Molecular engineering of face-on oriented dopant-free hole transporting material for perovskite solar cells with 19% PCE

Kasparas Rakstys a, Sanghyun Paek a, Peng Gao a, Paul Gratia a, Tomasz Marszalek b, Giulia Grancini a, Kyung Taek Cho a, Kristijonas Genevicius c, Vygintas Jankauskas c, Wojciech Pisula b and Mohammad Khaja Nazeeruddin *a
aGroup for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland
bOrganisch-Chemisches Institut, Ruprecht-Karls-Universitt, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
cDepartment of Solid State Electronics, Vilnius University, Sauletekio 3, Vilnius 10222, Lithuania

Received 24th February 2017 , Accepted 29th March 2017

First published on 29th March 2017

Through judicious molecular engineering, novel dopant-free star-shaped D–π–A type hole transporting materials coded KR355, KR321, and KR353 were systematically designed, synthesized and characterized. KR321 has been revealed to form a particular face-on organization on perovskite films favoring vertical charge carrier transport and for the first time, we show that this particular molecular stacking feature resulted in a power conversion efficiency over 19% in combination with mixed-perovskite (FAPbI3)0.85(MAPbBr3)0.15. The obtained 19% efficiency using a pristine hole transporting layer without any chemical additives or doping is the highest, establishing that the molecular engineering of a planar donor core, π-spacer and periphery acceptor leads to high mobility, and the design provides useful insight into the synthesis of next-generation HTMs for perovskite solar cells and optoelectronic applications.

Hybrid lead halide perovskite-based solar cells (PSCs) have attracted significant attention in photovoltaics due to inexpensive precursors, simple fabrication methods, and remarkably high power conversion efficiency (PCE), which already exceeds those of commercialized polycrystalline silicon solar cells.1–8 A typical PSC configuration consists of the electron transporting material (ETM) TiO2, infiltrated with a perovskite absorbing material and coated with a hole transporting material (HTM), which plays an important role in facilitating the extraction and transportation of holes from perovskite to the corresponding contact and is essential for achieving a high light-to-current conversion efficiency.9 To date, a great number of new promising molecular organic HTMs have been reported, but only very few candidates reached PCE values close to or exceeding 20%.10–15 However, although PSCs have achieved high PCE values, the stability remains an issue due to dopant-induced degradation of PSCs.16–18

Traditionally, the hole transporting layer (HTL) of PSCs is heavily doped with the bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), 4-tert-butylpyridine (TBP), and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[hexafluorophosphate] (FK209). TBP is commonly used as the HTL morphology controller, while LiTFSI and FK209 provide the necessary electrical conductivity.19,20 However, the use of additives is problematic, since the hygroscopic nature of the lithium salt makes the HTL highly hydrophilic and the Co(III) dopant shows a tendency towards chemical degradation, negatively influencing the stability of the entire device.21,22 Therefore, a promising solution for stabilizing PSCs is the appropriate choice of dopant-free HTMs. However, the PCE of pristine HTL based devices are consistently lying around 10%, with only very few examples over 15%.23–29 Therefore, development of dopant-free HTMs with both enhanced moisture resistance and charge transport properties is desired to probe their structure–performance correlations towards the realization of stable and high-efficiency PSCs.

In this work, we have systematically engineered three novel dopant-free star-shaped donor–π-bridge–acceptor (D–π–A) type HTMs. Such molecules feature a planar triazatruxene central core (D), inducing π-stacking for vertical hole conduction, thiophene-based multiple conjugated arms (π) and malononitrile (A). Due to strong intermolecular interaction, these molecules have great potential to show high charge carrier mobility, minimizing ohmic losses of the contact. Moreover, they combine the advantages of both small molecules, i.e. well-defined structures, and polymers, like good thermal, electrochemical and photochemical stability, together with high solubility and suitable wetting on the perovskite.30 All three molecularly engineered HTMs have been applied in PSCs and for the first time, we show that a highly ordered characteristic face-on organization could favor vertical charge carrier transport in the perovskite solar cell and a PCE over 19% with improved stability was achieved using KR321.

The general synthesis scheme for the preparation of 2,2′,2′′-(((5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole-3,8,13-triyl)tris(4-hexylthiophene-5,2-diyl))tris(methanylylidene))trimalononitrile (KR355), 2,2′,2′′-(((5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole-3,8,13-triyl)tris(3,3′′-dihexyl-[2,2′:5′,2′′-terthiophene]-5′′,5-diyl))tris(methanylylidene))trimalononitrile (KR321) and 2,2′,2′′-(((5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole-3,8,13-triyl)tris(4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-6,2-diyl))tris(methanylylidene))trimalononitrile (KR353) is shown in Fig. 1.

image file: c7ta01718a-f1.tif
Fig. 1 Synthesis route of star-shaped D–π–A HTMs. (a) Pd(PPh3)4, 2 M aq. K2CO3, THF, 80 °C; (b) POCl3/DMF, DCE, 0 °C-reflux; (c) CH2(CN)2, Et3N, DCM, RT.

