Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems Jo € el Teuscher, 1,2 Jan C. Brauer, 3 Andrey Stepanov, 4 Alicia Solano, 2,5 Ariadni Boziki, 2,5 Majed Chergui, 2,6 Jean-Pierre Wolf, 4 Ursula Rothlisberger, 2,5 Natalie Banerji, 3 and Jacques-E. Moser 1,2,a) 1 Photochemical Dynamics Group, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland 2 Lausanne Centre for Ultrafast Science (LACUS), Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland 3 FemtoMat Group, Department of Chemistry, Universit e de Fribourg, 1700 Fribourg, Switzerland 4 GAP-Biophotonics Group, Department of Applied Physics, Universit e de Gene `ve, 1205 Geneva, Switzerland 5 Laboratory of Computational Chemistry and Biochemistry, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland 6 Laboratory of Ultrafast Spectroscopy, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland (Received 17 July 2017; accepted 13 November 2017; published online 19 December 2017) Electron transfer and subsequent charge separation across donor-acceptor heterojunc- tions remain the most important areas of study in the field of third-generation photo- voltaics. In this context, it is particularly important to unravel the dynamics of individual ultrafast processes (such as photoinduced electron transfer, carrier trapping and association, and energy transfer and relaxation), which prevail in materials and at their interfaces. In the frame of the National Center of Competence in Research “Molecular Ultrafast Science and Technology,” a research instrument of the Swiss National Science Foundation, several groups active in the field of ultrafast science in Switzerland have applied a number of complementary experimental techniques and computational simulation tools to scrutinize these critical photophysical phenomena. Structural, electronic, and transport properties of the materials and the detailed mech- anisms of photoinduced charge separation in dye-sensitized solar cells, conjugated polymer- and small molecule-based organic photovoltaics, and high-efficiency lead halide perovskite solar energy converters have been scrutinized. Results yielded more than thirty research articles, an overview of which is provided here. V C 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/ licenses/by/4.0/).[https://doi.org/10.1063/1.4996409] I. INTRODUCTION AND SCOPE Dye-sensitized mesoscopic solar cells (DSSCs), organic photovoltaic devices (OPV), and emerging hybrid organic-inorganic lead halide perovskite cells (PSCs) belong to a new genera- tion of photovoltaic solar energy converters based on cheap, solution-processable materials. Contrary to the first-generation Si and GaAs solar cells, these systems separate the functions of light absorption and carrier transport. Light harvesting is carried out by an active material, in which photon absorption generates a local, generally short-lived charge separation. Charge transfer (CT) across specific contacts with a donor material able to transport positive carriers, on the one side, and an acceptor material constituting an electron transmitting medium, on the a) E-mail: je.moser@epfl.ch 2329-7778/2017/4(6)/061503/27 V C Author(s) 2017. 4, 061503-1 STRUCTURAL DYNAMICS 4, 061503 (2017)
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Charge separation and carrier dynamics in donor-acceptorheterojunction photovoltaic systems
Jo€el Teuscher,1,2 Jan C. Brauer,3 Andrey Stepanov,4 Alicia Solano,2,5
Ursula Rothlisberger,2,5 Natalie Banerji,3 and Jacques-E. Moser1,2,a)
1Photochemical Dynamics Group, Institute of Chemical Sciences and Engineering,�Ecole Polytechnique F�ed�erale de Lausanne, 1015 Lausanne, Switzerland2Lausanne Centre for Ultrafast Science (LACUS), �Ecole PolytechniqueF�ed�erale de Lausanne, 1015 Lausanne, Switzerland3FemtoMat Group, Department of Chemistry, Universit�e de Fribourg, 1700 Fribourg,Switzerland4GAP-Biophotonics Group, Department of Applied Physics, Universit�e de Geneve,1205 Geneva, Switzerland5Laboratory of Computational Chemistry and Biochemistry, Institute of Chemical Sciencesand Engineering, �Ecole Polytechnique F�ed�erale de Lausanne, 1015 Lausanne, Switzerland6Laboratory of Ultrafast Spectroscopy, Institute of Chemical Sciences and Engineering,�Ecole Polytechnique F�ed�erale de Lausanne, 1015 Lausanne, Switzerland
(Received 17 July 2017; accepted 13 November 2017; published online 19 December 2017)
Electron transfer and subsequent charge separation across donor-acceptor heterojunc-
tions remain the most important areas of study in the field of third-generation photo-
voltaics. In this context, it is particularly important to unravel the dynamics of
individual ultrafast processes (such as photoinduced electron transfer, carrier trapping
and association, and energy transfer and relaxation), which prevail in materials and
at their interfaces. In the frame of the National Center of Competence in Research
“Molecular Ultrafast Science and Technology,” a research instrument of the Swiss
National Science Foundation, several groups active in the field of ultrafast science in
Switzerland have applied a number of complementary experimental techniques and
computational simulation tools to scrutinize these critical photophysical phenomena.
