Dithieno[3,2‐b:2′,3′‐d]pyrrol‐Cored Hole Transport Material Enabling Over 21% Efficiency Dopant‐Free Perovskite Solar Cells

Dopant‐free hole transport materials (HTMs) are essential for commercialization of perovskite solar cells (PSCs). However, power conversion efficiencies (PCEs) of the state‐of‐the‐art PSCs with small molecule dopant‐free HTMs are below 20%. Herein, a simple dithieno[3,2‐b:2′,3′‐d]pyrrol‐cored small molecule, DTP‐C6Th, is reported as a promising dopant‐free HTM. Compared with commonly used spiro‐OMeTAD, DTP‐C6Th exhibits a similar energy level, a better hole mobility of 4.18 × 10^?4 cm² V^?1 s^?1, and more efficient hole extraction, enabling efficient and stable PSCs with a dopant‐free HTM. With the addition of an ultrathin poly(methyl methacrylate) passivation layer and properly tuning the composition of the perovskite absorber layer, a champion PCE of 21.04% is achieved, which is the highest value for small molecule dopant‐free HTM based PSCs to date. Additionally, PSCs using the DTP‐C6Th HTM exhibit significantly improved long‐term stability compared with the conventional cells with the metal additive doped spiro‐OMeTAD HTM. Therefore, this work provides a new candidate and effective device engineering strategy for achieving high PCEs with dopant‐free HTMs.     

Hole Transport Materials Based on 6,12‐Dihydroindeno[1,2‐b]fluorine with Different Periphery Groups: A New Strategy for Dopant‐Free Perovskite Solar Cells

Although several hole‐transporting materials (HTMs) have been designed to obtain perovskite solar cells (PSCs) devices with high performance, the dopant‐free HTMs for efficient and stable PSCs remain rare. Herein, a rigid planar 6,12‐dihydroindeno[1,2‐b]fluorine (IDF) core with different numbers of bulky periphery groups to construct dopant‐free HTMs of IDF‐SFXPh, IDF‐DiDPA, and IDF‐TeDPA is modified. Thanks to the contributions of the planar IDF core and the twisted SFX periphery groups, the dopant‐free IDF‐SFXPh‐based PSCs device achieves a device performance of 17.6%, comparable to the doped 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based device (17.6%), with much enhanced device stability under glovebox and ambient conditions.     

Heteroatom Effect on Star‐Shaped Hole‐Transporting Materials for Perovskite Solar Cells

Three new star‐shaped hole‐transporting materials (HTMs) incorporating benzotripyrrole, benzotrifuran, and benzotriselenophene central cores endowed with three‐armed triphenylamine moieties ( BTP‐1 , BTF‐1 , and BTSe‐1 , respectively) are designed, synthesized, and implemented in perovskite solar cells (PSCs). The impact that the heteroatom‐containing central scaffold has on the electrochemical and photophysical properties, as well as on the photovoltaic performance, is systematically investigated and compared with their sulfur‐rich analogue ( BTT‐3 ). The new HTMs exhibit suitable highest‐occupied molecular orbitals (HOMO) levels regarding the valence band of the perovskite, which ensure efficient hole extraction at the perovskite/HTM interface. The molecular structures of BTF‐1 , BTT‐3 , and BTSe‐1 are fully elucidated by single‐crystal X‐ray crystallography as toluene solvates. The optimized (FAPbI₃)_0.85(MAPbBr₃)_0.15‐based perovskite solar cells employing the tailor‐made, chalcogenide‐based HTMs exhibit remarkable power conversion efficiencies up to 18.5%, which are comparable to the devices based on the benchmark spiro‐OMeTAD. PSCs with BTP‐1 exhibit a more limited power conversion efficiency of 15.5%, with noticeable hysteresis. This systematic study indicates that chalcogenide‐based derivatives are promising HTM candidates to compete efficiently with spiro‐OMeTAD.     

