ZnO Thin Films Prepared By Spray Pyrolysis And Application In The Polymer Solar Cells

ZnO is a high performance electron transport material that can effectively improve the conversion efficiency of polymer solar cell(PSC) device.It is also one of the most promising electron transport layer(ETL) materials that are compatible with roll-to-roll fabrication. Spray pyrolysis(SP) technique can be successfully employed for the preparation of ZnO thin films with several advantages such as low-cost,large-area preparation. However there are few reports about PSC and other oganic photoelectric devices with ZnO thin films prepared by this technique. In this paper,properties of ZnO prepared by spray pyrolysis at various processing parameters and solution system were studied. The effects on performance to inverted PSC were investigated as well.X-ray diffraction(XRD) results show that all films deposited at 350-450℃ by SP using zinc nitrate solutions mixed with methanol as the precursor material were hexagonal wurtzite ZnO. Scanning electron microscopy(SEM) analysis indicated that the ZnO thin films consisted dense and uniform grain. ZnO thin films prepared at a higher substrate temperature had clearer boundary. The ZnO films prepared at higher substrate temperature has a better crystallinity according to XRD and SEM results.ZnO films prepared at 450℃ had higher(0 0 1) preferred orientation. Optical tests showed all the prepared ZnO films had a high transmittance up to 80% in visible range.The transmittance increased with the substrate temperature. Electrical test results showed that all the samples were high resistivity ZnO films. According to the test of PSC with as-prepared ZnO ETL,The best power conversion efficiency value of the inverted PSC was 2.80% when ZnO ETL prepared at the substrate temperature of 400℃.In order to reduce the substrate temperature of the preparation of ZnO, we developed a technique for preparing ZnO thin films at a low temperature of120-250 ℃. Firstly, thermodynamic properties of precursor and necessary substrate temperature were determined by the Thermogravimetry Differential scanning calorimetry(TG-DSC). The XRD results showed that the films were hexagonal wurtzite ZnO and have no preferential orientation. SEM and AFM results showed the grain size and roughness of ZnO film increase with the preparation substrate temperature. ZnO thin films prepared by this technique had a high averagetransmittance up to 80% in the 400-800 nm range. Both The grain size and transmittance of the films is higher than that prepared using zinc salt solution. The inverted PSC obtain the best power conversion efficiency value of 3.05%, in which ZnO ETL were prepared at the relative low substrate temperature of 200℃ with the thickness of 50 nm. Compared with ZnO ETL prepared using the zinc salt solution,the power conversion efficiency of the inverted PSC were improved obviously.However, the substrate temperature was reduced significantly to a relatively low level,which is compatible to flexible PSC fabrication.