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Comptes Rendus Physique
Volume 19, n° 3
pages 89-112 (mars 2018)
Doi : 10.1016/j.crhy.2018.03.001
Historical perspective on the physics of artificial lighting
Perspective historique sur la physique de l'éclairage
 

Fig. 1




Fig. 1 : 

Time evolution of lighting: (a) evolution of light source efficiency [5,6]; (b) evolution of yearly light production (Tlm⋅h) (from [3]); (c) evolution of lighting prices (in £ 2000 and lighting consumption per capita, UK (after [3]); (d) evolution of the share for lighting of new lighting energy sources (after [3]).


Fig. 2




Fig. 2 : 

Wallplug efficiency (WPE), luminous efficacy, and eye response curve (full line curve). Note that the luminous efficacy (lm/W) is the product of the WPE and the eye response, hence violet LEDs have a poor luminous efficacy in spite of their excellent WPE.


Fig. 3




Fig. 3 : 

Emission spectra of blackbodies at 1800 K and 1350 K, and of gas-heated mantles with either pure ThO2 (green), Ce2 O3 (blue) or Auer mantle, TiO2 /Ce2 O3 (red) (after [13]).


Fig. 4




Fig. 4 : 

Schematics of the modern explanation of Losev's type-I electroluminescence in case of an n-type semiconductor. (a) and (b): metal semiconductor rectifying contact, so-called Schottky contact (at the time, all metal-semiconductor were rectifying, as no such contact could be Ohmic because of the poor semiconductor doping). (a): no bias voltage applied; (b) small forward voltage applied (semiconductor negatively biased relative to the metal); a few electrons flow from the semiconductor into the metal; (c) strong negative bias applied through the contact, which acts as a blocking contact, regulating the injected current. Due to the high accelerating field in the depletion layer, electrons reach a kinetic energy larger than the bandgap, allowing electron–hole pairs to be generated by impact ionization. These electron–hole pairs with energies near the band edges can then recombine radiatively by emitting light; (d): under strong forward bias, minority holes are injected into the n-type semiconductor, recombining with electrons by emitting light. To avoid strong electron (majority carriers) current, electrons are injected by a current limiting contact, a Schottky contact.


Fig. 5




Fig. 5 : 

Schematics of a Destriau cell (after Destriau [51]).


Fig. 6




Fig. 6 : 

Schematics of carrier injection in a forward biased p–n junction leading to light emission. (a) Junction with no applied bias. Carriers are repelled from the depletion region by the built-in potential. (b) Under forward bias, the repelling potential is reduced and both electrons and holes are injected in regions where they can recombine (after [54]).


Fig. 7




Fig. 7 : 

Semiconductor optical transitions: (a) direct bandgap emission, (b) indirect bandgap phonon assisted emission, (c) indirect bandgap deep-donor assisted emission, (d) indirect bandgap deep acceptor assisted emission.


Fig. 8




Fig. 8 : 

Time evolution of the external quantum efficiency of visible and UV LEDs (after [87,88]).


Fig. 9




Fig. 9 : 

Bandgap map (forbidden bandgap vs. crystal lattice constant) of major semiconductors and their alloys.


Fig. 10




Fig. 10 : 

Schematics of real-space band diagram and carrier diffusion in forward biased, from (a) p–n homostructure, (b) p–n double heterostructure, (c) p–n multi-quantum well structure.


Fig. 11




Fig. 11 : 

Two specific aspects of nitride LEDs. Left: spontaneous polarization and strain induced piezo electric fields create strong internal fields in nitride heterostructures; right: despite the large dislocations densities in the 108–109 cm−2 range, InGaN materials exhibit high IQEs.


Fig. 12




Fig. 12 : 

Time evolution of LED markets (after Yole's development, 2014). Note that this is value of market, product of rapidly increasing number of devices and simultaneously rapidly decreasing unit costs.


Fig. 13




Fig. 13 : 

Time evolution of light output and cost per LED lamp package, for red and white lamps [after 120].


Fig. 14




Fig. 14 : 

Comparison of state of the art (SOTA) and expected future performance of blue LEDs and laser light sources (LDs). Lasers are expected to have a larger efficiency at high currents, because carrier densities and droop are clamped at their values at the laser threshold (from [81]).


Fig. 15




Fig. 15 : 

Illustration of light emission at the bandgap energy for a LED electrically injected at an electron–hole pair energy lower than the bandgap: electrons and holes are “boosted” to energies higher than their injected energies thanks to the high-energy thermal tail of e–h distributions created by the thermal phonons of the lattice at finite temperature.


Fig. 16




Fig. 16 : 

Relationship for EQE, WPE, and electrical efficiency V p /V on emitted optical power for the mid-IR LED reported in [144]. The WPE curve has two peaks, the conventional peak near the EQE peak, and the other at low bias, where the EQE saturates as V p /V continues to rise. At low bias, the WPE exceeds 100% at an EQE of just 0.03% and V p /V of ∼700%. The plot is based on 135 °C measurement data [from 144,145].

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