146 results found
Lanzetta Lopez L, Webb T, Zibouche N, et al., 2021, Degradation mechanism of hybrid tin-based perovskite solar cells and the critical role of tin (IV) iodide, Nature Communications, Vol: 12, ISSN: 2041-1723
Tin perovskites have emerged as promising alternatives to toxic lead perovskites in next-generation photovoltaics, but their poor environmental stability remains an obstacle towards more competitive performances. Therefore, a full understanding of their decomposition processes is needed to address these stability issues. Herein, we elucidate the degradation mechanism of 2D/3D tin perovskite films based on (PEA)0.2(FA)0.8SnI3 (where PEA is phenylethylammonium and FA is formamidinium). We show that SnI4, a product of the oxygen-induced degradation of tin perovskite, quickly evolves into iodine via the combined action of moisture and oxygen. We identify iodine as a highly aggressive species that can further oxidise the perovskite to more SnI4, establishing a cyclic degradation mechanism. Perovskite stability is then observed to strongly depend on the hole transport layer chosen as the substrate, which is exploited to tackle film degradation. These key insights will enable the future design and optimisation of stable tin-based perovskite optoelectronics.
Yoo B, Aziz A, Ghosh D, et al., 2021, Structural, electronic, and optical properties of the vacancy-ordered bismuth-antimony perovskites (CH3NH3)(3)(Bi1-xSbx)(2)I-9, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 125, Pages: 8938-8946, ISSN: 1932-7447
Lead-based perovskites have achieved excellent photovoltaic efficiencies in the last decade, but key intrinsic issues related to their instability and Pb toxicity need to be overcome for their successful commercialization. In this combined experimental–computational study, we investigate the structural and optoelectronic properties of the novel vacancy-ordered lead-free perovskites (CH3NH3)3(Bi1–xSbx)2I9. We find complete miscibility across the solid solution with less than 1% change in the lattice parameters. This miscibility extends to the full configurational disorder between Bi and Sb and a subsequent reduction in the experimentally observed photoluminescence quantum yields. We highlight the significance of the observed band-gap bowing as a means to fine-tune the electronic structure for optoelectronic devices. The substitution of Bi with Sb leads to lower calculated electron and hole effective masses that result in lower exciton binding energies. We show a clear shift from a strongly bound to a weakly bound excitonic regime with the substitution of Bi with Sb, which correlates with the increase in device performance.
Ding D, Lanzetta L, Liang X, et al., 2021, Ultrathin polymethylmethacrylate interlayers boost performance of hybrid tin halide perovskite solar cells, Chemical Communications, Vol: 57, Pages: 5047-5050, ISSN: 1359-7345
Introducing a polymethylmethacrylate (PMMA) layer at the (PEA)0.2(FA)0.8SnI3 perovskite/hole transport layer interface leads to a remarkable improvement in the photogenerated current density and fill factor, resulting in an increase in the power conversion efficiency from 6.5% to 10%. PMMA is proposed to mitigate interfacial charge losses and to induce a more favourable distribution of 2D perovskite phases, elucidating a pathway towards the development of high-performance tin-based perovskite solar cells.
Westbrook RJE, Xu W, Liang X, et al., 2021, 2D phase purity determines charge-transfer yield at 3D/2D lead halide perovskite heterojunctions, Journal of Physical Chemistry Letters, Vol: 12, Pages: 3312-3320, ISSN: 1948-7185
Targeted functionalization of 3D perovskite with a 2D passivation layer via R-NH3I treatment has emerged as an effective strategy for enhancing both the efficiency and chemical stability of ABX3 perovskite solar cells, but the underlying mechanisms behind these improvements remain unclear. Here, we assign a passivation mechanism where R-NH3I reacts with excess PbI2 in the MAPbI3 film and unsaturated PbI6 octahedra to form (R-NH3)2(MA)n-1PbnI3n+1. Crucially, we show that precise control of the 2D (R-NH3)2(MA)n-1PbnI3n+1 layer underpins performance improvements: n = 1 yields over a 2-fold improvement in hole injection to the HTL; n > 1 deteriorates hole injection. Ultrafast transient absorption spectroscopy suggests this n-dependence is rooted in the fact that fast (<6 ns) hole injection does not occur between the 3D and 2D layers. These results help explain contemporary empirical findings in the field and set out an important design rule for the further optimization of multidimensional perovskite optoelectronics.
