Ulrich Lemmer

Terahertz emission from lateral photo-Dember currents.

The photo-Dember effect is known as source of impulsive THz radiation after excitation with femtosecond optical pulses. The origin of the emission is the ultrafast separation of electron and holes in strong carrier gradients due to different diffusion coefficients. For a simple semiconductor surface the time dependent polarization is oriented perpendicular to the excited surface which complicates efficient out coupling of THz radiation. We investigate a new scheme for generating strong carrier gradients parallel to the surface. In that case the photo-Dember currents are oriented parallel to the surface and the generated THz radiation can be easily out coupled. This concept can be scaled up so that multiple phase coherent photo-Dember currents contribute to the THz emission. These passive THz emitters reach electric field amplitudes comparable to high-efficiency externally biased photoconductive emitters.

Femtosecond dynamics of stimulated emission and photoinduced absorption in a PPP-type ladder polymer

Abstract The dynamics of optical excitations in a PPP-type ladder polymer have been investigated by means of femtosecond pump-probe experiments. Stimulated emission (SE) is observed around the S 1 → S 0 transition while photoinduced absorption (PA) dominates the differential transmission at lower energies. The stimulated emission is due to singlet exciton recombination. Comparative studies of the temporal behaviour of the SE and the PA demonstrate that the SE and the PA signals originate from different species. Our results provide evidence that the PA is due to spatially separated electron-hole pairs (indirect excitons).

Single-Molecule Spectroscopy on a Ladder-Type Conjugated Polymer: Electron–Phonon Coupling and Spectral Diffusion

We employ low-temperature single-molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl-substituted ladder-type poly(para-phenylene) (MeLPPP) to investigate the electron-phonon coupling to low-energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron-phonon coupling to low-frequency vibrations of the surrounding matrix of the chromophores, and that low-energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line-broadening mechanism of the zero-phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material.

Two Step Charge Carrier Generation in a Ladder-Type Conjugated Polymer

Two pulse femtosecond photocurrent experiments are performed on a conjugated polymer photodiode. With a photon energy of 3.1eV a first laser pulse excites singlet excitons. Photons absorbed from a time-delayed second pulse with a photon energy of 1.26eV lead to the excitation of higher lying states. The high dissociation rate for these states leads to an efficient two step carrier generation.

Intense terahertz generation based on the photo-Dember effect

We demonstrate a new scheme for generating THz radiation based on the photo-Dember effect in lateral geometry. By micro-structuring a semiconductor surface we achieve strongly enhanced THz emission comparable to high-efficiency externally biased photoconductive emitters.

Inkjet Printed Perovskite Photovoltaics

In this contribution, we report on our recent advances on non-contact inkjet-printed perovskite photovoltaics. Digital inkjet printing offers rapid deposition of perovskite absorber layers for large area perovskite absorber layers as well as perovskite solar cells of arbitrary shape. By adjusting the drop spacing of the inkjet printer cartridge, we demonstrate excellent control of the perovskite layer formation and perovskite layer thickness. Our digitally inkjet-printed triple cation perovskite solar cells with 10% cesium in a mixed formamidinium/ methylammonium lead iodide/bromide composite reach initial efficiencies as high as 14.0%. Moreover, a continuous power output at constant voltage, resulting in a power conversion efficiency of 12.9% is demonstrated, representing a major improvement from previously reported results.

Hybrid electronic systems powered by printed thermoelectric generators (Conference Presentation)

Thermoelectric generators (TEGs) transform heat to electricity without any movable parts. These devices are considered to be an important means of energy harvesting for wearables and sensor nodes for the Internet-of-Things (IoT). Organic semiconductors have recently demonstrated ZT-values approaching the same order of magnitude of those of the best established inorganic materials. Conjugated polymers as well as printable inorganic nanomaterials offer the unique advantage of being processable on printing machines. This opens a pathway for the fabrication of thermoelectric generators with unprecedented low costs thus enabling mass consumer applications. We have developed novel printable PEDOT formulations and a device layout which allow for a roll-to-roll printing process on ultrathin plastic foils. The TEGs are then subsequently fabricated by an automated folding process which allows to adapt the geometry of the devices such that the desired thermal resistance is optimum for the specific thermal boundary condition. Using this approach in combination with designed low power electronics forms the basis for several wireless sensor nodes.

Soft lithography of 3D polymeric lasers and cavities

We demonstrate the realization of 3D photonic lasers and cavities in polymeric materials using soft lithography and characterize them experimentally. High-Q cavities and low-threshold lasers based on SU-8 and Solgel are demonstrated.

Multipass inkjet printing of methylammonium lead iodide for planar perovskite solar cells(Conference Presentation)

We show inkjet printed state-of-the-art perovskite solar cells with efficiencies of up to 12% which is an important step towards fully printed large scale production of photovoltaic perovskite devices. In comparison, the spin-coated reference achieves 13% efficiency. In both cases, the solar cell absorbers are prepared using a one-step process on a TiO2 compact layer without mesoporous intermediate layer as electron transport material and spiroMeOTAD as hole transport material. Moreover, we show that controlling printing parameters, like drop spacing and size, is essential to optimizing the final perovskite performance. Whereas parameters were initially controlled to be consistent with a final layer thicknesses known from literature, subsequent processes were aimed at also controlling crystallinity and roughness. To demonstrate the homogeneity of the printed devices, light beam induced current measurements (LBIC) were made. To evaluate the quality of the perovskite layer and the charge transfer efficiency in the device, time resolved photoluminescence measurements were conducted on the perovskite with and without electrical transport layers. Light soaking effects were also investigated and evaluated. Important differences between printed and spin-coated devices will be outlined, as well as other relevant parameters to optimize printed device performance.