Green-Chemistry-Inspired Synthesis of Cyclobutane-Based Hole-Selective Materials for Highly Efficient Perovskite Solar Cells and Modules.

Hybrid lead halide perovskite solar cells (PSCs) have emerged as potential competitors to silicon-based solar cells with an unprecedented increase in power conversion efficiency (PCE), nearing the breakthrough point toward commercialization. However, for hole-transporting materials, it is generally acknowledged that complex structures often create issues such as increased costs and hazardous substances in the synthetic schemes, when translated from the laboratory to manufacture on a large scale. Here, we present cyclobutane-based hole-selective materials synthesized using simple and green-chemistry inspired protocols in order to reduce costs and adverse environmental impact. A series of novel semiconductors with molecularly engineered side arms were successfully applied in perovskite solar cells. V1366-based PSCs feature impressive efficiency of 21 %, along with long-term operational stability under atmospheric environment. Most importantly, we also fabricated perovskite solar modules exhibiting a record efficiency over 19 % with an active area of 30.24 cm2 .

Synthesis, crystal structure and photophysical properties of bis-[2,6-di-fluoro-3-(pyridin-2-yl)pyridine-κN](tri-fluoro-methane-sulfonato-κO)silver(I).

In the title compound, [Ag(CF3SO3)(C10H6F2N2)2], the AgI centre adopts a highly distorted trigonal-planar coordination environment resulting from its coordination by one O atom of the tri-fluoro-methane-sulfonate anion and the pyridine N atoms of two crystallographically independent 2',6'-di-fluoro-2,3'-bi-pyridine ligands, which display very similar conformations to one another. Pairwise Ag⋯O-SO2CF3 - [Ag⋯O = 2.8314 (14) Å] inter-actions and inter-molecular C-H⋯O inter-actions between inversion-related units lead to the formation of an eight-membered cyclic dimer in which the silver atoms are separated by 6.2152 (3) Å. In the crystal, the dimers are linked through C-H⋯O hydrogen bonds, halogen⋯π and weak π-π stacking inter-actions, resulting in the formation of a three-dimensional supra-molecular network. The title compound exhibits a strong and broad emission band from 400 nm to 550 nm in solution and its photoluminescence quantum efficiency is estimated to be ca 0.2, indicating that the title compound could have applications as an emitting material in organic light-emitting diodes (OLEDs).

Synthesis, crystal structure and photophysical properties of chlorido-[2-(2',6'-di-fluoro-2,3'-bipyri-din-6-yl-κN 1)-6-(pyridin-2-yl-oxy-κN)phenyl-κC 1]platinum(II).

The title compound, [Pt(C21H12F2N3O)Cl], crystallizes with two crystallographically independent mol-ecules (A and B) in the asymmetric unit, which adopt similar conformations. The PtII atoms in both mol-ecules adopt distorted square-planar geometries, coordinated by one C and two N atoms from the tridentate 2',6'-di-fluoro-6-[3-(pyridin-2-yl-oxy)phen-yl]-2,3'-bi-pyridine ligand and a chloride anion: the C and Cl atoms are trans. In the crystal, C-H⋯Cl/F hydrogen bonds, F⋯π and weak π-π stacking inter-actions between adjacent A and B mol-ecules and between pairs of inversion-related B mol-ecules lead to the formation of a two-dimensional supra-molecular network lying parallel to the ab plane. The sheets are stacked along the c-axis direction and linked by F⋯π and weak π-π stacking inter-actions between pairs of inversion-related A mol-ecules, forming a three-dimensional supra-molecular network. The photoluminescence quantum efficiency of the title compound in the blue-green region of the visible region (λmax = 517 and 544 nm) is estimated to be ∼0.2-0.3, indicating that the title compound could be a suitable candidate as the emitting material in organic light-emitting diode (OLED) applications.

Molecular Design and Operational Stability: Toward Stable 3D/2D Perovskite Interlayers.

