Reduction of Nonradiative Loss in Inverted Perovskite Solar Cells by Donor-π-Acceptor Dipoles

Non-Fullerene Organic Electron Transport Materials toward Stable and Efficient Inverted Perovskite Photovoltaics

Inverted perovskite solar cells (PSCs) attract continuing interest due to their low processing temperature, suppressed hysteresis, and compatibility with tandem cells. Considerable progress has been made with reported power conversion efficiency (PCE) surpassing 26%. Electron transport Materials (ETMs) play a critical role in achieving high-performance PSCs because they not only govern electron extraction and transport from the perovskite layer to the cathode, but also protect the perovskite from contact with ambient environment. On the other hand, the non-radiative recombination losses at the perovskite/ETM interface also limits the future development of PSCs. Compared with fullerene derivatives, non-fullerene n-type organic semiconductors feature advantages like molecular structure diversity, adjustable energy level, and easy modification. Herein, the non-fullerene ETM is systematically summarized based on the molecular functionalization strategy. Various types of molecular design approaches for producing non-fullerene ETM are presented, and the insight on relationship of chemical structure and device performance is discussed. Meantime, the future trend of non-fullerene ETM is analyzed. It is hoped that this review provides insightful perspective for the innovation of new non-fullerene ETMs toward more efficient and stable PSCs.

Impact of Dipole Effect on Perovskite Solar Cells

Interface modification and bulk doping are two major strategies to improve the photovoltaic performance of perovskite solar cells (PSCs). Dipolar molecules are highly favored due to their unique dipolarity. This review discusses the basic concepts and characteristics of dipoles. In addition, the role of dipoles in PSCs and the corresponding conventional characterization methods for dipoles are introduced. Then, we systematically summarize the latest progress in achieving efficient and stable PSCs in dipole materials at several key interfaces. Finally, we look forward to the future application directions of dipole molecules in PSCs, aiming at providing deep insight and inspiration for developing efficient and stable PSCs.

Challenges in the design and synthesis of self-assembling molecules as selective contacts in perovskite solar cells

Self-assembling molecules (SAMs), as selective contacts, play an important role in perovskite solar cells (PSCs), determining the performance and stability of these photovoltaic devices. These materials offer many advantages over other traditional materials used as hole-selective contacts, as they can be easily deposited on a large area of metal oxides, can modify the work function of these substrates, and reduce optical and electric losses with low material consumption. However, the most interesting thing about SAMs is that by modifying the chemical structure of the small molecules used, the energy levels, molecular dipoles, and surface properties of this assembled monolayer can be modulated to fine-tune the desired interactions between the substrate and the active layer. Due to the important role of organic chemistry in the field of photovoltaics, in this review, we will cover the current challenges for the design and synthesis of SAMs PSCs. Discussing, the structural features that define a SAM, (ii) disclosing how commercial molecules inspired the synthesis of new SAMs; and (iii) detailing the pros- and cons- of the reported synthetic protocols that have been employed for the synthesis of molecules for SAMs, helping synthetic chemists to develop novel structures and promoting the fast industrialization of PSCs.

Interfacial dipole engineering in all-inorganic perovskite solar cells

Severe nonradiative recombination and energy level mismatch in perovskite solar cells (PSCs) are key factors affecting efficiency. Here, we report an effective strategy for surface passivation and interfacial dipole engineering of perovskite films. By precisely introducing electron-withdrawing and electron-donating groups on 7-azaindole, we have effectively controlled the passivation ability of N atoms and the polarity of the interfacial dipole, thereby regulating the perovskite surface's work function and obtaining the optimal energy level matching. This strategy yields an impressive efficiency of 10.76% for the CsPbBr3 PSC and exceptional stability.

