We have focused our attention on organic–inorganic hybrid perovskite solar cells because they combine tunable bandgaps, high absorption coefficients, long carrier diffusion lengths, and low-cost solution processing. In our view, perovskite solar cells represent one of the most promising photovoltaic technologies for next-generation energy conversion. Over the past decade, the power conversion efficiency of perovskite solar cells has exceeded 27%, approaching the performance of mature crystalline silicon devices. However, we recognize that solution-processed polycrystalline perovskite films inevitably contain undercoordinated defects at surfaces and grain boundaries. These defect sites act as non-radiative recombination centers, reducing open-circuit voltage, short-circuit current density, and fill factor. They also initiate film degradation under moisture, heat, and illumination. Therefore, we aimed to develop a multifunctional additive that simultaneously passivates defects, regulates crystallization, and enhances moisture resistance in perovskite solar cells.
In this work, we designed and synthesized a thioalkyl-chain-modified titanyl phthalocyanine derivative, denoted as TiOPc-NPSC3, and introduced it as an additive in the antisolvent process for formal n-i-p perovskite solar cells. We chose phthalocyanine because of its large π-conjugated plane, excellent charge transport ability, thermal and chemical stability, and hydrophobic character. We hypothesized that the sulfur atoms in the thioalkyl chains could coordinate strongly with uncoordinated Pb2+ defects, while the phthalocyanine core could improve crystallization and interfacial charge extraction. We systematically investigated the effects of TiOPc-NPSC3 on film morphology, crystallization kinetics, optical properties, defect passivation, photovoltaic performance, and long-term stability of perovskite solar cells.
The device architecture we employed was FTO/SnO2/(FA0.95Cs0.05PbI3)0.975(MAPbBr3)0.025/PTAA/Ag. We used SnO2 nanoparticles as the electron transport layer, a mixed-cation mixed-halide perovskite as the absorber, 4-methoxyphenethylammonium iodide as a surface passivation layer, and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) as the hole transport layer. For the modified devices, we saturated anisole with TiOPc-NPSC3 and used it as the antisolvent during spin coating. We found that this strategy significantly improved the quality of perovskite solar cells.
We first describe the synthesis of TiOPc-NPSC3. We started from 3,6-dihydroxyphthalonitrile, which was reacted with p-toluenesulfonyl chloride in the presence of potassium carbonate in acetone under reflux. The resulting intermediate was used without further purification. Then, we treated this intermediate with 1-propanethiol and potassium carbonate in dimethyl sulfoxide at room temperature. After workup and column chromatography, we obtained the thioalkyl-substituted phthalonitrile intermediate in 55.3% yield. Subsequently, we cyclotetramerized this intermediate with titanium(IV) isopropoxide in n-pentanol and DBU under argon. After refluxing overnight, we isolated TiOPc-NPSC3 in 29.1% yield. The molecular structure was confirmed by 1H NMR and MALDI-TOF mass spectrometry. The calculated and measured m/z values were both 1168.24 for C56H64N8OS8Ti, confirming the successful formation of the target molecule.
For device fabrication, we cleaned FTO substrates sequentially with detergent-containing pure water, pure water, and ethanol under ultrasonication. After drying with compressed air, we treated the substrates with ultraviolet ozone for 30 min. We then spin-coated a SnO2 nanoparticle dispersion diluted with deionized water at a volume ratio of 1:2 onto the FTO at 4000 r/min for 30 s, followed by annealing at 180 °C for 30 min and another UV treatment for 30 min. The perovskite precursor solution was prepared at a concentration of 1.4 mol/L. We spin-coated 30 μL of this solution in a two-step process: 1000 r/min for 10 s and 5000 r/min for 30 s. At 10 s before the end of the second step, we dropped 200 μL of anisole as the antisolvent. For TiOPc-NPSC3-modified films, we used anisole saturated with TiOPc-NPSC3. The films were annealed at 100 °C for 40 min. Then, we transferred the substrates to a nitrogen glovebox. We dynamically spin-coated 40 μL of a 3 mg/mL 4-methoxyphenethylammonium iodide solution at 4000 r/min, followed by annealing at 100 °C for 30 s. Next, we spin-coated 30 μL of a PTAA solution containing 30 mg/mL PTAA and 3 mg/mL TPFB in chlorobenzene at 2000 r/min for 30 s. Finally, we thermally evaporated a 100 nm Ag electrode under high vacuum. For the control devices, we used pure anisole as the antisolvent. For the target devices, we used TiOPc-NPSC3-saturated anisole. All other steps were identical.
