Investigation Of Energy Level Architecture In Bulk Heterojunction Of Organic/Inorganic Hybrid Solar Cells

The explorations and investigations of solar energy have aroused intensive interest over the past decade due to its safety and sustainability. Since the photoelectric conversion effect has become the main application of solar energy for researchers, solar cells have witnessed a long-period and tremendous progress. In recent years, Organic/Inorganic hybrid solar cells have attracted increasing attention as an alternative and promising solution for fossil energy depletion. This type of solar cells possesses unique advantages of the both components such as low-temperature processing together with low-cost of organic semiconductors and high electron mobility as well as high dielectric constant of inorganic semiconductors.D-π-A type small molecules is usually consist of triphenylamine as strong electron-donating unit, thiophene as π-conjugation bridge, alkyl cyanoacetate, dicyanovinyl or rhodanine as strong electron-accepting unit. Small molecules based on triphenylamine derivatives exhibit excellent photovoltaic properties, we designed and fabricated three new types of organic small molecules, C-TPA, D-TPA, R-TPA, The energy level alignment can be tuned by different end groups. Here, we investigated properties of small molecules by UV-vis absorption spectra, PL spectra, cyclic voltammetry and thermogravimetric analysis. The results show this type of small molecules possess widen absorption range and low redox potential along with fine thermal stability, which are beneficial to be an ideal candidate for photovoltaic material. Moreover, we tested the final hybrid solar cells with different small molecules, D-TPA demonstrates the best performance and PCE is 0.802% compared to R-TPA and C-TPA.In addition, anatase TiO₂ is well applied to Origanic/Inorganic hybird solar cells as a typical electron accepter material with the characteristics of high carrier mobility, high dielectric constant and long excited state lifetimes and etc. It is well known that the energy level of TiO₂ can be tuned effectively by doping rare earth ions with the purpose of a better matched energy level between the donor and accepter. SmPO4 NPs doped TiO₂/P3 HT bulk heterojunction（BHJ） solar cell shows an enhanced power conversion efficiency of approaching 3% as compared with pure TiO₂ without doping（1.98%）, which can be ascribed to the accelerating exciton generation, dissociation, and charge transport of BHJ. The femtosecond transient absorption spectroscopy results show that the hot electron transfer life-time was shortened from 30.2 ps to 17.9 ps, that is, more than 40% faster than the pure TiO₂ accepter while the hole transfer lifetime was boosted by almost 20%（from 6.92 ns to 5.58 ns）.Metal oxide nanocrystals have been pursued for various applications in photovoltaics as a buffer layer. But, it still meets a challenge of mismatched energy level in the donor-acceptor system. However, the grapheme quantum dots（GQDs） with bright blue photoluminescence can solve this problem. In our work, we successfully fabricated GQDs by a top-down strategy based on laser fragmentation with hydrothermal treatment. The GQDs demonstrate appropriate energy level between TiO₂ and P3 HT to form a ladder energy level architecture. The introduction of the GQDs as a buffer layer into a bulk heterojunction hybrid solar cell has led to an enhancement of the power conversion efficiency. After introduction of the GQDs buffer layer, the short-circuit current density（Jsc） increases from 5.89 7.98 mA·cm-2 to 7.98 mA·cm-2. Meanwhile, the open-circuit voltage（Voc） increases from 0.698 V to 0.757 V, thus leading to a notable PCE enhancement, from 2.04% to 3.16%.