High open-circuit voltage, high fill factor single-junction organic solar cells
Yuelin Peng†, Lushuai Zhang§, Trisha L. Andrew‡§*
[†]
Department of Electrical and Computer Engineering, University of Wisconsin-Madison,
Madison, Wisconsin 53706
[§]
Department of Materials Science and Engineering, University of Wisconsin-Madison,
Madison, Wisconsin 53706
[‡]
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706
[*]
corresponding author (email: tlandrew@wisc.edu)
SUPPLEMENTAL INFORMATION
Determination of Frontier Energy Levels with Cyclic Voltammetry
Electrochemical measurements on 150 nm-thick films of DPP grown on a platinum button
electrode (working electrode) were made in acetonitrile/NBu4PF6 electrolyte with a Pine
Instruments Wavenow potentiostat using a quasi-internal Ag wire reference electrode
(BioAnalytical Systems) submersed in 0.1M AgNO3 and a platinum wire counter electrode. The
ferrocene/ferrocenium (Fc/Fc+) redox couple in dichloromethane was used as an external
reference. An irreversible oxidation peak is observed (see Figure S1). The onset potential of the
first oxidation peak is measured to be 1.05 V vs. Ag/AgNO3, or equivalenly 0.96 V vs. Fc/Fc+
(the ferrocene redox couple occurs at 90 mV vs. Ag/AgNO3). To convert this oxidation potential
to the vacuum highest occupied molecular orbital (HOMO) level, we employ the linear
relationship between oxidation levels obtained by cyclic voltammetry and HOMO levels
obtained from ultraviolet photoemission spectroscopy that was previously established.S1 We
arrive at a HOMO level of 5.9 ± 0.2 eV. In order to determine the lowest occupied molecular
orbital (LUMO), we first use a previously-described empirical relationS2 to relate the optical
bandgap (λ = 650 nm) to the transport bandgap (Et) of DPP. We then add Et to the HOMO level.
Et is calculated to be 2.08 ± 0.2 eV and the LUMO level is calculated to be 3.8 ± 0.2 eV.
Figure S1. Cyclic voltammogram for a 150 nm-thick DPP film grown on a platinum button
electrode (working electrode) obtained in an acetonitrile/NBu4PF6 electrolyte with an Ag/AgNO3
reference electrode.
Measurement of Optical Characteristics
Figure S2. Absorption spectra for 20 nm-thick films of DBP (black) and DPP (red).
Figure S3. Wavelength-resolved 1-Reflection (R)-Transimission (T) for a DBP/C60 based solar
cell (blue triangles) and DBP/DPP based solar cell (black dots).
Understanding DBP and DPP Contributions to EQE
Figure S4. (a) Absorption spectra of the individual DBP (blue) and DPP (red) active layers
overlaid onto the wavelength-resolved EQE of the PHJ device (black). (b) Plot of the residual
arising from subtracting the DBP absorption spectrum from the EQE of the PHJ device. The
residual largely coincides with the DPP absorption spectrum in the 450–600 nm range, indicating
that DPP does contribute to the photoactivity of the PHJ solar cell.
Understanding DBP and DPP Morphology
Figure S5. X-ray Diffraction (XRD) data for MoO3 , DBP and DPP thin film layers grown on
ITO. The blank ITO XRD data is also provided. All the samples were sequentially deposited by
vacuum thermal evaporation in parallel. Film thicknesses were as follows: 5 nm MoO3, 150 nm
DBP, 150 nm DPP. The broad peak from 18° to 38° arises from the amorphous glass substrate.
Peaks at 21.8°, 30.9°, 35.8°, 45.9°, and 51.5° are assigned to ITO crystallites.S3 No unique
diffraction peaks are observed for either the MoO3 film or DBP film, suggesting that both these
layers are amorphous. Only a unique peak at 27.2° is observed for the DPP film, which
corresponds to a d-spacing of 3.4 Å. This peak most likely arises due to intermolecular πstacking within the DPP layer; no long-range order is observed for the DPP film. Amorphous
thin films with weak Van der Waals interactions between a donor and a acceptor were previously
reported to yield lower JSO than crystalline materials,S4 suggesting that the largely amorphous
nature of our DBP and DPP films also contribute to a high VOC.
Figure S6. (a) and (b) AFM phase images of the surface of a 150 nm-thick DBP film on
ITO\MoO3 (5 nm). (c) and (d) AFM phase images of the surface of a 150 nm-thick DPP film on
ITO\MoO3 (5 nm) \DBP (150 nm). All films were deposited by vacuum thermal evaporation in
parallel. The root mean square roughness of the DBP film is 0.63 nm and that of the DPP film is
3.4 nm. Distinct grains, indicating crystalline domains, can be observed on the surface of the
DPP film, which is consistent with the unique XRD peak (d-spacing 3.4 Å) observed for the DPP
film. In contrast, no such grains can be observed in AFM surface micrographs of the DBP film,
indicating that the DBP film is amorphous (this is also corroborated by the absence of any
diffraction peaks unique to the DBP film). The surface roughness observed in the AFMs of the
DBP film arises from the underlying substrate.
Diode Characteristics
Figure S7. Ideality factors calculated from dark current densities of DPP based solar cell (black
dots) and C60 based solar cell (blue triangles).
Exploring the molecular origins of JSO
We posit that the strength of intermolecular coupling at the D/A heterointerface is higher for a
DBP/C60 interface, compared to a DBP/DPP interface. To support this hypothesis, we used
Density Functional Theory (Gaussian, B97xD dispersion-corrected functional, aug-cc-pVDZ
basis set) to obtain geometry-optimized structures of a Van der Waals bonded DBP-C60 dimer
and a DBP-DPP dimer. We found that the intermolecular spacing in a DBP-DPP dimer is longer
than that of a DBP-C60 dimer. Since orbital overlap and exchange interactions are exponentially
dependent on distance, the longer intermolecular spacing in DBP-DPP is indicative of weaker
coupling, to first degree.
Figure S8. Geometry-optimized structure of DBP-C60 dimer obtained using Density Functional
Theory (Gaussian, B97xD dispersion-corrected functional, aug-cc-pVDZ basis set).
Figure S9. Geometry-optimized structure of DBP-DPP dimer obtained using Density Functional
Theory (Gaussian, B97xD dispersion-corrected functional, aug-cc-pVDZ basis set).
Supplemental Information References
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