Imperial College London

Dr Robert Hoye, FIMMM CEng CSci

Faculty of EngineeringDepartment of Materials

Lecturer in Materials
 
 
 
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Contact

 

+44 (0)20 7594 6048r.hoye Website

 
 
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Location

 

2.27Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

72 results found

Silva JPB, Vieira EMF, Gwozdz K, Kaim A, Goncalves LM, MacManus-Driscoll JL, Hoye RLZ, Pereira Met al., 2021, High-performance self-powered photodetectors achieved through the pyro-phototronic effect in Si/SnOx/ZnO heterojunctions, NANO ENERGY, Vol: 89, ISSN: 2211-2855

Journal article

Huang Y-T, Kavanagh SR, Scanlon DO, Walsh A, Hoye RLZet al., 2021, Perovskite-inspired materials for photovoltaics and beyond-from design to devices (vol 32, 132004, 2021), NANOTECHNOLOGY, Vol: 32, ISSN: 0957-4484

Journal article

Senanayak S, Dai L, Kusch G, Shivanna R, Zhang Y, Pradhan D, Ye J, Huang Y-T, Sirringhaus H, Oliver R, Greenham N, Friend R, Hoye Ret al., 2021, Understanding the role of grain boundaries on charge-carrier and ion transport in Cs2AgBiBr6 thin films, Advanced Functional Materials, ISSN: 1616-301X

Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air, and recent demonstrations of long charge-carrier lifetimes that can exceed 1 s. In particular, Cs2AgBiBr6 has been the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, we investigate the role of grain size on the optoelectronic properties of Cs2AgBiBr6 thin films. We show through cathodoluminescence measurements that grain boundaries are the dominant non-radiative recombination sites. We also demonstrate through field-effect transistor and temperature-dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, we find a positive correlation between carrier mobility and temperature, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths >1 m found in Cs2AgBiBr6 single crystals versus the limited performance achieved in their thin film counterparts. Our work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic single-crystal properties.

Journal article

Pecunia V, Occhipinti L, Hoye R, 2021, Emerging indoor photovoltaic technologies for sustainable internet of things, Advanced Energy Materials, Vol: 11, Pages: 131-131, ISSN: 1614-6832

The Internet of Things (IoT) provides everyday objects and environments with “intelligence” and data connectivity to improve quality of life and the efficiency of a wide range of human activities. However, the ongoing exponential growth of the IoT device ecosystem—up to tens of billions of units to date—poses a challenge regarding how to power such devices. This Progress Report discusses how energy harvesting can address this challenge. It then discusses how indoor photovoltaics (IPV) constitutes an attractive energy harvesting solution, given its deployability, reliability, and power density. For IPV to provide an eco-friendly route to powering IoT devices, it is crucial that its underlying materials and fabrication processes are low-toxicity and not harmful to the environment over the product life cycle. A range of IPV technologies—both incumbent and emerging—developed to date is discussed, with an emphasis on their environmental sustainability. Finally, IPV based on emerging lead-free perovskite-inspired absorbers are examined, highlighting their status and prospects for low-cost, durable, and efficient energy harvesting that is not harmful to the end user and environment. By examining emerging avenues for eco-friendly IPV, timely insight is provided into promising directions toward IPV that can sustainably power the IoT revolution.

Journal article

Dey A, Ye J, De A, Debroye E, Ha SK, Bladt E, Kshirsagar AS, Wang Z, Yin J, Wang Y, Quan LN, Yan F, Gao M, Li X, Shamsi J, Debnath T, Cao M, Scheel MA, Kumar S, Steele JA, Gerhard M, Chouhan L, Xu K, Wu X-G, Li Y, Zhang Y, Dutta A, Han C, Vincon I, Rogach AL, Nag A, Samanta A, Korgel BA, Shih C-J, Gamelin DR, Son DH, Zeng H, Zhong H, Sun H, Demir HV, Scheblykin IG, Mora-Sero I, Stolarczyk JK, Zhang JZ, Feldmann J, Hofkens J, Luther JM, Perez-Prieto J, Li L, Manna L, Bodnarchuk M, Kovalenko M, Roeffaers MBJ, Pradhan N, Mohammed OF, Bakr OM, Yang P, Muller-Buschbaum P, Kamat P, Bao Q, Zhang Q, Krahne R, Galian RE, Stranks SD, Bals S, Biju V, Tisdale WA, Yan Y, Hoye RLZ, Polavarapu Let al., 2021, State of the Art and Prospects for Halide Perovskite Nanocrystals, ACS NANO, Vol: 15, Pages: 10775-10981, ISSN: 1936-0851

