Determining the optical absorption edge in organic semiconductor composites with a bulk heterojunction by the constant photocurrent method

Spectral dependences of photoconductivity in thin layers of polyconjugated polymers (PCDTBT, PTB7) and their composites with a fullerene derivative (PC$_{70}$BM), which are promising for the development of organic solar cells, have been studied. It was found that the photoconductivity in the polymeric composite exceeds that in the polymer in the whole spectral range under study and the edge of the photoconductivity spectrum is shifted to the long-wavelength part of the spectrum. Use of the constant photocurrent method made it possible to obtain spectral dependences of the absorption coefficients and determine the optical gap width of the materials studied.