Bregt Verreet

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.

Microcrystalline Organic Thin‐Film Solar Cells

Microcrystalline organic films with tunable thickness are produced directly on an indium-tin-oxide substrate, by crystallizing a thin amorphous rubrene film followed by its use as a template for subsequent homoepitaxial growth. These films, with exciton diffusion lengths exceeding 200 nm, produce solar cells with increasing photocurrents at thicknesses up to 400 nm with a fill factor >65%, demonstrating significant potential for microcrystalline organic electronic devices.

The angular response of ultrathin film organic solar cells

We study the effect of oblique incident light on the optical interference effects and photocurrent of organic planar heterojunction solar cells. We find that the thin layers used in organic solar cells induce a complex light distribution within the device, further altered by incident angle. The responsivity can increase as a function of angle to values up to 15% larger than at normal incidence, and optical simulations are shown to match these trends. Furthermore, we show that the outdoor performance at different tilting angles is higher than expected, which improves the attractiveness of organic solar cells for building-integrated applications.

The characterization of chloroboron (III) subnaphthalocyanine thin films and their application as a donor material for organic solar cells

Chloroboron (III) subnaphthalocyanine (SubNc) films have been characterized by ellipsometry, absorption, photoluminescence measurements, and atomic force microscopy. The films strongly absorb red light, as the extinction coefficient k peaks at 1.4 at a wavelength of 686 nm. Planar bilayer heterojunctions with fullerene (C60) on top of SubNc are measured under AM 1.5 simulated illumination at various light intensities, leading to an open-circuit voltage (Voc) of 790 mV and a power conversion efficiency of 2.5%. The external and internal quantum efficiencies peaked at 36% and 70%, respectively. The combination of a strong red absorption and high Voc make SubNc an interesting material for organic solar cells, in particular for tandem cells.

Organic complementary oscillators with stage-delays below 1 μs

Recently, complex circuits of organic thin-film transistors have been shown. The use of complementary logic can significantly ease the design of large integrated circuits. However, the performance of complementary logic in organic thin-film technology has not been able to equivale that of unipolar logic, due to the difficulty to densely integrate and simultaneously optimize p-type and n-type transistors on a single substrate. Here, we develop an optimized complementary process for C60 n-type and pentacene p-type transistors, both having bottom-gate bottom-contact geometry. Using this complementary technology, we show ring-oscillators with a stage-delay below 1 μs at a supply-voltage of 20 V.

Improved cathode buffer layer to decrease exciton recombination in organic planar heterojunction solar cells

We show that an advanced cathode buffer design, consisting of bathocuproine/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag, increases the short-circuit current of organic planar heterojunction cells and reduces the J-V slope at reverse voltages. We study the physical origin of these effects by measuring reflectivity, voltage dependent external quantum efficiency, and voltage dependent photoluminescence. Our findings suggest that the observed effects are mainly associated with a voltage dependent polaron-induced exciton quenching in the C60 layer. Finally, this improved cathode buffer design is applied to a diindeno[1,2,3-cd:1′,2′,3′-lm]perylene/C70 based cell, leading to a considerable planar heterojunction efficiency of 5.7%.

Accurate spectral response measurements of a complementary absorbing organic tandem cell with fill factor exceeding the subcells

Single heterojunction organic photovoltaic cells based on co-evaporated donor–acceptor layers with power conversion efficiencies (η) above 5.5% are demonstrated, using either high (1.8 eV) or low (1.4 eV) optical gap materials. The high energy absorbing cell utilizes a high fullerene-C70 content, in combination with a high mobility amorphous donor, while the low energy absorbing cell consists of a donor–acceptor molecule paired with C60 as the acceptor. The integration of the two cells in an optimized tandem configuration leads to η =7.2%, verified by external quantum efficiency measurements of the subcells. Notably, the fill-factor of the tandem stack is higher than either one of the sub-cells.