Toward Highly Reproducible, Efficient, and Stable Perovskite Solar Cells via Interface Engineering with CoO Nanoplates

Synergistic Surface Reconstruction and Defect Passivation via Guanidine Sulfonate for High-Efficiency Perovskite Photovoltaics

Perovskite films have long suffered from various defects located at grain boundaries and surfaces (GBS), especially residual lead iodide (PbI2), which can seriously impair the photoelectric conversion efficiency (PCE) and long-term stability of the corresponding photovoltaic devices. Herein, guanidine sulfamate (GSM), with desired -NH2, S═O, and Gua+ functional groups, is introduced to the perovskite surface by a post-treatment process to achieve high-quality films with fewer defects. It was found that -NH2 and S═O in GSM contribute to the passivation of various defects in perovskites and suppress non-radiative recombination, thus improving the interfacial carrier transport efficiency. Meanwhile, the guanidine (Gua+) cations promote grain fusion during post-treatment to achieve large-sized grains and effectively reduce residual PbI2 content. Moreover, the optimized perovskite films also exhibited better energy level alignment and surface hydrophobicity. Consequently, the champion PCE of the optimized perovskite solar cells (PSCs) was increased from 21.69 to 23.85% at the appropriate GMS post-treatment concentration, along with a significant improvement in storage stability and light stability.

Inhibited Aggregation of Lithium Salt in Spiro-OMeTAD for Perovskite Solar Cells

High-efficiency and stable hole transport materials (HTMs) play an essential role in high-performance planar perovskite solar cells (PSCs). 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobi-fluorene (Spiro-OMeTAD) is often used as HTMs in perovskite solar cells because of its excellent characteristics, such as energy level matching with perovskite, good film-forming ability, and high solubility. However, the accumulation and hydrolysis of the common additive Li-TFSI in Spiro-OMeTAD can cause voids/pinholes in the hole transport layer (HTL), which reduces the efficiency of the PSCs. In order to improve the functional characteristics of HTMs, in this work, we first used CsI as a dopant to modify the HTL and reduce the voids in the HTL. A small amount of CsI is introduced into Spiro-OMeTAD together with Li-TFSI and 4-tert-butylpyridine (TBP). It is found that CsI and TBP formed a complex, which prevented the rapid evaporation of TBP and eliminated some cracks in Spiro-OMeTAD. Moreover, the uniformly dispersed TBP inhibits the agglomeration of Li-TFSI in Spiro-OMeTAD, so that the effective oxidation reaction between Spiro-OMeTAD and air produces Spiro-OMeTAD+ in the oxidation state, thereby increasing the conductivity and adjusting the HTL energy. Correspondingly, the PCE of the planar PSC of the CsI-modified Spiro-OMeTAD is up to 13.31%. In contrast, the PSC without CsI modification showed a poor PCE of 10.01%. More importantly, the PSC of Spiro-OMeTAD treated with CsI has negligible hysteresis and excellent long-term stability. Our work provides a low-cost, simple, and effective method for improving the performance of hole transport materials and perovskite solar cells.

Metal oxide charge transporting layers for stable high-performance perovskite solar cells

This review summarizes the recent progress in metal oxide charge transporting layers to achieve stable high-performance perovskite solar cells.

Enhancing efficiency of perovskite solar cells from surface passivation of Co2+ doped CuGaO2 nanocrystals

Before completely applying inorganic materials as hole transport materials (HTM) for perovskite solar cells (PSCs), modifying devices with inorganic oxides that have the potential as inorganic hole transporters is an effective way to improve device performance and stability. Co²⁺ doped CuGaO₂ nanocrystals (Co-CuGaO₂ NCs) with sizes about 20 nm are synthesized by hydrothermal method and used for surface passivation at the interface of perovskite (PVK)/2,2',7,7'-Tetrakis[N,N-di (4-methoxyphenyl) amino]-9,9'-spirobifluorene (spiroOMeTAD). Co-CuGaO₂ NCs have a larger bandgap with lower valance band compared with spiroOMeTAD, which is more beneficial to the conduction of holes and the blocking of electrons. Furthermore, the Co-CuGaO₂ has a lower valance band energy compared with the original CuGaO₂, which reduces the energy gap between Co-CuGaO₂ and PVK. Co-CuGaO₂ NCs fully cover the upper surface of PVK, which helps prevent direct contact between PVK and oxygen and moisture. The Co-CuGaO₂ NCs surface passivation also gives better hole transport as revealed by the ultraviolet photoelectron spectroscopy (UPS), steady-state photoluminescence (PL), and time-resolved photoluminescence (TRPL) data. When the concentration of Co-CuGaO₂ NCs solution is set to 7.5 mg mL^-1, the device exhibits a best PCE of 20.39% and maintains 84.34% of the initial power conversion efficiency (PCE) after stored 30 days under air atmosphere with 15 ± 5% humidity.

