Spin-dependent charge transfer state design rules in organic photovoltaics.

Charge transfer states play a crucial role in organic photovoltaics, mediating both photocurrent generation and recombination losses. In this work, we examine recombination losses as a function of the electron-hole spacing in fluorescent charge transfer states, including direct monitoring of both singlet and triplet charge transfer state dynamics. Here we demonstrate that large donor-acceptor separations minimize back transfer from the charge transfer state to a low-lying triplet exciton 'drain' or the ground state by utilizing external pressure to modulate molecular spacing.

Nanostructured singlet fission photovoltaics subject to triplet-charge annihilation.

Singlet exciton fission is an efficient multiple-exciton generation process that is vulnerable to a characteristic loss process: triplet-charge annihilation. This loss process is characterized in singlet-fission photovoltaics and losses as high as 40% are observed in poorly designed devices. Techniques are demonstrated to improve charge extraction and reduce triplet-charge annihilation to negligible levels at short-circuit conditions.

External quantum efficiency above 100% in a singlet-exciton-fission-based organic photovoltaic cell.

Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum.

Dry lithography of large-area, thin-film organic semiconductors using frozen CO(2) resists.

To address the incompatibility of organic semiconductors with traditional photolithography, an inert, frozen CO(2) resist is demonstrated that forms an in situ shadow mask. Contact with a room-temperature micro-featured stamp is used to pattern the resist. After thin film deposition, the remaining CO(2) is sublimed to lift off unwanted material.

Charge transfer state versus hot exciton dissociation in polymer-fullerene blended solar cells.

We examine the significance of hot exciton dissociation in two archetypical polymer-fullerene blend solar cells. Rather than evolving through a bound charge transfer state, hot processes are proposed to convert excitons directly into free charges. But we find that the internal quantum yields of carrier photogeneration are similar for both excitons and direct excitation of charge transfer states.

LED-Based Optical Device for Chronic In Vivo Cerebral Blood Volume Measurement.

We demonstrate a reflectivity-based cerebral blood volume sensor comprised of surface-mount light-emitting diodes on a flexible substrate with integrated photodetectors in a form factor suitable for direct brain contact and chronic implantation. This reflectivity monitor is able to measure blood flow through the change of the surface reflectivity and, through this mechanism, detect the cerebral-blood-volume changes associated with epileptic seizures with a signal-to-noise (SNR) response of 42 dB. The device is tested in an in vivo model confirming its compatibility and sensitivity.