I have focused my effort on inverted perovskite solar cells because their p-i-n architecture offers attractive opportunities for low-temperature processing, flexible device integration, and tandem photovoltaic applications. In the broader field of photovoltaic research, the perovskite solar cell has advanced at an extraordinary pace, with certified power conversion efficiency rising from 3.8% in early reports to more than 27% in optimized laboratory devices. This rapid progress originates from the outstanding optoelectronic properties of metal halide perovskites, including strong broadband absorption, long ambipolar carrier diffusion lengths, low exciton binding energy, and defect-tolerant electronic structure. Despite these advantages, the perovskite solar cell still suffers from nonradiative recombination losses that originate from point defects, grain boundaries, surfaces, and buried interfaces. These defects not only reduce open-circuit voltage and fill factor but also create pathways for moisture, oxygen, and ion migration, thereby limiting long-term operational stability.
In my view, defect passivation is not a single-interface problem but a multidimensional challenge that spans the bulk, grain boundaries, top surface, and buried interface of the perovskite solar cell. Residual lead iodide, undercoordinated lead cations, halide vacancies, and organic-cation vacancies act as deep-level traps. These traps capture free carriers, quench photoluminescence, increase dark leakage current, and accelerate thermal degradation. Additive engineering is one of the most practical routes to address these issues because it can be implemented directly in the precursor ink without adding complex processing steps. A well-designed additive can coordinate to lead-halide frameworks, hydrogen-bond to halide ions, compensate ionic defects, and simultaneously influence nucleation and crystal growth. The result is a perovskite solar cell with larger grains, fewer grain boundaries, lower trap density, and improved charge extraction.
I selected 4-hydrazinopyridine hydrochloride, abbreviated as 4HPCl, as a multifunctional hydrochloride salt additive for inverted perovskite solar cells. The molecule contains a pyridine nitrogen site, a hydrazine group, a protonated ammonium-like moiety, and a chloride counterion. This combination is chemically rich. The pyridine nitrogen and hydrazine group can donate electron density to undercoordinated Pb2+ sites through Lewis acid-base interactions. The protonated nitrogen sites can form hydrogen bonds with iodide ions and organic cations. The chloride anion can participate in local halide coordination, help redistribute halide vacancies, and influence the crystallization kinetics of the perovskite solar cell absorber. I therefore hypothesized that 4HPCl could provide multidimensional defect passivation, suppress nonradiative recombination, enlarge grains, and improve both efficiency and thermal stability.
Experimental Design and Device Architecture
I prepared mixed-cation perovskite films with the target composition Cs0.05MA0.10FA0.85PbI3. The precursor solution was formed by dissolving CsI, MAI, FAI, and PbI2 in a mixed solvent of N,N-dimethylformamide and dimethyl sulfoxide at a volume ratio of 5:1. I additionally introduced 15 mol% methylammonium chloride and 5 mol% excess PbI2 to regulate crystallization. The final concentration of the perovskite precursor was fixed at 1.6 mol L-1. After stirring for 4 h, the solution was filtered through a 0.22 μm polytetrafluoroethylene filter. For additive-modified samples, I transferred the filtered precursor into a vial containing a predetermined amount of 4HPCl and stirred for another 2 h. The additive concentration was varied from 0 to 1.00 mg mL-1, and I identified 0.50 mg mL-1 as the optimized condition.
I fabricated inverted perovskite solar cells with the stack glass/ITO/NiOx/Ph-4PACz/perovskite/PEAI/C60/BCP/Ag. The ITO substrates were cleaned sequentially with detergent, ultrapure water, acetone, and ethanol, each for 15 min, followed by nitrogen drying and ozone treatment for 15 min. A NiOx nanoparticle dispersion at 10 mg mL-1 was spin-coated at 3000 rpm for 30 s and annealed at 120 °C for 20 min. Then Ph-4PACz ethanol solution at 0.5 mg mL-1 was spin-coated at 3000 rpm for 30 s and annealed at 100 °C for 10 min. The perovskite layer was deposited by a two-step spin-coating procedure: 1000 rpm for 10 s and 5000 rpm for 35 s, with 220 μL ethyl acetate dripped during the last 15 s of the second step. The film was annealed at 100 °C for 30 min. A PEAI isopropanol solution at 1 mg mL-1 was then spin-coated at 5000 rpm for 30 s for surface passivation. Finally, C60 (25 nm), BCP (7 nm), and Ag (100 nm) were deposited by thermal evaporation.