Structural, electronic, and transport properties of the materials and the detailed mech-
anisms of photoinduced charge separation in dye-sensitized solar cells, conjugated
polymer- and small molecule-based organic photovoltaics, and high-efficiency lead
halide perovskite solar energy converters have been scrutinized. Results yielded
more than thirty research articles, an overview of which is provided here. VC 2017Author(s). All article content, except where otherwise noted, is licensed under aCreative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). [https://doi.org/10.1063/1.4996409]
I. INTRODUCTION AND SCOPE
Dye-sensitized mesoscopic solar cells (DSSCs), organic photovoltaic devices (OPV), and
emerging hybrid organic-inorganic lead halide perovskite cells (PSCs) belong to a new genera-
tion of photovoltaic solar energy converters based on cheap, solution-processable materials.
Contrary to the first-generation Si and GaAs solar cells, these systems separate the functions of
light absorption and carrier transport. Light harvesting is carried out by an active material, in
which photon absorption generates a local, generally short-lived charge separation. Charge
transfer (CT) across specific contacts with a donor material able to transport positive carriers,
on the one side, and an acceptor material constituting an electron transmitting medium, on the
FIG. 11. Comparison of NIR transient absorption (TA) and time-resolved terahertz (THz) kinetics for neat MAPbI3 show-
ing that THz mobility remains constant for at least 1 nanosecond.
061503-17 Teuscher et al. Struct. Dyn. 4, 061503 (2017)
of a decreasing carrier concentration. At the lowest excitation intensity (2.0� 1012 photons
cm�2 per pulse) where the charge carrier mobility is found to be highest, i.e., 25 cm2 V�1 s�1,
no decay is observed for the first 200 ps. This remarkably high THz mobility in solution-
processed perovskite is consistent with the mobility recently obtained by Wehrenfennig et al.107
THz photoconductivity spectra were also measured 10 ps after photoexcitation. Within
experimental error, both the amplitude and shape of the spectra of neat MAPbI3 and MAPbI3 jAl2O3 are identical. However, for MAPbI3 j TiO2, the spectral shape is qualitatively different,
and the signal amplitude is approximately four times lower than for MAPbI3 and MAPbI3 jAl2O3 [as also shown in the THz signals of Fig. 10(a), inset].
From the above observations, a number of conclusions can be drawn: First, the identical
THz spectra and photoconductivity kinetics of neat MAPbI3 and MAPbI3 j Al2O3 show that the
presence of Al2O3 does not alter the dynamics and mobility of the charges in the perovskite, on
the distance scale probed by the THz measurements (<100 nm). Second, the presence of TiO2
nanoparticles accelerates the formation of charge carriers, which leads to efficient electron
injection in <1 ps, due to favorable band energy alignment of TiO2 and the perovskite.
However, due to the low intrinsic mobility of conduction band electrons in TiO2, injection leads
to unbalanced transport of charges, lowering the overall mobility.
Unbalanced electron and hole mobilities, which differ by orders of magnitude in bulk het-
erojunction solar cells, result in space charge-limited photocurrents lowering the power conver-
sion efficiency.108 Therefore, it is important to assess the mobilities of both electrons and holes.
From the THz measurements of porous TiO2films, it was shown that its intrinsic electron
mobility is �1 cm2 V�1 s�1. In this case, the electron mobility of TiO2 in MAPbI3 j TiO2
should be the same, i.e., the THz response is mainly due to the holes in the perovskite phase
having a mobility of close to 7.5 cm2 V�1 s�1. Consequently, from the measured THz mobility
of �20 cm2 V�1 s�1 for MAPbI3 and MAPbI3 j Al2O3, we can conclude that the electron mobil-
ity in the perovskite phase is �12.5 cm2 V�1 s�1. The 12.5/7.5–2 ratio of electron and hole
mobilities in the perovskite phase is in agreement with the recent theoretical calculations of the
relative effective masses of electrons and holes.109 The finding that electron and hole mobility
is almost balanced is a key information for the understanding of why pristine perovskite or
perovskite j Al2O3 solar cells are so efficient.
