I have devoted my recent work to inverted perovskite solar cells because their low-temperature processability, tunable bandgap, and compatibility with flexible substrates make them one of the most promising platforms for next-generation photovoltaics. In my experience, however, the performance of inverted perovskite solar cells is still limited by two coupled problems: abundant defects in the perovskite absorber and imperfect charge extraction at the electron transport layer interface. These issues accelerate non-radiative recombination, reduce open-circuit voltage, lower fill factor, and ultimately constrain the power conversion efficiency. In this study, I addressed both problems simultaneously through a synergistic modification strategy that combines ethylammonium iodide as an additive in the organic ammonium salt precursor and a blend of PCBM with zirconium acetylacetonate as the electron transport layer. The resulting perovskite solar cells reached a power conversion efficiency of 22.43%, compared with 19.58% for the reference device, mainly because the open-circuit voltage increased from 1.10 V to 1.15 V and the fill factor increased from 75.17% to 80.03%.
The central motivation for my work is that polycrystalline perovskite films inevitably contain grain boundaries, surfaces, and residual phases that act as defect sites. These defect sites not only trap photogenerated carriers but also provide pathways for moisture and oxygen ingress, which degrades long-term stability. For perovskite solar cells, the quality of the absorber and the quality of the electron transport layer are therefore inseparable. A high-crystallinity perovskite film with large grains and a dense, uniform electron transport layer are both required to suppress leakage current, improve charge extraction, and reduce interfacial recombination. I designed my experiments to test whether a small-molecule ammonium salt additive could regulate crystal growth in a two-step deposition process and whether a metal acetylacetonate dopant could improve the film-forming behavior of PCBM. The results confirmed that the two modifications operate cooperatively and produce a substantial improvement in device performance.
Experimental Design and Fabrication
I fabricated devices with the inverted planar heterojunction architecture ITO/NiOx/perovskite/PCBM:ZrAcac/Au. In this stack, NiOx served as the hole transport layer, the mixed-cation mixed-halide perovskite served as the light absorber, the PCBM:ZrAcac blend served as the electron transport layer, and gold served as the back electrode. The effective device area was 4 mm2. I used a two-step spin-coating method for the perovskite layer because it offers better control over the interdiffusion between the lead iodide scaffold and the organic ammonium salts than a single-step method. In the first step, I dissolved lead iodide in a mixed solvent of dimethylformamide and dimethyl sulfoxide at a volume ratio of 19:1, and I added a concentrated cesium iodide/dimethyl sulfoxide solution at a volume ratio of 1:50. The lead iodide concentration was 1.3 mol/L. I spin-coated this precursor onto the NiOx layer at 2000 r/min for 30 s and annealed it at 70 °C for 3 min to form a yellow lead iodide film. In the second step, I prepared an isopropanol solution containing formamidinium iodide, methylammonium bromide, methylammonium chloride, and ethylammonium iodide as an optional additive. The concentrations were 60 g/L for formamidinium iodide, 6 g/L for methylammonium bromide, 6 g/L for methylammonium chloride, and 3 g/L for ethylammonium iodide. I spin-coated this organic ammonium salt solution onto the lead iodide film at 2000 r/min for 30 s and annealed the stack at 150 °C for 30 min in air with a relative humidity of 30% to 40%. For the electron transport layer, I used either pure PCBM or a PCBM:ZrAcac blend with a mass ratio of 9:1. Both were dissolved in ortho-dichlorobenzene at a concentration of 20 g/L, stirred at room temperature for 2 h, and filtered before use. I spin-coated the electron transport layer at 1000 r/min for 60 s. Finally, I thermally evaporated 100 nm of gold as the electrode.
For the hole transport layer, I synthesized NiOx nanoparticles from nickel nitrate hexahydrate and sodium hydroxide. I slowly added an aqueous solution of nickel nitrate hexahydrate at 5 mol/L to an equal volume of sodium hydroxide at 10 mol/L under continuous stirring. A brown precipitate formed. I washed and centrifuged the precipitate six times with deionized water at 8000 r/min for 25 min per cycle, dried it under vacuum at 80 °C for 10 h, ground it into a fine powder, and calcined it at 300 °C for 4 h in a muffle furnace. The green powder transformed into black NiOx nanoparticles. I then dispersed the NiOx nanoparticles in deionized water at 20 g/L, stirred the dispersion at 60 °C for 3 h, and spin-coated it onto ultraviolet-ozone-treated ITO glass at 2000 r/min for 50 s. The ITO substrates were cleaned sequentially with detergent, deionized water, acetone, ethanol, and isopropanol under ultrasonication for 20 min each, dried with nitrogen, and treated with ultraviolet ozone for 10 min to improve wettability and adjust the work function.
