Ligang Yuan

Energy barriers between the metal Fermi energy and the molecular levels of organic semiconductor devoted to charge transport play a fundamental role in the performance of organic electronic devices. Typically, techniques such as electron photoemission spectroscopy, Kelvin probe measurements, and in-device hot-electron spectroscopy have been applied to study these interfacial energy barriers. However, so far there has not been any direct method available for the determination of energy barriers at metal interfaces with n-type polymeric semiconductors. This study measures and compares metal/solution-processed electron-transporting polymer interface energy barriers by in-device hot-electron spectroscopy and ultraviolet photoemission spectroscopy. It not only demonstrates in-device hot-electron spectroscopy as a direct and reliable technique for these studies but also brings it closer to technological applications by working ex situ under ambient conditions. Moreover, this study determines that the contamination layer coming from air exposure does not play any significant role on the energy barrier alignment for charge transport. The theoretical model developed for this work confirms all the experimental observations.

In-device hot-electron spectroscopy is demonstrated as a direct and reliable technique for the determination of the energy barrier between a metal and a solution-processed electron-transporting organic semiconductor. With experimental advance, this work opens new possibilities to bring this technique closer to the organic electronics industry.

The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium and formamidinium mixed cations. Currently, high-quality mixed-cation perovskite thin films are normally made by use of antisolvent protocols. However, the widely used “antisolvent”-assisted fabrication route suffers from challenges such as poor device reproducibility, toxic and hazardous organic solvent, and incompatibility with scalable fabrication process. Here, a simple dual-source precursor approach is developed to fabricate high-quality and mirror-like mixed-cation perovskite thin films without involving additional antisolvent process. By integrating the perovskite films into the planar heterojunction solar cells, a power conversion efficiency of 20.15% is achieved with negligible current density–voltage hysteresis. A stabilized power output approaching 20% is obtained at the maximum power point. These results shed light on fabricating highly efficient perovskite solar cells via a simple process, and pave the way for solar cell fabrication via scalable methods in the near future.

A dual-source precursor approach is developed to fabricate a high-quality and mirror-like mixed-cation perovskite without involving additional antisolvent process. By integrating the perovskite films into the planar heterojunction solar cells, a power conversion efficiency of 20.15% is achieved with negligible hysteresis effect. A stabilized power output approaching 20% is obtained at the maximum power point.

An organic solar cell (OSCs) containing double bulk heterojunction (BHJ) layers, namely, double-BHJ OSCs is constructed via stamp transferring of low bandgap BHJ atop of mediate bandgap active layers. Such devices allow a large gain in photocurrent to be obtained due to enhanced photoharvest, without suffering much from the fill factor drop usually seen in thick-layer-based devices. Overall, double-BHJ OSC with optimal ≈50 nm near-infrared PDPP3T:PC₇₁BM layer atop of ≈200 nm PTB7-Th:PC₇₁BM BHJ results in high power conversion efficiencies over 12%.

An organic solar cell (OSCs) containing double bulk heterojunction (BHJ) layers, namely, double-BHJ OSCs is constructed via stamp-transferring of low bandgap BHJ layer atop of mediate bandgap active layers. Such devices obtain a large gain in photocurrent due to the enhanced photo harvest with little fill-factor drop. Overall, double-BHJ OSC results in high power conversion efficiencies over 12%.

