Fullerene-based organic solar cells with only a minute amount of donor show a substantial photocurrent while maintaining a large open-circuit voltage. At low concentrations the donor is fully dispersed within the fullerene and no percolation pathways of holes toward the anode exist; this morphology is in contrast to bulk-heterojunction donor:acceptor blends where percolation pathways for both electrons and holes are present within their respective transport phases. Therefore, the question of how holes contribute to the photocurrent arises. Here we demonstrate that the photocurrent is readily explained by photogenerated holes transferring back to the fullerene matrix due to Coulomb repulsion and the fullerene acting as an ambipolar conductor for both electrons and holes. The two critical parameters controlling this process are the values of the highest occupied molecular orbital level difference between the donor and the acceptor and of the recombination strength; both are found to agree between experimental measurements and kinetic Monte Carlo simulations. We provide evidence that the highest occupied molecular orbital level difference between donor and acceptor is smaller in a dilute donor configuration. Successive percolation pathways toward the contactsthe reason for introducing the bulk-heterojunction configurationare not an absolute requirement to obtain substantial photocurrents in organic solar cells.

Low charge carrier mobility is one key factor limiting the performance and applicability of devices based on organic semiconductors. Theoretical studies on mobility using the kinetic Monte Carlo or master equation are mainly based on a Gaussian energetic disorder and regular cubic lattices. The dependence of mobility on the electric field, temperature and charge carrier density is well studied for the Gaussian disorder model. In this work, we investigate the influence of spatially correlated site energies and spatial disorder in the lattice sites on the mobility using kinetic Monte Carlo simulations. Our analysis is based on both a regular cubic and a non-cubic Voronoi lattice. The latter is used to include spatial disorder in order to study its influence on the mobility for amorphous organic materials. Our results show that charge carrier mobility is strongly influenced by correlations in the site energies. Strong correlations even invert the field dependence of the mobility as observed experimentally in semi-crystalline polymers such as P3HT. Evaluation of local currents between localized states reveals the formation of current filaments with increasing correlation. Furthermore, the influence of the electric field and the energy landscape on the transport energy is studied by evaluation of active sites. A strong correlation between the transport energy, filaments in the local currents and the charge carrier mobility is observed. Our studies on the spatial disorder model do not indicate an inversion of the field dependence as observed by other researchers. The negative field-dependence in semi-crystalline materials may be explained by a higher correlation in the site energies as shown in a strongly correlated energetic landscape.

Abstract:In this paper, we present our generalized kinetic Monte Carlo (kMC) framework for the simulation of organic semiconductors and electronic devices such as solar cells (OSCs) and light-emitting diodes (OLEDs). Our model generalizes the geometrical representation of the multifaceted properties of the organic material by the use of a non-cubic, generalized Voronoi tessellation and a model that connects sites to polymer chains. Herewith, we obtain a realistic model for both amorphous and crystalline domains of small molecules and polymers. Furthermore, we generalize the excitonic processes and include triplet exciton dynamics, which allows an enhanced investigation of OSCs and OLEDs. We outline the developed methods of our generalized kMC framework and give two exemplary studies of electrical and optical properties inside an organic semiconductor.

In bulk-heterojunction organic solar cells the low permittivity in combination with the spatial and energetic disorder of the organic materials lead to a complex behavior of charge carriers within the active layer. Charges originate from exciton splitting at the heterojunction interface and the successive interplay between mutual Coulomb interactions and transport through the disordered organic can lead to insufficient separation from the interface, increased interface densities with respect to the bulk regions and, hence, affect recombination. To further understand the mechanisms of recombination, insight into the explicit spatial distribution of charge carriers within the blend is crucial. We performed kinetic Monte Carlo simulations on a bulk-heterojunction organic solar cell to assess the effect of Coulomb interactions and energetic disorder on the three-dimensional spatial distribution of charge carriers and highlight the correlation with both geminate and non-geminate recombination. We show that for materials with low permittivity and large energetic disorder the charge distribution is strongly inhomogeneous with accumulation along the heterojunction interface. In such cases recombination is not limited by recombination partners finding each other but rather an interface controlled process where geminate recombination dominates over nongeminate recombination.

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Charge pair separation in organic bulk-heterojunction (BHJ) solar cells is a complex interplay between numerous factors, such as the spatial geometry of the blend, the distribution of energetic disorder, the electric field, thermal fluctuations, and the mutual electron-hole Coulomb attraction. Insufficient separation from the interface and concomitant charge pair recombination is a main limitation in improving the PCE of organic BHJ solar cells and requires an in-depth understanding of the timescales involved. Here, a 3D kinetic Monte Carlo model of a BHJ organic solar cell is set up and the time-dependent evolution of mutual electron-hole pair distances separating from the heterojunction interface is investigated. Large fluctuations in separation times are found, in particular in dependence of the energetic disorder and the permittivity of the organic materials. At commonly observed values of energetic disorder, slight modifications of the permittivity can drastically influence the charge separation time and even outweigh orders of magnitude of geminate recombination rates, hence help to suppress geminate recombination. Thus, the results strongly support the recent trend of developing high-permittivity organic materials for solar cell applications.

The exploitation of ultrafast electron dynamics in quantum cascade lasers (QCLs) holds enormous potential for intense, compact mode-locked terahertz (THz) sources, squeezed THz light, frequency mixers, and comb-based metrology systems. Yet the important sub-cycle dynamics have been notoriously difficult to access in operational THz QCLs. Here, we employ high-field THz pulses to perform the first ultrafast two-dimensional spectroscopy of a free-running THz QCL. Strong incoherent and coherent nonlinearities up to eight-wave mixing are detected below and above the laser threshold. These data not only reveal extremely short gain recovery times of 2 ps at the laser threshold, they also reflect the nonlinear polarization dynamics of the QCL laser transition for the first time, where we quantify the corresponding dephasing times between 0.9 and 1.5 ps with increasing bias currents. A density-matrix approach reproducing the emergence of all nonlinearities and their ultrafast evolution, simultaneously, allows us to map the coherently induced trajectory of the Bloch vector. The observed high-order multi-wave mixing nonlinearities benefit from resonant enhancement in the absence of absorption losses and bear potential for a number of future applications, ranging from efficient intracavity frequency conversion, mode proliferation to passive mode locking.