Figure  12  AFM  scans  of  debond  surfaces  

Al  side

 ITO  side

 

No  annealing   150  °C,  30  min  

Figure   10   Four   point   bend   results.   Average   Gc  determined  from  Gc  values  at  criDcal  load  peaks  

Figure  11  Plot  of  fracture  energy  vs.  anneal    Dme  

Figure  8  Average  Gc  at  different  MoO3  deposiDon  rate.  Average  Gc  determined  from  three  samples  tested  per  cell

Cohesion  and  Reliability  of  Organic  Bulk  Heterojunc?on  Solar  Cells  Ellen  Tsay,  Christopher  Bruner,  and  Reinhold  H.  Dauskardt  

Department  of  Materials  Science  and  Engineering,  Stanford  University  

•  We   chose   to   study   P3HT:PC60BM   bulk   heterojuncDon  (BHJ)   cells   because   they   offer   high   internal   quantum  efficiency  and  have  been  thoroughly  researched  

•  Typically  these  cells  fail  cohesively  within  the  BHJ,  the  weakest  layer  

•  A   rougher   debond   surface   is   associated   with   greater  cohesion  for  several  reasons:  o  Stress  is  distributed  over  a  larger  area  and  in  

mulDple  direcDons  o  More  contact  between  asperiDes  behind  crack  Dp   Figure  4  Mechanical  tesDng  system  used  for  four  point  bend  test  

Fabrica?ng  the  Solar  Cells  1.  50  mm  x  50  mm  borosilicate  glass    substrates  coated  with  ITO  2.  Cleaning:  ultrasonic  baths  with  Extran,  IPA  and  Ethanol  and  UV  ozone  3.  Low  pressure  thermal  evaporaDon  of  MoO3  onto  ITO    4.  BHJ   soluDon   contained   1:1   weight   raDo   PCBM:P3HT   and   50   mg   of  

solute  per  1  ml  of  chlorobenzene  solvent  5.  Spin-­‐coated   BHJ   onto   the  MoO3   at   900   rpm   and   500   rpm/s   for   45   s.  

Devices  slow  dried  overnight  6.  Low  pressure  thermal  evaporaDon  of  Ca  and  Al  metal  electrodes  

Figure   3   A   diced   cell,   ITO   side   up.  Individual   samples  were  detached  and  prenotched  

1.  We   hypothesize   that   varying   the   deposiDon   rate   of   a  MoO3   hole   transport   layer   (HTL)   will   influence   its  surface   roughness   and   therefore   cohesion.   Since   the  BHJ   is  so  thin,   the   failure  path  may  follow  the  surface  of  the  underlying  MoO3  layer  in  order  to  stay  cohesive  within  the  BHJ  layer.  

2.  A   second   hypothesis   is   that   thermally   annealing   cells  will   increase   cohesion.   This   has   been   shown   in   cells  with   a   PEDOT:PSS   HTL,   and   we   aeempt   to   duplicate  the  result  with  MoO3  as  the  HTL.  

•  All  devices  failed  cohesively  within  the  bulk  heterojuncDon  •  Varying  the  rate  of  MoO3  deposiDon  had  no  significant  effect  on  surface  roughness,  and  there  was  no  correlaDon  between  MoO3  surface  roughness  and  cohesion  

•  Annealing  produced  a  70-­‐112%  increase  in  Gc,  from  3.63  J/m2  ±  0.642  up  to  7.71  J/m2  ±  0.162  •  The  opDmal  annealing  Dme  at  150  °C  was  30  min.  Annealing  results  closely  resembled  those  for  PEDOT:PSS  HTL  cells  

Introduc?on  

Organic  Bulk  Heterojunc?on  Solar  Cells  

Hypotheses  

Device  Prepara?on  and  Tes?ng   Effect  of  Deposi?on  Rate  

Conclusions  

References  and  Acknowledgements  

Annealed  four  samples  from  the  same  cell  at  150  °C  in  a  glovebox  for  5,  30,  60,  120  min  

