131 results found
Xu W, Du T, Sachs M, et al., 2022, Asymmetric charge carrier transfer and transport in planar lead halide perovskite solar cells, Cell Reports Physical Science, Vol: 3, Pages: 1-17, ISSN: 2666-3864
Understanding charge carrier extraction from the perovskite photoactive layer is critical to optimizing the design of perovskite solar cells. Herein, we demonstrate a simple time-resolved photoluminescence method to characterize the kinetics of charge transport across the bulk perovskite and charge transfer from the perovskite layer to the interlayers, elucidating the dependence of these dynamics on film thickness, grain boundaries (GBs), and interlayers. Using asymmetric laser excitation, we selectively probe charge transport by generating charges both close to and far from the heterojunction interface and correlate these results with device performance. We observe that both film thickness and GBs affect the asymmetry between electron and hole charge transport across the bulk perovskite and charge transfer from the bulk perovskite to the respective interlayers.
Ierides I, Ligorio G, McLachlan MA, et al., 2022, Inverted organic photovoltaics with a solution-processed Mg-doped ZnO electron transport layer annealed at 150 degrees C, Sustainable Energy & Fuels, Vol: 6, Pages: 2835-2845, ISSN: 2398-4902
The use of dopants is an effective strategy to improve ZnO electron transport layers (ETLs) for application in solution-processed opto-electronic devices. Mg, in particular, has shown significant promise as a dopant and Mg-doped ZnO ETLs have been used to enhance the performance of a number of solution-processed light-emitting diodes and photovoltaics. However, such a use of Mg to dope ZnO ETLs for organic photovoltaics (OPVs) has remained limited, and only investigated in connection with annealing temperatures of 300 °C or so. In this work, with a view to increase sustainability and compatibility with soft and flexible or foldable substrates, we present OPVs incorporating Mg-doped ZnO ETLs fabricated with annealing temperatures of 150 °C. We demonstrate that Mg doping (≈1% at%) in the ZnO ETL reduces leakage currents and recombination losses in our devices, whilst leaving the morphology of the active layer and the work function of the ETL unaffected. A concomitant increase of the short circuit current density, open circuit voltage and fill factor is also observed, thereby leading to a relative enhancement of the power conversion efficiency by ≈18% compared to devices prepared using undoped ZnO.
Du T, Macdonald TJ, Yang RX, et al., 2022, Additive-free, low-temperature crystallization of stable alpha-FAPbI(3) perovskite, Advanced Materials, Vol: 34, Pages: 1-10, ISSN: 0935-9648
Formamidinium lead triiodide (FAPbI3) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure α-FAPbI3 conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI3 is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow δ-FAPbI3 into black α-FAPbI3 at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α-FAPbI3 exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable α-FAPbI3. This overcomes the strain-induced lattice expansion that is known to cause the metastability of α-FAPbI3. Accordingly, pure FAPbI3 p–i–n solar cells are reported, facilitated by the low-temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.
Du T, Richheimer F, Frohna K, et al., 2022, Overcoming nanoscale inhomogeneities in thin-film perovskites via exceptional post-annealing grain growth for enhanced photodetection, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 22, Pages: 979-988, ISSN: 1530-6984
Antisolvent-assisted spin coating has been widely used for fabricating metal halide perovskite films with smooth and compact morphology. However, localized nanoscale inhomogeneities exist in these films owing to rapid crystallization, undermining their overall optoelectronic performance. Here, we show that by relaxing the requirement for film smoothness, outstanding film quality can be obtained simply through a post-annealing grain growth process without passivation agents. The morphological changes, driven by a vaporized methylammonium chloride (MACl)–dimethylformamide (DMF) solution, lead to comprehensive defect elimination. Our nanoscale characterization visualizes the local defective clusters in the as-deposited film and their elimination following treatment, which couples with the observation of emissive grain boundaries and excellent inter- and intragrain optoelectronic uniformity in the polycrystalline film. Overcoming these performance-limiting inhomogeneities results in the enhancement of the photoresponse to low-light (<0.1 mW cm–2) illumination by up to 40-fold, yielding high-performance photodiodes with superior low-light detection.
