Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites

Solar cells based on the organic-inorganic tri-halide perovskite family of materials have shown remarkable progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over three times smaller than has been previously assumed. In the room temperature phase we show that the binding energy falls to even smaller values of only a few millielectronvolts, which explains their excellent device performance due to spontaneous free carrier generation following light absorption. Additionally, we determine the excitonic reduced effective mass to be 0.104me (where me is the electron mass), significantly smaller than previously estimated experimentally but in good agreement with recent calculations. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties

Ultrafast Terahertz Photoconductivity of Bulk Heterojunction Materials Reveals High Carrier Mobility up to Nanosecond Time Scale

The few-picosecond (ps) decay of terahertz (THz) photoconductivity typically observed for conjugated polymer fullerene blends (at excitation fluencies similar to 10(15) photons/cm(2) per pulse) is shown to be a result of charge pair annihilation for two polymer:PCBM blends. At a factor of 100 lower excitation density, the THz decay is in the hundreds of ps time scale, implying that very high carrier mobility (similar to 0.1 cm(2) V-1 s(-1)) prevails for long time after charge formation, of importance for free charge formation in organic solar cells

Experimental and calculated terahertz spectra of naphthalene and 1,4-dihydroxynaphthalene in the 0.5 - 6 terahertz region

Far infrared (IR) spectra of naphthalene and 1,4-dihydroxynaphthalene (1,4-naphthol) were measured in the region from 0.5 to 6.3 THz at room temperature using a GaP THz wave generator. Quantum chemical calculations were performed to obtain normal mode coordinates and frequencies for an isolated molecule case of naphthalene and 1,4-naphthol. © 2008 IOP Publishing Ltd

Thermally activated exciton dissociation and recombination control the carrier dynamics in organometal halide perovskite.

Solar cells based on organometal halide perovskites have seen rapidly increasing efficiencies, now exceeding 15%. Despite this progress, there is still limited knowledge on the fundamental photophysics. Here we use microwave photoconductance and photoluminescence measurements to investigate the temperature dependence of the carrier generation, mobility, and recombination in (CH3NH3)PbI3. At temperatures maintaining the tetragonal crystal phase of the perovskite, we find an exciton binding energy of about 32 meV, leading to a temperature-dependent yield of highly mobile (6.2 cm(2)/(V s) at 300 K) charge carriers. At higher laser intensities, second-order recombination with a rate constant of gamma = 13 x 10(-10) cm(3) s(-1) becomes apparent. Reducing the temperature results in increasing charge carrier mobilities following a T-1.6 dependence, which we attribute to a reduction in phonon scattering (Sigma mu = 16 cm(2)/(V s) at 165 K). Despite the fact that Sigma mu increases, gamma diminishes with a factor six, implying that charge recombination in (CH3NH3)PbI3 is temperature activated. The results underline the importance of the perovskite crystal structure, the exciton binding energy, and the activation energy for recombination as key factors in optimizing new perovskite materials

Bulk-like transversal electron mobility in heavily n-doped InP nanowires probed by terahertz spectroscopy

Waveguiding of excitation beam and propagation of THz beam in a complex gradient environment were studied in an array of InP nanowires. Measurements by time-resolved THz spectroscopy accompanied by Monte-Carlo calculations of the response of localized charges enabled determination of transversal electron mobility

Observation of four-fold azimuthal angle dependence in the terahertz radiation power of (100) p-InAs

The azimuthal angle dependence of the terahertz (THz) radiation power of (100) p-type InAs under 1-T magnetic field is presented. Results exhibited four-fold symmetry for the s-polarized THz radiation power. Moreover, the two-fold symmetry for the p-polarized THz radiation was modified to four-fold symmetry for a 1-T applied magnetic field. A tentative explanation regarding a magnetic field-enhanced L-valley carrier scattering is proposed. The actual physical origin of these results is currently under investigation. ©2006 IEEE

Accurate modeling of inter- and intra-molecular interactions in 1,4-dihydroxynaphthalene in the 0.5-6 terahertz region

Semi-empirical calculations successfully predicted the terahertz (THz) absorption spectrum associated with intra- and intermolecular interactions in 1,4-dihydroxynaphthalene. Results are in excellent agreement with spectroscopy data in the 0.5-6 THz region using GaP wave THz source. © 2006 Optical Society of America

Insights into the Charge Carrier Terahertz Mobility in Polyfluorenes from Large-Scale Atomistic Simulations and Time-Resolved Terahertz Spectroscopy

Multiscale atomistic simulations were used to. understand the factors that determine the charge carrier mobility spectrum obtained from time-resolved terahertz (THz) spectroscopy in conjugated polymers. The simulation approach combines classical molecular dynamics simulations of atomic structure, large-scale electronic structure calculations, and the evaluation of the THz mobility using Kubo's linear response formula. We found that THz radiation probes a single carrier hop at low temperatures and high frequencies, while the transport over somewhat longer distances is probed in the opposite cases. Our results indicate that charge carrier transport at THz frequencies is thermally activated but with a much smaller activation energy compared to the dc case. The shape of the real and imaginary part of the mobility spectral curve reveals the presence of above THz hopping rates that are relevant for charge carrier transport. Strong differences of the mobilities in the polymer and the corresponding monomer material are largely caused by stronger energetic disorder of the monomer material

Low-loss single-mode THz waveguiding using CYTOP, a highly transparent plastic with potential as hybrid optics

A low-loss CYTOP planar photonic crystal THz waveguide with single-mode propagation is realized. The highly transparent nature of CYTOP from deep ultraviolet to the THz region indicates its potential usage as component of hybrid optics. © 2006 Optical Society of America