Bifunctional Hole-Transport Materials with Modification and Passivation Effect for High-Performance Inverted Perovskite Solar Cells

Hole-transport materials (HTMs) play an important role in perovskite solar cells (PSCs) to enhance the power conversion efficiency (PCE). The innovation of HTMs can increase the hole extraction ability and reduce interfacial recombination. Three organic small molecule HTMs with 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) as the central unit was designed and synthesized, namely, CPDTE-MTP (with the 2-ethylhexyl substituent and diphenylamine derivative end-group), CPDT-MTP (with the hexyl substituent and diphenylamine derivative end-group), and CPDT-PMTP (with the hexyl substituent and triphenylamine derivative end-group), which can form bifunctional and robust hole transport layer (HTL) on ITO and is tolerable to subsequent solvent and thermal processing. The X-ray photoelectron spectroscopy (XPS) results proved that CPDT-based HTMs can both interact with ITO through the nitrogen element in them and the tin element in ITO and passivate the upper perovskite layer. It is worth noting that the champion efficiency of MAPbI₃ PSCs based on CPDT-PMTP achieved 20.77%, with an open circuit voltage (V_OC) of 1.10 V, a short-circuit current (J_SC) of 23.39 mA cm^-2, and a fill factor (FF) of 80.83%, as three new materials were introduced into p-i-n PSCs as dopant-free HTMs.

Symmetry-Breaking Induced Dipole Enhancement for Efficient Spiro-Type Hole Transporting Materials: Easy Synthesis with High Stability

Spiral cores are crucial for designing efficient hole transporting materials (HTMs) for perovskite solar cells (PSCs), owing to their no-planar 3D architecture, high thermal stability, good solubility, and beneficial solid-state morphology. A lack of facile synthetic procedures for the spiral core limited the development of novel and stable spiral HTMs. In this regard, a one-step reaction is adopted to produce several novel acceptor-embedded spiral cores containing electron-withdrawing carbonyl group embedded orthogonal spiral conformation. After coupling with triphenylamine donors, symmetry-breaking spiral HTMs with uneven charge distribution can be obtained, bearing the advantages of adjustable dipole moment and enhanced structural stability. A combined theoretical and experimental study shows that the HTM with a stronger dipole moment can easily adsorb on the surface of perovskite via electrostatic potential, and the closer distance promoted facile hole transfer from perovskite to HTMs. In the end, PSCs based on strongly polarized spiro-BC-OMe achieved efficient hole extraction and thus an improved fill factor, promoting a power conversion efficiency (PCE) of 22.15%, and a module-based PCE of 18.61% with an active area of 16.38 cm² . This study provides a new avenue for designing HTMs with strong dipole moments for efficient PSCs.

Improved p-i-n MAPbI3 perovskite solar cells via the interface defect density suppression by PEABr passivation

Organic-inorganic hybrid perovskite solar cells (PSCs) are promising candidates for next-generation photovoltaics due to their excellent optoelectronic properties and process compatibility. In this report, numerical simulations show the effect of perovskite surface defect density on the inverted MAPbI₃ perovskite device. The Phenethylammonium bromide (PEABr) is introduced to passivate the MAPbI₃ layer surface of the perovskite solar cell devices, PEA⁺ diffuses into the grain boundaries of the 3D perovskite to form 2D/3D hybrid structure during the thermal annealing process, thus improve the surface morphology and decrease the interface defects between MAPbI₃ layer and PCBM layer. The power conversion efficiency (PCE) of the PSCs increased from 17.95% to 19.24% after PEABr treatment. In addition, the 2D/3D hybrid structure can also hinder the intrusion of water and oxygen, the stability of perovskite devices has been greatly improved.

Anchoring Vertical Dipole to Enable Efficient Charge Extraction for High-Performance Perovskite Solar Cells

Perovskite solar cells (PSCs) via two-step sequential method have received great attention in recent years due to their high reproducibility and low processing costs. However, the relatively high trap-state density and poor charge carrier extraction efficiency pose challenges. Herein, highly efficient and stable PSCs via a two-step sequential method are fabricated using organic-inorganic (OI) complexes as multifunctional interlayers. In addition to reduce the under-coordinated Pb2+ ions related trap states by forming interactions with the functional groups, the complexes interlayer tends to form dipole moment which can enhance the built-in electric field, thus facilitating charge carrier extraction. Consequently, with rational molecular design, the resulting devices with a vertical dipole moment that parallels with the built-in electric field yield a champion efficiency of 23.55% with negligible hysteresis. More importantly, the hydrophobicity of the (OI) complexes contributes to an excellent ambient stability of the resulting device with 91% of initial efficiency maintained after 3000 h storage.

Multifunctional Chemical Linker in Buried Interface for Stable and Efficient Planar Perovskite Solar Cells

The buried interface between perovskite light absorbing layer (PVK) and electron transport layer (ETL) plays an utmost important role for further improving the efficiency and stability of planar perovskite solar...