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In this study, we investigate the influence of molecular geometry of the donor polymers and the perylene diimide dimers (di-PDIs) on the bulk heterojunction (BHJ) morphology in the nonfullerene polymer solar cells (PSCs). The results reveal that the pseudo 2D conjugated poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) has better miscibility with both bay-linked di-PDI (B-di-PDI) and hydrazine-linked di-PDI (H-di-PDI) compared to its 1D analog, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), to facilitate more efficient exciton dissociation in the BHJ films. However, the face-on oriented π–π stacking of PTB7-Th is severely disrupted by the B-di-PDI due to its more flexible structure. On the contrary, the face-on oriented π–π stacking is only slightly disrupted by the H-di-PDI, which has a more rigid structure to provide suitable percolation pathways for charge transport. As a result, a very high power conversion efficiency (PCE) of 6.41% is achieved in the PTB7-Th:H-di-PDI derived device. This study shows that it is critical to pair suitable polymer donor and di-PDI-based acceptor to obtain proper BHJ morphology for achieving high PCE in the nonfullerene PSCs.

Suitable selection of perylene diimide dimer (di-PDI)-based acceptors and polymer donors is essential to optimize the bulk heterojunction morphology. When blended with the 2D polymer, PTB7-Th, the rigidly hydrazine-linked di-PDI affords the recorded power conversion efficiency of 6.4% due to excellent miscibility for efficient exciton dissociation and appropriate percolation pathways for charge transport.

Charge carrier transport through organic solar cells is fundamentally dispersive due to the disordered structure and complex film morphology within the photoactive layer. A novel application of transient photocurrent and short-circuit variable time-delayed collection field measurements is used to reconstruct the complete charge carrier mobility distribution for the photogenerated carriers in optimized organic solar cells.

Nanocomposite cathode structures with the aim of combining mechanical and electronic properties to achieve better performance in an organic flexible are examined by D. L. Carroll and team on page 4397. A flexible high-efficiency alternating current (AC) driven field-induced polymer electroluminescent) device is chosen as the platform system with the understanding that this approach to organic devices clearly points to organic light emitting diodes, organic thin-film transistors, and other flexible systems.

For organic photovoltaic (OPV) cells based on the bulk heterojunction (BHJ) structure, it remains challenging to rationally control the degree of phase separation and percolation within blends of donors and acceptors to secure optimal charge separation and transport. Reported is a bottom-up, supramolecular approach to BHJ OPVs wherein tailored hydrogen bonding (H-bonding) interactions between π-conjugated electron donor molecules encourage formation of vertically aligned donor π-stacks while simultaneously suppressing lateral aggregation; the programmed arrangement facilitates fine mixing with fullerene acceptors and efficient charge transport. The approach is illustrated using conventional linear or branched quaterthiophene donor chromophores outfitted with terminal functional groups that are either capable or incapable of self-complementary H-bonding. When applied to OPVs, the H-bond capable donors yield a twofold enhancement in power conversion efficiency relative to the comparator systems, with a maximum external quantum efficiency of 64%. H-bond promoted assembly results in redshifted absorption (in neat films and donor:C₆₀ blends) and enhanced charge collection efficiency despite disparate donor chromophore structure. Both features positively impact photocurrent and fill factor in OPV devices. Film structural characterization by atomic force microscopy, transmission electron microscopy, and grazing incidence wide angle X-ray scattering reveals a synergistic interplay of lateral H-bonding interactions and vertical π-stacking for directing the favorable morphology of the BHJ.

Tailored hydrogen-bonding interactions between molecular donors in blends with C₆₀ result in vertically aligned π-stacks, shorter π-stacking distances, higher charge collection efficiency, and redshifted absorption relative to non-H-bonding comparator molecules. For two quaterthiophene families, the benefits result in a twofold enhancement in power conversion efficiency and a maximum external quantum efficiency of 64% in photovoltaic devices.