Journal article

Pecunia V, Zhao J, Kim C, Tuttle BR, Mei J, Li F, Peng Y, Huq TN, Hoye RLZ, Kelly ND, Dutton SE, Xia K, MacManusDriscoll JL, Sirringhaus Het al., 2021, Assessing the impact of defects on lead‐free perovskite‐inspired photovoltaics via photoinduced current transient spectroscopy, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832

The formidable rise of lead‐halide perovskite photovoltaics has energized the search for lead‐free perovskite‐inspired materials (PIMs) with related optoelectronic properties but free from toxicity limitations. The photovoltaic performance of PIMs closely depends on their defect tolerance. However, a comprehensive experimental characterization of their defect‐level parameters—concentration, energy depth, and capture cross‐section—has not been pursued to date, hindering the rational development of defect‐tolerant PIMs. While mainstream, capacitance‐based techniques for defect‐level characterization have sparked controversy in lead‐halide perovskite research, their use on PIMs is also problematic due to their typical near‐intrinsic character. This study demonstrates on four representative PIMs (Cs3Sb2I9, Rb3Sb2I9, BiOI, and AgBiI4) for which Photoinduced Current Transient Spectroscopy (PICTS) offers a facile, widely applicable route to the defect‐level characterization of PIMs embedded within solar cells. Going beyond the ambiguities of the current discussion of defect tolerance, a methodology is also presented to quantitatively assess the defect tolerance of PIMs in photovoltaics based on their experimental defect‐level parameters. Finally, PICTS applied to PIM photovoltaics is revealed to be ultimately sensitive to defect‐level concentrations <1 ppb. Therefore, this study provides a versatile platform for the defect‐level characterization of PIMs and related absorbers, which can catalyze the development of green, high‐performance photovoltaics.

Journal article

Nasane MP, Rondiya SR, Jadhav CD, Rahane GR, Cross RW, Jathar S, Jadhav Y, Barma S, Nilegave D, Jadkar V, Rokade A, Funde A, Chavan PG, Hoye RLZ, Dzade NY, Jadkar Set al., 2021, An interlinked computational-experimental investigation into SnS nanoflakes for field emission applications, New Journal of Chemistry: a journal for new directions in chemistry, Vol: 45, Pages: 11768-11779, ISSN: 1144-0546

Layered binary semiconductor materials have attracted significant interest as field emitters due to their low work function, mechanical stability, and high thermal, and electrical conductivity. Herein, we report a systematic experimental and theoretical investigation of SnS nanoflakes synthesized using a simple, low-cost, and non-toxic hot injection method for field emission studies. The field emission studies were carried out on SnS nanoflake thin films prepared using a simple spin coating technique. The X-ray diffraction (XRD) and Raman spectroscopy analysis revealed an orthorhombic phase of SnS. Scanning electron microscopy (SEM) analysis revealed that as-synthesized SnS has a flakes like morphology. The formation of pure-phase SnS nanoflakes was further confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The UV-Visible-NIR spectroscopy analysis shows that SnS nanoflakes have a sharp absorption edge observed in the UV region and have a band gap of ∼1.66 eV. In addition, the first-principles density functional theory (DFT) calculations were carried out to provide atomic-level insights into the crystal structure, band structure, and density of states (DOS) of SnS nanoflakes. The field emission properties of SnS nanoflakes were also investigated and it was found that SnS nanoflakes have a low turn-on field (∼6.2 V μm−1 for 10 μA cm−2), high emission current density (∼104 μA cm−2 at 8.0 V μm−1), superior current stability (∼1 μA for ∼2.5 hrs), and a high field enhancement factor of 1735. First principles calculations predicted lower work function for different surfaces, especially for the most stable SnS (001) surface (ϕ = 4.32 eV), which is believed to be responsible for the observed facile electron emission characteristics. We anticipate that the SnS could be utilized for future vacuum nano/microelectronic and flat panel display applications due to the low turn-on field and flakes like structure.

