Recombination velocity less than 100 cm/s at polycrystalline Al2O3/CdSeTe interfaces

Abstract

Reducing recombination in polycrystalline solar cells by orders of magnitude is currently one of the greatest challenges for increasing thin-film solar cell efficiency to theoretical limits. The question of how to do this has been a challenge for the thin-film community for decades. This work indicates that effective interface passivation is critical. Here, polycrystalline Al2O3/CdSeTe/Al2O3/glass heterostructures are grown, and a combination of spectroscopic, microscopic, and time-resolved electro-optical measurements demonstrates that the interface recombination velocity at alumina/thin-film interfaces can be less than 100 cm/s. This is three orders of magnitude less than typical CdTe interfaces without passivation, commensurate with single-crystal epitaxial CdMgSeTe/CdSeTe/CdMgSeTe double heterostructures, and enables minority-carrier lifetimes in polycrystalline CdSeTe well above 100 ns. Microscopic interfacial electric-field measurements identify the field effect as a potential mechanism for polycrystalline Al2O3/CdSeTe interface passivation. The results provide guidance for modeling and interface passivation in devices and indicate future paths to realize highly efficient thin-film solar cells.

Radiative Efficiency and Charge‐Carrier Lifetimes and Diffusion Length in Polycrystalline CdSeTe Heterostructures

“…To develop passivated contacts, Kephart et al investigated CdSe x Te 1− x passivation with Al 2 O 3 . Alumina passivation reduces interface recombination velocity S int < 100 cm s −1 and correspondingly increases carrier lifetimes to more than 200 ns in polycrystalline double heterostructures (DHs) . The passivation mechanisms are related to interface oxides and charges …”

“…Two‐photon excitation (2PE) at 1120 nm with an average excitation power of 5 mW was used. This results in estimated injection of 1 × 10 15 cm −3 . All PL was integrated using a 188 nm bandpass filter centered at 795 nm.…”

“…[16] Alumina passivation reduces interface recombination velocity S int < 100 cm s À1 and correspondingly increases carrier lifetimes to more than 200 ns in polycrystalline double heterostructures (DHs). [17] The passivation mechanisms are related to interface oxides [18] and charges. [17,19] Here, we show that CdSe 0.2 Te 0.8 passivation with Al 2 O 3 increases PLQY by a factor of 1000x-to 2 Â 10 À3 (0.2%).…”

“…[17] The passivation mechanisms are related to interface oxides [18] and charges. [17,19] Here, we show that CdSe 0.2 Te 0.8 passivation with Al 2 O 3 increases PLQY by a factor of 1000x-to 2 Â 10 À3 (0.2%). Quasi-Fermi-level splitting (Δμ F ) of 950 mV is confirmed.…”

“…This results in estimated injection of 1 Â 10 15 cm À3 . [17] All PL was integrated using a 188 nm bandpass filter centered at 795 nm. For DHs with absorbers grown by CSS and sputter-deposited Al 2 O 3 , room-temperature minority carrier lifetimes were 430 [16] and 700 ns.…”