ZiQi Sun

Organic–inorganic hybrid perovskites (OIHPs) are new photoactive layer candidates for lightweight and flexible solar cells due to their low-temperature process capability; however, the reported efficiency of flexible OIHP devices is far behind those achieved on rigid glass substrates. Here, it is revealed that the limiting factor is the different perovskite film deposition conditions required to form the same film morphology on flexible substrates. An optimized perovskite film composition needs a different precursor ratio, which is found to be essential for the formation of high-quality perovskite films with longer radiative carrier recombination lifetime, smaller density of trap states, reduced precursor residue, and uniform and pin-hole free films. A record efficiency of 18.1% is achieved for the flexible perovskite solar-cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low temperature (≤100 °C) solution process.

A different precursor ratio is found to be essential for the formation of high-quality perovskite films on flexible substrates compared to those formed on rigid substrates. A high efficiency of 18.1% is achieved for flexible perovskite solar-cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low-temperature (≤100 °C) solution process.

A cross-linkable dual functional polymer hybrid electron transport layer (ETL) is developed by simply adding an amino-functionalized polymer dopant (PN4N) and a light crosslinker into a commercialized n-type semiconductor (N2200) matrix. It is found that the resulting hybrid ETL not only has a good solvent resistance, facilitating multilayers device fabrication but also exhibits much improved electron transporting/extraction properties due to the doping between PN4N and N2200. As a result, by using PTB7-Th:PC₇₁BM blend as an active layer, the inverted device based on the hybrid ETL can yield a prominent power conversion efficiency of around 10.07%. More interestingly, photovoltaic property studies of bilayer devices suggest that the absorption of the hybrid ETL contributes to photocurrent and hence the hybrid ETL simultaneously acts as both cathode interlayer material and an electron acceptor. The resulting inverted polymer solar cells function like a novel device architectures with a combination of a bulk heterojunction device and miniature bilayer devices. This work provides new insights on function of ETLs and may be open up a new direction for the design of new ETL materials and novel device architectures to further improve device performance.

Cross-linkable dual functional hybrid electron transport layers are developed and can work as both cathode interlayer and light harvesting layer in polymer solar cells, which enhance electron collection and contribute to photocurrent production in the resulting devices. These results may provide new directions for the design of multifunctional interface materials and novel device architectures.

Luminous efficacy (LE), which is given by the ratio of luminous flux to power, is commonly used to measure the power consumption of a light source. Unfortunately, the LE of white organic light-emitting diodes (OLEDs) still lags behind those of inorganic LED for practically used (>100 lm W⁻¹). In this paper, an ultraefficient white OLED is discussed based on a newly designed thermally activated delayed fluorescent exciplex host. The resulting white OLED delivers an unusually high forward-viewing LE of 105.0 lm W⁻¹ and external quantum efficiency (EQE) η_ext of ≈30% (without using any optical out-coupling techniques). As far as it is known, specifically, these efficiencies are the highest values among the published white OLEDs to date. Two-color warm white emission is realized with Commission International de I'Eclairage coordinates of (0.40, 0.48) at a brightness of 1000 cd m⁻². Furthermore, the well-matched energy alignment endows the device with an extremely low turn-on voltage (≈2.5 V). Such high efficiencies and excellent device performance should benefit from the advantages of exciplex material solely used as the host. Therefore, this study anticipates that the findings have great potential to boost the LE of OLEDs, and more importantly, fulfill the power efficacy requirement for lighting applications.

An outstanding blue exciplex, named mCP:B4PyMPM, is implemented as a host for white organic light-emitting diode (OLED). Super-high luminous efficacy of 105.0 lm W⁻¹ and external quantum efficiency of >28% are realized without employing optical out-coupling techniques. Efficiency roll-off of mCP:B4PyMPM based OLEDs is relatively mild. Additionally, extremely low turn-on voltages (≈2.5 V) and warm white emission are also achieved.

Transparent conducting electrodes (TCEs) are considered to be an essential structural component of flexible organic solar cells (FOSCs). Silver nanowire (AgNW) electrodes are widely used as TCEs owing to their excellent electrical and optical properties. The fabrication of AgNW electrodes has faced challenges in terms of forming large uniform interconnected networks so that high conductivity and reproducibility can be achieved. In this study, a simple method for creating an intimate contact between AgNWs that uses cold isostatic pressing (CIP) is demonstrated. This method increases the conductivity of the AgNW electrodes, which enables the fabrication of high-efficiency inverted FOSCs that have a power conversion efficiency of 8.75% on flexible polyethylene terephthalate with no short circuiting occurring as the CIP process minimizes the surface roughness of the AgNW electrode. This allows to achieve 100% manufacturing yield of FOSCs. Furthermore, these highly efficient FOSCs are proven to only be 2.4% less efficient even for an extreme bending radius of R ≈ 1.5 mm, compared with initial efficiency.

