Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 57

PEDOT:PSS and conjugated polymer-rich blend near the cathode side, which is
possibly detrimental to charge extraction efficiency [ 83 , 84 ]. Liang et al. hence
simply inserted a thin layer of high molecular weight P3HT between PEDOT:PSS
and P3HT:PC 61 BM blend layer to increase the electron-blocking ability of
PEDOT:PSS. The extra donor/acceptor interfaces adjacent to the bottom of the
composite were created as well, leading to an enhanced photoinduced electron
transfer efficiency and photocurrent density. Relative to the 3.98 % PCE of the
reference device, the extra P3HT inserted PSC delivered PCE of 5.05 % [ 85 ].
The insertion of a discotic liquid crystal of hexabutoxytriphenylene (HAT4) at the
interface between anodic buffer layers (PEDOT:PSS, MoO 3 or NiO) and organic
active layer was also found to be an effective method to improve the photovoltaic
performance of P3HT:PC 61 BM solar cell. Atomic force microscopy (AFM)
measurement indicate that the ordered hexagonal columnar phase formed in HAT4
layer provides a more efficient pass way for hole transporting, and thereby leads to
an enhanced J sc and FF [ 86 ].
The insertion of an in situ polymerized triphenylamine-containing polyperflu-
orocyclobutane (TPA-PFCB) thin layer between ITO and PEDOT:PSS was
revealed to be an effective approach to block the electron leakage at anode. After
the coverage of TPA-PFCB, the surface roughness of ITO was substantially
reduced. Compared to the control PEDOT:PSS solar cell, the implementation of
the TPA-PFCB layer increases both V oc and J sc , and thereby gives rise to enhanced
PCE. The HOMO and LUMO of TPA-PFCB was measured to be -5.2 and
-1.7 eV, respectively, which verified its good charge selective hole-transporting
and electron-blocking properties. In addition, FET measurement confirmed that
electron transporting is completely blocked by implementation of TPA-PFCB
between the organic active layer and the Al source and drain electrodes [ 87 ].
3.2.2.4 Carbon Nanotubes as Anode Interlayer
Carbon nanotubes (CNTs) possess high electrical conductivity, approximate work
function of *5.0 eV which match well the work function of ITO and HOMO of
most donor polymers, and outstanding optical transparency in a broad spectral
range from UV to deep infrared region, making CNTs potential hole-collecting
materials for BHJ-PSCs [ 88 ]. Chaudhary et al. had inserted CNTs at different
interfaces (ITO/PEDOT:PSS, PEDOT:PSS/P3HT:PC 61 BM, and P3HT:PC 61 BM/
Al) of P3HT:PC 61 BM solar cell, and had found that only CNTs were incorporated
into the interfaces of ITO/PEDOT:PSS or PEDOT:PSS/P3HT:PC 61 BM and the
solar cells exhibited obvious enhancement in PCE [ 89 ]. Most recently, Hatton
et al. had demonstrated that the use of partially oxidized CNTs as anodic buffer
layer to replace PEDOT:PSS can effectively facilitate the hole extraction.
P3HT:PC 61 BM solar cells with this neutral aqueous processed CNTs anodic buffer
layer exhibited comparable photovoltaic performance with that of PEDOT:PSS
control device [ 90 ].