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SUMMARY:On the role of excess energy on free charge generation and recombi
 nation in polymer:fullerene blends - Prof. Dr. Dieter Neher\, University o
 f Potsdam
DTSTART:20171114T111000Z
DTEND:20171114T115000Z
UID:TALK94447@talks.cam.ac.uk
CONTACT:Emrys Evans
DESCRIPTION:Pi-conjugated materials are among the most promising classe
 s of semiconductors for future thin film organic photovoltaics cells. Comp
 ared to the nowadays used inorganic semiconductors\, organic semiconductin
 g layers exhibit significantly higher absorption coefficients but also muc
 h lower mobilities\, the latter origin from significant positional and ene
 rgetic disorder in organic thin films. One prominent consequence of disord
 er is dispersive transport\, where charge carrier motion progressively slo
 ws down due to occupation of states in the lower part of the density of st
 ates distribution. Given the fact that the average energy of incident phot
 ons under sun illumination is much larger than energy of the final electro
 n-hole pair\, the question needs to be addressed whether non-thermalized c
 arrier contribute to free carrier generation and recombination under solar
  cell working conditions\, and whether concepts based on local equilibrium
  considerations are applicable to organic solar cells. \n\nIn this talk I 
 will present results of recent experiments regarding the generation and re
 combination of free charges in selected polymer:fullerene bulk heterojunct
 ion solar cells. Time-delayed collection experiments performed as function
  of field\, excitation energy and temperature reveal an overwhelming effec
 t of fullerene ordering on the efficiency of free charge generation\, whil
 e excess energy seems to be of minor importance. On the other hand\, free 
 charge recombination is dispersive in some highly-disordered BHJ layers\, 
 while the steady state recombination seems to be dominated by thermalized 
 carriers. We\, therefore\, propose that the charge generation and recombin
 ation mainly involves thermalized states. In accordance to this finding\, 
 the JV characteristics of most of our devices can be well explained by a m
 odified Shockley-equation.\n
LOCATION:MRC Seminar Room\, MRC\, Cavendish laboratory
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