Stretchable electronics are engineered with soft mechanical properties to enable conformal operation on complex biological surfaces. A typical skin-mounted device comprises thin semiconductor films, which act as the functional element, embedded into soft elastomeric layers. This layout enhances the overall stretchability of the device and protects the embedded rigid functional components from mechanical failure during flexing, twisting and bending.
The degree of protection and the device performance, however, are limited by the mechanical properties of the semiconductor layers. Replacing the rigid semiconductor films with skin-like elastomeric semiconductors that match the mechanics of soft tissues can increase the range of deformation. This could prove invaluable for future biomedical technologies that rely on photodetectors to convert light into electrical current.
With this aim, researchers from the Georgia Institute of Technology have developed an elastomeric bulk heterojunction (e-BHJ). The e-BHJ combines polymeric materials to create a photoactive layer with a Young’s modulus comparable to that of skin, and that can be stretched up to 189% before breaking. Writing in Science Advances, they describe the use of this e-BHJ in stretchable organic photodiodes (OPDs) that can detect ultralow light levels, even when stretched up to 60%.
“We have shown that you can build stretchability into semiconductors that retains the electrical performance needed to detect light levels that are around hundred million times fainter than produced by a light bulb used for indoor illumination,” explains corresponding author Canek Fuentes-Hernandez.
Organic materials as semiconductors
To create the e-BHJ semiconductor layer, the researchers combined the elastomer styrene-ethylene-butylene-styrene (SEBS) with a blend of synthetic polymers referred to as r-BHJ, which act as electron acceptor and donor materials and enable the photoactive properties.
Uniaxial tensile tests showed that the hyperelastic e-BHJ has a Young’s modulus between 2.4 and 6.9 MPa, comparable to that of human tissues. Further, the team found that the addition of SEBS to the r-BHJ increased the strain-at-break of the semiconductor layer from 6% to 189%, thereby making the e-BHJ an excellent candidate for photovoltaics applications using stretchable electronics.
The authors evaluated the electrical performance of the e-BHJ film using photodetector measurements of electronic noise in the dark. They also measured the specific detectivity, a common performance metric for photodetectors. The median RMS electronic noise was 72 fA at 0 V, at a measurement bandwidth of 1.5 Hz, which is comparable to industry-standard low-noise silicon photodiodes. The device showed a specific detectivity of 1010 Jones, high enough to detect weak light signals, at 653 nm.
Stretchable photodiodes
Next, the researchers fabricated elastomeric organic photodiodes (e-OPDs) by spin coating the e-BHJ film on top of a prestrained substrate, using a liquid metal alloy as the top electrode. The resulting e-OPD had an initial area of 0.09 cm2 and could be stretched up to 60% without significant degradation in performance. The device exhibited RMS electronic noise of 51 fA and an average specific detectivity of 2.3 x 1010 Jones at 653 nm.
Comparing the photoactive device before and after applying strain, the researchers found that the resistance of the electrode rapidly increased, causing the device to fail. They showed that increasing the prestrain in the substrate during fabrication increased the stretchability of the electrodes, enabling the e-OPDs to undergo strain of up to 100% without substantial degradation.
Moving forward, the researchers envision applications of their technology in bioelectronic systems that interact directly with the body.
“The soft device also could be attractive for implantable electronics for bioelectronic applications since the interfaces comply with the dynamic motion of the soft biological tissues, reducing the foreign body reaction,” explains co-author Kyungjin Kim.
