Carrier recombination in CH3NH3PbI3: why is it a slow process?

In methylammonium lead iodide (MAPbI3), a slow recombination process of photogenerated carriers has often been considered to be the most intriguing property of the material resulting in high-efficiency perovskite solar cells. In spite of intense research over a decade or so, a complete understanding of carrier recombination dynamics in MAPbI3has remained inconclusive. In this regard, several microscopic processes have been proposed so far in order to explain the slow recombination pathways (both radiative and non-radiative), such as the existence of shallow defects, a weak electron-phonon coupling, presence of ferroelectric domains, screening of band-edge charges through the formation of polarons, occurrence of the Rashba splitting in the band(s), and photon-recycling in the material. Based on the up-to-date findings, we have critically assessed each of these proposals/models to shed light on the origin of a slow recombination process in MAPbI3. In this review, we have presented the interplay between the mechanisms and our views/perspectives in determining the likely processes, which may dictate the recombination dynamics in the material. We have also deliberated on their interdependences in decoupling contributions of different recombination processes.

Formation of CuIn(1-x)GaxS2 Thin Films through a Solution Approach: Nonlinear Variation of Fermi Energy and Band Gap Bowing.

We report the formation of CuIn(1-x)GaxS2 (CIGS) thin films through a solution approach, namely, successive ionic layer adsorption and reaction (SILAR) technique. The obtained films possessed a high degree of crystallinity indicating the efficacy of the deposition process in forming CIGS films. A series of alloys have evidenced band gap bowing, that is, the optical band gap does not follow a linear relationship with the composition; the band gap of an intermediate compound is higher than that is interpolated from a linear relationship or Vegard's law. The composition-dependent band gap followed a quadratic relationship evidencing reverse band gap bowing, manifesting an upward convex behavior. With scanning tunneling spectroscopy (STS) and thereby the density of states of the disordered semiconductors, we have observed a bowing behavior in the transport gap as well and identified the roles of the conduction and valence bands in yielding the bowing phenomenon. The results are explained in terms of the anionic and cationic orbitals involved in forming the two bands. The STS studies have been analyzed further to derive the Urbach energy of the disordered semiconductors. When STS studies are combined with Kelvin probe force microscopy, which in effect provides the Fermi energy of the alloyed semiconductors, we could derive the band edges and the Fermi energy of the whole series in an absolute energy scale during the manifestation of the bowing phenomenon.

Spin-Selective Charge Transport in Lead-Free Chiral Perovskites: The Key towards High-Anisotropy in Circularly-Polarized Light Detection.

A pair of zero-dimensional lead-free chiral perovskites is introduced towards the detection of circularly polarized light (CPL). Although spin-polarized carriers are generated in the perovskites under the CPL, the absorption anisotropy remained low leading to mostly similar density of photogenerated carriers under the two CPLs. Interestingly, due to the intrinsic chirality in the perovskites, they exhibited chirality-induced spin-selectivity (CISS) allowing the transport of only one type of spin-half states. A high anisotropy in photocurrent along the out-of-plane direction has therefore appeared resulting in a spin-dependent photovoltaic effect in vertical heterojunction devices and making them suitable for CPL detection. While a self-powered CPL detector showed a limited (but one of the highest to date) anisotropy factor of 0.3 due to possible spin-flips during the transport process, the factor rose to 0.6 under bias prompting extension of the effective spin-diffusion length.

Rashba Band Splitting in CH3NH3PbI3: An Insight from Spin-Polarized Scanning Tunneling Spectroscopy.

We report an experimental observation of Rashba splitting in methylammonium lead iodide (MAPbI3). Because of a large spin-orbit coupling parameter in the noncentrosymmetric material, both of the bands were predicted to split having two different spin-textures leading to two different Z-components of total angular momentum (JZ). We use spin-polarized scanning tunneling spectroscopy to probe only one-type of JZ-matched bands throughout the film; dI/dV spectra recorded at many different points of a film however allowed us to spot both the Rashba split-levels and also deliberate on their spin-textures. We observe that the bands split in such a manner that the conduction and valence bands closer to the Fermi energy have the same type of spin-textures (a spin-allowed transition model). Still a low recombination rate of photogenerated carriers in MAPbI3 has been analyzed by considering Wannier-type excitons, a molecular nature of spin-domains from dI/dV images, and therefore a spin-forbidden nature of interdomain transition.