Creating an intimate contact between silver nanowires (AgNWs) that uses cold isostatic pressing (CIP) is demonstrated, which enables to develop highly efficient and reproducible flexible organic solar cells (FOSCs). The fabricated FOSCs using CIP-treated AgNW electrode show the high-power conversion efficiency of 8.75%, with only a 2.4% of reduction in efficiency at a bending radius of R ≈ 1.5 mm.

A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 10⁵m⁻¹ cm⁻¹, a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10⁻³ cm² V⁻¹ s⁻¹. The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.

A fused hexacyclic electron acceptor with strong near-infrared absorption and high electron mobility is designed, synthesized, and applied in semitransparent organic solar cells, which exhibit a champion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability.

This paper reports highly bright and efficient CsPbBr₃ perovskite light-emitting diodes (PeLEDs) fabricated by simple one-step spin-coating of uniform CsPbBr₃ polycrystalline layers on a self-organized buffer hole injection layer and stoichiometry-controlled CsPbBr₃ precursor solutions with an optimized concentration. The PeLEDs have maximum current efficiency of 5.39 cd A⁻¹ and maximum luminance of 13752 cd m⁻². This paper also investigates the origin of current hysteresis, which can be ascribed to migration of Br⁻ anions. Temperature dependence of the electroluminescence (EL) spectrum is measured and the origins of decreased spectrum area, spectral blue-shift, and linewidth broadening are analyzed systematically with the activation energies, and are related with Br⁻ anion migration, thermal dissociation of excitons, thermal expansion, and electron–phonon interaction. This work provides simple ways to improve the efficiency and brightness of all-inorganic polycrystalline PeLEDs and improves understanding of temperature-dependent ion migration and EL properties in inorganic PeLEDs.

Efficient and bright CsPbBr₃ perovskite light-emitting diodes are achieved using a one-step fabrication of uniform CsPbBr₃ polycrystalline layers on a self-organized buffer hole injection layer without synthesis of quantum dots. A study of the temperature dependence of current hysteresis and electroluminescence spectrum provides understanding of ion migration, nonradiative pathways, and electron–phonon interaction in the CsPbBr₃ perovskite light-emitting diodes.

Upon cooling and heating, organic–inorganic perovskite nanoparticles are reversibly decomposed and formed through the lower-critical-solution-temperature phenomenon. Akinori Saeki and co-workers demonstrate an aurora-like photoemissive object in article number 1700047, and reveal the roles of the involved chemicals (methylammonium cation, lead, halogen, and molecular additives). The cloud temperature is varied from ca. 30 to 80 °C by controlling the concentration, and multi-luminescent-color thermoresponsive solutions are achieved through compositional engineering of the halogen.

Organic–inorganic hybrid perovskite solar cells have resulted in tremendous interest in developing next generation photovoltaics due to high record efficiency exceeding 22%. For inverted structure perovskite solar cells, the hole extraction layers play a significant role in achieving efficient and stable perovskite solar cell by modifying charge extraction, interfacial recombination losses, and band alignment. Here, cesium doped NiO_x is selected as a hole extraction layer to study the impact of Cs dopant on the optoelectronic properties of NiO_x and the photovoltaic performance. Cs doped NiO_x films are prepared by a simple solution-based method. Both doped and undoped NiO_x films are smooth and highly transparent, while the Cs doped NiO_x exhibits better electron conductivity and higher work function. Therefore, Cs doping results in a significant improvement in the performance of NiO_x-based inverted planar perovskite solar cells. The best efficiency of Cs doped NiO_x devices is 19.35%, and those devices show high stability as well. The improved efficiency in devices with Cs:NiO_x is attributed to a significant improvement in the hole extraction and better band alignment compared to undoped NiO_x. This work reveals that Cs doped NiO_x is very promising hole extraction material for high and stable inverted perovskite solar cells.

