Electron-enriched thione enables strong Pb-S interaction for stabilizing high quality CsPbI3 perovskite films with low-temperature processing.

Cesium lead iodide (CsPbI3) perovskite is a promising photovoltaic material with a suitable bandgap and high thermal stability. However, it involves complicated phase transitions, and black-phase CsPbI3 is mostly formed and stabilized at high temperatures (200-360 °C), making its practical application challenging. Here, for the first time, we have demonstrated a feasible route for growing high quality black-phase CsPbI3 thin films under mild conditions by using a neutral molecular additive of 4(1H)-pyridinethione (4-PT). The resulting CsPbI3 thin films are morphologically uniform and phase stable under ambient conditions, consisting of micron-sized grains with oriented crystal stacking. With a range of characterization experiments on intermolecular interactions, the electron-enriched thione group in 4-PT is distinguished to be critical to enabling a strong Pb-S interaction, which not only influences the crystallization paths, but also stabilizes the black-phase CsPbI3 via crystal surface functionalization. The 4-PT based CsPbI3 achieves 13.88% power conversion efficiency in a p-i-n structured device architecture, and encapsulated devices can retain over 85% of their initial efficiencies after 20 days of storage in an ambient environment, which are the best results among fully low-temperature processed CsPbI3 photovoltaics.

Self-assembled naphthalimide derivatives as an efficient and low-cost electron extraction layer for n-i-p perovskite solar cells.

Based on self-assembly, novel electron extraction layers (EELs) composed of naphthalimide (NPI) derivatives are constructed for application in n-i-p perovskite solar cells. Upon molecular energy level modulation, the power conversion efficiencies have been largely improved from 5.4% to 16%. Such low-cost and highly regulable EELs are promising for future commercial applications.

Semi-Locked Tetrathienylethene as a Building Block for Hole-Transporting Materials: Toward Efficient and Stable Perovskite Solar Cells.

The construction of state-of-the-art hole-transporting materials (HTMs) is challenging regarding the appropriate molecular configuration for simultaneously achieving high morphology uniformity and charge mobility, especially because of the lack of appropriate building blocks. Herein a semi-locked tetrathienylethene (TTE) serves as a promising building block for HTMs by fine-tuning molecular planarity. Upon incorporation of four triphenylamine groups, the resulting TTE represents the first hybrid orthogonal and planar conformation, thus leading to the desirable electronic and morphological properties in perovskite solar cells (PSCs). Owing to its high hole mobility, deep lying HOMO level, and excellent thin film quality, the dopant-free TTE-based PSCs exhibit a very promising efficiency of over 20 % with long-term stability, achieving to date the best performances among dopant-free HTM-based planar n-i-p structured PSCs.

Low cost and stable quinoxaline-based hole-transporting materials with a D-A-D molecular configuration for efficient perovskite solar cells.

The use of expensive hole transporting materials (HTMs), such as spiro-OMeTAD, in perovskite solar cells (PSCs) is one of the critical bottlenecks to hinder their large-scale applications. Some low-cost alternatives have been developed by combining conjugated electron-rich cores with arylamine end-caps, usually in a donor-π spacer-donor (D-π-D) molecular configuration. However, incorporation of electron-rich cores can lead to undesirable up-shift in the HOMO energy level and low stability, and few of these new HTMs can outperform spiro-OMeTAD in terms of device efficiency. Given that electron-deficient units have shown many advantages in developing efficient and stable photovoltaic dyes and polymers, we herein present a couple of novel molecular quinoxaline-based HTMs (TQ1 and TQ2) with a donor-acceptor-donor (D-A-D) configuration, especially for rationally modulating the HOMO level, improving the stability and decreasing the cost. The TQ2-based PSCs exhibit a maximum efficiency of 19.62% (working area of 0.09 cm2), unprecedentedly outperforming that of spiro-OMeTAD (18.54%) under the same conditions. In comparison, TQ1 based devices only showed moderate efficiencies (14.27%). The differences in hole extraction and transportation between TQ1 and TQ2 are explored by photoluminescence quenching, mobility and conductivity tests, and single crystal analysis. The scaling-up of the TQ2 based device to 1.02 cm2 achieves a promising efficiency of 18.50%, indicative of high film uniformity and processing scalability. The significant cost advantage and excellent photovoltaic performance strongly indicate that the D-A-D featured TQ2 has great potential for future practical applications.