Journal article

Perini C, Doherty T, Stranks S, Correa-Baena J-P, Hoye Ret al., 2021, Pressing challenges in halide perovskite photovoltaics - from the atomic to module level, Joule, Vol: 5, Pages: 1024-1030, ISSN: 2542-4351

Journal article

Hoye R, Hidalgo J, Jagt RA, Correa-Baena J-P, Fix T, MacManus-Driscoll JLet al., 2021, The role of dimensionality on the optoelectronic properties of oxide and halide perovskites, and their halide derivatives, Advanced Energy Materials, ISSN: 1614-6832

Halide perovskite semiconductors have risen to prominence in photovoltaics and light-emitting diodes (LEDs), but traditional oxide perovskites, which overcome the stability limitations of their halide counterparts, have also recently witnessed a rise in potential as solar absorbers. One of the many important factors underpinning these developments is an understanding of the role of dimensionality on the optoelectronic properties and, consequently, on the performance of the materials in photovoltaics and LEDs. This review article examines the role of structural and electronic dimensionality, as well as form factor, in oxide and halide perovskites, and in lead-free alternatives to halide perovskites. Insights into how dimensionality influences the band gap, stability, charge-carrier transport, recombination processes and defect tolerance of the materials, and the impact these parameters have on device performance are brought forward. Particular emphasis is placed on carrier/exciton-phonon coupling, which plays a significant role in the materials considered, owing to their soft lattices and composition of heavy elements, and becomes more prominent as dimensionality is reduced. It is finished with a discussion of the implications on the classes of materials future efforts should focus on, as well as the key questions that need to be addressed.

Journal article

Napari M, Huq TN, Hoye RLZ, MacManusDriscoll JLet al., 2021, Nickel oxide thin films grown by chemical deposition techniques: Potential and challenges in next‐generation rigid and flexible device applications, InfoMat, Vol: 3, Pages: 536-576, ISSN: 2567-3165

Journal article

Polavarapu L, Ye J, Byranvand MM, Martínez CO, Hoye RL, Saliba Met al., 2021, Defect passivation in lead-halide perovskite nanocrystals and thin films: toward efficient LEDs and solar cells., Angewandte Chemie International Edition, ISSN: 1433-7851

Lead-halide perovskites (LHPs), in the form of both colloidal nanocrystals (NCs) and thin films, have emerged over the past decade as leading candidates for next-generation, efficient light-emitting diodes (LEDs) and solar cells. Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge-carriers to light and vice versa . However, despite the defect-tolerance of LHPs, defects at the surface of colloidal NCs and at grain boundaries in thin films play a critical role in charge-carrier transport and non-radiative recombination, which lowers PLQYs, device efficiency and stability. Therefore, understanding the defects that play a key role in limiting performance, and developing effective passivation routes is critical for achieving advances in performance, and therefore features at the forefront of perovskite research. This review presents the current understanding of the defects in perovskites (both colloidal NCs and thin films) and their influence on the optical and charge-carrier transport properties. The review also discusses the passivation strategies toward improving the efficiencies of perovskite-based LEDs and solar cells.

Journal article

Gan J, Yu M, Hoye RLZ, Musselman KP, Li Y, Liu X, Zheng Y, Zu X, Li S, MacManus-Driscoll JL, Qiao Let al., 2021, Defects, photophysics and passivation in Pb-based colloidal quantum dot photovoltaics, Materials Today Nano, Vol: 13, Pages: 1-17, ISSN: 2588-8420

Colloidal quantum dots (CQDs) are a class of third-generation materials for photovoltaics (PVs) that are promising for enabling high efficiency devices with potential for exceeding the Shockley-Queisser limit. This is due to their potential to decrease thermal dissipation via multiple exciton generation during charge conversion and collection, which could potentially lead to an increase in the photovoltage or photocurrent in colloidal quantum dot photovoltaics (CQD PVs). But despite a predicted upper efficiency limit of 42%–44%, the highest power conversion efficiencies of these PVs using lead sulfide colloidal quantum dots (PbS CQDs) remains at approximately 13% on a laboratory scale. For further improvements, the fundamental recombination mechanisms need to be studied to determine their effects on the open-circuit voltage (VOC) and charge-carrier lifetime as well as the diffusion length of the carriers. Also, surface defect passivation and interface engineering should be studied. In this work, we discuss different pathways for non-radiative recombination losses in lead sulfide colloidal quantum dot photovoltaics (PbS CQD PVs), as well as the strategies for reducing these losses by the passivation of the surface and interface defects. We also discuss routes to overcome limits in the diffusion length of the carriers through the engineering of charge transport layers. This work provides routes for the fabrication of highly efficient CQD PVs.

