Dopant-Free Hole-Transport Materials with Germanium Compounds Bearing Pseudohalide and Chalcogenide Moieties for Perovskite Solar Cells

Article abstract of DOI:10.1021/acs.inorgchem.0c02120

Hole-transport materials (HTMs) are key electronic components for the functioning of perovskite solar cells (PSCs) as they extract the photogenerated holes from the perovskite to be transported subsequently to the back electrode while minimizing the loss from electron recombination. Herein, we report the synthesis and characterization of novel germanium-based compounds with [{HC(CMeNAr)2}GeNCS] (2), [{HC(CMeNAr)2}Ge(S)NCS] (3), and [{HC(CMeNAr)2}Ge(Se)NCS] (4) compositions, with Ar = 2,6-iPr2C6H3 and the photovoltaic performance of 3 and 4 that is the same as for HTM in PSC. All compounds displayed excellent thermal properties and an appropriate alignment of energy levels for the perovskite with maximum optical absorption in the near-UV region. As revealed by space-charge limited-current (SCLC) measurements, compounds 3 and 4 have competing hole mobilities of about 1.37 × 10-4 and 4.88 × 10-4 cm2 V-1 s-1, respectively. Upon assessing PSC devices using 3 and 4 with triple-cation perovskite absorber Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3, the power conversion efficiencies (PCEs) were about 13.03 and 9.23%, respectively, both without doping and additives, and were compared with benchmark HTM spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene). Quantum chemical calculations with DFT showed that the optoelectronic properties are strongly influenced by the combined contributions of the germanium atom, the pseudohalide moiety (NCS-), and chalcogenides (S2- or Se2-). Fine tuning the electronic properties of germanium is thus a good strategy for the targeted synthesis of potential conducting molecules in PSCs.

Full text of DOI:10.1021/acs.inorgchem.0c02120

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Article
Dopant-Free Hole-Transport Materials with Germanium Compounds
Bearing Pseudohalide and Chalcogenide Moieties for Perovskite
Solar Cells
́
Tatiana Soto-Montero, Natalie Flores-Díaz, Desire Molina, Andrea Soto-Navarro,
Andres Lizano-Villalobos, Cristopher Camacho,* Anders Hagfeldt,* and Leslie W. Pineda*
́
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ABSTRACT: Hole-transport materials (HTMs) are key elec-
tronic components for the functioning of perovskite solar cells
(PSCs) as they extract the photogenerated holes from the
perovskite to be transported subsequently to the back electrode
while minimizing the loss from electron recombination. Herein, we
report the synthesis and characterization of novel germanium-
based compounds with [{HC(CMeNAr)2}GeNCS] (2), [{HC-
(CMeNAr)2}Ge(S)NCS] (3), and [{HC(CMeNAr)2}Ge(Se)-
NCS] (4) compositions, with Ar = 2,6-iPr2C6H3 and the
photovoltaic performance of 3 and 4 that is the same as for
HTM in PSC. All compounds displayed excellent thermal
properties and an appropriate alignment of energy levels for the
perovskite with maximum optical absorption in the near-UV
region. As revealed by space-charge limited-current (SCLC) measurements, compounds 3 and 4 have competing hole mobilities of
about 1.37 × 10−4 and 4.88 × 10−4 cm2 V−1 s−1, respectively. Upon assessing PSC devices using 3 and 4 with triple-cation perovskite
absorber Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3, the power conversion efficiencies (PCEs) were about 13.03 and 9.23%, respectively,
both without doping and additives, and were compared with benchmark HTM spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-
methoxyphenylamine)-9,9′-spirobifluorene). Quantum chemical calculations with DFT showed that the optoelectronic properties
are strongly influenced by the combined contributions of the germanium atom, the pseudohalide moiety (NCS−), and chalcogenides
(S2− or Se2−). Fine tuning the electronic properties of germanium is thus a good strategy for the targeted synthesis of potential
conducting molecules in PSCs.
the photogenerated charges (electrons and holes) to the
electrodes.15 In the case of the photogenerated holes, their
extraction and transport toward the back electrode are
accomplished with components known as hole-transport
materials (HTMs) that aim to minimize the electron
recombination by preventing direct contact between the
perovskite and the back-contact electrode.16
At present, the extensive assortments of doped and dopant-
free HTMs for perovskite solar devices rely mainly on organic
and polymeric backbones, originating from diverse molecular
designs. Examples of these scaffolds are 2,2′,7,7′-tetrakis(N,N-
di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMe-
INTRODUCTION
■
Perovskite solar cells (PSCs) are attracting a large amount of
attention due to their astonishing improvement in emerging
photovoltaic technologies. Since their first report in 2009,1
PSC have attained exceptional improvements in performance,
achieving certified power conversion efficiencies (PCEs) of up
to 25.2% in 2020,2 overtaking multicrystalline silicon solar cells
(23.3%). This rapid evolution of device efficiencies was due
mainly to the development of novel deposition techniques and
perovskite compositions.3 Organometal halide perovskites have
attractive optoelectronic features: large absorption coeffi-
cients,1 small exciton binding energies (<10 meV),4 large
charge-carrier diffusion lengths,5 and tunable band gaps.6 For
this reason, much research effort has been devoted to the
design and optimization of perovskite materials, the deposition
techniques for high-quality perovskite films (e.g., low-temper-
ature solution-processable approaches), and device architec-
tures.7−14
Received: July 17, 2020
As part of the principle of operation of PSC devices, two
selective charge-extraction layers are essential to transporting
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Scheme 1. Synthesis Scheme for Compounds 2−4
TAD) and polytriarylamines (PTAAs), both of which have
found widespread use as hole-transport layers in PSC despite
various disadvantages associated with multistep syntheses,
coupled with expensive and scanty metal-catalyzed cross-
coupling reactions and extensive product purification and
reproducibility.17 Moreover, because spiro-OMeTAD has poor
hole mobility in its pristine condition, dopant molecules are
added to enhance the hole mobility and the device
performance.18 Doping molecules such as lithium bis(tri-
fluoromethanesulfonyl)imide (LiTFSI) and 4-tert-butylpyri-
dine (tBP) are hygroscopic and cause damage to the perovskite
layer over time.19−21 In contrast, few examples of inorganic
compounds (e.g., CuI, CuSCN, and NiO) acting as HTM have
been reported, in part because of the poor solubility that
hampers their deposition using spin-coating methods, even
though some compounds exhibit excellent electrical and hole
conductivities.22 Some interesting findings have shown the
