2D-OIHP Quantum Wells: Structure & Optoelectronics

• 2D-OIHP quantum wells are layered semiconductors combining metal-halide sheets with organic barriers to achieve quantum confinement and tunable electronic properties.
• They exhibit high exciton binding energies, strong anisotropic light-matter coupling, and nonlinear optical responses suitable for diverse device applications.
• Design parameters such as organic cation length, metal-halide composition, and quantum well thickness enable precise control over bandgap, exciton dynamics, and carrier mobility.

Two-dimensional organic–inorganic hybrid perovskite (2D-OIHP) quantum wells are layered semiconductor materials constructed by alternating nanoscopic slabs of metal-halide frameworks and electronically insulating organic molecules. These systems represent a distinct class of quantum-confined, dielectric-heterostructure semiconductors, exhibiting a direct, tunable electronic bandgap, unusually large excitonic binding energies, significant anisotropy in optical and electronic response, and strong nonlinearities—all properties that have positioned 2D-OIHPs as leading candidates for next-generation optoelectronic, excitonic, and photonic devices.

1. Structural Architecture and Quantum-Well Model

2D-OIHP quantum wells are realized in members of the Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) perovskite families, with generic formulae (for RP) (RNH₃)₂Aₙ₋₁MₙX₃ₙ₊₁ and (for DJ) A'(A)ₙ₋₁MₙX₃ₙ₊₁ (R = organic spacer, A = small monovalent cation, M = metal cation such as Pb²⁺ or Sn²⁺, X = halide). The integer n gives the number of contiguous metal-halide octahedral sheets within each quantum well. For n=1, a single PbX₄ layer (W ≈ 0.6 nm) is sandwiched by 1–3 nm thick organic barriers; increasing n stepwise broadens the quantum well and tunes both electronic bandwidth and dielectric screening (Molina-Sánchez, 2018, Blancon et al., 2017, Song et al., 2020). The structural motif is a self-assembled superlattice: …[Organic]∣[PbX₄]∣[Organic]∣[PbX₄]…

The organic barrier serves dual roles: providing high-energy and low-dielectric barriers for carrier confinement, and enabling fine-tuning of quantum-well depth and lateral coherence via chain length, steric bulk, and electrostatic polarization. Substitutions at the organic site, metal site, or halide site offer extended compositional control (Vassilakopoulou et al., 2016, Liu et al., 2018).

2. Electronic Structure: Band Edges and Effective Masses

The conduction and valence band edges in 2D-OIHPs are strongly localized in the inorganic sheets, with the electronic states derived primarily from Pb p and halide p orbitals—organic cations do not contribute to states near the fundamental gap (Molina-Sánchez, 2018). DFT and GW calculations give in-plane effective masses m_e* ≈ 0.1–0.2 m₀, m_h* ≈ 0.15–0.25 m₀ for n=1–2, increasing with quantum confinement (Blancon et al., 2017, Molina-Sánchez, 2018, Todd et al., 2018). Out-of-plane carrier mobility is sharply suppressed due to the insulating barrier.

The bandgap can be modulated across a wide range (1.3–3.2 eV) by changing n, halide, and organic cation. For example, CH₃NH₃PbI₃ (3D) has E_g ≃ 1.7 eV, while the n=1 (RNH₃)₂PbI₄ analogue may reach E_g ≈ 2.4–2.8 eV (Vassilakopoulou et al., 2016, Huan et al., 2015). Quantum confinement and interface effects can even lead to indirect gaps and flat band edges near the fundamental transitions (Huan et al., 2015).

Spin–orbit coupling is critical, lowering the bandgap by ≈0.2–0.3 eV in Pb- or Sn-based systems, and splitting the conduction band at the M-point or R-point by 0.1–0.2 eV (Liu et al., 2018, Molina-Sánchez, 2018). Rashba-type spin splitting is observed in non-centrosymmetric compounds, with splittings up to 10 meV at modest excess energies—twenty times what is seen in III–V quantum wells (Todd et al., 2018).

3. Exciton Physics: Confinement, Binding Energy, and Fine Structure

Electrons and holes generated in the inorganic quantum well are tightly bound by both quantum and dielectric confinement. The discontinuity at the organic–inorganic interface produces strong image-charge effects: the low ε_r(organic) (≈2–3) compared to ε_r(inorganic) (≈5–8) reduces Coulomb screening, boosting the exciton binding energy and decreasing the Bohr radius (Abdel-Baki et al., 2015, 1803.02455, Molina-Sánchez, 2018).

Quantitative values:

n	E_b (meV)	μ (m₀)	Reference
1	~470–650	0.18–0.22	(Molina-Sánchez, 2018, Blancon et al., 2017)
2	340–400	≈0.20	(Blancon et al., 2017)
3	250	≈0.20	(Blancon et al., 2017)
4	185	≈0.20	(Blancon et al., 2017)
5	125	≈0.20	(Blancon et al., 2017)

At n=1, E_b approaches nearly one order of magnitude larger than in 3D perovskites (which have E_b ≈ 70 meV) and exceeds that in monolayer TMDs (E_b ∼ 0.5 eV) (Molina-Sánchez, 2018). The observed non-hydrogenic Rydberg series reflects the combination of quantum confinement, dielectric mismatch, and nonparabolic band edges. The lowest excitonic state sequence includes optically bright and dark states, with bright–dark splitting on the order of 20 meV (Molina-Sánchez, 2018). The large oscillator strength, f_ex, of the 1s exciton is f_ex ≈ 0.6 for n=1, falling with increasing n (Song et al., 2020).

