Double-Cation CsCH(NH₂)₂PbI₃ Absorbers

• Double-cation CsCH(NH₂)₂PbI₃ absorbers are hybrid perovskite materials featuring both Cs⁺ and FA⁺ at the A-site, enabling precise tuning of optical absorption and structural stability.
• The material leverages controlled hydrogen-bonding and oscillator strength to achieve intermediate visible-light absorption and enhanced carrier mobility compared to single-cation systems.
• Bulk and interface passivation strategies using additives like TPA-Py and TPATC improve defect tolerance and mechanical flexibility, resulting in higher efficiency and prolonged thermal stability.

Double-cation CsCH(NH₂)₂PbI₃ perovskite absorbers comprise both inorganic cesium (Cs⁺) and organic formamidinium (FA⁺, CH(NH₂)₂⁺) cations occupying the A-site in the ABX₃ lattice structure with lead (Pb²⁺) as the B-site and iodide (I⁻) as the X-site. Combining Cs⁺ and FA⁺ in the A-site enables simultaneous tuning of optoelectronic properties, structural stability, and defect tolerance, positioning this class of materials at the forefront of solution-processed photovoltaics. These absorbers leverage the complementary properties of CsPbI₃ (high absorption coefficient, enhanced stability) and FAPbI₃ (optimal band gap, mechanical flexibility), while offering new possibilities for interface and bulk engineering aimed at further improving efficiency and durability.

1. Influence of A-site Double Cation on Optical Absorption

In hybrid perovskites, the A-site cation plays a decisive role in determining visible-light absorption. When FA⁺ replaces methylammonium (MA⁺) in APbI₃, the absorption coefficient (α) in the visible strongly decreases—α is reduced to approximately half that of MAPbI₃. This is attributed to FA⁺'s dual amine configuration, enabling robust hydrogen bonding with I⁻ ions. Such strong A–X interaction results in "anti-coupling," which redistributes valence electron density (especially on I p-orbitals), decreases interband oscillator strength, and suppresses absorption amplitude (Kato et al., 2016). The fundamental band gap (E₉) remains nearly unchanged (FAPbI₃: ~1.55 eV; MAPbI₃: ~1.61 eV), but the reduced transition probability dominates α behavior.

Incorporating Cs⁺, which does not engage in hydrogen bonding, keeps the A–X interaction weak. CsPbI₃ thus retains high oscillator strength for the same interband transitions and exhibits one of the highest visible α values. These effects extend to double-cation systems: in CsCH(NH₂)₂PbI₃, the expected optical response is intermediate between pure FAPbI₃ and CsPbI₃, tunable by the Cs/FA mixing ratio. The spectral changes upon halide substitution (I⁻/Br⁻/Cl⁻) are quantitatively described by the sum rule:

$$\int_0^\infty E\,\epsilon_2(E)\,dE = \text{constant}$$

where $E$ is photon energy and $\epsilon_2(E)$ the imaginary part of the dielectric function. The area under $E\,\epsilon_2(E)$ remains conserved, reflecting a tradeoff: lighter halides shift $\epsilon_2(E)$ peaks to higher energy, lowering peak amplitude to maintain overall spectral weight (Kato et al., 2016).

2. Carrier Transport: Modulation via Polaron Coupling

The A-site cation also governs carrier transport via Fröhlich electron–phonon (e–ph) coupling. In CsCH(NH₂)₂PbI₃, the FA⁺ moiety can coordinate strongly to I⁻, shortening A–I distances and increasing the number of hydrogen bonds; Cs⁺ remains structurally central but largely inert. Enhanced coordination lowers the Born effective charge ($Z^*$) on Pb and I, diminishing polar LO phonon contribution and the polaron coupling constant ($\alpha$):

$$\alpha = \frac{e^2}{8\pi\epsilon_0\hbar\omega_\text{LO} \sqrt{2\omega_\text{LO}m_0/\hbar}} \left( \frac{1}{\epsilon_\infty} - \frac{1}{\epsilon_0} \right)$$

where $\omega_\text{LO}$ is the LO phonon frequency; $\epsilon_0$ and $\epsilon_\infty$ are static and high-frequency dielectric constants. Stronger coordination (shorter A–I, higher coordination number) buffers LO lattice vibrations and decreases the carrier scattering rate. Carrier mobility ($\mu$) consequently rises, as

$\mu = \frac{e\tau}{m^*}$

with $\tau$ the lifetime and $m^*$ the effective mass (Myung et al., 2017). In double-cation systems, molecular engineering of the organic component (e.g., FA⁺ analogs with even higher coordination) offers a pathway to maximize transport and, by implication, device efficiency.

3. Mechanical Flexibility and Ductility

The FA⁺ cation induces significant mechanical effects: its planar geometry distorts the PbI₃ octahedral network, lowering the shear modulus relative to systems with smaller or more isotropic A-site cations. For single-cation FAPbI₃, moduli values (bulk $B$, shear $G$, and Young’s $E$) quantify its flexibility and ductility. For example,

$$E = \frac{9BG}{3B+G},\quad \nu = \frac{3B-2G}{2(3B+G)}$$

with $B/G$ (Pugh’s ratio) typically between 2.37 and 2.88 and Poisson ratio ($\nu$) from 0.31 to 0.34, indicating strong ductility (Guo et al., 2019). The projected crystal orbital Hamilton population (pCOHP) method allows further analysis: Pb–I bond strength (ICOHP ≈ –2.115 eV) supports both mechanical integrity and defect tolerance. In CsCH(NH₂)₂PbI₃, the interplay between Cs⁺ (spherical, strengthens lattice) and FA⁺ (imparts flexibility and anisotropy) is expected to yield a material balancing robust mechanical stability with capacity for strain accommodation in flexible optoelectronic devices.