I characterized the perovskite solar cell films and devices using complementary techniques. X-ray diffraction provided crystal structure and phase information. Scanning electron microscopy revealed surface morphology and grain size. X-ray photoelectron spectroscopy probed chemical states and binding-energy shifts. Ultraviolet-visible absorption spectroscopy, steady-state photoluminescence, time-resolved photoluminescence, photoluminescence quantum yield, and two-dimensional photoluminescence mapping evaluated optical and recombination properties. Current density-voltage curves, external quantum efficiency, dark current-voltage measurements, space-charge-limited current analysis, electrochemical impedance spectroscopy, and Mott-Schottky analysis quantified photovoltaic performance, trap density, charge transport, and built-in potential.
Additive Concentration Screening and Photovoltaic Performance
I first screened the 4HPCl concentration in the precursor ink because additive loading strongly affects crystallization and defect passivation in a perovskite solar cell. The control device without 4HPCl delivered a power conversion efficiency of 22.38%, with an open-circuit voltage of 1.105 V, a short-circuit current density of 25.94 mA cm-2, and a fill factor of 78.06%. When I added 4HPCl, all photovoltaic parameters improved relative to the control. The best performance occurred at 0.50 mg mL-1. At this concentration, the champion device reached a power conversion efficiency of 25.31%, an open-circuit voltage of 1.166 V, a short-circuit current density of 26.04 mA cm-2, and a fill factor of 83.40%. Lower additive concentration provided only limited improvement, while higher concentration did not further enhance performance and likely disturbed crystal growth.
The power conversion efficiency of a perovskite solar cell is determined by the product of open-circuit voltage, short-circuit current density, and fill factor divided by incident power density:
$$ \eta = \frac{V_{OC} J_{SC} FF}{P_{in}} $$
where η is the power conversion efficiency, VOC is the open-circuit voltage, JSC is the short-circuit current density, FF is the fill factor, and Pin is the incident optical power density. Under standard AM 1.5G illumination, Pin is 100 mW cm-2. The fill factor itself is defined as:
$$ FF = \frac{V_{MPP} J_{MPP}}{V_{OC} J_{SC}} $$
where VMPP and JMPP are the voltage and current density at the maximum power point. The substantial increase in fill factor from 78.06% to 83.40% indicates that 4HPCl reduces series resistance and suppresses recombination losses in the perovskite solar cell. The open-circuit voltage gain of approximately 61 mV further confirms that nonradiative recombination is reduced. The short-circuit current density also increases slightly, which is consistent with improved light absorption and charge collection.
| 4HPCl concentration (mg mL-1) | Role in the perovskite solar cell | Key photovoltaic outcome |
|---|---|---|
| 0 | Reference without additive | PCE 22.38%, VOC 1.105 V, JSC 25.94 mA cm-2, FF 78.06% |
| 0.25 | Partial crystallization regulation | Improved but suboptimal passivation and grain growth |
| 0.50 | Optimal multidimensional passivation | PCE 25.31%, VOC 1.166 V, JSC 26.04 mA cm-2, FF 83.40% |
| 1.00 | Excess additive interferes with growth | No further gain; possible disruption of crystal quality |
I therefore selected 0.50 mg mL-1 as the optimized 4HPCl concentration for all subsequent film and device studies. The concentration trend is important for the rational design of additives in a perovskite solar cell: an effective additive must be present at sufficient concentration to passivate defects, but excessive loading can disrupt the stoichiometry and crystallization pathway. The optimized 4HPCl-modified perovskite solar cell benefits from a balanced interaction between the additive and the lead-halide framework.
Crystallization, Phase Purity, and Morphology
I used X-ray diffraction to evaluate how 4HPCl affects the crystal structure of the perovskite solar cell absorber. The pristine film showed a distinct diffraction peak near 12.7°, which is assigned to residual PbI2. This residual phase is common in two-step and antisolvent-assisted perovskite processing, and it can act as a defect source or a charge-transport barrier depending on its distribution. After 4HPCl modification, the PbI2 diffraction peak disappeared. I also quantified the intensity ratio between the PbI2 peak and the perovskite (100) peak. The control film had a ratio of approximately 0.7, while the 4HPCl-modified film showed a much lower ratio, confirming that the additive suppresses residual PbI2 in the perovskite solar cell.
