| Organic solar cells become the important object on the research and developmentof third-generation solar cells due to their widely sources of materials, simpleproduction process, low cost, low energy consumption, flexibility, as well as large-scaleproduction. In order to further improve the power conversion efficiency exceeding15%,much afford are needed to deepen the understanding and use of charge transfer state(CT) exciton which determines the photovoltaic properties of organic solar cells. Thus,the main research studies of this paper are establishing a theoretical analysis and CTexciton equivalent circuit model to simulate the generation and transportation ofphoto-induced excitons, as well as separation and recombination process of CT exciton.And these theories can be applied to analyse the photovoltaic mechanism and optimizethe device structure and photovoltaic properties. Thus, the main work include five partsas following:1. A detailed study on the four photovoltaic processes of the OSCs: thegeneration of excitons, exciton transfer, exciton separation and collection of the carriers.And the full expression of photocurrent density Jph(V) has been given. Finally, somebasic requirements at improving photovoltaic performance have been summerizied:(1)synthesizing organic photosensitive materials with a low band gap of1.1eV,~1.7eV toenhance the optical absorption of OSCs;(2) improving the exciton diffusion length oforganic materials to increase the utilization efficiency of excitons;(3) lowering theHOMO level of donor materials and heightening the LUMO level of acceptors, whichcan directly increase the bandgap of CT exciton and lead to the blue shift of absorptionspectrum, and thereby improve the VOC;(4) reducing the CT exciton binding energy andlengthening the combined radiuscan can improve the separation efficiency of theexciton and JSC;(5) reducing the roll-off of photocurrent density Jph(V) within the0-VOCregion can facilitate charge collection, as well as improve the fill factor FF.2. Proposed the CT exciton equivalent circuit model which can effectivelysimulate the CT exciton dissociation and recombination processes, and this part ofthe work is the core of independent innovations of this thesis. The explainations of CT circuit model are two electronic components added to simulate the CT exciton behaviorbased on the traditional equivalent circuit model, and their physical meanings are: diodeDextreflects the separation process and diode Drecpresents recombination process of CTexcitons. Then, the calculating method is given to extract the parameters of electroniccomponents in the equivalent circuit model. Thus, this circuit model is adopted tosimulate and modeling optical and electrical properties, and also capable of extractingenergy loss mechanisms and interpretation of the device, and finally provides the mostdirect indicator parameters for the optimization of device performance.3. Explaining the basic optimization problem of thickness dependence ofphotosensitive layer, and finding out the energy loss formulas of OSCs based on thesinglet-fission materials. This thesis, thickness optimization has been carried out usingconventional pentacene/C60planar heterojunction solar cell, and several results wereobtained:(1) when the thickness of pentacene is close to the exciton diffusion length LD~40nm, the utilization efficiency of the exciton reach the highest level, and themaximum JSCcan be achieved; when the thickness exceeds the LD, the binding energy(Eb) of CT exciton would increase, and this will reduce the separation efficiency of theexciton, leading to the reduction of JSC. However, the increase of EBwould cause thefurther improve at VOC. This is the fundamental explaination of the phenomenon thatoptimal JSCand optimal VOCcan hardly obtained at the same time just by adjusting thethickness of photosensitive layer.(2) finding out the reason that the actual measuredvalues of exciton density JPexceed the maximum current P0R0based on the opticaltheory. Therefore, this paper presents a practical energy loss formulaEl oss=1β J P/P0R0of photovoltaic devices with singlet fission characteristics, and thecorrection factor β in this article is1/1.5.4. Study of the main functions of cathode buffer layer: regulation of the lightfield, the exciton blocking, and organic protective layer effects. Firstly, the effect ofdifferent BCP layer on photoelectric characteristics of planar heterojunction CuPc/C60OSCs have been optimized thickness of about10nm. Secondly, the optical spacer effectof BCP layer has been discussed based on optical transmission matrix theory, i.e. theadded BCP layer can enhance optical field distribution in the organic thin film, as wellas improve the density of light absorption and excitons; the exciton-blocking effect wasalso explained based on the exciton transporting theory, i.e. the BCP layer can provent the exciton quenching at the C60/Ag interface, and thus, increase exciton utilization.Then, through the use of the the CT exciton equivalent circuit model and the theory ofOnsager-Braun, a detailed analysis of the BCP layer on the charge collection function H(V) and resistance characteristics have been carried out. Finally, diffusion thickness~11.4nm in the BCP layer when depositing Ag cathode was extracted through theanalysis of defect states, and also well explained the reason of optimal thickness at10nm.5. Study of OSCs with a multi-charge separating (MCS) interfaceconfiguration, and the equivalent circuit model and the method of analyzingphotocurrent of MCS structure were firstly proposed. And this part of work is alsoan independent innovation. In this thesis, the phosphorescent dye (t-bt)2Ir(acac) with atriplet exciton and fluorescent dye Rubrene were introduced as interface modificationlayer, the VOCand the photo-electric conversion efficiency of MCS devices have beeneffectively improved. In order to directly simulate the competition relations of CTexciton of the main layer and modified layer at the separation heterojunction, anequivalent circuit diagram for MCS devices was proposed. Then, combined with theoptical transmission matrix theory and the external quantum efficiency, the actualphotocurrent in each functional layer were extracted. Thus, this work paved theequivalent circuit and theoretical basis to fabricate highly efficient OSCs with singletfission materials based on MCS structure. |
PDF Full Text Request