Intermolecular CT excitons enable nanosecond excited-state lifetimes in NIR-absorbing non-fullerene acceptors for efficient organic solar cells

State-of-the-art Y6-type molecular acceptors exhibit nanosecond excited-state lifetimes despite their low optical gaps (~1.4 eV), thus allowing organic solar cells (OSCs) to achieve highly efficient charge generation with extended near-infrared (NIR) absorption range (up to ~1000 nm). However, the precise molecular-level mechanism that enables low-energy excited states in Y6-type acceptors to achieve nanosecond lifetimes has remained elusive. Here, we demonstrate that the distinct packing of Y6 molecules in film leads to a strong intermolecular charge-transfer (iCT) character of the lowest excited state in Y6 aggregates, which is absent in other low-gap acceptors such as ITIC. Due to strong electronic couplings between the adjacent Y6 molecules, the iCT-exciton energies are greatly reduced by up to ~0.25 eV with respect to excitons formed in separated molecules. Importantly, despite their low energies, the iCT excitons have reduced non-adiabatic electron-vibration couplings with the electronic ground state, thus suppressing non-radiative recombination and allowing Y6 to overcome the well-known energy gap law. Our results reveal the fundamental relationship between molecular packing and nanosecond excited-state lifetimes in NIR-absorbing Y6-type acceptors underlying the outstanding performance of Y6-based OSCs

Sodium-glucose Cotransporter 2 (SGLT2) Inhibitors vs. Dipeptidyl Peptidase-4 (DPP4) inhibitors for new-onset dementia: A propensity score-matched population-based study with competing risk analysis

The effects of sodium-glucose cotransporter 2 inhibitors (SGLT2I) and dipeptidyl peptidase-4 inhibitors (DPP4I) on new-onset cognitive dysfunction in type 2 diabetes mellitus remain unknown. This study aimed to evaluate the effects of the two novel antidiabetic agents on cognitive dysfunction by comparing the rates of dementia between SGLT2I and DPP4I users. This was a population-based cohort study of type 2 diabetes mellitus patients treated with SGLT2I and DPP4I between January 1, 2015 and December 31, 2019 in Hong Kong. Exclusion criteria were <1-month exposure or exposure to both medication classes, or prior diagnosis of dementia or major neurological/psychiatric diseases. Primary outcomes were new-onset dementia, Alzheimer's, and Parkinson's. Secondary outcomes were all-cause, cardiovascular, and cerebrovascular mortality. A total of 13,276 SGLT2I and 36,544 DPP4I users (total = 51,460; median age: 66.3 years old [interquartile range (IQR): 58-76], 55.65% men) were studied (follow-up: 472 [120-792] days). After 1:2 matching (SGLT2I: = 13,283; DPP4I: = 26,545), SGLT2I users had lower incidences of dementia (0.19 vs. 0.78%, < 0.0001), Alzheimer's (0.01 vs. 0.1%, = 0.0047), Parkinson's disease (0.02 vs. 0.14%, = 0.0006), all-cause (5.48 vs. 12.69%, < 0.0001), cerebrovascular (0.88 vs. 3.88%, < 0.0001), and cardiovascular mortality (0.49 vs. 3.75%, < 0.0001). Cox regression showed that SGLT2I use was associated with lower risks of dementia (hazard ratio [HR]: 0.41, 95% confidence interval [CI]: [0.27-0.61], < 0.0001), Parkinson's (HR:0.28, 95% CI: [0.09-0.91], = 0.0349), all-cause (HR:0.84, 95% CI: [0.77-0.91], < 0.0001), cardiovascular (HR:0.64, 95% CI: [0.49-0.85], = 0.0017), and cerebrovascular (HR:0.36, 95% CI: [0.3-0.43], < 0.0001) mortality. The use of SGLT2I is associated with lower risks of dementia, Parkinson's disease, and cerebrovascular mortality compared with DPP4I use after 1:2 ratio propensity score matching. [Abstract copyright: Copyright © 2021 Mui, Zhou, Lee, Leung, Lee, Chou, Tsang, Wai, Liu, Wong, Chang, Tse and Zhang.

Sodium-glucose cotransporter 2 (SGLT2) inhibitors vs. dipeptidyl peptidase-4 (DPP4) inhibitors for new-onset dementia: A propensity score-matched population-based study with competing risk analysis

