Research On Inverted Polymer Solar Cells

Every day we consume the limited non-renewable energy on the earth, mainly natural gas, coal, oil and other fossil fuels as the world's rapid industrial development and raise of the level of human life. Meanwhile, the concentration of carbon dioxide produced by the fossil fuels increased rapidly in the atmosphere, which has seriously affected the climate, causing global warming and melting of polar glaciers. So to find out clean, renewable energy as alternative energy sources as soon as possible is imperative. Hydro, wind, geothermal. tidal energy, nuclear energy, solar energy are all alternative energy sources. Solar energy gets more attention for its advantage of clean, inexhaustible, wide distribution. Photovoltaic solar cell is a device to change solar radiation directly into electricity, which is considered to be one of the best ways to use solar energy. And it is a green energy representative without releasing carbon dioxide. The inorganic solar cells, has a high conversion efficiency, but its promotion is limited because of the complexed preparation and high production cost. Organic polymer solar cells with the organic polymer material as the active layer, has become the focus in recent years, for the advantage of light weight, simple process, forming large area, flexible etc. It is reported that the traditional polymer solar cells. PEDOT: PSS is often used as a hole transportation layer. It is span and coated on ITO glass substrate as the cathode. However, studies have shown that the acidic PEDOT:PSS will corrode the ITO electrode, affecting the stability of the battery. Our inverted polymer solar cells are based on the TiO2 thin film.This paper is mainly about inverted semi-transparent polymer solar cells and parallel polymer solar cells. The stable N-type nanocrystalline anatase TiO2 thin films were prepared by sol-gel method. Here, as electron selection layer, TiO2 film will block the hole transfer from the donor material to ITO. and thus avoid the electron-hole pairs in the interface composite. The donor material of the active layer is PSBTBT, the acceptor material is PCBM. The organic solvent to dissolving active layer material was o-dichlorobenzene. We selected transition metal oxides MoO3 and WO3, which were optically transparent and suitable for collecting holes, to be anode modification layer. We used the Metal Ag to play a part of the anode electrode.First, the narrow gap polymer material PSBTBT as donor material is introduced into the inverted structure. we produced the inverted polymer solar cells with a structure of ITO/nc-TiO2/PSBTBT:PCBM/MoO3/Ag. PSBTBT is a narrow band Polymers, whose energy level is (-3.5eV.-5.0eV). bandwidth is 1.5eV, and it has a strong absorption of light in the 600-800nm band. The J-V characteristics of the device is measured. At standard solar illumination of AM 1.5G, the open circuit voltage VOC is about 0.65V. the short current density JSC is about 10.54 mA/cm2,the fill factor FF is about 47.88%, the power conversion efficiency PCE is about 3.28%. From the Solar spectrum, we know that most of the photon heavily concentrate at the wavelength which is larger than 600 nm in the band. The absorption of PSBTBT ranges in the band exactly. Based on the equation:external quantum=photon absorption efficiency* exciton dissociation efficiency* charge collection efficiency, we know that, used of PSBTBT has improved the efficiency of photon absorption. As a transition metal oxide, MoO3 can effectively block the electronics, transport the holes and improve the charge transport efficiency.Second, we first used a multi-layer structure of MoO3/Ag/WO3.V2O5/Ag/V2O5 with a high transmittance as the transparent anode and had made a semi-transparent polymer solar cell with a adjustable light transmission of anode. The structure of the cells is ITO/nc-TiO2/RR-P3HT:PCBM/MoO3/Ag/WO3 (structure 1). ITO/nc-TiO2/ RR-P3HT:PCBM/V2O5/Ag/V2O5 (structure 2). For structure (1). we first tested the transparent anode MoO3(1 nm)/Ag(10 nm)/W03(x nm) composite film. The inner MoO3 can improve battery performance by collecting the holes. The outer one is used to control the light transmittance of the composite film. By changing the thickness of the out layer WO3 (thickness changes from 0 nm to 80nm). Transmission curves of the film shows obvious red shift. The film has the maximum light transmittance at 400-650 nm in wavelength when the thickness of WO3 gets to 40 nm. Test of performance of the device shows the fill factor will be improved when increasing the out layer antireflection coating. The maximum light transmission of the transparent electrode is in the active layer absorption range (thickness of WO3 is 40 nm). when the light incident from the ITO side, the device short current density JSC has the minimum value and the power conversion efficiency PCE has the minimum value. When the incident light is from the MoO3/Ag/WO3 side, the short current density JSC of the device has largest value and the power conversion efficiency PCE has the maximum value. For Structure 2, a similar manner with Structure 1. we tested the transparent anode V2O5(10 nm)/Ag(13 nm)V2O5(x nm) composite film. By changing the thickness of the out layer V2O5 (thickness changes from 0 nm to 80 nm), Transmission curves of the film shows obvious red shift. The film has the maximum light transmittance at 400-700 nm in wavelength when the thickness of V2O5 gets to 40 nm. Similar to the structure 1, the device will then be tested. So we get two valid transparent electrode, whose light transmittance can be controlled. We can choose a certain thickness of the transparent electrode to control the light transmission. In the production of these cells, we can select a appropriate thickness of the transparent electrode to make the transmitted light perfect for the top active layer materials" absorption of the upper transparent electrode cell. and the reflected light is perfect for the of absorption the end active layer materials.Finally, we explored and produced the parallel inverted polymer solar cells preliminarily, which makes the light absorption range of the battery device expand to the entire visible spectrum and has improved the efficiency of light absorption. It is parallel batteries with a structure of ITO/nc-TiO2/RR-P3HT:PCBM/MoO3/Ag/WO3 /PSBTBT:PCBM/LiF/Al. Under standard solar illumination of AM1.5G. the open circuit voltage VOC is about 0.58 V. approximately equal to the open circuit voltage of the lesser single cell (0.59 V). The short current density JSC is about 6.56 mA/cm2. approximately equal to the sum of two single cells' short current density (6.76 mA/cm2). The fill factor FF is about 54.94%. The power conversion efficiency PCE (2.09%) is higher than that of each single cell (1.17% and 1.15%). For the (structure 2):By the introduction of PbPc and optimization of its thickness we can obtaine that in AM1.5G standard solar light, the device has a best performance when the thickness of PbPc is 5nm. The open circuit voltage Voc is 0.59 V, short circuit current density JSC is 8.61 mA/cm2, fill factor FF is 60.8% and energy conversion efficiency PCE is about 3.09% of. At the same time, through statistical analysis of the characteristic of the multiple parallel cells, we obtained a realistic and effective method to use of the solar energy.