Elucidating Compositional Differences in Halide Perovskites for Normal and Inverted Perovskite Solar Cells

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Improving the interface characteristics between the hole-transport layer (HTL) and perovskite absorber layer is crucial for achieving maximum efficiency in inverted perovskite solar cells (PSCs). This paper presents an effective functional compensation layer (FCL) composed of benzothiophene derivatives, particularly 5-(trifluoromethyl)-1-benzothiophene-2-carboxylic acid (TFMBTA); this layer is introduced between the MeO-2PACz HTL and perovskite absorber layer to improve the interfacial characteristics between them. This FCL improves charge transfer, hole extraction, and perovskite deposition by improving the surface morphology of the HTL and optimizing the energy level alignment. The functional groups of TFMBTA effectively passivate perovskite defects. As a result, the introduction of the TFMBTA FCL markedly reduces non-radiative recombination at the interface between the HTL and perovskite layer. The MeO-2PACz-based PSCs with the FCL demonstrated an impressive peak power conversion efficiency of 23.85%, accompanied by substantially enhanced open-circuit voltage (Voc), fill factor (FF), and long-term stability. Similarly, introducing the TFMBTA FCL between the PEDOT:PSS HTL and the perovskite layer enhanced both the stability and efficiency of PSCs, demonstrating the universality of FCLs across different types of HTLs.

An essential role is played by the charge selective layers, in particular, the hole transport layer (HTL) in determining the performance of perovskite solar cells (PSCs). HTL contribution is even more substantial in the inverted PSCs (p‐i‐n) as it also affects the crystallization process of the overlayer perovskite deposition. The influence of HTL on inverted PSCs was probed, the role through admittance spectroscopy quantified, and the photovoltaic performance correlation with the microstructure of the perovskite established. By measuring the low‐frequency impedance responses of inverted PSCs, the photo‐induced charge transfer dynamics in the PSCs were deciphered, revealing negative chemical inductor features at low frequencies in the Nyquist plots. The noted additional recombination pathways are attributed to the ionic vacancies in the interfacial states. The PSCs fabricated with poly(triaryl amine) showed a higher open‐circuit voltage, lower recombination, and higher charge carrier lifetime. The chemical inductance behavior was supported through variable temperature frequency‐dependent capacitance measurements.

Thermally-stable and highly-efficient bi-layered NiOx-based inverted planar perovskite solar cells by employing a p-type organic semiconductor

Revivification of nickel oxide-perovskite interfaces via nickel nitrate to boost performance in perovskite solar cells

To commercialize perovskite solar cells (PSCs), it is crucial to develop cost‐effective, dopant‐free hole transport layers (HTLs) that can be processed at low temperatures. Herein, a dopant‐free small molecular material 4,4′,4′‐Tris[2‐naphthyl(phenyl)amino]triphenylamine (2TNATA) was utilized in inverted PSCs as a HTL. The position of the highest occupied molecular orbital energy of 2TNATA is properly aligned with the perovskite valence band maximum. Moreover, 2TNATA can be processed at lower temperatures and shows excellent thermal stability. The lead (Pb) perovskite on 2TNATA exhibited superior crystallinity and morphology compared to the perovskite on poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). Furthermore, the carrier kinetics in 2TNATA‐based PSCs was superior to PTAA and PEDOT:PSS‐based PSCs. Consequently, an outstanding power conversion efficiency (PCE) of 20.58% was observed from the 2TNATA HTL‐based 0.09 cm2 PSCs, while PTAA and PEDOT:PSS HTLs‐based 0.09 cm2 PSCs showed PCE of 19.36% and 14.35%, respectively. Moreover, the 2TNATA HTL‐based 1.0 cm2 PSCs demonstrated an impressive PCE of 20.04%. The results indicate that 2TNATA might be a promising HTL for the inexpensive and efficient inverted PSCs.

The performances of perovskite solar cells (PSCs) largely depend on the perovskite compositions and the selection of electron and hole transport layers (ETLs and HTLs). The p‐type NiO x films are largely used as HTLs in p‐i‐n PSCs, thanks to their high transparency, processing versatility, cost‐effectiveness, and easy integration within tandem devices. Several studies have shown that surface modifications on NiO x films remove the surface defects, increase the NiO x conductivity, and alter the band offset, consequently improving the interfaces between NiO x films and the perovskite active layer. Indeed, besides improving the NiO x intrinsic properties, the surface treatments also lead, in many cases, to superior perovskite quality driving high photovoltaic performance.