capability of various compounds to work without dopant as
HTM, delivering highly efficient PSC.17 For instance, using
optimized undoped spiro-OMeTAD, processed from a non-
halogenated solvent (tetrahydrofuran), resulted in 17% PCE
with a device containing planar n−i−p architecture.23
Appending redox-active triphenyl amine (TPA) fragments to
a spiro-based HTM followed by annealing afforded, in an
inverted (p−i−n) device without dopant, 20.6% PCE and an
intrinsic hole mobility of 1 × 10−3 cm2 V−1 s−1.24 Also,
organometal-based phthalocyanine and porphyrin molecules,
which are structurally related macrocycles, have proven
effective as HTM with no dopant. The use of zinc(II)
phthalocyanine conjugated dimers with 2,5-thienyl-,2,7-
fluorenyl-,3,6-bisthienyldiketopyrrolopyrrole and 1,4-phenyl
bridges yielded the best efficiencies, about 15.5−16.8%.25
The use of Zn(II) porphyrin-based molecules with either
push−push (a 5,15-diethynylporphirin core and two lateral
N,N-dialkylaniline groups) or push−pull (N,N-dialkylaniline
and methyl 4-benzoate) traits delivered a PCE of up to
13.10%.26 Regardless of the chemical categorization, all
previous substances function as a hole-conductive layer if
they align the energy levels regarding the perovskite, yielding a
driving force for the hole injection necessary for hole mobility
and conductivity in such devices. Additionally, chemical and
physical requirements such as solubility in common organic
solvents, synthetically attainable protocols, and thermal
stability are pursued.27
electrode. We employed β-diketiminate ligands because they
resemble the molecular scaffold of porphyrins, which are
regarded as light-harvesting units and charge-transport
materials in photovoltaics.29−32 Furthermore, such bidentate
ligands are versatile, achieving diverse compounds exhibiting
low oxidation states and low coordination geometries because
of their steric hindrance and electronic-density donation
features, and likewise act as spectator ligands.33−38
In this work, we report the synthesis and characterization of
germanium compounds with chemical compositions of [{HC-
(CMeNAr)2}GeNCS] (2), [{HC(CMeNAr)2}Ge(S)NCS]
(3), and [{HC(CMeNAr)2}Ge(Se)NCS] (4), where Ar =
2,6-iPr2C6H3, and the evaluation of complete PSC devices
based on 3 and 4 as HTM. Compounds 2−4 were prepared
according to metathesis and oxidative paths, resulting in
straightforward synthesis procedures and simple product
purification, as indicated by our assessment of the E factor.39,40
To our knowledge, 3 and 4 are the first Ge-based compounds
evaluated as organometallic HTM in PSC, specifically having a
triple-cation perovskite composition of Cs0 . 0 5
(MA0.17FA0.83 0.95Pb(I0.83Br0.17)3.
-
)
RESULTS AND DISCUSSION
Synthesis. The metathesis reaction of precursor [{HC-
■
(CMeNAr)2}GeCl]41 (1) in the presence of KSCN in toluene
at −78 °C affords compound [{HC(CMeNAr)2}GeNCS] (2)
as a yellow solid in 83% yield. The oxidative addition reaction
of 2 in toluene with elemental sulfur or red selenium leads to
[{HC(CMeNAr)2}Ge(S)NCS] (3) and [{HC(CMeNAr)2}-
Ge(Se)NCS] (4) as pale-yellow (65% yield) and lemon-green
(77%) solids, respectively (Scheme 1). Compounds 2−4
solubilize well in solvents commonly used for HTM in the
assembly of PSC such as toluene and chlorobenzene.
The 1H NMR spectrum of 2 exhibits a singlet resonance for
the γ-CH proton of the ligand at δ = 5.02 ppm, which, by
comparison with that of 1 (δ = 5.14 ppm), shows an upfield
shift,41 highlighting the electronic feature of an inductive effect
of a pseudohalide moiety smaller than for a halide group; those
of 3 and 4 resonate at δ = 4.80 and 4.82 ppm, respectively. The
latter proton resonances are clearly both shifted to high field,
relative to 1 and 2, which is consistent with an increased
electronic density associated with the donor properties from
both tethered substituents (pseudohalide and chalcogenide)
and a change in the oxidation state of the germanium atom
(Quantum Chemical Calculations). The 77Se NMR spectrum
of 4 revealed a resonance signal at δ = −404.3 ppm, which is in
agreement with compounds exhibiting multiple-bond and
ylide-type character at the germanium−selenium bond as in
[{HC(CMeNAr)2}Ge(Se)OH] (δ = −439 ppm; Ar = 2,6-
iPr2C6H3),36 [{HC(CMeNAr)2}Ge(Se)F] (δ = −465 ppm),42
[{HC(CMeNAr)2}Ge(Se)X] (δ = −349 and −297 ppm; X =
Me, nBu),42 and [{HC(CMeNAr)2}Ge(Se)Cl] (δ = −287
Our interest in Ge-based molecules stems from the
intriguing electronic structure regarding the dual germanium
valency (Ge2+, Ge4+), which is associated with charge transport
in a semiconducting material.28 In this context, the
incorporation of strong and moderately strong donating
ligands, such as isothiocyanate (pseudohalide) and chalcoge-
nides, could result in donor−acceptor Ge-based systems with
large hole mobility to transport the holes to the counter
B
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ppm).42 The IR spectrum of 2 shows a line at 2041 cm−1
ascribed to the stretching vibrational mode of the NCS−
moiety, which is slightly shifted when elemental S8, as in 3
(2052 cm−1), or Se8, as in 4 (2025 cm−1), is incorporated (SI
Table S1). Regarding the UV−visible absorption spectra of 1−
4 in chlorobenzene solution, all show absorption bands in the
near-UV region, below 370 nm (SI Table S1 and Figure S1),
implying no competition for light absorption concerning the
perovskite absorber in the visible region.
As for 3, all attempts to isolate a monocrystalline material
suitable for X-ray structure analysis have so far proven
unsuccessful.
Thermal, Optoelectronic, and Electrochemical Prop-
erties. Thermogravimetric analysis (TGA) of compounds 1−4
revealed their excellent thermal stabilities up to 220 °C (Figure
2) and decomposition temperatures (Tdec) in the range of
As β-diketiminate ligands can be prepared without
complicated protocols and with readily available chemical
reagents, cost-analysis estimates40,43 for compounds 1−4 based
on laboratory-scale synthesis were undertaken (SI Schemes
S1−S5 and Tables S2−S6). Importantly, the E factor, defined
as kilograms of waste per kilograms of product,39 ranges from
72 to 170 for compounds 1−4 (detailed calculations in the SI),
in sharp contrast to 3600 for spiro-OMeTAD;40 our processes
are hence more efficient in terms of waste materials.
Crystal Structure Analysis. Pale-green single crystals of 4
suitable for X-ray structural analysis were grown from a
saturated solution in toluene and pentane kept at ambient
temperature for 3 days. Compound 4 crystallizes in monoclinic
space group P21/c with one molecule in the asymmetric unit.
Crystal data, data collection, and details of the structure
refinement are summarized in the SI (Tables S7−S10). The
molecular structure of 4 consists of a central germanium atom
tetrahedrally coordinated with two nitrogen atoms from the
monoanionic β-diketiminate ligand, a nitrogen atom from the
isothiocyanate group, and a selenium atom (Figure 1).
