Enhancing the Performance of the Half Tin and Half Lead Perovskite Solar Cells by Suppression of the Bulk and Interfacial Charge Recombination

Effect of ABX3 site changes on the performance of tin-lead mixed perovskite solar cells

Tin-lead mixed perovskite solar cells (TLMPSCs), with the advantage of approaching the Shockley-Queisser (S-Q) limit for photovoltaic applications, have been rapidly developed and achieved a power conversion efficiency (PCE) of 23.7%. Although the low toxicity of TLMPSCs is conducive to sustainable development, the oxidation of Sn2+ could destroy the perovskite structure easily. Thus, most researchers are devoted to improving the photoelectric performance and stability through additive engineering, interface engineering, device structure optimization, solvent engineering, etc. However, TLMPs with different A-sites and X-sites in the ABX3 model and an optimal ratio of Sn : Pb still need to be investigated; this is the basis of mechanistic analysis. In this paper, we introduce TLMPSCs with different A-sites, X-sites, and Sn-Pb ratios. The mechanism and properties of the cations are analyzed based on the performance of TLMPSCs. Finally, a series of prospects for optimizing ABX3 are put forward, with the hope of attracting the attention and interest of researchers.

Hole Transport Layer-Free Low-Bandgap Perovskite Solar Cells for Efficient All-Perovskite Tandems

Low-bandgap (LBG, Eg  ≈1.25 eV) tin-lead (Sn-Pb) perovskite solar cells (PSCs) play critical roles in constructing efficient all-perovskite tandem solar cells (TSCs) that can surpass the efficiency limit of single-junction solar cells. However, the traditional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layer (HTL) in LBG PSCs usually restricts device efficiency and stability. Here, a strategy of employing 2-aminoethanesulfonic acid (i.e., taurine) as the interface bridge to fabricate efficient HTL-free LBG PSCs with improved optoelectronic properties of the perovskite absorbers at the buried contacts is reported. Taurine-modified ITO substrate has lower optical losses, better energy level alignment, and higher charge transfer capability than PEDOT:PSS HTL, leading to significantly improved open-circuit voltage (VOC ) and short-circuit current density of corresponding devices. The best-performing LBG PSC with a power conversion efficiency (PCE) of 22.50% and an impressive VOC of 0.911 V is realized, enabling all-perovskite TSCs with an efficiency of 26.03%. The taurine-based HTL-free TSCs have highly increased stability, retaining more than 90% and 80% of their initial PCEs after constant operation under 1-sun illumination for 600 h and under 55 °C thermal stress for 950 h, respectively. This work provides a facile strategy for fabricating efficient and stable perovskite devices with a simplified HTL-free architecture.

A Newly Crosslinked-double Network PEDOT:PSS@PEGDMA toward Highly-Efficient and Stable Tin-Lead Perovskite Solar Cells

Until now, poly(3,4-ethylenedioxythiophene):poly(styrensulfonate) (PEDOT:PSS) is widely used in Sn-Pb perovskite solar cells (PSCs) due to its many advantages, including high optical transparency, suitable conductivity, superior wettability, and so on. However, the acidic and hydroscopic properties of the PSS component, as well as the incongruous energy level of the hole transport layer (HTL), may lead to unsatisfying interface properties and decreased device performance. Herein, by adding polyethylene glycol dimethacrylate (PEGDMA) into PEDOT:PSS, a newly crosslinked-double-network obtain of PEDOT:PSS@PEGDMA film, which could not only optimize nucleation and crystallinity of Sn-Pb perovskite films, but also suppress defect density and optimize energy level alignment at the HTL/perovskite interface. As a result, the achieves highly efficient and stable mixed Sn-Pb PSCs with an encouraging power conversion efficiency of 20.9%. Additionally, the device can maintain good stability under N2 atmosphere.

