Ligang Yuan

Multijunction organic solar cells provide higher power conversion efficiencies than the corresponding single junction solar cells by reducing thermalization and transmission losses and are fabricated by sequential layer deposition from solution. In recent years, important progress has been made in terms of novel materials and device design and the most salient advances are discussed.

Abstract

The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution‐processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these devices. Next to developing new materials and processing methods for the photoactive and interconnecting layers, specific layers or stacks are designed to increase light absorption and improve the photocurrent by utilizing optical interference effects. These activities have resulted in power conversion efficiencies that approach those of modern thin film photovoltaic technologies. Multijunction cells require more elaborate and intricate characterization procedures to establish their efficiency correctly and a critical view on the results and new insights in this matter are discussed. Application of multijunction cells in photoelectrochemical water splitting and upscaling toward a commercial technology is briefly addressed.

Formamidinium iodide (FAI) post‐treatment is used on MAPbI₃ surfaces to obtain high‐quality perovskite films, which leads to traps‐states or defects reduction, thus reducing hysteresis. The FAI post‐treated solar cells show improved device performance, with the average efficiency increasing from 16.86% to 18.40%, and the power conversion efficiency reaching 20.25%.

Abstract

Organolead trihalide perovskite films with a large grain size and excellent surface morphology are favored to good‐performance solar cells. However, interstitial and antisite defects related trap‐states are originated unavoidably on the surfaces of the perovskite films prepared by the solution deposition procedures. The development of post‐growth treatment of defective films is an attractive method to reduce the defects to form good‐quality perovskite layers. Herein, a post‐treatment tactic is developed to optimize the perovskite crystallization by treating the surface of the one‐step deposited CH₃NH₃PbI₃ (MAPbI₃) using formamidinium iodide (FAI). Charge carrier kinetics investigated via time‐resolved photoluminescent, open‐circuit photovoltage decay, and time‐resolved charge extraction indicate that FAI post‐treatment will boost the perovskite crystalline quality, and further result in the reduction of the defects or trap‐states in the perovskite films. The photovoltaic devices by FAI treatment show much improved performance in comparison to the controlled solar cell. As a result, a champion solar cell with the best power conversion efficiency of 20.25% is obtained due to a noticeable improvement in fill factor. This finding exhibits a simple procedure to passivate the perovskite layer via regulating the crystallization and decreasing defect density.

A new type of high‐performance, good solubility, and thermally stable emitters based on a novel unsymmetrical heterotruxene core, functionalized with carbazole and diphenylamine, is introduced. The electroluminescence efficiencies of solution‐processed devices reach 3.7% for a deep‐blue emitter and 7.0% for a green emitter with CIE color coordinates (0.16, 0.09) and (0.22, 0.40), respectively at 100 cd m⁻².

Abstract

The development of blue materials with good efficiency, even at high brightness, with excellent color purity, simple processing, and high thermal stability assuring adequate device lifetime is an important remaining challenge for organic light‐emitting didoes (OLEDs) in displays and lightning applications. Furthermore, these various features are typically mutually exclusive in practice. Herein, four novel green and blue light‐emitting materials based on a monothiatruxene core are reported together with their photophysical and thermal properties, and performance in solution‐processed OLEDs. The materials show excellent thermal properties with high glass transition temperatures ranging from 171 to 336 °C and decomposition temperatures from 352 to 442 °C. High external quantum efficiencies of 3.7% for a deep‐blue emitter with CIE color co‐ordinates (0.16, 0.09) and 7% for green emitter with color co‐ordinates (0.22, 0.40) are achieved at 100 cd m⁻². The efficiencies observed are exceptionally high for fluorescent materials with photoluminescence quantum yields of 24% and 62%, respectively. The performance at higher brightness is very good with only 38% and 17% efficiency roll‐offs at 1000 cd m⁻². The results indicate that utilization of this unique molecular design is promising for efficient deep‐blue highly stable and soluble light‐emitting materials.

The latest progress and issues toward scalable fabrication of perovskite solar cells are reviewed in an attempt to provide insights on the development of rational fabrication methods for large‐area perovskite films and solar modules.

