Bandgap absorber materials

EQE spectra can be analyzed to obtain the bandgap of absorber materials. The photocurrent response (Jphoto) of the semiconductor electrolyte junction is described by the Gartner equation [148] as... 

Fig. 7.16 General schematic of a three-cell stack of three absorber materials with bandgap-Eg and absorption coefficient a. Indicated are the photon fluxes in each cell (<1>), along with the generated photovoltages (V) and photocurrent densities (J). The total voltage in the series-connected stack is the sum of the individual cell voltages, while the stack current is the minimum of the individual cell currents...

PBG material A non-light-absorbing material which contains a bandgap for electromagnetic waves propagating in all directions. 

Klipstein et al. (2010) describes these devices succinctly A new XBn device architecture, based on heterostructures, has been proposed as an alternative to a homojunction photodiode. The main difference is that no depletion layer exists in any narrow bandgap region of the device. Instead, the depletion layer is confined to a wide bandgap barrier material. The Generation-Recombination (G-R) contribution to the dark current is then almost totally suppressed and the dark current becomes diffusion limited. This lowering of the dark current allows the device operating temperature to be raised relative to that of a standard photodiode made from the same photon absorbing material, with essentially no loss of performance. 

Since IR detector materials are direct bandgap materials (with no change in electron momentum required), they are very efficient absorbers (and emitters) of light - all IR photons are absorbed within the first few /rm of material. The reason that infrared detectors are 10 to 15 ptm thick is for structural and fabrication reasons, not for light absorption reasons. 

One major issue with p-GaP is its large bandgap, which allows only 17% of the solar spectrum to be absorbed. When a material with a smaller bandgap, namely p-CdTe, was examined for C02 reduction, Taniguchi et al. obtained faradaic efficiencies of 70% for CO formation in DMF with 5% water [107]. For this, the electrode was illuminated with visible fight at 600 nm with tetrabutylammonium perchlorate (TBAP) as the electrolyte. Although the quantum yields approached unity, the electrode was biased at -1.6 V (versus SCE), which indicated a require-... 

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