Materials Design Laboratory

A numerical modeling system was developed which can simulate the transport phenomena of a bending type square billet continuous casting process. Fluid flow and heat transfer were analyzed with a 3-dimensional finite volume method (FVM) with the aid of an effective heat capacity algorithm for the solidification. For a complex geometry of the bending type billet caster, a body-fitted-coordinate (BFC) system was employed. The bent structure of the caster allows a recirculating flow to develop in the upper and outer-radius region and the main stream to shift toward inner radius. This causes the thinner solid shell in the inner radius region than in the outer one. Besides standard operation conditions, we have analyzed the results when casting speed, caster shape, and tundish superheat changes. Lower casting speed makes the solid shell thicker by reducing heat flux from the mold. In the vertical caster, solid shell thickness are more uniform than that in the bending-type in entire region. When superheat increases by 5°C, solid shell thickness at the mold exit becomes thinner by 1 mm.

Using empirical embedded atom method (EAM) potential, activation barriers of adatom diffusion on the surface, diffusion along the step edge, and island corner diffusion are calculated for metal (111) surface (Pt, Ag) and the behavior of those barriers is investigated with respect to lattice strain. Based on the calculated barriers, we perform the kinetic Monte Carlo simulation for monolayer island growth and for how the variation of the activation barriers with lattice strain changes the island growth. Results from this work show that the lattice strain transforms the island growth shape from⟨1¯1¯2⟩ dendritic through random fractal to⟨112¯⟩ denditic at 100 K and from triangular through hexagonal to inverse triangular at 400 K.

Homoepitaxy provides an ideal testing ground for fundamental concepts in film growth. The rich variety of complex far-from-equilibrium morphologies which can form during deposition contrasts with the simple equilibrium structure of homoepitaxial films. These complex morphologies result from the inhibition on the time-scale of deposition of various equilibrating surface diffusion processes. A sophisticated framework for analysis of such phenomena derives from the concepts and methodology of Statistical Physics. Kinetic Monte Carlo (KMC) simulation of suitable atomistic lattice–gas models has elucidated the growth behavior of numerous specific systems. In this review, we describe in detail submonolayer nucleation and growth of two-dimensional islands during deposition. The traditional mean-field treatment is quite successful in capturing the behavior of mean island densities, but it fails to predict island size distributions. The latter are provided by simulation of appropriate atomistic models, as well as by suitable hybrid models. Recent developments towards providing reliable analytic beyond-mean-field theories are also discussed. Kinetic roughening of multilayer films during deposition is also described with particular emphasis on the formation of mounds (multilayer stacks of 2D islands) induced by step-edge barriers to downward transport. We describe results for mound evolution from realistic atomistic simulations, predictions of phenomenological continuum theories, and efforts to derive more reliable coarse-grained formulations. For both regimes, we demonstrate how atomistic modeling can be used extract key activation barriers by comparison with experimental data from scanning tunneling microscopy and surface sensitive diffraction. Significantly, suitable tailored atomistic models are often shown to have predictive capability for growth over a broad range of temperatures. Finally, we comment briefly on other deposition processes such as heteroepitaxial growth and chemisorption.

Multi-Scale Computational Framework (MSCF) integrating a Computational Fluid Dynamics software for reactor-scale processes, a Kinetic Monte Carlo solver for the growth of molecular structures, and a Molecular Dynamic simulator for the self-assembly of atoms into molecular structures is presented. The integration was achieved using equation-free Gap-tooth and Coarse Timestepper algorithms. The MSCF was demonstrated for a plasma-assisted synthesis of vertically aligned carbon nanotubes (CNTs) in an inductively coupled C2H4/H2 plasma system. Paths for delivering a supply of carbon onto catalyst/CNT interface, formation of single-wall and multi-wall CNTs, and time-dependences for probabilities of carbon incorporation into CNTs are discussed.

The influence of epitaxial strain on the surface and inter-layer diffusions are investigated using molecular statics and transition state theory with several types of embedded atom method potentials for Ag and Ni. Quantitatively to analyze the competition of the surface and inter-layer diffusions in the instability to the multi-layer growth, Ehrlich–Schwoebel barrier and the attempt frequencies of the surface and inter-layer diffusions by both hopping and exchange mechanisms are considered simultaneously. The attempt frequencies of exchange mechanism are larger by order one than those of hopping mechanism and especially, the difference becomes more severe for the inter-layer diffusion. Considering both the attempt frequency and activation energy barrier shows that the layer-by-layer growth is enhanced by compressive strain for Ag(0 0 1), which is confirmed by the existing linear stability theory and kinetic Monte Carlo simulations.

Amorphous-gallium-indium-zinc-oxide (⁠a -GIZO) thin filmtransistors (TFTs) are fabricated without annealing, using processes and equipment for conventional a-Si:H TFTs. It has been very difficult to obtain sound TFT characteristics, because the a -GIZO active layer becomes conductive after dry etching the Mo source/drain electrode and depositing the a-SiO2 passivation layer. To prevent such damages, N2O plasma is applied to the back surface of the a -GIZO channel layer before a-SiO2 deposition. N2O plasma-treated a -GIZO TFTs exhibit excellent electrical properties: a field effect mobility of 37cm2/Vs ⁠, a threshold voltage of 0.1V ⁠, a subthreshold swing of 0.25V /decade, and an Ion∕off ratio of 7

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.

We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle calculations and reveal that surface dangling bonds (SDBs) on Si nanostructures could be fundamental obstacles in nanoscale doping. In contrast to bulk Si, as the size of Si becomes smaller, SDBs on Si nanostructures prefer to be charged and asymmetrically deactivate n- and p-type doping. The asymmetric dopant deactivation in Si nanostructures is ascribed to the preference for negatively charged SDBs as a result of a larger quantum confinement effect on the conduction band. On the basis of our results, we show that the control of the growth direction of silicon nanowire as well as surface passivation is very important in preventing dopant deactivation.