Abstract

With the impressive record power conversion efficiency (PCE) of perovskite solar cells exceeding 23%, research focus now shifts onto issues closely related to commercialization. One of the critical hurdles is to minimize the cell‐to‐module PCE loss while the device is being developed on a large scale. Since a solution‐based spin‐coating process is limited to scalability, establishment of a scalable deposition process of perovskite layers is a prerequisite for large‐area perovskite solar modules. Herein, this paper reports on the recent progress of large‐area perovskite solar cells. A deeper understanding of the crystallization of perovskite films is indeed essential for large‐area perovskite film formation. Various large‐area coating methods are proposed including blade, slot‐die, evaporation, and post‐treatment, where blade‐coating and gas post‐treatment have so far demonstrated better PCEs for an area larger than 10 cm². However, PCE loss rate is estimated to be 1.4 × 10⁻²% cm⁻², which is 82 and 3.5 times higher than crystalline Si (1.7 × 10⁻⁴% cm⁻²) and thin film technologies (≈4 × 10⁻³% cm⁻²) respectively. Therefore, minimizing PCE loss upon scaling‐up is expected to lead to PCE over 20% in case of cell efficiency of >23%.

Perovskite crystallization kinetics can be finely controlled using a self‐doped conducting polymer as the hole injection layer of perovskite light‐emitting diodes (PeLEDs). Polar solvent‐soluble self‐doped polyaniline facilitates crystallization control by impeding the solvent evaporation from cast perovskite precursor pseudo‐films. The finely controlled crystallization contributes to achieving granular nanograin structure, which can strengthen the exciton confinement for boosting luminescence efficiency of PeLEDs.

Abstract

Organic–inorganic hybrid perovskites (OHPs) are promising emitters for light‐emitting diodes (LEDs) due to the high color purity, low cost, and simple synthesis. However, the electroluminescent efficiency of polycrystalline OHP LEDs (PeLEDs) is often limited by poor surface morphology, small exciton binding energy, and long exciton diffusion length of large‐grain OHP films caused by uncontrolled crystallization. Here, crystallization of methylammonium lead bromide (MAPbBr₃) is finely controlled by using a polar solvent‐soluble self‐doped conducting polymer, poly(styrenesulfonate)‐grafted polyaniline (PSS‐g‐PANI), as a hole injection layer (HIL) to induce granular structure, which makes charge carriers spatially confined more effectively than columnar structure induced by the conventional poly(3,4‐ethylenedioythiphene):polystyrenesulfonate (PEDOT:PSS). Moreover, lower acidity of PSS‐g‐PANI than PEDOT:PSS reduces indium tin oxide (ITO) etching, which releases metallic In species that cause exciton quenching. Finally, doubled device efficiency of 14.3 cd A^‐1 is achieved for PSS‐g‐PANI‐based polycrystalline MAPbBr₃ PeLEDs compared to that for PEDOT:PSS‐based PeLEDs (7.07 cd A^‐1). Furthermore, PSS‐g‐PANI demonstrates high efficiency of 37.6 cd A^‐1 in formamidinium lead bromide nanoparticle LEDs. The results provide an avenue to both control the crystallization kinetics and reduce the migration of In released from ITO by forming OIP films favorable for more radiative luminescence using the polar solvent‐soluble and low‐acidity polymeric HIL.

Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted CH₃NH₃PbI₃ perovskite solar cells.

Abstract

Interface engineering is critical to the development of highly efficient perovskite solar cells. Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted MAPbI₃ perovskite solar cells (PSCs). This treatment has significantly increased MAPbI₃ photovoltaic performance to 18.8% for the urea treated PEDOT:PSS PSCs from 14.4% for pristine PEDOT:PSS devices. The use of urea controls phase separation between PEDOT and PSS segments, leading to the formation of a unique fiber‐shaped PEDOT:PSS film morphology with well‐organized charge transport pathways for improved conductivity from 0.2 S cm⁻¹ for pristine PEDOT:PSS to 12.75 S cm⁻¹ for 5 wt% urea treated PEDOT:PSS. The urea‐treatment also addresses a general challenge associated with the acidic nature of PEDOT:PSS, leading to a much improved ambient stability of PSCs. In addition, the device hysteresis is significantly minimized by optimizing the urea content in the treatment.

Recent progresses in 2D/3D mixed systems for photovoltaics are reviewed. The evolution of crystal structure, optoelectronic properties, charge carrier dynamics, and their impact on the photovoltaic performances are herein discussed for the different 2D/3D perovskites reported in the literature. This review raises fundamental discussions on the challenges and the perspectives of 2D/3D perovskites toward high‐efficiency and stable perovskite solar cells.

