Effect of PEI cathode interlayer on work function and interface resistance of ITO electrode in the inverted polymer solar cells

In this paper, we investigated the effect of PEI cathode interlayer on the work function and the interface resistance of ITO electrode in the inverted polymer solar cells (PSCs) based on PBDTTT-C-T:PC₇₀BM. It is found that a very thin layer of PEI (⩽5.5 nm), either linear PEI (l-PEI) or branched PEI (b-PEI) with different molecular weights, is enough to lower the work function of the ITO electrode and to enhance the photovoltaic performance of the devices. The champion power conversion efficiency (PCE) of the devices with the PEI cathode interlayer is 7.84%, more than doubled of that without the interlayer. However, a thicker PEI interlayer (⩾10 nm) results in abrupt decrease of the PCEs due to the increase of the resistance. Interestingly, for the thicker interlayers, the l-PEI shows high photovoltaic performance than that of b-PEI, which can also be explained by their difference in the resistances. This work supplies an insight into the function of PEI cathode interlayer on improving the work function and resistance of ITO electrode in the inverted PSCs, and provides some instructions on the future design of interlayer materials in PSCs.     

Polymer Solar Cells Exceeding 10% Efficiency Enabled via a Facile Star‐Shaped Molecular Cathode Interlayer with Variable Counterions

A series of star‐shaped small molecular cathode interlayer materials are synthesized for PTB7:PC₇₁BM based polymer solar cells (PSCs), comprising neutral amino groups, quaternary ammonium ions, amino N‐oxides, and sulfobetaine ions as pendant polar functionalities, respectively. For the first time, the effect of these different pendant functional groups with or without mobile counterions on the cells' photovoltaic properties is investigated in detail. A large improvement in device performance is observed by inserting these cathode interfacial layers (CILs) between the PTB7:PC₇₁BM active layer and the Al electrode. The CILs could effectively lower the work function of the Al cathode, increase the built‐in potential, and decrease the series resistance of the related PSCs. poly(9,9‐dioctylfluorene‐co‐N‐[4‐(3‐methyl‐propyl)]‐diphenylamine) with pendant quaternary ammonium ions shows the best cathode modification ability, giving rise to the highest power conversion efficiency of 10.1%, even better than that of the typical poly[(9,9‐bis(3′‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)] based device. The design strategy and structure–property relationships concluded in this work will be helpful to develop more efficient cathode interface materials for high‐performance PSCs in the future.     

Amino N‐Oxide Functionalized Conjugated Polymers and their Amino‐Functionalized Precursors: New Cathode Interlayers for High‐Performance Optoelectronic Devices

A series of amino N‐oxide functionalized polyfluorene homopolymers and copolymers (PNOs) are synthesized by oxidizing their amino functionalized precursor polymers (PNs) with hydrogen peroxide. Excellent solubility in polar solvents and good electron injection from high work‐function metals make PNOs good candidates for interfacial modification of solution processed multilayer polymer light‐emitting diodes (PLEDs) and polymer solar cells (PSCs). Both PNOs and PNs are used as cathode interlayers in PLEDs and PSCs. It is found that the resulting devices show much better performance than devices based on a bare Al cathode. The effect of side chain and main chain variations on the device performance is investigated. PNOs/Al cathode devices exhibit better performance than PNs/Al cathode devices. Moreover, devices incorporating polymers with para‐linkage of pyridinyl moieties exhibit better performance than those using polymers with meta‐linked counterparts. With a poly[(2,7‐(9,9‐bis(6‐(N,N‐diethylamino)‐hexyl N‐oxide)fluorene))‐alt‐(2,5‐pyridinyl)] (PF6NO25Py) cathode interlayer, the resulting device exhibits a luminance efficiency of 16.9 cd A^?1 and a power conversion efficiency of 6.9% for PLEDs and PSCs, respectively. These results indicate that PNOs are promising new cathode interlayers for modifying a range of optoelectronic devices.     

