Session R8: Hybrid Organic Inorganic Perovskite Photovoltaics

Understanding the Unique Properties of Organometal Trihalide Perovskite with Single Crystals

Organometal Trihalide Perovskite has been discovered to be all-round optoelectronic materials many types of electronic devices. The understanding of this family of materials is however limited yet due to the complicated grain structures in polycrystalline films which are generally used in most of the devices. In this contribution, I will present our recent progress in understanding the fundamental properties, including optoelectronic properties and electromechanical properties, using the high quality organometal trihalide perovskite single crystals. I will report the crystallographic orientation dependent charge transport and collection, surface and bulk charge recombination process, and direction measuring of carrier diffusion length using the lasing induced photocurrent scanning. The polarity of the organometal trihalide perovskite crystals will also be examined.   

High Performance Tandem Perovskite/Polymer Solar Cells

Combining perovskites with other inorganic materials, such as copper indium gallium diselenide (CIGS) or silicon, is enabling significant improvement in solar cell device performance. Here, we demonstrate a highly efficient hybrid tandem solar cell fabricated through a facile solution deposition approach to give a perovskite front sub-cell and a polymer:fullerene blend back sub-cell. This methodology eliminates the adverse effects of thermal annealing during perovskite fabrication on polymer solar cells. The record tandem solar cell efficiency of 15.96{\%} is 40{\%} greater than the corresponding perovskite-based single junction device and 65{\%} greater than the polymer-based single junction device, while mitigating deleterious hysteresis effects often associated with perovskite solar cells. The hybrid tandem devices demonstrate the synergistic effects arising from the combination of perovskite and polymer-based materials for solar cells.   

Nanoimprinted Perovskite Solar Cells With Enhanced Photocurrent

We have developed a new method of Nanoimprint Lithography (NIL) to shape the morphology of organolead trihalide perovskite. With this hot stamping process we created ordered gratings or other micro or nanostructures of perovskite resembling 2D photonic crytals on the scale of 200 to 600 nm from a starting small grain spin-coated film of the same scale. With this new method of nanoimprinting, we demonstrate that perovskite PV device performance can be improved and controlled. Initial results comparing flat vs. NIL-PV structure devices show dramatic increase in photocurrent as well as better crystallinity. The origin of Isc enhancement is explained in terms of better morphology and larger grains, resulting in longer diffusion length of carriers, while better light absorption by photonic crystal nanopatterns cannot be excluded.   

Designing lead-free and stable perovskite materials for photovoltaic applications.

A critical barrier for large-scale deployment of the current perovskite solar materials is the use of Pb to achieve high power conversion efficiency. While this appears to be a technical issue, there are more fundamental reasons behind. The current research has mainly focused on the replacement of Pb by other elements, in particular, Sn. However, in halide perovskites (i.e., I-II-VII$_{\mathrm{3}}$ composition), Sn is in its less stable 2$+$ state. The formation of more stable 4$+$ centers in the Sn(II)-based materials under ambient conditions makes the device efficiency very low. Worse, there might be no other elements across the Periodic Table that can replace Pb while maintaining the desirable properties, such as band gap. Out-of-the-box ideas are therefore called for to stimulate the research in this field. In this talk, two approaches are proposed based on state-of-the-art first-principles calculations. Through a screening of chalcogenide perovskite materials, CaTiS$_{\mathrm{3}}$, BaZrS$_{\mathrm{3}}$, CaZrSe$_{\mathrm{3}}$, and CaHfSe$_{\mathrm{3}}$ have been predicted to have suitable band gaps for making solar cells. Among these materials, BaZrS$_{\mathrm{3}}$ have been synthesized experimentally. Another proposed approach is to introduce dual anions (i.e., splitting the anion sites) that allow the composition to satisfy charge neutrality, while replacing Pb by more environmentally benign elements. One of the candidate materials is CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$BiSI$_{\mathrm{2}}$, which is predicted to have band gap around 1.4 eV and high optical absorption.   

Light induced polaron formation in perovskite solar cell devices

The need for a low-cost, clean, and abundant source of energy has generated large amounts of research in solution processed solar cell materials. The lead halide perovskite has rapidly developed as a serious candidate for the active layer of photovoltaic devices. The efficiencies of devices made with this material have increased from 3.5{\%} to over 20{\%} in around 5 years. Despite the remarkable progress associated with perovskite materials, there are still fundamental questions regarding their lack of photo-stability over prolonged solar irradiation that need to be addressed. Recent experiments on photo-degradation under constant illumination have found fast self-healing by resting the device in the dark for less than 1 minute. Density functional theory and symmetry analysis show that localized charge states couple to local structural lattice distortions and methyl ammonium quasistatic configurations. Once translational symmetry is lost, additional bonding configurations become symmetry allowed, triggering localized charges in the vicinity over time under constant illumination, thus seeding the formation of macroscopic charged domains and preventing efficient charge extraction. Here we present an in-depth study of polaron formation and binding energy at the atomistic level.   

