Visualizing Interfacial Energy Offset and Defects in Efficient 2D/3D Heterojunction Perovskite Solar Cells and Modules

Organic–inorganic hybrid perovskite solar cells (PSCs) achieve high efficiencies, but interfacial recombination from imperfect passivation limits further progress. The underlying mechanisms and interplay between chemical passivation (defect reduction) and field-effect passivation (band bending/energy offset) at perovskite/transport-layer interfaces remain insufficiently understood. This study develops a device-physics framework to quantify how interfacial energy offset (bi) and defect density jointly determine recombination, carrier extraction, and efficiency. It hypothesizes that an appropriately positive energy offset at the perovskite/HTL interface can suppress minority-carrier concentration and interfacial recombination more effectively than chemical passivation alone, thereby enabling high PCE with greater defect tolerance. To realize a practical scheme, a 2D perovskite interlayer is introduced to form a 2D/3D heterojunction that provides both chemical and field-effect passivation, targeting record device and module performance with improved operational stability.

The paper situates its contribution within efforts to enhance PSC passivation quality and manage interfacial recombination at perovskite/ETL or HTL interfaces. Prior work demonstrates high-efficiency PSCs and the promise of 2D perovskites for interfacial modification due to their passivation capability and facile processing. However, clear quantitative relationships between interfacial defect densities, band alignment (energy offset), and photovoltaic metrics have been lacking. The study addresses this gap by coupling simulations (device physics, DFT, finite-element) with experiments to disentangle chemical versus field-effect passivation roles and to guide heterojunction design.

- Device architecture and simulations: Regular n–i–p PSC structure SnO2/perovskite/spiro-OMeTAD (perovskite = Cs0.05MA0.05FA0.9PbI3; HTL = spiro-OMeTAD). Electrical/device-physics simulations assessed how interfacial defect density and interfacial energy offset (bi) at the perovskite/HTL interface affect J–V, VOC, FF, carrier concentrations, and E-field distributions. Cut-line analyses evaluated PCE across defect densities (10^8–10^11 cm−2) and bi values (−0.2 to 0.7 eV). - 2D/3D heterojunction fabrication: A 2D perovskite layer (~30 nm) was formed on 3D perovskite via post-treatment with n-butylammonium halide precursors (BABr or BAI), generating a 2D/3D perovskite/HTL interface. Processing details are provided in Supporting Information. - Structural characterization: GIWAXS and XRD confirmed 2D layer formation and orientation predominantly parallel to the substrate with characteristic peaks at qz ≈ 0.30, 0.43, 0.62 Å−1 indexed to (020)n=2, (020)n=1, (040)n=2. XPS evidenced chemical interaction/grafting of BA cations at the interface. - Band alignment and surface potential: KPFM measured contact potential difference (CPD) on 2D/3D perovskite/ETL/FTO stacks; UPS determined valence-band maximum (VBM) shifts to quantify band alignment changes induced by BABr vs BAI. - Defect density and carrier dynamics: Capacitance-based carrier profiles estimated defect densities. Transient photoluminescence (TrPL) probed fast (τ1) and slow (τ2) decay components to assess extraction efficiency and non-radiative recombination. Steady-state PL corroborated passivation and extraction behavior. - Ultrafast charge transfer: Femtosecond transient absorption spectroscopy (TAS) with 800 nm excitation monitored ground-state bleaching (GSB) dynamics of 3D perovskite to resolve carrier transfer from 3D to 2D layers via opening-time delays. - Ion migration kinetics: Temperature-dependent photocurrent relaxation was fit to Arrhenius behavior to extract apparent activation energies for ion migration. - DFT and finite-element screening: DFT computed density of states and band positions for 2D perovskites (n=2) as a function of Br−/I− content to evaluate VBO relative to 3D. Finite-element device simulations linked VBO, interfacial defect density, and 2D-layer mobility to PCE, FF, VOC, and recombination/power losses. - Device and module testing: Small-area cells characterized by J–V, EQE, transient photovoltage, and impedance spectroscopy. Certified measurement provided for top cell. Stability tested under continuous 1-sun at MPP. Nine-subcell series-connected modules (29 cm2 designated area) were fabricated and characterized.

