Richard R. Lunt

Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching

We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuum-deposited polycrystalline films. The measurement of the singlet diffusion lengths is found to be in agreement with diffusion by Forster transfer, whereas triplet diffusion occurs primarily via Dexter transfer.

Energy level modification in lead sulfide quantum dot thin films through ligand exchange.

The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto underutilized means of control over the absolute energy levels of QDs within electronic devices. Here, we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices.

Relationship between Crystalline Order and Exciton Diffusion Length in Molecular Organic Semiconductors

One of the most fundamental properties of both organic and inorganic semiconductors is charge mobility. It has been unambiguously shown that the mobility in both of these materials systems is strongly linked to the degree of long range order—thatis,moreextendedcrystallinityleadstoalargercharge mobility, which ultimately determines such extrinsic properties as seriesresistance andresponse tocurrentand optical pulses. An equally fundamental property for organic semiconductors is the molecular excited state-, or exciton-, diffusion length which characterizes energy transport within these more correlated solids. While it has been predicted that exciton transport should also be linked to the extent of crystalline order, to our knowledge nosuchdependencehasyetbeenestablished.Here,weaccurately measure the exciton diffusion length of the archetypal organic semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and clearly show its relationship to thin-film crystal morphology. As in the case of charge mobility, we show that the exciton transport diffusion length is a monotonic function of the extent of crystalline order. This study provides insight into the control and ultimately the tunability of the exciton diffusion lengthinorganic systems, whichiscrucial forthemanagementof energy transport in a wide range of important organic electronic devices.

Open circuit voltage enhancement due to reduced dark current in small molecule photovoltaic cells

We demonstrate high open circuit voltage photovoltaic cells achieved by reducing the electron leakage current through the introduction of both organic and inorganic electron blocking layers between the donor layer and the anode contact. As an example, the blocking layers reduce the dark current in tin (II) phthalocyanine (SnPc)∕C60 solar cells with response across the visible and near infrared spectral region up to a wavelength of 1000nm, is decreased by two orders of magnitude compared to cells lacking the layers, resulting in a doubling of the open circuit voltage. The structure: indium tin oxide/electron blocker/SnPc (100A)∕C60 (400A)/bathocuproine (100A)∕Al, has a power conversion efficiency of (2.1±0.1)% at 1sun, standard AM1.5G solar illumination. This work demonstrates the importance of reducing dark current to achieve high organic thin film photovoltaic cell efficiencies.

Improved Current Extraction from ZnO/PbS Quantum Dot Heterojunction Photovoltaics Using a MoO3 Interfacial Layer

The ability to engineer interfacial energy offsets in photovoltaic devices is one of the keys to their optimization. Here, we demonstrate that improvements in power conversion efficiency may be attained for ZnO/PbS heterojunction quantum dot photovoltaics through the incorporation of a MoO(3) interlayer between the PbS colloidal quantum dot film and the top-contact anode. Through a combination of current-voltage characterization, circuit modeling, Mott-Schottky analysis, and external quantum efficiency measurements performed with bottom- and top-illumination, these enhancements are shown to stem from the elimination of a reverse-bias Schottky diode present at the PbS/anode interface. The incorporation of the high-work-function MoO(3) layer pins the Fermi level of the top contact, effectively decoupling the device performance from the work function of the anode and resulting in a high open-circuit voltage (0.59 ± 0.01 V) for a range of different anode materials. Corresponding increases in short-circuit current and fill factor enable 1.5-fold, 2.3-fold, and 4.5-fold enhancements in photovoltaic device efficiency for gold, silver, and ITO anodes, respectively, and result in a power conversion efficiency of 3.5 ± 0.4% for a device employing a gold anode.

Direct Monolithic Integration of Organic Photovoltaic Circuits on Unmodified Paper

There has been signifi cant recent interest in integrating electronics into low-cost paper substrates, including transistors, storage devices, displays, and circuitry. [ 1–4 ] Paper-based photovoltaics (PVs) could serve as an “on-chip” power source for these paper electronics, and also create attractive new paradigms for solar power distribution, including seamless integration into ubiquitous formats such as window shades, wall coverings, apparel, and documents. Module installation may be as simple as cutting paper to size with scissors or tearing it by hand and then stapling it to roof structures or gluing it onto walls. Moreover, paper is ∼ 1000 times less expensive ( ∼ 0.01

Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications

· m − 2 ) than traditional glass substrates ( ∼ 10

Broad Spectral Response Using Carbon Nanotube/Organic Semiconductor/C60 Photodetectors

· m − 2 ) [ 5 , 6 ]

Toward Efficient Carbon Nanotube/P3HT Solar Cells: Active Layer Morphology, Electrical, and Optical Properties

We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.

Practical Roadmap and Limits to Nanostructured Photovoltaics

We demonstrate that photogenerated excitons in semiconducting carbon nanotubes (CNTs) can be efficiently dissociated by forming a planar heterojunction between CNTs wrapped in semiconducting polymers and the electron acceptor, C(60). Illumination of the CNTs at their near-infrared optical band gap results in the generation of a short-circuit photocurrent with peak external and internal quantum efficiencies of 2.3% and 44%, respectively. Using soft CNT-hybrid materials systems combining semiconducting small molecules and polymers, we have fabricated broad-band photodetectors with a specific detectivity >10(10) cm Hz(1/2) W(1-) from lambda = 400 to 1450 nm and a response time of tau = 7.2 +/- 0.2 ns.