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A stringent limitation in many optoelectronic devices, such as solar cells and light emitting diodes, is the intrinsic need for a transparent electrode. Uniting relevant aspects, indium tin oxide (ITO) is often the material of choice, however, alternatives are sought and being in particular found in conductive polymers. In this work, we present a novel doping strategy to arrive at highly conducting polymeric material based on poly-3,4-ethylenedioxythiophene (PEDOT). Based on commercial high conductivity PEDOT:PSS (Clevios PH 1000), and a post processing with aqueous triflic acid delivers a material that is both transparent and of low resistivity (5.23 × 10−4 Ω cm). Furthermore, this material retains its conductive character over a large temperature range, indicating metallic behaviour. This is further supported by positive magnetoconductance effects at low temperatures (1.8–10 K) and extended mean free paths of the conduction electrons are observed—evidencing for a metallic state in this polymer. Graphical abstract Electronic supplementary material The online version of this article (doi:10.1007/s00706-017-1973-1) contains supplementary material, which is available to authorized users.

The preparation and characterization of novel electron donor compounds with two (D1) and four (D2) quaterthiophene-containing arms attached to the flexible carbosilane–siloxane core are described. These materials were investigated in organic field effect transistors and photovoltaic cells. A power conversion efficiency of ∼0.9% obtained for D1:[70]PCBM blends is one of the highest values achieved up to now for solution-processible small molecular bulk heterojunction solar cells with large band-gap oligothiophene-based donor materials comprising no other chromophore units.

Novel fullerene derivatives bearing thiophene and furan residues were synthesized and studied as electron acceptor materials in bulk heterojunction organic solar cells, together with poly(3-hexylthiophene) (P3HT) as the donor polymer. Some compounds showed large nanomorphological inhomogenities in blends with P3HT; in particular, clusters with dimensions in the range of 100-1000 nm were formed. However, some blends that showed such large clusters yielded at the same time high power conversion efficiencies in photovoltaic devices, approaching 3.7 %. This is in sharp contrast with previously studied systems, in which a substantial phase separation always resulted in a poor photovoltaic performance. We assume that the attachment of thienyl or furyl groups to the fullerene cage results in a certain ordering of the designed fullerene derivatives I-IX with P3HT in photoactive blends. Both the fullerene derivative and P3HT might assemble via pi-pi stacking of the thiophene units to form the nanostructures observed in the films by optical and atomic force microscopy. The presence of ordered donor and acceptor counterparts in these nanostructures results in superior photovoltaic device operation.

photovoltaicdevicesareclosetobeingcommercialized.However,introducing organic photodiodes to the sensor market showsmuch less progress. Organic photodiodes can be easily producedin large areas on flexible substrates and lucrative applications forsuch sensors will have to be found.Matrices of organic photodiodes combine high sensitivity andspatialresolutionwithlargeareaproduction

Processes are investigated for the fabrication of hybrid bilayer photovoltaic (PV) devices consisting of IV–VI nanocrystals (NCs) having near-infrared optical gaps and an organic conductive polymer. Our design utilizes PbS NCs spin-cast onto an indium tin oxide (ITO) coated glass slide, covered with a poly(3-hexylthiophene-2,5-diyl) (P3HT) layer. We demonstrate here that the removal of the surface ligand, by pre-rinsing the NCs and subsequent annealing, generates a smoother film with a greater degree of cross-linking between the NCs. Additionally, a post-production treatment enhances the interfacial layer and improves charge carrier separation and mobility, rendering better performing solar cells. Further, results using PbSe NCs indicate the method can be adapted to similar systems. The method proposed here was designed to optimize more independently the polymer and nanocrystal components of the device and, to an extent, the interface between them.

The phthalocyanine–fullerene dyad Pc-C60 is an intramolecular donor–acceptor system that can be used as photoactive material in photovoltaic devices. The photocurrent shows maxima below 400 and 700 nm, but only a small response in the range around 500 nm. To improve the spectral match to the solar emission spectrum, an energy transfer process from the conjugated polymer poly [2-methoxy-5-(3 0 ,7 0 -dimethyloctyloxy)-1-4-phenylene vinylene] MDMO-PPV (antenna system) to the dyad compound can be used. The photophysics of the energy transfer process was studied with luminescence measurements and sub-picosecond time-resolved transmission changes. The influence of the polymeric antenna system on the photovoltaic device properties was measured with spectrally resolved photocurrent (IPCE) measurements. Photocurrent in nearly the full visible spectral range was obtained. In addition, Pc-C60 was also added as third component to MDMO-PPV:PCBM mixtures. The photocurrent around 700 nm was increased, however with reduction in the short-circuit current under white light illumination.

