Research Background and Core Strategy

The design and optimization of heterojunctions in perovskite solar cells (PSCs) represent critical pathways for enhancing power conversion efficiency (PCE) and operational stability. Traditional in-situ fabrication methods face challenges in precisely controlling nanoscale interfacial phase purity and conformal coverage of heterojunctions, often leading to mixed-dimensional phases, interfacial inconsistencies, and morphological defects. These interfacial defects cause nanoscopic energetic disorder, exacerbating charge trapping and non-radiative recombination, thereby limiting device performance and long-term stability.

This research, jointly led by Professor Zonglong Zhu from City University of Hong Kong, Professor Nicholas J. Long from Imperial College London, and Professor Bo Li from Central South University, proposes a “soft-soft interaction-guided” strategy. The research team introduced a soft Lewis base additive, Dimethyl Sulfide (DMS), into the organic cation precursor solution to regulate the formation of perovskite heterojunctions.

DMS possesses unique physicochemical properties: high donor number (DN: 40-41), low dielectric constant (DC: 6-7), and low boiling point (BP: 38°C). According to Hard-Soft Acid-Base (HSAB) theory, DMS as a soft base can engage in dynamic soft-soft interactions with soft acid Pb²⁺ ions on the perovskite surface. This interaction selectively guides the formation of preferred phase heterojunctions and finely regulates nanoscale crystallization kinetics, achieving conformal, defect-minimized interfacial growth.

Using Cs_0.05FA_0.9MA_0.05PbI₃ (CsFAMA) as the bulk perovskite, DMS-regulated PSCs achieved a PCE of 26.70% (third-party certified at 26.47%) and demonstrated excellent operational stability. After 2000 hours of maximum power point tracking (MPPT) under continuous 1-sun illumination (following ISOS-L-1 protocol), the devices maintained over 94% of their initial efficiency.

Figure 4c, 4d: Long-term stability testing showing PCE retention over 2000 hours of continuous operation.

J-V Curve Performance Characterization and V_OC Analysis

Device Architecture and Measurement Conditions

The research employed p-i-n architecture PSCs (ITO/NiO_x/SAM/CsFAMA/LD heterojunction/C₆₀/BCP/Ag), utilizing NiO_x-modified self-assembled monolayers as the hole transport layer and C₆₀ as the electron transport layer. J-V characteristic curves were measured at room temperature in a nitrogen-filled glove box, using an Enlitech SS-F5 xenon lamp solar simulator (SS-X) providing 100 mW cm^⁻² light power density, calibrated with a silicon reference cell.

Measurements were performed using a Keithley 2400 source meter with a scan rate of 0.01 V s^⁻¹ and delay time of 10 ms, conducting both forward scans (-0.1 V to 1.35 V) and reverse scans (1.35 V to -0.1 V) without pre-conditioning.

Performance Data and Reproducibility Analysis

The champion device achieved a reverse-scan PCE of 26.70% (certified value 26.47%), corresponding to an open-circuit voltage (V_OC) of 1.196 V, short-circuit current density (J_SC) of 26.23 mA cm^⁻², and fill factor (FF) of 85.11%. In contrast, untreated heterojunction devices averaged a PCE of 25.27%. DMS-regulated devices achieved an average PCE of 26.19% with a standard deviation of only 0.26, significantly lower than the untreated group’s 0.35, confirming enhanced process reproducibility.

Figure 4b, 4c: Statistical distribution of device performance parameters showing improved reproducibility with DMS regulation.

V_OC Enhancement Mechanism and Loss Analysis

DMS-regulated devices showed V_OC improvement from 1.138 V to 1.196 V, primarily attributed to suppression of interfacial non-radiative recombination and enhanced electronic uniformity.

Table S1: Detailed photovoltaic parameters comparison between control and DMS-regulated devices.

Time-resolved photoluminescence (TRPL) spectroscopy revealed that carrier lifetime increased dramatically from 447.8 ns in pristine CsFAMA perovskite to 1896.3 ns in DMS-regulated heterojunctions. This significant increase in carrier lifetime confirms that the DMS-regulated layer effectively passivates surface defects and suppresses interfacial recombination.

Figure 3g: TRPL decay curves showing extended carrier lifetimes in DMS-regulated heterojunctions.

Defect analysis corroborated these results: DMS-treated films exhibited reduced trap-filled limit voltage (V_TFL), and trap density of states (tDOS) measured by thermal admittance spectroscopy showed decreased trap state density.

Figure S16: Thermal admittance spectroscopy analysis revealing reduced trap density in DMS-regulated films.

Kelvin probe force microscopy (KPFM) analysis demonstrated more uniform surface potential distribution in DMS-regulated heterojunctions. Under illumination KPFM measurements on the C₆₀ electron extraction layer, the DMS-regulated group displayed larger contact potential difference (CPD) shifts, reflecting enhanced photo-induced electron transport efficiency from the perovskite bulk to the C₆₀ layer, thereby improving interfacial charge separation efficiency.

