Foldable Hole-Transporting Materials for Merging Electronic States between Defective and Perfect Perovskite Sites

Enhanced Long-Term Stability of Perovskite Solar Cells Employing a Benzodithiophene Derivative-Based Dopant-Free Hole Transporting Layer

Perovskite solar cell (PSC) is characterized by high photoelectric conversion efficiency (PCE) and low material cost compared to the silicon solar cell that has already been commercialized. Many researchers are conducting various studies to improve efficiency and stability. To increase power conversion efficiency, hole transporting materials (HTMs) are one of the most important factors for achieving high performance in PSCs. HTMs exhibit an important role in PSCs to transfer the positive charges in between perovskite and counter electrodes. Among the various HTMs, Spiro-OMeTAD has been widely used in PSCs. However, due to its reliance on additives to enhance hole mobility, Spiro-OMeTAD suffers from degradation issues that compromise its long-term stability. So, it is necessary to develop new HTMs to replace Spiro-OMeTAD for their low stability and expensiveness in future application of PSCs. Therefore, we designed and synthesized the materials named PEH-23 and PEH-24 incorporating dialkoxyphenyl and dialkoxybenzodithiophene. As the conjugation length increases, the material demonstrates improved stability, suggesting its potential as an effective HTM with long-term reliability.

Toughened Interface by Engineering the Side Group of Conjugated Polymers to Stabilize Flexible Perovskite Solar Modules

Perovskite solar cells (PSCs) have attracted considerable attention due to their high power conversion efficiency (PCE), cost-effective manufacturing processes, as well as the potential flexibility. However, a significant challenge to the commercial applications of PSCs is their mechanical reliability. In this work, three naphthalene diimide polymers with distinct donor units are chosen to reduce surface trap states and enhance the long-term stability and mechanical reliability of photovoltaic devices. The champion rigid PSCs incorporating conjugated polymers achieved a 373% increase of adhesion toughness at the interface, with a champion PCE of 25.5% for a 0.16 cm2 single cell and 22.3% for a 30.9 cm2 module and retain 97% of the initial efficiency after 2000 h of continuous light soaking. Especially, the flexible PSCs exhibited improved mechanical stability, achieving a champion PCE of 24.8% for a 0.16 cm2 single cell and 20.3% for a 27.9 cm2 module, maintaining 95% of the initial efficiency after 5,000 bending cycles. This study highlights the potential of interfacial conjugated polymer in enhancing the efficiency and stability of PSCs.

P-Dopant with Spherical Anion for Stable n-i-p Perovskite Solar Cells

Li-TFSI/t-BP is the most widely utilized p-dopant for hole-transporting materials (HTMs) in state-of-the-art perovskite solar cells (PSCs). However, its nonuniformity of doping, along with the hygroscopicity and migration of dopants, results in the devices exhibiting limited stability and performance. This study reports on the utilization of a spherical anion derived from the p-dopant, regulated by its radius and shape, as an alternative to the linear TFSI- anion. The theoretical and experimental results reveal that the spherical anion significantly increases the doping effect of HTMs due to an enhanced electron transfer from larger dipole moments. The enhanced transfer leads to a shift in the Pb-6p defect orbitals, resulting in shallower trap states. Moreover, compared to the linear structure of the TFSI- anion, the anion of sodium tetrakis[3,5-bis(trifluoro methyl)phenyl]borate (Na-TFPB) with a larger van der Waals radius and spherical shape offers increased hydrophobicity and migration barriers, which can protect the perovskite crystal and facilitate stable p-doping of HTMs. The use of Na-TFPB results in enhanced thermal and ambient stability of PSCs. The devices fabricated with the shape- and radius-regulated p-dopant achieve remarkable efficiencies of 24.49 % and 24.31 % for CJ-01 and spiro-OMeTAD, respectively, representing the highest efficiency values for organic dopants to date. This study underscores the ingenious design of spherical anions of p-dopants in contrast to the conventional linear anions.

Quasi-Planar Core Based Spiro-Type Hole-Transporting Material for Dopant-Free Perovskite Solar Cells

