Development of simple hole-transporting materials for perovskite solar cells

Article abstract of DOI:10.1139/cjc-2015-0427

Three low-cost propeller-shaped small molecules based on a triphenylamine core and the high-performance donor molecule 7,7′-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl]bis[6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole] (DTS(FBTTh₂)₂) were investigated as hole-transporting materials in perovskite solar cells. Each hole-transporting material was designed with highly modular side arms, allowing for different bandgaps and thin-film properties while maintaining a consistent binding energy of the highest occupied molecular orbitals to facilitate hole extraction from the perovskite active layer. Perovskite solar cell devices were fabricated with each of the three triphenylamine-based hole-transporting materials and DTS(FBTTh₂)₂ and were compared to devices with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) hole-transporting layers. Each of our triphenylamine hole-transporting materials and DTS(FBTTh₂)₂ displayed surface morphologies that were considerably rougher than that of spiro-OMeTAD; a factor that may contribute to lower device performance. It was found that using inert, insulating polymers as additives with DTS(FBTTh₂)₂ reduced the surface roughness, resulting in devices with higher photocurrents.

Full text of DOI:10.1139/cjc-2015-0427

Page 1 of 22
Development of Simple Hole-transporting Materials for Perovskite Solar Cells
Andrew M. Namespetra,a Arthur D. Hendsbee,c Gregory C. Welch,c* Ian G. Hillb*
a Department of Chemistry, Dalhousie University, 6274 Coburg Road, Halifax, NS B3H 4R2
Canada
b Department of Physics and Atmospheric Sciences, Dalhousie University, 6310 Coburg Road,
Halifax, NS B3H 4R2, Canada
c Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB,
T2N 1N4
Gregory.welch@ucalgary.ca
Ian.hill@dal.ca
Key Words: Solar Energy, Perovskite Solar Cells, Hole-Transport Materials, Triphenyl Amine,
Polymer additives
Abstract
Three low-cost propeller-shaped small molecules based on a triphenylamine core and the high
performance donor molecule DTS(FBTTh2)2 were investigated as hole-transporting materials in
perovskite solar cells. Each hole-transporting material was designed with highly modular side-
arms, allowing for different bandgaps and thin-film properties while maintaining a consistent
binding energy of the highest occupied molecular orbitals in order to facilitate hole-extraction
from the perovskite active layer. Perovskite solar cell devices were fabricated with each of the
three triphenylamine-based hole-transporting materials and DTS(FBTTh2)2 and were compared
to devices with spiro-OMeTAD the hole-transporting layers. Each of our triphenylamine hole-
transporting materials and DTS(FBTTh2)2
displayed surface morphologies that were
considerably rougher than that of spiro-OMeTAD; a factor that may contribute to lower device
1

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performance. It was found that using inert, insulating polymers as additives with DTS(FBTTh2)2
reduced the surface roughness, resulting in devices with higher photocurrents.
Introduction
The use of soluble photoactive materials in third-generation photovoltaic (PV)
technologies enables low-cost, solution-processed manufacturing. In particular, perovskite solar
cells (PSC) combine low-cost materials, solution processing and high efficiencies for a
promising PV technology. Since the first report of organometal halide perovskites (e.g.,
CH3NH3PbI3) as light-harvesters in photovoltaic devices in 2009,1 vast improvements have been
made with power conversion efficiencies now reaching 19%.2 It has been shown that hole-
extraction from the perovskite active layer is a critical step in charge-separation and current
generation.3 Therefore, a key component in the perovskite solar cell is the hole-transporting
material (HTM). In fact, the development of an all-solid-state perovskite solar cell (PSC) was a
significant advancement toward commercialization of this technology and was made possible by
the application of a solid-state hole-transporting material: 2,2’,7,7’-tetrakis(N,N-di-p-
methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD).4
Effective hole-transport from the perovskite to the metal electrode is dependent on a
number of factors including electronic compatibility and nanomorphology. More specifically,
there must be favourable alignment of the perovskite valence band maximum (VBM) and the
highest occupied molecular orbital (HOMO) of the HTM to allow hole-extraction upon carrier
generation. Furthermore, the material must form a continuous film over the perovskite active
layer. Complete coverage of the perovskite layer can prove difficult as the perovskite tends to be
rough with large crystalline domains, especially in a planar device architecture with no
mesoporous framework.5–8 Spiro-OMeTAD has well-aligned energy levels,9 good hole
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mobility10 and the ability to form amorphous and uniform thin films;11 properties that make it a
perfect candidate as an HTM in PSCs and the reason for its continued use and high-performance
in PSCs.
