Chunhui Duan

Inverted polymer solar cells with 8.4% efficiency by conjugated polyelectrolyte

Bulk heterojunction (BHJ) polymer solar cells (PSCs) that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based solar cells. A combination of novel polymer development, nanoscale morphology control and processing optimization has led to over 8% power conversion efficiencies (PCEs) for BHJ PSCs with a conventional device structure. Attempts to develop PSCs with an inverted device structure as required for achieving high PECs and good stability have, however, met with limited success. Here, we report that a high PCE of 8.4% under AM 1.5G irradiation was achieved for BHJ PSCs with an inverted device structure. This high efficiency was obtained through interfacial engineering of solution-processed electron extraction layer, leading to facilitate electron transport and suppress bimolecular recombination. These results provided an important progress for solution-processed PSCs, and demonstrated that PSCs with an inverted device structure are comparable with PSCs with the conventional device structure.

Optical and electrical effects of gold nanoparticles in the active layer of polymer solar cells

The effects of Au nanoparticles (NPs) incorporated into the active layer of polymer solar cells (PSCs) with a newly synthesized donor polymer are investigated in detail. Our work shows that localized surface plasmon resonance (LSPR) introduced by the metallic NPs can experimentally and theoretically enhance the light absorption in the active layer of PSCs because the strong LSPR near field mainly distributes laterally along the active layer. The understanding can be applied to other metallic NP incorporated organic solar cells. Meanwhile, our results show that electrical properties can counter-diminish the optical enhancement from LSPR and thus reduce the overall performance improvement. It is important that both optical and electrical properties need to be studied and optimized simultaneously for achieving improved power conversion efficiency. The study contributes to better understanding the uses of Au NPs for enhancing PSC performances.

Highly efficient fullerene/perovskite planar heterojunction solar cells via cathode modification with an amino-functionalized polymer interlayer

A new amino-functionalized polymer, PN4N, was developed and applied as an efficient interlayer to improve the cathode interface of fullerene/perovskite (CH3NH3PbIxCl3−x) planar heterojunction solar cells. The PN4N polymer is soluble in IPA and n-BuOH, which are orthogonal solvents to the metallohalide perovskite films, and therefore they can be spuncast on the heterojunction layer before the deposition of the metal cathode. This simple modification of the cathode interface showed a remarkable enhancement of power conversion efficiency (PCE) from 12.4% to 15.0% and also reduced the hysteresis of photocurrent. We also found that conventional water–methanol-soluble polymer interlayer, such as PFN, was incompatible with the perovskite films because of the small molecular size of aprotic solvent such as MeOH, which could decompose the perovskite films to PbI2, resulting in considerably lower solar cell performance. This study provides new design guidelines for efficient interfacial materials and also demonstrates that interface engineering could be a key strategy to improve perovskite solar cells.

A Series of New Medium‐Bandgap Conjugated Polymers Based on Naphtho[1,2‐c:5,6‐c]bis(2‐octyl‐[1,2,3]triazole) for High‐Performance Polymer Solar Cells

A series of novel conjugated copolymers based on naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) are synthesized. These copolymers exhibit medium bandgaps of ≈1.9 eV. One of them demonstrates a high performance of up to 6.10% power conversion efficiency in a bulk-heterojunction (BHJ) solar-cell device. The performance can be further enhanced to 7.11% when applied in an inverted device architecture, using PF3 N-OX as an interfacial modifier.

Conjugated zwitterionic polyelectrolytes and their neutral precursor as electron injection layer for high-performance polymer light-emitting diodes.

