Organic photovoltaics (OPVs) hold great promise for next-generation photovoltaics in renewable energy because of the potential to realize low-cost mass production via large-area roll-to-roll printing technologies on flexible substrates. To achieve high-efficiency OPVs, one key issue is to overcome the insufficient photon absorption in organic photoactive layers, since their low carrier mobility limits the film thickness for minimized charge recombination loss. To solve the inherent trade-off between photon absorption and charge transport in OPVs, the optical manipulation of light with novel micro/nano-structures has become an increasingly popular strategy to boost the light harvesting efficiency. In this Review, we make an attempt to capture the recent advances in this area. A survey of light trapping schemes implemented to various functional components and interfaces in OPVs is given and discussed from the viewpoint of plasmonic and photonic resonances, addressing the external antireflection coatings, substrate geometry-induced trapping, the role of electrode design in optical enhancement, as well as optically modifying charge extraction and photoactive layers.

White organic light-emitting diodes (OLEDs) hold great promise for applications in displays and lighting due to high efficiency and superior white color balance. However, further improvement in efficiency remains a continuous and urgent demand due to limited energy flow extraction. A powerful method for drastically releasing the trapped energy flow in conventional white OLEDs is demonstrated by implementing unique quasi-periodic subwavelength nanofunnel arrays (NFAs) via soft nanoimprinting lithography, which is ideal for enhancing light extraction without any spectral distortion or angular dependence. The resulting efficiency is over 2 times that of a conventional OLED used as a comparison. The external quantum efficiency and power efficiency are raised to 32.4% and 56.9 lm W⁻¹, respectively. Besides, the substantial increase in efficiency over a broad bandwidth with angular color stability, the experimental proofs show that the NFA-based extraction structure affords the enticing capacity against scrubbing and the self-cleaning feature, which are critical to the commercial viability in practical applications.

An efficient tandem organic light-emitting diode is realized by employing a novel light out-coupling structure with deterministic quasi-periodic nanocone arrays. The enhancement of color spatial uniformity and operational stability is simultaneously realized along with the remarkable efficiency improvement in tandem devices by integrating multi-photon emission and light out-coupling techniques. The optimized tandem organic light-emitting diode with two emission units exhibits an enhancement factor of 4.99 and 10.49 for device efficiency and half-decay lifetime, respectively, as compared to the conventional device with a planar single-unit structure. Moreover, it is verified that the emission color with viewing angles was significantly stabilized with no apparent spectral distortion. Theoretical calculations clarify that the improved device performance is primarily attributed to the effective extraction of the waveguide and surface plasmonic modes of the confined light over all the emission wavelengths and viewing-angles.

Flexible organic light-emitting diodes are gaining increasing importance as a leading technology for high-quality displays and lighting in wearable electronics due to their low power consumption, excellent color gamut, and the desirable mechanical flexibility with soft materials and curvilinear surfaces. However, further enhancements in efficiency are still challenging because of the optical confinement and limited light out-coupling efficiency. Here, a simple and wavelength-independent light extraction scheme is demonstrated using the biomimetic quasirandom nanostructures that can simultaneously enhance the out-coupling of the waveguided light and allow the minimized ohmic losses without spectral distortion. Compared to periodic grating structures, the nanoimprinted quasirandom nanostructures can broaden the periodicity and randomize the emission directionality, leading to the superiority of color stability over the visible wavelength range for a large variation of viewing angles. The resulting external quantum efficiency and current efficiency are 1.51 and 1.43 times that of a conventional flexible organic light-emitting diode used as a comparison, respectively.

Lead halide perovskites are currently attracting a great deal of attentions due to their great promise as light absorbers in high-efficiency hybrid organic–inorganic solid-state solar cells. The reliable information about interface energetics of lead halide perovskite-based interfaces is indispensable to unraveling the photon harvesting and charge separation process for this emerging photovoltaic technology. Here, we provide the direct evidence on energy level alignments at the hybrid interfaces between lead halide perovskite and organic hole-transport materials (HTMs) using in situ ultraviolet and X-ray photoemission techniques. The measured alignment schemes at perovskite/HTM hybrid interfaces reveal four entirely different energy level offsets with respect to the variation of HTMs, including spiro-OMeTAD, NPB, F₁₆CuPc, HATCN, and MoO₃, and their impacts on charge separation are also elucidated. It is identified that the staggered-gap heterojunction in contact with a HTM of higher-lying occupied molecular orbital can facilitate the interfacial hole extraction. Our experimental findings provide the guideline of not only understanding the interfacial charge separation mechanisms but also optimizing the HTMs in perovskite-based solar cells.

