For quantum-dot photodiodes comprising an electron-transporting layer assembled of ZnO nanoparticles, the light emitter/absorber generally exhibits enhanced optoelectronic performance after the device is shelf-aged. To understand the so-called positive aging effect, the optoelectronic properties of ZnO nanoparticles are investigated at the thin film and device level as a function of aging time. It is evidenced that the aging process is driven by a surface-stabilizing mechanism of ZnO nanoparticles, in which the active surface adsorption sites for oxygen are gradually but irreversibly stabilized, i.e.. with surface termination of HO-ZnO, leading to reduced nonradiative recombination and increased built-in potential in the adjacent photoactive layer. This work provides insight into new synthetic routes for minimizing the negative impact caused by the aging process.

To industrialize printed full-color displays based on quantum-dot light-emitting diodes, one must explore the degradation mechanism and improve the operational stability of blue electroluminescence. Here, we report that although state-of-the-art blue quantum dots, with monotonically-graded core/shell/shell structures, feature near-unity photoluminescence quantum efficiency and efficient charge injection, the significant surface-bulk coupling at the quantum-dot level, revealed by the abnormal dipolar excited state, magnifies the impact of surface localized charges and limits operational lifetimes. Inspired by this, we propose blue quantum dots with a large core and an intermediate shell featuring nonmonotonically-graded energy levels. This strategy significantly reduces surface-bulk coupling and tunes emission wavelength without compromising charge injection. Using these quantum dots, we fabricate bottom-emitting devices with emission colors varying from near-Rec.2020-standard blue to sky blue. At an initial luminance of 1000 cd m−2, these devices exhibit T95 operational lifetimes ranging from 75 to 227 h, significantly surpassing the existing records.

ZnO-based electron-transporting layers (ETLs) have been universally used in quantum-dot light-emitting diodes (QLEDs) for high performance. The active surface chemistry of ZnO nanoparticles (NPs), however, leads to QLEDs with positive aging and unacceptably poor shelf stability. SnO 2 is a promising candidate for ETLs with less reactivity, but NP agglomeration in nonionic solvents makes the conventional device structure abandoned, resulting in QLEDs with extremely low operational lifetimes. The large barrier for electron injection also limits the electroluminescence efficiency. Here, we report one solution to all the above-mentioned problems. Owing to the strong HO−SnO 2 coordination and the steric effect provided by the hydrocarbon groups, tetramethylammonium hydroxide can stabilize SnO 2 NPs in alcohol, while its intrinsic dipole induces a favorable electronic-level shift for charge injection. The SnO 2 -based devices, with the conventional structure, exhibit not only the most efficient electroluminescence among ZnO-free QLEDs but also an operational lifetime (T 95 ) over 3200 h at 1000 cd m −2 , which is comparable with that of state-of-the-art ZnO-based devices. More importantly, the superior shelf stability means that the TMAH−SnO 2 NPs are promising to enable QLEDs with real stability.

For organic solar cells (OSCs), the charge generation mechanism and the recombination loss are heavily linked with charge transfer states (CTS). Measuring the energy of CTS (E CT) by the most widely used technique, however, has become challenging for the non-fullerene-based OSCs with a small driving force, resulting in difficulty in the understanding of OSC physics. Herein, we present a study of the PM6:Y6 bulk heterojunction. It is demonstrated that electro-absorption can not only reveal the dipolar nature of Y6 but also resolve the morphology-dependent absorption signal of CTS in the sub-bandgap region. The device with the optimum blending weight ratio shows an E CT of 1.27 eV, which is confirmed by independent measurements. Because of the charge transfer characteristics of Y6, the charge generation at PM6:Y6 interfaces occurs efficiently under a small but non-negligible driving force of 0.14 eV, and the total recombination loss is as low as 0.43 eV.

n-i-p normal structure, the deep-level states between the perovskite absorber and the metal oxide electron-transporting layer (ETL), with high density of states, are one of the major sources of recombination current. [18,19] Aiming to reduce surface recombination in PSCs, numerous types of additives and surface stabilizers have been attempted. [20,21] So far, most high-performance PSCs were achieved through the strategy of surface passivation. [4,8,22] Theoretically, the total recombination flux in a solar cell is not only determined by recombination centers, but also the spatial distribution of minority carriers. [23] If minority carriers accumulate in the vicinity of deep-level centers, i.e., the ETL/ perovskite interface, the cell is subject to significant V OC loss even if the defects are partially passivated. In fact, comparing to other types of solar cells, the issue of carrier distribution is barely studied in PSCs. As revealed by ultraviolet photoelectron spectroscopy (UPS), the perovskites grown on n-type metal oxides consistently exhibit n-type characteristics. [24][25][26][27] Therefore, if the ETL has a larger work function than the perovskite, a depletion region is formed at the perovskite surface. [26,28] The bending of valence band maximum (VBM) therefore confines the minority carriers, holes, to the front surface, resulting in aggravated recombination loss. Alternatively, if the ETL has a smaller work function than the perovskite, the positive offset of conduction band minimum (CBM) at the ETL/perovskite interface creates an extra energy barrier for electron extraction. In practice, unless one can find an ETL with a smaller work function but larger electron affinity than the perovskite, [29] the above-mentioned issue is inevitable for the absorbers with homogenous composition.Introducing bandgap gradient to the absorber enables independent modification of CBM and VBM. [30] As we and many others have demonstrated for CIGS cells, the widening of absorber bandgap at the front surface can significantly enhance Voc, while maintaining the photocurrent harvested in the remaining part of the absorber. [31][32][33][34] To be noted, such a gradient structure reduces surface recombination through the management of carrier distribution, therefore it has a distinctly different working principle comparing to those designed to improve charge extraction. [35,36] In the present work, by introducing wide-gap perovskite, CsPbBr 3 quantum dots (QDs), to the front surface, the VBM of the absorber is modified so as to lower the minority carrier concentration in the vicinity of ETL/ absorber interface, and therefore reduce the recombination loss. As a result, V OC enhancement is achieved for both CsFAMAThe recombination flux in a solar cell is determined by not only recombination centers, but also the spatial distribution of minority carriers. For halide perovskite solar cells (PSCs), although there has been a tremendous amount of work focusing on defect passivation, the issue of carrier distribution is not as well stud...