David Cheyns

8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer

In order to increase the power conversion efficiency of organic solar cells, their absorption spectrum should be broadened while maintaining efficient exciton harvesting. This requires the use of multiple complementary absorbers, usually incorporated in tandem cells or in cascaded exciton-dissociating heterojunctions. Here we present a simple three-layer architecture comprising two non-fullerene acceptors and a donor, in which an energy-relay cascade enables an efficient two-step exciton dissociation process. Excitons generated in the remote wide-bandgap acceptor are transferred by long-range Förster energy transfer to the smaller-bandgap acceptor, and subsequently dissociate at the donor interface. The photocurrent originates from all three complementary absorbing materials, resulting in a quantum efficiency above 75% between 400 and 720 nm. With an open-circuit voltage close to 1 V, this leads to a remarkable power conversion efficiency of 8.4%. These results confirm that multilayer cascade structures are a promising alternative to conventional donor-fullerene organic solar cells.

Strategies for Increasing the Efficiency of Heterojunction Organic Solar Cells: Material Selection and Device Architecture

Thin-film blends or bilayers of donor- and acceptor-type organic semiconductors form the core of heterojunction organic photovoltaic cells. Researchers measure the quality of photovoltaic cells based on their power conversion efficiency, the ratio of the electrical power that can be generated versus the power of incident solar radiation. The efficiency of organic solar cells has increased steadily in the last decade, currently reaching up to 6%. Understanding and combating the various loss mechanisms that occur in processes from optical excitation to charge collection should lead to efficiencies on the order of 10% in the near future. In organic heterojunction solar cells, the generation of photocurrent is a cascade of four steps: generation of excitons (electrically neutral bound electron-hole pairs) by photon absorption, diffusion of excitons to the heterojunction, dissociation of the excitons into free charge carriers, and transport of these carriers to the contacts. In this Account, we review our recent contributions to the understanding of the mechanisms that govern these steps. Starting from archetype donor-acceptor systems of planar small-molecule heterojunctions and solution-processed bulk heterojunctions, we outline our search for alternative materials and device architectures. We show that non-planar phthalocynanines have appealing absorption characteristics but also have reduced charge carrier transport. As a result, the donor layer needs to be ultrathin, and all layers of the device have to be tuned to account for optical interference effects. Using these optimization techniques, we illustrate cells with 3.1% efficiency for the non-planar chloroboron subphthalocyanine donor. Molecules offering a better compromise between absorption and carrier mobility should allow for further improvements. We also propose a method for increasing the exciton diffusion length by converting singlet excitons into long-lived triplets. By doping a polymer with a phosphorescent molecule, we demonstrate an increase in the exciton diffusion length of a polymer from 4 to 9 nm. If researchers can identify suitable phosphorescent dopants, this method could be employed with other materials. The carrier transport from the junction to the contacts is markedly different for a bulk heterojunction cell than for planar junction cells. Unlike for bulk heterojunction cells, the open-circuit voltage of planar-junction cells is independent of the contact work functions, as a consequence of the balance of drift and diffusion currents in these systems. This understanding helps to guide the development of new materials (particularly donor materials) that can further boost the efficiency of single-junction cells to 10%. With multijunction architectures, we expect that efficiencies of 12-16% could be attained, at which point organic photovoltaic cells could become an important renewable energy source.

Solution-Processed MoO3 Thin Films As a Hole-Injection Layer for Organic Solar Cells

We report on a sol-gel-based technique to fabricate MoO(3) thin films as a hole-injection layer for solution-processed or thermally evaporated organic solar cells. The solution-processed MoO(3) (sMoO(3)) films are demonstrated to have equal performance to hole-injection layers composed of either PEDOT:PSS or thermally evaporated MoO(3) (eMoO(3)), and the annealing temperature at which the sol-gel layer begins to work is consistent with the thermodynamic analysis of the process. Finally, the shelf lifetime of devices made with the sMoO(3) is similar to equivalent devices prepared with a eMoO(3) hole-injection layer.

Design of Transparent Anodes for Resonant Cavity Enhanced Light Harvesting in Organic Solar Cells