n-type absorber by Cd2+ doping achieves high-performance carbon-based CsPbIBr2 perovskite solar cells

High efficiency and stability have long been the key issues faced by perovskite solar cells (PSCs). It is found that the CsPbIBr₂ all-inorganic perovskite has a suitable band gap and satisfactory stability, so it has attracted much attention. However, the many defects in the CsPbIBr₂ film are one of the main problems hindering the improvement of power conversion efficiency (PCE) of the CsPbIBr₂ PSCs. The substitution of trace impurities is undoubtedly a simple, cost-effective and efficient strategy. In this work, an appropriate amount of Cd²⁺ (1.0% mol of Pb²⁺) is added into the CsPbIBr₂ precursor solution to fabricate high quality CsPbIBr₂ film with improved crystallinity, reduced trap density, suppressed photo-generated carrier recombination, displayed n-type doping and optimized energy level alignment. The corresponding carbon-based all-inorganic Cd²⁺-doped CsPbIBr₂ PSCs achieve a maximum PCE of 10.63% with a high open circuit voltage (V_OC) of 1.324 V, which are much higher than those of the control one with a PCE of 8.48% and an V_OC of 1.235 V. The unencapsulated device can still retain more than 92% of the initial PCE when stored at ambient atmosphere (25 °C, relative humidity about 30%) for 40 days.

Photoelectrical Dynamics Uplift in Perovskite Solar Cells by Atoms Thick 2D TiS2 Layer Passivation of TiO2 Nanograss Electron Transport Layer

A deficiency in the photoelectrical dynamics at the interface due to the surface traps of the TiO₂ electron transport layer (ETL) has been the critical factor for the inferiority of the power conversion efficiency (PCE) in the perovskite solar cells. Despite its excellent energy level alignment with most perovskite materials, its large density of surface defect as a result of sub lattice vacancies has been the critical hurdle for an efficient photovoltaic process in the device. Here, we report that atoms thick 2D TiS₂ layer grown on the surface of a (001) faceted and single-crystalline TiO₂ nanograss (NG) ETL have effectively passivated the defects, boosting the charge extractability, carrier mobility, external quantum efficiency, and the device stability. These properties allow the perovskite solar cells (PSCs) to produce a PCE as high as 18.73% with short-circuit current density (J_sc), open-circuit voltage (V_oc), and fill-factor (FF) values as high as 22.04 mA/cm², 1.13 V, and 0.752, respectively, a 3.3% improvement from the pristine TiO₂-NG-based PSCs. The present approach should find an extensive application for controlling the photoelectrical dynamic deficiency in perovskite solar cells.

Strong electron acceptor additive based spiro-OMeTAD for high-performance and hysteresis-less planar perovskite solar cells

As the most popular hole-transporting material (HTM), spiro-OMeTAD has been extensively applied in perovskite solar cells (PSCs). Unluckily, the pristine spiro-OMeTAD film has inferior conductivity and hole mobility, thus limiting its potential for application in high-performance PSCs. To ameliorate the electrical characteristics of spiro-OMeTAD, we employ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as a strong electron acceptor into spiro-OMeTAD in PSCs. The incorporation of DDQ with spiro-OMeTAD not only improves the conductivity and the Fermi energy level, but also reduces the trap states and nonradiative recombination, which accounts for the remarkable enhancement in both the fill factor (FF) and open-circuit voltage (V OC) of PSCs. Consequently, the champion PSC with DDQ doped hole transport layer (HTL) generates a boosted power conversion efficiency (PCE) of 21.16% with an FF of 0.796 and a V OC of 1.16 V. Remarkably, DDQ modified devices exhibit superb device stability, as well as mitigated hysteresis. This study provides a facile and viable strategy for dopant engineering of HTL to realize highly efficient PSCs.

Solution-processed p-type nanocrystalline CoO films for inverted mixed perovskite solar cells

Inorganic p-type materials show great potential as the hole transport layer in perovskite solar cells with the merits of low costs and enhanced chemical stability. As a p-type material, cobalt oxide (CoO) has received so far not that level of attention despite its high hole mobility. Herein, solution-processed p-type CoO nanocrystalline films are developed for inverted mixed perovskite solar cells. The ultrafine CoO nanocrystals are synthesized via an oil phase method, which are subsequently treated by a ligand exchange process using pyridine solvent to remove the long alkyl chains covering the nanocrystals. From this homogeneous colloidal solution CoO films are obtained, which exhibit a smooth and pin-hole free surface morphology with high transparency and good conductivity. The ultraviolet photoelectron spectrum also indicates that the energy levels of the CoO film match well with the mixed perovskite Cs_0.05(FA_0.83MA_0.17)_0.95(I_0.83Br_0.17)₃. Inverted solar cells based on crystalline CoO films with ligand exchange show a reasonable energy conversion efficiency, whereas devices based on CoO films without ligand exchange suffer from a strong S-shape JV-characteristic. Thus, the crystalline CoO films are foreseen to pave a new way of inorganic hole transport materials in the fields of perovskite solar cells.

