A
B
-∆T/T (x10-6)
Wavelength (nm)
Transport lifetime (s)
C
Photocurrent decay
Time (ms)
Fig. 3. (A) Schematic illustrating the charge transfer and charge transport in a
perovskite-sensitized TiO2 solar cell (left) and a noninjecting Al2O3-based solar cell
(right); a representation of the energy landscape is shown below, with electrons shown
as solid circles and holes as open circles. (B) Photoinduced absorbance (PIA) spectra of
the mesoporous TiO2 films (black circles) and Al2O3 films (red crosses) coated with
perovskite with (solid lines) and without (dashed lines) spiro-OMe TAD hole transporter,
Current density (mA cm-2)
under 496.5 nm excitation at 23 Hz repetition rate. (C) Charge transport lifetime
determined by small-perturbation transient photocurrent decay measurement of
perovskite-sensitized TiO2 cells (black circles) and Al2O3 cells (red crosses), both with
lines to aid the eye. Inset shows normalized photocurrent transients for Al2O3 cells (red
trace with crosses every 7th point) and TiO2 cells (black trace with circles every 7th
point), set to generate 5 mA cm−2 photocurrent from the background light bias.
the n-type material and holes in the p-type material, resulting in splitting of the quasi Fermi levels for both electrons and holes. For mesoporous
TiO2, there exist sites in the tail of the density
of states that extend into the band gap (38).
These fill with electrons under illumination; the
result is that the quasi–Fermi level for electrons
(EFn ) is farther from the conduction band, for
any given charge density, than would be the
case if these states did not exist (i.e., in a highly
crystalline semiconductor). The increased charge-storing capacity of materials with a high density
of sub–band gap states is termed “chemical capacitance” (38). There is, in essence, no chemical
capacitance of the Al2O3, and for the MSSCs
all the electronic charge resides in the perovskite, moving the EFn in this material nearer to
the conduction band for the same charge density. The higher voltage indicates that there are
fewer surface and sub–band gap states in the
perovskite films than in the mesoporous TiO2.
Hence, the increased voltage is caused by a substantial reduction of the chemical capacitance of
the solar cell. We used a compact layer of TiO2
as the electron-selective anode, but the chemical
capacitance of this extremely thin (50 to 100 nm)
TiO2 layer was very low because of the low
volume and surface area (i.e., flat). In addition,
the compact layer deposited via spray pyrolysis
has a donor density of ~1018 cm−3 (39), and the
sub–band gap sites responsible for the chemical
capacitance may be full.
A central question is whether the MSSC is
excitonic or a distributed p-n junction. The pe-
rovskites tend to form layered structures, with
continuous two-dimensional metal halide planes
perpendicular to the z axis and the lower di-
electric organic components (methyl amine) be-
tween these planes. The possible quasi–two-
dimensional confinement of the excitons can
result in an increased exciton binding energy,
which can be up to a few hundred millielectron
volts (40). The reasonably high photocurrents
from the planar-junction solar cells (Fig. 2B)
could be explained by either moderately delo-
calized and highly mobile excitons being quenched
at the perovskite–spiro-OMeTAD interface, or
the generation of free charges in the bulk of the
perovskite films with reasonably good electron
and hole migration out of the devices.
References and Notes
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