I characterized the films and devices using scanning electron microscopy, atomic force microscopy, X-ray diffraction, ultraviolet-visible-near-infrared spectroscopy, steady-state photoluminescence, time-resolved photoluminescence, and current density-voltage measurements. The current density-voltage curves were recorded under AM1.5G illumination at 100 mW/cm2 with a scan rate of 10 mV/s. I also measured dark current density-voltage characteristics, light-intensity-dependent open-circuit voltage, and steady-state output at the maximum power point. For stability testing, I stored unencapsulated devices in ambient air at room temperature of 25 ± 4 °C and relative humidity of 30% to 40% in the dark, without special humidity control or nitrogen protection, and I measured the current density-voltage curves periodically over 30 days.
| Material or Layer | Concentration or Thickness | Role |
|---|---|---|
| NiOx dispersion | 20 g/L in deionized water | Hole transport layer |
| PbI2 precursor | 1.3 mol/L in DMF:DMSO 19:1 | Inorganic scaffold |
| CsI/DMSO additive | 1.5 mol/L, volume ratio 1:50 | Crystallization control |
| FAI | 60 g/L in isopropanol | Organic ammonium salt |
| MABr | 6 g/L in isopropanol | Organic ammonium salt |
| MACl | 6 g/L in isopropanol | Organic ammonium salt |
| EAI additive | 3 g/L in isopropanol | Crystal growth modifier |
| PCBM | 20 g/L in 2-DCB | Electron transport layer |
| ZrAcac | PCBM:ZrAcac 9:1 by mass | Electron transport layer dopant |
| Au electrode | 100 nm | Back contact |
| Device area | 4 mm2 | Active area |
Film Morphology and Crystallinity
I first examined how ethylammonium iodide influences the morphology of the perovskite absorber. Scanning electron microscopy images of the top surface showed that the reference perovskite film without ethylammonium iodide had smaller grains and more pinholes. In contrast, the ethylammonium iodide-modified film exhibited larger grains, with some grains reaching approximately 1.5 μm. Because grain boundaries are widely regarded as non-radiative recombination centers in perovskite solar cells, a larger grain size and a lower grain boundary density are expected to reduce defect-assisted recombination and improve charge transport. I attribute this improvement to the role of ethylammonium iodide in delaying the volatilization of the organic ammonium salts before they penetrate the lead iodide scaffold. This extended penetration time promotes more complete grain growth and phase transformation. The cross-sectional image of the ethylammonium iodide-modified film showed a dense, compact layer with large grains spanning the film thickness and no obvious voids. Such a morphology is beneficial for forming ohmic contacts with adjacent layers, avoiding shunts caused by direct contact between the perovskite and the metal electrode, and improving overall device performance.
I used atomic force microscopy to quantify the surface roughness. The ethylammonium iodide-modified film had an average surface roughness of 31.4 nm, whereas the reference film had an average surface roughness of 39.2 nm. The smoother surface of the modified film is consistent with its larger grains and fewer grain boundaries. A rough perovskite surface can create poor interfacial contact with the electron transport layer or the electrode, which increases leakage current and contact resistance. Therefore, the reduced roughness in my modified film is beneficial for charge extraction and device reproducibility. The three-dimensional atomic force microscopy images further confirmed that the ethylammonium iodide film contained larger, more uniformly distributed grains. Since grain boundaries are major recombination sites, a smoother and more compact film with fewer boundaries should reduce interfacial leakage and non-radiative losses in perovskite solar cells.
| Perovskite Film | Average Roughness (nm) | Grain Size (μm) | Pinhole Density | Surface Quality |
|---|---|---|---|---|
| Without EAI | 39.2 | Small, sub-micron | High | Rough, many boundaries |
| With EAI | 31.4 | Up to ~1.5 | Low | Smooth, compact, large grains |
X-ray diffraction provided further insight into the crystallization behavior and phase evolution. Both the reference and ethylammonium iodide-modified films showed the characteristic three-dimensional perovskite diffraction peaks at 2θ = 14.2° and 28.4°, which correspond to the (110) and (220) planes, respectively. However, the reference film exhibited a strong peak at 12.7°, which is assigned to residual lead iodide. This residual lead iodide indicates that the interdiffusion between lead iodide and the organic ammonium salts was incomplete during film formation, leaving unreacted lead iodide at the bottom of the film. In the ethylammonium iodide-modified film, the lead iodide peak was significantly weaker, demonstrating that the modified perovskite film interacted more completely with the lead iodide scaffold and formed a more perfect cubic perovskite phase. Because excess lead iodide can impair device stability, this reduction is an important benefit. I also analyzed the ultraviolet-visible absorption spectra of the two films. The absorbance changed only slightly because the ethylammonium iodide amount was very small, but Tauc plot analysis revealed a small change in the optical bandgap. The reference film had a bandgap of 1.62 eV, whereas the ethylammonium iodide-modified film had a bandgap of 1.64 eV. This slight widening suggests that ethylammonium iodide was incorporated into the perovskite lattice and may contribute to a higher open-circuit voltage in the corresponding perovskite solar cells.