I further compared the full width at half maximum of the perovskite diffraction peaks. A smaller full width at half maximum generally indicates higher crystallinity, larger coherent crystalline domains, and lower microstrain. The 0.50 mg mL-1 4HPCl sample exhibited the narrowest full width at half maximum among the tested concentrations. At 0.25 mg mL-1, the value was similar to the control. At 1.00 mg mL-1, the full width at half maximum became broader than the control, indicating that excess additive degrades crystal quality. This result supports the concentration screening and shows that 0.50 mg mL-1 is optimal for the perovskite solar cell.
| Sample | Residual PbI2 peak near 12.7° | PbI2/perovskite (100) intensity ratio | Full width at half maximum trend | Average grain size from SEM |
|---|---|---|---|---|
| Control | Clear | Approximately 0.7 | Reference | Approximately 255 nm |
| 4HPCl 0.25 mg mL-1 | Reduced | Lower than control | Similar to control | Moderately increased |
| 4HPCl 0.50 mg mL-1 | Not observed | Strongly reduced | Narrowest | Approximately 701 nm |
| 4HPCl 1.00 mg mL-1 | Reduced | Lower than control | Broader than control | Less uniform |
Scanning electron microscopy provided direct morphological evidence. The control perovskite film had relatively small grains with an average size of about 255 nm. The surface was rough, and many grain boundaries contained bright residues that I assigned to unreacted or segregated PbI2. Such grain boundaries are detrimental to the perovskite solar cell because they promote nonradiative recombination and ion migration. After 4HPCl modification at 0.50 mg mL-1, the average grain size increased to about 701 nm. The surface became smoother, and the amount of residual PbI2 at grain boundaries decreased markedly. Larger grains reduce the total grain-boundary area, which lowers trap density and improves charge transport in the perovskite solar cell.
I attribute the improved morphology to the interaction between 4HPCl and the perovskite precursor. The pyridine nitrogen and hydrazine group can coordinate to Pb2+, while the chloride ion can associate with halide vacancies and influence the local ionic environment. These interactions retard nucleation and allow grains to grow more uniformly. The protonated nitrogen sites can also hydrogen-bond to iodide and organic cations, stabilizing the colloidal species in the precursor ink. The result is a perovskite solar cell absorber with higher phase purity, larger grains, and fewer defective boundaries.
Optical Absorption, Photoluminescence, and Carrier Lifetime
I measured ultraviolet-visible absorption spectra to examine the optical response of the perovskite solar cell films. The 4HPCl-modified film showed enhanced absorption compared with the control, especially near the band edge. This improvement is consistent with better film quality, reduced scattering from rough boundaries, and more complete conversion of residual PbI2 into the perovskite phase. Stronger absorption contributes to the slight increase in short-circuit current density observed in the optimized perovskite solar cell.
Steady-state photoluminescence is a sensitive probe of nonradiative recombination. The control film exhibited relatively weak photoluminescence, indicating a high density of trap states. After 4HPCl modification, the photoluminescence intensity increased significantly. This enhancement implies that nonradiative recombination pathways are suppressed and that the photoexcited carriers in the perovskite solar cell have a longer effective lifetime. I also measured photoluminescence quantum yield. The 4HPCl-modified film showed more than twice the photoluminescence quantum yield of the control film, which provides direct evidence for reduced nonradiative recombination.
I used time-resolved photoluminescence to quantify carrier dynamics. The decay curves were fitted with a biexponential model:
$$ I(t) = A + B_{1} e^{-t/\tau_{1}} + B_{2} e^{-t/\tau_{2}} $$
where I(t) is the photoluminescence intensity at time t, A is a constant, B1 and B2 are amplitudes, and τ1 and τ2 are the fast and slow decay time constants. The average carrier lifetime was calculated as:
$$ \tau_{ave} = \frac{B_{1}\tau_{1}^{2} + B_{2}\tau_{2}^{2}}{B_{1}\tau_{1} + B_{2}\tau_{2}} $$
The control film had an average carrier lifetime of 403.77 ns. After 4HPCl modification, the average lifetime increased to 1056.12 ns. This more than twofold increase confirms that 4HPCl passivates defects and reduces nonradiative recombination in the perovskite solar cell. The fast decay component is often associated with trap-assisted recombination at grain boundaries and surfaces, while the slow component reflects bulk recombination. The increase in both components after 4HPCl treatment indicates that the additive passivates multiple defect populations rather than a single type of site.