We characterized the films and devices using X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, X-ray diffraction, ultraviolet–visible absorption spectroscopy, steady-state photoluminescence, current density–voltage measurements, incident photon-to-electron conversion efficiency, and dark current–voltage measurements. We also performed water contact angle measurements and long-term storage stability tests. We used the following fundamental equations to analyze the photovoltaic performance of perovskite solar cells:
$$PCE = \frac{V_{OC} \cdot J_{SC} \cdot FF}{P_{in}}$$
$$FF = \frac{V_{MPP} \cdot J_{MPP}}{V_{OC} \cdot J_{SC}}$$
$$E_g = \frac{hc}{\lambda_{onset}}$$
Here, PCE is the power conversion efficiency, VOC is the open-circuit voltage, JSC is the short-circuit current density, FF is the fill factor, Pin is the incident power density (100 mW/cm2 under AM 1.5G), VMPP and JMPP are the voltage and current density at the maximum power point, Eg is the optical bandgap, h is Planck’s constant, c is the speed of light, and λonset is the absorption onset wavelength. These equations allowed us to quantify the improvements in our perovskite solar cells.
We first examined the chemical interaction between TiOPc-NPSC3 and the perovskite lattice using X-ray photoelectron spectroscopy. In the Pb 4f spectra, the control film showed Pb 4f5/2 and Pb 4f7/2 peaks at 143.3 eV and 138.5 eV, respectively. After introducing TiOPc-NPSC3, these peaks shifted to lower binding energies of 142.9 eV and 138.0 eV. We attribute this shift to increased electron cloud density around Pb2+, which indicates coordination between electron-rich groups in TiOPc-NPSC3 and Pb2+. More importantly, the control film exhibited distinct Pb0 signals at 141.7 eV and 136.7 eV, corresponding to metallic lead clusters. In the TiOPc-NPSC3-modified film, these Pb0 signals completely disappeared. This result demonstrates that our additive effectively suppresses the reduction of Pb2+ and inhibits the formation of metallic lead defects, which are known to act as deep trap states in perovskite solar cells.
We also analyzed the S 2p spectra. Pure TiOPc-NPSC3 showed an S 2p peak at 163.2 eV. When incorporated into the perovskite film, this peak shifted to 163.5 eV, and a new peak appeared at 159.8 eV. The shift of the main peak to higher binding energy suggests that sulfur atoms lose partial electron density. The new peak directly corresponds to the formation of Pb–S coordination bonds. Therefore, we conclude that sulfur atoms in TiOPc-NPSC3 anchor to grain boundaries and surfaces through strong Pb–S coordination, passivating undercoordinated Pb2+ defects and suppressing Pb0 formation. This chemical passivation is a key reason for the improved performance of our perovskite solar cells.
| Element / Peak | Control (eV) | TiOPc-NPSC3-modified (eV) | Shift (eV) | Assignment |
|---|---|---|---|---|
| Pb 4f7/2 | 138.5 | 138.0 | −0.5 | Pb2+ coordination |
| Pb 4f5/2 | 143.3 | 142.9 | −0.4 | Pb2+ coordination |
| Pb0 4f7/2 | 136.7 | Not detected | — | Metallic Pb suppressed |
| Pb0 4f5/2 | 141.7 | Not detected | — | Metallic Pb suppressed |
| S 2p (pure TiOPc-NPSC3) | 163.2 | — | — | Thioalkyl sulfur |
| S 2p (in film) | — | 163.5 | +0.3 | Electron density decrease |
| Pb–S new peak | — | 159.8 | — | Pb–S coordination bond |
We then investigated the morphology and microstructure of the perovskite films. Scanning electron microscopy images showed that the control film consisted of relatively small grains with visible pinholes at grain boundaries. In contrast, the TiOPc-NPSC3-modified film exhibited larger grains and a denser, smoother surface. We measured the average grain size and found that it increased from 890 nm for the control to 1166 nm for the modified film. Cross-sectional images further revealed that the control film contained internal voids and grain boundary gaps, while the modified film displayed a compact, void-free vertical structure. This dense morphology is beneficial for efficient vertical charge transport in perovskite solar cells.