We investigate the role of fluorine (F) in GaN-based high electron mobility transistors (HEMTs) with first principle calculations. Formation energy calculations of F in GaN and AlN reveal that energetically favored interstitial F (Fi) and substitutional F at N sites (FN) could play important roles in the performance of HEMTs. Fi is responsible for positive threshold voltage (Vth) shift by forming F– anion and depleting 2DEG carriers. The degradation of device performance at high temperature is ascribed to the defect energy state near conduction band edge of FN. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces half-emptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.

We investigate the electronic band structures of Ge/Si core–shell nanowires (CSNWs) and devise a way to realize the electron quantum well at Ge core atoms with first-principles calculations. We reveal that the electronic band engineering by the quantum confinement and the lattice strain can induce the type-I/II band alignment transition, and the resulting type-I band alignment generates the electron quantum well in Ge/Si CSNWs. We also find that the type-I/II transition in Ge/Si CSNWs is highly related to the direct to indirect band gap transition through the analysis of charge density and band structures. In terms of the quantum confinement, for [100] and [111] directional Ge/Si CSNWs, the type-I/II transition can be obtained by decreasing the diameters, whereas a [110] directional CSNW preserves the type-II band alignment even at diameters as small as 1 nm. By applying a compressive strain on [110] CSNWs, the type-I band alignment can be formed. Our results suggest that Ge/Si CSNWs can have the type-I band alignment characteristics by the band structure engineering, which enables both n-type and p-type quantum-well transistors to be fabricated using Ge/Si CSNWs for high-speed logic applications.

Crystalline Mg-based alloys with a distinct reduction in hydrogen evolution were prepared through both electrochemical and microstructural engineering of the constituent phases. The addition of Zn to Mg-Ca alloy modified the corrosion potentials of two constituent phases (Mg + Mg2Ca), which prevented the formation of a galvanic circuit and achieved a comparable corrosion rate to high purity Mg. Furthermore, effective grain refinement induced by the extrusion allowed the achievement of much lower corrosion rate than high purity Mg. Animal studies confirmed the large reduction in hydrogen evolution and revealed good tissue compatibility with increased bone deposition around the newly developed Mg alloy implants. Thus, high strength Mg-Ca-Zn alloys with medically acceptable corrosion rate were developed and showed great potential for use in a new generation of biodegradable implants.

Normally-off AlGaN/GaN HEMTs have been fabricated by employing a recessed-gate structure and oxygen plasma treatment and outstanding improvement of V th variation is observed. The origin of the observed positive V th shift and reduced variation window induced by oxygen plasma treatment is investigated by computational methods. Formation energy calculations for oxygen inclusions in III–N reveal that a negatively charged V Al -O N complex in the AlGaN passivation layer can be a major source of V th variation in AlGaN/GaN hetero-structured devices. Calculated trap energy levels are used as the parameters of a device simulation, which indicated that significant V th variation can be induced by a small fluctuation in the AlGaN layer thickness and defect densities. Our theoretical investigation shows that normally-off AlGaN/GaN HEMTs having reliable V th variation can be produced by oxygen inclusions accompanying a recessed-gate structure.

One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic–inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.

We investigate the role of atomic hydrogen in Ge/Si core–shell nanowires with first-principles calculations and present that the hole doping in the Ge/Si heterointerface is achievable through interstitial hydrogen mediated remote doping. This atomic hydrogen induced hole doping could generate one-dimensional hole gas in Ge/Si core–shell nanowires. Hydrogen prefers to be incorporated in the Si shell due to the lattice strain effect. As the charge transition energy level of the interstitial hydrogen in Si is lower than the valence band maximum of the Ge band, the electrons in the Ge core prefer to move toward the Si shell and become trapped by the interstitial hydrogen. This unique hydrogen energy level in the Ge/Si heterostructure between the Ge and Si valence band edges drives the electron transfer from the Ge core and induces holes states in the Ge core through remote hole doping. We also perform a quantum transport simulation and show that a high conductive hole channel in the Ge core can be generated when hydrogen is incorporated into the Si shell. Our investigation on the role of atomic hydrogen in the Ge/Si core–shell nanowire opens the possibility of manipulating the hole concentration by tuning the process conditions.

The dependency of dopant-distributions on channel diameters in realistically sized, highly phosphorus-doped silicon nanowires is investigated with an atomistic tight-binding approach coupled to self-consistent Schrödinger–Poisson simulations. By overcoming the limit in channel sizes and doping densities of previous studies, this work examines electronic structures and electrostatics of free-standing circular silicon nanowires that are phosphorus-doped with a high density of ∼2 × 1019 cm–3 and have 12 nm−28 nm cross-sections. Results of analysis on the channel energy indicate that the uniformly distributed dopant profile would be hardly obtained when the nanowire cross-section is smaller than 20 nm. Insufficient room to screen donor ions and shallower impurity bands are the primary reasons of the nonuniform dopant-distributions in smaller nanowires. Being firmly connected to the recent experimental study (Proc. Natl. Acad. Sci. U.S.A.2009, 106, 15254–15258), this work establishes the first theoretical framework for understanding dopant-distributions in over-10 nm highly doped silicon nanowires.

The retarded dopant diffusion in Si nanostructures is investigated using the first principles calculation. It is presented that weak dopant–dopant interaction energy (DDIE) in nanostructures is responsible for the suppressed dopant diffusion in comparison with that in bulk Si. The DDIE is significantly reduced as the diameter of the Si nanowire becomes smaller. The mechanical softening and quantum confinement found in nanostructures are the physical origin for the small interaction energy. Reduced dopant–dopant interaction slows down the diffusion process from heavily doped regions to undoped regions. Thus, we suggest that an additional annealing process is indispensable to make a desired dopant profile in the nanoscale semiconductor devices.

Organic–inorganic perovskites are promising materials for improving the efficiency of solar cells, but there are still uncovered issues on the understanding of their electronic band structures. Using first-principles calculations, we investigate the electronic band features of organo-lead iodide perovskites and present the efficient model to predict the band gap variation based on the orbital interaction scheme. The orbital interaction between Pb and I atoms can be controlled through the structural modification such as the change in lattice constant and the deviation of I atoms from cubic symmetry sites. The increase of the lattice constant and the positional distortion of I atoms from the cubic symmetry sites lead to the increase of the band gap. With our findings, puzzling band gap variation behaviors in previous experiments and simulations can be understood, and we suggest a pathway to precisely control their band gap. Our study can serve as the design rule for band gap engineering for various kinds of organic–inorganic hybrid perovskites.