Despite organic/inorganic lead halide perovskite solar cells becoming one of the most promising next-generation photovoltaic materials, instability under heat and light soaking remains unsolved. In this work, a highly hydrophobic cation, perfluorobenzylammonium iodide (5FBzAI), is designed and a 2D perovskite with reinforced intermolecular interactions is engineered, providing improved passivation at the interface that reduces charge recombination and enhances cell stability compared with benchmark 2D systems. Motivated by the strong halogen bond interaction, (5FBzAI)2PbI4 used as a capping layer aligns in in-plane crystal orientation, inducing a reproducible increase of ≈60 mV in the V oc, a twofold improvement compared with its analogous monofluorinated phenylethylammonium iodide (PEAI) recently reported. This endows the system with high power conversion efficiency of 21.65% and extended operational stability after 1100 h of continuous illumination, outlining directions for future work.

Carbazole-Terminated Isomeric Hole-Transporting Materials for Perovskite Solar Cells.

A set of novel hole-transporting materials (HTMs) based on π-extension through carbazole units was designed and synthesized via a facile synthetic procedure. The impact of isomeric structural linking on their optical, thermal, electrophysical, and photovoltaic properties was thoroughly investigated by combining the experimental and simulation methods. Ionization energies of HTMs were measured and found to be suitable for a triple-cation perovskite active layer ensuring efficient hole injection. New materials were successfully applied in perovskite solar cells, which yielded a promising efficiency of up to almost 18% under standard 100 mW cm-2 global AM1.5G illumination and showed a better stability tendency outperforming that of 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene. This work provides guidance for the molecular design strategy of effective hole-conducting materials for perovskite photovoltaics and similar electronic devices.

Elucidating the Doping Mechanism in Fluorene-Dithiophene-Based Hole Selective Layer Employing Ultrahydrophobic Ionic Liquid Dopant.

Perovskite solar cells have set a new milestone in terms of efficiencies in the thin film photovoltaics category. Long-term stability of perovskite solar cells is of paramount importance but remains a challenging task. The lack of perovskite solar cells stability in real-time operating conditions erodes and impedes commercialization. Further improvements are essential with a view to delivering longer-lasting photovoltaic (PV) performances. An ideal path in this direction will be to identify novel dopants for boosting the conductivity and hole mobility of hole transport materials (HTMs), and by so doing, the usage of hygroscopic and deliquescent additive materials can be avoided. The present work demonstrates the employment of ionic liquids into a dissymmetric fluorene-dithiophene, FDT (2',7'-bis(bis(4-methoxyphenyl)amino) spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene]) based HTM to understand the doping mechanisms. N-Heterocyclic hydrophobic ionic liquid, 1-butyl-3-methylpyidinium bis(trifluoromethylsulfonyl)imide (BMPyTFSI) as p-type dopant for FDT was found to increase the conductivity of FDT, to higher geometrical capacitance, to facilitate homogeneous film formation, and to enhance device stability. Our findings open up a broad range of hole-transport materials to control the degradation of the underlying water-sensitive active layer by substituting a hygroscopic element.

Doped but Stable: Spirobisacridine Hole Transporting Materials for Hysteresis-Free and Stable Perovskite Solar Cells.

Four spirobisacridine (SBA) hole-transporting materials were synthesized and employed in perovskite solar cells (PSCs). The molecules bear electronically inert alkyl chains of different length and bulkiness, attached to in-plane N atoms of nearly orthogonal spiro-connected acridines. Di-p-methoxyphenylamine (DMPA) substituents tailored to the central SBA-platform define electronic properties of the materials mimicking the structure of the benchmark 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD), while the alkyl pending groups affect molecular packing in thin films and affect the long-term performance of PSCs. Devices with SBA-based hole transporting layers (HTL) attain efficiencies on par with spiro-MeOTAD. More importantly, solar cells with the new HTMs are hysteresis-free and demonstrate good operational stability, despite being doped as spiro-MeOTAD. The best performing MeSBA-DMPA retained 88% of the initial efficiency after a 1000 h aging test under constant illumination. The results clearly demonstrate that SBA-based compounds are potent candidates for a design of new HTMs for PSCs with improved longevity.

