Understanding the limitations of NIR-to-visible photon upconversion in phthalocyanine-sensitized rubrene systems

Abstract

The low statistical probability factor of the rubrene emitter (f = 15.5 ± 3%) was determined to be the prime TTA-UC efficiency-limiting factor.

Triplet–triplet annihilation based near infrared to visible molecular photon upconversion

“…Indeed, the reported NIR to Vis TTA-UC systems have certainly proved it correct. The maximum UC quantum yield obtained till date for NIR to Vis TTA-UC system is F UC B 5.5% in deoxygenated organic solvent 18,40 and F UC = 3.5 AE 0.5% in a degassed solid device. 12 At the excitation intensity front, the lowest I th = 3.2 mW cm À2 is reported for the inorganic core-shell sensitizer-transmitter-annihilator system in deoxygenated organic solvent 36 and 240 mW cm À2 for S 0 -to-T 1 sensitized degassed cyclophane annihilator solid.…”

“…The encounter complex formation probabilities are weighed according to their spin multiplicities as 1 : 3 : 5. 18 Out of these, only A S 1 decay back to A S 0 state to give UC emission via singlet channel thus setting 11.1% (1/9) as the maximum statistical limit of f factor. This is due to the fact that triplet photons of quintet and triplet encounter complexes are entirely quenched since they cannot relax via internal conversion to A S 1 state due to the spin restriction.…”

“…The f factor of 50% (diphenyl anthracene), 33% (rubrene) and even 100% (perylene) are reported now, thus making these annihilators among the popular choices for TTA-UC studies. 18 This is possible by suitable engineering of the energy levels of annihilator via chemical modification.…”

“…Upon investigating, the Pd(II) phthalocyanine/rubrene pair in deoxygenated toluene, they concluded that the low statistical probability ( f = 15.5 AE 3%) of getting rubrene's singlet from two triplets after TTA is the key limiting factor for its poor UC quantum yield. 18 The low f value sets the maximum limit of 8% for F UC with rubrene based TTA-UC systems. They investigated different sensitizer/ annihilator couples and examined their comparative quantum yield considering f factor.…”

“…Therefore, besides poor photostability, they highlighted another limiting factor of low f value of rubrene based TTA-UC system. 18 One possible application of TTA-UC systems, is in photovoltaics to harvest the wasted sub-band gap photons to enhance the efficiency. [1][2][3] Schmidt's research group were the first to demonstrate the integration of red to yellow (670 nm to 570 nm) TTA-UC system of PQ 4 PdNA/rubrene couple to organic bulk heterojunction solar cells (OPV), hydrogenated amorphous silicon (a-Si:H) thin-film solar cell and dye sensitized solar cells (DSC) (Fig.…”