p‐Doping of Copper(I) Thiocyanate (CuSCN) Hole‐Transport Layers for High‐Performance Transistors and Organic Solar Cells

The ability to tune the electronic properties of soluble wide bandgap semiconductors is crucial for their successful implementation as carrier‐selective interlayers in large area opto/electronics. Herein the simple, economical, and effective p‐doping of one of the most promising transparent semiconductors, copper(I) thiocyanate (CuSCN), using C₆₀F₄₈ is reported. Theoretical calculations combined with experimental measurements are used to elucidate the electronic band structure and density of states of the constituent materials and their blends. Obtained results reveal that although the bandgap (3.85 eV) and valence band maximum (?5.4 eV) of CuSCN remain unaffected, its Fermi energy shifts toward the valence band edge upon C₆₀F₄₈ addition—an observation consistent with p‐type doping. Transistor measurements confirm the p‐doping effect while revealing a tenfold increase in the channel's hole mobility (up to 0.18 cm² V^?1 s^?1), accompanied by a dramatic improvement in the transistor's bias‐stress stability. Application of CuSCN:C₆₀F₄₈ as the hole‐transport layer (HTL) in organic photovoltaics yields devices with higher power conversion efficiency, improved fill factor, higher shunt resistance, and lower series resistance and dark current, as compared to control devices based on pristine CuSCN or commercially available HTLs.     

A Band‐Edge Potential Gradient Heterostructure to Enhance Electron Extraction Efficiency of the Electron Transport Layer in High‐Performance Perovskite Solar Cells

As the key component in efficient perovskite solar cells, the electron transport layer (ETL) can selectively collect photogenerated charge carriers produced in perovskite absorbers and prevent the recombination of carriers at interfaces, thus ensuring a high power conversion efficiency. Compared with the conventional single‐ or dual‐layered ETLs, a gradient heterojunction (GHJ) strategy is more attractive to facilitate charge separation because the potential gradient created at an appropriately structured heterojunction can act as a driving force to regulate the electron transport toward a desired direction. Here, a SnO₂/TiO₂ GHJ interlayer configuration inside the ETL is reported to simultaneously achieve effective extraction and efficient transport of photoelectrons. With such an interlayer configuration, the GHJs formed at the perovskite/ETL interface act collectively to extract photogenerated electrons from the perovskite layer, while GHJs formed at the boundaries of the interconnected SnO₂ and TiO₂ networks throughout the entire ETL layer can extract electron from the slow electron mobility TiO₂ network to the high electron mobility SnO₂ network. Devices based on GHJ ETL exhibit a champion power conversion efficiency of 18.08%, which is significantly higher than that obtained from the compact TiO₂ ETL constructed under the comparable conditions.     

PbTiO3 as Electron‐Selective Layer for High‐Efficiency Perovskite Solar Cells: Enhanced Electron Extraction via Tunable Ferroelectric Polarization

PbTiO₃ (PTO) is explored as a versatile and tunable electron‐selective layer (ESL) for perovskite solar cells. To demonstrate effectiveness of PTO for electron–hole separation and charge transfer, perovskite solar cells are designed and fabricated in the laboratory with the PTO as the ESL. The cells achieve a power conversion efficiency (PCE) of ≈12.28% upon preliminary optimization. It is found that the PTO ferroelectric layer can not only increase the PCE, but also tune the photocurrent via tuning PTO's ferroelectric polarization. Moreover, to understand the physical mechanism underlying the carrier transport by the ferroelectric polarization, the electronic structure of PTO/CH₃NH₃PbI₃ heterostructure is computed using the first‐principles methods, for which the triplet state is used to simulate charge transfer in the heterostructure. It is shown that the synergistic effect of type II band alignment and the specific ferroelectric polarization direction provide the effective extraction of electrons from the light absorber, while minimize recombination of photogenerated electron–hole pairs. Overall, the ferroelectric PTO is a promising and tunable ESL for optimizing electron transport in the perovskite solar cells. The design offers a different strategy for altering direction of carrier transport in solar cells.     