Crystal Orientation Drives the Interface Physics at Two/Three-Dimensional Hybrid Perovskites.

Combining halide perovskites with tailored dimensionality into two/three-dimensional (2D/3D) systems has revealed a powerful strategy to boost the performances of perovskite photovoltaics (PVs). Despite recent advances, a clear understanding of the intimate link between interface structure and physics is still missing, leading so far to a blind optimization of the 2D/3D PVs. Here, we reveal the impact of 2D/3D crystal alignment in driving interface charge-recombination dynamics. The 2D crystal growth and orientation are manipulated by specific fluorination of phenethylammonium (PEA), used here as the organic cation backbone of the 2D component. By means of time-resolved optoelectronic analysis from the femto- to microsecond regions, we demonstrate a static function of the 2D layer as an electron barrier and homogeneous surface passivant, together with a dynamic role in retarding back charge recombination. Our results reveal a crucial dependence of such beneficial effects with the 2D layer, leading to an enhanced open-circuit voltage (Voc), mostly attributed to the 2D phase which orients parallel on the 3D layer. Such findings provide a deep understanding and delineate precise guidelines for the smart design of multidimensional perovskite interfaces for advanced PVs and beyond.

Efficient Planar Perovskite Solar Cells Using Passivated Tin Oxide as an Electron Transport Layer.

Planar perovskite solar cells using low-temperature atomic layer deposition (ALD) of the SnO2 electron transporting layer (ETL), with excellent electron extraction and hole-blocking ability, offer significant advantages compared with high-temperature deposition methods. The optical, chemical, and electrical properties of the ALD SnO2 layer and its influence on the device performance are investigated. It is found that surface passivation of SnO2 is essential to reduce charge recombination at the perovskite and ETL interface and show that the fabricated planar perovskite solar cells exhibit high reproducibility, stability, and power conversion efficiency of 20%.

Approaches for Selective Synthesis of Ullazine Donor-Acceptor Systems.

Methods for effective synthesis for the four possible isomeric 3,9-diphenylullazine carboxaldehydes and reactive halogen intermediates are described. Ullazine donor-acceptor (D-A) dyes were studied using UV/Vis, photoluminescence (PL) spectroscopy and cyclic voltammetry. X-ray single crystal diffraction analysis independently confirmed the structures of two key intermediates. A D-A dye based on ullazine with dihexylmalonate acceptor was tested as a dopant-free hole-transporting material (HTM) in a perovskite solar cell, exhibiting promising power conversion efficiency (PCE) reaching 13.07 %.

Dopant-Free Hole-Transporting Materials for Stable and Efficient Perovskite Solar Cells.

Molecularly engineered novel dopant-free hole-transporting materials for perovskite solar cells (PSCs) combined with mixed-perovskite (FAPbI3 )0.85 (MAPbBr3 )0.15 (MA: CH3 NH3+ , FA: NH=CHNH3+ ) that exhibit an excellent power conversion efficiency of 18.9% under AM 1.5 conditions are investigated. The mobilities of FA-CN, and TPA-CN are determined to be 1.2 × 10-4 cm2 V-1 s-1 and 1.1 × 10-4 cm2 V-1 s-1 , respectively. Exceptional stability up to 500 h is measured with the PSC based on FA-CN. Additionally, it is found that the maximum power output collected after 1300 h remained 65% of its initial value. This opens up new avenue for efficient and stable PSCs exploring new materials as alternatives to Spiro-OMeTAD.

Femtosecond Charge-Injection Dynamics at Hybrid Perovskite Interfaces.

With a power conversion efficiency (PCE) exceeding 22 %, perovskite solar cells (PSCs) have thrilled photovoltaic research. However, the interface behavior is still not understood and is a hot topic of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, which makes perovskite interface physics intriguing. Herein, through femtosecond transient absorption spectroscopy with spectral coverage extending into the crucial IR region, the ultrafast interface-specific processes at standard and newly molecularly engineered perovskite interfaces in state-of-the-art PSCs are interrogated. Ultrafast interfacial charge injection occurs with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the timescale for the first step involved in the complex charge-transfer process. This is also true for 20 % efficient devices measured under real operation, for which the femtosecond injection is followed by a slower picosecond component. These findings provide compelling evidence for the femtosecond interfacial charge-injection step and demonstrate a robust method for the straightforward identification of interfacial non-equilibrium processes on the ultrafast timescale.

