Effects Of Active Layer Optimization And Electrode Interface Modification On The Performance Of Polymer Solar Cells

Polymer solar cells (PSCs) with light-weight, compatible with low-cost andlarge-area of wet-film manufacturing technology, and can be made flexible, specialshape of the device. Furthermore, through molecular design and synthesis of novelsemiconductor, device performance can be easily improved. Based on these aboveunique advantages, PSCs have been attracted much attentions of the research teamsat home and abroad, and have thus become one of the most popular fields of study.Nowadays, power conversion efficiency (PCE) over10%of PSC with a smallarea has been reported in the literature, achieving the requirements of thecommercial production, has thus become one of the most promising next-generationsolar cell products. So far, research on PSC is mainly concentrated in thedevelopment of versatile new polymer materials, structure design, and deviceperformance optimization. In this thesis, based on our laboratory conditions, weaddress to optimize the device performance in order to solve the existed problems ofPSCs and grasp the research direction of PSCs. The optimization of PSCs wasperformed by a step-by-step process. The first process is to optimize the bulkheterojunction active layer of PSCs; the second process is to optimize the interfacebetween the active layer and the electrode. The first step is the foundation of thesecond step, which expects to achieve the perfect optimization of the deviceperformance. Our research will offer the effective way to further enhance theperformance of PSCs in the future. The main contents and results of this thesis are asfollows:First, high performance and free-thermal annealing PSCs were fabricated basedon P3HT:PCBM active layer at room temperature in air conditions. Throughadjusting the speed and time of the spin-coating process, and room temperatureduring the solvent evaporation of the active layer, the rate of solvent evaporation wascontrolled, and the self-assembly function of the active layer material can beachieved. Self-assembly function of the active layer makes the ordered degree of crystallinity of P3HT and improved the hole carrier mobility, which balances thecarrier transport in the anctive layer. This improved the fill factor (FF) andshort-circuit current density (Jsc) of PSCs. The optimization of P3HT:PCBM deviceperformance is close to the international reported level (4-5%), and its specificparameters were as follows: open circuit voltage (Voc)=0.61V, FF=0.71, Jsc=10.87mA/cm2and PCE=4.71%. Since this process does not require heating annealing, theefficiency of flexible PSCs achieved4.06%. In view of the adjusted solventevaporation rate by room temperature, high-efficiency PSCs were also fabricatedusing dip coating process which is compatible with roll to roll technique.Second, the current expensive ITO was replaced by alternative transparentconductive film (TCO), the aim is to reduce the device cost and provide moreflexibility to the choice of electrodes. A series of different TCOs (FTO, IWO, AZO,etc.) as the electrode material is applied in PSCs. The results demonstrated that theTCOs can completely replace the ITO and does not reduce the device performancethrough modification and optimization of the interface between the electrode and theactive layer. The influence of electrical, optical properties, surface morphology andwork function of different TCOs on device performance parameters was analyzed. Itwas found that the electrical properties and surface morphology of TCO have acertain impact on the device FF and Jsc, while the device Vocis independent of thechoice of electrodes.Third, the textured electrode with the light-trapping effect was introduced intoPSCs, which replaces the planar electrodes in PSCs in order to strengthen theabsorption of the solar spectrum. The excellent interface contact between theelectrode and the active layer was achieved by rational optimization of the texturedelectrode surface, which ensures the increased Jscand keeps the device FF and Voc.Compared with the planar electrode cell, Jscand PCE of the textured cell enhancedby10%and8%, respectively. The study also showed that the "U" shaped texturedelectrode is more suitable than the "V" shaped textured electrode for application inPSCs. The tests of the reflection spectra and external quantum efficiency (EQE) ofPSCs demonstrate that the light-trapping electrode could significantly increase thelight absorption and utilization. Fourth, after PEDOT:PSS doped with different concentrations of EG andDMSO, the conductivity, transmittance and morphology of doped PEDOT: PSS filmwere investigated. The influence of the doped PEDOT:PSS film as the anodemodification layer on the device performance of PSCs was researched. When thedoping concentration of EG and DMSO was10wt%, the electrical conductivities ofthe film were increased about two and three orders of magnitude, respectively,compared with the undoped. The transmittance of the EG and DMSO-dopedPEDOT:PSS did not change, but the film surface roughness was increased. Theenhanced conductivity of the doped PEDOT:PSS film reduced the device seriesresistance (Rs), resulting in increased the device Jsc, but the doped PEDOT:PSS filmwith increased roughness also affected the device FF. The experimental resultsshowed that when the doping concentration of EG was10wt%, the device with thelowest Rsshowed a PCE of4.34%which enhacend by29%than the undoped. Whenthe doping concentration DMSO was10wt%, the device with the lowest Rsshoweda PCE of4.45%which enhacend by30%than the undoped.Fifth, the inverted structure of PSCs was fabricated by selecting the modifiedlayer of ZnO to optimize the electrode interface, which avoids the use of corrosiveand hydrophilic PEDOT:PSS resulting in the recession of the device performance.The ZnO nanorods arrays were prepared by water bath, and were applied in invertedPSCs. The impact of the different ZnO nano-morphology as the interfacemodification layer on the device performance was investigated. The study found thatthe sparse ZnO nano-pillar array was helpful for electronic collection and transport,thereforce, can significantly improve the device performance. Thickness series anddoping series of ZnO thin films were prepared by metal organic chemical vapormethod (MOCVD), and were applied in PSCs as the interface modification layer. Itwas found that an appropriate increase in the ZnO conductivity can improve thedevice performance at a fixed ZnO film thickness. Compared with the generalstructure, the stability of the inverted device useing ZnO as an interface modificationlayer significantly inproved.