Fahhad H. Alharbi

Revealing the role of organic cations in hybrid halide perovskite CH 3 NH 3 PbI 3

The hybrid halide perovskite CH3NH3PbI3 has enabled solar cells to reach an efficiency of about 20%, demonstrating a pace for improvements with no precedents in the solar energy arena. Despite such explosive progress, the microscopic origin behind the success of such material is still debated, with the role played by the organic cations in the light-harvesting process remaining unclear. Here van der Waals-corrected density functional theory calculations reveal that the orientation of the organic molecules plays a fundamental role in determining the material electronic properties. For instance, if CH3NH3 orients along a (011)-like direction, the PbI6 octahedral cage will distort and the bandgap will become indirect. Our results suggest that molecular rotations, with the consequent dynamical change of the band structure, might be at the origin of the slow carrier recombination and the superior conversion efficiency of CH3NH3PbI3.

An Efficient and Low-Cost Method for the Purification of Colloidal Nanoparticles**

Efficient purification is critical to the fundamental and practical exploitation of nanoparticles in the technological domain. Whether for lighting, bio-imaging, coatings, photovoltaics, or displays, a routine, low-cost purification method that is directly applicable post-synthesis and in reuse/recycling would be of tremendous benefit to the commercialization of nanoparticle-based devices. Moreover, purification is essential to understanding the functional properties of nanoparticles, which are strongly dependent on purification history. In nonaqueous media, the most common technique for nanoparticle purification is the precipitation–dissolution method. This technique requires significant time and materials, including expensive centrifuges that do not scale well in an industrial setting. Meanwhile, the efficiency of this technique varies with the morphology and the nature of the nanoparticles, in particular giving lower yields with certain smaller and shaped nanoparticles. Other techniques, such as dialysis, ultra-filtration, and diafiltration, remain problematic as they rely on controlled-pore-size materials that are expensive and suffer from fouling. Additionally, these techniques, along with size-exclusion and high-performance liquid chromatography, have high solvent burdens and suffer from slow dynamics. Other interesting techniques involving microemulsions or particular ternary solvent systems may be difficult to generalize. Collectively, the purification techniques developed to date for nanoparticles in nonaqueous media have issues with cost, scale-up, applicability, and/or the lack of green processing. These issues are tremendous obstacles for efficient manufacturing and, by consequence, to the widespread development and implementation of nanoparticle-based technologies. Herein, we report an effective nanoparticle purification method based on reversible electrophoretic deposition, defined here as electropurification, which overcomes most the disadvantages of traditional techniques. Electrophoretic deposition is the deposition of colloidal particles through the application of an electric field, a widelyadopted industrial process used, for example, in automobile coatings. Electrophoretic deposition has also been applied in several embodiments to deposit nanoparticles such as CdSe to form permanent fixed coatings. Herein, however, we describe a method to reversibly deposit nanoparticles onto an electrode surface. We can use the reversible nature of this process, achieved through the addition of particular nonsolvents to the electrodeposition solution, to selectively collect and separate nanoparticles from unwanted impurities in solution. The collected nanoparticles are then redispersed into clean solvent. Figure 1 shows how a simple lab-scale setup can be used to separate and collect nearly 100% of oleic acid (OA)-capped CdTe nanoparticles from solution within a matter of minutes. The setup comprises two electrodes, an aluminum bar and a stainless steel mesh, placed in a glass beaker and connected by a glovebox feed-through to a DC power supply. The initial solution is the unpurified reaction media diluted with about 1.5 volume equivalents of acetone, a non-solvent. This reaction media contains the primary solvent (octadecene) and contaminants, such as excess OA, tributylphosphine (TBP), leftover precursors (cadmium oleate and Te-TBP), and reaction byproducts. Within minutes, an applied DC potential of 500 V causes the nanoparticles to collect on the aluminum anode, leaving a nearly colorless residual solution. The adsorbed nanoparticles are further washed with acetone while still on the electrode. Thus purified, the nanoparticles may be collected either as a solid or redispersed in a good nonpolar solvent, such as chloroform, hexane, or toluene. A real-time video demonstration, showing both the collection and redissolution of CdTe nanoparticles in less than two minutes, is available in the Supporting Information. Figure 2 shows the absorption and emission of electropurified CdTe nanoparticles as compared to those before purification and those conventionally purified by repeated precipitation–dissolution. The essentially identical first exciton and emission peak positions and spectral broadening indicate that there are no significant variations between the electropurified nanoparticles and those purified by conventional precipitation–dissolution. The H NMR spectrum of these electropurified nanoparticles (Figure 3) reveals a clean product. Broad resonances consistent with bound oleic acid are observed at d= 1.1, 1.44, and 2.3, and 5.56 ppm. There is a notable absence of resonances attributable to the cadmium [*] Dr. J. D. Bass, Dr. X. Ai, Dr. P. M. Rice, Dr. T. Topuria, Dr. J. C. Scott, Dr. H.-C. Kim, Dr. Q. Song, Dr. R. D. Miller IBM Almaden Research Center 650 Harry Road, San Jose, CA 95120 (USA) Fax: (+1)408-927-2073 E-mail: jbass@us.ibm.com qsong@us.ibm.com