The Tauc relation I used to extract the optical bandgap can be written as
$$(\alpha h\nu)^2 = A(h\nu – E_g)$$
where α is the absorption coefficient, h is Planck’s constant, ν is the photon frequency, A is a proportionality constant, and Eg is the optical bandgap. A plot of (αhν)2 versus hν gives a linear region whose intercept with the energy axis provides Eg. The shift from 1.62 eV to 1.64 eV is small but consistent with the incorporation of ethylammonium iodide into the lattice and with the improved phase purity observed by X-ray diffraction.
| XRD Peak Position (2θ) | Assignment | Reference Film | EAI-Modified Film | Interpretation |
|---|---|---|---|---|
| 12.7° | PbI2 | Strong | Weak | Residual PbI2 reduced |
| 14.2° | (110) perovskite | Present | Present, stronger | Improved cubic phase |
| 28.4° | (220) perovskite | Present | Present, stronger | Higher crystallinity |
Charge Carrier Dynamics and Optical Properties
To investigate charge carrier dynamics, I measured time-resolved photoluminescence decay curves using a 479 nm laser and collected the emission at 790 nm. I fitted the decay curves with a double-exponential model and calculated the average lifetime using
$$\tau_{avg} = \frac{A_1\tau_1^2 + A_2\tau_2^2}{A_1\tau_1 + A_2\tau_2}$$
where τ1 is the fast decay time, τ2 is the slow decay time, and A1 and A2 are fitting constants. The reference film had an average lifetime of 2.93 μs. The ethylammonium iodide-modified film had an average lifetime of 3.76 μs. The longer lifetime indicates higher crystal quality, a lower defect density, and a reduced probability of defect-induced non-radiative recombination. For perovskite solar cells, a longer carrier lifetime is directly linked to a lower recombination rate and a higher open-circuit voltage. I also measured steady-state photoluminescence spectra. The ethylammonium iodide-modified film exhibited a higher photoluminescence intensity than the reference film. Because grain boundaries act as non-radiative recombination centers, a higher photoluminescence intensity is consistent with fewer grain boundaries and fewer defects. The enhanced emission suggests that non-radiative recombination was suppressed, which supports my conclusion that the ethylammonium iodide additive enlarged the grains and reduced the grain boundary defect density.
| Perovskite Film | A1 (%) | τ1 (μs) | A2 (%) | τ2 (μs) | τavg (μs) |
|---|---|---|---|---|---|
| Without EAI | 1.72 | 0.37 | 98.28 | 2.93 | 2.93 |
| With EAI | 5.52 | 1.38 | 94.48 | 3.81 | 3.76 |
In addition to the perovskite absorber, I modified the electron transport layer. I compared pure PCBM with a PCBM:ZrAcac blend at a mass ratio of 9:1. Both PCBM and ZrAcac are amorphous or nanocrystalline materials without obvious grain structures, but their film morphologies differed significantly. The pure PCBM film contained larger particles and was relatively rough. The PCBM:ZrAcac film was more uniform, smoother, and denser. This improvement in film-forming behavior is important because a rough electron transport layer can create poor interfacial contact with the perovskite, leading to leakage current and increased ohmic resistance. A denser and more uniform electron transport layer can form better ohmic contact with the perovskite, block direct contact between the electrode and the perovskite, prevent shunts, and improve electron extraction. I confirmed the improved electron extraction using photoluminescence quenching measurements. When PCBM was deposited on the perovskite, the perovskite photoluminescence was strongly quenched, indicating efficient electron extraction. When PCBM:ZrAcac was used instead, the quenching was even more pronounced, demonstrating that the blend extracted electrons more effectively and exhibited superior charge transport properties.
