18 Interfacial Materials for Organic Solar Cells
405
Cao and co-workers developed various alcohol-/water-soluble conjugated
polymers,
poly[(9,9-bis(3
-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7(9,9–dioctyldioctylfluorene)] (PFN) and poly[(9,9-bis(3
-((N,N-dimethyl)-Nethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]
dibromide
(PFN-Br) and used as the ITO surface modifier (Huang et al. 2004). Using an inverted
device architecture a certified PCE of 9.2 and 8.2% for conventional structure was
achieved which offered Ohmic contact for photogenerated charge carrier collection
and improve photon harvesting (He et al. 2012). The same group further developed
an alcohol soluble polyelectrolyte poly[(9,9-bis((N-(4-sulfonate-1-butyl)-N,Ndimethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFNSO)
containing sulfobetaine zwitterionic groups on their side chains to use as CIL. (Duan
et al. 2013b) A PCE of 8.74% could be achieved using PTB7:PC 71 BM photoactive
layer resulting from the small leakage current and lower series resistance. The
same group further developed alcohol soluble thermal cross-linkable, conjugated
polyfluorene, poly[(9,9-bis(6
-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7(9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene)] (PFN-OX). (Hu et al. 2014)
PFN-OX electron extraction layer can be completely crosslinked at 140 °C forming a
smooth layer over ITO and modify the WF of ITO to 3.5 eV to match with the LUMO
level of PC 71 BM. Inverted BHJ device using PBDT–DTNT:PC 71 BM exhibited a
PCE of 8.62% with a BHJ film thickness of 280 nm. While device without PFN-OX
layer gave very low PCE of 1.42% due to very low V OC (0.32 V) and FF (0.31).
Remarkable PCE of 7.24% has also been measured with BHJ thickness of 1000 nm
demonstrating high hole mobility and ordered molecular structure of donor polymer.
Cao and co-workers demonstrated the incorporation of PFN as CIL between
PCDTBT:PC 71 BM blend and Al simultaneously improved the V OC , J SC and FF,
thus the PCE from 4.02 to 6.73% (He et al. 2011b). The V OC was improved from
0.71 to 0.91 V and FF from 0.49 to 0.62. It has been shown that the device with
PFN/Al exhibited a high turn-on voltage of 0.8–1.0 V compared to bare Al device
(0.5–0.6 V), indicating the enhancement of V bi across the device with PFN layer
which is key to the V OC improvement. The role of PFN layer was further studied by
405
Cao and co-workers developed various alcohol-/water-soluble conjugated
polymers,
poly[(9,9-bis(3
-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7(9,9–dioctyldioctylfluorene)] (PFN) and poly[(9,9-bis(3
-((N,N-dimethyl)-Nethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]
dibromide
(PFN-Br) and used as the ITO surface modifier (Huang et al. 2004). Using an inverted
device architecture a certified PCE of 9.2 and 8.2% for conventional structure was
achieved which offered Ohmic contact for photogenerated charge carrier collection
and improve photon harvesting (He et al. 2012). The same group further developed
an alcohol soluble polyelectrolyte poly[(9,9-bis((N-(4-sulfonate-1-butyl)-N,Ndimethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFNSO)
containing sulfobetaine zwitterionic groups on their side chains to use as CIL. (Duan
et al. 2013b) A PCE of 8.74% could be achieved using PTB7:PC 71 BM photoactive
layer resulting from the small leakage current and lower series resistance. The
same group further developed alcohol soluble thermal cross-linkable, conjugated
polyfluorene, poly[(9,9-bis(6
-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7(9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene)] (PFN-OX). (Hu et al. 2014)
PFN-OX electron extraction layer can be completely crosslinked at 140 °C forming a
smooth layer over ITO and modify the WF of ITO to 3.5 eV to match with the LUMO
level of PC 71 BM. Inverted BHJ device using PBDT–DTNT:PC 71 BM exhibited a
PCE of 8.62% with a BHJ film thickness of 280 nm. While device without PFN-OX
layer gave very low PCE of 1.42% due to very low V OC (0.32 V) and FF (0.31).
Remarkable PCE of 7.24% has also been measured with BHJ thickness of 1000 nm
demonstrating high hole mobility and ordered molecular structure of donor polymer.
Cao and co-workers demonstrated the incorporation of PFN as CIL between
PCDTBT:PC 71 BM blend and Al simultaneously improved the V OC , J SC and FF,
thus the PCE from 4.02 to 6.73% (He et al. 2011b). The V OC was improved from
0.71 to 0.91 V and FF from 0.49 to 0.62. It has been shown that the device with
PFN/Al exhibited a high turn-on voltage of 0.8–1.0 V compared to bare Al device
(0.5–0.6 V), indicating the enhancement of V bi across the device with PFN layer
which is key to the V OC improvement. The role of PFN layer was further studied by