Mokhtar MZ, He J, Li M, et al., 2021, Bioinspired scaffolds that sequester lead ions in physically damaged high efficiency perovskite solar cells, CHEMICAL COMMUNICATIONS, Vol: 57, Pages: 994-997, ISSN: 1359-7345
Aziz A, Aristidou N, Bu X, et al., 2020, Understanding the Enhanced Stability of Bromide Substitution in Lead Iodide Perovskites, CHEMISTRY OF MATERIALS, Vol: 32, Pages: 400-409, ISSN: 0897-4756
Lanzetta L, Aristidou N, Haque SA, 2020, Stability of lead and tin halide perovskites: the link between defects and degradation, The Journal of Physical Chemistry Letters, Vol: 11, Pages: 574-585, ISSN: 1948-7185
The field of photovoltaic research has been lately dominated by the rapid evolution of low-cost and high-efficiency hybrid organic lead halide perovskite solar cells. Despite the considerable progress made in the efficiency of such devices, the achievement of long-term material and device stability remains a challenge. In this Perspective, insights into the role structural defects play in the stability of these perovskite absorbers are examined, highlighting the critical importance of vacancy type defects as the initiation sites for moisture-, oxygen-, and light-induced degradation and the approaches that are emerging to help overcome these issues. In the second part of the Perspective we consider the stability of tin-based perovskites. Here, the Sn4+ defects that arise upon material degradation are described along with the strategies being developed to enhance stability and decrease their formation. Finally, the discussion is extended to innately more stable layered tin-based perovskites, identifying them as a route to the development of efficient lead-free perovskite solar cells.
Yoo B, Ding D, Marin-Beloqui JM, et al., 2019, Improved Charge Separation and Photovoltaic Performance of BiI3 Absorber Layers by Use of an In Situ Formed BiSI Interlayer, ACS APPLIED ENERGY MATERIALS, Vol: 2, Pages: 7056-7061, ISSN: 2574-0962
Speller EM, Clarke AJ, Aristidou N, et al., 2019, Toward improved environmental stability of polymer:fullerene and polymer:non-fullerene organic solar cells: a common energetic origin of light and oxygen induced degradation, ACS Energy Letters, Vol: 4, Pages: 846-852, ISSN: 2380-8195
With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.
Lin C-T, Rossi F, Kim J, et al., 2019, Evidence for surface defect passivation as the origin of the remarkable photostability of unencapsulated perovskite solar cells employing aminovaleric acid as a processing additive, Journal of Materials Chemistry A, Vol: 7, ISSN: 2050-7496
This study addresses the cause of enhanced stability of methyl ammonium lead iodide when processed with aminovaleric acid additives (AVA-MAPbI3) in screen printed, hole transport layer free perovskite solar cells with carbon top electrodes (c-PSC). Employing AVA as an additive in the active layer caused a 40-fold increase in device lifetime measured under full sun illumination in ambient air (RH ~15%). This stability improvement with AVA was also observed in optical photobleaching studies of planar films on glass, indicating this improvement is intrinsic to the perovskite film. Employing low-energy ion scattering spectroscopy, photoluminescence studies as a function of AVA and oxygen exposure, and a molecular probe for superoxide generation, we conclude that even though superoxide is generated in both AVA-MAPbI3 and MAPbI3 films, AVA located at grain boundaries is able to passivate surface defect sites, resulting in enhanced resistivity to oxygen induced degradation. These results are discussed in terms of their implications for the design of environmentally stable perovskite solar cells.