Surface‐Modified Metallic Ti3C2Tx MXene as Electron Transport Layer for Planar Heterojunction Perovskite Solar Cells

MXenes are a large and rapidly expanding family of 2D materials that, owing to their unique optoelectronic properties and tunable surface termination, find a wide range of applications including energy storage and energy conversion. In this work, Ti₃C₂T_x MXene nanosheets are applied as a novel type of electron transport layer (ETL) in low‐temperature processed planar‐structured perovskite solar cells (PSCs). Interestingly, simple UV‐ozone treatment of the metallic Ti₃C₂T_x that increases the surface Ti? O bonds without any change in its bulk properties such as high electron mobility improves its suitability as an ETL. Improved electron transfer and suppressed recombination at the ETL/perovskite interface results in augmentation of the power conversion efficiency (PCE) from 5.00% in the case of Ti₃C₂T_x without UV‐ozone treatment to the champion PCE of 17.17%, achieved using the Ti₃C₂T_x film after 30 min of UV‐ozone treatment. As the first report on the use of pure MXene layer as an ETL in PSCs, this work shows the great potential of MXenes to be used in PSCs and displays their promise for applications in photovoltaic technology in general.     

Copper (I) Selenocyanate (CuSeCN) as a Novel Hole‐Transport Layer for Transistors,Organic Solar Cells,and Light‐Emitting Diodes

The synthesis and characterization of copper (I) selenocyanate (CuSeCN) and its application as a solution‐processable hole‐transport layer (HTL) material in transistors, organic light‐emitting diodes, and solar cells are reported. Density‐functional theory calculations combined with X‐ray photoelectron spectroscopy are used to elucidate the electronic band structure, density of states, and microstructure of CuSeCN. Solution‐processed layers are found to be nanocrystalline and optically transparent (>94%), due to the large bandgap of ≥3.1 eV, with a valence band maximum located at ?5.1 eV. Hole‐transport analysis performed using field‐effect measurements confirms the p‐type character of CuSeCN yielding a hole mobility of 0.002 cm² V^?1 s^?1. When CuSeCN is incorporated as the HTL material in organic light‐emitting diodes and organic solar cells, the resulting devices exhibit comparable or improved performance to control devices based on commercially available poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate as the HTL. This is the first report on the semiconducting character of CuSeCN and it highlights the tremendous potential for further developments in the area of metal pseudohalides.     

Solution‐Processable Ultrathin Black Phosphorus as an Effective Electron Transport Layer in Organic Photovoltaics

2D van der Waals crystals, possessing excellent electronic and physical properties, have been intriguing building blocks for organic optoelectronic devices. Most of the 2D materials are served as hole transport layers in organic devices. Here,it is reported that solution exfoliated few layers black phosphorus (BP) can be served as an effective electron transport layer (ETL) in organic photovoltaics (OPVs) for the first time. The power conversion efficiencies (PCEs) of the BP‐incorporated OPVs can be improved to 8.18% in average with the relative enhancement of 11%. The incorporation of BP flakes with the optimum thickness of ≈10 nm can form cascaded band structure in OPVs, which can facilitate electron transport and enhance the PCEs of the devices. This study opens an avenue in using solution exfoliated BP as a highly efficient ETL for organic optoelectronics.     

Copper(I) Thiocyanate (CuSCN) Hole‐Transport Layers Processed from Aqueous Precursor Solutions and Their Application in Thin‐Film Transistors and Highly Efficient Organic and Organometal Halide Perovskite Solar Cells

This study reports the development of copper(I) thiocyanate (CuSCN) hole‐transport layers (HTLs) processed from aqueous ammonia as a novel alternative to conventional n‐alkyl sulfide solvents. Wide bandgap (3.4–3.9 eV) and ultrathin (3–5 nm) layers of CuSCN are formed when the aqueous CuSCN–ammine complex solution is spin‐cast in air and annealed at 100 °C. X‐ray photoelectron spectroscopy confirms the high compositional purity of the formed CuSCN layers, while the high‐resolution valence band spectra agree with first‐principles calculations. Study of the hole‐transport properties using field‐effect transistor measurements reveals that the aqueous‐processed CuSCN layers exhibit a fivefold higher hole mobility than films processed from diethyl sulfide solutions with the maximum values approaching 0.1 cm² V^?1 s^?1. A further interesting characteristic is the low surface roughness of the resulting CuSCN layers, which in the case of solar cells helps to planarize the indium tin oxide anode. Organic bulk heterojunction and planar organometal halide perovskite solar cells based on aqueous‐processed CuSCN HTLs yield power conversion efficiency of 10.7% and 17.5%, respectively. Importantly, aqueous‐processed CuSCN‐based cells consistently outperform devices based on poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate HTLs. This is the first report on CuSCN films and devices processed via an aqueous‐based synthetic route that is compatible with high‐throughput manufacturing and paves the way for further developments.     

