In recent years, organic-inorganic hybrid perovskite materials have garnered significant attention in the photovoltaic field due to their exceptional optoelectronic properties, including high absorption coefficients, tunable bandgaps, and long carrier diffusion lengths. However, the inherent ionic crystal nature and low-temperature solution processing of perovskite films often lead to the formation of defects, particularly at grain boundaries and surfaces. These defects, such as uncoordinated Pb2+ and halide vacancies, induce non-radiative recombination of charge carriers and ion migration, resulting in substantial efficiency losses and compromised stability in perovskite solar cells. To address these challenges, surface passivation strategies have emerged as a promising approach. In this study, we propose a multifunctional synergistic passivation strategy using hexamethonium bromide (HMABr) to simultaneously mitigate various defect types in perovskite films. Through a combination of theoretical calculations and experimental investigations, we demonstrate that HMABr effectively passivates deep-level defect states, suppresses ion migration, and enhances moisture resistance, leading to significant improvements in both efficiency and stability of perovskite solar cells.
The HMABr molecule features bromide anions (Br–) and quaternary ammonium cations (HMA+), which interact synergistically with perovskite surface defects. The Br– ions fill iodine vacancies and coordinate with uncoordinated Pb2+ sites, while the HMA+ cations anchor free I– ions via Coulomb interactions. Additionally, the alkyl chain structure of HMABr imparts hydrophobic characteristics, thereby inhibiting moisture ingress. Our results show that HMABr passivation reduces the defect density from 3.18×1015 cm−3 to 2.43×1015 cm−3, extends the carrier lifetime from 620.94 ns to 706.44 ns, and increases the power conversion efficiency (PCE) from 22.21% to 23.39%. Furthermore, unencapsulated devices retain 90.1% of their initial PCE after 1,152 hours in ambient air, underscoring the effectiveness of this passivation approach.
To elucidate the interaction mechanisms between HMABr and perovskite surfaces, we performed density functional theory (DFT) calculations on the α-FAPbI3 (001) crystal plane with iodine vacancy defects. The adsorption energy of HMABr on the defect site was calculated using the formula: $$E_{\text{ads}} = E_{\text{total}} – E_{\text{perovskite}} – E_{\text{HMABr}}$$ where \(E_{\text{total}}\) is the total energy of the system, \(E_{\text{perovskite}}\) is the energy of the perovskite slab, and \(E_{\text{HMABr}}\) is the energy of an isolated HMABr molecule. The negative adsorption energy indicates a stable interaction. Differential charge density analysis revealed electron accumulation around uncoordinated Pb2+ sites and electron depletion around Br– ions, confirming the electron transfer from Br– to Pb2+. This coordination effectively passivates the deep-level defects associated with uncoordinated Pb2+.
X-ray diffraction (XRD) patterns of control and HMABr-passivated perovskite films showed characteristic peaks at 14.03° and 28.27°, corresponding to the (100) and (220) planes, respectively. No peak shifts or additional phases were observed, indicating that HMABr passivation does not alter the perovskite crystal structure. The surface morphology, examined by scanning electron microscopy (SEM), exhibited similar grain sizes and coverage for both films, suggesting that HMABr treatment does not adversely affect film formation. Atomic force microscopy (AFM) measurements indicated a reduction in root-mean-square (RMS) roughness from 48.75 nm to 43.09 nm after passivation, which improves the interface contact with the hole transport layer (HTL).
Steady-state photoluminescence (PL) spectra showed a significant increase in intensity for HMABr-passivated films without peak shifts, implying reduced non-radiative recombination. Time-resolved photoluminescence (TRPL) decay curves were fitted using a bi-exponential function: $$I(t) = A_1 \exp(-t/\tau_1) + A_2 \exp(-t/\tau_2)$$ where \(\tau_1\) and \(\tau_2\) represent the fast and slow decay lifetimes, and \(A_1\) and \(A_2\) are their respective amplitudes. The average carrier lifetime (\(\tau_{\text{avg}}\)) was calculated as: $$\tau_{\text{avg}} = \frac{A_1 \tau_1^2 + A_2 \tau_2^2}{A_1 \tau_1 + A_2 \tau_2}$$ The results, summarized in Table 1, demonstrate an increase in \(\tau_{\text{avg}}\) from 620.94 ns to 706.44 ns, corroborating the suppression of defect-mediated recombination.
| Sample | \(\tau_1\) (ns) | \(A_1\) (%) | \(\tau_2\) (ns) | \(A_2\) (%) | \(\tau_{\text{avg}}\) (ns) |
|---|---|---|---|---|---|
| Control | 157.40 | 34.54 | 865.56 | 65.46 | 620.94 |
| HMABr | 163.03 | 6.57 | 744.63 | 93.43 | 706.44 |
Fourier-transform infrared (FTIR) spectroscopy revealed new peaks at 1082 cm-1 and 1381 cm-1 for HMABr-passivated films, corresponding to N-C stretching vibrations and methyl symmetric bending vibrations, respectively. This confirms the chemical interaction between HMABr and the perovskite surface. X-ray photoelectron spectroscopy (XPS) analysis of I 3d and Pb 4f core levels showed shifts toward lower binding energies after passivation. For I 3d, the peaks shifted from 619.22 eV and 630.70 eV to 619.07 eV and 630.55 eV, indicating Coulomb interactions between HMA+ and I–. For Pb 4f, the peaks shifted from 138.30 eV and 143.20 eV to 138.15 eV and 143.05 eV, demonstrating electron transfer from Br– to Pb2+ and effective passivation of iodine vacancies.
