016).
[4] P. Singh et al., ACS Appl. Electron. Mater. 1, 1269 (2019)

Performance of perovskite solar cells (PSCs) are largely determined by the nature of the hole -collector material and its interface with the perovskite. The molecular solid Spiro-OMeTAD; inorganic p – type semiconductors such as CuSCN, CuI and NiO; conducting polymers; metals and forms of carbon have been used as the hole -collector materials. PSCs based on metals and carbon are often referred to as hole- collector free perovskite solar cells. A misleading statement, because it is not clear how the hole extraction property has arisen, as the of nature of the true interface remain uncertain. Metals are often coated with oxide layers and in many instances different forms of carbon had been blended other substances and reported as carbon electrodes without empathizing role of the binder.
Compared to metals, carbon and its blends turn out be more suitable to understand mechanism by which the holes photo-generated in perovskite are transferred to the cathode of the solar cell. Carbon when perfectly purified contains no solid surface layers; but liable to be populated with surface groups. Therefore in the case of carbon; the grain structure, grain size and surface groups are the main relevant parameters, characterizing the material.
An extensively studied carbon material which can be easily purified is coconut shell charcoal. In the present work; the rectification properties of the perovskite/carbon interface and IV characteristic of cells are examined in detail by pressing coconut shell derived carbon powder on the perovskite surface deposited on TiO₂ by the usual procedure. Results indicate that the grain electrical conductivity, grain size and pressure applied are the crucial factors determining the cell efficiency. Experimental details and possible electron-hole separation mechanisms at the perovskite/carbon will be discussed.

Over the two decades, amorphous oxide semiconductors (AOSs) and their thin film transistor (TFT) channel application have been intensely explored to realize high performance, transparent and flexible displays due to their high field effect mobility (μ_FE=5-20 cm²/Vs), visible range optical transparency, and low temperature processability (25-300 °C).[1-2] The metastable amorphous phase is to be maintained during operation by the addition of Zn and additional third cation species (e.g., Ga, Hf, or Al) as an amorphous phase stabilizer.[3-5] To limit TFT off-state currents, a thin channel layer (10-20 nm) was employed for InZnO (IZO)-based TFTs, or third cations were added to suppress carrier generations in the TFT channel. To resolve bias stress-induced instabilities in TFT performance, approaches to employ defect passivation layers or enhance channel/dielectric interfacial compatibility were demonstrated.[6-7]

Metallization contact is also a dominating factor that determines the performance of TFTs. Particularly, it has been reported that high electrical contact resistance significantly sacrifices drain bias applied to the channel, which leads to undesirable power loss during TFT operation and issues for the measurement of TFT field effect mobilities. [2, 8] However, only a few reports that suggest strategies to enhance contact behaviors are available in the literature. Furthermore, the previous approaches (1) require an additional fabrication complexity due to the use of additional treatments at relatively harsh conditions such as UV, plasma, or high temperatures, and (2) may lead to adverse effects on the channel material attributed to the chemical incompatibility between dissimilar materials, and exposures to harsh environments. Therefore, a simple and easy but effective buffer strategy, which does not require any additional process complexities and not sacrifice chemical compatibility, needs to be established to mitigate the contact issues and therefore achieve high performance and low power consumption AOS TFTs.

The present study aims to demonstrate an approach utilizing an interfacial buffer layer, which is compositionally homogeneous to the channel to better align work functions between channel and metallization without a significant fabrication complexity and harsh treatment conditions. Photoelectron spectroscopic measurements reveal that the conducting IZO buffer, of which the work function (Φ) is 4.37 eV, relaxes a relatively large Φ difference between channel IZO (Φ=4.81 eV) and Ti (Φ=4.2-4.3 eV) metallization. The buffer is found to lower the energy barrier for charge carriers at the source to reach the effective channel region near the dielectric. In addition, the higher carrier density of the buffer and favorable chemical compatibility with the channel (compositionally the same) further contribute to a significant reduction in specific contact resistance as much as more than 2.5 orders of magnitude. The improved contact and carrier supply performance from the source to the channel lead to an enhanced field effect mobility of up to 56.49 cm²/Vs and a threshold voltage of 1.18 V, compared to 13.41 cm²/Vs and 7.44 V of IZO TFTs without a buffer. The present work is unique in that an approach to lower the potential barrier between the source and the effective channel region (located near the channel/dielectric interface, behaving similar to a buried-channel MOSFET [9]) by introducing a contact buffer layer that enhances the field effect mobility and facilitates carrier supply from the source to the effective channel region.

This work is partially supported by the U.S. National Science Foundation (NSF) Award No. ECCS-1931088 and the KRISS Improvement of Measurement Standards and Technology for Mechanical Metrology, Award No. 20011028.

This presentation is based on our recent publication, M. Liu et al. ACS Applied Electronic Materials, 2021 (In press) (DOI: 10.1021/acsaelm.1c00284).

Perovskite solar cells (PSCs) with a standard sandwich structure suffer from optical transmission losses due to the substrate and its active layers. Developing strategies for compensating for the losses in light-harvesting is of significant importance to achieving a further enhancement in device efficiencies. In this work, the down-conversion effect of carbon quantum dots (CQDs) was employed to convert the UV fraction of the incident light into visible light. For this, thin films of poly(methyl methacrylate) with embedded carbon quantum dots (CQD@PMMA) were deposited on the illumination side of PSCs. Analysis of the device performances before and after application of CQD@PMMA photoactive functional film on PSCs revealed that the devices with the coating showed an improved photocurrent and fill factor, resulting in higher device efficiency.

Flexible electronics and electro-optics have drawn a significant attention in recent years. Successful fabrication of flexible mirrors or transparent electronics will require deposition of relatively rigid thin films on soft compliant substrates. However, direct deposition of a rigid film on soft substrates often suffers from wrinkling or buckling due to the interfacial instability arising from the substrate compression beyond a critical strain. Wrinkled surfaces cause severe reduction in reflectivity and low transmittance. In this work, we report a novel technique to fabricate wrinkle free deposited rigid thin films on soft substrate by suppressing the instability with a nm-layer of jamming nanoparticles. The thickness of the jammer can tailor the wrinkle periodicity and the roughness of the surface. The technique is robust and readily applicable to wide variety of materials. For demonstration system, we selected Poly(dimethylsiloxane) (PDMS), poly(monochloro-p-xylylene) (parylene C) as jammer and various metals (e.g., Cu, Al, Ti). Different jammer layers varying from 40nm to 2um are deposited on 1mm PDMS substrate followed by sputter deposition of 50nm thin metal film. The surface morphology of the samples was analyzed by atomic force microscopy (AFM). The results showed that as the jammer thickness increases, the wrinkle wavelength increases accordingly and when the thickness exceeds a threshold (e.g., ~150nm), wrinkles are suppressed. Nanoindentation measurements also show the interfacial stiffness increases by two orders of magnitude as the jammer thickness increases. When the layer reaches 150nm, the metal-jammer-PDMS composite becomes increasingly elastic and deviates substantially from viscoelastic behavior. A dual power law emerges in Young’s relaxation modulus.

