The iPhone's success has transformed the fortunes of dozens of suppliers, from glass manufacturers to the maker of robots that cut metal cases.

Now, as Apple Inc. prepares to introduce a new smartphone with an OLED screen, a Japanese refiner better known for its chain of gas stations is about to join the list.

Idemitsu Kosan Co. began experimenting with organic light-emitting diodes in the mid-1980s, seeking to reduce its reliance on petroleum after the global oil shock. Now, pick up Google's latest Pixel smartphone or a Galaxy model from Samsung Electronics Co., and chances are the blue pixels on the OLED screens are built with Idemitsu's materials or patents. Wider adoption of the advanced displays, which offer crisper images and use less battery power, is putting the spotlight on Japanese suppliers that up until now had virtually no stake in iPhone sales. Canon Tokki Corp., for example, has a near monopoly on the giant vacuum machines capable of making OLED screens. Dai Nippon Printing Co. and Toppan Printing Co. are leading manufacturers of fine metal meshes needed to imprint OLED pixels.