Abstract

The cost‐effective processability and high efficiency of the organic–inorganic metal halide perovskite solar cells (PSCs) have shown tremendous potential to intervene positively in the generation of clean energy. However, prior to an industrial scale‐up process, there are certain critical issues such as the lack of stability against over moisture, light, and heat, which have to be resolved. One of the several proposed strategies to improve the stability that has lately emerged is the development of lower‐dimensional (2D) perovskite structures derived from the Ruddlesden–Popper (RP) phases. The excellent stability under ambient conditions shown by 2D RP phase perovskites has made the scalability expectations burgeon since it is one of the most credible paths toward stable PSCs. In this review, the 2D/3D mixed system for photovoltaics (PVs) is elaborately discussed with the focus on the crystal structure, optoelectronic properties, charge carrier dynamics, and their impact on the photovoltaic performances. Finally, some of the further challenges are highlighted while outlining the perspectives of 2D/3D perovskites for high‐efficiency stable solar cells.

Perovskites are versatile ABX₃ crystals, hosting many intriguing physical properties. While most are inorganic compounds with cationic A‐ and B‐, and anionic X‐sites, recently, the introduction of organic ions (hybrid perovskites) and structures with inverted ionic charges (inverse (hybrid) perovskites) have been explored. Thus, the combinatorial space for design with optimized properties has new dimensions.

Abstract

Materials science evolves to a state where the composition and structure of a crystal can be controlled almost at will. Given that a composition meets basic requirements of stoichiometry, steric demands, and charge neutrality, researchers are now able to investigate a wide range of compounds theoretically and, under various experimental conditions, select the constituting fragments of a crystal. One intriguing playground for such materials design is the perovskite structure. While a game of mixing and matching ions has been played successfully for about 150 years within the limits of inorganic compounds, the recent advances in organic–inorganic hybrid perovskite photovoltaics have triggered the inclusion of organic ions. Organic ions can be incorporated on all sites of the perovskite structure, leading to hybrid (double, triple, etc.) perovskites and inverse (hybrid) perovskites. Examples for each of these cases are known, even with all three sites occupied by organic molecules. While this change from monatomic ions to molecular species is accompanied with increased complexity, it shows that concepts from traditional inorganic perovskites are transferable to the novel hybrid materials. The increased compositional space holds promising new possibilities and applications for the universe of perovskite materials.

Small‐molecule‐based ternary BHJ solar cells with the SM donor DR3, the SM acceptor ICC6, and the fullerene PC₇₁BM yield high power conversion efficiencies nearing 11% for active layer thicknesses >200 nm. With low geminate and nongeminate recombination, long carrier lifetimes, and high/balanced carrier mobilities, the ternary system maintains PCEs >8% over a wide range of active layer thicknesses within 200–500 nm.

Abstract

Solution‐processed small molecule (SM) solar cells have the prospect to outperform their polymer‐fullerene counterparts. Considering that both SM donors/acceptors absorb in visible spectral range, higher expected photocurrents should in principle translate into higher power conversion efficiencies (PCEs). However, limited bulk‐heterojunction (BHJ) charge carrier mobility (<10^‐4 cm² V^‐1 s^‐1) and carrier lifetimes (<1 µs) often impose active layer thickness constraints on BHJ devices (≈100 nm), limiting external quantum efficiencies (EQEs) and photocurrent, and making large‐scale processing techniques particularly challenging. In this report, it is shown that ternary BHJs composed of the SM donor DR3TBDTT (DR3), the SM acceptor ICC6 and the fullerene acceptor PC₇₁BM can be used to achieve SM‐based ternary BHJ solar cells with active layer thicknesses >200 nm and PCEs nearing 11%. The examinations show that these remarkable figures are the result of i) significantly improved electron mobility (8.2 × 10^‐4 cm² V^‐1 s^‐1), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC₇₁BM has been added as ternary component. Optically thick (up to ≈500 nm) devices are shown to maintain PCEs >8%, and optimized DR3:ICC6:PC₇₁BM solar cells demonstrate long‐term shelf stability (dark) for >1000 h, in 55% humidity air environment.

