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BEGIN:VEVENT
DTSTAMP:20260624T071423Z
DTSTART;VALUE=DATE-TIME:20180410T140000
DTEND;VALUE=DATE-TIME:20180410T150000
SUMMARY:CMP seminar: Prof. Sanford Ruhman: Following the emergence of fre
 e hot carriers in lead halide perovskites with 10 fs time resolution
TZID:Europe/London
UID:20180410-8a17841a6272400201628d49433c55ce@warwick.ac.uk
CREATED:20180403T205331Z
DESCRIPTION:Abstract The high energy conversion efficiency obtained in so
 lar cells based on lead halide perovskites (LHP) has been attributed to 
 intense absorption\, long carrier diffusion lengths and slow electron-ho
 le recombination rates. Much recent research has been concentrated on fi
 guring out how these structurally inhomogeneous and seemingly defect lad
 en thin semiconducting films nonetheless attain this performance. As par
 t of this effort\, exciton and free carrier dynamics have been followed 
 using ultrafast spectroscopy on various perovskite materials\, ranging f
 rom bulk thin films to single crystals of various dimensions. However al
 most all such studies have been conducted with pulse duration insufficie
 nt for characterizing the earliest stages of free-carrier generation. In
  this seminar sub-10fs resolution pump-probe experiments on methylammoni
 um lead halide perovskite films will be described [1]. The initial respo
 nse is assigned to localized hot excitons which dissociate to free carri
 ers. This is attested to by band integrals of the pump-probe spectra whe
 re photoinduced bleaching rises abruptly 20fs after photoexcitation. Lat
 er stages of spectral evolution are consistent with hot carrier cooling\
 , during which state filling induced bleaching of interband and exciton 
 transitions curiously more than doubles [2]. Electron coupling to optica
 l phonons is observed as periodic spectral modulations in the pump-probe
  data of both films. Fourier analysis identifies active phonons at ~100 
 and 300 wavenumbers pertaining to the lead-halide framework and organic 
 cation motions respectively. Coupling strengths estimated from the depth
  of these modulations are in the weak coupling limit. These findings sup
 port free carriers in these materials existing as large polarons. Finall
 y recent results will be presented demonstrating that pump-probe data in
  perovskites is dominated by changes in absorption and not reflectivity.
  References [1] Ghosh et al.\, Journal of the American Chemical Society 
 139 18262-18270 (2017). [2] Sharma et al.\, J. Mater. Chem. A\, 4\, 3546
 -3553 (2016).
LOCATION:MAS2.06
CATEGORIES:Condensed Matter Physics Seminar
LAST-MODIFIED:20180403T205331Z
ORGANIZER;CN=Jon Duffy:
END:VEVENT
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