Surface Passivation of Sputtered NiO x Using a SAM Interface Layer to Enhance the Performance of Perovskite Solar Cells

On the V OC loss in NiO-based inverted metal halide perovskite solar cells

Recent reports have shown that nickel oxide (NiO) when adopted as a hole transport layer (HTL) in combination with organic layers, such as PTAA or self-assembled monolayers (SAMs), leads to a higher device yield for both single junction as well as tandem devices. Nevertheless, implementing NiO in devices without PTAA or SAM is seldom reported to lead to high-performance devices. In this work, we assess the effect of key NiO properties deemed relevant in literature, namely- resistivity and surface energy, on the device performance and systematically compare the NiO-based devices with those based on PTAA. To this purpose, (thermal) atomic layer deposited (ALD) NiO (NiOBu-MeAMD), Al-doped NiO (Al:NiOBu-MeAMD), and plasma-assisted ALD NiO (NiOMeCp) films, characterized by a wide range of resistivity, are investigated. Although Al:NiOBu-MeAMD (∼400 Ω cm) and NiOMeCp(∼80 Ωcm) films have a lower resistivity than NiOBu-MeAMD (∼10 kΩ cm), the Al:NiOBu-MeAMD and NiOMeCp-based devices are found to have a modest open circuit voltage (V OC) gain of ∼30 mV compared to NiOBu-MeAMD-based devices. Overall, the best-performing NiO-based devices (∼14.8% power conversion efficiency (PCE)) still lag behind the PTAA-based devices (∼17.5%), primarily due to a V OC loss of ∼100 mV. Further investigation based on light intensity analysis of the V OC and FF and the decrease in V OC compared to the quasi-Fermi level splitting (QFLS) indicates that the V OC is limited by trap-assisted recombination at the NiO/perovskite interface. Additionally, SCAPS simulations show that the presence of a high interfacial trap density leads to a V OC loss in NiO-based devices. Upon passivation of the NiO/perovskite interface with Me-4PACz, the V OC increases by 170-200 mV and is similar for NiOBu-MeAMD and Al:NiOBu-MeAMD, leading to the conclusion that there is no influence of the NiO resistivity on the V OC once interface passivation is realized. Finally, our work highlights the necessity of comparing NiO-based devices with state-of-the-art HTL-based devices to draw conclusion about the influence of specific material properties on device performance.

Efficient wide-bandgap perovskite photovoltaics with homogeneous halogen-phase distribution

Wide-bandgap (WBG) perovskite solar cells (PSCs) are employed as top cells of tandem cells to break through the theoretical limits of single-junction photovoltaic devices. However, WBG PSCs exhibit severe open-circuit voltage (Voc) loss with increasing bromine content. Herein, inhomogeneous halogen-phase distribution is pointed out to be the reason, which hinders efficient extraction of carriers. We thus propose to form homogeneous halogen-phase distribution to address the issue. With the help of density functional theory, we construct a double-layer structure (D-2P) based on 2-(9H-Carbazol-9-yl)ethyl]phosphonic acid molecules to provide nucleation sites for perovskite crystallization. Homogeneous perovskite phase is achieved through bottom-up templated crystallization of halogen component. The efficient carrier extraction reduces the Shockley-Read-Hall recombination, resulting in a high Voc of 1.32 V. As a result, D-2P-treated device (1.75 eV) achieves a record power conversion efficiency of 20.80% (certified 20.70%), which is the highest value reported for WBG (more than 1.74 eV) PSCs.

Interfacial Dipole Engineering for Energy Level Alignment in NiOx-Based Quantum Dot Light-Emitting Diodes

The solution-derived non-stoichiometric nickel oxide (NiOx) is a promising hole-injecting material for stable quantum dot light-emitting diodes (QLEDs). However, the carrier imbalance due to the misalignment of energy levels between the NiOx and polymeric hole-transporting layers (HTLs) curtails the device efficiency. In this study, the modification of the NiOx surface is investigated using either 3-cyanobenzoic acid (3-CN-BA) or 4-cyanobenzoic acid (4-CN-BA) in the QLED fabrication. Morphological and electrical analyses revealed that both 4-CN-BA and 3-CN-BA can enhance the work function of NiOx, reduce the oxygen vacancies on the NiOx surface, and facilitate a uniform morphology for subsequent HTL layers. Moreover, it is found that the binding configurations of dipole molecules as a function of the substitution position of the tail group significantly impact the work function of underlying layers. When integrated in QLEDs, the modification layers resulted in a significant improvement in the electroluminescent efficiency due to the enhancement of energy level alignment and charge balance within the devices. Specifically, QLEDs incorporating 4-CN-BA achieved a champion external quantum efficiency (EQE) of 20.34%, which is a 1.8X improvement in comparison with that of the devices utilizing unmodified NiOx (7.28%). Moreover, QLEDs with 4-CN-BA and 3-CN-BA modifications exhibited prolonged operational lifetimes, indicating potential for practical applications.

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.

