High‑Performance Green Polymer POFP: Low‑Threshold Lasing and Superior Electron Transport for Diode‑Pumped Organic Solid Lasers

Background

Organic semiconductors have long attracted interest for optoelectronic devices such as OLEDs and OPVs, thanks to their mechanical flexibility, solution processability, and cost‑effectiveness.^[1, 2] Conjugated polymers, in particular, can be tuned to exhibit high PLQY, large stimulated emission cross‑sections, and a broad visible‑light emission spectrum, positioning them as promising candidates for optical amplifiers and electrically pumped lasers.^{[3, 4, 5, 6]} Since the first polymer‑based OSLs were demonstrated in 1996,^[9] researchers have focused on synthesising low‑threshold gain materials. For example, poly(9,9‑dioctylfluoren‑2,7‑diyl‑alt‑benzothiadiazole) (F8BT) achieved a lasing threshold of 6.1 µJ cm⁻², while fluorene‑based copolymers such as PPV and PF show ASE thresholds between 4.4 and 10.0 µJ cm⁻².^{[4, 10, 11]} Fluorine‑containing polymers with sub‑µJ cm⁻² thresholds remain highly desirable.

In addition to material design, several strategies have been employed to boost optical gain. Femtosecond pulsed pumps and two‑dimensional distributed feedback (DFB) resonators have lowered thresholds, while Förster resonance energy transfer (FRET) between host and guest molecules can enhance optical gain.^[12–15] However, electrically pumped OSLs have yet to achieve comparable performance, largely due to limited current densities (~kA cm⁻²) required for population inversion and challenges in integrating efficient optical resonators without compromising carrier transport.^[16, 17] A simple vertical waveguide microcavity, which can be fabricated with minimal complexity, offers a promising route to confine ASE and achieve spectral narrowing.^[18] Our earlier work introduced diode‑pumped OSLs that use an organic electroluminescent layer (EML) as the pump source and an organic laser dye as the gain medium.^[19]

Here we investigate POFP, a green‑emitting polymer derived from the PPV family. POFP achieves an ASE threshold of 4.0 µJ cm⁻² and a Q‑factor of 159, indicating exceptional light amplification. We also examine its electron‑transport characteristics and integrate it into an inverted microcavity structure to demonstrate diode‑pumped lasing behaviour, laying groundwork for future electrically pumped organic lasers.

Methods/Experimental

POFP (average molecular weight 40 k–80 k) was obtained from American H.W. SANDS. The polymer was dissolved in chloroform at 0.7 wt % and spin‑coated onto cleaned glass substrates to form films of varying thicknesses, followed by annealing at 60 °C for 20 min.

a Molecular structure of POFP. b Absorption, PL, and ASE spectra of POFP thin films.

Hole‑only and electron‑only devices were fabricated to probe carrier transport. The hole‑only stack was ITO/POFP/NPB/Al, while the reference used only NPB. For electron‑only devices, the stacks were Ag/BCP/POFP/Al and Ag/BCP/Bphen/Al. NPB, Bphen, and BCP were chosen as standard hole‑, electron‑transport, and hole‑blocking layers, respectively. For diode‑pumped OSLs, ZnS served as the electron‑injection layer (EIL) and MoO₃ as the hole‑injection layer (HIL). The complete device architecture was ITO/ZnS/POFP/AND:2 wt %DSA‑ph/NPB/2T‑NATA/MoO₃/Al, where AND:DSA‑ph is the blue‑emitting EML.

All devices were fabricated in a thermal‑evaporation chamber on ITO‑coated glass (150 nm, 15 Ω sq⁻¹). Substrates were sequentially cleaned with detergent, de‑ionised water, acetone, isopropanol, and UV‑ozone (15 min). Deposition rates were 0.6 Å s⁻¹ for organic layers, 0.1 Å s⁻¹ for Ag, and 5.0 Å s⁻¹ for Al. The active area was 4 mm².

ASE measurements employed a 355‑nm Nd:YAG laser (1 ns, 100 Hz). The beam was focused to 2.5 mm × 10 mm and neutral‑density filters tuned the pump fluence. Emission was collected from the film edge via optical fibre to a spectrometer. PL spectra were recorded with an FLSP 920 spectrometer, while absorption was measured on a Hitachi U‑3900H UV–vis spectrophotometer. EL spectra were obtained with a Photo Research PR‑650. Current–voltage characteristics were measured using a Keithley 2400 source‑meter. All measurements were performed in the dark at room temperature without encapsulation.