A study was conducted to demonstrated ordered vacancy compound (OVC) formation by controlling element redistribution in molecular-level precursor solution processed CuInSe2 (CIS) thin films. It was also possible to form the OVC phase relatively uniformly through the bulk of CIS films. The hydrazine solution process used in this work demonstrated the highest power conversion efficiencies (PCE) among the molecular-level precursor solution processed thin film solar cells.

Solar cells based on organic–inorganic hybrid metal halide perovskites have been proven to be one of the most promising candidates for the next generation thin film photovoltaic cells. Mixing Br or Cl into I-based perovskites has been frequently tried to enhance the cell efficiency and stability. One of the advantages of mixed halides is the modulation of band gap by controlling the composition of the incorporated halides. However, the reported band gap transition behavior has not been resolved yet. Here a theoretical model is presented to understand the electronic structure variation of metal mixed-halide perovskites through hybrid density functional theory. Comparative calculations in this work suggest that the band gap correction including spin–orbit interaction is essential to describe the band gap changes of mixed halides. In our model, both the lattice variation and the orbital interactions between metal and halides play key roles to determine band gap changes and band alignments of mixed halides. It is also presented that the band gap of mixed halide thin films can be significantly affected by the distribution of halide composition.

While oxide dispersion strengthening of Fe-based alloys has been actively studied, the theoretical investigation of dispersion strengthening in Ni-based alloys is still lacking. This study presents potential oxide candidates for oxide dispersion strengthened Ni-based alloys using density functional calculations. By comparing the formation and binding energies of various metal oxides and lanthanide oxides, we determined that the lanthanide oxides can be efficient oxide dispersion sources and have the potential to compete with yttrium oxides for Ni alloys. We also found that the vacancies in a Ni–lanthanide oxide system play an important role in stabilizing nuclei formation process.

In general, in thermoelectric materials the electrical conductivity σ and thermal conductivity κ are related and thus cannot be controlled independently. Previously, to maximize the thermoelectric figure of merit in state-of-the-art materials, differences in relative scaling between σ and κ as dimensions are reduced to approach the nanoscale were utilized. Here we present an approach to thermoelectric materials using tin disulfide, SnS2, nanosheets that demonstrated a negative correlation between σ and κ. In other words, as the thickness of SnS2 decreased, σ increased whereas κ decreased. This approach leads to a thermoelectric figure of merit increase to 0.13 at 300 K, a factor ∼1,000 times greater than previously reported bulk single-crystal SnS2. The Seebeck coefficient obtained for our two-dimensional SnS2 nanosheets was 34.7 mV K−1 for 16-nm-thick samples at 300 K.

Although the record efficiencies of perovskite hybrid solar cells are gradually reaching the efficiency of crystalline Si solar cells, perovskite hybrid solar cells often exhibit significant current density–voltage (J–V) hysteresis with respect to the forward and reverse scan direction and scan rate. The origin of the J–V hysteresis of perovskite hybrid solar cells has not, to date, been clearly elucidated. Dielectric polarization by the ferroelectric properties of perovskite (i), the ionic motion/migration of perovskite materials (ii), and charge trapping and detrapping at trap sites by the unbalanced electron and hole flux (iii) are considered the possible origins of J–V hysteresis. Here, we reviewed the origin of the J–V hysteresis of perovskite solar cells from the above three points of view and we then suggest how one may reduce the J–V hysteresis with respect to the scan direction and scan rate

Organic–inorganic hybrid perovskites have unique electronic properties in which deep level defects are rarely formed. This unique defect characteristic is the source of the long carrier diffusion length. This theoretical study shows what causes this characteristic formation of shallow level defects in lead tri-halide perovskites. Comparative studies between iodides and other halides showed that deep level defect states were generated for Cl based perovskites. Longer Pb–halide bond lengths and narrower band gaps are beneficial for preventing deep level defect states. Additionally, our study shows that the formation of shallow level defects does not change even when the lattice structures of the perovskites do not reach their equilibrium structures.

We report solutions (durable material and degradation prevention method) to minimize the performance degradation of cell components occurring in the solid oxide fuel cell (SOFC) operation. Reliability testing is carried out with the Ni[single bond]Nd0.1Ce0.9O2-? (NDC) anode-supported intermediate temperature-SOFCs. For the cathode materials, single perovskite structured Ba0.5Sr0.5Co0.8Fe0.2O3-? (BSCF) and double perovskite structured NdBa0.5Sr0.5Co1.5Fe0.5O5+? (NBSCF) are prepared and evaluated under harsh SOFC operating conditions. The double perovskite NBSCF cathode shows excellent stability in harsh SOFC environments of high humidity and low flow rate of air. Furthermore, we propose the concurrent fuel and air starvation mode, in which the cell potential is temporarily reduced due to the formation of both fuel-starvation (in the anode) and air-depletion (in the cathode) concurrently under a constant load. This is carried out in order to minimize the performance decay of the stable NBSCF-cell through the periodic and extra reduction of (and ) in the anode. The operating-induced degradation of SOFCs, which are ordinarily assumed to be unrecoverable, can be completely circumvented by the proposed periodical operation logic to prevent performance degradation (concurrent fuel-starvation and air-depletion mode).

Ternary metal halides (A3X2I9) have attracted considerable interest because they have good stability and reduced toxicity compared with Pb-based halide perovskites. The main issue with A3X2I9 is their band gap, which is relatively large for use in a single junction solar cell (1.9–2.2 eV for the Cs3Bi2I9). This theoretical study found that the band gap of Cs3Bi2I9 can be successfully modulated by using dual metal cations, i.e., by forming Cs3BiXI9 (X: trivalent cation). Among the various trivalent atoms investigated, In and Ga showed very promising band gap modulation behaviors. Additionally, the indirect band gap of Cs3Bi2I9 can be changed into a direct band gap.