Polyfluorene Derivatives are High‐Performance Organic Hole‐Transporting Materials for Inorganic−Organic Hybrid Perovskite Solar Cells

Photovoltaics based on organic?inorganic perovskites offer new promise to address the contemporary energy and environmental issues. These solar cells have so far largely relied on small‐molecule hole transport materials such as spiro‐OMeTAD, which commonly suffer from high cost and low mobility. In principle, polyfluorene copolymers can be an ideal alternative to spiro‐OMeTAD, given their low price, high hole mobility and good processability, but this potential has not been explored. Herein, polyfluorene derived polymers‐TFB and PFB, which contain fluorine and arylamine groups, are demonstrated and can indeed rival or even outperform spiro‐OMeTAD as efficient hole‐conducting materials for perovskite solar cells. In particular, under the one‐step perovskite deposition condition, TFB achieves a 10.92% power conversion efficiency that is considerably higher than that with spiro‐OMeTAD (9.78%), while using the two‐step perovskite deposition method, about 13% efficient solar cells with TFB (12.80%) and spiro‐OMeTAD (13.58%) are delivered. Photo­luminescence reveals the efficient hole extraction and diffusion at the interface between CH₃NH₃PbI₃ and the hole conducting polymer. Impedance spectroscopy uncovers the higher electrical conductivity and lower series resistance than spiro‐OMeTAD, accounting for the significantly higher fill factor, photocurrent and open‐circuit voltage of the TFB‐derived cells than with spiro‐MeOTAD.     

Multifunctional Molecular Design of a New Fulleropyrrolidine Electron Transport Material Family Engenders High Performance of Perovskite Solar Cells

[6,6]-phenyl-C₆₁-butyric acid methyl ester remains indispensable as the electron transport material (ETM) for perovskite solar cells (PSCs), but its synthesis involves complicated multisteps with low productivity. In contrast, the potential of synthesizing simpler fulleropyrrolidine derivatives has long been overlooked, and little has been understood regarding their structure-dependent effects on photovoltaic (PV) performance. Herein, seven novel fulleropyrrolidine derivatives (F1–F7) are deliberately designed, synthesized, and comprehensively characterized in both solution and thin-film states and subsequently investigated as ETMs for PSCs. Notably, the F4 delivers the highest power conversion efficiencies over 20% of devices, which surpass all reported fulleropyrrolidine ETMs due to its optimal photoelectric property. Moreover, the structure-dependent effects of the fullerenes on PV parameters are uncovered, including solubility, intermolecular interaction, packing structure, and charge-transfer ability, which can guide the future design of high-performance and stable fullerene ETMs for PSCs.     

Pyrite‐Based Bi‐Functional Layer for Long‐Term Stability and High‐Performance of Organo‐Lead Halide Perovskite Solar Cells

Organo‐lead halide perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for solar cell applications. Recently, a dramatic increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. However, as of now, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under conventional working conditions, including moist conditions. Furthermore, conventional organic HTMs are normally expensive because material synthesis and purification are complicated. It is herein reported, for the first time, octadecylamine‐capped pyrite nanoparticles (ODA‐FeS₂ NPs) as a bi‐functional layer (charge extraction layer and moisture‐proof layer) for organo‐lead halide PSCs. FeS₂ is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. A bi‐functional layer based on ODA‐FeS₂ NPs shows excellent hole transport ability and moisture‐proof performance. Through this approach, the best‐performing device with ODA‐FeS₂ NPs‐based bi‐functional layer shows a power conversion efficiency of 12.6% and maintains stable photovoltaic performance in 50% relative humidity for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.     

