In recent years, perovskite solar cells have emerged as a promising technology for next-generation photovoltaics due to their high efficiency, low cost, and ease of fabrication. The electron transport layer plays a critical role in the performance of perovskite solar cells, as it facilitates charge separation and electron extraction. Among various nanostructures, one-dimensional arrays such as ZnO nanorod arrays offer advantages like high surface area and direct radial electron transport pathways, which can enhance the photovoltaic properties of perovskite solar cells. In this study, we focus on optimizing the electron transport layer by incorporating Nb-doped TiO2 compact layers and ZnO nanorod arrays to improve the efficiency and stability of perovskite solar cells.
The development of perovskite solar cells has seen rapid progress, with power conversion efficiencies exceeding 25% in recent years. However, challenges such as charge recombination and instability under ambient conditions remain. The electron transport layer is crucial for addressing these issues, as it affects electron injection and transport. ZnO nanorod arrays, with their high electron mobility and ordered structure, provide an ideal platform for enhancing charge separation in perovskite solar cells. Additionally, the compactness of the underlying TiO2 layer is vital to prevent shunting and ensure efficient electron collection. By doping TiO2 with Nb, we aim to improve its compactness and electronic properties, thereby boosting the overall performance of perovskite solar cells.
In our experimental approach, we first prepared Nb-doped TiO2 compact layers on FTO conductive substrates using a hydrolysis-pyrolysis method. The precursor solution consisted of titanium isopropoxide and NbCl5 in isopropanol, with a molar ratio of Nb/Ti = 2.5%. This solution was spin-coated onto FTO substrates at 2000 rpm for 60 seconds, followed by annealing at 500°C for 30 minutes. The resulting Nb-TiO2 compact layers exhibited improved smoothness and density compared to undoped TiO2, as confirmed by scanning electron microscopy. This enhancement is critical for preventing the infiltration of subsequent layers and minimizing charge recombination in perovskite solar cells.
Next, we deposited ZnO seed layers on the Nb-TiO2 compact layers using zinc acetate dihydrate solutions in ethanol at concentrations of 10 mM, 30 mM, and 90 mM. The spin-coating parameters were optimized to achieve uniform coverage, with multiple coatings applied for lower concentrations to maintain a consistent total amount of precursor. For instance, the 10 mM solution was spin-coated 9 times, the 30 mM solution 3 times, and the 90 mM solution once, all at 2000 rpm for 20 seconds, followed by annealing at 200°C for 20 minutes. These seed layers served as the foundation for the hydrothermal growth of ZnO nanorod arrays.
The ZnO nanorod arrays were synthesized via a hydrothermal method at 120°C for 75 minutes using an aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine. After growth, the arrays were annealed at 450°C for 30 minutes to improve crystallinity. We investigated the influence of seed layer concentration on the morphology of the ZnO nanorod arrays, including their diameter, length, and areal density. The structural and optical properties were characterized using X-ray diffraction and UV-visible spectroscopy, which confirmed the formation of hexagonal wurtzite ZnO with preferential growth along the (002) plane and absorption edges around 380 nm and 410 nm for Nb-TiO2 and ZnO, respectively.
The perovskite active layer was deposited using a two-step sequential method. First, a PbI2·DMSO precursor solution (1.70 M) was spin-coated onto the ZnO nanorod arrays at 3000 rpm for 20 seconds. Then, a mixed methylammonium halide solution (CH3NH3I and CH3NH3Br in a molar ratio of 85:15, total concentration 0.465 M in isopropanol) was applied, followed by spin-coating at 5000 rpm for 30 seconds and annealing at 140°C for 10 minutes. This resulted in a smooth, black CH3NH3PbI3-xBrx perovskite film. The hole transport layer was formed by spin-coating a spiro-OMeTAD solution (72.3 mg in 1 mL chlorobenzene, with additives including 4-tert-butylpyridine, LiTFSI, and FK209 Co(III)-TFSI) at 4000 rpm for 30 seconds. Finally, a gold electrode (~60 nm) was thermally evaporated to complete the perovskite solar cell device with the structure FTO/Nb-TiO2/ZnO seed layer/ZnO nanorod array/CH3NH3PbI3-xBrx/spiro-OMeTAD/Au.
We evaluated the photovoltaic performance of the devices under standard AM1.5G illumination (100 mW/cm²). The key parameters, including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE), were measured for both planar and nanorod-based perovskite solar cells. The introduction of ZnO nanorod arrays significantly enhanced Jsc due to improved charge separation and electron transport. The efficiency of perovskite solar cells can be expressed by the equation:
$$ \eta = \frac{J_{sc} \times V_{oc} \times FF}{P_{in}} $$
where Pin is the incident light power density. For our optimized devices, we achieved a PCE of up to 13.97%, demonstrating the effectiveness of the ZnO nanorod arrays in perovskite solar cells.
