Ingmar Bruder

Enhancing Thermal Stability and Lifetime of Solid-State Dye-Sensitized Solar Cells via Molecular Engineering of the Hole-Transporting Material Spiro-OMeTAD

Thermal stability of hybrid solar cells containing spiro-OMeTAD as hole-transporting layer is investigated. It is demonstrated that fully symmetrical spiro-OMeTAD is prone to crystallization, and growth of large crystalline domains in the hole-transporting layer is one of the causes of solar cell degradation at elevated temperatures, as crystallization of the material inside the pores or on the interface affects the contact between the absorber and the hole transport. Suppression of the crystal growth in the hole-transporting layer is demonstrated to be a viable tactic to achieve a significant increase in the solar cell resistance to thermal stress and improve the overall lifetime of the device. Findings described in this publication could be applicable to hybrid solar cell research as a number of well-performing architectures rely heavily upon doped spiro-OMeTAD as hole-transporting material.

Branched methoxydiphenylamine-substituted fluorene derivatives as hole transporting materials for high-performance perovskite solar cells

Small-molecule hole transporting materials based on methoxydiphenylamine-substituted fluorene fragments were synthesized and incorporated into a perovskite solar cell, which displayed a power conversion efficiency of up to 19.96%, one of the highest conversion efficiencies reported. The investigated hole transporting materials were synthesized in two steps from commercially available and relatively inexpensive starting reagents, resulting in up to fivefold cost reduction of the final product compared with spiro-OMeTAD. Electro-optical and thermoanalytical measurements such as UV/Vis, thin-film conductivity, hole mobility, DSC, TGA, ionization potential and current voltage scans of the full perovskite solar cells have been carried out to characterize the new materials.

Double Donor-Thiophene Dendron-Perylene Monoimide: Efficient Light-Harvesting Metal-Free Chromophore for Solid-State Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) based on a stable largeband nanostructured semiconductor, such as titanium dioxide, are low cost and easily processable alternatives to conventional silicon wafers, and as such have lately drawn much attention. In particular, the solid-state DSCs show great potential owing to their increased stability compared to liquid DSCs. The reason for this stability is the exchange of the liquid electrolyte, which often bears the problem of leakage and electrode corrosion, for a solid hole-conducting material, mainly 2,2’,7,7’-tetrakis(N,N-para-dimethoxyphenylamino)-9,9’-spirobifluorene (spiro-MeOTAD). However, compared to the parent liquid DSCs, solid-state DSCs have shown much lower efficiencies. Whilst ruthenium-based sensitizers, and now a first porphyrin sensitizer, have shown efficiencies up to approximately 11 % in liquid cells, solidstate DSCs only reach values of up to 6 %. Most of the more efficient sensitizers are ruthenium-based, which have drawbacks such as cost, sustainability, and limited ease of band-gap manipulation. One very stable and metal-free alternative are sensitizers based on perylene monoimides, which are known for their excellent chemical, photochemical, and thermal stability as well as high absorptivity and acceptor ability. Another outstanding class of chromophores are thiophenes, in particular oligothiophenes, for their highly variable optical properties according to their architecture, extraordinary charge transport properties, and extinction. Both perylenes and thiophenes have found wide application in optoelectronic devices. Herein, we present a donor–acceptor perylene monoimide with a branched terthiophene spacer group and a triphenylamine donor moiety (1 a, Scheme 1) as well as a naphthalene analogue (1 b, Scheme 1). As reported by Thomas et al. and Fischer et al., the combination of a triphenylamine donor and a branched oligothiophene spacer in combination with a 2-cyanoacrylate acceptor gave good efficiencies of up to 6.15 % and 6.8 % in liquid DSCs and up to 2.6 % in a solid-state DSC. Furthermore, three moieties—triphenylamine, oligothiophene, and perylene monoimide—have recently caused a stir in a p-DSC (NiO), showing a sevenfold increase in energy conversion efficiencies compared to preceding sensitizers. However, we have designed our sensitizers for n-DSCs (TiO2), in which the perylene sensitizer 1 a in particular shows an outstanding efficiency of 3.8 % under 1.5 AM illumination (1 sun). To the best of our knowledge, this is an unprecedented performance for a perylene sensitizer, the best perylene sensitizer for solid-state DSCs so far being ID176 with an efficiency of 3.2 %. Both compounds were prepared by the initial introduction of the terthiophene spacer group by Suzuki coupling with the brominated perylene (or naphthalene) imide, successive Suzuki coupling with the triphenylamine donor, and finally saponification and imidization with glycine to yield the final product (Scheme 1). As described above, both sensitizers consist of an acceptor unit with a carboxylic acid anchor in the imide structure, a branched terthiophene (a a connection and a b connection of the thiophene units) and a triphenylamine donor. In [a] H. Wonneberger, Dr. C. Li, Prof. Dr. K. M llen Max-Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz (Germany) Fax: (+49) 6131-379-100 E-mail : lichen@mpip-mainz.mpg.de muellen@mpip-mainz.mpg.de [b] Dr. C.-Q. Ma, Prof. Dr. P. B uerle Institute of Organic Chemistry II and Advanced Materials University of Ulm Albert-Einstein-Allee 11, 89081 Ulm (Germany) [c] Dr. N. Pschirer, Dr. I. Bruder, Dr. J. Schçneboom, Dr. P. Erk BASF SE 67056 Ludwigshafen (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201000895.