We have determined the anisotropic dielectric function of thin spin-coated as well as drop-cast poly(3-octylthiophene)-films from near-normal reflectance and transmittance data and from spectroscopic ellipsometry. The influence of the growth parameters (i.e. spin frequency, concentration of the polymer in solvent, type of solvent) and of the substrate on the anisotropic dielectric function of the films was studied. The determined anisotropic dielectric functions were used to investigate the orientation of the polymer chains within these films. For the spin-coated films we found, that the optical anisotropy increases with increasing spin frequency and decreasing polymer concentration in the solution, indicating that polymer chains become more aligned parallel to the substrate. The drop-cast films were found to be even more anisotropic. In addition, the anisotropy increases considerably if the property of the Si substrate is changed from hydrophilic to hydrophobic (done by HF-treatment). Summing up, all results are discussed in dependence of the film thickness as the decisive parameter for the anisotropy.

The discovery of a photoinduced charge transfer interaction between conjugated polymers and fullerenes led to intensive spectroscopic investigations as well as to high efficiency photovoltaic devices. Up to now, most spectroscopic investigations were performed on thin films, whereas little is known about the behavior of the charge carriers under working conditions of the device with an applied electric field. In this work, we investigated thin film photovoltaic devices by means of photoinduced absorption spectroscopy. Significant changes were observed for the polaron signal as well as the lifetime of the positive polaron on the conjugated polymer backbone upon changing the applied voltage on the device. The results are discussed in comparison with current–voltage characteristics of the devices.

The photophysical properties of a new dyad molecule composed of a covalently linked Zn-phthalocyanine (antenna/donor) and a C60 derivative (acceptor) have been investigated. We report experimental evidence of long-lived charge separation in the solid state with a lifetime several orders of magnitude higher than in solution. Such a long lifetime, unusual for phthalocyanine–fullerene dyads, is the basis for possible photovoltaic applications. A first demonstration of a working solar cell using phthalocyanine–fullerene dyads as the active material is presented. Though the power conversion efficiency under simulated solar illumination of 80 mW cm−2 is found to be moderate (0.02%), it is an encouraging result for application of C60 dyad molecules to photovoltaics.

First attempts were made to use nanocrystalline CuInS 2 with the fullerene derivatives to form flat interface donor-acceptor heterojunction solar cells. We synthesized nanocrystalline CuInS 2 in a colloidal route, the particle surfaces were shielded by an organic surfactant. We present photovoltaic data ofnanocrystalline CuInS 2 Schottky diodes in comparison with heterojunctions of CuInS 2 and methanofullerene derivatives. Flat interface heterojunction solar cells show significant better photovoltaic response than solar cells consist of nanocrystalline CuInS 2 single-layers. In comparison to the single-layers, anincrease of cell efficiency about factor 25 is received.

Colloidal semiconductor nanocrystal routes represent an attractive alternative compared to various vacuum-based techniques in order to obtain optoelectronic devices. It is the solutionprocessibility, which opens up new prospects in implementing thin-fi lms on large areas. Photovoltaic cells, for example, have been successfully demonstrated using a colloidal approach— nanocrystallline CdTe or CuInSe cells exhibiting power conversion effi ciencies ≥10% have been achieved recently. [ 1,2 ] By virtue of the colloidal route bulk semiconductor properties can be tuned via the quantum size effect: Quantum dots (QD)—nanocrystals having dimensions below their semiconductor's exciton radii—liberate the material portfolio from the classic photovoltaic materials: The absorption-emission of low-gap semiconductors can be shifted from the deep infrared toward the visible range to match to the solar spectrum. Combined with an effi cient luminescence quantum yield QDs represent a promising absorber material. Lead-sulfi de (PbS) as an example holds the record within QD-approaches: Monodisperse, stable solutions have been size-shifted to a peak emission at 1.34 eV and implemented in thin-fi lm solar cells achieving power-conversion effi ciencies (PCEs) greater than 7%. [ 3–6 ] Although these photovoltaic cells apply optimized designs, the full potential is not harnessed yet. It is the QD’s thin-fi lm

We report on photoinduced charge transfer and energy transfer reactions using a new cyano-substituted naphthalene-vinylene type conjugated polymer. Optical absorption and emission spectra of thin films of mixtures of this polymer with the fullerene derivative 1-(3-(methoxycarbonyl)propyl)-1-phenyl-(6,6)C 61 (PCBM) exhibit several interesting features such as luminescence quenching and polaronic photoinduced absorption (PIA) of charges indicating photoinduced charge transfer. On the other hand this new polymer exhibits energy transfer phenomena in blends with MDMO-PPV (poly((2-methoxy-5-((3,7-dimethyloctyl)oxy)-1,4-phenylene)vinylene)). Photovoltaic devices and light emitting diode (LED) devices have been fabricated and characterized using this novel conjugated polymer in composite with fullerenes and MDMO-PPV’s, respectively.

