Tailored Band Alignment for Improved Carrier Transport in Composition-Controlled Sb2(S,Se)3.

Sb2(S,Se)3 is a highly available energy material with a tunable bandgap by adjusting the S/Se ratio. Increasing the Se ratio can enhance the efficiency of Sb2(S,Se)3 solar cells, with a higher short-circuit current (JSC). However, the accompanying decrease in the open-circuit voltage (VOC) restricts further improvement. The defect passivation is important, since it can reduce carrier recombination, enhancing the VOC. In this study, the relevance of the S/Se ratio, defect concentration, and VOC was investigated. The samples with or without the deposition of Se-rich Sb2(S,Se)3 onto S-rich Sb2(S,Se)3 were used for defect characterization. Different surface compositions were confirmed by Raman spectroscopy. The complicated subdefect states of S-rich Sb2(S,Se)3 were shown through photoluminescence and conductive atomic force microscopy, and a decrease in the defect concentration was observed through surface photovoltage. The improvement of JSC via bandgap grading and the simultaneous VOC improvement by defect passivation resulted in efficient cell performance.

Understanding the Space-Charge Layer in SnO2 for Enhanced Electron Extraction in Hybrid Perovskite Solar Cells.

Tin oxide (SnO2) has been widely used as an n-type metal oxide electron transport layer in perovskite solar cells (PSCs) owing to its superior electrical and optical properties and low-temperature synthesis process. In particular, the interfacial effect between indium tin oxide (ITO) and SnO2 is an important parameter that controls the charge transport properties and device performance of the PSCs. Therefore, understanding the interfacial effect of ITO/SnO2 and its role in PSCs is crucial, but it is not studied intensively. Herein, we investigated the space-charge effect at the interface of ITO/SnO2 using transfer length measurement and conductive atomic force microscopy as a function of SnO2 thickness. Moreover, optical, morphologic, and device measurements were performed to determine the optimal SnO2 thickness for PSCs. The space-charge effect was identified in ITO/SnO2 when the SnO2 layer was very thin due to electron depletion near the interface. Interestingly, a critical kink point was observed at approximately 10 nm SnO2 thickness, indicating the electron depletion and weak charge transfer behavior of the device. Thus, a thickness around 20 nm was favorable for the best PSC performance because charge transport behavior in the thin SnO2 layer was depressed by electron depletion. However, when the thickness of SnO2 exceeded 50 nm, the device performance deteriorated due to increased series resistance. This study provides a strategy to tune the electron transport layer and boost the charge transport behavior in PSCs, making important contributions to optimizing SnO2-based PSCs.

Improvement of Open-Circuit Voltage Deficit via Pre-Treated NH4 + Ion Modification of Interface between SnO2 and Perovskite Solar Cells.

Passivation is a popular method to increase power conversion efficiency (PCE), reduce hysteresis related to surface traps and defects, and adjust mismatched energy levels. In this paper, an approach is reported using ammonium chloride (AC) to enhance passivation effects by controlling chlorine (Cl) and ammonium ions (NH4 + ) on the front and back side of tin oxides (SnO2 ). AC pre-treatment is applied to indium tin-oxide (ITO) prior to SnO2 deposition to advance the passivation approaches and compare the completely separated NH4 + and Cl passivation effects, and sole NH4 + is successfully isolated on the SnO2 surface, the counterpart of AC-post-treatment, generating ammonia (NH3 ) and Cl. It is demonstrated that multifunctional healing effects of NH4 + are ascribed from AC-pre-treatment being the basis of SnO2 crystallization and adjusting bifacial interface energy levels at ITO/SnO2 and SnO2 /perovskite to enhance photo-carrier transport. As calculated by density functional theory, how the change of the passivation agent from Cl to NH4 + more effectively suppresses non-radiative recombination ascribed to hydrated SnO2 surface defects is explained. Consequently, enhancement of photo-carrier transport significantly improves a superior open-circuit voltage of 1.180 V and suppresses the hysteresis, which leads to the PCE of 22.25% in an AC-pre-treated device 3.000% higher than AC-post-treated devices.

Grain boundary passivation via balancing feedback of hole barrier modulation in HfO2-x for nanoscale flexible electronics.