High Efficiency Planar p‐i‐n Perovskite Solar Cells Using Low‐Cost Fluorene‐Based Hole Transporting Material

For commercial applications, it is a challenge to find suitable and low‐cost hole‐transporting material (HTM) in perovskite solar cells (PSCs), where high efficiency spiro‐OMeTAD and PTAA are expensive. A HTM based on 9,9‐dihexyl‐9H‐fluorene and N,N‐di‐p‐methylthiophenylamine (denoted as FMT) is designed and synthesized. High‐yield FMT with a linear structure is synthesized in two steps. The dopant‐free FMT‐based planar p‐i‐n perovskite solar cells (pp‐PSCs) exhibit a high power conversion efficiency (PCE) of 19.06%, which is among the highest PCEs reported for the pp‐PSCs based on organic HTM. For comparison, a PEDOT:PSS HTM‐based pp‐PSC is fabricated under the same conditions, and its PCE is found to be 13.9%.     

Fast and Controllable Electric‐Field‐Assisted Reactive Deposited Stable and Annealing‐Free Perovskite toward Applicable High‐Performance Solar Cells

Recently, organic–inorganic hybrid perovskite materials have drawn great attention for their outstanding performance in high‐efficiency solar cells. Successful synthesis has been realized either in solution‐based chemical deposition or vapor deposition. However, conflicts have never ceased among quality control, growth rate, process complexity, and instrument requirement, which have limited their development toward real applications. In this work, the first electrochemical fabrication of perovskite toward high‐efficiency and scalable perovskite solar cells (PSCs) is established. The morphology and crystallization of the CH₃NH₃PbI₃ film can be effectively controlled by simply modulating a few physical parameters. A detailed study on its optoelectronic properties reveals significantly improved film quality and interfacial conditions. Aided by this, the total process does not require standard annealing, which greatly reduces the total growth time from hours to minutes. Up to now, an efficiency of 15.65% has been achieved in planar PSCs under 1 sun AM 1.5 condition, with small hysteresis and efficiency loss under longtime exposure to air. Moreover, high efficiency (10.45%) can be easily attained for large cells (2 cm²). This result will hopefully facilitate research for applicable high‐efficiency PSCs and other multicomponent materials as well.     

Efficient and Stable Mesoscopic Perovskite Solar Cells Using PDTITT as a New Hole Transporting Layer

Stability is the main challenge in the field of organic–inorganic perovskite solar cells (PSCs). Finding low‐cost and stable hole transporting layer (HTL) is an effective strategy to address this issue. Here, a new donor polymer, poly(5,5‐didecyl‐5H‐1,8‐dithia‐as‐indacenone‐alt‐thieno[3,2‐b]thiophene) (PDTITT), is synthesized and employed as an HTL in PSCs, which has a suitable band alignment with respect to the double‐A cation perovskite film. Using PDTITT, the hole extraction in PSCs is greatly improved as compared to commonly used HTLs such as 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD), addressing the hysteresis issue. After careful optimization, an efficient PSC is achieved based on mesoscopic TiO₂ electron transporting layer with a maximum power conversion efficiency (PCE) of 18.42% based on PDTITT HTL, which is comparable with spiro‐OMeTAD‐based PSC (19.21%). Since spiro‐based PSCs suffer from stability issue, the operational stability in the PSC with PDTITT HTL is studied. It is found that the device with PDTITT retains 88% of its initial PCE value after 200 h under illumination, which is better than the spiro‐based PSC (54%).     

Defect Passivation in Perovskite Solar Cells by Cyano‐Based π‐Conjugated Molecules for Improved Performance and Stability

Defects at the surface and grain boundaries of metal–halide perovskite films lead to performance losses of perovskite solar cells (PSCs). Here, organic cyano‐based π‐conjugated molecules composed of indacenodithieno[3,2‐b]thiophene (IDTT) are reported and it is found that their cyano group can effectively passivate such defects. To achieve a homogeneous distribution, these molecules are dissolved in the antisolvent, used to initiate the perovskite crystallization. It is found that these molecules are self‐anchored at the grain boundaries due to their strong binding to undercoordinated Pb²⁺. On a device level, this passivation scheme enhances the charge separation and transport at the grain boundaries due to the well‐matched energetic levels between the passivant and the perovskite. Consequently, these benefits contribute directly to the achievement of power conversion efficiencies as high as 21.2%, as well as the improved environmental and thermal stability of the PSCs. The surface treatment provides a new strategy to simultaneously passivate defects and enhance charge extraction/transport at the device interface by manipulating the anchoring groups of the molecules.     

