Chengmei Zhong

Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells

Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in highly efficient polymer solar cells by incorporating an alcohol/water-soluble conjugated polymer as cathode interlayer is domonstrated. When combined with a low-bandgap polymer PTB7 as the electron donor material, the power efficiency of the devices is improved to a certified 8.370%. Due to the drastic improvement in efficiency and easy utilization, this method opens new opportunities for PSCs from various material systems to improve towards 10% efficiency.

High-Performance Solution-Processed Non-Fullerene Organic Solar Cells Based on Selenophene-Containing Perylene Bisimide Acceptor

Non-fullerene acceptors have recently attracted tremendous interest because of their potential as alternatives to fullerene derivatives in bulk heterojunction organic solar cells. However, the power conversion efficiencies (PCEs) have lagged far behind those of the polymer/fullerene system, mainly because of the low fill factor (FF) and photocurrent. Here we report a novel perylene bisimide (PBI) acceptor, SdiPBI-Se, in which selenium atoms were introduced into the perylene core. With a well-established wide-band-gap polymer (PDBT-T1) as the donor, a high efficiency of 8.4% with an unprecedented high FF of 70.2% is achieved for solution-processed non-fullerene organic solar cells. Efficient photon absorption, high and balanced charge carrier mobility, and ultrafast charge generation processes in PDBT-T1:SdiPBI-Se films account for the high photovoltaic performance. Our results suggest that non-fullerene acceptors have enormous potential to rival or even surpass the performance of their fullerene counterparts.

Simultaneous optimization of charge-carrier balance and luminous efficacy in highly efficient white polymer light-emitting devices.

The use of white organic light-emitting devices (WOLEDs) for solid-state lighting applications is becoming increasingly attractive, [ 1 − 5 ] given that legislation in more countries is banning the use of ineffi cient incandescent lamps. Moreover, since fl uorescent lamps involve the use of mercury and its disposal represents a great challenge, many scientists have been working aggressively to make the replacement of the fl uorescent light sources by WOLEDs a reality. Indeed, the effi ciency of multilayer vacuum-evaporated WOLEDs based on small molecules has been greatly improved in the past several years [ 6 − 10 ] and has already exceeded that of fl uorescent lamps. [ 11 ] In contrast, despite many unique advantages, such as low-cost manufacturing using solution-processing techniques, easy processability over large-areas by spin-coating or ink-jet printing, compatibility with fl exible substrates, a relatively small amount of wasted material, and precise control of the doping level, the application of white polymer light-emitting diodes (WPLEDs) is still severely hindered by the relatively low device effi ciency. [ 3 , 12 − 16 ]

Origin of the enhanced open-circuit voltage in polymer solar cells via interfacial modification using conjugated polyelectrolytes

It has been previously shown that incorporating alcohol/water-soluble conjugated polyelectrolytes (CPEs) as cathode interlayers can substantially enhance the open-circuit voltage (Voc) of bulk heterojunction polymer solar cells (PSCs). This improvement is attractive since there are only a few solution-based processing technologies that can enhance the Voc. In this article, the Voc enhancement effect of five distinct CPE materials was studied on three typical PSC device configurations, each using different donor materials. Significant Voc enhancement was observed in devices that contain a polyfluorene copolymer as the donor material. The origin of the Voc enhancement was investigated by examination of the dark currents.

Electrochemical route to fabricate film-like conjugated microporous polymers and application for organic electronics.

Film-like conjugated microporous polymers (CMPs) are fabricated by the novel strategy of carbazole-based electropolymerization. The CMP film storing a mass of counterions acting as an anode interlayer provides a significant power-conversion efficiency of 7.56% in polymer solar cells and 20.7 cd A(-1) in polymer light-emitting diodes, demonstrating its universality and potential as an electrode interlayer in organic electronics.