Figure 2. Thermogravimetric curves of compounds 1−4.
Table 1. Thermal Properties of Compounds 1−4 from TGA
and DSC
a
b
c
compound
Tdec/°C
Tm/°C
Tg/°C
1
2
3
4
293
271
220
240
180
221
212
172
215
164
158
230
a
b
Decomposition temperature from TGA. Melting temperature.
c
Glass-transition temperature from DSC.
220−293 °C (Table 1), demonstrating the robustness of these
compounds attributed to the chelate effect involving the
bidentate β-diketiminate ligand.45,33,37,38 All of our compounds
are thus far above the risk of degradation under typical
photovoltaic operating temperatures of between 70 and 90
°C.46
The presence of temperatures at which phase transitions of
compounds 1−4 occur was tested with a differential scanning
calorimeter (DSC) (SI Figure S2); detailed thermal features
are collated in Table 1. Notably, all thermal transitions in 1−4
exceed common operating temperatures of photovoltaic
devices46 and have a range of glass-transition temperatures
(Tg) surpassing that of spiro-OMeTAD (125 °C).47 Having a
high Tg affords stable amorphous phases; a decreased tendency
to crystallize could prevent device failure over time.48 Such
unwanted trouble is a major concern for spiro-OMeTAD as it
tends to crystallize under operating conditions in PSC devices,
particularly at elevated temperatures.49
We experimentally estimated the optical band gap (Eg) of
compounds 1−4 from the solid-state UV−visible diffuse
reflectance spectra (Figure 3a). On the basis of a modified
Tauc plot method50 (SI Figure S3 and Table S11), the
corresponding Eg values denoting directly allowed transitions
were calculated to be 2.94, 2.77, 2.84, and 2.86 eV for 1−4,
respectively.
Figure 1. Molecular structure of compound 4. All hydrogen atoms are
omitted for clarity, and thermal ellipsoids are drawn at the 50%
probability level.
The length, 2.174(4) Å, of the Ge−Se bond is comparable
to those in [{HC(CMeNAr)2}Ge(Se)OH] (2.206(1) Å),36
[{HC(CMeNAr)2}Ge(Se)Cl] (2.197(6) Å), and [{HC-
(CMeNAr)2}Ge(Se)(nBu)] (2.219(6) Å),42 indicating again
a resonance-structure contribution from a Ge−Se ylide-type
bond and multiple bond character rather than a singly bonded
germanium−selenium, of which the bond distances extend
from 2.24 to 2.77 Å. With respect to the length (1.901(3) Å)
of the Ge−NCS bond in 4, it is shorter than that in
[(iBu)2ATl]GeNCS (2.050(2) Å; ATl = aminotroponimi-
nate), in which the germanium atom is in oxidation state +2.44
The highest-occupied molecular orbital (HOMO) of
compounds 1−4 was assessed with an experimental determi-
nation in solution from cyclic voltammetry (CV) (Figure 3b).
C
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Figure 3. (a) UV−visible absorption spectra of compounds 1−4 in the solid state. (b) Cyclic voltammogram of compounds 1−4. Electrolyte:
80:20 LiTFSI (1.0 M) in acetonitrile/chlorobenzene. Scan rate: 100 mV/s.
The resulting HOMO energies are −5.13, −5.18, −5.16, and
−5.23 eV for 1−4, respectively. For spiro-OMeTAD under the
same experimental conditions, EHOMO is −5.03 eV.
The energy levels of compounds 1−4 (Figure 4) were
estimated from experimental HOMO from CV and the lowest
Table 2. HOMO, Optical Band Gap (Eg), and LUMO of
Compounds 1−4 from the Experimental Methods
HOMO/V
a
b
c
d
compound
vs SHE
HOMO/eV
Eg/eV
LUMO/eV
1
2
3
4
0.732
0.784
0.762
0.832
0.626
−5.13
−5.18
−5.16
−5.23
−5.03
2.94
2.77
2.84
2.86
−3.03
−2.19
−2.41
−2.32
−2.37
−2.00
e
spiro-OMeTAD
a
Values from the onset of the first oxidation potential from CV
51
b
+
measurements. SHE scale: HOMOvs Fc/Fc + 0.624. Vacuum scale:
c
−(4.4 + HOMOvs SHE). Values from a Tauc plot for directly allowed
transitions ((F(R)hν)2)/eV) according to the UV−visible spectra of
d
e
compounds in the solid state. (bHOMO + Eg). Spiro-OMeTAD:
HOMO value from CV experimental measurements; Eg from the
literature.22
c
respectively. (See the Cartesian coordinates of ground-state
optimized geometry in the SI Tables S13−S16). Frontier
molecular orbitals and calculated absorption spectra of
compounds 1 and 2 (SI Figure S4) and compounds 3 and 4
are shown in Figure 5. Compound 1 shows the main
absorption at 344 nm, corresponding to excitation from p
orbitals of the chlorine atom to π antibonding orbitals in the
central ring. Similarly, compound 2 presents three main
excitations at 352, 351, and 342 nm, corresponding to
excitations from π antibonding orbitals in the NCS group to
π antibonding orbitals in the central ring.
For compound 3, two main excitations at 324 and 323 nm
correspond to electronic transitions from p orbitals of the S
atom connected to the Ge atom toward the π antibonding
orbitals in the main ring. Following the same pattern as 3,
compound 4 has a main absorption at 325 nm ascribed to
excitations from π antibonding orbitals from the NCS group
and p orbitals from Se to the central ring.
Figure 4. Energy diagram of compounds 1−4 and spiro-OMeTAD
according to the experimental data. Valence-band (VB) and
conduction-band (CB) data for triple-cation perovskite (perovskite
TC) from the literature.52
unoccupied molecular orbital (LUMO) experimental values
(Table 2 and SI Table S12). All HOMO energies evaluated for
compounds 1−4 are well positioned compared to the triple-
cation perovskite valence band (−5.65 eV). Hole injection
from the perovskite to the hole carrier is expected to be
efficient because of satisfactory energy alignment. Calculated
LUMO levels are −2.19 eV for 1, −2.41 eV for 2, −2.32 eV for
3, and −2.37 eV for 4, which all lie above the perovskite
conduction band, −3.92 eV, which indicates their suitable
electron-blocking features.
