David Mühlbacher

Photovoltaic action of conjugated polymer/fullerene bulk heterojunction solar cells using novel PPE-PPV copolymers

The design of novel conjugated polymers suitable for use in plastic solar cells is one of todays challenges aiming towards improved key properties like the increase of photocurrent and open circuit voltage of such devices. In this work we present first results on arylene-ethynylene/arylene-vinylene hybrid polymers 3 (poly(-2,5-dioctyloxy-1,4-phenylene-diethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)) and 5 (poly(2,5-dioctyloxy-1,4-phenylene-ethynylene-9,10-anthracenylene-ethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)), demonstrating photovoltaic action in combination with the soluble C60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61 (PCBM). Devices with an active layer thickness of about 100 nm yielded power conversion efficiencies of up to 2% under 100 mW cm−2 AM 1.5 white light illumination. The coarse grained morphology of the active layers was identified as the main limitation for the photocurrent, revealed by AFM measurements. The photovoltaic devices were characterized by current–voltage and spectral photocurrent measurements. The results show that the open circuit voltage is weakly dependent on the HOMO (highest occupied molecular orbital) level of the conjugated polymer used as donor.

Comparison of the electrochemical and optical bandgap of low-bandgap polymers

The use of low bandgap polymers in organic photovoltaics for better light harvesting is getting more and more important. For determination of the electrochemical bandgap cyclovoltammetry (CV) and electrochemical voltage spectroscopy (EVS) was applied. The optical bandgap was determined by optical absorption measurements. The results are compared in terms of energy vs. vacuum level to predict the suitability of these materials for organic photovoltaics.

Polymer solar cells and infrared light emitting diodes: Dual function low bandgap polymer

Conjugated Polymers with a HOMO-LUMO transition <2 eV, i.e. a low bandgap, respectively, have interesting and desired properties for some thin film optoelectronic devices like light emitting diodes and solar cells. In this contribution we present the implementation of the novel copolymer PTPTB, consisting of alternating electron-rich N -dodecyl-2,5-bis(2′thienyl)pyrrole (TPT and electron-deficient 2,1,3-benzothiadiazole (B) units, in light emitting diodes (LEDs ) and photovoltaic devices. The LEDs show emission in the near infrared region, peaking at 800 nm. The electroluminescence yield can be significantly enhanced by blending with the wide bandgap polymer poly (para-phenylene-vinylene) derivative MDMO-PPV. Bulk heterojunction devices of PTPTB blended with the C 60 derivative PCBM shows AM 1.5 efficiencies around 1%. The low bandgap of PTPTB allows collecting photons up to 750 nm.

Plastic Solar Cells Based on Novel PPE-PPV-Copolymers

ABSTRACT In this study plastic solar cells based on arylene-ethynylene/arylene-vinylene hybrid polymers in combination with the soluble fullerene PCBM (1-(3-methoxycarbonyl) propyl-1-phenyl [6 6]C61) reaching 2% AM 1.5 solar power conversion efficiency at 80 mW/cm2 are reported. The polymers used are DE105 (poly(-2,5-dioctyloxy-1,4-phenylene-diethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)) and DE142 (poly(2,5-dioctyloxy-1,4-phenylene-ethynylene-9,10-anthracenylene-ethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)), whose main difference lies in the additional anthracene group in the latter one. Comparing results from electrochemical characterizations with IV-measurements reveals a weak dependency of the maximum open circuit voltage on the molecular HOMO level of the polymer used. A coarse grained morphology of the active layers was found responsible for limiting the photocurrent, as shown by AFM measurements.

Design Rules for Efficient Organic Solar Cells

There has been an intensive search for cost-effective photovoltaics since the development of the first solar cells in the 1950s [1-3]. Among all the alternative technologies to silicon-based pn-junction solar cells, organic solar cells are the approach that could lead to the most significant cost reduction [4]. The field of organic photovoltaics (OPV) is composed of organic/inorganic nanostructures, like the dyesensitized solar cell, multilayers of small organic molecules and mixtures of organic materials (bulk-heterojunction solar cell). A review of several so-called organic photovoltaic (OPV) technologies was recently presented [5].

Processible Cyclopentadithiophene Copolymers for Photovoltaic Applications

We designed and synthesized a series of conjugated polymers containing alternating 4H-cyclopenta[2,1-b:3,4-b′]dithiophene units and comonomers consisting of 2,2′-bithiophene, 3″, 4″ -dihexyl-α -pentathiophene, 3,4-ethylenedioxythiophene and 5,5′ -bis(2-thienyl)-4,4′ -dihexyl-2,2′ -bithiazole. These polymers possess optical bandgaps in the range of 1.75 to 2.0 eV. The desirable absorption attributes of these materials make then excellent candidates for use in photovoltaic cells. Electrochemical studies indicate desirable HOMO-LUMO levels for use with fullerene derivatives as electron transporters. Field effect transistors made of these materials show hole mobilities in the range of 7.5 × 10−4 cm2/Vs to 2.0 × 10−3 cm2/Vs. Due to the combination of these characteristics, power conversion efficiencies up to 3.1% were achieved on devices made of bulk heterojunction composites of these materials with soluble fullerene derivatives.