Bu X, Westbrook RJE, Lanzetta L, et al., 2019, Surface passivation of perovskite films via Iodide salt coatings for enhanced stability of organic lead halide perovskite solar cells, Solar RRL, Vol: 3, Pages: 1-9, ISSN: 2367-198X
Organic–inorganic halide perovskite materials have emerged as attractive alternatives to conventional solar cells, but device stability remains a concern. Recent research has demonstrated that the formation of superoxide species under exposure of the perovskite to light and oxygen leads to the degradation of CH3NH3PbI3 perovskites. In particular, it has been revealed that iodide vacancies in the perovskite are key sites in facilitating superoxide formation from oxygen. This paper shows that passivation of CH3NH3PbI3 films with an iodide salt, namely phenylethylammonium iodide (PhEtNH3I) can significantly enhance film and device stability under light and oxygen stress, without compromising power conversion efficiency. These observations are consistent with the iodide salt treatment reducing iodide vacancies and therefore lowers the yield of superoxide formation and improves stability. The present study elucidates a pathway to the future design and optimization of perovskite solar cells with greater stability.
Ding D, Rath T, Lanzetta L, et al., 2018, Efficient hybrid solar cells based on solution processed mesoporous TiO2 / Tin (II) sulfide heterojunctions, ACS Applied Energy Materials, Vol: 1, Pages: 3042-3047, ISSN: 2574-0962
Tin monosulfide (SnS) is emerging as a promising absorber material for the development of low-cost, solution-processable solar cells. Herein, we present a straightforward, solution-based route for the preparation of tin monosulfide (SnS) films employing a green solvent, namely, tetrahydrofuran (THF). We show that the surface coverage and the morphology of the SnS films can be tuned by modification of the precursor solution composition and film deposition conditions. Furthermore, we explore the effect of a PEDOT:PSS interlayer and fabricate solar cells based on the following architecture: FTO/planar-TiO2/mesoporous-TiO2/SnS/P3HT/PEDOT:PSS/Ag. Devices based on this architecture are shown to exhibit power conversion efficiencies (PCEs) of 3.0%, thus demonstrating the potential of our method for the development of low-cost and environmentally friendly solar cells.
Westbrook RJE, Haque S, Sanchez-Molina I, et al., 2018, The Effect of Interfacial Energetics on Charge Transfer from Lead Halide Perovskite to Organic Hole Conductors, Journal of Physical Chemistry C, Vol: 122, Pages: 1326-1332, ISSN: 1932-7447
The control and optimization of interfacial charge transfer processes is crucial to the design of efficient perovskite solar cells. Herein, we measure the yield and kinetics of hole transfer across the methylammonium lead triiodide perovskite|polymeric hole transport material heterojunction, as a function of the interfacial energy offset, ∆E between the highest occupied molecular orbital of the hole transport material and the valence band of the perovskite. A combination of steady-state and time-resolved photoluminescence, along with transient absorption spectroscopy revealed that only a small driving energy (∆E~0.07eV) is required to induce highly efficient hole transfer. The findings of this paper suggest that further improvements in the open-circuit voltage, and so the power conversion efficiency, of perovskite solar cells could be achieved by incorporating hole transport materials that provide an interfacial energy offset in the range 0 < ∆E < 0.18eV.
Aristidou, Eames C, Islam MS, et al., 2017, Insights into the increased degradation rate of CH3NH3PbI3 solar cells in combined water and O2 environments, Journal of Materials Chemistry A, Vol: 5, Pages: 25469-25475, ISSN: 2050-7496
Halide perovskites offer low cost and high efficiency solar cell materials but serious issues related to air and moisture stability remain. In this study we show, using UV-vis, fluorescence and time of flight secondary ion mass spectrometry (ToF-SIMS) techniques, that the degradation of methylammonium lead iodide solar cells is significantly accelerated when both air and moisture are present in comparison to when just air or moisture is present alone. Using ab initio computational techniques we identify the thermodynamic driving force for the enhanced reactivity and highlight the regions of the photoexcited material that are the most likely reaction centres. We suggest that water catalyses the reaction by stabilising the reactive superoxide species, enabling them to react with the methylammonium cation.