Ga₂O₃ is being actively explored for optoelectronic (phosphorus, and electroluminescent devices, solar-blind photodetectors) application due to its ultra-wide bandgap and low projected fabrication cost of large-size and high-quality crystals. Efficient n-type doping of Ga₂O₃ has been achieved, but p-type doping faces fundamental obstacles due to compensation, deep acceptor levels, and the polaron transport mechanism of free holes. However, aside from achieving p-type conductivity, plenty of opportunities exists to engineer the position of the Fermi level for improved design of Ga₂O₃ based devices. We use first principles defect theory and defect equilibrium calculations to stimulate a 3-step growth-annealing-quench synthesis protocol for hydrogen assisted Mg doping in beta Ga₂O₃, considering the gas phase equilibrium between H₂, O₂, and H₂O, which determines the H chemical potential. We predict Ga₂O₃ doping-type conversion to a net p-type regime after growth under reducing conditions in the presence of H₂ followed by O-rich annealing, which is similar process to the Mg acceptor activation by H removal in GaN. We show there is an optimal temperature that maximizes the Ga₂O₃ net acceptor density for given Mg doping level. After quenching to operating temperatures, the Ga₂O₃ Fermi level drops below mid-gap down to +1.5 eV above the valence band maximum, creating significant number of uncompensated neutral Mg_Ga⁰ acceptors. The resulting free hole concentration in Ga₂O₃ is very low due to deep energy level of these Mg acceptors, and hole conductivity is further impeded by the polaron hopping mechanism. However, the Fermi level reduction down to +1.5 eV and suppression of free electron density in this doping type converted Ga₂O₃ material is of significance and impact for the design of Ga₂O₃ based optoelectronic devices.

The UV-C (< 280 nm of wavelength) part of the solar spectrum is highly dangerous to living organisms, with exposure often causing changes in their genetic make-up. Thankfully, the ozone layer in the atmosphere absorbs almost all of this radiation, thereby providing us with natural protection. The complete absorption leads to a very low background signal of UV-C on earth. This unique characteristic can be exploited in the fabrication of highly sensitive photodetectors which are responsive to UV-C only from other sources on earth, leading to the development of the so-called solar-blind photodetectors (SBPD). Solar-blind photodetectors are deep-UV photodetectors that show sensitivity to only UV-C radiation. They show no response when placed in natural sunlight and thus have applications ranging from ozone-hole monitoring, missile tracking, flame detection, space exploration, UV sterilization systems, non-line-of-sight communications, etc. Traditionally, SBPDs were fabricated using mature technologies such as silicon. However, a band gap of 1.1 eV renders the devices to have a broadband photoresponse, thus necessitating the use of costly optical filters. Wide band gap semiconductors offer a solution to this problem by removing the need for any filters due to their suitable band gap. Gallium oxide is a new, ultra-WBG material with properties that are promising for its use as a functional material in the fabrication of SBPDs. It has an apt band gap of 4.6-5.3 eV (depending on the polymorph) making it intrinsically solar-blind. Moreover, it is highly chemically and thermally stable (M.P. ~ 1730°C), has a high breakdown voltage and is extremely radiation hardened and hence it can be employed for use in extreme environment applications (high temperature and high radiation conditions). Some of these applications include employment in nuclear, geothermal or space exploration, which require the material to be immune (at least partially) to all sorts of radiation. This requires extensive studies to investigate the effect that each type of radiation might have on the material as well as on the performance of its devices.
We have undertaken such a study on the radiation hardness and device performance of amorphous and polycrystalline gallium oxide thin film against heavy ion (Ag⁷⁺) irradiation with a high energy of 100 MeV. We found that the amorphous thin film shows better resilience to the radiation as compared to the polycrystalline film. Changes in the morphology and crystal structure of the material were thereby investigated. There occurs recrystallization and formation of nano-pores in the amorphous film while the polycrystalline film shows destruction of the larger crystallites. The photodetectors fabricated on the irradiated films exhibit good device integrity albeit with a decrease in performance. The change in performance is linked to the change in the material’s defect density which is confirmed by photoluminescence study. The optical defects, which are responsible for the high photoconductive gain in gallium oxide, undergo ionization-induced defect annealing and thus lead to an overall decrease in performance. The radiation-damaged performance of the photodetectors is recovered by annealing at moderate temperatures in air ambient, so much so that the amorphous based photodetector not only recovers but shows a two order of magnitude increase in its responsivity. This study not only shows the high radiation hardness of gallium oxide against Swift Heavy Ions but may also serve as a stepping stone for the use of high energy irradiation as a tool for defect engineering in gallium oxide based devices.

Despite high chemical stability in an aqueous environment, application of ZnO as a visible light photoanode in photoelectrochemical cells (PECs) is limited due to its large bandgap. Alloying ZnO with ZnSe reduces the electronic bandgap of ZnO, as observed experimentally at low alloy compositions. In this work, we have performed density of states and electronic band structure calculations using DFT to determine the band gap of wurtzite solid state solutions of ZnO_1-xSe_x over the entire composition range to develop deeper insight into the system. The band gap calculated using the delta-sol method shows a bowing behaviour, with a minimum bandgap at x=0.5, as predicted by the band anticrossing (BAC) model. Difference in the electronegativities of O and Se atoms in the lattice leads to hybridization of O-2p and Se-4p electronic states which can be clearly observed from the projected density of states and band structure plots. Interaction between these electronic states also leads to a split in the valence band edge at O-rich end and a split in the conduction band edge at the Se-rich end. These are observed as dips in the density of states plot at the respective band edges. In addition, the curvature of the valence band maxima and conduction band minima reduce at the O-rich end and Se-rich end, respectively. This implies an increased effective mass of the charge carriers and hence a decreased mobility. These fundamental insights will help in employing suitable alloy compositions in PECs for optimal photocurrent density.

Ultraviolet optoelectronics devices have attracted much interest for a variety of applications to use photolithography, biochemical and so on. Among the wide-bandgap semiconductors, Ga₂O₃ with the bandgap of about 5 eV is considered as one of the eco-friendly candidates used for ultraviolet optoelectronics devices. The development of the Ga₂O₃-based devices relies on the feasibility of growing impurity-doped p-type conductive Ga₂O₃ thin films .^[1] It has been reported that n-type Ga₂O₃ semiconductors have been prepared via impurity doping of 4+ cations like Si⁴⁺ .^[2] On the other hand, to fabricate p-type Ga₂O₃ semiconductors, potential dopants like Mg²⁺ and Zn²⁺ are presented. ^[3] However, as far as we know, there is no report on fabrication of impurity-doped p-type Ga₂O₃ thin films.
So far, we have investigated the pulsed excimer laser-induced room-temperature epitaxial crystallization of β-Ga₂O₃ thin films on the NiO-buffered sapphire substrates. ^[4,5] Different from the conventional high-temperature growth of β-Ga₂O₃ thin films, crystallization of amorphous Ga₂O₃ thin films via excimer laser annealing at room-temperature is expected to suppress phase separation and precipitation, and also to induce unusual impurity doping. In this study, for the purpose of controlling the electrical properties such as p- or n-type of β-Ga₂O₃ thin films by laser-induced impurity doping, we evaluated the effects of laser irradiation conditions and/or thin film compositions on the crystal growth and conductivity of the impurity-doped amorphous Ga₂O₃ thin films.
In the experiment, Li element was used as one of impurity dopants because Li is expected as monovalent p-type dopant .^[6] The amorphous 5 at% Li-doped Ga₂O₃ thin film (50-80 nm-thick) was grown on the atomically stepped α-Al₂O₃ (0001) substrates with about 2 nm thick MgO (111) buffer layer by pulsed laser deposition (PLD) equipped with pulsed KrF excimer laser (λ=248 nm, d=20 ns, and E~1.2 J/cm²) at room-temperature in ultra-high vacuum (~10^-6 Pa) using a sintered 5 at% Li-doped β-Ga₂O₃ target. The obtained amorphous Ga₂O₃ thin film were subsequently crystallized in the solid-phase by excimer laser annealing (ELA), in which KrF excimer laser beam (not focused, E~175 mJ/cm²) was irradiated from the substrate side at room-temperature in air. As a result, it was verified from the XRD and RHEED analyses that 5 at% Li-doped β-Ga₂O₃ (-201) epitaxial film was obtained on the MgO (111) (2 nm thick) buffered α-Al₂O₃ (0001) substrate. Li inclusion in the β-Ga₂O₃ thin film after ELA was confirmed from TOF-SIMS measurements (operated by the Open Facility Center of Tokyo Inst. Tech.). These results indicate that ELA process induces solid-phase epitaxy at room-temperature and suppress re-evaporation of Li in Ga₂O₃ crystallization. In addition, TOF-SIMS measurements also indicate the possible diffusion of Al⁺ and Mg⁺ elements into the thin film. The optical property of the epitaxial 5 at% Li-doped β-Ga₂O₃ thin films were evaluated by UV-vis measurements, which revealed a transmission over 90% of light in the visible region (360-830 nm) and rapid absorption in the UV region (~360 nm). In addition, the optical bandgap of the 5 at% Li-doped β-Ga₂O₃ thin film was estimated about 4.81 eV, being about 0.1 eV smaller than that of the non-doped β-Ga₂O₃ thin film (4.9 eV). It is suggested that the impurity Li atoms have been largely activated as acceptors in the β-Ga₂O₃ film to modify the energy level of the valence band top. The electrical properties are also reported in the presentation on the day.
[1] F. Alema et al., Phys. Status Solidi. 214, 1600688 (2017).
[2] F. B. Zhang et al., J. Mater. Sci.: Mater. Electron 26, 9624 (2015).
[3] Y. Su et al., J. Alloys Compd. 782, 299 (2019).
[4] D. Shiojiri et al., J. Cryst. Growth 424, 38 (2015).
[5] H. Morita et al., J. Vac. Sci. Technol. A 39, 043414 (2021).
[6] A. Kyrtsos et al., Appl. Phys. Lett. 112, 032108 (2018).

