Photoluminescence of Low‑Molecular‑Weight Poly(3‑hexylthiophene) Polymorphs: Insights into Interchain Interactions

Abstract

We investigated the structural and photoluminescence (PL) properties of poly(3‑hexylthiophene) (P3HT) thin films with molecular weights (MWs) of 3 000 and 13 300. While high‑MW P3HT invariably adopts a single lamellar packing (form I), low‑MW P3HT can crystallize into either form I or the alternative form II depending on processing. By optimizing fabrication conditions—particularly drop‑casting—we produced form II films with minimal form I contamination. Their PL spectra differ markedly from those of form I, confirming that the spectral shift arises from the longer interchain spacing (≈ 4.4 Å) in form II, which weakens interchain coupling.

Backgrounds

Poly(3‑alkylthiophene)s (P3ATs) are prototypical π‑conjugated polymers that can adopt multiple crystalline structures governed by processing conditions. High‑MW P3ATs typically form a lamellar π‑stacking phase (form I) with a 3.8 Å backbone spacing, facilitating charge delocalization. In contrast, low‑MW P3ATs can also crystallize into a tilted, interdigitated phase (form II) with a 4.4 Å spacing, reducing interchain coupling. Prior studies have documented form I and form II in P3BT and P3HT, yet the distinct optical signatures of form II remain underexplored, largely because high‑quality form II films are difficult to prepare.

Lu et al. demonstrated that form II of poly(3‑butylthiophene) (P3BT) can be promoted by slow solvent evaporation or vapor exposure, and that thermal annealing reverses the transformation back to form I. Analogous behavior is observed in poly(9,9‑dioctylfluorene) (F8), where vapor treatment yields the β‑phase while annealing restores the crystalline phase. Drawing on this analogy, we hypothesized that drop‑casting—an inherently slow evaporation technique—might favor the formation of high‑quality form II P3HT films.

Methods

Regioregular P3HTs of MW = 3 000 (PDI = 1.3) and MW = 13 300 (PDI = 1.3) were purchased and used without further purification. Thin films were fabricated on quartz by spin‑coating or drop‑casting from chloroform solutions, yielding thicknesses between 80 and 120 nm. Films were dried under vacuum for 30 min; selected samples underwent thermal annealing at 155 °C for 30 min or vapor exposure to saturated chloroform for 15 h. A precipitate of low‑MW P3HT was also prepared by adding methanol to the chloroform solution and drying on Si.

Optical measurements were performed at 6 K using a calibrated CCD detector and a 532 nm laser for PL. Absorption spectra were recorded with a Xenon lamp. X‑ray diffraction (XRD) employed Cu Kα radiation in the out‑of‑plane geometry.

Results and Discussion

Figure 1a displays out‑of‑plane XRD patterns of high‑MW P3HT, showing characteristic form I reflections and confirming that the stacking direction lies parallel to the substrate. Processing method had no effect on high‑MW films.

Figure 1b reveals that low‑MW P3HT spin‑coated and annealed films also exhibit form I diffraction. Drop‑cast low‑MW films, however, show additional peaks at 7.35° and 14.7°, matching the reported form II pattern. The relative intensity of form II increases as solvent evaporation is slowed, and vapor exposure can convert form I to form II. A precipitated low‑MW sample shows a 20.2° reflection corresponding to the 4.4 Å spacing of form II.

The polymorphic behavior arises because the energetic difference between form I and form II is small; the dominant stabilizing interactions differ. In form I, inter‑alkyl‑chain attractions prevail, while in form II backbone–backbone attractions dominate. Drop‑casting allows sufficient time for backbones to self‑organize before alkyl chains solidify, favoring form II. Thermal annealing, which equilibrates both backbone and alkyl chains, drives the system toward the thermodynamically preferred form I.

Absorption spectra (Figure 2) of high‑MW films are consistent with literature. Spin‑coated low‑MW films show a slight blue shift relative to high‑MW, attributable to a higher amorphous content rather than backbone length. Drop‑cast low‑MW films exhibit a baseline shift due to light scattering but lack distinct form II absorption features, likely because absorption bands of the two forms overlap.

PL spectra (Figure 3) reveal the key distinction: high‑MW films and spin‑coated low‑MW films display the characteristic form I PL, whereas drop‑cast low‑MW films show a blue‑shifted PL exceeding 0.1 eV, characteristic of form II. Thermal annealing or vapor treatment of low‑MW films produces mixed PL spectra, confirming the coexistence of both forms. Thus, drop‑cast low‑MW films provide the cleanest platform to study form II photophysics.

Comparing form I and form II PL yields direct evidence of interchain coupling. The 0.1 eV blue shift in form II arises from the increased inter‑stack distance (4.4 Å vs. 3.8 Å), reducing electronic interaction. Additionally, the 0–0 transition at 1.98 eV is more pronounced in form II, and the PL quantum yield is roughly three times higher than in form I—both predictions of the weakly coupled H‑aggregate model. The excitation spectra (Figure 4) are nearly identical for the two forms, indicating that the primary absorption bands overlap and explaining why absorption spectra alone cannot distinguish the polymorphs.

The ~0.05 eV shift between excitation spectra corresponds to half the PL blue shift, implying a reduced Stokes shift in form II due to a narrower distribution of energy levels and diminished exciton migration.

Conclusions

We demonstrated that drop‑cast thin films of low‑MW P3HT reliably form the form II polymorph, which exhibits a distinct, blue‑shifted PL spectrum and higher quantum yield relative to form I. Because both polymorphs adopt fully planar backbones, the spectral differences directly reflect the altered interchain spacing and strength of interchain coupling. These findings provide unambiguous experimental evidence of interchain interactions in P3HT and establish a clear strategy for tailoring photophysical properties via polymorphic control.

Abbreviations

F8

Poly(9,9‑dioctylfluorene)

MW

Molecular weight

P3AT

Poly(3‑alkylthiophene)

P3BT

Poly(3‑butylthiophene)

P3HT

Poly(3‑hexylthiophene)

PDI

Polydispersity index

PL

Photoluminescence

XRD

X‑ray diffraction