N‐type Sb³⁺ heterovalent doping is conducted to regulate the polarity of perovskite and to optimize the interface band structure of ETL‐free PSCs. With doping, the band bending at both FTO/perovskite and perovskite/spiro‐MeOTAD interfaces ensures efficient collection of majority carrier and blocking of minority carrier, contributing to 12.62% efficient ETL‐free device with a Schottky/p‐n cascade heterojunction, pointing out new insights into the understanding of such devices.

Electron transport layer (ETL)‐free perovskite solar cells (PSCs) are getting much more attention with their simpler structure and potentially low cost as well as higher stability. However, the elimination of ETL (such as TiO₂) with intrinsically deep valance band level leads to the absence of the hole blocking mechanism and thus serious charge recombination at the FTO/perovskite interface compared with ETL‐based devices. An interface band bending associated built‐in electric field is an essential driving force of charge separation. Here, by intentional polarity tailoring of perovskite via incorporation of Sb as a shallow donor, ETL‐free PSCs with optimized energy level alignment both at the front and rear interfaces are constructed, resulting in an enhanced built‐in electric field and thus efficient majority carrier collection and minority blocking as well as reduced interfacial charge recombination at both interfaces. Device simulation calculations also confirm the importance of polarity control for device performance improvement. The effect of doping on the perovskite films properties and device performance are systematically demonstrated. Correspondingly, ETL‐free PSCs with a champion power conversion efficiency of 12.62% is achieved.

Scalable room‐temperature sputtering deposition of the SnO₂ electron transport layer (ETL) with reduced gap states has been demonstrated. Perovskite solar cells using a SnO₂ ETL show an efficiency up to 20.2% and a T₈₀ lifetime of 625 h. Mini‐modules with a 22.8 cm₂ aperture area show efficiencies over 12% and a T₈₀ lifetime of 515 h, which indicates the upscalability of our method.

Abstract

Stability and scalability have become the two main challenges for perovskite solar cells (PSCs) with the research focus in the field advancing toward commercialization. One of the prerequisites to solve these challenges is to develop a cost‐effective, uniform, and high quality electron transport layer that is compatible with stable PSCs. Sputtering deposition is widely employed for large area deposition of high quality thin films in the industry. Here the composition, structure, and electronic properties of room temperature sputtered SnO₂ are systematically studied. Ar and O₂ are used as the sputtering and reactive gas, respectively, and it is found that a highly oxidizing environment is essential for the formation of high quality SnO₂ films. With the optimized structure, SnO₂ films with high quality have been prepared. It is demonstrated that PSCs based on the sputtered SnO₂ electron transport layer show an efficiency up to 20.2% (stabilized power output of 19.8%) and a T₈₀ operational lifetime of 625 h. Furthermore, the uniform and thin sputtered SnO₂ film with high conductivity is promising for large area solar modules, which show efficiencies over 12% with an aperture area of 22.8 cm² fabricated on 5 × 5 cm² substrates (geometry fill factor = 91%), and a T₈₀ operational lifetime of 515 h.

Two novel spiro‐type hole‐transporting materials (HTMs) SCZF‐5 and SAF‐5 are designed based on different spiro‐cores, SZCF and SAF, respectively, and are applied in the perovskite solar cells. An impressive power conversion efficiency of 20.10% is achieved in the SCZF‐5‐based device, which is obviously higher than that of commercial HTM spiro‐OMeTAD (19.11%) and SAF‐5 (13.93%).

Abstract

Hole‐transporting materials (HTMs) play a significant role in hole transport and extraction for perovskite solar cells (PeSCs). As an important type of HTMs, the spiro‐architecture‐based material is widely used as small organic HTM in PeSCs with good photovoltaic performances. The skeletal modification of spiro‐based HTMs is a critical way of modifying energy level and hole mobility. Thus, many spiro alternatives are developed to optimize the spiro‐type HTMs. Herein, a novel carbazole‐based single‐spiro‐HTM named SCZF‐5 is designed and prepared for efficient PeSCs. In addition, another single‐spiro HTM SAF‐5 with reported 10‐phenyl‐10H‐spiro[acridine‐9,9′‐fluorene] (SAF) core is also synthesized for comparison. Through varying from SAF core to SCZF core as well as comparing with the classic 9,9′‐spiro‐bifluorene, it is found that the new HTM SCZF‐5 exhibits more impressive power conversion efficiency (PCE) of 20.10% than SAF‐5 (13.93%) and the commercial HTM spiro‐OMeTAD (19.11%). On the other hand, the SCZF‐5‐based device also has better durability in lifetime testing, indicating the newly designed SCZF by integrating carbazole into the spiro concept has good potential for developing effective HTMs.

