A microscopic model for the behavior of nanostructured organic photovoltaic devices

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We present a Monte Carlo model of carrier separation and recombination in nanostructured organic photovoltaic ͑OPV͒ devices which takes into account all electrostatic interactions, energetic disorder, and polaronic effects. This permits a detailed analysis of the strong morphology dependence of carrier collection efficiency. We find that performance is determined both by the orientation of the heterojunction relative to the external electric field as well as by carrier confinement due to polymer intermixing. The model predicts that an idealized interdigitated structure could achieve overall efficiencies twice as high as blends. The model also reproduces the weakly sublinear intensity dependence of short-circuit photocurrent ͑I SC ͒ seen in experiment. We show that this is not the result of space-charge effects but of bimolecular recombination. Disconnected islands of polymer in coarser blends result in bimolecular recombination even at low intensities and should therefore be minimized. By including a microscopic description of dark injection, the model can describe the full current-voltage ͑J-V͒ characteristics of different OPV structures. We examine the effect of morphology, intensity, mobility, and recombination rate on key parameters such as short-circuit current, open-circuit voltage ͑V OC ͒, and fill factor ͑FF͒. The model reproduces the intensity-dependent contribution to V OC in a bilayer above that of a blend observed in experiment. We find that performance in both bilayers and blends is very sensitive to the recombination rate across the heterojunction. The model also predicts a striking dependence of performance on mobility. Indeed it is shown that a tenfold increase in mobility dramatically improves I SC and FF and doubles the maximum power output in a bilayer device. As well as informing routes for improving device performance, the model also offers an improved microscopic understanding of OPV operation.

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…93,94 Another parameter which is neglected in the BO theory is the morphology, which plays a critical role in determining charge generation in PSCs. 95,96 In addition, the BO theory focuses on how the CT state is separated, and ignores how it is created. Considering ultrafast generation of free (2) charge carriers (on the timescale < 100 fs), the dynamics of the CT state creation and free charge carrier generation might be difficult to separate.…”

Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends

“…As geminate recombination has been shown to be the limiting mechanism in these devices, particularly under lower light intensities, [7] photoexcitation of ordered P3HT segments is more likely to result in separated charges (and hence photocurrent) compared to photoexcitation of disordered P3HT as the higher local hole mobility in ordered P3HT segments aids charge separation. [23] Therefore the photoexcitation of the minority component of ordered P3HT regions that provides a high charge separation efficiency accounts for a more than representative contribution to photocurrent at these wavelengths. The reason for the relative decrease in photocurrent at 600 nm with annealing most likely results from the increase in the size of P3HTdomains, with fewer P3HT excitons reaching a donor-acceptor interface compared to F8TBTexcitons.…”

Computationally connecting organic photovoltaic performance to atomistic arrangements and bulk morphology

“…The exponential term contains the free-energy difference between the initial and final hopping states, ∆E ij = E j − E i , and therefore represents a penalty associated with hops across energies with magnitudes significantly greater than λ ij . The majority of other investigations that employ this semi-classical Marcus treatment use a lattice-based model of a complete active layer, the morphology of which can be inferred from MD simulations [93], or by numerical methods such as modified Cahn-Hilliard [94][95][96][97] or Ising model techniques [98][99][100]. In these cases, the free energy, ∆E ij , takes into account any local variation in the energy levels (which is often selected from a density of state (DoS) distribution), the Coulomb interactions between nearby charge carriers and their images, as well as any contributions from the applied electric field across the morphology [96,97,101,102].…”