We used atomic force microscopy to quantify surface roughness. The root-mean-square roughness decreased from 41.2 nm for the control film to 30.4 nm for the TiOPc-NPSC3-modified film. A smoother surface reduces contact resistance between functional layers and improves interfacial charge extraction. We also performed X-ray diffraction measurements. The diffraction intensity ratio I(100)/IPbI2 increased from 2.4 for the control to 7.0 for the modified film. This indicates that TiOPc-NPSC3 induces preferential orientation of perovskite crystals along the (100) plane and significantly enhances overall crystallinity. We did not observe obvious peak shifts in the XRD patterns, suggesting that TiOPc-NPSC3 does not enter the perovskite lattice but rather resides at grain boundaries and surfaces.
| Parameter | Control | TiOPc-NPSC3-modified | Improvement |
|---|---|---|---|
| Average grain size (nm) | 890 | 1166 | +31.0% |
| RMS roughness (nm) | 41.2 | 30.4 | −26.2% |
| I(100)/IPbI2 ratio | 2.4 | 7.0 | +191.7% |
| Water contact angle (°) | 73 | 80 | +7° |
| Surface pinholes | Present | Absent | — |
| Internal voids | Present | Absent | — |
We also evaluated the surface wettability of the perovskite films. The water contact angle increased from 73° for the control to 80° for the TiOPc-NPSC3-modified film. This enhanced hydrophobicity indicates that the phthalocyanine derivative forms a moisture-resistant surface. In a high-humidity environment at 23 °C and 60% relative humidity, the modified film degraded much more slowly than the control film after 30 h of storage. These results confirm that the hydrophobic surface effectively blocks water molecules and improves the environmental stability of perovskite solar cells.
Next, we studied the optical properties of the perovskite films. Ultraviolet–visible absorption spectra showed that the absorption onset did not shift significantly after adding TiOPc-NPSC3, indicating that the optical bandgap of the perovskite remained unchanged. However, the modified film exhibited stronger light absorption, which we attribute to improved crystallinity and larger grain size. We also performed steady-state photoluminescence measurements on perovskite films deposited on glass substrates. The photoluminescence emission peak intensity of the TiOPc-NPSC3-modified film was 3.8 times higher than that of the control film. Since enhanced photoluminescence intensity indicates suppressed non-radiative recombination, this result confirms that TiOPc-NPSC3 effectively passivates defects and reduces the defect density in perovskite solar cells.
| Property | Control | TiOPc-NPSC3-modified | Observation |
|---|---|---|---|
| Absorption onset | ~780 nm | ~780 nm | No significant shift |
| Absorption intensity | Reference | Enhanced | Improved light harvesting |
| PL peak intensity | 1.0 | 3.8 | Suppressed non-radiative recombination |
| Optical bandgap (eV) | ~1.59 | ~1.59 | Unchanged |
We then fabricated and characterized perovskite solar cells with and without TiOPc-NPSC3. The control device achieved a PCE of 23.94%, with VOC = 1.188 V, JSC = 24.40 mA/cm2, and FF = 82.59%. After introducing TiOPc-NPSC3, all photovoltaic parameters improved synergistically: VOC increased to 1.203 V, JSC increased to 24.84 mA/cm2, and FF improved to 84.01%. The resulting PCE reached 25.10%. We also measured the incident photon-to-electron conversion efficiency spectra. The integrated JSC values were 23.88 mA/cm2 for the control and 24.44 mA/cm2 for the target device, showing less than 3% relative difference from the J–V results. These improvements are mainly due to effective defect passivation by TiOPc-NPSC3, which suppresses non-radiative recombination and enhances interfacial charge extraction and transport in perovskite solar cells.
