Perovskite solar cells have garnered significant attention as a promising next-generation photovoltaic technology due to their high power conversion efficiency, low-cost fabrication, and tunable optoelectronic properties. However, the commercialization of perovskite solar cells is hindered by critical challenges such as environmental degradation and lead leakage, which compromise device stability and pose environmental and health risks. In this study, we introduce an internal encapsulation strategy using bio-based polymers—polydopamine (PDA) and chitosan—to address these issues. These materials leverage their biocompatibility, functional groups, and sustainable sourcing to enhance crystal quality, passivate defects, and mitigate lead leakage. Through comprehensive characterization and testing, we demonstrate that this approach significantly improves the performance and stability of perovskite solar cells, offering a viable path toward green and scalable manufacturing.
The inherent vulnerability of perovskite solar cells to moisture, heat, oxygen, and light-induced degradation often initiates at surface and grain boundary defects, leading to the release of toxic lead halides. Traditional physical encapsulation methods, such as those using ethylene-vinyl acetate or wax, provide limited protection under extreme conditions and lack environmental sustainability. In contrast, chemical encapsulation via functional materials like PDA and chitosan offers a more robust solution by forming stable interactions with perovskite components, thereby reducing defect densities and preventing ion migration. Our work focuses on integrating these biopolymers as internal encapsulants within the perovskite layer, leveraging their amino-rich structures for effective passivation and lead immobilization.
To evaluate the impact of biopolymer encapsulation on perovskite film morphology, we employed scanning electron microscopy (SEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). The SEM images revealed that encapsulation with PDA and chitosan increased the average grain size from 288.5 nm in the control films to 356.3 nm and 323.8 nm, respectively. This grain growth is attributed to the nucleation-inducing effects of the polymers’ functional groups, which coordinate with Pb²⁺ ions and promote recrystallization. The reduction in grain boundaries decreased non-radiative recombination sites, enhancing carrier transport. XRD analysis confirmed improved crystallinity, with the intensity ratio of the (110) perovskite peak to the PbI₂ peak increasing from 2.28 (control) to 2.35 (PDA) and 3.81 (chitosan), indicating fewer defects and better phase purity. AFM measurements further showed a decrease in surface roughness from 22.8 nm (control) to 14.8 nm (PDA) and 13.8 nm (chitosan), contributing to smoother films and improved interfacial contact.
The optical properties of the encapsulated perovskite solar cells were assessed using UV-Vis absorption and photoluminescence (PL) spectroscopy. While UV-Vis spectra showed minimal changes in absorption edges, encapsulated films exhibited slightly enhanced visible light absorption due to improved film quality. PL intensity increased significantly with PDA and chitosan treatment, with PDA showing the highest enhancement, indicating effective suppression of non-radiative recombination. This aligns with the defect passivation mechanism, where functional groups quench trap states. To quantify defect densities, we performed space-charge-limited current (SCLC) measurements on electron-only devices. The trap-fill limit voltage (V_TFL) decreased from 0.248 V (control) to 0.186 V (PDA) and 0.215 V (chitosan), corresponding to trap densities (N_t) of 2.34 × 10¹⁶ cm⁻³, 1.76 × 10¹⁶ cm⁻³, and 2.03 × 10¹⁶ cm⁻³, respectively. The lower N_t values confirm that biopolymer encapsulation reduces charge carrier traps, enhancing device performance.
Electrochemical impedance spectroscopy (EIS) under dark conditions provided insights into charge transport dynamics. The equivalent circuit fitting revealed that PDA-encapsulated devices had a lower charge transfer resistance (R_ct = 1.0 kΩ) compared to the control (1.3 kΩ), facilitating better carrier extraction. Additionally, the recombination resistance (R_rec) increased from 21.7 kΩ (control) to 23.2 kΩ (PDA), indicating suppressed charge recombination. Mott-Schottky analysis demonstrated a higher built-in potential (V_bi) for encapsulated devices—1.10 V for PDA versus 1.04 V for the control—which enhances the driving force for carrier collection and open-circuit voltage (V_OC). Dark current measurements further validated reduced leakage currents in encapsulated perovskite solar cells, underscoring the effectiveness of defect passivation.
