Simulated Lead-Free Perovskite Solar Cells Reach 28.6% Efficiency
The study, published in Frontiers of Optoelectronics, focuses on a lead-free design that eliminates the need for traditional hole and electron transport layers, thereby simplifying the device structure. By utilizing the wxAMPS simulation software, the team investigated the photoelectric characteristics of an inverted p-p-n planar heterojunction structure comprising triple Cs3Bi2I9, single MASnI3, and double Cs2TiBr6 absorber layers. The primary objective was to determine whether a multi-absorber configuration could enhance carrier extraction and broaden the absorption spectrum compared to standard two-absorber architectures, ultimately leading to higher efficiency in a non-toxic photovoltaic system.
The motivation for this research stems from the environmental and stability concerns associated with lead-based perovskite solar cells. While lead-based PSCs have achieved high efficiencies exceeding 26%, the chemical instability and toxicity of lead pose significant barriers to commercialization. Substituting lead with tin, as in MASnI3, offers a non-toxic alternative with a narrow band gap that extends optical absorption to 1050 nm. However, tin-based materials suffer from the oxidation of Sn2+ to Sn4+, which creates defects and increases carrier recombination. The proposed design addresses this by using Cs3Bi2I9 and Cs2TiBr6 as stabilizing layers that may act similarly to antioxidant additives, reducing the oxidation of the central MASnI3 layer while maintaining high light absorption properties.
The simulation methodology employed the wxAMPS software under AM 1.5G solar spectrum illumination at a temperature of 300 K. The device architecture consists of a transparent conductive oxide (FTO) electrode, a gold photocathode, and the three perovskite absorber layers. The crystal structures of the materials were defined as hexagonal for Cs3Bi2I9, cubic for MASnI3, and cubic for Cs2TiBr6. The energy level diagram revealed negative conduction band offsets of -0.77 eV and -0.28 eV at the Cs3Bi2I9/MASnI3 and MASnI3/Cs2TiBr6 interfaces, respectively. These offsets facilitate the transmission of electrons from the Cs3Bi2I9 layer to the photocathode and holes from the Cs2TiBr6 layer to the FTO, enabling efficient charge separation without the need for additional transport layers.
Initial comparisons between two-absorber and three-absorber configurations showed that the triple Cs3Bi2I9/single MASnI3/double Cs2TiBr6 structure outperformed the alternatives. However, the three-absorber configuration achieved a significantly higher short-circuit current density (Jsc) of 19.9235 mA/cm2, compared to 14.6741 mA/cm2 for the MASnI3/Cs2TiBr6 device and 12.4054 mA/cm2 for the Cs3Bi2I9/Cs2TiBr6 device. This enhancement in Jsc is due to the wider photon absorption range provided by the MASnI3 layer and the improved hole extraction from the valence band.
The study further compared two heterojunction structures: p-type-Cs3Bi2I9/p-type-MASnI3/n-type-Cs2TiBr6 (p-p-n) and p-type-Cs3Bi2I9/n-type-MASnI3/n-type-Cs2TiBr6 (p-n-n). The p-p-n structure demonstrated superior performance in terms of Voc and fill factor (FF), despite having a lower Jsc than the p-n-n configuration. The p-n-n structure achieved a higher Jsc of 22.3157 mA/cm2 due to a more favorable electric field direction for hole transport. However, the p-p-n structure exhibited less carrier recombination and higher carrier lifetime within the absorber layers, resulting in a higher overall PCE.
To optimize the material composition, the researchers evaluated various double perovskite materials, including Cs2TiBr6, Cs2TiF6, Cs2TiCl6, and Cs2TiI6. The Cs2TiF6 material emerged as the optimal choice, yielding a PCE of 16.1736% in the baseline three-absorber configuration. This improvement over the Cs2TiBr6-based device is attributed to ideal band alignment at the p-type-MASnI3/n-type-Cs2TiF6 interface and lower carrier recombination losses.
Thickness optimization of the absorber layers revealed that the thickness of the MASnI3 layer significantly impacts device performance. The total thickness of the three absorber layers was maintained at 0.6 µm, with seven different combinations tested. Case C, featuring a MASnI3 thickness of 0.4 µm, achieved the highest PCE of 21.4530%. This enhancement is due to the narrow band gap of MASnI3, which allows for a wider range of photon absorption, and the comprehensive coverage of the Cs3Bi2I9 layer, which prevents direct contact between the Cs2TiF6 and Cs3Bi2I9 layers. This configuration reduces series resistance and avoids leakage currents, leading to improved efficiency. Thinner MASnI3 layers resulted in lower Voc and Jsc due to increased defects and carrier recombination in the wider band gap layers.
