Perovskite solar cells have emerged as a promising third-generation photovoltaic technology due to their high power conversion efficiency, low material cost, simple fabrication processes, excellent weak-light response, and potential for flexible and semi-transparent applications. Recent advancements have pushed the certified efficiency of single-junction narrow-bandgap perovskite solar cells to over 27%, surpassing traditional silicon-based cells. However, the Shockley-Queisser limit constrains further improvements, necessitating the development of wide-bandgap perovskite solar cells for tandem structures to break this barrier. Wide-bandgap perovskite solar cells, with bandgaps ≥1.6 eV, face challenges such as rapid crystallization, increased defect density, and poor stability due to higher cesium or bromine content. Additive engineering offers a straightforward approach to mitigate these issues by incorporating functional molecules into the perovskite precursor solution. In this study, we introduce homoveratric acid as a bulk passivation additive to wide-bandgap perovskite solar cells, leveraging its carboxyl and methoxy groups to interact with Pb²⁺ ions, reduce defect states, and enhance crystallinity. We demonstrate a significant improvement in device performance and stability, providing a foundation for developing effective passivation strategies in wide-bandgap perovskite solar cells.
The wide-bandgap perovskite formulation used in this work is (FA0.77MA0.23)0.95Cs0.05Pb(I0.77Br0.23)3, with a bandgap of approximately 1.68 eV. Homoveratric acid was dissolved in dimethyl sulfoxide at various concentrations (0.5 mM, 1 mM, and 1.5 mM) and added to the perovskite precursor solution. The device structure consists of ITO/PTAA/PEAI/perovskite/PEAI/PCBM/BCP/Ag, fabricated using spin-coating and thermal annealing processes. Characterization techniques included scanning electron microscopy, X-ray diffraction, steady-state and time-resolved photoluminescence, X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy, electrochemical impedance spectroscopy, and transient photovoltage/current measurements. The performance of perovskite solar cells was evaluated under standard illumination conditions, and stability tests were conducted in a nitrogen-filled glovebox at room temperature and 85°C.
The incorporation of homoveratric acid into the perovskite precursor solution significantly influenced the photovoltaic parameters. We observed a clear trend where the open-circuit voltage, short-circuit current density, and fill factor initially increased with homoveratric acid concentration up to 0.5 mM, then decreased at higher concentrations. The optimal device achieved a power conversion efficiency of 20.60%, with a VOC of 1.215 V, JSC of 21.33 mA cm−2, and FF of 79.53%. In contrast, the control device without homoveratric acid exhibited a power conversion efficiency of 18.54%, with VOC = 1.189 V, JSC = 19.96 mA cm−2, and FF = 78.08%. The hysteresis index, calculated as HI = (PCEreverse – PCEforward) / PCEforward × 100%, decreased from 2.83% for the control to 1.58% for the homoveratric acid-modified device, indicating suppressed ion migration and reduced defect-assisted recombination. Statistical analysis of 50 independent devices confirmed the reproducibility of the homoveratric acid treatment, with tighter distributions of VOC, JSC, and power conversion efficiency values compared to the control.
| Homoveratric Acid Concentration (mM) | VOC (V) | JSC (mA cm−2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 0 | 1.189 | 19.96 | 78.08 | 18.54 |
| 0.5 | 1.215 | 21.33 | 79.53 | 20.60 |
| 1 | 1.196 | 21.15 | 77.90 | 19.70 |
| 1.5 | 1.207 | 20.82 | 77.76 | 19.54 |
The morphology and crystallinity of the perovskite films were examined using scanning electron microscopy and X-ray diffraction. The control film showed sparse grain distribution with visible PbI2 residues at grain boundaries, whereas the homoveratric acid-treated film exhibited denser packing, larger grain sizes, and suppressed PbI2 formation. Cross-sectional images revealed vertically aligned grains in the homoveratric acid-modified perovskite layer, facilitating efficient charge transport. X-ray diffraction patterns confirmed the reduction in PbI2 peaks and enhanced intensity of perovskite characteristic peaks, such as (110) and (220), indicating improved crystallinity. The intensity ratios of (110)/(310) and (220)/(310) increased with homoveratric acid addition, further supporting the optimized crystal growth.
Optical properties were investigated through ultraviolet-visible absorption and photoluminescence spectroscopy. The homoveratric acid-treated film demonstrated enhanced absorption in the wavelength range below 750 nm, attributed to reduced defect-mediated non-radiative recombination. The Tauc plot derived from absorption data yielded bandgap values of 1.6757 eV for the control and 1.6735 eV for the homoveratric acid-modified film, confirming that homoveratric acid did not alter the perovskite bandgap. Steady-state photoluminescence spectra showed a emission peak at 740 nm for both films, but the homoveratric acid-treated sample exhibited higher intensity, suggesting suppressed non-radiative recombination. Time-resolved photoluminescence decay curves were fitted with a bi-exponential model, revealing an average carrier lifetime of 306.29 ns for the homoveratric acid-treated film compared to 119.97 ns for the control. This prolonged lifetime indicates reduced trap-assisted recombination and improved charge carrier dynamics in the homoveratric acid-modified perovskite solar cells.
