continuously increase with light illumination while the shortcircuit current ( J SC ) experiences a quick increase and then a decrease upon light exposure. The C -V measurements fi nd that light soaking can decrease the charge accumulation at the electrode interfaces. Essentially, the light soaking-decreased charge accumulation at electrode interface can be attributed to following two possible processes. First, the photogenerated carriers can neutralize the interfacial defects at electrode interface upon light illumination. Second, the migration of ions can change the built-in electric fi eld and then affects the charge accumulation at electrode interfaces. In particular, these two processes can largely increase the V OC by increasing interfacial potential barrier at electrode interfaces during light illumination. The time-dependent PL and frequency-dependent capacitance ( Cf ) fi nd that the bulk defects within perovskite fi lm are mainly positively charged and can be neutralized by photogenerated electrons upon light illumination. In particular, our frequency-dependent capacitance provides the fi rst direct evidence that light soaking can decrease bulk-electrical polarization within organo-metal halide perovskites. Especially, decreasing the bulk-electrical polarization causes a decrease on J SC in light soaking. However, neutralizing the defects at electrode interfaces and bulk perovskite fi lm can enhance the transport of the dissociated charge carriers to respective electrodes, increasing the FF during light illumination. Clearly, our experimental studies provide an in-depth understanding on internal coupling between electrode and bulk parameters in light soaking and hysteresis phenomena in perovskite solar cells under deviceoperating condition. Figure 1 shows the light soaking effects on device performance for perovskite solar cells. On initial light exposure the device shows a lower photovoltaic performance with V OC = 0.51 V, J SC = 18.34 mA cm −2 , FF = 53.1%, and power conversion effi ciency (PCE) = 4.97%. With continuous light exposure, device performance is signifi cantly improved over time. After ≈20 min of light soaking, the enhanced V OC = 0.83 V, J SC = 18.18 mA cm −2 , FF = 69.5% are obtained, resulting in a PCE = 10.49%. As we know, a continuous light illumination can cause heating and charge trapping in the development of light soaking and hysteresis effects. Here, our studies indicate that the light soaking and hysteresis effects come from charge trapping rather than heating. Specifi cally, we observe that the surface temperature of device can be increased from 26 to 42 °C when the cells are continuously exposed to light illumination. However, the device performance only slightly decreases, according to the J -V characteristics (inset in Figure 2 ), when the temperature increased to 42 ºC equivalent to the temperature produced by continuous light illumination. After removing the heating and cooling the device to room temperature, we can see that the device performance is again signifi cantly

Blue emissions in organic light-emitting devices (OLEDs) are of great significance for their application in full color flat-panel displays and white lightings. [1] However, high-performance blue emitters are still relatively rare. In OLEDs, the injected electrons and holes recombine to form singlet and triplet excitons in the ratio of 1:3, according to the spin statistics, whereas only singlet exciton can decay radiatively in fluorescent materials. [2] Approximately 75% of the triplet excitons are wasted in nonradiative processes, leading to an upper limit of the internal quantum efficiency (IQE) of only 25% in conventional fluorescent devices. One of the methods to enhance the efficiency of OLEDs is to make use of the nonemissive triplet excitons. [3] Phosphorescent OLEDs (PhOLEDs) based on Ir, Pt, and Os organic-metal complexes can approach 100% IQE, which is attributed to the heavy-atom effect. [4] Yet, pure-blue and deep-blue phosphors with Commission Internationale de l'Eclairage (CIE) y values smaller than 0.15 are particularly scarce due to the inherently great challenge in their molecular design; similarly, proper host materials with a large band gap that allows for the refinement of the triplet excitons in devices are also rare. Therefore, it is important to find a way to develop efficient, stable, pure-and deep-blue fluorescent materials. In principle, new-generation, purely organic fluorescent materials can also utilize the nonemissive triplet excitons and achieve high efficiency by converting triplet excitons into singlet excitons. The main mechanisms involve triplet-triplet annihilation (TTA), thermally activated delayed fluorescence (TADF) and the "hot exciton" channel. [5] Essentially, both the TTA and TADF processes can promote the external quantum efficiency (EQE) of the devices by converting excitons from the lowest triplet excited state (T 1 ) to the lowest singlet excited state (S 1 ). Experimental results have confirmed that devices based on TTA and TADF materials can realize a high EQE with a breakthrough of the spin statistical limitation. [6] Although a high EQE has been obtained in TTA and TADF materials, pure-and deep-blue emitters with high efficiency and stability are still exiguous. Unlike TTA and TADF materials, the "hot exciton" materials reported by our group highlight the reverse intersystem crossing from Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet-triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T 1 ) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high-lying excited state (T 2 ), referred to here as a "hot exciton" path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S 1 and T 2 and rather large T 2 -T 1 energy gap, which are expected to...

Possible ionic motion becomes visible by an impedance spectroscopy technique in the different phases of CH3NH3PbI3.

