Perovskite solar cells have emerged as a promising technology for next-generation photovoltaics due to their high power conversion efficiencies, low-cost fabrication, and tunable optoelectronic properties. Among various perovskite materials, all-inorganic CsPbBr3 has gained significant attention for its excellent thermal and environmental stability compared to organic-inorganic hybrid counterparts. However, the performance of CsPbBr3-based perovskite solar cells is often limited by interfacial defects, particularly at the electron transport layer/perovskite interface, which lead to non-radiative recombination and poor carrier transport. In this work, we introduce a simple yet effective interface modification strategy using methylammonium chloride (MACl) to passivate defects at the TiO2/CsPbBr3 interface and enhance the crystallinity of the perovskite layer. Through comprehensive characterization and device optimization, we demonstrate that MACl treatment significantly improves the photovoltaic parameters, achieving a champion power conversion efficiency of 10.10% with enhanced stability.
The fabrication of perovskite solar cells typically involves multiple layers, including a transparent conductive oxide substrate, electron transport layer, perovskite absorber, hole transport layer, and back electrode. For CsPbBr3 perovskite solar cells, the TiO2 electron transport layer plays a crucial role in electron extraction and transport. However, inherent issues such as low conductivity, high defect density, and energy level misalignment at the TiO2/CsPbBr3 interface often hinder device performance. To address these challenges, we focused on interface engineering by applying a MACl solution onto the TiO2 surface prior to perovskite deposition. This approach aims to passivate interface defects and modulate the crystallization process of the subsequent CsPbBr3 layer.
The device structure and fabrication process are illustrated schematically. Briefly, fluorine-doped tin oxide (FTO) substrates were cleaned and coated with a compact TiO2 layer via chemical bath deposition and annealing. Various concentrations of MACl solution (3.0, 5.0, and 8.0 mg·mL⁻¹) were spin-coated onto the TiO2 layer, followed by thermal treatment. The CsPbBr3 perovskite layer was then deposited using a sequential two-step method involving PbBr2 and CsBr solutions, with intermediate annealing steps. Finally, a carbon electrode was applied by screen printing. The entire process was conducted under ambient conditions unless specified otherwise.
To investigate the impact of MACl modification on the morphology of the CsPbBr3 perovskite layer, we performed scanning electron microscopy (SEM) analysis. The pristine CsPbBr3 film exhibited numerous pinholes and non-uniform grain distribution, which are detrimental to device performance as they act as recombination centers. In contrast, the MACl-modified CsPbBr3 film showed a much denser and more homogeneous morphology with significantly larger grain sizes. Statistical analysis of grain size distribution revealed that the average grain size increased from approximately 800 nm for the pristine film to over 1200 nm for the MACl-treated film. This improvement in morphology is attributed to the role of MACl in promoting lateral crystal growth and reducing nucleation sites during perovskite formation.
The enhanced crystallinity was further confirmed by X-ray diffraction (XRD) measurements. The XRD patterns of both pristine and MACl-modified CsPbBr3 films displayed characteristic peaks corresponding to the cubic perovskite phase. However, the MACl-treated sample showed intensified diffraction peaks at 15.3°, 21.7°, 26.7°, 30.8°, and 38.1°, which are assigned to the (100), (110), (111), (200), and (211) planes of CsPbBr3, respectively. Additionally, the peaks associated with secondary phases such as CsPb2Br5 at 27.8° and 29.8° were significantly suppressed in the modified film. These results indicate that MACl modification enhances the phase purity and crystallinity of the CsPbBr3 perovskite layer, which is beneficial for charge transport and reduced recombination.
Energy-dispersive X-ray spectroscopy (EDS) mapping was employed to verify the successful incorporation of MACl at the TiO2 interface. The EDS analysis detected a higher atomic percentage of chlorine (0.11%) in the MACl-modified TiO2 layer compared to the pristine TiO2 (0.08%), confirming the presence of Cl⁻ ions at the interface. The uniform distribution of Cl elements suggests effective coverage of the TiO2 surface by MACl, which facilitates defect passivation and improves interfacial contact.
The optical properties of the TiO2 electron transport layer with and without MACl modification were evaluated using UV-visible spectroscopy. The transmittance spectra of both samples were nearly identical in the visible range, indicating that the thin MACl layer does not adversely affect light absorption by the perovskite layer. However, electrical conductivity measurements revealed a notable improvement in the conductivity of the MACl-modified TiO2 layer. This enhancement can be attributed to the passivation of oxygen vacancies and other surface defects by MA⁺ and Cl⁻ ions, leading to better electron transport and reduced recombination losses.
