Homo- and heterodimers of metalloporphyrins with a defined separation were immobilized on a Au electrode using a layer-by-layer technique. The photoelectrochemical measurements of these dimers quantitatively showed that resonance energy transfers between neighboring dyes affect the internal quantum efficiency (IQE). An increase in IQE was induced by unidirectional energy transfers in heterodimers, whereas a decrease in IQE was caused by bidirectional energy transfers in homodimers.