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Light Induced Charge Generation, Recombination, and Transport in Organic Solar Cells

Infahsaeng, Yingyot LU (2013)
Abstract
Light-to-electricity conversion processes in organic and dye sensitized solar cells have attracted much scientific effort, aiming to understand the fundamental nature of these processes and to improve solar cell efficiency. Power conversion efficiency of such solar cells has progressed steadily in the last few decades, suggesting applicability for commercial products in the nearest future. Yet, the overall light-to-charge carrier conversion processes as well as the factors controlling these processes are not fully understood. Dye sensitized semiconductor solar cells (DSSC) and bulk heterojunction (BHJ) organic solar cells are addressed in this Thesis. The main topics of the Thesis are... (More)
Light-to-electricity conversion processes in organic and dye sensitized solar cells have attracted much scientific effort, aiming to understand the fundamental nature of these processes and to improve solar cell efficiency. Power conversion efficiency of such solar cells has progressed steadily in the last few decades, suggesting applicability for commercial products in the nearest future. Yet, the overall light-to-charge carrier conversion processes as well as the factors controlling these processes are not fully understood. Dye sensitized semiconductor solar cells (DSSC) and bulk heterojunction (BHJ) organic solar cells are addressed in this Thesis. The main topics of the Thesis are light-induced charge generation, recombination, and transport in the active materials as well as solar cell devices. Electron injection and recombination in Zn-porphyrin sensitized TiO2 electrode were investigated by means of transient absorption (TA) spectroscopy and surface sensitive sum frequency generation. It was found that the sensitizer molecule is bound to the semiconductor (TiO2) surface at an angle, which is controlled by experimental factors such as natureof the sensitization solvent and time. This tilt angle controls the electron transfers (injection and recombination) between sensitizer and TiO2 and it was shown that transfer occurs through space rather than through the molecular spacer connecting the porphyrin core to the surface. A strong correlation between cell efficiency and electron injection and recombination dynamics was demonstrated. In particular, the role of the electron-Zn-porphyrin cation recombination was shown to be decisive for high solar cell efficiency – a very slow (> 50 ns) recombination resulted in the highest cell efficiency. In functioning BHJ solar cells, charge pair photo-generation and separation as well as recombination, transport and extraction of photo-generated charge carriers were studied by several techniques such as TA spectroscopy, electro-modulated differential absorption (EDA), time resolved electric field induced second harmonic generation (TREFISH), time-of-flight method (TOF), photo-carrier extraction by linearly increasing voltage (CELIV), transient photocurrent and photovoltage. Solar cells made of fullerene blends with several different polymers have been studied. Photo-induced dynamics has been observed over more than ten decades of time and three decades of photo-generated charge concentrations by a combination of various optical and electrical time-resolved methods. Light absorption has been shown to lead to formation of closely spaced bound rather than long-distance separated free charges. Mechanism of the charge pair separation dominated by fast three-dimensional diffusion at earlier time scale and by electric field-induced drift on later times has been purposed. Electric field dependence of charge pair separation and recombination in non-optimized and optimized solar cells was studied. The time dependent carrier mobility over picosecond to microsecond time scale was shown and considered in relation to charge pair separation and extraction in solar cells made of polymer:fullerene blends with different stoichiometry. Dominance of geminateand non-g eminate recombination has been shown for solar cell blends made of different polymers. Detail description of recombination mechanism over broad range of times and charge concentrations have been achieved by means of the combination of different optical and electrical techniques. A critical role of electrodes in functioning solar cells on the charge dynamics has been discussed. (Less)
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@phdthesis{5b215372-fc1c-45d7-9833-ab22e4a02bb3,
  abstract     = {{Light-to-electricity  conversion  processes  in  organic  and  dye  sensitized  solar  cells  have  attracted  much  scientific  effort,  aiming  to  understand  the  fundamental  nature  of  these  processes  and  to  improve  solar  cell  efficiency.  Power  conversion  efficiency of such solar cells has progressed steadily in the last few decades, suggesting applicability for commercial products in the nearest future. Yet, the overall light-to-charge carrier conversion processes as well as the factors controlling these processes are  not  fully  understood.  Dye  sensitized  semiconductor  solar  cells  (DSSC)  and  bulk  heterojunction  (BHJ)  organic  solar  cells  are addressed in this Thesis. The main topics of the Thesis are light-induced charge generation, recombination, and transport in the active materials as well as solar cell devices.  Electron  injection  and  recombination  in  Zn-porphyrin  sensitized  TiO2  electrode  were  investigated  by  means  of  transient  absorption  (TA)  spectroscopy  and  surface  sensitive  sum  frequency  generation.  It  was  found  that  the  sensitizer  molecule is bound to the semiconductor (TiO2) surface at an angle, which is controlled by experimental factors such as natureof  the  sensitization  solvent  and  time.  This  tilt  angle  controls  the  electron  transfers  (injection  and  recombination)  between  sensitizer and TiO2 and it was shown that transfer occurs through space rather than through the molecular spacer connecting the  porphyrin  core  to  the  surface.  A  strong  correlation  between  cell  efficiency  and  electron  injection  and  recombination  dynamics  was  demonstrated.  In  particular,  the  role  of  the  electron-Zn-porphyrin  cation  recombination  was  shown  to  be  decisive for high solar cell efficiency – a very slow (> 50 ns) recombination resulted in the highest cell efficiency.  In  functioning  BHJ  solar  cells,  charge  pair  photo-generation  and  separation  as  well  as  recombination,  transport  and  extraction  of  photo-generated  charge  carriers  were  studied  by  several  techniques  such  as  TA  spectroscopy,  electro-modulated differential  absorption  (EDA),  time  resolved  electric  field  induced  second  harmonic  generation  (TREFISH),  time-of-flight method  (TOF),  photo-carrier  extraction  by  linearly  increasing  voltage  (CELIV),  transient  photocurrent  and  photovoltage.  Solar  cells  made  of  fullerene  blends  with  several  different  polymers  have  been  studied.  Photo-induced  dynamics  has  been  observed over more than ten decades of time and three decades of photo-generated charge concentrations by a combination of various optical and electrical time-resolved methods. Light absorption has been shown to lead to formation of closely spaced bound  rather  than  long-distance  separated  free  charges.  Mechanism  of  the  charge  pair  separation  dominated  by  fast  three-dimensional  diffusion  at  earlier  time  scale  and  by  electric  field-induced  drift  on  later  times  has  been  purposed.  Electric  field  dependence  of  charge  pair  separation  and  recombination  in  non-optimized  and  optimized  solar  cells  was  studied.  The  time  dependent  carrier  mobility  over  picosecond  to  microsecond  time  scale  was  shown  and  considered  in  relation  to  charge  pair  separation and extraction in solar cells made of polymer:fullerene blends with different stoichiometry. Dominance of geminateand  non-g eminate  recombination  has  been  shown  for  solar  cell  blends  made  of  different  polymers.  Detail  description  of  recombination  mechanism  over  broad  range  of  times  and  charge  concentrations  have  been  achieved  by  means  of  the  combination of different optical and electrical techniques. A critical role of electrodes in functioning solar cells on the charge dynamics has been discussed.}},
  author       = {{Infahsaeng, Yingyot}},
  language     = {{eng}},
  month        = {{06}},
  school       = {{Lund University}},
  title        = {{Light Induced Charge Generation, Recombination, and Transport in Organic Solar Cells}},
  year         = {{2013}},
}