A real-time study of the benefits of co-solvents in polymer solar cell processing

Abstract

The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating and electron-accepting organic semiconductors. Controlling the degree of phase separation between these components is crucial to reach efficient solar cells. In solution-processed polymer-fullerene solar cells, small amounts of co-solvents are commonly used to avoid the formation of undesired large fullerene domains that reduce performance. There is an ongoing discussion about the origin of this effect. To clarify the role of co-solvents, we combine three optical measurements to investigate layer thickness, phase separation and polymer aggregation in real time during solvent evaporation under realistic processing conditions. Without co-solvent, large fullerene-rich domains form via liquid-liquid phase separation at B20 vol% solid content. Under such supersaturated conditions, co-solvents induce polymer aggregation below 20 vol% solids and prevent the formation of large domains. This rationalizes the formation of intimately mixed films that give high-efficient solar cells for the materials studied.

Exploring the origin of high optical absorption in conjugated polymers

“…Whilst the absorption of a polymer in the solid state will also be affected by intermolecular interactions, there is substantial evidence that microstructure in the solid state reflects the structure in solution (e.g. Ref 40 ). By employing these design considerations, our results indicate that polymers can be designed such that their absorption is less sensitive to conformation, thus allowing their full potential to be realised.…”

Improving the Performances of Random Copolymer Based Organic Solar Cells by Adjusting the Film Features of Active Layers Using Mixed Solvents

“…Furthermore, the performance of BHJ solar cell in part depends on whether polymer or [6,6]-phenyl-C71-butyric acid methyl ester (PC 71 BM) would be enriched on the top surface of the blend film [19]. Besides, mixed solvents can also be effectively used to tune the miscibility of the donor and acceptor components, the domain size and the morphology of active layer, which play an important role in optimizing the photovoltaic performances for PSCs [20][21][22].…”

Enhanced power conversion efficiency in iridium complex-based terpolymers for polymer solar cells

“…20,21 From the device engineering side, (iv) the introduction of ternary structures in the active layer with two-donors/one-acceptor or two-acceptors/one-donor to enhance the absorption and increase the short-circuit current (J sc ), 8,[22][23][24][25][26] (v) the use of tandem structures including two or more subcells connected in series with complementary absorption bands to reduce the thermalization losses and increase the open-circuit voltages (V oc ) [27][28][29] and (vi) the optimization of the film morphology of the active layer adopting solvent vapor annealing, thermal annealing, mixed solvent and/or solvent additives have also been proved to be successful methods to achieve high performance OSCs. [30][31][32] Besides, from the perspective of materials, the involvement of a suitable third component either through low ratio physical doping into the mixture of active layer, or as a third monomer by chemical bonding to the new random terpolymers and then used as new polymer donors 33,34 or non-fullerene polymer acceptors [35][36][37] have also been employed as facile and reliable approaches to enhance the PCE. For example, doping 10 wt% of an orange-emitting Ir complex [(2-pq) 2 Irpic-OH] to PTB7/PC 71 BM solar cell devices, resulted in the increase of PCE from 7.37 to 8.72%.…”