Determination of the optical constants of bulk heterojunction active layers from standard solar cell measurements

Optical Expediency of Back Electrode Materials for Organic Near-Infrared Photodiodes

“…The optical field distribution of the device stack is determined by the interplay of the layer thicknesses, the optical constants of all layers, and the incident illumination. , Therefore, it is crucial to know the optical constants, n and k , of all layers in the first step. The refractive indices ( n ) of the P3HT:PC 60 BM and PCE10:COTIC-4F blends used in this work are shown in Figure , as published in previous work and the literature. , The refractive index is defined as the ratio of the phase velocity of light in vacuum and the phase velocity of light in the material; the difference between the refractive indices of adjunct layers in the device defines the reflection of the light that passes through the device stack at each interface. , In both blends, n is in a range that is typical for organic semiconducting materials, ranging from about 1.2 to 2.6. ,,, The extinction coefficient k of both blends is shown in Figure j; k is directly proportional to the absorption coefficient α via the relationship , with λ being the wavelength, and describes the exponential decay of the light as it passes through the material. , It is evident that the fullerene-based system absorbs in the narrow range between 300 and 620 nm, with the peak absorption around 500 nm, whereas the ultra-narrow-band gap PCE10:COTIC-4F blend reaches its maximum at 961 nm and extends into the near-IR region to wavelengths of 1100 nm. The optical constants of the electrode materials Au, Ag, and Al show a characteristic dispersion for each of the metals (Figure k,l), which differs significantly from that of graphite. ,, The refractive indices of the metals reveal a strong wavelength dependence and are of larger values, as expected for the highly reflective metals.…”