Effects Of Electrode Modification On The Performance Of Inverted Organic Solar Cells

In recent years, organic solar cells (OSCs) have received considerable interest in the sustainableenergy conversion due to their advantages of low cost, light weight, flexibility, and roll-to-roll (R2R)processing with high speed and large area manufacturing. Great progress has been made by using differentactive materials and designing the different structures. The pHotoelectric conversion efficiency of organicsolar cells is achieved more than10%.Generally, the basic structure of organic solar cells is: ITO/PEDOT: PSS/organic active layer/LiF/Al.PEDOT: PSS is used as a hole transport layer. However, the PSS corrodes the surface of the ITO, resultingin rapid degradation in the performance of the conventional organic solar cells. In this work, zinc dopedTiOxand RGO/TiO2composite films were prepared by chemical methods. They were used as electrontransport layers in the inverted organic solar cells based on PEDOT: PSS or MoO3as hole transport layers.The effects of these buffer modified electrodes on the performance of inverted organic solar cells wereinvestigated by current-voltage, capacitance-voltage, and impedance spectroscopy measurements.This dissertation includes six sections, as follows:In the first section, a review was given on the latest research in organic solar cells. The significance ofthis work was stated.In the second section, Zn doped TiOxcomposite films with different zinc/Ti ratios on ITO glasssubstrate were prepared by a sol gel method as the electron transport layer in the inverted organic solar cells.The device structure is: ITO/ZnTiOx/P3HT: PCBM/PEDOT: PSS/Au. The performance of the invertedsolar cells with Zn/Ti ratio of0.8%is the best power conversion efficiency (PCE) of3.39%in all solar cellswith different zinc/Ti ratios. Compared with solar cell without Zn (PCE=2.51%), the PCE of the cell withZn/Ti ratio of0.8%increased35.1%. AFM images of ZnTiOxfilms show that the film with Zn/Ti ratio of0.8%has a lot of zinc on the vertical growth of nanorods on the ITO substrate, indicating a one dimensionalgeometry of electrode. This structure would increase the specific surface area of the active layer and theelectrode, and increase the rate of the electronic collection. Thus, after adding zinc, electron mobilityincreased. Electrochemical impedance analysis showed that the device with zinc/Ti ratio of0.8%had the smallest resistance. The small transport time constant of the electronic charge at the interface betweenorganic active ITO layer will increase the short circuit current density of organic solar cells, and improvethe PCE of a cell.In the third section, the inverted organic solar cells with ZnTiOxcomposite film as the electrontransport layer and MoO3as hole transport layer were prepared.Its price is high than oxide materialsbecause PEDOT:PSS preparation technology is complex, Therefore, PEDOT: PSS was replaced by MoO3.Moreover, Au was replaced by Ag as an anode with lower cost to prepare organic solar cells. The structureis ITO/ZnTiOx/P3HT:PCBM/MoO3/Ag. Organic solar cells with Zn/Ti ratio of0.8%had PCE of3.64%,and increased by20%, compared with the device without Zn (PCE=3.03%). Organic solar cells with Zn/Tiratio of0.8%showed better pHotoelectric properties. Electrochemical impedance analysis showed that thedevice with Zn/Ti ratio of0.8%.In the fourth section, rGO/TiO2composite films as the electron transport layer influence the propertiesof inverted organic solar cells. The device structure is: ITO/(rGO:TiO2)/P3HT:PCBM/PEDOT:PSS/Au. Theactive layer still is P3HT: PCBM, and electron transport layer is rGO/TiO2. AFM images show that themonolayer grapHene oxide (GO) with the thickness of about0.8nm, which is in keeping with a lot ofliterature reports. XPS spectra confirmed that grapHene oxide (GO) was converted into rGO. The invertedorganic solar cell with electron transport layer of rGO/TiO2composite film has PCE of2.7%, while thePCE of the device with TiO2as electron transport layer is2.5%. The impedance spectra showed that the cellwith rGO titanium oxide composite film had the larger open circuit voltage. Thus, it improved the PCE ofdevice. Under the laboratory’s conditions, after storage of90days, the PCE of the cell with rGO/TiO2of0.83wt%maintained the initial value of72.6%.In the fifth section, the inverted organic solar cells incorporating rGO/TiO2composite film as theelectronic transport layer and MoO3as hole transport layer were parpared. Cell structure is:ITO/rGO:TiO2/P3HT: PCBM/MoO3/Ag. In this section, NaOH solution was used as a reducing agent. TheGO was converted to rGO. The cell with rGO/TiO2of0.83wt%has the best performance in all deviceswith different weight percent of rGO and pure TiO2thin film. The PCE of cell with rGO/TiO2of0.83wt%is3.82%, and increased by26%, compared with the device without rGO (PCE=3.03%). This result isattributed to good rGO electrical conductivity. It can effectively prevent the charge recombination, and improve the efficiency of electronic collection, Thus, it can improve the efficiency of the organic solarcells.In the sixth section, the conclusions and perspective of this work on inverted organic solar cells weregiven.