Ligang Yuan

Researchers have recently revealed that hybrid lead halide perovskites exhibit ferroelectricity, which is often associated with other physical characteristics, such as a large nonlinear optical response. In this work, the nonlinear optical properties of single crystal inorganic–organic hybrid perovskite CH₃NH₃PbBr₃ are studied. By exciting the material with a 1044 nm laser, strong two-photon absorption-induced photoluminescence in the green spectral region is observed. Using the transmission open-aperture Z-scan technique, the values of the two-photon absorption coefficient are observed to be 8.5 cm GW⁻¹, which is much higher than that of standard two-photon absorbing materials that are industrially used in nonlinear optical applications, such as lithium niobate (LiNbO₃), LiTaO₃, KTiOPO₄, and KH₂PO₄. Such a strong two-photon absorption effect in CH₃NH₃PbBr₃ can be used to modulate the spectral and spatial profiles of laser pulses, as well as to reduce noise, and can be used to strongly control the intensity of incident light. In this study, the superior optical limiting, pulse reshaping, and stabilization properties of CH₃NH₃PbBr₃ are demonstrated, opening new applications for perovskites in nonlinear optics.

The two-photon absorption properties of CH₃NH₃PbBr₃ are investigated by exciting the material with a 1044 nm laser. Such a strong two-photon absorption effect can be used to modulate the spectral and spatial profiles of laser pulses. In this study, the superior optical limiting, pulse reshaping, and stabilization properties of CH₃NH₃PbBr₃ are demonstrated.

A novel small molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520–740 nm. The MeIC-based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short-circuit current (J_SC) of 18.41 mA cm⁻², significantly higher than that of the device based on J71:ITCPTC (11.63% with a J_SC of 17.52 mA cm⁻²). The higher J_SC of the PSC based on J71:MeIC can be attributed to more balanced μ_h/μ_e, higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X-ray diffraction and resonant soft X-ray scattering results. It is worth mentioning that the as-cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as-cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.

A novel small-molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits higher lowest unoccupied molecular orbital (LUMO) level, tighter molecular packing, and better crystallite quality. MeIC-based polymer solar cells with J71 as donor achieve high power conversion efficiency up to 12.54%, significantly higher than that of the device of ITCPTC.

Optimizing the interfacial contacts between the photoactive layer and the electrodes is an important factor in determining the performance of organic solar cells (OSCs). A charge-selective layer with tailored electrical properties enhances the charge collection efficiency and interfacial stability. Here, the potential of hydrogenated TiO₂ nanoparticles (H-TiO₂ NPs) as an efficient electron-selective layer (ESL) material in OSCs is reported for the first time. The H-TiO₂ is synthesized by discharge plasma in liquid at atmospheric pressure, which has the benefits of a simple one-pot synthesis process, rapid and mild reaction conditions, and the capacity for mass production. The H-TiO₂ exhibits high conductivity and favorable energy level formation for efficient electron extraction, providing a basis for an efficient bilayer ESL system composed of conjugated polyelectrolyte/H-TiO₂. Thus, the enhanced charge transport and extraction efficiency with reduced recombination losses at the cathode interfacial contacts is achieved. Moreover, the OSCs composed of H-TiO₂ are almost free of light soaking, which has been reported to severely limit the performance and stability of OSCs based on conventional TiO₂ ESLs. Therefore, H-TiO₂ as a new efficient, stable, and cost-effective ESL material has the potential to open new opportunities for optoelectronic devices.

This study demonstrates the potential of hydrogenated TiO₂ (H-TiO₂) as an efficient electron-selective layer in optoelectronic devices. The H-TiO₂ is simply one-pot mass-produced using a discharge plasma system in liquid at atmospheric pressure. The H-TiO₂ exhibits high conductivity and favorable energy level formation, resulting in the high-efficiency and light-soaking-free organic solar cells.

Three acceptor–donor–acceptor type nonfullerene acceptors (NFAs), namely, F–F, F–Cl, and F–Br, are designed and synthesized through a halogenation strategy on one successful nonfullerene acceptor FDICTF (F–H). The three molecules show red-shifted absorptions, increased crystallinities, and higher charge mobilities compared with the F–H. After blending with donor polymer PBDB-T, the F–F-, F–Cl-, and F–Br-based devices exhibit power conversion efficiencies (PCEs) of 10.85%, 11.47%, and 12.05%, respectively, which are higher than that of F–H with PCE of 9.59%. These results indicate that manipulating the absorption range, crystallinity and mobilities of NFAs by introducing different halogen atoms is an effective way to achieve high photovoltaic performance, which will offer valuable insight for the designing of high-efficiency organic solar cells.

