Abstract
Control over carrier type and doping levels in semiconductor materials is key for optoelectronic applications. In colloidal quantum dots (CQDs), these properties can be tuned by surface chemistry modification, but this has so far been accomplished at the expense of reduced surface passivation and compromised colloidal solubility; this has precluded the realization of advanced architectures such as CQD bulk homojunction solids. Here we introduce a cascade surface modification scheme that overcomes these limitations. This strategy provides control over doping and solubility and enables n-type and p-type CQD inks that are fully miscible in the same solvent with complete surface passivation. This enables the realization of homogeneous CQD bulk homojunction films that exhibit a 1.5 times increase in carrier diffusion length compared with the previous best CQD films. As a result, we demonstrate the highest power conversion efficiency (13.3%) reported among CQD solar cells.
Highlights
Control over carrier type and doping levels in semiconductor materials is key for optoelectronic applications
We first proceed with surface halogenation of colloidal quantum dots (CQDs) with lead halide anions to obtain n-type CQD inks, after which the dots are transferred to dimethylformamide (DMF) in which they form a stable colloid
We investigated the effect of doping on carrier transport properties using the space charge-limited current (SCLC) method[30]
Summary
Control over carrier type and doping levels in semiconductor materials is key for optoelectronic applications. We introduce a cascade surface modification scheme that overcomes these limitations This strategy provides control over doping and solubility and enables n-type and p-type CQD inks that are fully miscible in the same solvent with complete surface passivation. This enables the realization of homogeneous CQD bulk homojunction films that exhibit a 1.5 times increase in carrier diffusion length compared with the previous best CQD films. One may architect devices that favor charge transport and extraction in CQD solids by—in a bulk heterojunction—separating photoexcited electrons and holes into distinct phases and collecting them at charge-selective contacts This approach results in extended carrier lifetimes, reduced recombination rates, and longer effective diffusion lengths[19,20]. The density of states of ligand/ CQD systems is influenced by ligand functionalization (arising from their electron-donating vs. electron-withdrawing character)[6,25,26,27,28], and as a result, the use of different ligands provides another degree of freedom in control over the doping level in CQDs6,11
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