Highly efficient and stable near-infrared photodetectors enabled from passivated tin-lead hybrid perovskites

Tin-lead perovskite-based photodetectors have a wide light-absorption wavelength range, which spans 1000 nm. However, the preparation of the mixed tin-lead perovskite films faces two great obstacles, namely easy oxidation of Sn²⁺to Sn⁴⁺and fast crystallization from tin-lead perovskite precursor solutions, thus further resulting in poor morphology and high density of defects in tin-lead perovskite films. In this study, we demonstrated a high-performance of near-infrared photodetectors prepared from a stable low-bandgap (MAPbI₃)_0.5(FASnI₃)_0.5film modified with 2-fluorophenethylammonium iodide (2-F-PEAI). The addition engineering can efficiently improve the crystallization of (MAPbI₃)_0.5(FASnI₃)_0.5films through the coordination binding between Pb²⁺and N atom in 2-F-PEAI, and resulting in a uniform and dense (MAPbI₃)_0.5(FASnI₃)_0.5film. Moreover, 2-F-PEAI suppressed Sn²⁺oxidation and effectively passivated defects in the (MAPbI₃)_0.5(FASnI₃)_0.5film, thereby significantly reducing the dark current in the PDs. Consequently, the near-infrared photodetectors showed a high responsivity with a specific detectivity of over 10¹²Jones at 800 to near-1000 nm. Additionally, the stability of PDs incorporated with 2-F-PEAI has been significantly improved under air conditions, and the device with the 2-F-PEAI ratio of 400:1 retained 80% of its initial efficiency after 450 h storage in air without encapsulation. Finally, 5 × 5 cm²photodetector arrays were fabricated to demonstrate the potential utility of the Sn-Pb perovskite photodetector in optical imaging and optoelectronic applications.

Manipulation of the Buried Interface for Robust Formamidinium-based Sn-Pb Perovskite Solar Cells with NiOx Hole-Transport Layers

Low band gap tin-lead perovskite solar cells (Sn-Pb PSCs) are expected to achieve higher efficiencies than Pb-PSCs and regarded as key components of tandem PSCs. However, the realization of high efficiency is challenged by the instability of Sn²⁺ and the imperfections at the charge transfer interfaces. Here, we demonstrate an efficient ideal band gap formamidinium (FA)-based Sn-Pb (FAPb_0.5 Sn_0.5 I₃ ) PSC, by manipulating the buried NiO_x /perovskite interface with 4-hydroxyphenethyl ammonium halide (OH-PEAX, X=Cl^- , Br^- , or I^- ) interlayer, which exhibits fascinating functions of reducing the surface defects of the NiO_x hole transport layer (HTL), enhancing the perovskite film quality, and improving both the energy level matching and physical contact at the interface. The effects of different halide anions have been elaborated and a 20.53 % efficiency is obtained with OH-PEABr, which is the highest one for FA-based Sn-Pb PSCs using NiO_x HTLs. Moreover, the device stability is also boosted.

A Roadmap for Efficient and Stable All-Perovskite Tandem Solar Cells from a Chemistry Perspective

Multijunction tandem solar cells offer a promising route to surpass the efficiency limit of single-junction solar cells. All-perovskite tandem solar cells are particularly attractive due to their high power conversion efficiency, now reaching 28% despite being made with relatively easy fabrication methods. In this review, we summarize the progress in all-perovskite tandem solar cells. We then discuss the scientific and engineering challenges associated with both absorbers and functional layers and offer strategies for improving the efficiency and stability of all-perovskite tandem solar cells from the perspective of chemistry.

Compositional Variation in FAPb1-xSnxI3 and Its Impact on the Electronic Structure: A Combined Density Functional Theory and Experimental Study

