Stretchable Photodiodes with Polymer-Engineered Semiconductor Nanowires for Wearable Photoplethysmography

Healthcare systems worldwide have been stressed to provide sufficient resources to serve the increasing and aging population in our society. The situation became more challenging at the time of pandemic. Technology advancement, especially the adoption of wearable health monitoring devices, has provided an important supplement to current clinical equipment. Most health monitoring devices are rigid, however, human tissues are soft. Such a difference has prohibited intimate contact between the two and jeopardized wearing comfortableness, which hurdles measurement accuracy especially during longtime usage. Here, we report a soft and stretchable photodiode that can conformally adhere onto the human body without any pressure and measure cardiovascular variables for an extended period with higher reliability than commercial devices. The photodiode used a composite light absorber consisting of an organic bulk heterojunction embedded into an elastic polymer matrix. It is discovered that the elastic polymer matrix not only improves the morphology of the bulk heterojunction for obtaining the desired mechanical properties but also alters its electronic band structure and improves the electrical properties that lead to a reduced dark current and enhanced photovoltage in the stretchable photodiode. The work has demonstrated high fidelity measurements and longtime monitoring of heat rate variability and oxygen saturation, potentially enabling next-generation wearable photoplethysmography devices for point-of-care diagnosis of cardiovascular diseases in a more accessible and affordable way.

The impact of plasmonic electrodes on the photocarrier extraction of inverted organic bulk heterojunction solar cells

Nano-patterning the semiconducting photoactive layer/back electrode interface of organic photovoltaic devices is a widely accepted approach to enhance the power conversion efficiency through the exploitation of numerous photonic and plasmonic effects. Yet, nano-patterning the semiconductor/metal interface leads to intertwined effects that impact the optical as well as the electrical characteristic of solar cells. In this work we aim to disentangle the optical and electrical effects of a nano-structured semiconductor/metal interface on the device performance. For this, we use an inverted bulk heterojunction P3HT:PCBM solar cell structure, where the nano-patterned photoactive layer/back electrode interface is realized by patterning the active layer with sinusoidal grating profiles bearing a periodicity of 300 nm or 400 nm through imprint lithography while varying the photoactive layer thickness (L _PAL ) between 90 and 400 nm. The optical and electrical device characteristics of nano-patterned solar cells are compared to the characteristics of control devices, featuring a planar photoactive layer/back electrode interface. We find that patterned solar cells show for an enhanced photocurrent generation for a L _PAL above 284 nm, which is not observed when using thinner active layer thicknesses. Simulating the optical characteristic of planar and patterned devices through a finite-difference time-domain approach proves for an increased light absorption in presence of a patterned electrode interface, originating from the excitation of propagating surface plasmon and dielectric waveguide modes. Evaluation of the external quantum efficiency characteristic and the voltage dependent charge extraction characteristics of fabricated planar and patterned solar cells reveals, however, that the increased photocurrents of patterned devices do not stem from an optical enhancement but from an improved charge carrier extraction efficiency in the space charge limited extraction regime. Presented findings clearly demonstrate that the improved charge extraction efficiency of patterned solar cells is linked to the periodic surface corrugation of the (back) electrode interface.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00339-023-06492-6.

NiOx nanoparticles obtained from hydrothermally treated NiC2O4 as an electron blocking layer for organic photodetectors

A room-temperature p-type NiO_x film synthesized from a NiC₂O₄ precursor via hydrothermal treatment is employed as an electron blocking layer (EBL) to fabricate organic photodetectors (OPDs). A simple and efficient calcine process at 375 °C in air decomposes the NiC₂O₄ particles into NiO_x, removes organic components and crystal water, and releases CO₂ gas. Our experimental results indicate that this gaseous by-product prevents the agglomeration of NiO_x, which yields smaller nanoparticles (5-10 nm). The formation of an EBL at room temperature improves device performance. After optimization, the performance parameters obtained, including dark current density, responsivity, specific detectivity and response, are 1.13 × 10^-7 A cm^-2, 0.74 A W^-1, 3.86 × 10¹² Jones, and 0.5/8 ms, respectively. Additionally, the dark current is reduced by more than an order of magnitude after the insertion of the NiO_x layer. The proposed simple and easy method for producing an EBL could be beneficial for the commercial low-temperature and large-area preparation of OPDs.

