Environmental Engineering Reference
In-Depth Information
improve the PCE of inverted solar cell device with PCBSD interlayer [ 113 ],
yielding a record efficiency of 6.2 % for inverted organic solar cell devices at the
time.
3.3.1.5 Polymer or Cross-linkable Organic Thin Film as Single ETL
Besides acting as an interlayer between metal oxide layer and active layer,
polymer or cross-linkable organic thin films can also be implemented as inde-
pendent ETL for inverted solar cells to completely replace the metal oxide layer.
Zhou et al. reported the use of PEO thin film as single ETL on ITO for inverted
solar cells. The insertion of PEO interlayer between ITO and APFO3:PC 61 BM
active layer improved the PCE of the devices from 0.5 % to 0.7 % [ 114 ]. Na et al.
later reported the use of a water-soluble polyfluorene CPE as a single ETL which
can improve the PCE of P3HT:PC 61 BM inverted devices from 1.04 % without
interlayer to 3.38 % [ 115 ]. The same group found out that the CPE material can
also be an effective ETL for multilayered graphene (MLG) cathode [ 116 ]. An
inverted solar cell device with configuration of MLG/CPE/P3HT:PC 61 BM/PE-
DOT:PSS/Al has a PCE of 1.23 %, while the reference device without CPE
interlayer has virtually no photovoltaic effect at all. In these studies, the increase in
PCE is explained by the enhancement of built-in voltage due to work function
lowering effect of the interlayer, and this is generally supported by work function
measurement of ITO substrate with or without interlayer through ultraviolet
photoelectron spectroscopy (UPS) or Kelvin probe.
Besides common solution process, polymer ETL can also be fabricated by layer-
by-layer (LBL) method. Rider et al. reported an ETL generated by repeated LBL
deposition of cationic water-soluble polythiophene poly[3-(6-pyridiniumylhexyl)
thiophene bromide] (P3PHT + Br - ) with anionic (PEDOT:PSS) - Na + on ITO [ 117 ].
The inverted solar cell device of ITO/(P3PHT + /(PEDOT: PSS) - ) 5 /P3HT:PC 61 BM/
V 2 O 5 /Al has an efficiency of *2 % and operation lifetime [500 hours.
As the organic ETL materials generally suffer from their low electron con-
ductivity compared to metal oxides, some researchers try to compensate this by
doping the ETL layer. Cho et al. demonstrated that the n-doping of a cross-linkable
PC 61 BM derivative PCBM-S using decamethylcobaltocene (DMC) can signifi-
cantly increase the electron conductivity of the cross-linked ETL layer [ 118 ].
In the configuration of ITO/PCBM-S/P3HT:PC 61 BM/PEDOT:PSS/Ag, device
with PCBM-S layer of optimum DMC doping level yield a PCE of 2.53 %,
compared to 1.24 % for devices with undoped PCBM-S interlayer.
Before the end of this section it should be noted that the electronic properties of
the interface of ITO and metal oxide is complicated and our understanding is far
from complete. For example, Wang et al. reported an interesting case that the PCE
of the inverted solar cell device with the structure of ITO/P3HT:PC 61 BM/PE-
DOT:PSS/Ag changes irreversibly from 1.46 to 4.1 % when the device is exposed
to continuous illumination of simulated sunlight for 2 h [ 119 ]. Though the
mechanisms behind the large enhancement in PCE is not thoroughly explained, the
Search WWH ::




Custom Search