NOVELTY - Redox couple composition comprises a first specific sulfide-containing compound, and a second specific disulfide-containing compound.    USE - The composition is used as redox couple useful in photovoltaic cell or solar cell, fuel cell, battery, sensor or display (claimed). It is used in dye-sensitized solar cells (DSSCs) or electrochemical photovoltaic cells (EPCs).    ADVANTAGE - The composition and redox couple have a good solubility, colorless or optically clear at concentrations permitting a good conductivity, a low vapor pressure, a high thermal stability, a small difference between anodic (Epa) and cathodic (Epc) peak potentials (deltaEp), and a high degree of reversibility. They do not corrode other components when used in a device, e.g. solar cells or photovoltaic cells. They are highly electropositive, e.g. in DSSCs or EPCs using n-type semiconductors; and highly electronegative, e.g. in EPCs using p-type semiconductors.    DETAILED DESCRIPTION - Redox couple composition comprises a first compound of formulae (I) and A-S-M+ (III), and a second compound of formulae (II) or A-S-S-A (IV).    R1, R2=H, optionally halogenated 1-30C alkyl, 3-12C cycloalkyl, 1-12C heterocyclyl, 2-8C alkenyl, 2-8C alkynyl, 6-12C aryl, 6-30C aralkyl, 6-30C alkylaryl, 1-12C heteroaryl, 6-30C alkylheteroaryl, 6-30C alkylheterocyclyl, or part of a polymer chain or network;    R3, R4=H, optionally halogenated 1-30C alkyl, 3-12C cycloalkyl, 1-12C heterocyclyl, 2-8C alkenyl, 2-8C alkynyl, 6-12C aryl, 6-30C aralkyl, 6-30C alkylaryl, 1-12C heteroaryl, 6-30C alkylheteroaryl, 6-30C alkylheterocyclyl, F, Cl, Br, I, CF3, CN, SO3H, CnF2n+1, HCnF2n+1-, CF3O-, CnF2n+1O-, HCnF2n+1O-, CF3S-, CnF2n+1S-, HCnF2n+1-, ClCnF2n+1O-, ClCnF2n+1S-, BrCnF2n+1-, BrCnF2n+1O-, BrCnF2n+1S-, ICnF2n+1-, ICnF2n+1O-, ICnF2n+1S-, CH2=CHCnF2n+1-, CH2=CHCnF2n+1O-, CH2=CHCnF2n+1S-, R2OCnF2n+1-, R2OCnF2n+1O-, R2OCnF2n+1S-, CF3CH2-, CF3CH2O-, (CF3)2CH-, (CF3)2CHO-, CHF2-, CHF2O-, CHF2S-, CClF2-, CClF2O-, CClF2S-, CCl2F-, CCl2FO-, CClF2S-, CCl3-, CCl3O-, C6F5-, CF3C6F4-, C6F5O-, CF3C6F4O-, 3,5-(CF3)2C6H2-, C6Cl5-, C6Cl5O-, FSO2CF2-,ClSO2(CF2)n-, -SO3(CF2)n-, -CO2(CF2)n-, FSO2N(-)SO2(CF2)n-, CF3SO2N(-)SO2(CF2)n-, CnF2n+1SO2N(-)SO2(CF2)n-, R2SO2N(-)SO2(CF2)n-, FSO2(CF2)n-, ClSO2(CF2)n-,CnF2n+1SO2N(-)(CF2)n-, R2SO2N(-)(CF2)n-, or part of a polymer chain or network, or absent;    X-=(FSO2)2N-, (CF3SO2)2N-, (C2F5SO2)2N-, (CF3SO2)3C-, CF3SO3, CF3COO-, AsF6-, CH3COO-, (CN)2N-, (CN)3C-, NO3-, 2.3HF, Cl-, B-, I-, PF6-, BF4-, ClO4, or SCN-;    M=H, or inorganic or organic cation;    A=1-12C heteroaryl, 1-12C heterocyclyl, 6-12C aryl, 6-30C aralkyl, or 6-30C alkylaryl;    n=1-48;and    m=2-12;and    p=1-48.    The alkyl, cycloalkyl, heterocyclyl, alkenyl, alkynyl, aryl, aralkyl, alkylaryl, heteroaryl, alkylheteroaryl, and alkylheterocyclyl optionally includes a heteroatom in the form of -O-, =N-, -S-, =P-, =(P=O)-, -SO-, or SO2-. They are optionally substituted with 1-3 substituents of R3, R4, F, Cl, Br, I, OH, 1-6C alkoxy, 1-6C hydroxy alkyl, NO2, CN, CF3, SO3H, CnF2n+1, 1-12C alkyl, 6-12C aryl, CnH2n+1, Ph2P(O)-, Ph2P-, Me2P(O)-, Me2P, Ph2P(S), Me2P(S), Ph3P=N-, Me3P=N-, C6H5CpH2p-, CpH2p+1C6H4-, CpH2p+1C6H4CnH2n-, CH2=CHCpH2p-, CH2=CHC6H5-, CH2=CHC6H4CH2p+1-, or CH2=CHCpH2pC6H4-. INDEPENDENT CLAIMS are included for:    (1) redox couple prepared from oxidation reduction of formulae (I) and (II) or of formulae A-S-M+ and A-S-S-A;    (2) photovoltaic cell comprising an anode, a cathode, and the composition; and    (3) use of the composition and redox couple in a solar cell, a fuel cell, a battery, a sensor or a display.