Grain boundary engineering for improved thin silicon photovoltaics

The Mystery behind Dynamic Charge Disproportionation in BaBiO3

BaBiO₃(BBO) is known to be a valence-skipping perovskite, which avoids the metallic state through charge disproportionation (CD), the mechanism of which is still unresolved. A novel mechanism for CD is presented here in the covalent limit using a molecular orbital (MO) picture under two scenarios: (case i) Bi 6sp-O 2p and (case ii) Bi 6p-O 2p hybridizations that favor 5+ and 3+ states, respectively. The proposed model is further validated by using a combinatorial approach of X-ray spectroscopic experiments and first-principle calculations. The bulk X-ray photoemission spectrum reveals that, at room temperature, the CD is dynamic in nature, whereas, at 200 K, it approaches a quasi-static limit. Under compressive strain, the octahedral breathing mode is damped and drives the system to a quasi-static limit even at room temperature, giving rise to asymmetric CD.

Direct insight into the structure-property relation of interfaces from constrained crystal structure prediction

A major issue that prevents a full understanding of heterogeneous materials is the lack of systematic first-principles methods to consistently predict energetics and electronic properties of reconstructed interfaces. In this work we address this problem with an efficient and accurate computational scheme. We extend the minima-hopping method implementing constraints crafted for two-dimensional atomic relaxation and enabling variations of the atomic density close to the interface. A combination of density-functional and accurate density-functional tight-binding calculations supply energy and forces to structure prediction. We demonstrate the power of this method by applying it to extract structure-property relations for a large and varied family of symmetric and asymmetric tilt boundaries in polycrystalline silicon. We find a rich polymorphism in the interface reconstructions, with recurring bonding patterns that we classify in increasing energetic order. Finally, a clear relation between bonding patterns and electrically active grain boundary states is unveiled and discussed.

Hydrogen Passivated Silicon Grain Boundaries Greatly Reduce Charge Recombination for Improved Silicon/Perovskite Tandem Solar Cell Performance: Time Domain Ab Initio Analysis

By performing nonadiabatic molecular dynamics simulations, we demonstrate that grain boundaries (GBs) can induce the indirect-to-direct transition of the silicon band gap. However, missing a silicon atom creates an electron trap state in the GBs. Electron trapping by the silicon vacancy occurs on tens of picoseconds followed by recombination of the trapped electron and valence band hole on sub-100 ps, which operates parallel to recombination of the free electron and hole on a similar time scale. The recombination is greatly accelerated by 2 orders of magnitude compared to the GBs without a silicon vacancy. Hydrogen passivation eliminates the trap state and notably delays the charge recombination due to an increased band gap and a shortened coherence time, extending the excited-state lifetime to sub-10 ns. Our study provides an atomistic description of how charge recombination in the silicon can be efficiently reduced, suggesting a rational route to enhance silicon/perovskite tandem solar cells performance.

Interface structure prediction via CALYPSO method

The atomistic structures of solid-solid interfaces are of fundamental interests for understanding physical properties of interfacial materials. However, determination of interface structures faces a substantial challenge, both experimentally and theoretically. Here, we propose an efficient method for predicting interface structures via the generalization of our in-house developed CALYPSO method for structure prediction. We devised a lattice match toolkit that allows us to automatically search for the optimal lattice-matched superlattice for construction of the interface structures. In addition, bonding constraints (e.g., constraints on interatomic distances and coordination numbers of atoms) are imposed to generate better starting interface structures by taking advantages of the known bonding environment derived from the stable bulk phases. The interface structures evolve by following interfacially confined swarm intelligence algorithm, which is known to be efficient for exploration of potential energy surface. The method was validated by correctly predicting a number of known interface structures with only given information of two parent solids. The application of the developed method leads to prediction of two unknown grain boundary (GB) structures (r-GB and p-GB) of rutile TiO₂ Σ5(2 1 0) under an O reducing atmosphere that contained Ti³⁺ as the result of O defects. Further calculations revealed that the intrinsic band gap of p-GB is reduced to 0.7 eV owing to substantial broadening of the Ti-3d interfacial levels from Ti³⁺ centers. Our results demonstrated that introduction of grain boundaries is an effective strategy to engineer the electronic properties and thus enhance the visible-light photoactivity of TiO₂.

Segregation of Native Defects to the Grain Boundaries in Methylammonium Lead Iodide Perovskite

Native point and grain boundary (GB) defects are ubiquitous in methylammonium lead iodide (MAPbI₃) sensitizers employed in solar cells that are polycrystalline in nature. Here we use density functional theory (DFT) in conjunction with a thermodynamic approach to determine the stability and electronic properties of all native point defects and their interplays with Σ5-(210) GB in MAPbI₃. The transition levels of charged defects are investigated with inclusion of electrostatic charge corrections and spin-orbit coupling. We find that the GB region is a sink for most of the native point defects under different synthesis conditions. For the crystalline and bicrystalline MAPbI₃ with Σ5-(210) GB, we find respectively that the p-type antisite defects MA_I and Pb_I, where I substitutes for MA or Pb, introduce deep levels, and both are relatively stable under I-rich conditions. Hence, I-poor conditions are more preferable for synthesis of MAPbI₃ to have defects with electronically benign character.