| Property | Control perovskite solar cell film | 4HPCl-modified perovskite solar cell film | Physical interpretation |
|---|---|---|---|
| UV-visible absorption | Reference | Enhanced near band edge | Improved film quality and phase conversion |
| Steady-state PL intensity | Lower | Significantly higher | Reduced nonradiative recombination |
| Average carrier lifetime from TRPL | 403.77 ns | 1056.12 ns | Suppressed trap-assisted recombination |
| Photoluminescence quantum yield | Reference | More than twofold increase | Lower defect density and better radiative efficiency |
| Average lifetime from PL mapping | 27.16 ns | 48.07 ns | Improved spatial uniformity and local passivation |
Spatial Uniformity and Two-Dimensional Photoluminescence Mapping
I used two-dimensional photoluminescence mapping to evaluate spatial heterogeneity in the perovskite solar cell films. The control film exhibited a nonuniform photoluminescence intensity map with many dark regions. These dark regions correspond to local nonradiative recombination centers, likely associated with grain boundaries, residual PbI2, and regions of poor crystallinity. In contrast, the 4HPCl-modified film displayed a brighter and more continuous photoluminescence map. Dark regions were substantially reduced, indicating that defect passivation occurs across the entire film rather than only at isolated locations.
I also mapped the photoluminescence lifetime. The control film had an average lifetime of 27.16 ns with a broad distribution, including many short-lifetime areas. The 4HPCl-modified film had an average lifetime of 48.07 ns and a more uniform lifetime distribution. The correlation between high photoluminescence intensity and long lifetime was stronger in the modified film. This spatial correlation suggests that 4HPCl reduces local trap density and improves carrier dynamics throughout the perovskite solar cell absorber. For a high-performance perovskite solar cell, spatial uniformity is critical because local shunts and recombination hot spots can limit the overall open-circuit voltage and fill factor.
Chemical Interactions and X-ray Photoelectron Spectroscopy
I used X-ray photoelectron spectroscopy to investigate the chemical interaction between 4HPCl and the perovskite solar cell absorber. The Pb 4f, I 3d, and N 1s core levels all showed measurable changes after 4HPCl modification. The Pb 4f signal shifted, which I assigned to coordination between electron-donating nitrogen sites in 4HPCl and undercoordinated Pb2+ ions. The I 3d signal shifted to lower binding energy, indicating increased local electron density around iodide. This shift suggests that the nitrogen-containing groups in 4HPCl interact with iodide ions and help passivate iodine vacancies. The N 1s peak moved from 400.39 eV to 400.25 eV, confirming that the chemical environment of nitrogen changes upon interaction with the perovskite solar cell film.
The chloride counterion of 4HPCl can also contribute to defect passivation. Chloride ions are known to incorporate into halide sites, modulate crystal growth, and compensate halide vacancies. In the perovskite solar cell, chloride can help form a more stable halide coordination environment and reduce deep-level traps associated with undercoordinated lead and halide vacancies. Therefore, 4HPCl provides both cationic and anionic passivation functions. The cationic nitrogen sites interact with lead and iodide, while chloride participates in local halide compensation. This dual-site and dual-ion behavior explains why 4HPCl is effective at multiple length scales.
| Core level | Control binding energy or behavior | 4HPCl-modified binding energy or behavior | Proposed interaction in the perovskite solar cell |
|---|---|---|---|
| Pb 4f | Reference position | Measurable shift | Coordination of nitrogen sites to undercoordinated Pb2+ |
| I 3d | Reference position | Shift to lower binding energy | Nitrogen-iodide interaction and iodine vacancy passivation |
| N 1s | 400.39 eV | 400.25 eV | Changed nitrogen environment due to hydrogen bonding and coordination |
| Cl 2p | Weak or absent in control | Detectable after modification | Chloride incorporation and halide vacancy compensation |
I propose that 4HPCl passivates the perovskite solar cell through several synergistic pathways. First, the pyridine nitrogen and hydrazine group donate lone-pair electrons to Pb2+, forming Lewis acid-base adducts. Second, the protonated nitrogen sites form hydrogen bonds with iodide and organic cations. Third, the chloride anion participates in halide-site coordination and reduces halide vacancy density. Fourth, the aromatic pyridine ring provides hydrophobic character that may help resist moisture ingress. Together, these interactions reduce nonradiative recombination, improve charge transport, and enhance the stability of the perovskite solar cell.
