十二烷二酸修饰TiO_2电子传输层改善钙钛矿太阳电池的电流特性

在经典的平面异质结钙钛矿太阳电池中,TiO_2致密层的电子传输性能一直是获得优异光伏性能的决定性因素之一.相较于spriro-OMe TAD等常见的空穴传输材料优异的空穴传输能力,作为电子传输材料的TiO_2的导电性较弱,无法形成良好的电荷匹配.为了解决这个问题,我们使用自组装的十二烷二酸(DDDA)单分子层来修饰TiO_2致密层的表面,TiO_2致密层的导电性能得到大幅提升,并且其能带结构得到优化,促进了电子传输,降低了电子积聚和载流子复合,使得电池的短路电流密度(JSC)从修饰前的20.34 mA·cm~(-2)提升至修饰后的23.28 mA·cm~(-2),进而使得电池在标准测量条件下的光电能量转换效率从14.17%提升至15.92%.同时还发现,通过DDDA修饰TiO_2致密层,所制备的器件的光稳定性显著提升,器件未封装暴露在AM 1.5光强100 mW·cm~(-2)的模拟太阳光下超过720 min,保持初始效率的71%以上且趋于稳定.

Improvement of current characteristic of perovskite solar cells using dodecanedioic acid modified TiO2 electron transporting layer

In the classical planar heterojunction perovskite solar cells (PSCs), the electron conducting TiO₂ layer shows lower conductivity than the hole transporting materials such as spiro-OMeTAD, which becomes one of the key problems in improving the power conversion efficiency (PCE) of PSCs. In this study, the surface of compact TiO₂ layer is modified by a thin self-assembled dodecanedioic acid (DDDA) molecular layer. The TiO₂ substrates are immersed into the DDDA solution for 0.5, 2.5, 4.5, 22 h, respectively. It is found that the PCE of PSCs is improved when using the DDDA modified TiO₂, showing optimized PCE of 15.35%±0.75% under AM 1.5G illumination at 100 mW·cm^-2 after 4.5 h modification. The short current density (J_SC) of the best device is improved from 20.34 mA· cm^-2 to 23.28 mA· cm^-2, with the PCE increasing from 14.17% to 15.92%. And it is found that the hysteresis of the PSC is also reduced remarkably with hysteresis index decreasing from 0.4288 to 0.2430. In the meantime, the device with DDDA modification shows a significant improvement in light stability, keeping 71% of its initial PCE value after 720 min exposure under AM 1.5G illumination at 100 mW· cm^-2 without encapsulation. As a contrast, the device without DDDA modification keeps 59% of its initial PCE value under the same condition. To reveal the mechanism, we investigate the surface energy level change using ultraviolet photoemission spectroscopy. It is found that after DDDA modification, the valence-band maximum energy (E_VBM) of TiO₂ decreases from -7.25 eV to -7.32 eV, and the conduction-band minimum energy (E_CBM) of TiO₂ from -4.05 eV to -4.12 eV. The shifting of energy level optimizes the energy level alignment at the interface between the TiO₂ and perovskite. It promotes the transport of electrons from perovskite layer to compact TiO₂ layer and obstructs the transport of holes from perovskite layer to compact TiO₂ layer more effectively. In addition, the decrease of E_CBM implies the increase of conductivity of TiO₂. We further design a series of electrical experiments, and confirm that the modification improves the conductivity of TiO₂ obviously with both contact resistance and thin-film resistance decreasing. In summary, our results indicate the enormous potential of the compact TiO₂ layer with a thin self-assembled DDDA molecular layer modification to construct efficient and stable planar heterojunction PSCs for practical applications.