As a next-generation solar cell, perovskite solar cells (PSCs) have been attracting considerable attention. FAPbI3 is particularly considered as an optimal material with a proper band gap and thus has been employed as a base material for the PSCs with more than 20% efficiency; however, the competitive polymorphic growth of α- and δ-phases is a major hurdle in utilizing this material. To provide the theoretical model of the polymorphic phase competition of FAPbI3 for the first time, we here investigate how compositional engineering can pave a route to control the polymorphic growth of FAPbI3 using density functional theory combined with a statistical-mechanical treatment of the configurational space. We find that dual-site alloying of both cations and halides is critically important to achieve the specific stabilization of the α-phase while maintaining the good miscibility, thermodynamic stability, and optimal band gap property. Based on our first successful theoretical modeling of the FAPbI3 system and its polymorphic phase competition behavior during dual-site alloying, we anticipate deriving new rational guidelines on compositional engineering of organic–inorganic hybrid perovskite alloys for designing PSCs with high efficiencies and stabilities.

Silver nanowire (AgNWs) networks offer a promising approach for producing cost effective transparent electrode materials. However, high temperature annealing is necessary to achieve low sheet resistance, which limits the use of AgNWs for low temperature processing applications such as flexible electronics, and perovskit.es solar cell fabrication. This study shows that the low temperature processing of efficient AgNW electrodes can be achieved with the choice of an appropriate dispersion solvent. Ethanol shows promising behaviors over IPA by forming uniform percolation networks even without thermal annealing. The key factors affecting percolation uniformity are the solubility of organic contaminants and the viscosity of the solvent which determines the droplet sizes. This study contributes to future studies of transparent conductors and flexible devices.

We quantitatively and analytically investigate the properties of buffer/window junctions and their effects on the energy band alignment and the current-voltage characteristics of Cu(In,Ga)Se2 (CIGS) thin film solar cells with solution processed silver nanowire (AgNW) composite window layers. AgNWs are generally embedded in a moderately conductive matrix layer to ensure lateral collection efficiency of charge carriers photogenerated in the lateral gaps present between AgNWs. Studies on the junctions between a buffer and AgNW-composite window layers and their effects on the performances of CIGS thin film solar cells have seldom been addressed. Here, we show that solution processed AgNW-composite window layers could induce defect states at the buffer/window interface, resulting in poor energy band alignment impeding carrier transport in the solar cells. On the basis of our analysis, we suggest an analytical expression of cm to avoid losses in the power conversion efficiency of the solar cells. is the carrier concentration in a matrix layer embedding AgNWs, is the negative defect density at the buffer/window interface, and is the relative dielectric constant of the matrix layer embedding AgNWs.

Cr is a critical alloying element in Ni-based alloys. The role of Cr on the formation of oxide nuclei in Ni-Cr alloys is theoretically investigated with density functional theory. Our simulation indicates that Cr has strong binding energy with the nearest oxygen and that it enhances the stability of the metal-oxide nucleus. Therefore, oxide dispersion can be promoted by the incorporation of Cr into Ni alloys. Additionally, potential oxide candidates for the oxide strengthening of Ni-Cr alloys are theoretically explored. The findings, through an analysis of electronic structures, suggest that La and Y are most promising when attempting to realize stable oxide formation in Ni-Cr alloys and reveal that the metal used to create the primary oxides should have low electronegativity and a large atomic radius to create stable metal-oxide nuclei.

New light is shed on the previously known perovskite material, Cs2Au2I6, as a potential active material for high-efficiency thin-film Pb-free photovoltaic cells. First-principles calculations demonstrate that Cs2Au2I6 has an optimal band gap that is close to the Shockley–Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs2Au2I6 a double-perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs2Au2I6 has a direct-band-gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state-of-the-art multiscale simulations connecting from the material to the device, strongly suggests that Cs2Au2I6 will serve as the active material in highly efficient, nontoxic, and thin-film perovskite solar cells in the very near future.

An 8% stable planar type trivalent Bi-based Cs3Bi2I9 perovskite-sensitized solar cell (P-SSC) was demonstrated by fabricating a glass/FTO (F doped tin oxide)/TiO2/Cs3Bi2I9/PTAA (poly-triarylamine)/Au construction. The Cs3Bi2I9 perovskite film formed by Cs3Bi2I9/N,N-dimethylformamide/HI solution featured a pure crystalline phase and excellent thermal stability. The unencapsulated Cs3Bi2I9 P-SSC maintained its initial efficiency for >500 h under continuous light soaking by 1 Sun at 65 °C and 60–70% relative humidity. Its functional stability may be due to the trivalent Bi based perovskite structure being more robust than the divalent Pb based perovskite one.

Solution-processed silver nanowires are promising materials as transparent electrodes for optoelectronic devices to indium tin oxide thin film. For application to various devices, the sheet resistance of silver nanowire networks must be reduced further and thermal stability of the silver nanowire networks should be improved. By lowering the contact resistance between the nanowires, the sheet resistance of the silver nanowire networks can be further lowered. In this work, we improve the electrical conductivity as well as thermal stability of silver nanowire networks by the electrodeposition of silver. The electrical and optical properties are readily controlled by adjusting the electrodeposition time. The sheet resistance of silver nanowire networks is decreased by welding between each silver nanowire. The thermal stability of silver nanowire networks is improved by controlling the diameter of the silver nanowires.

Double perovskites using dual metal cations are promising candidates for Pb-free perovskites. This study shows that the electronic structures of double perovskites (A2B+B3+X6) can be significantly modulated by cation ordering changes. The bandgap of Cs2AgBiCl6 can be affected by changing octahedron alignments, and even zero gap states can be realized for the 2-dimensional BiCl6 (AgCl6) configuration. It is presented that different types of B+/B3+-site orderings in double perovskites could be the origin of bandgap dispersion. Comparative studies on the various compositions show that, among B+/B3+ cations, Tl/Bi could be promising for the suppression of ordering variation.

