Degradation by ultra-violet light and its mechanism in organic solar cells

• Optimum illumination wavelength is longer than 400 nm for organic solar cell.
• Initial power conversion efficiency is not affected by the absence of UV light.
• Device degradation by illumination is dominated by the presence of UV light.
• UV light causes a change of donor crystal structure near the ITO electrode.
• The change of donor crystal structure increases the resistance at the interface.

We experimentally investigate the optimum illumination wavelength range and the degradation mechanism by the ultra-violet (UV) light for small-molecule organic solar cells (OSCs) using various long-pass filters with different cutoff wavelengths λc. The OSC structure consists of an indium–tin–oxide (ITO, anode)/copper phthalocyanine (CuPc, donor)/fullerene (acceptor)/bathocuproine (buffer)/Ag (cathode). The initial power conversion efficiency ηp is not affected by the use of the long-pass filter with λc < 400 nm. As λc increases above 400 nm, the initial ηp decreases significantly accompanied by a significant decrease in the short-circuit current, since the total amount of light energy absorbed in the OSC is decreased. The degradation of the OSC is investigated by repeating 30 s cycles, each consisting of 3 s of illumination followed by 27 s in the dark. The curve of the current JL vs voltage V under illumination becomes strongly S-shaped at the 100th illumination cycle for λc < 400 nm, showing degradation. In contrast, for λc > 400 nm, no distinct degradation appears. Thus, the origin of the degradation is revealed to be UV light with wavelengths less than 400 nm. The degradation mechanism is attributed to the decrease in carrier transport efficiency at the interface between ITO and CuPc film. Reorganization of the CuPc crystal structure occurs in the near-surface region on ITO due probably to the heat generated by the UV light. This would deteriorate the electrical contact at the interface, resulting in an increase in series resistance there.

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