Flexible electronics has attracted considerable attention owing to its enormous potential for practical applications in various fields. However, the massive strain produced during bending degrades the device. Especially at grain boundaries, due to the accumulation of defects, this degradation is exacerbated in flexible electronic devices. The importance of electrically inactivated grain boundaries increases as devices scale down to the nanoscale. Here, we propose an HfO2-x thin film that can be used as an excellent material for flexible electronics with versatile functionality, especially for grain boundary passivation. Various electrical phases of HfO2-x thin films with conducting to insulating behavior, which originates from oxygen deficiency, have been fabricated on flexible substrates. Furthermore, owing to the most stable charge state of oxygen vacancies, oxygen-deficient HfO2-x shows p-type conductivity. Current mapping by conductive atomic force microscopy reveals that current flow is hindered at grain boundaries due to the formation of potential barriers. This phenomenon is also observed in bent flexible thin films on convex and concave molds, leading to tensile and compressive strains, respectively. Although the defect concentration increases because of lattice deformation during bending, more holes are trapped at the grain boundaries, resulting in an increased hole barrier height. We believe that grain boundary passivation through hole barrier modulation during bending would pave the way for advances in hafnia-based nanoscale flexible electronics.

Tailored Band Structure of Cu(In,Ga)Se2 Thin-Film Heterojunction Solar Cells: Depth Profiling of Defects and the Work Function.

An efficient carrier transport is essential for enhancing the performance of thin-film solar cells, in particular Cu(In,Ga)Se2 (CIGS) solar cells, because of their great sensitivities to not only the interface but also the film bulk. Conventional methods to investigate the outcoming carriers and their transport properties measure the current and voltage either under illumination or dark conditions. However, the evaluation of current and voltage changes along the cross-section of the devices presents several limitations. To mitigate this shortcoming, we prepared gently etched devices and analyzed their properties using micro-Raman scattering spectroscopy, Kelvin probe force microscopy, and photoluminescence measurements. The atomic distributions and microstructures of the devices were investigated, and the defect densities in the device bulk were determined via admittance spectroscopy. The effects of Ga grading on the charge transport at the CIGS-CdS interface were categorized into various types of band offsets, which were directly confirmed by our experiments. The results indicated that reducing open-circuit voltage loss is crucial for obtaining a higher power conversion efficiency. Although the large Ga grading in the CIGS absorber induced higher defect levels, it effectuated a smaller open-circuit voltage loss because of carrier transport enhancement at the absorber-buffer interface, resulting from the optimized conduction band offsets.

Halide perovskite single crystals: growth, characterization, and stability for optoelectronic applications.

Recently, metal halide perovskite materials have received significant attention as promising candidates for optoelectronic applications with tremendous achievements, owing to their outstanding optoelectronic properties and facile solution-processed fabrication. However, the existence of a large number of grain boundaries in perovskite polycrystalline thin films causes ion migration, surface defects, and instability, which are detrimental to device applications. Compared with their polycrystalline counterparts, perovskite single crystals have been explored to realize stable and excellent properties such as a long diffusion length and low trap density. The development of growth techniques and physicochemical characterizations led to the widespread implementation of perovskite single-crystal structures in optoelectronic applications. In this review, recent progress in the growth techniques of perovskite single crystals, including advanced crystallization methods, is summarized. Additionally, their optoelectronic characterizations are elucidated along with a detailed analysis of their optical properties, carrier transport mechanisms, defect densities, surface morphologies, and stability issues. Furthermore, the promising applications of perovskite single crystals in solar cells, photodetectors, light-emitting diodes, lasers, and flexible devices are discussed. The development of suitable growth and characterization techniques contributes to the fundamental investigation of these materials and aids in the construction of highly efficient optoelectronic devices based on halide perovskite single crystals.

Enhanced Light Absorption and Efficient Carrier Collection in MoS2 Monolayers on Au Nanopillars.

We fabricated hybrid nanostructures consisting of MoS2 monolayers and Au nanopillar (Au-NP) arrays. The surface morphology and Raman spectra showed that the MoS2 flakes transferred onto the Au-NPs were very flat and nonstrained. The Raman and photoluminescence intensities of MoS2/Au-NP were 3- and 20-fold larger than those of MoS2 flakes on a flat Au thin film, respectively. The finite-difference time-domain calculations showed that the Au-NPs significantly concentrated the incident light near their surfaces, leading to broadband absorption enhancement in the MoS2 flakes. Compared with a flat Au thin film, the Au-NPs enabled a 6-fold increase in the absorption in the MoS2 monolayer at a wavelength of 615 nm. The contact potential difference mapping showed that the electric potential at the MoS2/Au contact region was higher than that of the suspended MoS2 region by 85 mV. Such potential modulation enabled the Au-NPs to efficiently collect photogenerated electrons from the MoS2 flakes, as revealed by the uniform positive surface photovoltage signals throughout the MoS2 surface.