Mixed-cation CsxRb1–xPbX3 (X = Cl, Br, I) perovskite quantum dots (PeQDs) are developed and show high quantum yields of 93% and 86% for green and blue wavelengths, respectively. The stability is significantly improved under heat, UV, and water aging conditions. These improved properties are explained by the increased defect formation resistance and exciton binding energy with Rb incorporation. Perovskite films are fabricated using CsxRb1–xPbX3 PeQDs and cyclic olefin copolymer. The films have a wide color gamut covering up to 104.15% of the BT.2020-defined color space, with the white light color coordinates of (0.33, 0.32), luminance of 68.86 Cd m−2, and correlated color temperature of 5299 K at 20 mA.

Preserving the stability of Sn-based halide perovskites is a primary concern in developing photovoltaic light-absorbing materials for lead-free perovskite solar cells. Whereas the addition of SnX2 (X = F, Cl, Br) has been demonstrated to improve the photovoltaic performance of Sn halide perovskite solar cells, the mechanistic roles of SnX2 in the performance enhancement have not yet been studied appropriately. Here we perform a comparative study of CsSnI3 films and devices and examine how SnX2 additives affect their stability, and the results are corroborated by first-principles-based theoretical calculations. Unlike the conventional belief that the additives annihilate defects, we find that the additives effectively passivate the surface and stabilize the perovskite phase, promoting the stability of CsSnI3. Our mechanism suggests that SnBr2, which shows ca. 100 h of prolonged stability along with a high power conversion efficiency of 4.3%, is the best additive for enhancing the stability of CsSnI3.

We demonstrate the ability to fabricate high-quality nanoscale electrical contact between silver nanowires (AgNWs) and underlying semiconducting layers in chalcogenide thin film solar cells. AgNW electrodes have attracted many interests due to their ability for low temperature solution processing. However, they have a drawback that the interfacial defects can be generated between AgNWs and underlying rugged semiconductor layers making it difficult to form high-quality junction. To enhance the junction properties, conducting matrix layers have been adapted. Yet, the issues regarding the AgNW/semiconductor junction have not been fully resolved. We developed a facile method to form robust nanoscale contact between AgNWs and semiconducting thin films to achieve high performance chalcogenide thin film solar cells. The method is to deposit an ultra-thin semiconductor layer on devices using aqueous chemical bath deposition. The chemical bath deposition has capability to effectively fill even nanoscale gap and to form chemically stable bonds as well as an intimate junction. As a proof of concept, a CdS layer (~ 10 nm) was deposited using the chemical bath deposition on Cu(In,Ga)Se2 (CIGS) solar cells with a structure of AgNW/CdS/CIGS/Mo/Glass. We also identified that the key factor governing the current-voltage characteristic is the electrical contact between the AgNW electrode and the CdS buffer layer in CIGS thin film solar cells. The power conversion efficiency of the CIGS cell was dramatically improved from 4.9% to 14.2% owing to high-quality AgNW-CdS electrical contact produced by chemical bath deposition of the additional CdS layer as thin as 10 nm.

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn0.5Ge0.5I3) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

Halide perovskite solar cells have been attracting tremendous attention as next-generation solar cell materials because of their excellent optical and electrical properties. Formamidinium lead tri-iodide (FAPbIM3) exhibits the narrowest band gap among lead iodide perovskites and shows excellent thermal and chemical stability, also. However, the large-area coating of FAPbIM3 needed for commercialization has not been successful because of the instability of the black phase of FAPbIM3 at ambient temperature. This study presents a compositional engineering direction to control the polymorph of the FAPbIM3 thin film for the shear coating processes, without halide mixing. By adopting a hot substrate above 100 °C, our shear coating process can produce the black phase FA-based halide perovskites without halide mixing. We carefully investigate the Cs-FA and MA-FA mixed lead iodide perovskites’ phase stability by combining the study with thin-film fabrication and ab initio calculations. Cs-FA mixing shows promising behaviors for stabilizing α-FAPbIM3 (black phase) compared with MA-FA. Stable FA-rich perovskite films cannot be achieved via shear coating processes with MA-FA mixing. Ab initio calculations revealed that Cs-FA mixing is excellent for inhibiting phase decomposition and water incorporation. This study is the first report that FA-based halide perovskite thin films can be made with the shear coating process without MA-Br mixing. We reveal the origin of the stable film formation with Cs-FA mixing, and present future research directions for fabricating FA-based perovskite thin films using shear coating.

The excited-state quantum dynamics of the organic cation in hybrid perovskites are investigated using the time-dependent density functional theory. The bond fluctuation reveals that the energy relaxation follows different pathways depending on the chemical bonding characteristics within the cation molecule, which can fundamentally affect photostability. For the ammonium-group-containing cations, such as methylammonium (MA) or ethylammonium (EA), local vibrational modes survive for a long time. However, as their lowest unoccupied molecular orbitals (LUMOs) have π* characters, the amidinium-group-containing cations, such as formamidinium (FA) or guanidinium (GA), efficiently dissipate deposited energy via chaotic intramolecular vibrational energy redistribution. The distinct A-site molecules’ dynamics are closely related to the quantum ergodicity, which can bring enhanced photostability of FA and GA compared to MA and EA. Our theoretical investigation reveals the quantum chaos origin of better light stability of FA-based perovskites and serves as the future research direction of the A-site engineering for better solar cells and light-emitting devices.

The toxicity of lead in metal halide perovskite solar cells is considered the most critical technical barrier for their commercialization. Among the candidates to replace Pb, trivalent cations (e.g., Bi3+ and Sb3+) are attracting considerable interest owing to their excellent reliability and favorable optical performance. Trivalent cation-based lead-free halide perovskites have two polymorphs: 0D phase (dimer, P63/mmc) and 2D phase (layered, P3m1). The 2D phase has a lower band gap and is preferred for carrier transport; however, its phase stability at room temperature remains a challenge. This study investigated the effects of dual-site compositional mixing on the stabilization of the 2D phase of A3Bi2X9 (A = Cs, methylammonium (MA), formamidinium (FA); X = I, Br, Cl) and electronic structure modulation. Cl mixing was noted to be essential in creating a stable 2D phase. MA mixing can be applied to reduce the band gap widening for the Cl-mixed perovskites. We suggest the optimal criterion as Cs3−χMAχBi2I9–yCly, with χ < 0.1 and y > 0.3.