The triazatruxene donor and malononitrile acceptor groups were preserved throughout the series, while the π-bridge was modulated with 3-hexylthiophene, 3,3′′-dihexyl-2,2′:5′,2′′-terthiophene and 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene, respectively. The synthesis begins with the construction of functional conjugated arms on the triazatruxene core by the Suzuki cross-coupling reaction of desired building blocks. After formylation with the Vilsmeier complex, the aldehyde-terminated derivatives were converted into final low-bandgap chromophoric HTMs with malononitrile through the Knoevenagel condensation reaction, successfully forming electron-accepting moieties. All detailed synthetic procedures are described in the ESI.

The optimized geometries of the new HTMs, along with the HOMO and LUMO pictograms are presented in Fig. S7. In all three molecules, the HOMO orbitals are mostly developed in the central triazatruxene core along with some thiophene substituents. After three thiophene rings, the space extension of the total HOMO is almost negligible. The extension of the LUMO orbitals is mainly in the arms and localized around the 2-methylenemalononitrile. All calculated energy level values are in close agreement with those obtained experimentally.

The normalized UV-Vis absorption and photoluminescence (PL) spectra of the new compounds in DCM solutions are shown in Fig. 2. Typically, for the dipolar D–π–A type molecules, sharp charge-transfer absorption bands were found in the visible region, with the peak maxima centered at 487, 515 and 569 nm for KR355, KR321, and KR353, respectively. As expected, KR355 and KR321 have an extra π–π* transition induced absorption band in the UV region. Photoluminescence spectra show that all molecules have very large Stokes shifts of around 200 nm suggesting significant changes in the geometrical configuration of the molecules upon excitation. The optical bandgap (Eg) is estimated from the intersection of the corresponding normalized absorbance and photoluminescence spectra. It is known that reducing the bandgap of a semiconductor can enhance the intrinsic electrical conductivity by increasing the carrier concentration.31Eg values of 2.13, 2.05 and 1.96 eV were determined for KR355, KR321, and KR353, respectively. The HOMO energies of KR355, KR321 and KR353 were measured to lie at −5.55, −5.24 and −5.34 eV, respectively, by cyclic voltammetry (CV). The solid-state ionization potential (IP) was measured by electron photoemission in air on the thin films (Fig. S12), and the measured IP values are fully in agreement with the HOMO levels, −5.52, −5.18 and −5.38 eV, respectively. The oxidation potential variation can be attributed to the changes in the length of the π-bridge moiety. These values are in alignment with those of the photoactive perovskite layer (FAPbI3)0.85(MAPbBr3)0.15 having a valence band at −5.65 eV and should favor efficient photogenerated charge transfer at the interface. Also, the determined LUMO values below −4 eV should effectively block electron transfer from perovskite to the HTMs. All optical and electrochemical properties are summarized in Table 1.

image file: c7ta01718a-f2.tif
Fig. 2 UV-Vis absorption (solid lines) and photoluminescence (dashed lines) spectra normalized at the peak value (left); and cyclic voltammograms of the triazatruxene-based HTMs (right).
Table 1 Optical and electrochemical properties of the synthesized HTMs
ID λ abs (nm) λ em (nm) E HOMO (eV) E g (eV) E LUMO (eV) μ (cm2 V−1 s−1)
a Measured in DCM solution. b Measured in DCM/tetra-n-butylammonium hexafluorophosphate (0.1 M) solution, using a glassy carbon working electrode, Pt reference electrode and Pt counter electrode with Fc/Fc+ as an internal standard. Potentials were converted to the normal hydrogen electrode (NHE) by the addition of +0.624 V and −4.44 eV to the vacuum, respectively. c Estimated from the intersection of the normalized absorbance and emission spectra. d Calculated from ELUMO = EHOMO + Eg. e Measured in the space-charge limited current (SCLC) regime and fitted using the Mott–Gurney law.
KR355 365, 487 688 −5.55 2.13 −3.42 5.0 × 10−7
KR321 401, 515 710 −5.24 2.05 −3.19 2.6 × 10−4
KR353 569 721 −5.34 1.96 −3.38 1.1 × 10−5

