Designing a Novel Hole-Transporting Layer for FAPbI3-Based Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted significant interest as potential photovoltaic technologies for the next generation. The 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) compound is commonly employed as a hole-transporting layer (HTL) in efficient PSC devices. Nevertheless, spiro-OMeTAD in its natural state exhibits limited hole mobility and conductivity. Consequently, the incorporation of chemical additives becomes necessary to enhance the conductivity and, subsequently, efficiency. Unfortunately, the presence of hygroscopic additives has been found to significantly deteriorate the quality of the perovskite layer and impair the stability of the PSCs. Herein, the novel dopant-free compound 4,4′,5,5′-tetrakis(5′-hexyl-[2,2′-bithiophen]-5-yl)-2,2′-bi(1,3-dithiolylidene) coded MS-1 was designed with a symmetrical shape, with tetrathiafulvalene (TTF) as a core and two thiophene molecules with long alkyl chains substituted in positions 4, 4′, 5, and 5′. TTF has remarkable non-aromatic 14 π electrons, and it is easy to oxidize through the reversible process to form cation and dication species (TTF+ and TTF2+, respectively). Numerical optimization of the proposed PSC with respect to several important parameters, involving thickness, total defect density of perovskite, energy bandgap, effective density of states at the valence band, and acceptor concentration of the MS-1 layer, was conducted using SCAPS-1D software. Moreover, the existence of imperfections in the electron-transporting layer (ETL)/perovskite and HTL/perovskite interfaces was taken into account, and their influence on performance was also analyzed. The designed PSC after optimization has a practically achievable efficiency of 19.91%. By conducting more investigations into this aspect of design and persisting in research endeavors within this domain, we may reveal the full capabilities of FAPbI3-based solar cells as a very promising technology for the production of green and sustainable power.