However, the core starting material (e.g., 2,2′,7,7′-Tetrabromo-9,9′-spirobifluorene) is
inherently expensive due to its multifaceted synthesis.9 Furthermore, spiro-OMeTAD requires
the use of p-type dopants (e.g., tert-butylpyridine (tBP) and lithium trifluoromethanesulfonate
(Li-TFSI)) to increase conductivity and hole-mobility to yield high-performance devices.12–14
However, such dopants are hygroscopic and detrimental to the long-term stability of the cell and
thus not practical as agents in commercial solar cells.15
efficient, yet dopant-free,16 economical HTMs.
It is therefore important to develop
Development of alternative HTMs from low-cost starting materials is an active area of
research and a strategy for reducing the overall cost of perovskite solar cells. Simple, easy-to-
synthesize HTMs have emerged with efficiencies rivalling spiro-OMeTAD in PSCs.17–19 For
example, a simple HTM known as “H101” was synthesized with a 3,4-ethylenedioxythiophene
(EDOT) core and shown to achieve give respectable PCE of 10.6 % without the use of dopants.17
In a subsequent paper, the same group reported a derivative of H101 with a tetra-substituted
thiophene core (aka “H111”) that achieved a PCE of 14.9 %.19
Other materials that show promise as cheap building blocks for HTMs are based upon
triphenylamine (TPA). A polytriarylamine HTM has already demonstrated potential in PSCs,
out-performing other polymeric HTMs based on thiophene and carbazole units.20 On the other
hand, small molecules with a single TPA unit have been used as hole conductors in a variety of
the organic electronic applications.20–24 In these compounds, the central TPA “core” adopts a
pseudotetrahedral geometry.25 Each of the three phenyl groups can be functionalized at para
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positions forming the three “side-arm” units. The result is a three-dimensional (3D) propeller-
26,27
shaped molecule. This design motif has already been applied for HTMs in PSCs.
For
example, Choi et al. synthesized a TPA-based HTM that achieved a 7.45% without the use of
dopants. Sung et al. achieved an impressive PCE of 13.86% with a TPA-based HTM employing
carbazole branching units in PSCs.27 However, these cells were fabricated with the use of p-type
HTL dopants and the stand-alone performance of the HTMs is not revealed.
In this contribution we have developed a series of three propeller-shaped small-
molecules, each with a TPA core and different side-arm compositions, for use as HTMs in PSCs
and have studied the utility of the known high performance donor DTS(FBTTh2)2 as a HTM. All
three new compounds were synthesized in-house and built with side-arms consisting of different
donor (i.e., electron-donating) and acceptor (i.e., electron-withdrawing) functionalities (see
Figure 1 for molecular structures). By applying this design strategy, it was possible to keep the
HOMO at an energy level consistent with that of spiro-OMeTAD to allow for efficient hole-
extraction from the perovskite active layer while modifying the molecular structures. In this
way, the effect of both molecular composition and film-forming properties on the performance
TPA-based HTMs in perovskite solar cells can be investigated.
The molecular structures of all organic materials used in this study are given in Figure 1.
The first hole-transporting material is tris(4-(5-hexylthiophen-2-yl)phenyl)amine, designated as
“HTM01”. HTM01 is the simplest of the three materials with a TPA core flanked by a thiophene
unit for the purpose of extending π-conjugation. Although it has been previously synthesized via
lithiation and transmetallation,28 we report an alternative and facile synthetic approach yielding
HTM01 via direct arylation (synthetic details are provided in the ESI).
