Perovskite solar cells have emerged as a highly promising photovoltaic technology due to their exceptional optoelectronic properties and facile processability. However, the performance of these devices is often limited by defects at the interface between the electron transport layer (ETL) and the perovskite layer, which lead to non-radiative recombination and reduced open-circuit voltage. In this study, we introduce 1-adamantanamine hydrochloride (ADA) as an interfacial modifier between the tin dioxide (SnO2) ETL and the perovskite layer to form an electronic reflux barrier. This layer passivates interface defects, blocks electron backflow, and suppresses non-radiative recombination, thereby enhancing the overall performance of perovskite solar cells. Our results demonstrate that the amino lone pair electrons in ADA coordinate with uncoordinated Pb2+ ions, facilitating the formation of high-quality perovskite films with reduced defect density. The optimized perovskite solar cells achieve a power conversion efficiency (PCE) of 23.07%, with an open-circuit voltage (VOC) of 1.13 V, a short-circuit current density (JSC) of 24.67 mA·cm−2, and a fill factor (FF) of 82.59%. Furthermore, the modified devices retain over 85% of their initial PCE after 1000 hours of storage in ambient air, indicating significantly improved stability. This work highlights the potential of molecular interface engineering using cage-like structures to advance the development of efficient and stable perovskite solar cells.
The interface between the ETL and the perovskite layer plays a critical role in the charge extraction and transport processes in perovskite solar cells. SnO2 is widely used as an ETL due to its high electron mobility and suitable energy levels. However, low-temperature processing of SnO2 often introduces defects such as oxygen vacancies and tin interstitials, which act as recombination centers and impede electron transport. These defects are particularly prevalent at the buried interface, where solvent evaporation during perovskite annealing creates voids and increases trap density. To address this issue, we employed ADA, a cage-like molecule with a symmetric and thermally stable structure, as a modifier to passivate these defects and form a thin barrier layer that prevents electron reflux. The amino group in ADA acts as a Lewis base, coordinating with undercoordinated Pb2+ ions and SnO2 surface defects, while the chloride ions help passivate oxygen vacancies. This dual passivation mechanism reduces non-radiative recombination and enhances charge extraction, leading to improved photovoltaic parameters in perovskite solar cells.
We fabricated the perovskite solar cells with an ITO/SnO2/ADA/Perovskite/Spiro-OMeTAD/Ag structure. The SnO2 layer was deposited by spin-coating a colloidal dispersion, followed by annealing at 150°C. The ADA modifier was applied by spin-coating an isopropanol solution at various concentrations (0.1, 0.3, and 0.5 mg·mL−1) onto the SnO2 film, with optimization revealing that 0.3 mg·mL−1 yielded the best performance. The perovskite layer was formed using a two-step sequential deposition method, involving the spin-coating of PbI2 and mixed amine salts (FAI, MAI, and MACl), followed by thermal annealing. The hole transport layer (Spiro-OMeTAD) and silver electrode were subsequently deposited to complete the device. We characterized the films and devices using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, time-resolved PL (TRPL), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), electrochemical impedance spectroscopy (EIS), and J-V measurements under simulated sunlight.
The photovoltaic performance of the perovskite solar cells was evaluated through J-V measurements. The introduction of the ADA layer significantly enhanced the device parameters compared to the control SnO2-based cells. The optimized ADA-modified device achieved a PCE of 23.07%, with a VOC of 1.13 V, JSC of 24.67 mA·cm−2, and FF of 82.59%. In contrast, the control device exhibited a PCE of 20.22%, VOC of 1.06 V, JSC of 24.52 mA·cm−2, and FF of 78.16%. The improvement in VOC and FF is attributed to reduced interface recombination and enhanced charge extraction. The external quantum efficiency (EQE) spectra showed higher values across the visible range for the ADA-modified devices, consistent with the increased JSC. The integrated current density from the EQE spectra matched well with the JSC from J-V measurements, confirming the reliability of our results. The stability test demonstrated that the ADA-modified perovskite solar cells retained over 85% of their initial PCE after 1000 hours in air, whereas the control devices degraded more rapidly. This enhanced stability is linked to the improved film quality and defect passivation provided by the ADA layer.
