Organic-semiconductor-assisted dielectric screening effect for stable and efficient perovskite solar cells

Recent Progress in Dopant-Free and Green Solvent-Processable Organic Hole Transport Materials for Efficient and Stable Perovskite Solar Cells

Dopant-free hole transport layers (HTLs) are crucial in enhancing perovskite solar cells (pero-SCs). Nevertheless, conventional processing of these HTL materials involves using toxic solvents, which gives rise to substantial environmental concerns and renders them unsuitable for large-scale industrial production. Consequently, there is a pressing need to develop dopant-free HTL materials processed using green solvents to facilitate the production of high-performance pero-SCs. Recently, several strategies have been developed to simultaneously improve the solubility of these materials and regulate molecular stacking for high hole mobility. In this review, a comprehensive overview of the methodologies utilized in developing dopant-free HTL materials processed from green solvents is provided. First, the study provides a brief overview of fundamental information about green solvents and Hansen solubility parameters, which can serve as a guideline for the molecular design of optimal HTL materials. Second, the intrinsic relationships between molecular structure, solubility in green solvents, molecular stacking, and device performance are discussed. Finally, conclusions and perspectives are presented along with the rational design of highly efficient, stable, and green solvent-processable dopant-free HTL materials.

Band Structure Optimized by Electron-Acceptor Cations for Sensitive Perovskite Single Crystal Self-Powered Photodetectors

Low-dimensional perovskites afford improved stability against moisture, heat, and ionic migration. However, the low dimensionality typically results in a wide bandgap and strong electron-phonon coupling, which is undesirable for optoelectronic applications. Herein, semiconducting A-site organic cation engineering by electron-acceptor bipyridine (bpy) cations (2,2'-bpy2+ and 4,4'-bpy2+) is employed to optimize band structure in low-dimensional perovskites. Benefiting from the merits of lower lowest unoccupied molecular orbital (LUMO) energy for 4,4'-bpy2+ cation, the corresponding (4,4'-bpy)PbI4 is endowed with a smaller bandgap (1.44 eV) than the (CH3NH3)PbI3 (1.57 eV) benchmark. Encouragingly, an intramolecular type II band alignment formation between inorganic Pb-I octahedron anions and bpy2+ cations favors photogenerated electron-hole pairs separation. In addition, a shortening distance between inorganic Pb-I octahedral chains in (4,4'-bpy)PbI4 single crystal (SC) can effectively promote carrier transfer. As a result, a self-powered photodetector based on (4,4'-bpy)PbI4 SC exhibits 131 folds higher on/off ratio (3807) than the counterpart of (2,2'-bpy)2Pb3I10 SC (29). The presented result provides an effective strategy for exporting novel organic cation-based low-dimensional perovskite SC for high-performance optoelectronic devices.

Heterogeneous Nucleating Agent for High-Boiling-Point Nonhalogenated Solvent-Processed Organic Solar Cells and Modules

High-boiling-point nonhalogenated solvents are superior solvents to produce large-area organic solar cells (OSCs) in industry because of their wide processing window and low toxicity; while, these solvents with slow evaporation kinetics will lead excessive aggregation of state-of-the-art small molecule acceptors (e.g. L8-BO), delivering serious efficiency losses. Here, a heterogeneous nucleating agent strategy is developed by grafting oligo (ethylene glycol) side-chains on L8-BO (BTO-BO). The formation energy of the obtained BTO-BO; while, changing from liquid in a solvent to a crystalline phase, is lower than that of L8-BO irrespective of the solvent type. When BTO-BO is added as the third component into the active layer (e.g. PM6:L8-BO), it easily assembles to form numerous seed crystals, which serve as nucleation sites to trigger heterogeneous nucleation and increase nucleation density of L8-BO through strong hydrogen bonding interactions even in high-boiling-point nonhalogenated solvents. Therefore, it can effectively suppress excessive aggregation during growth, achieving ideal phase-separation active layer with small domain sizes and high crystallinity. The resultant toluene-processed OSCs exhibit a record power conversion efficiency (PCE) of 19.42% (certificated 19.12%) with excellent operational stability. The strategy also has superior advantages in large-scale devices, showing a 15.03-cm2 module with a record PCE of 16.35% (certificated 15.97%).