Hole-transporting material (HTMs) are crucial for obtaining the stability and high efficiency of perovskite solar cells (PSCs). However, the current state-of-the-art n-i-p PSCs relied on the use of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) exhibit inferior intrinsic and ambient stability due to the p-dopant and hydrophilic Li-TFSI additive. In this study, a new spiro-type HTM with a critical quasi-planar core (Z-W-03) is developed to improve both the thermal and ambient stability of PSCs. The results suggest that the planar carbazole structure effectively passivates the trap states compared to the triphenylamine with a propeller-like conformation in spiro-OMeTAD. This passivation effect leads to the shallower trap states when the quasi-planar HTMs interact with the Pb-dimer. Consequently, the device using Z-W-03 achieves a higher Voc of 1.178 V compared to the spiro-OMeTAD's 1.155 V, resulting in an enhanced efficiency of 24.02 %. In addition, the double-column π-π stacking of Z-W-03 results in high hole mobility (~10-4 cm2 V-1 s-1) even without p-dopant. Moreover, when the surface interface is modified, the undoped Z-W-03 device can achieve an efficiency of nearly 23 %. Compared to the PSCs using spiro-OMeTAD, those with Z-W-03 exhibit enhanced stability under N2 and ambient conditions. This superior performance is attributed to the quasi-planar core structure and the presence of multiple CH/π and π-π intermolecular stacking in Z-W-03. The multiple CH/π and π-π intermolecular contacts of HTMs can improve the hole hopping transport. Therefore, it is imperative to focus on further molecular structure design and optimization of spiro-type HTMs incorporating quasi-planar cores and carbazole moieties for the commercialization of PSCs.

Organic-inorganic hybrid material for hole transport in inverted perovskite solar cells

Hole mobility is critical to the power conversion efficiencies of perovskite solar cells (PSCs). Organic small-molecule hole-transporting materials (HTMs) have attracted considerable interest in PSCs due to their structural flexibility and operational durability, but they suffer from modest hole mobility. On the other hand, inorganic HTMs with good hole mobility are inflexible in structural variation and exhibit unsatisfactory cell efficiency. In this study, a ligand BT28 and its zinc-based coordination complex BTZ30 were synthesized, characterized, and investigated as HTMs for PSC applications. The mixed-halide perovskites can be grown uniformly with large crystalline grains on both HTMs, which exhibit similar optical and electrochemical properties. However, it was discovered that the BTZ30-based solar cell exhibited an open-circuit voltage of 1.0817 V and a high short-circuit current density of 23.1392 mA cm-2 with a champion power conversion efficiency of close to 20 %. The performance difference between the two HTMs can be attributed to the difference in their hole mobilities, which is 63.31 % higher for BTZ30 than BT28. The comparison of non-metal and metal HTMs revealed the importance of considering hybrid structures to overcome some shortcomings associated with organic and inorganic HTMs and achieve high-performance PSCs.

Multifunctional Trifluoroborate Additive for Simultaneous Carrier Dynamics Governance and Defects Passivation to Boost Efficiency and Stability of Inverted Perovskite Solar Cells

The main obstacles to promoting the commercialization of perovskite solar cells (PSCs) include their record power conversion efficiency (PCE), which still remains below the Shockley-Queisser limit, and poor long-term stability, attributable to crystallographic defects in perovskite films and open-circuit voltage (Voc) loss in devices. In this study, potassium (4-tert-butoxycarbonylpiperazin-1-yl) methyl trifluoroborate (PTFBK) was employed as a multifunctional additive to target and modulate bulk perovskite defects and carrier dynamics of PSCs. Apart from simultaneously passivating anionic and cationic defects, PTFBK could also optimize the energy-level alignment of devices and weaken the interaction between carriers and longitudinal optical phonons, resulting in a carrier lifetime of greater than 3 μs. Furthermore, it inhibited non-radiative recombination and improved the crystallization capacity in the target perovskite film. Hence, the target rigid and flexible p-i-n PSCs yielded champion PCEs of 24.99 % and 23.48 %, respectively. More importantly, due to hydrogen bonding between formamidinium and fluorine, the target devices exhibited remarkable thermal, humidity, and operational tracking at maximum power point stabilities. The reduced Young's modulus and residual stress in the perovskite layer also provided excellent bending stability for flexible target devices.

Crystallization Regulation by Self-Assembling Liquid Crystal Template Enables Efficient and Stable Perovskite Solar Cells

It is found that the disordered growth of bottom perovskite film deteriorates the buried interface of perovskite solar cells (PSCs), so developing a new material to modify the buried interface for regulating the crystal growth and defect passivation is an effective approach for improving the photovoltaic performance of PSCs. Here, we developed a new ionic liquid crystal (ILC, 1-Dodecyl-3-methylimidazolium tetrafluoroborate) as both crystal regulator and defect passivator to modify the buried interface of PSCs. The high lattice matching between this ILC and perovskite promotes preferential growth of perovskite film along [001] direction, while the oriented ILC with mesomorphic phase has a strong chemical interaction with perovskite to passivate the interface defect, as a result, the modified buried interface exhibits suppressed defects, improved band alignment, reduced nonradiative recombination losses, and enhanced charge extraction. The ILC-modified PSC delivers a power conversion efficiency of 24.92 % and maintains 94 % of the original value after storage in ambient for 3000 h.