The Fabrication And Optimization Of Polymer Solar Cells And Perovskite Solar Cells

It is well known that the photovoltaic performance of the polymer solar cells（PSCs） is seriously hindered by the film morphology of the photoactive layer. A number of strategies to control the film morphology have been continuously applied, such as controlling the donor/acceptor weight ratio, choosing appropriate minxed solvent, thermal annealing, solvent annealing and using the processing additives. Now, we detailedly studied the effect of spin-coating speed on the absorbance（absorption intensity per unit thickness）, the morphology, hole mobility of the photoactive layer and the photovoltaic performanc of the PSCs. It would provide certain theoretical guidance for controlling the morphology of the active layers. Perovskite solar cells are widely studied in the last 5 years, and the power conversion efficiency（PCE） has been rapidly promoted to 20.2%. Usually, the conventional mesoporous TiO₂ films has to be sintered at 450–550°C, which lead to its high cost and limitation in flexible devices. We tried to combine the low temperature deposition of the Ti O₂ electron transfer layer（electron transfer layer, ETL） and two-step deposition process of the perovskite to fabricate perovskite solar cells. Meanwhile, we investigated the effect of the annealing condition on crystallinity and morphology of the perovskite films.1. The PSCs devices based on the semicrystalline polymer（PTBT-HTID-DPP） have been fabricated and optimized by tuning the polymer/ /PC61BM（D/A） weight ratio, the mixed solvent volume proportion, thermal annealing and the spin-coating speed. The effects of spin-coating speeds on the photophysical property of active layer and the photovoltaic performance have been investigated. The results indicated that the spin-coating speed is not only a way to control the thickness of active layer but also an influencing factor on the morphology of the active layer. The blend film spin-coated at low spin-coating speed possesses stronger absorbance and higher monochromatic incident photon-to-electron conversion efficiency（IPCE） response in the wavelength range of visible light. When the active layer with similar thickness prepared at different spin-coating speed and the corresponding concentration, the active layer prepared at 800 rpm showed an enhanced absorbance、smaller microphase separation size, lower roughness surface and higher IPCE response in the wavelength range of visible light. Therefore, the correspondind device achieved the best PCE value of 4.31%.2. Perovskite solar cells have been fabricated by employing a compact nanocrystalline TiO₂ layer prepared by a low temperatures deposition method as electron transport layer（ETL） and a two-step process to form the CH₃NH₃PbI₃ layer（It consisted of spin coating a layer of PbI₂ onto TiO₂ surface, subsequently immersing the substrate in a solution of CH₃NH₃ I.）. When TiCl4 was hydrolyzed in an isopropyl alcohol and water mixed solvent, the TiO₂ layer showed less aggregation and crack than that hydrolyzed in pure water. As a result, the CH₃NH₃PbI₃ crystal deposited on the former showed smaller grain size and less apparent porosity on the CH₃NH₃PbI₃ film. With the increase of annealing temperature and annealing time, the grain size of the perovskite decreased, and the film was much smoother. However, these CH₃NH₃PbI₃ films remained much PbI₂ in all cases. To reduce the remaining PbI₂, the dipping time of PbI₂ in CH₃NH₃ I solution was prolonged. We were able to attain smooth and high coverage film by this way. It is necessary that a rather long reaction time（30 min） to completely convert to the CH₃NH₃PbI₃ film, which is a film with small grain size, smooth surface and little holes. However, the CH₃NH₃PbI₃ film began to peel-off from the substrate after it was dipping in CH₃NH₃ I solution for 20 min. The result indicates that this process technology can not be applied to fabricate high performance perovskite solar cells. Therefore, we tried to use a one-step method to deposite CH₃NH₃ Pb I_3-xCl_x perovskite layer by spin-coating PbCl2 and CH₃NH₃ I mix solution. The results indicated it still remained PbI₂ even though the film was annealed for more than 6 h at 75 °C. When the perovskite film was annealed at 95℃ for 50 min, no residual PbI₂ could be found. It showed the higesth surface coverage and the highest absorption intensity, and the corresponding solar cell presented the highest PCE value of 11.08%. In case the anneal temperature increased to 105℃, the perovskite film presented obvious pores, and the solar cell showed a inferior PCE. The perovskite solar cell showed the highest PCE value of 11.73% when the thickness of PC₆₁ BM as ETL was 120 nm.