High speed growth of MAPbBr3 single crystals via low-temperature inverting solubility: enhancement of mobility and trap density for photodetector applications.

There has been growing interest in organic-inorganic hybrid perovskites as a promising candidate for optoelectronic applications due to their superior physical properties. Despite this, most of the reported perovskite devices based on polycrystalline thin films suffer immensely from poor stability and high trap density owing to grain boundaries limiting their performance. Perovskite single crystal structures have been recently explored to construct stable devices and reduce the trap density compared to their thin-film counterparts. We present a novel method of growing sizable CH3NH3PbBr3 single crystals based on the high solubility characteristic of hybrid perovskites at low temperatures within inverse temperature crystallization. We compared both the crystallinity of perovskite single crystal structures and optoelectronic charge transport of single crystal photodetectors as a function of dissolution temperature. The performance of the photodetector fabricated with our large-scaled single crystal with high quality demonstrated low trap density, high mobility, and high photoresponse.

Reduced extrinsic recombination process in anatase and rutile TiO2 epitaxial thin films for efficient electron transport layers.

TiO2 is the most widely used material for the electron transport layers (ETLs) because it is characterized by proper band alignment with light absorbers, adequate optical transmittance, and high electron mobility. There are two thermodynamically stable crystal phases of TiO2: anatase and rutile. However, understanding which phase is more effective as the ETL is still required. In this paper, we demonstrate the different effects of using epitaxial anatase TiO2 and epitaxial rutile TiO2 (both grown using pulsed laser deposition) as the ETL material on the electrical and optical properties. Epitaxial Nb-doped TiO2 layers were used as the common electrode material for the both epitaxial ETLs for which the crystalline structural analysis revealed high crystalline qualities and good coherency for both phases. By analyzing the recombination kinetics, the anatase phase shows a preferable performance in comparison with the rutile phase, although both epitaxial phases show remarkably reduced extrinsic recombination properties, such as trap-assisted recombination. This study demonstrates not only a better electron transporting performance of anatase phase but also reduced extrinsic recombination through epitaxy growth.

Impact of Na Doping on the Carrier Transport Path in Polycrystalline Flexible Cu2ZnSn(S,Se)4 Solar Cells.

It is well-known that the alkali doping of polycrystalline Cu2ZnSn(S,Se)4 (CZTSSe) and Cu(In,Ga)(Se,S)2 has a beneficial influence on the device performance and there are various hypotheses about the principles of performance improvement. This work clearly explains the effect of Na doping on the fill factor (FF) rather than on all of the solar cell parameters (open-circuit voltage, FF, and sometimes short circuit current) for overall performance improvement. When doping is optimized, the fabricated device shows sufficient built-in potential and selects a better carrier transport path by the high potential difference between the intragrains and the grain boundaries. On the other hand, when doping is excessive, the device shows low contact potential difference and FF and selects a worse carrier transport path even though the built-in potential becomes stronger. The fabricated CZTSSe solar cell on a flexible metal foil optimized with a 25 nm thick NaF doping layer achieves an FF of 62.63%, thereby clearly showing the enhancing effect of Na doping.

Raman Scattering Studies of the Structural Phase Transitions in Single-Crystalline CH3NH3PbCl3.

Methylammonium lead trihalide perovskites CH3NH3PbX3 (X = Cl, Br, and I) have recently attracted huge attention as a promising candidate for highly efficient solar cell absorber materials. To understand the physical properties of halide perovskites, we investigated the CH3NH3PbCl3 single crystal by Raman scattering spectroscopy from 80 K to room temperature. Benchmarking the phonon modes and their Raman activities obtained by density functional calculations, we successfully assign the molecular vibrations of methylammonium in the frequency range from 400 to 3300 cm-1. In the temperature-dependent Raman scattering spectra, the internal vibrational modes of the CH3NH3+ cation are observed in the frequency range above 400 cm-1 and a number of peaks among them show characteristic changes that reflect the phase transition occurring at about 160 K in CH3NH3PbCl3. A noticeable Raman peak at 2900 cm-1 persists in a wide range of temperature, suggesting the existence of microcrystalline methylammonium chloride possibly left on the crystal surface from the growth process or spontaneously formed after synthesis.

