Improved cathode buffer layer to decrease exciton recombination in
organic planar heterojunction solar cells
Supplementary information
Experimental details
Glass substrates coated with indium tin oxide (ITO, Kintec, 85 nm, sheet resistance < 20 Ω □-1) are
cleaned by subsequent ultrasonic treatment in detergent, de-ionized water, acetone, and iso-propanol,
for 5 min each, followed by an ultraviolet-ozone treatment for 900 s. On the samples for diindeno[1,2,3cd:1',2',3'-lm]perylene (DIP)-based solar cells, Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS, HC Starck Clevios P) was deposited by spin-coating for 60 s at 6000 rotations per minute
and annealed on a hotplate for 10 minutes at T = 130°C. The MoO3 (~550°C), zinc phthalocyanine (ZnPc)
(~380°C), DIP (~240°C), C60 (~380°C), bathocuproine (BCP) (~150°C) and 3,4,9,10-perylenetetracarboxylic
bis-benzimidazole (PTCBI) (~380°C) materials are evaporated at a rate of ~1 Å s-1 in a vacuum chamber at
a pressure of ~10-7 Torr. Deposition rates are determined from calibration of the film thickness,
measured ex-situ by spectroscopic ellipsometry (Sopra GESP-5). All organic materials are purified before
loading into the chamber by thermal gradient sublimation. The cathode, Ag (~10-7 Torr; 1.5 Å s-1), is
deposited through a shadow mask, without breaking vacuum.
The surface morphology was studied with atomic force microscopy (AFM) using a Picoscan
PicoSPM LE scanning probe in tapping mode.
Electrical characterization of the solar cells is performed with a Keithley 2602 measurement unit.
Cells are measured in the dark (information available from authors) and under 100 mWcm-2 AM1.5G
simulated solar illumination. Solar illumination is applied using an Abet solar simulator, calibrated with a
Fraunhofer certified photovoltaic cell. For all studied device architectures, 6 cells with active areas of
0.134 cm2 were measured, where the best performing (highest power conversion efficiency) is reported
here.
A commercial set-up (Bentham) is used to measure the external quantum efficiency (EQE). Light
from a Xe arc lamp (300 - 670 nm) and a quartz halogen lamp (670-900 nm) is chopped, coupled into a
monochromator and aimed at the device. The resulting current is sent through a Bentham477 current
pre-amplifier, then arriving in the Bentham485 lock-in amplifier. A voltage source (Thandar TS1541S)
was put in series to the solar cell/pre-amplifier loop to provide a voltage bias. Calibration is done with a
certificated Si cell. The same optics and measurement setup is used with a DTR6 integrating sphere to
determine the reflection.
For the photoluminescence (PL) measurements, solar cells were glass-encapsulated in a nitrogen
atmosphere. Excitation was performed using a wavelength λ = 532 nm continuous laser (Roithner
LaserTechnik RLDD532-50-5) followed by a 700 nm cutoff short pass filter (Thorlabs). The photocurrent
of the solar cell matched operational conditions for the chosen laser intensity. The emission spectra
were recorded using a triple-grating monochromator coupled to an intensified CCD camera (PI-MAX
from Princeton Instruments). A voltage source (Keithley 2400) provided a voltage bias for the PL
measurements. Devices were burnt-in by this laser during 20 minutes before the measurements shown
in this article, to limit PL drift during the measurement. This resulted in limited device degradation (Fig.
S2 and S3). The PL drift during the measurement was found to be 5%, as determined by PL
measurements at 0 V, at the start and the end of the measurement series.
Absorption spectra
Figure S1: Absorption coefficient k as determined by ellipsometry on films deposited on Si/SiO2 substrates, for ZnPc (black),
DIP (red), C60 (magenta), C70 (blue), BCP (green) and PTCBI (yellow)
Photodegradation device during PL experiments
Figure S2: Current density-voltage curves for 100 mWcm-2 AM1.5G simulated solar illumination of device a: ITO/ MoO3/
ZnPc/ C60 / BCP/ Ag; measured before (black) and after (red) PL measurements. The laser illumination slightly degraded
device performance, which was accompanied with slight shifts in PL intensity.
Figure S3: Current density-voltage (J-V) curves for 100 mWcm-2 AM1.5G simulated solar illumination of device b: ITO/ MoO3/
ZnPc/ C60 / BCP/ PTCBI/ Ag; measured before (black) and after (red) PL measurements.