A tool for automatic recognition of [110] tilt grain boundaries in zincblende-type crystals

The local atomic structure of [110] tilt grain boundaries (GBs) formed in ∼100 nm-sized GaAs nanocrystals, which crystallize in the non-centrosymmetric zincblende-type structure with face-centred cubic lattice symmetry, was imaged and analysed by means of high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The nanocrystals were grown by metal–organic vapour phase epitaxy on top of (001) Si nanotips embedded in an oxide matrix. This paper introduces an automatic analysis method and corresponding processing tool for the identification of the GBs. The method comprises (i) extraction of crystallographic parameters,i.e.misorientation angles and transformation matrices for the different crystal parts (grains/twins) observed by HAADF-STEM, and (ii) determination of their common plane(s) by modelling all possible intersections of the corresponding three-dimensional reciprocal lattices. The structural unit model is also used to characterize the GB structures and to validate the data obtained by the developed algorithm.

Photo-Carrier Multi-Dynamical Imaging at the Nanometer Scale in Organic and Inorganic Solar Cells

Investigating the photocarrier dynamics in nanostructured and heterogeneous energy materials is of crucial importance from both fundamental and technological points of view. Here, we demonstrate how noncontact atomic force microscopy combined with Kelvin probe force microscopy under frequency-modulated illumination can be used to simultaneously image the surface photopotential dynamics at different time scales with a sub-10 nm lateral resolution. The basic principle of the method consists in the acquisition of spectroscopic curves of the surface potential as a function of the illumination frequency modulation on a two-dimensional grid. We show how this frequency-spectroscopy can be used to probe simultaneously the charging rate and several decay processes involving short-lived and long-lived carriers. With this approach, dynamical images of the trap-filling, trap-delayed recombination and nongeminate recombination processes have been acquired in nanophase segregated organic donor-acceptor bulk heterojunction thin films. Furthermore, the spatial variation of the minority carrier lifetime has been imaged in polycrystalline silicon thin films. These results establish two-dimensional multidynamical photovoltage imaging as a universal tool for local investigations of the photocarrier dynamics in photoactive materials and devices.

Dipole-allowed direct band gap silicon superlattices

Silicon is the most popular material used in electronic devices. However, its poor optical properties owing to its indirect band gap nature limit its usage in optoelectronic devices. Here we present the discovery of super-stable pure-silicon superlattice structures that can serve as promising materials for solar cell applications and can lead to the realization of pure Si-based optoelectronic devices. The structures are almost identical to that of bulk Si except that defective layers are intercalated in the diamond lattice. The superlattices exhibit dipole-allowed direct band gaps as well as indirect band gaps, providing ideal conditions for the investigation of a direct-to-indirect band gap transition. The fact that almost all structural portions of the superlattices originate from bulk Si warrants their stability and good lattice matching with bulk Si. Through first-principles molecular dynamics simulations, we confirmed their thermal stability and propose a possible method to synthesize the defective layer through wafer bonding.

Application of Transmitted Kikuchi Diffraction in Studying Nano-oxide and Ultrafine Metallic Grains

Transmitted Kikuchi diffraction (TKD) is an emerging SEM-based technique that enables investigation of highly refined grain structures. It offers higher spatial resolution by utilizing conventional electron backscattered diffraction equipment on electron-transparent samples. A successful attempt has been made to reveal nano-oxide grain structures as well as ultrafine severely deformed metallic grains. The effect of electron beam current was studied. Higher beam currents enhance pattern contrast and intensity. Lower detector exposure times could be employed to accelerate the acquisition time and minimize drift and carbon contamination. However, higher beam currents increase the electron interaction volume and compromise the spatial resolution. Lastly, TKD results were compared to orientation mapping results in TEM (ASTAR). Results indicate that a combination of TKD and EDS is a capable tool to characterize nano-oxide grains such as Al2O3 and Cr2O3 with similar crystal structures.

Investigation into Efficiency-Limiting Defects in mc-Si Solar Cells

First-principles quantum-chemical simulations are combined with TCAD device modelling to examine the impact of the intrinsic stacking faults and Σ5-(001) twist grain-boundaries on the performance of solar cell efficiency. We find from the combination of these computational methods, the optical properties of ideal stacking faults are similar to those of pure Si, whereas the optimised grainboundaryleads to a clear change in the real and imaginary parts of refractive index, increasing the solar-cell current density, and thus the solar cell efficiency. The impact at a device level is dependent upon the areal density of such material. So far as the optically absorption and carrier generation is concerned, segregation of diffusing iron at these planar defects has a negligible impact on device characteristics, but non-radiative recombination processes and carrier traps due to iron are expected to significantly affect efficiency in these regions.