Spiro‐Phenylpyrazole‐9,9′‐Thioxanthene Analogues as Hole‐Transporting Materials for Efficient Planar Perovskite Solar Cells

Perovskite solar cells have emerged as a promising technique for low‐cost, light weight, and highly efficient photovoltaics. However, they still largely rely on 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (Spiro‐OMeTAD) to serve as hole‐transporting materials (HTMs). Here, a series of HTMs with small molecular weight is designed, which are constructed on a spiro core involving phenylpyrazole and a second heteroaromatics, i.e., xanthene (O atom), thioxanthene (S atom), and acridine (N atom). Through varying from phenylpyrazole substituted xanthene ( PPyra‐XA ), thioxanthene ( PPyra‐TXA ), to acridine ( PPyra‐ACD ), their optical and electrochemical properties, hole mobilities, and the photovoltaic performance are optimized. As a consequence, PPyra‐TXA based device exhibits the highest power conversion efficiency (PCE) of 18.06%, outperforming that of Spiro‐OMeTAD (16.15%), which could be attributed to the enhancement of hole mobility exerted by the thioxanthene. In addition, the dopant‐free device shows PCE of 11.7%. These results open a new direction for designing spiro‐HTMs by simple modification of chemical structures.     

Molecular Engineering Using an Anthanthrone Dye for Low‐Cost Hole Transport Materials: A Strategy for Dopant‐Free,High‐Efficiency,and Stable Perovskite Solar Cells

In this report, highly efficient and humidity‐resistant perovskite solar cells (PSCs) using two new small molecule hole transporting materials (HTM) made from a cost‐effective precursor anthanthrone (ANT) dye, namely, 4,10‐bis(1,2‐dihydroacenaphthylen‐5‐yl)‐6,12‐bis(octyloxy)‐6,12‐dihydronaphtho[7,8,1,2,3‐nopqr]tetraphene (ACE‐ANT‐ACE) and 4,4′‐(6,12‐bis(octyloxy)‐6,12‐dihydronaphtho[7,8,1,2,3‐nopqr]tetraphene‐4,10‐diyl)bis(N,N‐bis(4‐methoxyphenyl)aniline) (TPA‐ANT‐TPA) are presented. The newly developed HTMs are systematically compared with the conventional 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamino)‐9,9′‐spirbiuorene (Spiro‐OMeTAD). ACE‐ANT‐ACE and TPA‐ANT‐TPA are used as a dopant‐free HTM in mesoscopic TiO₂/CH₃NH₃PbI₃/HTM solid‐state PSCs, and the performance as well as stability are compared with Spiro‐OMeTAD‐based PSCs. After extensive optimization of the metal oxide scaffold and device processing conditions, dopant‐free novel TPA‐ANT‐TPA HTM‐based PSC devices achieve a maximum power conversion efficiency (PCE) of 17.5% with negligible hysteresis. An impressive current of 21 mA cm^?2 is also confirmed from photocurrent density with a higher fill factor of 0.79. The obtained PCE of 17.5% utilizing TPA‐ANT‐TPA is higher performance than the devices prepared using doped Spiro‐OMeTAD (16.8%) as hole transport layer at 1 sun condition. It is found that doping of LiTFSI salt increases hygroscopic characteristics in Spiro‐OMeTAD; this leads to the fast degradation of solar cells. While, solar cells prepared using undoped TPA‐ANT‐TPA show dewetting and improved stability. Additionally, the new HTMs form a fully homogeneous and completely covering thin film on the surface of the active light absorbing perovskite layers that acts as a protective coating for underlying perovskite films. This breakthrough paves the way for development of new inexpensive, more stable, and highly efficient ANT core based lower cost HTMs for cost‐effective, conventional, and printable PSCs.     