Preparing  Cells  for  Fracture  Tes?ng  •  A  solar  cell  “sandwich”  was  created  by  adhering  a  second,  idenDcal  glass  substrate  to  the  top  electrode  with  epoxy  

•  Cured  sandwich  for  30  min  at  85  °C  to  harden  the  epoxy  •  Diced  cells  into  individual  samples  (see  Fig.  3)    The   four   point   bend   test   was   used   to  measure   the   cohesion,   or   criDcal  energy   release   rate,   of   the   cells   (see   Fig.   4).   The   criDcal   energy   release  rate,   Gc,   is   the   energy   released   per   unit   crack   area   and   was   calculated  using  the  equaDon  in  Fig.  5.  

P3HT  a  polymer  

electron  donor  

+  

PC60BM  a  fullerene  derivaDve  electron  acceptor  

=  

Figure  1  Typical  structure  of  a  bulk  heterojuncDon  solar  cell  

Organic   solar   cells  a r e   p r om i s i n g  a l t e r n aD ve s   t o  silicon   solar   cells,  but   in   order   to  become   a   viable  technology   their  m e c h a n i c a l  reliability  must  be    

improved.   Cracks   and   defects   shorten   device   lifeDmes  and   have   a   negaDve   impact   on   producDon   yields   and  cost.  We  study  the  effect  of  chemical  deposiDon  rate  and  thermal  annealing  on  the  mechanical  strength  of  organic  bulk  heterojuncDon  solar  cells  and  whether  these  factors  can   be   successfully   manipulated   to   improve   device  reliability.  

Brand,  Bruner  &  Dauskardt.  2012.  “Cohesion  and  device  reliability  in  organic  bulk  heterojuncDon  photovoltaic  cells.”    Solar  Energy  Materials  &  Solar  Cells  99:182–189.  

Brand,  Levi,  McGehee  &  Dauskardt.  2012.  “Film  stresses  and  electrode  buckling  in  organic  solar  cells.”  Solar  Energy    Materials  &  Solar  Cells  103:80-­‐85.  

Dupont,  Oliver,  Krebs  &  Dauskardt.  2012.  “Interlayer  adhesion  in  roll-­‐to-­‐roll  processed  flexible  inverted  polymer  solar    cells.”  Solar  Energy  Material  &  Solar  Cells  97:171-­‐175.  

 I  would  like  to  thank  the  Vice  Provost  for  Undergraduate  EducaDon  and  the  Materials  Science  and  Engineering  Department  for  funding  this  project.  Special  thanks  to  Christopher  Bruner,  the  Dauskardt  Group,  and  the  McGehee  Group  for  their  guidance  and  support.  

Figure  9  Debonded  devices  

Figure   7   MoO3   surface   roughness   for   different  deposiDon   rates.   Rrms   is   the   root   mean   square  value  of  the  surface  topography  

Figure   2   Structure   of   the   tested  solar  cell  “sandwiches”  

Science  Kn

owledge  2010  

Molecule  im

ages:  

Brand,  Brune

r  &  Dauskardt  2012  

Figure  5  EquaDon   for  Gc,  where  Pc   is   the  criDcal    load,  L  is  the  moment  arm  length,  E’   is   the  biaxial  modulus  and  b  and  h  are  the  specimen  width  and  half-­‐height  

Brand,  Brune

r  &  Dauskardt  2012  

•  Built   three   devices   at   MoO3   deposiDon  rates  of  0.5,  5,  20  Å/s  

•  Characterized  roughness  of  MoO3  surfaces  with  AFM  

Figure  6  XPS  results  for  the  ITO  side  of  the  cell  annealed  for  30  min  

•  Characterized   debond   surfaces  with   non-­‐contact   atomic   force  microscopy   (AFM)   and   X-­‐ray  photoelectron   spectroscopy  (XPS)  

•  XPS   results   for   all   devices  looked   similar,   showing   carbon  and  sulfur  peaks  

Debond  Path  Characteriza?on  

Effect  of  Thermal  Annealing