A fast, nondestructive, camera‐based method to capture optical bandgap images of perovskite solar cells with micrometer‐scale spatial resolution is developed. This technique allows for the probing of relative variations in optical bandgaps across various solar cells, and for the resolution of bandgap inhomogeneity within the same device due to material degradation and impurities. The results are independently confirmed with other optical‐based techniques.

Abstract

A fast, nondestructive, camera‐based method to capture optical bandgap images of perovskite solar cells (PSCs) with micrometer‐scale spatial resolution is developed. This imaging technique utilizes well‐defined and relatively symmetrical band‐to‐band luminescence spectra emitted from perovskite materials, whose spectral peak locations coincide with absorption thresholds and thus represent their optical bandgaps. The technique is employed to capture relative variations in optical bandgaps across various PSCs, and to resolve optical bandgap inhomogeneity within the same device due to material degradation and impurities. Degradation and impurities are found to both cause optical bandgap shifts inside the materials. The results are confirmed with micro‐photoluminescence spectroscopy scans. The excellent agreement between the two techniques opens opportunities for this imaging concept to become a quantified, high spatial resolution, large‐area characterization tool of PSCs. This development continues to strengthen the high value of luminescence imaging for the research and development of this photovoltaic technology.

Perovskite solar cells have experienced a rapid development since the first report in 2012 with the power conversion efficiency approaching the theoretical limit. Device stability is still one of the remaining challenges for commercialisation. In this Review, the authors address the important role the charge selective contacts play in the long‐term stability of perovskite solar cells.

Abstract

Lead halide perovskite solar cells have rapidly achieved high efficiencies comparable to established commercial photovoltaic technologies. The main focus of the field is now shifting toward improving the device lifetime. Many efforts have been made to increase the stability of the perovskite compound and charge‐selective contacts. The electron and hole selective contacts are responsible for the transport of photogenerated charges out of the solar cell and are in intimate contact with the perovskite absorber. Besides the intrinsic stability of the selective contacts themselves, the interfaces at perovskite/selective contact and metal/selective contact play an important role in determining the overall operational lifetime of perovskite solar cells. This review discusses the impact of external factors, i.e., heat, UV‐light, oxygen, and moisture, and measured conditions, i.e., applied bias on the overall stability of perovskite solar cells (PSCs). The authors summarize and analyze the reported strategies, i.e., material engineering of selective contacts and interface engineering via the introduction of interlayers in the aim of enhancing the device stability of PSCs at elevated temperatures, high humidity, and UV irradiation. Finally, an outlook is provided with an emphasis on inorganic contacts that is believed to be the key to achieving highly stable PSCs.

Efficient carbon‐based hole conductor free planar heterojunction perovskite solar cells (PHJ‐PVSCs) fabricated in ambient air have been reported, achieving a champion power conversion efficiency of 13.52% and a best V _OC of 1.07 V. These PHJ‐PVSCs show superior reproducibility and stability. This work makes the preliminary exploration of cost‐efficient and stable C‐based hole transport material‐free PHJ‐PVSCs and paves the way to their further commercialization.

Organic‐inorganic hybrid metal halide perovskite solar cells have gained tremendous research interest in the past few years. Here, highly reproducible and efficient planar heterojunction perovskite solar cells (PHJ‐PVSCs) with good stability are fabricated using one‐step processed compact TiO₂ as electron‐selective layers (ESLs) and carbon as counter electrodes with no requirements of hole conductors. The fabrication process is fully conducted in ambient air with relatively high humidity (≥45%). Our PVSCs have a simplified planar architecture with only a compact TiO₂ ESL and carbon layer sandwiched with a methylammonium lead triiodide light absorber. Our best‐performing cell shows a good power conversion efficiency of 13.52% and superb illumination stability for 1200 s with no decrease under one sun. Our unencapsulated devices also show great thermal stability, retaining 90% of their initial efficiencies for 72 h under thermal stress at 80 °C in air. Notably, the open‐circuit voltage of our planar PVSCs are always over 1 V, and the best one is 1.07 V, which is the highest value ever reported for carbon‐based hole conductor free PVSCs. Our research demonstrates the feasibility of a fully ambient air fabrication process for carbon‐based PHJ‐PVSCs in simplified device architecture and paves the way to their further commercialization.