Macdonald T, Clancy A, Xu W, et al., 2021, Phosphorene nanoribbon-augmented optoelectronics for enhanced hole extraction, Journal of the American Chemical Society, Vol: 143, Pages: 21549-21559, ISSN: 0002-7863
Phosphorene nanoribbons (PNRs) have been widely predicted to exhibit a range of superlative functional properties, however since they have only recently been isolated, these properties are yet to be shown to translate to improved performance in any application. PNRs show particular promise for optoelectronics, given their predicted high exciton binding energies, tunable band gaps, and ultrahigh hole mobilities. Here, we verify the theorized enhanced hole mobility in both solar cells and space-charge-limited-current devices, demonstrating the potential for PNRs improving hole extraction in universal optoelectronic applications. Specifically, PNRs are demonstrated to act as an effective charge-selective interlayer by enhancing hole extraction from polycrystalline methylammonium lead iodide (MAPbI3) perovskite to the poly(triarylamine) semiconductor. Introducing PNRs at the hole-transport/ MAPbI3 interface achieves fill factors above 0.83 and efficiencies exceeding 21% for planar p-i-n (inverted) perovskite solar cells (PSCs). Such efficiencies are typically only reported in single-crystalline MAPbI3-based inverted PSCs. Methylammonium-free PSCs also benefit from a PNR interlayer, verifying applicability to architectures incorporating mixed perovskite absorber layers. Device photoluminescence and transient absorption spectroscopy are used to demonstrate that the presence of the PNRs drives more effective carrier extraction. Isolation of the PNRs in space-charge-limited-current hole-only devices improves both hole mobility and conductivity; demonstrating applicability beyond PSCs. This work provides primary experimental evidence that the predicted superlative functional properties of PNRs indeed translate to improved optoelectronic performance.
Gao L, Cheng T, Wang Q, et al., 2021, Color-Stable and High-Efficiency Blue Perovskite Nanocrystal Light-Emitting Diodes via Monovalent Copper Ion Lowering Lead Defects, ACS APPLIED MATERIALS & INTERFACES, Vol: 13, Pages: 55369-55379, ISSN: 1944-8244
Daboczi M, Ratnasingham SR, Mohan L, et al., 2021, Optimal Interfacial Band Bending Achieved by Fine Energy Level Tuning in Mixed-Halide Perovskite Solar Cells, ACS ENERGY LETTERS, Vol: 6, Pages: 3970-3981, ISSN: 2380-8195
Lin C-T, Xu W, Macdonald T, et al., 2021, Correlating active layer structure and composition with device performance and lifetime in amino acid modified perovskite solar cells, ACS Applied Materials and Interfaces, Vol: 13, Pages: 43505-43515, ISSN: 1944-8244
Additive engineering is emerging as a powerful strategy to further enhance the performance of perovskite solarcells (PSCs), with the incorporation of bulky cations and amino acid (AA) derivatives being shown as a promisingstrategy for enhanced device stability. However, the incorporation of such additives typically results inphotocurrent losses owing to their saturated carbon backbones hindering charge transport and collection. Herewe investigate the use of amino acids with varying carbon chain lengths as zwitterionic additives that enhancePSC device stability, in air and nitrogen, under illumination. We discover thatstability is insensitive to chain lengthhowever, as anticipated photocurrent drops as chain length increases. Using glycine as an additive results in animprovement in open circuit voltage from 1.10 to 1.14 V and a resulting power conversion efficiency of 20.2%(20.1% stabilized). Using time-of-flight secondary ion mass spectrometry we confirm that the AAs reside at thesurfaces and interfaces of our perovskite films and propose the mechanisms by which stability is enhanced. Wehighlight this with glycine as an additive, whereby an 8-fold increase in device lifetime in ambient air at 1-sunillumination is recorded. Short circuit photoluminescence quenching of complete devices are reported and revealthat the loss in photocurrent density observed with longer carbon chain AAs results from inefficient chargeextraction from the perovskite absorber layer. These combined results demonstrate new fundamentalunderstandings in the photophysical processes of additive engineering using amino acids and provide asignificant step forward in improving the stability of high-performance PSCs.
