Transition Metal Oxides as Highly Efficient Interfacial Materials in Organic Optoelectronic Devices

Transition metal oxides (TMOs) have been extensively studied due to their exceptional optoelectronic properties for charge injection and extraction in organic optoelectronic devices. These unique properties have led to the performance enhancement of organic light emitting diodes (OLEDs) and organic solar cells (OSCs), along with improved device stability. Our group has been successfully employed TMOs such as MoO3, WO3, TiO2, and ZnO as interfacial layers in highly-efficient OLEDs and OSCs. We have also shown that hydrogenated metal oxides exhibit desirable electronic properties. The tuning of their electronic structure was achieved as a result of hydrogen incorporation within their lattice, evidenced through the hydroxyl group formation. As a consequence, favorable energy alignment at the metal oxide/organic interface was achieved, leading to ohmic contacts, facile charge transport and thus improved device performance. A high power conversion efficiency (PCE) of ~7.2% for the PCDTBT:PCBM-based OSCs was exhibited when we use the hydrogenated metal oxides as interfacial materials.

Ultrathin ALD Insulating Metal Oxides Introduced as Passivation Interlayers to Increase Stability in Organic Solar Cells

An ultrathin ALD dielectric layer (Al2O3, ZrO2 or HfO2) was inserted between the electron extraction layer (ZnO or TiO2) and the photoactive layer of OSCs in order to successfully address the defect-rich nature of metal oxide, and enhance the efficiency and lifetime of the devices under ambient air. The passivation effect of ALD dielectrics on ZnO or TiO2 offered a highly controllable platform for improving the selectivity of the modified cathode interface by reducing the electron extraction barrier and suppressing surface recombination.

An increase of 30–35% in the PCE of P3HT:IC60BA- and PTB7:PC70BM-based solar cells was obtained when using ZnO passivated via the application of ALD deposition by six cycles of Al2O3. Moreover, the un-encapsulated ALD-modified devices exhibited a remarkable stability against ambient air retaining 70–80% of their initial PCEs after storage in the dark for 350 h.

Porphyrin Compounds as Charge Transfer Mediators in Organic Optoelectronics

The use of appropriately engineered porphyrin molecules as cathode interfacial modifiers deposited as very thin films on TiO2 for efficiency enhancement in OSCs with an inverted architecture is studied. The results show that the optimized porphyrin modifier bearing two carboxylic acids as the anchoring groups and a triazine electron-withdrawing spacer significantly reduces the work function of TiO2 thereby reducing the electron extraction barrier. Moreover, the lower surface energy of the porphyrin-modified substrate results in better physical compatibility between the latter and the photoactive blend.

Upon employing porphyrin-modified TiO2 electron transport layers in PTB7:PC71BM-based organic solar cells we obtained an improved average power conversion efficiency up to 8.73%. Importantly, porphyrin modification significantly increased the lifetime of the devices which retained 80% of their initial efficiency after 500 h of storage in the dark. In addition we employed porphyrins as thin interlayers in OLEDs at the emissive layer/Al (cathode) interface to realize efficient electron injection/transport. The interfacial energy level alignment at the cathode interface was investigated in detail, revealing that upon modification with the functionalized porphyrins a significant reduction in the electron injection barrier occurs. Porphyrin interlayer incorporation significantly improved the OLED performance as manifested by the reduction of turn-on and operational voltage and the increase of the maximum luminous/power efficiency and luminance.