Exploitation of charged surface defects on halide perovskite beyond passivation
In inverted structure perovskite solar cells, microscopic chemical structure and molecular orientation at the interface are directly correlated with macroscopic device performance via cross-scale characterizations.
Interfacial dipole is one of the hottest concepts in the field of thin-film solar cells, which is effective on tuning the energy band alignment to enhance the built-in field, resulting in improved charge separation, transportation and collection as well as alleviated recombination. In order to enhance interfacial dipole strength, significant efforts have been made to enhance molecular dipole moment. However, whether the molecular electric dipole moment is brought together to form an interfacial electric dipole layer is critically dependent on the ordering of molecules. A completely random orientation scenario may even lead to no interfacial dipole. Usually, it is difficult to direct translate sub-nm molecule structure across ~ ten orders of magnitude into macroscale device behavior. It is urgent to develop cross-scale characterizations to understand relationship between molecule structure, molecule aggregation and device performance.
Our research groups (Yongfang Li, Yaowen Li at Soochow University and Liwei Chen, Qi Chen at Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences) have designed a fullerene electrolyte (PCBB-3N-3I) dipole interlayer based on charged surface defects of halide perovskite. By employing cross-scale characterization techniques, it is found that the positively charged surface defects may act as anchor sites for iodide of PCBB-3N-3I via electrostatic interaction, which can be passivated efficiently and further provide a driving force for PCBB-3N-3I assembly with preferred orientation, resulting in superior device performance and stability.
Fullerene derivatives consist of carbon buckyball (e.g. C60，C70 et al.) and flexible pendant group, which exhibit strong molecular electric dipole due to symmetry breaking of the carbon buckyball. It is very challenge to realize ordering packing of fullerene derivatives on even atomic flat substrates (e.g. mica, Si), let alone on the pretty rough halide perovskite polycrystalline films.
Surprisingly, perovskite solar cells with PCBB-3N-3I treatment on perovskite active layer show significant improvement in built-in potential (Vbi) than that w/o PCBB-3N-3I treatment as supported by macroscale (~ 10-3 m - 10-2 m) device characterization methods, such as Mott-Schottky analysis and photocurrent (Jph) Vs effective voltage (Veff) curves etc. It indicates formation of PCBB-3N-3I dipole interlayer, which is further investigated by cross-scale characterizations from molecular scale, nano-micron scale, to mesoscale:
- Mesoscale (10-6 m - 10-3 m)：Spectroscopies have been widely used in the study of chemical composition and structure, which exhibits ~ μm spatial resolution and match scale of molecular aggregates well. SFG spectra, a second-order nonlinear optical process, is only sensitive to surface and interface structure, which has been employed to probe the molecule assembly of PCBB-3N-3I. It is found that PCBB-3N-3I molecules with strong electric dipole moment are assembled on the perovskite, which is a prerequisite for the dipole interlayer.
- Nano-micron scale (10-9 m - 10-6 m)：Scanning probe microscopy (SPM) covers from nanometer scale to micron meter scale. The functional imaging modes of SPM can provide abundant information of energy level alignment and interfacial charge transfer. To understand the improvement in Vbi with PCBB-3N-3I layer, in-operando cross-sectional SKPM has been performed, which visualizes abrupt vacuum level (Evac) downshift, indicating strong PCBB-3N-3I dipole interlayer. As a result, the actual Vbi across the device is reinforced as a result of superposition of interfacial dipole and Vbi originated from the asymmetric contact at the electrodes, which is consisted with that measured by macroscale technique, e.g. Mott-Schottky analysis, Jph Vs Veff curves etc.
- Molecular scale (10-10 m - 10-9 m)：The density functional theory (DFT) calculation is effective on analyzing the configuration of molecules on the substrate, which is able to explore the mechanism of the dipole interlayer formation from the molecular scale. PCBB-3N-3I, an iodide quaternized derivative of a fullerene derivative terminated with tris(dimethylamine) (PCBB-3N), exhibits lower adsorption energy on under-coordinated Pb2+ than that of PCBB-3N. It is proposed that the drastically different behavior of PCBB-3N-3I and PCBB-3N indicate that the iodide counter ion plays a critical role, which binds to under-coordinated Pb2+ via electrostatic interaction and provides a driving force for assembly of PCBB-3N-3I with favorable orientation, resulting in the formation of PCBB-3N-3I dipole interlayer.
This work not only opens up a new window to boost perovskite solar cells via rational exploitation of charged defects beyond passivation, but also prove the effects of cross-scale characterization techniques on understanding relationship between molecular structure and device performance.
The related paper has been published in Nature Communications. Please see details:
Moyao Zhang, Qi Chen*, Rongming Xue, Yu Zhan, Cheng Wang, Junqi Lai, Jin Yang, Hongzhen Lin, Jianlin Yao, Yaowen Li*, Liwei Chen and Yongfang Li. Reconfiguration of interfacial energy band structure for high-performance inverted structure perovskite solar cells. Nat. Commun. 2019, 10:4593 DOI: 10.1038/s41467-019-12613-8
Figure. Perovskite solar cells treated with PCBB-3N-3I show superior device performance, which is attributed to not only efficient defect passivation, but also optimized interfacial energy band structure due to assembly of PCBB-3N-3I driven by electrostatic interaction with charged surface defects.