| Sample | VOC (V) | JSC (mA·cm−2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| Control SnO2 | 1.06 | 24.52 | 78.16 | 20.22 |
| SnO2-ADA | 1.13 | 24.67 | 82.59 | 23.07 |
The morphology of the perovskite films was investigated using SEM and AFM. The control perovskite film deposited on SnO2 exhibited irregular grains with numerous pinholes and voids, which can act as recombination sites. In contrast, the ADA-modified film showed dense, uniform, and pinhole-free perovskite coverage, with larger grain sizes and reduced surface roughness. The AFM analysis revealed a root-mean-square roughness of 3.52 nm for the SnO2-ADA film, compared to 3.72 nm for the control, indicating a smoother interface. This improved morphology facilitates better charge transport and reduces interface recombination in perovskite solar cells. The cross-sectional SEM images confirmed the well-defined layered structure of the devices, with no significant changes in thickness but enhanced adhesion between the SnO2 and perovskite layers due to the ADA modification.
The crystallinity of the perovskite films was assessed using XRD. Both the control and ADA-modified films showed characteristic peaks at around 14.1° and 28.4°, corresponding to the (110) and (220) planes of the perovskite structure, respectively. However, the ADA-modified film exhibited a lower intensity of the PbI2 peak at 12.7°, indicating reduced residual PbI2 and improved perovskite formation. This suggests that ADA passivates the PbI2 and promotes the growth of high-quality perovskite crystals. The enhanced crystallinity contributes to the superior performance of the perovskite solar cells by reducing defect states and improving charge carrier mobility.
Steady-state PL and TRPL measurements were conducted to study the charge carrier dynamics. The PL intensity of the ADA-modified perovskite film was significantly quenched compared to the control, indicating more efficient electron extraction from the perovskite to the SnO2 ETL. The TRPL decays were fitted with a bi-exponential function, yielding fast (τ1) and slow (τ2) decay components. The average carrier lifetime (τave) decreased from 98.60 ns for the control to 68.12 ns for the ADA-modified film, demonstrating faster charge transfer and reduced recombination. This is consistent with the improved JSC and VOC in the perovskite solar cells. The passivation of interface defects by ADA facilitates the separation and extraction of photogenerated carriers, thereby enhancing the overall efficiency of the perovskite solar cells.
| Sample | τ1 (ns) | τ2 (ns) | A1 (%) | A2 (%) | τave (ns) |
|---|---|---|---|---|---|
| Control SnO2 | 19.25 | 114.29 | 53.85 | 46.15 | 98.60 |
| SnO2-ADA | 8.42 | 85.45 | 74.67 | 25.33 | 68.12 |
XPS and UPS analyses were performed to investigate the chemical interactions and energy level alignment at the SnO2/perovskite interface. The XPS spectra of the SnO2-ADA film showed a shift in the O 1s peak to higher binding energy (530.7 eV) compared to the control (530.4 eV), indicating strong interaction between ADA and SnO2. The intensity of the lattice oxygen peak increased, while the oxygen vacancy peak decreased, confirming passivation of oxygen vacancies by chloride ions. The Cl 2p peaks at 197.4 eV and 199.0 eV were observed only in the ADA-modified sample, verifying the incorporation of chloride. The Pb 4f peaks shifted to higher binding energy in the ADA-modified perovskite, suggesting coordination between the amino group of ADA and uncoordinated Pb2+ ions. This dual passivation mechanism reduces trap states and suppresses non-radiative recombination in perovskite solar cells.
