High photovoltage in perovskite solar cells: New physical insights from the ultrafast transient absorption spectroscopy

25.71 %-Efficiency FACsPbI3 Perovskite Solar Cells Enabled by A Thiourea-based Isomer

Various isomers have been developed to regulate the morphology and reduce defects in state-of-the-art perovskite solar cells (PSCs). To insight the structure-function-effect correlations for the isomerization of thiourea derivatives on the performance of the PSCs, we developed two thiourea derivatives [(3,5-dichlorophenyl)amino]thiourea (AT) and N-(3,5-dichlorophenyl)hydrazinecarbothioamide (HB). Supported by experimental and calculated results, it was found that AT can bind with undercoordinated Pb2+ defect through synergistic interaction between N1 and C=S group with a defect formation energy of 1.818 eV, which is much higher than that from the synergistic interaction between two -NH- groups in HB and perovskite (1.015 eV). Moreover, the stronger interaction between AT and Pb2+ regulates the crystallization process of perovskite film to obtain a high-quality perovskite film with high crystallinity, large grain size, and low defect density. Consequently, the AT-treated FACsPbI3 device engenders an efficiency of 25.71 % (certified as 24.66 %), which is greatly higher than control (23.74 %) and HB-treated FACsPbI3 devices (25.05 %). The resultant device exhibits a remarkable stability for maintaining 91.0 % and 95.2 % of its initial efficiency after aging 2000 h in air condition or tracking at maximum power point for 1000 h, respectively.

Champion Device Architectures for Low-Cost and Stable Single-Junction Perovskite Solar Cells

High power conversion efficiencies (PCE), low energy payback time (EPBT), and low manufacturing costs render perovskite solar cells (PSCs) competitive; however, a relatively low operational stability impedes their large-scale deployment. In addition, state-of-the-art PSCs are made of expensive materials, including the organic hole transport materials (HTMs) and the noble metals used as the charge collection electrode, which induce degradation in PSCs. Thus, developing inexpensive alternatives is crucial to fostering the transition from academic research to industrial development. Combining a carbon-based electrode with an inorganic HTM has shown the highest potential and should replace noble metals and organic HTMs. In this review, we illustrate the incorporation of a carbon layer as a back contact instead of noble metals and inorganic HTMs instead of organic ones as two cornerstones for achieving optimal stability and economic viability for PSCs. We discuss the primary considerations for the selection of the absorbing layer as well as the electron-transporting layer to be compatible with the champion designs and ultimate architecture for single-junction PSCs. More studies regarding the long-term stability are still required. Using the recommended device architecture presented in this work would pave the way toward constructing low-cost and stable PSCs.

Multi-functional MXene quantum dots enhance the quality of perovskite polycrystalline films and charge transport for solar cells

Recently, two-dimensional (2D) transition metal carbides/nitrides (MXenes) find applications in perovskite solar cells (PSCs), due to their high conductivity, tunable electronic structures, and rich surface chemistry, etc. However, the integration of 2D MXenes into PSCs is limited by their large lateral sizes and relatively-small surface volume ratios, and the roles of MXenes in PSCs are still ambiguous. In this paper, zero-dimensional (0D) MXene quantum dots (MQDs) with an average size of 2.7 nm are obtained through clipping step by step combining a chemical etching and a hydrothermal reaction, which display rich terminals (i.e., -F, -OH, -O) and unique optical properties. The 0D MQDs incorporated into SnO₂ electron transport layers (ETLs) of PSCs exhibit multifunction: 1) increasing the electrical conductivity of SnO₂, 2) promoting better alignments of energy band positions at the perovskite/ETL interface, 3) improving the film quality of atop polycrystalline perovskite. Particularly, the MQDs not only tightly bond with the Sn atom for decreasing the defects of SnO₂, but also interact with the Pb²⁺ of perovskite. As a result, the defect density of PSCs is significantly decreased from 5.21 × 10²¹ to 6.4 × 10²⁰ cm^-3, leading to enhanced charge transport and reduced nonradiative recombination. Furthermore, the power conversion efficiency (PCE) of PSCs is substantially improved from 17.44% to 21.63% using the MQDs-SnO₂ hybrid ETL compared with the SnO₂ ETL. Besides, the stability of the MQDs-SnO₂-based PSC is greatly enhanced, with only ～4% degradation of the initial PCE after storage in ambient condition (25 °C, RH: 30-40%) for 1128 h, as compared to that of the reference device with a rapid degradation of ～60% of initial PCE after 460 h. And MQDs-SnO₂-based PSC also presents higher thermal stability than SnO₂-based device with continuous heating for 248 h at 85 °C. The unique MQDs exhibited in this work might also find other exciting applications such as light-emitting diodes, photodetectors, and fluorescent probes.

