NOVELTY - Metal complex (I) is new.    USE - (I) is useful as a redox-pair, in a photoelectric conversion device including a dye-sensitized solar cell or photovoltaic cell, and in an electrochemical or optoelectronic device including a photo-electrochemical device, an optoelectronic device, an electrochemical battery (e.g. a lithium ion battery), a double layer capacitor, a light emitting device, an electrochromic or photo-electrochromic device, an electrochemical sensor, a biosensor, an electrochemical display, an electrochemical capacitor (e.g. a super capacitor) (all claimed).    ADVANTAGE - The metal complex (I) increases the oxidation potential of the redox mediator, thus increasing the open circuit voltage of the solar cells, and the oxidation potential of the iodide/triiodide based redox couple does not change, and provides the device with improved power conversion efficiency and stability.    DETAILED DESCRIPTION - Metal complex of formula (M1((L1) a) n((X1) b) m) (I), is new.    M1=first row transition metal, preferably Co;    n=1-6;    a=consecutive number of a first set consisting of the integers of 1-n, so that there are n ligands (L1) 1-(L1) n;    m=0 or 1-5;    b=consecutive number of a second set consisting of 0 and integers of 1-m, so that if m is greater than zero, then there are m ligands (X1) 1-(X1) m, where n and m are appropriate number of ligands present on metal M1;    any (L1) a(i.e. (L1) 1-(L1) n)=mono-, bi- or tri-dentate ligand, where at least one of (L1) a(i.e. (L1) 1-(L1) n) comprises ring or ring system comprising 5-membered N-containing heteroring and/or 6-membered ring comprising at least two heteroatoms (all optionally substituted), at least one of heteroatom is N, and the 5- or 6-membered heteroring, respectively, comprises at least one double bond; and    (X1) b=monodentate co-ligand.    INDEPENDENT CLAIMS are also included for:    (1) an electrochemical or optoelectronic device comprising the complex (I);    (2) an electrochemical device comprising a first and a second electrode, and between the first and second electrode, an electrolyte and/or an intermediate layer, where the electrolyte or the intermediate layer comprises the metal complex (I);    (3) an electrochemical and/or optoelectronic device comprising a first and a second electrode, and between the first and second electrode, a charge transport layer, where the a charge transport layer comprises tetracyanoborate anion and a cationic metal complex of formula (M1((L2) a) n((X1) b) m) (Ia); and    (4) preparing a electrochemical device, comprising providing a first and a second electrode, providing a charge transport layer, and adding to the charge transport layer a salt comprising tetracyanoborate anion and a cationic metal complex (Ia).    (L2) a(i.e. (L2) 1-(L2) n)=mono-, bi-, and tridentate ligands, comprising ring or ring system (both optionally substituted, and optionally comprising at least one N).