Dirac States of 2D Topological Insulators: Effect of Heterovalent Dopant-Content.

We have studied Bi2Se3 at its 2D-limit using scanning tunneling spectroscopy (STS). Bulk Bi2Se3 is a well-known topological insulator having gapless surface states. In the 2D limit, the interior of the material exhibits a band gap, whereas the periphery shows a gapless metallic state having a Dirac point. We demonstrate a method to tune the Fermi energy and hence the Dirac point of Bi2Se3 nanoplates through doping at the anionic site. For this purpose, STS measurements were carried out on the Bi2Se3 system. We have used bromide as a dopant, which turns the material to n-type in nature. As a result, STS studies infer that the Fermi energy (EF) shifted toward the conduction band and consequently the Dirac point could be found to move away from Fermi energy. Through STS measurements, we have demonstrated a correlation between the shift of Dirac point position and the dopant content. The size, shape, and compositions of Bi2Se3 nanoflakes and concentration of bromine in the doped nanostructures were determined using transmission electron microscopy, associated energy dispersive X-ray spectroscopy analysis, and X-ray diffraction.

All-Organic Dual Spin Valves with Well-Resolved Four Resistive-States.

The formation of all-organic dual spin valves (DSVs) with three organic spin-selective layers, that is, spin-injection, spin-detection, and an additional spin-filtering layer at the intermediate, is reported. As spin-selective layers, manganese- and cobalt phthalocyanines, which are well-known single-molecule magnets, are used in their immobilized forms, so that all-organic DSVs can be prefabricated for characterization. The three spin-selective layers have provided four configurations with at most two spin-flip interfaces enforcing spin-flipping at the two nonmagnetic organic spacer layers, for which copper phthalocyanine is used. Since a couple of the four configurations have exhibited similar resistivities, the degeneracy in the resistive-states is broken through asymmetric spin-injection and spin-detection layers and also through asymmetric thickness of the nonmagnetic spacer layers. When both the spin-flip interfaces are made operative independently, a 2-bit logic with four distinct resistive states can be achieved.

Bi2Se3 topological insulator at the 2D-limit: role of halide-doping on Dirac point.

2D topological insulators exhibit insulating bulk and conducting edge states with a Dirac point, which at times is within the energy gap and could be on either side of the Fermi energy. In this study, we demonstrate a method to tune the energy of the Dirac edge state by introducing halides as dopants in Bi2Se3. We chose halides to substitute the anion, so that due to higher atomic number (of iodine, for example) with respect to selenium, the spin-orbit coupling parameter could be enhanced, leading to the significant separation of the Dirac point from the Fermi energy. With different halogens having different atomic numbers on either side of selenium, the Dirac point could hence be tuned towards both directions. The dopants, due to their heterovalent nature with respect to selenide, introduce carriers in the lattice and consequently, also shift the Fermi energy. We show that the Dirac point with respect to Fermi energy could be correlated to the dopant's atomic number and thus the atomic-number-induced spin-orbit coupling parameter. Strains developed in the lattice due to a mismatch in the effective ionic radii of the dopants and the host anion affected distribution of band energies, leaving the (distribution of) Dirac point unaffected due to its topologically protected nature.

Differential conductance (dI/dV) imaging of a heterojunction-nanorod.

Through scanning tunneling spectroscopy, we envisage imaging a heterostructure, namely a junction formed in a single nanorod. While the differential conductance spectrum provides location of conduction and valence band edges, dI/dV images record energy levels of materials. Such dI/dV images at different voltages allowed us to view p- and n-sections of heterojunction nanorods and more importantly the depletion region in such a junction that has a type-II band alignment. Viewing of selective sections in a heterojunction occurred due to band-bending in the junction and is correlated to the density of states spectrum of the individual semiconductors. The dI/dV images recorded at different voltages could be used to generate a band diagram of a pn junction.

