Mid-gap trap states in CdTe nanoparticle solar cells

Abstract

Thin film solar cells comprised of quantum-confined CdTe nanoparticles are shown to have a low intrinsic density of mid-gap trap states relative to their equivalent bulk film, indicating that the ligands are effective at electrically passivating surface states. Sintering the nanoparticles into a poly-crystalline thin film increases device performance but also increases the density of mid-gap trap states due to doping from the CdCl treatment and the formation of long range disorder such as grain boundaries and dislocations. Long term aging under illumination increases the density of mid-gap traps in the unsintered films due to degradation of the ligands.

Safer salts for CdTe nanocrystal solution processed solar cells: the dual roles of ligand exchange and grain growth

“…However, this may not be true for other metal chlorides (such as CdCl 2 and MgCl 2 ). 23,42 Based on these results, the chlorides were shown to produce the most effective sintering due to their efficiency of oleate ligand removal paired with enhanced reactivity with CdTe nanocrystals due to higher volatility of tellurium intermediates. The identity of the cation is also important, although the change from cadmium(II) to ammonium was not as pronounced as the change among the halides.…”

“…25 mA cm À1 . 12,19,42 However, these differences are minimal and are expected to improve aer further optimization of the ammonium halide systems. Fig.…”

Sharp exponential band tails in highly disordered lead sulfide quantum dot arrays

“…Early work on E U in QD arrays found evidence of broad Urbach tails in nanostructured CdSe [38] and ZnSe [39]. Optical absorption of colloidal CdTe QD films show a broad band tail (97 meV), which reduces (to 30 meV) when the film is sintered to form larger grains [36]. However, optical absorption does not necessarily correlate with carrier transport.…”

Electrical conduction mechanisms of CdTe quantum dots/p-si heterojunction

“…Similar observation by A. Bezryadina, et al [30] for nanostructure CdTe. Bezryadina, et al [31] stated that the low trap density in the nanoparticle films shows that the ligands are effective at electrically passivating the surface of the nanoparticles and the nanoparticles themselves have a low mid gap trap density. The very large surface area of the nanoparticle films does not appear to introduce more trap states relative to microstructure so Ligands in nanoparticle based films are very effective at reducing the number of mid-gap trap states in photodetectors devices.…”