Doped and Hybrid Metal Oxides as HTL

Based on the hydrogen molybdenum oxide bronze, Li et al. demonstrated high performance inverted OSCs by using solution-processed MoO with unique features of room-temperature and water-free processing. By introducing the vacuum treatment after the deposition of the MoO film on the 

Efficiency and stability have been the major concerns for OSCs and OLEDs when compared with the conventional semiconductor-based optoelectronie device counterpart. Great progress in metal oxide-based CTLs has been made in recent decades to improve the efficiency of both normal and inverted organic optoelectronic devices. The single junction bulk-heterojunction OSCs have now achieved PCEs of 9% or above and are continuously evolving. 

Moreover, the stability has experienced significant improvement such that the device lifetime of organic optoelectronic devices has increased from minutes to years owing to the adoption of new materials and fabrication methods. Also, the invention of inverted architecture has brought substantial enhancement in the stability. 

The metal oxide based CTLs (including ETLs and HTLs) play an important role in overall device performance for bulk-heterojunction OSCs. In this chapter, I have explained the functions and effects of the CTLs and have focused on CLTs using solution-processible materials in organic optoelectronic devices. Examples of different solution process methods of CTLs were presented. For various ETL and HTL materials, performance comparisons of each solution process method were discussed. By doping and introduction of metal NPs, the electrical properties of CTLs can be further improved by different concepts such as plasmonic-electrical effects, carrier accumulation, etc. 

Although all the different solution-processed metal oxide based materials exhibit great potential for performance optimization and commercialization, there are still some limitations regarding optical, electrical, chemical and mechanical properties for each solution-process method and materials. Therefore additional research is required in the field of CTL development to improve the performance of the OSCs and OLEDs further. 

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