Enhanced electronic contacts in SnO2-dye-P3HT based solid state dye sensitized solar cells.

We present an investigation on the optimisation of solid-state dye sensitized solar cells (SDSCs) comprising mesoporous tin oxide photoanodes infiltrated with poly(3-hexylthiophene-2,5-diyl) (P3HT) hole conductor and sensitized with an organic dye. We chose both the SnO(2) and P3HT for their high charge carrier mobilities and conductivities, but as a result preclude conventional device configurations because of high leakage current and low shunt-resistance. To minimize the hole leakage current through the FTO anode, we employed a double compact layer structure, and to minimize electron leakage current at the silver cathode, we developed a protocol for depositing an optimal P3HT capping layer. After optimisation of cell fabrication, the electron lifetime is increased considerably and the solar cells exhibited simulated AM1.5 full sun solar power conversion efficiencies in excess of 1%.

Molecular engineering of the hole-transporting material spiro-OMeTAD via manipulation of alkyl groups

Aliphatic substituent effects on the HOMO energy levels and the ability to transport charge and form stable molecular glasses of systematically modified spiro-OMeTAD analogues were investigated. It was determined that the thermal properties, energy levels and hole mobility values are dependent on the number of alkyl substituents and their position in the investigated spirobifluorene-based hole transporting materials (HTMs). The charge mobility of HTM3 possessing a seemingly insignificant m-methyl group in the diphenylamino moieties is the highest with a value of 2.8 × 10−3 cm2 V−1 s−1 at 6.4 × 105 V cm−1 field strength. It was found that moving one methoxy group into the m-position in the diphenylamino fragment ensured a stable amorphous phase of HTM1. Moreover, the long-term stability of a solid state dye-sensitized solar cell (ssDSSC) device comprising HTM1 was significantly enhanced over a cell with spiro-OMeTAD, in lifetime tests. The findings described in this publication could be applicable to hybrid solar cell research as a number of well-performing architectures rely heavily upon doped spiro-OMeTAD as a HTM.

Relationship between measurement conditions and energy levels in the organic dyes used in dye-sensitized solar cells

The energy levels of new metal-free organic dyes for dye-sensitized solar cells have been investigated by the photoemission in air, UV-Vis absorption and cyclic voltammetry methods in the solutions of the dye molecules, in films of the pure dyes and in the dyes adsorbed on nanoporous TiO2. Significant differences of the energy levels have been found depending on the dye environment. For the best level of tuning in a solar cell, the energy levels are to be determined for the dyes adsorbed on the TiO2 surface. The absorbed photon conversion to current efficiency (APCE) of the solar cells was evaluated and compared with the incident photon quantum efficiency (IPCE). The results obtained show that the IPCE is dependent on the light quanta energy and reaches a maximum value when the light quanta energy is about 0.3 eV higher than the light absorption threshold.

Focus-Induced Photoresponse: a novel way to measure distances with photodetectors

We present the Focus-Induced Photoresponse (FIP) technique, a novel approach to optical distance measurement. It takes advantage of a universally-observed phenomenon in photodetector devices, an irradiance-dependent responsivity. This means that the output from a sensor is not only dependent on the total flux of incident photons, but also on the size of the area in which they fall. If probe light from an object is cast on the detector through a lens, the sensor response depends on how far in or out of focus the object is. We call this the FIP effect. Here we demonstrate how to use the FIP effect to measure the distance to that object. We show that the FIP technique works with different sensor types and materials, as well as visible and near infrared light. The FIP technique operates on a working principle, which is fundamentally different from all established distance measurement methods and hence offers a way to overcome some of their limitations. FIP enables fast optical distance measurements with a simple single-pixel detector layout and minimal computational power. It allows for measurements that are robust to ambient light even outside the wavelength range accessible with silicon.

Focus-induced photoresponse: a fundamentally novel approach to optical distance measurements (Conference Presentation)

Focus-Induced Photoresponse (FIP) is a patented monocular technology for optical distance measurements [1]. It relies on physical phenomena which are fundamentally different from established technologies such as time-of-flight, stereo vision, structured light or systems based on image processing. In this presentation, the underlying principles of the technology as well as application examples are introduced. nFIP exploits the nonlinear transient photoresponse of various organic as well as inorganic semiconductors when exposed to optical radiation. When a light-emitting (or reflecting) object moves in and out of focus, the size of the image that it creates on a sensor surface determines the magnitude of the photoresponse. As the focal point shifts with the distance between the collecting lens and the object, the sensor response yields a unique signature for every distance.nThe device layout can hence be simple: the main components are modulated light sources, a lens and a non-pixelated sensor. Due to the unstructured sensor, resolution is not restricted by pixel size. FIP does not require large computational power as neither image processing nor stereo vision is required. By proper choice of optics and sensor type, the system can be adapted to any measuring task. We have successfully demonstrated functionality for wavelengths from visible light to IR and for distances up to 100 m.n [1] Bruder et al. (US 9,001,029 B2) DETECTOR FOR OPTICALLY DETECTING AT LEAST ONE OBJECT.