Insight to the Catalytic Activity of Atomically Precise Ag4Ni2 Nanoclusters on Silicon Carbide for Nitroarene Reduction

Atomically precise Ag4Ni2 nanoclusters with 2,4-dimethylbenzenethiol as the ligands were synthesized and characterized as a cocatalyst of SiC for the selective hydrogenation of nitroarenes to arylamine in the presence of NaBH4. The obtained Ag4Ni2/SiC samples exhibited extraordinary catalytic activity, and a self-accelerated catalytic process was observed with the reduction of nitrophenol to aminophenol as the model reaction. Experimental comparison between the Ag4Ni2/SiC samples before and after the catalysis showed that the transformation of Ag4Ni2 clusters to polydisperse Ag particles as well as amorphous NiOx on the surface of SiC in the catalysis was the key to their high activity. AIMD calculations revealed that the transformation of Ag4Ni2 was driven by the presence of multiple hydrides on the cluster, which induced the detachment of the thiol ligand of the nanoclusters.

Enhancing the Stability and Efficiency of Inverted Perovskite Solar Cells with a Mixed Ammonium Ligands Passivation Strategy

The perovskite solar cell (PSC), which has achieved efficiencies of more than 26%, is expected to be a promising technology that can alternate silicon-based solar cells. However, the performance of PSCs is still limited due to defects and ion migration that occur at the large number of grain boundaries present in perovskite thin films. In this study, the mixed ammonium ligands passivation strategy (MAPS) is demonstrated, which combines n-octylammonium iodide (OAI) and 1,3-diaminopropane (DAP) can effectively suppress the grain boundary defects and ion migration through grain boundaries by the synergistic effect of OAI and DAP, resulting in improved efficiency and stability of PSCs. It has also been revealed that MAPS not only enhances crystallinity and reduces grain boundaries but also improves charge transport while suppressing charge recombination. The MAPS-based opaque PSC shows the best power conversion efficiency (PCE) of 21.29% with improved open-circuit voltage (VOC ) and fill factor (FF), and retained 84% of its initial PCE after 1900 h at 65 °C in N2 atmosphere. Amazingly, the MAPS-based semi-transparent PSC (STP-PSC) retained 94% of their maximum power (21.00% at around 10% AVT) after 1000 h under 1 sun illumination and MAPS-based perovskite submodule (PSM) achieved a PCE of 19.59%, which is among the highest values reported recently.

Utilizing rubidium chloride as an effective and stable interface modification layer for high-efficiency solar cells

The presence of interface defects between the perovskite layer and the underlying substrate has a significant impact on the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). S n O 2 thin films are employed in PSCs as electron transport layers to achieve high PCE. However, the significant lattice mismatch between S n O 2 and the perovskite material leads to a large number of uncoordinated defects at the interface between perovskite and substrate, resulting in recombination losses at the interface. In this study, rubidium chloride (RbCl) was introduced as the interface modification layer between the perovskite layer and the S n O 2 electron transport layer to enhance the PCE of PSCs. The research showed that the RbCl interface modification layer effectively passivated the under-coordinated defects of Sn ions and optimized the energy level alignment between the perovskite layer and the S n O 2 film. Moreover, the fabricated PSCs exhibited an open-circuit voltage of 1.11 V and a power conversion efficiency of 21.64%. Furthermore, the device maintained 80% of initial efficiency after storage for 30 days in an inert gas environment and 60% of the value after storage for 30 days in ambient air.

Impact of the TCO Microstructure on the Electronic Properties of Carbazole-Based Self-Assembled Monolayers

Carbazole-based self-assembled monolayers (PACz-SAMs), anchored via their phosphonic acid group on a transparent conductive oxide (TCO), have demonstrated excellent performance as hole-selective layers in perovskite/silicon tandem solar cells. Yet, whereas different PACz-SAMs have been explored, the role of the TCO, and specifically its microstructure, on the hole transport properties of the TCO/PACz-SAMs stack has been largely overlooked. Here, we demonstrate that the TCO microstructure directly impacts the work function (WF) shift after SAM anchoring and is responsible for WF variations at the micro/nanoscale. Specifically, we studied Sn-doped In2O3 (ITO) substrates with amorphous and polycrystalline (featuring either nanoscale- or microscale-sized grains) microstructures before and after 2PACz-SAMs and NiOx/2PACz-SAMs anchoring. With this, we established a direct correlation between the ITO crystal grain orientation and 2PACz-SAMs local potential distribution, i.e., the WF. Importantly, these variations vanish for amorphous oxides (either in the form of amorphous ITO or when adding an amorphous NiOx buffer layer), where a homogeneous surface potential distribution is found. These findings highlight the importance of TCO microstructure tuning, to enable both high mobility and broadband transparent electrodes while ensuring uniform WF distribution upon application of hole transport SAMs, both critical for enhanced device performance.

