In recent years, perovskite solar cells have garnered significant attention due to their exceptional optoelectronic properties, including high absorption coefficients, long carrier lifetimes, and tunable bandgaps. However, the commercialization of perovskite solar cells is hindered by issues such as non-radiative recombination caused by crystal defects and insufficient environmental stability. Additive engineering has emerged as a simple and effective strategy to enhance the crystallinity and overall quality of perovskite films. In this study, we introduce 3-pyridyl thiourea (3-PTU) as a multifunctional additive to improve the performance and stability of perovskite solar cells. We systematically investigate the effects of 3-PTU on film morphology, crystallinity, optoelectronic properties, and device performance through various characterization techniques.
The fabrication of perovskite solar cells involved a normal planar structure with fluorine-doped tin oxide (FTO) as the transparent conductive substrate, followed by the deposition of a tin oxide (SnO2) electron transport layer. The perovskite layer was prepared using a one-step spin-coating method, with the precursor solution consisting of methylammonium bromide, lead bromide, lead iodide, methylammonium chloride, and formamidinium iodide in a mixed solvent of N,N-dimethylformamide and dimethyl sulfoxide. For the target samples, 3-PTU was added to the precursor solution at a concentration of 2 mg/mL. The spin-coating process was optimized with anti-solvent quenching using ethyl acetate, followed by annealing at 100°C for 60 minutes. Subsequently, a spiro-OMeTAD hole transport layer and a gold electrode were deposited to complete the device structure.
To evaluate the impact of 3-PTU on the perovskite films, we performed X-ray photoelectron spectroscopy (XPS) analysis. The XPS spectra revealed a distinct F 1s peak at 687.1 eV for the 3-PTU-treated films, confirming the successful incorporation of the additive. Additionally, shifts in the Pb 4f peaks from 142.5 eV and 137.6 eV in the control films to 142.7 eV and 137.8 eV in the treated films indicated coordination interactions between the electron-donating groups of 3-PTU and Pb2+ ions. This coordination is crucial for modulating crystallization kinetics and passivating defects in the perovskite layer.
X-ray diffraction (XRD) analysis was conducted to examine the crystallinity and phase purity of the perovskite films. The diffraction patterns showed characteristic peaks at 14.2°, 20.1°, 24.6°, 28.4°, 31.8°, and 34.9°, corresponding to the (100), (110), (111), (200), (210), and (211) planes of the formamidinium lead iodide perovskite, respectively. The peak at 12.8°, associated with the PbI2 impurity phase, was significantly reduced in intensity for the 3-PTU-treated films, indicating the additive’s role in suppressing unwanted phases. The relative intensity ratio of the (100) to (110) peaks, denoted as I(100)/I(110), increased from 5.88 for the control films to 7.14 for the treated films. This enhancement suggests improved crystallinity and preferential growth along the (100) direction, which is beneficial for larger grain sizes and reduced grain boundaries. The crystallite size can be estimated using the Scherrer equation:
$$ D = \frac{K \lambda}{\beta \cos \theta} $$
where \( D \) is the crystallite size, \( K \) is the shape factor (approximately 0.9), \( \lambda \) is the X-ray wavelength, \( \beta \) is the full width at half maximum (FWHM) of the diffraction peak, and \( \theta \) is the Bragg angle. For the (100) peak, the FWHM decreased upon 3-PTU treatment, indicating an increase in crystallite size.
Scanning electron microscopy (SEM) images provided further insights into the morphological changes induced by 3-PTU. The control films exhibited small grains with visible PbI2 impurities, whereas the 3-PTU-treated films showed larger, more uniform grains with reduced pinholes and smoother surfaces. Cross-sectional SEM revealed that the control films had a thickness of approximately 430 nm with numerous grain boundaries, while the treated films had a thickness of about 405 nm with vertically aligned grains. This morphological improvement is attributed to the coordination and hydrogen-bonding interactions between 3-PTU and the perovskite components, which regulate nucleation and growth kinetics. The grain size distribution can be quantified using image analysis software, and the results are summarized in Table 1.