Highly efficient polymer solar cells with PTB7-based narrow band-gap conjugated polyelectrolytes as cathode interlayers: Device performance dependence on the ionic pendants

Narrow band-gap conjugated polyelectrolytes (NBGCPs) combine the advantages of narrow band-gap conjugated polymers and polyelectrolytes. However, they are limited reported and seldom used in polymer solar cells (PSCs). Herein, we design and synthesized two PTB7-based NBGCPs, cationic PTB7-NBr and zwitterionic PTB7-NSO₃, as cathode interlayers (CILs) in conventional PSCs. Compared to poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-ethylhexyl-3-fuorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7), both PTB7-NBr and PTB7-NSO₃ have similar absorption while PTB7-NBr has a higher HOMO level than PTB7 and PTB7-NSO₃. With PTB7-NBr and PTB7-NSO₃ as CIL and Al as cathode, the devices both exhibited high efficiencies. The optimal power conversion efficiencies (PCEs) of PTB7-NBr device and PTB7-NSO₃ device are about 9%. However, PTB7-NBr devices are less sensitive to the CIL film thicknesses and the PCEs of PTB7-NSO₃ device decrease sharply when the thickness of PTB7-NSO₃ over 3 nm. What's more, when the devices with an Ag cathode PTB7-NBr exhibits a much better interfacial modification than PTB7-NSO₃. Ultraviolet photoelectron spectroscopy (UPS) indicate that the work functions of PTB7-NBr and PTB7-NSO₃ film on Al are identical while on Ag they are different. We suggest the insensitivity to CIL thickness and good performance for Ag device of PTB7-NBr devices are attributed to the bromide anions in PTB7-NBr. The report on PTB7-based NBGCPs broads the types of organic cathode interfacial materials and is beneficial to the deep insight of ionic effect in electrolyte materials.     

Properties of inverted polymer solar cells based on novel small molecular electrolytes as the cathode buffer layer

Novel small-molecule electrolytes were designed and synthesized for use in the cathode interlayer in bulk-heterojunction polymer solar cells (PSCs). The synthesized materials consist of polar quaternary ammonium bromide with the addition of multiple hydroxyl groups, which are N,N,N,N,N,N-hexakis(2-hydroxyethyl)butane-1,4-diaminium bromide (C4) and N,N,N,N,N,N-hexakis(2-hydroxyethyl)hexane-1,6-diaminium bromide (C6). The materials generate a favorable interface dipole through the quaternary ammonium bromide. In addition, the multiple polar hydroxyl groups increased the interface dipole magnitude. The power conversion efficiency of the devices with the interlayer was up to 9.20% with a J_sc of 17.22 mA/cm², a V_oc of 0.75 V, and an FF of 71.3%. The PCE of devices with an interlayer show better long-term stability than a device without an interlayer. Our strategy shows that it is possible to enhance the efficiency of PSCs by simple approaches without complicated syntheses.     

Carbon Nanotube Based Inverted Flexible Perovskite Solar Cells with All‐Inorganic Charge Contacts

Organolead halide perovskite solar cells (PSC) are arising as promising candidates for next‐generation renewable energy conversion devices. Currently, inverted PSCs typically employ expensive organic semiconductor as electron transport material and thermally deposited metal as cathode (such as Ag, Au, or Al), which are incompatible with their large‐scale production. Moreover, the use of metal cathode also limits the long‐term device stability under normal operation conditions. Herein, a novel inverted PSC employs a SnO₂‐coated carbon nanotube (SnO₂@CSCNT) film as cathode in both rigid and flexible substrates (substrate/NiO‐perovskite/Al₂O₃‐perovskite/SnO₂@CSCNT‐perovskite). Inverted PSCs with SnO₂@CSCNT cathode exhibit considerable enhancement in photovoltaic performance in comparison with the devices without SnO₂ coating owing to the significantly reduced charge recombination. As a result, a power conversion efficiency of 14.3% can be obtained on rigid substrates while the flexible ones achieve 10.5% efficiency. More importantly, SnO₂@CSCNT‐based inverted PSCs exhibit significantly improved stability compared to the standard inverted devices made with silver cathode, retaining over 88% of their original efficiencies after 550 h of full light soaking or thermal stress. The results indicate that SnO₂@CSCNT is a promising cathode material for long‐term device operation and pave the way toward realistic commercialization of 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.     

