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A detailed investigation of the open circuit voltage (V-OC) of organic bulk heterojunction solar cells comprising three different donor polymers and two different fullerene-based acceptors is presented. Bias amplified charge extraction (BACE) is combined with Kelvin Probe measurements to derive information on the relevant energetics in the blend. On the example of P3HT:PC70BM the influence of composition and preparation conditions on the relevant transport levels will be shown. Moderate upward shifts of the P3HT HOMO depending on crystallinity are observed, but contrarily to common believe, the dependence of V-OC on blend composition and thermal history is found to be largely determined by the change in the PCBM LUMO energy. Following this approach, we quantified the energetic contribution to the V-OC in blends with fluorinated polymers or higher adduct fullerenes.

We investigate the role of excess excitation energy on the nature of photoexcitations in donor-acceptor π-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b']dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor-acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics.

The authors investigated organic tandem solar cells with solution processed recombination contacts made from a TiO2 sol in combination with PEDOT:PSS. Tandem cells were prepared either with two P3HT:PCBM-based subcells or by combining PCDTBT:PC[70]BM and PCPDTBT:PC[70]BM. Optical modeling is used to predict experimental short circuit currents of the tandem solar cells as a function of layer thickness. Both types of tandem cells yield energy conversion efficiencies of ca. 3.3%. We propose that a significant improvement of the performance of the PCDTBT:PC[70]BM - PCPDTBT:PC[70]BM tandem is possible by optimization of the recombination contact and layer thicknesses of both subcells.

We demonstrate the importance of the nanocrystal surface treatment and the inorganic composition for hybrid solar cells. Mixtures of CdSe nanorods and CdSe quantum dots integrated in hybrid solar cells together with the conjugated polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) perform better than nanorod and quantum dot only based devices. In addition larger sized quantum dots show a similar improvement after integration in respective solar cells. Power conversion efficiency values exceeding 3% are observed. A first result on the shelf lifetime of such a device is highlighted.

The cell performance of organic-inorganic hybrid photovoltaic devices based on CdSe nanocrystals and the semiconducting polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) is strongly dependent on the applied polymer-to-nanocrystal loading ratio and the annealing temperature. It is shown here that higher temperatures for the thermal annealing step have a beneficial impact on the nanocrystal phase by forming extended agglomerates necessary for electron percolation to enhance the short-circuit current. However, there is a concomitant reduction of the open-circuit voltage, which arises from energy-level alterations of the organic and the inorganic component. Based on quantum dots and PCPDTBT, we present an optimized organic–inorganic hybrid system utilizing an annealing temperature of 210 °C, which provides a maximum power conversion efficiency of 2.8%. Further improvement is obtained by blending nanocrystals of two different shapes to compose a favorable n-type network. The blend of spherical quantum dots and elongated nanorods results in a well-interconnected pathway for electrons within the p-type polmer matrix, yielding maximum efficiencies of 3.6% under simulated AM 1.5 illumination.

We report on the efficiency enhancement for bulk-heterojunction hybrid solar cells based on hexanoic acid treated trioctylphosphine/oleic acid-capped CdSe quantum dots (QDs) and low bandgap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4 H -cyclopenta[2,1- b ;3,4- b ′]-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) compared to devices based on poly(3-hexylthiophene) (P3HT). Photovoltaic devices with optimized polymer:QD weight ratio, photoactive film thickness, thermal annealing treatment, and cathode materials exhibited a power conversion efficiency of 2.7% after spectral mismatch correction, which is the highest reported value for spherical CdSe QD based photovoltaic devices. The efficiency enhancement is attributed to the surface treatment of the QDs together with the use of the low bandgap polymer PCPDTBT leading to an increased short-circuit current density due to additional light absorption between 650 and 850 nm. Our results suggest that the hexanoic acid treatment is generally applicable to various ligand-capped CdSe and confirm that low bandgap polymers with adequate HOMO and LUMO levels are promising to be incorporated into hybrid solar cells for further device performance improvement.

A novel polytriarylamine (PTAA)-type alternating copolymer with on-chain crystal violet moieties has been generated which shows the expected strong dependence of its UV-Vis absorption spectrum from the pH value of the surrounding medium. Starting from the colorless leucocrystal violet form subsequent protonation/water elimination (by trifluoroacetic acid) leads to the formation of a blue colored, cationic copolymer with on-chain crystal violet units that displays a characteristic Vis absorption band peaking at 627 nm. Further protonation leads to the generation of the corresponding dicationic species which is accompanied by a red shift of the long wavelength absorption maximum to 713 nm. The novel pH-sensitive polytriarylamine derivatives will be investigated as potential p-type semiconductors for OFET (gas) sensors.

We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer.

Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor-acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor-acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor-acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping.

We present a study on the performance and analysis of hybrid solar cells comprising a planar heterojunction between between a conjugated donor polymer, P3HT or PCPDTBT, and hydrogenated amorphous silicon (a-Si:H). A comparison of the modeled absorption spectra of the layer stack with the measured external quantum efciency is used to investigate the contribution of the inorganic and organic material to the photocurrent generation in the device. Although both materials contribute to the photocurrent, the devices exhibit poor quantum e ciencies and low short circuit currents. Bandstructure simulations of the hybrid layer structure reveal that an unfavorable electric eld distributionwithin the planarmultilayer structure limits the performance. Using electroabsorption measurements we can show that the electric eld is extremelyweak in the amorphous siliconbut strong in the organicmaterial. The situation changes drasticallywhen the conjugated polymer is p-doped. Doping not only increases the conductivity of the organic material, but also restores the electric eld in the amorphous silicon layer. Optimized hybrid solar cells comprising thin doped P3HT layers exhibit energy conversion e ciencies (ECE) up to 2.8 %. || S. Schaefer, S. Albrecht, D. Neher: Universitat Potsdam, Institute of Physics and Astronomy, Soft Matter Physics, D-14476 Potsdam, Germany T. F. Schulze, E. Conrad, L. Korte, B. Rech: Department of Silicon Photovoltaics, Helmholtz Center Berlin for Materials and Energy, Kekulestr. 5, D-12489 Berlin, Germany J. Wordenweber, A. Gordijn: IEK5-Photovoltaik, Forschungszentrum Julich, D-52425 Julich, Germany U. Scherf, I. Dumsch: Bergische Universitat Wuppertal, Macromolecular Chemistry and Institute for Polymer Technology, GaussStrasse 20, D-42097 Wuppertal, Germany