Organic Photovoltaics

Organic photovoltaic (OPV) devices use semi-conducting π-conjugated organic materials to convert energy from the sun into electrical energy. In the Reynolds group, we develop solution processable molecules and polymers to be used as light harvesting active layers for OPVs.

                    

Left: The conventional architecture of an organic photovoltaic cell. The active layer typically consists of a conjugated polymer donor and a fullerene acceptor. Right: Donor-Acceptor energy level diagram

One main focus of our OPV investigation is the pairing of donor and acceptor moieties to fine tune the opto-electronic properties of the polymers. Copolymers of different donor and acceptor motifs  allow fine-tuning of energy levels, bandgap, and intermolecular interactions, while the use of solubilizing side chains can affect processability and intermolecular interactions.

Structure-property relationships in PT3-iI (polyterthiophene-alt-isoindigo) solar polymer

Another focus in our group is the polymer PDTG-TPD (polydithienogermole-alt-thienopyrrolodione). First introduced in 2011,1this donor-acceptor polymer has served as a reliable, high-efficiency material with favorable UV-Vis absorption and high solubility in common organic solvents.

Top Left: Longer C-Ge bond length splay the alkyl side chains further away from the conjugated backbone, allowing a larger surface for π-π stacking and improving charge transport. Bottom Left: Stille cross coupling polymerization was utilized to achieve high molecular weight PDTG-TPD. Right: Device and EQE data for P-Si and P-Ge devices in an inverted architecture (ITO/ZnO/Polymer:PC70BM/MoO3/Ag)

Collaborative work with the So group at NC State has produced high efficiency inverted OPV devices. When using UV-ozone treated ZnO-PVP, power conversion efficiencies (PCE) exceeding 8.5% were achieved, with an NREL certified PCE of 7.4%.2 For a short time, this was the highest certified PCE in the OPV field.

We have also demonstrated that PDTG-TPD has an exceptional ability to be processed in air with minimal loss in performance. 3 The Reynolds group puts a strong emphasis on moving towards devices that can be fully fabricated in air via high throughput roll-to-roll processing techniques.

Left: UV-ozone treatment on ZnO-PVP composite shows an improved EQE in OPV devices. Right: devices processed in ambient air achieve 7.7% PCE, compared to 8.5% processed in inert atmosphere.