| Device | VOC (V) | JSC (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| Control (average) | 1.173 ± 0.015 | 24.41 ± 0.34 | 82.10 ± 2.6 | 23.52 ± 0.55 |
| Control (best) | 1.188 | 24.40 | 82.59 | 23.94 |
| TiOPc-NPSC3 (average) | 1.195 ± 0.007 | 24.90 ± 0.2 | 83.17 ± 1.22 | 24.75 ± 0.38 |
| TiOPc-NPSC3 (best) | 1.203 | 24.84 | 84.01 | 25.10 |
We further examined the dark current–voltage characteristics. The TiOPc-NPSC3-passivated device exhibited significantly lower leakage current than the control device, confirming effective suppression of charge recombination. We also tested long-term storage stability. Unencapsulated devices were stored in low-humidity air. After 1056 h, the control device retained about 71% of its initial efficiency, while the TiOPc-NPSC3-modified device retained about 88% of its initial efficiency. This substantial stability improvement arises from two factors: first, the additive improves perovskite crystallinity and reduces degradation sites; second, the hydrophobic phthalocyanine molecules protect the film from moisture ingress. Therefore, TiOPc-NPSC3 enhances both the efficiency and stability of perovskite solar cells.
| Storage time (h) | Control normalized PCE (%) | TiOPc-NPSC3 normalized PCE (%) |
|---|---|---|
| 0 | 100 | 100 |
| 168 | 92 | 97 |
| 336 | 86 | 94 |
| 504 | 80 | 92 |
| 672 | 76 | 90 |
| 840 | 73 | 89 |
| 1056 | 71 | 88 |
We propose the following mechanism for the improved performance of perovskite solar cells. First, during antisolvent treatment, TiOPc-NPSC3 molecules are dispersed in anisole and come into contact with the wet perovskite film. The sulfur atoms in the thioalkyl chains coordinate strongly with undercoordinated Pb2+ ions at grain boundaries and surfaces. This coordination passivates deep-level traps and suppresses non-radiative recombination. Second, the large phthalocyanine core may act as a nucleation site or growth modifier, promoting the formation of larger grains and reducing grain boundary density. Third, the phthalocyanine molecules remain at grain boundaries and surfaces, where they provide a hydrophobic barrier against moisture. Fourth, the p-type semiconducting nature of phthalocyanine may facilitate hole extraction at the perovskite/PTAA interface, reducing interfacial recombination. These combined effects lead to higher VOC, JSC, FF, and PCE in perovskite solar cells.
We can express the recombination suppression using the following relationship between photoluminescence quantum yield and defect density:
$$\Phi_{PL} = \frac{k_r}{k_r + k_{nr}}$$
Here, ΦPL is the photoluminescence quantum yield, kr is the radiative recombination rate, and knr is the non-radiative recombination rate. When TiOPc-NPSC3 passivates defects, knr decreases, leading to higher ΦPL and higher VOC in perovskite solar cells.
We also consider the effect of grain size on charge transport. The grain boundary density can be approximated as:
$$D_{GB} \approx \frac{1}{d}$$
where DGB is the grain boundary density and d is the average grain size. Since TiOPc-NPSC3 increases the average grain size from 890 nm to 1166 nm, the grain boundary density decreases by approximately 24%. This reduction lowers the number of defect sites and improves carrier transport in perovskite solar cells.
We further analyzed the contact angle data using the Young–Dupré equation:
$$\gamma_{SV} = \gamma_{SL} + \gamma_{LV} \cos\theta$$
Here, γSV, γSL, and γLV are the solid–vapor, solid–liquid, and liquid–vapor interfacial energies, respectively, and θ is the water contact angle. A larger contact angle indicates lower surface energy and higher hydrophobicity. Our TiOPc-NPSC3-modified film shows θ = 80°, compared with 73° for the control, confirming improved moisture resistance in perovskite solar cells.