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The second compound in this study
is 5,5',5''-(5,5',5''-(nitrilotris(benzene-4,1-
diyl))tris(furan-5,2-diyl))tris(2-octylisoindoline-1,3-dione), or “HTM02”. HTM02 has two
structural differences compared to HTM01: i) a furyl unit replaces the thienyl unit, and ii)
phthalimide groups are added to the ends of the side-arms as additional acceptor units. The furyl
units were selected for two reasons. First, it is considered to be a sustainable and “green”
alternative to thiophene as a building block in functional materials since it is derived from
biological feedstocks.29–31 Second, substitution of furan for thiophene as electron-rich donor
units in organic semiconductors can result in an increase in hole mobility, as demonstrated by
our recent work32 and others.33,34 The phthalimide groups are electron-withdrawing and result in
a stabilization of the frontier energy levels of HTM02 relative to HTM01, and corresponding
decrease in the bandgap energy. Conveniently, solubility can be tuned using the phthalimide unit
via simple modification of the alkyl chain attached to the nitrogen atom.
The third compound, HTM03, has a diketopyrrollopyrrole (DPP) unit in the center of the
side-arm units. DPP-based polymers have proven effective as HTMs in PSCs, 35 making DPP a
good choice for a donor building block. The insertion of DPP into the side-arm further stabilizes
the frontier molecular orbitals and reduces the bandgap energy (Eg) of HTM03 relative to both
HTM01 and HTM02. Large branched alkyl chains (i.e., 2-ethylhexyl) help to disrupt strong π-π
intermolecular interactions between DPP units,36 which improves solubility. 1-ethylpropyl or
“swallowtail” end caps were used in place of the hexyl chains present in HTM01 and HTM02.
Detailed synthetic procedures as well as IUPAC names for HTM01, HTM02 and HTM03 are
provided in the ESI.
The fourth HTM utilized is 7,7’-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-
b’]dithiophene-2,6-diyl)bis(6-fluoro-4-(5’-hexyl-[2,2’-bithiophen]-5-yl)benzo[c]-
5

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[1,2,5]thiadiazole), which is also known as p-DTS(FBTTh2)2, but is labelled as “HTM04” herein.
HTM04 is well-known as a high-performance donor material in bulk-heterojunction small-
molecule organic solar cells.37 HTM04 was selected as an HTM in this study since has similar
HOMO energy levels to HTM01, HTM02 and HTM03 and displays high hole-mobility, but has a
very different molecular geometry.38
Herein, we investigate the four HTMs in terms of optical and electronic properties and
film-formation using a combination of UV-Visible absorption spectroscopy (UV-Vis), cyclic
voltammetry (CV), and atomic force microscopy (AFM). The performance of the PSCs
fabricated with the new HTMs is subsequently tested.
Figure 1 depicts the molecular structures of the HTMs while Figure 2 depicts the
relevant energy levels of all components in our PSCs. HOMO energy levels of the HTMs were
estimated using the oxidation potentials determined by CV (see Figures S6-S10 in the ESI for
CV plots of the HTMs) The valence-band maximum (VBM) energy of the perovskite (MAPbI3)
was previously determined by others using ultraviolet photoelectron spectroscopy (UPS) and is
shown for comparison.4 The conduction band minimum of −5.5 eV (versus the energy of an
electron in vacuum) for an air-exposed film of MoO3 is representative of n-type MoO3 charge-
transport layers used in organic electronic devices.39 In perovskite solar cells, MoO3 is used as
an n-type material to assist charge transfer from the HTMs to Ag, a high work-function
electrode. Dense TiO2 acts as the electron-transport/hole-blocking layer with a work function of
4.0 eV.40 The work function of fluorine-doped tin-oxide (FTO), the electron-collecting
electrode, is 4.5 eV based on measurements conducted using ultraviolet photoelectron
spectroscopy (UPS).41
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Figure 1. Molecular structures of the five HTMs investigated in this study. TPA and thiophene
units are highlighted in blue and red, respectively.
