New Science: The field of bulk heterojunction (BHJ) solar cells was created as a result of the discovery of ultrafast charge transfer. The length scale for the wavefunction describing the probability amplitude for finding a photoexcitation at a particular point in space was estimated using position–momentum uncertainty as expressed by the uncertainty principle. The problem can also be considered semi-classically and a very similar estimate of the length scale of the photoexcitation wavefunction is obtained via the resolution limit of a microscope; (λ/2πn) where n is the index of refraction. Finally, the BHJ solar cell should be especially sensitive to the effective interaction volume of the photon because it is comprised of a densely packed collection of strong absorbers. The photoexcitation process, therefore, generates a delocalized coherent superposition of the eigenfunctions of the Schrodinger equation that describes the nanostructured blend, with an immediate probability amplitude for finding a photoexcitation near a BHJ boundary enabling charge transfer in the femtosecond regime over relatively long distances. New Technology: A general strategy is presented to self-assemble unidirectional alignment and efficient charge transport for semiconducting polymer films deposited on textured Si/SiO2 substrates. By employing sandwich casting in a tilted tunnel system, we utilize capillary action, generated by functionalized spacers, to self-assemble semiconducting polymers along uniaxial nano-grooves on the substrate. The strength of capillary action can be tailored by different surface treatments of the glass spacers. PTS functionalization yields highly oriented crystalline films with compact structure with µh = 25.4 cm2 V−1 s−1 and µh = 22.2 cm2 V−1 s−1 for PCDTPT and CDTBTZ, respectively. These values are limited by the S–D contact resistance, Rc. Using longer channels, Rc is significantly less than the channel resistance and µ = 36.3 cm2 V−1 s−1 was measured. Extrapolating to infinite channel length, the intrinsic mobility for PCDTPT is obtained at this degree of chain alignment and structural order: µ = 47 cm2 V−1 s−1. The mobility is strongly anisotropic with 13.6- and 17.6-fold higher values parallel to the direction of polymer alignment.

We report on scalable solid-state neutron detector system that is specifically designed to yield high thermal neutron detection sensitivity. The basic detector unit in this system is made of a (6)Li foil coupled to two crystalline silicon diodes. The theoretical intrinsic efficiency of a detector-unit is 23.8% and that of detector element comprising a stack of five detector-units is 60%. Based on the measured performance of this detector-unit, the performance of a detector system comprising a planar array of detector elements, scaled to encompass effective area of 0.43 m(2), is estimated to yield the minimum absolute efficiency required of radiological portal monitors used in homeland security.

Charge generation in an organic photovoltaic blend was investigated using transient absorption spectroscopy. In films of pure electron donating material, subpicosecond spectral oscillations were observed and assigned to torsional modes associated with excited state relaxation and localization. These modes are systematically suppressed in the presence of fullerene, indicating that a significant fraction of charge transfer occurs prior to excited state localization.

ADVERTISEMENT RETURN TO ISSUEPREVCommentNEXTReply to "Comment on 'Coherence and Uncertainty in Nanostructured Organic Photovolatics'"Loren G. Kaake*†, Daniel Moses†, and Alan J. Heeger*†‡§View Author Information† ‡ § †Center for Polymers and Organic Solids, ‡Department of Physics, and §Materials Department University of California, Santa Barbara, California 93106, United States*L. G. Kaake: e-mail, [email protected]*A. J. Heeger: e-mail, [email protected]Cite this: J. Phys. Chem. A 2013, 117, 40, 10565Publication Date (Web):September 13, 2013Publication History Received30 August 2013Published online26 September 2013Published inissue 10 October 2013https://pubs.acs.org/doi/10.1021/jp408700mhttps://doi.org/10.1021/jp408700marticle-commentaryACS PublicationsCopyright © 2013 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views691Altmetric-Citations5LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (117 KB) Get e-AlertscloseSUBJECTS:Absorption,Delocalization,Excited states,Excitons,Solar cells Get e-Alerts