| Electron Transport Layer | Surface Morphology | Interfacial Contact | Electron Extraction | Leakage Current |
|---|---|---|---|---|
| Pure PCBM | Rough, large particles | Moderate | Good | Higher |
| PCBM:ZrAcac 9:1 | Uniform, smooth, dense | Excellent | Stronger | Lower |
Photovoltaic Performance of the Devices
I prepared three device configurations to isolate the effects of the two modifications. Device D1 was the reference: ITO/NiOx/perovskite without ethylammonium iodide/PCBM/Au. Device D2 used the ethylammonium iodide-modified perovskite but still used pure PCBM as the electron transport layer: ITO/NiOx/EAI-modified perovskite/PCBM/Au. Device D3 combined both modifications: ITO/NiOx/EAI-modified perovskite/PCBM:ZrAcac/Au. The current density-voltage characteristics showed a clear stepwise improvement. D1 had a short-circuit current density of 23.68 mA/cm2, an open-circuit voltage of 1.10 V, a fill factor of 75.17%, and a power conversion efficiency of 19.58%. D2 improved to 24.18 mA/cm2, 1.13 V, 76.49%, and 20.90%. D3 reached 24.37 mA/cm2, 1.15 V, 80.03%, and 22.43%. The most significant gains were in the open-circuit voltage and fill factor, which is consistent with reduced non-radiative recombination and improved charge extraction.
| Device | Perovskite | Electron Transport Layer | Voc (V) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|---|---|
| D1 | Reference | PCBM | 1.10 | 23.68 | 75.17 | 19.58 |
| D2 | EAI-modified | PCBM | 1.13 | 24.18 | 76.49 | 20.90 |
| D3 | EAI-modified | PCBM:ZrAcac | 1.15 | 24.37 | 80.03 | 22.43 |
I also measured the steady-state output of D3 at its maximum power point under a fixed bias of 0.99 V. The steady-state short-circuit current density and power conversion efficiency were 22.65 mA/cm2 and 22.42%, respectively. This result indicates that the device can deliver a stable power output over the measurement period and that the efficiency is not merely a transient artifact of the current density-voltage scan. For perovskite solar cells, stable maximum-power-point output is a more reliable indicator of practical performance than a single scan.
To understand the origin of the improved fill factor and open-circuit voltage, I analyzed the current density-voltage curves using a single-diode model. The current density-voltage relationship can be written as
$$J = J_{sc} – J_0 \left\{ \exp\left[\frac{q(V+JR_s)}{mk_BT}\right] – 1 \right\}$$
where J0 is the reverse saturation current density, q is the elementary charge, Rs is the series resistance, m is the diode ideality factor, kB is the Boltzmann constant, and T is the thermodynamic temperature. Neglecting the constant term, I obtained
$$-\frac{dV}{dJ} = \frac{mk_BT}{q}(J_{sc}-J)^{-1} + R_s$$
and
$$\ln(J_{sc}-J) = \frac{q}{mk_BT}(V+JR_s) + \ln J_0$$
At open circuit, J = 0 and V = Voc, so
$$\ln J_{sc} = \frac{q}{mk_BT}V_{oc} + \ln J_0$$
and therefore
$$V_{oc} = \frac{mk_BT}{q}\ln L$$
where L is the light intensity. I fitted the (Jsc – J)-1 versus -dV/dJ curves to extract the series resistance. The series resistances of D1, D2, and D3 were 1.53, 1.17, and 0.55 Ω cm2, respectively. The series resistance includes contact resistance between layers, bulk resistance, and electrode resistance. The lower series resistance of D3 indicates that the large-grain perovskite reduced the bulk resistance and that the PCBM:ZrAcac layer formed a better contact with the perovskite and the electrode. I also fitted the V + JRs versus ln(Jsc – J) curves to extract the reverse saturation current density. The values for D1, D2, and D3 were 3.14 × 10-6, 1.85 × 10-6, and 3.12 × 10-7 mA/cm2, respectively. The reverse saturation current density of D3 was one order of magnitude lower than that of D1. Because defects, impurities, and grain boundaries act as recombination centers that promote electron-hole recombination and generate reverse saturation current, a lower J0 indicates suppressed charge recombination. Under similar short-circuit current density, a lower J0 leads to a higher open-circuit voltage, which explains the Voc improvement in D3.
| Device | Rs (Ω cm2) | J0 (mA/cm2) | m | Dominant Recombination |
|---|---|---|---|---|
| D1 | 1.53 | 3.14 × 10-6 | 1.59 | Defect-assisted non-radiative |
| D2 | 1.17 | 1.85 × 10-6 | 1.39 | Reduced non-radiative |
| D3 | 0.55 | 3.12 × 10-7 | 1.02 | Nearly ideal, diffusion-limited |
The diode ideality factor is a useful parameter for diagnosing the dominant recombination mechanism in perovskite solar cells. When m is close to 2, non-radiative recombination dominates. When m is close to 1, diffusion current dominates and the device behaves more like an ideal diode with free-charge radiative recombination. In real devices, the depletion region often contains recombination centers, so m typically lies between 1 and 2. I extracted m from the light-intensity dependence of the open-circuit voltage. The ideality factors of D1, D2, and D3 were 1.59, 1.39, and 1.02, respectively. The lower m of D3 confirms that the optimized perovskite and electron transport layer effectively passivated defect states and suppressed non-radiative recombination. Because the fill factor depends on J0, Rs, m, and Jsc, reducing J0 and Rs while keeping Jsc nearly constant naturally improves the fill factor. This explains the internal mechanism behind the enhanced fill factor in D3.
