Hexanary blends: a strategy towards thermally stable organic photovoltaics.

Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 °C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells.

In Situ Optical Spectroscopy Demonstrates the Effect of Solvent Additive in the Formation of All-Polymer Solar Cells.

1-Chloronaphthalene (CN) has been a common solvent additive in both fullerene- and nonfullerene-based organic solar cells. In spite of this, its working mechanism is seldom investigated, in particular, during the drying process of bulk heterojunctions composed of a donor:acceptor mixture. In this work, the role of CN in all-polymer solar cells is investigated by in situ spectroscopies and ex situ characterization of blade-coated PBDB-T:PF5-Y5 blends. Our results suggest that the added CN promotes self-aggregation of polymer donor PBDB-T during the drying process of the blend film, resulting in enhanced crystallinity and hole mobility, which contribute to the increased fill factor and improved performance of PBDB-T:PF5-Y5 solar cells. Besides, the nonradiative energy loss of the corresponding device is also reduced by the addition of CN, corresponding to a slightly increased open-circuit voltage. Overall, our observations deepen our understanding of the drying dynamics, which may guide further development of all-polymer solar cells.

Nonconjugated Terpolymer Acceptors with Two Different Fused-Ring Electron-Deficient Building Blocks for Efficient All-Polymer Solar Cells.

The ternary polymerization strategy of incorporating different donor and acceptor units forming terpolymers as photovoltaic materials has been proven advantageous in improving power conversion efficiencies (PCEs) of polymer solar cells (PSCs). Herein, a series of low band gap nonconjugated terpolymer acceptors based on two different fused-ring electron-deficient building blocks (IDIC16 and ITIC) with adjustable photoelectric properties were developed. As the third component, ITIC building blocks with a larger π-conjugation structure, shorter solubilizing side chains, and red-shifted absorption spectrum were incorporated into an IDIC16-based nonconjugated copolymer acceptor PF1-TS4, which built up the terpolymers with two conjugated building blocks linked by flexible thioalkyl chain-thiophene segments. With the increasing ITIC content, terpolymers show gradually broadened absorption spectra and slightly down-shifted lowest unoccupied molecular orbital levels. The active layer based on terpolymer PF1-TS4-60 with a 60% ITIC unit presents more balanced hole and electron mobilities, higher photoluminescence quenching efficiency, and improved morphology compared to those based on PF1-TS4. In all-polymer solar cells (all-PSCs), PF1-TS4-60, matched with a wide band gap polymer donor PM6, achieved a similar open-circuit voltage (Voc) of 0.99 V, a dramatically increased short-circuit current density (Jsc) of 15.30 mA cm-2, and fill factor (FF) of 61.4% compared to PF1-TS4 (Voc = 0.99 V, Jsc = 11.21 mA cm-2, and FF = 55.6%). As a result, the PF1-TS4-60-based all-PSCs achieved a PCE of 9.31%, which is ∼50% higher than the PF1-TS4-based ones (6.17%). The results demonstrate a promising approach to develop high-performance nonconjugated terpolymer acceptors for efficient all-PSCs by means of ternary polymerization using two different A-D-A-structured fused-ring electron-deficient building blocks.

An experimental setup for dip-coating of thin films for organic solar cells under microgravity conditions.

We report the design and testing of a custom-built experimental setup for dip-coating from volatile solutions under microgravity conditions onboard an aircraft. Function and safety considerations for the equipment are described. The equipment proved to work well, both concerning the safety and the preparation of thin films. No leakage of the solvents, nor the solvent vapors, was detected, not even in a situation with a fluctuating gravitational field due to bad weather conditions. We have shown that the equipment can be used to prepare thin films of polymer blends, relevant for organic solar cells, from solution in a feasible procedure under microgravity conditions. The prepared films are similar to the corresponding films prepared under 1 g conditions, but with differences that can be related to the absence of a gravitational field during drying of the applied liquid coating. We report on some introductory results from the characterization of the thin films that show differences in film morphology and structure sizes.

Using Solubility Parameters to Model More Environmentally Friendly Solvent Blends for Organic Solar Cell Active Layers.

To facilitate industrial applications, as well as for environmental and health purposes, there is a need to find less hazardous solvents for processing the photoactive layer of organic solar cells. As there are vast amounts of possibilities to combine organic solvents and solutes, it is of high importance to find paths to discriminate among the solution chemistry possibilities on a theoretical basis. Using Hansen solubility parameters (HSP) offers such a path. We report on some examples of solvent blends that have been found by modelling HSP for an electron donor polymer (TQ1) and an electron acceptor polymer (N2200) to match solvent blends of less hazardous solvents than those commonly used. After the theoretical screening procedure, solubility tests were performed to determine the HSP parameters relevant for the TQ1:N2200 pair in the calculated solvent blends. Finally, thin solid films were prepared by spin-coating from the solvent blends that turned out to be good solvents to the donor-acceptor pair. Our results show that the blend film morphology prepared in this way is similar to those obtained from chloroform solutions.