It is known that the charge within the active layers of a thin film solar cell will preferentially transfer through the vertical direction. At the same time, C3h symmetrical molecules have the advantage of forming face-on stacking and columnar geometry on the surface, which will favor vertical charge transport along the π–π stacking direction. To prove the concept, the supramolecular organization of the new HTMs on the silica substrate surface was determined by Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). The corresponding patterns in Fig. 3 and S10 indicate significant differences in self-assembly between the compounds. The highest order of the series and a distinct surface arrangement has been found only for KR321. Interestingly, KR321 shows an ideally characteristic face-on organization with columnar stacks standing on the surface as illustrated in the inset of Fig. 3a. For this molecular arrangement, an intense π-stacking reflection appears out-of-plane of the pattern and is related to a d-spacing of 0.38 nm. Furthermore, we found that the in-plane scattering intensities correspond to the rectangular lattice of the intercolumnar organization of KR321 (lattice parameters of a = 6.04 nm and c = 1.46 nm). The narrow azimuthal intensity distribution of these reflections suggests a pronounced out-of-plane surface alignment of the columnar stacks (Fig. 3b). The observed face-on orientation is expected to favor the vertical charge carrier transport and improve the solar cell efficiency in comparison to those of KR353 and KR355 (Fig. S10). An identical enhancement in solar cell performance was observed for face-on arranged donor–acceptor polymers.32 However, the alignment mechanisms of organic semiconductors during solution deposition are still under discussion. One hypothesis is that low aggregation and the existence of monomeric species in the solution result in a face-on organization, while aggregates are arranged edge-on in the film.33,34 The molecular structure plays a fundamental role in the solution aggregation and hence for the surface ordering. Normally a face-on orientation was achieved for various small molecular weight disc-shaped liquid crystalline molecules by cooling from their isotropic melt between two surfaces.35–37 In contrast to KR321, KR353 and KR355 are poorly ordered leading to patterns with only one isotropic intercolumnar peak (the d-spacing of the first peak is 2.27 nm for KR353 and 2.02 nm for KR355, Fig. S10). The decrease in order and lack of vertical surface alignment of KR353 and KR355 could lead to the decline of their device performance.

image file: c7ta01718a-f3.tif
Fig. 3 (a) GIWAXS pattern of the KR321 film coated from tetrachloroethane on a silica wafer (schematic illustration of the molecular surface arrangement), and (b) azimuthal integration of the π-stacking and intercolumnar reflections (the star indicates scattering at the beam stop).

The charge transport properties (Fig. S13,Table 1) of the novel derivatives were studied using the space-charge limited current (SCLC) regime by fabricating hole-only devices on an ITO/PEDOT:PSS/HTM/Au architecture. The calculated vertical hole mobility value of KR321 is 2.6 × 10−4 cm2 V−1 s−1, which is one order of magnitude higher than that of KR353 (μ = 1.1 × 10−5 cm2 V−1 s−1) and three orders of magnitude higher than that of KR355 (μ = 5.0 × 10−7 cm2 V−1 s−1), indicating improved charge hopping properties through face-on oriented columnar stacks.

To gain insight into the interface processes and in particular on the hole transfer at the perovskite/HTM interface, we monitored the steady-state photoluminescence (PL). Fig. S8 shows the comparison of the PL spectra between the pristine perovskite and the perovskite interfaced with spiro-OMeTAD as well as with the series of new molecules presented. The perovskite samples have a comparable thickness of 300 nm. This enables us to retain a constant density of absorbed photons for all the samples investigated. The CW PL spectra have been registered upon excitation at 650 nm; this enabled us to selectively excite the perovskite and not the HTM. The perovskite/HTMs show a reduction of the PL signal with respect to the pristine perovskite film. This suggests that interfacial hole transfer happens and quenches the PL signal. In particular, KR321 shows a similar quenching to that observed for spiro-OMeTAD.

To demonstrate the function of the novel compounds as dopant-free HTMs, we prepared PSCs with mixed perovskite absorber (FAPbI3)0.85(MAPbBr3)0.15 (MA: CH3NH3+, FA: NH[double bond, length as m-dash]CHNH3+). The solar cell preparation is fully described in the ESI. Fig. S11 displays the cross-sectional image of the PSC containing KR321, analyzed using a field-emission scanning electron microscope. The device is made using 700 nm thick perovskite atop a 200 nm thick mesoporous TiO2 layer, which was deposited on FTO glass coated with 50 nm of compact TiO2. The device is completed by depositing a 70 nm thick HTM layer and 80 nm of gold as the back contact.

