Dr Xueping Liu

Metal oxide charge transport materials are preferable for realizing long-term stable and potentially low-cost perovskite solar cells (PSCs). However, due to some technical difficulties (e.g., intricate fabrication protocols, high-temperature heating process, incompatible solvents, etc.), it is still challenging to achieve efficient and reliable all-metal-oxide-based devices. Here, we developed efficient inverted PSCs (IPSCs) based on solution-processed nickel oxide (NiOx) and tin oxide (SnO2) nanoparticles, working as hole and electron transport materials respectively, enabling a fast and balanced charge transfer for photogenerated charge carriers. Through further understanding and optimizing the perovskite/metal oxide interfaces, we have realized an outstanding power conversion efficiency (PCE) of 23.5% (the bandgap of the perovskite is 1.62 eV), which is the highest efficiency among IPSCs based on all-metal-oxide charge transport materials. Thanks to these stable metal oxides and improved interface properties, ambient stability (retaining 95% of initial PCE after 1 month), thermal stability (retaining 80% of initial PCE after 2 weeks) and light stability (retaining 90% of initial PCE after 1000 hours aging) of resultant devices are enhanced significantly. In addition, owing to the low-temperature fabrication procedures of the entire device, we have obtained a PCE of over 21% for flexible IPSCs with enhanced operational stability.

Perovskite solar cells (PSCs) have shown rapid development recently, whereas nonideal stability remains the chief obstacle toward commercialization. Thus, it is of utmost importance to probe the degradation pathway for the entire device. Here, the extrinsic stability of inverted PSCs (IPSCs) is investigated by using standard shelf-life testing based on the International Summit on Organic Photovoltaic Stability protocols (ISOS-D-1). During the long-term assessment of 1700 h, the degraded power conversion efficiency is mainly caused by the fill factor (53% retention) and short-circuit current density (71% retention), while the open-circuit voltage still maintains 97% of the initial values. Further absorbance evolution and density functional theory calculations disclose that the perovskite rear-contact side, in particular for the perovskite/fullerene interface, is the predominant degradation pathway. This study contributes to understanding the aging mechanism and enhancing the durability of IPSCs for future applications.

Miniaturized flexible photo-rechargeable systems show bright prospects for wide applications in internet of things, self-powered health monitoring and emergency electronics. However, conventional systems still suffer from complex manufacturing processes, slow photo-charging and discharging rate, and mismatch between photovoltaic and energy storage components in size, mechanics and voltage, etc. Here, we demonstrate a facile inkjet printing and electrodeposition approach for fabricating a highly integrated flexible photo-rechargeable system by combining stable and ultra-high-rate quasi-solid-state Zn-MnO2 micro-batteries (ZMBs) with flexible perovskite solar cells (FPSCs). In particular, Ni protective layer is first introduced into ZMBs to stabilize battery configuration and facilitate enhanced electrochemical performance. The optimized ZMB exhibits ultrahigh volumetric energy density of 148 mWh cm−3 (16.3 μWh cm−2) and power density of 55 W cm−3 (6.1 mW cm−2) at the current density of 400 C (5 mA cm−2), enabling them comparable with the state-of-the-art micro-batteries or supercapacitors fabricated by conventional methods. The embedded FPSCs show excellent photovoltaic performance, sufficient to charge ZMBs and create a self-charging system capable to offer energy autonomy in miniaturized wearable electronics. The integrated systems can achieve an ultrafast photo-charging within 30 s, with sufficient energy to power other functional electronics (e.g., LED bulb and pressure sensor) for tens of minutes. This prototype offers a promising scheme for next-generation miniaturized flexible photo-rechargeable systems.

The unprecedented advancement in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has rendered them a promising game-changer in photovoltaics. However, unsatisfactory environmental stability and high manufacturing cost of window electrodes are bottlenecks impeding their commercialization. Here, a strategy is introduced to address these bottlenecks by replacing the costly indium tin oxide (ITO) window electrodes via a simple transfer technique with single-walled carbon nanotubes (SWCNTs) films, which are made of earth-abundant elements with superior chemical and environmental stability. The resultant devices exhibit PCEs of ≈19% on rigid substrates, which is the highest value reported to date for ITO-free PSCs. The facile approach for SWCNTs also enables application in flexible PSCs (f-PSCs), delivering a PCE of ≈18% with superior mechanical robustness over their ITO-based counterparts due to the excellent mechanical properties of SWCNTs. The SWCNT-based PSCs also deliver satisfactory performances on large-area (1 cm2 active area in this work). Furthermore, these SWCNT-based PSCs can retain over 80% of original PCEs after exposure to air over 700 h while ITO-based devices only sustain ≈60% of initial PCEs. This work paves a promising way to accelerate the commercialization of ITO-free PSCs with reduced material cost and prolonged lifetimes.

