NOVELTY - A monolithic, multi-bandgap photovoltaic converter comprises: (a) a first sub-cell with a first bandgap and a first lattice constant; (b) a second sub-cell comprising GaInAs(P) with a second bandgap less than the first bandgap and a second lattice constant greater than the first lattice constant; and (c) a lattice constant transition material positioned between the first sub-cell and the second sub-cell.    USE - Converting solar and/or thermal energy to electricity.    ADVANTAGE - Provides a monolithic, multi-bandgap photovoltaic converter for absorbing and converting infrared (IR) radiation of multiple wavelengths to electricity.    DETAILED DESCRIPTION - A monolithic, multi-bandgap photovoltaic converter comprises:    (a) a first sub-cell comprising gallium indium arsenide (phosphide) GaInAs(P) with a first bandgap and a first lattice constant;    (b) a second sub-cell comprising GaInAs(P) with a second bandgap and a second lattice constant; and    (c) a lattice constant transition material positioned between the first sub-cell and the second sub-cell,    The second bandgap is less than the first bandgap and the second lattice constant is greater than the first lattice constant. The second lattice constant is equal to a lattice constant of a indium arsenide phosphide (InAsyP1-y) alloy with a bandgap greater than the first bandgap. The lattice constant transition material comprises InAsyP1-y alloy with a lattice constant that changes gradually from the first lattice constant to the second lattice constant.    INDEPENDENT CLAIMS are also included for:    (1) a monolithic, integrated module (MIM) comprising monolithic, multi-bandgap photovoltaic converters, and a common substrate with a substrate bandgap and a substrate lattice constant, where the common substrate is positioned between the first sub-cell and the lattice constant transition material of each converter, the substrate bandgap being at least as large as the first bandgap and the substrate lattice constant being equal to the first lattice constant; and    (2) a method of fabricating a monolithic, multibandgap photovoltaic converter by growing a first sub-cell comprising GaInAs(P) epitaxially on an indium phosphide (InP) substrate in a GaInAs(P) formulation that provides a first bandgap that is less than 1.35 eV and a first lattice constant which equals in InP lattice constant; growing a lattice constant transition layer on the first sub-cell; and growing a second sub-cell comprising GaInAs(P) epitaxially on the lattice constant transition layer in a GaInAs(P) formulation that has a lattice constant equal to the second lattice constant and a second bandgap that is less than the first bandgap.    DESCRIPTION OF DRAWING(S) - The figure shows a monofacial, inverted, multigap, monolithic photovoltaic device with two lattice-matched, double heterostructure sub-cells grown on an indium phosphide substrate and series connected, where the second sub-cell having a bandgap less than the bandgap of the first sub-cell and less than 0.74 eV is lattice-mismatched to the substrate but is grown on a transparent, lattice constant transition layer positioned between the two sub-cells to accommodate the lattice mismatch.