Organic Semiconductor Based on N, S-Containing Crown Ether Enabling Efficient and Stable Perovskite Solar Cells

The uncoordinated lead cations are ubiquitous in perovskite films and severely affect the efficiency and stability of perovskite solar cells (PSCs). In this work, 15-crown-5 with various heteroatoms are connected to the organic semiconductor carbazole diphenylamine, and two new compounds, CDT-S and CDT-N, are developed to modify the Pb2+ defects in perovskite films through the anti-solvent method. Apart from the oxygen atoms, there are also N atoms on crown ether ring in CDT-N, and both S and N heteroatoms in CDT-S. The heteroatoms enhance the interaction between the crown ether-based semiconductors and the undercoordinated Pb2+ defect in perovskite. Particularly, the stronger interaction between S atoms and Pb2+ further enhances the defect passivation effect of CDT-S than CDT-N, thereby more effectively suppressing the non-radiative recombination of charge carriers. Finally, the efficiency of the device treated with CDT-S is up to 23.05 %. Moreover, the unencapsulated device based on CDT-S maintained 90.5 % of the initial efficiency after being stored under dark conditions for 1000 hours, demonstrating good long-term stability. Our work demonstrates that crown ethers are promising in perovskite solar cells, and the crown ether containing multiple heteroatoms could effectively improve both efficiency and stability of devices.

High-performance pulse light stable perovskite indoor photovoltaics

Perovskite solar cells offer great potential as a sustainable power source for distributed electronic devices that operate indoors. However, the impact of advanced lighting technology, especially the widely used pulse width modulation (PWM) technology, on perovskite photovoltaics has been ignored. Herein, for the first time in photovoltaics, we find that the light impact emitted by the PWM lighting system caused dynamic strain in perovskite thin films, induced phase separation, and accelerated the generation of metallic lead (Pb0) defects, leading to irreversible degradation of the cell performance after 27 h (T80). To address this issue, formamidinium triiodide (FAI3) is chosen to treat the surface of the perovskite and release residual stress, resulting in reduced lattice deformation during dynamic strain processes. Meanwhile, it suppresses harmful Pb0 defects and reduces Voc loss at low light intensity. The champion device achieves impressive power conversion efficiency (PCE) of 35.14% and retains 99.5% of the initial PCE after continuous strobe light soaking for 2160 h.

Green Solvent Processable, Asymmetric Dopant-Free Hole Transport Layer Material for Efficient and Stable n-i-p Perovskite Solar Cells and Modules

The use of dopant-free hole transport layers (HTLs) is critical in stabilizing n-i-p perovskite solar cells (pero-SCs). However, these HTL materials are often processed with toxic solvents, which is not ideal for industrial production. Upon substituting them with green solvents, a trade-off emerges between maintaining the high crystallinity of the HTL materials and ensuring high solubility in the new solvents. In this paper, we designed a novel, linear, organic small molecule, BDT-C8-3O, by introducing an asymmetric polar oligo(ethylene glycol) side chain. This method not only overcomes the solubility limitations in green solvents but also enables stacking the conjugated main chains in two patterns, which further enhances crystallinity and hole mobility. As a result, the n-i-p pero-SCs based on chlorobenzene- or green (natural compound) solvent 3-methylcyclohexanone-processed BDT-C8-3O HTL that without any dopant delivered world-recorded power conversion efficiencies of 24.11 % (certified of 23.82 %) and 23.53 %, respectively. The devices also demonstrated remarkable operational and high-temperature stabilities, maintaining over 84 % and 79.5 % of their initial efficiency for 2000 h, respectively. Encouragingly, dopant-free BDT-C8-3O HTL exhibits significant advantages in large-area fabrication, achieving state-of-the-art PCEs exceeding 20 % for 5×5 cm2 modules (active area: 15.64 cm2 ), even when processed using green solvents.

Perovskite Grain-Boundary Manipulation Using Room-Temperature Dynamic Self-Healing "Ligaments" for Developing Highly Stable Flexible Perovskite Solar Cells with 23.8% Efficiency

Flexible perovskite solar cells (pero-SCs) are the best candidates to complement traditional silicon SCs in portable power applications. However, their mechanical, operational, and ambient stabilities are still unable to meet the practical demands because of the natural brittleness, residual tensile strain, and high defect density along the perovskite grain boundaries. To overcome these issues, a cross-linkable monomer TA-NI with dynamic covalent disulfide bonds, H-bonds, and ammonium is carefully developed. The cross-linking acts as "ligaments" attached on the perovskite grain boundaries. These "ligaments" consisting of elastomers and 1D perovskites can not only passivate the grain boundaries and enhance moisture resistance but also release the residual tensile strain and mechanical stress in 3D perovskite films. More importantly, the elastomer can repair bending-induced mechanical cracks in the perovskite film because of dynamic self-healing characteristics. The resultant flexible pero-SCs exhibit promising improvements in efficiency, and record values (23.84% and 21.66%) are obtained for 0.062 and 1.004 cm²  devices; the flexible devices also show overall improved stabilities with T₉₀  >20 000 bending cycles, operational stability with T₉₀  >1248 h, and ambient stability (relative humidity = 30%) with T₉₀  >3000 h. This strategy paves a new way for the industrial-scale development of high-performance flexible pero-SCs.