Triple-Junction Perovskite-Kesterite Device

• Triple-junction hybrid perovskite-kesterite is a lead-free tandem device using KSnI₃, FASnI₃, and ACZTSe absorbers to extend spectral absorption from 300 to 1200 nm, achieving a simulated PCE of 30.69%.
• Advanced optical (FDTD) and electrical (drift-diffusion) modeling guide precise layer thickness, doping, and band alignment optimizations for efficient carrier extraction and current matching.
• Robust material design with low-toxicity, earth-abundant components results in high fill factors (over 84%) and strong tolerance to absorber thickness variations for reliable performance.

A triple-junction hybrid perovskite-kesterite architecture is a lead-free, monolithic tandem photovoltaic device employing three optoelectronically distinct absorber materials: potassium tin iodide (KSnI₃), formamidinium tin triiodide (FASnI₃), and Ag-doped copper zinc tin selenide (ACZTSe). The architecture is designed to exploit broader sections of the solar spectrum with low toxicity and high earth abundance, achieving high simulated power conversion efficiency (PCE) via advanced optical and electrical modeling, interface and band-structure engineering, and meticulous optimization of layer thickness and doping profiles (Rahman et al., 8 Nov 2025).

1. Device Structure and Layer Engineering

The device consists of a vertically stacked series of layers, where each subcell targets a specific spectral region:

Top Subcell (KSnI₃ Absorber)

• Antireflection coating (ARC): MgF₂, 70 nm
• Transparent conducting oxide (TCO): FTO, 100 nm
• Electron transport layer (ETL): TiO₂, 40 nm, n-doped 5×10¹⁹ cm⁻³
• Absorber: KSnI₃, 400 nm, intrinsic; $E_g ≈ 1.84$ eV, $\chi ≈ 3.44$ eV, $\epsilon_r ≈ 10.4$
• Hole transport layer (HTL): CuSCN, 100 nm, p-doped 5×10¹⁸ cm⁻³
• Intercell recombination/contact: ITO, 50 nm

Middle Subcell (FASnI₃ Absorber)

• Window/buffer: ZnO, 10 nm, n-doped 8×10¹⁸ cm⁻³
• ETL: TiO₂, 20 nm, n-doped 5×10¹⁹ cm⁻³
• Absorber: FASnI₃, 450 nm, p-doped 7×10¹⁶ cm⁻³; $E_g ≈ 1.40$ eV, $\chi ≈ 3.90$ eV, $\epsilon_r ≈ 8.2$
• HTL: Spiro-OMeTAD, 80 nm, p-doped 7×10¹⁸ cm⁻³
• Recombination/TCO: ITO, 20 nm

Bottom Subcell (ACZTSe Absorber)

• Transparent front contact: aluminum-doped ZnO (AZO), 10 nm, n-doped 8×10¹⁸ cm⁻³
• Window: ZnO, 60 nm, n-doped 1.5×10¹⁷ cm⁻³
• Buffer/ETL: ZnS, 90 nm, n-doped 5×10¹⁶ cm⁻³
• Absorber: ACZTSe (kesterite), 900 nm, p-doped 9×10¹⁴ cm⁻³; $E_g ≈ 1.088$ eV, $\chi ≈ 4.05$ eV, $\epsilon_r ≈ 8.5$
• Back-surface-field (BSF): CZTSe, 220 nm, p-doped 6×10¹⁶ cm⁻³
• Back contact: Mo, 100 nm

Band Alignment and Interface Design

Band offsets are systematically engineered for efficient carrier extraction and minimized recombination:

• Top cell: TiO₂/KSnI₃ conduction band offset (CBO) ≈ 0.56 eV (blocks holes); CuSCN/KSnI₃ valence band offset (VBO) ≈ 0 eV (enables hole extraction)
• Middle cell: FASnI₃/Spiro-OMeTAD VBO ≈ –0.06 eV (downhill hole transport), CBO ≈ 1.56 eV (blocks electrons)
• Bottom cell: Favorable alignment at ZnS/ACZTSe, ACZTSe/CZTSe, and CZTSe/Mo interfaces for carrier confinement and selective collection

