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Singlet fission (SF) has the potential to bypass the Shockley–Queisser limit for solar cell efficiency through the production of two electron–hole pairs per photon. However, in polycrystalline pent...

Singlet fission (SF), the generation of two triplet excitons per the absorption of one photon, is a promising strategy for increasing the efficiency of solar cells beyond the theoretical Shockley–Queisser limit of 34%. Upon photon absorption by a SF molecule, the initially created singlet excited state (S1) interacts with a neighboring chromophore and is first transformed into a triplet pair (TT), which can be subsequently separated into independent triplet excitons (2T1). These independent triplet excitons can be harvested through triplet charge extraction or triplet energy transfer to an acceptor. Research on SF systems has revealed rates and efficiencies of triplet formation and triplet pair decorrelation that are strongly dependent on interchromophore coupling, which is dictated by molecular structure and the resulting geometrical arrangement of chromophores adopted in covalent (e.g., dimers) and noncovalent (e.g., films and crystals) systems. Incorporation of SF materials into realistic device archit...

Solution-processed bulk heterojunction organic solar cells fabricated with 1,3-bis[4-(N,N-diisopentylamino)-2,6-dihydroxyphenyl]squaraine and phenyl-C61-butyric acid methyl ester were found to exhibit unexpectedly low external quantum efficiency in the squaraine regions upon annealing. X-ray diffraction (XRD), spectral response, and time-resolved microwave absorption were all used to characterize the materials used and the devices prepared from them. An explanation for the drop in efficiency is proposed using Marcus–Hush theory to tie together the changes in coherent crystal domain size found by XRD and the external quantum efficiency results. Exciton dissociation at the interface was determined to be the rate-limiting step in efficient current generation for these devices.

The high-frequency mobility of positive charges (holes) moving along the backbones of an extensive data set of 7 oligomers (n = 2–16) and 6 polymers (⟨n⟩ = 26–138) of fluorene was measured using pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) in benzene. As expected, at 8.9 GHz, the measured isotropic ac mobility, μac,measiso, was observed to be strongly dependent on the lengths of the chains due to the charges encountering chain ends during one microwave cycle. Values of the measured mobility, μac,measiso, ranged from 5 × 10–4 cm2/(V s) for an n = 2 repeat unit oligomer to 0.18 cm2/(V s) for a polymer with an average length of ⟨n⟩ = 86 repeat units. Global fits to the entire set of lengths extracted the chain-length-independent intramolecular mobility, μacintra, using the Kubo formula, assuming normal diffusion along the contour of the molecule with reflecting boundary conditions at the ends. The effects of chain conformation, chain defects, polymer length distributions, and a finite pola...

During the past decade, time-resolved microwave conductivity (TRMC) has evolved to an established, powerful technique to study photoactive layers. With this feature paper, we aim to fulfill two goals: (1) give a full description of the photoinduced TRMC technique, including experimental details and data analysis, and discuss to what extent the TRMC technique differs from more conventional DC techniques and (2) illustrate the potential of this technique for probing charge carrier dynamics in photoactive materials. For these reasons recent studies on conjugated polymer:fullerene blends will be presented and discussed. The findings from these studies have advanced the insight into the mechanism of charge carrier generation and decay in polymer:fullerene blends, which allows us to improve the efficiency of organic photovoltaic cells based on this active layer architecture. In short, it is shown how the TRMC technique can be used as a versatile method to screen the potential of new photovoltaic materials.

The gas-phase electron affinity (EA) of phenyl–C61–butyric acid methyl ester (PCBM), one of the best-performing electron acceptors in organic photovoltaic devices, was measured by low-temperature photoelectron spectroscopy for the first time. The obtained value of 2.63(1) eV is only ca. 0.05 eV lower than that of C60 (2.683(8) eV), compared to a 0.09 V difference in their E1/2 values measured in this work by cyclic voltammetry. Literature E(LUMO) values for PCBM that are typically estimated from cyclic voltammetry and commonly used as a quantitative measure of acceptor properties are dispersed over a wide range between −4.38 and −3.62 eV; the reasons for such a huge discrepancy are analyzed here, and a protocol for reliable and consistent estimations of relative fullerene-based acceptor strength in solution is proposed.

A general but limiting characteristic in excitonic photovoltaics is that a portion of the incident photon energy appears necessary for converting excitons into electrical charges, resulting in a loss of efficiency. Currently, the mechanism underlying this process is unclear. Here, we describe the development of an experimental method for measuring charge creation yields in organic solar cell materials. We use this method to examine a series of conjugated polymer:fullerene blend films and observe two unexpected features: the existence of an optimal driving force and a loss in conversion efficiency if this force is exceeded. These observations have implications for the design of excitonic photovoltaic devices and can be explained by a simple Marcus formulation that introduces the importance of reorganization energy.

