Project LOSPHOGEN

Project LOSPHOGEN; protocol nr. EXCELLENCE/1216/0010

Low Photon-Energy Up-Conversion induced Sensitized Photocurrent Generation in Organic Photodiodes

The Project EXCELLENCE/1216/0010 is co-financed by the European Regional Development Fund and the Republic of Cyprus through the Research and Innovation Foundation

SPECIFIC OBJECTIVES

1. To establish a methodology for designing proper device electronic structures and developing accurate device engineering protocols that will enable visible light-induced photocurrent generation by UV-only sensitive organic photodiodes functionalized with solid-state TTA-interlayers. The extension of the photodiodes sensitivity in the green and in the red part of the visible spectral range, in which the photodiodes are otherwise blind, will be pursued by separately using two different metal-organic complexes as model triplet sensitizers for the development of the device solid-state TTA-interlayers
2. To perform detailed time-resolved spectroscopic characterization experiments for resolving the mechanism of an unexpected green-to-blue photon energy up-conversion process induced by triplet-triplet annihilation in organic composite films prepared by blends comprising a blue-light emitting poly(dioctyl fluorene) (PFO) derivative mixed with the green-light absorbing metal-organic triplet sensitizer of (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP). The PFO:PtOEP system belongs to a family of unconventional TTA-UC composites for which the occurrence of TTA-UC still remains a puzzling phenomenon that is in conflict with the internationally agreed picture of the TTA-UC mechanism.
Conventionally, TTA-UC organic composites are obtained by mixing a sensitizer species such as the PtOEP metal-organic complex with an emitter e.g. the organic lumophore of di-phenyl anthracene (DPA). The sequence of photophysical events leading to TTA-UC luminescence is: i) low-photon energy photoexcitation selectively activates the first singlet-excited sensitizer state (S1-sensitizer), ii) fast intersystem crossing (ISC) to an energetically lower triplet-excited sensitizer state (T1-sensitizer), iii) exothermic triplet energy transfer (TET) from triplet-excited sensitizer to ground-state emitter, iv) TTA between two triplet-excited emitter states and activation of an energetically higher singlet-excited emitter state (S1-emitter), v) photoluminescence (PL) activation at photon energy higher than that used for photoexcitation. Figure 1a visualizes these steps whereas Figure 1b presents the chemical structures of PtOEP and DPA.

Figure 1: The sequence of photophysical events that lead to photon up-converted (TTA-UC) delayed luminescence in conventional TTA-UC organic composites, b) the chemical structures of the organometallic PtOEP triplet sensitizer and the blue-light emitting DPA emitter.

For a rather unconventional TTA-UC composite type based on porphyrinoid sensitizers such as PtOEP mixed with blue light-emitting polymeric matrices, e.g. poly(fluorene)s (PFO) no exothermic triplet energy transfer from the triplet-excited PtOEP sensitizer (ET-PtOEP) to the triplet energy level of the emitter (ET-PF) is possible due to the positive energetic barrier ΔET= 240 meV. Unexpectedly, the TTA-UC luminescence of the PFO emitter can be still observed after selective excitation of PtOEP at 530 nm. More importantly, the PFO TTA-UC luminescence intensity increases as the temperature decreases down to 100 K, suggesting that the misalignment of the triplet energy levels of the system is irrelevant to the mechanism that drives photon up-conversion in the PFO:PtOEP system.

Figure 2: a) The sequence of photophysical events that lead to photon up-converted (TTA-UC) delayed luminescence in unconventional TTA-UC organic composites. The nature of the depicted excited state D* remains still elusive, b) the chemical structures of the blue-light emitting PFO polymeric emitter and the two distinct main polymer backbone conformations that can be arrested in the solid state.

Figure 2a visualizes the steps leading to this aspect of the TTA-UC process. To this date, this peculiar type of the TTA-UC process is not well-understood and several experimental and theoretical attempts have been made for explaining its operative mechanism. The detailed time-resolved spectroscopic characterization of this intriguing photophysical process is expected to gain a deep insight in the mechanism of TTA-UC in organic composites and to highlight the possibility for exploiting alternative excited state pathways for enabling photon energy pooling and storage in organic materials.

IMPLEMENTATION

Workpackage 1: Project Management

Workpackage 2: Dissemination Activities

Workpackage 3: Processing Protocols and Morphology Engineering

Workpackage 4: Fabrication and electrical characterization of organic photodiodes

Workpackage 5: Spectroscopic Characterization of photon up-converting composites