Introduction: The effects of sodium-glucose cotransporter 2 inhibitors (SGLT2I) and dipeptidyl peptidase-4 inhibitors (DPP4I) on new-onset cognitive dysfunction in type 2 diabetes mellitus remain unknown. This study aimed to evaluate the effects of the two novel antidiabetic agents on cognitive dysfunction by comparing the rates of dementia between SGLT2I and DPP4I users. Methods: This was a population-based cohort study of type 2 diabetes mellitus patients treated with SGLT2I and DPP4I between January 1, 2015 and December 31, 2019 in Hong Kong. Exclusion criteria were <1-month exposure or exposure to both medication classes, or prior diagnosis of dementia or major neurological/psychiatric diseases. Primary outcomes were new-onset dementia, Alzheimer's, and Parkinson's. Secondary outcomes were all-cause, cardiovascular, and cerebrovascular mortality. Results: A total of 13,276 SGLT2I and 36,544 DPP4I users (total n = 51,460; median age: 66.3 years old [interquartile range (IQR): 58–76], 55.65% men) were studied (follow-up: 472 [120–792] days). After 1:2 matching (SGLT2I: n = 13,283; DPP4I: n = 26,545), SGLT2I users had lower incidences of dementia (0.19 vs. 0.78%, p < 0.0001), Alzheimer's (0.01 vs. 0.1%, p = 0.0047), Parkinson's disease (0.02 vs. 0.14%, p = 0.0006), all-cause (5.48 vs. 12.69%, p < 0.0001), cerebrovascular (0.88 vs. 3.88%, p < 0.0001), and cardiovascular mortality (0.49 vs. 3.75%, p < 0.0001). Cox regression showed that SGLT2I use was associated with lower risks of dementia (hazard ratio [HR]: 0.41, 95% confidence interval [CI]: [0.27–0.61], P < 0.0001), Parkinson's (HR:0.28, 95% CI: [0.09–0.91], P = 0.0349), all-cause (HR:0.84, 95% CI: [0.77–0.91], P < 0.0001), cardiovascular (HR:0.64, 95% CI: [0.49–0.85], P = 0.0017), and cerebrovascular (HR:0.36, 95% CI: [0.3–0.43], P < 0.0001) mortality. Conclusions: The use of SGLT2I is associated with lower risks of dementia, Parkinson's disease, and cerebrovascular mortality compared with DPP4I use after 1:2 ratio propensity score matching

Charge Carrier Transport and Injection Across Organic Heterojunctions

The discovery of highly efficient organic light-emitting diodes (OLEDs) in the 1980s has stimulated extensive research on organic semiconductors and devices. Underlying this breakthrough is the realization of the organic heterojunction (OH). Besides OLEDs, the implementation of the OH also significantly improves the power conversion efficiency in organic photovoltaic cells (OPVs). The continued technological advancements in organic electronic devices depend on the accumulation of knowledge of the intrinsic properties of organic materials and related interfaces. Among them, charge-carrier transport and carrier injection are two key factors that govern the performance of a device. This thesis mainly focuses on the charge carrier injection and transport at organic heterojunctions. The carrier transport properties of different organic materials used in this study are characterized by time-of-flight (TOF) and admittance spectroscopy (AS). An injection model is formulated by considering the carrier distribution at both sides of the interface. Using a steady-state simulation approach, the effect of accumulated charges on energy level alignment at OH is revealed. Instead of a constant injection barrier, it is found that the barrier varies with applied voltage. Moreover, an escape probability function in the injection model is modified by taking into account the total hopping rate and available hopping sites at the interface. The model predicts that the injection current at low temperature can be dramatically modified by an extremely small density of deep trap states. More importantly, the temperature dependence of the injection current is found to decrease with increasing barrier height. This suggests that extracting the barrier height from the J vs 1/T plot, as commonly employed in the literature, is problematic. These theoretical predictions are confirmed by a series of experiments on heterojunction devices with various barrier heights. In addition, the presence of deep trap states is also consistent with carrier mobility measurements at low temperature. From the point of view of application, an interface chemical doping method is proposed to engineer the carrier injection at an organic heterojunction. It is found that the the injection current can be effectively increased or suppressed by introducing a thin (2 nm) doped organic layer at the interface. This technique is further extended to study the impact of an injection barrier at the OH. in OLEDs, on device performance. It is shown that a 0.3 eV injection barrier at the OH, that is normally negligible at metal/organic interface, can reduce the device efficiency by 25 %. This is explained by the carrier distribution in the density-of-states at the OH. Furthermore, the carrier transport properties in a bulk heterojunction system are investigated. The bulk heterojunction consists of an interpenetrating network of a polymeric electron donor and a molecular electron acceptor. This material system has been studied in the last few years as an attractive power conversion efficiency (5% under AM 1.5) of OPV cells has been demonstrated. It is found that the electron mobility is greatly dependent on the thermal treatment of the film. Interfacial dipole effect at the heterojunction between the donor and the acceptor is proposed to be the determining factor that alters the carrier mobility in different nano-scale structures.Ph

An evaluation on the implementation of environmental protection policies in Hong Kong

published_or_final_versionPublic AdministrationMasterMaster of Public Administratio

Self-organized phase segregation between inorganic nanocrystals and PC61BM for hybrid high-efficiency bulk heterojunction photovoltaic cells

