5. RESULTADOS Y DISCUSIÓN
5.1 Encuesta sobre el uso del celular
Chapter 8: UV-Vis-NIR TA spectrometer with 40 fs time
resolution
Fig. 28: Experimental layout of the ultrafast UV-Vis-NIR pump-probe spectrometer. CPA: chirped-pulse amplifier, NOPA: noncollinear optical parametric amplifier, SCG: supercontinuum generation, PC: prism compressor, DM: dichroic mirror, FS: fused silica, VA: variable attenuator, WG: wire-grid polarizer, PDA: photodiode array. To investigate the nature of the photoexcitations in thin film photovoltaics and their inherent kinetics from fs up to several ns after visible excitation we developed a novel ultrafast pump-probe spectrometer. The gained understanding within the first part of my thesis work – i.e. generation of tunable ultrashort light pulses via NOPA/NOPCPA as well as supercontinuum generation – was a prerequisite for this development. The setup is described in Fig. 28. It allows for ultrafast ultrabroadband (UV-Vis-NIR) transient absorption (TA) experiments at room temperature. The time resolution was verified to be 40 fs, which is significantly better than for TA measurements in solution due to reduced group-velocity mismatch between pump and probe in the 100 nm thin film. The UV-Vis part of the setup employing a Ti:Sa chirped-pulse amplifier system (CPA 2001, ClarkMXR) with 1 kHz repetition rate as fundamental light source for pump and probe generation has been described in Ref. [38]. It was extended in the course of this work to allow the investigation of solid thin film samples and to expand the detection range into the NIR (Fig. 28, Fig. 29).
The details of this novel ultrafast broadband TA spectrometer are described in the corresponding publication in Appendix B7:
Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption
Daniel Herrmann, Sabrina Niesar, Christina Scharsich, Anna Köhler, Martin Stutzmann, and Eberhard Riedle
Chapter 8: UV-Vis-NIR TA spectrometer with 40 fs time resolution
Briefly, a single-stage noncollinear optical parametric amplifier (NOPA) was used as pump source providing ultrashort pump pulses of 15 fs FWHM duration while the pump wavelength was chosen as 450, 518, 535, 555, 600 and 720 nm for site-selective excitation of the samples. To prevent photodegradation and to mimic the solar exposure of 1 kW/m2 average power,
low-excitation energies in the range of 4-60 nJ (4-60µJ/cm2 excitation fluence) were used for P3HT/Si, while higher values (>300 nJ) were needed for DIP:C60 films due to its reduced absorbance. We routinely verified that the samples did not degrade due to the prolonged illumination by the pump pulses and only observed signal degradation when storing the samples over several months at ambient conditions in the lab.
Pump central wavelength Pump duration (FWHM) UV-Vis probe Vis-NIR probe Spectral resolution Repetition rate Time resolution Detection sensitivity Pump diameter (1/e2) Probe diameter (1/e2)
450-750 nm 15 fs 280-740 nm 420-1150 nm <3 nm 1 kHz 40 fs ΔOD~10-4 210 µm 110 µm
Tab. 1: Experimental parameters of the ultrafast pump, the broadband probe and the pump- probe measurements.
Fig. 29: (a) Broadband Vis-NIR probe spectrum and RMS noise (b) Experimental sensitivity of the Vis-NIR TA spectrometer calculated by taking the transient signal standard deviation per pixel at negative delays.
For gap-free TA measurements from 420 to 1150 nm (Fig. 29(a)), a novel probe setup was developed employing a cascade of supercontinuum generation (SCG) in YAG, OPA at
Chapter 8: UV-Vis-NIR TA spectrometer with 40 fs time resolution
TA signals employing a fused silica (FS) prism polychromator and a silicon photodiode array (PDA) based camera with 512 pixels is described in Ref. [38]. Typically, a probe RMS noise below 2%, a pump RMS noise of 1.5% and a probe detection sensitivity of ΔOD~10-4 over the entire spectral range were achieved (Fig. 29). The wavelength calibration was performed with various filters and HR mirrors leading to a spectral resolution at the detector of better than 3 nm/pixel. To minimize pump straylight originating from the solid film samples, a broadband wire-grid polarizer (WG) was placed in front of the detector. This WG was aligned with maximum transmission for the probe polarization and the pump polarization was adjusted to be perpendicular to the probe polarization in this case. The residual straylight was deleted via subtracting the average signal over negative delays for each pixel, which also offers a tool to check for proper delay stage alignment.
If one assumes a Poisson distribution for the number of detected photons, the sensitivity is limited by:
( )
( )
( )
( )
* 0 min 0 0 I ln 1 I I I 1 N 1 1 OD log I ln 10 I ln 10 N ln 10 n m ln 10 Δ ⎛ + ⎞ ⎜ ⎟ ⎛ ⎞ ⎝ ⎠ Δ Δ = − ⎜⎜ ⎟⎟ = ≈ = = ⋅ ⋅ ⎝ ⎠ ,where the minimum detectable change in photon number Δ =I I* −I0 has to overcome the the standard deviation N of the Poisson distribution. N=nÿm, where n is the detected shot photon number per pixel and m is the number of averages. In the case of Fig. 29(a) with n=40000 and m=2000 the calculated sensitivity results in
( )
(
)
1 5min
OD 40000 2000 ln 10 − 4, 9 10−
Δ = ⋅ ⋅ = ⋅ ,
which is well within the experimentally determined sensitivity in Fig. 29(b).
Fig. 30: A fraction of the pump straylight behind the sample is propagating along with the probe beam and leads to interferences at the detector. This shows that even the straylight still has the same defined phase relationship with the probe as the pump has, despite the cascaded optical nonlinearities and the stray processes in the sample. Apart from providing the required high sensitivity for these experiments with low excitation fluence, an accurate calibration of the time zero is needed in order to resolve the primary photoexcitations in photovoltaics. For this reason, the raw data was corrected for the chirp of
Chapter 8: UV-Vis-NIR TA spectrometer with 40 fs time resolution
the probe pulses by adjusting the zero time at half rise of GSB and singlet-exciton signals or at coherent artifacts which occur due to two-photon absorption or wave-mixing [109]. Subsequently, a polynomial fit for the data points was applied. Additionally, we could show that the UV-Vis and the Vis-NIR pump-probe setup are interferometrically stable and thus offer the alternative to employ a periodic modulation due to spectral interference between pump straylight and probe at the detector to extract the delay via Fourier-transformation (Fig. 31). We have explicitly compared both options and found that the deviation of their result is negligible and enable calibration of the zero delay with a few-fs accuracy (Fig. 31). Overall, the time resolution of the ultrabroadband TA measurements was 40 fs, in particular because we do not need to measure over the spectral region of the SCG fundamental where a time jump of about 400 fs is present in the broadband probe pulse.
Fig. 31: Correlation between the nominal pump-probe delay and the delay retrieved from the interference fringes (insets) in the region of pump straylight on the detector employing Fourier transformation for the (a) UV-Vis and the (b) Vis-NIR TA spectrometer.