Patrik Henriksson

An Easily Accessible Isoindigo-Based Polymer for High-Performance Polymer Solar Cells

A new, low-band-gap alternating copolymer consisting of terthiophene and isoindigo has been designed and synthesized. Solar cells based on this polymer and PC(71)BM show a power conversion efficiency of 6.3%, which is a record for polymer solar cells based on a polymer with an optical band gap below 1.5 eV. This work demonstrates the great potential of isoindigo moieties as electron-deficient units for building donor-acceptor-type polymers for high-performance polymer solar cells.

An isoindigo-based low band gap polymer for efficient polymer solar cells with high photo-voltage

A new low band gap polymer (E(g) = 1.6 eV) with alternating thiophene and isoindigo units was synthesized and characterized. A PCE of 3.0% and high open-circuit voltage of 0.89 V were realized in polymer solar cells, which demonstrated the promise of isoindigo as an electron deficient unit in the design of donor-acceptor conjugated polymers for polymer solar cells.

2D π-conjugated benzo[1,2-b:4,5-b′]dithiophene- and quinoxaline-based copolymers for photovoltaic applications

Two medium gap semiconducting polymers, P(1)-Q-BDT-4TR and P(2)-FQ-BDT-4TR, based on alternate units of alkyl-dithiophene substituted benzodithiophene (BDT) and quinoxaline units (without or with fluorine substitution), are synthesized and fully characterized. The polymers exhibit optical and electrical properties favorable for being employed as donors in BHJ OPV devices, such as: absorption spectra extending up to around 720 nm for a high solar spectrum coverage, deep lying HOMO energy levels for a high device open circuit voltage and LUMO energy levels higher than those of PC61BM and PC71BM for an efficient exciton dissociation. In particular, the presence of alkyl-dithiophene side chains allows us to obtain a high 2D π-conjugation which promotes red shifted absorption profiles, low HOMO energy levels (<−5.6 eV) and enhanced environmental and thermal stability. Moreover, the introduction of the fluorine atom in the polymer backbone allows us to obtain efficient OPV devices, based on as-cast P(2)-FQ-BDT-4TR:PC61BM blend, showing a JSC of −10.2 mA cm−2, VOC of 0.90 V, FF of 58% and PCE of 5.3%, without the need for any additional thermal treatment.

Efficient red electroluminescence from diketopyrrolopyrrole copolymerised with a polyfluorene

We report the synthesis, characterization, and device incorporation of copolymers based on a common green-emitting polyfluorene but containing a small proportion of a low energy gap donor-acceptor-donor unit for red emission in photo- and electro-luminescence. At just 1%–3% random incorporation, the low-gap unit is not present on all chains, yet we demonstrate that efficient charge and energy transfer can yield electroluminescent devices with 1% quantum efficiency and a color that can be tuned by adjusting the density of low-gap units to achieve primary red (National Television System Committee). The high current density tail off in the efficiency is reduced by replacing the hole-injection layer with a photochemically cross-linked electron‑blocking layer.

Light-harvesting capabilities of low band gap donor–acceptor polymers

A series of nine donor-acceptor polymers, including three new and six polymers from previous work, have been investigated experimentally and theoretically. The investigation focuses on narrow band gaps and strong absorptions of the polymers, where experimentally determined first peak absorption energies range from 1.8 to 2.3 eV, and peak absorption coefficients vary between 19-67 L g(-1) cm(-1). An overall assessment of each polymers light-harvesting capability is made, and related to the chemical structure. Oligomer calculations using density functional theory are extrapolated to obtain size-converged polymer properties, and found to reproduce the experimental absorption trends well. Accurate theoretical predictions of absorption energies to within 0.06 eV of experiments, and absorption strength to within 12%, are obtained through the introduction of an empirical correction scheme. The computational and experimental results provide insight for the design of polymers with efficient absorption, concerning the intrinsic properties of the constituent units and the use of bulky side-groups.

Synthesis and characterization of benzodithiophene and benzotriazole-based polymers for photovoltaic applications

Summary Two high bandgap benzodithiophene–benzotriazole-based polymers were synthesized via palladium-catalysed Stille coupling reaction. In order to compare the effect of the side chains on the opto-electronic and photovoltaic properties of the resulting polymers, the benzodithiophene monomers were substituted with either octylthienyl (PTzBDT-1) or dihexylthienyl (PTzBDT-2) as side groups, while the benzotriazole unit was maintained unaltered. The optical characterization, both in solution and thin-film, indicated that PTzBDT-1 has a red-shifted optical absorption compared to PTzBDT-2, likely due to a more planar conformation of the polymer backbone promoted by the lower content of alkyl side chains. The different aggregation in the solid state also affects the energetic properties of the polymers, resulting in a lower highest occupied molecular orbital (HOMO) for PTzBDT-1 with respect to PTzBDT-2. However, an unexpected behaviour is observed when the two polymers are used as a donor material, in combination with PC61BM as acceptor, in bulk heterojunction solar cells. Even though PTzBDT-1 showed favourable optical and electrochemical properties, the devices based on this polymer present a power conversion efficiency of 3.3%, considerably lower than the efficiency of 4.7% obtained for the analogous solar cells based on PTzBDT-2. The lower performance is presumably attributed to the limited solubility of the PTzBDT-1 in organic solvents resulting in enhanced aggregation and poor intermixing with the acceptor material in the active layer.