Solution-processed organic-inorganic hybrid perovskites are promising emitters for next-generation optoelectronic devices. Multiple-colored, bright light emission is achieved by tuning their composition and structures. However, there is very little research on exploring optically active organic cations for hybrid perovskites. Here, unique room-temperature phosphorescence from hybrid perovskites is reported by employing novel organic cations. Efficient room-temperature phosphorescence is activated by designing a mixed-cation perovskite system to suppress nonradiative recombination. Multiple-colored phosphorescence is achieved by molecular design. Moreover, the emission lifetime can be tuned by varying the perovskite composition to achieve persistent luminescence. Efficient room-temperature phosphorescence is demonstrated in hybrid perovskites that originates from the triplet states of the organic cations, opening a new dimension to the further development of perovskite emitters with novel functional organic cations for versatile display applications.

A single walled carbon nanotube (SWCNT) possesses excellent hole conductivity. This work communicates an investigation of perovskite solar cells using a mesoscopic TiO2/Al2O3 structure as a framework in combination with a certain amount of SWCNT-doped graphite/carbon black counter electrode material. The CH3NH3PbI3-based device achieves a power conversion efficiency of 14.7% under AM 1.5G illumination. Detailed investigations show an increased charge collection in this device compared to that without the SWCNT additive.

White organic light‐emitting diodes (WOLEDs) are highly attractive in the fields of solid‐state lighting. The biggest challenge that is facing at present is how to maximize the exciton utilization to further enhance the efficiency, while taking into account the stability. Here, highly efficient all‐fluorescence and fluorescence/phosphorescence (F/P) hybrid WOLEDs with low efficiency roll‐off by designing exciplex‐sandwich emissive architecture and precisely manipulating the exciton allocation are demonstrated. The resulting complementary‐color hybrid WOLEDs realize the maximum external quantum efficiency of 28.3% and power efficiency of 102.9 lm W−1, and remain 26.9% and 73.5 lm W−1 at 500 cd m−2 and yet as high as 25.8% and 63.5 lm W−1 at 1000 cd m−2, respectively, revealing very low roll‐off. By using the efficient blue exciplex combined with red and green phosphorescent emitters, the three‐color WOLEDs yield a high color rendering index of 86, an external quantum efficiency of 29.4%, and a power efficiency of 75.5 lm W−1. It is anticipated that the exciplex engineering will open an efficient avenue to precisely allocate excitons, and finally producing high‐performance WOLEDs for next‐generation solid‐state lighting technology.

Most organic semiconductors have closed-shell electronic structures, however, studies have revealed open-shell character emanating from design paradigms such as narrowing the bandgap and controlling the quinoidal-aromatic resonance of the π-system. A fundamental challenge is understanding and identifying the molecular and electronic basis for the transition from a closed- to open-shell electronic structure and connecting the physicochemical properties with (opto)electronic functionality. Here, we report donor-acceptor organic semiconductors comprised of diketopyrrolopyrrole and naphthobisthiadiazole acceptors and various electron-rich donors commonly utilized in constructing high-performance organic semiconductors. Nuclear magnetic resonance, electron spin resonance, magnetic susceptibility measurements, single-crystal X-ray studies, and computational investigations connect the bandgap, π-extension, structural, and electronic features with the emergence of various degrees of diradical character. This work systematically demonstrates the widespread diradical character in the classical donor-acceptor organic semiconductors and provides distinctive insights into their ground state structure-property relationship.

wileyonlinelibrary.compast few years, hybrid WOLEDs, combining the blue fl uorophors and longwavelength phosphors, have attracted substantial attention owing to the unique merits of high effi ciency and excellent stability. [ 2 ] In principle, to achieve a theoretical maximum internal quantum efficiency for hybrid WOLEDs, a prerequisite key is that all electrically generated singlet and triplet excitons must be effectively utilized for the white emission. [ 2,3 ] Enormous efforts have been devoted to simultaneously harvest both the singlet and triplet excitons in single-emissive-layer (single-EML), [ 4 ] and multi-emissive-layer (multi-EML) hybrid WOLEDs. [ 5 ] In the single-EML hybrid WOLEDs, the precise manipulation of phosphorescent emitter concentration in blue fl uorophore host is very necessary to suppress the singlet exciton transfer from the blue fl uorophore to the phosphors via Förster energy transfer. In this case, the phosphorescent dopant concentration and the property of used blue fl uorophore host have obvious effects on the device performance and there exist the problems of notorious spectrum shift with the increased operational voltages. Alternatively, the multi-EML counterparts provide a reliable strategy Thermally activated delayed fl uorescence (TADF)-based white organic lightemitting diodes (WOLEDs) are highly attractive because the TADF emitters provide a promising alternative route to harvest triplet excitons. One of the major challenges is to achieve superior effi ciency/color rendering index/ color stability and low effi ciency roll-off simultaneously. In this paper, highperformance hybrid WOLEDs are demonstrated by employing an effi cient blue TADF emitter combined with red and green phosphorescent emitters. The resulting WOLED shows the maximum external quantum effi ciency, current effi ciency, and power effi ciency of 23.0%, 51.0 cd A −1 , and 51.7 lm W −1 , respectively. Moreover, the device exhibits extremely stable electroluminescence spectra with a high color rendering index of 89 and Commission Internationale de L'Eclairage coordinates of (0.438, 0.438) at the practical brightness of 1000 cd m −2 . The achievement of these excellent performances is systematically investigated by versatile experimental and theoretical evidences, from which it is concluded that the utilization of a blue-green-red cascade energy transfer structure and the precise manipulation of charges and excitons are the key points. It can be anticipated that this work might be a starting point for further research towards high-performance hybrid WOLEDs.