Recently, perovskite solar cells have attracted interests, and are of significance in photovoltaic energy conversion. Nickel oxide (NiO) with wide-bandgap of E_g~3.4 eV is one of the p-type semiconductors utilized as hole transport layers in the devices ^[1]. Aliovalent doping of such as Li⁺ have commonly applied to enhance its p-type conduction according to improved carrier concentration. Meanwhile, there are some researches of uv-ozone treatment of polycrystalline NiO films using mercury lamps, that altered resistance and electronic states was reported ^[2,3]. Modification of p-type conduction without impurity doping would advance development and properties of multilayered optoelectronic devices; although it still requires further research on the influence of film structure as well as illuminance and ambience of the irradiation. Application of vacuum-ultraviolet (VUV) light with larger photon energy and efficient production of active oxygen species would contribute to further modification of properties. In this study, noticeable conductivity improvement of epitaxial NiO thin films by excimer VUV-light irradiation was verified, and the effect of the treatment on structure and morphology of the films was investigated.
The epitaxial NiO (111) thin films were grown on atomically stepped α-Al₂O₃ (0001) single crystal substrates by pulsed laser deposition (PLD) technique using a KrF excimer laser (λ=248 nm, E~1.2 J/cm²) and a pure NiO sintered target. The thin film growth took place at room-temperature (not-heated intentionally) in 1×10^–3 Pa O₂ (base pressure ~3×10^–6 Pa). The NiO thin films were subsequently introduced to VUV-light treatment procedure using a Xe₂ excimer lamp (λ=172 nm, E~65 mW/cm² at lamp surface). The VUV-light was irradiated to thin film surfaces in air, vacuum, or inert gas at the distance of 0.5–2.0 mm. The electric property of NiO (111) thin films was measured by four-probe DC method. The resistivity of the PLD-grown 40 nm-thick epitaxial NiO (111) thin film was ~7×10³ Ωcm, although VUV-light irradiation in air at 0.5 mm drastically reduced the value by three-digits, that ~4×10⁰ Ωcm and ~2×10⁰ Ωcm were obtained after the treatment for 60 and 120 minutes, respectively. The epitaxial structure was retained even after VUV-light irradiation of 120 min according to the XRD and RHEED results, while observed slight reduction of the (111) plane spacing suggested redox reaction of the film. Thus, the demonstrated conductivity improvement of NiO (111) thin films due to VUV-light irradiation would advance optoelectronic application of the material. In addition, influence of conditions of thin film preparation and VUV-light treatment on the structures and properties would also be presented.
[1] S. E. Habas et al., Chem. Rev. 110 (2010) 6571–6594.
[2] G. H. Aydogdu et al., J. Appl. Phys. 108 (2010) 113702.
[3] R. Islam et al., ACS APPl. Mater. Interfaces 9 (2017) 17201–17207.

We review here the recent trends in silicon heterojunction solar cells using low-temperature passivated contacts. Beyond the archetypal amorphous silicon and indium tin oxide (ITO), several novel materials have recently demonstrated excellent performances when integrated in devices. Nanocrystalline silicon and silicon oxide alloys relax in part the trade-off between optical and electrical performances. Both n- and p-type contacts can be made with such material, with different strategies being necessary to enable excellent performance while maintaining surface passivation. Beyond silicon, several materials are investigated as more transparent alternative, including metal-oxides or nitrides. We will discuss what makes such materials attractive and what is still limiting the performance of devices incorporating them.

Titanium oxide (TiO_x, x≈2) has been shown to be an effective passivating contact interlayer in a range of high efficiency crystalline silicon (c-Si) solar cells. It has been incorporated in a broad range of configurations; with and without passivating interlayers (i.e. silicon oxide SiO_x, and amorphous silicon a-Si:H); with and without overlaying low work function electrodes (i.e. calcium, lithium fluoride / aluminium stacks LiF/Al); and in partial area and full area contact architectures. Passivating contacts incorporating TiO_x have enabled <10 fA/cm² scale recombination currents and <10 mΩcm² contact resistivities which are no inhibitor to cell efficiencies above 24%, with efficiencies above 23% already demonstrated in the laboratory. Though most of these high-efficiency structures employ TiO_x at the electron side of the cells, some studies have shown its potential tunability, which has enabled its use in hole contacts on c-Si as well, simply by varying deposition conditions and consequently changing its material properties. This talk explores the tunability of TiO_x thin films and their incorporation into passivated contacts. Interface properties, doping, thermal stability and post-metallisation stability will all be covered. Finally, remaining challenges for the incorporation of TiO_x in industrial c-Si cells will be discussed.

Metal oxides offer a diverse range of physical properties and can serve various roles in photovoltaic (PV) cells. The wide band gap of these materials allows them to be used on the front side of devices without adding parasitic optical absorption. There is a long track record of depositing metal oxides with atomic layer deposition (ALD), a manufacturing friendly process that results in highly conformal film growth and excellent film thickness control. This presentation will review how ALD metal oxides can be used as passivating, carrier-selective contacts for crystalline silicon (c-Si) PV cells. This begins with a breakdown of desirable characteristics for passivating contact materials, including transparency, interface defect passivation, carrier-selectivity, and carrier transport, as well as thermal stability and compatibility with high-volume manufacturing processes. Next, the performance of the more common metal oxides are surveyed and their process-structure-property relationships discussed, including how chemical interactions between layers can impact performance for better or worse. Much of this discussion will focus on aluminum oxide as a tunneling passivation layer, molybdenum oxide as a hole-selective material, and titanium oxide as an electron-selective material. Finally, remaining challenges and areas of opportunity for future research are covered.