Journal article

Napari M, Huq TN, Meeth DJ, Heikkilä MJ, Niang KM, Wang H, Iivonen T, Wang H, Leskelä M, Ritala M, Flewitt AJ, Hoye R, MacManus-Driscoll JLet al., 2021, Role of ALD Al2O3 surface passivation on the performance of p-type Cu2O thin film transistors, ACS Applied Materials and Interfaces, Vol: 13, Pages: 4156-4164, ISSN: 1944-8244

High-performance p-type oxide thin film transistors (TFTs) have great potential for many semiconductor applications. However, these devices typically suffer from low hole mobility and high off-state currents. We fabricated p-type TFTs with a phase-pure polycrystalline Cu2O semiconductor channel grown by atomic layer deposition (ALD). The TFT switching characteristics were improved by applying a thin ALD Al2O3 passivation layer on the Cu2O channel, followed by vacuum annealing at 300 °C. Detailed characterization by transmission electron microscopy–energy dispersive X-ray analysis and X-ray photoelectron spectroscopy shows that the surface of Cu2O is reduced following Al2O3 deposition and indicates the formation of a 1–2 nm thick CuAlO2 interfacial layer. This, together with field-effect passivation caused by the high negative fixed charge of the ALD Al2O3, leads to an improvement in the TFT performance by reducing the density of deep trap states as well as by reducing the accumulation of electrons in the semiconducting layer in the device off-state.

Journal article

Huang Y-T, Kavanagh S, Scanlon D, Walsh A, Hoye Ret al., 2021, Perovskite-inspired materials for photovoltaics and beyond – from design to devices, Nanotechnology, Vol: 32, Pages: 1-60, ISSN: 0957-4484

NanotechnologyACCEPTED MANUSCRIPT • The following article is Open accessPerovskite-Inspired Materials for Photovoltaics and Beyond – From Design to DevicesYi-Teng Huang1, Seán R. Kavanagh2, David O Scanlon3, Aron Walsh4 and Robert Hoye5Accepted Manuscript online 1 December 2020 • © 2020 The Author(s). Published by IOP Publishing Ltd.What is an Accepted Manuscript?Download Accepted Manuscript PDFDownload PDFArticle has an altmetric score of 6Turn on MathJaxShare this article Share this content via email Share on Facebook Share on Twitter Share on Google+ Share on MendeleyArticle informationAbstractLead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials i

Journal article

Li Z, Kavanagh S, Napari M, Palgrave RG, Abdi-Jalebi M, Andaji-Garmaroudi Z, Davies DW, Laitinen M, Julin J, Isaacs MA, Friend RH, Scanlon DO, Walsh A, Hoye Ret al., 2020, Bandgap lowering in mixed alloys ofCs2Ag(SbxBi1-x)Br6 double perovskite thin films, Journal of Materials Chemistry A, Vol: 8, Pages: 21780-21788, ISSN: 2050-7488

Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air and long charge-carrier lifetimes. However, most double perovskites, including Cs2AgBiBr6, have wide bandgaps, which limits photoconversion efficiencies. The bandgap can be reduced through alloying with Sb3+, but Sb-rich alloys are difficult to synthesize due to the high formation energy of Cs2AgSbBr6, which itself has a wide bandgap. We develop a solution-based route to synthesize phase-pure Cs2Ag(SbxBi1−x)Br6 thin films, with the mixing parameter x continuously varying over the entire composition range. We reveal that the mixed alloys (x between 0.5 and 0.9) demonstrate smaller bandgaps than the pure Sb- and Bi-based compounds. The reduction in the bandgap of Cs2AgBiBr6 achieved through alloying (170 meV) is larger than if the mixed alloys had obeyed Vegard's law (70 meV). Through in-depth computations, we propose that bandgap lowering arises from the type II band alignment between Cs2AgBiBr6 and Cs2AgSbBr6. The energy mismatch between the Bi and Sb s and p atomic orbitals, coupled with their non-linear mixing, results in the alloys adopting a smaller bandgap than the pure compounds. Our work demonstrates an approach to achieve bandgap reduction and highlights that bandgap bowing may be found in other double perovskite alloys by pairing together materials forming a type II band alignment.