Intermediate Phase Engineering with 2,2-Azodi(2-Methylbutyronitrile) for Efficient and Stable Perovskite Solar Cells

Sequential deposition has been widely employed to modulate the crystallization of perovskite solar cells because it can avoid the formation of nucleation centers and even initial crystallization in the precursor solution. However, challenges remain in overcoming the incomplete and random transformation of PbI₂ films with organic ammonium salts. Herein, a unique intermediate phase engineering strategy has been developed by simultaneously introducing 2,2-azodi(2-methylbutyronitrile) (AMBN) to both PbI₂ and ammonium salt solutions to regulate perovskite crystallization. AMBN not only coordinates with PbI₂ to form a favorably mesoporous PbI₂ film due to the coordination between Pb²⁺ and the cyano group (C≡N), but also suppresses the vigorous activity of FA⁺ ions by interacting with FAI, leading to the full PbI₂ transformation with the preferred orientation. Therefore, perovskites with favorable facet orientations are obtained, and the defects are largely suppressed owing to the passivation of uncoordinated Pb²⁺ and FA⁺ . As a result, a champion power conversion efficiency over 25% with a stabilized efficiency of 24.8% is achieved. Moreover, the device exhibits an improved operational stability, retaining 96% of initial power conversion efficiency under 1000 h continuous white-light illumination with an intensity of 100 mW cm^-2 at ≈55 °C in N₂ atmosphere.

Effects of polyethylene oxide particles on the photo-physical properties and stability of FA-rich perovskite solar cells

In this paper, we use Polyethylene Oxide (PEO) particles to control the morphology of Formamidinium (FA)-rich perovskite films and achieve large grains with improved optoelectronic properties. Consequently, a planar perovskite solar cell (PSC) is fabricated with additions of 5 wt% of PEO, and the highest PCE of 18.03% was obtained. This solar cell is also shown to retain up to 80% of its initial PCE after about 140 h of storage under the ambient conditions (average relative humidity of 62.5 ± 3.25%) in an unencapsulated state. Furthermore, the steady-state PCE of the PEO-modified PSC device remained stable for long (over 2500 s) under continuous illumination. This addition of PEO particles is shown to enable the tuning of the optoelectronic properties of perovskite films, improvements in the overall photophysical properties of PSCs, and an increase in resistance to the degradation of PSCs.

Roles of MACl in Sequentially Deposited Bromine-Free Perovskite Absorbers for Efficient Solar Cells

So far, the combination of methylammonium bromide/methylammonium chloride (MABr/MACl) or methylammonium iodide (MAI)/MACl is the most frequently used additives to stabilize formamidinium lead iodide (FAPbI₃ ) fabricated by the sequential deposition method. However, the enlarged bandgap due to the addition of bromide and the ambiguous functions of these additives in lead iodide (PbI₂ ) transformation are still worth considering. Herein, the roles of MACl in sequentially deposited Br-free FA-based perovskites are systematically investigated. It is found that MACl can finely regulate the PbI₂ /FAI reaction, tune the phase transition at room temperature, and adjust intermediate-related perovskite crystallization and decomposition during thermal annealing. Compared to FAPbI₃ , the perovskite with MACl exhibits larger grain, longer carrier lifetime, and reduced trap density. The resultant solar cell therefore achieves a champion power conversion efficiency (PCE) of 23.1% under reverse scan with a stabilized power output of 23.0%. In addition, it shows much improved photostability under 100 mW cm^-2 white illumination (xenon lamp) in nitrogen atmosphere without encapsulation.

