Carrier Dynamics Measured by Ultrafast Time-Resolved Terahertz Spectroscopy

Summary

Non-contact far-infrared (or THz, 25 to 300 micron wavelength) femtosecond laser methods are employed to measure 2D, bulk semiconductors, photovoltaic (PV) materials spectra (using Time-Domain Spectroscopy, TDS) and carrier dynamics in candidate polymeric, chalcogenide, perovskite and nanolayered donor-acceptor films. This region of the spectrum is particularly sensitive to detailed nano-structural and environmental properties as well as charge migration, doping and trapping in conductive materials. Ultrafast time-resolved THz spectroscopy (TRTS) is employed to directly monitor initially generated excitons, electron-hole separation, recombination and free carrier dynamics in novel photo-excited nanofilms. Development of this methodology is relevant for non-contact conductivity measurement of carrier recombination dynamics without examining actual devices.  Scattering THz-AFM is also being developed as a dynamical conductivity probe of nanometer materials and device features.

Description

We employ ultrafast pulsed optical measurement techniques to directly monitor carrier generation, migration and relaxation dynamics in bulk semiconductors, polymers and mixed organic/organometallic photovoltaic nanofilms. These studies are conducted to measure (without contact) ultrafast carrier dynamics in novel electronic photovoltaic materials being considered and developed for future solar cell, transparent conductor, LED and energy capture applications. By applying UV-Visible excitation with time-delayed THz probe pulses (TRTS), we identify candidate nanolayered materials and bulk organo-metallic mixtures that exhibit rapid exciton quenching, thermal energy relaxation and long-lived free carriers amenable for high efficiency transparent conductors, 2D transistors, and solar-to-electricity applications.  Dispersions of donor-acceptor polymers in transparent liquids are also studied to rapidly identify candidate systems for efficient PV applications:

 

Schematic of UV pump, THz probe optical detection scheme used to monitor photogenerated carriers in polymeric thin films (ca. 200 microns) of P3HT (spin cast, AC) and its structurally constrained analog PBTTT. The time-dependent dynamical behavior of highly mobile carriers in these films are shown on the right.

Recent studies examined bulk semiconductor mobility in commercial samples of silicon, gallium-arsenide (GaAs) and other II-VI crystals using two-photon below bandgap excitation.  This approach creates a homogeneous distribution of electron and hole excitation throughout the entire bulk sample, avoiding surface and other interfacial effects.  TRTS measurements were compared to standard magneto-Hall contact mobility of the same samples showing correlation for rapid sample evaluation.  We also internally collaborate with other NIST and external groups to study candidate 2D materials (e.g., MoS₂, MoSe₂), semiconductors (e.g, InSe), Perovskites (cation exchanged and doped) as well as high efficiency donor-acceptor polymers and heterojunction mixtures.

 

Real (circles/solid lines) and imaginary (squares/dashed lines) THz conductivity for peak (blue) and time-delayed tail (red) for (a) near-isotropic PCDTPT film and (b) PCDTPT/toluene dispersion. Spectra collected with 800 nm excitation and DC-tail conductivities are multiplied by 15 for visual comparison to peak data.

Extracted total charge DC mobility as a function of one and two-photon generated carrier density for undoped Si (left).  Mobility correlates with standard Hall measurements and fits the expected low-density limit while evaluation of undoped GaAs (right) shows lowest density values do not fit the Caughey/Thomas plateau limit suggesting the mobility continues to increase below 1012 cm-3.

These THz investigations use state-of-the-art, KHz repetition-rate amplified 45 femtosecond pulsed Ti: Sapphire lasers and optical parametric amplifiers to produce tunable sample excitation pulses and broadband (0.2-2.5) THz probe pulses generated and detected using ZnTe nonlinear and electro-optic crystals.

Major Accomplishments

• Monitored exciton/polaron formation, charge recombination, thermalization and measured conductivity and mobility of free-carriers in donor-acceptor semiconducting polymer nanofilm and liquid dispersion samples.
• First to employ two-photon below bandgap excitation of semiconductor samples to evaluate bulk conductivity/mobility and doping effects using non-contact TRTS probe methods.
• Demonstrated that nanolayers of photoconductive materials, including SnS, MoS2, MoSe2 and doped 2D/3D Perovskites are amenable to TRTS mobility measurements.
• Measurement and comparison between Zn-pthtalocyanine and α-sexithiophene doner with C60 acceptor nanolayers using different excitation energies implies initial picosecond TRTS relaxation is dominated by Auger recombination and carrier cooling.