Chen Weijie

Black phosphorus (BP) as a promising two-dimensional (2D) material has gained great attention in nanoelectronic devices because of its intrinsic semiconductor characteristics. However, the poor material availability and solution processability have been major roadblocks that hinder its wider application in microelectronics. Herein, readily available, lost-cost BP was utilized as an effective component that was integrated via a facile solution process for the fabrication of bulk heterojunction organic solar cells (OSCs). An impressive fill factor (FF) of 74.2% and power conversion efficiency (PCE) of 10.5% were realized in the OSCs incorporating 10 wt% of BP in the active layer of the benchmark polymer donor PTB7-Th and PC71BM acceptor, corresponding to a 20% PCE enhancement compared to the BP-free binary devices. The efficiency enhancement can be attributed to BP's high hole carrier mobility that facilitates better carrier extraction and suppression of the recombination. BP has been further demonstrated to be effective in additional solar cells based on other photovoltaic (PV) materials, including non-fullerene OSCs. The successful incorporation of the inexpensive BP in the solution-based fabrication of OSCs opens the door for its application in high performance, low cost flexible electronics.

Perovskite photovoltaics have recently attracted extensive attention due to their unprecedented high power conversion efficiencies (PCEs) in combination with primitive manufacturing conditions. However, the inherent polycrystalline nature of perovskite films renders an exceptional density of structural defects, especially at the grain boundaries (GBs) and film surfaces, representing a key challenge that impedes the further performance improvement of perovskite solar cells (PSCs) and large solar module ambitions towards commercialization. Here, a novel strategy is presented utilizing a simple ethylammonium chloride (EACl) additive in combination with a facile solvent bathing approach to achieve high quality methyammonium lead iodide (MAPbI₃) films. Well-oriented, micron-sized grains were observed, which contribute to an extended carrier lifetime and reduced trap density. Further investigations unraveled the distinctively prominent effects of EACl in modulating the perovskite film quality. The EACl was found to promote the perovskite grain growing without undergoing the formation of intermediate phases. Moreover, the EACl was also revealed to deplete at relative low temperature to enhance the film quality without compromising the beneficial bandgap for solar cell applications. This new strategy boosts the power conversion efficiency (PCE) to 20.9% and 19.0% for devices with effective areas of 0.126 cm² and 1.020 cm², respectively, with negligible current hysteresis and enhanced stability. Besides, perovskite films with a size of 10 × 10 cm², and an assembled 16 cm² (5 × 5 cm² module) perovskite solar module with a PCE of over 11% were constructed.

Very recently, non-fullerene acceptors (NFAs) based on low-bandgap small molecules have emerged as a new class of acceptors that rival the dominance of fullerene-based acceptors. Such discovery also stimulates promising device architectures such as ternary solar cells, with a handful that have achieved high power conversion efficiencies above 12%. The primary effort, however, has been focusing on low-bandgap NFAs that exploit complementary absorption and energy-level cascade. Herein we report a rare example of a wide-bandgap NFA that leads to high-performance ternary solar cells without relying on full absorption complementarity of all three components. Detailed studies revealed the multiple roles of this acceptor in blend films, which contribute synergistically to improved device characteristics. This work may inspire new design principles of potent wide-bandgap NFAs, which will open the door to high-efficiency organic photovoltaic devices through new opportunities such as multi-component solar cells.

Ternary structure is an important design strategy to obtain high-efficiency non-fullerene organic photovoltaics (OPVs). However, the role of the third component to the standard binary system is still unclear. Here, a wide-bandgap small-molecule acceptor, denoted IDT-T, is synthesized and used together with a wide-bandgap donor polymer, PBDB-T, and a low-bandgap acceptor, ITIC, for fullerene-free ternary solar cells. The ternary cell features an enhanced power conversion efficiency (PCE) up to 12.2%, together with improved photocurrent density, open-circuit voltage (V_OC), and fill factor. Studies of the thin films indicate that IDT-T functions as an energy-level mediator, a fluorescence resonance energy-transfer donor, an electron acceptor, and a crystallization modulator in the blend, which contribute synergistically in the ternary blend to deliver a higher V_OC, more efficient exciton generation, suppressed bimolecular charge recombination and enhanced charge transport, and an overall high photovoltaic performance.

In this work, two wide bandgap polymers of PDTT-TZNT and PDTF-TZNT were developed by Stille-coupling of naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) acceptor unit with bithiophene (DTH) and fluorinated bithiophene (DTF), respectively. These polymers exhibited a wide bandgap over 1.84 eV. The fluorinated PDTF-TZNT had lower highest occupied molecular orbital HOMO level (− 5.24 eV), higher molar absorption coefficient (1.28 × 10⁵ M⁻¹ cm⁻¹), and higher molecular packing order. Using a low bandgap 3,9-bis(2-methylene-(5&6-methyl-(3-(1,1-dicyanomethylene)-indanone)))−5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (IT-M) as the electron acceptor, the PDTF-TZNT:IT-M devices generated a higher power conversion efficiency (PCE) of 10.05%. To make up the weak absorption of above binary blend of PDTF-TZNT:IT-M in the short wavelength region and increase the device performance further, a large bandgap small molecular acceptor of 5,5,10,10,15,15-hexabutyl-2,7,12-tri(4-(3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile-benzothiadiazole)-truxene (meta-TrBRCN) was added as the second acceptor material to fabricate ternary blend PSCs. The meta-TrBRCN could not only expand the absorption range but also fine-tune the blend morphology by stepwise changing its content. When 0.2 of meta-TrBRCN was added, the PCE of PDTF-TZNT:IT-M devices was improved to 11.48%.

Two novel naphthobistriazole-based wide bandgap polymers, PDTH-TZNT and PDTF-TZNT, has been developed for non-fullerene polymer solar cells. The fluorinated PDTF-TZNT exhibited a higher PCE of 10.05% in binary PDTT-FTAZ:IT-M devices. By adding 0.2 of meta-TrBRCN as the second acceptor, the related ternary blend devices exhibited a significantly improved PCE of 11.48%.