The Importance of Pendant Groups on Triphenylamine‐Based Hole Transport Materials for Obtaining Perovskite Solar Cells with over 20% Efficiency

Tremendous progress has recently been achieved in the field of perovskite solar cells (PSCs) as evidenced by impressive power conversion efficiencies (PCEs); but the high PCEs of >20% in PSCs has so far been mostly achieved by using the hole transport material (HTM) spiro‐OMeTAD; however, the relatively low conductivity and high cost of spiro‐OMeTAD significantly limit its potential use in large‐scale applications. In this work, two new organic molecules with spiro[fluorene‐9,9′‐xanthene] (SFX)‐based pendant groups, X26 and X36, have been developed as HTMs. Both X26 and X36 present facile syntheses with high yields. It is found that the introduced SFX pendant groups in triphenylamine‐based molecules show significant influence on the conductivity, energy levels, and thin‐film surface morphology. The use of X26 as HTM in PSCs yields a remarkable PCE of 20.2%. In addition, the X26‐based devices show impressive stability maintaining a high PCE of 18.8% after 5 months of aging in controlled (20%) humidity in the dark. We believe that X26 with high device PCEs of >20% and simple synthesis show a great promise for future application in PSCs, and that it represents a useful design platform for designing new charge transport materials for optoelectronic applications.     

LiTFSI‐Free Spiro‐OMeTAD‐Based Perovskite Solar Cells with Power Conversion Efficiencies Exceeding 19%

To date, the most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) hole‐transporting material (HTM), which achieves optimum conductivity with the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. However, this additive along with its oxidation process leads to poor reproducibility and is detrimental to stability. Herein, a dicationic salt spiro‐OMeTAD(TFSI)₂, is employed as an effective p‐dopant to achieve power conversion efficiencies of 19.3% and 18.3% (apertures of 0.16 and 1.00 cm²) with excellent reproducibility in the absence of LiTFSI and air exposure. As far as it is known, these are the highest‐performing n–i–p PSCs without LiTFSI or air exposure. Comprehensive analysis demonstrates that precise control of the proportion of [spiro‐OMeTAD]⁺ directly provides high conductivity in HTM films with low series resistance, fast hole extraction, and lower interfacial charge recombination. Moreover, the spiro‐OMeTAD(TFSI)₂‐doped devices show improved stability, benefitting from well‐retained HTM morphology without forming aggregates or voids when tested under an ambient atmosphere. A facile approach is presented to fabricate highly efficient PSCs by replacing LiTFSI with spiro‐OMeTAD(TFSI)₂. Furthermore, this study provides an insight into the relationship between device performance and the HTM doping level.     

Di‐Spiro‐Based Hole‐Transporting Materials for Highly Efficient Perovskite Solar Cells

Hole‐transporting materials (HTMs) are essential for enabling highly efficient perovskite solar cells (PVSCs) to extract and transport the hole carriers. Among numerous HTMs that are studied so far, the single‐spiro‐based compounds are the most frequently used HTMs for achieving highly efficient PVSCs. In fact, all the new spiro‐based HTMs reported so far that render PVSCs over 20% are based on spiro[fluorene‐9,9′‐xanthene] or spiro [cyclopenta [2,1‐b:3,4b′]dithiophene‐4,9′‐fluorene] cores; therefore, there is a need to diversify the design of their structures for further improving their function and performance. In addition, the fundamental understanding of structure–performance relationships for the spiro‐based HTMs is still lagging, for example, how molecular configuration, spiro numbers, and heteroatoms in spiro‐rings impact the efficacy of HTMs. To address these needs, two novel H‐shaped HTMs, G1 and G2 based on the di‐spiro‐rings as the cores are designed and synthesized. The combined good film‐forming properties, better interactions with perovskite, slightly deeper highest occupied molecular orbital, higher mobility and conductivity, as well as more efficient charge transfer for G2 help devices reach a very impressive power conversion efficiency of 20.2% and good stability. This is the first report of demonstrating the feasibility of using di‐spiro‐based HTMs for highly efficient PVSCs.     

High‐Performance and Stable Perovskite Solar Cells Based on Dopant‐Free Arylamine‐Substituted Copper(II) Phthalocyanine Hole‐Transporting Materials

A power conversion efficiency (PCE) as high as 19.7% is achieved using a novel, low‐cost, dopant‐free hole transport material (HTM) in mixed‐ion solution‐processed perovskite solar cells (PSCs). Following a rational molecular design strategy, arylamine‐substituted copper(II) phthalocyanine (CuPc) derivatives are selected as HTMs, reaching the highest PCE ever reported for PSCs employing dopant‐free HTMs. The intrinsic thermal and chemical properties of dopant‐free CuPcs result in PSCs with a long‐term stability outperforming that of the benchmark doped 2,2′,7,7′‐Tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐Spirobifluorene (Spiro‐OMeTAD)‐based devices. The combination of molecular modeling, synthesis, and full experimental characterization sheds light on the nanostructure and molecular aggregation of arylamine‐substituted CuPc compounds, providing a link between molecular structure and device properties. These results reveal the potential of engineering CuPc derivatives as dopant‐free HTMs to fabricate cost‐effective and highly efficient PSCs with long‐term stability, and pave the way to their commercial‐scale manufacturing. More generally, this case demonstrates how an integrated approach based on rational design and computational modeling can guide and anticipate the synthesis of new classes of materials to achieve specific functions in complex device structures.     

