10 research outputs found
Origin of efficiency enhancement in Nb2O5 coated titanium dioxide nanorod based dye sensitized solar cells
[EN] Ordered one-dimensional metal oxides are considered promising architectures for dye-sensitized solar cells (DSC). Here we explore the properties determining the photovoltaic performance of DSC based on titanium dioxide nanorods prepared by a hydrothermal method. Ultrathin conformal coating with other oxides such as Al2O3 or Nb2O5 has been widely used in the literature to improve the conversion efficiency. Usually the effects attributed to the coating are either to prevent recombination by barrier effect or to change the position of titanium dioxide conduction band. Here we show that coating TiO2 nanorods with Nb2O5 increases the photocurrent and conversion efficiency of the DSC. However, impedance spectroscopy results indicate that neither recombination is reduced by the coating nor the conduction band position is moved. The improvement in the performance of the cell has been attributed to an enhancement in the charge injection efficiency promoted by Nb2O5.We are thankful for financial support from Ministerio de Ciencia e Innovacion under Projects HOPE CSD2007-00007 and MAT 2010-19827, Generalitat Valenciana under Project PROM-ETEO/2009/058 and Programa de Mobilidad del Personal Investigador de Bancaixa. Servei Central d'Instrumentacio Cientifica of Universitat Jaume I is acknowledged for the SEM and TEM measurements, and Sixto Gimenez for fruitful discussions.Barea, EM.; Xu, X.; González-Pedro, V.; Ripolles-Sanchis, T.; Fabregat-Santiago, F.; Bisquert Mascarell, J. (2011). Origin of efficiency enhancement in Nb2O5 coated titanium dioxide nanorod based dye sensitized solar cells. Energy and Environmental Science. 4(9):3414-3419. https://doi.org/10.1039/c1ee01193fS341434194
Interplay between Fullerene Surface Coverage and Contact Selectivity of Cathode Interfaces in Organic Solar Cells
Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces has an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques such as variable-angle spectroscopic ellipsometry and capacitance–voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in three additional donor/acceptor systems: PCPDTBT, P3HT, PCDTBT, and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene-rich interface at the cathode is a prerequisite to enhance contact selectivity and consequently power conversion efficiency
Molecular Electronic Coupling Controls Charge Recombination Kinetics in Organic Solar Cells of Low Bandgap Diketopyrrolopyrrole, Carbazole, and Thiophene Polymers
Low-bandgap
diketopyrrolopyrrole- and carbazole-based polymer bulk-heterojunction
solar cells exhibit much faster charge carrier recombination kinetics
than that encountered for less-recombining poly(3-hexylthiophene).
Solar cells comprising these polymers exhibit energy losses caused
by carrier recombination of approximately 100 mV, expressed as reduction
in open-circuit voltage, and consequently photovoltaic conversion
efficiency lowers in more than 20%. The analysis presented here unravels
the origin of that energy loss by connecting the limiting mechanism
governing recombination dynamics to the electronic coupling occurring
at the donor polymer and acceptor fullerene interfaces. Previous approaches
correlate carrier transport properties and recombination kinetics
by means of Langevin-like mechanisms. However, neither carrier mobility
nor polymer ionization energy helps understanding the variation of
the recombination coefficient among the studied polymers. In the framework
of the charge transfer Marcus theory, it is proposed that recombination
time scale is linked with charge transfer molecular mechanisms at
the polymer/fullerene interfaces. As expected for efficient organic
solar cells, small electronic coupling existing between donor polymers
and acceptor fullerene (<i>V</i><sub>if</sub> < 1 meV)
and large reorganization energy (λ ≈ 0.7 eV) are encountered.
Differences in the electronic coupling among polymer/fullerene blends
suffice to explain the slowest recombination exhibited by poly(3-hexylthiophene)-based
solar cells. Our approach reveals how to directly connect photovoltaic
parameters as open-circuit voltage to molecular properties of blended
materials
Nanoscale mapping by electron energy-loss spectroscopy reveals evolution of organic solar cell contact selectivity
How the Charge-Neutrality Level of Interface States Controls Energy Level Alignment in Cathode Contacts of Organic Bulk-Heterojunction Solar Cells
Electronic equilibration at the metal–organic interface, leading to equalization of the Fermi levels, is a key process in organic optoelectronic devices. How the energy levels are set across the interface determines carrier extraction at the contact and also limits the achievable open-circuit voltage under illumination. Here, we report an extensive investigation of the cathode energy equilibration of organic bulk-heterojunction solar cells. We show that the potential to balance the mismatch between the cathode metal and the organic layer Fermi levels is divided into two contributions: spatially extended band bending in the organic bulk and voltage drop at the interface dipole layer caused by a net charge transfer. We scan the operation of the cathode under a varied set of conditions, using metals of different work functions in the range of ∼2 eV, different fullerene acceptors, and several cathode interlayers. The measurements allow us to locate the charge-neutrality level within the interface density of sates and calculate the corresponding dipole layer strength. The dipole layer withstands a large part of the total Fermi level mismatch when the polymer:fullerene blend ratio approaches ∼1:1, producing the practical alignment between the metal Fermi level and the charge-neutrality level. Origin of the interface states is linked with fullerene reduced molecules covering the metal contact. The dipole contribution, and consequently the band bending, is highly sensitive to the nature and amount of fullerene molecules forming the interface density of states. Our analysis provides a detailed picture of the evolution of the <i>potentials</i> in the bulk and the interface of the solar cell when forward <i>voltage</i> is applied or when photogeneration takes place