Very slow microsecond time scale recombination at ambient solar intensities, assessed by
additional time-resolved microwave conductivity measurements, in combination with high and
almost equal electron and hole mobilities guarantees very efficient charge collection and thus
high solar cell efficiency. The results also show that, as a consequence of electron injection
from the perovskite to the TiO2 with very low electron mobility, the overall mobility is low-
ered. We note also that the lower Fermi level of TiO2 decreases the open circuit voltage leading
to lower overall efficiency. A possible improvement of solar cell performance would be to engi-
neer the active materials such that both electron and hole mobilities are on the level of the elec-
tron mobility in the perovskite.
Picosecond X-ray absorption spectroscopy was used to investigate the fate of charge car-
riers in Cs-based inorganic perovskites nanoparticles with atomic selectivity. A resolution of
80 ps was used and it was found that by then, hole forms small polarons at Br atoms, while
electrons remain delocalized in the conduction band. Finally, no effect whatsoever was noted
on the Cs atoms, in line with theoretical predictions.110
C. Charge transport and transfer through D-A heterojunctions
Methylammonium lead tribromide MAPbBr3 perovskite nanoparticles suspensions in chlo-
robenzene contain various nanostructures: quasi-2D nanoplatelets of variable thickness and 3D
bulk-like nanoparticles. These structures exhibit several optical signatures that were previously
reported and assigned. The nanoplatelets are blue-shifted compared with the bulk perovskite,
due to a significant confinement regime. Using a combination of steady-state, excitation-depen-
dent ultrafast transient absorbance and time-correlated single photon counting (TCSPC) meas-
urements, Bouduban et al. unraveled the presence of significant inter-structures interactions in
061503-18 Teuscher et al. Struct. Dyn. 4, 061503 (2017)
the form of a cascade of energy and charge transfer, the latter being mediated by the formation
of inter-particle charge transfer states.111 Upon photoexcitation, localized excitons are formed
within one nanostructure. They either rapidly recombine, yielding a short-lived emission on the
picosecond timescale, or turn into charge transfer excitons following the injection of one type
of carrier into a narrower band-gap, neighboring nanostructure (Fig. 12). These charge transfer
excitons possess a permanent dipole and submit the material at close proximity to an electric
field, which produces a significant photoinduced electroabsorption contribution to the transient
absorption spectra. Carrier pairs contained in CTE eventually recombine, resulting in the long-
lived, microsecond emission observed by TCSPC.
Similar cascade charge transfer processes in highly efficient light-emitting diode (LEDs)
and photovoltaic devices are likely to occur in materials characterized by a multigrain morphol-
ogy. As much as long-distance radiative energy transfer within the active film of a perovskite
solar cell (photon recycling), non-radiative energy transfer and inter-domain charge transfer
mediated by interfacial CT states could play an important role in slowing down the recombina-
tion of photocarriers and increasing their diffusion length.
The time-resolved electroabsorption spectroscopy (TREAS) technique was successfully
applied for the first time to a methylammonium lead triiodide perovskite multigrain film. The
active material was prepared by vapor deposition and appeared to be polycrystalline with an
average grain size of 40 nm. MAPbI3 subjected to an externally applied electric field on the
order of 10 MV m�1 displayed a blue shift of its excitonic absorption edge at 780 nm, corre-
sponding to a quadratic electroabsorption response compatible with both Stark and Franz-
Keldysh-Aspnes models. The electroabsorption signal was exploited to probe optically the time
evolution of the local electric field experienced by the perovskite.112
Upon band-gap irradiation, electron–hole pairs were formed. Their initial spatial separation
was observed from the differential electroabsorption signal dynamics to take place with a time
constant of 0.94 6 0.1 ps, until charges were trapped at grain boundaries (Fig. 13). An average
intra-grain dc mobility of the carriers of l6¼ 23 6 4 cm2 V�1 s�1 was extracted from this
result, in good agreement with terahertz spectroscopy measurements. A second charge separa-
tion step was observed optically with a time constant of 24 6 4 ps. This kinetic component was
assigned to the detrapping of carriers and their migration to the opposite insulated film surfaces,
where they accumulated, producing a Burstein-Moss blue shift of the absorption spectrum of
the MAPbI3 material. A value of the mobility, limited by trapping-detrapping processes at grain
FIG. 12. Cartoon illustrating the energy- and charge transfer processes occurring between the various nanostructures consti-
tuting MAPbBr3 perovskite colloidal aggregates. Left: Energy- and/or charge transfer cascade (curved blue arrows)
between q-2D nanoplatelets of increasing thicknesses and eventually a 3D bulk-like nanoparticle. Right: Energetic scheme
of some examples of photophysical processes taking place in a nanoparticle aggregate: Upon photoexcitation of a thin q-
2D (m¼ 3) nanoplatelet, interfacial hole transfer can take place to the adjacent particle (process I). Electrostatic interaction
of the hole with an electron remaining on the other side of the interface yields a CT exciton (green ellipse). Subsequent
electron transfer (process II) leads to the excitation of the m¼ 4 q-2D nanoplatelet. Energy transfer to a neighbouring nano-
structure characterized by a narrower bandgap is then possible (process III). Interfacial electron transfer (process IV) finally
enables the formation of a new interfacial CT excitonic species.
