Self‐Assembled Monolayers as Interface Engineering Nanomaterials in Perovskite Solar Cells

Abstract

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...

Charge transfer rates and electron trapping at buried interfaces of perovskite solar cells

“…Usually, since the SAMs serve as an HTL, at first sight, it may seem that a shorter PL decay is an indication for a faster hole transfer or quenching of holes in the HaP absorber, an interpretation that is widely common in the literature. [38][39][40] However, a faster PL decay can also be caused by other processes such as electron trapping, as will be shown in the following by comparing PL and SPV transients measured under identical conditions. Figure 3 shows in (A) and (B) a comparison between tr-PL and tr-SPV for two different samples, ITO/MeO-2PACz/HaP and ITO/Me-4PACz/HaP, using a repetition rate of f rep = 1 kHz.…”

18.4 % Organic Solar Cells Using a High Ionization Energy Self‐Assembled Monolayer as Hole‐Extraction Interlayer

“…In the area of hybrid perovskite solar cells (PSCs), selfassembled monolayers (SAMs) of organic small molecules have recently been exploited as a replacement to commonly used HTL materials resulting in cells with improved efficiency and stability. [24][25][26][27] Molecularly-thin SAM-based interlayers offer numerous potential advantages over traditional thin-film technologies, the most important of which include the need for tiny amounts of material (10-100 times less than conventional interlayer technologies), and tunable, well-defined chemistry that could enable additional functionalities and further device performance improvements. Surprisingly, the application of SAM interlayers in OPVs has so far attracted limited attention [28,29] with the majority of studies focusing on the use of molecular monolayers as interfacial modifiers in ETLs.…”

Effect of surface modification of nickel oxide hole‐transport layer via self‐assembled monolayers in perovskite solar cells

“…As the device power conversion efficiency is a composite result of open circuit voltage (V oc ), short circuit current (J sc ) and fill factor (FF), which can all affected by the morphology/crystallinity of perovskite and its contact with other layers and in turn be modulated by SAM modification, minimizing the number of variables involved may lead to better correlation. [39,40] Here in this report, we modified the nickel oxide surface with a series of para-substituted phenylphosphonic acid SAMs as the HTL in the fabrication of PSC to investigate their effect on the device performance. The different substituents (electron-donating CH 3 O-, H-and electronwithdrawing CN-groups) impart different molecular dipoles to the molecules so that when anchored to the NiO x surface, the work function (WF) of the NiO x HTL can be tuned with respect to the valence level of the perovskite layer.…”