Eco‐Friendly Solvent‐Processed Fullerene‐Free Polymer Solar Cells with over 9.7% Efficiency and Long‐Term Performance Stability

Abstract

A wide‐bandgap polymer, (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(2,5‐(methyl thiophene carboxylate))]) (3MT‐Th), is synthesized to obtain a complementary broad range absorption when harmonized with 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITIC). The synthesized regiorandom 3MT‐Th polymer shows good solubility in nonhalogenated solvents. A film of 3MT‐Th:ITIC can be employed for forming an active layer in a polymer solar cell (PSC), with the blend solution containing toluene with 0.25% diphenylether as a nonhalogenated additive. The corresponding PSC devices display a power conversion efficiency of 9.73%. Moreover, the 3MT‐Th‐based PSCs exhibit excellent shelf‐life time of over 1000 h and are operationally stable under continuous light illumination. Therefore, methyl thiophene‐3‐carboxylate in 3MT‐Th is a promising new accepting unit for constructing p‐type polymers used for high‐performance nonfullerene‐type PSCs.

Morphology Control in Organic Solar Cells

“…Interestingly, OSCs fabricated from nonchlorinated solvents toluene/diphenyl ether (DPE) afford higher J SC , FF, and PCE than those from CF with DCB or DPE as additive, which may originate from the morphology difference in the blend films . Recently, highly efficient nonfullerene polymer solar cells have been processed from environmental‐friendly solvent o ‐xylene ( o ‐XY) or toluene which are low‐toxic aromatic solvent and favorable for roll‐to‐roll fabrication . Taking the PTB7‐Th:ITIC system as an example, the nonhalogenated processing solvent o ‐XY affords higher J SC and PCE than those halogenated solvents DCB (14.97 vs 11.31 mA cm −2 and 8.11% vs 5.78%), attributed to the smaller and well‐defined nanoscale phase separation …”

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

“…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 . Given similarities in the BHJ components' chemical structures, solvents additives may need to exploit smaller differences in solubility.…”

Environmentally Friendly Solvent‐Processed Organic Solar Cells that are Highly Efficient and Adaptable for the Blade‐Coating Method

“…Chlorinated and aromatic solvents, which are detrimental to the environment and human health, are usually used for making the solutions . Although few studies have demonstrated that low toxic solvents are applicable for highly efficient OSCs (the detailed information of the halogen‐free solvents and the photovoltaic parameters of the corresponding devices are listed in Table S2 in the Supporting Information), all of the devices in these works have been fabricated using the spin‐coating method . As is known, wet films will be dried with the help of rapid airflow due to the centrifugal effect during the spin‐coating process, while wet films will be dried differently when using other coating methods, such as blade‐coating, slot‐die coating, and gravure, which are suitable for making large area devices .…”