Journal article

Peng Y, Huq T, Mei J, Portilla L, Jagt RA, Occhipinti L, MacManus-Driscoll JL, Hoye R, Pecunia Vet al., 2020, Lead-free perovskite-inspired absorbers for indoor photovoltaics, Advanced Energy Materials, Vol: 11, Pages: 1-12, ISSN: 1614-6832

With the exponential rise in the market value and number of devices part of the Internet of Things (IoT), the demand for indoor photovoltaics (IPV) to power autonomous devices is predicted to rapidly increase. Lead‐free perovskite‐inspired materials (PIMs) have recently attracted significant attention in photovoltaics research, due to the similarity of their electronic structure to high‐performance lead‐halide perovskites, but without the same toxicity limitations. However, the capability of PIMs for indoor light harvesting has not yet been considered. Herein, two exemplar PIMs, BiOI and Cs3Sb2ClxI9‐x are examined. It is shown that while their bandgaps are too wide for single‐junction solar cells, they are close to the optimum for indoor light harvesting. As a result, while BiOI and Cs3Sb2ClxI9‐x devices are only circa 1%‐efficient under 1‐sun illumination, their efficiencies increase to 4–5% under indoor illumination. These efficiencies are within the range of reported values for hydrogenated amorphous silicon, i.e., the industry standard for IPV. It is demonstrated that such performance levels are already sufficient for millimeter‐scale PIM devices to power thin‐film‐transistor circuits. Intensity‐dependent and optical loss analyses show that future improvements in efficiency are possible. Furthermore, calculations of the optically limited efficiency of these and other low‐toxicity PIMs reveal their considerable potential for IPV, thus encouraging future efforts for their exploration for powering IoT devices.

Journal article

Raninga RD, Jagt RA, Béchu S, Huq TN, Li W, Nikolka M, Lin Y-H, Sun M, Li Z, Li W, Bouttemy M, Frégnaux M, Snaith HJ, Schulz P, MacManus-Driscoll JL, Hoye RLZet al., 2020, Strong performance enhancement in lead-halide perovskite solar cells through rapid, atmospheric deposition of n-type buffer layer oxides, Nano Energy, Vol: 75, Pages: 104946-104946, ISSN: 2211-2855

Journal article

Jagt RA, Huq TN, Hill SA, Thway M, Liu T, Napari M, Roose B, Gałkowski K, Li W, Lin SF, Stranks SD, MacManus-Driscoll JL, Hoye RLZet al., 2020, Rapid vapor-phase deposition of high-mobility p-Type buffer layers on perovskite photovoltaics for efficient semi-transparent devices, ACS Energy Letters, Vol: 5, Pages: 2456-2465, ISSN: 2380-8195

Perovskite solar cells (PSCs) with transparent electrodes can be integrated with existing solar panels in tandem configurations to increase the power conversion efficiency. A critical layer in semi-transparent PSCs is the inorganic buffer layer, which protects the PSC against damage when the transparent electrode is sputtered on top. The development of n-i-p structured semi-transparent PSCs has been hampered by the lack of suitable p-type buffer layers. In this work we develop a p-type CuOx buffer layer, which can be grown uniformly over the perovskite device without damaging the perovskite or organic hole transport layers. The CuOx layer has high hole mobility (4.3 ± 2 cm2 V-1 s-1), high transmittance (>95%), and a suitable ionization potential for hole extraction (5.3 ± 0.2 eV). Semi-transparent PSCs with efficiencies up to 16.7% are achieved using the CuOx buffer layer. Our work demonstrates a new approach to integrate n-i-p structured PSCs into tandem configurations, as well as enable the development of other devices that need high quality, protective p-type layers.

Journal article

Hoye R, Fakharuddin A, Congreve D, Wang J, Schmidt-Mende Let al., 2020, Light emission from perovskite materials, APL Materials, Vol: 8, Pages: 070401-1-070401-3, ISSN: 2166-532X

Journal article

Napari M, Huq TN, Maity T, Gomersall D, Niang KM, Barthel A, Thompson JE, Kinnunen S, Arstila K, Sajavaara T, Hoye RLZ, Flewitt AJ, MacManusDriscoll JLet al., 2020, Antiferromagnetism and p‐type conductivity of nonstoichiometric nickel oxide thin films, Infomat, Vol: 2, Pages: 769-774, ISSN: 2567-3165

Plasma‐enhanced atomic layer deposition was used to grow non‐stoichiometric nickel oxide thin films with low impurity content, high crystalline quality, and p‐type conductivity. Despite the non‐stoichiometry, the films retained the antiferromagnetic property of NiO.

