In the last 10 years, there has been a boom in the organic photovoltaic (OPV) community, with new devices achieving power conversion efficiencies (PCEs) of ~18%. This significant increase in device performance is due to a switch from polymer:fullerenes, such as P3HT:PCBM, to polymer:non-fullerenes, like PM6:Y6. These films are often created with the use of solvent additives which, when correctly chosen, increase the device performance. However, because many of these polymer:non-fullerene systems have only recently been developed the exact effect that these solvent additives have on the morphology, performance and electronic properties is not well understood. Here we use a combination of photoluminescence and time-resolved spectroscopy along with grazing-incidence wide-angle x-ray scattering to fully understand what is occurring at both an electronic and morphological level in a series of PM6:Y6 films with varying amounts of acetone used as a solvent additive. From these data we find that acetone changes the degree of mixing and crystal grain size, which leads to changes in polaron yield and recombination, which is reflected in device performance.
The performance of photovoltaic and light-emitting devices that utilize singlet fission sensitization has been modest to date, despite spectroscopic measurements reporting high triplet exciton multiplication yields in the singlet fission active layer of these devices. This contrast highlights the need to characterize the factors that underpin device performance, such as triplet separation and diffusion. Here, we use ultrafast infrared spectroscopy to probe the dynamics and separation of correlated triplet pair intermediates following singlet fission in amorphous and crystalline pentacene films. The ultrafast vibrational measurements reveal that triplet-pair separation occurs on similar timescales in both types of films, despite differences in intermolecular coupling strength. Conversely, ultrafast electronic spectroscopy measurements of diffusion-controlled triplet-triplet annihilation reveal that triplet diffusion in the amorphous film is an order of magnitude lower than the crystalline analog. Together, these results suggest that sparse triplet traps limit the transport of triplet excitons in the amorphous film. Therefore, device developers should seek to identify the structural origins of these states to identify molecular structures that self-assemble in patterns that avoid triplet trap state formation.
Ultrafast visible pump - infrared probe spectroscopy is used to examine the dynamics of free carrier formation following
photoinduced electron transfer in an organic photovoltaic polymer blend. The carbonyl (C=O) stretch of the
functionalized fullerene, PCBM, is probed as a local vibrational reporter of the dynamics in a blend with a conjugated
polymer, CN-MEH-PPV. It has been determined that PCBM molecules at the interfaces of PCBM and polymer phases
possess higher frequency carbonyl vibrational modes while molecules in the centers of PCBM domains have lower
frequency modes. The shift in frequency of the carbonyl stretching mode is used to directly resolve the dynamics of free
carrier formation that occur on the few picosecond timescale. The fast dynamics suggest that the presence of an
interfacial dipole causes the charge carriers to experience a smaller Coulombic potential than that which would be
predicted from the dielectric properties of the materials. The free carrier formation dynamics are temperature
independent indicating that excess vibrational energy remaining for a short period of time after the
The dynamics of photoinduced charge separation, recombination and trapping are examined in a polymer blend
photovoltaic material with ultrafast visible pump - infrared probe and time-resolved infrared spectroscopy. The carbonyl
(C=O) stretch of methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is probed as a local vibrational
reporter of the dynamics in a blend with a poly(p-phenylenevinylene) (PPV) -based conjugated polymer, CN-MEH-PPV.
Following interfacial electron transfer, geminate electron - hole pair dissociation occurs on ultrafast timescales.
Subsequent to this charge separation process, charge carriers become trapped on the microsecond timescale resulting in
the formation of a distinct peak in the vibrational spectra corresponding to the anion of PCBM. The charge trapping
dynamics correspond to the carrier lifetime of similar PPV-based polymer blends as reported in photocurrent transients
from the literature.
Electron transfer (ET) dynamics between molecular adsorbates and semiconductor nanoparticles has been a subject of intense recent interest because of relevance to many applications of nanomaterials, such as dye-sensitized solar cells, molecular electronics and sensors. However, it is still unclear how the charge transfer rate depends on the properties of molecules and semiconductors. In this paper we examine electron injection from Ru and Re polypyridyl
complexes to metal oxide (TiO2, SnO2 and ZnO) nanocrystalline thin films. Adsorbates with different energetics and electronic coupling are compared to identify molecular properties that influence ET dynamics. Different semiconductor nanomaterials are compared to understand the dependence on conduction band composition and energetics. ET dynamics were found to be biphasic consisting of ultrafast (<100fs) and slower components, with varying partitioning between them and rates of slow components. These kinetics can be well described by a two-state injection model, which includes injection from both unthermalized and thermalized excited states and competition between electron injection and intramolecular relaxation from the unthermalized state. The dependence of ET rates on various molecular and semiconductor properties is also discussed.
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