Achieving balanced intermixed and pure crystalline phases in PDI-based non-fullerene organic solar cells via selective solvent additives

Abstract

Herein, balanced intermixed and pure crystalline phases in N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI)-based non-fullerene organic solar cells (OSCs) were achieved via selective solvent additives (SAs). Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (F-DTS) possessing different compatibilities with EP-PDI were selected as model systems to investigate the guideline of SAs selection for different non-fullerene-based systems. According to the solubility parameter difference (Δδ) between EP-PDI and SAs, five different SAs were divided into two types: (I) strong intermolecular interactions with EP-PDI molecules (with Δδ values less than 5 MPa(1/2)), (II) weak intermolecular interactions with EP-PDI molecules (with large Δδ values). For PTB7:EP-PDI system with large and obvious phase separation, the introduction of type (II) SAs provided extra interactions with EP-PDI molecules, thus effectively reducing EP-PDI aggregate domains and increasing intermixed fractions. The incorporation of type (II) SAs resulted in a greater yield of dissociated polarons, and the final device efficiency increased from 0.02% to 1.65%. On the contrary, for finely mixed F-DTS:EP-PDI systems, type (I) SAs were considerably more effective because of the fact that the required pure crystalline phases were readily induced by the unfavorable interactions. The charge transport pathways optimized by type (I) SAs improved device efficiency from 0.18% to 2.82%. Hence, by processing selective SAs, the fraction of intermixed and pure crystalline phases for PDI-based non-fullerene OSCs can be well regulated; therefore, the final performance for both systems can be significantly improved.

Solvent Additives: Key Morphology‐Directing Agents for Solution‐Processed Organic Solar Cells

“…Solvent additives often play a prominent role in optimizing donor:NFA morphologies, as NFAs can be prone to overcrystallization (for rigid planar structures) or intimate mixing (when there are fewer chemical differences between donor and acceptor). Li et al confirmed that additives can be used to address both L–L and S–L demixing processes in donor:NFA systems using PTB7:PDI and p ‐DTS(FBTTh 2 ) 2 :PDI, respectively . The greater and tunable solubility of NFAs will likely widen the selection of deposition solvents and solvent additives .…”

Recent Advances in Morphology Optimization for Organic Photovoltaics

“…The use of type II additives in the PTB7:EP‐PDI system reduces the severe aggregation of EP‐PDI molecules and increases the size of the donor:acceptor intermixed phase. These results show that a balance between the areas of the intermixed and pure phase can be achieved by selecting the solvent additive that is appropriate to the strengths of the intermolecular interactions in the donor:acceptor system …”

New 3,3′-(ethane-1,2-diylidene)bis(indolin-2-one) (EBI)-based small molecule semiconductors for organic solar cells

“…For BHJ OSCs, the device performance is highly dependent on the solid-state microstructure and phase morphology of the active layer which is ideally composed of three phases: an intermixed donor/acceptor phase, a pure crystalline donor phase and a pure crystalline acceptor phases. The intermixed network promotes efficient and fast charge separation [34][35][36] whereas pure crystalline domains ensure charge transport to the electrodes. 37 Hence, indication of the mutual miscibility of the donor and the acceptor, as obtained via establishing phase diagrams is important.…”