Quantum Chemical Calculations. To acquire insight
into the structural and electronic properties of compounds 1−
4, we undertook calculations with density functional theory
(DFT) at the BP86/def2-TZVP and TD-BP86/def2-TZVP
levels of theory for ground and electronically excited states,
Natural bond order (NBO) analysis of compounds 1−4
shows the germanium atom having charges of 0.99, 1.13, 1.63,
and 1.52, respectively. The charge deficiency on the
germanium atom for compounds 3 and 4 originates from the
electronegativity of its neighboring atoms, as a sulfur atom
possesses a greater capability to extract electron density from
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Figure 5. Simulated absorption spectra and frontier molecular orbitals of compounds 3 and 4 computed at the TD-BP86/def2-TZVP and BP86/
def2-TZVP levels.
the germanium atom than from the selenium atom. As
observed from the frontier molecular orbitals of 3 and 4 and
the corresponding electronic transitions generating the features
appearing in the window of interest in terms of wavelength in
the simulated UV−visible spectra (Figure 5), the germanium
atom acts as a bridge to move charge from the isothiocyanate
group to the central ring. The charge deficiency of the bridge-
like Ge atom is enhanced by the attachment of the
chalcogenides by oxidative addition, altering the oxidation
state from Ge(II) to Ge(IV) as in 3 and 4.
Periodic calculations of compounds 1−4 show two distinct
spatial arrangements among them. Compound 1 shows
stacking on the central ring, containing the germanium atom,
creating a layered structure in which the chlorine atoms align
antiparallel, which allows for stabilization by minimizing their
repulsion. In addition to those mentioned above, the
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Figure 6. Band structure, DOS, and unit-cell frontier molecular orbitals of compounds 3 and 4.
Figure 7. Current density−voltage curves from space−charge-limited current measurements of compounds 3 and 4 on hole-only devices. Inset:
device architecture.
substituents on the benzene rings move apart from the Cl
atoms, producing an out-of-plane arrangement of the Ge atoms
with respect to the central ring. In contrast, 2−4 display a
distorted bicolumnar arrangement of the central rings with the
NCS substituent aligning in an antiparallel configuration with
rotation 90° with respect to the stacking axis. Optimized cell
parameters are given in SI Table S17, according to which the
inclusion of S and Se atoms evidently increases the packing of
their unit cells, allowing for enhanced intramolecular
interactions.
Theoretically computed band gaps provided in SI Table S18
reveal a trend in which the gap decreases with the increment in
the number of electrons per unit cell. The density of states
(DOS) plots and their corresponding atomic contributions (SI
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Figure S5 and Figure 6) show that all bands near the Fermi
level have a majority p character for the four systems studied.
From the frontier molecular orbital analysis of the unit cells of
1−4, p orbitals evidently constitute valence bands from the
substituents bonded to the Ge atom. Moreover, as a second
substituent is attached to the Ge atom, the orbitals tend to be
localized on the second substituent, namely, S and Se.
Plots of the density of states and molecular orbitals make
clear that conduction bands are always localized in the central
ring for 1−4, with π antibonding character. An analysis of the
electronic charge shows that increasing the electronegativity of
the atom attached to the Ge atom decreases its electron
density, which facilitates the charge mobility from the valence
band to the π system in the central ring. The latter computed
results imply that compounds 3 and 4 bestow effective
electronic properties for hole extraction due to the
contribution of the central atom, with pseudohalide and
chalcogenide donating ligands in comparison with compounds
1 and 2. In practice, the latter two compounds give a short-
circuit response upon measuring the hole mobility exper-
imentally (no data were collected). Likewise, the PCE in
complete perovskite devices was less than 1%.
Charge-Transport Properties: Hole Mobility. The
electrical properties of compounds 3 and 4 were measured
with space−charge-limited current (SCLC) in a hole-only
device with configuration FTO/HTM/Au.53 Figure 7 depicts
typical J−V curves of SCLC in which ohmic and space−charge
regions are divided by the specific voltage (VT). The inset
shows the sandwich-type architecture containing only hole-
transport compounds; hole mobility values obtained are shown
in Table 3.
these tests, solutions of compounds 3 (45 mM) and 4 (15
mM) in chlorobenzene were used for spin-coating depositions.
The film thickness of compounds 3 and 4 was accessed with
images from a high-resolution cross-sectional scanning electron
microscope (SEM) of the complete devices, glass/FTO/-
TiO2/perovskite/Ge-compound/Au (Figure 8). These cross-
sectional images reveal a uniform film thickness and excellent
coverage of the deposited layers of 3 and 4, similar to those of
spiro-OMeTAD.55 The films of compounds 3 and 4 yield thin
layers of approximately 200 nm sandwiched between the
perovskite and the gold contact layer, with the perovskite films
showing crystals of appropriate size and homogeneity.
As J−V curves in Figure 9 and Table 4 show, compound 3
shows the best performance with a PCE of 13.03%, a
photocurrent density of 22.1 mA cm−2, a photovoltage of
0.94 V, and a fill factor of about 0.64. The best device for
compound 4 had a PCE of 9.23%, a photocurrent density of
18.2 mA cm−2, a photovoltage of 0.91 V, and a fill factor of
0.56. All J−V curves presented in Figure 9 correspond to a
reverse scan. Preliminary measurements of forward and reverse
scans for compound 3 (SI Figure S7 and Table S19) show that
the best results were obtained with measurements in reverse
scan mode; further study is required to understand the origins
of this effect and to improve the hysteresis.
Photovoltaic parameters of multiple devices are presented as
box plots in SI Figure S8.
Compound 3 exceeds the efficiency of undoped spiro-
OMeTAD, whereas 4 achieves comparable efficiency (Table
4). The fill factors obtained with compounds 3 and 4 are
superior to that of pristine spiro-OMeTAD, which improved
only upon doping. Our compounds instead show that upon
doping the performance diminished, proving that they work
better as dopant-free HTM. Such findings are ascribed to some
extent to the hole mobility attained for compounds 3 and 4,
which is ∼10 times that reported for pristine spiro-
OMeTAD.19
Table 3. Hole Mobility of Compounds 3 and 4
HTM
hole mobility/cm2 V−1 s−1
3
4
1.37 × 10−4
4.88 × 10−4
2.69 × 10−5
3.78 × 10−3
Nevertheless, complete devices for both novel compounds
did not surpass the pristine spiro-OMeTAD photocurrent
density. For compound 3, the overall large efficiency of the
incident photon to current (IPCE) is between 80 and 60% for
the entire visible wavelength range; that of compound 4 is
about 80% over the entire range (SI Figure S9). In these IPCE
plots, the difference between 3 and 4 is seen mainly at
wavelength greater than 550 nm; such behavior can be
tentatively associated with interfacial processes between the
perovskite and germanium-HTM, which in turn affect the
efficiency of charge collection, mainly when charge recombi-
nation occurs during charge-carrier transport. The suppression
of the recombination by appropriate interfacial engineering is
therefore important.56 In this context, we also do not rule out
that the presence of sulfur in 3 might have a beneficial effect on
PSC performance; sulfur-doped all-inorganic perovskite
materials have demonstrated superior stability with efficient
devices relative to control devices without the aid of doping.57
Halide perovskite devices with sulfur treatments of interfacial
layers resulted in the passivation of the charge traps, the
mitigation of interfacial recombination, and changes in the
charge-transport kinetics at the interface.58 These results
indicate that there is scope for improving the complete devices
of compounds 3 and 4 with further optimization of their
interfacial engineering and the processes involved (e.g.,
experiments on photoexcited charge-carrier recombination
dynamics).
a
spiro-OMeTAD (pristine)
spiro-OMeTAD + LiTFSI + tBP
a
a
Data from the literature.19
Notably, the hole mobility was 1.37 × 10−4 cm2 V−1 s−1 for 3
and 4.88 × 10−4 cm2 V−1 s−1 for 4, both about 10 times that of
pristine spiro-OMeTAD (2.69 × 10−5 cm2 V−1 s−1) and
various hole-transport materials with high efficiencies in PSC.22
Our preliminary results for compounds 3 and 4 are noteworthy
for having charge transport without dopant addition as both
surpass pristine spiro-OMeTAD, whereas doping with LiTFSI
or tBP yielded no further improvement.
