The Influence of Conjugated Polymer Side Chain Manipulation on the Efficiency and Stability of Polymer Solar Cells

The stability of polymer solar cells (PSCs) can be influenced by the introduction of particular moieties on the conjugated polymer side chains. In this study, two series of donor-acceptor copolymers, based on bis(thienyl)dialkoxybenzene donor and benzo[c][1,2,5]thiadiazole (BT) or thiazolo[5,4-d]thiazole (TzTz) acceptor units, were selected toward effective device scalability by roll-coating. The influence of the partial exchange (5% or 10%) of the solubilizing 2-hexyldecyloxy by alternative 2-phenylethoxy groups on efficiency and stability was investigated. With an increasing 2-phenylethoxy ratio, a decrease in solar cell efficiency was observed for the BT-based series, whereas the efficiencies for the devices based on the TzTz polymers remained approximately the same. The photochemical degradation rate for PSCs based on the TzTz polymers decreased with an increasing 2-phenylethoxy ratio. Lifetime studies under constant sun irradiance showed a diminishing initial degradation rate for the BT-based devices upon including the alternative side chains, whereas the (more stable) TzTz-based devices degraded at a faster rate from the start of the experiment upon partly exchanging the side chains. No clear trends in the degradation behavior, linked to the copolymer structural changes, could be established at this point, evidencing the complex interplay of events determining PSCs’ lifetime

A stability study of polymer solar cells using conjugated polymers with different donor or acceptor side chain patterns

The position and nature of side chains in the donor–acceptor copolymer were investigated in terms of stability in polymer solar cells.</p

Screen printing of transparent conductive features based on silver nanowire networks

Metal nanowire networks are a promising concept for replacing ITO in transparent electrodes for a range of optoelectronic devices. In these networks, the nanowires conduct charge carriers, while the open areas allow the transmission of light. Metal nanowires are both printable and achieve a performance equivalent to ITO upon thermal processing at temperatures below 150°C which make them ideal for depositing transparent electrodes on plastic substrates. For this contribution, silver nanowires (AgNWs) were synthesized using a polyol synthesis method. Various formulations containing AgNWs were prepared and their rheological behavior was assessed in view of screen printing. The most promising formulations were selected to print test features on PET substrates and the electrical and optical characteristics of these features were measured by a Van der Pauw method and UV-Vis spectroscopy respectively. By the addition of AgNWs to a PEDOT-based formulation, the sheet resistance of the printed features can be decreased from ca. 200 Ohm/sq to values below 40 Ohm/sq, indicating that a nanowire network improves the conductivity. However these features appear blue due to the absorption of (infra-)red light by PEDOT. The optical properties can be significantly enhanced by replacing the PEDOT-based formulation by a cellulose-based formulation.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 641864

Isomeric modulation of thermally activated delayed fluorescence in dibenzo[ a, c ]phenazine-based (deep) red emitters †

A series of four emissive regio-isomers are synthesized based on the dibenzo[a, c]phenazine-11,12-dicarbonitrile (DBPzCN) acceptor scaffold and a triphenylamine (TPA) donor. Density functional theory is utilized to compare the relative differences in molecular conformation, excited state distributions, and orbital interactions. Steady-state and time-resolved emission spectroscopy reveal strongly contrasting emissive properties and triplet harvesting of the four materials. In zeonex host emission maxima range widely, with differences of over 100 nm. Additionally, isomers 3-TPA-DBPzCN and 4-TPA-DBPzCN show photoluminescence quantum yields (PLQYs) of 46 and 62%, while 1-TPA-DBPzCN and 2-TPA-DBPzCN instead show values <1 and 24%, respectively. Relevant to thermally activated delayed fluorescence (TADF), very small singlet–triplet energy gaps are observed for isomers 2-TPA-DBPzCN and 4-TPA-DBPzCN, with corresponding reverse intersystem crossing (rISC) rates of 0.6 and 1.6 × 105 s−1, respectively. Unique in possessing both fast rISC and a relatively high PLQY, the unconventional 4-TPA-DBPzCN regio-isomer turns out to be an efficient TADF emitter, highlighting the important role of donor–acceptor substitution position in the design of efficient TADF materials targeting specific wavelength ranges