Solvent‐Assisted Low‐Temperature Crystallization of SnO2 Electron‐Transfer Layer for High‐Efficiency Planar Perovskite Solar Cells

A high‐quality polycrystalline SnO₂ electron‐transfer layer is synthesized through an in situ, low‐temperature, and unique butanol–water solvent‐assisted process. By choosing a mixture of butanol and water as a solvent, the crystallinity is enhanced and the crystallization temperature is lowered to 130 °C, making the process fully compatible with flexible plastic substrates. The best solar cells fabricated using these layers achieve an efficiency of 20.52% (average 19.02%) which is among the best in the class of planar n–i–p‐type perovskite (MAPbI₃) solar cells. The strongly reduced crystallization temperature of the materials allows their use on a flexible substrate, with a resulting device efficiency of 18%.     

Efficiency and Stability Enhancement in Perovskite Solar Cells by Inserting Lithium‐Neutralized Graphene Oxide as Electron Transporting Layer

This work proposes a new perovskite solar cell structure by including lithium‐neutralized graphene oxide (GO‐Li) as the electron transporting layer (ETL) on top of the mesoporous TiO₂ (m‐TiO₂) substrate. The modified work‐function of GO after the intercalation of Li atoms (4.3 eV) exhibits a good energy matching with the TiO₂ conduction band, leading to a significant enhancement of the electron injection from the perovskite to the m‐TiO₂. The resulting devices exhibit an improved short circuit current and fill factor and a reduced hysteresis. Furthermore, the GO‐Li ETL partially passivates the oxygen vacancies/defects of m‐TiO₂ by resulting in an enhanced stability under prolonged 1 SUN irradiation.     

Self‐Assembled Quasi‐3D Nanocomposite: A Novel p‐Type Hole Transport Layer for High Performance Inverted Organic Solar Cells

Hole transport layer (HTL) plays a critical role for achieving high performance solution‐processed optoelectronics including organic electronics. For organic solar cells (OSCs), the inverted structure has been widely adopted to achieve prolonged stability. However, there are limited studies of p‐type effective HTL on top of the organic active layer (hereafter named as top HTL) for inverted OSCs. Currently, p‐type top HTLs are mainly 2D materials, which have an intrinsic vertical conduction limitation and are too thin to function as practical HTL for large area optoelectronic applications. In the present study, a novel self‐assembled quasi‐3D nanocomposite is demonstrated as a p‐type top HTL. Remarkably, the novel HTL achieves ≈15 times enhanced conductivity and ≈16 times extended thickness compared to the 2D counterpart. By applying this novel HTL in inverted OSCs covering fullerene and non‐fullerene systems, device performance is significantly improved. The champion power conversion efficiency reaches 12.13%, which is the highest reported performance of solution processed HTL based inverted OSCs. Furthermore, the stability of OSCs is dramatically enhanced compared with conventional devices. The work contributes to not only evolving the highly stable and large scale OSCs for practical applications but also diversifying the strategies to improve device performance.     

Secondary Bonds Modifying Conjugate‐Blocked Linkages of Biomass‐Derived Lignin to Form Electron Transfer 3D Networks for Efficiency Exceeding 16% Nonfullerene Organic Solar Cells

Fabricating high‐efficient electron transporting interfacial layers (ETLs) with isotropic features is highly desired for all‐directional electron transfer/collection from an anisotropic active layer, achieving excellent power conversion efficiency (PCEs) on nonfullerene acceptor (NFA) organic solar cells (OSCs). The complicated synthesis and cost‐consumption in exploring versatile materials arouse great interest in the development of binary‐doping interlayers without phase separation and flexible manipulation. Herein, for the first time, a novel cathode interfacial layer based on biomass‐derived demethylated kraft lignin (D_MeKL) is proposed. Features of multiple phenolic‐hydroxyl (PhOH) and uniform‐distributed render D_MeKL to exhibit an excellent bonding capacity with amino terminal substituted perylene diiminde (PDIN), and successfully form a high‐efficient isotropic electron transfer 3D network. Synchronously, secondary bonds completely modify conjugate‐blocked linkages of D_MeKL, significantly enhance the electron transporting performance on cross‐section and vertical‐sections, and repair the contact of PDIN with active layer. The D_MeKL/PDIN‐based 3D‐network exhibits well‐matched work function (WF) (–4.34 eV) with cathode (–4.30 eV) and energy level of electron acceptor (–4.11 eV). D_MeKL/PDIN‐based NFAs‐OSC shows excellent short‐circuit current density (26.61 mA cm^–2) and PCE (16.02%) beyond the classic PDIN‐based NFA‐OSC (25.64 mA cm^–2, 15.41%), which is the highest PCEs among biomaterials interlayers. The results supply a novel method to achieve high‐efficient cathode interlayer for NFAs‐OSCs.     

