This study experimentally evaluates the long-term impact of an early childhood psychosocial
stimulation intervention on earnings in a low-income country. Twenty years after the intervention was conducted, we find that the earnings
of the stimulation group are 25% higher than
those of the control group and caught up to the
earnings of a nonstunted comparison group.
These findings show that a simple psychosocial
stimulation intervention in early childhood for
disadvantaged children can have a substantial
effect on labor market outcomes and can compensate for developmental delays. The estimated
impacts are substantially larger than the impacts
reported for the U.S.-based interventions, suggesting that ECD interventions may be an especially effective strategy for improving long-term
outcomes of disadvantaged children in developing countries.
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We gratefully acknowledge research support from the World Bank
Strategic Impact Evaluation Fund; the American Bar Foundation;
The Pritzker Children’s Initiative; grants R37HD065072 and
R01HD54702 from the Eunice Kennedy Shriver National Institute of
Child Health and Human Development; the Human Capital and
Economic Opportunity Global Working Group—an initiative of the
Becker Friedman Institute for Research in Economics funded by
the Institute for New Economic Thinking (INET); a European
Research Council grant hosted by University College Dublin;
DEVHEALTH 269874; and an anonymous funder. We have
benefited from comments of participants in seminars at the
University of Chicago; University of California, Berkeley;
Massachusetts Institute of Technology; the 2011 LACEA Meetings
in Santiago, Chile; and the 2013 AEA Meetings. We thank the
study participants for their continued cooperation and willingness
to participate, and S. Pellington for conducting the interviews.
The authors have not received any compensation for the research
nor do they have any financial stake in the analyses reported
here. Replication data for this article have been deposited at
Interuniversity Consortium for Political and Social Research
(ICPSR) and can be accessed at http://doi.org/10.3886/E2402V1.
Materials and Methods
Figs. S1 and S2
Tables S1 to S17
22 January 2014; accepted 6 May 2014
Coherent ultrafast charge transfer
in an organic photovoltaic blend
Sarah Maria Falke,1,2 Carlo Andrea Rozzi,3 Daniele Brida,4,5 Margherita Maiuri,4
Michele Amato,6 Ephraim Sommer,1,2 Antonietta De Sio,1,2 Angel Rubio,7,8
Giulio Cerullo,4 Elisa Molinari,3,9† Christoph Lienau1,2†
Blends of conjugated polymers and fullerene derivatives are prototype systems for organic
photovoltaic devices. The primary charge-generation mechanism involves a light-induced
ultrafast electron transfer from the light-absorbing and electron-donating polymer to
the fullerene electron acceptor. Here, we elucidate the initial quantum dynamics of this
process. Experimentally, we observed coherent vibrational motion of the fullerene moiety
after impulsive optical excitation of the polymer donor. Comparison with first-principle
theoretical simulations evidences coherent electron transfer between donor and acceptor
and oscillations of the transferred charge with a 25-femtosecond period matching that
of the observed vibrational modes. Our results show that coherent vibronic coupling
between electronic and nuclear degrees of freedom is of key importance in triggering
charge delocalization and transfer in a noncovalently bound reference system.
The currently accepted model for the basic working principle of a bulk-heterojunction organic solar cell (1, 2), comprising a con- jugated polymer donor and an electron ac- ceptor material, relies on four elementary
steps: (i) photon absorption, creating a spatially
localized, Coulomb-bound electron-hole pair (
exciton) in the donor phase; (ii) exciton diffusion to
the donor/acceptor interface; (iii) exciton dissociation at the interface leading to the formation
of a charge-separated state (3, 4), often called
charge-transfer exciton or polaron pair; and (iv)
dissociation of the polaron pair into free charges
and their transport to the electrodes.
In this work, we focused on the dynamics of
the primary light-induced steps, (i) and (iii),
which lead to a charge-separated state in organic
photovoltaic (OPV) materials and represent
the key process in OPV cells. Over the past years,
charge photogeneration has been investigated in
several technologically relevant materials, such
as blends of polyphenylene-vinylene (5, 6), poly-
thiophene (7, 8), or low band gap polymers (9, 10)
with fullerene derivatives. In all of these systems,
it is now accepted that charge separation is an
ultrafast process occurring on a sub-100-fs time
scale. So far the experimental studies on charge
photogeneration in OPV materials have mainly
been described within the framework of an in-
coherent transfer model (11, 12), giving a rate
constant for the transfer process. These rate con-
stants may be enhanced by hot exciton dissociation
RESEARCH | REPORTS