Energy-Level Alignment Tuning at Tetracene/c-Si Interfaces

The rational combination of tetracene (Tc) with crystalline silicon (c-Si) could greatly enhance c-Si solar cell efficiencies via singlet fission. The Tc/c-Si energy-level alignment (ELA) is thought to be central to controlling the required interface transfer processes. We modified hydrogen-terminated c-Si (H-Si) with 2,2'-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ), C-60 , or NF3 and probed the effect on the c-Si surface chemistry, the Tc/c-Si ELA, the Tc morphology, and solar cell characteristics using ultraviolet and X-ray photoelectron spectroscopy, atomic force microscopy, X-ray diffraction, photoluminescence transients, device measurements, and transfer matrix-optical modeling. Submonolayer interlayers of F6TCNNQ shifted the Tc/H-Si(111) ELA by up to 0.55 eV. C-60 showed no notable effect on the ELA and proved detrimental for the Tc film morphology and solar cell performance. Neither F6TCNNQ nor C-60 improved the Tc-related photocurrent significantly. NF3 CVD substituted the H-termination of H-Si(100) with more electronegative species and resulted in work functions as high as 6 eV. This changed the Tc/H-Si(100) ELA by up to 0.45 eV. NF3 plasma from a remote source caused pronounced c-Si oxidation and a diminished c-Si photoluminescence lifetime, which was not observed for NF3 plasma created in close proximity to the c-Si surface or neutral NF3. We discuss possible reasons for why the improved ELA does not lead to an improved singlet fission harvest.