Nanocrystalline Films

• Determining the Internal Quantum Efficiency of PbSe Nanocrystal Solar Cells with the Aid of an Optical Model
  Authors: M. Law, M. C. Beard, S. Choi, J. M. Luther, M. C. Hanna, A. J. Nozik
  NANO LETTERS 8 (11) 3904–3910 (2008)
  DOI: 10.1021/nl802353x
  Abstract: We determine the internal quantum efficiency (IQE) of the active layer of PbSe nanocrystal (NC) back-contact Schottky solar cells by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. The model is parametrized with the complex index of refraction of each layer in the stack as calculated from ellipsometry data. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the NC films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the NCs. Using a series of devices fabricated from 5.1 ± 0.4 nm diameter PbSe NCs, we show that thin NC cells achieve an EQE and an active layer IQE as high as 60 ± 5% and 80 ± 7%, respectively, while the QE of devices with NC layers thicker than about 150 nm falls, particularly in the blue, because of progressively greater light absorption in the field-free region of the films and enhanced recombination overall. Our results demonstrate that interference effects must be taken into account in order to calculate accurate optical generation profiles and IQE spectra for these thin film solar cells. The mixed modeling/experimental approach described here is a rigorous and powerful way to determine if multiple exciton generation (MEG) photocurrent is collected by devices with EQE < 100%. On the basis of the magnitudes and shapes of the IQE spectra, we conclude that the 1,2-ethanedithiol treated NC devices studied here do not produce appreciable MEG photocurrent.



• Structural and optical properties of nanocrystalline Er2O3 thin films deposited by a versatile low-pressure MOCVD approach
  Authors: Losurdo M, Giangregorio MM, Capezzuto P, Bruno G, Malandrino G, Fragala IL, Armelao L, Barreca D, Tondello E
  JOURNAL OF THE ELECTROCHEMICAL SOCIETY 155 (2) G44-G50 (2008)
  DOI: 10.1149/1.2820621
  Abstract: Nanocrystalline Er2O3 thin films have been grown on Si(100) by low-pressure metallorganic chemical vapor deposition in the temperature range of 450-700 degrees C using the tris(isopropylcyclopentadienyl)erbium precursor and O2. The growth kinetics has been investigated in real time using laser reflectance interferometry as a function of growth parameters, and three different steps have been highlighted: (i) the incubation, (ii) the nucleation, and (iii) the growth stages. It is demonstrated that functionalization of the Si substrate surface is important for shortening the incubation time and, consequently, reducing subcutaneous oxidation of the Si substrate and for preventing nonhomogeneous nucleation that yields rough films. The growth kinetics has been correlated with film properties. X-ray diffraction patterns show strongly (111) oriented Er2O3 thin films even at temperature as low as 400 degrees C; however, a deposition temperature of 600 degrees C is optimal for obtaining films with highest refractive index and lowest surface roughness (root-mean-square = 0.4 nm). Transmission electron microscopy shows very sharp interfaces and compact films. Spectroscopic ellipsometry analysis of the optical properties shows a very high refractive index comparable to that of Er2O3 single crystal and a very high transparency in the visible-ultraviolet energy photon range.
  Email: maria.losurdo@ba.imip.cnr.it



