Influences of Idling Time and Free Carrier Generation in Organic Photovoltaic Devices


Student thesis: Doctoral Thesis

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Award date5 Sep 2017


Conversion of sun light into electricity is becoming increasingly important to meet the future global energy demand. Among various photovoltaic technologies, organic photovoltaic (OPV) is considered as a promising candidate for flexible, clean and affordable renewable energy.

While photon absorption often results in direct free charge carriers in inorganic semiconductors, it results in strongly bound electron-hole pairs (excitons) in organic semiconductors. In order to dissociate these excitons into free charge carriers, a device structure consisting of a heterojunction between a donor and acceptor material is usually employed. However, highly localized nature of the excitons in organic materials remains as a limiting factor for improving the device performance.

In this work, we have shown that some bipolar organic materials (e.g. subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc)) can generate free charge carriers much more effectively than typical organic semiconductors upon photo-excitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than those of reported single-layer organic devices. In addition, internal quantum efficiencies (IQEs) of bilayer devices with opposite stacking sequences (i.e. SubNc/SubPc Vs SubPc/SubNc) are found to be the sum of IQEs of single layer devices. These results confirm that SubNc and SubPc can directly generate free carriers upon photo-excitation without assistance from a P/N junction. These allow them to be stacked onto each other with reversible sequence or simply stacking onto another P/N junction and contribute to the photo-carrier generation. These findings will have important fundamental implications in advanced OPV device architectures.

Efforts are also devoted to explore effects of ‘idling time’, a long ignored control parameter in OPV devices. It is found that the metal penetration through exciton blocking layer (EBL) can be controlled by optimizing the idling time between the depositions of EBL and cathode layers. Both electrical and optical data show that the resistance of BPhen layer (EBL) to metal (Mg:Ag) penetration increase with increasing idling time. Also, significant variation in power conversion efficiency (PCE) is observed in SubPc/fullerene (C60) based OPV cells by simply adding idling time as an additional control parameter. These observations are attributed to slow processes of self-organization in BPhen after their depositions.

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