The UPS measurements revealed the work function and energy levels of the films. The valence band maximum (VBM) of the SnO2-ADA film was shifted closer to the Fermi level, indicating improved band alignment with the perovskite layer. The optical bandgap (Eg) was determined from Tauc plots of the UV-visible absorption spectra, using the equation for direct bandgap materials:
$$(αhν)^2 = A(hν – E_g)$$
where α is the absorption coefficient, hν is the photon energy, and A is a constant. The Eg values were similar for both films (around 1.55 eV), but the ADA modification led to a slight upward shift in the conduction band minimum (CBM) of SnO2, creating a better energy match with the perovskite layer. This optimized alignment facilitates electron injection and blocks hole recombination, contributing to the higher VOC and FF in the perovskite solar cells. The energy level diagram illustrates how the ADA layer acts as an electronic reflux barrier, preventing back-transfer of electrons from SnO2 to the perovskite and thus reducing recombination losses.
Electrochemical characterization through EIS and Mott-Schottky analysis provided insights into the charge transport and recombination processes. The Nyquist plots showed a smaller charge transfer resistance (Rct) and larger recombination resistance (Rrec) for the ADA-modified devices compared to the control, indicating enhanced charge extraction and suppressed recombination. The Mott-Schottky plots were used to calculate the built-in potential (Vbi) from the equation:
$$\frac{1}{C^2} = \frac{2}{A^2 ε ε_0 q N_D} (V_{bi} – V)$$
where C is the capacitance, A is the device area, ε is the relative permittivity, ε0 is the vacuum permittivity, q is the elementary charge, ND is the carrier density, and V is the applied bias. The Vbi increased from 0.92 V for the control to 1.0 V for the ADA-modified device, which helps in reducing charge accumulation and improving the VOC. The light intensity dependence of VOC showed a lower slope for the ADA-modified devices, suggesting reduced trap-assisted recombination. The theoretical limit of FF was calculated using the equation:
$$FF_{max} = \frac{v_{oc} – \ln(v_{oc} + 0.72)}{v_{oc} + 1}$$
where voc = qVOC/nkT, n is the ideality factor, k is Boltzmann’s constant, and T is temperature. The experimental FF values were closer to the theoretical limit for the ADA-modified perovskite solar cells, indicating minimized non-radiative losses. Transient photovoltage (TPV) measurements revealed longer carrier lifetimes in the modified devices, consistent with reduced defect density and improved interface quality.
In conclusion, we have demonstrated that the introduction of an ADA interfacial layer between SnO2 and the perovskite significantly enhances the performance and stability of perovskite solar cells. The ADA molecule passivates interface defects through coordination with Pb2+ and SnO2 vacancies, while the chloride ions further reduce oxygen vacancies. This dual passivation, combined with the formation of an electronic reflux barrier, suppresses non-radiative recombination and improves charge extraction. The optimized perovskite solar cells achieve a PCE of 23.07% with excellent stability, retaining over 85% of their initial efficiency after 1000 hours in air. Our findings underscore the importance of molecular interface engineering using cage-like structures for developing high-efficiency and durable perovskite solar cells. Future work will focus on exploring other derivatives and scaling up the technology for commercial applications.
The general formula for the power conversion efficiency of a solar cell is given by:
$$PCE = \frac{J_{SC} \times V_{OC} \times FF}{P_{in}} \times 100\%$$
where Pin is the incident light power density (100 mW·cm−2 in standard testing conditions). For the ADA-modified perovskite solar cells, the high values of JSC, VOC, and FF collectively contribute to the superior PCE. The fill factor can be expressed in terms of series (Rs) and shunt (Rsh) resistances:
$$FF ≈ \frac{v_{oc} – \ln(v_{oc} + 0.72)}{v_{oc} + 1} \times \left(1 – \frac{R_s}{R_{sh}}\right)$$
where voc is the normalized VOC. The reduced Rs and increased Rsh in the ADA-modified devices lead to the higher FF, as confirmed by the J-V analysis. This comprehensive study provides a foundation for further optimization of interface materials in perovskite solar cells, paving the way for their widespread adoption in renewable energy technologies.