Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (o1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.

Photo-induced charge transfer at molecular heterojunctions has gained particular interest due to the development of organic solar cells (OSC) based on blends of electron donating and accepting materials. While charge transfer between donor and acceptor molecules can be described by Marcus theory, additional carrier delocalization and coherent propagation might play the dominant role. Here, we describe ultrafast charge separation at the interface of a conjugated polymer and an aggregate of the fullerene derivative PCBM using the stochastic Schrödinger equation (SSE) and reveal the complex time evolution of electron transfer, mediated by electronic coherence and delocalization. By fitting the model to ultrafast charge separation experiments, we estimate the extent of electron delocalization and establish the transition from coherent electron propagation to incoherent hopping. Our results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces.

Charge transport dynamics in solar cell devices based on as-spun and annealed P3HT:PCBM films are compared using ultrafast time-resolved optical probing of the electric field by means of field-induced second harmonic generation. The results show that charge carriers drift about twice as far during the first 3 ns after photogeneration in a device where the active layer has been thermally annealed. The carrier dynamics were modelled using Monte-Carlo simulations and good agreement between experimental and simulated drift dynamics was obtained using identical model parameters for both cells, but with different average PCBM and polymer domain sizes. The calculations suggest that small domain sizes in as-spun samples limit the carrier separation distance disabling their escape from geminate recombination.

We derive the stochastic Schrödinger equation for the system wave vector and use it to describe the excitation energy transfer dynamics in molecular aggregates. We suggest a quantum-measurement based method of estimating the excitation transfer time. Adequacy of the proposed approach is demonstrated by performing calculations on a model system. The theory is then applied to study the excitation transfer dynamics in a photosynthetic pigment-protein Fenna-Matthews-Olson (FMO) aggregate using both the Debye spectral density and the spectral density obtained from earlier molecular dynamics simulations containing strong vibrational high-frequency modes. The obtained results show that the excitation transfer times in the FMO system are affected by the presence of the vibrational modes, however the transfer pathways remain the same.

cation process, [9][10][11][12] fundamental understanding of the perovskite properties ensuring high photovoltaic performance still lags behind.Basic research has primarily focused on the elucidation of charge transfer, transport, and collection properties. [13][14][15][16] This in turn allowed determination of important parameters such as electron and hole diffusion lengths, lifetimes, surface recombination rates, etc. In addition, significant efforts have been made aiming to determine whether the photoexcitation in organometal trihalide perovskites creates free charges or bound electron-hole pairs (excitons). [17][18][19][20][21][22][23] Previous investigations have shown that excitons split into charge carriers within a few picoseconds. [24,25] Signatures of the direct free charge generation in MAPbI 3 perovskite materials were reported in refs. [18,22] and [23]. Meanwhile, existence of exciton-carrier duality was demonstrated by Sheng et al. [19] Despite significant controversy concerning this issue, [17,23,[26][27][28] the current prevailing view favors branching between free carriers and excitons [19,20,29] with binding energy within a 2-150 meV range [2,27,[30][31][32] depending on temperature, dielectric constant, and crystal size. [20,29,33,34] However under operating conditions free charges represent the dominant species in perovskite solar cells. [20,23,35] Despite many studies on the subject, the evidence on the exact mechanism of the charge carrier generation remains controversial.In this work, we aim to elucidate the nature and initial dynamics of photogenerated charge carriers in methylammonium lead iodide perovskites (MAPbI 3 ) by means of picosecond time-resolved photoluminescence (PL) spectroscopy at various temperatures (20-300 K). While photoluminescence originates from recombination of the carriers, its time and temperature dependencies allow indirect evaluation of photogenerated carrier dynamics. Our studies allowed us to separate geminate and nongeminate recombination contributions, and we argue that geminate recombination dynamics is determined by the loss of spatial correlation between initially generated charge pairs. ResultsWe investigated perovskite photoluminescence dynamics in their films deposited on glass, fluorine-doped tin oxide (FTO), Charge carrier dynamics in organolead iodide perovskites is analyzed by employing time-resolved photoluminescence spectroscopy with several ps time resolution. The measurements performed by varying photoexcitation intensity over five orders of magnitude enable separation of photoluminescence components related to geminate and nongeminate charge carrier recombination and to address the dynamics of an isolated geminate electronhole pair. Geminate recombination dominates at low excitation fluence and determines the initial photoluminescence decay. This decay component is remarkably independent of the material structure and experimental conditions. It is demonstrated that dependences of the geminate and nongeminate radiative recombination components on...

Abstract. Excitation energy transfer in a photosynthetic FMO complex has been simulated using the stochastic Schrödinger equation. Fluctuating chromophore transition energies are simulated from the quantum correlation function which allows to properly include the finite temperature. The resulting excitation dynamics shows fast thermalization of chromophore occupations into proper thermal equilibrium. The relaxation process is characterized by entropy dynamics, which shows nonclassical behavior.

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