Field and temperature dependence of the photocurrent in polymer/fullerene bulk heterojunction solar cells

Abstract

The photocurrent in polymer/fullerene blends is characterized as a function of bias at temperatures ranging from 125to300K. Assuming a constant generation rate and bimolecular recombination, the results are numerically modeled within the drift-diffusion approximation. Bimolecular recombination is found to be a dominant factor in the field dependence of the photocurrent in the entire measured voltage range. Inclusion of field dependent geminate pair dissociation and recombination via the Onsager expressions gives a much stronger field dependence than experimentally observed. From the temperature dependence of the extracted mobilities, we can simultaneously estimate the broadening of the transporting highest occupied and lowest unoccupied molecular orbital levels.

Mobility dependent efficiencies of organic bulk heterojunction solar cells: Surface recombination and charge transfer state distribution

“…Note that Eq. ͑1͒ is often [6][7][8][9][10][11]14 written in the form p diss = k d / ͑k d + k f ͒, where the dissociation constant k d relates to k rec and 0 via k d = k rec n i 2 / 0 . However, using the form of Eq.…”

“…This exciton then ͑ii͒ has to diffuse to the interface between the donor and acceptor molecules where it is separated. A frequently used model [6][7][8][9][10][11] for the simulation of current/voltage curves of organic bulk heterojunction solar cells assumes that the dissociated exciton ͑iii͒ first creates a charge transfer state 12,13 at the interface, before ͑iv͒ the electron and hole are separated and ͑v͒ transported to their respective junctions. A feature very uncommon to the classical picture of inorganic solar cells results from the interplay between electric field dependent dissociation and nonradiative recombination of the charge transfer state as outlined in Ref.…”

A microscopic model for the behavior of nanostructured organic photovoltaic devices

“…The majority of modeling thus far has employed a continuum approach. [20][21][22] While computationally efficient, this is unable to describe carrier interactions accurately and reduces morphological effects to bulk parameters. A proper inclusion of these effects requires a microscopic approach, which is computationally much more demanding, and hence only rarely adopted.…”

Electro-optical modeling of bulk heterojunction solar cells

“…21 The model takes into account exciton transport, dissociation, as well as the field and temperature dependent dissociation of a bound electron/hole pair. The widely used [22][23][24][25][26][27] model of Koster et al, 14 taking into account only the bound pair but not the exciton diffusion, is a special case of our model. We combine our model for the generation of free carriers in bhj-solar cells with the device simulator ASA, 28 which is able to calculate the generation rate of excitons with a thin film optics approach.…”