Du T, Ratnasingham SR, Kosasih FU, et al., 2021, Aerosol assisted solvent treatment: a universal method for performance and stability enhancements in perovskite solar cells, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832
Metal-halide perovskite solar cells (PSCs) have had a transformative impact on the renewable energy landscape since they were first demonstrated just over a decade ago. Outstanding improvements in performance have been demonstrated through structural, compositional, and morphological control of devices, with commercialization now being a reality. Here the authors present an aerosol assisted solvent treatment as a universal method to obtain performance and stability enhancements in PSCs, demonstrating their methodology as a convenient, scalable, and reproducible post-deposition treatment for PSCs. Their results identify improvements in crystallinity and grain size, accompanied by a narrowing in grain size distribution as the underlying physical changes that drive reductions of electronic and ionic defects. These changes lead to prolonged charge-carrier lifetimes and ultimately increased device efficiencies. The versatility of the process is demonstrated for PSCs with thick (>1 µm) active layers, large-areas (>1 cm2) and a variety of device architectures and active layer compositions. This simple post-deposition process is widely transferable across the field of perovskites, thereby improving the future design principles of these materials to develop large-area, stable, and efficient PSCs.
Mohan L, Ratnasingham SR, Panidi J, et al., 2021, Determining out-of-plane hole mobility in CuSCN via the time-of-flight technique to elucidate its function in perovskite solar cells, ACS Applied Materials and Interfaces, Vol: 13, Pages: 38499-38507, ISSN: 1944-8244
Copper(I) thiocyanate (CuSCN) is a stable, low-cost, solution-processable p-type inorganic semiconductor used in numerous optoelectronic applications. Here, for the first time, we employ the time-of-flight (ToF) technique to measure the out-of-plane hole mobility of CuSCN films, enabled by the deposition of 4 μm-thick films using aerosol-assisted chemical vapor deposition (AACVD). A hole mobility of ∼10–3 cm2/V s was measured with a weak electric field dependence of 0.005 cm/V1/2. Additionally, by measuring several 1.5 μm CuSCN films, we show that the mobility is independent of thickness. To further validate the suitability of our AACVD-prepared 1.5 μm-thick CuSCN film in device applications, we demonstrate its incorporation as a hole transport layer (HTL) in methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs). Our AACVD films result in devices with measured power conversion efficiencies of 10.4%, which compares favorably with devices prepared using spin-coated CuSCN HTLs (12.6%), despite the AACVD HTLs being an order of magnitude thicker than their spin-coated analogues. Improved reproducibility and decreased hysteresis were observed, owing to a combination of excellent film quality, high charge-carrier mobility, and favorable interface energetics. In addition to providing a fundamental insight into charge-carrier mobility in CuSCN, our work highlights the AACVD methodology as a scalable, versatile tool suitable for film deposition for use in optoelectronic devices.
Wilson RL, Macdonald TJ, Lin C-T, et al., 2021, Chemical vapour deposition (CVD) of nickel oxide using the novel nickel dialkylaminoalkoxide precursor [Ni(dmamp′)2] (dmamp′ = 2-dimethylamino-2-methyl-1-propanolate), RSC Advances: an international journal to further the chemical sciences, Vol: 11, Pages: 22199-22205, ISSN: 2046-2069
Nickel oxide (NiO) has good optical transparency and wide band-gap, and due to the particular alignment of valence and conduction band energies with typical current collector materials has been used in solar cells as an efficient hole transport-electron blocking layer, where it is most commonly deposited via sol–gel or directly deposited as nanoparticles. An attractive alternative approach is via vapour deposition. This paper describes the chemical vapour deposition of p-type nickel oxide (NiO) thin films using the new nickel CVD precursor [Ni(dmamp′)2], which unlike previous examples in literature is synthesised using the readily commercially available dialkylaminoalkoxide ligand dmamp′ (2-dimethylamino-2-methyl-1-propanolate). The use of vapour deposited NiO as a blocking layer in a solar-cell device is presented, including benchmarking of performance and potential routes to improving performance to viable levels.
