Electrochemical analysis

Electrochemical measurements have been carried out for several organic compounds allowing to estimate HOMO and LUMO levels as well as giving insight to the optical properties of charged species formed at certain bias potentials.

Cyclic voltammetry

An Autolab PGSTAT 302N potentiostat/galvanostat was used for cyclic voltammetry analysis. Measurements were performed in a three electrode setup using a Pt counter electrode, a Ag/AgCl reference electrode and an 0.1 M solution of tetrabutylammoni-

umhexauorophosphate in acetonitrile as electrolyte. As working electrode, thin organic lms coated on ITO glass were used. HOMO and LUMO levels were calculated with respect to a reference measurement of a0.01 Mferrocene solution.

Spectral electrochemistry

Spectral electrochemistry measurements were conducted using a quartz glass equipped sample chamber (Autocell). The chamber holds a three electrode setup and is equipped with quartz windows on both sides - a semi-transparent Pt mesh as counter electrode and a dened path length of the chamber facilitates detailed electrochemical measure- ments. Dicloromethan (DCM) containing 0.3 mg/mL tetrabutylammonim perchlorate

(TBAP) was used as solvent for the experiments. The calibration of the Ag wire elec- trode against the vacuum potential has been obtained using a 5 mM ferrocene solution in acetonitrile. Potentials desired for the formation of charged species in solution are applied to the mesh grid electrode and uv-vis spectra are obtained using a Shimamadsu UV-vis-NIR absorption spectrometer.

Assembly of Discotic Molecules

In this chapter the intermolecular packing and supramolecular organization of small molecule donor-acceptor heterojunction systems is analyzed. The inuence of chemical substitutions and processing conditions on photophysical properties and performance of photovoltaic devices is analyzed and conclusions for an optimized donor-acceptor design allowing for increased charge generation and charge extraction are drawn. For all studies HBC molecules specically synthesized by the Professor Müllen group were used as donor materials.

In a rst study solution processable derivatives of the HBC donor molecules are blended with perylene-diimide (PDI) and bulk heterojunction solar cells are fabricated. Film morphology, supramolecular packing and photophysical properties are analyzed and correlated with the performance of photovoltaic devices. Possible recombination mech- anisms are discussed based on studies analyzing the decay dynamics of charge carriers. Several results of this section have been published in the paper: Discotic materials for organic solar cells: Eects of chemical structure on assembly and performance [54]. The results are also correlated with a separate study on the supramolecular packing of HBC and PDI molecules based on X-ray analysis of thin blend lms [53].

Separately, the properties of sublimated HBCs are investigated in bi-layered and co- evaporated photovoltaic devices. Appealing properties of the un-substituted HBC molecule include a long exciton diusion length and high thermal stability. Dierent acceptor molecules - PDI and PCBM processed form solution and HBC-6F sublimated from the vapor phase - are compared as acceptor compounds in bi-layered devices.

4.1 HBC alkyl side chain modications

HBC and PDI seem to be an appealing donor-acceptor couple for solar light harvesting, as the molecules show a complementary absorption spectrum and suitable energy levels. PDI as electron conducting acceptor is known for its strong absorbance and the donor

4.1 HBC alkyl side chain modications

material HBC has the tendency to self assemble in a columnar fashion facilitating the formation of 1D molecular wires (as further described in Section 2.3.3). Exceptionally high charge carrier mobility values have been shown repeatedly for several derivatives of both classes of molecules when aligned in a suitable manner [20, 118]. In the year 2001 Schmidt-Mende and al. showed a remarkably high IPCE for bulk heterojunction devices based on the molecules HBC-Ph-C12 and PDI [22]. More recent studies on this

donor-acceptor system have shown, that the supramolecular organization of the blend lms plays a detrimental role and that slight changes in molecule design and processing conditions may strongly inuence lm morphology, charge carrier percolation pathways and thus also the photophysical properties of the blend lms [86, 87].

The aim of this Chapter is to gain further insight to morphology as well as exciton separation and charge extraction mechanisms in the active layer of blended discotic molecules. The interplay of the donor and acceptor compounds and the inuence of their supramolecular assembly on the photovoltaic performance is studied. HBC donor molecules are altered in shape by the attachment of a variety of residual groups. The impact of the structural changes are analyzed in blend devices with the common ac- ceptor PDI. Similar approaches of residue modications allowed to draw crucial conclu- sions on the structure-performance relationship in the past and helped severely for the conceptualization of novel and promising materials for OPV devices in several recent studies [79, 176]. This conceptual approach seems promising also for small molecules as has been shown recently [177].

Alkyl chains with dierent lengths (6, 8, 12 and 16 carbon atoms), a triple bond linker between HBC core and residual phenyl group and a swallow tailed dialkylphenyl chain are attached to the donor core molecule. Changes in both photophysical and electronic properties of the molecules and especially blend mixtures with PDI as acceptor are analyzed. Detailed insight to device physics and morphology is gained by analysis of photoluminescence quenching, transient photovoltage and photocurrent decay exper- iments and atomic force microscopy. The changes in blend morphology due to the variation of side chain length and structure are correlated to the solar cell performance of the resulting devices.

The investigations explain why using short alkyl side chains higher currents and con- sequently an increased device performance can be achieved. An external quantum e- ciency of over 27% is reported for the photovoltaic devices. For the solar cell production an inverted structure with electron collecting TiO2 bottom and Ag top electrode was

4.1.1 Experimental details