Green-Solvent-Processable Low-Cost Fluorinated Hole Contacts with Optimized Buried Interface for Highly Efficient Perovskite Solar Cells

The Renaissance of Poly(3-hexylthiophene) as a Promising Hole-Transporting Material Toward Efficient and Stable Perovskite Solar Cells

To push the commercialization of the promising photovoltaic technique of perovskite solar cells (PSCs), the three-element golden law of efficiency, stability, and cost should be followed. As the key component of PSCs, hole-transporting materials (HTMs) involving widely-used organic semiconductors such as 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD) or poly(triarylamine) (PTAA) usually suffer high-cost preparation and low operational stability. Fortunately, the studies on the classical p-type polymer poly(3-hexylthiophene) (P3HT) as an alternative HTM have recently sparked a broad interest due to its low-cost synthesis, excellent batch-to-batch purity, superior hole conductivity as well as controllable and stable film morphology. Despite this, the device efficiency still lags behind P3HT-based PSCs mainly owing to the mismatched energy level and poor interfacial contact between P3HT and the perovskite layer. Hence, in this review, the study timely summarizes the developed strategies for overcoming the corresponding issues such as interface engineering, morphology regulation, and formation of composite HTMs from which some critical clues can be extracted to provide guidance for further boosting the efficiency and stability of P3HT-based devices. Finally, in the outlook, the future research directions either from the viewpoint of material design or device engineering are outlined.

A Multifunctional Dye Molecule as the Interfacial Layer for Perovskite Solar Cells

In perovskite solar cells (PSCs), defects in the interface and mismatched energy levels can damage the device performance. Improving the interface quality is an effective way to achieve efficient and stable PSCs. In this work, a multifunctional dye molecule, named ThPCyAc, was designed and synthesized to be introduced in the perovskite/HTM interface. On one hand, various functional groups on the acceptor unit can act as Lewis base to reduce defect density and suppress nonradiative combinations. On the other hand, the stepwise energy-level alignment caused by ThPCyAc decreases the accumulation of interface carriers for facilitating charge extraction and transmission. Therefore, based on the ThPCyAc molecule, the devices exhibit elevated open-circuit voltage and fill factor, resulting in the best power conversion efficiency (PCE) of 23.16%, outperforming the control sample lacking the interface layer (PCE = 21.49%). Excitingly, when attempting to apply it as a self-assembled layer in inverted devices, ThPCyAc still exhibits attractive behavior. It is worth noting that these results indicate that dye molecules have great potential in developing multifunctional interface materials to obtain higher-performance PSCs.

A Fluorination Strategy and Low-Acidity Anchoring Group in Self-Assembled Molecules for Efficient and Stable Inverted Perovskite Solar Cells

Herein, we synthesized two donor-acceptor (D-A) type small organic molecules with self-assembly properties, namely MPA-BT-BA and MPA-2FBT-BA, both containing a low acidity anchoring group, benzoic acid. After systematically investigation, it is found that, with the fluorination, the MPA-2FBT-BA demonstrates a lower highest occupied molecular orbital (HOMO) energy level, higher hole mobility, higher hydrophobicity and stronger interaction with the perovskite layer than that of MPA-BT-BA. As a result, the device based-on MPA-2FBT-BA displays a better crystallization and morphology of perovskite layer with larger grain size and less non-radiative recombination. Consequently, the device using MPA-2FBT-BA as hole transport material achieved the power conversion efficiency (PCE) of 20.32 % and remarkable stability. After being kept in an N2 glove box for 116 days, the unsealed PSCs' device retained 93 % of its initial PCE. Even exposed to air with a relative humidity range of 30±5 % for 43 days, its PCE remained above 91 % of its initial condition. This study highlights the vital importance of the fluorination strategy combined with a low acidity anchoring group in SAMs, offering a pathway to achieve efficient and stable PSCs.

Self-Assembled Monolayer-Based Hole-Transporting Materials for Perovskite Solar Cells

Ever since self-assembled monolayers (SAMs) were adopted as hole-transporting layers (HTL) for perovskite solar cells (PSCs), numerous SAMs for HTL have been synthesized and reported. SAMs offer several unique advantages including relatively simple synthesis, straightforward molecular engineering, effective surface modification using small amounts of molecules, and suitability for large-area device fabrication. In this review, we discuss recent developments of SAM-based hole-transporting materials (HTMs) for PSCs. Notably, in this article, SAM-based HTMs have been categorized by similarity of synthesis to provide general information for building a SAM structure. SAMs are composed of head, linker, and anchoring groups, and the selection of anchoring groups is key to design the synthetic procedure of SAM-based HTMs. In addition, the working mechanism of SAM-based HTMs has been visualized and explained to provide inspiration for finding new head and anchoring groups that have not yet been explored. Furthermore, both photovoltaic properties and device stabilities have been discussed and summarized, expanding reader's understanding of the relationship between the structure and performance of SAMs-based PSCs.