Development of Dopant‐Free Organic Hole Transporting Materials for Perovskite Solar Cells

There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long‐term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′‐tetrakis(N,N′‐di‐pmethoxyphenylamino)‐9,9′‐spirbiuorene(Spiro‐OMeTAD) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). HTMs are essential for high‐performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping‐induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant‐free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low‐cost, effective, and scalable materials for devices with long‐term stability. It summarizes recent literature reports on non‐doped, alternative, and more stable HTMs used in PSCs as essential components for high‐efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant‐free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant‐free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.     

Copper Salts Doped Spiro‐OMeTAD for High‐Performance Perovskite Solar Cells

The development of effective and stable hole transporting materials (HTMs) is very important for achieving high‐performance planar perovskite solar cells (PSCs). Herein, copper salts (cuprous thiocyanate (CuSCN) or cuprous iodide (CuI)) doped 2,2,7,7‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (spiro‐OMeTAD) based on a solution processing as the HTM in PSCs is demonstrated. The incorporation of CuSCN (or CuI) realizes a p‐type doping with efficient charge transfer complex, which results in improved film conductivity and hole mobility in spiro‐OMeTAD:CuSCN (or CuI) composite films. As a result, the PCE is largely improved from 14.82% to 18.02% due to obvious enhancements in the cell parameters of short‐circuit current density and fill factor. Besides the HTM role, the composite film can suppress the film aggregation and crystallization of spiro‐OMeTAD films with reduced pinholes and voids, which slows down the perovskite decomposition by avoiding the moisture infiltration to some extent. The finding in this work provides a simple method to improve the efficiency and stability of planar perovskite solar cells.     

Tetrathienoanthracene and Tetrathienylbenzene Derivatives as Hole‐Transporting Materials for Perovskite Solar Cell

The synthesis and characterization of two related families of star‐shaped thiophene‐containing hole‐transporting materials (HTMs) based on fused tetrathienoanthracene and nonfused tetrathienylbenzene cores are reported. All of them are endowed with four terminal (4,4′‐dimethoxy)diphenylamino groups that are either linked directly to the core or showed a different type of bridges (i.e., thiophene‐phenyl or phenyl rings). The novel HTMs are tested in mixed‐ion perovskite (Cs_0.1FA_0.74MA_0.13PbI_2.48Br_0.39) solar cells, and power conversion efficiencies of up to 18.8% are measured under 1 sun irradiation, comparable with the efficiency obtained for the reference cell using 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene as an HTM.     

Low‐Cost N,N′‐Bicarbazole‐Based Dopant‐Free Hole‐Transporting Materials for Large‐Area Perovskite Solar Cells

Despite the recent unprecedented development of efficient dopant‐free hole transporting materials (HTMs) for high‐performance perovskite solar cells (PSCs) on small‐area devices (≤0.1 cm²), low‐cost dopant‐free HTMs for large‐area PSCs (≥1 cm²) with high power conversion efficiencies (PCEs) have rarely been reported. Herein, two novel HTMs, 3,3′,6,6′ (or 2,2′,7,7′)‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐N,N′‐bicarbazole (3,6 BCz‐OMeTAD or 2,7 BCz‐OMeTAD), are synthesized via an extremely simple route from very cheap raw materials. Owing to their excellent film‐forming abilities and matching energy levels, 3,6 BCz‐OMeTAD and 2,7 BCz‐OMeTAD can be successfully employed as a perfect ultrathin (≈30 nm) hole transporting layer in large‐area PSCs up to 1 cm². The 3,6 BCz‐OMeTAD and 2,7 BCz‐OMeTAD based large‐area PSCs show highest PCEs up to 17.0% and 17.6%, respectively. More importantly, high performance large‐area PSCs based on 2,7 BCz‐OMeTAD retain 90% of the initial efficiency after 2000 h storage in an ambient environment without encapsulation.     