AFM
Figure S4: Topographic AFM image (2500 x 2500 nm) of an ITO/PEDOT:PSS/DIP(10 nm) sample
Root mean square (rms) value = 2.2 nm
Folding ratio FR = surface area / projected area = 6.32 μm2 / 6.25 μm2 = 1.01
Simulation vs. measured reflection and EQE
The reflection and the EQE at 0 V are shown in black in respectively Fig. S5 and Fig. S6 for the following
devices (thicknesses given in nm):
c) ITO/PEDOT:PSS/DIP(10)/C60(30)/BCP(10)/Ag
d) ITO/PEDOT:PSS/DIP(10)/C60(30)/BCP(10)/PTCBI(5)/Ag
These spectra were compared with modeling results, combining optical transfer matrix modeling with
the exciton diffusion equation. 1,2 As input, the transfer matrix modeling required layer thicknesses and
refractive indices, which were measured by ellipsometry. The thickness of ITO and PEDOT:PSS were set
at respectively 80 nm and 40 nm. The ambient atmosphere was assumed to be glass. The optical
absorption profiles were used as input for the exciton diffusion equation. Here, we assumed perfect
exciton dissociation at the DIP/C60 interface ( CT (0 V) CC (0 V) = 100%, where  CT (V ) is the charge
transfer efficiency and CC (V ) is the charge collection efficiency). The ITO/DIP interface was assumed to
be exciton quenching, while C60/BCP was assumed to be exciton blocking. The exciton diffusion length of
DIP was set at LD = 90 nm3, although the exact value here is not very critical since the DIP films are only
10 nm thick and absorb minimal incident light.
Figure S5: Absorption spectra (100-reflection, in %, dashed curves) and external quantum efficiency (EQE) at 0 V (solid
curves), according to measurement (black) and optical simulation (red) of device (c) ITO/ PEDOT:PSS/ DIP/ C60 / BCP/ Ag.
The measured EQE and the simulated EQE (Fig. S5, red) of device c closely match for an exciton diffusion
length of C60 set at LD = 24 nm. Integration of the resulting simulated EQE over the solar spectrum leads
to JEQE (total) = 4.4 mA/cm2. The model further allows to discriminate in between the JEQE contributions
of DIP and C60, resulting in JEQE (DIP) = 0.5 mA/cm2 and JEQE (C60) = 3.9 mA/cm2. By comparing these
currents with the total amount of photons absorbed in DIP and C60 respectively, we calculated the
internal quantum efficiencies (IQE) for those layers to be IQE(DIP) = 41% and IQE(C60) = 65%.
Figure S6: Absorption spectra (100-reflection, in %, dashed curves) and external quantum efficiency (EQE) at 0 V (solid
curves), according to measurement (black) and optical simulation (red) of device (d) ITO/ PEDOT:PSS/ DIP/ C60 / BCP/ PTCBI/
Ag.
The simulation parameters for device d were the same, except for the added PTCBI layer. Simulation and
modeling now match for an exciton diffusion length of C60 at LD = 48 nm. We propose that higher C60
exciton lifetimes (less quenching), lead to this increase in effective exciton diffusion length. Integration
of the resulting simulated EQE over the solar spectrum leads to JEQE (total) = 5.4 mA/cm2, JEQE (DIP) = 0.5
mA/cm2 and JEQE (C60) = 4.9 mA/cm2. The simulation further on resulted in IQE(DIP) = 42% and IQE(C60) =
84%.
Figure S7: Absorption spectra (100-reflection, in %, dashed curves) and external quantum efficiency (EQE) at 0V (solid
curves), according to measurement (black) and optical simulation (red) of device (e) ITO/ PEDOT:PSS/ DIP/ C70 / BCP/ PTCBI/
Ag. No reflection measurements were performed here.
Finally, the EQE of the following device was measured and simulated:
e) ITO(80)/PEDOT:PSS(20)/DIP(30)/C70(30)/BCP(10)/ PTCBI(5)/Ag
Comparable EQE graphs were obtained for LD = 29 nm for C70. This simulation leads to JEQE (total)= 9.3
mA/cm2, JEQE (DIP) = 1.0 mA/cm2 , JEQE (C70) = 8.3 mA/cm2, IQE (DIP) = 49% and IQE (C70) = 71%.
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