In recent years, perovskite solar cells have emerged as a promising photovoltaic technology due to their rapid efficiency improvements, low-cost fabrication, and high theoretical power conversion efficiency. However, interface defects between functional layers, particularly at the electron transport layer (ETL) and perovskite interface, remain a critical challenge that limits both efficiency and stability. To address this, we introduce a novel interface modification strategy using NbTaOx layers between the SnO2 ETL and perovskite absorber in n-i-p structured perovskite solar cells. This approach aims to reduce interfacial recombination, enhance electron transport, and improve the overall performance of perovskite solar cells.
Interface modification in perovskite solar cells is essential for optimizing charge carrier dynamics and minimizing non-radiative losses. In this study, we fabricated NbTaOx layers via a low-temperature solution process, where ethanol solutions of niobium and tantalum ethoxides were spin-coated onto SnO2 ETLs, followed by slow hydrolysis and annealing. This resulted in in-situ growth of NbTaOx on SnO2, which we characterized using various techniques to understand its impact on the perovskite solar cell properties. Our findings demonstrate that NbTaOx modification significantly boosts the power conversion efficiency (PCE) from 19.14% to 21.51%, while also enhancing stability under continuous illumination.
The morphology and surface properties of the modified films were examined using atomic force microscopy (AFM) and water contact angle measurements. The AFM results revealed that the SnO2/NbTaOx films exhibited increased surface roughness with uniform protrusions compared to the smooth SnO2 films, which likely facilitates better perovskite deposition. Water contact angle measurements showed a decrease from 44.4° for SnO2 to 38.6° for SnO2/NbTaOx, indicating improved hydrophilicity that promotes denser perovskite layer formation. This enhanced wettability is crucial for achieving high-quality perovskite films in perovskite solar cells, as it reduces pinholes and improves interfacial contact.
X-ray diffraction (XRD) analysis was performed to evaluate the crystallinity of perovskite films deposited on modified and unmodified SnO2 ETLs. The diffraction peaks at 14.16°, 24.39°, and 28.32° correspond to the (100), (111), and (200) planes of the perovskite structure, respectively. No shift in peak positions was observed, indicating that the NbTaOx modification does not alter the perovskite crystal structure. However, the full width at half maximum (FWHM) of the (100) peak decreased from 0.324° for unmodified SnO2 to 0.242° for SnO2/NbTaOx, suggesting improved crystallinity and reduced defects in the perovskite layer. This enhancement is vital for minimizing charge recombination in perovskite solar cells.
To further investigate the chemical states and energy level alignment, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were employed. The XPS spectra of Sn 3d core levels showed a shift of 0.4 eV to higher binding energy after NbTaOx modification, implying a more stable chemical environment with reduced oxygen vacancies. The O 1s spectra were deconvoluted into lattice oxygen (O2-) and hydroxyl groups (OH-), with the proportion of O2- increasing from approximately 61% for SnO2 to 72% for SnO2/NbTaOx. This reduction in oxygen vacancies contributes to better electron transport and suppressed non-radiative recombination in perovskite solar cells. UPS measurements combined with Tauc plots derived the energy level diagrams, showing that the Fermi level (Ef) of SnO2 shifted from -4.45 eV to -4.43 eV after modification, while the conduction band minimum (Ec) moved from -4.29 eV to -4.22 eV. This upward shift in energy levels reduces the energy barrier at the ETL/perovskite interface, facilitating more efficient electron extraction and higher open-circuit voltage in perovskite solar cells.
The optical properties were assessed using steady-state photoluminescence (PL) spectroscopy. The PL intensity of perovskite films on SnO2/NbTaOx was significantly quenched compared to those on unmodified SnO2, with the highest quenching observed for a Nb:Ta molar ratio of 3:2. This indicates enhanced charge extraction and reduced carrier recombination at the interface, which is critical for improving the performance of perovskite solar cells. The PL quenching can be quantitatively described by the following equation for carrier lifetime: $$\tau = \frac{1}{k_r + k_{nr}}$$ where $\tau$ is the carrier lifetime, $k_r$ is the radiative recombination rate, and $k_{nr}$ is the non-radiative recombination rate. The reduction in PL intensity suggests a decrease in $k_{nr}$ due to defect passivation by NbTaOx.