Spray coating of the PCBM electron transport layer significantly improves the efficiency of p-i-n planar perovskite solar cells

The p-i-n structure for perovskite solar cells has recently shown significant advantages in minimal hysteresis effects, and scalable manufacturing potential using low-temperature solution processing. However, the power conversion efficiency (PCE) of the perovskite p-i-n structure remains low mainly due to limitations using a flat electron transport layer (ETL). In this work, we demonstrate a new approach using spray coating to fabricate the [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) ETL. By creating a rough surface, we effectively improve the light trapping properties inside the PCBM ETL. We reveal that the spray coated PCBM can form a cross-linked network, which may facilitate better charge transport and enhance extraction efficiency. By improving the contact between the perovskite film and the PCBM ETL, a reduction in the trap states is observed resulting in a PCE increase from 13% to >17%.

Recent Advances in Morphology Optimization for Organic Photovoltaics

Organic photovoltaics are an important part of a next-generation energy-harvesting technology that uses a practically infinite pollutant-free energy source. They have the advantages of light weight, solution processability, cheap materials, low production cost, and deformability. However, to date, the moderate photovoltaic efficiencies and poor stabilities of organic photovoltaics impede their use as replacements for inorganic photovoltaics. Recent developments in bulk-heterojunction organic photovoltaics mean that they have almost reached the lower efficiency limit for feasible commercialization. In this review article, the recent understanding of the ideal bulk-heterojunction morphology of the photoactive layer for efficient exciton dissociation and charge transport is described, and recent attempts as well as early-stage trials to realize this ideal morphology are discussed systematically from a morphological viewpoint. The various approaches to optimizing morphologies consisting of an interpenetrating bicontinuous network with appropriate domain sizes and mixed regions are categorized, and in each category, the recent trends in the morphology control on the multilength scale are highlighted and discussed in detail. This review article concludes by identifying the remaining challenges for the control of active layer morphologies and by providing perspectives toward real application and commercialization of organic photovoltaics.

MnO2 modified TiN nanotube arrays on Ti mesh for flexible supercapacitors electrode

The vertically grown TiN nanotube arrays maintain the structural integrity during cycling process and dramatically enhance faradic reaction of MnO₂ by offering superhighways for electrons and ions transport.

Highly efficient polymer solar cells by step-by-step optimizing donor molecular packing and acceptor redistribution

The dynamic drying process of the active layer should play a vitally important role in determining the performance of polymer solar cells (PSCs). Donor molecular packing and acceptor redistribution can be optimized by two successive post-treatments on the active layer. The blend films were freshly prepared by spin-coating method and then immediately transferred to a covered glass Petri dish to allow self-assembly of the donor molecules. The films were then treated with methanol or PFN-methanol solution to adjust the acceptor redistribution. In this study, power conversion efficiencies (PCEs) of PSCs with PffBT4T-2OD:PC₇₁BM as the active layer were improved from 6.74% to 8.75% by employing 80 min for self-assembly and 20 s of methanol soaking. The PCE was improved even further to 9.72% by inserting a PFN interfacial layer. The performance improvement was mainly attributed to the optimized PffBT4T-2OD molecular packing during the self-assembly process, ideal vertical phase separation driven by methanol soaking and efficient charge collection by insertion of a PFN interfacial layer. The molecular packing and vertical phase separation were characterized by grazing incidence X-ray diffraction (GIXD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The experimental results solidly supported the effectiveness of the step-by-step optimization strategy.

A Simple Approach to Fabricate an Efficient Inverted Polymer Solar Cell with a Novel Small Molecular Electrolyte as the Cathode Buffer Layer

A novel small-molecule electrolyte, 1,1'-bis(4-hydroxypropyl)-[4,4'-bipyridine]-1,1'-diium bromide (V-OH), containing a mixture of PTB7:PC₇₁BM has been designed and synthesized as a cathode buffer layer for inverted polymer solar cells (iPSCs). The molecular structure of this new compound comprises a viologen skeleton with hydroxyl group terminals. While the viologen unit is responsible for generating a favorable interface dipole, the two terminal hydroxyl groups of V-OH may generate a synergy effect in the magnitude of the interface dipole. Consequently, the devices containing the V-OH interlayer exhibited a power conversion efficiency (PCE) of 9.13% (short circuit current = 17.13 mA/cm², open circuit voltage = 0.75 V, fill factor = 71.1%). The PCE of the devices with V-OH exhibited better long-term stability compared to that of the devices without V-OH. Thus, we found that it is possible to enhance the efficiency of PSCs by a simple approach without the need for complicated methods of device fabrication.

