N‐methyl‐2‐pyrrolidone Iodide as Functional Precursor Additive for Record Efficiency 2D Ruddlesden‐Popper (PEA)₂(Cs)_n−1Pb_nI_3n+1 Solar Cells

Abstract

2D perovskite (PEA)2(Cs)n−1PbnI3n+1 (PEA: phenylethylammonium) exhibits more strengthened phase stability than its 3D components under ambient conditions and hence gained great attention in recent years. However, uncontrollable crystallization kinetics in (PEA)2(Cs)n−1PbnI3n+1 leads to difficulty in controlling film morphology and phase‐orientation regulation, resulting in poor power conversion efficiency (PCE). Herein, by incorporating precursor additive N‐methyl‐2‐pyrrolidone iodide (NMPI), the crystallization rate during the deposition of (PEA)2(Cs)n−1PbnI3n+1 film is efficiently regulated. As a result, the 2D or quasi‐2D perovskite solar cell (PSC) delivers record PCEs in all reported 2D or quasi‐2D CsPbX3 families, for instance, the quasi‐2D (n = 20) CsPbI3 PSC exhibits a record PCE of 14.59%, showing significantly enhanced stability. Detailed characterization reveals that the NMPI forms hydrogen bonds with dimethylammonium iodide (DMAI) in the precursor to control crystallization rate for a smooth morphology with small fluctuation, which leads to improved carrier lifetime and reduced trap‐density. More importantly, femtosecond transient absorption (fs‐TA) measurements confirm an improved phase purity and the suppressed nonradiative recombination in quasi‐2D perovskite film. It is believed that this simple additive strategy paves a new route for solving phase transition and crystallization kinetic problems in 2D and quasi‐2D CsPbX3.

Imidazole additives in 2D halide perovskites: impacts of –CN versus –CH3 substituents reveal the mediation of crystal growth by phase buffering

“…16 Additional organic spacers L ′ ineluctably retard the 2D perovskite crystal growth relative to 3D counterparts, resulting in small-sized grains and numerous grain boundaries, which make it difficult to obtain pinhole- and defect-free thin films. 17 In particular, 2D perovskite crystallization involves various degrees of [BX 6 ] 4− octahedral aggregation along with random orientation, which lead to disordered phase arrangements. 18 The resulting multiple- n valued low-dimensional phases with varying [BX 6 ] 4− distortion in terms of the B–X–B angles 19 give rise to the accumulation of lattice strain on interphase boundaries and therefore largely disturb the long-range ordering of perovskite lattices.…”

Vertically-aligned quasi-2D cesium lead halide perovskite solar cells

“…High coverage will make the perovskite films more stable since it blocks the rapid entry of moisture into the interior of the films. 25 However, the film thickness was reduced significantly when the DMF/DMSO ratio decreased to 3 : 7 or in the case of DMSO only. This tendency could originate from the extremely slow rate of transforming from precursor solutions to solid films which may lead to much precursor solution being sacrificed and then result in thinner films.…”

“…The trap densities were calculated using the formula N t = (2 ε r ε 0 V TFL )/qL 2 , where N t represents the trap state density, ε 0 , ε r , q , and L are the vacuum permittivity, relative dielectric constant, elementary charge, and film thickness, respectively, and V TFL is the voltage value of the intersection of the ohmic region and the TFL region. 25 The electron and hole mobilities were calculated using Mott–Gurney's law with the formula J D = 9 ε r ε 0 μV b 2 /8 L 3 , where J D , μ , and V b represent the dark current density, the carrier mobility, and the applied voltage, respectively. 34,35 The calculated trap densities and carrier mobilities of the perovskite films with different solvent ratios are shown in Fig.…”

“…controlled the crystallization process of perovskites by incorporating the precursor additive N -methyl-2-pyrrolidone iodide, and the solar cells of PEA 2 Cs n −1 Pb n I 3 n +1 ( n = 4) exhibited an improved PCE of 9.22% due to the better film morphology and phase-orientation. 25 Controlling the crystallization process has been verified to be an effective way of preparing well-performed quasi-2D A 2 Cs n −1 Pb n I 3 n +1 solar cells; however, research studies about regulating the orientation and phase compositions of quasi-2D A 2 Cs n −1 Pb n I 3 n +1 are still limited.…”

Current state-of-the-art characterization methods for probing defect passivation towards efficient perovskite solar cells

“…Li et al used femtosecond TA measurements to confirm the enhanced phase purity and suppressed nonradiative recombination in quasi-2D perovskite films. 109 Fig. 7a and d show two signals: negative (blue) GSB and positive (yellow) ESA.…”