Ratnasingham SR, Mohan L, Daboczi M, et al., 2021, Novel scalable aerosol-assisted CVD route for perovskite solar cells, Materials Advances, Vol: 2, Pages: 1606-1612
Organo-metal halide perovskite research has progressed rapidly, with photovoltaic (PV) devices achieving over 25% power conversion efficiency (PCE). However, scalable production of these devices is an ongoing challenge. We demonstrate the growth of methylammonium lead triiodide (MAPI) films via a novel two-step aerosol-assisted chemical vapour deposition (AACVD) method leading to the first ever perovskite-based PV devices using active layers deposited by AACVD. This is a scalable deposition process, requiring less complex equipment than conventional CVD. Furthermore, our method utilises methanol (MeOH) as the only solvent, as opposed to harmful solvents typically used in perovskite processing. Structural and optical characterization confirms successful formation of MAPI with no secondary phases and an optical bandgap of ∼1.58 eV. The final film had large grains (order of μm), with thickness ranging from 500–1100 nm. These films were used to fabricate working PV devices resulting in a champion PCE of 5.4%. While films demonstrated high structural and compositonal quality, we identified large film roughness as a limiting factor in device PCE, and elucidate the origin of this via detailed study of the film growth, which reveals a unique multi-step film formation process.
Isakov I, Faber H, Mottram AD, et al., 2020, Quantum Confinement and Thickness-Dependent Electron Transport in Solution-Processed In(2)O(3)Transistors, ADVANCED ELECTRONIC MATERIALS, Vol: 6, ISSN: 2199-160X
Du T, Xu W, Xu S, et al., 2020, Light-intensity and thickness dependent efficiency of planar perovskite solar cells: charge recombination versus extraction, Journal of Materials Chemistry C, Vol: 8, Pages: 12648-12655, ISSN: 2050-7526
Photoactive layer thickness is a key parameter for optimization of photovoltaic power conversion efficiency (PCE), yet its impact on charge extraction and recombination hasn’t been fully understood in perovskite solar cells (PSCs). Herein we find that in planar PSCs the perovskite thickness yielding maximal PCE is strongly light-intensity dependent. Whilst under 1 sun irradiation the PCE is relatively invariant for perovskite thicknesses between 250 to 750 nm, at lower light intensities (0.1–0.5 sun) the thickest devices yield strongly enhanced PCE, but at higher light intensities (>1 sun) the thinnest devices give optimal PCE. Our results unravel that increased perovskite thickness leads to enhanced light absorption, reduced interfacial recombination at open circuit but greater bimolecular recombination losses at short circuit thus is suitable for devices working under weak illumination, typical of many real-world applications. Reducing perovskite thickness, however, shows the contrast trend and is suitable for PSCs working under concentrated illumination.
Mohan L, Ratnasingham SR, Panidi J, et al., 2020, Low Temperature Scalable Deposition of Copper(I) Thiocyanate Films via Aerosol-Assisted Chemical Vapor Deposition, Crystal Growth & Design, Vol: 20, Pages: 5380-5386, ISSN: 1528-7483
Copper(I) thiocyanate (CuSCN) is a stable, wide bandgap (>3.5 eV), low-cost p-type semiconductor widely used in a variety of optoelectronic applications, including thin film transistors, organic light-emitting diodes, and photovoltaic cells. For CuSCN to have impact in the commercial fabrication of such devices, large-area, low-cost deposition techniques are required. Here, we report a novel technique for deposition of CuSCN that addresses these challenges. Aerosol-assisted chemical vapor deposition (AACVD) is used to deposit highly crystalline CuSCN films at low temperature. AACVD is a commercially viable technique due to its low cost and inherent scalability. In this study, the deposition temperature, CuSCN concentration and carrier gas flow rate were studied and optimized, resulting in homogeneous films grown over areas approaching 30 cm2. At the optimized values, i.e., 60 °C using a 35 mg/mL solution and a carrier gas flow rate of 0.5 dm3/min, the film growth rate is around 100 nm/min. We present a thorough analysis of the film growth parameters and the subsequent morphology, composition, and structural and optical properties of the deposited thin films.