Integrated Design of Organic Hole Transport Materials for Efficient Solid‐State Dye‐Sensitized Solar Cells

A series of triphenylamine‐based small molecule organic hole transport materials (HTMs) with low crystallinity and high hole mobility are systematically investigated in solid‐state dye‐sensitized solar cells (ssDSCs). By using the organic dye LEG4 as a photosensitizer, devices with X3 and X35 as the HTMs exhibit desirable power conversion efficiencies (PCEs) of 5.8% and 5.5%, respectively. These values are slightly higher than the PCE of 5.4% obtained by using the state‐of‐the‐art HTM Spiro‐OMeTAD. Meanwhile, transient photovoltage decay measurement is used to gain insight into the complex influences of the HTMs on the performance of devices. The results demonstrate that smaller HTMs induce faster electron recombination in the devices and suggest that the size of a HTM plays a crucial role in device performance, which is reported for the first time.     

Enhanced Stability of Perovskite Solar Cells Incorporating Dopant‐Free Crystalline Spiro‐OMeTAD Layers by Vacuum Sublimation

The main handicap still hindering the eventual exploitation of organometal halide perovskite‐based solar cells is their poor stability under prolonged illumination, ambient conditions, and increased temperatures. This article shows for the first time the vacuum processing of the most widely used solid‐state hole conductor (SSHC), i.e., the Spiro‐OMeTAD [2,2′,7,7′‐tetrakis (N,N‐di‐p‐methoxyphenyl‐amine) 9,9′‐spirobifluorene], and how its dopant‐free crystalline formation unprecedently improves perovskite solar cell (PSC) stability under continuous illumination by about two orders of magnitude with respect to the solution‐processed reference and after annealing in air up to 200 °C. It is demonstrated that the control over the temperature of the samples during the vacuum deposition enhances the crystallinity of the SSHC, obtaining a preferential orientation along the π–π stacking direction. These results may represent a milestone toward the full vacuum processing of hybrid organic halide PSCs as well as light‐emitting diodes, with promising impacts on the development of durable devices. The microstructure, purity, and crystallinity of the vacuum sublimated Spiro‐OMeTAD layers are fully elucidated by applying an unparalleled set of complementary characterization techniques, including scanning electron microscopy, X‐ray diffraction, grazing‐incidence small‐angle X‐ray scattering and grazing‐incidence wide‐angle X‐ray scattering, X‐ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy.     

The Effect of Hole Transport Material Pore Filling on Photovoltaic Performance in Solid‐State Dye‐Sensitized Solar Cells

A detailed investigation of the effect of hole transport material (HTM) pore filling on the photovoltaic performance of solid‐state dye‐sensitized solar cells (ss‐DSCs) and the specific mechanisms involved is reported. It is demonstrated that the efficiency and photovoltaic characteristics of ss‐DSCs improve with the pore filling fraction (PFF) of the HTM, 2,2’,7,7’‐tetrakis‐(N, N ‐di‐ p ‐methoxyphenylamine)9,9’‐spirobifluorene(spiro‐OMeTAD). The mechanisms through which the improvement of photovoltaic characteristics takes place were studied with transient absorption spectroscopy and transient photovoltage/photocurrent measurements. It is shown that as the spiro‐OMeTAD PFF is increased from 26% to 65%, there is a higher hole injection efficiency from dye cations to spiro‐OMeTAD because more dye molecules are covered with spiro‐OMeTAD, an order‐of‐magnitude slower recombination rate because holes can diffuse further away from the dye/HTM interface, and a 50% higher ambipolar diffusion coefficient due to an improved percolation network. Device simulations predict that if 100% PFF could be achieved for thicker devices, the efficiency of ss‐DSCs using a conventional ruthenium‐dye would increase by 25% beyond its current value.     