To quantify the photovoltaic performance, we fabricated perovskite solar cells with different MACl concentrations and measured their current density-voltage (J-V) characteristics under standard AM 1.5G illumination. The key parameters, including open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and power conversion efficiency (PCE), are summarized in Table 1. The pristine device without MACl modification exhibited a VOC of 1.51 V, JSC of 7.54 mA·cm⁻², FF of 73.99%, and PCE of 8.42%. With the introduction of MACl, all parameters improved progressively, reaching optimal values at a concentration of 5.0 mg·mL⁻¹: VOC = 1.58 V, JSC = 7.89 mA·cm⁻², FF = 81.09%, and PCE = 10.10%. Further increasing the MACl concentration to 8.0 mg·mL⁻¹ led to a slight decline in performance, likely due to excessive insulating layer formation that impedes charge transport.
| MACl Concentration (mg·mL⁻¹) | VOC (V) | JSC (mA·cm⁻²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 0 | 1.51 | 7.54 | 73.99 | 8.42 |
| 3.0 | 1.56 | 7.56 | 77.82 | 9.17 |
| 5.0 | 1.58 | 7.89 | 81.09 | 10.10 |
| 8.0 | 1.56 | 7.69 | 81.90 | 9.82 |
The improvement in JSC was corroborated by incident photon-to-electron conversion efficiency (IPCE) measurements. The integrated JSC values from the IPCE spectra were 6.76 mA·cm⁻² and 7.17 mA·cm⁻² for the pristine and MACl-modified devices, respectively, consistent with the J-V results. The enhanced JSC is attributed to better light absorption and reduced recombination in the modified perovskite layer, as evidenced by UV-visible absorption spectroscopy, which showed increased absorption in the 400-520 nm range for the MACl-treated film.
Steady-state photoluminescence (PL) spectroscopy was used to probe the charge carrier dynamics in the perovskite films. The MACl-modified CsPbBr3 film exhibited a much stronger PL intensity compared to the pristine film, indicating suppressed non-radiative recombination. This is due to the passivation of trap states at the grain boundaries and interfaces by MACl, which enhances the radiative recombination efficiency and facilitates better carrier extraction.
Transient photovoltage (TPV) measurements provided further insights into the carrier recombination lifetime. The decay curves were fitted with a bi-exponential function, and the average carrier lifetime (τavg) was calculated using the formula:
$$ \tau_{\text{avg}} = \frac{A_1 \tau_1^2 + A_2 \tau_2^2}{A_1 \tau_1 + A_2 \tau_2} $$
where A₁ and A₂ are the amplitudes, and τ₁ and τ₂ are the fast and slow decay time constants, respectively. The MACl-modified device showed a prolonged τavg of 5.56 ms, compared to 3.64 ms for the pristine device, confirming reduced non-radiative recombination and improved charge carrier stability.
To understand the recombination mechanisms, we analyzed the dependence of JSC and VOC on light intensity. The relationship between JSC and light intensity (I) can be expressed as JSC ∝ Iα, where α is the recombination factor. The MACl-modified device exhibited an α value closer to 1 (0.98) than the pristine device (0.95), indicating weaker bimolecular recombination. Similarly, the VOC vs. light intensity data were fitted to the equation:
$$ V_{\text{OC}} = \frac{n k_B T}{q} \ln I + B $$
where n is the ideality factor, kB is Boltzmann’s constant, T is temperature, and q is the elementary charge. The MACl-treated device had a lower n value (1.75) compared to the pristine device (1.91), suggesting reduced trap-assisted recombination in the modified perovskite solar cells.
Electrochemical impedance spectroscopy (EIS) was performed to investigate the interfacial charge transfer and recombination processes. The Nyquist plots showed a larger semicircle radius for the MACl-modified device, corresponding to a higher recombination resistance (Rrec) of 5702 Ω versus 856 Ω for the pristine device. This indicates that MACl modification effectively suppresses charge recombination at the TiO2/CsPbBr3 interface, leading to better device performance.
Capacitance-voltage (C-V) measurements under dark conditions revealed lower capacitance for the MACl-modified device across the applied voltage range, signifying reduced interfacial charge accumulation and better charge separation. The Mott-Schottky analysis provided the built-in potential (Vbi), which increased from 1.16 V for the pristine device to 1.55 V for the MACl-modified device. The higher Vbi enhances the driving force for charge carrier separation and collection, contributing to the improved VOC.
