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Perovskite light-emitting diodes (PeLEDs) have recently shown significant progress with external quantum efficiencies (EQEs) exceeding 20%. However, PeLEDs with pure-red (620-660 nm) light emission, an essential part for full-color displays, remain a great challenge. Herein, a general approach of spacer cation alloying is employed in Ruddlesden-Popper perovskites (RPPs) for efficient red PeLEDs with precisely tunable wavelengths. By simply tuning the alloying ratio of dual spacer cations, the thickness distribution of quantum wells in the RPP films can be precisely modulated without deteriorating their charge-transport ability and energy funneling processes. Consequently, efficient PeLEDs with tunable emissions between pure red (626 nm) and deep red (671 nm) are achieved with peak EQEs up to 11.5%, representing the highest values among RPP-based pure-red PeLEDs. This work opens a new route for color tuning, which will spur future developments of pure-red or even pure-blue PeLEDs with high performance. Ministry of Education (MOE) National Research Foundation (NRF) Published version This work was financially supported by the ERC Starting Grant (No. 717026), the Swedish Research Council VR (No. 2018–07109), the Swedish Foundation for International Cooperation in Research and Higher Education (No. CH2018-7736), and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU no. 2009-00971). The computational work for this article was fully performed on resources of the National Supercomputing Centre (NSCC), Singapore (https://www.nscc.sg). S.R., Q.X., X.-K.L., and T.C.S. acknowledge the support from Nanyang Technological University under its start-up grant (M4080514); the Ministry of Education, Singapore, under its AcRF Tier 2 grants (MOE2019-T2-1-006 and MOE-T2EP50120-0004); and the National Research Foundation (NRF), Singapore, under its NRF Investigatorship (NRF-NRFI2018-04). L.H. thanks the NSFC Project (61774077), the Key Projects of Joint Fund of Basic and Applied Basic Research Fund of Guangdong Province (2019B1515120073, 2019B090921002, 2020A1414010036), and the Guangzhou Key laboratory of Vacuum Coating Technologies and New Energy Materials Open Projects Fund (KFVE20200006) for financial support. Z.C. thanks the project funded by China Postdoctoral Science Foundation (2020M673055). J.Q. acknowledges the support by the Science and Technology Planning Project of Guangzhou, China (Grant No. 201605030008), and the Fundamental Research Funds for the Central Universities (Grant No. 21621008)

We investigate the effect of commonly used solution-processed TiOx, SnO2 and ZnO interlayers on the perovskite film crystallization process. We find that the ZnO/perovskite interface can efficiently catalyze the perovskite crystallization even without thermal annealing. Funding Agencies|ERC [717026]; Carl Tryggers Stiftelse; European Commission [691210]; China Scholarship Council; VINNMER Marie Curie Fellowships

Inorganic/organic ternary hybrid structures are emerging as promising candidates for the fabrication of novel solar cells. As one of the most important inorganic solar cell material, silicon (Si) is worth careful exploration during the preparation of the hybrid structures. Here we incorporate the nanoscale form of Si, Si nanocrystals (NCs), into the classical bulk-heterojunction organic solar cells based on poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). It is found that the solar cell efficiency may increase by ~ 40% when ~ 5% of PCBM in the original P3HT:PCBM blend is replaced with Si NCs. The efficiency enhancement is enabled by the improved short-wavelength absorption, optimized film structure and cascade energy-level alignment.

Semiconductor quantum dots (QDs) are among the most promising next-generation optoelectronic materials. QDs are generally obtained through either epitaxial or colloidal growth and carry the promise for solution-processed high-performance optoelectronic devices such as light-emitting diodes (LEDs), solar cells, etc. Herein, a straightforward approach to synthesize perovskite QDs and demonstrate their applications in efficient LEDs is reported. The perovskite QDs with controllable crystal sizes and properties are in situ synthesized through one-step spin-coating from perovskite precursor solutions followed by thermal annealing. These perovskite QDs feature size-dependent quantum confinement effect (with readily tunable emissions) and radiative monomolecular recombination. Despite the substantial structural inhomogeneity, the in situ generated perovskite QDs films emit narrow-bandwidth emission and high color stability due to efficient energy transfer between nanostructures that sweeps away the unfavorable disorder effects. Based on these materials, efficient LEDs with external quantum efficiencies up to 11.0% are realized. This makes the technologically appealing in situ approach promising for further development of state-of-the-art LED systems and other optoelectronic devices. Funding Agencies|ERC Starting Grant [717026]; Carl Tryggers Stiftelse; European Commission Marie SklodowskaCurie Actions [691210]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; China Scholarship Council