Solution-processed phosphorescent tandem organic light-emitting devices (OLEDs) exhibit extremely high efficiencies (94 cd A⁻¹) and 26% external quantum efficiency (EQE) at 5000 cd m⁻² for green phosphorescent devices and 69 cd A⁻¹ and 28% EQE at 5000 cd m⁻² for white phosphorescent devices. Development of these highly efficient solution-processed tandem-OLEDs with inverted device structure paves the way to printable, low-cost, and large-area white lighting.

A new copolymer PM6 based on fluorothienyl-substituted benzodithiophene is synthesized and characterized. The inverted polymer solar cells based on PM6 exhibit excellent performance with V_oc of 0.98 V and power conversion efficiency (PCE) of 9.2% for a thin-film thickness of 75 nm. Furthermore, the single-junction semitransparent device shows a high PCE of 5.7%.

A novel host material, MTXSFCz, with the thermally actived delayed fluorescent property is reported by Y. Wang, P. Wang, and co-workers on page 4041 for the construction of red and orange phosphorescent organic light-emitting diodes (PHOLEDs) with low efficiency roll-off. The efficient reverse intersystem crossing of the MTXSFCz host from triplet to singlet facilitates the reduction of the triplet density on the host and then diminished triplet–triplet annihilation and triplet–photon annihilation, leading to the low efficiency roll-off of the PHOLEDs.

A novel photoactive polymer with two different molecular weights is reported, based on a new building block: thieno[3,2-b][1]benzothiophene isoindigo. Due to the improved crystallinity, optimal blend morphology, and higher charge mobility, solar-cell devices of the high-molecular-weight polymer exhibit a superior performance, affording efficiencies of 9.1% without the need for additives, annealing, or additional extraction layers during device fabrication.

A nanostructured composite electrode consisting of a high-index indium-tin-oxide nanomesh and low-index high-conductivity conducting polymer effectively enhances coupling of internal radiation of organic light-emitting devices into their substrates. When combining this internal extraction structure and the external extraction scheme, a very high external quantum efficiency of nearly 62% is achieved with a green phosphorescent device.

A high-quality mixed-organic-cation perovskite (MA)_x(FA)_1−xPbI₃ is prepared from a phase-pure non-stoichiometric intermediate complex (FAI)_1−x−PbI₂. The phase-pure (FAI)_1−x−PbI₂ probably facilitates homogenous nucleation and modulates the growth kinetics during the crystallization of (MA)_x(FA)_1−xPbI₃. This strategy can be expected to pave the way for the development of mixed-organic-cation perovskite solar cells.

In-plane heterostructure engineering provides unique opportunities to control device properties. Here, a single-sheet solar cell made of a graphene sheet functionalized into 1D channels is explored. Compared to vertical heterostructure architectures based on 2D materials, the single-sheet solar cell shows potential for improved robustness against defects, enhancement of polaron dissociation, extra freedom for functionalization, and coverage of the entire solar spectrum. The partition width, device length, and functionalizations can be tuned independently to optimize the key optoelectronic properties for photovoltaic performance.

The device performance of a single-sheet solar cell made of graphene sheet functionalized into 1D stripes is analyzed using a combination of ab initio simulation and scaling analysis, as a prototype system of in-plane heterostructure engineering. The results suggests highly correlated optoelectronic properties can be optimized simultaneously via independent tuning of the partition width, device length, and functionalizations.

Two novel naphtho[1,2-d]imidazole derivatives are developed as deep-blue, light-emitting materials for organic light-emitting diodes (OLEDs). The 1H-naphtho[1,2-d]imidazole based compounds exhibit a significantly superior performance than the 3H-naphtho[1,2-d]imidazole analogues in the single-layer devices. This is because they have a much higher capacity for direct electron-injection from the cathode compared to their isomeric counterparts resulting in a ground-breaking EQE (external quantum efficiency) of 4.37% and a low turn-on voltage of 2.7 V, and this is hitherto the best performance for a non-doped single-layer fluorescent OLED. Multi-layer devices consisting of both hole- and electron-transporting layers, result in identically excellent performances with EQE values of 4.12–6.08% and deep-blue light emission (Commission Internationale de l'Eclairage (CIE) y values of 0.077–0.115) is obtained for both isomers due to the improved carrier injection and confinement within the emissive layer. In addition, they showed a significantly better blue-color purity than analogous molecules based on benzimidazole or phenanthro[9,10-d]imidazole segments.

