Side‐Chain Engineering for Enhancing the Thermal Stability of Polymer Solar Cells

Abstract

An effective strategy of engineering side chains is proposed for enhancing solar-cell-device thermal stability. As the conjugated length of the side chains increases, the morphological stability of the blend film is enhanced. The thermal stability of corresponding devices is consequently improved.

Side‐Chain Engineering for Enhancing the Properties of Small Molecule Solar Cells: A Trade‐off Beyond Efficiency

“…This trend is further confirmed by the change in PL intensity, i.e., ΔI = (I t −I 0 )/I 0 , where I 0 and I t are the PL intensities before and after annealing t hours, respectively. [30,31] As presented in Figure S22 and S23b in the SI, the initial PL quenching rate of 93% is significantly decreased after thermal annealing for 80 h, with BDTT-TR and BDTT-O-TR gaining PL intensity more accelerated than BDTT-S-TR. Enhanced molecular ordering and/or fullerene aggregations explain the general degradation pattern observed [30] and underline that BDTT-S-TR is thermally slightly more stable due to its moderate morphological deterioration at higher temperatures.…”

“…[30,31] As presented in Figure S22 and S23b in the SI, the initial PL quenching rate of 93% is significantly decreased after thermal annealing for 80 h, with BDTT-TR and BDTT-O-TR gaining PL intensity more accelerated than BDTT-S-TR. Enhanced molecular ordering and/or fullerene aggregations explain the general degradation pattern observed [30] and underline that BDTT-S-TR is thermally slightly more stable due to its moderate morphological deterioration at higher temperatures. [61] The concept of molecular side chain engineering for fixing or "freezing" the blend morphology was highlighted previously.…”

“…Nevertheless, the meta-alkylthio substitution seems to positively influence the thermal stability of the BHJ composites' microstructure. [30,59] Optical microscopy ( Figure S19 in the SI) and AFM ( Figure S20 in the SI) were further used to study the origin of the thermal instability but did not give strong evidences. The surface morphology of BDTT-TR, BDTT-O-TR, and BDTT-S-TR blends show distinct but small changes during thermal annealing (24 and 80 h) but no evidence of major phase separation.…”

“…Yang and co-workers have reported a series of polymers with different conjugated side chains on benzo[1,2-b:4,5-b′]-dithiophene (BDT) unit, and devices with a long alkyl-thieno [3,2-b]thiophene (alkyl-TT) substitution exhibited outstanding efficiency and thermal stability, which was attributed to the relatively small morphological changes of the active layer. [30] In addition, McGehee and co-workers and Brabec and co-workers found that a high degree of crystallinity could significantly reduce the initial light-induced degradation, which is caused by light-induced traps. [37,44] Despite the remarkable thermal stability and photoinduced stability in these studies, the precise correlation between the molecular structure and device stability remains phenomenological, based on the reports for several thousands of photovoltaic materials.…”

“…[30,37] An important intrinsic mechanism is thermal degradation because an optimal BHJ morphology was frequently found to be a metastable state which readily moves toward a less efficient thermodynamic equilibrium state. [30,38,39] Note that such morphological transformation is easily accelerated by accumulated heat from sunlight irradiation in a real device. [40,41] This has to be distinguished from intrinsic light-induced degradation as a severe limitation for device efficiency.…”