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Figure 2. Energy level diagram for all components in the PSCs. The HOMO and LUMO levels
for HTM01-04 were determined by CV, with coloured bars representing the bandgap. Only the
HOMO level was measured for spiro-OMeTAD. The CBM and VBM for CH3NH3PbI3 were
determined by others.7
Results and Discussion
Optical Characterization. UV-Vis spectra for thin films of the five hole-transporting materials
are shown in Figure 3, with a summary of electronic parameters given in Table 1. The optical
bandgap energy (Eg) is defined by the onset wavelength of light absorption (λonset). Spiro-
OMeTAD and HTM01 have similar absorption profiles with maximum absorption in the near-
UV region (373 and 377 nm, respectively). HTM02 has an intermediate optical bandgap, with
maximum absorption at 445 nm for the thin film. HTM03 and HTM04 have the smallest Eg with
onset of absorption at 715 and 759 nm, respectively.
CV was used to determine the oxidation potential of each of the HTMs. Individual CV
traces for each HTM are included in the ESI and show that all of the HTMs undergo multiple
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reversible oxidations. These values (Eox) are reported in Table 1 with respect to the oxidation
potential of ferrocene (Fc), which has been measured previously to be 4.80 eV.42 It was
confirmed that the HOMO levels of all five HTMs were similar (~5 eV below the vacuum) and
at an appropriate level for hole-extraction at the perovskite interface.
Figure 3. UV-Vis absorption spectra acquired for thin-film samples of each HTM.
Table 1. Summary of optical and electrochemical properties of the HTMs used in this study.
EHOMO (eV)
Eox vs. Fc
(V)
HTM
(relative to Eg,film (eV) λonset,film (nm)
Evac
)
Spiro-OMeTAD
HTM01
0.20
0.14
0.26
−5.00
−4.94
−5.08
2.96
2.95
2.38
419
420
520
HTM02
HTM03
HTM04
0.24
0.23
−5.04
−5.03
1.73
1.63
715
759
PSC Device Fabrication and Characterization.
The photovoltaic properties of the HTMs were investigated by fabricating planar
heterojunction PSC devices (see ESI for a description of the fabrication procedure). Control
devices were fabricated with both doped (tBP and Li-TFSI) and undoped spiro-OMeTAD as the
9

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HTMs. Our best-performing devices made with doped spiro-OMeTAD delivered a power
conversion efficiency (PCE) of 13.4%, which is comparable to PSC devices made by others
using a similar method of perovskite deposition.43 It was confirmed that the use of dopants with
spiro-OMeTAD afforded devices with lower series resistance (by a factor of two) and higher
PCE and fill-factor (FF) (Figure 4a)). However, no p-type dopants were used with the four new
HTMs in this study, and therefore the PV performance is compared only to undoped spiro-
OMeTAD.
The J-V curves of the best-performing devices with each of the undoped HTMs are
shown in Figure 4b) with corresponding performance metrics listed in Table 2. The best device
with spiro-OMeTAD reached a PCE of 6.6%, an open-circuit voltage (Voc) of 0.88 V, a short-
circuit photocurrent density (Jsc) of 17.2 mA cm-2 and a FF of 0.43. HTM01 performed the best
among the TPA-based HTMs with a PCE of 3.5 % and Jsc of 8.8 mA cm-2. Devices made with
HTM02 and HTM03 display lower Jsc and correspondingly lower PCE. HTM04 devices offered
the highest Jsc among HTMs at 10.0 mA cm-2. However, the high series resistance and low FF
ultimately resulted in low PCEs (2.8%) for even the best-performing HTM04 devices.