High-Performance ITO-Free Perovskite Solar Cells Enabled by Single-Walled Carbon Nanotube Films

The unprecedented advancement in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has rendered them a promising game-changer in photovoltaics. However, unsatisfactory environmental stability and high manufacturing cost of window electrodes are bottlenecks impeding their commercialization. Here, a strategy is introduced to address these bottlenecks by replacing the costly indium tin oxide (ITO) window electrodes via a simple transfer technique with single-walled carbon nanotubes (SWCNTs) films, which are made of earth-abundant elements with superior chemical and environmental stability. The resultant devices exhibit PCEs of ≈19% on rigid substrates, which is the highest value reported to date for ITO-free PSCs. The facile approach for SWCNTs also enables application in flexible PSCs (f-PSCs), delivering a PCE of ≈18% with superior mechanical robustness over their ITO-based counterparts due to the excellent mechanical properties of SWCNTs. The SWCNT-based PSCs also deliver satisfactory performances on large-area (1 cm² active area in this work). Furthermore, these SWCNT-based PSCs can retain over 80% of original PCEs after exposure to air over 700 h while ITO-based devices only sustain ≈60% of initial PCEs. This work paves a promising way to accelerate the commercialization of ITO-free PSCs with reduced material cost and prolonged lifetimes.     

Enhanced Performance of Planar Perovskite Solar Cells Induced by Van Der Waals Epitaxial Growth of Mixed Perovskite Films on WS2 Flakes

Organic–inorganic metal halide perovskite solar cells (PSCs) have attracted much research interest owing to their high power conversion efficiency (PCE), solution processability, and the great potential for commercialization. However, the device performance is closely related to the quality of the perovskite film and the interface properties, which cannot be easily controlled by solution processes. Here, 2D WS₂ flakes with defect‐free surfaces are introduced as a template for van der Waals epitaxial growth of mixed perovskite films by solution process for the first time. The mixed perovskite films demonstrate a preferable growth along (001) direction on WS₂ surfaces. In addition, the WS₂/perovskite heterojunction forms a cascade energy alignment for efficient charge extraction and reduced interfacial recombination. The inverted PSCs with WS₂ interlayers show high PCEs up to 21.1%, which is among the highest efficiency of inverted planar PSCs. This work demonstrates that high‐mobility 2D materials can find important applications in PSCs as well as other perovskite‐based optoelectronic devices.     

In-situ synthesis of metal nanoparticle-polymer composites and their application as efficient interfacial materials for both polymer and planar heterojunction perovskite solar cells

A new approach for the synthesis of gold nanoparticles (Au NPs) via a simple and fast in-situ generation method using an amine-containing polymer (PN4N) as both stabilizer and reducing agent is reported. The application of the Au NPs-PN4N hybrid material as efficient interfacial layer in different types of solar cells was also explored. The synthesized Au NPs show good uniformity in size and shape and the Au NPs doped PN4N hybrid composites exhibit high stability. Amine-containing polymers are good cathode interfacial materials (CIMs) in polymer solar cells (PSCs) and planar heterojunction perovskite solar cells (PVKSCs). The performance of the PSCs with Au NPs doped PN4N CIMs is largely improved when compares to devices with pristine PN4N CIM due to the enhanced electronic properties of the doped PN4N. Furthermore, by incorporating larger Au NPs into PEDOT:PSS to enhance absorption of the light harvesting layer, power conversion efficiencies (PCEs) of 6.82% and 13.7% are achieved for PSC with PCDTBT/PC₇₁BM as the light harvesting materials and PVKSC with a ∼280 nm-thick CH₃NH₃PbI_3−xCl_x perovskite layer, respectively. These results indicate that Au NPs doped into both PEDOT:PSS and PN4N interlayers exhibited a synergistic effect in performance improvement of PSCs and PVKSCs.     

