ESR11: Design of small organic molecules as hole transporting material in photovoltaic devices / Thesis

Abengoa Research (AR), Spain

Thesis Laura Caliò/ Download the thesis here
Universidad Pablo de Olavide

Abstract /Thesis download here
Third generation  photovoltaics  (PV) have demonstrated to have all the  requirements to replace
conventional  inorganic  silicon-based  solar  cells  due  to  their  appealing  characteristics, such  a easy  processability,  low  cost  and  reduced  material  usage.  In  particular,  organic photovoltaics have been intensely investigated  in the latest  decades since it  can  lead to clean, affordable, and flexible  devices.  Nevertheless,  their  limited  power  conversion
efficiency  needs  to  be  addressed, in  order  to  pave  the  way  for  a  large-scale commercialization.  Lately,  the  major  breakthroug among  PV  technology  was  indeed  achieved  with  the  introduction  of hybrid  organic-inorganic perovskite  solar  cells,  due   to   the   astonishing  properties  of  perovskite,  such  as  broad  light absorption,  easy  fabrication,  versatile  deposition  process  and   high  solar-to-electricity  power conversion  efficiencies  (PCE).  In  just   a  few  years,  certified  values  of  >22%  of  PCE  were obtained,  representing   an   astonishing   rise   from   their   first   introduction   in   2009,   and  an unprecedent  escalation  among  the  PV  technology.  These  results  leaded  perovskite  under  the limelight  and  entitled  them  as  strong  bet  for  future  PV  technology.  Although  considerable progress  has  been  achieved  in this field during last  eight  years,  some  issues still remain  to be addressed,  like  high  cost  materials  and  device  stability,  which  represents  a  big  limitation  for the  large-scale  production.
The use of hole transport  material  (HTM) is indispensable  in order to fabricate efficient deviceE Spiro-OMeTAD  (2,2′,7,7′-tetrakis(N,N-di-
methoxyphenylamine)-9,9′-spirobifluoren represents   the  state  of   the    art   HTM,   working  as  electron   blocking  and   p-type   material . However,  its  high  price due  to  the  multistep  synthetic  route and complex  purification,  hinder progress  toward  commercialization of
perovskite  PV  technology.  Furthermore,  it  needs p-type additives  in order  to  efficiently  extract  and  conduct  the  holes  through  the  devices,  due  to  it relatively  poor  hole  mobility  and  conductivity.  The  use  of  such  dopants  and  additives  in  the hole  transporting  layer  reduces  series  resistance  due  to  an  enhancement  in  the  hole injection.
Though,  device  long  term  stability  remains  a  matter  of  concern,  since  the   usage  of  highly hygroscopic  dopants,  such  as lithium  salts,  can  dissolve  the  water  sensitive  perovskite,  which is very  moisture  sensitive.

In  the   present   work,  three  main  objectives  were  addressed  in  order  to  overcome the cited drawbacks.  Firstly,  novel small organic molecules  have  been  designed  and  synthesized,  and they  have  been  effectively  implemented  in  perovskite  based  solar  devices as  innovative  and cost-effective  HTMs.  The  rationally  designed  molecules  present  a  thiophene-based core  and  a triphenylamine-  or  carbazole-derivative side  arms.  The  opto-electrical  properties  of  the   novel molecules  were measured and  tuned as such to yield better  photovoltaic  parameters for efficient device  fabrication.  Moreover,  two  novel  phthalocyanine-based  HTMs  were  implemented  and tested  in  PSCs.  Advantages  such  as  easy
synthesis,  low  cost  and  encouraging  results  in  term of device performance  provide a potential application as the replacement  of the  expensive Spiro­ OMeTAD.

Secondly,   some  of  the   newly  synthesized   thiophene-based   molecules   were  implemented  as exciton  blocking  layer  in  DBP/C70-based   planar  heterojunction  organic  solar  cells  (OPVs). The   enhancement   in  the   photocurrent   demonstrated   the  efficient  integration  of  the   novel molecules  and   will  allow  to   design  and  integrate  other  similar  small
molecules  as  exciton blocking layers in  OPVs in  the  future.

Finally,  pyridine-  and  imidazolium-based  ionic liquids  were introduced  as unique  hydrophobic p-type dopants, with the aim of boosting the conductivity and hole mobility of Spiro-OMeTAD, and  thus  avoiding  the  use  of  hygroscopic  and  corrosive  additive  materials,  with  the  aim  of
enhancing  the  long-term  stability  of  the  PSCs.  The  carried  studies  prove  the  effectiveness  of the  use of the  presented  pyridinium-based hydrophobic  ionic liquid as novel and  unique dopant for PSCs,  used  in  a very minute quantity.  The impressive  results  pave the  way  to  the  use of  a range  of  organic  semiconductors  as  hydrophobic  dopant,  which  can
work  as  effective  dopant for  HTMs  and  can effectively  enhance  the  lifetime of PSCs devices.

Open access article:
Cu(II) and Zn(II) based Phthalocyanine as Hole selective layers for Perovskite Solar Cells, L. Calio, J. Follana-Berná, S. Kazim, M. Madsen, H.-G. Rubahn, Á. Sastre-Santos and S. Ahmad, Sustainable Energy Fuels, 2017
DOI: 10.1039/C7SE00367F (2017)