A specially designed n-type semiconductor consisting of Ca-doped ZnO (CZO) nanoparticles is used as the electron transport layer (ETL) in high-performance multicolor perovskite light-emitting diodes (PeLEDs) fabricated using an all-solution process. The band structure of the ZnO is tailored via Ca doping to create a cascade of conduction energy levels from the cathode to the perovskite. This energy band alignment significantly enhances conductivity and carrier mobility in the CZO ETL and enables controlled electron injection, giving rise to sub-bandgap turn-on voltages of 1.65 V for red emission, 1.8 V for yellow, and 2.2 V for green. The devices exhibit significantly improved luminance yields and external quantum efficiencies of, respectively, 19 cd A⁻¹ and 5.8% for red emission, 16 cd A⁻¹ and 4.2% for yellow, and 21 cd A⁻¹ and 6.2% for green. The power efficiencies of these multicolor devices demonstrated in this study, 30 lm W⁻¹ for green light-emitting PeLED, 28 lm W⁻¹ for yellow, and 36 lm W⁻¹ for red are the highest to date reported. In addition, the perovskite layers are fabricated using a two-step hot-casting technique that affords highly continuous (>95% coverage) and pinhole-free thin films. By virtue of the efficiency of the ETL and the uniformity of the perovskite film, high brightnesses of 10 100, 4200, and 16,060 cd m⁻² are demonstrated for red, yellow, and green PeLEDs, respectively. The strategy of using a tunable ETL in combination with a solution process pushes perovskite-based materials a step closer to practical application in multicolor light-emitting devices.

A specially designed n-type semiconductor consisting of Ca-doped ZnO nanoparticles is used as the electron transport layer (ETL) in high-performance multicolor perovskite light-emitting diodes fabricated using an all-solution process. The strategy of using a tunable ETL in combination with a solution process pushes perovskite-based materials a step closer to practical application in multicolor light-emitting devices.

In article number 1605290, Hao-Wu Lin and co-workers report efficient all-vacuum-deposited inorganic cesium lead halide perovskite solar cells of which the stoichiometric ratios of the precursors were carefully calibrated by ellipsometry. The incorporation of bromine was exploited to further enhance the device performance and stability. The results provide a paragon for the use of inorganic precursors en route to efficient vacuum-deposited perovskite devices.

2D photonic crystals (2D-PCs) are directly patterned into methylammonium lead iodide perovskite layers by thermal nanoimprint lithography (NIL) at moderate temperatures of only 100 °C, as described in article number 1605003 by Thomas Riedl and co-workers. The imprinted layers are significantly smoothened and surface defects are eliminated upon thermal imprint. 2D-PCs afford lasing with ultra-low lasing thresholds of 3.8 μJ/cm² at room temperature, which is indicative of excellent material quality of the perovskite after imprint.

High-performance nonfullerene polymer solar cells (PSCs) are developed by integrating the nonfullerene electron-accepting material 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophne) (ITIC) with a wide-bandgap electron-donating polymer PTzBI or PTzBI-DT, which consists of an imide functionalized benzotriazole (TzBI) building block. Detailed investigations reveal that the extension of conjugation can affect the optical and electronic properties, molecular aggregation properties, charge separation in the bulk-heterojunction films, and thus the overall photovoltaic performances. Single-junction PSCs based on PTzBI:ITIC and PTzBI-DT:ITIC exhibit remarkable power conversion efficiencies (PCEs) of 10.24% and 9.43%, respectively. To our knowledge, these PCEs are the highest efficiency values obtained based on electron-donating conjugated polymers consisting of imide-functionalized electron-withdrawing building blocks. Of particular interest is that the resulting device based on PTzBI exhibits remarkable PCE of 7% with the thickness of active layer of 300 nm, which is among the highest values of nonfullerene PSCs utilizing thick photoactive layer. Additionally, the device based on PTzBI:ITIC exhibits prominent stability, for which the PCE remains as 9.34% after thermal annealing at 130 °C for 120 min. These findings demonstrate the great promise of using this series of wide-bandgap conjugated polymers as electron-donating materials for high-performance nonfullerene solar cells toward high-throughput roll-to-roll processing technology.

High-performance nonfullerene polymer solar cells with power conversion efficiencies of around 10% are achieved by integrating the wide-bandgap polymers PTzBI or PTzBI-DT with a nonfullerene acceptor ITIC. The extension of conjugation can affect the optical and electronic properties, molecular aggregation properties, and charge separation in the bulk-heterojunction films, and thus the overall photovoltaic performances.

The concept of topology is becoming more and more relevant to the properties and functions of electronic materials including various transport phenomena and optical responses. A pedagogical introduction is given here to the basic ideas and their applications to optoelectronic processes in solids.

