The Interface Modification And Stability Of The Organic Photovoltaic Cells

Organic photovoltaic has considerable technological potentials as an alternative, clean,renewable source of energy, and offers the combined attraction of light-weight,flexibility and low cost to manufacture by large-area roll-to-roll and free vacuumcoating processes. However, the relatively low power conversion efficiency (PCE),poor stability and lifetime severely are considered to be the major obstacle forcommercialization. How to simplify the fabrication process and reduce the productioncost is also important. With the purpose of improving the PCE and the stability oforganic photovoltaic device, following research work is made in this thesis.1. The organic photovoltaic cells with cesium carbonate (CH3COOCs) as cathodebuffer layer prepared by vacuum thermal deposition were studied. Effect of theCH3COOCs thickness on the performance and storeage time were investigated. Thedevice performance was greatly optimized at the CH3COOCs thickness of0.8nmwith PCE of (4.16+0.02)%,short circuit current (Isc) of (10.20+0.01) mA/cm2; Fillfactor (FF) of (63.6+0.01)%; open circuit voltage (Voc) of（0.64+0.24）V. Theimproved performance may attribute to the dissociation of semi-conductingCH3COOCs upon deposition to liberate Cs with a low work function, which reducesthe interface resistance between the active layer and the cathode and enhances theinterior electric field that may result in efficient charge transportion.2. The organic photovoltaic cells with CH3COOCs as cathode buffer layer preparedby spin-coating method were studied. The effect of different concentration ofCH3COOCs alcohol solution on the device performance was investigated. Spin-coatedCH3COOCs buffer layer can improve the device performance. The best performance(Voc=(0.66+0.01)V, Jsc=(9.82+0.34) mA/cm2, FF=(62.8+0.02)%，PCE=(4.02+0.18)%) was obtained. Compared with thedevices with CH3COOCs cathode buffer layerprepared by vacuum thermal deposition, the spin-coating method can simplify thefabrication process of organic photovoltaic devices and lowers the cost. Therefore,CH3COOCs is a promising candidate as an interlayer to enhance the PCE of organicphotovoltaic devices.3. Polymer solar cells based on P3HT: ICBA were investigated. The device with thestructure of ITO/PEDOT: PSS/P3HT: ICBA/CH3COOCs/Al was fabricated. Effect ofdifferent ratios of the donor and acceptor on the device performance was studied. Thebest performance (Voc=0.83V, Jsc=10.86mA/cm2, FF=61.1%, PCE=5.51%) wasobtained. The parameters are much higher than that of the device with PCBM as theacceptor (Jsc=9.02mA/cm2, PCE=3.22%, Voc=0.66V, FF=54.1%). Theimprovement of the performance is mainly because of the0.17eV higher of theLUMO value than that of PCBM, which is helpful for electronic transport and collection. The stability of the ICBA based devices were investigated by transientspectrum. The results revealed that the ICBA based devices have poor stability. Twokinds of method were used to improve the stability. First, Ditert butyl peroxide(DTBP) was introduced as an additive to fix the morphology. Second, PCBM wasused as a second acceptor. The results showed that both methods can improve thestability of the device. The latter is more effective.