Comparing the PV data between all of the HTLs, it is evident that all of the devices display a
similar Voc as a result of the similar HOMO levels of the HTMs. Others have shown that the
Voc of PSC devices is strongly dependent on the HOMO level of the HTM.44 Although it is
possible to enhance the Voc of devices by utilizing HTMs with low-lying HOMOs, such materials
offer poor electron-blocking capabilities leading to recombination and low current densities poor
device efficiencies.45
In the present study, the Jsc is the parameter that shows the most variability among
devices utilizing different HTMs. One can reason that factors influencing photocurrent,
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Page 11 of 22
including HTL morphology, hole-transport properties, and HTL-perovskite interfacing are
critical for realizing high-performance devices. It was also observed that all devices displayed
some degree of hysteresis with scan direction (see Figure S2 in the ESI), a phenomenon that is
described by others throughout the literature.46–48
Figure 4. a) J-V curves of the best-performing PSCs fabricated with doped and undoped spiro-
OMeTAD. b) J-V curves of the best-performing PSCs fabricated with four new HTMs and
undoped spiro-OMeTAD.
Table 2. Summary of performance metrics for champion cells with different HTLs. The plots
were obtained by scanning from +2.0 V to 0.0 V at a rate of 1 V per second under 100 mW/cm2
AM1.5 G illumination. The average metrics for five sample devices (each with a different HTL)
are also provided with standard deviations in the brackets. The shunt and series resistances listed
are for the best-performing devices only.
Shunt
Resistance
(ꢀ/cm2)
Series
Resistance
(ꢀ/cm2)
HTL
Voc (V) Jsc (mA cm-2) PCE (%)
FF
Doped spiro-
OMeTAD
Best
1.06
17.8
17.3(0.4)
17.2
13.4
0.71
1.0×106
1.9×10-2
Average 1.04(0.02)
12.2(0.7) 0.68(0.03)
-
-
Undoped spiro-
OMeTAD
Best
Average 0.85(0.04)
Best 0.97
Average 0.94(0.03)
Best 0.92
Average 0.88(0.08)
Best 0.85
Average 0.87(0.04)
0.88
6.6
0.43
1.6×106
3.7×10-2
16.4(1.3)
8.8
5.6(0.7)
3.5
0.40(0.03)
0.41
-
-
HTM01
HTM02
HTM03
2.5×105
3.6×10-2
8.0(0.6)
4.2
2.8(0.4)
2.0
0.38(0.02)
0.52
-
-
5.7×102
2.0×101
3.9(1.0)
3.4
1.6(0.4)
0.9
0.48(0.13)
0.31
-
-
8.5
-
3.4×105
3.3(0.4)
0.9(0.1)
0.30(0.01)
-
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HTM04
Best
0.89
10.0
2.8
0.31
1.7×105
-
2.3×10-2
-
Average 0.80(0.08)
7.5(3.0)
1.8(0.9)
0.28(0.03)
Hole-transporting layer (HTL) Surface Morphology.
The surface morphologies of the HTLs were probed to further investigate the source of the
differences in PV performance. Figure 5 shows atomic force microscope (AFM) images of the
HTL surfaces acquired after device fabrication and testing. An image of the bare perovskite
layer is shown for comparison. The bare perovskite film consists of micron-sized crystalline
grains with large height deviations that resemble “mountains” and “valleys.”
Due to the large height deviations in the thin-film samples, acquisition of AFM tapping-mode
images proved challenging and tip convolution artifacts are apparent in the roughest images.
Nevertheless, information about the morphology of the HTL surfaces and quantification of the
roughness parameters was possible. It is observed that spiro-OMeTAD forms the smoothest,
most planar film over the underlying perovskite layer with the lowest value of root-mean square
roughness (RRMS) (see Table 3). Among the three propeller HTMs, the roughness of the films
increases from HTM03 to HTM02 to HTM01. Considering the trend in PV performance of the
devices shown in Figure 4, Jsc also increases from HTM03 to HTM02 to HTM01. This result is
counterintuitive as one would expect that the smoothest film (i.e., one that is similar to the spiro-
OMeTAD HTL) would lead to higher Jsc values and overall better PCE. This suggests that many
other factors are at play. Indeed, the layers formed by HTM03 and HTM04 have surfaces of
comparable roughness (RRMS = 17 and 20, respectively), however, PSC devices with HTM04 can
generate threefold higher photocurrent than those with HTM03, indicating that surface roughness
cannot be the only factor contributing to photovoltaic performance. Other important structural
details, including the film morphology at the HTL-perovskite interface and the HTL thickness,
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cannot be determined by AFM, yet are likely critical factors. Interestingly, both HTM01 and
HTM04 bear similar alkyl-thiophene endcapping units as opposed to phthalimide-based
endcapping units, suggesting a possible material composition effect. Further experiments are
needed.