Synthesis of Highly Fluorescent All-Conjugated Alternating Donor–Acceptor (Block) Copolymers via GRIM Polymerization

Although controlled polymerization procedures for conjugated polymers have considerable advantages with respect to molar mass and end group control, the material scope has been very limited, in particular considering block copolymers and donor–acceptor type all-conjugated polymers, imposing considerable challenges upon the synthetic polymer community. In this work, a push–pull monomer consisting of a thiophene (donor) and a pyridine (acceptor) unit is synthesized and subsequently polymerized via Kumada catalyst-transfer polymerization using a nickel catalyst (GRIM polymerization). In this way, an alternating donor–acceptor copolymer is obtained via a chain-growth mechanism. Furthermore, an all-conjugated block copolymer containing a poly­(3-hexyl­thiophene) block and the alternating copolymer is successfully prepared in a one-pot procedure as well. The diblock structure is confirmed by comparison of the thermal, electrochemical, and spectroscopic properties of the block copolymer and its constituting polymer parts

Low bandgap copolymers based on monofluorinated isoindigo towards efficient polymer solar cells

\u3cp\u3eTo explore the effectiveness of monofluorinated isoindigo as an electron-deficient building block in push-pull conjugated polymers for organic solar cell applications, four low bandgap copolymers are effectively synthesized and characterized. The effects of fluorine introduction, thiophene spacer length and polymer molar mass on the general electro-optical polymer characteristics, thin film blend microstructure and electronic performance are investigated. Isoindigo monofluorination effectively improves the power conversion efficiency from 2.8 up to 5.0% upon molar mass optimization, without using any processing additives or post-treatments.\u3c/p\u3

Designing small molecule organic solar cells with high open-circuit voltage

\u3cp\u3eThree extended 2,5-dithienylthiazolo[5,4-d]thiazole-based small molecule chromophores are prepared via a sustainable direct arylation approach and their physicochemical and opto-electrical material characteristics are analyzed toward integration in solution-processed bulk heterojunction organic photovoltaics. Efficient charge separation and high values of the charge transfer state energy are derived from sensitive ground and excited state absorption and photoluminescence measurements on blends of the thiazolo[5,4-d]thiazole-based electron donor components with the PC\u3csub\u3e71\u3c/sub\u3eBM fullerene acceptor. Upon implementation in organic solar cells, a maximum power conversion efficiency of 2.7% and particularly high open-circuit voltages (0.93−0.98 V) are observed, which are correlated to the charge transfer state energies as derived from photoluminescence, Fourier transform photocurrent spectroscopy and combined electrochemical and photophysical data. Furthermore, several loss processes at the origin of the modest short-circuit current densities and fill factors are elucidated.\u3c/p\u3

Designing small molecule organic solar cells with high open-circuit voltage

Three extended 2,5-dithienylthiazolo[5,4-d]thiazole-based small molecule chromophores are prepared via a sustainable direct arylation approach and their physicochemical and opto-electrical material characteristics are analyzed toward integration in solution-processed bulk heterojunction organic photovoltaics. Efficient charge separation and high values of the charge transfer state energy are derived from sensitive ground and excited state absorption and photoluminescence measurements on blends of the thiazolo[5,4-d]thiazole-based electron donor components with the PC71BM fullerene acceptor. Upon implementation in organic solar cells, a maximum power conversion efficiency of 2.7% and particularly high open-circuit voltages (0.93−0.98 V) are observed, which are correlated to the charge transfer state energies as derived from photoluminescence, Fourier transform photocurrent spectroscopy and combined electrochemical and photophysical data. Furthermore, several loss processes at the origin of the modest short-circuit current densities and fill factors are elucidated