Dark Current, Trap Density, and Charge Transport
I measured dark current-voltage characteristics to evaluate leakage pathways in the perovskite solar cell. The control device had a dark leakage current density of 7.14 × 10-6 mA cm-2. After 4HPCl modification, the leakage current density decreased to 1.16 × 10-6 mA cm-2. This reduction indicates that shunt paths and trap-assisted leakage are suppressed. Lower dark current is beneficial because it reduces the loss of charge carriers under forward bias and contributes to a higher open-circuit voltage and fill factor in the perovskite solar cell.
I then used space-charge-limited current measurements on electron-only devices to quantify trap density. The trap-filled limit voltage decreased from 0.51 V in the control to 0.33 V in the 4HPCl-modified perovskite solar cell. The defect density was calculated using:
$$ N_{trap} = \frac{2 \epsilon_{0} \epsilon_{r} V_{TFL}}{e L^{2}} $$
where Ntrap is the trap density, ε0 is the vacuum permittivity, εr is the relative dielectric constant of the perovskite, VTFL is the trap-filled limit voltage, e is the elementary charge, and L is the film thickness. The calculated trap density decreased from 4.68 × 1015 cm-3 to 3.02 × 1015 cm-3. This reduction of approximately 35% directly confirms that 4HPCl passivates defect states in the perovskite solar cell.
I also used electrochemical impedance spectroscopy to examine charge transport and recombination. The series resistance decreased from 14.19 Ω to 11.61 Ω, while the recombination resistance increased from 8086 Ω to 16621 Ω. The decrease in series resistance indicates improved charge extraction and reduced contact resistance. The increase in recombination resistance indicates that the perovskite solar cell has fewer active recombination centers. These two changes are consistent with lower trap density, larger grains, and better interfacial quality.
| Parameter | Control perovskite solar cell | 4HPCl-modified perovskite solar cell | Trend and implication |
|---|---|---|---|
| Dark leakage current density | 7.14 × 10-6 mA cm-2 | 1.16 × 10-6 mA cm-2 | Reduced shunting and trap-assisted leakage |
| Trap-filled limit voltage | 0.51 V | 0.33 V | Lower trap density |
| Trap density | 4.68 × 1015 cm-3 | 3.02 × 1015 cm-3 | Effective multidimensional passivation |
| Series resistance | 14.19 Ω | 11.61 Ω | Improved charge transport |
| Recombination resistance | 8086 Ω | 16621 Ω | Suppressed nonradiative recombination |
Built-in Potential and Device Physics
I performed Mott-Schottky analysis to determine the built-in potential of the perovskite solar cell. The built-in potential increased from 0.66 V in the control to 0.87 V after 4HPCl modification. The Mott-Schottky relationship is commonly written as:
$$ \frac{1}{C^{2}} = \frac{2(V_{bi} – V)}{e \epsilon_{0} \epsilon_{r} N_{A}} $$
where C is the capacitance, Vbi is the built-in potential, V is the applied bias, e is the elementary charge, ε0 is the vacuum permittivity, εr is the relative dielectric constant, and NA is the acceptor density. A larger built-in potential provides a stronger driving force for charge separation and extraction in the perovskite solar cell. This explains part of the open-circuit voltage improvement. The increase in built-in potential is also consistent with reduced interfacial recombination and better energy-level alignment after 4HPCl treatment.
I interpret the device physics as follows. The 4HPCl-modified perovskite solar cell has fewer trap states, so nonradiative recombination is suppressed. The larger grains reduce the number of grain boundaries, which lowers the density of recombination centers. The improved surface chemistry reduces interfacial defects and enhances charge extraction. The chloride anion participates in halide vacancy compensation, which lowers ionic disorder. The pyridine and hydrazine groups coordinate to lead, which reduces deep-level Pb-related traps. The combined effect is a higher open-circuit voltage, a higher fill factor, and a slight increase in short-circuit current density.
Current Density-Voltage Characteristics, Hysteresis, and External Quantum Efficiency
I measured current density-voltage curves under standard AM 1.5G illumination. The control perovskite solar cell delivered a reverse-scan power conversion efficiency of 22.38%, with a forward-scan efficiency of 20.87%. The hysteresis index was 6.75%. After 4HPCl modification, the reverse-scan efficiency increased to 25.31% and the forward-scan efficiency increased to 24.82%. The hysteresis index decreased to 1.93%. Reduced hysteresis is important because it indicates that ion migration and interfacial charge accumulation are suppressed in the perovskite solar cell. The 4HPCl-modified device therefore exhibits more reliable and stable photovoltaic behavior.