Sodium-assisted passivation of grain boundaries and defects in Cu2ZnSnSe4 thin films.

The long stagnation of the photo-conversion efficiency of kesterites below 13% is a source of frustration in the scientific community. In this study, we investigated the effects of sodium on the passivation of grain boundaries and defects in Cu2ZnSnSe4 (CZTSe) grown on a soda-lime glass (SLG) and borosilicate (BS) glass. Because BS glass does not inherently contain sodium, we placed a thin layer of NaF between CZTSe and Mo. The composition of the samples is Cu-poor and Zn-rich. The distribution of sodium and its contributions to phase formation and defects were examined by cross-sectional energy-dispersive X-ray profiling, Raman scattering spectroscopy and imaging, surface potential and photoluminescence. From the experimental results, it can be strongly claimed that sodium ions segregate predominantly near the grain boundaries and reduce CuZn-related defects. These local surface imaging analyses provided the exact locations of the secondary phases. In particular, the photo-assisted scanning probe method enabled us to observe the changes in the optoelectrical properties of the thin films and the carrier behavior within the materials. Further studies with distinct alkali ions and optimal processing conditions will pave a way to improve the performance of kesterite solar cells.

Reconfigurable Dipole-Induced Resistive Switching of MoS2 Thin Layers on Nb:SrTiO3.

The controllable band gap and charge-trapping capability of MoS2 render it suitable for use in the fabrication of various electrical devices with high-k dielectric oxides. In this study, we investigated reconfigurable resistance states in a MoS2/Nb:SrTiO3 heterostructure by using conductive atomic force microscopy. Low-resistance and high-resistance states were observed in all MoS2 because of barrier height modification resulting from redistribution of charge and oxygen vacancies in the vicinity of interfaces. In a thin layer of the MoS2 film, the carrier density was high, and layer-dependent transport properties appeared because of the charge separation in MoS2. The hysteresis and switching voltage of the MoS2/Nb:SrTiO3 heterostructure could be varied by controlling the number of layers of MoS2.

Enhanced carrier transport over grain boundaries in lead-free CH3NH3Sn(I1-x Br x )3 (0 ≤ x ≤ 1) perovskite solar cells.

This paper reports on grain boundary (GB) roles in lead-free tin halide perovskite thin films. Nano scale spatial mapping of charge separation efficiency in methylammonium tin halide (MASn(I1-x Br x )3, MA = CH3NH3) thin films were constructed by Kelvin probe force microscopy and conductive atomic force microscopy (C-AFM). We observed downward band bending at GBs under dark conditions and higher surface photovoltage along the GBs, confirmed by C-AFM which showed high local current flows along the GBs. The band bending degree and local current intensity were affected by the Br/I ratio. Photo-generated carriers were more effectively separated and collected at GBs with increased Br content, and hysteresis was observed in Br-rich Sn-halide perovskite.

Virtual Out-of-Plane Piezoelectric Response in MoS2 Layers Controlled by Ferroelectric Polarization.

The MoS2 carrier distribution can be controlled with the use of a dielectric environment substrate. Ferroelectric thin films are used to investigate the electrical responses at the MoS2 layer. The MoS2/(111)-PbTiO3 vertical heterostructure is investigated, and the electrical responses, including piezoelectricity, are obtained using piezoresponse force microscopy. The piezoelectric response modifications obtained at the MoS2 layer on the ferroelectric thin films are a result of the depolarizing effect. In particular, the piezoelectricity enhancement is observed at the 19-layer MoS2 because of an induced dipole effect. By considering the polarization effects of ferroelectric thin films, the electrical responses at the MoS2 layers can be controlled, and the interfacial carrier distribution at the interface results in different electrical performances at the MoS2.

Memory effect behavior with respect to the crystal grain size in the organic-inorganic hybrid perovskite nonvolatile resistive random access memory.

The crystal grain size of CH3NH3PbI3 (MAPbI3) organic-inorganic hybrid perovskite (OHP) film was controllable in the range from ~60 nm to ~600 nm by non-solvents inter-diffusion controlled crystallization process in dripping crystallization method for the formation of perovskite film. The MAPbI3 OHP non-volatile resistive random access memory with ~60 nm crystal grain size exhibited >0.1 TB/in2 storage capacity, >600 cycles endurance, >104 s data retention time, ~0.7 V set, and ~-0.61 V re-set bias voltage.