Though nickel oxide (NiOx) has been widely used as a hole transport layer in inverted perovskite solar cells (PSCs), the performance of NiOx-based PSCs is limited by low conduction of NiOx, surface defects, lattice mismatch, poor energy level alignment, and redox reactions at the NiOx/perovskite interface. To address these issues, a series of small dicarboxylic acid molecules (DAMs) are introduced as a buffer layer between NiOx and the perovskite films in this study. Utilizing the double carboxylic acid moieties to orderly anchor NiOx, DAMs effectively passivate defects of NiOx and inhibit interfacial redox reactions. Among the DAMs, pyridine-3,5-dicarboxylic acid (P35DA) owns unique pyridine rings, which induce more favorable dipole moments for energy level alignment and interact with uncoordinated lead ion, thus regulating the crystallization of perovskite, reducing the interfacial tensile strain, and suppressing nonradiative recombination. Consequently, the devices based on NiOx/P35DA exhibit the champion power conversion efficiency (PCE) of 24.05% and 21.48% for 1.56 and 1.68 eV PSCs, respectively. Meanwhile, the unencapsulated devices maintain 81% of their initial PCE after being stored in air with 50–60% relative humidity for 1200 h, exhibiting remarkable environmental stability.

Inverted (p-i-n) perovskite solar cells (PSC) have demonstrated significant promise compared to the conventional n-i-p architecture, owing to their improved operational stability, reduced hysteresis, and low-temperature fabrication process. These advantages make inverted devices particularly attractive for tandem and flexible applications. Self-assembled molecules (SAM) have been used lately in hole transport layer (HTL) of inverted PSC while the shortcomings like bad coverage and homogeneity, misalignment and weaker compactness with the substrate have appealed for its hybrid self-assembled molecular (HSAM) approaches. The compactness, coverage, and homogeneity of SAM can be further enhanced through HSAM, which leads to higher crystallinity, better band alignment, improved nucleation, and reduced non-radiative recombination losses in inverted PSC. Herein, the effect of HSAM is summarized at the HTL of inverted perovskite through their structural characteristics, interface passivation effects, crystallization modulation effect, dipole effect, redox behavior effects and the prevention of SAM aggregation in perovskite films for providing insights to broaden the impact of HSAM in inverted PSC for future research and development.

In perovskite solar cells (PSCs), the hole transport layer (HTL) is a crucial component, especially in inverted PSCs, which require enhanced cost efficiency, high mobility, excellent transmittance, and stability. The performance of inverted perovskite solar cells remains lower than the regular PSCs because of interfacial defects, poor electrical conductivity, and unfavorable band alignment between the perovskite and the HTL (NiOx). In this work, fluorine‐incorporated nickel oxide (F doped NiOx) NPs are prepared using a co‐precipitation approach and subsequently used as a HTL in the PSCs. Fluorine is effectively incorporated into the NiOx, resulting in strong bonding and a more stable structure, which passivates the hydroxyl groups from the NiOx surface and decreases defect sites. Moreover, fluorine is established as an efficient dopant for nickel oxide, which minimizes the formation of nickel vacancies while simultaneously enhancing the density of Ni3+ ions. Additionally, the electronic conductivity was enhanced, and the work function was increased with the F‐doped NiOx film, which can facilitate hole extraction and reduce the recombination rate. Therefore, the F‐doped NiOx PSCs achieved a remarkable power conversion efficiency of up to 20.78%. The F‐doped NiOx also lead to improvement in stability under several environmental conditions, such as air, light soaking, and heating (65°C).

Hole transport layers (HTLs) play a key role in perovskite solar cells (PSCs), particularly in the inverted PSCs (IPSCs) that demand more in its stability. In this study, samarium‐doped nickel oxide (Sm:NiOx) nanoparticles are synthesized via a chemical precipitation method and deposited as a hole transport layer in the IPSCs. Sm3+ doping can reduce the formation energy of Ni vacancy and naturally increase the density of Ni vacancies, thereby rendering increased hole density. Thenceforth, the electronic conductivity is enhanced significantly, and work function enlarged in the Sm:NiOx film in favor of extracting holes and suppressing charge recombination. Consequently, the Sm:NiOx‐based IPSCs attain outstanding power conversion efficiencies as high as 20.71%. Even when it is applied in flexible solar cells, it still outputs efficiency as high as 17.95%. More importantly, the Sm:NiOx is compatible with large‐scale processing whereby the large area IPSCs of 1.0 cm2 and 40 × 40 mm2 deliver high efficiencies of 18.51% and 15.27%, respectively, all are among the highest for the inorganic HTLs based IPSCs. This research demonstrates that, while revealing the doping effect in depth, Sm:NiOx can be a promising hole transport material for fabricating efficient, large‐area, and flexible IPSCs in the future.