The current density–voltage (JV) characteristics of the champion PSCs using the dopant-free HTMs under AM1.5G irradiation at 100 mW cm−2 are shown in Fig. 4, and the corresponding device output parameters are summarized in Table 2. The PCE histograms are shown in Fig. S15 in the ESI. The device with dopant-free KR321 as the HTM, which was determined to have ideal columnar stacks standing on the surface showed an excellent PCE of 19%. The device exhibited an open-circuit voltage (VOC) of 1.13 V, a short-circuit current density (JSC) of 21.7 mA cm−2, and a fill factor (FF) of 0.78, indicating its identical photovoltaic performance to the heavily doped spiro-OMeTAD reference. In contrast, devices using dopant-free KR353 and KR355 only yield very low PCEs of 14.87% and 8.8%, respectively, which is attributed to the significantly decreased JSC and FF, most likely due to the lower lying LUMO level leading to poor electron blocking and greater charge recombination.

image file: c7ta01718a-f4.tif
Fig. 4 Current–voltage curves of the novel dopant-free HTMs and doped spiro-OMeTAD as the reference (left), and IPCE spectra of the devices (right).
Table 2 Photovoltaic performance of the devices based on KR321, KR353, KR355 and spiro-OMeTAD under AM1.5G illumination (100 mW cm−2)
ID J SC (mA cm−2) V OC (V) FF PCE (%)
KR355 16.01 1.05 0.53 8.88
KR321 21.70 1.13 0.78 19.03
KR353 19.31 1.11 0.69 14.87
Spiro-OMeTAD 22.25 1.12 0.76 19.01

This is also fully in agreement with the result of 2D-WAX measurement, proving the advantage of ordered face-on stacking on the charge transport and hence the device performance. To show the impact of hysteresis on device performance, we reported the JV traces collected by scanning the applied voltage at 0.01 V s−1 from forward bias (FB) to short circuit (SC) and the other way around (Fig. S14). The incident photon-to-electron conversion efficiency (IPCE) spectra with integrated JSC values are shown in Fig. 4. The integrated photocurrents calculated from the overlap integral of the IPCE spectra are 21.2, 18.4, 16.6, and 22.4 mA cm−2 for KR321, KR353, KR355 and spiro-OMeTAD, respectively, and are consistent with those obtained from the experimental JV measurements.

In Fig. 5, the maximum power point tracking (MPO) using pristine KR321 and doped spiro-OMeTAD layers for hole transport in corresponding perovskite devices is shown. During the measurement, unsealed devices were kept in argon ambience under a constant illumination of 100 mW cm−2. The devices were maintained at the maximum power point during aging and the current–voltage curve was recorded automatically every 2 h. The efficiency of the devices initially decreased in early time decay. Similar dynamics have recently been observed demonstrating that a rapid degradation mechanism is activated by metal electrode migration through HTMs and contact with the perovskite layer.38,39 The general trend showed significantly improved durability of the device prepared with dopant-free KR321, which maintained 60% of its initial PCE after 650 h, while the PCE of devices with doped spiro-OMeTAD dropped by 80% under identical conditions.

image file: c7ta01718a-f5.tif
Fig. 5 Normalized maximum power point tracking of perovskite solar cells prepared in a single experiment, using KR321 and spiro-OMeTAD as HTMs. The measurement was performed under UV-filtered simulated sunlight in an argon atmosphere without any encapsulation for 650 h.


To conclude, we have successfully synthesized three symmetrical dopant-free hole transporting materials based on the D–π–A type architecture. For the first time, we show that face-on formed columnar stacks of HTM molecules are beneficial for charge transfer within a perovskite solar cell. The optimized structure of KR321 showed a highly ordered characteristic face-on organization leading to increased vertical charge carrier transport and a power conversion efficiency over 19% with improved stability. This result is on par with the heavily doped spiro-OMeTAD reference, clearly showing the importance of appropriate molecular engineering and the great prospects of dopant-free HTMs for perovskite solar cells, and outperforms most of the other dopant-free HTMs reported to date.


The authors acknowledge financial support from the SNSF NRP 70 project, number: 407040_154056, CTI 15864.2 PFNM-NM, Solaronix, Aubonne, Switzerland, the European Commission H2020-ICT-2014-1, SOLEDLIGHT project, grant agreement No. 643791, the Swiss State Secretariat for Education, Research and Innovation (SERI) and NPRP award [NPRP 6-175-2-070] from the Qatar National Research Fund. The authors thank Manual Tschumi for designing the stability measurement system.

Notes and references

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Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta01718a
These authors contributed equally to this work.

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