We present polarized, doping-induced absorption spectra obtained from conjugated polymers oriented by gel-processing in ultrahigh molecular weight polyethylene. Upon doping with iodine vapor, both poly(2-methoxy, 5-(2 ′ -ethyl)-hexyloxy) paraphenylene vinylene), MEH-PPV, and poly (3-octyl thiophene), P3OT, show absorption features in the mid-infrared (the infrared-active vibrational (IRAV) modes and the low energy electronic absorption) that are strongly polarized parallel to the chain axis. All observed features have predominantly parallel polarization. We suggest that carrier-induced and electrode-induced losses in electrically pumped waveguide structures could be greatly reduced by using highly oriented films, with molecular axes orthogonal to the electric field polarization of the waveguide mode.

In this section the energy level alignment at the semiconductor–dielectric interface in organic field-effect transistors is discussed. We focus at the comparison of pristine oxide structures with advanced organic interlayer structures. The interface is particularly interesting, since the performance of an organic field-effect transistor is significantly influenced by the energy level alignment. The study targets on an understanding of interfacial effects, hence complementary techniques are presented to investigate the interface. Most appropriately a combination of device analysis with a device-related photoemission spectroscopy study provides insight on the energy level alignment. Especially the role of commonly used organic interlayers is discussed, which enhance transistor performances. On organic–organic interfaces dipoles arise, which shift the relative positions of energy levels. Interestingly the magnitude and the direction of the shift is reflected in the device parameter analysis and in the corresponding photoemission spectra explaining the enhanced transistor performance.

Biological materials in organic electronics stand for low-cost production of biocompatible, biodegradable, and sustainable electronic devices. In this chapter, we discuss such materials and their implementation in the fabrication of electronic circuits. We briefly introduce applications of such biocompatible and biodegradable materials for interfacing electronics with living tissue. Research on bio-organic materials may ultimately result in establishing robust, environmentally safe, “green electronics” alternatives.

In this review we discuss the photophysical properties of the supramolecular composites of two π-electron semiconductors; conjugated polymers as electron donors and Buckminsterfullerene as electron acceptor. Conjugated, polymeric semiconductors have been found to be effective donors upon photoexcitation of the valence band electrons across the bandgap into the conduction bands. The Buckminsterfullerene, C60, acts as a strong acceptor upon photoexcitation. Thus, the supramolecular composite of these two conjugated materials exhibit ultrafast, reversible, metastable photoinduced electron transfer and charge separation. This process, similar to the primary steps of photosynthesis, has been utilized in conjugated polymer/C60 based heterojunction as well as in metal-semiconductor-metal tunnel devices for effective conversion of the solar photon energy into electricity. Other related applications of the photophysics, including photolithographic and xerographic processes, are reviewed. Furthermore, recent developments on light emitting diodes and solar cells with anomalous behavior based on Buckminsterfullerene are critically reviewed. Quantum well-like heterostructures based on organic donor-acceptor layers are proposed to exhibit interesting photoinduced phenomena.

The insertion of thin interlayers of LiF under the negative metal electrode (Al and Au) of bulk heterojunction solar cells significantly enhances the fill factor and stabilizes high open circuit voltages. Compared to devices without the LiF interfacial layer, the white light efficiencies increase by over 20% up to ηeff∼3.3%. Substitution of the LiF by another insulating interlayer SiOx results in lower overall efficiencies. In the case of a LiF/Au electrode, substantial efficiency enhancement is observed compared to a pristine Au electrode and white light efficiencies up to ηeff∼2.3% are reported.

Here, we report on a novel light sensing scheme based on a silicon/fullerene-derivative (methano-fullerene [6,6] phenylC61 butyric acid methyl ester – PCBM) heterojunction that allows the realization of optoelectronic devices for the detection of nearto mid-IR light. Despite the absent absorption of silicon and the fullerene-derivative in the IR a heterojunction of these materials absorbes and generates a photocurrentin the spectral range from 1.1 to 0.55 eV. Besides its scientific relevance, the simple fabrication process of the heterojunction (e.g., the fullerene-derivative is deposited by spin coating on Si) as well as its compatibility with the established complementary metaloxide semiconductor (CMOS) technology makes the presented hybrid approach a promising candidate for widespread applications.

Organic field effect transistors (O-FETs) are of considerable interest for the development of flexible displays and a variety of other macro-electronic applications. We investigated a DNA-based biopolymer material as a gate dielectric for fabricating O-FET backplanes using the organic semiconductor material Pentacene having small geometry devices and improved performance characteristics required for high resolution displays. The results to date continue to show the potential of this approach for low-cost O-FET backplanes for the next generation flexible displays and other macro-electronic applications.

A soluble low band gap polymer was synthesized and blended with a soluble fullerene derivative for the fabrication of plastic solar cells. The devices showed a remarkable harvest of photons, within the wide range of 300−900 nm, that were converted into current.