Modifying perovskite surface using various organic ammonium halide cations has proven to be an effective approach for enhancing the overall performance of perovskite solar cells. Nevertheless, the impact of the structural symmetry of these ammonium halide cations on perovskite interface termination has remained uncertain. Here, this work investigates the influence of symmetry on the performance of the devices, using molecules based on symmetrical bis(2-chloroethyl)ammonium cation (B(CE)A+) and asymmetrical 2-chloroethylammonium cation (CEA+) as interface layers between the perovskite and hole transport layer. These results reveal that the symmetrical B(CE)A+ cations lead to a more homogeneous surface potential and more comprehensive chelation with uncoordinated Pb2+ compared to the asymmetrical cations, resulting in a more favorable energy band alignment and strengthened defect healing. This strategy, leveraging the spatial geometrical symmetry of the interface cations, promotes hole carrier extraction between functional layers and reduces nonradiative recombination on the perovskite surface. Consequently, perovskite solar cells processed with the symmetrical B(CE)A+ cations achieve a power conversion efficiency (PCE) of 25.60% and retain ≈91% of their initial PCE after 500 h of maximum power point operation. This work highlights the significant benefits of utilizing structurally symmetrical cations in promoting the performance and stability of perovskite solar cells.

Interfacial modulation utilizing functional materials is proven to be crucial for obtaining high photovoltaic performance in lead halide perovskite solar cells (PSCs). This study investigates, for the first time, the utilization of a pyrene-based sp2 carbon-conjugated covalent organic framework (sp2c-COF) as an interfacial layer in inverted PSCs. Functionalized with cyano (-CN) Lewis base groups, the sp2c-COF exhibits a dual effect, simultaneously passivating both the NiOx and the perovskite layers. Detailed characterization results highlight the role of sp2c-COF in reducing the Ni3+ defect density in NiOx films and forming Lewis acid-base adducts with undercoordinated Pb2+ on the perovskite surfaces, thereby inhibiting interfacial redox reactions and suppressing non-radiative recombination. Moreover, sp2c-COF leads to improved crystallinity of perovskite films. Benefiting from the synergistic effects, sp2c-COF-modified devices delivered a champion efficiency of 17.64%. These findings underscore the potential of sp2c-COF as a functional interface material for PSCs, offering enhanced efficiency and stability. The study contributes to advancing the understanding and application of covalent organic frameworks in photovoltaic technologies.

Solution-processed organo-lead halide perovskite solar cells with deep pinholes in the perovskite layer lead to shunt-current leakage in devices. Herein, we report a facile method for improving the performance of perovskite solar cells by inserting a solution-processed polymer layer between the perovskite layer and the hole-transporting layer. The photovoltaic conversion efficiency of the perovskite solar cell increased to 18.1% and the stability decreased by only about 5% during 20 days of exposure in moisture ambient conditions through the incorporation of a poly(methyl methacrylate) (PMMA) polymer layer. The improved photovoltaic performance of devices with a PMMA layer is attributed to the reduction of carrier recombination loss from pinholes, boundaries, and surface states of perovskite layer. The significant gain generated by this simple procedure supports the use of this strategy in further applications of thin-film optoelectronic devices.

Inverted (p‐i‐n) perovskite solar cells have drawn great attention due to their outstanding stability and low‐temperature processibility. However, their power conversion efficiency (PCE) still lags behind conventional (n‐i‐p) devices mainly due to the lack of strategies to stabilize α‐FAPbI3 without changing the bandgap. In this work, a facile and effective strategy is reported to regulate the residual strain via pseudo halide‐based ionic liquids incorporation to stabilize α‐FAPbI3 perovskite in inverted perovskite solar cells (PVSCs). The employment of methylamine formate (MAFa) ionic liquid enables a homogenously stronger compressive strain to restrain the transition of shared‐corner PbI6 octahedron into shared‐face δ‐FAPbI3, as well as affecting the dynamic behavior of carriers and defects to achieve a record PCE (24.08%) among the reported inverted FAPbI3 perovskite solar cells up to now. In addition, the MAFa incorporation results in enhanced device stability, unencapsulated PVSC retains over 90% of its initial efficiency after stored in ambient environment (RH:30 ± 5%) for 1000 h. This work provides an efficient strategy to realize efficient and stable α‐FAPbI3 based inverted PVSCs to further catch up with the conventional ones.