Trapping charges at grain boundaries and degradation of CH3NH3Pb(I1-x Br x )3 perovskite solar cells.

The electrical properties of CH3NH3Pb(I1-x Br x )3 (x = 0.13) perovskite materials were investigated under ambient conditions. The local work function and the local current were measured using Kelvin probe force microscopy and conductive atomic force microscopy, respectively. The degradation of the perovskite layers depends on their grain size. As the material degrades, an additional peak in the surface potential appears simultaneously with a sudden increase and subsequent relaxation of the local current. The potential bending at the grain boundaries and the intragrains is the most likely reason for the change of the local current surface of the perovskite layers. The improved understanding of the degradation mechanism garnered from this study helps pave the way toward an improved photo-conversion efficiency in perovskite solar cells.

Luliconazole Retention in Stratum Corneum and Prevention of Fungal Infection in a Guinea Pig Tinea Pedis Model.

OBJECTIVE: To compare drug concentrations in the stratum corneum following daily application of luliconazole and terbinafine cream in a guinea pig tinea pedis model. METHODS: Luliconazole 1% cream or terbinafine 1% cream were topically applied once daily to hind limbs of guinea pigs for 14 days. Drug concentration in stratum corneum of plantar skin was measured by HPLC-UV on days 1, 3, 7, 10, and 14. Separately, creams were applied daily for 5 days to the hind limbs of guinea pigs and skin drug release determined. In addition, drug retention in the stratum corneum was assessed by infecting guinea pigs with Trichophyton mentagrophytes, 14 and 21 days after a single application of luliconazole or terbinafine creams. RESULTS: Luliconazole stratum corneum concentrations were higher than those of terbinafine throughout the study. Concentrations of luliconazole and terbinafine were 71.6µg/g and 36.6µg/g, respectively, after a single application (P<.05), reaching steady state after 10 days. Cumulative release of luliconazole from the stratum corneum was 4.5 times greater than with terbinafine. Unlike terbinafine, no fungal invasion of the stratum corneum was seen 14 days post-treatment with luliconazole. CONCLUSIONS: Drug concentrations of luliconazole in the stratum corneum and subsequent release are greater than those achieved with terbinafine and may contribute to clinical efficacy. Luliconazole may also provide greater protection against disease recurrence.

Efficient Carrier Separation and Intriguing Switching of Bound Charges in Inorganic-Organic Lead Halide Solar Cells.

We fabricated a mesoporous perovskite solar cell with a ∼14% conversion efficiency, and we investigated its beneficial grain boundary properties of the perovskite solar cells through the use of scanning probe microscopy. The CH3NH3Pb(I0.88,Br0.12)3 showed a significant potential barrier bending at the grain boundary and induced passivation. The potential difference value in the x = 0.00 sample is ∼50 mV, and the distribution of the positive potential is lower than that of the x = 0.12 sample. We also investigated the polarization and hysteretic properties of the perovskite thin films by measuring the local piezoresponse. Specifically, the charged grain boundaries play a beneficial role in electron-hole depairing and in suppressing recombination in order to realize high-efficiency perovskite solar cells.

Nanostructured p-type CZTS thin films prepared by a facile solution process for 3D p-n junction solar cells.

Nanoporous p-type semiconductor thin films prepared by a simple solution-based process with appropriate thermal treatment and three-dimensional (3D) p-n junction solar cells fabricated by depositing n-type semiconductor layers onto the nanoporous p-type thin films show considerable photovoltaic performance compared with conventional thin film p-n junction solar cells. Spin-coated p-type Cu2ZnSnS4 (CZTS) thin films prepared using metal chlorides and thiourea show unique nanoporous thin film morphology, which is composed of a cluster of CZTS nanograins of 50-500 nm, and the obvious 3D p-n junction structure is fabricated by the deposition of n-type CdS on the nanoporous CZTS thin films by chemical bath deposition. The photovoltaic properties of 3D p-n junction CZTS solar cells are predominantly affected by the scale of CZTS nanograins, which is easily controlled by the sulfurization temperature of CZTS precursor films. The scale of CZTS nanograins determines the minority carrier transportation within the 3D p-n junction between CZTS and CdS, which are closely related with the photocurrent of series resistance of 3D p-n junction solar cells. 3D p-n junction CZTS solar cells with nanograins below 100 nm show power conversion efficiency of 5.02%, which is comparable with conventional CZTS thin film solar cells.