| Device | Scan direction | VOC (V) | JSC (mA cm-2) | FF (%) | PCE (%) | Hysteresis index (%) |
|---|---|---|---|---|---|---|
| Control | Reverse | 1.105 | 25.94 | 78.06 | 22.38 | 6.75 |
| Control | Forward | — | — | — | 20.87 | — |
| 4HPCl 0.50 mg mL-1 | Reverse | 1.166 | 26.04 | 83.40 | 25.31 | 1.93 |
| 4HPCl 0.50 mg mL-1 | Forward | — | — | — | 24.82 | — |
I also evaluated steady-state power output at the maximum power point. The control perovskite solar cell produced a steady-state current density of 23.72 mA cm-2 at 0.94 V, corresponding to a steady-state efficiency of 22.25%. The 4HPCl-modified perovskite solar cell produced a steady-state current density of 24.74 mA cm-2 at 1.02 V, corresponding to a steady-state efficiency of 25.23%. The steady-state values are close to the values obtained from the current density-voltage curves, which confirms that the performance improvement is stable and not an artifact of scan-rate-dependent hysteresis.
External quantum efficiency measurements showed that the 4HPCl-modified perovskite solar cell has a broad photoresponse from 300 to 900 nm. The integrated current density increased from 25.12 mA cm-2 to 25.27 mA cm-2. This increase is consistent with the short-circuit current density trend from the current density-voltage measurements. The external quantum efficiency improvement is modest but meaningful because it reflects better charge collection and reduced recombination losses in the perovskite solar cell.
| Measurement | Control perovskite solar cell | 4HPCl-modified perovskite solar cell | Difference |
|---|---|---|---|
| Champion PCE from reverse scan | 22.38% | 25.31% | +2.93 percentage points |
| Champion PCE from forward scan | 20.87% | 24.82% | +3.95 percentage points |
| Open-circuit voltage | 1.105 V | 1.166 V | +61 mV |
| Short-circuit current density | 25.94 mA cm-2 | 26.04 mA cm-2 | +0.10 mA cm-2 |
| Fill factor | 78.06% | 83.40% | +5.34 percentage points |
| Steady-state PCE at maximum power point | 22.25% | 25.23% | +2.98 percentage points |
| Integrated JSC from EQE | 25.12 mA cm-2 | 25.27 mA cm-2 | +0.15 mA cm-2 |
Thermal Stability and Operational Reliability
I evaluated the thermal stability of unencapsulated perovskite solar cells in a nitrogen atmosphere at 65 °C. The control device degraded to approximately 55% of its initial efficiency after 900 h. In contrast, the 4HPCl-modified perovskite solar cell retained about 90% of its initial efficiency after 900 h. This T90 lifetime of more than 900 h at 65 °C demonstrates that 4HPCl substantially improves the thermal durability of the perovskite solar cell. The stability improvement is consistent with the defect passivation, improved crystallinity, and stronger chemical bonding observed in the film characterization.
I attribute the enhanced stability to several factors. First, 4HPCl reduces residual PbI2 and halide vacancies, which are initiation sites for degradation. Second, the larger grains and reduced grain-boundary density limit ion migration and moisture diffusion pathways. Third, the nitrogen-containing groups and chloride anion strengthen interfacial bonding, which helps preserve the perovskite lattice under thermal stress. Fourth, the aromatic pyridine ring may provide a degree of hydrophobic protection at surfaces and grain boundaries. Together, these effects make the 4HPCl-modified perovskite solar cell more resistant to thermal degradation.
| Stability condition | Control perovskite solar cell | 4HPCl-modified perovskite solar cell | Interpretation |
|---|---|---|---|
| Unencapsulated, N2, 65 °C, 900 h | Approximately 55% of initial PCE retained | Approximately 90% of initial PCE retained | Enhanced thermal stability |
| T90 lifetime at 65 °C | Not achieved within 900 h | More than 900 h | Suppressed degradation pathways |
| Proposed degradation suppression | Residual PbI2, halide vacancies, grain boundaries | Passivated defects, larger grains, stronger bonding | Improved lattice and interface robustness |
Mechanistic Summary of Multidimensional Passivation
I summarize the role of 4HPCl in the perovskite solar cell as a multidimensional passivation strategy. The molecule contains several functional sites that act at different locations and defect types. The pyridine nitrogen is a Lewis base that coordinates to undercoordinated Pb2+. The hydrazine group is electron-rich and can further donate electron density to lead or hydrogen-bond to halide. The protonated nitrogen sites can interact with iodide and organic cations through hydrogen bonding and electrostatic attraction. The chloride anion can occupy halide vacancies, participate in local halide coordination, and help stabilize the perovskite lattice.
