2026-07-25
In 1930, a Berlin physicist named Bruno Lange filed German patent DE 588,247 (with a U.S. counterpart US 2,097,556 granted in 1937) for what he called a "photoelement" — a thin sandwich of iron, selenium, and a translucent metal film that generated an electric current when light hit it. No battery. No external voltage. Just photons in, electrons out.
This was radical. Every photoelectric device before Lange — including Becquerel's 1839 wet cell and the selenium resistors used in Bell's Photophone — either needed an external power source or worked by changing resistance under light. Lange's cell was a true photovoltaic device: it was its own generator. Point it at the sun and a galvanometer needle swung. Point it at a candle and it still twitched.
Lange discovered that when he pressed a semi-transparent layer of gold or platinum onto a slab of crystalline selenium backed by iron, the junction between the metal and the semiconductor formed what we would now call a Schottky barrier — a rectifying interface where photons could kick electrons across an internal electric field. This is the same physics that would later win Walter Schottky his fame and, decades on, define the p-n junction solar cells at Bell Labs in 1954.
The efficiency was terrible — under 1% — but the sensitivity was extraordinary. A Lange cell responded to light levels a human eye could barely detect, and its output was linear across many orders of magnitude of brightness.
Within two years, Weston Electrical Instrument Company in Newark had licensed the design and built the Weston Model 617 exposure meter (1932) — the first practical handheld light meter for photography. Every Leica, Rolleiflex, and Speed Graphic photographer of the 1930s carried one. Ansel Adams's Zone System, published in 1948, is built on the assumption that photographers have a Lange-descended selenium meter in their pocket.
The same cells went into:
Lange's photoelement is the direct ancestor of the photodiode — the tiny semiconductor detector inside every barcode scanner at your grocery store, every optical mouse, every pulse oximeter clipped to a hospital patient's finger, and every fiber-optic receiver on the transatlantic cables that carry this webpage to your screen. Modern silicon and InGaAs photodiodes hit quantum efficiencies above 90% and respond in picoseconds, but the topology — a semiconductor with a metal contact forming a rectifying junction that turns photons into current — is Lange's.
Even more directly: the CdTe and CIGS thin-film solar panels now competing with silicon on utility-scale solar farms are conceptual descendants of Lange's evaporated-layer sandwich. Roll-to-roll production of flexible solar cells uses essentially his manufacturing philosophy — deposit thin films onto a substrate, form a heterojunction, harvest photocurrent — updated with 90 years of materials science.
Lange himself vanishes from the record after WWII. His patent expired quietly. But every time your phone camera decides an exposure, an OCT scanner images your retina, or a datacenter transceiver converts laser pulses back into bits, a self-powered metal-on-semiconductor junction is doing the work he first built on a Berlin lab bench.