Metal microgrid-based transparent conductors enable the acceleration of wearable electronic applications development by constructing the high transparency, low sheet resistance and mechanically stable metal grid within flexible substrates. In this study, we demonstrate polymer-embedded silver microgrid structures with low porosity using a particle-free silver ink for flexible transparent electrodes. The conductive grids indicate a high transparency of 91.8% and a low sheet resistance of 0.88 Ω/sq, corresponding to a figure of merit value σ_dc/σ_op of nearly 4500 with gridlines of 10 μm width. The optimized gridline width may be fabricated down to 3.5 μm. Furthermore, the embedded structure of Ag microgrids shows 3 times lower surface roughness compared to particle-based conductive Ag inks and better mechanical as indicated by bending, folding and tape peeling test. The results demonstrate (1) a scalable approach for microgrid manufacturing, (2) microgrids with nearly the best transparent electrode performance properties, and (3) the benefit of reactive silver inks as opposed to particle-based ones. Metal microgrids may address technical and economical limitations with indium tin oxide (ITO) as well as enable the development of a various flexible and wearable optoelectronic applications.

Heterostructure interfaces are of critical importance for next generation device architectures. Defects at these interfaces significantly impact the resulting properties and ultimately device performance. Many studies focus on the growth of thin films by Atomic Layer Deposition (ALD) but neglect the unique surface chemistry and atomic structures formed at the resulting interface. This limits the device applications of heterostructures made with these types of thin films due to the limitations in crystallinity and lattice match. In an ALD-based approach we call selective interface reactions (SIRs) we target specific undesirable defect sites on a surface to achieve uniform behavior at interfaces formed on these surfaces. TiO₂ is considered the prototypical oxide surface and has been intensely studied due to its photocatalytic and optoelectronic properties. It is well established that the defects on the TiO₂ surface affect the optical and electronic properties, interfaces, and performance of resulting devices. DFT calculations are used to predict the difference in reactivity of “perfect” TiO₂ and minority atomic configurations such as step edges. We translate these differences in site reactivity to differences in thermal dehydration which is confirmed by STM. SIR is ultimately used to exploit these differences in thermal dehydration to selectively deposit material specifically on defect sites to prevent defect formation.

ZnO thin films typically grow with the c-axis normal to the substrate leading to a columnar microstructure with the (002) ZnO planes parallel to the substrate surface. Tuning the incident angle of the species reaching the substrate changes the morphology and microstructure of the film. In this work Nd-doped ZnO thin films were grown by pulsed-electron beam deposition (PED) on c-cut sapphire substrates under oblique angle incidence, at 10^-2 mbar oxygen pressure and for a substrate temperature of 500°C. Transmission electron microscopy, X-ray diffraction and pole figures analyses were performed to obtain the film microstructure together with their electrical and optical properties. The Nd-doped ZnO thin films were smooth, compact and constituted with columnar grains inclined at about 17°±4° from the normal to the substrate surface. From X-ray diffraction and transmission electron microscopy experiments, the wurtzite phase of Nd-doped ZnO thin films was observed with the c-axis inclined around 35° with respect to the normal axis of the substrate, with a three-fold azimuthal symmetry. Epitaxial relationships between Nd-doped ZnO films and sapphire substrate were determined from the pole figures and compared with those obtained for the films grown under non-oblique angle incidence. Classical in-plane epitaxy with a 30° rotation between the two hexagonal unit cells of ZnO and c-cut sapphire was observed under non-oblique incidence [1, 2]. For the films grown under oblique incidence, the 35° angle tilt of the ZnO c-axis from the normal to the substrate is explained by the epitaxy of the (229) ZnO plane on the (002) sapphire plane. This particular orientation, which has never been reported previously, is obtained from the rotation of the [001] ZnO direction around the [1-10] ZnO one, which allows the (229) ZnO plane to be parallel to the (002) sapphire substrate one [2]. A new well-defined epitaxial relationship was observed between the (229) ZnO on (002) Al₂O₃ plane although the large lattice mismatch and symmetry mismatch. This unusual epitaxy will be discussed, as well as the effect of tilted growth on the electrical and optical properties of Nd-doped ZnO films. These epitaxial c-axis tilted Nd-doped ZnO thin films are also interesting for sensitive and disposable biosensors working as film bulk acoustic resonators in the shear acoustic mode for biological and medical applications. [1] M. Nistor, L Mihut, E. Millon, C. Cachoncinlle, C. Hebert, J. Perrière, RSC Adv. 6, 41465 (2016); [2] M. Nistor, E. Millon, C. Cachoncinlle, C. Ghica, C. Hebert, J. Perrière, Appl. Surf. Sci. 563, 150287 (2021)

Oxides play a critical role in a wide range of optoelectronic devices as the charge-selective layer. Whilst the electronic properties of oxides generally improve with increasing growth temperature, the processing window is limited when depositing over device stacks, owing to the thermal sensitivity of the active layer. A prominent example of a thermally-sensitive active layer is methylammonium lead iodide (MAPbI₃) perovskite.^[1] This talk explores the use of atmospheric pressure chemical vapor deposition (AP-CVD) as a means to broaden the processing window of oxide overlayers on optoelectronic device stacks. AP-CVD produces oxide films with similar uniformity, density and conformality as films grown by atomic layer deposition (ALD), but with orders of magnitude higher growth rates, and without the need for a vacuum chamber. We investigate the growth of TiO₂ over MAPbI₃ films. We show that growth rates of 1.19±0.04 nm s^-1 are achievable, which allows 7 nm TiO₂ films to be grown in 6 s (compared to >30 min for ALD). The rapid processing allows us to deposit TiO₂ directly onto MAPbI₃ films at 150 °C without any structural, chemical or electronic damage, as found from X-ray diffraction, X-ray photoemission spectroscopy and time-resolved photoluminescence measurements. In MAPbI₃ photovoltaic devices, we show that TiO₂ growth temperatures can exceed 180 °C without any drop in efficiency. This is >70 °C larger than achievable with ALD or solution-based methods for growing oxide overlayers, resulting in more conductive TiO₂ films which lead to increases in device performance. We show that these results can be generalised to triple-cation perovskites, as well as to AP-CVD SnO₂ overlayers. In particular, we show that the high density of the oxide overlayers improves fill factors by reducing dark currents, reaching up to 84% (with 19.4% steady-state power conversion efficiency) for triple-cation perovskite devices covered with 60 nm SnO₂.^[1]

More broadly, we demonstrate the wider applicability of AP-CVD to grow oxide electron transport layers on the perovskite-inspired material bismuth oxyiodide (BiOI). We show through transmission electron microscopy that AP-CVD ZnO films are highly conformal to the textured BiOI surface, resulting in photovoltaic devices with external quantum efficiencies reaching 80% at 450 nm wavelength.^[2] We demonstrate the benefits of this oxide-based device structure for achieving improved performance and stability across multiple light-harvesting applications.

References:

[1] Raninga, Jagt, …, Hoye, Nano Energy, 2020, 75, 104946
[2] Hoye, et al., Adv. Mater., 2017, 29, 1702176

Following the emergence of lead halide perovskites as record-breaking materials for emerging photovoltaic applications, lead-free perovskites are gaining interest as novel materials which are compatible with inkjet-printing based technologies^1,2,3. Bulk properties are of the prime importance, but charge transport can be rapidly altered across the interface of perovskite/ (hole or electron) transport layer (HTL or ETL). Yet, due to the size and complexity of the interface systems, a thorough ab initio modeling of the interfaces remains elusive to-date. Within this work, to explore the atomic-scale details of the structural and electronic properties of these materials, we employ state-of-the-art density functional theory (DFT) based calculations of model tin-based heterostructures. We investigate in particular three-dimensional corner-sharing Sn-based perovskites, their surface and interface properties and calculate the interaction of the materials with charge transport layers (CTLs). Our approach reveals the electronic band alignments and dielectric mismatches between perovskite layers and the most important hole and electron transport layers that have been used in prototype thin film device architectures^4,5. In an effort to understand the fundamental physico-chemical mechanisms that govern the interactions between the photo-active absorber material and CTLs, we also probe the influence of surface termination on structural and electronic properties at FASnI₃/C₆₀ (as ETL) interfaces and FASnI₃/NiO_x (as ETL) interfaces. Our findings give a detailed insight into the interface engineering necessary for optimizing the next generation of environmental-friendly high-performing opto-electronic devices.
This DROP-IT project¹ has received funding from the European Union’s Horizon 2020 research and innovation Program under the grant agreement No 862656. The information and views set out in the abstracts and presentations are those of the authors and do not necessarily reflect the official opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained herein.