- Field-effect passivation via energy offset: Simulations show that introducing a positive interfacial energy offset (bi ≈ +0.2 eV) at the perovskite/HTL interface yields high defect tolerance, maintaining PCE >27% across defect densities from 10^8 to 10^11 cm−2. Electron (minority carrier) concentration at the HTL/perovskite interface decreases by ~5 orders (10^17→10^12 cm−3) as bi increases from −0.2 to +0.7 eV, increasing VOC and FF. The local E-field near the HTL rises from <10^4 to ~10^5 V cm−1 when bi goes from −0.2 to +0.2 eV. Excessive offset (>~0.4 eV) introduces transport barriers, lowering FF and PCE. - 2D/3D heterojunction effects and band alignment: KPFM CPD shifts indicate reduced work function and interfacial band bending: CPD (mV) changes from −602 (3D) to −528 (BABr) and −455 (BAI). UPS VBMs are −5.80 eV (3D), −5.75 eV (BABr), −5.65 eV (BAI), indicating shallower VBMs and favorable VBO for hole extraction. - Defect passivation and carrier dynamics: Defect densities (cm−3) decrease from 5.58×10^16 (control) to 4.53×10^16 (BABr) and 3.67×10^16 (BAI). TrPL lifetimes: control τ1/τ2 = 177 ns/1.1 μs; BABr 108 ns/3.6 μs; BAI 46 ns/2.6 μs. Shorter τ1 indicates faster extraction; longer τ2 evidences reduced non-radiative loss. TAS GSB opening times (ps): 0.668 (control), 0.905 (BABr), 0.755 (BAI), confirming carrier transfer from 3D to 2D. - Ion migration suppression: Arrhenius analysis yields ion-migration activation energies (eV): 0.43 (control), 0.50 (BABr), 0.52 (BAI), consistent with suppressed ion migration in 2D/3D heterojunctions. - DFT/device modeling of halide tuning: Increasing Br content widens the 2D bandgap and lowers VBM; VBO shifts from +0.23 to −0.26 eV as Br increases from 0% to 100%. Positive VBO (<~0.42 eV) enables high PCE even at high interface defect density. Within the experimental window (0<VBO<~0.15 eV), PCE improves with increasing VBO and decreasing interface defects. Adequate VBO (e.g., 0.15 eV) mitigates PCE loss from low 2D mobility (10→0.1 cm2 V−1 s−1). - Device performance: Champion small-area cells: control PCE 23.02% (JSC 26.04 mA cm−2, VOC 1.120 V, FF 78.9%); BABr 24.16% (VOC 1.134 V, FF 81.8%); BAI 25.32% (VOC 1.159 V, FF 83.9%). Certified BAI device: PCE 25.04% (JSC 25.986 mA cm−2, VOC 1.152 V, FF 83.68%). - Stability and modules: Under continuous 1-sun at MPP: control T80 ≈ 100 h; BABr T80 ≈ 500 h; BAI retains 90% of initial efficiency after 2000 h (T90 = 2000 h). Nine-subcell series module (29 cm2) achieved PCE 21.39% (JSC 2.83 mA cm−2, VOC 9.416 V, FF 80.3%).

Quantitative simulations and experiments jointly show that interfacial band alignment (positive VBO/bi) is a dominant lever for suppressing interfacial recombination by reducing minority-carrier concentration at the perovskite/HTL interface. While chemical passivation lowers defect density, adding a modest positive energy offset markedly increases defect tolerance (by ~three orders), improving VOC and FF across a broad defect-density range. Implementing a 2D/3D perovskite heterojunction (via BA-based 2D layers) simultaneously delivers chemical passivation and the desired field-effect passivation. BAI-based 2D layers provide a slightly larger VBO (shallower VBM) than BABr, yielding stronger field-effect passivation, faster extraction (short τ1), lower non-radiative losses (extended τ2), and higher PCE. The heterojunction also raises the apparent activation energy for ion migration, correlating with enhanced operational stability (T90 = 2000 h without encapsulation). Modeling further clarifies that too large an offset can impede transport (reducing FF), and that proper VBO can compensate for lower 2D vertical mobility, relaxing material selection for 2D layers and HTLs. Overall, the results validate the hypothesis that tailored interfacial energy offsets, in concert with chemical passivation, are critical to achieving record efficiencies and durable operation in PSCs and modules.

This work quantifies the relative roles of interfacial energy offset and defect density in 2D/3D perovskite heterojunction PSCs. An optimized positive offset (bi ≈ 0.15–0.2 eV) drastically suppresses minority-carrier concentration and interfacial recombination, increasing defect tolerance by three orders of magnitude. A practical 2D/3D architecture using BA-based 2D perovskites provides both chemical and field-effect passivation, enabling certified small-area PCE of 25.04% and module PCE up to ~21.4%, with strong operational stability (T90 = 2000 h). These insights offer clear design rules—tuning VBO via halide composition and 2D layer selection—to engineer interfaces for efficient, stable, scalable PSCs. Future directions include: systematic exploration of other spacer cations and halide ratios to finely tune VBO; optimizing 2D layer thickness and orientation for balanced transport; integrating with diverse HTLs guided by the defect-tolerance map; and extending the approach to tandem and large-area manufacturing processes.

- Excessive positive energy offset (bi > ~0.4 eV) introduces an interfacial transport barrier under MPP, lowering FF and PCE. - 2D perovskites exhibit lower vertical electrical conductivity due to layered structure and insulating spacers, which can limit extraction if VBO is not optimized. - Complete elimination of interfacial defects via conventional dielectrics is impractical; performance relies on balancing chemical passivation with field-effect passivation. - Reported performance depends on specific materials (BAI/BABr) and processing; transferability to other 2D chemistries or large-scale fabrication may require further optimization.