Tang W, Zhang J, Ratnasingham S, et al., 2020, Substitutional doping of hybrid organic-inorganic perovskite crystals for thermoelectrics, Journal of Materials Chemistry A, Vol: 8, Pages: 13594-13599, ISSN: 2050-7488
Hybrid organic–inorganic perovskites have generated considerable research interest in the field of optoelectronic devices. However, there have been significantly fewer reports of their thermoelectric properties despite some promising early results. In this article, we investigate the thermoelectric properties of bismuth-doped CH3NH3PbBr3 (MAPbBr3) single crystals. The high-quality Bi-doped crystals were synthesized by inverse temperature crystallization and it was found that Bi substitutes onto the B-site of the ABX3 perovskite lattice of MAPbBr3 crystals with very little distortion of the crystal structure. Bi doping does not significantly alter the thermal conductivity but dramatically enhances the electrical conductivity of MAPbBr3, increasing the charge carrier density by more than three orders of magnitude. We obtained a negative Seebeck coefficient of −378 μV K−1 for 15% (x = 0.15) Bi-doped MAPb(1−x)BixBr3 confirming n-type doping and also measured the figure of merit, ZT. This work highlights routes towards controlled substitutional doping of halide perovskites to optimise them for thermoelectric applications.
Daboczi M, Kim J, Lee J, et al., 2020, Towards efficient integrated perovskite/organic bulk heterojunction solar cells: interfacial energetic requirement to reduce charge carrier recombination losses, Advanced Functional Materials, Vol: 30, Pages: 1-8, ISSN: 1616-301X
Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.
Zhang J, Yang H, Zhang X, et al., 2020, Effect of processing temperature on film properties of ZnO prepared by the aqueous method and related organic photovoltaics and LEDs, Inorganic Chemistry Frontiers, Vol: 7, Pages: 2809-2817, ISSN: 2052-1553
Here the influence of processing temperature on the properties of zinc oxide (ZnO) thin films fabricated using a carbon-free solution process is investigated. Our results show that the film processing temperature influences a wide range of structural and electro-optical properties. Films processed at 100 °C are shown to be formed of coalesced ZnO nanoparticles, whose dimensions increase with the processing temperature, accompanied by an increase in electron mobility. ZnO films processed at different temperatures were incorporated as electron transport layers (ETLs) in organic photovoltaic devices with PCDTBT:PC71BM as the active layer. We find that the ETLs processed at low temperature (100–200 °C) exhibit good device performance compared with those prepared at elevated temperatures, an effect we attribute to shifts in the work function and electrical conductivity. Interestingly a similar trend is observed when our ZnO is used as an electron injection layer in organic light emitting diodes, where the EILs processed at >200 °C show higher turn-on voltages and lower efficiencies than those annealed in the 100–200 °C range.
Lin C-T, Ngiam J, Xu S, et al., 2020, Enhancing the operational stability of unencapsulated perovskite solar cells through Cu-Ag bilayer electrode incorporation, Journal of Materials Chemistry A, Vol: 8, Pages: 8684-8691, ISSN: 2050-7488
We identify a facile strategy that significantly reduces electrode corrosion and device degradation in unencapsulated perovskite solar cells (PSCs) operating in ambient air. By employing Cu-Ag bilayer top electrodes PSCs, we show enhanced operational lifetime compared with devices prepared from single metal (Al, Ag and Cu) analogues. Time-of-flight secondary ion mass spectrometry depth profiles indicate that the insertion of the thin layer of Cu (10nm) below the Ag (100nm) electrode significantly reduces diffusion of species originating in the perovskite active layer into the electron transport layer and electrode. X-ray diffraction (XRD) analysis reveals the mutually beneficial relationship between the bilayer metals, whereby the thermally evaporated Ag inhibits Cu oxidation and the Cu prevents interfacial reactions between the perovskite and Ag. The results here not only demonstrate a simple approach to prevent the electrode and device degradation that enhance lifetime and stability but also give an insight into ageing related ion migration and structural reorganisation.
