We reveal that 2D/3D interfaces are dynamical in nature which is detrimental for long term perovskite solar cells stability.

Engineering 2D/3D perovskite interfaces is a common route to realizing efficient and stable perovskite solar cells. Whereas 2D perovskite’s main function in trap passivation has been identified and is confirmed here, little is known about its 2D/3D interface properties under thermal stress, despite being one of the main factors that induces device instability. In this work, we monitor the response of two typical 2D/3D interfaces under a thermal cycle by in situ X-ray scattering. We reveal that upon heating, the 2D crystalline structure undergoes a dynamical transformation into a mixed 2D/3D phase, keeping the 3D bulk underneath intact. The observed 3D bulk degradation into lead iodide is blocked, revealing the paramount role of 2D perovskite in engineering stable device interfaces.

Engineering two-dimensional (2D) / three-dimensional (3D) perovskites has emerged as an attractive route to efficient and durable perovskite solar cells. Beyond improving the surface stability of the 3D layer and acting as a trap passivation agent, the exact function of 2D/3D device interface remains vague. Here, we provide evidence that 2D/3D perovskite interface that forms a p-n junction is capable to reduce the electron density at the hole-transporting layer interface and ultimately suppress interfacial recombination. By a novel ultraviolet photoelectron spectroscopy (UPS) depth-profiling technique, we show that engineering of the 2D organic cations, in this case by simply varying the halide counter ions in thiophene methylammonium-salts, modifies the 2D/3D perovskite energy alignment. These measurements enable the true identification of the energetic across the 2D/3D interface, so far unclear. When integrated in solar cells, due to the electron blocking nature of the 2D layer, the optimized 2D/3D structures suppress the interfacial recombination losses, leading to opencircuit voltage (VOC) which approaches the potential internal Quasi-Fermi Level Splitting (QFLS) voltage of the perovskite absorber. The devices exhibit an improved fill factor (FF) driven by the enhanced hole extraction efficiency and reduced electron density at the 2D/3D interface. We thus identify the essential parameters and energetic alignment scenario required for 2D/3D perovskite systems in order to surpass the current limitations of hybrid perovskite solar cell performances.Understanding and exploiting interfacial physics is key in perovskite solar cell engineering and optimization. 1,2 That is especially true when interface losses play a dominant role and complex interface functionalization is essential to minimize them. In the field of hybrid perovskite engineering, much attention has been lately focused on multi-dimensional perovskite interfaces consisting of a wider band gap layered (namely, two dimensional-2D) perovskite deposited between the bulk 3D perovskite and the hole transporting layer (HTL) in a standard mesoporous configuration. [3][4][5][6][7][8][9] Such configuration is currently among the most effective strategies to enhance both the efficiency and stability of perovskite solar cells. 3,10,11 It is generally considered that the 2D perovskite acts as both an efficient mean to passivate the surface traps (leading to reduced defect recombination) and an electron blocking layer due to its wider band gap. [12][13][14][15] However, despite these empirical observations, the energetic alignment at the interface and the relative function of the 2D/3D interface is only qualitatively depicted with a only a partial understanding of these

Approaches to boost the efficiency and stability of perovskite solar cells often address one singular problem in a specific device configuration. In this work, we utilize a poly(ionic-liquid) (PIL) to...

Recently, perovskite solar cells (PSC) with high power-conversion efficiency (PCE) and long-term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in-depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self-crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self-crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long-term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′-tetrakis-(N,N-di-pmethoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD) compared to the devices without the M2P.light absorbers have suffered from high defect density at the surfaces and grain boundaries, which can lead to nonradiative recombination and decrease PCEs of PSC. [6,8] Also, the volatile organic cation in the 3D perovskite lattice such as methylammonium (MA + ) and formamidinium (FA + ) degrades the stability of the perovskite itself and lifetime of PSCs. [9,10] In this regard, 2D perovskites have recently received substantial attention as they contain less-volatile bulkier organic cations, which can trigger passivation effect and lead to better water resistance, resulting in higher PCE and extended long-term stability of PSCs. Therefore, the deposition of 2D perovskite on top of the 3D perovskite has presented significant enhancement both in PCEs and stability of PSCs because the 3D/2D perovskite can take advantage of features of 2D perovskite while it can maintain the excellent optoelectronic properties of 3D perovskites. [11][12][13][14][15][16][17][18][19] In this study, we present the incorporation of self-crystallized multifunctional 2D perovskite (M2P) on top of a 3D perovskite (Cs 0.08 FA 0.77 MA 0.12 PbI 2.62 Br 0.35 ) light absorber. The self-crystallized M2P can facilitate hole transfer due to its randomly oriented crystalline structure and reduce the trap density in underlying 3D perovskite through a trap passivation effect. Therefore, the introduction of the M2P layer between 3D perovskite light absorber and hole transport material (HTM) in a PSC led to improvement in the PCE from 19.75% to 20.79% and long-term stability under simulated continuous sunlight, compared to the device without the M2P layer. Furthermore, we demonstrated the use of M2P as a hole-transporting layer (HTL) in a PSC without using the organic HTM, 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD), which resulted in PCE of 15.17%, which was greatly higher than PCE of the device without M2P (6.22%). Results and DiscussionWe developed layer-by-layer deposited self-crystallized 2D perovskite grown on top of 3D per...

Two structural isomers of carbazole decorated with triarylamine have been designed and synthesized with a facile synthetic procedure. The impact of triarylamine substitution on the isomeric structural linkage of carbazole on the optical, thermal, electrochemical, and photovoltaic properties has been extensively studied by combining experimental and simulation methods. Car­[2,3] showed a red shift in the absorption maximum compared to that of Car­[1,3], indicating the linear conjugation along the 2,7-position of carbazole in the former. The high thermal decomposition temperature (>420 °C) of these compounds could be attributed to the rigid structure of the carbazole core. Perovskite solar cells fabricated with Car­[2,3] as the hole transporting material (HTM) displayed the highest power conversion efficiency (PCE) of 19.23%. It can be attributed to the suitable energy alignment of the highest occupied molecular orbital (HOMO) of HTM with the adjacent perovskite valence band energy level, which results in efficient hole transport. Furthermore, the molecular dynamic simulation demonstrates that the triphenylamine substitution on the 2,3,6,7 positions of Car­[2,3] results in a more planar molecular alignment on top of the perovskite surface, promoting an efficient hole extraction. Essentially, when Car­[1,3] and Car­[2,3] were applied in perovskite solar cells, they showed enhanced long-term stability by retaining >80% of their initial PCEs after 1000 h of continuous illumination.