How PMMA Confinement Enhances Photoluminescence in P3HT Aggregates: Band‑Gap Redshift and Exciton Bandwidth Reduction

Background

Conjugated polymers such as P3HT have attracted sustained interest over the past decade because their excitonic dynamics directly affect the performance of organic photovoltaic devices [1–4]. Exciton generation, radiative recombination, and charge transfer processes in P3HT aggregates and nanocrystallites dictate the efficiency of light‑to‑current conversion. Notably, isolated P3HT molecules emit via a relaxed intrachain exciton state with reduced torsional disorder [5], whereas aggregates emit from an interchain singlet exciton that migrates through energy‑transfer cascades to the lowest‑energy domain [6]. In densely packed lamellar films, PL QY is markedly suppressed relative to solution‑phase molecules due to interchain delocalization and exciton quenching [7]. Nonetheless, QY can be tuned by temperature control [8] or by manipulating regioregularity [9], underscoring the importance of structural ordering on optical performance.

Despite extensive studies on solvent and processing effects, the role of an inert polymer host in modulating the optical properties of P3HT has been less explored. Prior work has shown that polar environments such as water, poly(ethylene oxide) (PEO), or poly(ethylene glycol) can shift absorption spectra and modify oscillator strengths [10–13]. Yet, the influence on PL QY, especially in the solid state, remains underreported. Recent reports suggest that diluting conjugated polymers with moderate inert hosts can enhance PL by converting aggregates to isolated chains [14]. Building on these insights, our study demonstrates that blending P3HT with PMMA—a polar, high‑dielectric constant polymer—induces systematic photophysical changes by forming micro‑ and nanoscale P3HT particles.

The central objective of this work is to elucidate how varying the P3HT/PMMA ratio alters the electronic absorption, band‑gap, and PL QY of P3HT aggregates. We show that decreasing the P3HT content leads to a red‑shifted band‑gap, improved ordering, and higher PL QY, likely driven by planarization of the polymer backbone within the PMMA matrix via hydrophobic interactions.

Methods

Sample Preparation

P3HT (regioregularity ≈93 %, Mw 15–45 kDa, Sigma‑Aldrich) was dissolved at 1.0 wt % in chlorobenzene (CB). Binary blends were prepared by adding PMMA (Mw 120 kDa, Sigma‑Aldrich) to the P3HT solution, followed by 30 min sonication. Films were spin‑coated on glass at 1500 rpm for 30 s. For TEM, films were scraped into acetone, left for several hours to dissolve PMMA, and drop‑cast onto carbon grids; the resulting insoluble P3HT aggregates were imaged.

Spectroscopy Measurements

Absorption spectra were recorded with SPECORD M40 and OLIS Cary 14 spectrophotometers, using bare glass as reference. PL spectra were collected on a SPEX Fluorolog 1680 with a 468 nm Xe lamp excitation. Spectra were normalized to maximum absorbance and corrected for system sensitivity, yielding relative QY values. Transient absorption (TA) was performed on a Ti:sapphire laser system (410 nm pump, 1 kHz, 100 fs pulses) with a white‑light probe; the pump was modulated at 500 Hz, and ΔOD was detected by lock‑in amplification. The TA signal was expressed as TA = −ΔT/T = (T_on – T_off)/T_off.

Microscopy Measurements

Optical micrographs were captured with a ULAB XY‑B2 microscope, while TEM imaging employed a JEOL JEM‑1400 at 80 kV.

Results

Photophysical Studies

The electronic absorption spectra of P3HT/PMMA films (Fig. 1a) display the canonical onset at ≈650 nm (1.9 eV) followed by vibronic replicas at 605, 560, and 525 nm (0‑0, 0‑1, 0‑2 transitions). As the P3HT content decreases, the 0‑0/0‑1 amplitude ratio rises, the short‑wavelength side narrows—indicating reduced amorphous disorder—and the 0‑0 maximum shifts from 602 to 608 nm, translating to a band‑gap reduction from 1.92 to 1.89 eV.

a Normalized electronic absorption spectra, b PL spectra (relative QY), and c comparison of normalized absorption and PL spectra for varying P3HT:PMMA ratios

PL spectra (Fig. 1b) exhibit a Stokes shift of ≈0.15 eV and mirror the absorption sideband sequence. The increasing 0‑0/0‑1 ratio with decreasing P3HT content reflects enhanced ordering, consistent with literature on lamellar ordering in conjugated polymers [16–19]. The exciton bandwidth (W) can be estimated via Equation (1), revealing a narrowing trend as the P3HT/PMMA ratio decreases (Fig. 2a). Correspondingly, the relative PL QY rises by a factor of four (Fig. 2b). This behavior indicates that reduced interchain coupling and increased intrachain order favor exciton localization, thereby suppressing non‑radiative pathways.

