Synergistic nanophotonic and work-function engineering in HTL-free carbon-based perovskite solar cells employing a double electron transport layer

Hole transport layer-free perovskite solar cells (HTL-free PSCs) are promising candidates for low-cost and scalable photovoltaic technologies. Yet, their performance is still limited by insufficient light harvesting and imperfect contact energetics. In this work, we focus on carbon-based HTL-free PSCs (C-PSCs) and numerically design and study an architecture that combines a double electron transport layer (DETL), a printable carbon back electrode, and a nanostructured aluminum (Al) rear contact. On the front side, a bilayer electron transport stack is employed to support efficient electron extraction and favorable band alignment with the perovskite absorber. On the rear side, a subwavelength graded Al grating is introduced and decorated with plasmonic Al nanoparticles (NPs) located near the perovskite/carbon interface. Three-dimensional finite-difference time-domain (FDTD) simulations are performed to resolve the optical field distribution and the resulting generation profile in the perovskite layer, enabling us to assess how the nanostructures affect the device photocurrent directly. Adding only the graded Al grating increases the short-circuit current density (J SC ) from 16.71 (mA/cm 2 ) for a planar reference device to 23.09 (mA/cm 2 ). When Al NPs are incorporated into the grating, J SC further rises to 23.58 (mA/cm 2 ), and the power conversion efficiency (PCE) improves from 10.09% to 14.41%, while the fill factor (FF) remains close to 81%. Finally, we investigate the role of allowable additives in the printable carbon ink by systematically increasing its work function from 5.0 to 5.5 (eV). This work-function engineering mainly affects the open-circuit voltage, which increases from 0.743 to 1.08 (V), whereas J SC remains nearly constant at about 23.7 (mA/cm 2 ). As a result, the PCE increases from 10.09% to 21.61%, with a maximum FF of 84.26%. Overall, the results indicate that combining a double electron transport layer with Al-based nanophotonic light trapping and additive-assisted work-function tuning of carbon electrodes provides a promising route toward high-efficiency HTL-free C-PSCs.