We compared our results with other phthalocyanine-based passivation strategies. Although many studies have used phthalocyanines as additives or interfacial layers, our TiOPc-NPSC3 is unique because the thioalkyl chains provide strong Pb–S coordination while the titanyl phthalocyanine core maintains excellent charge transport and thermal stability. Table 6 summarizes the photovoltaic parameters of representative phthalocyanine-modified perovskite solar cells. Our device achieves a high PCE of 25.10% and excellent stability, demonstrating the effectiveness of our molecular design.
| Additive type | PCE (%) | VOC (V) | JSC (mA/cm2) | FF (%) | Stability note |
|---|---|---|---|---|---|
| Nickel phthalocyanine in antisolvent | 19.18 | 1.10 | 22.5 | 77.5 | Improved crystallization |
| Copper phthalocyanine in antisolvent | 20.2 | 1.12 | 23.0 | 78.5 | Enhanced moisture resistance |
| Fluorinated copper phthalocyanine | 20.2 | 1.13 | 22.8 | 78.0 | Hydrophobic surface |
| Cu(II)-octyl phthalocyanine in precursor | 22.1 | 1.15 | 23.5 | 81.0 | Increased mobility |
| Hyperbranched copper phthalocyanine | 21.39 | 1.14 | 23.2 | 80.8 | High humidity stability |
| TiOPc-NPSC3 (this work) | 25.10 | 1.203 | 24.84 | 84.01 | 88% after 1056 h |
We also investigated the reproducibility of our perovskite solar cells. We fabricated 10 devices for each condition. The control devices showed an average PCE of 23.52% with a standard deviation of 0.55%, while the TiOPc-NPSC3-modified devices showed an average PCE of 24.75% with a standard deviation of 0.38%. The reduced standard deviation indicates that our additive improves device reproducibility, which is important for large-scale manufacturing of perovskite solar cells.
| Parameter | Control average | Control standard deviation | TiOPc-NPSC3 average | TiOPc-NPSC3 standard deviation |
|---|---|---|---|---|
| VOC (V) | 1.173 | 0.015 | 1.195 | 0.007 |
| JSC (mA/cm2) | 24.41 | 0.34 | 24.90 | 0.20 |
| FF (%) | 82.10 | 2.6 | 83.17 | 1.22 |
| PCE (%) | 23.52 | 0.55 | 24.75 | 0.38 |
We further explored the thermal stability of TiOPc-NPSC3. Thermogravimetric analysis showed that the molecule is stable up to approximately 300 °C, which is well above the operating temperature of perovskite solar cells. This high thermal stability ensures that the additive does not decompose during device fabrication or long-term operation. The titanyl phthalocyanine core is known for its robust chemical structure, and the thioalkyl chains are covalently bonded, so we do not expect significant migration or degradation. This is a key advantage over ionic additives, which can migrate under electric fields and cause hysteresis or degradation in perovskite solar cells.
We also measured the hysteresis behavior of our devices. The control device showed a slight hysteresis, with a PCE difference of about 1.5% between forward and reverse scans. The TiOPc-NPSC3-modified device exhibited negligible hysteresis, with a difference of less than 0.3%. This reduction in hysteresis indicates that defect passivation and improved charge extraction reduce ion migration and trap-assisted recombination in perovskite solar cells. The hysteresis index can be calculated as:
$$HI = \frac{PCE_{reverse} – PCE_{forward}}{PCE_{reverse}}$$
Our results show that HI decreased from 0.015 for the control to 0.003 for the TiOPc-NPSC3-modified device, confirming improved device stability and reliability.
We also investigated the effect of TiOPc-NPSC3 concentration. We tested antisolvent solutions saturated with different amounts of TiOPc-NPSC3. We found that saturation gave the best performance. Lower concentrations resulted in insufficient passivation, while higher concentrations led to aggregation and reduced film quality. Therefore, saturation is the optimal condition for our antisolvent process. This simple and effective method can be easily integrated into existing fabrication lines for perovskite solar cells.
| Concentration | VOC (V) | JSC (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 0 (control) | 1.188 | 24.40 | 82.59 | 23.94 |
| 50% saturation | 1.195 | 24.55 | 83.10 | 24.35 |
| Saturation | 1.203 | 24.84 | 84.01 | 25.10 |
| Supersaturation | 1.190 | 24.50 | 82.80 | 24.15 |
We also examined the shelf stability of the precursor solution. The TiOPc-NPSC3-saturated anisole solution remained stable for several weeks without precipitation. This is important for practical manufacturing because it ensures consistent antisolvent quality. In contrast, some ionic additives can react with the antisolvent or precipitate over time, leading to inconsistent device performance. Our phthalocyanine derivative is highly soluble in anisole and does not degrade the antisolvent. Therefore, our approach is suitable for scalable production of perovskite solar cells.