Equation (1) where A_0‑0/A_0‑1 ≈ (n_0‑1/n_0‑0)·[(1 – 0.24W/E_p)/(1 + 0.073W/E_p)]² with n ratios ≈0.97 and E_p = 0.18 eV (C=C stretch) [21]. The exciton bandwidth asymptotically approaches a saturation value of 45 ± 5 meV, implying a limit to the size of ordered P3HT domains even at minimal loadings.

a Exciton bandwidth, b relative PL QY vs. PMMA:P3HT ratio (assuming neat P3HT QY ≈ 0.5 %)

TA spectroscopy further substantiates increased ordering. The neat P3HT film shows a pronounced stimulated emission (SE) band at ≈700 nm, characteristic of intrachain excitons in disordered chains, whereas the P3HT/PMMA blend displays a delocalized polaron absorption at ≈660 nm, indicative of crystalline order (Fig. 3). Relaxation dynamics derived from the 0‑0 and 0‑1 bands reveal faster torsional relaxation (≈1.8 ps) and shorter exciton lifetimes in the blend, consistent with reduced torsional disorder and more efficient intrachain energy transfer.

TA spectra of a P3HT:PMMA (1:50) and b neat P3HT films. Time delays are shown in femtoseconds.

Morphology Studies

Optical and TEM imaging reveal a pronounced phase separation between P3HT and PMMA. At high P3HT loadings (≥10 wt %), a percolating network forms, whereas at lower loadings the P3HT fraction segregates into micron‑ to sub‑micron particles. TEM images (Fig. 5a) show nearly spherical particles as small as ≈30 nm, with SAED patterns indicating a crystalline core surrounded by an amorphous shell (~10 nm thick) [35]. This morphology suggests repulsive forces between the polar PMMA matrix and the hydrophobic P3HT chains, driving the formation of compact aggregates.

Optical micrographs of a neat P3HT, b P3HT/PMMA (10 wt % P3HT), and c P3HT/PMMA (2 wt % P3HT).

TEM images of a spherical P3HT particles, b crystalline domains, and their SAED patterns; c neat PMMA (right) and P3HT/PMMA blend (left) for comparison.

Assignment of the Crystal Structure

Edge‑on orientation of P3HT chains dominates in these samples, as evidenced by SAED patterns revealing (010) and (001) reflections (Fig. 6). Stacking periods along the b‑axis were measured as 0.45 ± 0.1 nm for spherical domains (Fig. 7) and 0.48 ± 0.1 nm for lamellae (Fig. 5b). The inter‑chain spacing of 1.23 nm, extracted from (h00) rings (Fig. 8), corresponds to the crystalline form II. The measured b‑axis distances (0.417–0.445 nm) indicate a mixture of intermediate form I′ (≈0.42 nm) and form II (≈0.44 nm), consistent with literature on P3HT polymorphism [42–44].

Crystal structure schematic of P3HT.

a TEM image and b SAED of a spherical P3HT domain in a 1:50 P3HT/PMMA blend.

SAED of a P3HT domain in a 1:50 P3HT/PMMA blend.

Discussion

The key outcome of this study is that PL QY can be enhanced in the condensed phase of P3HT by reducing aggregate size to micro‑ and nanoparticles, rather than by dissociating aggregates into isolated chains. Two primary mechanisms underpin this enhancement: (1) the increased interfacial area between P3HT and PMMA elevates the contribution of interfacial molecules to emission, and (2) the rearrangement of P3HT chains within crystalline domains, driven by hydrophobic forces from PMMA, reduces interchain coupling and torsional disorder.

Dielectric constant effects were investigated by gradually replacing CB with PMMA in solution. A modest 14 % QY increase was observed, indicating that while dielectric screening contributes, it cannot account for the fourfold QY rise seen in films. Instead, the dominant factor is structural reordering within the PMMA matrix. The shift from form I′ to form II (Fig. 10) reflects a slow relaxation toward the thermodynamically favored crystalline phase, driven by the polar environment and the kinetics of phase separation.

PL spectra of P3HT in CB before (red) and after (green) mixing with PMMA (5.4 wt %). (b) PL evolution upon progressive dissolution of PMMA powder in the P3HT solution.

Electronic absorption of as‑prepared P3HT/PMMA (1:50) and (1:4) samples (solid curves) and after 2 weeks (dotted curves).

Conclusions

Embedding P3HT in a PMMA matrix leads to a remarkable increase in PL QY—up to 400 %—despite the presence of aggregates. This enhancement is attributed to two intertwined effects: a modest dielectric‑constant–induced boost (~14 %) and, more importantly, a pronounced rearrangement of P3HT chains. Spectroscopic evidence and exciton‑bandwidth calculations confirm superior ordering and reduced torsional disorder in the nanoscopic aggregates. The observed structural evolution from form I′ to form II, accompanied by narrowing exciton bandwidth, explains the increased PL efficiency, as narrowed bands favor radiative recombination. These findings highlight the potential of polymer blending strategies to tailor the photophysical performance of conjugated systems for advanced organic electronic applications.