Ambroz F, Sathasivam S, Lee R, et al., 2020, Corrigendum: Influence of lithium and lanthanum treatment on TiO2 nanofibers and their application in n-i-p solar cells, ChemElectroChem, Vol: 7, Pages: 2173-2173, ISSN: 2196-0216
Wang S, Li W, Morbidoni M, et al., 2020, Building on soft hybrid perovskites: highly oriented metal oxides as electron transport and moisture resistant layers, APPLIED NANOSCIENCE, ISSN: 2190-5509
Lin C-T, Lee J, Kim J, et al., 2020, Origin of open-circuit voltage enhancements in planar Perovskite solar cells induced by addition of bulky organic cations, Advanced Functional Materials, Vol: 30, ISSN: 1616-301X
The origin of performance enhancements in p‐i‐n perovskite solar cells (PSCs) when incorporating low concentrations of the bulky cation 1‐naphthylmethylamine (NMA) are discussed. A 0.25 vol % addition of NMA increases the open circuit voltage (Voc) of methylammonium lead iodide (MAPbI3) PSCs from 1.06 to 1.16 V and their power conversion efficiency (PCE) from 18.7% to 20.1%. X‐ray photoelectron spectroscopy and low energy ion scattering data show NMA is located at grain surfaces, not the bulk. Scanning electron microscopy shows combining NMA addition with solvent assisted annealing creates large grains that span the active layer. Steady state and transient photoluminescence data show NMA suppresses non‐radiative recombination resulting from charge trapping, consistent with passivation of grain surfaces. Increasing the NMA concentration reduces device short‐circuit current density and PCE, also suppressing photoluminescence quenching at charge transport layers. Both Voc and PCE enhancements are observed when bulky cations (phenyl(ethyl/methyl)ammonium) are incorporated, but not smaller cations (Cs/MA)—indicating size is a key parameter. Finally, it demonstrates that NMA also enhances mixed iodide/bromide wide bandgap PSCs (Voc of 1.22 V with a 1.68 eV bandgap). The results demonstrate a facile approach to maximizing Voc and provide insights into morphological control and charge carrier dynamics induced by bulky cations in PSCs.
Georgiadou DG, Lin Y, Lim J, et al., 2020, High Responsivity and Response Speed Single‐Layer Mixed‐Cation Lead Mixed‐Halide Perovskite Photodetectors Based on Nanogap Electrodes Manufactured on Large‐Area Rigid and Flexible Substrates, Advanced Functional Materials, Vol: 30, Pages: 1909758-1909758, ISSN: 1616-301X
Liu T, Yue S-Y, Ratnasingham S, et al., 2019, Unusual Thermal Boundary Resistance in Halide Perovskites: A Way To Tune Ultralow Thermal Conductivity for Thermoelectrics, ACS APPLIED MATERIALS & INTERFACES, Vol: 11, Pages: 47507-47515, ISSN: 1944-8244
Ambroz F, Xu W, Gadipelli S, et al., 2019, Room Temperature Synthesis of Phosphine-Capped Lead Bromide Perovskite Nanocrystals without Coordinating Solvents, PARTICLE & PARTICLE SYSTEMS CHARACTERIZATION, Vol: 37, ISSN: 0934-0866
Daboczi M, Hamilton I, Xu S, et al., 2019, The origin of open-circuit voltage losses in perovskite solar cells investigated by surface photovoltage measurement, ACS Applied Materials & Interfaces, Vol: 11, Pages: 46808-46817, ISSN: 1944-8244
Increasing the open circuit voltage (Voc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction and their loss mechanisms inside a device upon illumination. Herein we report the important origin of Voc losses in methylammonium lead iodide perovskite (MAPI) based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the PEDOT:PSS hole transport layer. We show strong correlation between the thickness-dependent surface photovoltage and device performance, unraveling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in Voc for the PEDOT:PSS/MAPI inverted devices (180 mV reduction in 50-nm-thick device compared to 230-nm-thick one). In contrast, accumulated positive charges at the TiO2/MAPI interface modify interfacial energy band bending, which leads to an increase in Voc for the TiO2/MAPI conventional devices (70 mV increase in 50-nm-thick device compared to 230-nm-thick one). Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.
