Static and Dynamic Postannealing Strategies for Roll-to-Roll Fabrication of DC Magnetron Sputtered Bismuth Telluride Thin Films onto Polymer Webs.

High-throughput roll-to-roll processes are desirable to scale up the manufacture of flexible thermoelectric generators. While vacuum deposition onto a heated dynamic substrate presents a considerable engineering challenge, viable postdeposition in-line annealing processes are considered as an alternative to improve the functional performance of as-deposited films. The effect of infrared and electron-beam irradiations of 1 µm thick bismuth telluride thin films, produced by a vacuum roll-to-roll process for use as thermoelectric materials, was examined. A static vacuum oven and pulsed high-energy electron beam were also studied as control groups. All annealing strategies increased the crystallite size and decreased the Te content. Only the static vacuum oven treatment was shown to significantly improve the film's crystallinity. After 1 h annealing, the power factor improved by 400% (from 2.8 to 14 × 10-4 W/mK2), which, to the knowledge of the authors, is the highest reported thermoelectric performance of postannealed or hot-deposited Bi-Te films. As for in-line annealing, infrared and electron-beam post treatments improved the power factor by 146% (from 2.8 to 6.9 × 10-4 W/mK2) and 64% (from 2.8 to 4.6 × 10-4 W/mK2), respectively.

Selective ozone treatment of PDMS printing stamps for selective Ag metallization: A new approach to improving resolution in patterned flexible/stretchable electronics.

HYPOTHESIS: Selective ozone treatment of Polydimethylsiloxane (PDMS) print-stamps may facilitate local de-wetting of Krytox®1506 oil; the resulting printed pattern can be used as a masking liquid during roll-to-roll vacuum-metallization, exemplified with Ag. This novel method may exploit high-throughput manufacture without chemical etchants or elevated temperatures for thin-film electronics. EXPERIMENTS: The mechanism for selective wetting arose from O3 treatment of PDMS through a shadow-mask to vary surface-energy due to formation of polar silanol (Si-OH) replacing surface methyl groups leading to contact angle reduction from 40°-9° for oil on PDMS. Oiled PDMS was (1) metalized itself and (2) used as a stamp to print onto polyethylene-terephthalate, consisting of oil pick-up/de-wetting/transfer-to-substrate/metallization. FINDINGS: Ag (520-568 nm) thick was deposited outside oiled regions, surpassing ~20 µm resolution of commercial printing. On metalized PDMS, minimum line widths were 2.6 µm (with 10 µm edge-grading from centrifugal oil spreading) or widths of 24 µm (no Ag grading) following spin-coating/roll-coating oil respectively. The progressive effect of thinning oil via five successive stamp-to-substrate impressions, produced line widths of 14 µm (with graded edge of 7.6 µm via spreading from stamp-substrate compression). Developments may reduce reliance on laser engraving/photocuring, and could enhance micro-contact printing through liquid dynamics vs. topographical relief structures.

Towards Wearable and Flexible Sensors and Circuits Integration for Stress Monitoring.

Excessive stress is one of the main causes of mental illness. Long-term exposure of stress could affect one's physiological wellbeing (such as hypertension) and psychological condition (such as depression). Multisensory information such as heart rate variability (HRV) and pH can provide suitable information about mental and physical stress. This paper proposes a novel approach for stress condition monitoring using disposable flexible sensors. By integrating flexible amplifiers with a commercially available flexible polyvinylidene difluoride (PVDF) mechanical deformation sensor and a pH-type chemical sensor, the proposed system can detect arterial pulses from the neck and pH levels from sweat located in the back of the body. The system uses organic thin film transistor (OTFT)-based signal amplification front-end circuits with modifications to accommodate the dynamic signal ranges obtained from the sensors. The OTFTs were manufactured on a low-cost flexible polyethylene naphthalate (PEN) substrate using a coater capable of Roll-to-Roll (R2R) deposition. The proposed system can capture physiological indicators with data interrogated by Near Field Communication (NFC). The device has been successfully tested with healthy subjects, demonstrating its feasibility for real-time stress monitoring.

Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics.

The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction.

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.

Poly(3-hexylthiophene-2,5-diyl) as a Hole Transport Layer for Colloidal Quantum Dot Solar Cells.

Lead sulfide colloidal quantum dot (CQD) solar cells demonstrate extremely high short-circuit currents (Jsc) and are making decent progress in power conversion efficiencies. However, the low fill factors (FF) and open-circuit voltages have to be addressed with urgency to prevent the stalling of efficiency improvements. This paper highlights the importance of improving hole extraction, which received much less attention as compared to the electron-accepting component of the device architecture (e.g., TiO2 or ZnO). Here, we show the use of semiconducting polymer poly(3-hexylthiophene-2,5-diyl) to create efficient CQD devices by improving hole transport, removing interfacial barriers, and minimizing shunt pathways, thus resulting in an overall improvement in device performance stemming from better Jsc and FF.