X-ray photoelectron spectroscopy was employed to probe the chemical interactions between homoveratric acid and the perovskite lattice. The Pb 4f and I 3d core-level spectra exhibited shifts toward lower binding energies upon homoveratric acid incorporation. Specifically, the Pb 4f7/2 and Pb 4f5/2 peaks shifted by approximately 0.2 eV, indicating coordination between the carboxyl group of homoveratric acid and undercoordinated Pb²⁺ ions. Similarly, the I 3d5/2 and I 3d3/2 peaks shifted downward, suggesting interactions that inhibit iodide migration and reduce halide-related defects. These interactions passivate surface and bulk defects, enhancing the stability and performance of perovskite solar cells.
To quantify the defect density, we fabricated electron-only and hole-only devices using the space-charge-limited-current method. The trap-filled limit voltage was extracted from the J-V curves and used to calculate the defect density according to the formula:
$$N_{\text{trap}} = \frac{2 \varepsilon \varepsilon_0 V_{\text{TFL}}}{e L^2}$$
where ε is the relative permittivity, ε0 is the vacuum permittivity, VTFL is the trap-filled limit voltage, e is the elementary charge, and L is the film thickness. For electron-only devices, the trap-filled limit voltage decreased from 0.584 V (control) to 0.400 V (homoveratric acid-treated), corresponding to electron defect densities of 5.56 × 1015 cm−3 and 3.81 × 1015 cm−3, respectively—a reduction of 31.45%. Similarly, for hole-only devices, the trap-filled limit voltage dropped from 0.632 V to 0.434 V, yielding hole defect densities of 6.01 × 1015 cm−3 and 4.13 × 1015 cm−3, a decrease of 31.28%. The dark J-V characteristics of complete perovskite solar cells showed a lower reverse saturation current for the homoveratric acid-modified device (5.96 × 10−4 mA cm−2) compared to the control (5.33 × 10−5 mA cm−2), consistent with reduced trap-assisted recombination.
| Device Type | Sample | VTFL (V) | Defect Density (cm−3) | Reduction (%) |
|---|---|---|---|---|
| Electron-only | Control | 0.584 | 5.56 × 1015 | 31.45 |
| Homoveratric Acid | 0.400 | 3.81 × 1015 | ||
| Hole-only | Control | 0.632 | 6.01 × 1015 | 31.28 |
| Homoveratric Acid | 0.434 | 4.13 × 1015 |
Electrochemical impedance spectroscopy revealed a lower series resistance for the homoveratric acid-treated perovskite solar cells (33.78 kΩ) compared to the control (58.66 kΩ), facilitating improved charge extraction. Transient photovoltage measurements showed extended carrier recombination lifetimes of 132.2 μs for the homoveratric acid-modified device versus 71.2 μs for the control, indicating suppressed non-radiative recombination. Conversely, transient photocurrent decay times decreased from 1.69 μs (control) to 0.81 μs (homoveratric acid-treated), demonstrating enhanced charge collection efficiency. These results collectively affirm that homoveratric acid passivation reduces defect states, improves charge transport, and minimizes recombination losses in wide-bandgap perovskite solar cells.
The stability of unencapsulated perovskite solar cells was evaluated under two conditions: storage in a nitrogen-filled glovebox at room temperature and at 85°C. After 1440 hours, the control device retained 83.3% of its initial power conversion efficiency, while the homoveratric acid-modified device maintained 91.2%. Under thermal stress at 85°C for 504 hours, the control degraded to 40.2% of its initial efficiency, whereas the homoveratric acid-treated device retained 71.8%. Water contact angle measurements showed an increase from 55.79° for the control film to 67.80° for the homoveratric acid-treated film, indicating enhanced hydrophobicity that mitigates moisture ingress. This improved stability is attributed to the suppression of ion migration and decomposition pathways through homoveratric acid passivation.
In conclusion, homoveratric acid serves as an effective bulk passivation additive for wide-bandgap perovskite solar cells. By coordinating with Pb²⁺ ions through its carboxyl and methoxy groups, homoveratric acid reduces defect density, enhances crystallinity, and improves charge carrier dynamics. The optimized device achieved a power conversion efficiency of 20.60% with suppressed hysteresis and superior stability under thermal and environmental stress. This work underscores the potential of molecular additives in advancing wide-bandgap perovskite solar cells for tandem applications and provides insights into the design of passivation strategies for high-performance and stable photovoltaics.