Marginal solvents preferentially improve the molecular order of thin polythiophene films

The impact of the solvent exposure was more pronounced in thin P3HT films, especially at the center of the film. Concomitant with the improved ordering, the charge carrier transport increased in the resulting field-effect transistors.

Surface treatment by binary solvents induces the crystallization of a small molecular donor for enhanced photovoltaic performance

The surface treatment of p-DTS(FBTTh₂)₂:PC₇₁BM films with binary solvents of methanol and 1-chloronaphthalene enhanced the PCE from 2.4% to 6.5%.

Small molecule based N-phenyl carbazole substituted diketopyrrolopyrroles as donors for solution-processed bulk heterojunction organic solar cells

We report two acetylene-bridged small molecules DPP5 and DPP6 with low HOMO–LUMO gaps as donors along with PC₇₁BM as an acceptor for the fabrication of solution-processed bulk heterojunction solar cells.

Microscopic Investigations into the Effect of Surface Treatment of Cathode and Electron Transport Layer on the Performance of Inverted Organic Solar Cells

Surface treatments of various layers in organic solar cells play a vital role in determining device characteristics. In this manuscript, we report on the influence of surface treatment of indium tin oxide (ITO) electrode and electron transport layer (ETL), ZnO, on the photovoltaic performance of inverted organic solar cells (IOSC) and their correlation with the surface chemistry and surface potential as studied using X-ray photoelectron spectroscopy (XPS) and Kelvin probe force microscopy (KPFM), using the device structure glass/ITO/ZnO/P3HT: PCBM/MoO3/(Au or Ag) (P3HT, poly(3-hexylthiophene-2,5-diyl), and PCBM, phenyl-C61-butyric acid methyl ester). Our results show that although ozonization of ITO leads to an improvement in the device power conversion efficiency, the ozonization of a subsequent ZnO layer results in a decreased performance mainly because of a decrease in the fill factor (FF). However, subsequent methanol (CH3OH) treatment of ZnO layer on an ozonized ITO electrode shows substantial improvement with device efficiencies exceeding ∼4% along with superior reproducibility of the devices. Furthermore, a detailed analysis of the surface wettability, chemistry, and surface potential using contact angle measurements, XPS, and KPFM attribute the improvements to the elimination of surface defects and the changes in the surface potential. Finally, impedance analysis suggests that methanol treatment of the ZnO layers leads to the development of a favorable nanophase structure with higher conductivity, which is otherwise indiscernible using microscopic methods.

Uncovering the role of cathode buffer layer in organic solar cells

Organic solar cells (OSCs) as the third generation photovoltaic devices have drawn intense research, for their ability to be easily deposited by low-cost solution coating technologies. However the cathode in conventional OSCs, Ca, can be only deposited by thermal evaporation and is highly unstable in ambient. Therefore various solution processible cathode buffer layers (CBLs) are synthesized as substitute of Ca and show excellent effect in optimizing performance of OSCs. Yet, there is still no universal consensus on the mechanism that how CBL works, which is evidently a critical scientific issue that should be addressed. In this article detailed studies are targeted on the interfacial physics at the interface between active layer and cathode (with and without treatment of a polar CBL) by using ultraviolet photoelectron spectroscopy, capacitance-voltage measurement, and impedance spectroscopy. The experimental data demonstrate that CBL mainly takes effect in three ways: suppressing surface states at the surface of active layer, protecting the active layer from being damaged by thermally evaporated cathode, and changing the energy level alignment by forming dipole moments with active layer and/or cathode. Our findings here provide a comprehensive picture of interfacial physics in devices with and without CBL.

The effect of with/without resonance-mediated interactions on the organic solar cell performance of new 2D π-conjugated polymers

Bis-tolane as an integrated part of the benzodithiophene donor unit for OSCs.

Organic photovoltaic cells with stable top metal electrodes modified with polyethylenimine

Efficient organic photovoltaic cells (OPV) often contain highly reactive low-work-function calcium electron-collecting electrodes. In this work, efficient OPV are demonstrated in which calcium electrodes were avoided by depositing a thin layer of the amine-containing nonconjugated polymer, polyethylenimine (PEIE), between the photoactive organic semiconductor layer and stable metal electrodes such as aluminum, silver, or gold. Devices with structure ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly(3-hexylthiophene):indene-C60-bis-adduct (P3HT:ICBA)/PEIE/Al demonstrated overall photovoltaic device performance comparable to devices containing calcium electron-collecting electrodes, ITO/PEDOT:PSS/P3HT:ICBA/Ca/Al, with open-circuit voltage of 775±6 mV, short-circuit current density of 9.1±0.5 mA cm(-2), fill factor of 0.65±0.01, and power conversion efficiency of 4.6±0.3%, averaged over 5 devices at 1 sun.

