Advanced Functional Materials and Devices (AFMD) Group

Abstract:

<p>We use the electron spin as a probe to gain insight into the mechanism of molecular doping in a p-doped zinc phthalocyanine host across a broad range of temperatures (80–280 K) and doping concentrations (0–5 wt% of F6-TCNNQ).</p>

Abstract:

Stability is now a critical factor in the commercialization of organic photovoltaic (OPV) devices. Both extrinsic stability to oxygen and water and intrinsic stability to light and heat in inert conditions must be achieved. Triplet states are known to be problematic in both cases, leading to singlet oxygen production or fullerene dimerization. The latter is thought to proceed from unquenched singlet excitons that have undergone intersystem crossing (ISC). Instead, we show that in bulk heterojunction (BHJ) solar cells the photo-degradation of C₆₀ via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C₆₀ excited triplet state. We demonstrate this to be the principal pathway from a combination of steady-state optoelectronic measurements, time-resolved electron paramagnetic resonance, and temperature-dependent transient absorption spectroscopy on model systems. BHT is a much more serious concern than ISC because it cannot be mitigated by improved exciton quenching, obtained for example by a finer BHJ morphology. As BHT is not specific to fullerenes, our results suggest that the role of electron and hole back transfer in the degradation of BHJs should also be carefully considered when designing stable OPV devices.

Abstract:

The β-phase, in which the intermonomer torsion angle of a fraction of chain segments approaches ∼180°, is an intriguing conformational microstructure of the widely studied light-emitting polymer poly(9,9-dioctylfluorene) (PFO). Its generation can in turn be used to significantly improve the performance of PFO emission-layer-based light-emitting diodes (LEDs). Here, we report the generation of β-phase chain segments in a copolymer, 90F8:10BT, containing 90% 9,9-dioctylfluorene (F8) and 10% 2,1,3-benzothiadiazole (BT) units and show that significant improvements in performance also ensue for LEDs with β-phase 90F8:10BT emission layers, generalizing the earlier PFO results. The β-phase was induced by both solvent vapor annealing and dipping copolymer thin films into a solvent/nonsolvent mixture. Subsequent absorption spectra show the characteristic fluorene β-phase peak at ∼435 nm, but luminescence spectra (∼530 nm peak) and quantum yields barely change, with the emission arising following efficient energy transfer to the lowest-lying excited states localized in the vicinity of the BT units. For ∼5% β-phase chain segment fraction relative to 0% β-phase, the LED luminance at 10 V increased by ∼25% to 5940 cd m^-2, the maximum external quantum efficiency by ∼61 to 1.91%, and the operational stability from 64% luminance retention after 20 h of operation to 90%. Detailed studies addressing the underlying device physics identify a reduced hole injection barrier, higher hole mobility, correspondingly more balanced electron and hole charge transport, and decreased carrier trapping as the dominant factors. These results confirm the effectiveness of chain conformation control for fluorene-based homo- and copolymer device optimization.

Abstract:

Interfaces in perovskite solar cells (PSCs) are closely related to their power conversion efficiency (PCE) and stability. It is highly desirable to minimize the interfacial nonradiative recombination losses through rational interfacial engineering. Herein we develop an effective and easily reproducible interface engineering strategy where three mercaptobenzimidazole (MBI)-based molecules are employed to modify the perovskite/electron transport layer (ETL) interface. MBI and MBI-OCH₃ can not only passivate defects at surface and grain boundaries (GBs) of perovskite films but can also improve energy level alignment (ELA), which leads to enhanced PCE and stability. Consequently, the PCE is improved from 19.5% for the control device to 21.2% for MBI-modified device, which is among the best reported inverted MAPbI₃-based PSCs. In contrast, incorporation of MBI-NO₂ increases defect density and negligibly influences the energy level alignment. This work indicates that defect passivation and ELA modulation can be achieved simultaneously through modulating functional groups in interface modification molecules.