The improvement of efficiency for organic photovoltaic cell with controlled nano structures

Abstract

The photovoltaic devices based on conjugated polymers with semi-conducting properties have received considerable attention from industrial companies and academic research groups because of their possibility of cost-effective fabrication, light-weight, and mechanical flexibility. However, low devices efficiency and durability comes from low conductivity and non-ohmic contact between organic layer are the obstacles to be commercialized. In this point of view, the control of nano-morphology and structure can be an effective way to solve these problems. In this thesis, efficient photovoltaic devices were fabricated by control of nano-morphology and nano-structures in various novel methods. In chapter 2, We present the effects of a concentration graded Li:Al cathode when it is made by one step evaporation method using single alloy sources on the performance of organic solar cells. The concentration profile of the Li:Al cathode and related interface energy levels were investigated by means of secondary ion mass spectroscopy and ultraviolet photoelectron spectroscopy, in comparison with those of a common Al cathode. The results indicate that interfacial lithium accumulation introduces a cascade decrease of the work function (WF) of the cathode. The WF graded cathode applied to bulk heterojunction solar cells resulted in increased short circuit current and power conversion efficiency. Furthermore, the Li:Al cathode avoids the formation of interface Al-C complex which may cause disruption of electron transport. In chapter 3, An organophilic cadmium selenide (CdSe) quantum dot (QD) interlayer was prepared on the active layer in organic solar cells by a stamping transfer method. The mother substrate composed of a UV-cured film on a polycarbonate film with strong solvent resistance makes it possible to spin-coat QDs on it and dry transfer onto an active layer without damaging the active layer. The QD interlayers have been optimized by controlling the concentration of the ...