The defect density in the perovskite films was evaluated using the space-charge-limited current (SCLC) method. The trap-filled limit voltage (VTFL) was determined from the J-V curves in the dark, and the defect density (Nt) was calculated using the formula:
$$ N_t = \frac{2 \epsilon_0 \epsilon_r V_{\text{TFL}}}{q d^2} $$
where ε0 and εr are the vacuum and relative permittivities, respectively, and d is the film thickness. The MACl-modified film showed a lower VTFL (1.56 V) and defect density (1.35 × 10¹⁵ cm⁻³) compared to the pristine film (VTFL = 1.74 V, Nt = 1.50 × 10¹⁶ cm⁻³), demonstrating effective passivation of bulk and interfacial defects.
To assess the role of different chloride salts, we compared MACl with KCl and NH4Cl at the same concentration (5.0 mg·mL⁻¹). The photovoltaic parameters are summarized in Table 2. The MACl-modified devices outperformed those with KCl and NH4Cl, achieving higher VOC, JSC, FF, and PCE. This superior performance is attributed to the synergistic effect of MA⁺ and Cl⁻ ions: MA⁺ improves wettability and regulates crystallization, while Cl⁻ passifies defects and enhances electronic properties.
| Salt | VOC (V) | JSC (mA·cm⁻²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| MACl | 1.58 | 7.89 | 81.09 | 10.10 |
| KCl | 1.56 | 7.64 | 78.69 | 9.37 |
| NH4Cl | 1.55 | 7.62 | 76.19 | 8.99 |
Contact angle measurements revealed that the MACl modification increased the hydrophilicity of the TiO2 surface, with the water contact angle decreasing from 32.5° to 19.5°. This improved wettability facilitates the uniform deposition of the subsequent PbBr2 layer, leading to a more homogeneous and high-quality perovskite film with larger grain sizes and fewer defects.
The stability of the perovskite solar cells was evaluated by monitoring the normalized PCE over 30 days under ambient conditions (25°C, 25% relative humidity). The MACl-modified devices retained over 80% of their initial efficiency, whereas the pristine devices degraded more rapidly. The enhanced stability is ascribed to the improved crystallinity and reduced defect density, which mitigate moisture ingress and ion migration.
Statistical analysis of 30 devices each for pristine and MACl-modified perovskite solar cells confirmed the reproducibility of the enhancement. The box plots of VOC, JSC, FF, and PCE showed higher median values and narrower distributions for the MACl-treated devices, underscoring the consistency of the interface modification approach.
In conclusion, we have demonstrated that MACl interface modification is a highly effective strategy for improving the performance and stability of CsPbBr3 perovskite solar cells. By passifying defects at the TiO2/CsPbBr3 interface and enhancing the crystallinity of the perovskite layer, MACl treatment leads to significant improvements in VOC, JSC, FF, and PCE. The optimal MACl concentration of 5.0 mg·mL⁻¹ resulted in a champion PCE of 10.10%, with enhanced carrier lifetime, reduced recombination, and better interfacial charge transport. This work highlights the importance of interface engineering in perovskite solar cells and provides a simple, scalable method for achieving high-efficiency, stable all-inorganic perovskite photovoltaics.
The power conversion efficiency of a perovskite solar cell can be expressed as:
$$ \text{PCE} = \frac{J_{\text{SC}} \times V_{\text{OC}} \times \text{FF}}{P_{\text{in}}} $$
where Pin is the incident light power density. For our best device, with Pin = 100 mW·cm⁻², the calculated PCE matches the experimental value. The fill factor is influenced by the series and shunt resistances, which are optimized through interface modification. The diode equation for a solar cell is given by:
$$ J = J_{\text{SC}} – J_0 \left( \exp\left(\frac{q(V + J R_s)}{n k_B T}\right) – 1 \right) – \frac{V + J R_s}{R_{\text{sh}}} $$
where J0 is the reverse saturation current, Rs is series resistance, Rsh is shunt resistance, and n is the ideality factor. The MACl modification reduces Rs and increases Rsh, leading to a higher FF.
The optical bandgap of the CsPbBr3 perovskite was determined from the Tauc plot using the equation:
$$ (\alpha h\nu)^2 = A (h\nu – E_g) $$
where α is the absorption coefficient, hν is photon energy, and Eg is the bandgap. Both pristine and MACl-modified films exhibited similar Eg values of approximately 2.3 eV, confirming that MACl does not alter the intrinsic optical properties of CsPbBr3.
In summary, the integration of MACl as an interface modifier in CsPbBr3 perovskite solar cells effectively addresses key challenges related to interfacial defects and film quality. This approach not only boosts efficiency but also enhances device stability, making it a valuable contribution to the advancement of perovskite solar cell technology. Future work could explore the application of this strategy to other perovskite compositions and device architectures to further push the boundaries of photovoltaic performance.