Compound	Pugh's Ratio (B/G)	Young's Modulus (E, GPa)	Poisson Ratio (ν)
FAPbI₃	2.88	Flexible	0.34
MAPbI₃	Lower	Stiffer	<0.31
CsCH(NH₂)₂PbI₃	Intermediate	Tunable	Intermediate

4. Bulk and Interface Passivation Strategies

Engineering grain boundaries and device interfaces in CsCH(NH₂)₂PbI₃ has emerged as an essential tool for enhancing stability and photovoltaic performance. Incorporation of pyridine-functionalized triphenylamine (TPA-Py) directly into bulk perovskite (inter-grain passivation) results in robust coordination and dipole-dipole interactions. TPA-Py coordinates with under-coordinated Pb²⁺ ions or PbI₂ at grain boundaries:

$$\text{Pb}^{2+} + :N\text{–(TPA-Py)} \rightarrow \text{Pb–N(TPA-Py)}$$

This reduces defect sites, locally bends energy bands (dipole ~4 D), and suppresses formation of non-photoactive phases such as PbI₂ segregates. The effect is substantiated by increased open-circuit voltage ($V_{oc} \approx 1.14$ V), enhanced power conversion efficiency (PCE up to 21.3%), and extended T₈₀ lifetime under sustained 85°C heating (T₈₀ ≈ 600 h vs. 200 h reference) (Ilicheva et al., 9 Oct 2025). Additionally, TPA-Py traps mobile ionic defects, as evidenced by activation energy for ion migration ($E_a ≈ 0.45$ eV), slowing ion transport and screening migration pathways.

At the hole-selective interface, incorporating TPATC (triphenylamine-based with carboxyl group) as an ultrathin self-assembled monolayer increases the work function by ~0.2 eV, enhances charge extraction (transient rise time reduced from 33 to 18 μs), and lowers defect concentration by an order of magnitude (from ~$1.0×10^{15}$ to $7.2×10^{14}$ cm⁻³) (Sukhorukova et al., 2023). The interface engineering suppresses ionic migration, stabilizes Pb and I states, and translates into improved PCE (up to 20.58%), as well as high operational stability (minimal 2% performance loss after 1000 h light soaking).

5. Phase Stability, Decomposition, and Interfacial Chemistry

Thermal and phase stability remain focal challenges for CsCH(NH₂)₂PbI₃ absorbers. Integrating quasi-2D AVA₂FAPb₂I₇ additives at grain boundaries modifies local energetics, increasing the activation energy for phase transitions and suppressing the nucleation of non-photoactive δ phases:

$k = A \exp{\left(-\frac{E_a}{k_BT}\right)}$

where $E_a$ is elevated by the additive, and $k$ reduced accordingly (Luchnikov et al., 28 Feb 2025). Ionic diffusion of Cs⁺ and FA⁺ across boundaries is simultaneously decreased, curtailing phase segregation and decomposition (e.g., PbI₂ formation). Surface and boundary passivation further counter non-radiative recombination by increasing Shockley–Read–Hall lifetimes and reducing recombination centers.

Metal–perovskite interfaces—crucial in device stacks—benefit significantly from such stabilization. In conventional CsFAPbI₃ films, copper contact evolves rapidly to the corrosive Cu(II) state via progressive oxidation (Cu(0) → Cu(I) → Cu(II)), driven by reactive Pb and I byproducts. AVA₂FAPb₂I₇-modified films maintain a predominant Cu(0)/Cu(I) interface, minimizing interfacial corrosion and supporting efficient charge extraction.

6. Performance and Scalability in Photovoltaic Devices

Device-level improvements realized via bulk and interface modifications are confirmed across cell and module scales. Applying the TPATC interlayer, large-area perovskite modules (active area 64.8 cm², 12 sub-cells) achieve increased PCE (from 13.22% to 15.64%), with improved open-circuit voltage and fill factor under low light. Phase stabilization and defect suppression underpin long-term durability and reliable high output (Sukhorukova et al., 2023).

Thermal cycling studies (–10°C to +100°C) show that double-cation CsCH(NH₂)₂PbI₃ absorbers with appropriate grain boundary additives (e.g., AVA₂FAPb₂I₇) retain phase composition, resist PbI₂ decomposition, and minimize delta-phase nucleation, maintaining optical quality and functional integrity (Luchnikov et al., 28 Feb 2025). Such resilience is essential for the deployment of perovskite modules in environments subject to temperature fluctuations and operational stress.

7. Outlook: Material Engineering and Device Design

Research in double-cation perovskite absorbers demonstrates that rational design—tuning the A-site cation mixture, controlling hydrogen bonding and coordination environments, and applying targeted bulk/interface passivation—can directly modulate optical absorption, carrier transport, mechanical properties, and environmental stability.

These findings collectively point to the following design principles:

• Optimize Cs⁺/FA⁺ ratio for desired absorption and stability—balancing anti-coupling effects and oscillator strength.
• Engineer organic cations with high coordination numbers and short A–I bond distances to boost carrier mobility.
• Apply bulk passivants (e.g., TPA-Py, quasi-2D AVA₂FAPb₂I₇) for phase and defect control.
• Use interface layers (TPATC) to optimize energy alignment, suppress ionic defect propagation, and preserve metal–perovskite contacts.

A plausible implication is that continued advances in understanding A-site chemistry, grain boundary engineering, and interfacial stabilization will enable further leaps in perovskite solar cell efficiency, reliability, and manufacturability. These developments are steering double-cation CsCH(NH₂)₂PbI₃ absorbers toward commercial viability for high-performance, stable, and scalable solution-processed photovoltaics.