These interactions produce a cascade of beneficial effects in the perovskite solar cell. At the precursor stage, 4HPCl modulates colloidal chemistry and retards nucleation, which promotes larger grains. During film formation, it suppresses residual PbI2 and passivates grain boundaries. After film formation, it reduces surface and bulk trap density, suppresses nonradiative recombination, and improves charge extraction. Under thermal stress, it strengthens bonding and reduces ion migration, which improves stability. This multidimensional action distinguishes 4HPCl from simple single-site additives and explains why it improves multiple photovoltaic parameters simultaneously.
| Functional site in 4HPCl | Target defect or process | Proposed interaction | Benefit to the perovskite solar cell |
|---|---|---|---|
| Pyridine nitrogen | Undercoordinated Pb2+ | Lewis acid-base coordination | Reduced Pb-related deep traps |
| Hydrazine group | Pb2+ and halide vacancies | Electron donation and hydrogen bonding | Lower nonradiative recombination |
| Protonated nitrogen sites | Iodide and organic cations | Hydrogen bonding and electrostatic interaction | Stabilized lattice and improved film quality |
| Chloride anion | Halide vacancies | Halide-site compensation and coordination | Reduced ionic disorder and improved stability |
| Aromatic pyridine ring | Surface and grain boundaries | Hydrophobic and steric protection | Better resistance to moisture and thermal stress |
Discussion of Performance Gains and Loss Reduction
I can analyze the performance gains of the perovskite solar cell using the detailed balance framework. The open-circuit voltage of a real perovskite solar cell is reduced from the radiative limit by nonradiative recombination. The voltage loss can be expressed as:
$$ V_{OC} = V_{OC,rad} – \frac{k_{B}T}{q} \ln\left(\frac{J_{0,nonrad}}{J_{0,rad}} + 1\right) $$
where VOC,rad is the radiative open-circuit voltage, kB is the Boltzmann constant, T is temperature, q is the elementary charge, J0,nonrad is the nonradiative saturation current density, and J0,rad is the radiative saturation current density. When 4HPCl reduces nonradiative recombination, J0,nonrad decreases, so the voltage loss decreases. This is consistent with the observed open-circuit voltage increase from 1.105 V to 1.166 V.
The fill factor is also sensitive to recombination and series resistance. The relationship between fill factor, open-circuit voltage, and the ideal fill factor can be approximated by:
$$ FF \approx \frac{v_{OC} – \ln(v_{OC} + 0.72)}{v_{OC} + 1} $$
where vOC is the normalized open-circuit voltage. While this expression is simplified, it shows that reducing recombination losses and series resistance improves the fill factor. The 4HPCl-modified perovskite solar cell showed a fill factor increase from 78.06% to 83.40%, which agrees with reduced trap density, lower leakage current, and improved charge transport. The short-circuit current density increased slightly because of enhanced absorption and better carrier collection.
I also note that the improvement in the perovskite solar cell is not limited to one metric. The simultaneous increase in open-circuit voltage, short-circuit current density, and fill factor indicates that 4HPCl addresses multiple loss mechanisms. If only series resistance were reduced, the fill factor would improve but the open-circuit voltage might not change much. If only surface passivation occurred, the open-circuit voltage might improve but the short-circuit current density might remain similar. The fact that all parameters improve suggests that 4HPCl acts in the bulk, at grain boundaries, and at interfaces.
Comparison with Other Additive Strategies
I place 4HPCl in the broader context of additive engineering for the perovskite solar cell. Many additives have been explored, including inorganic salts, ammonium halides, long-chain surfactants, aromatic ammonium iodides, and polymers. Each class has advantages and limitations. Inorganic salts can passivate halide vacancies but may not strongly coordinate to lead. Long-chain surfactants can improve moisture resistance but may hinder charge transport if insulating chains accumulate at interfaces. Aromatic ammonium halides can form two-dimensional perovskite phases, but controlling phase distribution is challenging. 4HPCl is attractive because it combines a small aromatic heterocycle, a strong electron-donating hydrazine group, protonated nitrogen sites, and a halide counterion in one molecule.