Conjugated zwitterionic polyelectrolyte-based interface modification materials for high performance polymer optoelectronic devices

A series of new water/alcohol-soluble conjugated polymers (WSCPs) poly[(9,9-bis((N-(4-sulfonate-1-butyl)-N,N-dimethylammonium)propyl)-2,7-fluorene)-alt-N-phenyl-4,4′-diphenylamine)] (PFNSO-TPA), poly[(9,9-bis((N-(4-sulfonate-1-butyl)-N,N-dimethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFNSO) and poly[(9,9-bis((N-(4-sulfonate-1-butyl)-N,N-dimethylammonium)propyl)-2,7-fluorene)-alt-4,7-(2,1,3-benzothiadiazole)] (PFNSO-BT), comprising identical sulfobetaine zwitterionic groups on their side chains but different conjugated main chain structures, were designed and developed as interface modification materials to improve electron collection in bulk-heterojunction polymer solar cells (PSCs), and to improve electron injection/transporting in polymer light-emitting diodes (PLEDs). The resulting WSCPs possess integrated advantages of excellent alcohol processability, interface modification functions and mobile ion free nature. The relationships between the WSCPs main chain structures and properties (including optical/electrical properties and interface modification functions in resulting devices) were investigated systematically. In PSCs, it was found that the WSCPs interface modification properties led to varying differences, but all of them can boost the photovoltaic performances of PSCs; encouragingly, a high power conversion efficiency (PCE) of 8.74% could be achieved. In PLEDs, the interface modification functions of the WSCPs strongly depend upon their conjugated main chain structures. The WSCPs should possess suitable energy levels to match well with the light-emitting layer (EML), even though the electron injection from metal cathode was efficient. Our results show promising potentials of WSCPs as interface modification layers in organic/polymer optoelectronic devices, and provide new insights for the development of new interface modification materials in the future.

In Situ Electrochemical Deposition and Doping of C60 Films Applied to High‐Performance Inverted Organic Photovoltaics

Novel C(60)-based cross-linked films formed by electrodeposition are produced and used as the electron-collection layer in inverted polymer solar cells (PSCs). The electrodeposited films exhibit a low work function of 4.2 eV and the PSCs perform well, with power conversion efficiencies of up to 6.31%. This new kind of electrodeposited film affords more opportunities to develop modified electrodes with a low work function.

Highly efficient green-emitting electrophosphorescent hyperbranched polymers using a bipolar carbazole-3,6-diyl-co-2,8-octyldibenzothiophene-S,S-dioxide-3,7-diyl unit as the branch

Green-emitting hyperbranched polymers with a bipolar 3,6-carbazole-co-2,8-dioctyl-dibenzothiophene-S,S-dioxide-3,7-diyl [Cz-co-DOSO] as the branch and fac-tris(2-phenylpyridine)iridium[Ir(ppy)3] as the core were synthesized by Suzuki polycondensation. The Cz-co-DOSO branch with a triplet energy level above 2.5 eV can effectively prevent energy feedback from the phosphorescent core. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the copolymers reduce dramatically by over 0.6 and 0.8 eV with increasing DOSO content up to 45.5 mol% in the polymers, respectively. A maximum luminous efficiency of 50.5 cd A−1 and a maximum external quantum efficiency of 15.3% were obtained for PCzDOSO40Ir3 based on a single layer device of ITO/PEDOT:PSS/polymer/CsF/Al, which were improved by over 13 times compared with PCzIr3. The hole- and electron-only devices show that the hole and electron flux are well balanced, which demonstrates that PCzDOSO40Ir3 is a bipolar polymer with a balanced charge carrier transport.

Ordered Polymer Nanofibers Enhance Output Brightness in Bilayer Light-Emitting Field-Effect Transistors

Polymer light emitting field effect transistors are a class of light emitting devices that reveal interesting device physics. Device performance can be directly correlated to the most fundamental polymer science. Control over surface properties of the transistor dielectric can dramatically change the polymer morphology, introducing ordered phase. Electronic properties such as carrier mobility and injection efficiency on the interface can be promoted by ordered nanofibers in the polymer. Moreover, by controlling space charge in the polymer interface, the recombination zone can be spatially extended and thereby enhance the optical output.

Ultrafast Charge Generation in an Organic Bilayer Film

The dynamics of charge generation in a high performing molecular photovoltaic system, p-SIDT(FBTTh2)2 (see Figure 1 ) is studied with transient absorption. The optimized bulk heterojunction material shows behavior observed in many other systems; the majority of charges are generated at short time scales (<150 fs), and a slower contribution from incoherently diffusing excitons is observed at low pump fluence. In a separate experiment, the role of bulk heterojunction material morphology on the process of ultrafast charge generation was investigated with bilayers made with solution processed donor molecules on a photopolymerized C60 layer. The majority of carriers are again produced at short time scales, ruling out the idea that subpicosecond charge generation can be understood wholly in terms of localized excitons. We evaluate possible causes of this behavior and propose that the excited state is highly delocalized on short time scales, providing ample probability density at the charge generating interface.