Aza[5]helicene Rivals N‐Annulated Perylene as π‐Linker of D−π−D Typed Hole‐Transporters for Perovskite Solar Cells

The superior role of helical π‐linkers is demonstrated for the design of donor?π linker?donor typed molecular semiconductors in perovskite solar cells (PSCs). Flat N‐annulated perylene (NP) and contorted aza[5]helicene (A5H) are side‐functionalized with methoxyphenyl and end‐capped with dimethoxydiphenylamine electron‐donor to afford two small‐molecule hole‐transporters J3 and J4. For methoxyphenyl functionalized π‐linkers, intermolecular π???π interactions in planar NP exist more extensively than those in helical A5H. However, for the dimethoxydiphenylamine derived hole‐transporters with high highest occupied molecular orbital energy levels, a part of the π???π interaction remains for J4 with A5H, while this desirable effect for charge transport is completely deprived for J3 with NP. Thus, the theoretically predicted hole mobility of J4 single‐crystal is even over two times higher than that of J3 one. Because of the larger size of the molecular aggregate, the hole mobility of the spin‐coated J4 thin film is also over three times as high as that of the J3 analog. Due to the reduced transport resistance and enhanced recombination resistance, PSCs with J4 exhibit a power conversion efficiency of 21.0% at standard air mass 1.5 global conditions, which is higher than that of 19.4% with J3 and that of 20.3% with spiro‐OMeTAD control.     

Inorganic Electron Transport Materials in Perovskite Solar Cells

In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to the substantial efforts in improving the power conversion efficiency from 3.8% to 25.5% for single-junction devices and even perovskite-silicon tandems have reached 29.15%. This is a result of improvement in composition, solvent, interface, and dimensionality engineering. Furthermore, the long-term stability of PSCs has also been significantly improved. Such rapid developments have made PSCs a competitive candidate for next-generation photovoltaics. The electron transport layer (ETL) is one of the most important functional layers in PSCs, due to its crucial role in contributing to the overall performance of devices. This review provides an up-to-date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. The three most prevalent inorganic ETMs (TiO₂, SnO_2, and ZnO) are examined with a focus on the effects of synthesis and preparation methods, as well as an introduction to their application in tandem devices. The emerging trends in inorganic ETMs used for PSC research are also reviewed. Finally, strategies to optimize the performance of ETL in PSCs, effects the ETL has on J–V hysteresis phenomenon and long-term stability with an outlook on current challenges and further development are discussed.     

Water‐Soluble Triazolium Ionic‐Liquid‐Induced Surface Self‐Assembly to Enhance the Stability and Efficiency of Perovskite Solar Cells

Despite being a promising candidate for next‐generation photovoltaics, perovskite solar cells (PSCs) exhibit limited stability that hinders their practical application. In order to improve the humidity stability of PSCs, herein, a series of ionic liquids (ILs) “1‐alkyl‐4‐amino‐1,2,4‐triazolium” (termed as RATZ; R represents alkyl chain, and ATZ represents 4‐amino‐1,2,4‐triazolium) as cations are designed and used as additives in methylammonium lead iodide (MAPbI₃) perovskite precursor solution, obtaining triazolium ILs‐modified PSCs for the first time (termed as MA/RATZ PSCs). As opposed to from traditional methods that seek to improve the stability of PSCs by functionalizing perovskite film with hydrophobic molecules, humidity‐stable perovskite films are prepared by exploiting the self‐assembled monolayer (SAM) formation of water‐soluble triazolium ILs on a hydrophilic perovskite surface. The mechanism is validated by experimental and theoretical calculation. This strategy means that the MA/RATZ devices exhibit good humidity stability, maintaining around 80% initial efficiency for 3500 h under 40 ± 5% relative humidity. Meanwhile, the MA/RATZ PSCs exhibit enhanced thermal stability and photostability. Tuning the molecule structure of the ILs additives achieves a maximum power conversion efficiency (PCE) of 20.03%. This work demonstrates the potential of using triazolium ILs as additives and SAM and molecular design to achieve high performance PSCs.     

