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Photovoltaic Carrier Dynamics Measured by Time-Resolved Terahertz Spectroscopy

Summary:

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. Ultra 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 examing actual PV devices.

Description:

We employ novel pulsed optical measurement techniques to directly monitor carrier generation, migration and dynamics in semiconducting polymer and mixed organic 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-dependent THz probe pulses, we are able to identify nanolayered materials and bulk organic mixtures that exhibit rapid exciton quenching and long-lived free carriers that could be amenable for high efficiency solar-to-electricity 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 analog PBTTT. The time-dependent dynamical behavior of highly mobile carriers in these films is shown on the right.

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 analog PBTTT. The time-dependent dynamical behavior of highly mobile carriers in these films are shown on the right.

Recent studies examined nano-layered Zn-pthlalocyanine electron domer with C60 as a function of layer thickness and number of interfaces grown at NRL under chemical vapor deposition conditions. As the layer thickness decreases (to ca. 1-2 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.

Alternating nanolayered structure of B) 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.

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 is shown.

These investigations use state-of-the-art, KHz rep-rate amplified 45 femtosecond pulsed Ti: Sapphire lasers and optical parametric amplifiers to produce turnable excitation pulse and broadband (0.2-2.5) THz probe pulse generation and detection using ZnTe nonlinear and electro-optic crystals.

Major Accomplishments:

  • Monitored exciton formation, recombination and 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 thiner 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 intitial picosecond TRTS relaxation is dominated by carrier cooling.

Selected Publications:

  • P.A.Lane, P.D. Cunningham, J.S.Melinger, G.P.Kushto, O.Esenturk and E.J. Heilweil, "Photoexcitation Dynamics in Nanolayered 60Co-Zinc Phthalocyanine Films," Physical Review Letters 108,077402 (2012)
  • O. Esenturk, J. S. Melinger, P. A. Lane, and E. J. Heilweil, "Ultrafast Photoinduced Carrier Dynamics of Organic Semiconductors Measured by Time-Resolved Terahertz Spectroscopy," Chapter 17 in ACS Symposium series #1039 (2010) entitled "Organic Thin Films for Photonics Applications," Warren N. Herman, S. R. Flom and S. H. Fouler, eds., April 28, 2010.
  • O. Esenturk, J. S. Melinger, P. A. Lane, and E. J. Heilweil, "Relative Photon-to-Carrier Efficiencies of Alternating Nanolayers of Zinc Phthalocyanine and C60 Films Assessed by Time-Resolved Terahertz Spectroscopy," J. Phys. Chem. C, 113(43), 18842 (2009).
  • O. Esenturk, J. Melinger and E. J. Heilweil, "Terahertz Mobility Measurements on P3HT films: Device Comparison, Molecular Weight and Film Processing Effects," J. Applied Physics, 103, 023102 (2008).
  • O. Esenturk, R. J. Kline, D. DeLongchamp and E. J. Heilweil, "Mobility versus Effective Conjugation Length of Polythiophenes Probed by Time Resolved Terahertz Spectroscopy," J. Phys. Chem. C Letters, 112(29), 10587 (2008).
  • O. Esenturk, E. J. Heilweil, P. Lane and J. S. Melinger, "Ultrafast Carrier Dynamics in Blended and Layered Zn-Pthalocyanine/C60 Polymer Films Measured by Time-Resolved Terahertz Spectroscopy," Polymer Preprints 49(2), 1026 (2008).

Lead Organizational Unit:

pml

Customers/Contributors/Collaborators:

Dr. Joseph S. Melinger, NRL

Dr. Paul A. Lane, NRL

Staff:

Radiation Physics Division
Contact

Photovoltaic Carrier Dynamics Measured by Time-Resolved Terahertz Spectroscopy:
Edwin Heilweil, Project Leader
301-975-2370 Telephone
301-975-6991 Facsimile

100 Bureau Drive, M/S 8443
Gaithersburg, MD 20899-8443