Triple cation CsFAMA perovskite films fabricated via a one-step method have recently gained attention as an outstanding light-harvesting layer for photovoltaic devices. However, questions remain over the suitability of one-step processes for the production of large-area films, owing to difficulties in controlling the crystallinity, in particular, scaling of the frequently used anti-solvent washing step. This can be mitigated through the use of the two-step method which has recently been used to produce large-area films via techniques such as slot dye coating, spray coating or printing techniques. Nevertheless, the poor solubility of Cs containing salts in IPA solutions has posed a challenge for forming triple cation perovskite films using the two-step method. In this study, we tackle this challenge through fabricating perovskite films on a caesium carbonate (Cs2CO3) precursor layer, enabling Cs incorporation within the film. Synergistically, we find that Cs2CO3 passivates the SnO2 electron transport layer (ETL) through interactions with Sn 3d orbitals, thereby promoting a reduction in trap states. Devices prepared with Cs2CO3 treatment also exhibited an improvement in the power conversion efficiency (PCE) from 19.73% in a control device to 20.96% (AM 1.5G, 100 mW cm−2) in the champion device. The Cs2CO3 treated devices (CsFAMA) showed improved stability, with un-encapsulated devices retaining nearly 80% efficiency after 20 days in ambient air.

Interest in photovoltaics (PVs) based on Earth-abundant halide perovskites has increased markedly in recent years owing to the remarkable properties of these materials and their suitability for energy-efficient and scalable solution processing. Formamidinium lead triiodide (FAPbI )-rich perovskite absorbers have emerged as the frontrunners for commercialization, but commercial success is reliant on the stability meeting the highest industrial standards and the photoactive FAPbI phase suffers from instabilities that lead to degradation - an effect that is accelerated under working conditions. Here, we critically assess the current understanding of these phase instabilities and summarize the approaches for stabilizing the desired phases, covering aspects from fundamental research to device engineering. We subsequently analyse the remaining challenges for state-of-the-art perovskite PVs and demonstrate the opportunities to enhance phase stability with ongoing materials discovery and in operando analysis. Finally, we propose future directions towards upscaling perovskite modules, multijunction PVs and other potential applications.

Over the last decade, 2,2 '',7,7 ''-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 '-spirobifluorene (spiro-OMeTAD) has remained the hole transporting layer (HTL) of choice for producing high efficiency perovskite solar cells (PSCs). However, PSCs incorporating spiro-OMeTAD suffer significantly from dopant induced instability and non-ideal band alignments. Herein, a new approach is presented for tackling these issues using the functionality of organometallocenes to bind to Li+ dopant ions, rendering them immobile and reducing their impact on the degradation of PSCs. Consequently, significant improvements are observed in device stability under elevated temperature and humidity, conditions in which ion migration occurs most readily. Remarkably, PSCs prepared with ferrocene retain 70% of the initial power conversion efficiency (PCE) after a period of 1250 h as compared to only 8% in the control. Synergistically, it is also identified that ferrocene improves the hole extraction yield at the HTL interface and reduces interfacial recombination enabling PCEs to reach 23.45%. This work offers a pathway for producing highly efficient spiro-OMeTAD devices with conventional dopants via addressing the key challenge of dopant induced instability in leading PSCs.

Successful manipulation of halide perovskite surfaces is typically achieved via the interactions between modulators and perovskites. Herein, it is demonstrated that a strong-interaction surface modulator is beneficial to reduce interfacial recombination losses in inverted (p-i-n) perovskite solar cells (IPSCs). Two organic ammonium salts are investigated, consisting of 4-hydroxyphenethylammonium iodide and 2-thiopheneethylammonium iodide (2-TEAI). Without thermal annealing, these two modulators can recover the photoluminescence quantum yield of the neat perovskite film in contact with fullerene electron transport layer (ETL). Compared to the hydroxyl-functionalized phenethylammonium moiety, the thienylammonium facilitates the formation of a quasi-2D structure onto the perovskite. Density functional theory and quasi-Fermi level splitting calculations reveal that the 2-TEAI has a stronger interaction with the perovskite surface, contributing to more suppressed non-radiative recombination at the perovskite/ETL interface and improved open-circuit voltage (V-OC) of the fabricated IPSCs. As a result, the V-OC increases from 1.11 to 1.20 V (based on a perovskite bandgap of 1.63 eV), yielding a power conversion efficiency (PCE) from approximate to 20% to 21.9% (stabilized PCE of 21.3%, the highest reported PCEs for IPSCs employing poly[N,N ''-bis(4-butylphenyl)-N,N ''-bis(phenyl)benzidine] as the hole transport layer, alongside the enhanced operational and shelf-life stability for unencapsulated devices.

Recent advances in heterojunction and interfacial engineering of perovskite solar cells (PSCs) have enabled great progress in developing highly efficient and stable devices. Nevertheless, the effect of halide choice on the formation mechanism, crystallography and photoelectric properties of the low-dimensional phase still requires further detailed study. In this work, we present key insights into the significance of halide choice when designing passivation strategies comprising large organic spacer salts, clarifying the effect of anions on the formation of quasi2D/3D heterojunctions. To demonstrate the importance of halide influences, we employ novel neo-pentylammonium halide salts with different halide anions (neoPAX, X = I, Br or Cl). We find that regardless of halide selection, iodide-based (neoPA)2(FA)(n-1)PbnI(3n+1) phases are formed above the perovskite substrate, while the added halide anions diffuse and passivate the perovskite bulk. In addition, we also find the halide choice has an influence on the degree of dimensionality (n). Comparing the three halides, we find that chloride-based salts exhibit superior crystallographic, enhanced carrier transport and extraction compared to the iodide and bromide analogs. As a result, we report high power conversion efficiency in quasi-2D/3D PSCs, which are optimal when using chloride salts, reaching up to 23.35% and improving long-term stability.