Extremely Low‐Threshold Amplified Spontaneous Emission of 9,9′‐Spirobifluorene Derivatives and Electroluminescence from Field‐Effect Transistor Structure

By doping 2,7‐bis[4‐(N‐carbazole)phenylvinyl]‐9,9′‐spirobifluorene (spiro‐SBCz) into a wide energy gap 4,4′‐bis(9‐carbazole)‐2,2′‐biphenyl (CBP) host, we demonstrate an extremely low ASE threshold of E_th = (0.11 ± 0.05) μJ cm^–2 (220 W cm^–2) which is the lowest ASE threshold ever reported. In addition, we confirmed that the spiro‐SBCz thin film functions as an active light emitting layer in organic light‐emitting diode (OLED) and a field‐effect transistor (FET). In particular, we succeeded to obtain linear electroluminescence in the FET structure which will be useful for future organic laser diodes.     

Organic Ionic Plastic Crystals as Hole Transporting Layer for Stable and Efficient Perovskite Solar Cells

Organic ionic plastic crystals (OIPCs) are synthesized through a simple metal‐free, cost‐effective approach. The strategized synchronization of electron‐rich phenoxazine with benzimidazolium iodide (OIPC‐I) and bromide (OIPC‐Br) salts lead to enhanced hole mobility and conductivity of OIPCs which is suitable for an efficient alternative to conventional organic hole transporting materials (HTMs) for stable perovskite solar cells (PSCs). The fabricated PSCs with OIPC‐I as hole transporting layer yielded a power conversion efficiency of 15.0% and 18.1% without and with additive (Li salt) respectively, which are comparable with spiro‐OMeTAD based devices prepared under similar conditions. Furthermore, the PSCs with OIPCs show good stability compared to the spiro‐OMeTAD with or without additives. Here, first time benzimidazolium‐based OIPCs have been used as an alternative organic HTM for perovskite solar cells, which opens a window for the design of effective OIPCs for highly efficient PSCs with long‐term stability.     

Self‐Crystallized Multifunctional 2D Perovskite for Efficient and Stable Perovskite Solar Cells

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.     

Efficient 4,4′,4″‐tris(3‐methylphenylphenylamino)triphenylamine (m‐MTDATA) Hole Transport Layer in Perovskite Solar Cells Enabled by Using the Nonstoichiometric Precursors

To overcome the drawbacks of the acidic and hygroscopic nature of the poly(3,4‐ethylenedio‐xythiophene):poly(styrenesulfonate) hole transport layer (HTL), the p‐type polymeric hole transport materials have attracted great attention and have been applied into perovskite solar cells. Here, a starburst amine molecule 4,4′,4″‐tris(3‐methylphenylphenylamino)triphenylamine (m‐MTDATA) without any additive is demonstrated as an effective hole transport material in perovskite solar cells. Meanwhile, considering the different surface affinity of precursor's composition on m‐MTDATA, the influence of the molar ratios between lead iodine (PbI₂) and methylammonium iodide in precursor solution on the perovskite characteristics and device performance is investigated in‐depth. Ultimately, an enhanced efficiency of 17.73% and a high fill factor of 79.6% are achieved, which attribute to the strong passivation effect of traps and small resistance loss from appropriate unreacted PbI₂ left in the perovskite layer. This work not only provides a remarkable HTL, but also reveals that the adjustment of precursor ratio is necessary for the one‐step solution approach, because the affinities of precursor's composition may be different with the underlying transport layers.     