Solution Processable Inorganic–Organic Double‐Layered Hole Transport Layer for Highly Stable Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have reached their highest efficiency with 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). However, this material can cause problems with respect to reproducibility and stability. Herein, a solution‐processable inorganic–organic double layer based on tungsten oxide (WO₃) and spiro‐OMeTAD is reported as a hole transport layer in PSCs. The device equipped with a WO₃/spiro‐OMeTAD layer achieves the highest efficiency (21.44%) in the tin (IV) oxide planar structure. The electronic properties of the double layer are thoroughly analyzed using photoluminescence, space‐charge–limited current, and electrochemical impedance spectroscopy. The WO₃/spiro‐OMeTAD layer exhibits better hole extraction ability and faster hole mobility. The WO₃ layer particularly improves the open‐circuit voltage (V_OC) by lowering the quasi‐Fermi energy level for holes and reducing charge recombination, resulting in high V_OC (1.17 V in the champion cell). In addition, the WO₃ layer as a scaffold layer promotes the formation of a uniform and pinhole‐free spiro‐OMeTAD overlayer in the WO₃/spiro‐OMeTAD layer. High stability under thermal and humid conditions stems from this property. The study presents a facile approach for improving the efficiency and stability of PSCs by stacking an organic layer on an inorganic layer.     

A Dopant‐Free Polymeric Hole‐Transporting Material Enabled High Fill Factor Over 81% for Highly Efficient Perovskite Solar Cells

Although perovskite solar cells (PVSCs) have achieved rapid progress in the past few years, most of the high‐performance device results are based on the doped small molecule hole‐transporting material (HTM), spiro‐OMeTAD, which affects their long‐term stability. In addition, some defects from under‐coordinated Pb atoms on the surface of perovskite films can also result in nonradiative recombination to affect device performance. To alleviate these problems, a dopant‐free HTM based on a donor‐acceptor polymer, PBT1‐C, synthesized from the copolymerization between the benzodithiophene and 1,3‐bis(4‐(2‐ethylhexyl)thiophen‐2‐yl)‐5,7‐bis(2‐alkyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione units is introduced. PBT1‐C not only possesses excellent hole mobility, but is also able to passivate the surface traps of the perovskite films. The derived PVSC shows a high power conversion efficiency of 19.06% with a very high fill factor of 81.22%, which is the highest reported for dopant‐free polymeric HTMs. The results from photoluminescence and trap density of states measurements validate that PBT1‐C can effectively passivate both surface and grain boundary traps of the perovskite.     

Spray Deposition of Silver Nanowire Electrodes for Semitransparent Solid‐State Dye‐Sensitized Solar Cells

Transparent top electrodes for solid‐state dye‐sensitized solar cells (ssDSCs) allow for fabrication of mechanically stacked ssDSC tandems, partially transparent ssDSCs for building integration, and ssDSCs on metal foil substrates. A solution‐processed, highly transparent, conductive electrode based on PEDOT:PSS [poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)] and spray‐deposited silver nanowires (Ag NWs) is developed as an effective top contact for ssDSCs. The electrode is solution‐deposited using conditions and solvents that do not damage or dissolve the underlying ssDSC and achieves high performance: a peak transmittance of nearly 93% at a sheet resistance of 18 Ω/square – all without any annealing that would harm the ssDSC. The role of the PEDOT:PSS in the electrode is twofold: it ensures ohmic contact between the ssDSC 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)9,9′‐spirobifluorene (Spiro‐OMeTAD) overlayer and the silver nanowires and it decreases the series resistance of the device. Semitransparent ssDSCs with D35 dye fabricated using this Ag NW/PEDOT:PSS transparent electrode show power conversion efficiencies of 3.6%, nearly as high as a reference device using an evaporated silver electrode (3.7%). In addition, the semitransparent ssDSC shows high transmission between 700–1100 nm, a necessity for use in efficient tandem devices. Such an electrode, in combination with efficient ssDSCs or hybrid perovskite‐sensitized solar cells, can allow for the fabrication of efficient, cost‐effective tandem photovoltaics.     

Efficient and Hysteresis‐Free Perovskite Solar Cells Based on a Solution Processable Polar Fullerene Electron Transport Layer

Fullerene derivatives, which possess extraordinary geometric shapes and high electron affinity, have attracted significant attention for thin film technologies. This study demonstrates an important photovoltaic application using carboxyl‐functionalized carbon buckyballs, C60 pyrrolidine tris‐acid (CPTA), to fabricate electron transport layers (ETLs) that replace traditional metal oxide‐based ETLs in efficient and stable n‐i‐p‐structured planar perovskite solar cells (PSCs). The uniform CPTA film is covalently anchored onto the surface of indium tin oxide (ITO), significantly suppressing hysteresis and enhancing the flexural strength in the CPTA‐modified PSCs. Moreover, solution‐processable CPTA‐based ETLs also enable the fabrication of lightweight flexible PSCs. The maximum‐performing device structures composed of ITO/CPTA/CH₃NH₃PbI₃/2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)/Au yield power conversion efficiencies of more than 18% on glass substrates and up to 17% on flexible substrates. These results indicate that the CPTA layers provide new opportunities for solution‐processed organic ETLs by substantially simplifying the procedure for fabricating PSCs for portable applications.     