Conjugated polyelectrolytes (CPEs) have attracted considerable attention in recent years due to their signifi cant application potential in organic optoelectronic devices [ 1–4 ] and chemo-/ bio-sensors. [ 5 , 6 ] Their pendant ionic groups offer them unique solubility in high polarity solvents, which is orthogonal to most of the commonly used conjugated polymer active materials, and allow the fabrication of solution-processed multilayer devices to maximize the performance. Moreover, it was proved that the CPE’s pendant polar groups also can effectively improve electron injection from high work-function metals (such as Al, Ag, Au), which open a way to achieve all printable roll-to-roll polymer based organic electronic devices and the fi rst fully solution processed polymer light-emitting diodes (PLEDs) have been realized based on them. [ 7 ] Despite their application potential in devices, most of CPE contain mobile counter-ions, which can migrate into the emission layer (EML) and may affect the longterm stability of devices. [ 8 ] In addition, the response time of the devices and the mobility of the CPE are also greatly infl uenced by the nature of their counter-ions. [ 9 , 10 ] To overcome these, one alternative way is to develop water/alcohol soluble conjugated polymers with neutral high polarity pendant groups. [ 11 ] However, it is very challenging to get highly water/alcohol soluble neutral conjugated polymers due to that conjugated polymers always contain a very rigid, highly hydrophobic main chain and the polarity of nonionic pendant groups is usually not high enough to ensure them good solubility in water/alcohol. Zwitterionic polyelectrolytes contain both anionic and cationic groups, which usually exhibit good water/alcohol solubility and there is no mobile counter-ions among them. [ 12 ] Moreover, recent research has shown that zwitterionic groups can also enhance the electron injection from high work-function metal cathodes as those traditional ionic groups (such as ammonium) or other high polarity groups (such as amino, diethanol amino etc.) [ 13 ]

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.

Wide-Bandgap Benzodithiophene-Benzothiadiazole Copolymers for Highly Efficient Multijunction Polymer Solar Cells.

Novel wide-bandgap semiconducting polymers are designed and synthesized for multijunction polymer solar cell (PSC) applications. In single-junction PSCs, BDT-FBT-2T exhibits efficiencies exceeding 6.5% for active layer thicknesses between 90 and 250 nm, with the highest efficiency of 7.7% at 100 and 250 nm. This enables tandem PSCs to be created with an efficiency of 8.9%.

Control of efficiency, brightness, and recombination zone in light-emitting field effect transistors.

The split-gate light emitting field effect transistors (SG-LEFETs) demonstrate a new strategy for ambipolar LEFETs to achieve high brightness and efficiency simultaneously. The SG architecture forces largest quantity of opposite charges on Gate 1 and Gate 2 area to meet in the center of the channel. By actively and independently controlling current injection from separated gate electrodes within transporting channel, high brightness can be obtained in the largest injection current regime with highest efficiency.

Solution processed thick film organic solar cells

Solution processed bulk-heterojunction (BHJ) organic solar cells (OSCs) are an emerging next-generation photovoltaic technology. Laboratory-scale power conversion efficiencies (PCEs) of OSCs exceeding 10% in single-junction devices and approaching 12% in multijunction devices, respectively, have been achieved. However, the translation of this technology to industrial high throughput manufacturing needs the development of practically useful photoactive materials and processing methods that can produce efficient devices with a large active layer thickness. In this review, we introduce the factors that determine the optimal thickness of the active layer in OSCs at first. The significant advances in materials development and processing methods toward efficient thick film OSCs are summarized subsequently.

Effect of side chain length on the charge transport, morphology, and photovoltaic performance of conjugated polymers in bulk heterojunction solar cells

The effect of side chain length on the photovoltaic properties of conjugated polymers is systematically investigated with two sets of polymers that bear different alkyl side chain lengths based on benzodithiophene and benzo[2,1,3]thiadiazole or 5,6-difluorobenzo[2,1,3]thiadiazole. Characterization of the photovoltaic cells reveals a strong interdependency between the side chain length of conjugated polymers and photovoltaic performances (power conversion efficiency, short-circuit current, and fill factor) of the resulting bulk-heterojunction (BHJ) solar cells. Charge carrier transport and external quantum efficiency (EQE) measurements in combination with morphology characterization suggest that too long side chains lead to deteriorated charge transport, suboptimal BHJ morphology, considerable bimolecular recombination, and consequently poor photovoltaic performances. On the other hand, when the side chains are too short, they cannot afford a high enough solubility and molecular weight for the resulting polymers and produce poor solar cell performance as well. This study shows that side chain optimization is of significant importance to maximize the potential of photovoltaic active conjugated polymers, which indicates the fruitful molecular design rules toward highly efficient BHJ polymer solar cells.