2. Optical Modeling and Photogeneration

Optical behavior is modeled using a two-dimensional finite-difference time-domain (FDTD) approach:

• Mesh: Uniform 2–5 nm in the vertical direction, optimized for resolving 300–1200 nm wavelengths.
• Simulation domain: Periodic boundary (x-axis), PML (y-axis)
• Illumination: AM1.5G spectrum (300–1200 nm) at normal incidence
• Material inputs: Complex refractive index $n(\lambda) + i \kappa(\lambda)$ values extracted from literature

Photogeneration Calculations

• Absorbed power density at $(x,\lambda)$: $$P_\mathrm{abs}(x, \lambda) = -\frac{1}{2} \omega |E(x, \lambda)|^2 \operatorname{Im} \{\epsilon(\lambda)\}$$
• Local generation rate: $$G(x) = \int [P_\mathrm{abs}(x, \lambda) / (\hbar \omega)]\, d\lambda$$
• Short-circuit current density in subcell $i$: $$J_\mathrm{sc,i} = q \int_{\lambda_\mathrm{min}}^{\lambda_\mathrm{max}} A_i(\lambda)\, \Phi_\mathrm{inc}(\lambda)\,d\lambda$$, where $A_i(\lambda)$ is the spectral absorptance, and $\Phi_\mathrm{inc}(\lambda)$ is the incident photon flux

Current Matching Constraint

In two-terminal monolithic architectures, $J_\mathrm{sc}$ is limited by the lowest-$J_\mathrm{sc}$ subcell

3. Electrical Modeling and Recombination Mechanisms

Electrical response is simulated by finite-element solution of coupled drift-diffusion and Poisson equations, utilizing spatially resolved $G(x)$:

• Poisson equation: $$-\nabla\cdot[\epsilon_0 \epsilon_r \nabla \phi] = q(p - n + C_d - C_a)$$
• Electron current: $J_n = q\mu_n n (-\nabla\phi) + qD_n\nabla n$
• Hole current: $J_p = q\mu_p p (-\nabla\phi) - qD_p\nabla p$
• Continuity: $$\frac{\partial n}{\partial t} = \frac{1}{q}\nabla\cdot J_n - R$$, $$\frac{\partial p}{\partial t} = -\frac{1}{q} \nabla\cdot J_p - R$$

Recombination Channels

• Shockley–Read–Hall (SRH): $$R_\mathrm{SRH} = \frac{np - n_i^2}{\tau_p (n + n_i) + \tau_n (p + n_i)}$$
• Radiative: $R_\mathrm{rad} = B (np - n_i^2)$
• Surface recombination included at each interface with specified velocity $S$

Key Metrics Extraction

• $J_\mathrm{sc}$: From $J$–$V$ sweep under illumination at $V=0$
• $$V_\mathrm{oc} \approx \frac{kT}{q} \ln(J_\mathrm{sc}/J_0 + 1)$$, $J_0$ the reverse-bias saturation current
• Fill factor: $$FF = (V_\mathrm{mpp} J_\mathrm{mpp})/(V_\mathrm{oc} J_\mathrm{sc})$$
• Efficiency: $$\eta = (V_\mathrm{mpp} J_\mathrm{mpp})/P_\mathrm{in}$$

4. Subcell and Device Optimization Approaches

A systematic multistep optimization yields high η:

• Single-junction tuning:
  • Absorber thickness varied to maximize absorption vs. minimize recombination and series resistance for optimal tradeoff
  • Transport layer (ETL/HTL) thicknesses optimized ($\sim$40/100 nm for top, $\sim$20/160 nm for middle subcells)
  • Doping in transport layers increased up to $10^{19}$ cm⁻³ to improve built-in field and FF, with further increases suppressed to avoid tunneling recombination (max PCE gain $<$0.5%)
  • ARC (MgF₂) thickness fine-tuned (70–90 nm) for reflection minimization (300–800 nm window)
• Tandem stacking strategy:
  • Current-matching enforced through volumetric sweeps of absorber thickness. Dual-junction optimum: 600 nm (KSnI₃) / 800 nm (FASnI₃), $J_\mathrm{sc} \approx 14.74$ mA/cm²
  • ITO tunnel junctions maintained at ≤20 nm to ensure tunneling and suppress optical losses/parasitic absorption
• Triple-junction optimization:
  • Bottom (ACZTSe) absorber thickness swept 400–1400 nm, with top/middle fixed, maximizing η
  • Doping in ACZTSe tuned to $9 \times 10^{14}$ cm⁻³ for optimal $V_\mathrm{oc}$
  • Volumetric sweeps of all absorbers in (350–550 nm, 400–700 nm, 850–1150 nm) to refine current matching and maximize η
  • Sensitivity analysis indicates robust η (Δη $<$0.3%) under ±50 nm absorber thickness perturbations

Trade-offs emerge: thinning top absorbers reduces parasitic absorption in deeper subcells but lowers top $J_\mathrm{sc}$, while thickening the bottom cell enhances IR harvesting at the cost of additional recombination.

5. Performance Benchmarks and Comparative Outcomes

The triple-junction architecture demonstrates key performance advancements over both single- and dual-junction variants. Device metrics are strictly achieved by simulation with all recombination and optical loss mechanisms included:

Device	Thicknesses (nm)	$J_\mathrm{sc}$ (mA/cm²)	$V_\mathrm{oc}$ (V)	FF (%)	$\eta$ (%)
1J KSnI₃	400	16.62	1.232	79.5	16.28
1J FASnI₃	450	29.80	1.029	84.84	26.03
2J KSnI₃/FASnI₃	600/800	14.74	2.227	83.14	27.29
3J KSnI₃/FASnI₃/ACZTSe	400/450/900	13.18	2.766	84.18	30.69

Notable performance drivers:

• Addition of ACZTSe subcell extends absorption to the near-infrared (900–1200 nm), augmenting current from the bottom cell ($J_\mathrm{sc,bottom} \approx 12.11$ mA/cm²) and boosting $V_\mathrm{oc}$ by ≈0.54 V over the dual-junction.
• Fill factor remains consistently high ($>84\%$), indicative of effective band alignment/tunnel junction design and negligible interfacial recombination losses.
• Sensitivity analysis shows total efficiency is minimally impacted ($<$0.3%) by $\pm$50 nm changes in absorber thickness, supporting robust tolerance to thickness variability.

6. Materials Considerations and Sustainability

The architecture leverages materials with low toxicity profiles and high natural abundance:

• Tin halide perovskites (KSnI₃, FASnI₃) evade environmental/health risks associated with lead-based perovskite tandems.
• ACZTSe (Ag-doped copper zinc tin selenide) is a kesterite absorber based on earth-abundant metals.
• All key materials' properties and relevant doping/conduction/valence bands are tuned to maximize carrier selectivity and stability, further aligning with sustainable photovoltaic technology initiatives.

The combination of lead-free perovskites and kesterite advances the goal of high-performance, environmentally benign photovoltaic devices.

7. Significance and Prospects

The simulated champion triple-junction device achieves a PCE of 30.69% ($J_\mathrm{sc}$ 13.18 mA/cm², $V_\mathrm{oc}$ 2.766 V, FF 84.18%) under AM1.5G illumination. This aligns with top-performing lead-containing multijunctions while addressing sustainability constraints (Rahman et al., 8 Nov 2025).

The architecture's robustness, high FF, and potential for further material/architectural refinement position it as a promising foundation for sustainable, high-efficiency tandem photovoltaics.