Multi‐channel interface dipole of hyperbranched polymers with quasi‐immovable hydrion to modification of cathode interface for high‐efficiency polymer solar cells

The single‐junction polymer solar cells (PSCs) with high power conversion efficiency (PCE) are demonstrated by first incorporating two hyperbranched polymers (carboxylic acid functionalized hyperbranched poly(ether ketone) (CHBPEK) and sulfonic acid functionalized hyperbranched poly(ether sulfone) (SHBPES)) with carboxylic and sulfonic acid groups to modify the cathode interface. The effect of cathode modification by CHBPEK or SHBPES for conventional PSCs based on different active‐layer materials is systematically investigated. Compared with traditional LiF/Al, Ca/Al, and PFN/Al devices, significant improvement in short circuit current and fill factors are achieved by employing CHBPEK or SHBPES. In particular, the device with SHBPES as cathode interlayer can be achieved with a highest PCE of 9.12%, which are among the high performance reported for single‐junction PSCs modified with hyperbranched interlayers. Importantly, the influence of the hyperbranched polymer interlayer modification on the cathode interface is also discussed and found to be the formation of the stable multi‐channel interface dipole based on quasi‐immovable counterions in these polymers. The results supply a feasible means to obtain the improved J_SC and FF for high efficiency PSCs. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

π‐Extended Spiro Core‐Based Nonfullerene Electron‐Transporting Material for High‐Performance Perovskite Solar Cells

Electron transport materials (ETMs) play a significant role in perovskite solar cells (PSCs). However, conventional solution processable organic ETMs are mainly restricted to fullerene derivatives and it is challenging to obtain nonfullerene ETMs with satisfactory properties. In this work, a new organic semiconductor SPS‐4F is synthesized by utilizing the classical spiro[fluorine‐9′9‐thioxanthene] unit to construct a π‐extended core. Although spiro is normally used in hole transport materials, the new spiro derivative SPS‐4F is successfully used as an ETM in inverted PSCs with power conversion efficiency over 20%. In addition, SPS‐4F can strongly coordinate with MAPbI₃ perovskite and lead to efficient surface trap passivation. The resultant PSCs exhibit excellent stability in air because of the hydrophobic property of SPS‐4F. This work opens up opportunities to obtain a new family of ETMs based on spiro and paves a way to the fabrication of high‐performance PSCs with low cost.     

Overcoming the PCBM/Ag Interface Issues in Inverted Perovskite Solar Cells by Rhodamine-Functionalized Dodecahydro-Closo-Dodecaborate Derivate Interlayer

Efficient modification of the interface between metal cathode and electron transport layer are critical for achieving high performance and stability of the inverted perovskite solar cells (PSCs). Herein, a new alcohol-soluble rhodamine-functionalized dodecahydro-closo-dodecaborate derivate, RBH, is developed and applied as an efficient cathode interlayer to overcome the (6,6)-phenyl-C₆₁ butyrie acid methyl ester (PCBM)/Ag interface issues. By introducing RBH cathode interlayer, the functions of the interface traps passivation, interfacial hydrophobicity enhancement, interface contact improvement as well as built-in potential enhancement are realized at the same time and thus correspondingly improve the device performance and stability. Consequently, a power conversion efficiency (PCE) of 21.08% and high fill factor of 83.37% are achieved, which is one of the highest values based on solution-processed MAPbI₃/PCBM heterojunction PSCs. Moreover, RBH can act as a shielding layer to slow down moisture erosion and self-corrosion. The PCE of the RBH devices still maintain 84% for 456 h (85 °C @ N₂), 87% for 360 h (23 °C @ relative humidity (RH) 35%) of its initial PCE value, while the control device can only maintain ≈23%, 58% of its initial PCE value under the same exposure conditions, respectively.     

Fluorene-based cathode interlayer polymers for high performance solution processed organic optoelectronic devices