Given their comparatively narrow band gap, mixed Pb-Sn iodide perovskites are interesting candidates for bottom cells in all-perovskite tandems or single junction solar cells, and their luminescence around 900 nm offers great potential for near-infrared optoelectronics. Here, we investigate mixed FAPb_1-xSn_xI₃ offering the first accurate determination of the crystal structure over a temperature range from 293 to 100 K. We demonstrate that all compositions exhibit a cubic structure at room temperature and undergo at least two transitions to lower symmetry tetragonal phases upon cooling. Using density functional theory (DFT) calculations based on these structures, we subsequently reveal that the main impact on the band gap bowing is the different energy of the s and p orbital levels derived from Pb and Sn. In addition, this energy mismatch results in strongly composition-dependent luminescence characteristics. Whereas neat and Sn-rich compounds exhibit bright and narrow emission with a clean band gap, Sn-poor compounds intrinsically suffer from increased carrier recombination mediated by in-gap states, as evidenced by the appearance of pronounced low-energy photoluminescence upon cooling. This study is the first to link experimentally determined structures of FAPb_1-xSn_xI₃ with the electronic properties, and we demonstrate that optoelectronic applications based on Pb-Sn iodide compounds should employ Sn-rich compositions.

Narrowband Near-Infrared Photodetectors Based on Perovskite Waveguide Devices

Organic-inorganic perovskites have attracted great interest for developing wavelength-selective photodetectors. Currently, the spectral response range of narrowband photodetectors is mainly concentrated within the visible light region, lacking near-infrared photodetectors. Here, we present perovskite narrowband near-infrared photodetectors achieved by transverse propagation of light in perovskite waveguide devices for the first time. The response spectra of photodetectors are continuously tuned from 750 to 1000 nm by changing the perovskite component and the position of the incident light with the full width at half-maximum of 25-80 nm. The theoretical and experimental results reveal that the typical perovskite photodetectors serve as an optical waveguide to confine and propagate the light, in which the long wavelength light propagates a longer distance; oppositely, the short wavelength light with the higher loss in devices fails to produce a photoresponse, owing to the wavelength dependent absorption of perovskite. This provides a new concept for designing narrowband photodetectors and will simulate new applications.

Efficient and Stable Methylammonium-Free Tin-Lead Perovskite Solar Cells with Hexaazatrinaphthylene-Based Hole-Transporting Materials

Incorporating non-aqueous hole-transporting materials (HTMs) to replace the widely used PEDOT:PSS is favorable for improving the stability of tin-lead perovskite solar cells (Sn-Pb PSCs). Herein, hexaazatrinaphthylene (HATNA) is found to be a promising HTM building block for Sn-Pb PSCs. By introducing triphenylamine (TPA) and methoxy-triphenylamine into the HATNA core, molecular energy levels and surface wettability can be well regulated, and a high hole mobility and thermal stability can be maintained. Moreover, a homogeneous Sn-Pb perovskite film with low Sn⁴⁺ contents and vertically orientated grains can be prepared on the substrate TPA-HATNA. Compared with PEDOT:PSS, the optimal TPA-HATNA-based methylammonium-free device enables a 70 mV increase in V_OC, delivering a remarkable PCE exceeding 18% (certified 16.4%). Impressively, the TPA-HATNA-based devices without encapsulation retain 90% efficiency after aging for 600 min under maximum-power-point tracking. Our work provides alternative HTMs for boosting the performance of Sn-Pb PSCs.

Optoelectronic Properties of Low-Bandgap Halide Perovskites for Solar Cell Applications

Riding on the coat tails of rapid developments in single-junction halide perovskite solar cells, all-perovskite multijunction solar cells have recently garnered significant attention, with the highest power-conversion efficiency already reaching 25.6%. Much of this progress has been fueled by the rapid rise in the photovoltaic performance of low-bandgap halide perovskite absorbers, materials, which, to date, have only been achievable by the partial or complete substitution of lead with tin. However, much room still exists to develop a more critical understanding of key material properties in these low-bandgap perovskites. Herein, the key optoelectronic properties of absorption, carrier generation, recombination, and transport in these tin-containing perovskites are discussed, showing that intrinsic doping distinctively impacts many of these properties, thereby rendering this class of halide perovskites unique within the family. Current understanding of the mechanisms that degrade optoelectronic performance in these materials and the corresponding devices are also summarized. These collective results highlight an important interplay between doping, defects, and degradation that will need to be controlled. Finally, the current gaps in understanding of these low-bandgap perovskites are outlined, thereby providing guidelines for further research, which will unlock their full potential for realizing a plethora of high-performance optoelectronic devices.