Hybrid heterojunctions between a 2D transition metal dichalcogenide and metal phthalocyanines: their energy levels vis-à-vis current rectification.

We form junctions between a monolayer of a range of metal-phthalocyanines and a monolayer of 2D transition metal dichalcogenides (TMD) through an electrostatic adsorption process. The energy levels of the components, as drawn from scanning tunneling spectroscopy (STS) and the density of states (DOS) thereof, indicated that the hybrid junctions would act as current rectifiers. We have observed that the central metal atom affected the energies of the metalorganics and thereby the rectification ratio of the junctions. In addition, since planar single molecule magnets (SMMs) were used in which magnetization appears due to the 3d-electrons of the metal, we could align the molecules followed by their structural immobilization. We have observed that such an alignment changed their molecular orbitals and hence affected the energy levels at the interface. The rectification ratio of molecule|TMD hybrid junctions depends on the metal in metalorganics and also on their alignment with the substrate electrode. In effect, the rectification ratio in a range of junctions has been correlated to the energy-level diagram at the interface.

Simultaneous observation of surface- and edge-states of a 2D topological insulator through scanning tunneling spectroscopy and differential conductance imaging.

A 2D form of Bi2Se3 which acts as a topological insulator was grown through colloidal synthesis method. The surface-states and edge-states of the nanoplates were simultaneously probed through scanning tunneling spectroscopy (STS). At the interior, density of states (DOS) revealed the location of conduction and valence band edges. The DOS at the edges, on the other hand, brought out gapless conducting states along with a Dirac point at a non-zero value below the Fermi energy representing the Dirac cone of a 2D topological insulator. In differential tunnel conductance (dI/dV), images are recorded at different voltages and the two sections of the topological insulator can be viewed selectively or simultaneously with a clear contrast in illumination. Upon increasing the 2D-nanoplates thickness, the material turned into a 3D topological insulator with gapless surface states.

Band mapping across a pn-junction in a nanorod by scanning tunneling microscopy.

We map band-edges across a pn-junction that was formed in a nanorod. We form a single junction between p-type Cu2S and n-type CdS through a controlled cationic exchange process of CdS nanorods. We characterize nanorods of the individual materials and the single junction in a nanorod with an ultrahigh vacuum scanning tunneling microscope (UHV-STM) at 77 K. From scanning tunneling spectroscopy and correspondingly the density of states (DOS) spectra, we determine the conduction and valence band-edges at different points across the junction and the individual nanorods. We could map the band-diagram of nanorod-junctions to bring out the salient features of a diode, such as p- and n-sections, band-bending, depletion region, albeit interestingly in the nanoscale.

Hybrid pn-junction solar cells based on layers of inorganic nanocrystals and organic semiconductors: optimization of layer thickness by considering the width of the depletion region.

We report the formation and characterization of hybrid pn-junction solar cells based on a layer of copper diffused silver indium disulfide (AgInS2@Cu) nanoparticles and another layer of copper phthalocyanine (CuPc) molecules. With copper diffusion in the nanocrystals, their optical absorption and hence the activity of the hybrid pn-junction solar cells was extended towards the near-IR region. To decrease the particle-to-particle separation for improved carrier transport through the inorganic layer, we replaced the long-chain ligands of copper-diffused nanocrystals in each monolayer with short-ones. Under illumination, the hybrid pn-junctions yielded a higher short-circuit current as compared to the combined contribution of the Schottky junctions based on the components. A wider depletion region at the interface between the two active layers in the pn-junction device as compared to that of the Schottky junctions has been considered to analyze the results. Capacitance-voltage characteristics under a dark condition supported such a hypothesis. We also determined the width of the depletion region in the two layers separately so that a pn-junction could be formed with a tailored thickness of the two materials. Such a "fully-depleted" device resulted in an improved photovoltaic performance, primarily due to lessening of the internal resistance of the hybrid pn-junction solar cells.

Introducing Chiro-optical Activities in Photonic Synapses for Neuromorphic Computing and In-Memory Logic Operations.