Chen M, Mokhtar MZ, Whittaker E, et al., 2017, Reducing hole transporter use and increasing perovskite solar cell stability with dual-role polystyrene microgel particles, NANOSCALE, Vol: 9, Pages: 10126-10137, ISSN: 2040-3364
Lanzetta Lopez L, Marin-Beloqui JM, Sanchez-Molina I, et al., 2017, Two-dimensional organic tin Halide Perovskites with tunable visible emission and their use in light-emitting devices, ACS Energy Letters, Vol: 2, Pages: 1662-1668, ISSN: 2380-8195
Hybrid organic lead trihalide perovskites continue to generate significant interest for use in optoelectronic devices such as solar cells and light-emitting devices. However, the toxicity of lead is considered one of the main obstacles to the commercialization of this technology. Although challenging, the replacement of lead by tin is currently the most promising alternative. Herein, we explore a class of low-dimensional, lead-free perovskite materials (2D (PEA)2SnIxBr4–x, where PEA ≡ C6H5CH2CH2NH3+) with tunable optical properties in the visible region of the spectrum. Specifically, we show that 2D (PEA)2SnI4 perovskite exhibits superior photoluminescence properties to conventional 3D CH3NH3SnI3 and that (PEA)2SnI4 can act as a sensitizer on mesoporous TiO2. We go on to demonstrate visible (∼630 nm) electroluminescence from a device employing a (PEA)2SnI4 emitter sandwiched between ITO/PEDOT:PSS and F8/LiF/Al as hole and electron injection electrodes, respectively. These devices reach a luminance of 0.15 cd/m2 at 4.7 mA/cm2 and an efficacy of 0.029 cd/A at 3.6 V. This proof-of-principle device indicates a viable path to low-dimensional, lead-free perovskite optoelectronics.
Aristidou N, Haque S, Eames C, et al., 2017, Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells, Nature Communications, Vol: 8, Pages: 1-10, ISSN: 2041-1723
Methylammonium lead halide perovskites are attracting intense interest as promising materials for next-generation solar cells, but serious issues related to long-term stability need to be addressed. Specifically, perovskite films based on CH3NH3PbI3 undergo fast oxygen- and light-induced degradation. However, the mechanism of such degradation and its relation to particle size and oxygen transport are poorly understood. Here, we report new mechanistic insights through the combined use of isothermal gravimetric analysis, photoluminescence, secondary ion mass spectrometry and ab initio simulation techniques. We find fast oxygen diffusion into CH3NH3PbI3 films is accompanied by superoxide formation, which are critical to degradation by oxygen in the atmosphere.Perovskite films composed of small crystallites show high yields of photo-induced superoxide species and low stability. Ab initio simulations indicate that iodide vacancies are the preferred sites in mediating the photo-induced formation of superoxide species from oxygen. We also show that thin-film passivation with iodide salts can lead to enhanced film stability and thus device stability. The understanding of degradation phenomena gained from this study is important for the future design and optimisation of perovskite solar cells with greater stability.
Bruno A, Borriello C, Di Luccio T, et al., 2017, Oxadiazole-carbazole polymer (POC)-Ir(ppy)(3) tunable emitting composites, OPTICAL MATERIALS, Vol: 66, Pages: 166-170, ISSN: 0925-3467
Pont S, Bryant D, Lin CH, et al., 2017, Tuning CH 3 NH 3 Pb(I 1-x Br x ) 3 Perovskite Oxygen Stability in Thin Films and Solar Cells, Journal of Materials Chemistry A, Vol: 5, ISSN: 2050-7488
The rapid development of organic–inorganic lead halide perovskites has resulted in high efficiency photovoltaic devices. However the susceptibility of these devices to degradation under environmental stress has so far hindered commercial development, requiring for example expensive device encapsulation. Herein, we have investigated the stability of CH3NH3Pb(I1−xBrx)3 [x = 0–1] thin films and solar cells under controlled humidity, light, and oxygen conditions. We show that higher bromide ratios increase tolerance to moisture, with x = 1 thin films being stable to 120 h of moisture stress. Under light and dry air, partial bromide (x < 1) substitution does not enhance film stability significantly, with the corresponding solar cells degrading within two hours. In contrast, CH3NH3PbBr3 films show excellent stability, with device stability being limited by the organic interlayer. For these x = 1 films, we show that charge carriers are quenched in the presence of oxygen and form superoxide; however in contrast to perovskites containing iodide, this superoxide does not degrade the crystal. Our observations show that iodide limits the oxygen and light stability of CH3NH3Pb(I1−xBrx)3 perovskites, but that CH3NH3PbBr3 provides an opportunity to develop inherently stable high voltage photovoltaic devices and 4-terminal tandem solar cells.