I further examined dark current density-voltage characteristics. In the low-voltage region, the dark current follows Ohm’s law and corresponds to leakage current. In the high-voltage region, it follows an exponential diode behavior. The leakage current followed the trend D3 < D2 < D1. Compared with D1, D2 had fewer pinholes and larger grains, which suppressed leakage current. D3, with its denser electron transport layer and better ohmic contact, exhibited the lowest leakage current and therefore the largest shunt resistance, leading to higher efficiency. This result is fully consistent with the morphology and photoluminescence data.
| Device | Leakage Current Trend | Shunt Resistance | Ohmic Contact Quality | PCE (%) |
|---|---|---|---|---|
| D1 | Highest | Lowest | Poor | 19.58 |
| D2 | Intermediate | Intermediate | Improved | 20.90 |
| D3 | Lowest | Highest | Excellent | 22.43 |
Optimization of EAI Content and ZrAcac Ratio
I optimized the ethylammonium iodide content in the organic ammonium salt solution by varying the amount from 1 mg to 4 mg. When the ethylammonium iodide amount increased from 1 mg to 3 mg, the open-circuit voltage, short-circuit current density, and fill factor all increased, and the power conversion efficiency improved from 19.45% to 20.90%. When the amount increased further to 4 mg, the fill factor decreased and the efficiency dropped to 20.09%. This indicates that an appropriate amount of ethylammonium iodide improves perovskite film quality, while an excessive amount may introduce defects or degrade interfacial contact. I also optimized the PCBM:ZrAcac mass ratio using ratios of 19:1, 9:1, and 17:3. The 9:1 ratio produced the best overall performance, with an open-circuit voltage of 1.15 V, a short-circuit current density of 24.37 mA/cm2, a fill factor of 80.03%, and a power conversion efficiency of 22.43%. This demonstrates that an appropriate ZrAcac fraction improves the film quality and interfacial contact of the electron transport layer in perovskite solar cells. Taken together, the optimization of ethylammonium iodide content and PCBM:ZrAcac ratio jointly promoted the significant efficiency improvement.
| EAI Amount (mg) | Voc (V) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 1 | 1.11 | 23.55 | 74.41 | 19.45 |
| 2 | 1.13 | 23.67 | 76.72 | 20.52 |
| 3 | 1.13 | 24.18 | 76.49 | 20.90 |
| 4 | 1.14 | 24.25 | 72.67 | 20.09 |
| PCBM:ZrAcac Mass Ratio | Voc (V) | Jsc (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 19:1 | 1.14 | 24.53 | 79.03 | 22.10 |
| 9:1 | 1.15 | 24.37 | 80.03 | 22.43 |
| 17:3 | 1.13 | 24.43 | 75.82 | 20.93 |
I also performed a statistical analysis of multiple batches of devices to assess reproducibility. The short-circuit current density varied only slightly among D1, D2, and D3. D1 had low open-circuit voltage and fill factor, which led to the lowest power conversion efficiency. Doping with ethylammonium iodide in D2 significantly improved both the open-circuit voltage and the fill factor, resulting in a higher efficiency. D3 reached approximately 22%, clearly higher than D1 at approximately 19% and D2 at approximately 20%, mainly because of the improvements in open-circuit voltage and fill factor. This statistical distribution confirms that the improvements are reproducible and arise from the combined optimization of the perovskite film and the electron transport layer.
| Device | Average Jsc (mA/cm2) | Average Voc (V) | Average FF (%) | Average PCE (%) |
|---|---|---|---|---|
| D1 | 23.6 ± 0.2 | 1.09 ± 0.02 | 74.8 ± 1.0 | 19.3 ± 0.4 |
| D2 | 24.1 ± 0.2 | 1.12 ± 0.02 | 76.1 ± 0.8 | 20.7 ± 0.4 |
| D3 | 24.3 ± 0.2 | 1.14 ± 0.01 | 79.6 ± 0.7 | 22.1 ± 0.4 |
Stability of the Modified Devices
I evaluated the long-term stability of unencapsulated devices stored in ambient air at room temperature of 25 ± 4 °C and relative humidity of 30% to 40% in the dark. D1 and D3 were stored and tested side by side to eliminate environmental variables. Over 30 days, D3 exhibited a slower efficiency decay and retained approximately 90% of its initial power conversion efficiency. In contrast, D1 decayed more rapidly in the early stage and eventually declined to approximately 80% of its initial efficiency. The improved stability of D3 is attributed to the higher film quality and the effective passivation of interfacial defects, which delay the degradation process. For perovskite solar cells, suppressing defects and improving film compactness are critical not only for efficiency but also for operational lifetime. The combination of ethylammonium iodide and ZrAcac therefore offers a dual benefit: higher efficiency and better stability.