Journal article

Jagt RA, Huq TN, Börsig KM, Sauven D, Lee LC, MacManus-Driscoll JL, Hoye RLZet al., 2020, Controlling the preferred orientation of layered BiOI solar absorbers, Journal of Materials Chemistry C, Vol: 15 jun 2020, Pages: 10791-10797, ISSN: 2050-7526

Bismuth oxyiodide (BiOI) has gained attention for photovoltaics, photocatalysis and photodetectors owing to its composition of non-toxic elements, tolerance to point defects, and highly-suitable optical properties. But like many other bismuth-based compounds, BiOI is a layered material with anisotropic transport properties, making control over the preferred orientation critical for achieving optimal device performance. In this work, we develop new insights into the growth mechanism of BiOI synthesized by chemical vapor deposition (CVD) and show how the preferred orientation can be controlled. By adjusting the precursor and substrate temperatures to tune whether or not we are in a nucleation- or growth-controlled regime, we reproducibly vary the ratio of the (001) and (110) orientations by over two orders of magnitude. As a result, we achieve highly c-axis oriented films, which leads to less shunting than a/b-axis oriented films, resulting in improved open-circuit voltages from a median value of 0.7 V (a/b-axis oriented) to 0.9 V (c-axis oriented) in BiOI solar cells. More broadly, the described mechanisms can be used to control the preferred orientation in other low-dimensional materials, which will be important for achieving improved performance across a wide variety of devices.

Journal article

Gan J, Hoye R, Ievskaya Y, Vines L, Marin A, MacManus-Driscoll J, Monakhov Eet al., 2020, Elucidating the origin of external quantum efficiency losses in cuprous oxide solar cells through defect analysis, Solar Energy Materials and Solar Cells, Vol: 209, Pages: 1-8, ISSN: 0165-1633

Heterojunction Cu2O solar cells are an important class of earth-abundant photovoltaics that can be synthesized by a variety of techniques, including electrochemical deposition (ECD) and thermal oxidation (TO). The latter gives the most efficient solar cells of up to 8.1 %, but is limited by low external quantum efficiencies (EQE) in the long wavelength region. By contrast, ECD Cu2O gives higher short wavelength EQEs of up to 90 %. We elucidate the cause of this difference by characterizing and comparing ECD and TO films using impedance spectroscopy and fitting with a lumped circuit model to determine the trap density, followed by simulations. The data indicates that TO Cu2O has a higher density of interface defects, located approximately 0.5 eV above the valence band maximum (NV),and lower bulk defect density thus explaining the lower short wavelength EQEs and higher long wavelength EQEs. This work shows that a route to further efficiency increases of TO Cu2O is to reduce the density of interface defect states.

Journal article

Peng Y, Li F, Wang Y, Li Y, Hoye RLZ, Feng L, Xia K, Pecunia Vet al., 2020, Enhanced photoconversion efficiency in cesium-antimony-halide perovskite derivatives by tuning crystallographic dimensionality, Applied Materials Today, Vol: 19, Pages: 1-11, ISSN: 2352-9407

Lead-based perovskites have reached prominence in optoelectronic and photovoltaic research, yet their toxicity has prompted the search for alternative lead-free compounds. All-inorganic antimony-/bismuth-halide perovskite derivatives have been identified as a promising class of materials. Despite attractive bulk optoelectronic properties, their optoelectronic device performance has been lagging behind. Here we examine one of their most promising embodiments, the all-inorganic cesium-antimony-halide system. Through solution-based halide mixing, we achieve its structural conversion from a zero-dimensional to a layered phase at processing temperatures <150 °C, i.e., much lower than those relied upon in the prior literature (≥230 °C) of all-inorganic cesium-antimony halides. In order to evaluate the technological significance of this finding, we integrate our layered films into a sandwich-type device structure, and characterize their external quantum efficiency and photovoltaic behavior. We find that the structural conversion leads to a considerable enhancement in the optoelectronic device performance. Additionally, photocurrent-power characterization and Hall effect measurements reveal that this performance enhancement is brought about by an improvement in charge carrier transport, which can be exploited due to the unoriented nature of our low-temperature-processed layered films. Such performance boost and mechanistic insight constitute an important step in realizing the full potential of these (and related) compounds for their application in lead-free optoelectronic and photovoltaic devices, e.g., for top-cell in tandem photovoltaics, indoor photovoltaics, and photodetectors.

