The objective of this paper is to show the main motive force of technological progress on solar cells by analyzing correlation between R&D expenditures and conversion efficiency improvement. In the analysis, instead of R&D expenditures, expected conversion efficiency improvement, which is defined as product of cost efficiency from the viewpoint of conversion efficiency improvement and actual RED expenditures, is used. Based on results of the analysis, expected and actual conversion efficiency improvement show good correlation. This means R&D expenditures are the main motive force of technological progress on solar cells and they are appropriate variables for modeling technological progress of solar cells.