Eduard Brier

Organic solar cells on indium tin oxide and aluminum doped zinc oxide anodes

The authors compare organic solar cells using two different transparent conductive oxides as anode: indium tin oxide (ITO) and three kinds of aluminum doped zinc oxide (ZAO). These anodes with different work functions are used for small molecule photovoltaic devices based on an oligothiophene derivative as donor and fullerene C60 as acceptor molecule. It turns out that cells on ITO and ZAO have virtually identical properties. In particular, the authors demonstrate that the work function of the anode does not influence the Voc of the photovoltaic device due to the use of doped transport layers.

Organic thin film photovoltaic cells based on planar and mixed heterojunctions between fullerene and a low bandgap oligothiophene

We present the material α-ω-bis-(dicyanovinylen)-sexithiophen (DCV6T) as donor material in organic solar cells. A systematic study on the potential of DCV6T is given for different active layer concepts. DCV6T is a member of a class of acceptor-substituted oligothiophenes, which showed efficiencies of up to 3.4% and open circuit voltages (Voc) of 1.0V, which were recently reported [K. Schulze et al. Adv. Mater. (Weinheim, Ger.) 18, 2875 (2006)]. To verify the potential of the material (DCV6T), organic solar cells with planar heterojunctions, bulk heterojunctions, and a hybrid-planar-mixed heterojunction are investigated. The planar heterojunction solar cells of DCV6T and C60 show the highest Voc of 0.90V. The mixed heterojunction solar cells have improved currents but a lower Voc of 0.82V. The solar cell using the hybrid-planar-mixed heterojunction achieves the best combination of parameters. It has a Voc of 0.88V, a short circuit current (jsc) of 5.7±0.4mAcm−2, a fill factor of 41.6%, and a power conversi...

Dicyanovinyl-quinquethiophenes with varying alkyl chain lengths: Investigation of their performance in organic devices

We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl side chain lengths. Both materials show comparable open circuit voltages Voc in organic solar cells with fullerene C60 as acceptor, as expected since they have the same highest occupied molecular orbital energy. However, differences in the current-voltage-characteristics, particularly in the fill factor, are observed. We analyze both derivatives in hole-only devices and find a difference in the hole injection between the doped hole transport layer and the oligothiophenes. Additionally, we determine the hole mobility of the two materials and explain the different behaviors of the two materials in solar cells.We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl side chain lengths. Both materials show comparable open circuit voltages Voc in organic solar cells with fullerene C60 as acceptor, as expected since they have the same highest occupied molecular orbital energy. However, differences in the current-voltage-characteristics, particularly in the fill factor, are observed. We analyze both derivatives in hole-only devices and find a difference in the hole injection between the doped hole transport layer and the oligothiophenes. Additionally, we determine the hole mobility of the two materials and explain the different behaviors of the two materials in solar cells.

Fused Thiophene-Pyrrole-Containing Ring Systems up to a Heterodecacene†

A new class of π-conjugated polycyclic hydrocarbons that promises interesting electronic properties is presented. The synthesis and extension of the S,N-heteroacene series consisting of only five-membered heterocyclic rings up to a very long, stable, and still soluble decacene SN10 is realized by multiple Pd-catalyzed aminations of halogenated thiophene precursors as key reactions. These novel heteroacenes were characterized by optical spectroscopy and electrochemistry providing interesting structure-property relationships. Nearly complete bond-length equalization in the inner part of the conjugated backbone and an unusual herringbone packing in the solid state underline the structural features of these novel systems.

Exciton diffusion and energy transfer in organic solar cells based on dicyanovinyl-terthiophene

We discuss exciton transport and energy transfer in organic solar cells based on dicyanovinyl-terthiophene (DCV3T). Time-resolved surface luminescence quenching experiments on double layers of DCV3T and zinc-phthalocyanine as luminescence quencher are analyzed in the framework of a three-level luminescence model with an initially absorbing state of short lifetime that relaxes to a longer living emitting state. Luminescence quenching of the emitting state is assigned to Forster-type energy transfer with an apparent Forster radius of 2.1 nm, which is in accordance with the Forster radius obtained from the spectral overlap integral. A diffusion based analysis for the emitting state yields a diffusion length of Le=6.9 nm. The short living initial state is quenched by diffusion of the excitons to the interface with a diffusion length of La=5.3 nm. External quantum efficiency measurements of the photocurrent in a corresponding organic solar cell structure evaluated with a two-level diffusion model give a diffus...

Optimization of solution-processed oligothiophene:fullerene based organic solar cells by using solvent additives.

Summary The optimization of solution-processed organic bulk-heterojunction solar cells with the acceptor-substituted quinquethiophene DCV5T-Bu 4 as donor in conjunction with PC61BM as acceptor is described. Power conversion efficiencies up to 3.0% and external quantum efficiencies up to 40% were obtained through the use of 1-chloronaphthalene as solvent additive in the fabrication of the photovoltaic devices. Furthermore, atomic force microscopy investigations of the photoactive layer gave insight into the distribution of donor and acceptor within the blend. The unique combination of solubility and thermal stability of DCV5T-Bu 4 also allows for fabrication of organic solar cells by vacuum deposition. Thus, we were able to perform a rare comparison of the device characteristics of the solution-processed DCV5T-Bu 4:PC61BM solar cell with its vacuum-processed DCV5T-Bu 4:C60 counterpart. Interestingly in this case, the efficiencies of the small-molecule organic solar cells prepared by using solution techniques are approaching those fabricated by using vacuum technology. This result is significant as vacuum-processed devices typically display much better performances in photovoltaic cells.

Self-organizing carbohydrate-oligothiophene-hybrids for eukaryotic membrane-labelling

We report synthesis and photophysical characterization of d-(+) and l-(-) mannose terminated oligothiophene hybrids. In polar environment fluorescently quenched suprastructures were formed. Fluorescence was recovered by integration of the hybrids into phospholipidic bilayers of artificial vesicles as well as membrane compartments of myeloid cells, a defined hematopoetic lineage.

Recent progress in organic solar cells based on small molecules

We report on a series of organic solar cells based on heterojunctions of oligothiophene derivatives with varying chain length and C60 fullerenes. Devices are based on either p-i-n or p-i-i structure. In the first the intrinsic photovoltaic active layer is sandwiched between a p-type and n-type doped organic wide-gap layer for hole and electron transport respectively. In the latter the electron transport layer is replaced by a thin layer of wide-gap material as exciton blocker. Through optimization of transport and absorber layers we are able to reach in devices with single heterojunctions an open circuit voltage Voc of about 1V, a short circuit current density Jsc of about 5.6mA/cm2 and a fill factor FF above 50% under an AM1.5 illumination with 1000W/m2. However, still only a small part of the available solar spectrum is used. Thus, based on these materials stacked solar cells have been made to further improve the light absorption. The thickness of each layer is optimized using optical simulations to match the currents delivered by each of the solar cells in the stack. Through the incorporation of a very efficient recombination zone between the stacked solar cells the resulting Voc nearly reaches the sum of the Voc of the two serially connected solar cells.