Chemical doping is often used to enhance electric conductivity of the conjugated molecule as hole-transporting material (HTM) for the application in optoelectronics. However, chemical dopants can promote ion migration at the electrical field, which deteriorates the device efficiency as well as increases the fabrication cost. Here, two star HTMs, namely 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine) 9,9′-spirobifluorene (Spiro-OMeTAD) and poly(triarylamine) are subjeted to chemical combination to yield dopant-free N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-10-phenyl-10H-spiro[acridine-9,9′-fluorene]-2,2′,7,7′-tetraamine (SAF-OMe). The power conversion efficiencies (PCEs) of 12.39% achieved by solar cells based on pristine, dopant-free SAF-OMe are among the highest reported for perovskite solar cells and are even comparable to devices based on chemically doped Spiro-OMeTAD (14.84%). Moreover, using a HTM comprised of SAF-OMe with an additional dopant results in a record PCE of 16.73%. Compared to Spiro-OMeTAD-based devices, SAF-OMe significantly improves stability.

Layered bismuth selenide (L-Bi2Se3) nanoplates were implemented as hole transporting layers (HTLs) for inverted organic solar cells. Device based on L-Bi2Se3 showed increasing power conversion efficiency (PCE) during ambient condition storage process. A PCE of 4.37% was finally obtained after 5 days storage, which outperformed the ones with evaporated-MoO3 using poly(3-hexylthiophene) (P3HT) as donor material and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor. The improved device efficiency can be attributed to the high conductivity and increasing work function of L-Bi2Se3. The work function of L-Bi2Se3 increased with the storage time in ambient condition due to the oxygen atom doping. Ultraviolet photoelectron spectroscopy and high resolution X-ray photoelectron spectroscopy were conducted to verify the increased work function, which originated from the p-type doping process. The device based on L-Bi2Se3 exhibited excellent stability in ambient condition up to 4 months, which was much improved compared to the device based on traditional HTLs.

A new naphthodifuran (NDF) building block has been designed and synthesized. The distinguishing feature of the monomer is two dialkyl substituted thieno groups which are placed in the middle of the ring-fused structure as solubilizing groups. The steric interactions between repeating units in the polymer can be reduced by placing the solubilising groups in the middle of the ring-fused structure. Those dialkyl substituted thieno groups not only improved the solubility of the polymer but also provided additional conjugation in the second dimension. The new monomer (NDFT) was applied to the synthesis of the conjugated polymer (PNDFT-DTBT). The photovoltaic properties of the polymer were evaluated in bulk hetero-junction polymer solar cell devices with PC61BM as acceptors under AM 1.5G illumination at 100 mW cm−2. The device showed an open circuit voltage (Voc) of 0.89 V, a fill factor (FF) of 0.72, a short-circuit current density (Jsc) of 8.21 mA cm−2, and a power conversion efficiency (PCE) of 5.22%.

Solution-processed phosphomolybdic acid (PMA) was investigated as a hole extraction layer for organic solar cells without any post-annealing processes. PMA is a stable, cost-effective, and commercial soluble oxide cluster, which dramatically simplifies the device fabrication process as well as reduces the cost. The PMA-based device with an active layer of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexl)carbonyl]thieno [3,4-b]thiophenediyl]] (PTB7) mixed with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) achieved a power conversion efficiency (PCE) as high as 8.32% compared with 7.46% for an evaporated MoO3 (e-MoO3)-based device. Transient electrical output characteristics, optical measurements as well as scanning Kelvin probe microscopy were utilized to investigate the improvement in the performance of the device. The results indicated that the enhanced PCE was due to suppressed charge recombination as well as an improved optical path across the device, which led to an enhanced short-circuit current density and fill factor. In addition, the PMA-based device displayed improved long term stability in ambient conditions compared to the e-MoO3-based device.

Design and synthesis of novel monomers may create new opportunities for high performance semiconducting polymers. The synthetic modification of an isoindigo related building block based on rational design has been carried out in this study. An expanded isoindigo unit (IBTI) with 5,5′-divinyl-2,2′-bithiophene as a conjugated bridge has been successfully incorporated into a donor–acceptor conjugated polymer, PBDT–IBTI. The new polymer showed broad absorption and a large extinction coefficient in the visible region. The optical bandgap, HOMO and LUMO levels of this new polymer are 1.71 eV, −5.24 eV and −3.72 eV, respectively. The polymer solar cells based on PBDT–IBTI and (6,6)-phenyl-C61 butyric acid methyl ester (PC61BM) blends achieved a PCE of 6.41%. This work has demonstrated that IBTI is a promising building block for organic photovoltaic (OPV) polymers. These findings have also revealed the versatility of oxindole as the basic building element for isoindigo-like conjugated monomers.