Again, for compounds 1 and 2 ohmic contact issues and
short circuits were observed in hole-only devices; further
experimental analysis would be necessary to assess such
behavior.54
Photovoltaic Characterization. The performance of 3
and 4 as HTM was evaluated with a triple-cation perovskite
employing a regular mesoporous architecture, FTO/cp-
TiO2/mp-TiO2/perovskite/Ge-HTM/Au, measured under
solar simulator AM 1.5 G conditions with 100 mW cm−2
light intensity and with a 0.16 cm2 mask of active area. To
achieve excellent film-forming properties and efficient photo-
voltaic performance with the germanium compounds, opti-
mizations were undertaken in terms of solvents and
concentrations (further details in SI Figure S6). According to
G
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Figure 8. Cross-sectional scanning electron microscope (SEM) images of regular architecture triple-cation perovskite solar cells with (a) compound
3, (b) compound 4, and (c) spiro-OMeTAD as HTM.
Table 4. Photovoltaic Parameters of the Best-Performing
Complete Devices Employing Compounds 3 and 4 and
spiro-OMeTAD, Pristine as Well as Doped with LiTFSI and
tBP
JSC
compound
(mA cm−2
)
VOC (V)
FF
PCE (%)
3
4
22.1
18.2
20.4
12.7
22.8
23.6
0.94
0.91
0.78
0.81
1.06
1.10
0.64
0.56
0.40
0.40
0.41
0.74
13.03
9.23
6.36
4.04
9.81
19.29
3 + LiTFSI + tBP
4 + LiTFSI + tBP
spiro-OMeTAD
spiro-OMeTAD + LiTFSI +
tBP
Figure 9. Current−voltage (J−V) curves of the best-performing
devices of compounds 3 and 4 and spiro-OMeTAD as HTM, pristine,
and doped with LiTFSI and tBP. Devices employing triple-cation
demonstrated by computational, optical, and electrochemical
means, rendering excellent band alignments for application as
HTM. The latter electronic synergy enables enhanced hole
mobility of compounds 3 and 4, about 10 times that of the
undoped spiro-based counterpart. Moreover, these germa-
nium-based compounds are readily synthesized with the
advantage of being solution-processable to form hole-transport
layers in perovskite solar cells.
perovskite [Cs0.05(MA0.17FA0.83
scans.
)0.95Pb(I0.83Br0.17)3], with reverse
CONCLUSIONS
■
We report the first synthesis of organogermanium compounds
bearing strong or moderately strong σ-donating ligands such as
pseudohalides (isothiocyanate) and chalcogenides (S and Se).
Our innovative approach takes advantage of the dual
germanium valency to tune the donor−acceptor properties as
In assessing compounds 3 and 4 as HTM in perovskite solar
devices, we found that, without dopants or additive substances,
satisfactory efficiency was attained for compounds 3 (13.03%)
H
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constant stirring at 45 °C for 3 days, the light-yellow solution turned
pale yellow. After the removal of all volatiles, the remaining crude
product was rinsed with hexane (3 × 10 mL) and dried under reduced
pressured to yield pure 3. Storage of a solution of 3 (0.5 g), dissolved
in a minimum amount of toluene and pentane (2 mL) in a freezer
(−20 °C) for 7 days, yielded colorless crystals. Yield: 3.44 g (65%).
Mp: 193−195 °C (dec). Elemental anal. calcd for C30H41N3GeS2
(580.44 g mol−1): C, 62.10; H, 7.12; N, 7.24. Found: C, 62.05; H,
and 4 (9.23%) relative to benchmark compound pristine spiro-
OMeTAD.
Designing molecules with novel electronic interactions to
function as HTM could benefit from the variety of elements in
the periodic table, as highlighted by the bridge-like germanium
behavior, transporting charge from donating moieties to an
electron-deficient central core. This strategy provides oppor-
tunities to bond germanium to arylamines, diphenylamines,
triarylamines, bis(trifluoromethanesulfonyl)imide and porphyr-
ins, which are all potential HTMs in perovskite devices.
7.15; N, 7.23. UV−vis (λmax, chlorobenzene) nm {ε/dm3 mol−1
−1
cm }: 293 {13 391} and 367 {16 882}. IR (ATR): ν
̃
2052 cm−1
(NCS−). H NMR (400 MHz, C6D6, Me4Si): δ 7.03−7.12 (m, 6H,
m-, p-ArH), 4.80 (s, 1H, γ-CH), 3.40 (sept, 2H, JHH = 6.8 Hz,
1
3
3
CH(Me)2), 3.11 (sept, 2H, JHH = 6.8 Hz, CH(Me)2), 1.60 (d, 6H,
EXPERIMENTAL SECTION
3
3JHH = 6.8 Hz, CH(Me)2), 1.40 (d, 6H, JHH = 6.8 Hz, CH(Me)2),
■
1.36 (s, 6H, Me), 1.25 (d, 6H, 3JHH = 6.8 Hz, CH(Me)2), 0.95 (d, 6H,
3JHH = 6.8 Hz, CH(Me)2). 13C NMR (100 MHz, C6D6, Me4Si): δ
170.8 (CN), 146.0, 143.9, 135.8, 129.1, 124.9, 124.3 (i-, o-, m-, p-Ar),
100.1 (γ-CH), 29.2 (CH(Me)2), 28.8 (CH(Me)2), 25.9 (Me), 24.1
(CH(Me)2), 24.0 (CH(Me)2), 23.6 (CH(Me)2), 23.4 (CH(Me)2).
Preparation of Compound [{HC(CMeNAr)2}Ge(Se)NCS] (4).