The dynamics of charge generation in a high performing molecular photovoltaic system, p-SIDT(FBTTh2)2 (see Figure 1 ) is studied with transient absorption. The optimized bulk heterojunction material shows behavior observed in many other systems; the majority of charges are generated at short time scales (<150 fs), and a slower contribution from incoherently diffusing excitons is observed at low pump fluence. In a separate experiment, the role of bulk heterojunction material morphology on the process of ultrafast charge generation was investigated with bilayers made with solution processed donor molecules on a photopolymerized C60 layer. The majority of carriers are again produced at short time scales, ruling out the idea that subpicosecond charge generation can be understood wholly in terms of localized excitons. We evaluate possible causes of this behavior and propose that the excited state is highly delocalized on short time scales, providing ample probability density at the charge generating interface.

study on carrier-collection efficiency in various organic photovoltaic systems now reveals that ultrafast relaxation of photoexcitations within the manifold of charge-transfer states does not impede mobile charge carrier generation.

We report time-resolved photoinduced absorption and time-resolved photoconductivity in conducting polymer-C60 composites. Photoinduced electron transfer occurs at times < 1 ps. Two subgap features are observed in the photoinduced absorption of pure poly(3-octylthiophene) (P3OT); these form earlier than 1 ps and decay with τ<5 ps. P3OT mixed with C60 shows similar spectral features at early times (<1 ps); however, a new spectrum evolves after ≈1 ps as a result of electron transfer from P3OT to C60. Ultrafast photoinduced electron transfer improves the quantum efficiency for photogeneration of charge carriers; in the ps domain the photoconductivity of the conducting polymer host is enhanced by more than an order of magnitude upon mixing in a few percent C60.

The effects of fundamental uncertainty present a compelling rationale for a highly delocalized photoexcitation on ultrafast time scales. This delocalized photoexcitation enables an immediate probability of charge-transfer over distances compatible with the uncertainty principle. We perform transient absorption measurements on organic bulk heterojunction solar cells to investigate charge-transfer dynamics in a variety of materials. A startling generality emerges indicating that the majority of charge carriers are generated at times within the temporal resolution of our instrument (∼100 fs).

The photocarrier transport properties in two leading molecular bulk-heterojunction systems, p-DTS(PTTh2)2:PC71BM and p-DTS(FBTTh2)2:PC71BM, are studied using transient photoconductivity measurements. We find that in these systems shallow trap states are distributed at relatively narrow energy range, giving rise to time-independent carrier mobility in the nanosecond time regime that evidently contributes to the high performance of molecular-based OPV devices.

The unique benefit of solution-based fabrication of solid-state p-n junctions is demonstrated for radiation detection. In particular, an in situ inorganic semiconductor synthesis and film deposition facilitates a novel neutron detector configuration consisting of a host inorganic semiconductor matrix impregnated with a guest neutron sensitizing material. Spectroscopic investigations of the structural order of the top detector active layer indicate that it consists of interpenetrating networks of the host semiconductor nanocrystals and sensitizing guest material that self-assemble during film formation. The host semiconductor network exhibits a good charge transport as evidenced by steady-state photoconductivity measurements. The detectors developed indicate high sensitivity to ionizing radiation and a demonstrated ability to detecting thermal neutrons.

The transport and relaxation of photogenerated carriers in a bulk heterojunction (BHJ) material made of a blend of PCDTBT and PC(60) BM are studied as a function of the concentration of PC(84)BM impurities. For low concentrations of PC(84)BM, the increasing activation energy with delay time indicates transport dominated by trap-limited carrier drift while the photocarriers relax through a manifold of disorder-induced localized states near the band edge. In the BHJ material with high concentration of PC(84)BM, transport is dominated by carrier hopping within the PC(84)BM impurity band.

Spectrally dependent steady-state photoconductivity is a convenient method to gain insight into the charge generation and transport processes within a given material. In this work, we report on the photoconductive response of solution-processed neat films and blends of the fullerene, PC(71)BM, and the donor-acceptor small-molecule, p-DTS(PTTh(2))(2), as function of the processing additive, diiodooctance (DIO). The results, when considered in the context of their structural, optical, and electronic properties give insight into the dominant carrier generation and charge transport mechanisms in each of these molecular systems.

