While it has been argued that field‐dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT‐type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p‐DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV‐characteristics as a function of intensity and charge‐extraction‐by‐linearly‐increasing‐voltage‐type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p‐DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s‐shape of the JV‐curves and the concomitant additional decrease of the FF with increasing layer thickness.

We have investigated how the addition of 1,8-diiodooctane (DIO) alters the formation of disordered and ordered phases in a film of poly(3-hexylthiophene-2,5-diyl) (P3HT). By combining in situ time-resolved absorption spectroscopy with 60 ms time resolution, optical and transmission electron microscopy and spatially resolved photoluminescence spectroscopy, we show that, in addition to the excitonic coupling, the film formation process during spin-coating as well as the subsequent long-time film drying process differ significantly when DIO is added to a solution of P3HT. During spin-coating, the addition of DIO reduces the actual time for transformation from disordered to ordered phase, even though it increases the time until the disorder−order transition sets in. In place of a solidification front, we observe an all-over solidification throughout the entire film. The phase separation between nonaggregated and aggregated phase increases when using DIO, with compositional variation in the content of aggregated phase on a micrometer scale.

Aggregates -that is short-ranged ordered moieties in the solid-state of p-conjugated polymers -play an important role in the photophysics and performance of various optoelectronic devices. We have previously shown that many polymers change from a disordered to a more ordered conformation when cooling a solution below a characteristic critical temperature T c . Using in situ time-resolved absorption spectroscopy on the prototypical semiconducting polymers P3HT, PFO, PCPDTBT, and PCE11 (PffBT4T-2OD), we show that spincoating at a temperature below T c can enhance the formation of aggregates with strong intra-chain coupling. An analysis of their time-resolved spectra indicates that the formation of nuclei in the initial stages of film formation for substrates held below T c seems responsible for this. We observe that the growth rate of the aggregates is thermally activated with an energy of 310 meV, which is much more than that of the solvent viscosity (100 meV). From this we conclude that the rate controlling step is the planarization of a chain that is associated with its attachment to a nucleation center. The success of our approach for the rather dynamic deposition method of spin-coating holds promise for other solution-based deposition methods.So far, however, only limited approaches have been reported to induce aggregate formation in a controlled fashion, including slow solidification in marginal solvents, 26-29 control of entanglements and sonication, 30-34 and blending 35 -many using poly(3-hexyl thiophene) (P3HT) as model system and many relying on relatively time-consuming methodologies. Approaches to control the formation of aggregates during the solution deposition should ideally be based on considering thermodynamics of the solution as well as by taking the kinetics of film formation into account. 9 In particular with respect to the latter, several methods have been reported, such as varying the boiling point of the solvent, 9,21,36 varying Additional Supporting Information may be found in the online version of this article.

Using optical spectroscopy in solution and thin film, and supported by quantum chemical calculations, we investigated the aggregation process of the donor-acceptor type molecule p-DTS(FBTTH 2 ) 2 . We demonstrate that cooling a solution induces a disorder-order phase transition that proceeds in three stages analogous to the steps observed in semi-rigid conjugated polymers. By analyzing the spectra we are able to identify the spectral signature of monomer and aggregate in absorption and emission. From this we find that in films the fraction of aggregates is near 100 % which is in contrast to films made from semi-rigid conjugated polymers.3

We investigated the aggregation behavior of the donor-acceptor molecules p-DTS(FBTTh) (T1) and p-SIDT(FBTTh) (H1) in MTHF solutions. Using optical spectroscopy, we found that T1 forms aggregates in solution while H1 aggregates only when processed as a thin film, but not in solution. Free energy molecular dynamics (MD) simulations based on force fields derived from quantum-mechanical density functional theory fully reproduce this difference. Our simulations reveal that this difference is not due to the lengthy carbon side chains. Rather, the molecular symmetry of T1 allows for an aggregated state in which the central donor units are spatially well-separated while a similar configuration is sterically impossible for H1. As a consequence, any aggregation of H1 necessarily involves aggregation of the central donors which requires, as a first step, stripping the central donor of its protective MTHF solvation shell. This unfavorable process leads to a significant kinetic hindrance for aggregation and explains the strongly differing aggregation behavior of T1/H1 in MTHF despite their otherwise similar structures.

Charge separation at the donor–acceptor interface is a key step for high efficiency in organic solar cells. If interfacial hybrid states exist already in the dark it is plausible that they can have a major impact on the dissociation of optically generated excitations. In this work we probe such interfacial states via steady state absorption spectroscopy. A substantial bleaching of the absorption spectrum is found near the absorption edge when an electron‐accepting layer of either trinitrofluorenone (TNF), C60, or a perylene‐diimide derivative is deposited on top of a layer of electron‐donating conjugated polymers, such as MEH‐PPV or various poly‐phenylene. This is in part attributed to the formation of ground state complexes with low oscillator strength. The experiments bear out a correlation between the reduction of the absorbance with the energy gap between the donor‐HOMO and acceptor‐LUMO, the effective conjugation length of the donor, and the efficiency of exciton dissociation in the solar cell. The effect originates from mixing of the donor‐HOMO and the acceptor LUMO. Calculations using density functional theory support this reasoning. Implications for efficiency of organic solar cells will be discussed.

No abstract

No abstract