Improving Performance and Stability of Flexible Planar‐Heterojunction Perovskite Solar Cells Using Polymeric Hole‐Transport Material

For realizing flexible perovskite solar cells (PSCs), it is important to develop low‐temperature processable interlayer materials with excellent charge transporting properties. Herein, a novel polymeric hole‐transport material based on 1,4‐bis(4‐sulfonatobutoxy)benzene and thiophene moieties (PhNa‐1T) and its application as a hole‐transport layer (HTL) material of high‐performance inverted‐type flexible PSCs are introduced. Compared with the conventionally used poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the incorporation of PhNa‐1T into HTL of the PSC device is demonstrated to be more effective for improving charge extraction from the perovskite absorber to the HTL and suppressing charge recombination in the bulk perovskite and HTL/perovskite interface. As a result, the flexible PSC using PhNa‐1T achieves high photovoltaic performances with an impressive power conversion efficiency of 14.7%. This is, to the best of our knowledge, among the highest performances reported to date for inverted‐type flexible PSCs. Moreover, the PhNa‐1T‐based flexible PSC shows much improved stability under an ambient condition than PEDOT:PSS‐based PSC. It is believed that PhNa‐1T is a promising candidate as an HTL material for high‐performance flexible PSCs.     

All‐Carbon‐Electrode‐Based Endurable Flexible Perovskite Solar Cells

Endured, low‐cost, and high‐performance flexible perovskite solar cells (PSCs) featuring lightweight and mechanical flexibility have attracted tremendous attention for portable power source applications. However, flexible PSCs typically use expensive and fragile indium–tin oxide as transparent anode and high‐vacuum processed noble metal as cathode, resulting in dramatic performance degradation after continuous bending or thermal stress. Here, all‐carbon‐electrode‐based flexible PSCs are fabricated employing graphene as transparent anode and carbon nanotubes as cathode. All‐carbon‐electrode‐based flexible devices with and without spiro‐OMeTAD (2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene) hole conductor achieve power conversion efficiencies (PCEs) of 11.9% and 8.4%, respectively. The flexible carbon‐electrode‐based solar cells demonstrate superior robustness against mechanical deformation in comparison with their counterparts fabricated on flexible indium–tin oxide substrates. Moreover, all carbon‐electrode‐based flexible PSCs also show significantly enhanced stability compared to the flexible devices with gold and silver cathodes under continuous light soaking or 60 °C thermal stress in air, retaining over 90% of their original PCEs after 1000 h. The promising durability and stability highlight that flexible PSCs are fully compatible with carbon materials and pave the way toward the realization of rollable and low‐cost flexible perovskite photovoltaic devices.     

A Dual‐Retarded Reaction Processed Mixed‐Cation Perovskite Layer for High‐Efficiency Solar Cells

Mixed‐cation perovskite solar cells (PSCs) have become of enormous interest because of their excellent efficiency, which is now crossing 23.7%. Their broader absorption, relatively high stability with low fabrication cost compared to conventional single phase ABX₃ perovskites (where A: organic cation; B: divalent metal ion; and X: halide anion) are key properties of mixed‐halide mixed‐cation perovskites. However, the controlling reaction rate and formation of extremely dense, textured, smooth, and large grains of perovskite layer is a crucial task in order to achieve highly efficient PSCs. Herein, a new simple dual‐retarded reaction processing (DRP) method is developed to synthesize a high‐quality mixed‐cation (FAPbI₃)_0.85(MAPbBr₃)_0.15 (where MAPbBr₃ stands for methylammonium lead bromide and FAPbI₃ stands for formamidinium lead iodide) perovskite thin film via intermediate phase and incorporation of nitrogen‐doped reduced graphene oxide (N‐rGO). The reaction rate is retarded via two steps: first the formation of intermediate phase and second the interaction of the nitrogen groups on N‐rGO with hydrogen atoms from formamidinium cations. This DRP process allows for the fabrication of PSCs with maximum conversion efficiency higher than 20.3%.