Abstract
SnO2 electron transport layer (ETL) has been spotlighted with its excellent electron extraction and stability over TiO2 ETL for perovskite solar cells (PSCs), rapidly approaching the highest power conversion efficiency. Thus, how to boost the performance of ETL is of utmost importance and of urgent need in developing more efficient PSCs. Here we elucidate the atomistic origin of efficient electron extraction and long stability of SnO2-based PSCs through the analysis of band alignment, carrier injection, and interfacial defects in the SnO2/MAPbI3(MA = CH3NH3+) interface using unprecedentedly high level of first-principles calculations at the PBE0 + spin-orbit-coupling + dispersion-correction level for all possible terminations and MA directions. We find that Sn-s orbital plays a crucial role in carrier injection and defect tolerance. SnO2/MAPbI3 shows favorable conduction band alignments at both MAI- and PbI2-terminations, which makes the solar cell performance of SnO2/MAPbI3 excel that of TiO2/MAPbI3. Different electron transfer mechanisms of dipole interaction and orbital hybridization at the MAI- and PbI2-terminations indicate that post-transition metal (sp valence) oxide ETLs would outperform transition metal (d valence) oxide ETLs for PSCs.
Highlights
Lead halide perovskite (LHP) solar cells (PSCs) based on ABX3 [A = Cs+, CH3NH3+ (MA+), CHN2H4+ (FA+), B = Pb2+, Sn2+, Ge2+, X = I−, Br−, Cl−] have become one of the most promising large-scale photovoltaic materials by achieving the power conversion efficiency over 25%1–7
TiO2 has been widely used as the electron transport layer (ETL) material for organic/inorganic PSCs10,11
PbI2-termination: 3 MA molecules, 4 Pb atoms, and 11 I atoms), where the lattice mismatches between MAPbI3 and SnO2 or TiO2 are as small as ~3% with a vacuumpsffiiffize pofffiffi~40 Å
Summary
The conduction band minimum (CBM) of TiO2 is slightly higher than that of MAPbI317, which hinders the electron extraction from ETL20. High temperature annealing for processing TiO2 hampers elaborate device fabrication[15]. SnO2 has shown an excellent chemical stability, UV-resistance, superior band alignment, high charge extraction, and less photocatalytic activity compared with TiO2 or other ETLs21–26,28,29. We show a comparative study of rutile SnO2/MAPbI3 and rutile TiO2/MAPbI3 interfaces to uncover the mechanism behind the superior SnO2-based PSCs by employing first-principles calculations at the hybrid Perdew–Burke–Ernzerhof (PBE0) + spin-orbit-coupling (SOC) + Tkatchenko–Scheffler (TS) dispersion correction (PBE0-SOC-TS) level. The SnO2/MAPbI3 shows superior features to TiO2/MAPbI3, including CBM band alignments, large electron carrier injection, and the suppression of mid-gap defect states. We discuss a fundamental difference in electron extraction mechanisms between MAIterminated (dipole polarization) and PbI2-terminated (orbital hybridization) MAPbI3
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