A pedagogical review on the geometry in the quantum theory of electrons in solids is presented. The Bloch wavefunction in momentum space is characterized by the Berry phase, i.e., the connection between the neighboring crystal momenta, which acts as the intracell coordinate. The optical transition induces the shift of this intracell coordinate to result in a DC current in noncentrosymmetric crystals called the shift current.

Inspired by the remarkable promotion of power conversion efficiency (PCE), commercial applications of organic photovoltaics (OPVs) can be foreseen in near future. One of the most promising applications is semitransparent (ST) solar cells that can be utilized in value-added applications such as energy-harvesting windows. However, the single-junction STOPVs utilizing fullerene acceptors show relatively low PCEs of 4%–6% due to the limited sunlight absorption because it is a dilemma that more photons need to be harvested in UV–vis–near-infrared (NIR) region to generate high photocurrent, which leads to the significant reduction of device transparency. This study describes the development of a new small-bandgap electron-acceptor material ATT-2, which shows a strong NIR absorption between 600 and 940 nm with an E_g^opt of 1.32 eV. By combining with PTB7-Th, the as-cast OPVs yield PCEs of up to 9.58% with a fill factor of 0.63, an open-circuit voltage of 0.73 V, and a very high short-circuit current of 20.75 mA cm⁻². Owing to the favorable complementary absorption of low-bangap PTB7-Th and small-bandgap ATT-2 in NIR region, the proof-of-concept STOPVs show the highest PCE of 7.7% so far reported for single-junction STOPVs with a high transparency of 37%.

A small-bandgap electron acceptor, ATT-2, is designed and synthesized. By combining PTB7-Th donor, the power conversion efficiencies reach 9.58% and 7.74% for opaque and semitransparent devices, respectively. The highest PCE among single-junction STOPVs can be attributed to the beneficial complementary near-infrared absorption of the low-bandgap donor and small-bandgap acceptor. Non-fullerene acceptors are thus very promising for the development of high-performance STOPVs.

Carrier transport in methylammonium lead iodide (MAPbI₃)-based hybrid organic–inorganic perovskites (HOIPs) is obscured by vacancy-mediated ion migration. Thus, the nature of migrating species (cation/anion) and their effect on electronic transport in MAPbI₃ has remained controversial. Temperature-dependent pulsed voltage–current measurements of MAPbI₃ thin films are performed under dark conditions, designed to decouple ion-migration/accumulation and electronic transport. Measurement conditions (electric-field history and scan rate) are shown to affect the electronic transport in MAPbI₃ thin films, through a mechanism involving ion migration and accumulation at the electrode interfaces. The presence of thermally activated processes with distinct activation energies (E_a) of 0.1 ± 0.001 and 0.41 ± 0.02 eV is established, and are assigned to electromigration of iodine vacancies and methylammonium vacancies, respectively. Analysis of activation energies obtained from electronic conduction versus capacitive discharge shows that the electromigration of these ionic species is responsible for the modification of interfacial electronic properties of MAPbI₃, and elaborates previously unaddressed issues of “fast” and “slow” ion migration. The results demonstrate that the intrinsic behavior of MAPbI₃ material is responsible for the hysteresis of the solar cells, but also have implications for other HOIP-based devices, such as memristors, detectors, and energy storage devices.

It is demonstrated that hybrid perovskite (MAPbI₃)-based M–S–M devices show I–V anomalies (hysteresis and field/scan rate dependent dynamic rectification) even under dark conditions. Using temperature-dependent pulsed current–voltage measurements, the origin of such I–V anomalies in migration and accumulation of two ionic species with different activation energies (0.1 and 0.41 eV) is established.

The first complete electrochemical fabrication of perovskite has been achieved for perovskite solar cells, as presented by Hong Liu, Wenzhong Shen, and co-workers in article number 1606156. The cover shows the three stages in the fabrication from top to bottom: before reaction, first step, and second step (where perovskite is formed). The rectangle in the middle stands for the substrate in different stages.