Novel naphtho[1,2-d]imidazole derivatives are developed as light-emitting materials for OLEDs. 1H-naphtho[1,2-d]imidazole based compounds exhibit a significantly superior performance than their isomeric counterparts in the single-layer devices owing to the much higher electron injection ability directly from the cathode. However, in the multi-layer devices, uniformly high efficiencies are obtained with a desirable bluecolor that is more pure than that of their benzimidazole and phenanthro[9,10-d]imidazole analogues.

Organic–inorganic metal halide perovskite solar cells have emerged in the past few years to promise highly efficient photovoltaic devices at low costs. Here, temperature-sensitive core–shell Ag@TiO₂ nanoparticles are successfully incorporated into perovskite solar cells through a low-temperature processing route, boosting the measured device efficiencies up to 16.3%. Experimental evidence is shown and a theoretical model is developed which predicts that the presence of highly polarizable nanoparticles enhances the radiative decay of excitons and increases the reabsorption of emitted radiation, representing a novel photon recycling scheme. The work elucidates the complicated subtle interactions between light and matter in plasmonic photovoltaic composites. Photonic and plasmonic schemes such as this may help to move highly efficient perovskite solar cells closer to the theoretical limiting efficiencies.

Adding plasmonic core–shell nanoparticles (Ag@TiO₂) to perovskite solar cells is shown to improve the photo­current and thus the overall efficiency. A theoretical model, introducing a novel photon recycling scheme, predicts that highly polarizable nanoparticles act as antennas for light re-emitted from radiative recombination. The work elucidates the complicated, subtle interactions between light and matter in plasmonic photovoltaic composites.

As new members of carbon material family, carbon and graphene quantum dots (CDs, GQDs) have attracted tremendous attentions for their potentials for biological, optoelectronic, and energy related applications. Among these applications, bio-imaging has been intensively studied, but optoelectronic and energy devices are rapidly rising. In this Feature Article, recent exciting progresses on CD- and GQD-based optoelectronic and energy devices, such as light emitting diodes (LEDs), solar cells (SCs), photodetctors (PDs), photocatalysis, batteries, and supercapacitors are highlighted. The recent understanding on their microstructure and optical properties are briefly introduced in the first part. Some important progresses on optoelectronic and energy devices are then addressed as the main part of this Feature Article. Finally, a brief outlook is given, pointing out that CDs and GQDs could play more important roles in communication- and energy-functional devices in the near future.

Carbon dots and graphene quantum dots have been investigated for several years and researchers' interest is moving from photoluminescence towards device applications. In this Feature Article, applications in optoelectronic and energy devices of carbon dots and graphene quantum dots are summarized, as well as their optical properties. Future directions, challenges, and other possible applications are also put forward.

Four new molecular donors are reported using a D1-A-D2-A-D1 structure, where D1 is an oligiothiophene, A is a benzothiadiazole, and D2 is indacenodithieno[3,2-b]thiophene. The resulting materials provide efficiencies as high as 6.5% in organic solar cells, without the use of solvent additives or thermal/solvent annealing. A strong correlation between the end group (D1-A) dipole moment and the fill factor (FF), mobility, and loss in the open-circuit voltage (V_OC) is observed. Indacenodithieno[3,2-b]thiophene-fluorobenzothiadiazole-terthiophene (IDTT-FBT-3T) possesses the largest end group dipole moment, and in turn, has the highest mobility, FF, and power conversion efficiency in devices. It also has a similarly high V_OC (0.95 V) to the other materials (0.93–0.99 V), despite possessing a much higher highest occupied molecular orbital (HOMO) energy level.

Four new ladder-type molecular donors are reported with varying end groups and their photovoltaic properties are explored. It is found that utilizing end groups with larger ground-state dipole moments results in significantly enhanced fill factors and decreased voltage losses in devices.