In this paper, we reported the in situ fabrication of highly luminescent formamidinium lead bromide (FAPbBr) nanocrystal thin films by dropping toluene as an anti-solvent during the spin-coating with a perovskite precursor solution using 3,3-diphenylpropylamine bromide (DPPA-Br) as a ligand. The resulting films are uniform and composed of 5-20 nm FAPbBr perovskite nanocrystals. By monitoring the solvent mixing of anti-solvent and precursor solution on the substrates, we illustrated the difference between the ligand-assisted reprecipitation (LARP) process and the nanocrystal-pinning (NCP) process. This understanding provides a guideline for film optimization, and the optimized films obtained through the in situ LARP process exhibit strong photoluminescence emission at 528 nm, with quantum yields up to 78% and an average photoluminescence lifetime of 12.7 ns. In addition, an exciton binding energy of 57.5 meV was derived from the temperature-dependent photoluminescence measurement. More importantly, we achieved highly efficient pure green perovskite based light-emitting diode (PeLEDs) devices with an average external quantum efficiency (EQE) of 7.3% (maximum EQE is 16.3%) and an average current efficiency (CE) of 29.5 cd A (maximum CE is 66.3 cd A) by adapting a conventional device structure of ITO/PEDOT:PSS/TFB/perovskite film/TPBi/LiF/Al. It is expected that the in situ LARP process provides an effective methodology for the improvement of the performance of PeLEDs.

The in situ fabricated perovskite nanocrystals (PNCs) obtained through spin‐coating a precursor solution are suitable candidates to achieve efficient perovskite light emitting diodes (PeLEDs). In this work, the solvent effects of on‐chip crystallization are investigated by correlating the nucleation and growth process of in situ fabricated formamidinium lead iodide (FAPbI3) nanocrystals with their optical and electronic properties. The FAPbI3 nanocrystals obtained from a precursor solution in γ‐butyrolactone (GBL) are smaller than those obtained from N,N‐dimethylformamide and dimethyl sulfoxide, and the relatively weak coordination between GBL and the precursor molecules enables reduced defect states in the resulted PNCs with enhanced photoluminescence properties. A modified LaMer model is proposed to describe the solvent effects in the on‐chip crystallization process. Based on these understandings, red emissive FA0.87Cs0.13PbI3 nanocrystal films with absolute photoluminescence quantum yields up to 70% are realized. Finally, an efficient PeLED with maximum luminance of 218 cd m−2 and peak external quantum efficiency of 15.8% is achieved with good reproducibility.

Solution‐processed optoelectronic devices based on conjugated polymers, colloidal quantum dots (CQDs), halide perovskites, and so on are now emerging as a new‐generation semiconductor technology which prevails its conventional counterparts in terms of low fabrication cost, ease of scalable manufacturing, and abundant material designability. However, the solution‐processed thin films obtained through spin‐coating, spray, inkjet printing, and doctor blading usually suffer from low film quality and a high defect density especially at the interfaces of different functional layers. Currently, the most significant subject is to address the non‐ideal interfaces for achieving improved performance of the devices. Impedance spectroscopy (IS) is a universal technique that can help to examine the charge behavior at the interfaces in an electrochemical or solid‐state multilayered device. Owing to its ability to elucidate the charge transfer, charge transport, and accumulation within the interfaces of electrochemical or multilayered devices with minimal effects to the devices themselves, the use of IS has increased vividly in the last decades. This review provides the basic principles of IS and its applications on solution‐processed optoelectronic devices.

The feasibility of in situ fabrication of metal halide perovskite films for optoelectronic devices has attracted increasing research interest as it eliminates the material preparation step and thus simplifies the integration procedure. The success of in situ fabrication requires delicate regulations of the underlayers. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a universal hole transporting material for solution-processed optoelectronic devices; however, it is usually hard to cast high-quality active layers directly onto the pristine PEDOT:PSS layer due to its unfavored surface properties. The physical changes in the film morphology and optical and electric properties and the chemical change in the molecular composition of the oxygen plasma-treated PEDOT:PSS surface are investigated in this work. It is found that the reduced film thickness and PSS-rich surface after plasma treatment facilitate the growth of upper layers of formamidinium lead bromide (FAPbBr 3 ) nanocrystal films, leading to higher photoluminescence quantum yields. The magnificent performance of perovskite-based light-emitting diodes (Pe-LEDs) with oxygen plasma-treated PEDOT:PSS shows a highest current efficiency of 54.0 cd/A, which is more than 4 times than those of the analogous Pe-LEDs fabricated with pristine PEDOT:PSS.

Colloidal quantum dots (QDs) are talented materials and have been extensively investigated in the field of photonics and optoelectronics due to their size-dependent optical properties. The core/shell structure of QDs with a wide band gap shell has been adopted for obtaining stable emission and high photoluminescent (PL) quantum efficiency. However, when employed in active devices such as light-emitting diodes (LEDs), the thick-shell structure of QDs may impede the transportation of carriers, thus deteriorating the device performance. In this work, the effect of the shell thickness of CdSe/ZnS QDs on the device performance is systematically studied through impedance spectroscopy, by constructing the electron-only symmetric device architecture. It is found that the evolution of capacitance in the symmetric device under applied voltage reflects the charge accumulation within the device and predicts the LED performance. The lowest capacitance is evaluated in the symmetric device containing QDs with a medium shell size of 2.1 nm, showing improved performance in the LED with the highest luminance and current efficiency of 26 370 cd/m2 and 8.3 cd/A, respectively.