A simple method for obtaining highly efficient and stable inverted perovskite solar cells (PeSCs) is suggested. A defect‐free perovskite film with large‐sized grains is achieved by adding an organic conjugated molecule, which improves the charge extraction and reduces defect sites in perovskite crystals, resulting in highly efficient and stable PeSCs.

Abstract

The addition of chemical additives is considered as a promising approach for obtaining high‐quality perovskite films under mild conditions, which is essential for both the efficiency and the stability of organic–inorganic hybrid perovskite solar cells (PeSCs). Although such additive engineering yields high‐quality films, the inherent insulating property of the chemical additives prevents the efficient transport and extraction of charge carriers, thereby limiting the applicability of this approach. Here, it is shown that organic conjugated molecules having rhodanine moieties (i.e., SA‐1 and SA‐2) can be used as semiconducting chemical additives that simultaneously yield large‐sized perovskite grains and improve the charge extraction. Using this strategy, a high power conversion efficiency of 20.3% as well as significantly improved long‐term stability under humid air conditions is achieved. It is believed that this approach can provide a new pathway to designing chemical additives for further improving the efficiency and stability of PeSCs.

Sn dopants trigger extrinsic self‐trapping of excitons in bulk 2D perovskite crystals, and afford broadband red‐to‐near‐infrared emission, with luminescence quantum yield increase from 0.7% to 6.0% (8.6‐fold). Random potential wells that the Sn dopants create preferentially localize excitons through the fast (sub‐picosecond) exciton diffusion, suppressing the original weak emissions from free and bound excitons.

Abstract

As emerging efficient emitters, metal‐halide perovskites offer the intriguing potential to the low‐cost light emitting devices. However, semiconductors generally suffer from severe luminescence quenching due to insufficient confinement of excitons (bound electron–hole pairs). Here, Sn‐triggered extrinsic self‐trapping of excitons in bulk 2D perovskite crystal, PEA₂PbI₄ (PEA = phenylethylammonium), is reported, where exciton self‐trapping never occurs in its pure state. By creating local potential wells, isoelectronic Sn dopants initiate the localization of excitons, which would further induce the large lattice deformation around the impurities to accommodate the self‐trapped excitons. With such self‐trapped states, the Sn‐doped perovskites generate broadband red‐to‐near‐infrared (NIR) emission at room temperature due to strong exciton–phonon coupling, with a remarkable quantum yield increase from 0.7% to 6.0% (8.6 folds), reaching 42.3% under a 100 mW cm⁻² excitation by extrapolation. The quantum yield enhancement stems from substantial higher thermal quench activation energy of self‐trapped excitons than that of free excitons (120 vs 35 meV). It is further revealed that the fast exciton diffusion involves in the initial energy transfer step by transient absorption spectroscopy. This dopant‐induced extrinsic exciton self‐trapping approach paves the way for extending the spectral range of perovskite emitters, and may find emerging application in efficient supercontinuum sources.

Ultra‐low concentration of 2–3 nm gold nanoparticles embedded with an Ohmic contact into a NiO_x film triples its hole concentration. This leads to a significant conductivity increase of the NiO_x hole transport layer and the consequent decrease of the series resistance of the HTL based perovskite solar cells, pronouncedly improving the efficiency of PVSCs from 17.8 to 20.2%.

NiO_x is a promising hole transport material for p‐i‐n perovskite solar cells (PVSCs) on account of its high mobility, excellent stability and prospect for large‐scale fabrication. However, the typical conductivity of NiO_x is low because of the low carrier concentration, which consequently compromises its photovoltaic performance. Herein, we show that the carrier concentration of NiO_x can be improved by as much as three times through embedding a small concentration (0.11 At%) of gold nanoparticles (Au‐NPs), 2–3 nm in diameter, into the NiO_x thin film. The enhancement is due to the formation of Ohmic contact between Au and NiO_x while avoiding direct contact between Au and perovskite. All key parameters of the PVSC, namely J _sc, V _oc, and FF have improved, and the overall efficiency shows a significant improvement from 17.8% to 20.2%. The small size, low concentration, and Ohmic contact nature of the embedded Au‐NPs, together with this simple method point to a new promising direction for developing high performance PVSCs.