We investigate zirconium (Zr) incorporation into the titanium dioxide (TiO2) electron-transporting layer used in organometal halide perovskite photovoltaics. Compared to Zr-free controls, solar cells employing electrodes containing Zr exhibit increased power conversion efficiency (PCE) and decreased hysteresis. We use transient photovoltage and photocurrent extraction to measure carrier lifetimes and densities and observe longer carrier lifetimes and higher charge densities in devices on Zr-containing electrodes at microsecond times as well as longer persistent photovoltages extending from ∼milliseconds to tens of seconds. We characterize the surface stoichiometry and change in work function and reduction potential of the TiO2 upon incorporation of Zr and discuss the charge recombination at the TiO2 interface in the context of these variables. Finally, we show that the combination of Zr-TiO2 electrode modification with device pyridine treatment leads to a cumulative improvement in performance.

We use photoinduced absorption (PIA) spectroscopy to investigate pathways for photocurrent generation in hybrid organic/inorganic quantum dot bulk heterojunction solar cells. We study blends of the conjugated polymer poly(2,3-bis(2-(hexyldecyl)quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole) (PDTPQx-HD) with PbS quantum dots and find that positively charged polarons are formed on the conjugated polymer following selective photoexcitation of the PbS quantum dots. This result provides a direct spectroscopic fingerprint demonstrating that photoinduced hole transfer occurs from the photoexcited quantum dots to the host polymer. We compute the relative yields of long-lived holes following photoexcitation of both the polymer and quantum dot phases and estimate that more long-lived polarons are produced per photon absorbed by the polymer phase than by the quantum dot phase.

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The size of a quantum-confined nanocrystal determines the energies of its excitonic transitions. Previous work has correlated the diameters of PbS nanocrystals to their excitonic absorption; however, we observe that PbS quantum dots synthesized in saturated dispersions of PbCl2 can deviate from the previous 1Sh-1Se energy vs diameter curve by 0.8 nm. In addition, their surface differs chemically from that of PbS quantum dots produced via other syntheses. We find that these nanocrystals are coated in a shell that is measurable in transmission electron micrographs and contains lead and chlorine, beyond the monatomic chlorine termination previously proposed. This finding has implications for understanding the growth mechanism of this reaction, the line width of these quantum dots’ photoluminescence, and electronic transport within films of these nanocrystals. Such fundamental knowledge is critical to applications of PbS quantum dots such as single-photon sources, photodetectors, solar cells, light-emitting diodes, lasers, and biological labels.

Control of quantum dot surface chemistry offers a direct approach to tune the molecular interface between donor and acceptor constituents in hybrid bulk heterojunction photovoltaics incorporating organic semiconductors and colloidal quantum dots. We investigate the effects of altering the quantum dot surface chemistry via ligand exchange in blends of PbS quantum dots with the conjugated polymer poly­((4,8-bis­(octyloxy)­benzo­(1,2-b:4,5-b′)­dithiophene-2,6-diyl)­(2-((dodecyloxy)­carbonyl)­thieno­(3,4-b)­thio­phenediyl)) (PTB1). We study organic ligands with both thiol and carboxylic acid functional groups including 1,2-ethanedithiol (EDT), 3-mercapto­propionic acid (MPA), and malonic acid (MA), in addition to inorganic halide ions such as tetrabutylammonium iodide (TBAI). We show that the different ligand treatments influence hybrid solar cell efficiency primarily through changes in open-circuit voltage (V OC) and fill factor (FF). We use photoinduced absorption (PIA) spectroscopy to probe the generation of long-lived polarons resulting from charge transfer between the donor and acceptor constituents. We further characterize the recombination dynamics in the hybrid devices using transient photovoltage (TPV) and charge extraction (CE) techniques. Both methods show that ligand exchange with MPA yields superior device performance by promoting longer carrier recombination lifetimes under open-circuit conditions.