A hydrophilic polyfluorene-based conjugated polyelectrolyte (CPE) Poly[9,9-bis(4′-(6″-(diethanolamino)hexyloxy) phenyl)fluorene], PPFN-OH (Scheme 1) has been synthesized and utilized as cathode interlayer for both polymer light emitting diodes (PLEDs) and solar cells (PSCs). For comparison, another CPE namely Poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH) has also been investigated. They comprise the same polyfluorene backbone structures with, respectively, diethanolaminohexyl (PFN-OH) and diethanolaminohexoxyphenyl (PPFN-OH) substituents attached to the C9 carbon of the fluorene repeat unit. In comparison to reference devices with more reactive Ca/Al cathodes, utilizing these CPEs as interlayers allowed an Al cathode to be used for blue light emission PLEDs, yielding 51% and 92% enhancement of maximum luminous efficiency (LE) for PFN-OH and PPFN-OH, respectively. The PLEDs with PPFN-OH showed both higher maximum LE and maximum luminance (L) (LE = 2.53 cd/A at 6.2 V, L = 9917 cd/m² at 8.3 V) than devices with PFN-OH (2.00 cd/A at 4.1 V, 3237 cd/m² at 7.2 V). The PPFN-OH PLEDs also showed no significant roll-off in efficiency with increasing current density up to 400 mA/cm², indicating excellent electron injection ability and stability for this interlayer. The insertion of alkoxy-phenyl groups at the C9-position in PPFN-OH is clearly advantageous. This simple modification significantly improves the CPE cathode interlayer performance. Parallel investigations of the electron extraction properties of PPFN-OH in inverted architecture PSCs with PCDTBT:PC₇₀BM bulk heterojunction active layers demonstrated a power conversion efficiency enhancement of ∼19% (from 4.99% to 5.95%) for indium tin oxide cathode devices compared with reference devices using Ca/Al cathodes. These results confirm PPFN-OH to be a promising interlayer material for high performance solution processed organic optoelectronic devices.     

Side-Chain Functionalized Polymer Hole-Transporting Materials with Defect Passivation Effect for Highly Efficient Inverted Quasi-2D Perovskite Solar Cells

Compared with inverted 3D perovskite solar cell (PSCs), inverted quasi-2D PSCs have advantages in device stability, but the device efficiency is still lagging behind. Constructing polymer hole-transporting materials (HTMs) with passivation functions to improve the buried interface and crystallization properties of perovskite films is one of the effective strategies to improve the performance of inverted quasi-2D PSCs. Herein, two novel side-chain functionalized polymer HTMs containing methylthio-based passivation groups are designed, named PVCz-SMeTPA and PVCz-SMeDAD, for inverted quasi-2D PSCs. Benefited from the non-conjugated flexible backbone bearing functionalized side-chain groups, the polymer HTMs exhibit excellent film-forming properties, well-matched energy levels and improved charge mobility, which facilitates the charge extraction and transport between HTM and quasi-2D perovskite layer. More importantly, by introducing methylthio units, the polymer HTMs can enhance the contact and interactions with quasi-2D perovskite, and further passivating the buried interface defects and assisting the deposition of high-quality perovskite. Due to the suppressed interfacial non-radiative recombination, the inverted quasi-2D PSCs using PVCz-SMeTPA and PVCz-SMeDAD achieve impressive power conversion efficiency (PCE) of 21.41% and 20.63% with open-circuit voltage of 1.23 and 1.22 V, respectively. Furthermore, the PVCz-SMeTPA based inverted quasi-2D PSCs also exhibits negligible hysteresis and considerably improved thermal and long-term stability.     

Potassium-neutralized perylene derivative (K4PTC) and rGO-K4PTC composite as effective and inexpensive electron transport layers for polymer solar cells

The polymer solar cell (PSC) with Ca/Al electrode always suffers from low stability mainly due to the incorporation of oxygen and moisture-sensitive Ca electron-transport interlayer (ETL). To alleviate this problem, air-stable alternatives to Ca ETL are highly desired. Herein, we report two solution-processable, air-stable, effective and inexpensive ETLs based on potassium-neutralized perylene tetracarboxylic derivative (K₄PTC) and its rGO composite (rGO-K₄PTC), respectively. These ETL materials were facilely prepared and characterized by means of UV-vis, FL, FTIR, XPS and UPS. Importantly, both ETLs exhibited a low work function (WF) of 4.0 eV, which well matches the LUMO level of fullerene acceptors and allows their use as ETL in PSCs. As a result, the P3HT and PTB7-th-based devices with respective ETL remarkably outperformed those without ETL yielding increases of ∼35% in power conversion efficiencies (PCEs), which indicates good electron-transporting capabilities of K₄PTC and rGO-K₄PTC interlayers. The high-performance PSCs with the ETL gave average PCEs of 6.17–6.18% (for PTB7-th:PC₆₁BM-based devices) and 7.26% (for PTB7-th:PC₇₁BM-based devices), respectively, fairly comparable to those of Ca/Al devices (6.50% and 7.50%). Furthermore, the rGO-K₄PTC device exhibited stability higher than that of the K₄PTC device probably due to the fact that the rGO-K₄PTC layer can provide more efficient protection for the active layer against degradation. Thus, rGO-K₄PTC layer might be more suitable for real applications as compared to the K₄PTC layer.     

