Dissertation
Dissertation > Industrial Technology > Electrotechnical > Independent power supply technology (direct power) > Photocell > Solar cells

The Investigation of Organic Solar Cells and Relative System Controlling Based on Phthalocyanine and Fullerene

Author XiZuo
Tutor ShiZhengRong
School Jiangnan University
Course Control Theory and Control Engineering
Keywords Small molecule organic solar cells Copper Phthalocyanine Fullerene Interface Modification Stability
CLC TM914.4
Type PhD thesis
Year 2012
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Organic solar cells, as a kind of new photovoltaic devices, compared to inorganicsolar cells, have many advantages, such as light weight, low cost, good mechanicalflexibility, large area fabrication, etc. Nowadays, since the power problem is more andmore serious, there is a wide developing space, an important practical value and a brightutilizing foreground for organic solar cells.The organic solar cells based on copper phthalocyanine and fullerene are studied inthis thesis. The investigation includes the optimization of thickness combination of CuPcand C60, LiF cathode buffer layer and its mechanism, the mechanisms and theirimprovements of the anode buffer layers, the improvements of electron acceptor materials,and the stability and degradation of the cells.Firstly, the light intensity distribution inside the CuPc/C60cells is studied. Based onthis result, a new parameter called “Effective Exciton Generation Velocity” is defined,which describes the amount of the excitons that can diffuse to the donor/acceptor interfacein a unit area and a unit time. The photon distribution of AM1.5G sunlight and thetransmisstivity of the substrate are introduced into the calculation process. In this thesis,the optimized thickness combination of CuPc and C60is obtained, and it is verified bypractical experiment. The error is in an acceptable range. So it is a useful way to searchingthe best thickness combination of the active layers by utilizing the method of effectiveexciton generation velocity. And this method is one of innovation points in this thesis.After obtaining the best thickness of CuPc and C60, the cells’ characteristics can beimproved by inserting an ultrathin LiF layer between the cathode and acceptor. In thisthesis, the thickness of LiF cathode buffer layer is optimized and the best thickness is1.5nm. The mechanism of this cathode buffer is also discussed. An MIS structure is used hereto explain the increase of both short circuit current density and open circuit voltage of thedevices.Actually, a lot of defects also exist in the anode interface. The characteristics of thecells are improved a lot after the utilization of the anode buffer layers. Firstly, LiF anodebuffer layer is introduced. And an MIS structure can also successfully explain themechanism here. So it can be concluded that the MIS method is a universal way to explainthis kind problem. Secondly, a PEDOT:PSS layer is introduced. The characteristics of the cells, especially the short circuit current density are improved a lot. The function of thisPEDOT:PSS layer is to smooth the anode surface, to improve the output of carriers and todecrease the interface resistance. At last, a PEDOT:PSS/LiF anode buffer layer system isused. The performances of the cells with this system are better than those of the cells witha single LiF layer and with a single PEDOT:PSS layer. And this is another innovationpoint in this thesis.Beside the investigation on the improvements at the electrode interfaces, the work onthe acceptor improvement is also studied. C70is used to replace C60as an acceptor in thisthesis. The cells using C70as an acceptor show higher Jsc, Voc, FF and efficiency than thatof the ones using C60-acceptor, because of its higher responsibility in visible region, betterelectron transport and quantum current distribution in C70molecule, and its higherconductivity. This is the third innovation point in this thesis.The stability and degradation of the cells in this thesis are also discussed.For a CuPc/C60cell without any electrode buffers, the performances improve a littleafter placing in the clean air for a while (about an hour). That is caused by an ultrathinAl2O3which is formed between C60and Al. Its function is as a cathode buffer layer. Then,with the increase of the thickness of Al2O3, the performances of the devices degrade. Sothe encapsulation time should be about an hour later after fabrication. This is the fourthinnovation point in this thesis.Contrary, the performances of the cells with electrode buffers directly and simplydecline. The increase of the performances in the first hour cannot be seen. The order of thestability from the best to the worst is as follow,①the cells with PEDOT:PSS/LiF anodebuffer system and LiF cathode buffer layer;②the cells with LiF anode buffer layer andLiF cathode buffer layer;③the cells only with LiF cathode buffer layer;④the cells withPEDOT:PSS anode buffer layer and LiF cathode buffer layer. The mechanisms whichcause the different stability are discussed.At last, the stability of the cells based on CuPc/C60and CuPc/C70is comparativelystudied. The cells using C70as an acceptor also show a superior stability than that of thecells using C60-acceptor. This comes from the higher electron affinity and ionizationpotential, and the lower symmetry of C70. Using C70as an acceptor not only can increasethe photovoltaic characteristics, but also can improve the stability of the cells dramatically.Additionally, some relative jobs based on copper phthalocyanine/fullerene organicsolar cells are also done, such as obtaining the thickness control parameters of theexperimental materials in thermal vacuum evaporation, the design of LED sun simulatorfor low cost test, building the equivalent circuit model of organic solar cells and the modelparameters identification by two different methods, and at the end, analyzing the influencefactors of mismatch in solar cell modules and giving a suggestion of gradating organicsolar cells.In this thesis, the emphases are focused on the improvements of the interface betweenthe electrode and active layer, the improvement of acceptor material (i.e. the choice of the two kinds of fullerene molecules), and the investigation on the stability and degradation oforganic solar cells. The research and analysis on the mechanisms and some basic problemsof small molecule organic solar cells are the emphases. It is hoped that the understandingof small molecule organic solar cells based on phthalocyanine and fullerene could be moreintensive from this thesis, and some instructional suggestions may be given to the laterinvestigation.

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