To a suspension of elemental red selenium (0.29 g, 3.65 mmol) in
toluene (20 mL) was added via cannula a solution of 2 (2.0 g, 3.65
mmol) in toluene (40 mL). A lemon-green solution appeared after 30
min, which remained unchanged after stirring for 2 days. Subsequent
filtration and removal of solvent gave a dark-yellow solid. 4 (0.5 g)
was dissolved in a minimum amount of toluene and pentane (2 mL)
and kept at ambient temperature for 3 days to afford lemon-green
crystals. Yield: 1.77 g (77%). Mp: 180−182 °C (dec). Elemental anal.
calcd for C30H41N3GeSeS (627.33 g mol−1): C, 57.44; H, 6.59; N,
All experimental manipulations were undertaken with standard
Schlenk line and glovebox techniques under a dry nitrogen
atmosphere. 2,6-Diisopropylaniline (Sigma-Aldrich), 2,4-pentane-
dione (Sigma-Aldrich), GeCl2·diox (Sigma-Aldrich), nBuLi (solution
2.5 M in hexanes, Sigma-Aldrich), KSCN (Sigma-Aldrich), PbBr2
(99.99%, TCI), PbI2 (99.99%, TCI), formamidinium iodide (FAI,
Dyesol), methylammonium bromide (MABr, Dyesol), CsI (99.998,
ABCR), RbI (99.9%, Sigma-Aldrich), LiTFSI (99.95%, Sigma-
Aldrich), spiro-OMeTAD (Dyesol), tBP (Sigma-Aldrich), chloroben-
zene (99.8% extra dry, Acros), DMF (99.8% extra dry, Acros), DMSO
(99.8% extra dry, Acros), and acetonitrile (99.8% extra dry, Acros)
were used as received. Red amorphous selenium (Se8) was prepared
according to the literature procedure.59 Sulfur (S8) was recrystallized
from toluene before use. Solvents were purified according to
conventional procedures and were freshly distilled over appropriate
drying agents under a dry nitrogen atmosphere before use; toluene,
pentane, and deuterated benzene (C6D6) were dried with a sodium
benzophenone mixture; tetrahydrofuran and diethyl ether were dried
with MBraun Solvent Purification Systems (MS-SPS). The samples
for spectral measurements were prepared in a glovebox (Lab MBraun
workstation). An NMR spectrometer (Bruker 400 MHz) was used to
record 1H and 13C chemical shifts, both reported with reference to the
resonances of the solvent used. The 77Se NMR spectrum was acquired
at 14.1T (600 MHz for 1H) at 298 K on a Bruker AVIII spectrometer
equipped with a 5 mm N2-cooled cryoprobe summing 256 transients.
6.70. Found: C, 57.42; H, 6.56; N, 6.68. UV−vis (λmax
,
chlorobenzene) nm {ε/dm3 mol−1 cm−1}: 351 {15 918}. IR
1
(ATR): ν
̃
2025 cm−1 (NCS−). H NMR (400 MHz, C6D6, Me4Si):
δ 7.03−7.13 (m, 6H, m-, p-ArH), 4.82 (s, 1H, γ-CH), 3.40 (sept, 2H,
3JHH = 6.8 Hz, CH(Me)2)), 3.08 (sept, 2H, JHH = 6.8 Hz,
3
3
CH(Me)2)), 1.62 (d, 6H, JHH = 6.8 Hz, CH(Me)2)), 1.42 (d, 6H,
3JHH = 6.8 Hz, CH(Me)2)), 1.38 (s, 6H, Me), 1.25 (d, 6H, 3JHH = 6.8
3
Hz, CH(Me)2)), 0.95 (d, 6H, JHH = 6.8 Hz, CH(Me)2)). 13C NMR
(100 MHz, C6D6, Me4Si): δ 170.4 (CN), 146.1, 143.9, 136.0, 129.1,
124.9, 124.3 (i-, o-, m-, p-Ar), 100.4 (γ-CH), 29.3, (CH(Me)2), 28.9
(CH(Me)2), 26.0 (Me), 24.1(CH(Me)2), 24.0 (CH(Me)2), 23.7
1
Residual H resonance from deuterated C6D6 solvent was used to
1
1
(CH(Me)2), 23.6 ppm (CH(Me)2). 77Se NMR (600 MHz for H,
reference the H spectrum with the methyl resonance of TMS at 0.0
ppm. The 77Se chemical shift was referenced using the unified
chemical shift scale, according to an IUPAC-recommended method.60
Compounds [{HC(CMeNAr)2}Li(Et2O)] and [{HC(CMeNAr)2}-
GeCl] (1) were prepared according to the literature procedures.61,41
Preparation of Compound [{HC(CMeNAr)2}GeNCS] (2). To a
suspension of KSCN (0.53 g, 5.42 mmol) in toluene (20 mL) at −78
°C was added via cannula a solution of 1 (2.85 g, 5.42 mmol) in
toluene (40 mL). A light-yellow solution appeared after 2 h, which
remained unchanged after stirring for 2 days. Subsequent filtration
and removal of solvent gave a light-yellow solid. Keeping a solution of
2 (0.5 g) dissolved in a minimum amount of toluene and pentane (2
mL) in a freezer (−20 °C) for 7 days gave light-yellow crystals. Yield:
2.47 g, (83%). Mp: 223−225 °C. Elemental anal. calcd for
C30H41N3GeS (548.37 g mol−1): C, 65.71; H, 7.54; N, 7.66.
Found: C, 65.90; H, 7.52; N, 7.65. UV−vis (λmax, chlorobenzene)
C6D6, 25 °C): −404.3 ppm.
Preparation of Substrates. A glass substrate (Nippon sheet
glass, 10 Ω sq−1) with FTO substrates was cleaned vigorously but
without scratching, using a cleaning solution (Hellmanex 10%)
diluted with water, and then subjected to cleaning three times in an
ultrasonic bath: (1) in an aqueous solution (2% Hellmanex, Hellma
GmbH) for 20 min, (2) in an acetone bath for 15 min, and (3) in an
ethanol bath for 15 min. After ultrasonication, the substrates were
rapidly dried in a strong flow of air. UV-ozone cleaning for 15 min at
maximum power was carried out before an application of the TiO2
compact layer.
Bottom Selective Contacts. FTO substrates were rapidly
warmed to 450 °C (within 15 min) and then left for 15 min at 450
°C before deposition of the compact TiO2 layer with aerosol spray
pyrolysis. The sprayed solution was made with a precursor solution of
anhydrous ethanol (EtOH, 9 mL), 2,4-pentanedione (400 μL), and
titanium diisopropoxide bis(acetylacetonate) (600 μL); the mixture
was stirred manually for 1 min and then added with a nozzle. The
substrates were kept at 450 °C for 10 min and then slowly cooled to
near 23 °C. The mesoporous TiO2 layer was prepared with TiO2
paste (30 N-RD Dyesol) diluted in ethanol as 1:6 w/w. The resulting
dispersion was stirred vigorously overnight before use. To prepare the
mesoporous TiO2 layer, a dispersion (50 μL) was added dropwise
over the TiO2 compact layer and then spin-coated using acceleration
at 4000 and 2000 rpm/s for 20 s. Immediately after the spin coating,
the layer was dried at 100 °C for 10 min and subsequently sintered
with multiple temperature steps up to 450 °C.
