Light Trapping in charge transfer states for improved organic photovoltaic performance

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Photovoltaics (PVs) convert sunlight directly into electricity without any production of greenhouse gases. Most commercial PVs are made from silicon, which is expensive to process, heavy to transport, and brittle. This fundamental research project contributes to the development of PV devices that are made from organic materials that are low-cost, light-weight, and mechanically flexible. One issue for all PV devices is that the sunlight must be absorbed and not reflected from the surface for any angle of incidence on the surface. This problem has been addressed for silicon PV devices by roughening the front and back surfaces of the silicon in a specific pattern to cause the light to be absorbed into the silicon instead of being reflected back to space. This research project addresses a similar process to roughen the back surface of the organic PV layer to enhance absorption of light specifically in the near infrared portion of the solar spectrum, which contains a large proportion of the solar energy. Through this surface roughening process, the efficiency of organic PV devices will be increased, making them a better commercial option for clean energy production. The pattern will also make the organic PV absorb light more efficiently at high incidence angles, which is similar to sunlight in the morning and evening. Undergraduate and Ph.D. graduate students will be trained with research skills that are valued in the solar, polymer, and semiconducting industries. Student recruiting and outreach activities are designed to enhance inclusion of underrepresented minorities in research science.There is a critical need to engineer light‐trapping structures into organic photovoltaic (OPV) devices that can greatly increase the charge‐transfer (CT) state absorbance in the near infrared (NIR). The goal of this project is to enhance CT‐state absorbance in OPV devices using lateral light‐trapping structures. The overall objective is to develop a roll‐to‐roll (R2R) compatible optical patterning process to scribe lateral light‐trapping structures into OPV layers that can increase the external quantum efficiency (EQE) of CT‐state absorbance above 20% across a broad wavelength range. The central hypothesis of this project is that 700‐1000 nm 2D lateral patterning of the OPV layer combined with a thick active layer will achieve this goal of 20% EQE in the CT‐states. The rationale that underlies the research is to mimic light‐trapping structures used in inorganic thin‐film PV devices using solution processing methods that make OPV potentially both inexpensive and scalable. The University of California-Davis team brings expertise in OPV device fabrication, optical modeling, and conjugated polymer synthesis to the project. The project is structured into three aims. Aim 1: Create NIR light‐trapping structures using rapid optical processing. The working hypothesis is that deep light‐trapping structures will optimize waveguide modes below the excitonic band gap, leading to enhanced absorbance in the NIR range. Aim 2: Synthesize OPV materials with controlled solubility for optimized patterning. The pattern fidelity is maximized by high molecular weight and low dispersity polymers that are active donors for OPV applications. And Aim 3: Develop, test and model patterned OPV devices with record power conversion efficiency (PCE). The team will use large‐area solution patterning to fabricate patterned OPV devices with the most promising polymeric active materials. This fundamental research will enable a departure from flat OPV layers to focus on light capture in sub‐band gap states. The expected significance extends beyond the individual device efficiency as other researchers will be able to adopt the patterning method and industry will be able to expand its use to large area organic device applications.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.