Polaronic and vibronic features are superimposed on the excitonic series, as hybridization with low-energy phonons leads to line broadening, fine structure splitting (~35 meV subbands), and the development of Huang–Rhys sidebands, all influenced by organic cation chemistry (1803.02455).

4. Optical Properties and Light–Matter Coupling

2D-OIHPs exhibit extreme optical anisotropy and intense excitonic resonances in both absorption and emission. The absorption coefficient at the exciton peak can reach α ∼ 10⁵–5×10⁵ cm⁻¹, far surpassing conventional III–V or TMD quantum wells. The electric loss tangent tan δ at resonance may exceed 16 for n=1, highlighting giant energy dissipation and polarizability (Song et al., 2020).

Optical response is dominated by excitonic effects, with pronounced in-plane polarization peaks (ε∥) and a much smaller out-of-plane response (ε⊥), a consequence of quantum confinement and depolarization corrections. The highly tunable dielectric environment (including adjacent substrates or encapsulation) enables spectral engineering of absorption and luminescence (Molina-Sánchez, 2018, Fieramosca et al., 2018).

Strong light–matter coupling is ubiquitous in single-crystal flakes and thin films, giving rise to large vacuum Rabi splittings—Ω_R ≈ 170–260 meV—in both cavity and bare slab configurations (Fieramosca et al., 2018, Fieramosca et al., 2018). The out-of-plane component of the excitonic dipole moment (up to 30% for thin barriers) allows for unconventional plasmonic or TM-polarized polariton coupling—absent in van der Waals 2D materials (Fieramosca et al., 2018). Extraordinary birefringence (Δn up to 0.18) is measured, with sub-micron thickness sufficient for full polarization rotation (Fieramosca et al., 2018).

5. Dynamic Processes and Nonlinearities

Exciton recombination in 2D-OIHPs shows biexponential kinetics: a fast component τ_F ≈ 100 ps (radiative lifetime), and a slower component τ_D ≳ several ns (dark or trapped states) (Abdel-Baki et al., 2015). Ultrafast intraband relaxation (τ_intra < 150 fs) following above-gap excitation enables highly efficient non-adiabatic carrier cooling (Abdel-Baki et al., 2015).

Nonlinear optical response at low exciton densities includes linear scaling of bleaching and broadening with density, exceeding inorganic GaAs quantum wells in magnitude due to stronger Coulomb and dielectric confinement (Abdel-Baki et al., 2015). Exciton–exciton interaction strengths, as measured from polaritonic blueshift, reach g_exc,layer ≈ 1–3 μeV μm² at room temperature—values that match or exceed GaAs quantum wells operated at cryogenic temperature, and which are more than tenfold higher than room-temperature TMD or organic systems (Fieramosca et al., 2018).

6. Defect States, Surface/Edge Physics, and Stability

Edge and surface physics are crucial for optoelectronic performance. When the perovskite slab thickness exceeds a critical value, interface strain relaxes, and edge states emerge several tens to >100 meV below the conduction band minimum (Kepenekian et al., 2018, Hong et al., 2020). These edge-localized states, stabilized by either elastic relaxation or internal electric fields from polarized organic cations, trap electrons preferentially at surfaces, spatially separating them from holes and suppressing radiative recombination (Kepenekian et al., 2018, Hong et al., 2020). The result is an increased photoluminescence lifetime and enhanced charge separation—key for photovoltaic operation.

Dielectric engineering, defect variation (notably in fluorinated cations or stoichiometry), and heterostructure design (e.g., deposition on MoS₂) further control exciton dissociation, charge percolation, and environmental stability (Singh et al., 2018, Singh et al., 2021, Vareli et al., 2018). Incorporation of monolayer MoS₂, for instance, can stabilize 2D-OIHPs thermodynamically (ΔE_f ≈ –0.32 eV) while maintaining desirable optoelectronic properties (Singh et al., 2018, Singh et al., 2021).

7. Device Applications and Materials Design Principles

2D-OIHP quantum wells are implemented in LEDs, photodetectors, solar cells, modulators, and polariton devices. Green and blue LEDs functioning at room temperature have been demonstrated using single or defect-engineered (RNH₃)₂PbX₄ films (turn-on voltages 5–10 V, emission λ_EL ≈ 417–555 nm) (Vassilakopoulou et al., 2016, Vareli et al., 2018). The high exciton binding energy enables low-threshold polariton lasing and nonlinear optical switching without cryogenic cooling (Fieramosca et al., 2018). Anisotropic and tunable optical responses support polarization-sensitive photonics, while tailored edge/surface engineering enables high-efficiency photovoltaics with controlled carrier dissociation (Hong et al., 2020, Kepenekian et al., 2018).

Design rules emerging from ab initio and experimental studies emphasize the role of:

• Layer number n: tuning Eg and Eb (Eb ∝ 1/n)
• Organic cation type: ligand length and dipole moment setting dielectric contrast, confinement, and polarization fields
• Halide/metal composition: band edge energies, spin–orbit coupling, and oscillator strength
• Interface/heterostructure engineering: environmental stability, charge transport, and exciton dissociation

The balance between strong quantum/dielectric confinement, oscillator strength, and charge mobility can be tuned by careful compositional and structural design, enabling devices that span high-brightness LEDs/lasers, efficient solar cells, and waveguide-integrated polaritonic circuits (Molina-Sánchez, 2018, Blancon et al., 2017, Liu et al., 2018, Singh et al., 2021).