Through a halogenation strategy onto the end-capping group in the FDICTF-based small-molecule acceptor, red-shifted absorptions, increased crystallinities, and higher charge mobilities are achieved. The device based on F–Br with power conversion efficiency of 12.05% and remarkable FF of 76% is one of only a few organic solar cells with efficiencies over 12% reported to date.

Novel nonspiro, fluorene-based, small-molecule hole transporting materials (HTMs) V1050 and V1061 are designed and synthesized using a facile three-step synthetic route. The synthesized compounds exhibit amorphous nature with a high glass transition temperature, a good solubility, and decent thermal stability. The planar perovskite solar cells (PSCs) employing V1050 generated an excellent power conversion efficiency of 18.3%, which is comparable to 18.9% obtained with the state-of-the-art Spiro-OMeTAD. Importantly, the devices based on V1050 and V1061 show better stability compared to devices based on Spiro-OMeTAD when aged without any encapsulation under uncontrolled humidity conditions (relative humidity around 60%) in the dark and under continuous full sun illumination.

Novel, non-spiro, amorphous hole-transporting materials (HTMs) (V1050 and V1061) with fluorene fragments are synthesized using a facile three-step synthetic route. Planar perovskite solar cells with the V1050 HTM exhibited an excellent power conversion efficiency (PCE) of 18.3%. Furthermore, V1050- and V1061-based devices show better stability compared to devices based on Spiro-OMeTAD.

The application of nanophotonic structures for organic solar cells (OSCs) is quite popular and successful, and has led to increased optical absorption, better spectral overlap with solar irradiances, and improved charge collection. Significant improvements in the power conversion efficiency (PCE) have also been reported, exceeding 11%. Nonetheless, with the given material properties of OSCs with low optical absorption, narrow spectrum, short transport length of carriers, and nonuniform photocarrier generations resulting from the nanophotonic structure, the PCE of single-junction OSCs has been stagnant over the past few years, at a barrier of 12%. Here, an ultrathin inverted OSC structure with the highest efficiency of ≈13.0%, while being made from widely used organic materials, is demonstrated. By introducing a smooth spatial corrugation to the vertical plasmonic cavity enclosing the active layer, in-plane propagation modes and hybridized Fabry–Perot cavity modes inside the corrugated cavity are derived to achieve an ultralow Q, uniform coverage of optical absorption, in addition to uniform photocarrier generation and transport. As the first demonstration of ultra-broadband absorption with the introduction of spatial corrugation to the ultrathin metal film electrode–cathode Fabry–Perot cavity, future applications of the same concept in other light-harvesting devices utilizing different materials and structures are expected.

By introducing a smooth in-plane spatial corrugation to the ultrathin metal film-cathode Fabry–Perot cavity, an organic solar cell with ≈13.0% power conversion efficiency, made from widely used organic materials is demonstrated. Ultralow Q, highly uniform coverage of the optical absorption, is derived from the hybridized Fabry–Perot cavity mode and multipeak in-plane propagation modes.

Perovskite solar cells have shown a meteoric rise of power conversion efficiency and a steady pace of improvements in their stability of operation. Such rapid progress has triggered research into approaches that can boost efficiencies beyond the Shockley–Queisser limit stipulated for a single-junction cell under normal solar illumination conditions. The tandem solar cell architecture is one concept here that has recently been successfully implemented. However, the approach of solar concentration has not been sufficiently explored so far for perovskite photovoltaics, despite its frequent use in the area of inorganic semiconductor solar cells. Here, the prospects of hybrid perovskites are assessed for use in concentrator solar cells. Solar cell performance parameters are theoretically predicted as a function of solar concentration levels, based on representative assumptions of charge-carrier recombination and extraction rates in the device. It is demonstrated that perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they are able to exceed the Shockley–Queisser limit and exhibit strongly elevated open-circuit voltages. It is therefore concluded that sufficient material and device stability under increased illumination levels will be the only significant challenge to perovskite concentrator solar cell applications.