For enhancing the performance and long-term stability of perovskite solar cell (PSC) devices, interfacial engineering between the perovskite and hole-transporting material (HTM) is important. We developed a fluorinated conjugated polymer PFPT3 and used it as an interfacial layer between the perovskite and HTM layers in normal-type PSCs. Interaction of perovskite and PFPT3 via Pb–F bonding effectively induces an interfacial dipole moment, which resulted in energy-level bending; this was favorable for charge transfer and hole extraction at the interface. The PSC device achieved an increased efficiency of 22.00% with an open-circuit voltage of 1.13 V, short-circuit current density of 24.34 mA/cm2, and fill factor of 0.80 from a reverse scan and showed an averaged power conversion efficiency of 21.59%, which was averaged from forward and reverse scans. Furthermore, the device with PFPT3 showed much improved stability under an 85% RH condition because hydrophobic PFPT3 reduced water permeation into the perovskite layer, and more importantly, the enhanced contact adhesion at the PFPT3-mediated perovskite/HTM interface suppressed surface delamination and retarded water intrusion. The fluorinated conjugated polymeric interfacial material is effective for improving not only the efficiency but also the stability of the PSC devices.

In this study, green and blue perovskite light-emitting diodes (PeLEDs) were systematically investigated using various organic spacers of different molecular lengths to control the number of dimensions of the PeLED material. The primary perovskite used was CsPbBr3, and the organic spacers tested were aniline hydrobromide (AnBr), benzylamine hydrobromide (BABr), phenylethylamine hydrobromide (PEABr), phenylpropylamine hydrobromide (PPABr), and phenylbutylamine hydrobromide (PBABr). By specifically testing different amounts of each spacer, green to blue PeLEDs were prepared and analyzed using XRD and SEM. The best optical and electrical performances were obtained with PEABr as the organic spacer. Using 0.4 mol of PEABr for 1 mol of CsPbBr3 yielded a green electroluminescence (EL) emission with a CIE of (0.078, 0.745), a maximum current efficiency of 43.9 cd/A, and an EQE of 8.67%, and using 1.6 mol of PEABr per mole of CsPbBr3 yielded a blue EL emission with a CIE of (0.151, 0.084), a maximum current efficiency of 0.87 cd/A, and an EQE of 0.39%.

Metal-halide perovskites (MHPs) have attracted tremendous attention as active materials in optoelectronic devices. For light-emitting diode (LED) applications, nanostructuring of MHPs is considered to be inevitable, but its light-enhancement mechanism is still elusive because the particle (or grain) size is often beyond the quantum confinement regime. As motivated by the experimental finding that the nanostructuring can change the preferred crystalline symmetry from the orthorhombic phase to the high-symmetric cubic phase, we here investigated the carrier dynamics in various polymorphic phases of CsPbBr3 using ab initio quantum dynamics simulation. We found that the cubic phase shows a smaller inelastic phonon scattering than the orthorhombic phase; the suppression of the octahedral tilt minimizes the longitudinal Br fluctuation and helps disentangle the A-site cation dynamics from the nonadiabatic carrier dynamics. We thus anticipate that our present work will offer a material design principle to enhance the quantum yield of MHPs via symmetry engineering, which will help develop highly luminescent LED technology based on MHPs.

We synthesized colloidal cesium metal halide CsMX (M = Fe, Co, Ni; X = Cl, Br) nanoparticles (NPs) and assessed their crystal stability by density functional theory (DFT) calculations. We successfully synthesized Cs3FeCl5, Cs3FeBr5, Cs3CoCl5, Cs3CoBr5, CsNiCl3, and CsNiBr3 NPs. CsMX NPs with Fe and Co exhibited Cs3M1X5 and Cs2M1X4 structures depending on the reaction conditions; however, CsNiX NPs exhibited only the CsNiX3 structure. The differences in structural stability by central metal ions were explained using spin-polarized DFT calculations. The analysis revealed tetragonal Cs3M1X5 and orthorhombic Cs2M1X4 structures to have similar thermodynamic stabilities in the case of Fe and Co, whereas the hexagonal CsMX3 structure in the case of Ni was the most stable. Moreover, the calculation results were the same as the experimental results. In particular, cobalt-related Cs3CoBr5 NPs easily developed into Cs2CoCl4 nanorods with an increase in temperature.

Toxicity is the main bottleneck for the commercialization of Pb halide perovskites. Bi has been considered a promising metal cation to replace Pb because of its comparable electronic structures with Pb and better stability. Although experimental and theoretical studies have proposed various Bi-based halides, the present achievements in photovoltaic cells and other photoelectronic device fields do not compete with Pb analogs. Thermodynamic stability, bandgap control, and enhancement of carrier transport are fundamental challenges in the context of intrinsic material properties for developing highly efficient Bi-based devices. This study evaluates the potential of Bi-based halide compounds with good stability and electronic properties through high-throughput density functional theory calculations. Lattice structures and compositions are selected based on previous reports and an open material database. Then, we expanded our dataset to cover all possible compositional variations of A- and X-sites and alloying to B-sites. We examined over six-hundred candidates and found ten new candidates that have not been reported previously. Rb3SbBiI9 exhibits the best-expected efficiency for high-efficiency solar cells among selected compounds, and other compounds can be used as visible-light-generation sources. Analysis of the screening procedure revealed that vacancy-ordered (A3B2X9)-type Bi-halides exhibit significantly favorable characteristics when compared with those of double perovskites and rudorffite-like structures for Bi-based photoelectronic devices.