| Sample | Average Grain Size (nm) | Film Thickness (nm) | PbI2 Peak Intensity (a.u.) |
|---|---|---|---|
| Control | 150 ± 20 | 430 ± 10 | 0.15 |
| 3-PTU Treated | 220 ± 30 | 405 ± 10 | 0.05 |
Ultraviolet-visible (UV-Vis) absorption spectroscopy and photoluminescence (PL) measurements were employed to study the optoelectronic properties of the perovskite films. The absorption spectra showed a similar absorption edge at around 790 nm for both control and 3-PTU-treated films, but the treated films exhibited enhanced absorption intensity in the visible region. The PL spectra displayed a peak at approximately 800 nm for both samples, with a significant increase in PL intensity for the 3-PTU-treated films. This enhancement indicates a reduction in non-radiative recombination centers, likely due to defect passivation by the additive. The PL quenching efficiency can be calculated using the formula:
$$ \eta_{\text{quench}} = 1 – \frac{I_{\text{film}}}{I_{\text{ref}}} $$
where \( I_{\text{film}} \) is the PL intensity of the perovskite film and \( I_{\text{ref}} \) is the reference intensity. For the 3-PTU-treated films, the lower quenching efficiency suggests improved charge extraction and reduced recombination.
The current density-voltage (J-V) characteristics of the perovskite solar cells were measured under simulated AM 1.5 illumination. The control devices achieved a power conversion efficiency (PCE) of 22.46% with an open-circuit voltage (Voc) of 1.17 V, a short-circuit current density (Jsc) of 24.69 mA/cm2, and a fill factor (FF) of 77.68% under reverse scan. In contrast, the 3-PTU-treated devices exhibited a PCE of 23.75% with a Voc of 1.21 V, a Jsc of 24.95 mA/cm2, and an FF of 78.52%. The hysteresis index (HI) was calculated to assess the hysteresis effect:
$$ \text{HI} = \frac{\text{PCE}_{\text{reverse}} – \text{PCE}_{\text{forward}}}{\text{PCE}_{\text{reverse}}} $$
For the control devices, the HI was 0.071, indicating noticeable hysteresis, while for the 3-PTU-treated devices, the HI was negligible at 0.017. This reduction in hysteresis is consistent with suppressed ion migration and reduced defect density in the perovskite layer. The photovoltaic parameters are summarized in Table 2.
| Device | Scan Direction | Voc (V) | Jsc (mA/cm2) | FF (%) | PCE (%) | Hysteresis Index |
|---|---|---|---|---|---|---|
| Control | Forward | 1.18 | 24.35 | 72.73 | 20.87 | 0.071 |
| Reverse | 1.17 | 24.69 | 77.68 | 22.46 | ||
| 3-PTU Treated | Forward | 1.21 | 24.67 | 78.08 | 23.34 | 0.017 |
| Reverse | 1.21 | 24.95 | 78.52 | 23.75 |
The stability of the perovskite solar cells was evaluated under ambient conditions with a relative humidity of 35% at 25°C. Water contact angle measurements revealed that the control films had a contact angle of 75.44°, while the 3-PTU-treated films exhibited a higher contact angle of 93.71°. This increase in hydrophobicity is attributed to the hydrophobic pyridine ring in 3-PTU, which helps to mitigate moisture ingress. The normalized PCE of unencapsulated devices was monitored over 950 hours. The control devices retained only 68% of their initial PCE, whereas the 3-PTU-treated devices maintained 85% of their initial efficiency. The degradation kinetics can be modeled using a first-order reaction equation:
$$ \frac{PCE(t)}{PCE_0} = e^{-kt} $$
where \( PCE(t) \) is the efficiency at time \( t \), \( PCE_0 \) is the initial efficiency, and \( k \) is the degradation rate constant. The lower \( k \) value for the 3-PTU-treated devices confirms their enhanced stability. The stability data are presented in Table 3.