Mokhtar MZ, Chen M, Whittaker E, et al., 2017, CH3NH3PbI3 films prepared by combining 1-and 2-step deposition: how crystal growth conditions affect properties, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 19, Pages: 7204-7214, ISSN: 1463-9076
Bryant D, Aristidou N, Pont S, et al., 2016, Correction: Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells, Energy & Environmental Science, Vol: 9, Pages: 1850-1850, ISSN: 1754-5706
Bryant D, Aristidou N, Pont S, et al., 2016, Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells, Energy and Environmental Science, Vol: 9, Pages: 1655-1660, ISSN: 1754-5692
Here, we demonstrate that light and oxygen-induced degradation is the main reason for the low operational stability of methylammonium lead triiodide (MeNH3PbI3) perovskite solar cells exposed to ambient conditions. When exposed to both light and dry air, unencapsulated MeNH3PbI3 solar cells rapidly degrade on timescales of minutes to a few hours. This rapid degradation is also observed under electrically bias driven current flow in the dark in the presence of O2. In contrast, significantly slower degradation is observed when the MeNH3PbI3 devices are exposed to moisture alone (e.g. 85% relative humidity in N2). We show that this light and oxygen induced degradation can be slowed down by the use of interlayers that are able to remove electrons from the perovskite film before they can react with oxygen to form O2-. These observations demonstrate that the operational stability of electronic and optoelectronic devices that exploit the electron transporting properties of MeNH3PbI3 will be critically dependent upon the use of suitable barrier layers and device configurations to mitigate the oxygen sensitivity of this remarkable material.
Cappel UB, Moia D, Bruno A, et al., 2016, Evidence for photo-induced charge separation between dye molecules adsorbed to aluminium oxide surfaces, Scientific Reports, Vol: 6, ISSN: 2045-2322
Excited state dynamics and photo-induced charge transfer of dye molecules have been widely studied due to their relevance for organic and dye-sensitised solar cells. Herein, we present a femtosecond transient absorption spectroscopy study of the indolene dye D131 when adsorbed to inert Al2O3 substrates for different surface concentration of the dye. Surprisingly, we find that at high surface concentrations, the first singlet excited state of the dye is converted into a new state with an efficiency of about 80%. We assign the absorption features of this state to the oxidised dye and discuss the possibility of photo-induced charge separation between neighboring dye molecules. Our study is the first to show that this process can be highly efficient without the use of donor and acceptor molecules of different chemical structures.
Hu Y, Sanchez-Molina I, Haque SA, et al., 2015, Ruthenium Dyes with Azo Ligands: Light Harvesting, Excited-State Properties and Relevance to Dye-Sensitised Solar Cells, EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Pages: 5864-5873, ISSN: 1434-1948
Rath T, MacLachlan AJ, Brown MD, et al., 2015, Structural, optical and charge generation properties of chalcostibite and tetrahedrite copper antimony sulfide thin films prepared from metal xanthates, Journal of Materials Chemistry A, Vol: 3, Pages: 24155-24162, ISSN: 2050-7496
Herein, we report on a solution based approach for the preparation of thin films of copper antimony sulfide, an emerging absorber material for third generation solar cells. In this work, copper and antimony xanthates are used as precursor materials for the formation of two different copper antimony sulfide phases: chalcostibite (CuSbS2) and tetrahedrite (Cu12Sb4S13). Both phases were thoroughly investigated regarding their structural and optical properties. Moreover, thin films of chalcostibite and tetrahedrite were prepared on mesoporous TiO2 layers and photoinduced charge transfer in these metal sulfide/TiO2 heterojunctions was studied via transient absorption spectroscopy. Photoinduced charge transfer was detected in both the chalcostibite as well as the tetrahedrite sample, which is an essential property in view of applying these materials as light-harvesting agents in semiconductor sensitized solar cells.