Cesium doping of NiO_x enhances the conductivity of the oxide film and the hole extraction from the perovskite film in inverted planar perovskite solar cells. Significantly improved photovoltaic performance is obtained with the best efficiencies of 16.04% and 19.35% for NiO_x and Cs:NiO_x, respectively. The devices exhibit negligible hysteresis and good stability.

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

Perovskite solar cells bearing spiro-phenylpyrazole-9,9′-thioxanthene exhibit power conversion efficiency of 18.06%, outperforming that of Spiro-OMeTAD (16.15%), which could be attributed to the enhancement of hole mobility exerted by the thioxanthene.

A wide-bandgap polymer, (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(2,5-(methyl thiophene carboxylate))]) (3MT-Th), is synthesized to obtain a complementary broad range absorption when harmonized with 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). The synthesized regiorandom 3MT-Th polymer shows good solubility in nonhalogenated solvents. A film of 3MT-Th:ITIC can be employed for forming an active layer in a polymer solar cell (PSC), with the blend solution containing toluene with 0.25% diphenylether as a nonhalogenated additive. The corresponding PSC devices display a power conversion efficiency of 9.73%. Moreover, the 3MT-Th-based PSCs exhibit excellent shelf-life time of over 1000 h and are operationally stable under continuous light illumination. Therefore, methyl thiophene-3-carboxylate in 3MT-Th is a promising new accepting unit for constructing p-type polymers used for high-performance nonfullerene-type PSCs.

A wide-bandgap polymer, (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(2,5-(methyl thiophene carboxylate))]) (3MT-Th), displays a high efficiency of 9.73% with 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene acceptor using toluene as a nonhalogenated solvent. The 3MT-Th-based PSCs exhibit excellent shelf-life stability of over 1000 h and operationally stable under continuous light illumination compared with the PTB7-Th-based PSCs.

This paper presents a systematic study of the influence of electron-transport materials on the operation stability of the inverted perovskite solar cells under both laboratory indoor and the natural outdoor conditions in the Negev desert. It is shown that all devices incorporating a Phenyl C₆₁ Butyric Acid Methyl ester ([60]PCBM) layer undergo rapid degradation under illumination without exposure to oxygen and moisture. Time-of-flight secondary ion mass spectrometry depth profiling reveals that volatile products from the decomposition of methylammonium lead iodide (MAPbI₃) films diffuse through the [60]PCBM layer, go all the way toward the top metal electrode, and induce its severe corrosion with the formation of an interfacial AgI layer. On the contrary, alternative electron-transport material based on the perylendiimide derivative provides good isolation for the MAPbI₃ films preventing their decomposition and resulting in significantly improved device operation stability. The obtained results strongly suggest that the current approach to design inverted perovskite solar cells should evolve with respect to the replacement of the commonly used fullerene-based electron-transport layers with other types of materials (e.g., functionalized perylene diimides). It is believed that these findings pave a way toward substantial improvements in the stability of the perovskite solar cells, which are essential for successful commercialization of this photovoltaic technology.

Diffusion of CH₃NH₃I and other volatile products of photodegradation of CH₃NH₃PbI₃ into the [60]PCBM electron-transport layer represents the key failure mechanism of inverted hybrid perovskite solar cells.

High-efficiency small-molecule-based organic photovoltaics (SM-OPVs) using two electron donors (p-DTS(FBTTh₂)₂ and ZnP) with distinctively different absorption and structural features are reported. Such a combination works well and synergically improves device short-circuit current density (J_sc) to 17.99 mA cm⁻² and fill factor (FF) to 77.19%, yielding a milestone efficiency of 11%. To the best of our knowledge, this is the highest power conversion efficiency reported for SM-OPVs to date and the first time to combine high J_sc over 17 mA cm⁻² and high FF over 77% into one SM-OPV. The strategy of using multicomponent materials, with a selecting role of balancing varied electronic and structural necessities can be an important route to further developing higher performance devices. This development is important, which broadens the dimension and versatility of existing materials without much chemistry input.

High-efficiency all-small-molecule organic solar cells using two electron donors with distinctively different absorption and structural features are reported. Such a combination works well and synergically improves device current and fill factor, yielding a milestone efficiency of 10.97%.

In article number 1606909, Ming-Fai Lo, Chun-Sing Lee, and co-workers report unusual free-carrier generation in some bipolar organic materials (SubNc and SubPc) upon photoexcitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than previously reported single-layer devices. Interestingly, these photoactive layers can be stacked onto each other with any sequence to generate a photocurrent in OPV devices.