Thick-Film Low Driving-Force Indoor Light Harvesters

Organic indoor light harvesters require large-area active layers for power-greedy internet of thing (IoT) things due to much weaker light emissions from indoor sources than the standard 1-Sun case. This presents new challenges for the thick-film indoor photovoltaic (IPV) device fabrication to meet the requirements of current leading roll-to-roll (R2R) techniques. Herein, a concept of low driving-force bulk heterojunction (BHJ) into the IPV field is introduced, and a P3TEA:FTTB-PDI4 solar cell is demonstrated with negligible energy offset between donor and acceptor materials for the electron transport that can achieve high efficiencies over 20% in the thick-film region, which is comparable with their optimized thin-film counterpart. In the low driving-force P3TEA:FTTB-PDI4 BHJ film, negative Poole–Frenkel (PF) effect of the electron transport can be observed. Such an effect originates from alternative electron pathways opened up by the donor polymer P3TEA. The negative PF effect results in improved electron extraction in the thick-film BHJs, leading to comparable fill factors (FFs) and efficiencies of thick-film and thin-film devices. From the photothermal deflection spectroscopy measurement, similar but low Urbach energy values of the low driving-force BHJ and the neat acceptor film are observed, indicating reduced defect states for the electron transport in such low driving-force devices.