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Photovoltaic Carrier Dynamics Measured by Time-Resolved Terahertz Spectroscopy


Far-infrared (or THz, 25 to 300 micron wavelength) femtosecond laser methods are employed to measure photovoltaic (PV) materials spectra and photocarrier dynamics in candidate polymeric and nanolayered donor-acceptor films. This region of the spectrum is particularly sensitive to detailed structural and environmental properties as well as charge migration in PV 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 photovoltaic nanofilms. Development of this non-contact methodology is directly relevant to comparing conductivity in the active donor-acceptor layer and carrier recombination that directly affect solar cell efficiencies without examining actual PV devices.


We employ novel pulsed optical measurement techniques to directly monitor carrier generation, migration and relaxation dynamics in semiconducting polymer and mixed organic/organometallic photovoltaic films. These studies are being conducted to measure (without contact) ultrafafast carrier dynamics in novel electronic photovoltaic materials being considered and developed for future solar cell, LED and energy capture applications. By applying UV-Visible excitation with time delayed THz probe pulses, we are able to identify candidate nanolayered materials and bulk organic mixtures that exhibit rapid exciton quenching, thermal energy relaxation and long-lived free carriers that could be amenable for higher efficiency solar-to-electricity applications.

schematic of UV pump
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 samples of nano-layered Zn-pthlalocyanine electron donor with C60 acceptors as a function of layer thickness and number of interfaces grown at NRL using chemical vapor deposition conditions. As the layer thickness decreases (to <5 nm), initially generated bi-excitons thermally relax recombine or are trapped within a few picoseconds, while free carriers provide a longer-lived signal proportional to their population measured by the transient THz absorption. This signal is directly proportional to the material conductivity suggesting that as the layer thickness decreases, initially generated excitons more readily dissociate to free carriers which are more highly mobile within the donor-acceptor interface.

carrier dynamics
Alternating nanolayered structure of Zinc Phthalocyanine and C60 for solar cell photovoltaic applications. Below, carrier decay dynamics measured with 400 nm excitation and THz probe pulses as a function of layer thickness.
These investigations use state-of-the-art, kHz repetition-rate amplified 45 femtosecond pulsed Ti: Sapphire lasers and optical parametric amplifiers to produce turnable excitation pulses and broadband (0.2-2.5) THz probe pulse generated and detected using ZnTe nonlinear and electro-optic crystals.

Major Accomplishments

  • Monitored exciton formation, recombination and thermalization of free-carrier dynamics in prototypical semiconducting polymer thin-film samples and showed that the relative mobilities correlate with device measurements. 
  • Determined charge carrier dynamics in mixed-blend and nanolayered organic donor-acceptor systems showing that thinner layered materials (1-5 nm) have higher conductivity than mixed-blend samples of the same materials.
  • Measurements and comparisons between Zn-pthtalocyanine and α-sexithiophene doner with C60 acceptor layers using different excitation energies strongly suggested the initial picosecond TRTS relaxation is dominated by carrier cooling.
Created January 6, 2014, Updated April 29, 2019