UV-visible absorption spectra showed no significant changes in the absorption edge, and Tauc plot analysis yielded a bandgap of 1.55 eV for both films, confirming that HMABr passivation does not alter the optical bandgap. The electrical properties were further investigated using electrochemical impedance spectroscopy (EIS). The Nyquist plots were fitted with an equivalent circuit model comprising series resistance (\(R_s\)), charge transfer resistance (\(R_{tr}\)), and recombination resistance (\(R_{rec}\)). The HMABr-passivated devices exhibited lower \(R_s\) and \(R_{tr}\) and higher \(R_{rec}\), indicating improved charge extraction and reduced recombination. The parameters are listed in Table 2.
| Sample | \(R_s\) (Ω) | \(R_{tr}\) (Ω) | \(R_{rec}\) (Ω) |
|---|---|---|---|
| Control | 12.5 | 45.3 | 320.6 |
| HMABr | 10.8 | 38.7 | 385.4 |
Space-charge-limited current (SCLC) measurements were conducted on electron-only devices with the structure ITO/SnO2/Perovskite/PCBM/Ag. The defect density (\(N_t\)) was determined from the trap-filled limit voltage (\(V_{\text{TFL}}\)) using the Mott-Gurney law: $$J = \frac{9}{8} \epsilon_0 \epsilon_r \mu \frac{V^2}{L^3}$$ where \(J\) is the current density, \(\epsilon_0\) is the vacuum permittivity, \(\epsilon_r\) is the relative permittivity, \(\mu\) is the carrier mobility, \(V\) is the applied voltage, and \(L\) is the film thickness. The defect density is given by: $$N_t = \frac{2 \epsilon_0 \epsilon_r V_{\text{TFL}}}{e L^2}$$ where \(e\) is the elementary charge. The \(V_{\text{TFL}}\) decreased from 0.27 V to 0.22 V after passivation, corresponding to a reduction in \(N_t\) from 3.18×1015 cm−3 to 2.43×1015 cm−3.
To analyze the recombination dynamics, we measured the open-circuit voltage (\(V_{\text{OC}}\)) as a function of light intensity. The data were fitted to the equation: $$V_{\text{OC}} = \frac{n k T}{q} \ln(I) + \text{constant}$$ where \(n\) is the ideality factor, \(k\) is Boltzmann’s constant, \(T\) is temperature, \(q\) is the elementary charge, and \(I\) is the light intensity. The ideality factor decreased from 1.91 to 1.80 after HMABr passivation, indicating suppressed Shockley-Read-Hall (SRH) recombination. Similarly, the short-circuit current density (\(J_{\text{SC}}\)) versus light intensity was fitted to a power law: $$J_{\text{SC}} \propto I^\alpha$$ where \(\alpha\) approaches 1 when bimolecular recombination is minimized. The value of \(\alpha\) increased from 0.979 to 0.994, confirming reduced recombination losses.
The current density-voltage (\(J-V\)) characteristics of perovskite solar cells were measured under standard AM 1.5G illumination. The HMABr-passivated devices exhibited a \(V_{\text{OC}}\) of 1.17 V, \(J_{\text{SC}}\) of 25.17 mA/cm2, fill factor (FF) of 79.40%, and PCE of 23.39%, compared to 1.15 V, 24.45 mA/cm2, 78.98%, and 22.21% for the control devices. External quantum efficiency (EQE) spectra showed enhanced response in the 350-500 nm range, and the integrated \(J_{\text{SC}}\) values agreed well with the \(J-V\) measurements. Statistical analysis of 16 devices confirmed the reproducibility of the performance enhancement.
| Device | \(V_{\text{OC}}\) (V) | \(J_{\text{SC}}\) (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| Control | 1.15 | 24.45 | 78.98 | 22.21 |
| HMABr | 1.17 | 25.17 | 79.40 | 23.39 |
The stability of perovskite solar cells was evaluated under various conditions. Thermal stability tests at 85°C and 10%±7% relative humidity (RH) showed that HMABr-passivated films retained their color and absorption properties for up to 8 days, while control films degraded rapidly. Water contact angle measurements increased from 58.3° to 66.9° after passivation, demonstrating enhanced hydrophobicity. Long-term stability tests in ambient air (20±5°C, 10%±5% RH) revealed that unencapsulated HMABr devices maintained 90.1% of their initial PCE after 1,152 hours, whereas control devices retained only 81.8%. The improved stability is attributed to the synergistic effects of defect passivation, ion migration suppression, and moisture blocking.
In conclusion, we have developed a multifunctional passivation strategy using HMABr to address the critical issues of defect-mediated recombination and instability in perovskite solar cells. The Br– anions effectively passivate uncoordinated Pb2+ and iodine vacancies, while the HMA+ cations anchor free I– ions, reducing ion migration. The alkyl chains provide a hydrophobic barrier against moisture. As a result, HMABr passivation significantly lowers defect density, extends carrier lifetime, and enhances device efficiency and stability. This approach offers a promising pathway for the commercialization of high-performance perovskite solar cells.
Future work will focus on optimizing the HMABr concentration for large-area devices and exploring its application in tandem perovskite solar cells. Additionally, the long-term stability under operational conditions, such as continuous illumination and thermal cycling, will be investigated to further validate the practicality of this passivation strategy.