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Effect of Polyelectrolyte Electron Collection Layer Counteranion on the Properties of Polymer Solar Cells
journal contribution
posted on 2015-02-11, 00:00 authored by Thu Trang Do, Hee Seob Hong, Ye Eun Ha, Juyun Park, Yong-Cheol Kang, Joo Hyun KimPolyviologen (PV) derivatives are
known materials used for adjusting
the work function (WF) of cathodes by reducing the electron injection/collection
barrier at the cathode interface. To tune and improve device performance,
we introduce different types of counteranions (CAs), such as bromide,
tetrafluoroborate, and tetraphenylborate, to a PV derivative. The
effective WF of the Al cathode is shown to depend on the size of the
CA, indicating that a Schottky barrier can be modulated by the size
of the CA. Through the increased size of the CA from bromide to tetraphenylborate,
the effective WF of the Al cathode is gradually decreased, indicating
a decreased Schottky barrier at the cathode interface. In addition,
the change of the power conversion efficiency (PCE) and the short
circuit current (Jsc) value show good
correlation with the change of the WF of the cathode, signifying the
typical transition from a Schottky to an Ohmic contact. The turn-on
electric field of the electron-only device without PV was 0.21 MV/cm,
which is dramatically higher than those of devices with PV-X (0.07
MV/cm for PV-Br, 0.06 MV/cm for PV-BF4, and 0.05 MV/cm
for PV-BPh4) This is also coincident with a decrease in
the Schottky barrier at the cathode interface. The device ITO/PEDOT/P3HT:PCBM/PV/Al,
with a thin layer of PV derivative and tetraphenylborate CA as the
cathode buffer layer, has the highest PCE of 4.02%, an open circuit
voltage of 0.64 V, a Jsc of 11.6 mA/cm2, and a fill factor of 53.0%. Our results show that it is
possible to improve the performance of polymer solar cells by choosing
different types of CAs in PV derivatives without complicated synthesis
and to refine the electron injection/collection barrier height at
the cathode interface.