Organic light emitting diode (OLED) has lots of advantages in display technology such as self-emissive, light-weight, and compatible for flexible substrates. Compared to the red and green OLEDs, blue one has shorter lifetime due to high-energy polaron quenching. Triplet-triplet annihilation up conversion (TTAUC) is a promising way to reduce the driving voltage and improve the operation lifetime for a blue OLED. This system includes two materials, sensitizer and emitter, which has a narrower bandgap and triplet-triplet annihilation (TTA) characteristic, respectively. When the sensitizer is excited, its triplet exciton can transfer the energy to the triplet of emitter with lower triplet energy and fuse into one singlet exciton with a higher energy photon than the sensitizer.
In this research, we demonstrate that a convention green fluorescent material, tris(8-hydroxyquinolinato)aluminum (Alq3) can be used as a sensitizer for a blue TTA emitter, 9,10-Bis(2-naphthyl)anthraces (ADN). In a conventional Alq3-based OLED, when the electron and hole coming from cathode and anode recombined at Alq3 as the recombination layer, 25% of singlet and 75% of triplet were generated. All triplet exciton experienced non-radiative recombination, which resulted in low efficiency due to the poor reverse intersystem crossing (RISC) rate. On the other hand, in TTAUC-OLED, triplet exciton of Alq3 (ET=2.0 eV) transferred the energy to the triplet of ADN (ET=1.67 eV) via Dexter energy transfer and two of them fused into one singlet (ES=2.83 eV) with blue emission. This recycled the useless triplet exciton and resulted in a higher external quantum efficiency (EQE) for TTAUC-OLED (2.1%) compared to the Alq3 (1.2%) and ADN (1.67%) control devices. Moreover, the recombination zone was shifted from ADN to Alq3, the operation lifetime of blue component can be increased by 3x times longer than the ADN control device. From transient electroluminescence (TrEL) measurement, Alq3-control device showed a fast decay within 1us which implied only singlet exciton involved. For the TTAUC-OLED, blue emission showed only delayed component which meant the emission came from only TTA process without direct recombination.
The spectrum of 400 to 1100 nm sunlight can be divided into three bands, each absorbed by organic photovoltaic devices that are particularly efficient under the band in question, to achieve higher photoelectric conversion efficiency. The three bands are the absorption bands of fullerene (C70), chloroaluminum phthalocyanine (ClAlPc), and tin naphthalocyanine dichloride (SnNcCl2), which have peak values of 500, 731, and 863 nm, respectively. C70 is a well-known acceptor, whereas ClAlPc and SnNcCl2 serve as donors. In combination with another donor made of 4,4’-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC), which is almost transparent in 400 to 1100 nm, three devices were fabricated and had active layers of TAPC : C70, ClAlPc : C70, and SnNcCl2 : C70. After the doping proportions of these materials had been optimized, the maximum power conversion efficiency (PCE) values of the three devices were 4.52%, 4.3%, and 1.33%, respectively. The properties of donor material dominated the differences among these device behaviors. Subsequently, the overall PCE of a simulated multiple reflection module was calculated using these three devices, which, depending on the arranged sequence in which they were exposed to light and reflected the light to another, generated different absorption spectrum and thus influenced the overall PCE of the photovoltaic integrator. The highest overall simulated and experimental PCE of the photovoltaic integrator was 6.12% and 5.9%, respectively.