Bruno A, Commodo M, Haque SA, et al., 2015, Spectroscopic investigation of flame synthesized carbon nanoparticle/P3HT blends, CARBON, Vol: 94, Pages: 955-961, ISSN: 0008-6223
Aristidou N, Sanchez-Molina I, Chotchuangchutchaval T, et al., 2015, The role of oxygen in the degradation of methylammonium lead trihalide perovskite photoactive layers, Angewandte Chemie-International Edition, Vol: 54, Pages: 8208-8212, ISSN: 1521-3773
In this paper we report on the influence of light and oxygen on the stability of CH3NH3PbI3 perovskite-based photoactive layers. When exposed to both light and dry air the mp-Al2O3/CH3NH3PbI3 photoactive layers rapidly decompose yielding methylamine, PbI2, and I2 as products. We show that this degradation is initiated by the reaction of superoxide (O2−) with the methylammonium moiety of the perovskite absorber. Fluorescent molecular probe studies indicate that the O2− species is generated by the reaction of photoexcited electrons in the perovskite and molecular oxygen. We show that the yield of O2− generation is significantly reduced when the mp-Al2O3 film is replaced with an mp-TiO2 electron extraction and transport layer. The present findings suggest that replacing the methylammonium component in CH3NH3PbI3 to a species without acid protons could improve tolerance to oxygen and enhance stability.
Rath T, Gury L, Sanchez-Molina I, et al., 2015, Formation of porous SnS nanoplate networks from solution and their application in hybrid solar cells, Chemical Communications, Vol: 51, Pages: 10198-10201, ISSN: 1364-548X
Herein, we present a facile solution-based route towards nanostructured, hybrid absorber layers based on tin mono-sulfide (SnS), an emerging, non-toxic absorber material for low-cost and large-scale PV applications. Charge photogeneration properties in the hybrid system are studied using transient absorption spectroscopy and fabricated solar cells show efficient photocurrent generation over a broad spectral range.
Cao Y, Bernechea M, Maclachlan A, et al., 2015, Solution Processed Bismuth Sulfide Nanowire Array Core/Silver Sulfide Shell Solar Cells, CHEMISTRY OF MATERIALS, Vol: 27, Pages: 3700-3706, ISSN: 0897-4756
O'Mahony FTF, Lee YH, Jellett C, et al., 2015, Improved environmental stability of organic lead trihalide perovskite-based photoactive-layers in the presence of mesoporous TiO2, Journal of Materials Chemistry A, Vol: 3, Pages: 7219-7223, ISSN: 2050-7496
Impressive hybrid photovoltaic device performances have been realised with the methylammonium lead triiodide (MAPbI3) perovskite absorber in a wide range of device architectures. However, the question as to which of these systems represents the most commercially viable long-term prospect is yet to be answered conclusively. Here, we report on the photoinduced charge transfer processes in MAPbI3 based films measured under inert and ambient conditions. When exposed to ambient conditions, the coated mesoporous Al2O3 and bilayer systems show a rapid and significant degradation in the yield of long-lived charge separation. This process, which does not affect sensitized-mesoporous TiO2 films, is only found to occur when both light and oxygen are present. These observations indicate that the presence of a mesostructured TiO2 electron acceptor to rapidly extract the photoexcited electron from the perovskite sensitizer may be crucial for fundamental photovoltaic stability and significantly increases innate tolerance to environmental conditions. This work highlights a significant advantage of retaining mesoscale morphological control in the design of perovskite photovoltaics.
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