Doping concentration optimization further enhanced the device performance. Increasing the Cs3Bi2I9 acceptor doping concentration to E15 cm-3 improved the Jsc and Voc by narrowing the depletion layer width and facilitating carrier movement. The Cs2TiF6 donor doping concentration was set to 1.5E19 cm-3, which maximized the Voc, FF, and PCE by increasing the electron concentration and electric field strength at the MASnI3/Cs2TiF6 interface. These optimized doping densities resulted in the highest-performance device with a PCE of 28.6193%.
The simulation results highlight the potential of CTLs-free PSCs with multi-absorber layers for future photovoltaic applications. The elimination of charge transport layers simplifies the fabrication process and reduces costs, while the lead-free composition addresses environmental concerns. The study demonstrates that the spatial distribution of internal carriers and the charge transport mechanism can be effectively managed through the strategic selection of absorber materials, thicknesses, and doping concentrations. The p-p-n heterojunction structure, combined with the optimized Cs2TiF6 double perovskite material, provides a robust framework for achieving high efficiency in non-toxic perovskite solar cells.
Beyond simplifying device design, eliminating traditional charge transport layers avoids detrimental factors such as perovskite degradation and stability limits. Conventional electron transport layers such as titanium dioxide, tin oxide, and zinc oxide introduce notable manufacturing and operational challenges. Titanium dioxide requires high post-annealing temperatures that increase fabrication complexity, whereas immediate contact with zinc oxide or tin oxide spawns degradation of the perovskite absorber layer. Concurrently, standard hole transport materials, including Spiro-OMeTAD and PEDOT:PSS, require higher preparation costs and are subject to stability limits. Removing both transport layers avoids these detrimental factors, while the inverted configuration offers superior fixity and avoids evident current density–voltage hysteresis effects.
The underlying physical modeling in wxAMPS accounts for intra-band tunneling and trap-assisted tunneling by coupling Poisson’s equation with electron and hole continuity equations. Across all functional layers, the simulation specified equal electron and hole thermal velocities of 10^7 cm/s. Defect states were modeled using neutral Gaussian distributions positioned at the center of the band gap, each assigned carrier capture cross sections of 10^-14 cm^2. Beyond the conduction band steps, the heterojunction interfaces create favorable valence band offsets of -0.15 eV between Cs3Bi2I9 and MASnI3, and -0.42 eV between MASnI3 and Cs2TiBr6. These negative energy offsets establish an energetic pathway that assists the transport of photogenerated holes through the absorber stack toward the transparent conductive oxide electrode.
Analysis of spectral response and internal electrostatic potentials highlights how the multi-absorber architecture expands photon harvesting. In two-absorber cells pairing Cs3Bi2I9 with Cs2TiBr6, the external quantum efficiency response cuts off around 770 nm. Introducing the narrow band gap MASnI3 layer broadens absorbance into the near-infrared spectrum to approximately 870 nm, providing the highest external quantum efficiency across visible wavelengths. Furthermore, the calculated built-in potential, determined by the conduction band difference across the entire absorber stack, reached 1.48 V in the three-layer device, compared to 1.46 V for Cs3Bi2I9/Cs2TiBr6 and 1.34 V for MASnI3/Cs2TiBr6. While an excessive band offset at the Cs3Bi2I9 and Cs2TiBr6 interface hampers carrier transit in the two-absorber variant, the three-absorber stack maintains a strong internal field that drives charge separation.
Subject of Research: Design and simulation investigations on charge transport layers-free in lead-free three absorber layer all-perovskite solar cells
Article Title: Design and simulation investigations on charge transport layers-free in lead-free three absorber layer all-perovskite solar cells
Article References: Li, G., Xu, M., & Chen, Z. (2024). Design and simulation investigations on charge transport layers-free in lead-free three absorber layer all-perovskite solar cells. Frontiers of Optoelectronics, 17(1), Article 18. https://doi.org/10.1007/s12200-024-00119-1
Image Credits: AI Generated
DOI: 10.1007/s12200-024-00119-1
Keywords: Design, simulation, investigations, charge, transport, layers-free, lead-free, three, absorber, layer, all-perovskite, solar, cells
News Source: Denise Maddox. (November 13, 2026). Simulated Lead-Free Perovskite Solar Cells Reach 28.6% Efficiency. Scienmag.
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