| Device | Storage Condition | Initial PCE (%) | PCE After 30 Days (%) | Retention (%) |
|---|---|---|---|---|
| D1 | Ambient air, dark, 25 ± 4 °C, 30%–40% RH | 19.58 | 15.66 | ~80 |
| D3 | Ambient air, dark, 25 ± 4 °C, 30%–40% RH | 22.43 | 20.19 | ~90 |
Mechanistic Discussion
The improvements observed in my experiments can be explained by the complementary roles of ethylammonium iodide and ZrAcac. In the two-step deposition process, the lead iodide film is first formed, and the organic ammonium salts must diffuse into the lead iodide scaffold to convert it into the perovskite phase. Ethylammonium iodide, when added to the organic ammonium salt solution, delays the volatilization of the organic species and extends the penetration time. This allows a more complete reaction between lead iodide and the organic ammonium salts, which reduces residual lead iodide and promotes the formation of a cubic perovskite phase with larger grains. Larger grains mean fewer grain boundaries, and because grain boundaries are the main sites of defect-assisted recombination, the non-radiative recombination rate decreases. The longer carrier lifetime and higher photoluminescence intensity that I measured directly support this mechanism. The slight widening of the optical bandgap also suggests that ethylammonium iodide modifies the lattice and may contribute to a higher open-circuit voltage.
On the electron transport layer side, pure PCBM can suffer from aggregation, rough surface morphology, and imperfect contact with the perovskite. These problems increase series resistance and leakage current in perovskite solar cells. When ZrAcac is blended with PCBM at a mass ratio of 9:1, the film becomes more uniform, smoother, and denser. The improved morphology leads to better interfacial contact with the perovskite and the electrode, more efficient electron extraction, and lower leakage current. The photoluminescence quenching results show that PCBM:ZrAcac extracts electrons more strongly than pure PCBM. The electrical analysis shows that the series resistance decreases from 1.53 Ω cm2 for D1 to 0.55 Ω cm2 for D3, and the reverse saturation current density decreases from 3.14 × 10-6 mA/cm2 to 3.12 × 10-7 mA/cm2. The diode ideality factor decreases from 1.59 to 1.02, indicating that the dominant recombination mechanism shifts from defect-assisted non-radiative recombination toward a more ideal diode behavior. These electrical improvements explain the significant gains in open-circuit voltage and fill factor.
The synergy between the two modifications is essential. The ethylammonium iodide-modified perovskite provides a high-quality absorber with fewer defects and larger grains, which reduces bulk recombination and improves charge transport. The PCBM:ZrAcac electron transport layer provides a dense, uniform, and well-contacted extraction layer, which reduces interfacial recombination and leakage current. When both modifications are combined, the device benefits from both bulk and interfacial improvement. This is why D3 outperforms D1 and D2 in every photovoltaic parameter and why the power conversion efficiency reaches 22.43%. The fact that the steady-state maximum-power-point efficiency of D3 is 22.42% further confirms that the improvement is stable and not an artifact of the scan.
| Parameter | D1 Reference | D2 EAI Only | D3 EAI + ZrAcac | Improvement Mechanism |
|---|---|---|---|---|
| Voc (V) | 1.10 | 1.13 | 1.15 | Reduced non-radiative recombination |
| Jsc (mA/cm2) | 23.68 | 24.18 | 24.37 | Better light absorption and transport |
| FF (%) | 75.17 | 76.49 | 80.03 | Lower Rs and J0 |
| PCE (%) | 19.58 | 20.90 | 22.43 | Synergistic bulk and interface optimization |
| Rs (Ω cm2) | 1.53 | 1.17 | 0.55 | Improved contact and lower bulk resistance |
| J0 (mA/cm2) | 3.14 × 10-6 | 1.85 × 10-6 | 3.12 × 10-7 | Suppressed charge recombination |
| m | 1.59 | 1.39 | 1.02 | Closer to ideal diode behavior |
I also considered the role of the NiOx hole transport layer. NiOx is a wide-bandgap p-type semiconductor that forms a favorable energy alignment with the perovskite absorber. In my devices, the NiOx layer was prepared from solution-processed nanoparticles and spin-coated onto ultraviolet-ozone-treated ITO. The ultraviolet-ozone treatment improves wettability and adjusts the work function, which helps to match the energy levels with the perovskite. Although the main focus of this study was the perovskite absorber and the electron transport layer, the quality of the hole transport layer also influences the overall device performance. The large-grain perovskite film with a smooth surface allows better contact with NiOx, which reduces interfacial recombination and improves hole extraction. This is another reason why the dual modification strategy works well in the inverted architecture.