The photovoltaic performance of perovskite solar cells with PDA and chitosan encapsulation was evaluated through current density-voltage (J-V) measurements under standard AM 1.5G illumination. The optimized PDA-encapsulated devices achieved a power conversion efficiency (PCE) of 24.09%, with a V_OC of 1.21 V, short-circuit current density (J_SC) of 24.35 mA cm⁻², and fill factor (FF) of 81.41%. In comparison, control devices exhibited a PCE of 21.11%, while chitosan-encapsulated devices reached 23.07%. The steady-state power output at the maximum power point (MPP) confirmed these improvements, with PDA devices maintaining a PCE of 23.73% versus 20.27% for controls. These enhancements stem from synergistic effects: improved crystallinity, reduced defect-assisted recombination, and optimized charge transport. The table below summarizes the key performance parameters:
| Device Type | V_OC (V) | J_SC (mA cm⁻²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| Control | 1.15 | 23.50 | 78.20 | 21.11 |
| PDA-Encapsulated | 1.21 | 24.35 | 81.41 | 24.09 |
| Chitosan-Encapsulated | 1.19 | 24.00 | 80.50 | 23.07 |
Lead leakage assessments were conducted by immersing perovskite films in deionized water at pH 5.5 to simulate acid rain conditions. Inductively coupled plasma optical emission spectroscopy (ICP-OES) revealed that PDA encapsulation reduced lead leakage from 13.2 mg/L (control) to 7.2 mg/L, highlighting its efficacy in immobilizing Pb²⁺ ions. Contact angle measurements showed increased hydrophobicity for encapsulated films—70.4° for PDA versus 46° for the control—which mitigates moisture ingress and degradation. Accelerated aging tests under water vapor exposure demonstrated that PDA-encapsulated films retained their black perovskite phase for over 4 minutes, while control films degraded within 2 minutes, turning yellow due to δ-phase formation. These results underscore the role of encapsulation in enhancing environmental stability.
Cytotoxicity evaluations using L-929 mouse fibroblast cells revealed that PDA encapsulation significantly improved biocompatibility. After 5 hours of film immersion, cell viability remained at 84.3% for PDA-encapsulated devices, compared to 57.6% for controls, as determined by CCK-8 assays. Fluorescence imaging showed predominantly green viable cells in encapsulated samples, whereas controls exhibited extensive red dead cells. Ion migration tests under a 1.20 V bias showed that PDA encapsulation reduced the initial current density from 29.56 × 10⁻⁵ mA cm⁻² (control) to 15 × 10⁻⁵ mA cm⁻², indicating suppressed ion diffusion. Long-term stability tests in 85% relative humidity (RH) demonstrated that PDA-encapsulated perovskite solar cells retained 87.6% of their initial PCE after 500 hours, far exceeding the 50.5% retention for controls. The table below compares stability and leakage metrics:
| Parameter | Control | PDA-Encapsulated |
|---|---|---|
| Lead Leakage (mg/L) | 13.2 | 7.2 |
| Cell Viability (%) | 57.6 | 84.3 |
| Current Density (×10⁻⁵ mA cm⁻²) | 29.56 | 15.00 |
| PCE Retention after 500 h (%) | 50.5 | 87.6 |
The enhanced performance of encapsulated perovskite solar cells can be modeled using the diode equation for solar cells:
$$J = J_{SC} – J_0 \left( \exp\left(\frac{q(V + J R_s)}{n k T}\right) – 1 \right) – \frac{V + J R_s}{R_{sh}}$$
where \(J\) is the current density, \(J_0\) is the reverse saturation current, \(q\) is the electron charge, \(V\) is the voltage, \(R_s\) is the series resistance, \(n\) is the ideality factor, \(k\) is Boltzmann’s constant, \(T\) is the temperature, and \(R_{sh}\) is the shunt resistance. Encapsulation reduces \(J_0\) and \(R_s\) by passivating defects, leading to higher FF and PCE. The defect passivation efficiency (\(\eta_{\text{pass}}\)) can be expressed as:
$$\eta_{\text{pass}} = 1 – \frac{N_t}{N_{t0}}$$
where \(N_t\) and \(N_{t0}\) are the trap densities with and without encapsulation, respectively. For PDA, \(\eta_{\text{pass}} \approx 0.25\), indicating a 25% reduction in traps.
In conclusion, our internal encapsulation strategy using PDA and chitosan significantly advances the development of efficient and stable perovskite solar cells. By improving crystallinity, passivating defects, and reducing lead leakage, this approach addresses key commercialization barriers. The scalability and eco-friendliness of biopolymers make this strategy particularly attractive for sustainable energy applications. Future work will focus on optimizing polymer formulations for large-area perovskite solar cell modules and exploring their integration into tandem architectures.