Towards the Organic Double Heterojunction Solar Cell.

A perspective on the operating principles of organic bulk heterojunction solar cells is outlined and used to suggest an alternative device configuration, employing two type II semiconductor heterojunctions in series. Guiding principles to the implementation of this configuration, called a double heterojunction, are summarized. Assuming an exciton binding energy of 0.3 eV or less, results in a maximum achievable power conversion efficiency of well over 25 %. Achieving a high efficiency organic double heterojunction requires a specific energy level alignment, charge separation in the absence of driving forces, high phase purity and excellent diode quality. Fully conjugated triblock polymers of the form [D1 -A1 ]-[D1 -A2 ]-[D2 -A2 ] appear to be a system that can fulfill these requirements. Going forward, the primary challenge is the identification and development of synthetically tractable materials which have the necessary properties.

Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics.

Photocurrent generation in organic photovoltaics (OPVs) relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between electron-hole pairs that should in principle oppose the generation of free charges. The exact mechanism by which electrons and holes overcome this Coulomb trapping is still unsolved, but increasing evidence points to the critical role of hot charge-transfer (CT) excitons in assisting this process. Here we provide a real-time view of hot CT exciton formation and relaxation using femtosecond nonlinear optical spectroscopies and non-adiabatic mixed quantum mechanics/molecular mechanics simulations in the phthalocyanine-fullerene model OPV system. For initial excitation on phthalocyanine, hot CT excitons are formed in 10(-13) s, followed by relaxation to lower energies and shorter electron-hole distances on a 10(-12) s timescale. This hot CT exciton cooling process and collapse of charge separation sets the fundamental time limit for competitive charge separation channels that lead to efficient photocurrent generation.

Electrostatic Correlations Lead to High Capacitance in Zwitterion-Containing Thin Films.

A novel zwitterion composed of an imidazolium tethered to an anionic sulfonyl(trifluoromethane sulfonyl)imide group was prepared as an alternative dielectric material to traditional ionic liquids. The zwitterion not only melted below 100 °C but also proved to be nonhygroscopic. High-capacitance organic dielectric materials were obtained by blending this compound with poly(methyl methacrylate) over a range of concentrations and thicknesses. Above a specific temperature and concentration, films exhibit a capacitance nearly equivalent to that of an electrostatic double layer, approximately 10 µF/cm2, regardless of their thickness. Grazing-incidence wide-angle X-ray scattering experiments suggest that the zwitterions adopt a lamellar ordering at their surface above a critical concentration. The observed ordering is correlated with a 1000-fold increase in capacitance. The behavior suggests that the zwitterions exhibit strong electrostatic correlations throughout the film bulk, pointing the way toward a novel class of organic dielectric materials.

Photoinduced charge generation in a molecular bulk heterojunction material.

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).

Physical Supercritical Fluid Deposition: Patterning Solution Processable Materials on Curved and Flexible Surfaces.

We provide the initial demonstration of a general thin film deposition technique that leverages the unique solubility properties of supercritical fluids. The technique is the solution-phase analogue of physical vapor deposition and allows thin films of a semiconducting polymer to be grown without the need for in situ chemical reactions. Film growth is approximately linear with time, indicating that film thickness can be controlled in a straightforward manner by varying the time of deposition. To further demonstrate the flexibility of the technique, we demonstrate precise control over the location of material deposition using a combination of photolithography and resistive heating. The potential for scalable manufacturing is demonstrated by use of a master to control deposition onto a flexible polymer film. Finally, we demonstrate a unique deposition capability of this technique by depositing patterns onto the curved interior of a hemisphere made from a silicone elastomer. This capability is not possible with any printing or line-of-sight deposition technique. More generally, the ability to control the deposition of solution processed materials with high accuracy provides the long sought after bridge between top-down and bottom-up self-assembly.

Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption.

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we demonstrate that bimetallic Ni salen complexes form bis-ligand radicals upon two-electron oxidation, whose NIR absorption energies depend on the geometry imposed in the bis-ligand radical complex. Relative to the oxidized monomer [1˙]+ (E = 4500 cm-1, ε = 27 700 M-1 cm-1), oxidation of the cofacially constrained analogue 2 to [2˙˙]2+ results in a blue-shifted NIR band (E = 4830 cm-1, ε = 42 900 M-1 cm-1), while oxidation of 5 to [5˙˙]2+, with parallel arrangement of chromophores, results in a red-shifted NIR band (E = 4150 cm-1, ε = 46 600 M-1 cm-1); the NIR bands exhibit double the intensity in comparison to the monomer. Oxidation of the intermediate orientations results in band splitting for [3˙˙]2+ (E = 4890 and 4200 cm-1; ε = 26 500 and 21 100 M-1 cm-1), and a red-shift for [4˙˙]2+ using ortho- and meta-phenylene linkers, respectively. This study demonstrates for the first time, the applicability of exciton coupling to ligand radical systems absorbing in the NIR region and shows that by simple geometry changes, it is possible to tune the energy of intense low energy absorption by nearly 400 nm.

Ultrafast Charge Generation in an Organic Bilayer Film.

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.

Influence of Processing Additives on Charge-Transfer Time Scales and Sound Velocity in Organic Bulk Heterojunction Films.

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.