Over 19% Efficiency Perovskite Solar Modules by Simultaneously Suppressing Cation Deprotonation and Iodide Oxidation

Perovskite solar cells (PSCs) based on sputtered nickel oxide (NiOx) hole transport layer have emerged as promising configuration due to their good stability, cost-effectiveness, and scalability. However, the adverse chemical redox reaction at the NiOx/perovskite interface remains an ever-present problem that has not yet been well solved. To address this issue before, the problems that cation deprotonation and iodide oxidation that occurred in precursor solution easily result in the interfacial chemical reaction should be prevented. Hence, we report an efficient strategy to simultaneously suppress the interfacial reaction and stabilize the precursor solution by incorporating a reducing and weakly acidic stabilizer, l-ascorbic acid (l-AA). l-AA can reduce I2 generated in the precursor solution and during the interfacial reaction to I-. Furthermore, the protons ionized by adjacent enol hydroxyl groups in l-AA effectively impede the deprotonation of organic cations in the precursor solution as well as at the NiOx/perovskite interface resulting from the chemical reaction. Attributing to the improved crystallization of the perovskite film and the suppression of the interfacial reaction by l-AA, the inverted PSC based on such good light absorber achieves an impressive power conversion efficiency (PCE) of 22.72% along with a high open-circuit voltage of 1.19 V. Notably, further introducing l-AA into the large-area solar modules by the slot-die coating method in air enables a remarkable PCE of 19.17%, which reaches one of the highest PCEs reported for inverted perovskite solar modules (PSMs) (active area >50 cm2) to date. l-AA located at the buried interface also forms a barrier layer that can prevent undesirable chemical reactions at the NiOx/perovskite interface, significantly enhancing the device stability of solar cells and PSMs. These findings in our work provide important guidance for improving the NiOx/perovskite interface and the fabrication of highly efficient, low-cost, and large-area PSMs.

Towards low-temperature processing of efficient γ-CsPbI3 perovskite solar cells

Inorganic cesium lead iodide (CsPbI3) perovskite solar cells (PSCs) have attracted enormous attention due to their excellent thermal stability and optical bandgap (∼1.73 eV), well-suited for tandem device applications. However, achieving high-performance photovoltaic devices processed at low temperatures is still challenging. Here we reported a new method for the fabrication of high-efficiency and stable γ-CsPbI3 PSCs at lower temperatures than was previously possible by introducing the long-chain organic cation salt ethane-1,2-diammonium iodide (EDAI2) and regulating the content of lead acetate (Pb(OAc)2) in the perovskite precursor solution. We find that EDAI2 acts as an intermediate that can promote the formation of γ-CsPbI3, while excess Pb(OAc)2 can further stabilize the γ-phase of CsPbI3 perovskite. Consequently, improved crystallinity and morphology and reduced carrier recombination are observed in the CsPbI3 films fabricated by the new method. By optimizing the hole transport layer of CsPbI3 inverted architecture solar cells, we demonstrate efficiencies of up to 16.6%, surpassing previous reports examining γ-CsPbI3 in inverted PSCs. Notably, the encapsulated solar cells maintain 97% of their initial efficiency at room temperature and under dim light for 25 days, demonstrating the synergistic effect of EDAI2 and Pb(OAc)2 in stabilizing γ-CsPbI3 PSCs.

Rational design of Lewis base molecules for stable and efficient inverted perovskite solar cells

Lewis base molecules that bind undercoordinated lead atoms at interfaces and grain boundaries (GBs) are known to enhance the durability of metal halide perovskite solar cells (PSCs). Using density functional theory calculations, we found that phosphine-containing molecules have the strongest binding energy among members of a library of Lewis base molecules studied herein. Experimentally, we found that the best inverted PSC treated with 1,3-bis(diphenylphosphino)propane (DPPP), a diphosphine Lewis base that passivates, binds, and bridges interfaces and GBs, retained a power conversion efficiency (PCE) slightly higher than its initial PCE of ~23% after continuous operation under simulated AM1.5 illumination at the maximum power point and at ~40°C for >3500 hours. DPPP-treated devices showed a similar increase in PCE after being kept under open-circuit conditions at 85°C for >1500 hours.

NiN-Passivated NiO Hole-Transport Layer Improves Halide Perovskite-Based Solar Cell

The interfaces between inorganic selective contacts and halide perovskites (HaPs) are possibly the greatest challenge for making stable and reproducible solar cells with these materials. NiO_x, an attractive hole-transport layer as it fits the electronic structure of HaPs, is highly stable and can be produced at a low cost. Furthermore, NiO_x can be fabricated via scalable and controlled physical deposition methods such as RF sputtering to facilitate the quest for scalable, solvent-free, vacuum-deposited HaP-based solar cells (PSCs). However, the interface between NiO_x and HaPs is still not well-controlled, which leads at times to a lack of stability and V_oc losses. Here, we use RF sputtering to fabricate NiO_x and then cover it with a Ni_yN layer without breaking vacuum. The Ni_yN layer protects NiO_x doubly during PSC production. Firstly, the Ni_yN layer protects NiO_x from Ni³⁺ species being reduced to Ni²⁺ by Ar plasma, thus maintaining NiO_x conductivity. Secondly, it passivates the interface between NiO_x and the HaPs, retaining PSC stability over time. This double effect improves PSC efficiency from an average of 16.5% with a 17.4% record cell to a 19% average with a 19.8% record cell and increases the device stability.