Hydrothermally Treated SnO2 as the Electron Transport Layer in High‐Efficiency Flexible Perovskite Solar Cells with a Certificated Efficiency of 17.3%

Perovskite solar cells (PSCs) are one of the most promising solar energy conversion technologies owing to their rapidly developing power conversion efficiency (PCE). Low‐temperature solution processing of the perovskite layer enables the fabrication of flexible devices. However, their application has been greatly hindered due to the lack of strategies to fabricate high‐quality electron transport layers (ETLs) at the low temperatures (≈100 °C) that most flexible plastic substrates can withstand, leading to poor performances for flexible PSCs. In this work, through combining the spin‐coating process with a hydrothermal treatment method, ligand‐free and highly crystalline SnO₂ ETLs are successfully fabricated at low temperature. The flexible PSCs based on this SnO₂ ETL exhibit an excellent PCE of 18.1% (certified 17.3%). The flexible PSCs maintained 85% of the initial PCE after 1000 bending cycles and over 90% of the initial PCE after being stored in ambient air for 30 days without encapsulation. The investigation reveals that hydrothermal treatment not only promotes the complete removal of organic surfactants coated onto the surface of the SnO₂ nanoparticles by hot water vapor but also enhances crystallization through the high vapor pressure of water, leading to the formation of high‐quality SnO₂ ETLs.     

PEDOT:PSS‐Assisted Exfoliation and Functionalization of 2D Nanosheets for High‐Performance Organic Solar Cells

Here, a facial and scalable method for efficient exfoliation of bulk transition metal dichalcogenides (TMD) and graphite in aqueous solution with poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to prepare single‐ and few‐layer nanosheets is demonstrated. Importantly, these TMD nanosheets retain the single crystalline characteristic, which is essential for application in organic solar cells (OSCs). The hybrid PEDOT:PSS/WS₂ ink prepared by a simple centrifugation is directly integrated as a hole extraction layer for high‐performance OSCs. Compared with PEDOT:PSS, the PEDOT:PSS/WS₂‐based devices provide a remarkable power conversion efficiency due to the “island” morphology and benzoid–quinoid transition. This study not only demonstrates a novel method for preparing single‐ and few‐layer TMD and graphene nanosheets but also paves a way for their applications without further complicated processing.     

SnO2‐C60 Pyrrolidine Tris‐Acid (CPTA) as the Electron Transport Layer for Highly Efficient and Stable Planar Sn‐Based Perovskite Solar Cells

For solar cell applications, Sn‐based hybrid perovskites have drawn particular interest due to their environmental friendliness. Here, a thin layer of C₆₀ pyrrolidine tris‐acid (CPTA) is found essential for achieving high efficiency with planar solar cells of Sn‐based perovskites. As a result, a power conversion efficiency of 7.40% is achieved for {en}FASnI₃ solar cells with a planar n–i–p architecture, and the device exhibits excellent stability in air. For the first time, highly efficient Sn‐based hybrid perovskite solar cells on n–i–p architecture are achieved. A V_oc of 0.72 V is highlighted as the highest V_oc ever reported for FASnI₃ solar cells.     

High Performance PbS Colloidal Quantum Dot Solar Cells by Employing Solution‐Processed CdS Thin Films from a Single‐Source Precursor as the Electron Transport Layer

CdS thin films are a promising electron transport layer in PbS colloidal quantum dot (CQD) photovoltaic devices. Some traditional deposition techniques, such as chemical bath deposition and RF (radio frequency) magnetron sputtering, have been employed to fabricate CdS films and CdS/PbS CQD heterojunction photovoltaic devices. However, their power conversion efficiencies (PCEs) are moderate compared with ZnO/PbS and TiO₂/PbS heterojunction CQD solar cells. Here, efficiencies have been improved substantially by employing solution‐processed CdS thin films from a single‐source precursor. The CdS film is deposited by a straightforward spin‐coating and annealing process, which is a simple, low‐cost, and high‐material‐usage fabrication process compared to chemical bath deposition and RF magnetron sputtering. The best CdS/PbS CQD heterojunction solar cell is fabricated using an optimized deposition and air‐annealing process achieved over 8% PCE, demonstrating the great potential of CdS thin films fabricated by the single‐source precursor for PbS CQDs solar cells.