The photovoltaic performance of perovskite solar cells with and without NbTaOx modification was evaluated through current density-voltage (J-V) measurements. The key parameters, including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and PCE, are summarized in Table 1. The unmodified cell achieved a PCE of 19.14%, with Voc = 1.08 V, Jsc = 24.43 mA/cm², and FF = 72.52%. In contrast, the cell with a 3:2 Nb:Ta ratio exhibited a PCE of 21.51%, with Voc = 1.10 V, Jsc = 25.33 mA/cm², and FF = 77.58%. This improvement underscores the role of NbTaOx in enhancing electron transport and reducing interfacial losses. The diode equation for solar cells can be expressed as: $$J = J_{sc} – J_0 \left( \exp\left(\frac{qV}{nkT}\right) – 1 \right)$$ where $J$ is the current density, $J_0$ is the reverse saturation current, $q$ is the electron charge, $V$ is the voltage, $n$ is the ideality factor, $k$ is Boltzmann’s constant, and $T$ is the temperature. The increased Voc and FF in modified cells indicate a lower $J_0$ and reduced recombination, aligning with the interface passivation effects.
| Sample | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| Unmodified | 1.08 | 24.43 | 72.52 | 19.14 |
| 4:1 NbTaOx | 1.10 | 25.33 | 75.58 | 21.09 |
| 3:2 NbTaOx | 1.10 | 25.33 | 77.58 | 21.51 |
| 2:3 NbTaOx | 1.09 | 24.94 | 76.56 | 20.83 |
| 1:4 NbTaOx | 1.08 | 24.40 | 77.32 | 20.35 |
Stability testing under continuous one-sun illumination for 30 hours revealed that the NbTaOx-modified perovskite solar cells retained 94.42% of their initial PCE, compared to 89.54% for unmodified cells. This enhanced stability can be attributed to the reduced interfacial defects and improved charge extraction, which mitigate degradation mechanisms. The normalized PCE over time follows a decay function that can be modeled as: $$\text{PCE}(t) = \text{PCE}_0 \exp(-t/\tau_s)$$ where $\text{PCE}_0$ is the initial efficiency, $t$ is time, and $\tau_s$ is the stability time constant. The higher $\tau_s$ for modified cells indicates slower degradation, emphasizing the importance of interface engineering in perovskite solar cells.
In conclusion, our study demonstrates that NbTaOx interface modification effectively enhances the performance and stability of perovskite solar cells. By reducing defects, optimizing energy level alignment, and improving perovskite crystallinity, this approach leads to a significant increase in PCE from 19.14% to 21.51%. The synergistic effects of niobium and tantalum in the oxide layer provide excellent electron transport and passivation properties, making NbTaOx a promising material for advancing perovskite solar cell technology. Future work will focus on scaling up this method and exploring its application in other perovskite-based optoelectronic devices.
The mechanisms behind the improved performance can be further elucidated through additional equations and models. For instance, the electron transport in the ETL can be described by the drift-diffusion equation: $$J_n = q \mu_n n E + q D_n \frac{dn}{dx}$$ where $J_n$ is the electron current density, $\mu_n$ is the electron mobility, $n$ is the electron concentration, $E$ is the electric field, and $D_n$ is the diffusion coefficient. The NbTaOx layer likely increases $\mu_n$ and reduces traps, leading to higher Jsc and FF. Additionally, the interface recombination velocity $S$ can be expressed as: $$S = \sigma v_{th} N_t$$ where $\sigma$ is the capture cross-section, $v_{th}$ is the thermal velocity, and $N_t$ is the trap density. The reduction in $N_t$ due to NbTaOx passivation lowers $S$, thereby enhancing Voc and overall efficiency in perovskite solar cells.
Overall, the integration of NbTaOx as an interface modifier offers a versatile strategy to address key challenges in perovskite solar cells, paving the way for more efficient and stable photovoltaic devices. The combination of experimental results and theoretical models provides a comprehensive understanding of how interface engineering can revolutionize the field of perovskite solar cells.