References:
(1) DROPIT https://www.uv.es/dropit/ (accessed June 22, 2021).
(2) Volonakis, G.; Filip, M. R.; Haghighirad, A. A.; Sakai, N.; Wenger, B.; Snaith, H. J.; Giustino, F. Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals. J. Phys. Chem. Lett. 2016, 7 (7), 1254–1259. https://doi.org/10.1021/acs.jpclett.6b00376.
(3) Volonakis, G.; Haghighirad, A. A.; Milot, R. L.; Sio, W. H.; Filip, M. R.; Wenger, B.; Johnston, M. B.; Herz, L. M.; Snaith, H. J.; Giustino, F. Cs2InAgCl6: A New Lead-Free Halide Double Perovskite with Direct Band Gap. J. Phys. Chem. Lett. 2017, 8 (4), 772–778. https://doi.org/10.1021/acs.jpclett.6b02682.
(4) Traore, B.; Pedesseau, L.; Blancon, J.-C.; Tretiak, S.; Mohite, A. D.; Even, J.; Katan, C.; Kepenekian, M. Importance of Vacancies and Doping in Hole Transporting Nickel Oxide Interface with Halide Perovskites. ACS Appl. Mater. Interfaces 2020. https://doi.org/10.1021/acsami.9b19457.
(5) Canicoba, N. D.; Zagni, N.; Liu, F.; McCuistian, G.; Fernando, K.; Bellezza, H.; Traoré, B.; Rogel, R.; Tsai, H.; Le Brizoual, L.; et al. Halide Perovskite High-k Field Effect Transistors with Dynamically Reconfigurable Ambipolarity. ACS Materials Lett. 2019, 1 (6), 633–640. https://doi.org/10.1021/acsmaterialslett.9b00357.

The effect of terbium doping on the electrical, optical and light emission properties of sputtered indium tin oxide thin films was investigated. The films were prepared by radio frequency dual magnetron sputtering maintaining a high optical transmittance in the ultraviolet and visible spectral regions and an electrical resistivity ranging from 5E-3 to 0.3 Ωcm . Terbium-related luminescence is achieved after thermal treatments at 470 °C in air at atmospheric pressure. Electrical resistivity and optical transmittance were registered after each annealing step to evaluate the compromise between the achieved light emission intensity, electrical and optical properties. Additionally, Tb-related luminescence thermal quenching is assessed by temperature-dependent photoluminescence measurements, from -190 °C to 300 °C, under non-resonant excitation. Thermal quenching activation energies suggest an effective energy transfer mechanism from the ITO host to the rare-earth ions. This indirect excitation mechanism is tentatively modeled using a spherical potential well, as well as a tight-binding one-band approximation, approach for a short-range charge trapping process and subsequent formation of bound excitons to rare-earth ion clusters.

Zinc oxide is an extensively studied n-type semiconductor for various applications such as light emitting devices, detection of chemicals or solar cells,¹ which often needs to be doped, modified or protected.

Doped-ZnO films can replace conventional transparent conductive oxides (TCO), especially in high aspect ratio structures. For example, radial junction silicon nanowire (Si NW) based solar cells, which allow to decouple light absorption and charge carrier collection, are highly interesting but challenging to top contact.² Use of ALD-Ti:ZnO (TZO) as the top electrode has proven the applicability of ALD-TZO as TCO and the unique capabilities of ALD, showing superior optoelectrical properties, conformally covering the Si NWs, and yielding a PV diode behavior with external quantum efficiency response surpassing the conventional ITO top electrode.³
In other cases, it is beneficial to modify the properties of the ZnO surface, for instance by the grafting of organic molecules.⁴ ALD-ZnO surfaces were modified by phosphonic acid derivatives with different spacer and functionalizing groups (2-aminoethylphosphonic acid, 2-AEPA ; 4-aminobenzylphosphonic acid, 4-ABzPA; and 4-fluorobenzylphosphonic acid, 4-FBzPA). The resulting ZnO surfaces were characterized, used as a mean to passivate the reactive interface between ALD-ZnO and a hybrid organic inorganic metal halide perovskite, and complete solar cell devices were prepared.⁵
Finally, ZnO-based films often need to be protected to prevent their degradation. Al:ZnO (AZO) window layer is reported as the primary component responsible for the degradation of CIGS solar cells. The feasibility to prevent AZO degradation and encapsulate module-level (10×10 cm²) CIGS solar devices by a 10 nm ALD-Al₂O₃ barrier layer was demonstrated.⁶ However, solar panels in field operation are also exposed to various chemical air pollutants such as in (NH₄)₂SO₄ in rural, and NaCl in marine environments. Their effects were studied by placing AZO w/ and w/o encapsulation in specific climatic test conditions. This demonstrated the necessity to consider atmospheric chemistry when evaluating barrier protection capacities of encapsulants and assessing the durability of PV materials and devices.⁷

References
¹ Z.L. Wang, J. Phys. Condens. Matter 16, R829 (2004).
² S. Misra, L. Yu, M. Foldyna, and P. Roca i Cabarrocas, IEEE J. Photovolt. 5, 40 (2015).
³ D. Coutancier, S.-T. Zhang, S. Bernardini, O. Fournier, T. Mathieu-Pennober, F. Donsanti, M. Tchernycheva, M. Foldyna, and N. Schneider, ACS Appl. Mater. Interfaces (2020).
⁴ I. Lange, S. Reiter, M. Pätzel, A. Zykov, A. Nefedov, J. Hildebrandt, S. Hecht, S. Kowarik, C. Wöll, and G. Heimel, Adv. Funct. Mater. 24, 7014 (2014).
⁵ O. Fournier, C.D. Bapaume, D. Messou, M. Bouttemy, P. Schulz, F. Ozanam, L. Lombez, N. Schneider, and J. Rousset, ACS Appl. Energy Mater. 4, 5787 (2021).
⁶ S.-T. Zhang, M. Guc, O. Salomon, R. Wuerz, V. Izquierdo-Roca, A. Pérez-Rodríguez, F. Kessler, W. Hempel, T. Hildebrandt, and N. Schneider, Sol. Energy Mater. Sol. Cells 222, 110914 (2021).
⁷ S.-T. Zhang, A. Maltseva, G. Herting, J.-F. Guillemoles, N. Schneider, I. Odnevall Wallinder, and P. Volovitch, (submitted).

Solar photovoltaic (PV) and photoelectrochemical (PEC) energy conversion share the same fundamental requirements of photon absorption, charge separation, and selective carrier collection [1]. In addition, in order to produce storable chemical energy, PEC systems must drive multi-electron transfer reactions, i.e. water oxidation (OER, a 4 electron process) for a photoanode and hydrogen evolution (HER) or carbon diode reduction (CO₂R) for a photocathode (2-18 electron processes) [2]. In a small number of cases it has been possible to have a single material perform all of the requisite steps and remain stable under operation: water oxidation photoanodes composed of TiO₂ or Fe₂O₃ are examples. However, in the vast majority of cases, it has been beneficial to construct multi-material PEC systems, with different components chosen for optimal light absorption, selective carrier collection/passivation, and catalysis.

