Cell phone cameras are famous for taking grainy, low-resolution pictures. Part of the problem is the lens, which is usually cheaply made and has limited resolution and ability to collect light. But another problem is the light sensor: a silicon chip containing photodetectors. When shrunk to fit in a phone, these chips are limited in the amount of light they can capture.
Now, InVisage, a Menlo Park, CA-based startup, is demonstrating a way to improve the quality of pictures, at least on the sensor side, without adding size, significant complexity, or cost. At the DEMO conference in Palm Springs, CA, today, the company’s executives announced a new technology called QuantumFilm that lets small camera sensors, like those in cell phones, capture more light than ever before. QuantumFilm is simply a layer of quantum dots–tiny crystals that efficiently absorb light and emit either photons or electrons–in a top layer of the sensor. The electrons emitted by QuantumFilm are collected and sorted the chip’s circuitry.
The result is a sensor that collects twice the light of the standard chip, converts it to electricity twice as efficiently, and is just as cheap to make, says Ted Sargent, chief technology officer of InVisage and professor of electrical and computer engineering and the University of Toronto, where the early research for QuantumFilm began. “Silicon image sensors have a really severe problem in that they just throw away photons left right and center,” says Sargent. Quantum dots, he says, provide a “fundamental solution to the problem.”
In today’s digital cameras, a silicon sensor does double duty. It serves as a photodetector that absorbs incoming light and converts it to an electrical signal. But it also acts as the foundation for the electronics that store the signal from the photodetector and route it away from the chip, where it is processed by separate electronics. The problem is that the photodetector part of the silicon often sits below layers of transistors, metal wires, and a color filter. Because of these obstructions, only about half the original light reaches the photodetector.
There are some commercial technologies that try to tackle the problem of the obstructed photodetector. For instance, manufacturers have added microlenses that focus the light into a tiny space. But even with a bigger photodetector area, silicon still isn’t the best light collector: it registers less than half of the photons that hit it.