| Device | Initial PCE (%) | PCE after 950 h (%) | Retention (%) | Degradation Rate Constant (h-1) | Water Contact Angle (°) |
|---|---|---|---|---|---|
| Control | 22.46 | 15.27 | 68 | 0.00042 | 75.44 |
| 3-PTU Treated | 23.75 | 20.19 | 85 | 0.00017 | 93.71 |
To further understand the role of 3-PTU in defect passivation, we analyzed the trap density of states (tDOS) using space-charge-limited current (SCLC) measurements. The trap-filled limit voltage (VTFL) was determined from the J-V curves in the dark, and the trap density (nt) was calculated using the formula:
$$ n_t = \frac{2 \epsilon_0 \epsilon_r V_{TFL}}{e L^2} $$
where \( \epsilon_0 \) is the vacuum permittivity, \( \epsilon_r \) is the relative permittivity of the perovskite, \( e \) is the elementary charge, and \( L \) is the film thickness. The 3-PTU-treated films showed a lower trap density compared to the control films, indicating effective passivation of deep-level defects. This reduction in trap density contributes to the improved Voc and FF in the perovskite solar cells.
Electrochemical impedance spectroscopy (EIS) was performed to investigate the charge transport and recombination dynamics. The Nyquist plots exhibited a single semicircle, which was fitted to an equivalent circuit model consisting of a series resistance (Rs) and a recombination resistance (Rrec). The 3-PTU-treated devices displayed a higher Rrec value, suggesting suppressed charge recombination at the interfaces. The characteristic frequency corresponding to the recombination process was also lower for the treated devices, consistent with longer carrier lifetimes. The carrier lifetime (τ) can be estimated from the Bode phase plots using the relation:
$$ \tau = \frac{1}{2 \pi f_{\text{max}}} $$
where \( f_{\text{max}} \) is the frequency at the maximum phase angle. The increased carrier lifetime in the 3-PTU-treated devices aligns with the enhanced PL intensity and reduced non-radiative recombination.
In conclusion, the incorporation of 3-PTU as a multifunctional additive in perovskite solar cells significantly improves the crystallinity, morphology, and optoelectronic properties of the perovskite films. The additive promotes preferential crystal growth, reduces PbI2 impurities, and enhances hydrophobicity, leading to higher PCE, reduced hysteresis, and improved stability. The coordination between 3-PTU and Pb2+ ions, along with hydrogen bonding, plays a key role in modulating the crystallization kinetics and passivating defects. These findings underscore the potential of 3-PTU as a versatile additive for advancing the performance and durability of perovskite solar cells. Future work will focus on optimizing the concentration of 3-PTU and exploring its effects in large-area devices and tandem configurations.
The performance of perovskite solar cells is often limited by non-ideal charge transport and recombination losses. To quantify these effects, we can use the diode equation under illumination:
$$ J = J_{sc} – J_0 \left( e^{\frac{q(V + J R_s)}{n k T}} – 1 \right) – \frac{V + J R_s}{R_{sh}} $$
where \( J \) is the current density, \( V \) is the voltage, \( J_0 \) is the reverse saturation current density, \( n \) is the ideality factor, \( k \) is Boltzmann’s constant, \( T \) is the temperature, \( R_s \) is the series resistance, and \( R_{sh} \) is the shunt resistance. For the 3-PTU-treated devices, the lower \( J_0 \) and higher \( R_{sh} \) values indicate reduced recombination and improved diode characteristics.
Furthermore, the external quantum efficiency (EQE) spectra were measured to verify the Jsc values. The integrated Jsc from the EQE spectra agreed well with the J-V measurements, confirming the reliability of the performance data. The EQE spectra showed enhanced response in the long-wavelength region for the 3-PTU-treated devices, which is consistent with the improved absorption and reduced recombination.
In summary, the use of 3-PTU additive in perovskite solar cells offers a promising pathway to achieve high efficiency and stability. The multifunctional nature of 3-PTU, combining crystallization control, defect passivation, and hydrophobicity enhancement, addresses key challenges in perovskite photovoltaics. We believe that this approach can be extended to other perovskite compositions and device architectures, contributing to the development of next-generation solar energy technologies.