Low-Cost Perovskite Solar Cells Employing Dimethoxydiphenylamine-Substituted Bistricyclic Aromatic Enes as Hole Transport Materials.

The synthesis, characterization and photovoltaic performance of series of novel molecular hole transport materials (HTMs) based on bistricyclic aromatic enes (BAEs) are presented. The new derivatives were obtained following a simple and straightforward procedure from inexpensive starting reagents mimicking the synthetically challenging 9,9'-spirobifluorene moiety of the well-studied spiro-OMeTAD. The novel HTMs were tested in mixed cations and anions perovskite solar cells (PSCs) yielding a power conversion efficiency (PCE) of 19.2 % under standard global 100 mW cm-2 AM1.5G illumination using 9-{2,7-bis[bis(4-methoxyphenyl)amino]-9H-fluoren-9-ylidene}-N2 ,N2 ,N7 ,N7 -tetrakis(4-methoxyphenyl)-9H-thioxanthene-2,7-diamine (coded as KR374). The power conversion efficiency data confirms the easily attainable heteromerous fluorenylidenethioxanthene structure as valuable core for low-cost and highly efficient HTM design and paves the way towards cost-effective PSC technology.

Methoxydiphenylamine-substituted fluorene derivatives as hole transporting materials: role of molecular interaction on device photovoltaic performance.

The molecular structure of the hole transporting material (HTM) play an important role in hole extraction in a perovskite solar cells. It has a significant influence on the molecular planarity, energy level, and charge transport properties. Understanding the relationship between the chemical structure of the HTM's and perovskite solar cells (PSCs) performance is crucial for the continued development of the efficient organic charge transporting materials. Using molecular engineering approach we have constructed a series of the hole transporting materials with strategically placed aliphatic substituents to investigate the relationship between the chemical structure of the HTMs and the photovoltaic performance. PSCs employing the investigated HTMs demonstrate power conversion efficiency values in the range of 9% to 16.8% highlighting the importance of the optimal molecular structure. An inappropriately placed side group could compromise the device performance. Due to the ease of synthesis and moieties employed in its construction, it offers a wide range of possible structural modifications. This class of molecules has a great potential for structural optimization in order to realize simple and efficient small molecule based HTMs for perovskite solar cells application.

PbI2-HMPA Complex Pretreatment for Highly Reproducible and Efficient CH3NH3PbI3 Perovskite Solar Cells.

Interfacial engineering of the meso-TiO2 surface through a modified sequential deposition procedure involving a novel PbI2-HMPA complex pretreatment is conducted as a reproducible method for preparing MAPbI3 based perovskite solar cells providing the highest efficiencies yet reported with the polymer HTM layer. Grazing-incidence X-ray diffraction depth profiling confirms the formation of a perovskite film with a PbI2-rich region close to the electron transport layer (ETL) due to the strong interaction of HMPA with PbI2, which successfully retarded the dissolution of the PbI2 phase when depositing the perovskite layer on top. These results are further confirmed by energy-dispersive X-ray spectroscopy performed in a scanning transmission electron microscope, which reveals that the I/Pb ratio in samples treated with the complex is indeed reduced in the vicinity of the ETL contact when compared to samples without the treatment. The engineered interface leads to an average power conversion efficiency of 19.2% (reverse scan, standard deviation SD < 0.2) over 30 cells (best cell at 19.5% with high FF of 0.80).

Beneficial Role of Reduced Graphene Oxide for Electron Extraction in Highly Efficient Perovskite Solar Cells.

