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SUMMARY:Advanced spectroscopy of organic and hybrid photovoltaic materials
  and thin-film solar cells
DTSTART:20170518T160000
DTEND:20170518T170000
DTSTAMP:20260919T012824Z
UID:63756b8c8167421d73415d7dbf6cd567e4470116c0bcef3f657a76a7
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Vladimir Dyakonov\nExperimental Physics\, Faculty of Phy
 sics and Astronomy\, Julius-Maximillian University of Würzburg and Bavari
 an Center for Applied Energy Research\, Würzburg\, Germany\nChE-602 - Rec
 ent Events in Energy seminar series\n\nOrganic and recently particularly h
 ybrid (perovskite) thin-film solar cells are progressing very fast\, showi
 ng extraordinary performance. However\, there is still a lack of fundament
 al understanding of charge carrier transport and recombination losses in t
 hese types of solar cells. The charge carrier lifetime and the mobility ar
 e two very important parameters in solar cells\, as they define the diffus
 ion length of the charge carriers and thus the efficiency. To determine th
 em\, various optical and electrical techniques can be applied. We will dis
 cuss the peculiarities and limitations of transient photo-voltage and phot
 o-current techniques\, such as open circuit voltage decay (OCVD)\, the sma
 ll-perturbation transient photo-voltage (TPV) and charge extraction (CE) a
 nd the time-of flight (TOF). To fully understand recombination mechanisms 
 in solar cells\, it is essential to evaluate them under as large a range o
 f illumination intensities as possible to assess which recombination pathw
 ay becomes dominant under each corresponding carrier concentration. As an 
 example\, we will discuss the OCVD technique. There we observed several ti
 me domains in the voltage transient. On a short timescale\, a voltage drop
  due to free charge carrier recombination is observed and transients are q
 ualitatively similar in organic and perovskite solar cells. [1] On longer 
 times\, a rapid voltage drop is often observed\, however\, these losses ar
 e not associated with the absorber material itself\, but rather with the e
 ntire device. They are due to so-called shunts\, which act as additional r
 ecombination pathway. By the proper choice of transport materials\, it is 
 however possible to remove the shunts and to study the devices under reduc
 ed illumination conditions\, as charge carriers are efficiently blocked fr
 om recombining at the electrodes. [2]\n \nFinally\, to address the possib
 le influence of electronic trap states on device performance\, we probed t
 he traps in organic and perovskite solar cells via thermally stimulated cu
 rrent (TSC) analysis. In this method the sample is initially cooled down\,
  where trap states are subsequently filled optically. Then\, the current f
 low is monitored upon heating the device up again. This very small current
  is attributed to charge carriers being thermally released from trap sites
  in the semiconductor bulk or at the interfaces\, which allows to draw con
 clusions about their energy distribution\, depth and density. [3] We will 
 also discuss the influence of the chemical composition as well as morpholo
 gy on the energetic trap landscape.\n \n[1] A. Baumann et al.\, Appl. Phy
 s. Lett. Mater. 2\, 081501 (2014).\n[2] K. Tvingstedt et al.\, ACS Energy 
 Lett. 2\, 424 (2017).\n[3] A. Baumann et al.\, J. Phys. Chem. Lett. 6\, 23
 50 (2015).
LOCATION:Zeuzier https://www.google.com/maps/place/EPFL+Valais+Wallis/?ref
 =zeuzier
STATUS:CONFIRMED
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