Perovskite solar cells have emerged as a promising photovoltaic technology due to their high efficiency and low-cost fabrication. However, in n-i-p type perovskite solar cells, the commonly used electron transport layer, tin oxide (SnO2), often suffers from defects such as oxygen vacancies and uncoordinated Sn4+ ions, which can lead to charge recombination and reduced performance. To address this, we developed a fulleropyrrolidine derivative, CN-PPA, as an interface modification material. This derivative was synthesized via a one-step Prato reaction and applied between the SnO2 layer and the perovskite absorber in perovskite solar cells. Our study demonstrates that CN-PPA effectively passivates defects, enhances electron transport, and improves the overall photovoltaic performance of perovskite solar cells.
The synthesis of CN-PPA involved reacting C60 with N-(2-cyanoethyl)glycine and 3-(4-formylphenyl)propionic acid in toluene under reflux conditions. The molecular structure was confirmed using nuclear magnetic resonance and mass spectrometry. For device fabrication, we employed a conventional n-i-p structure: ITO/SnO2/CN-PPA/perovskite/Spiro-OMeTAD/Ag. The SnO2 layer was spin-coated from a colloidal dispersion, followed by annealing. The CN-PPA solution (0.1 mg/mL in chlorobenzene) was then spin-coated onto SnO2 and annealed. The perovskite layer, composed of FA0.95Cs0.05PbI3, was deposited using a two-step method, and a hole transport layer of Spiro-OMeTAD was applied before evaporating the silver electrode.
To investigate the passivation effect of CN-PPA on SnO2, we performed X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). The XPS spectra revealed a shift in the Sn 3d peaks to lower binding energies after CN-PPA modification, indicating increased electron density around Sn atoms due to electron transfer from the carboxyl groups of CN-PPA to uncoordinated Sn4+ ions. The O 1s spectra showed a reduction in oxygen vacancy (OV) content from 59.24% in bare SnO2 to 36.30% in SnO2/CN-PPA, confirming that CN-PPA fills oxygen vacancies. The N 1s spectrum displayed a signal from the cyano group, affirming the anchoring of CN-PPA on SnO2. FTIR analysis further supported this, with a shift in the C=O stretching vibration from 1703 cm−1 in pure CN-PPA to 1726 cm−1 in SnO2/CN-PPA, indicating interaction between the carboxyl groups and SnO2.
The impact of CN-PPA on charge carrier transport was evaluated using space-charge-limited current (SCLC) measurements and photoluminescence (PL) spectroscopy. For SCLC, we fabricated electron-only devices with the structure ITO/SnO2(or SnO2/CN-PPA)/Ag. The current-density-voltage (J-V) characteristics were analyzed to determine electrical conductivity and electron mobility. The conductivity (σ) and electron mobility (μe) can be derived from the Ohmic and trap-free regions using the equations:
$$ J = \frac{9}{8} \epsilon_0 \epsilon_r \mu_e \frac{V^2}{L^3} $$
where J is the current density, ε0 is the vacuum permittivity, εr is the relative permittivity, μe is the electron mobility, V is the applied voltage, and L is the film thickness. The results are summarized in Table 1.
| Sample | Conductivity (mS/cm) | Electron Mobility (cm²/(V·s)) |
|---|---|---|
| SnO2 | 1.10 × 10−3 | 4.78 × 10−4 |
| SnO2/CN-PPA | 1.77 × 10−3 | 7.45 × 10−4 |
The enhanced electron mobility in SnO2/CN-PPA films facilitates more efficient electron extraction in perovskite solar cells. Steady-state PL and time-resolved PL (TRPL) measurements were conducted on perovskite films deposited on SnO2 and SnO2/CN-PPA. The PL intensity decreased significantly with CN-PPA modification, indicating improved charge extraction. The TRPL decay curves were fitted using a bi-exponential model:
$$ I(t) = A_1 \exp\left(-\frac{t}{\tau_1}\right) + A_2 \exp\left(-\frac{t}{\tau_2}\right) $$
where I(t) is the PL intensity at time t, A1 and A2 are amplitudes, and τ1 and τ2 are decay lifetimes. The average lifetime (τavg) is given by:
$$ \tau_{\text{avg}} = \frac{A_1 \tau_1^2 + A_2 \tau_2^2}{A_1 \tau_1 + A_2 \tau_2} $$
The calculated average lifetimes were 2.31 μs for SnO2 and 1.17 μs for SnO2/CN-PPA, confirming faster carrier extraction in the modified interface.
The photovoltaic performance of perovskite solar cells was evaluated under AM 1.5G illumination. The J-V curves for devices based on SnO2, SnO2/PCBM, and SnO2/CN-PPA are shown in Figure 1, and the parameters are listed in Table 2. The power conversion efficiency (PCE) is calculated as:
$$ \text{PCE} = \frac{J_{\text{sc}} \times V_{\text{oc}} \times \text{FF}}{P_{\text{in}}} \times 100\% $$
where Jsc is the short-circuit current density, Voc is the open-circuit voltage, FF is the fill factor, and Pin is the incident light power density (100 mW/cm²).
| Device | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| SnO2 | 1.11 | 24.49 | 78.61 | 21.07 |
| SnO2/PCBM | 1.12 | 24.58 | 79.74 | 21.95 |
| SnO2/CN-PPA | 1.12 | 24.95 | 81.43 | 22.79 |
The external quantum efficiency (EQE) spectra were measured to verify the Jsc values. The integrated current densities from EQE matched the J-V results, with SnO2/CN-PPA devices showing higher response across the spectrum. The stability of unencapsulated devices was tested under ambient conditions. After 500 hours, the SnO2/CN-PPA-based perovskite solar cells retained about 76% of their initial PCE, compared to 43% for bare SnO2 devices, highlighting the role of CN-PPA in enhancing device stability.
In conclusion, our study demonstrates that the fulleropyrrolidine derivative CN-PPA serves as an effective interface modifier for perovskite solar cells. By passivating defects in SnO2, it improves electron mobility, reduces charge recombination, and boosts photovoltaic performance. The champion PCE of 22.79% achieved with CN-PPA modification surpasses that of control devices, underscoring the potential of simple, solution-processable fullerene-based materials for advancing perovskite solar cell technology. Future work could explore the application of similar derivatives in large-scale or flexible perovskite solar cells to further enhance efficiency and durability.