Results and Discussion

Figure 1b shows that POFP emits strongly in the green (512 nm peak, shoulder at 550 nm) with an absorption maximum at 452 nm. The PL full width at half maximum (FWHM) is 60 nm. Under 355‑nm excitation, the ASE peaks at 548 nm, confirming efficient blue‑light absorption suitable for blue OLED pumping.

Figure 2a depicts the evolution of FWHM and ASE intensity for a 135 nm POFP film as the pump fluence increases from 1 to 20 µJ cm⁻². The FWHM narrows from 27.3 to 3.5 nm, while the ASE intensity rises super‑linearly, signalling the ASE threshold. Table 1 lists thresholds for films 60–165 nm thick; the lowest value (4.0 µJ cm⁻²) occurs at 135 nm, balancing absorption and scattering losses. Figure 2b demonstrates clear gain narrowing with increasing pump.

a FWHM and peak intensity vs. pump fluence for 135 nm POFP. b ASE spectra at 3, 4, and 16 µJ cm⁻².

The calculated Q‑factor of 159 for POFP exceeds that of many inorganic resonators (e.g., CaF₂ = 109) and polymer analogues (e.g., pyrene‑capped starburst = 65), underscoring its high optical feedback capability.^[21, 22]

Carrier‑transport measurements (Figure 3) reveal that POFP’s hole‑transport is inferior to NPB, whereas its electron‑transport surpasses Bphen. This suggests POFP is best employed as an electron‑transport layer in OSLs.

a J–V curves for hole‑only devices. b J–V curves for electron‑only devices. Device structures are inset.

The AND:DSA‑ph EML emits at 468 nm with a shoulder at 500 nm, overlapping strongly with POFP’s absorption band, enabling efficient energy transfer and pumping. Figure 4 illustrates this overlap.

EL spectrum of AND:2 wt %DSA‑ph and absorption spectrum of POFP.

Using thin‑film interference theory, the optimal microcavity thickness for constructive interference is d = λ/2n. With λ = 512 nm and n ≈ 1.7, the minimum constructive thickness is ≈ 150 nm, while destructive interference occurs at ≈ 75 nm.

We fabricated inverted diode‑pumped OSLs with microcavity thicknesses of 75 nm (device E) and 150 nm (device F). Figures 5a–b show device schematics and EML structures.

a Device E and F structures. b EML molecular structures.

Electroluminescence spectra (Figure 6) show both devices peak at 512 nm, matching POFP PL, indicating excitation of POFP by the blue EML. Device F (150 nm microcavity) exhibits FWHM narrowing from 60 to 32 nm and a radiance surge above 34 W cm⁻², while device E shows only minor narrowing. These features reflect waveguiding and microcavity interference rather than full lasing; nevertheless, they demonstrate POFP’s potential as a gain medium under diode pumping.

EL spectra vs. voltage for devices E (a) and F (b). Insets show radiance and FWHM vs. power density.

These results confirm that POFP can act as an efficient gain medium in diode‑pumped OSLs, achieving spectral narrowing and radiance enhancement that are hallmark signatures of laser action. Comparative studies with MEH‑PPV (Supporting Information, Fig. S3) indicate that POFP offers superior performance, encouraging further exploration of pulsed‑voltage pumping or Bragg‑mirror resonators to realise fully electrically pumped organic lasers.

Conclusions

We have demonstrated that the green conjugated polymer POFP exhibits an exceptionally low ASE threshold of 4.0 µJ cm⁻², a high Q‑factor of 159, and superior electron‑transport properties compared to conventional ETLs. Integrating POFP into an inverted waveguide microcavity yields diode‑pumped OSLs that display clear gain narrowing and radiance enhancement, showcasing its promise for future electrically pumped organic lasers.

Abbreviations

ASE

Amplified spontaneous emissions

DFB

Distributed feedback

EL

Electroluminescence

EML

Electroluminescent layer

FRET

Förster resonance energy transfer

FWHM

Full width at half maximum

OLEDs

Organic light‑emitting diodes

OPV

Organic photovoltaic

OSL

Organic solid lasers

PL

Photoluminescence

PLQY

Photoluminescence quantum yield

Q‑factor

Quality factor