The Fascinating Properties of Tin-Alloyed Halide Perovskites

Tin-alloyed halide perovskites are progressively becoming more popular as slowly their optoelectronic properties start to rival those of the potentially risky pure lead analogues. However, to push this attractive semiconductor toward realistic applications, several major issues need to be solved. This Perspective will start with a description of the fundamental properties of tin-alloyed halide perovskites, continue discussing their weak points with special attention on the structural and electronic instabilities, and conclude examining the effects of the above-mentioned properties on devices. Finally we propose a plausible roadmap to further boost tin-alloyed halide perovskite devices to practical applications. We believe this roadmap should start from an understanding of this family of semiconductors from an atomistic viewpoint, proceeding to the control of thin-film fabrication, the structural properties, and finally the device optimization. We hope this Perspective can help to inject new enthusiasm and facilitate the progress in tin-alloyed halide perovskites, catalyzing their transition from the cradle of the laboratories to the reality of their fabrication.

The role of sodium in stabilizing tin-lead (Sn-Pb) alloyed perovskite quantum dots

Narrow-bandgap CsSn x Pb1-x I3 perovskite quantum dots (QDs) show great promise for optoelectronic applications owing to their reduced use of toxic Pb, improved phase stability, and tunable band gaps in the visible and near-infrared range. The use of small ions has been proven beneficial in enhancing the stability and photoluminescence quantum yield (PLQY) of perovskite QDs. The introduction of sodium (Na) has succeeded in boosting the PLQY of CsSn0.6Pb0.4I3 QDs. Unfortunately, the initial PLQY of the Na-doped QDs undergoes a fast degradation after one-day storage in solution, hindering their practical applications. Using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, we study the effect of Na ions on the strength of surface bonds, defect formation energies, and the interactions between surface ligands and perovskite QDs. Our results suggest that Na ions enhance the covalent bonding of surface tin-iodine bonds and form strong ionic bonding with the neighboring iodine anions, thus suppressing the formation of I and Sn vacancies. Furthermore, Na ions also enhance the binding strength of the surface ligands with the perovskite QD surface. However, according to our AIMD simulations, the enhanced surface ligand binding is only effective on a selected surface configuration. While the position of Na ions remains intact on a CsI-terminated surface, they diffuse vigorously on an MI2-terminated surface. As a result, the positive effect of Na vanishes with time, explaining the relatively short lifetime of the experimentally obtained high PLQYs. Our results indicate that engineering the surface termination of the QDs could be the next step in maintaining the favorable effect of Na doping for a high and stable PLQY of Sn-Pb QDs.

Impact of the Hole Transport Layer on the Charge Extraction of Ruddlesden-Popper Perovskite Solar Cells

Recent works demonstrate that polyelectrolytes as a hole transport layer (HTL) offers superior performance in Ruddlesden-Popper perovskite solar cells (RPPSCs) compared to poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The factors contributing to such improvement need to be systematically investigated. To achieve this, we have systematically investigated how the two HTLs affect the morphology, crystallinity, and orientation of the Ruddlesden-Popper perovskite (RPP) films as well as the charge extraction of the RPPSCs. PEDOT:PSS as a HTL leads to RPP films of low crystallinity and with a number of large pinholes. These factors lead to poor charge carrier extraction and significant charge recombination in the RPPSCs. Conversely, a PCP-Na HTL gives rise to highly crystalline and pinhole-free RPPSC films. Moreover, a PCP-Na HTL provides a better energy alignment at the perovskite/HTL interface because of its higher work function compared to PEDOT:PSS. Consequently, devices using PCP-Na as HTLs are more efficient in extracting charge carriers.