In this context, the choice of electron transfer layers (ETLs) for PEC CO₂R reduction will be discussed. While it is attractive to design semiconductor absorbers based on the position of their conduction band relative to the standard state redox potentials of a given CO₂ reaction, e.g. CO₂/CO, CO₂/HCOOH, CO₂/CH₄, etc., this analysis neglects the fact that these multi-electron reactions occur via a series of elementary proton coupled electron transfers, with each step having its own redox potential [3]. For this reason, either solid state or molecular CO₂R co-catalysts are usually integrated with the absorber in PEC CO₂R studies [4,5].

These concepts will be illustrated through the design and operation of Si-based CO₂R photocathodes. Adroit use of charge selective contacts allows for considerable flexibility in design. For example, in contrast to conventional photocathode designs which employ p-type absorbers, we used a back illumination geometry with an n-type Si absorber to permit the use of absorbing metallic catalysts which would otherwise block the light. Hole collection was enabled by p⁺ implant on the illumination side while TiO₂ as applied as an ETL on the catalyst side. Selectivity to C-C coupled products was achieved by using hierarchical Au- Ag-Cu nanostructures as electrocatalysts. The photovoltage, 550- 600 mV under simulated 1-sun illumination, and photocurrents exceeding 30 mA cm^-2 confirm the carrier selectivity and passivation of the front and back interfaces. Under simulated diurnal illumination conditions, over 60% faradaic efficiency to C₂₊ hydrocarbon and oxygenate products (mainly ethylene, ethanol, propanol) is maintained for several days [6].

As further examples of the PEC design space for CO₂R, use of other oxides as ETLs (e.g. Al₂O₃ and TaO_x) and the prospects of achieving cascade CO₂R [7] on the PEC surface will be discussed.

This material is based on work performed by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266.

Wurfel, U.; Cuevas, A.; Wurfel, P. Charge Carrier Separation in Solar Cells. IEEE J. Photovoltaics 2015, 5, 461–469.
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Gurudayal; Beeman, J. W.; Bullock, J.; Wang, H.; Eichhorn, J.; Towle, C.; Javey, A.; Toma, F. M.; Mathews, N.; Ager, J. W. Energy Environ. Sci. 2019, 12, 1068–1077.
Lum, Y.; Ager, J. W. Energy Environ. Sci. 2018, 11, 2935–2944.

One of the primary challenges facing solar conversion of CO₂ to fuel is the selectivity of the CO₂ reduction reaction to a single, ideally liquid, product. One method for improving selectivity is to use a cascade approach, where sequential steps of the multi-step reduction reaction are driven in series using different catalysts and adjacent microenvironments[1]. Each of these catalysts should be supplied with the ideal photocurrent and photovoltage to drive its respective reaction step. As a model system, we are considering the conversion of CO₂ to ethylene using a patterned, multi-terminal photovoltaic device as a photocathode. This two-step reaction (CO₂ to CO, then CO to C₂H₄) can be driven by two terminals of a three-terminal tandem (3TT) photovoltaic cell, where the third terminal drives the oxidation reaction. We have previously demonstrated 3TTs for photovoltaic applications[2], and will present the operational principles of these devices as well as experimental results using III-V semiconductors as a model system. We show that we can achieve the necessary photovoltages and photocurrents from the rear “R” and middle “Z” contact of these devices to drive the two CO₂ reduction reactions, using the top “T” contact to drive oxidation[3]. Then, we will present a model for how this device is expected to operate in an integrated photoelectrochemical cell[4]. Finally, we will discuss surface protection schemes using a transparent, conductive encapsulant[5] with embedded catalysts to protect the semiconductor from degradation, while providing the required electrical conduction and catalyst integration.

This material is based on work performed by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266.

[1] Y. Lum and J.W. Ager, Energy Environ. Sci. 2018
[2] J.F. Geisz et al., submitted
[3] E.L. Warren et al., ACS Energy Letters 2020
[4] C.J. Kong et al., in prep
[5] T.R. Klein et al., J. Phys. D. 2021

In this work, a sequential catalysis approach by light was employed to achieve high product selectivity for CO₂ reduction. By employing light, regions with high photovoltage and low photovoltages was achieved by using photo-absorbers with different opto-electronic properties. In the low photovoltage region, CO₂ would be reduced to CO and at the high photovoltage region, CO would be reduced to useful products such as ethylene and ethanol with high selectivity. Different photovoltage regions was achieved on Silicon photocathodes by tuning the band edge of Silicon using surface dipoles. The surface dipoles were fabricated on Silicon photocathodes using a solution processed technique like spin coating. The regions of different photovoltages were investigated using surface photovoltage (SPV) and kelvin probe force microscopy (KPFM). SPV shows the increase in the photovoltage of Silicon photocathode upon deposition of the surface dipole. KPFM measurements also reveal an increase in the surface potential with the dipole on Silicon. By accomplishing voltage tunability using this strategy, high product selectivity could be achieved by sequential catalysis using light. The results and outcome of this work would bring a paradigm shift in the methodology of obtaining high product selectivity in CO₂ reduction research.

Vertical GaN nanowires have emerged over the past decade as promising candidates for next-generation optoelectronics, which are of great importance for applications such as novel display systems, micro-LEDs, and nanowire LEDs. The small footprint of these nanowire devices allows for increased device scaling and three-dimensional (3D) integration. Typically, GaN nanowires can be formed through either bottom-up or top-down fabrication processes. Metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) are often used to grow nanowires in the bottom-up approach, however these techniques have many drawbacks, including complex growth conditions and non-uniformities in nanowire diameter, height, and spacing, which can only be partially corrected using expensive selective area epitaxy. Top-down fabrication on the other hand, makes use of the well-developed processes to form nanowires from a monolithic, planar epitaxial stack. Our previous works have explored top-down fabrication approaches for GaN nanowires extensively, for device applications such as vertical nanowire LEDs and vertical transistors. Our top-down fabrication approach consists of patterning a 200 nm thick Ni hardmask on top of a 300 nm thick layer of SiO₂ which has been deposited on the GaN epistack. The Ni layer is then used to mask Cl-based dry etches of the underlying SiO₂ and GaN to form approximately 3 µm tall nanowires with diameters of approximately 1 µm down to 20 nm. Following and hydroxyl-based wet etch and deposition of the n-metal contact at the base of the nanowires, an HF-based wet etch is used to remove the Ni etch mask from the tops of the nanowires by etching the SiO₂ to allow for formation of a Ni/Au p-contact. Ni/Au is typically preferred as a p-contact as it exhibits superior contact resistance to Ni, due to the high work function of Au (5.1 eV). While this process has proved successful, the use of a SiO₂ liftoff layer adds complexity to the fabrication process in the form of additional deposition, dry etch, and wet etch steps. It is therefore desirable to develop a method of embedding an ohmic contact beneath the Ni dry etch mask so that it does not require a lift-off process. This can be achieved through exploration of a novel metal stack which acts as both an ohmic contact to p-GaN as well as an effective etch mask for dry etching of higher aspect ratio GaN nanowire structures.
In this work, we have developed a process to embed the low-resistance ohmic contact with the dry etch hard mask, which leads to the realization of vertical GaN nanowire transistors with promising current injection. This novel structure is composed of a 20 nm Ni layer deposited via electron beam evaporation, a 20 nm Au layer directionally deposited via thermal evaporation. This metal stack was then annealed at 500°C for 10 minutes under oxygen. After this anneal, the bottom Ni/Au layers form a low resistance ohmic contact with the p-GaN through diffusion and the formation of NiO. Following the anneal step, a final and conformal 200 nm Ni capping layer deposited via electron beam evaporation. Electrical performance of this full contact stack was measured via the transmission line model (TLM) using 10 µm x 30 µm contact pads at separation distances ranging from 25 µm to 75 µm. Our measurements indicate that the inclusion of this additional capping layer still provides a good ohmic contact with GaN, showing a specific contact resistance as low as 0.14 Ω/cm². By including an additional capping layer in our electrical contacts, the metal stack can serve both as a low resistance ohmic contact and additionally as a hard mask for Cl-based inductively coupled plasma (ICP) etching. These promising results suggest that our proposed process could be used for the successful fabrication of high aspect ratio vertical GaN nanowire optoelectronic devices.