Using flash photolysis, time-resolved microwave conductivity we report the sub-200 ns photoconductivity transients for neat poly(3-hexylthiophene), P3HT, and four associated blends containing 1%, 5%, 20%, and 50%, by weight, of the soluble fullerene, [6,6]-phenyl-c61-butyric acid methyl ester, PCBM. We propose a detailed kinetic scheme that when solved numerically is consistent with all the data recorded as a function of excitation density and that describes the fate of mobile and trapped carriers in the system. In the neat polymer, mobile holes are the only contributor to the photoconductance transients, which decay according to first-order kinetics at all light intensities due to the presence of a large concentration of dark carriers present in the polymer. The signal decays with a characteristic rate constant (∼1 × 107 s–1) that describes the re-equilibration of trapped and mobile holes. In all four blends, the microwave absorption contains a significant contribution due to electrons in the PCBM cluste...

The combination of electron-rich and electron-poor moieties in conjugated molecules is frequently utilized in order to red shift the absorption spectrum and improve photon harvesting in bulk hetero...

The generation of excitons and their interaction with holes in films of neat regioregular poly(3-hexylthiophene) and the polymer blended with 1 wt% of the electron-acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been studied using flash-photolysis time-resolved microwave conductivity. The sublinear relationship between the photogenerated hole density and the incident light intensity, in both the neat polymer and the donor−acceptor blend, can be attributed to the quenching of excitons by holes, at a rate characterized by a second-order rate constant (γ2) of 3 × 10−8 cm3/s. This value is larger than that found for other, luminescent conjugated polymers; the difference may be attributed to a greater collision probability, due to the higher mobility of the interacting species, or to an enhancement of the quenching rate once they are in close proximity. The phenomenon has consequences for the ultimate efficiency of organic photovoltaic solar cells that are based on the simple polymer:PCBM bulk ...

We use the contactless time-resolved microwave conductivity (TRMC) technique to investigate the photoconductance of CdS-coated mesoporous TiO2 and ZrO2. The CdS domains were grown directly on the surface of the oxide by a chemical bath deposition method. Mobile charge carriers are generated with a low yield following photoexcitation of CdS grown on ZrO2; there is no injection into the oxide and the short-lived photoconductance signal is attributed to the hole in CdS. In contrast, illumination of CdS domains grown on TiO2 results in very efficient electron injection into the TiO2 nanoparticles, producing long-lived charge carriers. We show that the quantum yield for electron injection into TiO2 per photon absorbed by the CdS sensitizer is close to unity. The photoconductance action spectrum follows the absorption of the CdS domains, which indicates that the charge injection efficiency does not depend on wavelength in the range where the CdS absorbs (425–550 nm).

We study the effects of adding H2O2 to acid-purified and unpurified single-walled carbon nanotubes (SWNTs) in aqueous suspensions using photoluminescence (PL) and optical absorption spectroscopies. The addition of H2O2 to suspensions of unpurified SWNTs results in a rapid (1−2 h) quenching of the photoluminescence from all tubes, whereas H2O2 addition to acid-purified SWNTs causes the nanotube PL to grow in intensity over a period of several days before decaying in a tube-specific manner that depends on the binding strength of the surfactant sheath. With the appropriate choice of surfactants, the PL for specific acid-purified SWNTs can be protected such that novel mid-gap and phonon-assisted absorption and emission transitions can be observed without the obscuring effects associated with emission from other nanotubes. The H2O2 treatment also results in a reduction of the high-energy absorption background that has been associated with either carbonaceous impurities or the SWNT π-plasmon oscillation. An und...

Thin films of single-wall carbon nanotubes were used as the transparent top electrical contact in Cu(In,Ga)Se2- based solar cells. Specifically, we demonstrate that thin layers of carbon nanotubes in combination with insulating polymer layers can effectively replace the metal oxide layers typically used in polycrystalline thin-film solar cells. Replacing the standard n-type ZnO layer with a thin film of carbon nanotubes yielded energy conversion efficiencies up to 13%. The optical and electrical transport properties of the single-wall carbon nanotubes suggest that suitable applications for these materials include multiple-junction solar cells, thermophotovoltaics, and other applications benefiting from a p-type transparent conductor with high near-infrared transmission.

A combined experimental and theoretical study of the photophysical properties and excited-state dynamics of semiconducting single-wall carbon nanotubes (SWNTs) is reported. Steady-state and time-resolved fluorescence data as a function of temperature are explained on the basis of a manifold of four low-lying singlet exciton states with kinetically controlled interconversion. Relaxation among these levels is slow and therefore Kasha's rule is not obeyed. Quantum chemical calculations based on time-dependent density functional theory complement the experimental findings. The temperature-dependence of the radiative and nonradiative rate constants are examined.

We have performed steady-state photoluminescence, time-correlated single photon counting, and Raman spectroscopy measurements on single-wall carbon nanotubes from 4 to 293 K. We observe novel photoluminescence spectra that cannot be attributed to vibronic transitions and verify the existence and energy levels of weakly emissive excitonic states. By combining photoluminescence intensity and lifetime data, we determine how nonradiative and radiative excitonic decay rates change as a function of temperature and contrast this with theoretical predictions. The results suggest that recombination kinetics are influenced by multiple excitonic states, including a dark lower state.