• Tailoring nanostructure of ZnO thin films by plasma assisted and Au-catalyst assisted MOCVD
  Authors: Losurdo M, Giangregorio MM, Malandrino G, Capezzuto P, Fragala IL, Bruno G
  JOURNAL OF NON-CRYSTALLINE SOLIDS 354 (19-25) 2821-2825 (2008)
  DOI: 10.1016/j.jnoncrysol.2007.09.068
  Abstract: The structural, morphological and optical properties of ZnO thin films grown by metal organic chemical vapor deposition (MOCVD) also plasma-assisted (PA-MOCVD) and Au-catalyst assisted (Au-MOCVD) are investigated. A new precursor Zn(TTA)(2)TMED (HTTA = 2-thenoyltrifluoroacetone, TMED = N,N,N',N'- tetramethylethylendiamine) and O2 have been used to grow film on various substrates, including Si(100), alpha-Al2O3(0001) and quartz. Spectroscopic ellipsometry is used for investigating the correlation between film nanostructure and optical properties (exciton of ZnO appears in dielectric function). It has been found that the O2 plasma assisted growth results in highly (0001) oriented ZnO films on all investigated substrates, with better optical quality as indicated by a sharp and intense exciton in the dielectric function spectra. Conversely, highly textured and rough films are deposited by MOCVD and also using the Au-catalyst. In the latter case, coupling ZnO with Au nanoparticles used as catalyst results in an enhancement of the Au surface plasmon resonance peak at 2.2 eV.
  Email: maria.losurdo@ba.imip.cnr.it



• Relationship between nanostructure and optical properties of ZnO thin films
  Authors: Malandrino G, Blandino M, Fragala ME 1, Losurdo M, Bruno G
  JOURNAL OF PHYSICAL CHEMISTRY C 112(26) 9595-9599 (2008)
  DOI: 10.1021/jp8001492
  Abstract: Nanostructured ZnO thin films have been grown on quartz substrates by metal organic chemical vapor deposition. Zn(tta)2 center dot tmeda (H-tta = 2-thenoyltrifluoroacetone, tmeda = N,N,N',N'-tetramethylethylenediamine) has been used as Zn precursor. The impact of deposition temperature, in the range 400-750 degrees C, on film nanostructure and optical properties has been investigated. The X-ray diffraction patterns show that film crystallinity improves upon increasing the substrate temperature. This is consistent with morphologies observed through scanning electron microscopy (SEM) and atomic force microscopy (AFM), which indicate that grain dimensions increase ranging from 80 nm rounded grains at 400 degrees C to barlike grains of about 200-400 nm at the highest investigated temperature of 750 degrees C. The AFM investigation shows that the roughness is also a function of the substrate temperatures and parallels the grains size of ZnO films. The UV-vis transmission spectrum shows that ZnO films grown at 600 degrees C on quartz are highly transparent in the visible region. Furthermore, spectroscopic ellipsometry is used for investigating the dependence of the ZnO dielectric function and optical gap on film nanostructure.
  Email: maria.losurdo@ba.imip.cnr.it
  
  
• Microstructural and optical investigations of sol-gel derived ferroelectric BaTiO3 nanocrystalline films determined by spectroscopic ellipsometry
  Authors: Hu, Z. G., Li, Y. W., Zhu, M., Zhu, Z. Q., Chu, J. H.
  PHYSICS LETTERS A 372 (24) 4521-4526 (2008)
  DOI: 10.1016/j.physleta.2008.04.001
  Abstract: Ferroelectric BaTiO3 nanocrystalline films (BTNFs) with the crystalline sizes of about 30 nm were grown on Pt/Ti/SiO2/Si substrates by a modified sol-gel method. Spectroscopic ellipsometry (SE) was used to characterize the films in the photon energy range of 1.5-5.0 eV with a five-phase layered model (air/surface rough layer/BaTiO3/interface layer/Pt). The optical properties in the transparent and absorption regions have been investigated with the Forouhi-Bloomer dispersion relation. With the aid of the structural and dielectric function models, the microstructure and electronic structure of the BTNFs can be readily obtained. it was found that the refractive index reaches the value of 2.20 in the transparent region. Based on the Sellmeier dispersion analysis, the single-oscillator energy is about 4.7 eV for the BTNFs. The long wavelength refractive index n(0) can be estimated to about 2.00 at zero point. The direct optical band gap energy approaches approximately 4.2 eV and Urbach band tail energy is 262 2 and 268 +/- 1 meV respectively with increasing crystalline size. A higher band gap observed can be owing to the known quantum confinement effect in the nanocrystalline formation and different fraction of amorphous and crystalline components. The theoretical analysis based on the effective mass approximation theory is well used to explain these experimental data.