Organic solar cells offer an attractive approach to low-cost solar energy conversion, due to a combination of abundant materials and high throughput fabrication processes.[1] However, organic semiconductors suffer from short exciton diffusion lengths and low charge carrier mobility, which necessitates the use of thin photoactive films and intercalated networks of donor and acceptor molecules in a so-called bulk heterojunction.[2,3] On the other hand, thinner photoactive regions in organic solar cells cause a reduction in optical absorption. This leads to the well known tradeoff in organic solar cells between internal quantum efficiency (IQE) and absorption efficiency. Several approaches have been previously used to enhance absorption in organic solar cells,[4] including plasmonics,[5] photonic crystal approaches[6] and external coatings.[7] Moreover, in recent years significant research efforts have been directed towards replacing the tin-doped indium oxide (ITO) transparent electrode in optoelectronic devices, owing to its poor mechanical flexibility, the necessary performance enhancing thermal treatment unsuitable for low temperature substrates, as well as the increasing cost of indium.[8] Potential alternative transparent conductors include high conductivity polymers,[9] unpatterned metal films,[10] patterned metal grids,[11] random metal nanowire meshes,[12] graphene,[13] and random carbon nanotube meshes.[14] In addition, several groups have shown promising results for tri-layer dielectricmetal-dielectric electrodes.[15–18] Here, building on the latter approach, a tri-layer electrode is proposed composed of a thin film of silver (Ag) sandwiched between two layers of molybdenum trioxide (MoO3). The MoO3/Ag/MoO3 transparent electrode is ITO-free, compatible with low-temperature substrates, and capable of alleviating the absorption-IQE trade-off by creating a resonant optical cavity to coherently trap light in the photoactive absorber. The thin Ag film dominates the lateral conductive properties of the electrode and therefore provides a means to obtain a sheet resistance below 10 Ω per square. Silver is known to prefer 3D island growth and therefore the percolation threshold of Ag layers

Organic tandem solar cells with complementary absorbing layers and a high open-circuit voltage

Tandem organic solar cells with peak conversion efficiencies (η) of 5.15% are demonstrated. This is achieved by stacking two different planar heterojunction devices, each with a high open-circuit voltage (Voc). The phthalocyanine based donor materials in the employed subcells possess complementary absorption, a quality of critical importance to optimize photocurrent in a series connected tandem cell. The tandem structure produces Voc values of nearly 2 V, while fill factor remains above 60%. The measured η corresponds to a 40% increase compared to η of the optimal single cells.

Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells

The use of non-fullerene acceptors in organic photovoltaic (OPV) devices could lead to enhanced efficiencies due to increased open-circuit voltage (VOC) and improved absorption of solar light. Here we systematically investigate planar heterojunction devices comprising peripherally substituted subphthalocyanines as acceptors and correlate the device performance with the heterojunction energetics. As a result of a balance between VOC and the photocurrent, tuning of the interface energy gap is necessary to optimize the power conversion efficiency in these devices. In addition, we explore the role of the charge transport layers in the device architecture. It is found that non-fullerene acceptors require adjusted buffer layers with aligned electron transport levels to enable efficient charge extraction, while the insertion of an exciton-blocking layer at the anode interface further boosts photocurrent generation. These adjustments result in a planar-heterojunction OPV device with an efficiency of 6.9% and a VOC above 1 V.

Enhanced photocurrent and open-circuit voltage in a 3-layer cascade organic solar cell

We demonstrate a cascade architecture for organic solar cells with two planar donor/acceptor (DA) heterojunctions operating in series. In a 3-layered structure, subphthalocyanine (SubPc) acts as an ambipolar interlayer between a tetracene (Tc) donor and a C60 acceptor. The Tc/SubPc and SubPc/C60 interfaces are both able to contribute to the photocurrent, which results in a short-circuit current in the 3-layer cascade cell larger than in any of the constituent bi-layer DA combinations. Furthermore, the open-circuit voltage is increased due to reduced recombination losses at the DA interface.

Ambipolar injection in a submicron-channel light-emitting tetracene transistor with distinct source and drain contacts

We report on organic light-emitting transistors with a submicron-channel length, gold source, and calcium drain contacts. The respective contact metals allow efficient injection of holes and electrons in the tetracene channel material. Transistor characteristics were measured in parallel with electroluminescence being recorded by a digital camera focused on the transistor channel. In the case of submicron-channel lengths, the transistor source-drain current at higher gate voltages was determined by the source-drain voltage. At larger channel lengths, the source-drain current was limited by the injection of electrons from the calcium contact, as hole ejection to this contact was fully blocked. The hole blocking is explained in terms of a chemical reaction occurring at the Ca/tetracene interface.

Pentacene devices and logic gates fabricated by organic vapor phase deposition

An organic vapor phase deposition (OVPD) tool has been developed and optimized for the deposition of pentacene thin films. Pentacene is grown with a good thickness uniformity, a good material consumption efficiency, and deposition rates up to 9.5 A/s. Top-contact transistors based on OVPD-grown pentacene show high mobilities (up to 1.35 cm(2)/V s) and excellent characteristics, even at high deposition rates. Elementary circuit blocks have also been produced using an OVPD-deposited pentacene film. A five-stage ring oscillator features a stage delay of 2.7 mu s at a supply voltage of 22 V. (c) 2006 American Institute of Physics.

Numerical simulation of tetracene light-emitting transistors: A detailed balance of exciton processes

We assess the possibility to use an ambipolar organic light-emitting transistor structure as gain medium for an electrically pumped laser. Singlet and triplet continuity equations are solved together with Poissons and drift-diffusion equations in two dimensions. The solution allows for a detailed balance between the exciton decay, quenching and generation mechanisms. Simulations of a tetracene light-emitting transistor show that triplets are most dominant in quenching singlets. Singlet–triplet quenching can ultimately prevent pure tetracene crystals or films—when provided with a realistic optical feedback structure, to reach the threshold for stimulated emission.