Figure 5. 2D AFM images of HTL surfaces of the five materials under study. An image of a
neat perovskite film (i.e., no HTL) is shown for comparison.
Table 3. Roughness parameters of HTL films on perovskite obtained from 2D AFM images. .
The parameters were calculated over an area of 25 µm2.
No HTL
Spiro-
OMeTAD
(Neat
perovskite)
26
HTM01 HTM02 HTM03 HTM04
Surface Area (µm²)
RRMS a (nm)
24
12
10
86
28
58
24
28
25
17
24
20
35
28
Ra b (nm)
40
21
13
14
Rmax c (nm)
246
445
228
132
170
a
b
RRMS, the root mean square (RMS) average of height deviations relative to the mean image data plane. Ra, the
c
average of the absolute values of the surface height deviations relative to the mean plane.
distance between the highest and lowest data points in the image.
Rmax, the vertical
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As shown in Figure 4b), HTM04 displays the highest Jsc among the new HTMs and is
known to have excellent hole mobility.38 HTM04 is also unique among the HTMs in this study;
it is the only HTM that displays crystallinity (see thin-film XRD patterns in Figure S3 in the
ESI). AFM images in Figure 5 reveal that HTM04 has a rougher, textured surface morphology
over the perovskite layer when compared to the Spiro-OMeTAD HTL. Therefore, it is of
interest to investigate methods for improving the planarity of the HTL. Since HTM04 is spin-
casted from a hot solution at its solubility limit, it is not possible to increase the concentration in
an attempt to generate a thicker, more-planar film. Alternatively, high-molecular weight non-
conjugated polymers can be used as inert solvent additives to increase the viscosity of the spin-
coating solution in an attempt to increase the thickness and coverage of the HTL.
Following this approach, two non-conjugated insulating polymers were selected as
additives: polystyrene (PS) and trimethyl-terminated polydimethylsiloxane (PDMS). The
concentration of HTM04 was kept at 25 mg/mL and the amount of polymer additive was
equivalent to 2% of the HTM by weight (i.e., 0.5 mg/mL). PS was selected because it has been
shown to improve the performance of small-molecule bulk-heterojunction organic solar cells
with HTM04 by inducing phase separation.49 PDMS is also known to improve the morphology
and PV performance of small-molecule bulk-heterojunction active layers in organic solar cells50
and has also been demonstrated as an inert additive in HTLs of PSCs for the purpose of surface
planarization.15
PSC devices were fabricated with HTM04 as the hole-transporting material combined
with either PS or PDMS as inert additives. The J-V curves for the best-performing devices
containing HTM04 with and without polymer additives are shown in Figure 6, with
corresponding performance metrics listed in Table 4. Addition of PS to the HTL film results in
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Page 15 of 22
a slight decrease in Jsc and Voc compared to the devices with a neat HTM04 layer. On the other
hand, devices with PDMS as an HTM-additive display both higher photocurrent (Jsc = 11.9
mA/cm2) and a higher FF than those without the additive, resulting in an increase in PCE (3.3 %
versus 2.8 % for the HTM04 neat film).
Figure 6. A comparison J-V curves of best-performing perovskite solar cell devices with
HTM04 as the HTM, with and without non-conjugated polymer additives. The plots were
obtained by scanning from +2.0 V to 0.0 V at a rate of 1 V per second under 100 mW/cm2 (1.0
sun) AM1.5 G illumination.
Table 4. A summary of J-V characteristics of perovskite solar cell devices with HTM04 as the
HTM, with and without non-conjugated polymer additives. The average metrics for five sample
devices (each with a different HTL) are also provided with standard deviations in the brackets.
The shunt and series resistances listed are for the best-performing devices only.
