Dicyandiamide-Driven Tailoring of the n-Value Distribution and Interface Dynamics for High-Performance ACI 2D Perovskite Solar Cells

Abstract

Organic-inorganic hybrid perovskite solar cells achieve remarkable efficiencies (> 26%) yet face stability challenges. Quasi-2D alternating-cation-interlayer perovskites offer enhanced stability through hydrophobic spacer cations but suffer from vertical phase segregation and buried interface defects. Herein, we introduce dicyanodiamide (DCD) to simultaneously address these dual limitations in GA(MA)_nPb_nI_3n+1 perovskites. The guanidine group in DCD passivates undercoordinated Pb²⁺ and MA⁺ vacancies at the perovskite/TiO₂ interface, while cyano groups eliminate oxygen vacancies in TiO₂ via Ti⁴⁺-CN coordination, reducing interfacial trap density by 73% with respect to the control sample. In addition, DCD regulates crystallization kinetics, suppressing low-n-phase aggregation and promoting vertical alignment of high-n phases, which benefit for carrier transport. This dual-functional modification enhances charge transport and stabilizes energy-level alignment. The optimized devices achieve a record power conversion efficiency of 21.54% (vs. 19.05% control) and retain 94% initial efficiency after 1200 h, outperforming unmodified counterparts (84% retention). Combining defect passivation with phase homogenization, this work establishes a molecular bridge strategy to decouple stability-efficiency trade-offs in low-dimensional perovskites, providing a universal framework for interface engineering in high-performance optoelectronics.

Keywords: Alternating-cation-interlayer 2D perovskite solar cell; Buried interface; Interface dynamics; Phase modulation.

Fig. 1

a Schematic illustration of the interfacial layer mechanism of DCD. XPS spectra of b Ti 2p, c O 1s of the TiO₂ and DCD modified TiO₂ films. d ¹H NMR spectra of GAI, DCD and mixed powder of DCD and GAI. e FTIR spectra of pure DCD and DCD-TiO₂. f Theoretical calculation of -CN and GA absorbed on TiO₂ surface. g ¹H NMR spectra of PbI₂ and mixed powder of DCD and PbI₂. XPS spectra of h Pb 4f and i I 3d of the exposed buried perovskite interface

Fig. 2

XRD patterns of a top surface, d bottom surface of perovskite films with and without modification. b SEM image of the top surface of perovskite films without DCD modified. c KPFM image of TiO₂. e SEM image of the top surface of perovskite films with DCD modified. f KPFM image of DCD-TiO₂. g, h UPS spectra of secondary electron cut-off and valence bands of TiO₂ and DCD-TiO₂ ETL. i Dark J-V cures of devices structured as FTO/ETLs/Ag

Fig. 3

Transient absorption (TA) spectra at different delay times (0, 1, 2, 3, 5, 7, 10, and 100 ps) of the quasi-2D ACI perovskite (< n >  = 5) films without DCD in a front-side and b, c back-side photoexcitation. TA spectra with different delay time of the quasi-2D ACI perovskite film with DCD modified in d front-side and e, f back-side photoexcitation. g, h Schematic diagrams of the phase distribution of the with or without DCD film as conductive channel

Fig. 4

a Energy level diagrams of PSCs. b, c SCLC measurements for the DCD-modified and control devices. d Steady-state PL spectra of perovskite on TiO₂ and TiO₂-DCD. e Transient photocurrent (TPC) curves, f the transient photovoltage (TPV) curves. g Relationship between Voltage and Light intensity of Control and DCD modified devices. h The dark current–voltage curves of control and DCD modified perovskite devices. i Nyquist plots of PSCs

Fig. 5

a J-V curve and efficiency data of the PSCs with and without DCD. b External quantum efficiency of the PSCs with and without DCD. c Stabilized power output of the PSCs with and without DCD treatment, measured at the maximum power point (MPP) under AM 1.5G one sun illumination. d Statistical distribution of PCEs obtained from 25 devices. e PCE evolution of unencapsulated with and without DCD PSCs stored under ambient air. f The stability tested at 65 °C in a nitrogen environment