EAGER: Scalable Organic Shortwave Infrared Photodiodes

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Photodetectors responsive to shortwave infrared (SWIR) light are critical to a wide range of spectroscopic systems and optoelectronics that form the foundation for scientific, industrial, medical, and defense applications. SWIR technologies remain largely dependent on inorganic crystals, which require complex manufacturing processes that are cost-prohibitive and not affordable for widespread use. New alternative materials are highly sought after, especially the ones that allow direct deposition to replace complex manufacturing processes. Organic semiconductors allow direct deposition from solutions and enable scalability but so far are limited to the visible and near-infrared spectrum. This project will investigate a promising class of novel SWIR polymeric photodiodes to reveal the fundamental properties necessary for extending the utility of organics into the SWIR spectrum currently dominated by inorganic materials. If successful, the proposed research will offer a transformative capability to realize widely deployable SWIR systems that are not achievable with conventional semiconductors and will potentially revolutionize a wide variety of applications that rely on SWIR sensing. This project provides graduate and undergraduate students opportunities for exchange visits and internships with an industrial partner. The team will showcase research results in outreach activities, which include hands-on workshops for high-school students and teachers from under-served schools and summer research programs for students from community colleges.The project goal is to find the fundamental limits on the spectral range and detectivity of organic bulk heterojunction photodiodes for detecting shortwave infrared light. This project will study structure-processing-property relationships and unveil the essential characteristics for effective photocurrent generation from low-energy charge-transfer (CT) states. Specifically, this study will measure the device photoresponse and dark current to quantify the extent of spectral range and noise increase with bandgap reduction. In Task 1, the objective is to examine if CT exciton dissociation will become efficient with high permittivity materials, because large permittivity presents a highly polarizable environment that will reduce the Coulombic attraction between the electron-hole pair and thus facilitate dissociation. The permittivity of the bulk heterojunction will be adjusted by blending in high-k camphoric molecules and by using fullerene derivatives with ethylene glycol units in solubilizing side-chains. The dissociation efficiency will be obtained from transient photoconductivity measurements in relation to a material?s permittivity. More effective dissociation will enable photocurrent generation from low CT-state bandgaps and extend spectral range. In Task 2, the objective is to determine the minimum CT-state energy that still allows exciton dissociation before recombination dominates, to understand and predict spectral range. The same SWIR polymer will be paired with various fullerene derivatives to systematically lower CT-state bandgaps and thus increase spectral range. As the CT-state bandgap is significantly reduced, more thermal generation-recombination events will likely lead to an increase in dark current noise. The charge recombination lifetime will be measured via impedance spectroscopy to examine the balance between spectral range and noise. The resulting knowledge will provide better understandings of the photogeneration requirements for future development of organic infrared photodetectors.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.