Sequential solvent casting for improving the structural ordering and electrical characteristics of polythiophene thin films

We developed a facile post-deposition method for preparing high-performance organic transistors using direct solvent exposure.

Conjugated polymeric zwitterions as efficient interlayers in organic solar cells

Conjugated polymeric zwitterions, when utilized as interlayer materials in bulk heterojunction organic solar cells, lead to significantly enhanced power conversion efficiencies. The electrostatic model of self-aligning dipolar side groups in the vicinity of the metal surface rationalizes the effects of reduced cathode work function, a key factor behind the observed enhanced efficiency.

The effect of methanol treatment on the performance of polymer solar cells

Significant performance enhancement was observed for the bulk-heterojunction polymer solar cell of ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3' -benzodithiazole)]:[6,6]-phenyl C71 butyric acid methyl ester (PCDTBT:PC71BM)/Al when the as-cast active layer was rinsed with methanol before the deposition of the metal electrode. Comparison of independent anode interfacial layers of PEDOT:PSS and MoO3 indicated that the effects of methanol treatment on the improvement of device performance are more pronounced for PEDOT:PSS-based devices. No discernible changes can be observed in film thickness, surface topography and UV-vis absorption profiles of the photoactive layer, indicating the absence of film reconstruction and the improvement of device performance are hence attributed to the modification of the interface between the PEDOT:PSS and the fresh active layer. Further examination of the devices containing a cathode interlayer of poly[(9,9-bis(3'-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) also demonstrates the potential modification of the interface between the PEDOT:PSS and the active layer by methanol in addition to the widely observed PFN functionality.

High-efficiency polymer solar cells enhanced by solvent treatment

A significant enhancement of efficiency in thieno[3,4-b]-thiophene/benzodithiophene:[6,6]-phenyl C71-butyric acid methyl ester (PTB7:PC70 BM) solar cells can be achieved by methanol treatment. The effects of methanol treatment are shown in an improvement of built-in voltage, a decrease in series resistance, an enhanced charge-transport property, an accelerated and enlarged charge extraction, and a reduced charge recombination, which induce a simultaneous enhancement in open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) in the devices.

High-performance triethylsilylethynyl anthradithiophene transistors prepared without solvent vapor annealing: the effects of self-assembly during dip-coating

Solution-processable small-molecule organic semiconductors have recently attracted significant attention for use as the active channel layers in organic field-effect transistors due to their good intrinsic charge carrier mobility and easy processability. Dip-coating is a good method for optimizing the film morphology and molecular ordering of the small-molecular semiconductors because the drying speed can be quantitatively controlled at the air-solution-substrate contact line. Here, we report the preparation of highly crystalline triethylsilylethynyl-anthradithiophene (TES-ADT) crystal arrays that exhibit an excellent field-effect mobility (up to 1.8 cm(2)/(V s)) via an optimized one-step dip-coating process. High-quality TES-ADT crystals were grown without solvent vapor annealing postprocessing steps, which were previously thought to be essential for improving the morphology, crystallinity, and electrical characteristics of TES-ADT thin films. An interesting correlation between the optimal pull-out rate and the self-assembly tendencies of some soluble acene semiconductors was observed, and the origin of the correlation was investigated. Our work demonstrates an alternative simple approach to achieving highly crystalline TES-ADT thin films, and further proposes a prospective method for optimizing the formation of thin films via the molecular self-assembly of soluble acenes.

Strong carrier lifetime enhancement in GaAs nanowires coated with semiconducting polymer

The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surface-state density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.

Post-deposition treatment of an arylated-carbazole conjugated polymer for solar cell fabrication

An arylated-carbazole conjugated polymer with a deep HOMO level has been developed. Solar cells based on blends of PCX3 and PC(71) BM show efficiency of 3.9% with a V(oc) of 0.96 V. The device performance can be improved to 5.1% by using polar solvent treatment, most likely as a result of modified interfacial properties.