Concentrator solar cells based on hybrid organic–inorganic perovskites are predicted to be a facile and promising technique to further boost power conversion efficiencies. High open-circuit voltages close to 1.4 V and power conversion efficiencies >30% can be expected under 100 sun illumination, assuming monomolecular recombination and bimolecular recombination dominate the losses and material degradation is absent.

Fullerenes and their derivatives are widely used as electron acceptors in bulk-heterojunction organic solar cells as they combine high electron mobility with good solubility and miscibility with relevant semiconducting polymers. However, studies on the use of fullerenes as the sole photogeneration and charge-carrier material are scarce. Here, a new type of solution-processed small-molecule solar cell based on the two most commonly used methanofullerenes, namely [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₀BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM), as the light absorbing materials, is reported. First, it is shown that both fullerene derivatives exhibit excellent ambipolar charge transport with balanced hole and electron mobilities. When the two derivatives are spin-coated over the wide bandgap p-type semiconductor copper (I) thiocyanate (CuSCN), cells with power conversion efficiency (PCE) of ≈1%, are obtained. Blending the CuSCN with PC₇₀BM is shown to increase the performance further yielding cells with an open-circuit voltage of ≈0.93 V and a PCE of 5.4%. Microstructural analysis reveals that the key to this success is the spontaneous formation of a unique mesostructured p–n-like heterointerface between CuSCN and PC₇₀BM. The findings pave the way to an exciting new class of single photoactive material based solar cells.

A new type of solution-processed solar cells based on fullerene derivatives and copper (I) thiocyanate (CuSCN) acting as the light absorbing and hole-extracting materials, respectively, is reported. The resulting cells are semitransparent (active layer transmission of ≈56% @ 300–800 nm) and exhibit power conversion efficiencies of 5.4%. The high performance is attributed to the spontaneous formation of a mesostructured p–n heterointerface.

Naphthalene dimide (NDI) is the most effective building block to construct n-type photovoltaic polymers, but all the polymers are obtained by copolymerized NDI with the symmetric building blocks. Here, the first NDI-based asymmetric-building block-containing polymer (ABC-polymer) is synthesized by molecular tailoring of benzodithiophene (BDT) unit. The final polymer of PNDI-T-BTh realizes a high FET electron mobility even with a face-on orientation, and achieves the highest power conversion efficiency (PCE) of 5.99%, which is obviously higher than that of the analogue PNDI-BDT polymers (PCE = 2.06%) and PNDI-T-P-T (PCE = 4.12%).

The research on transparent conductive electrodes is in rapid ascent in order to respond to the requests of novel optoelectronic devices. The synergic coupling of silver nanowires (AgNWs) and high-quality solution-processable exfoliated graphene (EG) enables an efficient transparent conductor with low-surface roughness of 4.6 nm, low sheet resistance of 13.7 Ω sq⁻¹ at high transmittance, and superior mechanical and chemical stabilities. The developed AgNWs–EG films are versatile for a wide variety of optoelectronics. As an example, when used as a bottom electrode in organic solar cell and polymer light-emitting diode, the devices exhibit a power conversion efficiency of 6.6% and an external quantum efficiency of 4.4%, respectively, comparable to their commercial indium tin oxide counterparts.

Solution-processed hybrid transparent electrodes are developed by spray-coating of silver nanowires and exfoliated graphene. Mechanical, chemical, and electronic properties of the silver nanowire network are improved upon the coverage of graphene layer. Uniform morphology, low surface roughness, and high conductivity at high transmittance of the film contribute to highly efficient optoelectronic devices, such as organic solar cells and polymer light-emitting diode.

In this essay, the authors use two properly encapsulated high-efficiency mesoscopic perovskite solar cells (PSCs), which use a state-of-the-art perovskite composition (HC(NH₂)₂PbI₃)_0.85(CH₃NH₃PbBr₃)_0.15 with excess PbI₂ as the active layer, to demonstrate the potential effect of dynamical electroluminescence responses on the analysis and interpretation of PSCs electrical characteristic. The essay does not aim to determine how to overcome this issue, nor to investigate its physical/chemical origin, although tentative propositions are made; but rather, to warn researchers in the field about the interpretation and reporting the results obtained from luminescence imaging measurements and the effect of image collection timing on the results. This is a critical message since the authors predict that luminescence imaging techniques will soon become one of the key tools for PSCs characterization, both for long-term stability assessment and fabrication process optimization.

