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.
KEYWORDS: Absorption, Solar cells, Heterojunctions, Interfaces, Excitons, Solar energy, Thin film solar cells, Organic photovoltaics, Thin films, Molecules
Recently, we have demonstrated an open circuit voltage of 1.0V and a power conversion efficiency of 3.4% in thin
film solar cells, utilizing a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor and
C60 as acceptor. Stimulated by this result, we systematically study the energy and electron transfer processes
taking place at the oligothiophene:fullerene heterojunction along a homologous series of these oligothiophenes.
The heterojunction is modified by tuning the HOMO level using different oligothiophene chain lengths, while
the LUMO level is essentially fixed by the choice of the acceptor-type end-groups (dicyanovinyl) attached to
the oligothiophene. We study electron transfer at the heterojunction to C60 using photoinduced absorption.
The observed transitions are unambiguously identified by TD-DFT calculations. With increasing the effective
energy gap of the donor-acceptor pair, charge carrier dissociation following the photoinduced electron transfer is
eventually replaced by recombination into the triplet state, which alters the photovoltaic operation conditions.
Therefore, the optimum open-circuit voltage of a solar cell is a trade-off between an efficient charge separation at
the interface and a maximized effective gap. We conclude that values between 1.0 and 1.1 V for the open-circuit
voltage in our solar cell devices present an optimum, as higher voltages were only achieved with concomitant
losses in charge separation efficiency.
In this contribution we report two-photon excited time-resolved fluorescence and fluorescence anisotropy for several
macromolecular systems of different symmetry such as cyclic (single ring) thiophene-based annulenes, triphenylaminecentered
branched trimers with pyridine acceptor groups, and linear chromophores possessing high TPA-cross-section.
We have compared the ultrafast fluorescence anisotropy dynamics for one- and two-photon excitation routes using timeresolved
fluorescence up-conversion setup with femtosecond time resolution. For linear systems the initial anisotropy
was found to be about factor 1.4 higher than that for one-photon excitation as predicted by theory. For macromolecular
strongly coupled planar systems under two-photon excitations we observed a relatively low initial fluorescence
anisotropy (~0.1) and specific femtosecond anisotropy dynamics. Two-photon excited fluorescence anisotropy is
strongly correlated to the orientation and value of the transition moment from the excited state to the second and higher
lying states and can be used as a direct indicator of strong coupling regime.
In this work, we report on efficient heterojunction organic solar cells containing a new oligothiophene derivative α,α'-bis-(2,2-dicyanovinyl)-quinquethiophene (DCV5T) as donor (D) and fullerene C60 as acceptor (A). The oligothiophene carries electron withdrawing substituents which increase the ionization energy and even more strongly the electron affinity. In thin films, the absorption is significantly broadened compared to solution and the optical gap is reduced to 1.77 eV. Nevertheless, the material shows strong fluorescence with low Stokes shift (peak at 1.71 eV), i.e. low energy loss upon reorganisation in the excited state.
At the heterointerface between the low band-gap oligothiophene and fullerene C60, photogenerated excitons from both materials are efficiently separated into electrons on the LUMO of C60 and holes on the low-lying HOMO of the oligothiophene. This step involves only low energetic losses since both the HOMO and the LUMO offset of the two materials are below 0.6 eV, close to the expected exciton binding energy. We can thus reach high open circuit voltages of up to 1.0 V. The most efficient solar cells with power efficiencies around 4 % are obtained when the photoactive heterojunction is embedded between a p-doped hole transport layer on the anode side and a combination of a thin exciton blocking layer and aluminium on the cathode side. However, due to the high ionization energy of the oligothiophene (approx. (5.6 ± 0.1) eV), hole injection from any anode or hole transport layer is difficult and the IV curves thus show a characteristic S-shape which reduces the fill factor FF. It is found that the actual FF sensitively depends on the work function of the p-doped hole transport layer, that can be influenced by doping.
The absolute photoluminescence quantum yields ((Phi) PL) of three end-capped oligothiophene derivatives dispersed in N,N'-((alpha) -naphthyl)-N,N'-diphenyl-1,1'-biphenyl ((alpha) -NPD) have been evaluated and the most efficient of the emitters was used as a dopant in molecular organic LEDs. Composite films of 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl) thien-2-yl]-furan (EC5FUR); 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl) thien-2-yl]-oxazole (EC5OXZ) and 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl)thien-2-yl]-1,3,4- oxadiazole (EC5OXD) doped into (alpha) -NPD were found to have (Phi) PL values of 78, 62 and 28%, respectively. MOLED devices were fabricated using an EC5FUR/(alpha) -NPD composite as the emitting layer and the external quantum efficiencies ((eta) EL) of these devices were evaluated. The results of the device characterization show that the inclusion of EC5FUR in the NPD hole transport layer increases the device (eta) EL to 1.45% at a current density of 10 mA/cm2. In addition, the concentration dependence of the (eta) EL on the EC5FUR dopant in certain device structures when considered in conjunction with the current results of ultraviolet photoemission spectroscopic experiments suggests that this dopant species may be acting as both a hole and electron trap in the (alpha) -NPD host.
Homologous series of mixed oligoheterocycles based on end- capped oligothiophenes ECnT 1 were synthesized by introduction of electronegative heteroatoms like oxygen and nitrogen into the conjugated (pi) -system. This led to novel structures 2-11 in which thiophene units of the parent compounds are substituted by other heterocycles with more pronounced acceptor character. Other moieties like phenylsubstituted thiophenes, benzo(c)thiophene, and spiro- bithiophenes have also been implemented resulting in oligothiophenes 12-16. The characterization of the optical and electrochemical properties clearly reveals the influence of the heteroatoms on the electronic properties. Thus e.g., due to the electron withdrawing character of the central heterocycle oxidation of the oligomer is rendered more difficult while reduction is facilitated. In some cases, a hypsochromic shift of the longest wavelength absorption and emission is observed and additionally a significant enhancement of the fluorescence quantum yield in solution and in the solid state. The HOMO/LUMO energy differences determined from the optical measurement correspond qualitatively well with the values obtained from electrochemical data. First experiments on single layer organic light emitting diodes show that these mixed oliogoheterocycles can be used as emitting materials.
Changes as small as 1 ppm in the real and imaginary components of the complex permittivity resulting from flash photoexcitation of dilute solutions of (pi) -bond conjugated oligomers and polymers can be measured with nanosecond time resolution using the time-resolved microwave conductivity technique. The results provide information on the extent of delocalization of singlet and triplet state photoexcitations. Data are presented for oligomeric and polymeric derivatives of phenylene vinylene, paraphenylene and thiophene. For the polymers excess polarizability volumes, (Delta) Vp, of the singlet exciton of 2000, 180 and 570 angstroms3 are found, respectively. (Delta) Vp for the triplet exciton of the polythiophene derivative is found to be only 25 angstroms3.
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