Recent advances in alternative material photovoltaics

Abstract Alternative material photovoltaics (PVs) have started gaining more attention recently. Although the field is not new, it just started growing a few years ago. The PV market has been dominated by various silicon technologies, besides a few other popular thin films, such as CdTe, copper–indium–galium–selenide varieties and some III–V materials. This has been reflected in research as well. Successful developments of efficient solar cells using alternative absorbers will significantly enrich the PV industry and reduce the market gap with other energy sources. Hence, in this review, recent advances and trends to develop PVs using alternative materials are presented and discussed. The focus will be mainly on binary as well as environmentally friendly compounds and thin film devices. Nonetheless, some other more complex materials and structures will be briefly addressed.

Transfer molding of nanoscale oxides using water-soluble templates.

We report a facile method for creating nanoscopic oxide structures over large areas that is capable of producing high aspect ratio nanoscale structures with feature sizes below 50 nm. A variety of nanostructured oxides including TiO(2), SnO(2) and organosilicates are formed using sol-gel and nanoparticle precursors by way of molding with water-soluble polymeric templates generated from silicon masters. Sequential stacking techniques are developed that generate unique 3-dimensional nanostructures with combinatorially mixed geometries, scales, and materials. Applicable to a variety of substrates, this scalable method allows access to a broad range of new thin film morphologies for applications in devices, catalysts, and functional surface coatings.

Domain Walls Conductivity in Hybrid Organometallic Perovskites and Their Essential Role in CH3NH3PbI3 Solar Cell High Performance.

The past several years has witnessed a surge of interest in organometallic trihalide perovskites, which are at the heart of the new generation of solid-state solar cells. Here, we calculated the static conductivity of charged domain walls in n- and p- doped organometallic uniaxial ferroelectric semiconductor perovskite CH3NH3PbI3 using the Landau-Ginzburg-Devonshire (LGD) theory. We find that due to the charge carrier accumulation, the static conductivity may drastically increase at the domain wall by 3 – 4 orders of magnitude in comparison with conductivity through the bulk of the material. Also, a two-dimensional degenerated gas of highly mobile charge carriers could be formed at the wall. The high values of conductivity at domain walls and interfaces explain high efficiency in organometallic solution-processed perovskite films which contains lots of different point and extended defects. These results could suggest new routes to enhance the performance of this promising class of novel photovoltaic materials.

Delocalized quantum states enhance photocell efficiency.

The high quantum efficiency of photosynthetic complexes has inspired researchers to explore new routes to utilize this process for photovoltaic devices. Quantum coherence has been demonstrated to play a crucial role in this process. Herein, we propose a three-dipole system as a model of a new photocell type which exploits the coherence among its three dipoles. We have proved that the efficiency of such a photocell is greatly enhanced by quantum coherence. We have also predicted that the photocurrents can be enhanced by about 49.5% in such a coherent coupled dipole system compared with the uncoupled dipoles. These results suggest a promising novel design aspect of photosynthesis-mimicking photovoltaic devices.