| Additive strategy | Primary passivation mechanism | Potential advantage | Potential limitation in a perovskite solar cell |
|---|---|---|---|
| Inorganic halide salts | Halide vacancy compensation | Simple and effective for halide defects | Limited coordination to undercoordinated Pb2+ |
| Long-chain ammonium halides | Surface passivation and hydrophobic barrier | Improved moisture stability | Insulating chains may impede charge transport |
| Aromatic ammonium iodides | 2D/3D interface formation | Strong surface passivation and energy-level tuning | Phase control can be difficult |
| Lewis base molecules | Coordination to Pb2+ | Reduces Pb-related deep traps | May not address halide vacancies |
| 4HPCl | Multisite coordination, hydrogen bonding, halide compensation, crystallization control | Simultaneous efficiency and stability improvement | Requires optimized concentration to avoid excess additive effects |
I believe that the multifunctional nature of 4HPCl is a key design principle for future additives in the perovskite solar cell. The ideal additive should not only passivate one type of defect but also regulate crystallization, improve charge transport, and enhance stability. This requires a careful balance of functional groups and molecular size. If the molecule is too large or insulating, it may block charge transport. If it is too small or volatile, it may not remain in the film. If it lacks halide-containing groups, it may not compensate halide vacancies. 4HPCl satisfies these criteria by combining a compact aromatic core with multiple nitrogen sites and a chloride counterion.
Implications for Future Perovskite Solar Cell Development
I see several implications of my work for the future development of the perovskite solar cell. First, multidimensional defect passivation should be pursued through rational molecular design rather than trial-and-error screening. The molecule should contain complementary functional groups that target both cationic and anionic defects. Second, additive engineering should be integrated with crystallization control. Defect passivation alone cannot fully optimize a perovskite solar cell if the film has small grains and residual secondary phases. Third, stability should be evaluated under realistic thermal and operational conditions. The ability of 4HPCl to retain about 90% of initial efficiency after 900 h at 65 °C is a promising indicator for practical applications.
I also note that the 4HPCl strategy may be compatible with other optimization methods. For example, it could be combined with interface passivation layers, solvent engineering, compositional engineering, or tandem device design. In a tandem perovskite solar cell, defect passivation of the wide-bandgap or narrow-bandgap subcell is critical for voltage matching and overall efficiency. The multifunctional passivation approach described here could be adapted to different perovskite compositions by adjusting the additive concentration and processing conditions.
From a manufacturing perspective, 4HPCl is introduced directly into the precursor ink, which avoids additional deposition steps. This is advantageous for scalable fabrication of the perovskite solar cell. The additive is soluble in the precursor solvent system and does not require a separate post-treatment. The concentration window is also relatively low, which minimizes material cost. These features make 4HPCl a practical candidate for large-area coating, blade coating, slot-die coating, and other scalable deposition methods. However, future work should evaluate uniformity over large areas and compatibility with high-throughput manufacturing.
Conclusion
I have demonstrated that 4HPCl is an effective multifunctional additive for inverted perovskite solar cells. The molecule interacts with the perovskite absorber through pyridine nitrogen, hydrazine, protonated nitrogen sites, and chloride anions. These interactions passivate undercoordinated Pb2+, iodide vacancies, and halide-related defects. They also modulate crystallization, increase grain size from approximately 255 nm to approximately 701 nm, reduce residual PbI2, and improve film uniformity. As a result, the 4HPCl-modified perovskite solar cell exhibits reduced nonradiative recombination, longer carrier lifetime, lower trap density, lower leakage current, higher recombination resistance, and a larger built-in potential.
The optimized perovskite solar cell achieved a champion power conversion efficiency of 25.31%, an open-circuit voltage of 1.166 V, a short-circuit current density of 26.04 mA cm-2, and a fill factor of 83.40%. The forward-scan efficiency was 24.82%, and the hysteresis index decreased to 1.93%. The steady-state efficiency was 25.23%. In addition, the unencapsulated 4HPCl-modified perovskite solar cell retained approximately 90% of its initial efficiency after 900 h of thermal aging at 65 °C in nitrogen, compared with only approximately 55% for the control. These results show that 4HPCl simultaneously improves efficiency, reduces hysteresis, and enhances thermal stability.
I conclude that multidimensional defect passivation is a powerful strategy for advancing the perovskite solar cell. The design of 4HPCl illustrates how a single small molecule can regulate crystallization, passivate multiple defect types, improve charge transport, and strengthen device stability. I expect that this approach will inspire further molecular engineering of additives for high-performance and durable perovskite solar cells, including single-junction devices, tandem devices, and scalable photovoltaic modules. The perovskite solar cell remains one of the most promising platforms for next-generation photovoltaics, and rational defect management will be central to its continued progress.