ABSTRACT
During the past decade, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has risen rapidly, and it now approaches the record for single crystal silicon solar cells. However, these devices still suffer from a problem of stability. To improve PSC stability, two approaches have been notably developed: the use of additives and/or post-treatments that can strengthen perovskite structures and the use of a nontypical architecture where three mesoporous layers, including a porous carbon backcontact without hole transporting layer, are employed. This paper focuses on 5-ammonium valeric acid iodide (5-AVAI or AVA) as an additive in methylammonium lead iodide (MAPI). By combining scanning electron microscopy (SEM), X-ray diffraction (XRD), time-resolved photoluminescence (TRPL), current-voltage measurements, ideality factor determination, and in-depth electrical impedance spectroscopy (EIS) investigations on various layers stacks structures, we discriminated the effects of a mesoscopic scaffold and an AVA additive. The AVA additive was found to decrease the bulk defects in perovskite (PVK) and boost the PVK resistance to moisture. The triple mesoporous structure was detrimental for the defects, but it improved the stability against humidity. On standard architecture, the PCE is 16.9% with the AVA additive instead of 18.1% for the control. A high stability of TiO2/ZrO2/carbon/perovskite cells was found due to both AVA and the protection by the all-inorganic scaffold. These cells achieved a PCE of 14.4% in the present work.
ABSTRACT
Hexagonal selenium with a direct band gap has been developed for optoelectronic applications for more than one century. The major advances in Se solar cells have been made using vacuum or solution-based processing methods. In this work, we demonstrate a new two-stage melt processing (TSMP) method for incorporating Se in printable triple mesoscopic solar cells in the ambient conditions. It is observed that polymerization and depolymerization between several types of selenium chains are simultaneously triggered during the melt processing, from which phase-pure hexagonal selenium is formed in the mesopores of solar cells with high crystallinity. The TSMP method has positive effects on the conduction-band energy level, band gap, and crystal phase of as-deposited Se, as revealed UV electron spectroscopy, UV-vis absorption spectroscopy, and in situ X-ray diffraction. The TSMP-based printable mesoscopic selenium solar cells show a power conversion efficiency of 2%, which is eight times that for devices based on the single-stage melting processing. These findings open up a new research direction of melting processing toward more efficient photovoltaic devices.
