In recent years, perovskite solar cells have emerged as a leading technology in photovoltaics due to their high efficiency and tunable optoelectronic properties. Among various perovskite materials, CsPbI3 has garnered significant attention for its ideal bandgap and excellent photophysical characteristics, making it a promising candidate for all-inorganic perovskite solar cells. However, the instability of the black phase (α-CsPbI3) at room temperature, which tends to transform into a non-photoactive yellow phase (δ-CsPbI3), remains a major challenge. To address this, we designed and synthesized two novel small molecule triphenylamine derivatives, H432 and H462, featuring D-π-A structures with pyridine-based functional groups. These molecules were incorporated into CsPbI3 perovskite solar cells via crystallization modification and surface post-treatment methods. Our investigations reveal that surface post-treatment with these derivatives significantly enhances the power conversion efficiency (PCE) of the devices, underscoring their potential in stabilizing and improving perovskite solar cell performance.
The synthesis of H432 and H462 was achieved through Suzuki-Miyaura and Ullmann coupling reactions, yielding compounds with distinct electronic properties. The molecular structures were confirmed using nuclear magnetic resonance (NMR) spectroscopy, and their electrochemical and photophysical behaviors were characterized to determine energy levels. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were calculated using cyclic voltammetry and UV-visible absorption spectroscopy. For H462, the HOMO and LUMO energies were determined to be -5.40 eV and -3.16 eV, respectively, with a bandgap (ΔE) of 2.36 eV. Similarly, H432 exhibited HOMO and LUMO energies of -5.36 eV and -3.11 eV, with a ΔE of 2.37 eV. The energy level alignment between these molecules and the CsPbI3 perovskite is crucial for efficient charge transport and reduced recombination in perovskite solar cells.
To fabricate the perovskite solar cells, we deposited TiO2 seed and mesoporous layers on fluorine-doped tin oxide (FTO) substrates, followed by the formation of CsPbI3 films using a solution-based method. The small molecule derivatives were introduced via two approaches: (1) crystallization modification, where H432 or H462 was blended with the CsPbI3 precursor solution, and (2) surface post-treatment, where a solution of H432 or H462 in chlorobenzene was spin-coated onto the as-deposited CsPbI3 film. The devices were completed with a Spiro-OMeTAD hole transport layer and a silver electrode. The structural and morphological properties of the films were analyzed using scanning electron microscopy (SEM), which revealed that both modification methods led to denser and more uniform CsPbI3 films with reduced grain boundaries compared to the control. This improvement is attributed to the influence of the triphenylamine derivatives on the crystallization process, which enhances film quality and coverage.
The photoluminescence (PL) spectra of the modified CsPbI3 films provided insights into the charge carrier dynamics. The PL intensity for H462-modified films was lower than that for H432-modified and control films, indicating suppressed charge recombination and improved charge separation efficiency. This is further supported by transient photocurrent measurements, where H462-modified electrodes exhibited the highest photocurrent density, highlighting its role in enhancing charge extraction. The electrochemical impedance spectroscopy (EIS) Nyquist plots showed larger charge recombination resistances (R_rec) for surface post-treated devices, particularly at optimal concentrations, which correlates with reduced recombination losses and higher PCE in perovskite solar cells.
The current density-voltage (J-V) characteristics of the perovskite solar cells were evaluated under standard illumination conditions. The control device exhibited a PCE of 12.44%, with an open-circuit voltage (V_OC) of 0.95 V, short-circuit current density (J_SC) of 17.40 mA/cm², and fill factor (FF) of 65.22%. In contrast, devices modified with H462 and H432 via surface post-treatment showed significant improvements. For instance, at a concentration of 0.05 mol/L, H462-modified devices achieved a PCE of 15.66%, with V_OC = 1.08 V, J_SC = 17.49 mA/cm², and FF = 74.61%. Similarly, H432-modified devices reached a PCE of 15.54% under the same conditions. The enhancement in V_OC and FF is attributed to better energy level alignment, reduced trap states, and improved interfacial properties. The performance parameters for various devices are summarized in Table 1.
