We demonstrate an ionically gated planar PS-PV solar cell with ultra-thick
fullerene ETL with a porous CNT electron collector on top of it. Perovskite
photovoltaic devices usually have undoped electron transport layers, usually
thin like C60 due to its high resistance. Metallic low work function cathodes
are extremely unstable in PS-PV due to reaction with halogens I-/Br-, and it
would be desirable to have stable carbon cathodes on top of thick low
resistance ETL for enhancing the stability of PS-PVs. We show that gating such
top CNT cathode in ionic liquid, as part of a supercapacitor charged by Vg
tunes the Fermi level of CNT by EDL charging, and causes lowering of a barrier
at of C60/C70 ETL. Moreover, at higher gating voltage ions further propagates
into fullerene by electrochemical n-doping, which increases dramatically PV
performance by raising mostly two parameters: Isc and FF, resulting in PCE
efficiency raised from 3 % to 11 %. N-doping of ETL strongly enhances charge
collection by ETL and CNT raising Isc and lowering series resistance and thus
increasing strongly PCE. Surprisingly Voc is not sensitive in PS-PV to external
Vg gating, on the contrary, to strongly enhanced Voc in ionically gated organic
PV, where it is the main gating effect. This insensitivity of Voc to lowering
of the work function of Vg gated CNT electrode is a clear indication that Voc
in PS-PV is determined by inner p-i-n junction formation in PS itself, via
accumulation of its intrinsic mobile ionic species halogens and cations and
their vacancies.
arXiv Physics
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