Hydrogen Bonding and Stability of Hybrid Organic-Inorganic Perovskites.

In the past few years, the efficiency of solar cells based on hybrid organic-inorganic perovskites has exceeded the level needed for commercialization. However, existing perovskites solar cells (PSCs) suffer from several intrinsic instabilities, which prevent them from reaching industrial maturity, and stabilizing PSCs has become a critically important problem. Here we propose to stabilize PSCs chemically by strengthening the interactions between the organic cation and inorganic anion of the perovskite framework. In particular, we show that replacing the methylammonium cation with alternative protonated cations allows an increase in the stability of the perovskite by forming strong hydrogen bonds with the halide anions. This interaction also provides opportunities for tuning the electronic states near the bandgap. These mechanisms should have a universal character in different hybrid organic-inorganic framework materials that are widely used.

Electronic transport in organometallic perovskite CH3NH3PbI3: The role of organic cation orientations

Density functional theory in combination with the nonequilibrium Greens function formalism is used to study the electronic transport properties of methylammonium lead-iodide perovskite CH3NH3PbI3. Electronic transport in homogeneous ferroelectric and antiferroelectric phases, both of which do not contain any charged domain walls, is quite similar. The presence of charged domain wall drastically (by about an order of magnitude) enhances the electronic transport in the lateral direction. The increase of the transmission originates from the smaller variation of the electrostatic potential profile along the charged domain walls. This fact may provide a tool for tuning transport properties of such hybrid materials by manipulating molecular cations having dipole moment.

Cuckoo search inspired hybridization of the Nelder-Mead simplex algorithm applied to optimization of photovoltaic cells

A new hybridization of the Cuckoo Search (CS) is developed and applied to optimize multi-cell solar systems; namely multi-junction and split spectrum cells. The new approach consists of combining the CS with the Nelder-Mead method. More precisely, instead of using single solutions as nests for the CS, we use the concept of a simplex which is used in the Nelder-Mead algorithm. This makes it possible to use the flip operation introduces in the Nelder-Mead algorithm instead of the Levy flight which is a standard part of the CS. In this way, the hybridized algorithm becomes more robust and less sensitive to parameter tuning which exists in CS. The goal of our work was to optimize the performance of multi-cell solar systems. Although the underlying problem consists of the minimization of a function of a relatively small number of parameters, the difficulty comes from the fact that the evaluation of the function is complex and only a small number of evaluations is possible. In our test, we show that the new method has a better performance when compared to similar but more compex hybridizations of Nelder-Mead algorithm using genetic algorithms or particle swarm optimization on standard benchmark functions. Finally, we show that the new method outperforms some standard meta-heuristics for the problem of interest.

Enhancing Intrinsic Stability of Hybrid Perovskite Solar Cell by Strong, yet Balanced, Electronic Coupling

In the past few years, the meteoric development of hybrid organic–inorganic perovskite solar cells (PSC) astonished the community. The efficiency has already reached the level needed for commercialization; however, the instability hinders its deployment on the market. Here, we report a mechanism to chemically stabilize PSC absorbers. We propose to replace the widely used methylammonium cation (CH3NH3+) by alternative molecular cations allowing an enhanced electronic coupling between the cation and the PbI6 octahedra while maintaining the band gap energy within the suitable range for solar cells. The mechanism exploits establishing a balance between the electronegativity of the materials’ constituents and the resulting ionic electrostatic interactions. The calculations demonstrate the concept of enhancing the electronic coupling, and hence the stability, by exploring the stabilizing features of CH3PH3+, CH3SH2+, and SH3+ cations, among several other possible candidates. Chemical stability enhancement hence results from a strong, yet balanced, electronic coupling between the cation and the halides in the octahedron. This shall unlock the hindering instability problem for PSCs and allow them to hit the market as a serious low-cost competitor to silicon based solar cell technologies.