| Layer | Function | Key Requirement | Modification in This Work |
|---|---|---|---|
| ITO | Transparent electrode | High transparency, low resistance | Ultraviolet-ozone treatment |
| NiOx | Hole transport layer | Good energy alignment, compact film | Solution-processed nanoparticles |
| Perovskite | Light absorber | Large grains, low defects, smooth surface | EAI additive in organic salt precursor |
| PCBM:ZrAcac | Electron transport layer | Dense, uniform, good contact | 9:1 blend in 2-DCB |
| Au | Back electrode | Low resistance, stable contact | Thermal evaporation, 100 nm |
The implications of my findings for the broader field of perovskite solar cells are significant. First, the use of a small amount of ethylammonium iodide as an additive is a simple and low-cost method to regulate crystal growth in two-step deposition. It does not require complex processing or expensive materials. Second, the PCBM:ZrAcac blend provides a straightforward way to improve the electron transport layer without introducing a completely new material. Third, the combination of the two modifications demonstrates that bulk and interface engineering should be considered together. Many studies focus on either the perovskite or the transport layer, but my results show that the best performance comes from optimizing both. The power conversion efficiency of 22.43% is among the higher values reported for inverted perovskite solar cells with PCBM-based electron transport layers, and the stability retention of approximately 90% after 30 days in ambient air is encouraging.
In terms of future work, I plan to investigate the thermal stability of the ethylammonium iodide-modified films at higher temperatures and to test the devices under continuous illumination at maximum power point for longer periods. I also intend to study the optimal ZrAcac fraction more systematically, perhaps using a wider range of mass ratios and different metal acetylacetonates. In addition, I would like to apply the same dual modification strategy to larger-area perovskite solar cells and modules, where uniformity and defect control become even more critical. The scalability of the two-step deposition method and the solution-processed electron transport layer makes this strategy attractive for practical manufacturing. Because the modifications are compatible with roll-to-roll processing and low-temperature fabrication, they may help accelerate the commercialization of perovskite solar cells.
| Challenge in Perovskite Solar Cells | Consequence | My Solution | Observed Result |
|---|---|---|---|
| Small grains and high grain boundary density | Non-radiative recombination | EAI additive | Larger grains, longer lifetime |
| Residual PbI2 | Instability and defects | EAI-assisted interdiffusion | Weaker PbI2 XRD peak |
| Rough perovskite surface | Poor contact, leakage | EAI-modified film | Ra reduced from 39.2 to 31.4 nm |
| Rough PCBM film | High series resistance | ZrAcac blending | Dense, smooth ETL |
| Interfacial recombination | Low Voc and FF | PCBM:ZrAcac 9:1 | Voc 1.15 V, FF 80.03% |
| Leakage current | Low shunt resistance | Dense ETL and large grains | Lowest dark current in D3 |
To summarize the electrical analysis, I can express the relationship between the open-circuit voltage and the reverse saturation current density as
$$V_{oc} = \frac{mk_BT}{q}\ln\left(\frac{J_{sc}}{J_0}\right)$$
This equation shows that for a given short-circuit current density and ideality factor, reducing J0 directly increases Voc. In my devices, J0 decreased by an order of magnitude from D1 to D3, which is fully consistent with the observed increase in Voc from 1.10 V to 1.15 V. Similarly, the fill factor can be approximated by empirical expressions that depend on Rs and J0. Lower Rs and lower J0 both contribute to a higher fill factor, as observed in my experiments. The ideality factor of 1.02 for D3 indicates that the device is close to the ideal diode limit, meaning that defect-assisted recombination has been largely suppressed. This is a strong indication that the dual modification strategy is effective at both the bulk and interface levels.
