NOVELTY - Nitrogen-containing heterocyclic metal complexes (I), are new.    USE - (I) are useful in organic solar cell (claimed).    ADVANTAGE - (I) have deep highest occupied molecular orbital level, small band gap, high charge carrier mobility and excellent thermal stability, and improve open circuit voltage, efficiency and lifetime of the organic solar cell.    DETAILED DESCRIPTION - Nitrogen-containing heterocyclic metal complexes of formula (I), are new.    M = Os, Ir or Pt;    Ar1 = at least one N-containing heterocyclic ring (optionally substituted);    Ar2 = arylene or N, O, S, Ge, Si or Se-containing heterocyclic ring (both optionally substituted with G1);    G1 = alkyl, cycloalkyl, alkoxy, aryloxy, alkylthioxy, arylthioxy, alkyl sulfoxide, aryl sulfoxide, alkenyl, silyl, boronyl, alkyl amine, aralkyl amine, aryl amine, heteroaryl amine, monocyclic or bicyclic aryl, N, O, S or Se-containing monocyclic or bicyclic heterocyclyl (all optionally substituted), D, halo, nitrile, NO2, imide, amide or OH;    ring A = 5-6 membered ring (optionally substituted) (where the ring contains M as ring member), preferably substituted (Z)-3-oxoprop-1-en-1-olate moiety of formula (II); and    R1-R3 = alkyl, cycloalkyl, alkoxy, aryloxy, alkylthioxy, arylthioxy, alkyl sulfoxide, aryl sulfoxide, alkenyl, silyl, boronyl, alkyl amine, aralkyl amine, aryl amine, heteroaryl amine, aryl, N, O or S-containing heterocyclyl (all optionally substituted), H, D, halo, nitrile, NO2, imide, amide or OH.    An INDEPENDENT CLAIM is also included for organic solar cell comprising a first electrode (102), a second electrode (105) arranged opposite to the first electrode and at least one an organic layer comprising a photoactive layer (104) arranged between the first and second electrodes, where the organic layer made of (I).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic cross-sectional view of the organic solar cell.    Substrate (101)    First electrode (102)    Hole transport layer (103)    Photoactive layer (104)    Second electrode (105)