It is expected that luminescence imaging will become a key tool for perovskite solar cells (PSCs) stability assessment and fabrication process optimization, especially for large-area devices. Even state-of-the-art mesoscopic PSCs, with a small photocurrent hysteresis, show dynamic electroluminescence signal, which complicates immediate and accurate analysis of luminescence imaging measurements results. This article demonstrates this while focusing on the effect of image collection timing.

Adding cesium (Cs) and rubidium (Rb) cations to FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time-of-flight, time-resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs-incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open-circuit voltage and fill factor of Cs-containing devices. By comparison, Rb-addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs-Rb-FA-MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple-cation perovskite solar cells, which are essential for the design of high-performance devices.

Time-resolved microwave conductivity, time-of-flight, and thermally stimulated current measurements reveal that Cs reduces the trap density in hybrid lead halide perovskites. Rb additives enhance the charge carrier mobility, but show minor effects on the trap landscape. The increase in open-circuit voltage in multiple-cation perovskite solar cells can be related to a reduced trap density through Cs-incorporation.

Organic/inorganic hybrid solar cells, typically mesoscopic and perovskite solar cells, are regarded as promising candidates to replace conventional silicon or thin film photovoltaics. There have been intensive investigations on the development of advanced materials for improved power conversion efficiencies, however, economical feasibilities and reliabilities of the organic/inorganic photovoltaics are yet to reach at a sufficient level for practical utilizations. In this study, cobalt nitride (CoN) nanofilms prepared by room-temperature vapor deposition in an inert N₂ atmosphere, which is a facile and highly reproducible procedure, are proposed as a low-cost counter electrode in mesoscopic dye-sensitized solar cells (DSCs) and a hole transport material in inverted planar perovskite solar cells (PSCs) for the first time. The CoN film successfully replaces conventional Pt in DSCs, resulting in a power conversion efficiency comparable to the ones based on Pt. In addition, PSCs employing the CoN manifest high efficiency even up to 15.0%, which is comparable to state-of-the-art performance in the cases of PSCs employing inorganic hole transporters. Furthermore, flexible solar cell applications of the CoN are performed in both mesoscopic and perovskite solar cells, verifying the advantages of the room-temperature deposition process and feasibilities of the CoN nanofilms in various fields.

CoN nanofilms prepared by room-temperature vapor deposition are applied as electrocatalysts and hole transport materials in organic/inorganic hybrid solar cells. The CoN counter electrode in place of Pt manifests a comparably high performance, and power conversion efficiency achieved in perovskite solar cells employing the CoN hole transporter is among the state-of-the-art results from inorganic hole transport materials.

In this contribution, a facile and universal method is successfully reported to fabricate perovskite solar cells (PSCs) with enhanced efficiency and stability. Through dissolving functional conjugated polymers in antisolvent chlorobenzene to treat the spinning CH₃NH₃PbI₃ perovskite film, the resultant devices exhibit significantly enhanced efficiency and longevity simultaneously. In-depth characterizations demonstrate that thin polymer layer well covers the top surface of perovskite film, resulting in certain surface passivation and morphology modification. More importantly, it is shown that through rational chemical modification, namely molecular fluorination, the air stability and photostability of the perovskite solar cells are remarkably enhanced. Considering the vast selection of conjugated polymer materials and easy functional design, promising new results are expected in further enhancement of device performance. It is believed that the findings provide exciting insights into the role of conjugated polymer in improving the current perovskite-based solar cells.

By dissolving conjugated polymer in antisolvent, the antisolvent is updated to antisolution in perovskite solar cell fabrication. Benefiting from a favorable arrangement of polymer layers, the device containing polymer exhibits improved photovoltaic efficiency (18.7%) and long-term stability. More importantly, it is shown that molecular fluorination of these functional polymers enhances device performance further.

