# Tuning Molar Mass of the D18 Polymer via Stille Polymerization: Impact on Morphology and Large-Area Blade-Coated Organic Solar Cells

## TL;DR

This paper shows how adjusting the molar mass of a polymer used in solar cells affects their performance and film quality.

## Contribution

The study systematically tunes D18 polymer molar mass via Stille polymerization and links it to OSC performance and morphology.

## Key findings

- High-molar-mass D18 polymers produce smoother films and better phase separation in solar cells.
- Low-molar-mass D18 results in poor solar cell performance with power conversion efficiencies below 2%.
- Halogenated solvents are needed to optimize high-molar-mass D18 performance, while o-xylene works better for low-molar-mass D18.

## Abstract

Achieving reproducible,
high-performance organic solar cells (OSCs)
requires precise control over the molar mass of donor polymers, as
it governs film formation, morphology, and charge transport. Here,
we systematically investigate the influence of Stille polymerization
conditions on the molar mass, optoelectronic properties, morphology,
and device performance of the benchmark donor polymer D18. By varying
reaction time, catalyst type, and catalyst loading, we access D18
batches with weight-average molar masses (M
w) ranging from approximately 12 to 93 kg·mol–1. Gel permeation chromatography, UV–vis absorption, cyclic
voltammetry, optical microscopy, profilometry, and AFM indicate that
higher-M
w polymers exhibit enhanced aggregation
signatures, as well as smoother, more compact films, and improved
donor–acceptor phase separation compared to low-M
w analogues. Bulk heterojunction OSCs with an inverted
architecture (R2R-patterned PET/IMI)/ZnO/D18:Y6:PC70BM/PEDOT:PSS/Ag)
are fabricated by blade coating under ambient atmosphere over large
active areas (0.55 cm2). Devices based on low-M
w D18 (M
w ≈ 12–14
kg·mol–1) show poor performance (PCE < 2%),
which correlates with low shunt resistance and unfavorable morphology,
whereas high-M
w D18 samples (M
w ≈ 83–93 kg·mol–1) reach power conversion efficiencies of 7.8–8.0%, approaching
that of a commercial D18 reference (8.9%). A solvent study further
reveals that halogenated solvents (chloroform, chlorobenzene) are
required to fully realize the potential of high-M
w D18, while o-xylene yields more homogeneous
films and competitive efficiencies primarily for low-M
w material. These findings highlight molar mass control
and solvent selection as interdependent parameters for optimizing
morphology and device performance in scalable, blade-coated D18-based
organic solar cells.