Hybrid materials consist of inorganic nanoparticles embedded in polymer matrices. An advantage of these materials is to combine the unique properties of one or more kinds of inorganic nanoparticles with the film forming properties of polymers. Most of the polymers can be processed from solution at room temperature enabling the manufacturing of large area, flexible and light weight devices. To exploit the full potential for the technological applications of the nanocrystalline materials, it is very important to endow them with good processing attributes. The surface of the inorganic cluster can be modified during the synthesis by organic surfactants. The surfactant can alter the dispersion characteristic of the particles by initiating attractive forces with the polymer chains, in which the particles should be homogenously arranged. In this review, we present wet chemical methods for the synthesis of nanoparticles, which have been used as photovoltaic materials in polymer blends. The photovoltaic performance of various inorganic/organic hybrid solar cells, prepared via spin-coating will be the focus of this contribution.

The use of conjugated polymers as antenna systems in photovoltaic devices with phthalocyanine‐fullerene dyad compounds is described. With luminescence measurements and time resolved transmission changes, energy transfer processes are studied. The influence on photovoltaic device properties is presented.

Polymer-fullerene based solar cells with power conversion efficiencies up to 3 % have been reported in the recent years. In this contribution we will present the recent developments in the field of these devices and discuss the basic physical processes, on which these devices are based.

Thermal deposition of small amounts of various salts at the interface between the active layer and the aluminum cathode was shown to alter the performance of bulk heterojunction photovoltaic devices. LiF and LiBr were found to enhance the power conversion efficiency as compared to devices with no interfacial salt, but Cs and K compounds were found to severely diminish the device performance. It is suggested that the Li compounds preferentially align to produce a bulk dipole moment at the interface, whereas the Cs and K compounds do not.

We report the first large-scale preparation of fluorinated trannulene C60F15[C(COOMe)3]3 (TMCMT) and its application as a photoactive material in organic solar cells. Two types of photovoltaic devices were fabricated: bulk heterojunction cells comprising blends of TMCMT with a polyfluorene polymer and bilayer cells based on a planar heterojunction of TMCMT with a perylene diimide. Photoluminescence quenching studies suggested that a photoinduced electron transfer occurs from polyfluorene and perylenediimide to TMCMT, therefore it behaves as photoinduced electron acceptor. Both types of solar cells comprising TMCMT as photoactive material showed clear photovoltaic effects. This proves for the first time the feasibility of the previously anticipated idea of using trannulenes as light-harvesting materials in photovoltaic devices.

Graphical abstract, Highlights • We demonstrate the fabrication and characterization of a silicon/organic hybrid heterojunction based infrared photodetector. • Photovoltaic effect up to 2.7 μm at room temperature is demonstrated. • Fabrication is CMOS compatible., The authors report on the fabrication of a silicon/organic heterojunction based IR photodetector. It is demonstrated that an Al/p-Si/perylene-derivative/Al heterostructure exhibits a photovoltaic effect up to 2.7 μm (0.46 eV), a value significantly lower than the bandgap of either material. Although the devices are not optimized, at room temperature a rise time of 300 ns, a responsivity of ≈0.2 mA/W with a specific detectivity of D∗ ≈ 7 × 107 Jones at 1.55 μm is found. The achieved responsivity is two orders of magnitude higher compared to our previous efforts [1,2]. It will be outlined that the photocurrent originates from an absorption mechanism involving excitation of an electron from the Si valence band into the extended LUMO state in the perylene-derivative, with possible participation of intermediate localized surface state in the organic material. The non-invasive deposition of the organic interlayer onto the Si results in compatibility with the CMOS process, making the presented approach a potential alternative to all inorganic device concepts.

Using the ultrafast photoinduced electron transfer with long-living charge separation in the conjugated polymer/fullerene thin films, photovoltaic devices have been fabricated. The photoinduced charge separation happens with internal quantum efficiency near unity. The performance of such “bulk heterojunction” photovoltaic devices is critically dependent on the transport properties of the interpenetrating network. The influence of the variation of different donor / acceptor materials on the sample morphology is monitored by atomic force microscopy (AFM), while I/V characteristics have been studied to evaluate the conversion efficiency.

Organic field-effect transistors (FETs) have been fabricated using an active region consisting of a conjugated polymer poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) blended with a soluble derivative of C60 (6,6)-phenyl C61-butyric acid methyl ester (PCBM). Such blends are promising for use in organic photovoltaic devices. Fabrication of the FET provides a technique for estimating both the hole and electron field-effect mobilities of the material in the active region by measuring the saturation current as a function of the gate voltage. The hole field-effect mobility for pristine MDMO-PPV was found to be 9×10−4 cm2/(Vs). Saturation of electron current could not be attained for pristine MDMO-PPV. Evidence for the saturation of electron current could be seen only at very high PCBM doping levels.