Tin-halide perovskite solar cells (THPSCs) are attractive in the photovoltaic field as promising candidates to address the issue of potential lead toxicity and approach the theoretical efficiency limit in lead-halide perovskite photovoltaics. Nevertheless, THPSCs suffer from fast crystallization, low defect tolerance, mismatched energy levels, as well as severe oxidation from Sn2+ to Sn4+, leading to the low performance of devices. Herein, bromide is incorporated in the PEA0.15EA0.15FA0.70SnI1−XBrX perovskite precursor, which produces 2D/3D hybrid cations tin-halide perovskite films with highly vertical oriented crystallization, favorable band-level alignment, and suppressed tin oxidation. This leads to the decrease of trap density and charge recombination losses and the enhancement of charge carrier extraction in THPSCs. Consequently, the power conversion efficiency of the optimal THPSC (X = 0.30) surges to 10.12% in contrast to 7.13% of the control device (X = 0.00), along with a nearly eliminated current–voltage hysteresis. Furthermore, bromine-incorporated THPSCs exhibit outstanding light soaking and humidity stability. These results are also in good agreement with the density functional theory calculations. This compositional engineering with Br could become a promising approach for improving the efficiency and stability of THPSCs.

Degradation in CH3NH3PbI3 (MAPbI(3)), when in contact with commonly used metal oxide transport layer materials in optoelectronic devices, is examined experimentally and theoretically. On the basis of the decomposition temperature, the interfacial stability decreases in the following order: MAPbI(3) + TiO2 similar to MAPbI(3) alone > MAPbI(3) + SnO2 > MAPbI(3) + NiO, consistent with thermodynamic data. When MAPbI(3) contacts NiO or SnO2, experimental results unequivocally show interfacial decomposition occurs at a lower temperature than bulk decomposition and produces different degradation products. Density functional theory calculations reveal an altered reaction pathway on oxide surfaces and elucidate the difference between NiO and TiO2. These findings pinpoint the importance of understanding the interaction between halide perovskite and other materials used in a device to achieve intrinsically stable devices.

With their dramatic improvement of photoconversion efficiency, metal-halide perovskite (MHP) solar cells are receiving great attention. For successful deployment of these materials as next-generation solar cells, many research efforts are being undertaken to develop highly efficient and stable perovskite solar cells. Because compositional engineering in particular has provided a powerful route to optimize the material properties, MHPs with high efficiency and stability often include a number of different components. In this study, using ab initio thermodynamics for ternary mixtures at the A-site (FA, MA, and Cs) and varying Br/I content at the X-site, we provide thermodynamic modeling on how mixtures of different cations and halides at A- and X-sites can modify the stability of MHPs. Our in-depth calculation reveals that Br mixing is inevitable to stabilize the corner-shared perovskite structure of highly efficient FAPbI(3) with low bandgap. To maintain the minimal content of Br, which widens the bandgap, MA co-mixing is required, while Cs mixing contributes to prevent the decomposition of MHPs into precursors. We anticipate that the present study will provide thermodynamic insight into the distinctive roles of different components of MHPs and offer a design guideline for future compositional engineering of MHPs.

The cesium lead triiodide (CsPbI3) perovskite is a promising candidate for stable light absorbers and red-light-emitting sources due to its outstanding stability. Phase engineering is the most important approach for the commercialization of CsPbI3 because the optically inactive nonperovskite structure is more stable than three-dimensional (3-D) perovskite lattices at ambient temperature. This study presents an in-depth evaluation to find the optimum surface ligand and to reveal the mechanism of phase stabilization by surface ligands. Thermodynamic evaluations combined with density functional theory calculations indicate the criteria for forming stable 3-D CsPbI3 perovskites under surface and volume free energy competition between perovskite and nonperovskite phases. Comparative calculations for ammonium, alcohol, and thiol groups show that ammonium groups enhance the phase stability of 3-D perovskites the most. In addition, ammonium-passivated CsPbI3 is relatively robust against defect formation and H2O adsorption.

Structure engineering of trivalent metal halide perovskites (MHPs) such as A(3)Sb(2)X(9) (A = a monovalent cation such as methyl ammonium (MA), cesium (Cs), and formamidinium (FA) and X = a halogen such as I, Br, and Cl) is of great interest because a two dimensional (2D) layer structure with direct bandgap has narrower bandgap energy than a zero dimensional (0D) dimer structure with indirect bandgap. Here, we demonstrated 2D layer structured FACs(2)Sb(2)I(6)Cl(3) MHP by dual-site (A and X site) mixing. Thanks to the lattice-symmetry change by I-Cl mixed halide, the shortest ionic radius of Cs, and the lower solution energy due to dual-site mixing, the FACs(2)Sb(2)I(6)Cl(3) MHP had 2D layer structure and thereby the MHP solar cells exhibited improved short-circuit current density.

Sn-based halide perovskites have attracted much interest due to their highly valuable electrical and optical properties. The promising optical and electrical properties of Sn-based perovskites have enticed a lot of research to focus on developing the strategies and explore the in-depth material characteristics. Sn-halide perovskites exhibit apparent merits and demerits. The ideal electrical and optical properties are even better than that of Pb-analogs, namely close-to-optimal bandgap, strong optical absorption, and good carrier mobilities. However, the present achievement of Sn-halide perovskite solar cells is not satisfactory, which is commonly attributed to relatively low defect tolerance, fast crystallization, and oxidative instability. The efficiency of Sn-based perovskites is far ahead, with a 9% power conversion efficiency (PCE), than the other (Ge, Bi, Sb, Cu, etc.) Pb-free options but simultaneously lagging far behind Pb-based analogs that have a 25.2% PCE. This review is aimed at presenting milestone works and revealing the pros and cons of Sn-halide perovskites. In addition, the defect physics of Sn-based perovskites is described. The improvement of open-circuit voltage is a critical issue for Sn-halide perovskites to compete with Pb-based perovskites. The understanding of defect physics plays an instrumental role in designing strategies for efficient and robust Sn-halide perovskite solar cells.