Photoelectrochemical (PEC) water splitting, which is the direct electrolysis of water with one of the half-reactions occurring on an immersed semiconductor electrode, has made good progress in solar-to-hydrogen (STH) efficiency in the last several years. III-V semiconductor materials have demonstrated the highest STH efficiencies mostly due to their excellent optoelectronic properties, bandgap tunability, and ease of monolithic multijunction fabrication. Many unique semiconductor layers must work in harmony to achieve a device that can spontaneously split water upon illumination. The layers that constitute the solid-state device establish the current and voltage generated under illumination while the strata closer to the electrolyte greatly influence the electrochemical performance.
During this talk we will describe the process of designing and testing a multijunction semiconductor structure optimized for maximum STH efficiency. We grew III-V semiconductors with metalorganic vapor phase epitaxy using the inverted metamorphic approach to obtain non-lattice-matched absorber junctions. We used over a dozen distinct III-V layers to achieve a tandem absorber with a 1.8 eV band gap top junction and 1.2 eV bottom junction. While the photovoltaic performance of these devices was exceptional, they were unable to perform photoelectrolysis without a bias due to an unoptimized interface with the electrolyte. We grew a highly doped emitter layer on the wide band gap material to create a buried junction which improved the open-circuit voltage of the PEC structures by > 500 mV. This configuration achieved 14% STH efficiency, which was short of our target efficiency (15%), so we grew a solid-state, surface-passivating “window layer” that is latticed matched to the wide band gap material. While this device structure had improved voltage and current, the window layer is chemically unstable in our acidic electrolyte. We grew a thin, highly-doped, lattice-matched capping layer to protect the underlying window layer. Finally, we sputtered PtRu nanoparticulate cocatalysts on the photoelectrode surface to lower the kinetic overpotential of the hydrogen evolution reaction. We rigorously benchmarked the STH efficiency of the resulting device and observed greater than 16% conversion of sunlight into hydrogen fuel [1]. Still more work remains to be done to engineer the semiconductor/electrolyte interface to address insufficient stability.

[1] NATURE ENERGY 2, 17028 (2017).

Dopant-free heterocontacts are frequently utilized as charge-carrier selective contacts of hybrid silicon solar cells with high power conversion efficiencies. Although these interfaces are well characterized from a device perspective, the physics and chemistry of contact formation mechanisms are still not well understood. We use X-ray photoelectron spectroscopy (XPS) to track core-level shifts upon contact formation and accessing the built-in potential. Furthermore, vacuum-level shifts are measured by ultraviolet photoelectron spectroscopy (UPS). By spin-coating ultrathin layers of the conducting polymer PEDOT:PSS on hydrogen-terminated n-Si we can track the Si core-levels upon contact formation and get direct evidence for inversion layer formation. In addition, our XPS and UPS data allows us to speculate that PEDOT passivates Si surface states. Furthermore, we demonstrate that the built-in potential measured with XPS correlates well with the open-circuit voltage of model solar cells. The same holds for devices with dopant-free MoO_x/n-Si or LiF/p-Si heterocontacts as main building blocks. Strikingly, the insulator LiF forms a Schottky contact with p-Si and facilitates an ohmic contact in n-Si.

Tuning the carrier injection barrier between organic semiconductors and electrodes in organic optoelectronic devices is of pivotal importance for their future development. The electron affinity (EA) of materials used in organic light-emitting diodes (OLEDs) is smaller than that of materials used in other organic devices. Therefore, chemically reactive alkaline elements such as Li/Cs have been essential for fabricating the electron injection layer (EIL) near the cathode. Because such alkali elements are highly chemically reactive and easily oxidize in the presence of ambient oxygen and moisture, there is a strong desire to eliminate reactive materials from devices, especially flexible optoelectronic devices.
The method of fabricating a low-work-function (WF) electrode that eliminates reactive alkali elements, which are widely used for electron injection, from organic devices has been the subject of intense study in recent years. Zhou et al. reported electrodes with a WF of about 3 eV by utilizing polyethyleneimine, and some organic devices without reactive alkali elements have been demonstrated [1]. Very recently, an electrode with a WF of 2.4 eV has been reported by Tang et al. [2]. However, alkali elements are still essential in most OLEDs to achieve a barrier-free contact between the cathode and the organic layer, since the EA of materials used in OLEDs is about 2.0 eV.
Here, we succeeded in injecting electrons into various organic materials owing to the discovery of an EIL that can significantly reduce the WF around the cathode [3]. The novel electron injection material found in this study is the superbase 2,6-bis(1,3,4,6,7,8-tetrahydro-2H-pyrimido[1,2-a] pyrimidin-1-yl)pyridine (Py-hpp₂), which can reduce the WF near an Al cathode from 4.3 to 2.0 eV by utilizing both a coordination reaction [4] and the formation of hydrogen bonds [5]. To investigate the correlation between the EA of the electron transport materials and the electron injection properties, OLEDs were fabricated using 17 different materials for the electron transport layer (ETL). The actual EA of these materials was measured by low-energy inverse photoemission spectroscopy (LEIPS). We found that the operating voltage was low when the actual EA was above 2.0 eV, even if the molecular structures were different, such as a compound with nitrogen-containing heterocycles and aromatic hydrocarbons. A compound with nitrogen-containing heterocycles has conventionally been considered to be essential for delivering electrons from the cathode to the light-emitting layer in OLEDs. However, by tuning the WF near the cathode to about 2.0 eV using a superbase, it is possible to deliver electrons from the cathode to the light-emitting layer independent of the molecular structure. On the basis of these results, we fabricated a simple-structure blue OLED with 9-(naphthalen-1-yl)-10-(naphthalen-2-yl)anthracene (1-2ADN) as the emitting host material, and we also used it as the ETL. Then, an OLED with low operating voltage, high efficiency and high operational stability was realized in spite of the simple device structure. This liberation of the materials used in OLEDs by utilizing superbases can enable the realization of device structures that was previously impossible.
Our breakthrough discovery represents a departure from the conventional wisdom and is expected to contribute to significant progress in the development of various devices such as OLEDs, perovskite LEDs, organic/perovskite solar cells, organic semiconductor laser diodes, and organic transistors.

[1] Y. Zhou et al., Science vol.336, p.327 (2012).
[2] C. G. Tang et al., Nature vol.573, p.519 (2019)
[3] T. Sasaki et al., Nat. Commun. vol.12, p.2706 (2021).
[4] H. Fukagawa et al., Nat. Commun. vol.11, p.3700 (2020).
[5] H. Fukagawa et al., Adv. Mater. vol.31, e1904201 (2019).

Owning to their good photophysical properties, perylene diimides have shown great potential in many fields, such as photo-redox catalysis, bio-imaging and optoelectronic devices. In fact, imine functionalized PDI derivatives have recently been used as cathode interlayers in organic solar cells (OSCs) significantly improving their efficiency. To create the most suitable interface between the bulk heterojunction (BHJ) and the electrode, these materials are processed with an alcohol-based solvent. This combination enables multiple properties that were examined to understand the full extent of how this cathode enhances charge carrier dynamics as well as the exciton dissociation in the OSC’s active layer.

A PTQ10:IDIC conventional organic solar cell was reproduced and used as a reference for this project. Several OSCs were fabricated under different conditions and studied through a series of standard PV and spectroscopic characterizations. Indeed, pristine BHJ, alcohol treated BHJ and the addition of a PDI based interlayer were compared. However, one of the key characterization methods used was light intensity variation measurements that were conducted to enlighten us on the types of recombination. This method enables the extraction of the ideality factor through various approaches and can greatly expand our knowledge of the OSC’s inner workings.

