Session S8: Electrical Transport and Photoexcitations in Organic/Inorganic Perovskites

Charge Transport in Hybrid Halide Perovskite Field-Effect Transistors

Hybrid organic-inorganic trihalide perovskite (HTP) materials exhibit a strong optical absorption, tunable band gap, long carrier lifetimes and fast charge carrier transport. These remarkable properties, coupled with their reduced complexity processing, make the HTPs promising contenders for large scale, low-cost thin film optoelectronic applications. But in spite of the remarkable demonstrations of high performance solar cells, light-emitting diodes and field-effect transistor devices, all of which took place in a very short time period, numerous questions related to the nature and dynamics of the charge carriers and their relation to device performance, stability and reliability still remain. This presentation describes the electrical properties of HTPs evaluated from field-effect transistor measurements. The electrostatic gating of provides an unique platform for the study of intrinsic charge transport in these materials, and, at the same time, expand the use of HTPs towards switching electronic devices, which have not been explored previously. We fabricated FETs on SiO$_{\mathrm{2}}$ and polymer dielectrics from spin coating, thermal evaporation and spray deposition and compare their properties. CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3-x}}$Cl$_{\mathrm{x}}$ can reach balanced electron and hole mobilities of 10 cm$^{\mathrm{2}}$/Vs upon tuning the thin-film microstructure, injection and the defect density at the semiconductor/dielectric interface. The work was performed in collaboration with Yaochuan Mei (Wake Forest University), Chuang Zhang, and Z. Valy Vardeny (University of Utah).   

Understanding charge transport in organometal halide field effect transistors.

Organometal halide based perovskite are emerging materials for wide range of electronic applications. A range of optoelectronic applications like high efficiency solar cells, color pure LEDs and optical pumped lasers have been demonstrated. Here, we report the demonstration of a high performance field effect transistor fabricated from iodide perovskite material at room temperature. The devices exhibit clean saturation behavior with electron $\mu_{\mathrm{FET\thinspace }}$\textgreater 3 cm$^{\mathrm{2}}$V$^{\mathrm{-1}}$s$^{\mathrm{-1}}$ and current modulation in the range of 10$^{\mathrm{6}}$ -- 10$^{\mathrm{7}}$ which are till date the best performance achieved with these class of materials. This high performance is attributed to a combination of novel film fabrication technique and device engineering strategies. Detailed understanding of the observed band-like transport phenomenon is developed by tuning the different sources of dynamic and static disorder prevalent in the system. These finding are expected to pave way for developing next generation electronic application from perovskite materials.   

Charge carrier transport properties of methyl-ammonium-lead-trihalide perovskites investigated by the time-of-flight method

We studied the charge transport properties of methyl-ammonium-lead-trihalide perovskites using the photocurrent transient time-of-flight method. Various morphologies that include single-crystals and thin films with different crystalline grain sizes and surface roughness were investigated. The photocurrent transients were recorded as a function of excitation wavelength, intensity, and applied electric field as well as the sample temperature. We found that surface recombination leads to a photocurrent response that is sharply peaked at the band edge. While the carrier mobility depends on the sample preparation and sample temperature, typical values are on the order of 1cm$^{\mathrm{2}}$/Vs, consistent with previous reports using similar methods. This value is high compared to other solution-processed semiconductors such as pi-conjugated polymers and quantum dots; however it is relatively low compared to inorganic semiconductors. Therefore determining the mobility limiting factors in hybrid perovskite devices is important for progress in their optoelectronic device performance.   

Electronic Structure Approach to Tunable Electronic Properties of Hybrid Organic-Inorganic Perovskites

We present a study of the electronic structure of layered hybrid organic-inorganic perovskite (HOIP) materials using all-electron density-functional theory. Varying the nature of the organic and inorganic layers should enable systematically fine-tuning the carrier properties of each component. Using the HSE06 hybrid density functional including spin-orbit coupling (SOC), we validate the principle of tuning subsystem-specific parts of the electron band structures and densities of states in $\mathrm{CH_3NH_3PbX_3}$ (X=Cl, Br, I) compared to a modified organic component in layered $\mathrm{(C_6H_5C_2H_4NH_3)_2PbX_4}$ (X=Cl, Br, I) and $\mathrm{C_{20}H_{22}S_4N_2PbX_4}$ (X=Cl, Br, I). We show that tunable shifts of electronic levels indeed arise by varying Cl, Br, I as the inorganic components, and $\mathrm{CH_3NH_3^+}$, $\mathrm{C_6H_5C_2H_4NH_3^+}$, $\mathrm{C_{20}H_{22}S_4N_2^{2+}}$ as the organic components. SOC is found to play an important role in splitting the conduction bands of the HOIP compounds investigated here. The frontier orbitals of the halide shift, increasing the gap, when Cl is substituted for Br and I.   

Electronic properties of CH3NH3PbBr3 (001) surface

The energetics and electronic properties of cubic CH3NH3PbBr3 (001) surfaces are studied using a first-principles method. We find that the uncompensated intrinsic dipole moment of a CH3NH3 molecule induces a band bending, being larger for the PbBr2-terminated surface than for the CH3NH3Br-terminated surface. When the intrinsic dipole are fully compensated, the surface electronic structure shows new states near band edges for both MABr and PBBr2 terminations. We find that for the PbBr2-terminated surface, less dispersive surface states appear just above the bulk valence bands at the center of Brillouin zone ($\Gamma $? point), while for the CH3NH3Br-terminated surface, more dispersive surface states appear below the conduction bands at the M¯ point. These states effectively reduce the band gap and improve optical absorption properties. The PbBr2-terminated surface states are of the Pb-p character, and hence are strongly affected by spin-orbit coupling, whereas the CH3NH3Br-terminated surface states are of the Pb-s character and hence are not affected by the spin-orbit coupling effect. Our study suggests a way to tune the spin-orbit coupling by selecting an appropriate surface.   

