Effect of Interfacial Molecular Orientation on Power Conversion Efficiency of Perovskite Solar Cells

Abstract

A wide variety of charge carrier dynamics, such as transport, separation, and extraction, occur at the interfaces of planar heterojunction solar cells. Such factors can affect the overall device performance. Therefore, understanding the buried interfacial molecular structure in various devices and the correlation between interfacial structure and function has become increasingly important. Current characterization techniques for thin films such as X-ray diffraction, cross section scanning electronmicroscopy, and UV-visible absorption spectroscopy are unable to provide the needed molecular structural information at buried interfaces. In this study, by controlling the structure of the hole transport layer (HTL) in a perovskite solar cell and applying a surface/interface-sensitive nonlinear vibrational spectroscopic technique (sum frequency generation vibrational spectroscopy (SFG)), we successfully probed the molecular structure at the buried interface and correlated its structural characteristics to solar cell performance. Here, an edge-on (normal to the interface) polythiophene (PT) interfacial molecular orientation at the buried perovskite (photoactive layer)/PT (HTL) interface showed more than two times the power conversion efficiency (PCE) of a lying down (tangential) PT interfacial orientation. The difference in interfacial molecular structure was achieved by altering the alkyl side chain length of the PT derivatives, where PT with a shorter alkyl side chain showed an edge-on interfacial orientation with a higher PCE than that of PT with a longer alkyl side chain. With similar band gap alignment and bulk structure within the PT layer, it is believed that the interfacial molecular structural variation (i.e., the orientation difference) of the various PT derivatives is the underlying cause of the difference in perovskite solar cell PCE.

Intensive Exposure of Functional Rings of a Polymeric Hole‐Transporting Material Enables Efficient Perovskite Solar Cells

“…It should be mentioned that such a pronounced increasing amplitude has seldom been reported for the HTM/perovskite interfacial optimization, and the J SC and PCE values of DTB device are the highest values ever released for the PSCs based on dopant‐free organic HTMs and the mesoporous PSCs based on dopant‐free polymeric HTMs, respectively (Tables S3 and S4, Supporting Information). Moreover, this is the first report that the unusual edge‐on orientation of polymer chains is realized for highly efficient polymers according to our knowledge …”

“…Accordingly, both the bulk of charge‐transporting material and its interface with the perovskite layer are crucial to the performance of PSCs; however, the latter one was primarily optimized by introducing additional materials as stated above, and those highly efficient charge‐transporting materials that could simultaneously serve as interfacial optimizers were rarely reported and only limited to the function of defect passivation but without charge extraction . Recently Xiao et al found that an edge‐on orientation where the π–π stacking direction is in parallel to the perovskite layer showed a superior photovoltaic performance over the face‐on one for the homopolymer polythiophene based PSCs mainly due to its higher charge‐extracting ability. Although this edge‐on orientation could favor the hole‐transfer across the HTM/perovskite interface, the PCE it yielded was still lower than 14% .…”

“…Recently Xiao et al found that an edge‐on orientation where the π–π stacking direction is in parallel to the perovskite layer showed a superior photovoltaic performance over the face‐on one for the homopolymer polythiophene based PSCs mainly due to its higher charge‐extracting ability. Although this edge‐on orientation could favor the hole‐transfer across the HTM/perovskite interface, the PCE it yielded was still lower than 14% . Therefore, it would be really desirable if the PCE of these HTMs with edge‐on arrangement could be raised and this concept could be intentionally integrated to design small molecular or other polymeric HTMs beyond the above homopolymer.…”

Flow Alters the Interfacial Reactions of Upconversion Nanocrystals Probed by In Situ Sum Frequency Generation

“…The interfaces of inorganic fluorides have been regarded as model systems for exploring many important reactions such as water‐mineral interactions and fluorine dissolution. [ 4c,5 ] Although the fluoride can be regarded as chemically stable in aqueous solution, slight dissolution of still occurs unless solubility equilibrium is achieved. Moreover, the hydration energy of fluoride (F − ) is much smaller that the metal ions, thus preferential leakage of F − would occur at the fluoride/water interface and more F − ions would be dissolved that the metal ions at solubility equilibrium.…”

“…The sum frequency generation (SFG) vibrational spectroscopy, a second‐order nonlinear optical technique, has been proved to be powerful for probing the interfacial molecules in view of its inherent interface selectivity and high sensitivity. [ 4 ] In this work, combined in situ spectroscopies based on SFG and photoluminescence (PL) characterization were adopted to reveal the unprecedented effect of particle moving to the interfacial adsorption of colloidal fluoride nanocrystals in great detail. Comparative results of the proof‐of‐concept experiments for the ligand‐free NaYF 4 :Yb,Er upconverting (UC) nanocrystals reveal that the particle moving in aqueous solution reversibly alter the surface charge.…”