Polyhedral oligomeric silsesquioxane (POSS)[1] based organic light-emitting devices (OLEDs) have significant potential applications in displays and solid-state lighting[2], based on favorable charge transport, light absorption, and emission, resulting high efficiency OLEDs[3]. The conventional method of attaching the emitters with the POSS core is metal-assisted hydrosilylation, which often involves the use of expensive metals, such as Pt. Since metal incorporation in POSS(R)₈ affects the cost of the device, and also increases the environmental footprint of the synthetic process, a metal-free synthetic strategy is highly desirable. In this work, we propose a highly efficient, easily accessible, metal-free Lewis acid-catalyzed hydrosilylation process for the synthesis of POSS(R)₈ under very mild conditions, and report on initial results for a blue emitter. A Lewis acid, B(C₆F₅)₃ [tris(pentafluorophenyl)borane], is used to activate the Si–H bond group as a constituent of frustrated Lewis pairs (FLP). For the blue emitter[2,4], we have modified the traditional diphenyl group with a more conjugated pyrene moiety. The characteristic peaks of alkenes at δ 5.74 (ddt, J = 17.1, 9.6, 6.8 Hz, 1H), 4.96 – 4.84 (m, 2H), 2.19 – 2.09 (m, 2H), the methyl group at δ 1.36 – 1.24 (m, 3H) ppm from 1H-NMR and the quaternary carbon peak at δ 40.56 ppm from 13C-NMR confirms the successful synthesis of the emitter. Currently, we are optimizing the reaction conditions for the synthesis of different types of POSS(R)₈ starting from different emitters where B(C₆F₅)₃ can be used as a metal-free catalyst. This reaction will be monitored by using thin-layer chromatography (TLC) and FT-IR spectroscopy. The synthesized products will be characterized by 1H-NMR, 13C-NMR, 29Si-NMR, high-resolution mass spectrometry (HRMS), FT-IR, and
thermogravimetric techniques. The newly developed protocol would provide a more industrially relevant and more environmentally benign process, and contribute significantly to commodity chemical production of [POSS(R)8] due to their significant applications in OLEDs. Characterization data of blue emitter (B2): Yellow fluorescent solid (125 mg, 89% yield). 1H-NMR (500 MHz, CDCl₃) δ 8.38 – 8.23 (m, 4H), 8.23 – 7.96 (m, 14H), 7.73 – 7.66 (m, 3H), 5.74 (ddt, J = 17.1, 9.6, 6.8 Hz, 1H), 4.96 – 4.84 (m, 2H), 2.19 – 2.09 (m, 2H), 1.97 (q, J = 7.4 Hz, 2H), 1.53 (s, 2H), 1.36 – 1.24 (m, 3H) ppm; 13C-NMR (101 MHz, CDCl₃) δ 152.62, 140.25, 139.08, 138.87, 138.25, 131.57, 131.04, 130.60, 129.67, 128.65, 127.74, 127.49, 127.42, 126.05, 125.39, 125.25, 125.14, 125.00, 124.83, 124.70, 119.93, 40.56, 33.47, 30.92, 29.31, 26.86 ppm.
[1] (a) Wu, J.; Mather, P. T. J. Macromol. Sci., Polym. Rev. 2009, 49, 25–63; (b) Kuo, S. W.; Chang, F. C. Prog. Polym. Sci. 2011, 36, 1649–1696.
[2] Froehlich, J. D.; Young, R.; Nakamura, T.; Ohmori, Y.; Li, S.; Mochizuki, A. Chem. Mater. 2007, 19, 4991–4997.
[3] (a) Yang, X.; Froehlich, J. D.; Chae, H. S.; Harding, B. T.; Li, S.; Mochizuki, A.; Jabbour, G. E. Chem. Mater. 2010, 22, 4776–4782; (b) Carmoa, D. R.; Filhob, N. L. D.; Stradiottoa, N. R. Material Research 2004, 7, 499–504.
[4] Hazra, C. K.; Gandhamsetty, N.; Park, S.; Chang, S. Nat. Commun. 2016, 7, 13431–13440.

Lack of stability is one of the major limitations towards halide perovskite solar cell (HPSC) commercialization. As charge transport layers are critical for device performance and stability, the development of charge transport layers with excellent chemical stability and good energy level alignment for efficient charge transfer is of high research interest. TiO₂ and SnO₂ are widely employed as electron transport layers. However, HPSCs with TiO₂ layer suffer from poor stability due to ultraviolet (UV) light-induced O^2- desorption leading to degradation of halide perovskites. SnO₂ shows temperature-induced degradation and exhibits a lower conduction band compared to halide perovskites resulting in a voltage loss. To achieve better stability and efficiency of HPSCs, it is essential to identify alternative wide band gap materials that are UV-stable and show favorable energy level alignment.
Here, we report on the flux-mediated synthesis of morphology- and surface chemistry-controlled NaTaO₃ thin films and their application as an electron transport layer for HPSCs. The synthesized NaTaO₃ films have a band gap of ~ 4 eV, which makes them more chemically robust under UV illumination and slows down the degradation of halide perovskites, leading to enhanced stability of HPSCs. By varying the precursor chemistry, the resulting microstructure of NaTaO₃ films can be varied from smooth thin films to nanocubes. The latter is possibly useful to increase photon recycling. More efficient photoluminescence quenching of the halide perovskite emission, as a measure for charge extraction efficiency, is observed with the NaTaO₃ in comparison to the SnO₂ layer. In addition, the precursor chemistry can be used to tune the charge extraction ability of NaTaO₃ thin films via surface chemistry modifications.

Single-junction perovskite solar cells (~ 1.5 eV) have reached certified efficiency of 25.5%, on par with c-Si technology. Due to the broad bandgap tunability of perovskites, all-perovskite tandem solar cell (TSC), which consists of a wide-bandgap (WBG, 1.75-1.85 eV) perovskite top subcell and a narrow-bandgap (NBG, 1.2-1.3 eV) perovskite bottom subcell, hold great promise to break the Shockley-Queisser limit of single-junction solar cells via reduced thermalization loss. With only a few years' developments, efficiencies over 25% have been demonstrated in both 4-terminal and 2-terminal all-perovskite TSCs. Despite the promising progress, all-perovskite tandems are still far from reaching their practical efficiency potential (>30%), and the long-term (>1000 hours) stability under operational conditions remains a major concern.
All-perovskite TSCs are complex multilayer devices featuring over 10 thin film layers. Each of these layers has its own functionality, such as charge extraction and transport, defect passivation, broadband transparent conducting electrode, etc., and are equally important to achieve high efficiency and stability. In this contribution, we present comprehensive studies on factors limiting the efficiency and stability of WBG and NBG perovskite subcells. We use advanced materials and device characterizations, including scanning transmission electron microscopy, time-of-flight secondary ion mass spectrometry, time-resolved photoluminescence (TRPL), (hard) X-ray photoelectron spectroscopy, thermal admittance spectroscopy, etc., to gain insights on optical, compositional, structural, and electronic properties in WBG and NBG perovskite absorbers and at interfaces to adjacent layers. We will present interfacial engineering (such as tuning band alignment and implementing surface passivation strategies) tailored for WBG and NBG perovskite layers and associated interfaces to mitigate the photovoltaic performance and stability loss mechanisms. With this approach, we demonstrate over 22% all-perovskite TSCs on flexible polymer foils with promising operational stability. In the end, we will discuss the opportunities and challenges towards 25% flexible all-perovskite TSCs on both 4-terminal and 2-terminal architecture.