NOVELTY - The forming method involves forming a bottom subcell (cell A), forming a heterojunction middle subcell (cell B) having a base (112) and an emitter (111), forming a continuously-graded interlayer composed of indium-gallium-aluminum-arsenide (InGaAlAs) and forming a homojunction top subcell (cell C) having a base (120) and an emitter (119). The graded interlayer is compositionally graded to lattice match the middle subcell at one side and the bottom subcell at an opposing side.    USE - Method of forming a multijunction solar cell such as an inverted metamorphic multijunction (IMM) solar cell for photovoltaic devices. Can also be used for optoelectronic devices such as thermophotovoltaic (TPV) cells, photodetectors and light-emitting diodes (LEDs).    ADVANTAGE - By using a high band gap heterojunction middle subcell, dark saturated current is reduced, providing a larger open circuit voltage. A triple-junction solar cell structure having a high band gap heterojunction middle subcell provides higher open circuit voltage and higher short circuit current i.e., the sunlight or photogenerated photocurrent increases by utilizing a higher band gap emitter heterojunction, since the amount of photons that can be absorbed in the emitter region is relatively low compared to the absorption in the base region. The emitter with high band gap semiconductor material is more efficient to pass the sub-band-gap sunlight to the base region. A high band gap heterojunction middle subcell provides a larger short circuit current since it offers higher average collection probability of photogenerated carriers.    DETAILED DESCRIPTION - The graded interlayer is preferably formed by providing a metal organic chemical vapor deposition (MOCVD) system to independently control the flow of source gases trimethylindium (InMe3), trimethylgallium (GaMe3), arsine (AsH3), and trimethylaluminium (Al2Me6), and selecting a reaction time and temperature and a flow rate for each source gas to form the continuously-graded interlayer disposed on the bottom subcell. The set of compositions of the formula (InxGa1-x)yAl1-yAs defined by specific values of x and y, in which 0 not greater than x not greater than 1 and 0 not greater than y not greater than 1, each composition having about the same bandgap is identified using a computer program.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the solar cell after the deposition of semiconductor layers on the growth substrate.    Emitter of middle subcell (111)    Base of middle subcell (112)    Emitter of top subcell (119)    Base of top subcell (120)    Bottom subcell (cell A)    Heterojunction middle subcell (cell B)    Homojunction top subcell (cell C)