nm {ε/dm3 mol−1 cm−1}: 362 {16 686}. IR (ATR): ν
̃
2041 cm−1
1
(NCS−). H NMR (400 MHz, C6D6, Me4Si): δ 6.98−7.13 (m, 6H,
3
m-, p-ArH), 5.02 (s, 1H, γ-CH), 3.63 (sept, 2H, JHH = 6.8 Hz,
3
CH(Me)2), 2.95 (sept, 2H, JHH = 6.8 Hz, CH(Me)2), 1.58 (d,
3
6H,3JHH = 6.8 Hz, CH(Me)2), 1.48 (s, 6H, Me), 1.30 (d, 6H, JHH
=
3
6.8 Hz, CH(Me)2), 1.14 (d, 6H, JHH = 6.8 Hz, CH(Me)2), 0.96 (d,
6H, 3JHH = 6.8 Hz, CH(Me)2). 13C NMR (100 MHz, C6D6, Me4Si): δ
165.4 (CN), 146.6, 143.0, 139.2, 127.9, 125.1, 123.7 (i-, o-, m-, p-Ar),
101.2 (γ-CH), 29.0 (CH(Me)2), 28.4 (CH(Me)2), 26.7 (Me), 24.2
(CH(Me)2), 24.0 (CH(Me)2), 23.6 (CH(Me)2), 22.7 (CH(Me)2).
Preparation of Compound [{HC(CMeNAr)2}Ge(S)NCS] (3). A
solution of 2 (5 g, 9.12 mmol) in toluene (40 mL) was slowly added
to a suspension of freshly recrystallized elemental sulfur (0.29 g, 9.12
mmol) in toluene (15 mL) via cannula at ambient temperature. After
Preparation and Deposition of the Perovskite Precursor
Solution. Stock solutions (1.5 M) of PbI2 and PbBr2 in 4:1
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dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) were
prepared by heating the solutions from 23 to 180 °C for 10 min or
until the dissolution of the salts was complete (clear solution). A CsI
stock solution (1.5 M) in DMSO was prepared on heating to 150 °C
until the solid was completely dissolved. Methylammonium bromide
(MABr) and formamidinium iodide (FAI) powders were weighed in
two separate vials. To determine the amount of solid from MABr and
FAI, one must consider the 1:1.09 stoichiometry of FAI/PbI2 (MABr/
PbBr2) (i.e., 9% excess lead) for final solutions (1.22 M) of FAPbI3
and MAPbBr3. Finally, CsI/FAPbI3/MAPbBr3 0.3:5:1 v/v/v solutions
were mixed. The perovskite solution was deposited using the
following two-step spin-coating protocol: a static slow step (1000
rpm, 200 rpm/s, 10 s) after adding perovskite solution (50−60 μL) to
the middle of the substrate for appropriate surface coverage and a
subsequent rapid step (6000 rpm, 2000 rpm/s, 20 s). In the second
step, the antisolvent (chlorobenzene, 100 μL) was dropped 5 min
before the end of the step onto the middle of the rotating substrate
with a pipet. Finally, substrates were placed on a preheated hot plate
at 100 °C for 40−60 min. After the annealing, substrates were left to
cool to ca. 23 °C (15−20 min).
Deposition of a Hole-Transport Layer. For the hole-transport
layers of 3 and 4, several parameters such as concentrations and
solvents (chlorobenzene and toluene) were tested and optimized to
achieve the best PV properties. Concentrations of 3 and 4 were varied
from 10 to 100 mg/μL; we found that 45 mg/μL (3) and 15 mg/ μL
(4) gave superior PV properties. A solution (60 μL) in chlorobenzene
(45 mM for 3 and 15 mM for 4) was hence added to the middle of
the perovskite film with spin coating (4000 rpm, 2000 rpm/s, 20 s) in
a one-step protocol.
Cyclic Voltammetry (CV). Cyclic voltammetry (CV) measure-
ments were performed in a three-electrode system: glassy carbon
(0.07 cm2) as the working electrode (WE), graphite as the counter
electrode (CE), and Ag/AgCl (0.1 M LiTFSI in MeCN) as the
pseudoreference electrode (RE) with ferrocene/ferrocenium (Fc/
Fc+) in the electrolyte as a reference. Acetonitrile (80%),
chlorobenzene (20%), and LiTFSI (1.0 M) served as supporting
electrolytes. (Each germanium compound (0.5 mM) was dissolved in
each solution.) This solvent mixture was due to the small solubility of
the compounds in MeCN.
Experimental Determination of HOMO and LUMO. The
highest occupied molecular orbital (HOMO) energy levels were
derived from the ground-state oxidation potential obtained from the
onset values of the first oxidation potential of the cyclic voltammetry
measurements of compounds in solutions described above. In
contrast, the lowest unoccupied molecular orbital (LUMO) energy
levels were estimated on adding the optical band gap (Eg) (obtained
from the recording of the UV−visible solid-state spectra) to the
HOMO energy levels. Potentials were converted to the standard
hydrogen electrode (SHE) with correction +0.624 V51 and then to
the vacuum scale with correction 4.4 eV.
Calculations with Density Functional Theory (DFT). Molec-
ular and periodic calculations for compounds 1−4 were undertaken
with the TURBOMOLE quantum chemical package. Molecular
calculations were performed at the BP86/def2-TZVP and TD-BP86/
def2-TZVP levels of theory for ground and electronically excited
states, respectively. Initial structures for the optimizations of
molecular geometries of 1 and 4 were based on their corresponding
X-ray crystal structures; for compounds 2 and 3, the initial structure
was based on that of 4. All geometries were tightly converged. All
minimum structures were corroborated on computing analytical
second derivatives of the energy with respect to the nuclear
coordinates. Calculations on the periodic systems were performed
with the BP86 pure functional, in conjunction with all-electron basis
set def2-TZVP for all atoms except hydrogen, for which def2-SV(P)
was used. Making use of the same strategy, which was taken for the
molecular calculations, the structures of unit cells for the geometry
optimizations of 1 and 4 were based on their corresponding X-ray
crystal structures. In contrast, for compounds 2 and 3, the initial
structures were based on that of 4. All geometry optimizations and
band structure calculations were undertaken with a 6 × 6 × 6 k-point
mesh grid size. The density of states and the respective atomic orbital
contributions were obtained with a convolution of Gaussian functions
on the discrete energy levels. Both molecular and periodic calculations
were conducted with the resolution of identity approximation (RI
approximation) for the computation of the four-center integrals. At
the same time, the calculation for periodic systems also included the
continuous fast multipole method (CFMM) for the electronic
Coulomb term.
For the reference cell, spiro-OMeTAD in chlorobenzene served as
the hole-transport material. Devices were prepared with and without
LiTFSI and tBP additives. For the spiro-OMeTAD standard solutions,
spiro-OMeTAD powder (45 mg) was dissolved in chlorobenzene
(500 μL) and doped with a LiTFSI stock solution (10.31 μL, 52 mg
of LiTFSI in 100 μL of MeCN) and tBP (17.75 μL). A spiro-
OMeTAD doped solution (50 μL) was added to the middle of the
perovskite film using spin coating (4000 rpm, 2000 rpm/s, 20 s) in a
one-step protocol.