Journal article

Huq T, Lee L, Eyre L, Li W, Jagt R, Kim C, Fearn S, Pecunia V, Deschler F, MacManus-Driscoll J, Hoye Ret al., 2020, Electronic structure and optoelectronic properties of bismuth Ooxyiodide robust against percent-level iodine-, oxygen- and bismuth-related surface defects, Advanced Functional Materials, Vol: 30, Pages: 1-11, ISSN: 1616-301X

In the search for nontoxic alternatives to lead‐halide perovskites, bismuth oxyiodide (BiOI) has emerged as a promising contender. BiOI is air‐stable for over three months, demonstrates promising early‐stage photovoltaic performance and, importantly, is predicted from calculations to tolerate vacancy and antisite defects. Here, whether BiOI tolerates point defects is experimentally investigated. BiOI thin films are annealed at a low temperature of 100 °C under vacuum (25 Pa absolute pressure). There is a relative reduction in the surface atomic fraction of iodine by over 40%, reduction in the surface bismuth fraction by over 5%, and an increase in the surface oxygen fraction by over 45%. Unexpectedly, the Bi 4f7/2 core level position, Fermi level position, and valence band density of states of BiOI are not significantly changed. Further, the charge‐carrier lifetime, photoluminescence intensity, and the performance of the vacuum‐annealed BiOI films in solar cells remain unchanged. The results show BiOI to be electronically and optoelectronically robust to percent‐level changes in surface composition. However, from photoinduced current transient spectroscopy measurements, it is found that the as‐grown BiOI films have deep traps located ≈0.3 and 0.6 eV from the band edge. These traps limit the charge‐carrier lifetimes of BiOI, and future improvements in the performance of BiOI photovoltaics will need to focus on identifying their origin. Nevertheless, these deep traps are three to four orders of magnitude less concentrated than the surface point defects induced through vacuum annealing. The charge‐carrier lifetimes of the BiOI films are also orders of magnitude longer than if these surface defects were recombination active. This work therefore shows BiOI to be robust against processing conditions that lead to percent‐level iodine‐, bismuth‐, and oxygen‐related surface defects. This will simplify and reduce the cost of fabricating BiOI‐based electronic devices

Journal article

Gonzalez Rodriguez R, Costa V, Delport G, Frohna K, Hoye R, Stranks S, Coffer Jet al., 2020, Structural and spectroscopic studies of a nanostructured silicon – perovskite interface, Nanoscale, Vol: 12, Pages: 4498-4505, ISSN: 2040-3364

While extensively investigated in thin film form for energy materials applications, this work investigates the formation of APbBr3 structures (A = CH3NH3+ (MA), Cs+) in silicon and oxidized silicon nanotubes (SiNTs) with varying inner diameter. We carefully control the extent of oxidation of the nanotube host and correlate the relative Si/Si oxide content in a given nanotube host with the photoluminescence quantum efficiency (PLQE) of the perovskite. Complementing these measurements is an evaluation of average PL lifetimes of a given APbBr3 nanostructure, as evaluated by time-resolved confocal photoluminescence measurements. Increasing Si (decreasing oxide) content in the nanotube host results in a sensitive reduction of MAPbBr3 PLQE, with a concomitant decrease in average lifetime (τave). We interpret these observations in terms of decreased defect passivation by a lower concentration of oxide species surrounding the perovskite. In addition, we show that the use of selected nanotube templates leads to more stable perovskite PL in air over time (weeks). Taken in concert, such fundamental observations have implications for interfacial carrier interactions in tandem Si/perovskite photovoltaics.

Journal article

Li F, Wang Y, Xia K, Hoye R, Pecunia Vet al., 2020, Microstructural and photoconversion efficiency enhancement of compact films of lead-free perovskite derivative Rb3Sb2I9, Journal of Materials Chemistry A, Vol: 8, Pages: 4396-4406, ISSN: 2050-7488

