# What is the meaning of the “colon” notation in chemical formulas?

For example PEDOT:PSS and P3HT:PCBM.

All it means is that there's an intimate physical mixture of the specified components in a solid or liquid phase (no chemical bonding between the components as happens in a copolymer, for example).

In the context of solar cells, LEDs and related devices, they are often used as part of shorthand device architecture notation. Slashes ( / ) are used to separate different layers of the device, while colons ( : ) are used to enunciate all separate components in a single layer. Do note that every so often a slash is used incorrectly in place of a colon. Checking the experimental procedure should clarify whether two consecutively listed materials are in the same or separate layers.

For example, consider the notation ITO/PEDOT:PSS/P3HT:MEH-PPV:PCBM/Ca/Al for an organic polymer solar cell. There are five layers in total, deposited in sequence from left to right:

1. A layer of ITO (usually on top of a glass substrate which is omitted).
2. A layer containing a binary mixture of PEDOT and PSS, usually deposited via spin casting of an aqueous solution with both materials dissolved.
3. A layer containing a ternary mixture of P3HT, MEH-PPV and PCBM, often deposited by spin casting from a solution of all materials in a chlorinated solvent.
4. A layer of pure calcium metal, likely deposited by thermal evaporation.
5. A layer of pure aluminium metal, deposited on top of the calcium layer.

Sometimes the thickness, component proportion and concentration of the spinning solution for each layer is reported inside of parentheses, such as in ITO/PEDOT:PSS (40 nm)/P3HT:PCBM (1:0.8, 150 nm)/Ca (80 nm). Be sure to pay attention to small differences in notation resulting in completely different cell architecture. For example, a solar cell with a ITO/PEDOT:PSS/P3HT:$\ce{C_{60}}$/Al represents a bulk heterojunction solar cell, while ITO/PEDOT:PSS/P3HT/$\ce{C_{60}}$/Al represents a bilayer (or planar heterojunction) solar cell.