Evaporation of Metal Electrodes. Gold layer contacts (thick-
ness 80 nm) were deposited over the HTM in the device with a mask
according to the method of thermal evaporation.
Thermogravimetric Analysis. Thermogravimetric analysis
(TGA) was performed (TGA Q 500 instrument). Samples were
subjected to a heating cycle from ambient temperature to 300 °C at
scanning rate of 10 °C/min under a nitrogen atmosphere.
Decomposition temperatures (Tdec) were derived from the plots.
The glass-transition temperature Tg and melting temperature Tm were
obtained with a differential scanning calorimeter (PerkinElmer
DSC8000). Samples were subjected to two consecutive heating and
cooling cycles from ambient temperature to 220−240 °C at scanning
rate of 10 °C/min under a nitrogen atmosphere.
Ultraviolet−Visible Spectral Measurements. Optical proper-
ties were measured with a UV−visible spectrophotometer (Evolution
600, Thermo Scientific). UV−visible absorption spectra were
recorded on samples in a solution of chlorobenzene at a concentration
of 1.0 × 10−5 M with a quartz cuvette (optical path 1 cm). Spectra
were recorded in the wavelength range of 300−800 nm. UV−visible
diffuse reflectance spectra were also recorded with a UV−visible
spectrophotometer (Evolution 600) and an integrating sphere (DRA-
EV-600, constructed of Spectralon, standard). Labsphere’s highest
reflectance material served to calibrate the equipment. The optical
absorption was measured in the range of 250−850 nm. Reflectance
values were converted to a magnitude of F(R) and plotted versus
energy according to the Kubelka−Munk method: F(R) = (1 − R)2/
2R, where R corresponds to the reflectance values. To estimate values
of optical band gaps (Eg), we applied the Tauc plot method as (F(R)
hν)n, where h is the Planck constant (J·s) and ν is the frequency (s−1)
of light; the value of n depends on the specific transition coefficient:
we used n = 1/2 for a directly allowed transition (plotted as α(hν)2
versus E).
Space−Charge-Limited Current (SCLC). The method of
space−charge-limited current (SCLC) was applied to hole-only
devices (FTO/Ge-compound/Au) using current−voltage (J−V)
curves. Vertical hole mobility values were fitted using the Mott−
Gurney law and calculated according to J = 9/8ere0μV2L−3; J, er, e0, μ,
V, and L denote the current, relative permittivity (er = 3 was used
here), vacuum permittivity, hole mobility, applied voltage, and film
thickness, respectively.
Images from a Scanning Electron Microscope (SEM). These
SEM images were collected (high-resolution Gemini SEM 300) with a
field-emission gun.
Current Density−Voltage (J−V) Curves. Current density−
voltage (J−V) curves and the time-dependent efficiency of power
conversion (PCE) of the devices were measured with a xenon lamp
(450 W, Oriel). To calibrate the light intensity, we used a Si
photodiode equipped with an IR cutoff filter (KG3, Schott) during
each measurement as well as 1.5 AM conditions. J−V parameters of
the devices were obtained on applying an external voltage bias while
measuring the current response with a digital source meter (Keithley
2400). A black metal mask (area 0.16 cm2) was used during the
measurements to avoid overestimation from scattered light.
J
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́
́
X-ray Diffraction Analysis. Monocrystals of compound 4 were
selected from the mother solution and covered with perfluorinated
polyether oil on a microscope slide, which was cooled with flowing
dinitrogen gas. The data of compound 4 were collected at 100 K in a
dual-source configuration (Mo and Cu, Bruker D8 Venture), three-
circle diffractometer equipped with a CCD detector (Photon CMOS
APEX III) and a Mo Kα (λ = 0.71073 Å) microfocus source
(INCOATEC)62 with Quazar mirror optics (INCOATEC Q). All
data were integrated with SAINT;63 semiempirical absorption
corrections were applied with SADABS.64 The structure was solved
with direct methods (SHELXS-97)65 and refined against all data with
full-matrix least-squares methods on F2 (SHELXL2013)66,67 inside
the SHELXLE GUI.68 All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were refined
on calculated positions using a riding model. Their Uiso values were
constrained to 1.5Ueq of their pivot atoms for terminal tetrahedrally
coordinated carbon atoms and to 1.2Ueq for all other carbon atoms.
All bond lengths and angles of 4 are shown in the Supporting
Information (SI).
́
́
Desire Molina − Area de Quimica Organica, Instituto de
́
́
Bioingenieria, Universidad Miguel Hernandez, 03202 Elche,
Spain
́
Andrea Soto-Navarro − Centro de Investigacion en
Electroquimica y Energia Quimica (CELEQ), Universidad de
Costa Rica, 11501-2060 San Jose, Costa Rica
Andres Lizano-Villalobos − Escuela de Quimica, Universidad
de Costa Rica, 11501-2060 San Jose, Costa Rica
́
́
́
́
́
́
́
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02120
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is dedicated to Professor Herbert W. Roesky on the
occasion of his 85th birthday. Centro de Electroquimica y
́
́
́
́
Energia Quimica (CELEQ, UCR) and Escuela de Quimica
(D8 Venture SC XRD, UCR) supported this work. T.S.-M. is
grateful for an Orlando Bravo scholarship (CELEQ) for
support and Sistema de Estudios de Posgrado (SEP, UCR) and
Vicerrectoria de Investigacion (VI, UCR, grant 804-B7-271)
for a research stay at EPFL. C.C. acknowledges financial
support from VI, UCR (grant 115-B9-461) and thanks
Roberto A. Gonzalez−Leon for technical assistance. D.M.
thanks the European Union through the “Programa Operativo
del Fondo Social Europeo (FSE) de la Comunitat Valenciana
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02120.
■
sı
́
́
1H, 13C, and 77Se NMR spectra, cost-analysis estimation,
and E factor; table of crystal data, structure refinement
details, and selected bond lengths and angles for
compound 4; quantum chemical calculations and DFT
results, additional DSC figures, UV−visible spectra, and
CV; hysteresis; and statistics and optimization plots
(PDF)
́
́
́
2014-2020” for grant APOSTD/2017/026. Dr. Aurelien
Bornet from the NMR team of ISIC, EPFL, is greatly
acknowledged for his assistance with the 77Se NMR experiment
Accession Codes
́
and the NMR unit of Escuela de Quimica at UCR.
CCDC 1867214 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
ABBREVIATIONS
■
HTM, hole-transport material; VB, valence band; CB,
conduction band; UV, ultraviolet; DFT, density functional
theory; IR, infrared; NMR, nuclear magnetic resonance
AUTHOR INFORMATION
Corresponding Authors
Cristopher Camacho − Escuela de Quimica, Universidad de
Costa Rica, 11501-2060 San Jose, Costa Rica; orcid.org/
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