E-ink Screen · Kindle Deconstructed

Kindle 101

A screen that never dies

A Kindle's screen doesn't glow like other popular devices. It uses an electrophoretic (e-ink) display. Millions of microcapsules sit between two electrodes, and each capsule holds positively and negatively charged black and white pigment particles in a clear fluid. When a voltage is applied across a capsule, one colour of particle is pulled toward the top of the capsule, so that spot shows black or white.

Power is only needed to switch a capsule. Once the particles have moved they stay put with no power at all, which is why a page of text costs almost nothing to display and a Kindle battery lasts for weeks.

Diagram of three e-ink capsules between a transparent top electrode and a bottom electrode; the black and white particles move depending on electrode polarity — **Figure 1.** How a capsule switches: the polarity of the bottom electrode decides whether black or white pigment rises to the transparent top electrode. Adapted from [1].

Microscope image of e-ink capsules next to a ruler with 0.1 mm divisions — **Figure 2.** Our microscope image of the capsule layer next to a scale. Each division is 0.1 mm, so each capsule is only a few hundredths of a millimetre across.

The screen

A conductor you can see through

For the circuit in Figure 1 to work, the top electrode has to conduct electricity while letting you see the ink underneath. The Kindle does this with a thin plastic film coated in indium tin oxide (ITO). The coating is invisible, but it adds a conductive layer that closes the circuit across every capsule.

Translucent ITO-coated film peeled from the Kindle display — **Figure 3.** The ITO-coated top electrode film, removed from our Kindle.

Is there really indium in it? (XRF)

The usual ITO recipe is about 90% indium oxide and 10% tin oxide by weight, which works out to roughly 74% indium, 8% tin and 18% oxygen [7]. We scanned the coated film twice with the X-ray fluorescence (XRF) analyser, expecting indium to stand out.

**Table 1.** XRF results from two scans of the ITO-coated film, normalised to 100%. Oxygen can't be detected by XRF, and indium did not appear in either scan.
Element	Scan 1 (%)	Scan 2 (%)
Si (silicon)	9.1	65.4
Sc (reported); likely Ca	51.4	13.0
P (phosphorus)	21.2	6.0
S (sulfur)	4.6	11.8
Al (aluminium)	13.4	3.5
Mn (manganese)	0.2	0.3
In (indium)	—	—

No indium appeared. The two scans are completely different, and they seem like the material underneath the coating, not the coating itself. The XRF signal goes far deeper than a coating only a few nanometres thick. The "scandium" is almost certainly calcium, a false reading due to the two elements' X-ray peaks overlapping. As a check, we scanned a piece of confirmed ITO coated glass from the materials science lab, and it didn't show indium either. Our conclusion is that the XRF can't detect a coating this thin, but it doesn't mean the indium is missing.

Is it conductive? (two-point probe)

To check that the coating conducts, we placed the film on a two-point probe connected to a power supply. Current flowed through the film, which confirms there is a conductive layer on the plastic even though we can't see it.

Two probe needles resting on the clear film, connected to a power supply by red leads — **Figure 4.** Testing the conductivity of the top film with the two-point probe.

Structure and bonding: why ITO?

ITO’s base is indium oxide (In₂O₃), an ionic–covalent compound with a cubic crystal structure. Its usefulness comes from two properties that normally don't go together:

Transparency. In₂O₃ has a wide optical band gap of about 3.5 eV. That is the minimum photon energy needed to push an electron across the gap and absorb the light. Visible-light photons carry only about 1.6–3.1 eV, so they pass straight through [21].
Conductivity. In ITO, some In³⁺ ions in the crystal are replaced by Sn⁴⁺ ions, in a process called tin doping. Each tin ion brings one extra electron that the lattice doesn't need for bonding, and that free electron can carry current. The indium-to-tin ratio is tuned to give enough free electrons to conduct without making the film cloudy [22].

The element indium has its own story. As a pure metal it forms a body-centred tetragonal crystal held together by fairly weak metallic bonds, with a cohesive energy of about 2.52 eV/atom compared with 3.49 eV/atom for copper [23]. Weak bonds explain why indium is so soft and melts so easily.

156.6 °C

Melting point of indium

~11 GPa

Young's modulus (copper ≈ 120 GPa) [24]

Once indium is locked into the oxide ceramic, those metallic properties disappear. It is the oxide's wide band gap and tin's extra electrons that make the screen work.

Sources

Cross-sectional diagram of e-ink microcapsules, ResearchGate. researchgate.net
Indium tin oxide, Wikipedia. en.wikipedia.org/wiki/Indium_tin_oxide
Walsh, A., Da Silva, J. L. F., & Wei, S.-H. (2011). Multi-component transparent conducting oxides: Progress in materials modelling. Journal of Physics: Condensed Matter, 23, 334210. arXiv:1101.5720
González, G. B., Mason, T. O., Quintana, J. P., Warschkow, O., Ellis, D. E., Hwang, J.-H., Hodges, J. P., & Jorgensen, J. D. (2004). Defect structure studies of bulk and nano-indium-tin oxide. Journal of Applied Physics, 96(7), 3912. doi:10.1063/1.1783610
KnowledgeDoor. Cohesive Energy. The Elements Handbook. knowledgedoor.com
Ansys Granta EduPack, record for indium (Young's modulus).