High‐Efficiency Perovskite Solar Cells Using Molecularly Engineered,Thiophene‐Rich,Hole‐Transporting Materials: Influence of Alkyl Chain Length on Power Conversion Efficiency

The synthesis and characterization of a series of novel small‐molecule hole‐transporting materials (HTMs) based on an anthra[1,2‐b:4,3‐b′:5,6‐b′′:8,7‐b′′′]tetrathiophene (ATT) core are reported. The new compounds follow an easy synthetic route and have no need of expensive purification steps. The novel HTMs are tested in perovskite solar cells and power conversion efficiencies (PCE) of up to 18.1% under 1 sun irradiation are measured. This value is comparable with the 17.8% efficiency obtained using 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene as a reference compound. Similarly, a significant quenching of the photoluminescence in the first nanosecond is observed, indicative of effective hole transfer. Additionally, the influence of introducing aliphatic alkyl chains acting as solubilizers on the device performance of the ATT molecules is investigated. Replacing the methoxy groups on the triarylamine sites by butoxy‐, hexoxy‐, or decoxy‐substituents greatly improves the solubility of the compounds without changing the energy levels, yet at the same time significantly decreasing the conductivity as well as the PCE, 17.3% for ATT‐OBu, 15.7% for ATT‐OHex, and 9.7% for ATT‐ODec.     

Rationalizing the Molecular Design of Hole‐Selective Contacts to Improve Charge Extraction in Perovskite Solar Cells

Two new hole selective materials (HSMs) based on dangling methylsulfanyl groups connected to the C‐9 position of the fluorene core are synthesized and applied in perovskite solar cells. Being structurally similar to a half of Spiro‐OMeTAD molecule, these HSMs (referred as FS and DFS) share similar redox potentials but are endowed with slightly higher hole mobility, due to the planarity and large extension of their structure. Competitive power conversion efficiency (up to 18.6%) is achieved by using the new HSMs in suitable perovskite solar cells. Time‐resolved photoluminescence decay measurements and electrochemical impedance spectroscopy show more efficient charge extraction at the HSM/perovskite interface with respect to Spiro‐OMeTAD, which is reflected in higher photocurrents exhibited by DFS/FS‐integrated perovskite solar cells. Density functional theory simulations reveal that the interactions of methylammonium with methylsulfanyl groups in DFS/FS strengthen their electrostatic attraction with the perovskite surface, providing an additional path for hole extraction compared to the sole presence of methoxy groups in Spiro‐OMeTAD. Importantly, the low‐cost synthesis of FS makes it significantly attractive for the future commercialization of perovskite solar cells.     

Few‐Layer MoS2 Flakes as Active Buffer Layer for Stable Perovskite Solar Cells

Solution‐processed few‐layer MoS₂ flakes are exploited as an active buffer layer in hybrid lead–halide perovskite solar cells (PSCs). Glass/FTO/compact‐TiO₂/mesoporous‐TiO₂/CH₃NH₃PbI₃/MoS₂/Spiro‐OMeTAD/Au solar cells are realized with the MoS₂ flakes having a twofold function, acting both as a protective layer, by preventing the formation of shunt contacts between the perovskite and the Au electrode, and as a hole transport layer from the perovskite to the Spiro‐OMeTAD. As prepared PSC demonstrates a power conversion efficiency (η) of 13.3%, along with a higher lifetime stability over 550 h with respect to reference PSC without MoS₂ (Δη/η = ?7% vs. Δη/η = ?34%). Large‐area PSCs (1.05 cm² active area) are also fabricated to demonstrate the scalability of this approach, achieving η of 11.5%. Our results pave the way toward the implementation of MoS₂ as a material able to boost the shelf life of large‐area perovskite solar cells in view of their commercialization.