Perovskite Single-Crystal Microwire-Array Photodetectors with Performance Stability beyond 1 Year

Compared with thin-film morphology, 1D perovskite structures such as micro/nanowires with fewer grain boundaries and lower defect density are very suitable for high-performance photodetectors with higher stability. Although the stability of perovskite microwire-based photodetectors has been substantially enhanced in comparison with that of photodetectors based on thin-film morphology, practical applications require further improvements to the stability before implementation. In this study, a template-assisted method is developed to prepare methylammonium lead bromide (MAPbBr₃ ) micro/nanowire structures, which are encapsulated in situ by a protective hydrophobic molecular layer. The combination of the protective layer, high crystalline quality, and highly ordered microstructures significantly improve the stability of the MAPbBr₃ single-crystal microwire arrays. Consequently, these MAPbBr₃ single-crystal microwire-array-based photodetectors exhibit significant long-term stability, maintaining 96% of the initial photocurrent after 1 year without further encapsulation. The lifetime of such photodetectors is hence approximately four times longer than that of the most stable previously reported perovskite micro/nanowire-based photodetector; this is thought to be the most stable perovskite photodetector reported thus far. Furthermore, this work should contribute further toward the realization of perovskite 1D structures with long-term stability.

Roadmap on halide perovskite and related devices

Since the first report on solid-state perovskite solar cells (PSCs) with ∼10% power conversion efficiency (PCE) and 500 h-stability in 2012, tremendous effort has been being devoted to develop PSCs with higher PCE, longer stability and recycling hazardous lead waste. As a result, PCE over 23% was recorded in 2018 and stability over 10 000 h was reported. Beyond photovoltaics, lead halide perovskite materials demonstrated superb properties when they were applied to flat-panel x-ray detectors and non-volatile resistive switching memory. In this review, the progress of the lead halide perovskite in photovoltaics, x-ray imaging and memristors is investigated. Pb-based PSCs and non-Pb-based PSCs are compared, where technologies of non-Pb-based PSCs are not matured for commercialization. Pb-based PSCs were found to be highly suitable for both terrestrial and space photovoltaics. Higher sensitivity under low dose rate observed from the lead halide perovskite suggests a bright future for perovskite x-ray imaging systems. Moreover, high on/off ratio and low energy consumption observed in resistive switching enables perovskite to be a promising candidate for high density memristors.

Perovskite and Conjugated Polymer Wrapped Semiconducting Carbon Nanotube Hybrid Films for High-Performance Transistors and Phototransistors

Although organic-inorganic halide perovskites continue to generate considerable interest due to great potentials for various optoelectronic devices, there are some critical obstacles to practical applications, including lead toxicity, relatively low field-effect mobility, and strong hysteresis during operation. This paper proposes a universal approach to significantly improve mobility and operational stability with reduced dual-sweep hysteresis for perovskite-based thin film transistors (TFTs) by coupling low-dimensional lead-free perovskite material (C₆H₅C₂H₄NH₃)₂SnI₄ (hereafter abbreviated as (PEA)₂SnI₄) with embedded conjugated polymer wrapped semiconducting carbon nanotubes (semi-CNTs). In (PEA)₂SnI₄/semi-CNT hybrid TFTs, semi-CNTs can provide highway-like transport paths, enabling smoother carrier transport with less trapping and scattering. We also demonstrate the performance of (PEA)₂SnI₄/semi-CNT hybrid phototransistors with ultrahigh photoresponsivity ( R) of 6.3 × 10⁴ A/W and detectivity ( D*) of 1.12 × 10¹⁷ Jones, which is about 2 or 3 orders of magnitude higher than that of the best devices available to date. The results indicate promising potentials for solution-processed perovskite/semi-CNT hybrid platforms, and the developed strategy can be applied for high-performance perovskite nanomaterial optoelectronics.

Carrier lifetimes of >1 μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells

All-perovskite–based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite–based tandem devices has been limited by the lack of high-efficiency, low–band gap tin-lead (Sn-Pb) mixed-perovskite solar cells (PSCs). We found that the addition of guanidinium thiocyanate (GuaSCN) resulted in marked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low–band gap (~1.25 electron volt) perovskite films. The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers. These improved properties enable our demonstration of >20% efficient low–band gap PSCs. When combined with wider–band gap PSCs, we achieve 25% efficient four-terminal and 23.1% efficient two-terminal all-perovskite–based polycrystalline thin-film tandem solar cells.