Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids

Abstract

Solid films of colloidal quantum dots show promise in the manufacture of photodetectors and solar cells. These devices require high yields of photogenerated charges and high carrier mobilities, which are difficult to achieve in quantum-dot films owing to a strong electron-hole interaction and quantum confinement. Here, we show that the quantum yield of photogenerated charges in strongly coupled PbSe quantum-dot films is unity over a large temperature range. At high photoexcitation density, a transition takes place from hopping between localized states to band-like transport. These strongly coupled quantum-dot films have electrical properties that approach those of crystalline bulk semiconductors, while retaining the size tunability and cheap processing properties of colloidal quantum dots.

Trap-Induced Dispersive Transport and Dielectric Loss in PbS Nanoparticle Films

“…However, it has also been argued that the increase in carrier density upon illumination cannot explain the relatively high values of photoconductivity observed in PbS NP films [19]. At high carrier densities increased carrier delocalization has been reported [34]. These results indicate that more detailed studies of carrier transport under different illumination conditions are necessary to fully understand electrical loss mechanisms in PbS NP films for opto-electronic applications.…”

High-Mobility Toolkit for Quantum Dot Films

“…Nevertheless surprisingly high mobilities were reported lately by several groups, [12][13][14][15][16][17][18] in high quality films made of different materials, suggesting that band-like transport through extended states is indeed achievable in CQD arrays, provided the surface traps are effectively passivated [18][19][20][21][22][23][24][25] and the separation between dots is reduced sufficiently by the use of extremely short ligands or inorganic capping. 26 This hypothesis is supported by the observed temperature dependence of mobility and conductivity [12][13][14][15][16][17][18] whereas the spectral broadening and red shifts of the 1S exciton peak observed in these systems, 13,27 may be indicative of strong electronic coupling between QDs, as are the remarkable values of diffusion lengths and lifetimes of charge carriers measured in QD solids. 28 In this work, we carried out a comprehensive and systematic study of the electronic structure and transport properties of CQD films made of different semiconductor materials, representatives of groups III-V, II-VI, and IV-VI, having different bulk crystal structures, varying from zinc blende to wurtzite to rocksalt, considering building blocks (dots) of different sizes, surface morphologies and stoichiometries, placed at different distances from each other and ordered according to different lattice types.…”

Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime

“…In solar cells, these materials have recently exceeded 7% certified power conversion efficiency (PCE) 5,12,13 , offering a promising path towards efficient, low-cost and roll-to-roll processed photovoltaics (PVs). Recent efforts have concentrated on eliminating trap states detrimental to carrier lifetime 5,14,15 , investigating the impact of size polydispersity on an ensemble of CQDs 16,17 , improving charge collection 13,18,19 , characterizing field-effect mobility in these materials [20][21][22][23] and developing novel doping strategies to enable new high-efficiency device architectures 6,[24][25][26] . However, present-day devices still suffer from current densities and fill factors that are well below their theoretical potential.…”

“…If these features can be combined to produce a high mobility-lifetime product, increased transport lengths should permit the construction of thick absorber layers capable of absorbing the available solar light while maintaining the efficient harvesting of the resultant photocarriers. Despite numerous reports of field effect 23,29,30 and terahertz 22,31 mobilities on the order of 1-30 cm 2 V À 1 s À 1 , which should in principle result in much greater efficiencies than seen today 32 , a PV device has yet to be made that benefits from these increased charge carrier mobilities. Instead, the best certified CQD solar cells reported to date employ active layer materials with fieldeffect mobilities of 10 À 3 -10 À 2 cm 2 V À 1 s À 1 (refs 5,33,34).…”