Environmental Engineering Reference
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results about the use of thermally-evaporated MoO
3
as the anodic buffer layer of
BHJ-PSCs with the configuration of ITO/MoO
3
/PCDTBT:PC
71
BM/TiO
x
/Al. The
implementation of MoO
3
improved the light absorption with the organic active
layer, and thereby resulted in a PCE over 6 % at BHJ layer thickness up to
200 nm. A further enhancement in PCE up to 7.2 % was achieved by using an
antireflection coating. In addition, BHJ-PSC with MoO
3
as anodic buffer layer
demonstrated much better long-term air stability than that of solar cell fabricated
with PEDOT:PSS. The PCE remains at approximately 50 % of the original value
after the storage in air for 720 h, while the PCE of control PEDOT:PSS device fell
to \10 % of the original value after storage in air for 480 h [
72
].
Deposition of a thin layer of p-type NiO by pulsed laser onto ITO to replace
PEDOT:PSS as anodic interlayer led to significant performance enhancement of
BHJ-PSC based on P3HT:PCBM blend. A 5-10 nm NiO layer gives rise to the
PCE as high as 5.2%, while the PCE of control PEDOT:PSS device is only 2.4 %.
The enhancement was initially attributed to the ideal work function of NiO (5.0-
5.4 eV) to match well with the HOMO level of P3HT (5.0), and large band gap
(ca. 3.6 eV) of NiO to deliver high transparency and sufficient barrier for electron
blocking [
73
]. After that, Irwin et al. via a multifaceted analysis further revealed
that NiO grows as smooth, crystalline, and oriented thin films on ITO substrates to
form an optically transparent, electrically uniform, and passivated semiconducting
anode coating, which prevents anode electron injection and facilitates anode hole
injection [
74
]. However, the pulsed laser deposition of NiO layer is neither scal-
able nor a cost effective method. Steirer et al. deposited a thin layer of NiO onto
ITO by spin coating a diluted nickel metal organic ink followed by thermal
annealing at 250 C. The BHJ-PSCs from this solution-processed NiO exhibited
comparable performances with that of PSCs from pulsed laser deposited NiO and
PEDOT:PSS [
75
].
Thermally evaporated thin layers of V
2
O
5
and WO
3
were employed as effective
buffer layers on ITO to improve the performance of BHJ-PSCs based on
P3HT:PC
61
BM blend. The devices based on both oxides exhibited comparable
performances with those of PEDOT:PSS control device. It was suggested that the
ideal work function (4.7 eV) and the relatively high-positioning of the lowest energy
level of the conduction band (2.4 eV) of V
2
O
5
are beneficial for forming efficient
hole-collection injection contact with the organic active layer and to provide suffi-
cient barrier for electron leakage at anode [
70
]. Han et al. revealed that the uniform
amorphous film of WO
3
can effectively planarize an originally rough ITO. P3HT
films, grown on WO
3
film, have a higher degree of ordering and larger hole
mobility than those grown on PEDOT:PSS [
76
]. An ultra-thin layer of AgO
x
generated through plasma oxidized Ag (1 nm) deposited on ITO was found to be
able to improve the contact property of ITO/PEDOT:PSS interface. The
enhancement of device performance is suggested to the formation of an interface
energy step between ITO and PEDOT:PSS that could improve the charge collection
efficiency and the overall efficiency of solar cell devices [
77
].
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