Band Alignment Engineering between Planar SnO2 and Halide Perovskites via Two-Step Annealing

Managing defects in SnO₂ is critical for improving the power conversion efficiency (PCE) of halide perovskite-based solar cells. However, typically reported SnO_2-based perovskite solar cells have inherent defects in the SnO₂ layer, which lead to a lower PCE and hysteresis. Here, we report that a dual-coating approach for SnO₂ with different annealing temperatures can simultaneously form a SnO₂ layer with high crystallinity and uniform surface coverage. Along with these enhanced physical properties, the dual-coated SnO₂ layer shows favorable band alignment with a mixed halide perovskite. After careful optimization of the dual-coating method, the average PCE of the perovskite solar cell based on the dual-coated SnO₂ layer increases from 18.07 to 19.23% with a best-performing cell of 20.03%. Note that a facile two-step coating and annealing method can open new avenues to develop SnO₂-based perovskite solar cells with stabilized and improved photovoltaic performances.

Chemical Dopant Engineering in Hole Transport Layers for Efficient Perovskite Solar Cells: Insight into the Interfacial Recombination

Chemical doping of organic semiconductors has been recognized as an effective way to enhance the electrical conductivity. In perovskite solar cells (PSCs), various types of dopants have been developed for organic hole transport materials (HTMs); however, the knowledge of the basic requirements for being efficient dopants as well as the comprehensive roles of the dopants in PSCs has not been clearly revealed. Here, three copper-based complexes with controlled redox activities are applied as dopants in PSCs, and it is found that the oxidative reactivity of dopants presents substantial impacts on conductivity, charge dynamics, and solar cell performance. A significant improvement of open-circuit voltage ( V_oc) by more than 100 mV and an increase of power conversion efficiency from 13.2 to 19.3% have been achieved by tuning the doping level of the HTM. The observed large variation of V_oc for three dopants reveals their different recombination kinetics at the perovskite/HTM interfaces and suggests a model of an interfacial recombination mechanism. We also suggest that the dopants in HTMs can also affect the charge recombination kinetics as well as the solar cell performance. Based on these findings, a strategy is proposed to physically passivate the electron-hole recombination by inserting an ultrathin Al₂O₃ insulating layer between the perovskite and the HTM. This strategy contributes a significant enhancement of the power conversion efficiency and environmental stability, indicating that dopant engineering is one crucial way to further improve the performance of PSCs.

The Role of Rubidium in Multiple-Cation-Based High-Efficiency Perovskite Solar Cells

Perovskite solar cells (PSCs) based on cesium (Cs)- and rubidium (Rb)-containing perovskite films show highly reproducible performance; however, a fundamental understanding of these systems is still emerging. Herein, this study has systematically investigated the role of Cs and Rb cations in complete devices by examining the transport and recombination processes using current-voltage characteristics and impedance spectroscopy in the dark. As the credibility of these measurements depends on the performance of devices, this study has chosen two different PSCs, (MAFACs)Pb(IBr)₃ (MA = CH₃ NH₃⁺ , FA = CH(NH₂ )₂⁺ ) and (MAFACsRb)Pb(IBr)₃ , yielding impressive performances of 19.5% and 21.1%, respectively. From detailed studies, this study surmises that the confluence of the low trap-assisted charge-carrier recombination, low resistance offered to holes at the perovskite/2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene interface with a low series resistance (R_s ), and low capacitance leads to the realization of higher performance when an extra Rb cation is incorporated into the absorber films. This study provides a thorough understanding of the impact of inorganic cations on the properties and performance of highly efficient devices, and also highlights new strategies to fabricate efficient multiple-cation-based PSCs.