Colloidal nanocrystals (NCs) are promising materials for electronics and optoelectronics, due to their (i) solution processability; (ii) size-dependent electronic band structures; and (iii) large surface-to-volume ratio. Exploring these features has led to great advances in our fundamental understanding of carrier mobility and lifetime in NC assemblies, which governs the performance of their devices. To continue to push forward NC technologies requires further development and optimization of materials and device design and fabrication process, and efforts are needed to replace the toxic elements found in the most studied NC compositions for many device applications. In this thesis, we develop solution-processable, all NC field-effect transistors (FETs) with metallic (Ag, In), semiconducting (CdSe), and insulating (Al2O3) NCs. We modify NC surfaces and the materials, yielding a chemically compatible, structurally stable, and physically cooperative device integration on flexible polymer substrates. All NC FETs show similar device performances with FETs built with NC semiconducting channels and conventionally vacuum-grown materials in other device layers. To address toxicity concerns, we develop a post-deposition cation exchange process to achieve high-performance CuInSe2 NC FETs and circuits. We start with high-quality CdSe NCs, and exchange Cd2+ with Cu+ and In3+. We integrate the high-quality CuInSe2 NC thin film into FETs and proof-of-concept circuits, by applying surface modifications to introduce fusion among NCs and to add In-doping, which show comparable device and circuit performances with the starting CdSe NC thin-film devices. We continue to optimize the NC device design to achieve shortwave infrared (SWIR) photodetection. We build photo FETs with CdSe NCs by surface modification to introduce In doping. We achieve strong quantum confinement in high mobility CdSe NC thin films, which we hypothesize makes intra sub-band transitions possible and allows us to achieve high photoresponsivity broad spectrum photodetection, including at SWIR wavelengths. To improve the compatibility with complementary metal oxide semiconductor (CMOS) technology, we build Cd- and Pb-free photodiodes by integrating InAs NCs with crystalline Si. We introduce a two-step surface modification process to enhance interparticle coupling, passivate trap states, and introduce doping. We show promising external quantum efficiencies (EQEs) at SWIR wavelengths.