Metal halide perovskites (MHPs) have numerous advantages as light emitters such as high photoluminescence quantum efficiency with a direct bandgap, very narrow emission linewidth, high charge-carrier mobility, low energetic disorder, solution processability, simple color tuning, and low material cost. Based on these advantages, MHPs have recently shown unprecedented radical progress (maximum current efficiency from 0.3 to 42.9 cd A⁻¹) in the field of light-emitting diodes. However, perovskite light-emitting diodes (PeLEDs) suffer from intrinsic instability of MHP materials and instability arising from the operation of the PeLEDs. Recently, many researchers have devoted efforts to overcome these instabilities. Here, the origins of the instability in PeLEDs are reviewed by categorizing it into two types: instability of (i) the MHP materials and (ii) the constituent layers and interfaces in PeLED devices. Then, the strategies to improve the stability of MHP materials and PeLEDs are critically reviewed, such as A-site cation engineering, Ruddlesden–Popper phase, suppression of ion migration with additives and blocking layers, fabrication of uniform bulk polycrystalline MHP layers, and fabrication of stable MHP nanoparticles. Based on this review of recent advances, future research directions and an outlook of PeLEDs for display applications are suggested.

Recent progress in understanding the origins of low stability of metal halide perovskite (MHP) materials and light-emitting diodes (PeLEDs) is reviewed. Various strategies to overcome the low stability are discussed with a special focus on the MHP material stability and operational stability of the PeLEDs. Future research directions to improve the stability are also suggested.

In most current state-of-the-art perovskite solar cells (PSCs), high-temperature (≈500 °C)-sintered metal oxides are employed as electron-transporting layers (ETLs). To lower the device processing temperature, the development of low-temperature-processable ETL materials (such as solution-processed ZnO) has received growing attention. However, thus far, the use of solution-processed ZnO is limited because the reverse decomposition reaction that occurs at ZnO/perovskite interfaces significantly degrades the charge collection and stability of PSCs. In this work, the reverse decomposition reaction is successfully retarded by sulfur passivation of solution-processed ZnO. The sulfur passivation of ZnO by a simple chemical means, efficiently reduces the oxygen-deficient defects and surface oxygen-containing groups, thus effectively preventing reverse decomposition reactions during and after formation of the perovskite active layers. Using the low-temperature-processed sulfur-passivated ZnO (ZnO–S), perovskite layers with higher crystallinity and larger grain size are obtained, while the charge extraction at the ZnO/perovskite interface is significantly improved. As a result, the ZnO–S-based PSCs achieve substantially improved power-conversion-efficiency (PCE) (19.65%) and long-term air-storage stability (90% retention after 40 d) compared with pristine ZnO-based PSCs (16.51% and 1% retention after 40 d). Notably, the PCE achieved is the highest recorded (19.65%) for low-temperature ZnO-based PSCs.

Air-stable high efficiency perovskite solar cells are developed using sulfur-passivated ZnO electron transport layers. Sulfur passivation of ZnO effectively prevents the interfacial reverse reaction from perovskite to PbI₂, while the surface hydrophobicity of ZnO is increased. The results show that the quality of perovskite layers is improved and the interfacial charge recombination is reduced.

All-inorganic CsPbBrI₂ perovskite has great advantages in terms of ambient phase stability and suitable band gap (1.91 eV) for photovoltaic applications. However, the typically used structure causes reduced device performance, primarily due to the large recombination at the interface between the perovskite, and the hole-extraction layer (HEL). In this paper, an efficient CsPbBrI₂ perovskite solar cell (PSC) with a dimensionally graded heterojunction is reported, in which the CsPbBrI₂ material is distributed within bulk–nanosheet–quantum dots or 3D–2D–0D dimension-profiled interface structure so that the energy alignment is optimized in between the valence and conduction bands of both CsPbBrI₂ and the HEL layers. Specifically, the valence-/conduction-band edge is leveraged to bend with synergistic advantages: the graded combination enhances the hole extraction and conduction efficiency with effectively decreased recombination loss during the hole-transfer process, leading to an enhanced built-in electric field, hence a high V_OC of as much as 1.19 V. The profiled structure induces continuously upshifted energy levels, resulting in a higher J_SC of as much as 12.93 mA cm⁻² and fill factor as high as 80.5%, and therefore record power conversion efficiency (PCE) of 12.39%. As far as it is known, this is the highest PCE for CsPbBrI₂ perovskite-based PSC.

Here, a 3D–2D–0D multi-graded interface based on CsPbBrI₂ bulk, nanosheets, and quantum dots is first designed for CsPbBrI₂ perovskite solar cells. Such a multigraded surface favorably reduces the recombination at the CsPbBrI₂/poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] interface, resulting in a record stabilized power conversion efficiency of 12.39%, a nearly 20% increase compared with 10.38% for ungraded devices.