While lead-based perovskites have held center stage in photovoltaic and optoelectronic research over the past decade, their toxicity has raised significant concerns, spurring the search for lead-free alternatives with similar potential. While a number of lead-free antimony-/bismuth-based compounds have been proposed, they have typically exhibited limited charge extraction efficiency, which has prompted the widespread adoption of a mesoporous device architecture. With a focus on compact films of Rb3Sb2I9—an emerging lead-free two-dimensional perovskite derivative—this study presents two strategies to enhance their microstructure: one relying on the reduction of the supersaturation level during the annealing step, and the other involving high-temperature annealing in an SbI3 atmosphere. Both strategies lead to a considerable improvement in film morphology and microstructure, with a twofold increase in apparent grain size. Such high-quality compact films deliver a pronounced rise in external quantum efficiency, as well as in short-circuit photocurrent under solar illumination—all this without the aid of a mesoporous architecture for charge extraction. Hall effect and photocurrent-power characterization show that this performance improvement results from an increase in charge carrier mobility and a reduction in the number of recombination centers. The microstructural improvement, photoconversion efficiency boost, and mechanistic insight provide valuable indications on the status and prospects of Rb3Sb2I9 and related derivatives—as relevant to the future exploration of these compounds for lead-free top-cells in tandem photovoltaics, indoor photovoltaics, and other optoelectronic application domains.

Journal article

Li Y, Hoye RLZ, Gao H, Yan L, Zhang X, Zhou Y, MacManus-Driscoll JL, Gan Jet al., 2020, Over 20% efficiency in methylammonium lead Iodide perovskite solar cells with enhanced stability via “in-situ solidification” of the TiO2 compact layer, ACS Applied Materials & Interfaces, Vol: 12, Pages: 7135-7143, ISSN: 1944-8244

In methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs), the device performance is strongly influenced by the TiO2 elec-tron transport layer (ETL). Typically, the ETL needs to simultaneously be thin and pinhole-free in order to have high transmittance and avoid shunting. In this work, we develop an “in-situ solidification” process following spin coating, in which the titanium-based precursor (ti-tanium (diisopropoxide) bis (2,4-pentaneclionate)) is dried under vacuum to rapidly achieve continuous TiO2 layers. We refer to this as gas-phase quenching. This results in thin (60±10 nm), uniform and pinhole-free TiO2 films. The PSCs based on the gas-phase quenched TiO2 exhibits improved power conversion efficiency, with a median value of 18.23% (champion value of 20.43%), compared to 9.03% and 14.09% for the untreated devices. Gas-phase quenching is further shown to be effective in enabling efficient charge transfer at the MAPbI3/TiO2 heterointerface. Furthermore, the stability of the gas-phase quenched devices is enhanced in ambient air as well as under 1-sun illumination. In addition, we achieve 12.1% efficiency in upscaled devices (1.1 cm2 active area).

Journal article

Shin M, Nam S-W, Sadhanala A, Shivanna R, Anaya M, Jiménez-Solano A, Yoon H, Jeon S, Stranks SD, Hoye RLZ, Shin Bet al., 2019, Understanding the Origin of Ultrasharp Sub-bandgap Luminescence from Zero-Dimensional Inorganic Perovskite Cs4PbBr6, ACS Applied Energy Materials, ISSN: 2574-0962

Journal article

Stranks SD, Hoye RLZ, Di D, Friend RH, Deschler Fet al., 2019, The physics of light emission in halide perovskite devices, Advanced Materials, Vol: 31, Pages: 1-11, ISSN: 0935-9648

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light‐emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state‐of‐the‐art metal‐halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.

Journal article

Ahmad S, Sadhanala A, Hoye RLZ, Andrei V, Modarres MH, Zhao B, Rongé J, Friend R, De Volder Met al., 2019, Triple-cation-based perovskite photocathodes with AZO protective layer for hydrogen production applications, ACS Applied Materials & Interfaces, Vol: 11, Pages: 23198-23206, ISSN: 1944-8244

Metal halide perovskites are actively pursued as photoelectrodes to drive solar fuel synthesis. However, currently, these photocathodes suffer from limited stability in water, which hampers their practical application. Here, we report a high-performance solution-processable photocathode composed of cesium formamidinium methylammonium triple-cation lead halide perovskite protected by an Al-doped ZnO (AZO) layer combined with a Field’s metal encapsulation. Careful selection of charge transport layers resulted in an improvement in photocurrent, fill factor, device stability and reproducibility. The dead pixels count reduced from 25 to 6% for the devices with an AZO layer, and in photocathodes with an AZO layer the photocurrent density increased by almost 20% to 14.3 mA cm–2. In addition, we observed a 5-fold increase in the device lifetime for photocathodes with AZO, which reached up to 18 h before complete failure. Finally, the photocathodes are fabricated using low-cost and scalable methods, which have promise to become compatible with standard solution-based processes.

Journal article

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