In artificial synaptic devices aimed at mimicking neuromorphic computing systems, electrical or optical pulses, or both, are generally used as stimuli. In this work, we introduce chiral materials for tailoring the characteristics of photonic synaptic devices to achieve handedness-dependent neuromorphic computing and in-memory logic gates. In devices based on a pair of chiral perovskites, the use of circularly polarized light (CPL) as the optical stimuli mimicked a series of electrical and opto-synaptic functionalities in order to emulate the multifunctional complex behavior of the human brain. Upon illumination in this two-terminal device, anisotropy in current has been observed due to the out-of-plane carrier transport, originating from spin-selective carrier transport. More importantly, the logic gate achieved in devices based on optoelectronic memristors turned out to be chirality-dependent; while an R-device functioned as an AND gate, the device based on the same perovskite of the opposite chirality (S-device) acted as a NOR gate toward in-memory logic operations. These findings in chiral perovskite-based artificial synapses can identify further strategies for future neuromorphic computing, vision simulation, and artificial intelligence.

Does an intrinsic strain contribute to the effect of quantum confinement phenomenon? An alloyed transition metal dichalcogenide series, Mo(S1-xSex)2 as a case study.

It is well known that the bandgap of 2D transition metal dichalcogenides (TMDs) in the quantum confinement regime increases with a decrease in the number of layers. In this work, we show the effect of lattice strain on the dependence of the gap. We have designed an ideal system in the form of common-cationic alloyed-TMDs, Mo(S1-xSex)2, for such studies. With a large difference between the ionic radii of the two chalcogens, the nanoflakes of the alloys possessed a lattice strain and have been found to yield a lower bandgap than those of both the end-members, MoS2 and MoSe2. More importantly, the dependence of the bandgap on the layer number in the nanoflakes of the alloys turned out to be steeper than in conventional binary TMDs. The experimental results imply that the lattice strain in 2D semiconductors has contributed to the effect of the quantum confinement phenomenon in addition to decreasing the bandgap, the latter being earlier predicted from a theoretical model. We have derived the electronic bandgap and the band-edge energies of the series of alloyed-TMDs in their nanoflake forms and the dependences on the number of layers from the density of states (DOS), as obtained from scanning tunneling spectroscopy (STS) recorded in a scanning tunneling microscope (STM) in an extremely localized manner.

Magnetic moment assisted layer-by-layer film formation of a Prussian Blue analog.

We formed magnetic moment assisted layer-by-layer (LbL) films of a Prussian Blue analogue (PB). We applied an external magnetic field to each monolayer of PB to orient the magnetic moment of the compound perpendicular to the substrate. Aligned moments or orientation of the magnetic compounds themselves were immobilized in each monolayer, so that the moments could augment formation of the subsequent monolayers of LbL adsorption process. We hence could form multilayered LbL films of PB molecules with their magnetic moments oriented perpendicular to the substrate. We also formed LbL films of the compound with their moments oriented parallel to the substrate and facing one particular direction. We have measured conductivity and dielectric constant of the two types of films and compared the parameters with that of conventional LbL films deposited without orienting magnetic moments of the molecules.

Combining negative photoconductivity and resistive switching towards in-memory logic operations.

A family of rudorffites based on silver-bismuth-iodide shows a transition from a conventional positive photoconductivity (PPC) to an unusual negative photoconductivity (NPC) upon variation in the precursor stoichiometry while forming the rudorffites. The NPC has arisen in silver-rich rudorffites due to the generation of illumination-induced trap-states which prompted the recombination of charge carriers and thereby a decrease in the conductivity of the compounds. In addition to photoconductivity, sandwiched devices based on all the rudorffites exhibited resistive switching between a pristine high resistive state (HRS) and a low resistive state (LRS) under a suitable voltage pulse; the switching process, which is reversible, is associated with a memory phenomenon. The devices based on NPC-exhibiting rudorffites switched to the HRS under illumination as well. That is, the resistive state of the devices could be controlled through both electrical and optical inputs. We employed such interesting optoelectronic properties of NPC-exhibiting rudorffites to exhibit OR logic gate operation. Because the devices could function as a logic gate and store the resistive state as well, we concluded that the materials could be an ideal candidate for in-memory logic operations.

