NOVELTY - A photoactive layer (30) of a solar cell is formed by in-situ forming of trunk parts on a metal conducting layer (20), performing a deposition process to deposit 1st branch parts on the trunk parts, providing a solution containing a conjugate polymer filler (32) and 2nd branch parts, and infiltrating the solution into spaces between trunk parts to have 2nd branch parts deposited on the 1st branch parts of the surfaces of the trunk parts so as to form a tree-like nanostructure array and to have the conjugate polymer filler cover and fill in the tree-like nanostructure array at the same time.    USE - A method of forming a photoactive layer of a solar cell (claimed).    ADVANTAGE - Energy conversion efficiency of the solar cell is improved, and device performance is enhanced.    DETAILED DESCRIPTION - A photoactive layer (30) of a solar cell is formed by in-situ forming of trunk parts on a metal conducting layer (20), performing a deposition process to deposit 1st branch parts on the trunk parts, providing a solution containing a conjugate polymer filler (32) and 2nd branch parts, and infiltrating the solution into spaces between trunk parts to have the 2nd branch parts deposited on the 1st branch parts of the surfaces of the trunk parts so as to form a tree-like nanostructure array and to have the conjugate polymer filler cover and fill in the tree-like nanostructure array at the same time, wherein the tree-like nanostructure array is used as an electron acceptor, the conjugate polymer filler is an electron donor, the trunk part (34B) is used to provide a long straight transport pathway to transport electrons, the large contact area between the branch part (34A) and the conjugate polymer filler provides electron-hole separation, and the tree-like nanostructure array and the conjugate polymer filler function together as the photoactive layer of the solar cell. An INDEPENDENT CLAIM is included for method of forming a solar cell with a tree-like structure.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a solar cell.    Metal conducting layer (20)    Photoactive layer (30)    Polymer filler (32)    Branch part (34A)    Trunk part (34B)