Christian Uhrich

Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.

Origin of open circuit voltage in planar and bulk heterojunction organic thin-film photovoltaics depending on doped transport layers

The aim of this article is to investigate the origin of the open circuit voltage (Voc) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C60 as acceptor material and an oligophenyl-derivative 4,4′-bis-(N,N-diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the Voc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, Voc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, Voc is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orb...

Interrelation between Crystal Packing and Small-Molecule Organic Solar Cell Performance†

X-ray investigations on single crystals of a series of terminally dicyanovinyl-substituted quaterthiophenes and co-evaporated blend layers with C(60) give insight into molecular packing behavior and morphology, which are crucial parameters in the field of organic electronics. Structural characteristics on various levels and length scales are correlated with the photovoltaic performance of bulk heterojunction small-molecule organic solar cells.

Controlled current matching in small molecule organic tandem solar cells using doped spacer layers

Current matching of the subcells is crucial to optimize the performance of tandem solar cells. Due to the thin film optics of organic solar cells, the position of the two subcells relative to the reflecting electrode becomes a very important issue. This is demonstrated for an indium tin oxide (ITO)/pin/pii/Al structure with thin intrinsic absorbing layers consisting of zinc-phthalocyanine and fullerene C60 and a metal-free lossless recombination contact between the subcells. By keeping the thickness of the absorbing layers constant and changing only the thickness of the inner p-doped transparent layer in 16 steps from 0to186nm, the distance of the ITO-sided subcell from the reflecting electrode (Al) is systematically varied. Thus, the p-doped layer works as an optical spacer between both subcells. The influence of its thickness on the thin film optics is shown in optical simulations and confirmed with current-voltage measurements. If both subcells are separated only by the recombination contact, they are ...

Surface Engineering Using Kumada Catalyst-Transfer Polycondensation (KCTP): Preparation and Structuring of Poly(3-hexylthiophene)-Based Graft Copolymer Brushes

Poly(4-vinylpyridine)-block-poly(4-iodo-styrene), P4VP-b-PS(I), block copolymers obtained by iodination of readily available P4VP-b-PS block copolymers strongly adhere to variety of polar substrates including Si wafers, glasses, or metal oxide surfaces by a polar P4VP block, forming polymer brushes of moderately stretched PS(I) chains. Kumada catalyst-transfer polycondensation (KCTP) from the P4VP-b-PS(I) brushes results into planar brushes of the graft copolymer in which relatively short ( approximately 10 nm) poly(3-hexylthiophene), P3HT, grafts emanate from the surface-tethered PS(I) chains. Grafting of the P3HT leads to significant stretching of the PS(I) backbone as a result of increased excluded volume interactions. Specific adsorption of the P4VP block to polar surfaces was utilized in this work to pattern the P4VP(25)-b-PS(I)(350) brush. The microscopically structured P4VP(25)-b-PS(I)(350) brush was converted into the respectively patterned P4VP-PS(I)-g-P3HT one using KCTP. We also demonstrated that KCTP from functional block copolymers is an attractive option for nanostructuring with polymer brushes. P4VP(75)-b-PS(I)(313) micelles obtained in selective solvent for the PS(I) block form a quasi-ordered hexagonal array on Si wafer. The P4VP(75)-b-PS(I)(313) monolayer preserves the characteristic quasi-regular arrangement of the micelles even after extensive rinsing with various solvents. Although the grafting of P3HT from the nanopatterned P4VP(75)-b-PS(I)(313) brush destroys the initial order, the particulate morphology in the resulting film is preserved. We believe that the developed method to structured brushes of conductive polymers can be further exploited in novel stimuli-responsive materials, optoectronic devices, and sensors.

Highly efficient semitransparent tandem organic solar cells with complementary absorber materials

We present highly efficient, semitransparent small molecule organic solar cells. The devices employ an indium tin oxide-free top contact, consisting of thin metal films. An additional organic layer is used to enhance light outcoupling. The solar cell incorporates two stacked subcells, each containing a donor:acceptor bulk heterojunction. The two subcells have complementary absorbers, with separate blue (C60), red (fluorinated zinc phthalocyanine), and green (dicyanovinyl oligothiophene derivative) absorbing molecules. A power conversion efficiency of 4.9 ± 0.2% is obtained for the device having an average transmission of 24% in the visible range.

A-D-A-D-A-type oligothiophenes for vacuum-deposited organic solar cells.

Novel A-D-A-D-A-type oligothiophenes incorporating electron-withdrawing benzo[c][1,2,5]thiadiazole (BTDA) as core and trifluoroacetyl (TFA) as terminal acceptor groups have been developed. Vacuum-processed planar heterojunction organic solar cells incorporating these new oligomers as donor and C(60) as acceptor showed very high open circuit voltages up to 1.17 V, resulting in power conversion efficiencies of 1.56% under AM1.5G conditions.

Increase in internal quantum efficiency in small molecular oligothiophene: C60 mixed heterojunction solar cells by substrate heating

We present small molecule solar cells with α,ω-bis-(dicyanovinylene)-sexithiophene:C60 mixed heterojunctions, reaching power conversion efficiencies of 4.9±0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%.

Efficient p-i-n type organic solar cells incorporating 1,4,5,8-naphthalenetetracarboxylic dianhydride as transparent electron transport material

The implementation of proper charge carrier transport materials in p-i-n type organic solar cells strongly influences the device performance. Our investigation focuses on the substitution of the standard layer sequence used at the side of electron transport, usually consisting of either C60/exciton blocking layer/metal or n-C60/metal by a layer sequence including a window layer. Here, we evaluate the transparent electron transport material 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), which guarantees a loss free charge extraction from the active materials due to a good energy level alignment as well as exciton blocking due to its wide bandgap. It is demonstrated that upon the exchange of the electron transport layer n-C60 for n-NTCDA, the solar power conversion efficiency of a p-i-n device can be increased by 10%.

Efficient semitransparent small-molecule organic solar cells

We present semitransparent small-molecule organic solar cells (OSC) deposited by thermal evaporation onto indium tin oxide (ITO)-coated glass substrates. The devices employ ITO-free ultrathin metal layers as top electrodes, containing 1nm metal surfactant interlayer for improved morphology. Using a bulk heterojunction of zinc phthalocyanine and C60, sandwiched in between doped dedicated transport layers for efficient charge carrier extraction, power conversion efficiencies comparable to conventional OSC with an intransparent thick back electrode and similar device layout are achieved: the semitransparent OSC yield power conversion efficiencies well above 2% with external quantum efficiencies above 30%–40%. Organic light incoupling layers improve the transmission to up to 50% in the visible part of the optical spectrum.