SUPPLEMENTARY INFORMATION
A. Band diagram:
The associated band-diagram is shown in Fig. S1. Here, the Fermi level of DTS-(FBTTh2)2 and
PC70BM is assumed to be aligned with their HOMO and LUMO level, respectively. Though, the active
layer is not completely depleted when no reverse bias voltage is applied, for simplicity this is not shown
in this diagram. The work function of Al is assumed 4.3eV. The work function of 20 nm thick Ca is
interpreted to be pinned at the 4.3eV which is also the LUMO level of PC70BM.
Fig. S1. Illustration of the band-diagram of the complete device. The stated energy values are from the
vacuum level.
In donor material, exciton is generated from photon energy. The exciton gets dissociated into free
carriers at the donor-acceptor interface. In the active layer bulk-heterojunction, the electron is
transferred to the cathode from the LUMO of PC70BM. Similarly, the hole is transferred to the HTL
from the HOMO of DTS-(FBTTh2)2. Finally, the hole is transferred to the anode (ITO) from HTL.
The Schottky-junction created at the cathode-donor interface between metal and DTS-(FBTTh2)2 is
illustrated in Fig. S2.
LUMO
E𝝎2
EMidgap
ET
EF
E𝝎1
HOMO
Metal
P-type Organic Semiconductor
Fig. S2. Band-diagram showing the Schottky-junction formed at the metal-DTS-(FBTTh2)2 interface.
Here, EF is the Fermi energy level, EMid-gap the midgap energy level, Eω1 the demarcation energy level at
high frequency, Eω2 the demarcation energy level at low frequency, and ET is an assumed trap energy
level.
At a lower small-signal frequency, the demarcation-energy level is higher as illustrated by Eω2 in Fig.
S2. With this small-signal frequency employed, the trap states located between the Fermi level and Eω2
respond to the signal and contribute to the measured capacitance.1,2 Trap states above the demarcationenergy level can not respond to the signal as the frequency is higher than their trap emission rate. As the
small-signal frequency is increased, the demarcation-energy level moves down to the Fermi level. At
very high frequency employed, the demarcation-energy level is below the Fermi level as illustrated by
Eω1 in Fig. S2. In this case, the frequency is too high for the trap states to respond.1,2
B. Nyquist plot from impedance spectroscopy
Nyquist plots were obtained by plotting the imaginary part of the impedance with the real part of the
impedance at small-signal frequencies ranging from 1 MHz to 100 mHz. The Nyquist plots of both
regular and thick-film of DTS-(FBTTh2)2:PC70BM, derived applying forward bias equal to the
corresponding open circuit voltage, are illustrated in Fig. S3. The Nyquist plot of the thick-film is right
shifted compared to the regular-film.
2
75
-Zimaginary ()
60
45
30
DTS-(FBTTh2)2:PC70BM Regular-film
15
DTS-(FBTTh2)2:PC70BM Thick film
60
90
120
150
180
210
Zreal ()
Fig. S3. Illustration of Nyquist plot of both regular and thick-film of DTS-(FBTTh2)2:PC70BM. The
negative of the imaginary part of impedance (Zimaginary) is plotted with the real part of impedance (Zreal).
Data points are obtained applying small-signal frequency ranging from 1 MHz to 100 mHz. The black
arrows indicate the peak of the semicircles and the green arrows indicate the quasi-straight line portion
of the profile.
The IS measurements were conducted by applying a forward bias equal to the corresponding opencircuit voltage; this ensures no extra charge injection and no band-bending which results in no internal
electric-field.3-8 In this situation, there is no drift mechanism in the system due to the absence of internal
electric-field. The semicircle portion of the Nyquist plot is the signature of a recombination mechanism
and the inverse of the small-signal frequency corresponding to the peak of the semicircles, indicated by
black arrows in Fig. S3, gives the electron lifetime in p-doped DTS-(FBTTh2)2.3-8 The quasi-straight line
portions, indicated by green arrows in Fig. S3, are the signature of a diffusion mechanism and the
inverse of the frequency corresponding to the point where the straight line intersects the semicircle gives
the electron transit time (τd).3-8 The electron diffusion-coefficient (Dn) and the electron mobility (μn) are
obtained subsequently by,4
𝑑2
𝐷𝑛 =
𝜏𝑑
(S1)
𝜇𝑛 =
3
𝑞𝐷𝑛
𝐾𝐵 𝑇
(S2)
The capacitance vs. frequency curves and negative differential susceptance vs. frequency curves
obtained from impedance spectroscopy measurements employed on DTS-(FBTTh2)2:PC70BM films are
shown below.
Fig. S4. Illustration of (a) capacitance versus frequency curves and (b) negative differential susceptance
versus frequency curves obtained from impedance spectroscopy measurements employed on DTS(FBTTh2)2:PC70BM films.
C. Additional Material
Conductance versus voltage curves for DTS-(FBTTh2)2:PC70BM and DTS-(FBTTh2)2 obtained from
CV-profiling is illustrated below.
4
Fig. S5: Illustration of G/ω vs. voltage curves of DTS-(FBTTh2)2:PC70BM and DTS-(FBTTh2)2 regularfilms at different small-signal frequencies.
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