In the pursuit of developing highly efficient polymer solar cells, it is indispensable to experimentally determine the molecular electronic and geometrical structures of distributed donor/acceptor bulk heterojunctions for understanding the processes inside the cell. In this article, substrate effect on interface energetics and film morphology of the poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl-C₇₀ butyric acid methyl ester (PCDTBT:PC₇₀BM) blends with various blending ratios on various conductive substrates is clarified based on the characterization of photoelectron spectroscopy and atomic force microscope, where the PCDTBT:PC₇₀BM blend film serves as an important model system due to efficient charge generation and transport with low recombination. The energetics of the PCDTBT:PC₇₀BM blend film is demonstrated to be highly dependent on the substrate work function, showing the transition from vacuum level alignment to Fermi level pinning with the variation of PC₇₀BM ratio in the blend film. The resulting morphology is in good agreement with the observed formation of a PCDTBT-rich layer at the top of the PCDTBT:PC₇₀BM blend film irrespective of the variation of the PC₇₀BM blending ratio and annealing temperature. The results show the possibility of tuning the interfacial electronic structures by utilizing the substrate effects and potential applications on performance enhancement in polymer solar cells.

Advanced light manipulation is extremely attractive for applications in organic optoelectronics to enhance light harvesting efficiency. A novel method of fabricating high-efficiency organic solar cells (OSCs) is proposed using biomimetic moth eye nanostructures in a quasi-periodic gradient shape active layer and an antireflective coating. A 24.3% increase in photocurrent is realized without sacrificing dark electrical properties, yielding a 22.2% enhancement in power conversion efficiency to a record of 7.86% for OSCs with a poly(3-hexylthiophene-2,5-diyl):indene-C60 bis-adduct (P3HT:ICBA) active layer. The experimental and theoretical characterizations verify that the substantial improvement of OSCs is mainly ascribed to the self-enhanced absorption resulting from the broadband polarization-insensitive light trapping in biomimetic nanostructured active layer, the reduction in reflectance by the antireflective coating, and surface plasmonic effect excited by corrugated metallic electrode. It is noteworthy that the pathway described here is promising for opening up opportunities to realize high-performance OSCs towards the future photovoltaic applications.

Highly power-efficient white organic light-emitting diodes (OLEDs) are still challenging to make for applications in high-quality displays and general lighting due to optical confinement and energy loss during electron-photon conversion. Here, an efficient white OLED structure is shown that combines deterministic aperiodic nanostructures for broadband quasi-omnidirectional light extraction and a multilayer energy cascade structure for energy-efficient photon generation. The external quantum efficiency and power efficiency are raised to 54.6% and 123.4 lm W⁻¹ at 1000 cd m⁻². An extremely small roll-off in efficiency at high luminance is also obtained, yielding a striking value of 106.5 lm W⁻¹ at 5000 cd m⁻². In addition to a substantial increase in efficiency, this device structure simultaneously offers the superiority of angular color stability over the visible wavelength range compared to conventional OLEDs. It is anticipated that these findings could open up new opportunities to promote white OLEDs for commercial applications.

Due to the highly anisotropic nature of π -conjugated molecules, the molecular structure of organic semiconductors can significantly affect the device performance of organic optoelectronics. Here, the molecular structure dependence on charge injection and doping efficiencies is investigated by characterizing the typical hole transport material of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) and its derivatives N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-9,9-dimethyl-fluorene (DMFL-NPB) and N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-9,9-diphenyl-fluorene (DPFL-NPB)]. Using photoelectron spectroscopy data and density functional theory calculation, it is identified that the side chain substitution in NPB and its derivatives plays a crucial role in the intrinsic injection and transport properties, and doping efficiency. The inner twist of the two main benzene rings in NPB is changed from out-of-plane to in-plane due to the alkyl or phenyl side chains of DMFL-NPB or DPFL-NPB, which reduces the ionization energies and thus decreases the hole injection barriers at the indium tin oxide/organic interface. The doping efficiency in 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) doped systems is also highly dependent on the degree of intermolecular orbital energy hybridization with respect to the side chain substitution. These findings show that the rational design of molecular structures with suitable side chains is crucial for achieving high-performance organic devices.

The charge generation and separation process in transition metal oxide (TMO)-based interconnectors for tandem organic light-emitting diodes (OLEDs) is explored using data on electrical and spectral emission properties, interface energetics, and capacitance characteristics. The TMO-based interconnector is composed of MoO₃ and cesium azide (CsN₃)-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) layers, where CsN₃ is employed to replace the reactive metals as an n-dopant due to its air stability and low deposition temperature. Experimental evidences identify that spontaneous electron transfer occurs in a vacuum-deposited MoO₃ layer from various defect states to the conduction band via thermal diffusion. The external electric-field induces the charge separation through tunneling of generated electrons and holes from MoO₃ into the neighboring CsN₃-doped BPhen and hole-transporting layers, respectively. Moreover, the impacts of constituent materials on the functional effectiveness of TMO-based interconnectors and their influences on carrier recombination processes for light emission have also been addressed.