Electric polarization of $CH_3NH_3PbI_3$ and enhancement by Cl substitution

As a prototype of organic-inorganic hybrid perovskite, CH$_3$NH$_3$PbI$_3$ has attracted extensive attention recently due to its applications in high power-conversion-efficiency solar cells. In comparison with its inorganic perovskite counterparts such as CsPbI$_3$, the organic cation CH$_3$NH$_3$+ is expected to play multiple important roles in distorting crystal structures and thus band structures as well as creating local electrically polarized domains to help separate charge carriers. Using first-principles method and berry phase theory, the electric polarization vectors of CH$_3$NH$_3$PbI$_3$ have been calculated. The off-center displacement of Pb within the PbI$_6$ octahedral is shown to introduce major intrinsic polarization, with additional contributions from off-center displacement of CH$_3$NH$_3$+ within PbI$_3$ cage and charge polarization within the organic cation. With chlorine substitution of iodine, the electronegativity difference between halogen and Pb becomes larger, and the lattice distortion and hence the electric polarization increases, which provides a possible mechanism to further assist charge carrier separation in solar cell devices. This is consistent with enhanced photovoltaics properties of CH$_3$NH$_3$PbI$_{3-x}$Cl$_x$ found in recent experiments.   

Hybrid organic-inorganic halide perovskites: Electronic structure, dielectric properties, native defects, and the role of ns$^{\mathrm{2}}$ ions

CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ possesses an interesting combination of properties, i.e., efficient carrier transport, high density of defects (which are nevertheless benign in terms of carrier trapping), large static dielectric constant, and significant ion migration. These properties have important effects on the solar cell performance and are unusual for a photovoltaic material. However, they are not unique; they have been reported for other halides. In the talk, we discuss the underlying physics behind these material properties in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ and other halide electronic materials. The large static dielectric constant resulting from the presence of the ns$^{\mathrm{2}}$ ions and the molecular dipoles is related to the high defect concentration, defect tolerance, and the defect diffusion. We will also show the calculations of the defect and impurity diffusion and discuss their impact on the solar cell performance.   

Quasiparticle band gap of organic-inorganic hybrid perovskites: Crystal structure, spin-orbit coupling, and self-energy effects

The quasiparticle band gaps of organic-inorganic hybrid perovskites are often determined (and can be controlled) by various factors, complicating predictive materials optimization. Here we report a comprehensive investigation on the band gap formation mechanism in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ by decoupling various contributing factors which ultimately determine their electronic structure and quasiparticle band gap. Four major factors, namely, quasiparticle self-energy, spin-orbit coupling, volume (lattice constant) effects, and structural distortions due to the presence of organic molecules, and their influences on the quasiparticle band structure of organometal hybrid perovskites are illustrated. We find that although methylammonium cations do not contribute directly to the electronic states near band edges, they play an important role in defining the band gap through a lattice distortion mechanism and by controlling the overall lattice constants (thus the chemical bonding of the optically active PbI$_{\mathrm{3}}^{\mathrm{-}})$. The spin-orbit coupling effects drastically reduce the electron and hole effective masses in these systems, which is beneficial for high carrier mobilities and small exciton binding energies.   

Light-induced defects in hybrid lead halide perovskite

One of the main challenges facing organohalide perovskites for solar application is stability. Solar cells must last decades to be economically viable alternatives to traditional energy sources. While some causes of instability can be avoided through engineering, light-induced defects can be fundamentally limiting factor for practical application of the material. Light creates large numbers of electron and hole pairs that can contribute to degradation processes. Using \textit{ab initio} theoretical methods, we systematically explore first steps of light induced defect formation in methyl ammonium lead iodide, MAPbI$_3$. In particular, we study charged and neutral Frenkel pair formation involving Pb and I atoms. We find that most of the defects, except negatively charged Pb Frenkel pairs, are reversible, and thus most do not lead to degradation. Negative Pb defects create a mid-gap state and localize the conduction band electron. A minimum energy path study shows that, once the first defect is created, Pb atoms migrate relatively fast. The defects have two detrimental effects on the material. First, they create charge traps below the conduction band. Second, they can lead to degradation of the material by forming Pb clusters.   