Macdonald TJ, Batmunkh M, Lin C-T, et al., 2019, Origin of performance enhancement in TiO2-carbon nanotube composite perovskite solar cells, Small Methods, Vol: 3, Pages: 1-10, ISSN: 2366-9608
Carbon nanotubes are shown to be beneficial additives to perovskite solar cells, and the inclusion of such nanomaterials will continue to play a crucial role in the push toward developing efficient and stable device architectures. Herein, titanium dioxide/carbon nanotube composite perovskite solar cells are fabricated, and device performance parameters are correlated with spectroscopic signatures of the materials to understand the origin of performance enhancement. By probing the charge carrier dynamics with photoluminescence and femtosecond transient absorption spectroscopy, the results indicate that charge transfer is not improved by the presence of the carbon nanotubes. Instead, carbon nanotubes are shown to passivate the electronic defect states within the titanium dioxide, which can lead to stronger radiative recombination in the titanium dioxide/carbon nanotube films. The defect passivation allows the perovskite solar cells made using an optimized titanium dioxide/carbon nanotube composite to achieve a peak power conversion efficiency of 20.4% (19% stabilized), which is one of the highest values reported for perovskite solar cells not incorporating a mixed cation light absorbing layer. The results discuss new fundamental understandings for the role of carbon nanomaterials in perovskite solar cells and present a significant step forward in advancing the field of high‐performance photovoltaics.
Panidi J, Kainth J, Paterson AF, et al., 2019, Introducing a nonvolatile N-type dopant drastically improves electron transport in polymer and small-molecule organic transistors, Advanced Functional Materials, Vol: 29, Pages: 1-10, ISSN: 1616-301X
KGaA, Weinheim Molecular doping is a powerful yet challenging technique for enhancing charge transport in organic semiconductors (OSCs). While there is a wealth of research on p-type dopants, work on their n-type counterparts is comparatively limited. Here, reported is the previously unexplored n-dopant (12a,18a)-5,6,12,12a,13,18,18a,19-octahydro-5,6-dimethyl- 13,18[1′,2′]-benzenobisbenzimidazo [1,2-b:2′,1′-d]benzo[i][2.5]benzodiazo-cine potassium triflate adduct (DMBI-BDZC) and its application in organic thin-film transistors (OTFTs). Two different high electron mobility OSCs, namely, the polymer poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8- bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2′-bithiophene)] and a small-molecule naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) are used to study the effectiveness of DMBI-BDZC as a n-dopant. N-doping of both semiconductors results in OTFTs with improved electron mobility (up to 1.1 cm2 V−1 s−1), reduced threshold voltage and lower contact resistance. The impact of DMBI-BDZC incorporation is particularly evident in the temperature dependence of the electron transport, where a significant reduction in the activation energy due to trap deactivation is observed. Electron paramagnetic resonance measurements support the n-doping activity of DMBI-BDZC in both semiconductors. This finding is corroborated by density functional theory calculations, which highlights ground-state electron transfer as the main doping mechanism. The work highlights DMBI-BDZC as a promising n-type molecular dopant for OSCs and its application in OTFTs, solar cells, photodetectors, and thermoelectrics.
Cha H, Fish G, Luke J, et al., 2019, Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity, ADVANCED ENERGY MATERIALS, Vol: 9, ISSN: 1614-6832
Du T, Xu W, Daboczi M, et al., 2019, p-Doping of organic hole transport layers in p–i–n perovskite solar cells: correlating open-circuit voltage and photoluminescence quenching, Journal of Materials Chemistry A, Vol: 7, Pages: 18971-18979, ISSN: 2050-7488
Doping is a widely implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic (PV) devices. Here, in direct contrast to existing understanding, we find that a reduction in p-doping of the organic hole transport layer (HTL) leads to substantial improvements in PV performance in planar p–i–n perovskite solar cells (PSCs), driven by improvements in open circuit voltage (VOC). Employing a range of transient and steady state characterisation tools, we find that the improvements of VOC correlate with reduced surface recombination losses in less p-doped HTLs. A simple device model including screening of bulk electric fields in the perovskite layer is used to explain this observation. In particular, photoluminescence (PL) emission of complete solar cells shows that efficient performance is correlated to a high PL intensity at open circuit and a low PL intensity at short circuit. We conclude that desirable transport layers for p–i–n PSCs should be charge selective contacts with low doping densities.
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