| Device | Modification | V_OC (V) | J_SC (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|---|
| Control | None | 0.95 | 17.40 | 65.22 | 12.44 |
| A | H462 (Crystallization) | 1.06 | 17.38 | 71.25 | 14.86 |
| B | H432 (Crystallization) | 0.99 | 17.33 | 68.34 | 14.26 |
| C | H462 (Surface Post-treatment) | 1.08 | 17.42 | 72.56 | 14.94 |
| D | H432 (Surface Post-treatment) | 1.05 | 17.40 | 69.38 | 14.58 |
| F | H462 (0.05 mol/L Surface) | 1.08 | 17.49 | 74.61 | 15.66 |
| I | H432 (0.05 mol/L Surface) | 1.08 | 17.47 | 73.34 | 15.54 |
The influence of derivative concentration on device performance was systematically studied. For H462, concentrations of 0.02, 0.05, and 0.1 mol/L were tested, with 0.05 mol/L yielding the highest PCE. This optimal concentration balances the benefits of defect passivation and charge transport without hindering perovskite crystallization. The J_SC values followed the order: H462 (0.05 mol/L) > H432 (0.05 mol/L) > control, indicating enhanced light harvesting and charge collection. The fill factor improvement is modeled using the equation for series and shunt resistances: $$FF = \frac{V_{MPP} \cdot J_{MPP}}{V_{OC} \cdot J_{SC}}$$ where V_MPP and J_MPP are the voltage and current at the maximum power point. The increased FF in modified devices suggests lower series resistance and higher shunt resistance, as confirmed by EIS.
X-ray diffraction (XRD) analysis confirmed that the incorporation of H462 or H432 did not alter the crystal structure of CsPbI3, as the characteristic peaks at 2θ ≈ 14.0° and 28.6° corresponding to the (002) and (004) planes remained unchanged. However, the intensity of these peaks increased in modified films, indicating improved crystallinity. This is crucial for achieving high-performance perovskite solar cells, as better crystallinity reduces defect density and enhances charge carrier mobility. The stability of the modified films was assessed through contact angle measurements, which showed higher values for H462- and H432-modified films (84.8° and 79.4°, respectively) compared to the control (63.2°). This demonstrates the hydrophobic nature of the triphenylamine derivatives, which protects the perovskite layer from moisture-induced degradation, thereby enhancing the longevity of the perovskite solar cell.
To quantify the charge transport properties, we analyzed the EIS data using an equivalent circuit model comprising a series resistance (R_s) and a charge transfer resistance (R_ct) in parallel with a constant phase element (CPE). The Nyquist plots were fitted to extract R_rec values, which were highest for surface post-treated devices at optimal concentrations. The recombination lifetime (τ_rec) can be estimated from the peak frequency (f_max) of the EIS semicircle using the relation: $$\tau_{rec} = \frac{1}{2\pi f_{max}}$$ The longer τ_rec for modified devices indicates suppressed recombination, contributing to higher V_OC and PCE. Additionally, the ideality factor (n) of the diodes was calculated from the dark J-V curves using the equation: $$J = J_0 \left( \exp\left(\frac{qV}{nkT}\right) – 1 \right)$$ where J_0 is the reverse saturation current, q is the electron charge, k is Boltzmann’s constant, and T is temperature. The lower n values for modified devices suggest reduced trap-assisted recombination, aligning with the PL and EIS results.
The bandgap energy of CsPbI3 and the derivatives plays a critical role in the device performance. The bandgap of CsPbI3 is approximately 1.73 eV, which is ideal for single-junction perovskite solar cells. The energy level diagram, constructed from electrochemical data, shows that the LUMO levels of H462 and H432 are higher than the conduction band of CsPbI3, facilitating electron injection, while their HOMO levels are well-aligned with the perovskite’s valence band, promoting hole extraction. The charge separation efficiency (η_sep) can be expressed as: $$\eta_{sep} = \frac{J_{SC}}{q \cdot \Phi \cdot (1 – R)}$$ where Φ is the photon flux and R is the reflectance. The higher J_SC in modified devices implies improved η_sep due to better interfacial engineering.
| Derivative | HOMO (eV) | LUMO (eV) | ΔE (eV) | PL Peak (nm) |
|---|---|---|---|---|
| H432 | -5.36 | -3.11 | 2.37 | 565 |
| H462 | -5.40 | -3.16 | 2.36 | 600 |
Further analysis of the J-V curves under reverse and forward scans revealed minimal hysteresis in the modified devices, indicating reduced ion migration and trap states. The hysteresis index (HI) is defined as: $$HI = \frac{PCE_{reverse} – PCE_{forward}}{PCE_{reverse}}$$ where PCE_reverse and PCE_forward are the PCE values measured in reverse and forward scan directions, respectively. The low HI values (below 0.05) for H462- and H432-modified perovskite solar cells confirm the effectiveness of these derivatives in passivating surface defects and improving interfacial charge transport.
In conclusion, we have demonstrated that pyridine-containing small molecule triphenylamine derivatives, H432 and H462, significantly enhance the performance of CsPbI3 perovskite solar cells through surface post-treatment modification. The optimal concentration of 0.05 mol/L for both derivatives resulted in PCEs of 15.54% and 15.66%, respectively, compared to 12.44% for the control device. The improvements are attributed to better film morphology, reduced charge recombination, enhanced energy level alignment, and increased hydrophobicity. These findings provide a promising strategy for developing efficient and stable perovskite solar cells, and future work will focus on optimizing the molecular design and exploring large-scale fabrication techniques.