Pyro-Phototronic Effect in All-Inorganic Two-Dimensional Ruddlesden-Popper Ferroelectric Perovskite Thin-films and Photodetection.

Ferroelectric perovskites, where ferroelectricity is embedded in the structure, are being considered for different device applications. In this study, we introduce Cs2PbI2Cl2, an all-inorganic 2D Ruddlesden-Popper (RP) halide perovskite, as a ferroelectric material suitable for pyro-phototronic applications. Thin-films of the all-inorganic perovskite are successfully cast, and they demonstrate ferroelectric properties. Unlike hybrid materials, the ferroelectricity in Cs2PbI2Cl2 does not rely on the organic moiety possessing an electric dipole moment. Instead, the 2D-layer-forming octahedra are twisted and tilted due to a distortion in the bond lengths, leading to the emergence of spontaneous electric polarization. Based on the properties, we fabricate p-i-n heterojunctions by integrating the perovskite with carrier-transport layers. To determine the band-energies of the materials, scanning tunneling spectroscopy and Kelvin probe force microscopy are employed. The band-edges evidence a type-II band-alignment at both interfaces, enabling the material to exhibit both photovoltaic and pyroelectric behaviors when subjected to pulsed illumination. The devices based on the all-inorganic RP perovskite developed in this study exhibit pyro-phototronic effects and serve as self-powered photodetectors without any need for an external bias.

Cu2ZnSnS4 (CZTS) nanoparticle based nontoxic and earth-abundant hybrid pn-junction solar cells.

A heterojunction between a layer of CZTS nanoparticles and a layer of fullerene derivatives forms a pn-junction. We have used such an inorganic-organic hybrid pn-junction device for solar cell applications. As routes to optimize device performance, interdot separation has been reduced by replacing long-chain ligands of the quantum dots with short-chain ligands and thickness of the CZTS layer has been varied. We have shown that the CZTS-fullerene interface could dissociate photogenerated excitons due to the depletion region formed at the pn-junction. From capacitance-voltage characteristics, we have determined the width of the depletion region, and compared it with the parameters of devices based on the components of the heterojunction. The results demonstrate solar cell applications based on nontoxic and earth-abundant materials.

Quasi-2D Ruddlesden-Popper Lead Halide Perovskites: How Edge Matters.

A band-mapping technique is introduced to investigate the formation of low-energy edge states in quasi-2D Ruddlesden-Popper (RP) perovskites, (BA)2(MA)n-1PbnI3n+1, through a localized mode of measurement, namely, scanning tunneling spectroscopy. The local band structures measured at different points reveal the formation of 3D CH3NH3PbI3 (MAPbI3) at the edges of the perovskite nanosheets; for thin films, the 3D phase (n = ∞) could be seen to form at grain boundaries. The presence of MAPbI3 at the edges or grain boundaries of the perovskites has led to self-forming type-II band alignment in BA2MA2Pb3I10 (n = 3). The rationale behind achieving a high-efficiency solar cell based on the material, which has a large exciton binding energy, has been inferred. Kelvin probe force microscopy studies under illumination have yielded a higher surface photovoltage at the edges compared to the interior and supported the inference of exciton dissociation due to internal type-II band alignment in the quasi-2D RP perovskites.

Necessity of Quantifying Urbach Energy through Scanning Tunneling Spectroscopy.

In this Letter, we introduce scanning tunneling spectroscopy (STS) to quantify the Urbach energy (EU) in disordered semiconductors. The technique enabled us to gain precise information on the extending component of conduction and valence band-edges responsible for Urbach tailing, individually; such information has been obtained from the width of band-energy-histograms drawn from STS studies at many different points. STS, as a probing method at the microscopic scale to derive EU, is in contrast to commonly employed optical spectroscopy studies which provide information at the macroscopic scale. A comparison between Urbach energy values from optical studies and distribution of band-edges obtained from STS revealed the inherent inaccuracies involved in the optical characterization process. We have considered copper oxide (CuxO) thin films in this regard; we show that through STS and the associated density of state (DOS) spectra, we can derive accurate information on the band-edges' distribution leading to EU in different phases of the binary oxide thin films.