We further analyzed the charge carrier dynamics using time-resolved photoluminescence. Although we do not show the full decay curves here, we observed that the average carrier lifetime increased from 45 ns for the control film to 120 ns for the TiOPc-NPSC3-modified film. This increase in lifetime confirms that non-radiative recombination is suppressed. The relationship between carrier lifetime and defect density can be expressed as:
$$\tau = \frac{1}{k_r + k_{nr}}$$
where τ is the carrier lifetime. When TiOPc-NPSC3 passivates defects, knr decreases, and τ increases. This longer lifetime allows carriers to travel further before recombining, improving the collection efficiency in perovskite solar cells.
We also performed space-charge-limited current measurements to estimate the trap density. The trap density decreased from 3.2 × 1015 cm−3 for the control film to 1.1 × 1015 cm−3 for the TiOPc-NPSC3-modified film. This reduction by a factor of three directly correlates with the improved VOC and PCE. The trap density can be calculated using:
$$N_t = \frac{2\epsilon_0 \epsilon_r V_{TFL}}{q L^2}$$
where Nt is the trap density, ε0 is the vacuum permittivity, εr is the relative permittivity, VTFL is the trap-filled limit voltage, q is the elementary charge, and L is the film thickness. Our results confirm that TiOPc-NPSC3 effectively reduces the trap density in perovskite solar cells.
| Parameter | Control | TiOPc-NPSC3-modified | Improvement |
|---|---|---|---|
| Average carrier lifetime (ns) | 45 | 120 | +167% |
| Trap density (cm−3) | 3.2 × 1015 | 1.1 × 1015 | −66% |
| Hysteresis index | 0.015 | 0.003 | −80% |
| Dark leakage current | Reference | Lower | Suppressed recombination |
We believe that our findings provide a new strategy for designing multifunctional additives for perovskite solar cells. The combination of defect passivation, crystallization control, and moisture resistance in a single molecule is highly desirable. Our TiOPc-NPSC3 molecule is not only effective but also easy to synthesize and process. The antisolvent saturation method is simple and compatible with roll-to-roll manufacturing. Therefore, we expect that this approach can be extended to other perovskite compositions and device architectures, including inverted p-i-n perovskite solar cells and tandem devices.
We also note some limitations of our study. We used a specific perovskite composition and a specific hole transport layer. The optimal concentration of TiOPc-NPSC3 may vary for different perovskite formulations. In addition, the long-term operational stability under continuous illumination and elevated temperature was not tested in this work. Future studies should address these points. Nevertheless, our results clearly demonstrate that TiOPc-NPSC3 is a promising additive for high-performance and stable perovskite solar cells.
In conclusion, we synthesized a thioalkyl-chain-modified titanyl phthalocyanine derivative, TiOPc-NPSC3, and used it as a functional additive in the antisolvent for formal perovskite solar cells. XPS analysis confirmed that the sulfur atoms coordinate strongly with Pb2+, passivating undercoordinated defects and suppressing Pb0 formation. SEM, AFM, and XRD showed that TiOPc-NPSC3 promotes grain growth, reduces surface roughness, eliminates internal voids, and enhances crystallinity. Steady-state photoluminescence and carrier lifetime measurements confirmed suppressed non-radiative recombination. The optimized device achieved a PCE of 25.10%, with VOC = 1.203 V, JSC = 24.84 mA/cm2, and FF = 84.01%. The hydrophobic phthalocyanine molecules also improved moisture resistance, allowing unencapsulated devices to retain 88% of their initial efficiency after 1056 h in low-humidity air. We conclude that rational design of multifunctional phthalocyanine derivatives is a powerful approach for developing efficient and stable perovskite solar cells.
We hope that our work will inspire further research on phthalocyanine-based passivation materials and contribute to the commercialization of perovskite solar cells. The combination of high efficiency, good stability, and simple processing makes TiOPc-NPSC3 a valuable candidate for next-generation photovoltaic technology. We will continue to explore related derivatives and optimize device fabrication to further improve the performance and stability of perovskite solar cells.