We demonstrate a simple approach to generate phase segregation between colloidal PbS nanocrystals (NCs) and organic [6,6]-phenyl C\u2086\u2081 butyric acid methyl ester (PC\u2086\u2081BM). Continuous vertical phase segregation is observed in cross-linked composite films of NCs and PC\u2086\u2081BM. Hybrid bulk heterojunction photovoltaic cells fabricated with the phase segreated composite layer have achieved the state-of-art power conversion efficiency of 3.7% under one sun of simulated Air Mass 1.5 Global solar irradiation. The presented method can be generally applied in other NC/organic systems for the development of hybrid heterojunction photovoltaic cells.Peer reviewed: YesNRC publication: Ye

Alkyl side chain impact on the charge transport and photovoltaic properties of benzodithiophene and diketopyrrolopyrrole-based copolymers

To investigate the side chain effect on the photovoltaic performance of conjugated copolymers with alternating electron push-pull structures , three alternating copolymers (O-HD, BO-BO, and PU-O) of benzodithiophene and dithienyldiketopyrrolopyrrole were designed and synthesized. They were nomenclated based on the side chains on the benzodithiophene (BDT) and the diketopyrrolopyrrole (DPP) units, with are octyl (O) and 2-hexyldecyl (HD); 3-butyloctyl (BO), and 20cutyloctyl (BO): and 3-pentylundecyl (PU) and octyl (O) groups, respectively. The total C number of the side chains in each repeat unit was kept at 48 to control the dilute effect. The solubility, optical, and electrochemical properties, and crystalline structure of the polymers were depended on the combination of these linear or branched alkyl chains. Thin film transistor (TFT) characterization showed that PU-O had the best hole mobility up to 1.6 x 10(to the power of)-3 cm(squared)V(to the power of)-1 s(to the power of)-1. The best photovoltaic performance was observed from O-HD with power conversion efficiency (PCE) up to 4.1%. However, it only showed a modest hole mobility of 3.8 x 10(to the power of)-4 cm(squared) V(to the power of)-1 s(to the power of)-1, about 4-fold lower than PU-O. This dramatically different performance of these polymers for TFT and photovoltaic devices was explained by the interaction at the interface of the polymer electron donor and the PCBM acceptor domains.Peer reviewed: YesNRC publication: Ye

Impact of the growth conditions of colloidal PbS nanocrystals on photovoltaic device performance

Here, we present a detailed investigation on the influence of the growth conditions of colloidal lead sulfide (PbS) nanocrystals on photovoltaic device performance. The PbS nanocrystals were synthesized in a noncoordinating solvent, 1-octadecene, using oleic acid (OA) as the ligand. It was found that both the feeding molar ratio of OA to Pb and the reactant concentration were critical for controlling the growth rate of nanocrystals. Transient photocurrent (TPC) measurements revealed reduced trap density in thin films using the slow-growth nanocrystals. Solar cells based on the slow-growth nanocrystals showed a high power conversion efficiency (PCE) of 3.8% under simulated Air Mass 1.5 Global (AM 1.5G) irradiation (100 mW/cm2), a 2-fold increase in PCE, compared to the fast-growth nanocrystals, because of the remarkable improvement in the open-circuit voltage and fill factor in the PV devices.Peer reviewed: YesNRC publication: Ye

Batch-to-batch variation of polymeric photovoltaic materials: Its origin and impacts on charge carrier transport and device performances

A detailed investigation of the impact of molecular weight distribution of a photoactive polymer, poly[N-9\u2032-heptadecanyl-2,7-carbazole-alt-5,5-(4\u2032,7\u2032-di-2-thienyl-2\u2032,1\u2032,3\u2032-benzothiadiazole)] (PCDTBT), on photovoltaic device performance and carrier transport properties is reported. It is found that different batches of as-received polymers have substantial differences in their molecular weight distribution. As revealed by gel permeation chromatography (GPC), two peaks can generally be observed. One of the peaks corresponds to a high molecular weight component and the other peak corresponds to a low molecular weight component. Photovoltaic devices fabricated with a higher proportion of low molecular weight component have power conversion efficiencies (PCEs) reduced from 5.7% to 2.5%. The corresponding charge carrier mobility at the short-circuit region is also significantly reduced from 2.7 7 10-5 to 1.6 7 10-8 cm2 V-1 s-1. The carrier transport properties of the polymers at various temperatures are further analyzed by the Gaussian disorder model (GDM). All polymers have similar energetic disorders. However, they appear to have significant differences in carrier hopping distances. This result provides insight into the origin of the molecular weight effect on carrier transport in polymeric semiconducting materials. Batch-to-batch variation of the photovoltaic performance of devices based on commercial samples of the polymer poly[N-9\u2032-heptadecanyl-2,7-carbazole-alt-5,5- (4\u2032,7\u2032-di-2-thienyl-2\u2032,1\u2032,3\u2032-benzothiadiazole)] (PCDTBT) is reported, with efficiency ranging from 5.7% to 2.5%. As revealed by gel permeation chromatography, bimodal distributions are observed in the molecular weight. Charge transport data suggest that low molecular weight components increase the average hopping distance, resulting in lower mobility and poorer photovoltaic performance. \ua9 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Peer reviewed: YesNRC publication: Ye