Dye Sensitization and Local Surface Plasmon Resonance-Enhanced Upconversion Luminescence for Efficient Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have achieved rapid progress in this decade. However, the limited solar spectral utilization has restricted the further improvement of performance of the PSCs. One promising approach to solving this problem is utilizing IR to visible upconversion nanoparticles (UCNPs) in the PSC devices. Despite being confined by the lower quantum yield (QY) and smaller absorption cross section of the traditional UCNPs, their application is still a great challenge. In this work, the IR-783 dye-sensitized core/shell NaYF₄:Yb³⁺, Er³⁺@NaYF₄:Yb³⁺, and Nd³⁺ UCNPs were synthesized and coupled with plasmonic Au nanorods films. Thereby, the upconversion luminescence (UCL) intensity was enhanced by about 120-fold, whereas the luminescent QY was improved from 0.2 to 1.2%. Then, the composite UCNPs were assembled on the SnO₂ layer of the PSCs, which resulted in the power conversion efficiency (PCE) increasing from 19.4 to 20.5% under simulated 100 mw/cm² AM 1.5G irradiation. Up to now, it is the highest PCE for the PSCs based on various upconversion devices. Under the irradiation of a sun concentrator (1 W/cm²), the PCE of the device can be further improved to 21.1%. In-depth studies indicate that under standard sunlight irradiation, the improvement of PCE is due to both the IR to visible UCL and the scattering effect of the UCNPs. Under irradiation of a sun concentrator, the UCL contributes dominantly to the further improvement of PCE. This work provides an effective method for increasing the luminescent QY utilized in the PSCs and is of great significance for future PSCs that use sunlight concentrator.

Highly Efficient CsPbBr3 Planar Perovskite Solar Cells via Additive Engineering with NH4SCN

Improving stability is a major aspect for commercial application of perovskite solar cells (PSCs). The all-inorganic CsPbBr₃ perovskite material has been proven to have excellent stability. However, the CsPbBr₃ film has a small range of light absorption and serious charge recombination at the interface or inside the device, so the power conversion efficiency is still lower than that of the organic-inorganic hybrid one. Here, we successfully fabricate high-quality CsPbBr₃ films via additive engineering with NH₄SCN. By incorporating NH₄⁺ and pseudo-halide ion SCN^- into the precursor solution, a smooth and dense CsPbBr₃ film with good crystallinity and low trap state density can be obtained. At the same time, the results of a series of photoluminescence and electrochemical analyses including electrical impedance spectroscopy, space-charge limited current method, Mott-Schottky data, and so on reveal that the NH₄SCN additive can greatly reduce the trap state density of the CsPbBr₃ film and also effectively inhibit interface recombination and promote charge transport in the CsPbBr₃ planar PSC. Finally, the CsPbBr₃ planar PSC prepared with a molar ratio of 1.5% NH₄SCN achieves a champion efficiency of 8.47%, higher than that of the pure one (7.12%).

In Situ Passivation on Rear Perovskite Interface for Efficient and Stable Perovskite Solar Cells

Despite the rocketing rise in power conversion efficiencies (PCEs), the performance of perovskite solar cells (PSCs) is still limited by the carrier transfer loss at the interface between perovskite (PVSK) absorbers and charge transporting layers. Here, we propose a novel in situ passivation strategy by using [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) to improve the charge dynamics at the rear PVSK/CTL interface in the n-i-p structure device. A pre-deposited PCBM-doped PbI₂ layer is redissolved during PVSK deposition in our routine, establishing a bottom-up PCBM gradient that is facile for charge extraction. Meanwhile, the surface defects are in situ-passivated via PCBM-PVSK interaction, which substantially suppresses the trap-assisted recombination at the rear interface. Due to the synergistic effect of charge-extraction promotion and trap passivation, the fabricated PSCs deliver a champion PCE of 20.10% with attenuated hysteresis and improved long-term stability, much higher than the 18.39% of the reference devices. Our work demonstrates a promising interfacial engineering strategy for further improving the performance of PSCs.