We report here an assessment of carrier multiplication (CM) yields in PbSe and PbS nanocrystals (NCs) by a quantitative analysis of biexciton and exciton dynamics in transient photoluminescence decays. Interest in CM, the generation of more than one electron and hole in a semiconductor after absorption of one photon, has renewed in recent years because of reports suggesting greatly increased efficiencies in nanocrystalline materials compared to the bulk form, in which CM was otherwise too weak to be of consequence in photovoltaic energy conversion devices. In our PbSe and PbS NC samples, however, we estimate using transient photoluminescence that at most 0.25 additional eh pairs are generated per photon even at energies ω > 5Eg, instead of the much higher values reported in the literature. We argue by comparing NC CM estimates and reported bulk values on an absolute energy basis, which we justify as appropriate on physical grounds, that the data reported thus far are inconclusive with respect to the importance of nanoscale-specific phenomena in the CM process. arXiv: 0806.1966v1 [cond-mat.mtrl-sci]

We fabricate PbS colloidal quantum dot (QD)-based solar cells using a fullerene derivative as the electron-transporting layer (ETL). A thiol treatment and oxidation process are used to modify the morphology and electronic structure of the QD films, resulting in devices that exhibit a fill factor (FF) as high as 62%. We also show that, for QDs with a band gap of less than 1 eV, an open-circuit voltage (VOC) of 0.47 V can be achieved. The power conversion efficiency reaches 1.3% under 1 sun AM1.5 test conditions and 2.4% under monochromatic infrared (lambda=1310 nm) illumination. A consistent mechanism for device operation is developed through a circuit model and experimental measurements, shedding light on new approaches for optimization of solar cell performance by modifying the interface between the QDs and the neighboring charge transport layers.

Quantum dots show great promise for fabrication of hybrid bulk heterojunction solar cells with enhanced power conversion efficiency, yet controlling the morphology and interface structure on the nanometer length scale is challenging. Here, we demonstrate quantum dot-based hybrid solar cells with improved electronic interaction between donor and acceptor components, resulting in significant improvement in short-circuit current and open-circuit voltage. CdS quantum dots were bound onto crystalline P3HT nanowires through solvent-assisted grafting and ligand exchange, leading to controlled organic-inorganic phase separation and an improved maximum power conversion efficiency of 4.1% under AM 1.5 solar illumination. Our approach can be applied to a wide range of quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general scheme for improving the efficiency of nanocrystal hybrid solar cells.

This article presents a perspective on the experimental and theoretical work to date on the efficiency of carrier multiplication (CM) in colloidal semiconductor nanocrystals (NCs). Early reports on CM in NCs suggested large CM efficiency enhancements. However, recent experiments have shown that CM in nanocrystalline samples is not significantly stronger, and often is weaker, than in the parent bulk when compared on an absolute photon energy basis. This finding is supported by theoretical consideration of the CM process and the competing intraband relaxation. We discuss the experimental artifacts that may have led to the apparently strong CM estimated in early reports. The finding of bulklike CM in NCs suggests that the main promise of quantum confinement is to boost the photovoltage at which carriers can be extracted. With this in mind, we discuss research directions that may result in effective use of CM in a solar cell.

lead iodide perovskite top cell on Cu 2 ZnSn(S,Se) 4 kesterite and Si-based bottom cells. [ 10,12 ] The remarkable effi ciencies of perovskite devices above 15%, the possibility to process at temperatures below 150 °C, and the highly tunable band gap range from 1.6 to 2.25 eV make these materials especially attractive for monolithic tandem integration with CIGS. [12][13][14][15][16][17][18][19][20] Here, we report perovskite-CIGS tandem solar cells in which each cell was customized for monolithic integration in the following sequence: transparent conducting electrode (TCE)/phenyl-C61-butyric acid methyl ester (PCBM)/perovskite/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/ITO/CdS/CIGS/Mo/Si 3 N 4 /glass. By modifying our solution-based process, [ 21 ] the CIGS absorber band gap was reduced to 1.04 eV for improved photon management. A 30 nm thick ITO was selected as a transparent recombination layer that was deposited directly onto the CdS without the intrinsic ZnO layer commonly used in CIGS devices. The elimination of ZnO from the device structure was critical for achieving functional perovskite tandems, as we discovered that the presence of ZnO in proximity to the perovskite layer degrades device performance. Perovskite devices with electron-selective ZnO layers have been reported previously, [22][23][24] however we found that processing at temperatures above 60 °C resulted in deterioration of the perovskite layer. In contrast, perovskite layers processed on our ZnO-free CIGS structure could withstand annealing treatments for several hours at 120 °C without any damage.Considering the importance of precise band gap control in monolithic tandem devices, we designed a reactor for continuous in situ monitoring and precise control of the optical properties of the perovskite layer via vapor-based halide exchange reactions, as illustrated in Figure 1 a. The system provided temperature and pressure control as well as spectroscopic measurement capability. In a typical process, a sample of spincoated lead halide PbX 2 (X = I and/or Br) is placed inside the temperature-controlled vacuum chamber and annealed in the presence of methylammonium halide (CH 3 NH 3 X) vapor. A transparent port at the top of the chamber provides an optical path for a broad-spectrum light source to illuminate the sample in the reactor, and a port beneath the sample leads to a spectrophotometer, which collects the transmission spectra. The real-time feedback is used for precise engineering of the perovskite absorber properties by adjusting the temperature, pressure, and precursor type. The process was designed to have better compatibility with the CIGS device than previously reported vapor-assisted approaches that employ temperatures in excess of 120 °C and focus on pure iodide perovskite with fi xed band gap. [ 25,26 ] In our reactor the conversion temperature was