Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 65

deposit PEDOT:PSS onto an active layer is to use spray coating technique [ 128 ]. By
incorporating D-sorbitol into PEDOT:PSS, a transparent ''electric glue'' can be
formed, which is capable of laminating films together both mechanically and
electrically [ 129 ]. Implementing the unique property of the ''electric glue'' into the
inverted device, a semitransparent polymer solar cell based on the P3HT:PC 61 BM
blend was fabricated by the lamination process with a 3 % PCE [ 130 ]. This method
took advantage of the solution process, which also featured self-encapsulation and
provided an alternative to the roll-to-roll production of inverted solar cells.
PEDOT:PSS in its high conductivity form can be directly used as anode for
inverted solar cells. Lim et al. spray coated a layer of modified PH500 (300 S
cm -1 ) high conductivity PEDOT:PSS onto ITO/Cs 2 CO 3 /P3HT:PC 61 BM to form
the anode, and the PCE of the device was 2 % [ 63 ]. Though the conductivity of
PEDOT:PSS layer formed by spray coating was not optimized compared to spin-
coating method, the advantage of spray deposition is that very thick films can be
obtained, which makes up for the low conductivity. Zhou et al. went one step
further to use PH1000 (900 S cm -1 ) high conductivity PEDOT:PSS as both anode
and cathode for semitransparent inverted solar cells [ 131 ]. The device structure
was

PH1000/ZnO/P3HT:PC 61 BM/CPP-PEDOT (a low conductivity PEDOT:

PSS)/PH1000, and the PCE was 1.8 %.

3.3.2.2 Solution Processable Metal Oxide HTL

Though the best inverted solar cells are fabricated with thermo-evaporated tran-
sitional metal oxide HTL such as MoO 3 or V 2 O 5 , solution processable HTL is
preferred since vacuum evaporation could detract from the advantage of the ease
of polymer solar cell fabrication. Therefore, some efforts are made to design
solution-processing route for transitional metal oxide materials. Huang et al. dis-
persed V 2 O 5 powder in isopropanol through ultrasonic agitation and spin cast the
dispersion onto P3HT: PC 61 BM active layer to form HTL [ 124 ]. The device with
configuration ITO/ZnO nanorod/P3HT:PC 61 BM/V 2 O 5 /Ag has a PCE of 3.56 %
under optimum V 2 O 5 thickness, compared to 2.24 % for reference device without
the V 2 O 5 layer. Later, it was found that NiO NPs can also be dispersed in
isopropanol and spin cast onto active layer to form HTL. Lim et al. reported
solution-processed NiO as single HTL [ 132 ] and Lin et al. reported the solution
process of NiO/plexcore HTL double layer HTL [ 133 ]. A sol-gel solution process
of copper (I) oxide (Cu 2 O) HTL similar to the sol-gel process of ZnO was recently
reported [ 134 ]. Deionized water was added into the copper (II) acetate monohy-
drate precursor solution to trigger sol-gel reaction and the solution is then spin cast
onto active layer to form Cu 2 O layer in situ. PCE of the devices with the Cu 2 O
HTL are improved from 3.34 to 4.02 %.