Ole Hagemann

Photochemical stability of π-conjugated polymers for polymer solar cells: a rule of thumb

A comparative photochemical stability study of a wide range of π-conjugated polymers relevant to polymer solar cells is presented. The behavior of each material has been investigated under simulated sunlight (1 sun, 1000 W m−2, AM 1.5G) and ambient atmosphere. Degradation was monitored during ageing combining UV-visible and infrared spectroscopies. From the comparison of the collected data, the influence of the polymer chemical structure on its stability has been discussed. General rules relative to the polymer structure–stability relationship are proposed.

Incorporation of ester groups into low band-gap diketopyrrolopyrrole containing polymers for solar cell applications

To increase the open circuit voltage (VOC) of polymer solar cells based on diketopyrrolopyrrole (DPP) containing polymers, the weakly electron-withdrawing thiophene-3,4-dicarboxylate unit was introduced into the polymer backbone. Two ester group functionalized DPP containing polymers, PCTDPP with a random structure and PDCTDPP with a regular structure, were designed and synthesized by the Stille coupling reaction. The resulting copolymers exhibit broad and strong absorption bands from 350 to 1000 nm with low optical band gaps below 1.40 eV. Through cyclic voltammetry measurements, it is found that regular PDCTDPPs HOMO energy level is 0.18 V lower than that of the corresponding random PCTDPP (−5.14 eV for PCTDPP and −5.32 eV for PDCTDPP). Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor were investigated. The PSC based on a PCTDPP:PCBM blend shows a power conversion efficiency (PCE) up to 3.52%, with a VOC of 0.66 V, a short circuit current (ISC) of 8.53 mA cm−2, and a fill factor (FF) of 0.63. For the PDCTDPP:PCBM blend, the highest VOC reaches a value of 0.84 V, and a final PCE (0.92%) is limited by the poor hole mobility of the active layer.

Low Band Gap Polymers for Roll-to-Roll Coated Organic Photovoltaics – Design, Synthesis and Characterization

Abstract In this paper we present the design and synthesis of 25 new low band gap polymers. The polymers were characterized by UV-vis spectroscopy which showed optical band gaps of 2.0–0.9 eV. The polymers which were soluble enough were applied in organic photovoltaics, both small area devices with a spin coated active layer and in large area modules where all layers including the active layer were either roll-to-roll coated or printed. These experiments showed that the design of polymers compatible with roll-toroll coating is not straightforward and that there are various issues such as donor/acceptor fitting within the polymer, side chains to ensure solubility and HOMO/LUMO level alignment with the acceptor (e.g. [60]PCBM) to take into consideration.

Syntheses of asymmetric porphyrins for photovoltaics

Porphyrins have attracted a lot of interest as potential light harvesting dyes in polymer solar cells due to a broad absorption range, wherein the porphyrin serves as a synthetic surrogate for chlorophyll. Asymmetric porphyrins are essential building blocks in one of our polymer solar cell projects. Synthesizing these asymmetric porphyrins on large scale, in good yield, with few scrambling byproducts and without the necessity for chromatographic workup proved to be a challenge. Here we present different approaches to the synthesis of asymmetric trans-A2B2-porphyrins and trans-AB2C-porphyrins on a large scale and detail problems associated with the synthetic work. The products were characterized using SEC, MALDI-TOF, UV-vis and NMR.

Light harvesting and energy transfer in large multidomain molecules

Light harvesting and energy transfer in two oligomer-dye assemblies has been investigated. In both cases the oligomer was a poly(terphenylenecyanovinylene) derivative while two different dyes was used, a porphyrin and an ionic dye. It is well known that the efficiency of solar cells consisting of a single homopolymer is limited. To increase overall efficiency different strategies have been used. One possible strategy aims at covalently linking different domains. With careful design, this type of assemblies is envisaged to show improved charge separation and charge transport properties. We have shown how photophysical measurements can be used to determine what happens to an exciton formed on any of the domains. From fluorescence and absorption measurements on the assemblies, along with model compounds, it was possible to quantify the number of excitons that are emitted (fluorescence), transferred between domains or lost in internal transfer processes. Both steady state and lifetime measurements were performed in solution and on solid films. The effect of acid was investigated in the cases of the oligomer-porphyrin assembly. We found that in solution the effect of acid was an increase in the time of energy transfer, probably due to acid induced structural change of the porphyrin moiety. It was possible to make LB-films of the ionic dye-assembly, which made it possible to investigate a monolayer of the assembly.