We have pioneering research on the thermomechanical reliability of materials in plastic electronics and photovoltaics. Materials systems we have investigated range from organic bulk heterojunction photovoltaics to conducting films in organic electronics.
Mechanical Reliability and Relation to Molecular Structure and Performance of Organic Bulk Heterojunction Photovoltaics
Organic bulk heterojunction (BHJ) photovoltaics (PVs) are one of many promising thin film technologies being developed in order to accommodate our ever-growing energy demands. Organic PVs possess certain advantages over traditional Si based technologies which include relative ease of processing and manufacturing as well as being able to conform to different shapes and surfaces. This would allow for high throughput production and easily portable designs. However, despite great gains in electrical performance, photochemical and thermomechanical reliability in the organic PV layers, interfaces and barrier coatings generally remains a challenge.
We use thin-film testing techniques such as the four-point bend (FPB) method in order to measure the internal cohesion of these organic PVs. Our initial findings have shown that for the most common organic PV system, cohesive failure occurs within the photoactive layer which consists of a semiconducting polymers such as poly-(3-hexylthiophene-2,5-diyl) (P3HT) and charge carrier acceptor molecule such as phenyl-C61-butyric acid methyl ester (PC60BM) as indicated by cohesive failure surface analysis using techniques such as X-ray photoelectron spectroscopy (XPS) and water contact angle measurements.
For many of the PV devices measured, cohesion ranged anywhere between ~0.5 J/m2 to 20 J/m2 depending on processing conditions, polymer batch, and molecular geometry. For example, we show the increase in BHJ layer cohesion as a function of thermal annealing time along with the evolution of the cohesive surface topography (from atomic force microscopy) at 150oC. It is the devices with cohesion on the lower end of this spectrum (<5 J/m2) that are cause for concern, as mechanical failure becomes more likely during manufacturing. As we learn more about this growing field, we come closer to understanding what device parameters we can manipulate to insure greater mechanical reliability.
Interlayer Adhesion and Mechanical Behavior of Inverted Flexible Polymer Solar Cells
Flexible roll-to-roll (R2R) polymer solar cells have great potential for low cost production, although concerns exist regarding their reliability. It is well established that the processing yield and the long-term reliability of multi-layer electronic devices are strongly influenced by the adhesive and cohesive properties of internal bi-materials and thin films, respectively. Thermomechanical stresses developed in the OPV during manufacturing and device operation provide the mechanical driving force for delamination of weak interfaces or decohesion of weak layers. This results in a loss of mechanical integrity and device performance. We use a thin-film adhesion technique that enables us to precisely measure the energy required to separate adjacent layers. The subsequent quantitative analysis allows us to study the impact of various material, processing, structural and environmental variables on adhesion and cohesion enables the design of more reliable polymer solar cells. A few examples are given below:
- The P3HT:PCBM/PEDOT:PSS interface was found to be the weakest in Inverted OPV’s
- The BHJ composition has a great influence on the adhesion between the BHJ and the PEDOT:PSS HTL. Fullerene-rich layers lead to poor adhesion.
- The adhesive P3HT:PCBM/PEDOT:PSS interfacial failure debond path is independent of the solution-based deposition technique. Solution processed PEDOT:PSS has a higher adhesion than thermally evaporated MoO3.
- The effects of environmental factors, such as temperature, moisture and solar irradiation are extensively studied. Annealing time and temperature increases the fracture energy dramatically. Solar irradiation on fully encapsulated solar cells has no damaging but in contrast an enhancing effect on the adhesion properties, due to the heat generated from IR irradiation.
Moisture assisted decohesion of PEDOT:PSS conducting films
The highly conductive polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) is widely used as a hole transport layer or electrode in organic electronic devices because of its good film forming properties and high visible light transmissivity. The cohesion of the PEDOT:PSS layer is significantly influenced by moisture along with temperature and mechanical loads. Due to its hygroscopic nature the PEDOT:PSS absorbs water from the environment, weakening the layer. The water easily diffuses through the layer and breaks the hydrogen bonds formed between PEDOT:PSS grains resulting in an accelerated decohesion rate.