Aims: To investigate The effect of concentration on organic solar cell efficiency.
 Study Design: Polymer and natural dye Eriochrome black T were deposited on ITO glass substrate, and then the resulted cells were tested for characteristics.
 Place and Duration of Study: International University of Africa Faculty of Pure and applied science-Department of Physics collaboration with University Medical Science and Technology- Alawia Centre and AlNilain University – Faculty of Science and Technology – Department of Physics, between March 2016 and May 2017.
 Methodology: we have dye of different colour, we select three colours the dye, and then UV-visible spectrometer used for absorption spectra. After that, the relation between absorption and wavelength, absorption coefficient, and energy band gap were found graphically. Finally, cells were designed on ITO glass substrate by using spin coating deposition. Characteristics measured. 
 Results: The relation between the Eriochrome black T concentration of dye for three samples are 6.67, 6.03, and 5.43 g/L, and the corresponding efficiencies for dark blue are 0.091, 0.090 and 0.229 respectively. The efficiencies of yellow Eriochrome Black T are 0.228, 0.193 and 0.181. And the efficiencies of red Eriochrome Black T are 0.246, 0.235 and 0.193. The decreases of efficiency due to the decreases of Eriochrome Black T concentration is related to the direct relation between Eriochrome Black T concentration and solar cell.
 Conclusion: The efficiency of dye-sensitised solar cells can be increased by increasing the Eriochrome Black T dye concentrations at least within the examined range. The Eriochrome Black T dye type also affect efficiency. This includes the transparency of the Eriochrome Black T dye beside the value of the Fermi energy. In addition to the relative positions of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbits (LUMO) to nearby layers.