Transfer printed silver nanowire transparent conductors for PbS-ZnO heterojunction quantum dot solar cells.

Transfer-printed silver nanowire transparent conducting electrodes are demonstrated in lead sulfide-zinc oxide quantum dot solar cells. Advantages of using this transparent conductor technology are increased junction surface energy, solution processing, and the potential cost reduction of low temperature processing. Joule heating, device aging, and film thickness effects are investigated to understand shunt pathways created by nanowires protruding perpendicular to the film. A V(oc) of 0.39 ± 0.07 V, J(sc) of 16.2 ± 0.2 mA/cm(2), and power conversion efficiencies of 2.8 ± 0.4% are presented.

Effect of oxygen, moisture and illumination on the stability and reliability of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) OTFTs during operation and storage.

We report a systemic study of the stability of organic thin film transistors (OTFTs) both in storage and under operation. Apart from a thin polystyrene buffer layer spin-coated onto the gate dielectric, the constituent parts of the OTFTs were all prepared by vacuum evaporation. The OTFTs are based on the semiconducting small molecule dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) deposited onto the surface of a polystyrene-buffered in situ polymerized diacrylate gate insulator. Over a period of 9 months, no degradation of the hole mobility occurred in devices stored either in the dark in dry air or in uncontrolled air and normal laboratory fluorescent lighting conditions. In the latter case, rather than decreasing, the mobility actually increased almost 2-fold to 1.5 cm(2)/(V · s). The devices also showed good stability during repeat on/off cycles in the dark in dry air. Exposure to oxygen and light during the on/off cycles led to a positive shift of the transfer curves due to electron trapping when the DNTT was biased into depletion by the application of positive gate voltage. When operated in accumulation, negative gate voltage under the same conditions, the transfer curves were stable. When voltage cycling in moist air in the dark, the transfer curves shifted to negative voltages, thought to be due to the generation of hole traps either in the semiconductor or its interface with the dielectric layer. When subjected to gate bias stress in dry air in the dark for at least 144 h, the device characteristics remained stable.

Polystyrene templated porous titania wells for quantum dot heterojunction solar cells.

Polystyrene spheres are used to template TiO2 with a single layer of 300 nm wells which are infilled with PbS quantum dots to form a heterojunction solar cell. The porous well device has an efficiency of 5.7% while the simple planar junction is limited to 3.2%. Using a combination of optical absorption and photocurrent transient decay measurement we determined that the performance enhancement comes from a combination of enhanced optical absorption and increased carrier lifetime.

Low temperature phase selective synthesis of Cu(2)ZnSnS(4) quantum dots.

The application of indium-free quaternary chalcogenides, such as Cu(2)ZnSnS(4) (CZTS), in photovoltaics has created tremendous interest in recent years. In this paper we develop a method to synthesize high quality CZTS nanoparticles with thermodynamically stable kesterite and wurtzite phases via a simple, one-pot, low-cost solution method.

The transitional heterojunction behavior of PbS/ZnO colloidal quantum dot solar cells.

The nature of charge separation at the heterojunction interface of solution processed lead sulphide-zinc oxide colloidal quantum dot solar cells is investigated using impedance spectroscopy and external quantum efficiency measurements to examine the effect of varying the zinc oxide doping density. Without doping, the device behaves excitonically with no depletion region in the PbS layer such that only charge carriers generated within a diffusion length of the PbS/ZnO interface have a good probability of being harvested. After the ZnO is photodoped such that the doping density is near or greater than that of the PbS, a significant portion of the depletion region is found to lie within the PbS layer increasing charge extraction (p-n operation).

Vacuum-deposited planar heterojunction polymer solar cells.

The vacuum thermal evaporation of poly(3-hexylthiophene) (P3HT) for application in photovoltaic cells is demonstrated. Structural changes before and after evaporation are determined using GPC, UV-vis absorption spectroscopy, NMR, and FTIR. GPC showed that the polymer molecular weight is reduced during evaporation, leading to a blue-shift of the absorption spectra. FTIR and NMR were used to examine the change in chemical structure: it was found that conjugation remains mostly intact; however, the conjugation length decreases and side chains dissociate from the backbone. Bilayer heterojunction solar cells were fabricated by sequential deposition of P3HT and C60 and the photovoltaic response measured.