In this work we systematically investigated the role of reduced graphene oxide (rGO) in hybrid perovskite solar cells (PSCs). By mixing rGO within the mesoporous TiO2 (m-TiO2 ) matrix, highly efficient solar cells with power conversion efficiency values up to 19.54 % were realized. In addition, the boosted beneficial role of rGO with and without Li-treated m-TiO2 is highlighted, improving transport and injection of photoexcited electrons. This combined system may pave the way for further development and optimization of electron transport and collection in high efficiency PSCs.

Highly Efficient Perovskite Solar Cells Employing an Easily Attainable Bifluorenylidene-Based Hole-Transporting Material.

The 4,4'-dimethoxydiphenylamine-substituted 9,9'-bifluorenylidene (KR216) hole transporting material has been synthesized using a straightforward two-step procedure from commercially available and inexpensive starting reagents, mimicking the synthetically challenging 9,9'-spirobifluorene moiety of the well-studied spiro-OMeTAD. A power conversion efficiency of 17.8 % has been reached employing a novel HTM in a perovskite solar cells.

A dual-functional asymmetric squaraine-based low band gap hole transporting material for efficient perovskite solar cells.

We demonstrate for the first time an asymmetric squaraine-based low band-gap hole transporting material, which acted as both light harvesting and hole transporting layers in methylammonium lead triiodide perovskite solar cells. Opto-electrochemical characterization revealed extremely high molar extinction coefficients of the absorption bands in the low energy region and prominent space charge delocalization due to its electronically asymmetric nature. A suitable band alignment of the squaraine HOMO level with the valence band edge of the perovskite, and the conduction band of the TiO2 with LUMO of the perovskite allowed a cascade of hole extraction and electron injection, respectively. Red-shifted absorption was observed for both HTMs in thin films coated on the perovskite, and the optimized devices exhibited an impressive PCE of 14.7% under full sunlight illumination (100 mW cm(-2), AM1.5 G). The efficiency value is comparable to that of the devices using a state-of-the-art spiro-OMeTAD hole transport layer under similar conditions. Ambient stability after 300 h revealed that 88% of the initial efficiency remained for , and almost no change for , indicating that the devices had good long-term stability thus suggesting that the asymmetric squaraines have great potential as a dual-functional HTM for high performance perovskite solar cells.

Efficient Hole-Transporting Materials with Triazole Core for High-Efficiency Perovskite Solar Cells.

Efficient hole-transporting materials (HTMs), TAZ-[MeOTPA]2 and TAZ-[MeOTPATh]2 incorporating two electron-rich diphenylamino side arms, through direct linkage or thiophen bridges, respectively, on the C3- and C5-positions of a 4-phenyl-1,2,4-triazole core were synthesized. These synthetic HTMs with donor-acceptor type molecular structures exhibited effective intramolecular charge transfer for improving the hole-transporting properties. The structural modification of HTMs by thiophene bridging might increase intermolecular π-π stacking in the solid state and afford a better spectral response because of their increased π-conjugation length. Perovskite-based cells using TAZ-[MeOTPA]2 and TAZ-[MeOTPATh]2 as HTMs afforded high power conversion efficiencies of 10.9 % and 14.4 %, respectively, showing a photovoltaic performance comparable to that obtained using spiro-OMeTAD. These synthetically simple and inexpensive HTMs hold promise for replacing the more expensive spiro-OMeTAD in high-efficiency perovskite solar cells.

Molecular Engineering of Functional Materials for Energy and Opto-Electronic Applications.

This review presents an overview of the dedicated research directions of the Group for Molecular Engineering of Functional Materials (GMF). This includes molecular engineering aspects of sensitizers constructed from ruthenium complexes, organic molecules, porphyrins and phthalocyanines. Manipulation of organometal trihalide perovskites, and charge transporting materials for high performance perovskite solar cells and photo-detectors are also described. Controlling phosphorescence color, and quantum yields in iridium complexes by tailoring ligands for organic light emitting diodes are demonstrated. Efficient reduction of CO(2) to CO using molecular catalyst on a protected Cu(2)O photocathode, and cost-effective water-splitting cell using a high efficiency perovskite solar cell are presented.