Molecular Engineering of Quinoxaline-Based D-A-π-A Organic Sensitizers: Taking the Merits of a Large and Rigid Auxiliary Acceptor.

The continuing efforts of creating novel D-A-π-A structured organic sensitizers with excellent optoelectronic properties have resulted in substantial improvement of power conversion efficiency (PCE) as well as stability of dye-sensitized solar cells (DSSCs). Here, we report a new molecular engineering strategy for enhancing optical gain and improving excited-state features in D-A-π-A structured organic sensitizers by improving the conjugation size and rigidity of the auxiliary acceptor functional group. A series of phenanthrene-fused-quinoxaline (PFQ)-based D-A-π-A organic sensitizers (WS-82, WS-83, and WS-84) are designed and synthesized for applications in DSSCs. Compared to 2,3-diphenylquinoxaline (DPQ)-based dye IQ-4, PFQ dyes show extended absorption spectra and improved open-circuit voltage performance. Upon a systematical engineering of alkyl chains and π-spacer structure, the unfavorable issues of PFQ dyes including low solubility and high energy barrier in intramolecular charge transition are successfully eliminated. When applied in iodine electrolyte-based DSSCs, the best performing PFQ dye WS-84 shows a PCE of 10.11%, which is much higher than that of our previous champion DPQ dye IQ-4 under the same conditions.

Molecular engineering and sequential cosensitization for preventing the "trade-off" effect with photovoltaic enhancement.

In dye-sensitized solar cells (DSSCs), it is essential to use rational molecular design to obtain promising photosensitizers with well-matched energy levels and narrow optical band gaps. However, the "trade-off" effect between the photocurrent and photovoltage is still a challenge. Here we report four benzoxidazole based D-A-π-A metal-free organic dyes (WS-66, WS-67, WS-68 and WS-69) with different combinations of π-spacer units and anchoring-acceptor groups. Either extending the π-spacer or enhancing the electron acceptor can efficiently modulate the molecular energy levels, leading to a red-shift in the absorption spectra. The optimal dye, WS-69, containing a cyclopentadithiophene (CPDT) spacer and cyanoacetic acid acceptor, shows the narrowest energy band gap, which displays a very high photocurrent density of 19.39 mA cm-2, but suffers from a relatively low photovoltage of 696 mV, along with the so-called deleterious "trade-off" effect. A cosensitization strategy is further adopted for enhancing the device performance. Optimization of the dye loading sequence is found to be capable of simultaneously improving the photocurrent and photovoltage, and distinctly preventing the "trade-off" effect. The superior cosensitized cell exhibits an excellent power-conversion efficiency (PCE) of 10.09% under one-sun irradiation, and 11.12% under 0.3 sun irradiation, which constitutes a great achievement in that the efficiency of a pure metal-free organic dye with iodine electrolyte can exceed 11% even under relatively weak light irradiation. In contrast with the previous cosensitization strategy which mostly focused on compensation of light-harvesting, we propose a novel cosensitization architecture, in which the large molecular-sized, high photocurrent dye WS-69 takes charge of broadening the light-harvesting region to generate a high short-circuit current (JSC) while the small molecular-sized, high photovoltage dye WS-5 is responsible for retarding charge recombination to generate a high open-circuit voltage (VOC). In addition, adsorption amount and photo-stability studies suggest that the cyano group in the anchoring acceptor is important for the stability since it is beneficial towards decreasing the LUMO levels and enhancing the binding of dyes onto TiO2 nanocrystals.