NOVELTY - An electron-donating polymer (P1) comprising 1) 1,6-dithia-dicyclopenta(b,g)naphthalene or 1,7-dithia-dicyclopenta(b,i)anthracene repeating units and 2) a thieno(3,4-b)thiophen-4,6-ylene repeating unit, is new.    USE - In photoactive layer, which is used in electronic device; in organic solar cell (claimed); and in other electronic devices such as an organic light emitting diode (OLED), and an organic thin film transistor (OTFT).    ADVANTAGE - The electron-donating polymer is capable of absorbing light with a wide wavelength region and has improved hole mobility. The organic solar cell increases light absorption rate, short circuit current density, and hole mobility; and improves efficiency. The electron-donating polymer increases interaction among polymers and thus decreases a band gap; absorbs solar light in a wider wavelength region; and has improved fill factor (FF). When the electron-donating polymer is applied to an organic solar cell, then the organic solar cell exhibits improved photoelectric conversion efficiency.    DETAILED DESCRIPTION - An electron-donating polymer (P1) comprising at least one 1,6-dithia-dicyclopenta(b,g)naphthalene unit of formula (I) or 1,7-dithia-dicyclopenta(b,i)anthracene repeating unit of formula (II); and at least one thieno(3,4-b)thiophen-4,6-ylene repeating unit of formula (III), is new.    X1-X10=1-20C aliphatic organic group, 1-20C alkoxy, 1-20C ester (all optionally substituted), H, halo, hydroxy, thiol or -SR100;    R100=1-20C aliphatic organic group, 2-30C aromatic organic group or 2-30 heterocycloalkyl (all optionally substituted);    R1-R4=1-20C linear or branched alkyl, 3-20C cycloalkyl, 1-20C alkoxy, 6-30C aryl or 2-30C heteroaryl (all optionally substituted), or H;    X11 and X12=1-20C aliphatic organic group, 1-20C alkoxy, 1-20C ester (all optionally substituted), H, halo, hydroxyl, thiol or -SR103;    R103=1-20C aliphatic organic group, 2-30C aromatic organic group or 2-30 heterocycloalkyl (all optionally substituted).    INDEPENDENT CLAIMS are included for the following:    (1) photoactive layer (L1) comprising the electron-donating polymer (P1);    (2) electronic device comprising the photoactive layer (L1); and    (3) an organic solar cell comprising: a substrate, an anode and a cathode facing each other and on the substrate; and a photoactive layer disposed between the anode and the cathode, and containing an electron donor and an electron acceptor, where the electron donor contains the electron-donating polymer (P1).