Figure 2f, 2j: KPFM measurements showing enhanced surface potential uniformity and improved charge separation in DMS-regulated heterojunctions.

Kinetic Analysis and Crystallization Control Mechanism

The research team conducted quantitative studies on the formation kinetics of low-dimensional (LD) layers (n=2 phase) using in-situ grazing-incidence wide-angle X-ray scattering (GIWAXS) and the Avrami-Erofeev model. Results showed that DMS addition decreased the Avrami exponent n from 0.325 to 0.295, confirming a diffusion-controlled nucleation process.

Figure 1h: Avrami-Erofeev kinetic analysis showing the effect of DMS on LD phase formation.

DMS regulation significantly increased the activation energy for LD layer formation from 21.3 kJ/mol to 37.7 kJ/mol, effectively reducing the rate of LD phase formation and enabling a prolonged growth period. This slow dimensional reduction process is critical for forming high-purity, highly conformal LD interfaces. This kinetic control not only broadens the processing window for interface layer preparation but also promotes efficient electron extraction from the perovskite absorber layer to the C₆₀ electron transport layer.

Figure 1i: Activation energy analysis demonstrating DMS-induced kinetic control of LD phase formation.

Photoluminescence and EQE-EL Characterization Analysis

Photoluminescence Spatial Distribution Analysis

PL spatial mapping measurements at 560 nm (corresponding to n=2 phase) and approximately 810 nm (corresponding to 3D CsFAMA bulk phase) revealed that DMS-regulated heterojunctions exhibited more uniform PL intensity over millimeter-scale dimensions. This result directly confirms that DMS promotes more conformal low-dimensional layer formation and improves defect passivation in the 3D bulk perovskite.

Figure 3d: PL spatial mapping showing improved uniformity in DMS-regulated heterojunctions at both n=2 and 3D phase wavelengths. 

EQE and Radiative Efficiency Assessment

External quantum efficiency (EQE) curves were measured using a QE-R EQE system, with the DMS-regulated champion device showing excellent agreement between EQE-integrated current values and J-V measurements.

To ensure high signal-to-noise ratio, the research employed the PECT-600 (FTPS) system for high-sensitivity EQE measurements, amplifying photocurrent signals through lock-in amplifiers.

Figure S24: High-sensitivity EQE measurements using FTPS system.

For radiative efficiency assessment, the study utilized the Enlitech ELCT-3010 system (REPS) for EQE-EL characterization. By applying external voltage or current bias, emitted photons were collected and quantified to determine radiative efficiency.

At an injection current equivalent to the short-circuit current density, DMS-regulated heterojunctions achieved an EQE-EL efficiency of 11.76%, significantly higher than the 6.23% of untreated heterojunctions and 3.74% of pristine CsFAMA perovskite. This result is consistent with TRPL and V_OC enhancement data, confirming that the DMS strategy effectively suppresses interfacial non-radiative recombination losses.

Figure S25: EQE-EL measurements demonstrating enhanced radiative efficiency in DMS-regulated devices.

The 11.76% EQE-EL efficiency achieved by DMS-regulated devices provides direct electrical evidence of V_OC enhancement and suppressed non-radiative recombination losses, serving as crucial validation of interfacial passivation effectiveness.

Research Achievements and Application Prospects

The research successfully developed a DMS regulation strategy that controls the crystallization kinetics of LD/3D perovskite heterojunction interfaces through soft-soft interactions, improving the uniformity and conformality of LD interfaces. Through strong coordination with Pb²⁺, DMS increases the activation energy for LD phase formation, slowing the bulk-to-low-dimensional phase transition rate, thereby achieving more uniform and efficient charge transport.

Core performance metrics include: champion device PCE of 26.70%, certified value of 26.48%, with efficiency retention exceeding 94% after 2000 hours of continuous MPPT tracking. The enhanced V_OC (1.196 V), high FF (85.11%), and low standard deviation establish the effectiveness of DMS regulation in suppressing interfacial recombination losses.

Figure 4d: Long-term operational stability showing sustained high performance over 2000 hours.

The DMS regulation strategy demonstrates excellent universality, having been validated across various perovskite compositions (including Br-containing formulations) and different organic ligand systems, all showing PCE improvements. The DMS-induced uniform LD layers enhance operational stability by increasing the energy barrier for ion migration. This work reveals the mechanism of controlling perovskite heterojunction formation kinetics through solvent engineering and soft-soft interactions, providing a universal interfacial engineering strategy for future high-performance, high-stability PSC fabrication, with potential for further exploration in high-efficiency tandem solar cell architectures.

Original Publication Information

Article Title: Nanoscale soft interaction-engineered perovskite heterojunctions for highly efficient and reproducible solar cells
Journal: Nature Communications
Publication Date: October 28, 2025
DOI: https://doi.org/10.1038/s41467-025-64550-4