High-speed continuous deposition of solution-processed perovskite thin films with controlled microstructural morphology has been intensively pursued toward practical commercialization of high-efficiency perovskite solar cells. In order to meet this urgent demand in a timely manner, we present a rapid (similar to 5 mm.s(-1)) scalable meniscus-controlled coating method to deposit large-grained ( > 100 mu m) perovskite thin films over large area. Shearing movement of the meniscus of perovskite solution, confined by the hydrophobic-treated top and temperature-controlled bottom plates enables a rapid crystallization of perovskite grains, also facilitating the continuous influx of solutes at the meniscus for continuous film growth with less consumption of perovskite solution. Optical and structural analyses revealed that the rapid-shear-deposited perovskite thin films possess a large-grained, densely-packed and highly-crystalline phase. Consequently, perovskite solar cell based this film exhibits the power conversion efficiency of 10.9% even without compositional engineering and additive controls. Therefore, it can be claimed that this rapid shearing deposition of perovskite solution paves a way to achieve a cost-competitive production of the high-efficiency solar cell.

Quantum confinement-driven band structure engineering of metal halide perovskites (MHPs) is examined for realistically sized structures that consist of up to 10(5) atoms. The structural and compositional effects on band gap energies are simulated for crystalline CH3NH3PbX3 (X = I/Br/Cl) with a tight-binding approach that has been well-established for electronic structure calculations of multimillion atomic systems. Solid maps of band gap energies achievable with quantum dots, nanowires, and nanoplatelets concerning sizes, shapes, and halide compositions are presented, which should be informative to experimentalists for band gap designs. The pathway to suppress band gap instability that appeared in mixed halide perovskites is proposed, revealing that the red shift induced by halide phase separation can be hugely diminished by reducing sizes and adopting halides of lower electronegativity. Our modeling results on finite MHP structures of over 10 nm dimensions show a blueprint for designs of stable light-emitting sources with precisely controlled wavelengths.

Cr-rich shells are known to decrease the lattice misfit of Y-Ti-O nanoparticles and contribute to high thermal stability. In the present work, the formation of a Cr-rich shell on the Y2Ti2O7/Fe interface is studied via density functional theory and examined with electron microscopy analyses. The density functional theory calculation showed that Cr substitution in Y2Ti2O7 is not allowed. Instead, Cr is favorably segregated on the interface of the Fe site owing to the strong binding energy between Cr and O. Atom probe tomography observations confirmed the formation of a Cr shell at the interface between Y2Ti2O7 nanoparticles and the Fe matrix. Contrary to the density functional theory calculation, a considerable Cr concentration was detected inside the oxide. However, additional transmission electron microscopy observation of very coarse Y2Ti2O7 nanoparticles demonstrated that the Cr concentration is almost negligible inside the oxide, which is consistent with the calculated prediction.

Sn-based halide perovskites are the most promising alternatives for developing Pb-free perovskite solar cell materials. However, the stability of Sn halide perovskites is the biggest concern for future developments. The phase stability and the doping-level control should be resolved for Sn perovskites to compete with Pb-based analogs. Herein, interstitial engineering is used to enhance the stability of Sn-based halide perovskites using alkali metals through ab initio calculations and controlled experiments. This study reveals that alkali metal interstitials can promote the performance of Sn perovskites by controlling their phase stability, suppressing free carrier density, and locking lattice vibration. K(+)shows the most promising behavior among alkali-metal cations in terms of phase stabilization and defect formation energy.

The strain aging behavior of a low carbon dual phase steel was examined in two conditions: representing room temperature strain aging (100 ºC × 1 hr after 7.5 % prestrain) and bake hardening process (170 ºC × 20 min after 2 % prestrain), basing on carbon effective diffusion and dislocation distribution. The first principle calculations revealed that (Mn or Cr)-vacancy-C complexes exhibit the strongest attractive interaction compared to other complexes, therefore, act as strong trapping sites for carbon. For room temperature strain aging condition, the carbon effective diffusion distance is smaller than the dislocation distance in the high dislocation density region near ferrite/martensite interfaces as well as ferrite interior considering the carbon trapping effect of the (Mn or Cr)-vacancy-C complexes, implying ineffective Cottrell atmosphere formation. Under bake hardening condition, the carbon effective diffusion distance is larger compared to the dislocation distance in both regions. Therefore, formation of the Cottrell atmosphere is relatively easy resulting in to a relatively large increase in yield strength under bake hardening condition.

We investigated the effects of materials and the film thickness of hole transport layers (HTLs) on inverted type perovskite solar cells using optics-charge transport coupled simulations. Power conversion efficiencies (PCEs), and the variations in efficiency induced by the film thickness dispersion, were intensively studied, to compare potential HTLs candidates like NiOx, PEDOT:PSS, CuSCN, and CuI. The optimum thickness of the solar cell layers differed based on the chosen combination of HTL and perovskite. It is suggested that the optoelectronic properties of HTLs like band gap, extinction coefficient, and refractive index can be used to determine the best ideal efficiencies, and sensitivity to process fluctuation. CuSCN showed the most promising behaviors, in that it can produce over-25% PCEs, and the lowest efficiency dispersion for various HTL thickness conditions. The best performance by CuSCN can be ascribed to its having a proper refractive index with the perovskite layer, and wide band gap characteristic. NiOx and CuI showed PCEs comparable to the CuSCN, but their efficiencies were sensitive to the varying thickness of the HTL. PEDOT:PSS exhibited the lowest simulated PCEs due to its small band gap. Our study suggests the best HTL candidates for inverted type perovskite solar cells, and demonstrates the importance of sophisticated numerical material studies, and device design, when developing highly efficient and robust perovskite solar cells.

ZnO thin films are of considerable interest because they can be customized by various coating technologies to have high electrical conductivity and high visible light transmittance. Therefore, ZnO thin films can be applied to various optoelectronic device applications such as transparent conducting thin films, solar cells and displays. In this study, ZnO rod and thin films are fabricated using aqueous chemical bath deposition (CBD), which is a low-cost method at low temperatures, and environmentally friendly. To investigate the structural, electrical and optical properties of ZnO for the presence of citrate ion, which can significantly affect crystal form of ZnO, various amounts of the citrate ion are added to the aqueous CBD ZnO reaction bath. As a result, ZnO crystals show a nanorod form without citrate, but a continuous thin film when citrate is above a certain concentration. In addition, as the citrate concentration increases, the electrical conductivity of the ZnO thin films increases, and is almost unchanged above a certain citrate concentration. Cu(In, Ga)Se