RESUMO
Controlled delamination of thin-film photovoltaics (PV) post-growth can reveal interfaces that are critical to device performance yet are poorly understood because of their inaccessibility within the device stack. In this work, we demonstrate a technique to lift off thin-film solar cells from their glass substrates in a clean, reproducible manner by first laminating a polymeric backsheet to the device and then thermally shocking the system at low temperatures ( T ≤ -30 °C). To enable clean delamination of diverse thin-film architectures, a theoretical framework is developed and key process control parameters are identified. Focusing on cadmium telluride (CdTe) devices, we show that the lamination temperature and device architecture control the quality of lift-off, while the rate at which the film stack is removed is controlled by the delamination temperature. Crack-free CdTe devices are removed and successfully recontacted, recovering up to 80% of the original device efficiency. The areal density of these devices is â¼0.4 kg m-2, a reduction of over an order of magnitude relative to their initial weight on glass. The framework developed here provides a pathway toward both the development of inexpensive, flexible PV with high specific power and the study of previously buried interfaces in thin-film architectures.
RESUMO
The development of state-of-the-art barrier films and encapsulation schema for displays and photovoltaics requires precise measurement of water vapor permeation as quickly as possible. We have demonstrated improvements to our electrical, Ca-trace-based water vapor transmission rate measurement technique without introducing any additional cost or sample handling concerns. Most importantly, the contacting scheme was changed so that the effective length of the sensor traces can be more precisely determined making the contact resistance between the Ca and Au/Ti films far less likely to affect the results. A 4-pt contacting pattern was also applied to the internal (non-data) witness trace. This expanded the potential utility of the witness trace from just an indicator for the integrity of the sample assembly, to also being used to compensate for measurement error. Lastly, we increased the relative precision of our resistance measurements by implementing a Ca sensor trace with significantly higher resistance. Principally, these changes produce significant measurement improvements for permeation rates less than 10(-4) g/m(2)/day, by lowering the noise floor, reducing required measurement time, and increasing the reproducibility of this test method.
RESUMO
The development of flexible organic light emitting diode displays and flexible thin film photovoltaic devices is dependent on the use of flexible, low-cost, optically transparent and durable barriers to moisture and∕or oxygen. It is estimated that this will require high barriers with water vapor transmission rates (WVTR) between 10(-4) and 10(-6) g∕m(2)∕day. Thus, there is a need to develop a relatively fast, low cost, and quantitative method to evaluate such low permeation rates. Prior works have demonstrated that Ca films, because they change optically and electrically upon reaction with moisture, can be used as a sensor, enabling one to calculate a WVTR between 10 and 10(-6) g∕m(2)∕day or better. In this work, we analyze the accuracy of an electrical Ca test method. We focus on the effects of the addition of a diffusion spacer and the effects of interactions of edge-seal material with changes to the spacer contacting surface on the overall accuracy. Furthermore, we examine a series of factors that can lead to different errors resulting in qualitative rather than quantitative Ca test behavior. We demonstrate that accurate, relatively high throughput, and reproducible measurements are possible for very low WVTR films in the 10(-6) g∕m(2)∕day range.
RESUMO
The development of flexible organic light emitting diode displays and flexible thin film photovoltaic devices is dependent on the use of flexible, low-cost, optically transparent and durable barriers to moisture and/or oxygen. It is estimated that this will require high moisture barriers with water vapor transmission rates (WVTR) between 10(-4) and 10(-6) g/m(2)/day. Thus there is a need to develop a relatively fast, low-cost, and quantitative method to evaluate such low permeation rates. Here, we demonstrate a method where the resistance changes of patterned Ca films, upon reaction with moisture, enable one to calculate a WVTR between 10 and 10(-6) g/m(2)/day or better. Samples are configured with variable aperture size such that the sensitivity and/or measurement time of the experiment can be controlled. The samples are connected to a data acquisition system by means of individual signal cables permitting samples to be tested under a variety of conditions in multiple environmental chambers. An edge card connector is used to connect samples to the measurement wires enabling easy switching of samples in and out of test. This measurement method can be conducted with as little as 1 h of labor time per sample. Furthermore, multiple samples can be measured in parallel, making this an inexpensive and high volume method for measuring high moisture barriers.
RESUMO
Rheological properties of triblock copolymers dissolved in a nematic liquid crystal (LC) solvent demonstrate that their microphase separated structure is heavily influenced by changes in LC order. Nematic gels were created by swelling a well-defined, high molecular weight ABA block copolymer with the small-molecule nematic LC solvent 4-pentyl-4'-cyanobiphenyl (5CB). The "B" midblock is a side-group liquid crystal polymer (SGLCP) designed to be soluble in 5CB and the "A" endblocks are polystyrene, which is LC-phobic and microphase separates to produce a physically cross-linked, thermoreversible, macroscopic polymer network. At sufficiently low polymer concentration a plateau modulus in the nematic phase, characteristic of a gel, abruptly transitions to terminal behavior when the gel is heated into its isotropic phase. In more concentrated gels, endblock aggregates persist into the isotopic phase. Dramatic changes in network structure are observed over small temperature windows (as little as 1 °C) due to tccche rapidly changing LC order near the isotropization point. The discontinuous change in solvent quality produces an abrupt change in viscoelastic properties for three polymers having different pendant mesogenic groups and matched block lengths.
RESUMO
Liquid crystals are often combined with polymers to influence the liquid crystals' orientation and mechanical properties, but at the expense of reorientation speed or uniformity of alignment. We introduce a new method to create self-assembled nematic liquid-crystal gels using an ABA triblock copolymer with a side-group liquid-crystalline midblock and liquid-crystal-phobic endblocks. In contrast to in situ polymerized networks, these physical gels are homogeneous systems with a solubilized polymer network giving them exceptional optical uniformity and well-defined crosslink density. Furthermore, the unusually high-molecular-weight polymers used allow gels to form at lower concentrations than previously accessible. This enables these gels to be aligned by surface anchoring, shear, or magnetic fields. The high content of small-molecule liquid crystal (>/=95%) allows access to a regime of fast reorientation dynamics.
