Organic Photovoltaic Materials: Squarylium and Cyanine-TCNQ Dyes

The photovoltaic properties of Schottky barrier sandwich cells consisting of sublimed and solution-cast thin films of selected squarylium (bis-anilino derivatives of cyclobuta-1,3-diene-2,4-dione) and cyanine-tetracyanoquinodimethanide (TCNQ) dyes have been measured. For hydroxy squarylium (OHSq), maximum power conversion efficiencies (η) were 0.2% for 850-nm light (1 mW/cm
²
); 0.05% for 633-nm light (94 mW/cm
²
); 0.06% for “white” light (21 mW/cm
²
); 0.15% for low intensity (0.14 mW/cm
²
) simulated AM0 light (sunlight under outer space conditions), and 0.02% for high intensity (140 mW/cm
²
) AM0 light. Efficiencies of selected OHSq cells were observed to increase fivefold when the cells were doped with bromine or 1-phenyl-3-p-N, N-diethylaminostyryl-5-p-N,N-diethylaminophenyl-Δ
²
-pyrazoline (DEASP), e.g., 0.05 to 0.23% (Br); 0.004 to 0.021% (DEASP). The efficiency of a solution-cast cell of amorphous 2,2′-dicarbocyanine-TCNQ was 0.02% when 933-nm light (approximately (1 mW/cm
²
) was used. Amorphous solid solutions of 1,1′-diethyl-2,2′-dicarbocyanine-and oxa-2,2′-dicarbocyanine-TCNQ salts were also tested. The effects of various material and device properties on the performance of organic photovoltaic cells are discussed, and it is proposed that organic solar cells having efficiencies of one percent or more can be made by using existing technologies.