Ab initio study of the polarization dependence of the optoelectronic properties of hybrid halide perovskites

With efficiencies as high as 20\%, hybrid organic-inorganic halide perovskites have garnered much of the photovoltaic community's attention. In light of recent experimental results [1], we investigate the coupling mechanism between polarization and optoelectronic properties of methylammonium (MA) lead iodide, (CH$_3$NH$_3$)PbI$_3$, and related halide perovskites. In particular, we study the conditions that promote a combined effect of strong spin-orbit coupling and inversion symmetry breaking and that lead to a sizable Rashba/Dresselhaus effect. Using density functional theory calculations, we elucidate the emergence of Rashba/Dresselhaus splitting associated with local distortions and long-range coherent alignment of MA moieties in the material. We examine the extent to which the magnitude of the splitting, as well as other important electronic and optical properties [1], can be altered by increasing the macroscopic polarization. This opens avenues for manipulation of optoelectronic properties by an external electric field and/or chemical substitution of the MA molecule. \newline [1] S.Y. Leblebici, L. Leppert, et al, Facet-dependent photovoltaic efficiency variations in single perovskite grains, submitted (2015).   

Degradation of Co-evaporated Perovskite Thin Films

Methylammonium lead halide perovskites have been developed as highly promising materials to fabricate efficient solar cells in the past few years. We have investigated degradation of co-evaporated CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ films using x-ray photoelectron spectroscopy (XPS), small angle x-ray diffraction (XRD), and atomic force microscopy (AFM). The CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ films have an excellent atomic ratio and crystal structure. The films were exposed to oxygen, air and water, respectively. The results indicate that CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ film is not sensitive to oxygen and dry air. The XPS results of H$_{\mathrm{2}}$O exposure are similar to those of ambient exposure except for the higher intensity of C and O. The XRD results indicate that the perovskite turned to PbI$_{\mathrm{2}}$ after ambient exposure. The AFM measurements reveal that the morphology of the film changed drastically from smooth to rough by ambient exposure. The experiment indicated that H$_{\mathrm{2}}$O plays a dominated role in the degradation of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ films. The degradation can be characterized by almost complete removal of N, substantial reduction of I, residual of PbI$_{\mathrm{2}}$, C, O, and I compounds on the surface.   

Use of Nanoconfinement to Control Metal-Halide Perovskite Crystallization and Stability

We present a systematic study of the effect of nanoconfinement on the crystallization of methylammonium lead halide (MAPbI$_{\mathrm{3}})$ perovskite crystallization. MAPbI$_{\mathrm{3}}$ was spin coated onto anodized aluminum oxide (AAO) templates with uniaxially-aligned pores ranging from 20 -- 200 nm in diameter and examined using 2-D X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns revealed the presence of a transient precursor phase that converts to the MAPbI$_{\mathrm{3}}$ crystal structure upon thermal annealing. The orientation of the precursor phase and conversion rate to the MAPbI$_{\mathrm{3}}$ crystal structure were found to depend on the pore size of the AAO template. The stability of MAPbI$_{\mathrm{3}}$ in air also depends on the extent of nanoconfinement. When deposited on flat SiO$_{\mathrm{2}}$ surfaces, MAPbI$_{\mathrm{3}}$ degraded into PbI$_{\mathrm{2}}$ and MA after 21 days. When deposited in AAO templates exhibiting 20-nm pore sizes, however, MAPbI$_{\mathrm{3}}$ crystals were stable for longer than 16 days. These findings suggest that nanoconfinement of MAPbI$_{\mathrm{3}}$ crystals may be a promising strategy for improving the stability of perovskite-based solar cells.   

Structural Effects on the Bandstructure of Methylammonium Lead Iodide

Metal-organic halide perovskites possess peculiar physical properties. The carrier diffusion length in methylammonium lead iodide (MAPbI) exceeds 1 $\mu $m, but this unusually high value for a solution-processed material is poorly understood. We developed first-principles calculations of carrier lifetimes and diffusion lengths in semiconductors, which require accurate knowledge of the bandstructure. In this talk, we show that in MAPbI the structure strongly affects the bandstructure and band edges, and that density functional theory (DFT) is unable to predict the room temperature tetragonal structure due to the polymorphism of MAPbI. The Rashba splitting induced by the spin-orbit interaction, and the DFT band gap and effective masses, all depend strongly on the chosen structure, a point that previous work failed to address. Working with multiple stochastic realizations of large unit cells with random methylammonium orientations, we compute average effective masses and show that the effective mass depends linearly on the band gap. The average Rashba coefficient we find is an order of magnitude smaller than previously reported, and the band edges are almost parabolic. Our structures possess the correct symmetry and are free of the spurious Pb off-centering assumed in previous work. We identify the correct starting point for GW bandstructure calculations and to compute the carrier lifetime and diffusion length.