NOVELTY - The device has a recombination region i.e. graded recombination layer, forming stepwise gradient in work function from a transparent and conductive hole-accepting layer in ohmic contact with a light-absorbing semiconductor layer of a top-stacked photovoltaic junction to a transparent and conductive electron-accepting layer in ohmic contact with an electron-accepting semiconductor layer of bottom-stacked photovoltaic junction. The recombination region contains thickness within an order of magnitude of sum of debye lengths of the layers of the recombination region.    USE - Multi-junction energy-converting device e.g. colloidal-quantum-dot-based tandem multi-junction photovoltaic device such as colloidal-quantum-dot-based tandem solar cell, for converting light energy into electrical energy e.g. photo-generated electron and photo-generated hole. Can also be used for a single junction tandem colloidal-quantum-dot-based photovoltaic device.    ADVANTAGE - The recombination region forms the stepwise gradient in the work function from the transparent and conductive hole-accepting layer in ohmic contact with the light-absorbing semiconductor layer of the top-stacked photovoltaic junction to the transparent and conductive electron-accepting layer in ohmic contact with the electron-accepting semiconductor layer of the bottom-stacked photovoltaic junction, so that the gradient in the work function produces increase in power conversion efficiency of multi-junction photovoltaics by providing a low-energy-pathway for recombination of electron and hole current from the photovoltaic junctions. The light-absorbing semiconductor layer of the top-stacked photovoltaic junction can be tuned to absorb light of wavelengths so as to result in optimal power-conversion efficiency.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a scanning electron microscope image of a colloidal-quantum-dot-based tandem multi-junction photovoltaic with a graded recombination layer and an electronic energy band-diagram of the colloidal-quantum-dot-based tandem multi-junction photovoltaic.