We introduce indium arsenide colloidal quantum dot films for photovoltaic devices, fabricated by two-step surface modification. Native ligands and unwanted oxides on the surface are peeled off followed by passivating with incoming atomic or short ligands. The near-infrared-absorbing n-type indium arsenide colloidal quantum dot films can be tuned in energy-level positions up to 0.4 eV depending on the surface chemistry, and consequently, they boost collection efficiency when used in various emerging solar cells. As an example, we demonstrate p–n junction between n-type indium arsenide and p-type lead sulfide colloidal quantum dot layers, which leads to a favorable electronic band alignment and charge extraction from both colloidal quantum dot layers. A certified power conversion efficiency of 7.92% is achieved without additionally supporting carrier transport layers. This study provides richer materials to explore for high-efficiency emerging photovoltaics and will broaden research interest for various optoelectronic applications using the n-type covalent nanocrystal arrays.

Indium oxide nanocrystals (In 2 O 3 NCs) with sizes of 5.5 nm-10 nm were synthesized by hot injection of the mixture precursors, indium acetate and oleic acid, into alcohol solution (1-octadecanol and 1-octadecence mixture). Field emission transmission electron microscopy (FE-TEM), High resolution X-Ray diffraction (Xray), Nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FT-IR) were employed to investigate the size, surface molecular structure, and crystallinity of the synthesized In 2 O 3 NCs. When covered by oleic acid as a capping group, the In 2 O 3 NCs had a high crystallinity with a cubic structure, demonstrating a narrow size distribution. A high mobility of 2.51 cm 2 /V·s and an on/off current ratio of about 1.0 × 10 3 were observed with an In 2 O 3 NCs thin film transistor (TFT) device, where the channel layer of In 2 O 3 NCs thin films were formed by a solution process of spin coating, cured at a relatively low temperature, 350°C. A size-dependent, non-monotonic trend on electron mobility was distinctly observed: the electron mobility increased from 0.43 cm 2 /V·s for NCs with a 5.5 nm diameter to 2.51 cm 2 /V·s for NCs with a diameter of 7.1 nm, and then decreased for NCs larger than 7.1 nm. This phenomenon is clearly explained by the combination of a smaller number of hops, a decrease in charging energy, and a decrease in electronic coupling with the increasing NC size, where the crossover diameter is estimated to be 7.1 nm. The decrease in electronic coupling proved to be the decisive factor giving rise to the decrease in the mobility associated with increasing size in the larger NCs above the crossover diameter.

We developed a new class of organic surface ligands; 2-aminopyridine (2AP), 4-aminobenzoic acid (4ABA), and benzoic acid (BA); for use in the solution ligand exchange of nanocrystals (NCs) in the presence of nitric acid (HNO3). Here, colloidal NCs synthesis is used for the first time. These short, air-stable, easy-to-model ligands bind to the surface of the indium oxide nanocrystal (In2O3 NC) and provide the electrostatic stabilization of NC semiconductor dispersions in N,N-dimethylformamide, allowing for a solution-based deposition of NCs into thin-film transitors (TFTs). The shorter organic ligands greatly facilitate electronic coupling between the NCs. For example, thin films made from 2AP-capped In2O3 NCs exhibited a high electron mobility of μ≈9.5 cm2/(V·s), an on-off current ratio of about >10(7), and a subthreshold swing of 2.34 V/decade. As the ligand length decreased, the electron mobility increased exponentially. Furthermore, we also report on the temperature-dependent behavior of the electron transport of In2O3 NCs films, in the case in which thin films were cured at 150 °C, as the 2AP, BA, and 4ABA ligand molecules were sustained on the NC. We demonstrated a hopping transport mechanism instead of a band-like transport, and the thermally activated carrier transport process governed the charge transport in our In2O3 NC thin-film solid.

Barium titanate nanocrystals (BT NCs) were prepared under solvothermal conditions at 200 °C for 24 h. The shape of the BT NCs was tuned from nanodot to nanocube upon changing the polarity of the alcohol solvent, varying the nanosize in the range of 14-22 nm. Oleic acid-passivated NCs showed good solubility in a nonpolar solvent. The effect of size and shape of the BT NCs on the ferroelectric properties was also studied. The maximum polarization value of 7.2 μC/cm(2) was obtained for the BT-5 NC thin film. Dielectric measurements of the films showed comparable dielectric constant values of BT NCs over 1-100 kHz without significant loss. Furthermore, the bottom gate In2O3 NC thin film transistors exhibited outstanding device performance with a field-effect mobility of 11.1 cm(2) V(-1) s(-1) at a low applied gate voltage with BT-5 NC/SiO2 as the gate dielectric. The low-density trapped state was observed at the interface between the In2O3 NC semiconductor and the BT-5 NCs/SiO2 dielectric film. Furthermore, compensation of the applied gate field by an electric dipole-induced dipole field within the BT-5 NC film was also observed.

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Indium-doped zinc oxide nanocrystals (IZO NCs), capped with stearic acid (SA) of different sizes, were synthesized using a hot injection method in a noncoordinating solvent 1-octadecene (ODE). The ligand exchange process was employed to modify the surface of IZO NCs by replacing the longer-chain ligand of stearic acid with the shorter-chain ligand of butylamine (BA). It should be noted that the ligand-exchange percentage was observed to be 75%. The change of particle size, morphology, and crystal structures were obtained using a field emission scanning electron microscope (FE-SEM) and X-ray diffraction pattern results. In our study, the 5 nm and 10 nm IZO NCs capped with stearic acid (SA-IZO) were ligand-exchanged with butylamine (BA), and were then spin-coated on a thermal oxide (SiO 2 ) gate insulator to fabricate a thin film transistor (TFT) device. The films were then annealed at various temperatures: 350 °C, 400 °C, 500 °C, and 600°C. All samples showed semiconducting behavior and exhibited n-channel TFT. Curing temperature dependent on mobility was observed. Interestingly, mobility decreases with the increasing size of NCs from 5 to 10 nm. Miller-Abrahams hopping formalism was employed to explain the hopping mechanism insight our IZO NC films. By focusing on the effect of size, different curing temperatures, electron coupling, tunneling rate, and inter-NC separation, we found that the decrease in electron mobility for larger NCs was due to smaller electronic coupling.

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