A Versatile Buffer Layer for Polymer Solar Cells: Rendering Surface Potential by Regulating Dipole

Spin‐coated film of poly(vinylidenefluoride‐hexafluoropropylene) (P(VDF‐HFP)) acts as a cathode/anode buffer layer in polymer solar cells (PSCs) with conventional/inverted device structures. Such devices show optimized performances comparable with the controlled device, making P(VDF‐HFP) a good substitute for LiF/MoO₃ as a cathode/anode buffer layer. Ultraviolet photoelectron spectroscopy (UPS) and Kelvin force microscope (KFM) measurements show that increased surface potential of active layers improves cathode contact. In piezoresponse force microscopy (PFM) measurement, P(VDF‐HFP) responds to applied bias in phase curve, showing tunable dipole. This tunable dipole renders surface potential under applied bias. As a result, open‐circuit voltage of devices alters instantly with poling voltage. Moreover, positive poling of P(VDF‐HFP) together with simultaneous oxidation of Ag gradually improves performance of inverted structure device. Integer charge transfer (ICT) model elucidates improved electrode contacts by dipole tuning, varying surface potential and vacuum level shift. Understanding the function of dipole makes P(VDF‐HFP) a promising and versatile buffer layer for PSCs.     

Tailoring Multifunctional Self-Assembled Hole Transporting Molecules for Highly Efficient and Stable Inverted Perovskite Solar Cells

The self-assembled hole transporting molecules (SAHTMs) bearing anchoring groups have been established as the hole transporting layers (HTLs) for highly efficient p–i–n perovskite solar cells (PSCs), yet their stability and engineering at the molecular level remain challenging. A topological design of highly anisotropic aligned SAHTM-based HTLs for operationally stable PSCs that exhibit exceptional solar-to-electric power conversion efficiencies (PCEs) is demonstrated. The judiciously designed multifunctional self-assembled molecules comprise the donor–acceptor subunit for hole transporting and the phosphonic acid group for anchoring, realizing face-on π-stacking parallel to the transparent conductive oxide substrate. The high affinity of SAHTMs to the multi-crystalline perovskite thin film benefits passivating the perovskite buried interface, strengthening interfacial contact while facilitating interfacial hole transfer. Consequently, highly efficient p–i–n PSC devices are obtained with a champion PCE of 23.24% and outstanding operational stability toward various environmental factors including long-term full sunlight soaking at evaluated temperatures. Perovskite solar modules with a champion efficiency approaching 20% are also fabricated for an active device area above 17 cm².     

Wide Bandgap Molecular Acceptors with a Truxene Core for Efficient Nonfullerene Polymer Solar Cells: Linkage Position on Molecular Configuration and Photovoltaic Properties

Two wide bandgap star‐shaped small molecular acceptors, para‐TrBRCN and meta‐TrBRCN , are synthesized for efficient nonfullerene polymer solar cells (PSCs). The tiny structural variation by just changing the linkage positions affects largely the inherent properties of the resulting molecules. Both molecules have a nonplanar 3D structure, which can prevent the excessively aggregation to realize the optimized morphology and ideal domain size in their active blends. Compared to para‐TrBRCN , meta‐TrBRCN exhibits the smaller distortions between the truxene skeleton and the benzothiadiazole units, which would also lead to the enhanced π–π stacking and charge transfer. When blending with PTB7‐Th, high power conversion efficiencies (PCEs) of 10.15% and 8.28% are obtained for meta‐TrBRCN and para‐TrBRCN devices, respectively. To make up the weak absorption of above binary active blend in the longer wavelength region and increase the whole device performance further, low bandgap 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylthienyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]‐dithiophene (ITIC‐Th) is added as the second acceptor material to fabricate ternary blend PSCs. After adding 20 wt% of ITIC‐Th, the resulting devices exhibit the well‐balanced optical absorption and fine‐tuned morphology, giving rise to the significantly improved PCE of 11.40% with much higher J _sc of 18.25 mA cm^?2 and fill factor of 70.2%.