Evolution of Long Range Bandgap Tunable Lead Sulfide Nanocrystals with Photovoltaic Properties Ali Hossain Khan, Umamahesh Thupakula, Amit Dalui, Subrata Maji, Anupam Debangshi, and Somobrata Acharya* Centre for Advanced Materials, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, India. * S Supporting Information ABSTRACT: Monodispersed bandgap tunable lead sulfide nanocrystals ranging from 0.6 to 1.7 eV have been synthesized by adjusting the reaction temperature and growth time. An evolution from cuboctahedra to perfect cube takes place at higher reaction temperature with longer annealing time. The nanocrystals absorb light both in the visible and IR spectral range. Bandgap dependent photovoltaic studies reveal optimal device performance for a critical size nanocrystal with ∼1.2 eV bandgap revealing the role of optimum bandgap on the photovoltaic performance. T he lead sulfide (PbS) nanoparticles are crucial from both fundamental scientific studies and technological applica- tions owing to the large Bohr exciton radius (20 nm), 1 size- tunable direct bandgap, 2-10 symmetric conduction and valence bands, 11 and multiple exciton generation (MEG) properties. 12 Significant electronic coupling between neighboring PbS nano- crystals can be achieved due to their relatively low effective electron and hole masses. 13 These properties ultimately probed PbS as promising candidates for a variety of electronic and optoelectronic applications. 14-25 PbS nanoparticle-based photo- voltaics have seen rapid advances in recent years, progressing from the first report of an infrared solar cell 14 to recent reports showing ∼6% solar AM1.5 power conversion efficiency. 26 The recent studies have explored engineering of ligand passivation, cell architectures, and multiple exciton generation possibilities for improving device performance. 26-28 One opportunity for further improvement in solution cast photovoltaic cells relies in making better use of the sun’s full spectrum. Since the majority of the solar spectrum remains in the visible and infrared region, the key promise remains in tuning the bandgap of lead chalcogenide nanoparticles to cover the maximum of the solar spectrum. Photovoltaic cell fabrication employing lead chalcogenide nanoparticles motivates investigations of devices having tunable quantum-confined bandgaps. 29-31 The potential to realize the effect of bandgap and to find an optimal bandgap of lead sulfide nanoparticles holds importance for photovoltaic device applications. Here we describe a novel synthesis route to design bandgap tunable PbS nanocrystals by adjusting only the reaction temperature and growth time. Five different bandgaps of PbS nanocrystals are synthesized, which absorb light both in the visible and IR range. The sizes of these nanocrystals fall in the range from 2.3 to 10 nm. We fabricate devices using these bandgap tunable nanocrystals to study the role of bandgap energy on the photovoltaic properties. Our results reveal that the photovoltaic performance can be significantly improved for a critical bandgap of the nanocrystals. We synthesize PbS nanocrystals by using lead nitrate and thiourea in controlled ratios in mixed ligands cum solvents hexadecylamine (HDA) and trioctylphosphine oxide (TOPO) (see Supporting Information for detailed synthesis processes and Table S1). First, TOPO was heated along with lead nitrate and thiourea at 110 °C to make a milky white turbid precursor solution to which molten HDA was injected in the second step. The bandgap of the nanocrystals was controlled by adjusting the injection temperature of the HDA and the growth time of the nanocrystals. Figure 1a shows the transmission electron microscope (TEM) images of 5.5 ± 0.25 nm nanocrystals obtained by injecting HDA into the milky white and turbid precursor solution at 150 °C followed by annealing for 30 min. All the nanocrystals show a narrow size distribution without any post synthesis technique. The bright-field high-resolution TEM (HRTEM) image reveals cube-shaped PbS nanocrystals of 5.5 ± 0.25 nm in size (Figure 1b). The cubes are of high crystallinity with well-resolved lattice planes corresponding to an interplanar spacing of 0.29 ± 0.02 nm, consistent with the (200) d-spacing of the PbS bulk rocksalt structure. The selected area electron diffraction (SAED) patterns (Inset of Figure 1b) confirm the rock-salt cubic structure of the PbS cubes with predominant 200 diffraction ring, corresponding to the interplanar distance of 0.29 ± 0.02 nm of Received: February 27, 2013 Revised: March 22, 2013 Published: March 25, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 7934 dx.doi.org/10.1021/jp402030p | J. Phys. Chem. C 2013, 117, 7934-7939