The role of processing additives in organic bulk heterojunction thin films was investigated by means of transient absorption spectroscopy. The rate of ultrafast charge transfer was found to increase when a small amount of diiodooctane was used during film formation. In addition, coherent acoustic phonons were observed, and their velocity was determined. A strong correlation between the sound velocity and the charge-transfer time scale was observed, both of which could be explained by a subtle increase in thin film density.

Understanding the charge generation dynamics in organic photovoltaic bulk heterojunction (BHJ) blends is important for providing the necessary guidelines to improve overall device efficiency. Despite more than 15 years of experimental and theoretical studies, a universal picture describing the generation and recombination processes operating in organic photovoltaic devices is still being forged. We report here the results of ultrafast transient absorption spectroscopy measurements of charge photogeneration and recombination processes in a high-performing solution-processed molecular BHJ. For comparison, we also studied a high-performing polymer-based BHJ material. We find that the majority of charge carriers in both systems are generated on <100 fs time scales and posit that excited state delocalization is responsible for the ultrafast charge transfer. This initial delocalization is consistent with the fundamental uncertainty associated with the photon absorption process (in the visible, λ/4π > 30 nm) and is comparable with the phase-separated domain size. In addition, exciton diffusion to charge-separating heterojunctions is observed at longer times (1-500 ps). Finally, charge generation in pure films of the solution processed molecule was studied. Polarization anisotropy measurements clearly demonstrate that the optical properties are dominated by molecular (Frenkel) exictons and delocalized charges are promptly produced (t < 100 fs).

Incorporating [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in solution-processed composites comprising the two small molecular semiconductors 9,10-Diphenylanthracene (DPA) and 5,6,11,12-tetraphenylnaphthacene (Rubrene) extends significantly the onset wavelength of the steady-state photoconductivity from 610 nm in the pristine DPA:Rubrene composite to 900 nm in the DPA:Rubrene:PCBM composite. The experimental data indicate carrier generation in the near IR spectral region arising from inter-molecular charge transfer (IMCT) excitation that potentially could be useful for extending solar radiation light harvesting. Pump/probe photoinduced absorption (PIA) measurements indicate instantaneous carrier generation at sub-band-gap photon energies, confirming the viability of IMCT excitations as the underlying carrier generation mechanism at the near IR spectral region.

A fruitful paradigm in the development of low-cost and efficient photovoltaics is to dope or otherwise photosensitize wide band gap semiconductors in order to improve their light harvesting ability for light with sub-band-gap photon energies.(1-8) Here, we report significant photosensitization of TiO2 due to the direct injection by quantum tunneling of hot electrons produced in the decay of localized surface-plasmon polaritons excited in gold nanoparticles (AuNPs) embedded in the semiconductor (TiO2). Surface plasmon decay produces electron-hole pairs in the gold.(9-15) We propose that a significant fraction of these electrons tunnel into the semiconductor's conduction band resulting in a significant electron current in the TiO2 even when the device is illuminated with light with photon energies well below the semiconductor's band gap. Devices fabricated with (nonpercolating) multilayers of AuNPs in a TiO2 film produced over 1000-fold increase in photoconductance when illuminated at 600 nm over what TiO2 films devoid of AuNPs produced. The overall current resulting from illumination with visible light is ∼50% of the device current measured with UV (ℏω>Eg band gap) illumination. The above observations suggest that plasmonic nanostructures (which can be fabricated with absorption properties that cover the full solar spectrum) can function as a viable alternative to organic photosensitizers for photovoltaic and photodetection applications.

We show that polybis(thienyl)thienodia-thiazolethiophene (PDDTT), a high-performance semiconducting polymer for photodetectors and field-effect transistors, has strong performance dependence on annealing temperature. An unprecedented increase of 3 orders of magnitude is observed in both transistor and photoconductive properties. XRD and AFM evidence points to increased ordering in PDDTT films with annealing. This correlation highlights the importance that order has in determining performance in PDDTT and has possible implications in the design of polymers.