Phase stability, electronic structure and phonons in CsGeI$_3$

Because Ge is smaller than Sn and Pb, CsGeI$_3$ is promising to overcome the stability problems of the perovskite forms of CsSnI$_3$ and CsPbI$_3$ halides toward the denser yellow phase in which octahedra are edge as well as cornersharing in one dimensional chains. This phase has higher gaps and is unsuitable for photovoltaics. CsGeI$_3$ and other trihalide germanates are found to exist in the cubic perovskite phase at high temperature but in a rhombohedral phase in which the Ge is displaced toward three of the halogen neighbors in its surrounding octahedron, accompanied by a rhombohedral distortion of the lattice vectors. We will present density functional total energy calculations and band structures obtained within the quasi-particle self-consistent $GW$ method for both the cubic and rhombohedral phase of CsGeI$_3$. For the latter, we find a gap of 1.6 eV in excellent agreement with recent experiments on its absorption edge. We will also present optical dielectric function and effective mass results for this material and discuss the trends for different types of distortions in halides depending on the chemical composition. The phonons at the Brillouin zone center are calculated and compared to experimental Raman spectra.   

Interface Engineering in Metal Halides Perovskites: from molecules to devices.

In this talk we review our recent studies which aim to clarify the relationship between structural and electronic properties from a molecular to mesoscopic level. First we identify the markers for local disorder at molecular level by using Raman Spectroscopy as a probe. Then, we exploit such a tool to explore the role of microstructure on the absorption and emission properties of the semiconductor looking both at polycrystalline thin films and single crystals. We address the controversy surrounding electron – hole interactions and excitonic effects. We show that in hybrid lead-halide perovskites dielectric screening also depends on the local microstructure of the hybrid crystals and not only on its chemical composition. This leads to the possibility of band gap engineering and the consequent control of the elementary photo-excitation dynamics that determine the perovskites’ performances in different optoelectronic devices. Finally, the role of interface engineering, the effect of ion migration, and interface doping on charge extraction will be elucidated to provide a guideline for the design of hysteresis free solar cells. 1)G. Grancini & AR Srimath Kandada et al, Nature Photonics, 9 (10), 695-701, 2015 2) C. Tao et al,” Energy Environ. Sci.,8, 2365-2370, 2015   

Transient Spectroscopy of Photoexcitations and Morphology Control of Organometal Trihalide Perovskites

We studied the photoexcitation dynamics in various hybrid perovskites by using broadband ps transient photomodulation (PM) spectroscopy and variable stripe length (VSL) technique. We observed both excitonic and free carriers spectral features in MAPbI$_{3}$ but mainly excitonic transition in MAPbI$_{1.1}$Br$_{1.9}$ and MAPbI$_{3-x}$Cl$_{x}$ films. We also fabricated MAPbBr$_{3}$ films with nano-crystal pinning (NCP) treatment, which allows for smaller crystalline grain size. The transient spectra show a narrower and longer-lived photobleaching band in NCP treated films consistent with the increase in the photoluminescence efficiency. In addition the net optical gain measured by VSL is markedly increased up to 300 cm$^{-1}$, and the lasing threshold is concurrently reduced. Measurement of the waveguide losses in the NCP films shows that the improvement in lasing properties can partly be attributed to the reduced optical scattering.   

Photoluminescence and lasing properties of MAPbBr$_{3}$ single crystals grown from solution

Recent studies of solution-grown single crystals of inorganic-organic hybrid lead-trihalide perovskites have suggested that surface traps may play a significant role in their photophysics. We study electron-hole recombination in single crystal MAPbBr$_{3}$ through such trap states using cw photoluminescence (PL) and ps transient photoinduced absorption (PA) spectroscopies. By varying the depth of the collecting optics we examined the contributions from surface and bulk radiative recombination. We found a surface dominated PL band at the band-edge that is similar to that observed from polycrystalline thin films, as well as a weaker red-shifted emission band that originates from the bulk crystal. The two PL bands are distinguished in their temperature, excitation intensity and polarization dependencies, as well as their ps dynamics. Additionally, amplified spontaneous emission and crystal-related cavity lasing modes were observed in the same spectral range as the PL band assigned to the surface recombination.   

Ultrafast Extreme Ultraviolet Spectroscopy of Lead Iodide and Methylammonium Lead Iodide

Methylammonium lead iodide (perovskite) is a leading candidate for use in next-generation solar cell devices. However, the photophysics of perovskite responsible for its strong photovoltaic qualities are not fully understood. Ultrafast extreme ultraviolet (XUV) spectroscopy was used to investigate relaxation dynamics in perovskite and its precursor, lead iodide, with carrier-specific signals arising from transitions from a common inner-shell level (I 4d) to the valence and conduction bands. Ultrashort (30 fs) pulses of XUV radiation in a broad spectrum (40-70 eV) were obtained using high-harmonic generation in a tabletop instrument. Transient absorption measurements with visible pump (3.1 eV) and XUV probe directly observed the relaxation of charge carriers after above band excitation for both perovskite and lead iodide in the femtosecond and picosecond time ranges.