The mixed halide perovskites have emerged as outstanding light absorbers for efficient solar cells. Unfortunately, it reveals inhomogeneity in these polycrystalline films due to composition separation, which leads to local lattice mismatches and emergent residual strains consequently. Thus far, the understanding of these residual strains and their effects on photovoltaic device performance is absent. Herein we study the evolution of residual strain over the films by depth-dependent grazing incident X-ray diffraction measurements. We identify the gradient distribution of in-plane strain component perpendicular to the substrate. Moreover, we reveal its impacts on the carrier dynamics over corresponding solar cells, which is stemmed from the strain induced energy bands bending of the perovskite absorber as indicated by first-principles calculations. Eventually, we modulate the status of residual strains in a controllable manner, which leads to enhanced PCEs up to 20.7% (certified) in devices via rational strain engineering.

The poor crystalline quality of tin-based perovskite films with unfavorable trap states is the biggest challenge to achieve highly efficient tin-based perovskite solar cells. Here, we reveal the surface-controlled growth of FASnI 3 perovskites and further precisely control the crystallization process by reducing the surface energy of the solution-air surface with a tailor-made pentafluorophen-oxyethylammonium iodide (FOEI). A highly oriented and smooth FASnI 3 -FOEI perovskite film with longer carrier lifetime is achieved with a certificated efficiency of 10.16% from an accredited institute.

Tin‐based perovskites with narrow bandgaps and high charge‐carrier mobilities are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, the crystalline rate of tin‐based perovskites is much faster, leading to abundant trap states and much lower open‐circuit voltage (Voc). Here, hydrogen bonding is introduced to retard the crystalline rate of the FASnI3 perovskite. By adding poly(vinyl alcohol) (PVA), the OH…I− hydrogen bonding interactions between PVA and FASnI3 have the effects of introducing nucleation sites, slowing down the crystal growth, directing the crystal orientation, reducing the trap states, and suppressing the migration of the iodide ions. In the presence of the PVA additive, the FASnI3–PVA PSCs attain higher power conversion efficiency of 8.9% under a reverse scan with significantly improved Voc from 0.55 to 0.63 V, which is one of the highest Voc values for FASnI3‐based PSCs. More importantly, the FASnI3–PVA PSCs exhibit striking long‐term stability, with no decay in efficiency after 400 h of operation at the maximum power point. This approach, which makes use of the OH…I− hydrogen bonding interactions between PVA and FASnI3, is generally applicable for improving the efficiency and stability of the FASnI3‐based PSCs.

Tin perovskite solar cells (TPSCs) are rising as the most promising candidates for lead-free PSCs. However, the randomly crystallized tin halide perovskite with high concentration of defects is still the...

process under mild conditions. The abovementioned advantages make perovskites a promising family of materials for the next-generation photovoltaic solar cells. Since the first report on the perovskite solar cell (PSC) by Miyasaka and coworkers in 2009, [1a] considerable efforts have been made to improve the power conversion efficiency (PCE) of the PSC to rival those of commercially available silicon solar panels. [1b-e] In a strikingly short period of time, the highest certified PCE has reached a value as high as 25.2% in a single-junction PSC. [2] Self-assembled monolayers (SAMs) are 2D nanomaterials with the thickness of one or few molecules. [3] Self-assembly of molecules on the surface is a thermodynamically favorable process where molecules interact with each other to form organized structures. Typically, molecules are vertically aligned on the surface with some tilt angle relative to the surface normal. The molecules are designed to have three parts: the anchoring, the spacer, and the terminal groups (Figure 1). 1) Anchoring group: the anchoring group is responsible for the interaction between the molecule and the surface. Various anchoring groups that bind to specific substrates are available, which provides users the option to select the type of electrode and molecule to suit their intended purpose. The most widely studied class of SAMs is derived from the anchoring chemistries between thiol and coinage metals or between silane and oxide substrates. In SAM-inserted PSCs, various oxide substrates are commonly used for bottom contacts, and few Brønsted-Lowry acids (e.g., carboxylic acid, phosphonic acid, and boronic acid) are extensively utilized (see below for details). Anchoring chemistry matters for the tilt angle of molecule with respect to the surface normal, work function (WF) of substrate, interfacial dipole, contact resistance, and energy offset between the Fermi level and energy of frontier molecular orbital. All of them are essential for the electronic function and performance of PSCs. These effects of SAMs on interfacial properties of PSCs are discussed below. 2) Spacer group: the spacer group is the backbone of the molecule, and it bridges terminal and anchoring groups. The length of the backbone is important for electronically isolating one contact from another. The spacer group is responsible for lateral interaction between molecules during the selfassembly process, which affects the final packing structure. Self-assembled monolayers (SAMs), owing to their unique and versatile abilities to manipulate chemical and physical interfacial properties, have emerged as powerful nanomaterials for improving the performance of perovskite solar cells (PSCs). Indeed, in the last six years, a collection of studies has shown that the application of SAMs to PSCs boosts the performance of devices compared to the pristine PSCs. This review describes recent studies that demonstrate the direct advantages of SAM-based interfacial engineering to power conversion efficiency (PCE) of PSCs. This review includes 1) a b...