The electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation. We show that this process is facilitated through the formation of excited, delocalized band states. In our experiments on organic photovoltaic cells, these states were accessed for a short time (<1 picosecond) via infrared (IR) optical excitation of electron-hole pairs bound at the heterojunction. Atomistic modeling showed that the IR photons promote bound charge pairs to delocalized band states, similar to those formed just after singlet exciton dissociation, which indicates that such states act as the gateway for charge separation. Our results suggest that charge separation in efficient organic photoconversion systems occurs through hot-state charge delocalization rather than energy-gradient-driven intermolecular hopping.

A. Experimental part: general General Chemicals. IR-780 iodide (99%), 4-mercaptobenzoic acid (99%), yttrium(III) oxide (Y 2 O 3 , 99.99%), ytterbium (III) oxide (Yb 2 O 3, 99.9%), erbium (III) oxide (Er 2 O 3, 99.9%), trifluoroacetic acid (CF 3 COOH, 99%), sodium trifluoroacetate (CF 3 COONa, 98%), N,Ndimethylformamide (DMF, anhydrous, 99.8%), and chloroform (ACS spectrophotometric grade, ≥99.8%, with amylenes as stabilizer) were obtained from Aldrich. Oleylamine (C 18-content 80-90%) was purchased from Acros. Dichloromethane (DCM, AR grade) and diethyl ether (AR grade) were obtained from LAB-SCAN. Ethanol (absolute) was purchased from Merck. All chemicals were used as received. Lanthanide trifluoroacetate trihydrates (RE(CF 3 COO) 3 •3H 2 O) were prepared by a method described in the literature. [1] Instrumentation. 1 H and 13 C NMR spectra were recorded on a Varian AMX400 (400 and 100.59 MHz, respectively) using CDCl 3 as solvent at room temperature. Spectra were referenced to the solvent line (CHCl 3 : 7.26 ppm for 1 H, 77.0 ppm for 13 C) relative to tetramethylsilane. Data are reported as follows: chemical shifts, multiplicity (s = singlet, d = doublet, dd = doublet of doublets, t = triplet), coupling constants (Hz), and integration. FT-IR spectra were recorded on a Nicolet Nexus FT-IR spectrometer, using the SMART iTR for ATR measurements (diamond). Mass spectra were recorded on a LTQ Orbitrap XL (ESI+). Transmission electron microscopy (TEM) was performed on a Philips CM10 transmission electron microscope operating at an accelerating voltage of 100 kV. Images were recorded on a Gatan slow-scan CCD camera. UV-Vis absorption spectra were measured on a Perkin/Elmer Lambda 900 UV-Vis-NIR Spectrometer in chloroform in a quartz cuvette with a path length of 1 cm. Standard CW fluorescence measurements of solutions in chloroform were performed on a Fluorolog 3 (Jobin Yvon Horiba). For the up-conversion luminescence measurements, a Ti:Sapphire laser system (MIRA-900-F) was used as the excitation source. Luminescence spectra and lifetime measurements were performed in the cw and pulsed mode, respectively. In the latter case, the repetition rate was 76 MHz. The excitation light was focused into a 1 mm thick sample cell by a 75 mm focal length lens. This resulted in a focal spot of 120±10 μm full-width at half-maximum level. The excitation power was controlled by a gradient neutral density filter and set at 2 mW for all experiments with the exception of the power-dependence measurements. The emission was collected at the right-angle geometry via an f/2 collimating lens and subsequently focused by an f/4 lens onto the slit of the spectrograph. The (time-resolved) emission detection was performed by a streak camera system equipped with a spectrograph (Hamamatsu C5680) running with the vertical time axis sweep off while recording cw emission spectra.

We show how in the framework of the multimode Brownian oscillator model the system-bath correlation function can be derived from conventional and time-gated stimulated photon echo experiments and consideration of the linear optical spectra. Experiments are performed on the infrared dye DTTCI in room temperature solutions of ethylene glycol, methanol, and acetonitrile. The obtained correlation function is the sum of several Brownian oscillators, of which four are attributed to intrachromophore vibrational dynamics and the other three to solute-solvent dynamics. The ultrafast part of the correlation function on the time scale of the excitation pulses is interpreted as a free induction decay-like effect due to impulsive excitation of spectrally broad underlying vibrational structure in the dye's electronic transition. The slower parts are assigned to multiple time scale solute-solvent dynamics. The effect of vibrational coherences on the echo measurements is also analyzed; this analysis permits the dissection of the correlation function into a part due to intrachromophore dynamics and a part due to solvation dynamics. The spectral densities associated with these latter oscillators are located in the far infrared, in the same spectral region as probed by the optical Kerr effect. The measurements, however, provide no definite answer to the question of whether these spectral densities are the same.

Chemical reaction and optical dynamics in the liquid phase are strongly affected by specific solute-solvent interactions. The dynamical part of this coupling leads to energy fluctuations. The associated energy gap dynamics can be probed by using various nonlinear optical spectroscopies. We discuss various forms of photon echo--time-integrated, time-gated, and heterodyne-detected photon echo--as well as Fourier transform spectral interferometry. It is shown that for solutions of the dye molecule DTTCI, a system-bath correlation function can be acquired that provides a quantitative description of all (non)linear spectroscopic experiments. The deduced correlation function is projected onto the multimode Brownian oscillator model, which allows for a physical interpretation of the multiple-time correlation function and a determination of the spectral density relevant to the solvation process. The following applications of photon echo to condensed phase dynamics are discussed: enhanced vibrational mode suppression, Liouville pathways interference, and dynamical Stokes shift. Recent results of echo-peak shift experiments on the hydrated electron are also presented. The review concludes that photon echo should be useful as a novel tool to explore transition state dynamics.

While the susceptibility of CH 3 NH 3 PbI 3 to water is well-documented, the influence of water on device performance is not well-understood. Herein, we use infrared spectroscopy to show that water infiltration into CH 3 NH 3 PbI 3 occurs much faster and at a humidity much lower than previously thought. We propose a molecular model in which water molecules have a strong effect on the hydrogen bonding between the methylammonium cations and the Pb−I cage. Furthermore, the exposure of CH 3 NH 3 PbI 3 to the ambient environment increases the photocurrent of films in lateral devices by more than 1 order of magnitude. The observed slow component in the photocurrent buildup indicates that the effect is associated with enhanced proton conduction when light is combined with water and oxygen exposure.

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