| Electrical Parameter | Symbol | D1 | D2 | D3 | Trend |
|---|---|---|---|---|---|
| Series resistance | Rs | 1.53 Ω cm2 | 1.17 Ω cm2 | 0.55 Ω cm2 | Decreases |
| Reverse saturation current | J0 | 3.14 × 10-6 mA/cm2 | 1.85 × 10-6 mA/cm2 | 3.12 × 10-7 mA/cm2 | Decreases |
| Ideality factor | m | 1.59 | 1.39 | 1.02 | Approaches 1 |
| Open-circuit voltage | Voc | 1.10 V | 1.13 V | 1.15 V | Increases |
| Fill factor | FF | 75.17% | 76.49% | 80.03% | Increases |
| Power conversion efficiency | PCE | 19.58% | 20.90% | 22.43% | Increases |
Another important observation is that the short-circuit current density increased only modestly from 23.68 to 24.37 mA/cm2, while the open-circuit voltage and fill factor increased substantially. This means that the main gains came from reduced recombination and improved charge extraction rather than from a large increase in light absorption. The ultraviolet-visible absorption spectra showed that the absorbance changed little because the ethylammonium iodide amount was small. Therefore, the efficiency improvement is primarily an electrical and interfacial effect, not an optical one. This is consistent with the morphology, photoluminescence, and diode analysis. For perovskite solar cells, this is an important distinction because it shows that defect passivation and interface engineering can provide significant performance gains even when the optical absorption remains nearly unchanged.
I also want to emphasize the reproducibility of the results. The statistical analysis of multiple batches showed that the average power conversion efficiency of D3 was approximately 22.1% with a standard deviation of about 0.4%, while D1 averaged approximately 19.3% and D2 averaged approximately 20.7%. The narrow distribution indicates that the fabrication process is reliable and that the modifications do not introduce excessive variability. This is important for practical applications because high efficiency must be accompanied by high yield and reproducibility. The fact that the same trends were observed across multiple batches strengthens my confidence in the conclusions.
| Device | Best PCE (%) | Average PCE (%) | Standard Deviation (%) | Reproducibility |
|---|---|---|---|---|
| D1 | 19.58 | 19.3 | 0.4 | Good |
| D2 | 20.90 | 20.7 | 0.4 | Good |
| D3 | 22.43 | 22.1 | 0.4 | Good |
Conclusion
I have demonstrated that the synergistic modification of the perovskite absorber with ethylammonium iodide and the electron transport layer with a PCBM:ZrAcac blend significantly improves the performance of inverted perovskite solar cells. The ethylammonium iodide additive promotes the complete reaction between lead iodide and the organic ammonium salts, leading to larger grains, reduced residual lead iodide, a smoother surface, a longer carrier lifetime, and stronger photoluminescence. The PCBM:ZrAcac blend forms a denser, more uniform electron transport layer with better interfacial contact, stronger electron extraction, and lower leakage current. The combination of these effects reduces both bulk and interfacial non-radiative recombination. The reference device achieved a power conversion efficiency of 19.58%, while the dual-modified device achieved 22.43%, with the open-circuit voltage increasing from 1.10 V to 1.15 V and the fill factor increasing from 75.17% to 80.03%. The series resistance decreased from 1.53 to 0.55 Ω cm2, the reverse saturation current density decreased from 3.14 × 10-6 to 3.12 × 10-7 mA/cm2, and the ideality factor decreased from 1.59 to 1.02. The dual-modified device also retained approximately 90% of its initial efficiency after 30 days of storage in ambient air. These results show that combining bulk crystal growth control with interface engineering is a powerful strategy for fabricating high-efficiency and stable perovskite solar cells. I believe that this approach can be extended to other perovskite compositions and device architectures, and that it provides a practical route toward the commercialization of perovskite solar cells.
| Metric | Reference Device | Dual-Modified Device | Relative Change |
|---|---|---|---|
| Power conversion efficiency | 19.58% | 22.43% | +14.6% |
| Open-circuit voltage | 1.10 V | 1.15 V | +4.5% |
| Short-circuit current density | 23.68 mA/cm2 | 24.37 mA/cm2 | +2.9% |
| Fill factor | 75.17% | 80.03% | +6.5% |
| Series resistance | 1.53 Ω cm2 | 0.55 Ω cm2 | -64.1% |
| Reverse saturation current | 3.14 × 10-6 mA/cm2 | 3.12 × 10-7 mA/cm2 | -90.1% |
| Ideality factor | 1.59 | 1.02 | -35.8% |
| 30-day efficiency retention | ~80% | ~90% | +12.5% |
In my future research, I will continue to explore the fundamental mechanisms of defect passivation in perovskite solar cells and to develop new materials and processing strategies that can further improve efficiency, stability, and scalability. The results presented here provide a solid foundation for further optimization and demonstrate that careful control of both the absorber and the electron transport layer is essential for achieving high-performance perovskite solar cells.