Size mismatch induces cation segregation in CsPbI3: Forming energy level gradient and 3D/2D heterojunction promotes the efficiency of carbon-based perovskite solar cells to over 15%

Flexible perovskite solar cells (f-PSCs) have been attracting tremendous attention due to their potentially commercial prospects in flexible energy system and mobile energy system. Reducing the energy barriers and charge extraction losses at the interfaces between perovskite and charge transport layers is essential to improve both efficiency and stability of f-PSCs. Herein, 4-trifluoromethylphenylethylamine iodide (CF3 PEAI) is introduced to form a 2D perovskite at the interface between perovskite and hole transport layer (HTL). It is found that the 2D perovskite plays a dual-functional role in aligning energy band between perovskite and HTL and passivating the traps in the 3D perovskite, thus reducing energy loss and charge carrier recombination at the interface, facilitating the hole transfer from perovskite to the Spiro-OMeTAD. Consequently, the photovoltaic performance of f-PSCs is significantly improved, leading to a power conversion efficiency (PCE) of 21.1% and a certified PCE of 20.5%. Furthermore, the long-term stability of f-PSCs is greatly improved through the protection of 2D perovskite layer to the underlying 3D perovskite. This work provides an excellent strategy to produce efficient and stable f-PSCs, which will accelerate their potential applications.

Creating a Dual-Functional 2D Perovskite Layer at the Interface to Enhance the Performance of Flexible Perovskite Solar Cells

A record power conversion efficiency of 20.1% is achieved for quasi-2D Ruddlesden–Popper perovskite solar cells. The quantum wells are reversely graded in the film, and the quantum confinement effect inside the film is significantly weakened. 

A finely regulated quantum well structure in quasi-2D Ruddlesden–Popper perovskite solar cells with efficiency exceeding 20%

Mixed lead–tin perovskite solar cells (LTPSCs) with an ideal bandgap are demonstrated as a promising candidate to reach higher power conversion efficiency (PCE) than their Pb‐counterparts. Herein, a Br‐free mixed lead–tin perovskite material, FA0.8MA0.2Pb0.8Sn0.2I3, with a bandgap of 1.33 eV, as a perovskite absorber, is selected. Through density functional theory calculations and optoelectronic techniques, it is demonstrated that both Pb‐ and Sn‐related A‐site vacancies are pushed into deeper energetic depth, causing severe nonradiative recombination. Hence, a selective targeting anchor strategy that employs phenethylammonium iodide and ethylenediamine diiodide as co‐modifiers to selectively anchor with Pb‐ and Sn‐related active sites and passivate bimetallic traps, respectively, is established. Furthermore, the selectivity of the molecular oriented anchor passivation is demonstrated through energetic depth specificity of Pb‐ and Sn‐related traps. As a result, a substantially enhanced open‐circuit voltage (VOC) from 0.79 to 0.90 V for the LTPSCs is achieved, yielding a champion PCE of 22.51%, which is the highest PCE among the reported ideal‐bandgap PSCs. The VOC loss is reduced to 0.43 V.

A Selective Targeting Anchor Strategy Affords Efficient and Stable Ideal‐Bandgap Perovskite Solar Cells

Graded 2D/3D (CF3-PEA)2FA0.85MA0.15Pb2I7/FA0.85MA0.15PbI3 heterojunction for stable perovskite solar cell with an efficiency over 23.0%

The two-dimensional (2D)/three-dimensional (3D) heterojunction perovskite solar cell (PSC) has recently been recognized as a promising photovoltaic structure for achieving high efficiency and long-...

Tuning the Interfacial Dipole Moment of Spacer Cations for Charge Extraction in Efficient and Ultrastable Perovskite Solar Cells

In the journey to obtain well-crystallized mixed tin (Sn)–lead (Pb) iodide perovskite films for solar cell application, great difficulties have been presented due to very different crystallization rates between Sn- and Pb-based perovskite components. Herein, we report a new strategy to grow highly crystallized Sn–Pb perovskite (FA0.7MA0.3Sn0.5Pb0.5I3) for perovskite solar cells (PSCs). An iso-pentylammonium tetrafluoroborate ([PNA]BF4) ionic salt layer is introduced on top of poly(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) to function as anchoring agent to bond Pb2+ to the surface of PEDOT:PSS, which can facilitate a quick crystallization of Pb-containing perovskite components and homogeneously distribute Sn/Pb elements inside the perovskite film in a vertical direction, uncovered by focused ion beam time-of-flight secondary ion mass spectrometry. Additionally, greatly reduced surface residual stress was also confirmed by X-ray diffraction. Lastly, these ionic salt molecules are able to encapsulate the acidic and hygroscopic surface of PEDOT:PSS to further ensure device stability. As a result, our strategies enabled a champion PCE of 20.11% for mixed Sn–Pb PSCs with improved thermal stability at 85 °C over 240 hours and shelf storage stability over 1200 hours. This work provides a new strategy to regulate the crystallization process of mixed Sn–Pb perovskites for both high performance and stability.

Lingti Kong

Papers

Tuning the Interfacial Dipole Moment of Spacer Cations for Charge Extraction in Efficient and Ultrastable Perovskite Solar Cells

Balancing crystallization rate in a mixed Sn–Pb perovskite film for efficient and stable perovskite solar cells of more than 20% efficiency

Organic nanocrystals induced surface passivation towards high-efficiency and stable perovskite solar cells