1.5V to light up LED
You need to boost the voltage somehow if you want to emit visible light via a LED.

The obvious choice is a boost converter.  You can find boost chips that are intended to run from a single primary cell, so can work with your 1.5 V.

If you don't need a lot of current, like just a mA or so to light up a indicator, a charge pump might work.  However, at these low voltages the diode drops will be significant.  You might be able to find a charge pump with synchronous rectification.  Or, get clever and use the diode characteristics of the LED to advantage, with the other diode being a Schottky.

Another option is to not use a LED to make the visible light.  LEBs (light emitting bulbs) intended to work from a single 1.5 V battery were commonly available for "pen lights" and the like.  However, the efficiency will be poor, even compared to relatively bad boost converters and charge pumps driving a LED.