Shunt Series
Resistance Resistance
HTL
Voc (V) Jsc (mAcm-2) PCE (%)
FF
(ꢀ/cm2)
(ꢀ/cm2)
HTM04 neat
Best
Average 0.80(0.08)
Best 0.87
Average 0.77(0.06)
Best 0.85
0.89
10.0
7.5(3.0)
9.4
2.8
0.31
1.7×105
7.5×10-2
1.8(0.9) 0.28(0.03)
2.7 0.33
2.0(0.5) 0.29(0.03)
3.3 0.33
-
-
HTM04 + 2% PS
2.1×105
7.3×10-2
9.2(1.4)
11.9
-
-
HTM04 + 2% PDMS
2.2×105
7.6×10-2
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Average 0.89(0.03)
10.1(1.6)
3.1(0.5) 0.34(0.01)
-
-
AFM was used to further probe the effect of non-conjugated polymers of the morphology
of the HTLs (see Figure 7). AFM images of the HTL surfaces reveal that films with HTM04
and PDMS on a perovskite active layer are 25% less rough than HTM04 neat films on perovskite
(see Table 5 for RRMS values). The images suggest that the addition of PDMS reduces the
roughness of the HTL by filling in the deep “valleys” in the perovskite film. Further evidence
for changes in self-assembly and crystallinity within the HTL are provided in the ESI; Figure S4
shows that the XRD peak intensity for HTM04 decreases upon addition of either polymer.
Interestingly, addition of either polymer improved the shunt resistance of the devices, which
provides further evidence that the blends provide better coverage of the perovskite layer, thus
passivating shunt pathways.
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Figure 7. 2D AFM images of HTL surfaces of HTM04 with and without polymer additives on a
perovskite film. An image of a neat perovskite film is shown for comparison.
Table 5. Roughness parameters for HTL films on perovskite obtained from 2D AFM images.
The parameters were calculated over an area of 25 µm2.
HTM04 + 2% HTM04 + 2%
No HTL HTM04 Neat
PS
24
PDMS
25
Surface Area (µm²)
RRMS (nm)
26
35
23
20
17
14
Ra (nm)
28
14
12
11
Rmax (nm)
246
170
139
139
Conclusions and Future Work
We have investigated three new triphenylamine-based materials and the popular donor
DTS(FBTTh)2 as hole transport materials for perovskite solar cells. The highly modular
molecular framework of the triphenylamine-based materials allows for facile synthesis and
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straight forward structural modifications. All of the HTMs investigated had a HOMO level on
par with spiro-OMeTAD, but with different optical and electrochemical properties.
Unfortunately devices utilising HTM 01-03 demonstrated inferior performance compared to the
control device using spiro-OMeAD as the HTM.J-V measurements of perovskite solar cell
devices fabricated with each HTM shows that low photocurrent is the primary reason for the
poor performance relative to spiro-OMeTAD. The low photocurrents are attributed to the rough
HTL surfaces (as revealed by AFM), potentially incomplete coverage, and resulting shunt
pathways. Further optimization of the triphenylamine-based HTMs, HTM01, HTM02 and
HTM03, will include adjustments in molecular design to enhance solubility, thus allowing for
spin-casting at greater solution concentrations, generating thicker and more planar HTLs with
more complete coverage. The fourth HTM investigated, HTM04 (DTS(FBTTh)2), showed
slightly better performance than HTM 01-03, but still worse than spiro-OMeTAD. Through the
addition of non-conjugated polymer additives to the HTM04 casting solution, we have
demonstrated that the roughness of the HTL layer can be decreased resulting in an improvement
in photocurrent generation. . This is a simple yet effective strategy for increasing solution
viscosity for HTMs with low-solubility, resulting in smoother HTL films and better device
performance.
Acknowledgments
GCW acknowledges the Canada Research Chairs Program and the Natural Science and
Engineering Council of Canada (NSERC RGPIN 435715-2013) for financial support. IGH
acknowledges NSERC CREATE DREAMS and NSERC (RGPIN 298170-2014). AMN
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acknowledges NSERC and NSERC CREATE DREAMS for financial support. ADH
acknowledges NSERC for financial support.
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