The morphological properties of the ZnO nanorod arrays were analyzed based on the seed layer conditions. The table below summarizes the diameter and areal density of ZnO nanorod arrays grown on different seed layers:
| Seed Layer Type | Diameter (nm) | Areal Density (μm⁻²) |
|---|---|---|
| 10 mM-9 | 43 | 225 |
| 30 mM-3 | 40 | 235 |
| 90 mM-1 | 38 | 287 |
As shown, the 90 mM-1 seed layer yielded the smallest diameter (38 nm) and highest areal density (287 μm⁻²), which contributed to a larger interfacial area for charge separation in perovskite solar cells. This optimization is crucial for enhancing the performance of perovskite solar cells, as it directly affects electron injection and collection efficiency.
The electronic properties of the ZnO nanorod arrays can be described by the electron mobility (μ), which is related to the drift velocity (vd) and electric field (E) as follows:
$$ \mu = \frac{v_d}{E} $$
In perovskite solar cells, high electron mobility in ZnO facilitates rapid electron transport, reducing recombination losses. The Nb doping in TiO2 further improves the compactness, which can be quantified by the film density. The effective mass of electrons in ZnO is given by:
$$ m^* = \frac{\hbar^2}{2E} \left( \frac{d^2E}{dk^2} \right)^{-1} $$
where ħ is the reduced Planck’s constant, E is the energy, and k is the wave vector. These parameters influence the overall efficiency of perovskite solar cells.
We also studied the optical absorption of the ZnO nanorod arrays using UV-visible spectroscopy. The absorption coefficient (α) can be calculated using the Tauc relation for direct bandgap materials:
$$ (\alpha h\nu)^2 = A (h\nu – E_g) $$
where hν is the photon energy, Eg is the bandgap, and A is a constant. For ZnO, Eg is approximately 3.2 eV, which aligns with our observations. This strong absorption in the UV region supports the use of ZnO nanorod arrays in perovskite solar cells for efficient light harvesting.
The photovoltaic performance parameters of the fabricated perovskite solar cells are compared in the table below:
| Device Structure | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| Planar (without ZnO array) | 0.89 ± 0.02 | 19.16 ± 0.39 | 71.35 ± 3.81 | 11.44 ± 0.92 |
| With ZnO nanorod array | 0.89 ± 0.02 | 21.28 ± 0.63 | 71.11 ± 1.56 | 13.72 ± 0.87 |
The data clearly indicate that the incorporation of ZnO nanorod arrays leads to a significant improvement in Jsc and PCE, underscoring their role in enhancing the performance of perovskite solar cells. The fill factor and open-circuit voltage remained relatively stable, suggesting minimal recombination in the optimized devices.
To further understand the charge transport dynamics in perovskite solar cells, we can consider the diode equation under illumination:
$$ J = J_0 \left( \exp\left(\frac{qV}{nkT}\right) – 1 \right) – J_{sc} $$
where J0 is the reverse saturation current, q is the electron charge, V is the voltage, n is the ideality factor, k is Boltzmann’s constant, and T is the temperature. In perovskite solar cells with ZnO nanorod arrays, the reduced series resistance and enhanced shunt resistance contribute to higher fill factors and efficiencies.
The stability of perovskite solar cells is another critical aspect. We tested the devices under ambient conditions with approximately 55% relative humidity. The ZnO nanorod arrays, combined with the Nb-TiO2 compact layer, helped maintain performance over time by reducing moisture ingress and charge recombination. This highlights the potential of such structures for developing durable perovskite solar cells.
In conclusion, our research demonstrates that Nb-doped TiO2 compact layers and ZnO nanorod arrays significantly enhance the efficiency of perovskite solar cells. The optimized morphology of the ZnO nanorod arrays, achieved through seed layer control, improves charge separation and electron transport. We achieved a maximum power conversion efficiency of 13.97% under ambient conditions, showcasing the promise of this approach for advancing perovskite solar cell technology. Future work will focus on further optimizing the interface engineering and scaling up the fabrication process for commercial applications of perovskite solar cells.
The integration of nanostructured electron transport layers like ZnO nanorod arrays opens new avenues for high-performance perovskite solar cells. By continuing to refine materials and interfaces, we can overcome existing limitations and push the boundaries of photovoltaic efficiency and stability in perovskite solar cells.