061503-19 Teuscher et al. Struct. Dyn. 4, 061503 (2017)
boundaries, of ln¼ 5.5 6 1 cm2 V�1 s�1 was estimated for electrons drifting across the entire
film thickness. Importantly, charge recombination was observed to be entirely suppressed
between field-separated carriers generated at initial densities of n0� 2� 1016 cm�3.
The TREAS technique also proved quite powerful in characterizing the kinetics of the
charge transmission between the perovskite absorber material and the carrier-extracting layers
in fully operational photovoltaic devices. In particular, electron accumulation at the junction
between the vapor-deposited MAPbI3 film and a mesoporous nanocrystalline TiO2 layer was
observed before the charge extraction could take place at the subnanosecond time scale.112
Paraecattil et al. carried out ultrafast TREAS on mixed halide, mixed cation perovskite
solar cells.101,113 These measurements present the first application of this technique to the investi-
gation of complete functional perovskite solar cells. A quadratic electroabsorption response was
observed in perovskite solar cells submitted to externally applied voltages (reverse bias) as low
as 1 V [Fig. 14(a)]. It allowed the investigation of the electric field screening dynamics by photo-
generated charge carriers with a monochromatic pump pulse at a low excitation fluence of 0.1 lJ
cm�2 (carrier density 3� 1015 cm�3). Experimental conditions resembled the electric field and
optical excitation densities experienced by the device under operating conditions with sunlight.
The screening dynamics of the electroabsorption signal were related to electron and hole drift to
the perovskite j acceptor and perovskite j HTM interfaces, respectively [Fig. 14(b)]. The insights
gained from measurements allowed to calculate carrier mobilities of 3 6 1 cm2 V�1 s�1 for holes
and 21.9 6 5 cm2 V�1 s�1 for electrons in state-of-the-art perovskite solar cells. No significant
difference in carrier dynamics was observed between substituting mesoporous TiO2 with SnO2 as
the electron-accepting layer in devices. The TREAS technique measures electron and hole trans-
port to the acceptor interface and not carrier injection from the perovskite into the acceptor. For
both architectures, the observed signal was dominated by carrier transport across the bulk perov-
skite layer, which is comparable in the two devices. Transient absorption measurements on the
perovskite devices revealed remarkably similar TA spectra at 500 fs after photo-excitation than
with steady-state EA spectra of the same devices. Observations suggest that photoexcitation
FIG. 13. Time-evolution of electro-modulated differential absorption (EDA) spectra of insulated MAPbI3 films excited at
k¼ 545 nm and submitted to an external electric field E0¼ 1.7� 105 V cm�1. Inset: Time-dependence of the differential
absorbance change recorded in the same conditions at kprobe¼ 762 nm.
061503-20 Teuscher et al. Struct. Dyn. 4, 061503 (2017)
results in a transient photoinduced Stark shift of the perovskite ground state absorption spectrum,
due to the electric field generated between electrons and holes and warrants further investigation.
The fundamental understanding of the working principle of perovskite-based solar cells
requires understanding, on the one hand, the charge generation inside the perovskite as well as
the charge transfer at the electron and hole extracting interfaces to optimize the device perfor-
mance. Brauer et al. showed that upon band-gap resonant excitation, the formation of free
charges within the MAPbI3 perovskite layer occurs via an excitonic state that dissociates into
free charges in approximately 200 fs.114 This fast dissociation is consistent with previously pub-
lished exciton binding energies of a few meV. When excess energy is provided during excitation,
free charges are formed directly and cool down to the band edge in a few hundreds of picosec-
onds while the excitonic signature is not obeserved.114 Following the excitation of perovskite, the
FIG. 14. (a) Electro-absorption spectra of the perovskite film contained in a complete solar cell upon application of increas-
ing voltages between both electrodes. Inset shows the quadratic dependence of the differential absorbance signal upon the
applied external field intensity and direction. (b) Time-dependence of the EA differential absorbance change recorded upon
application of an external electric field E0¼ 4.9� 104 V cm�1 (U0¼ 3.5 V), E0¼ 3.5� 104 V cm�1 (U0¼ 2.5 V), and
E0¼ 1.4� 104 V cm�1 (U0¼ 1.0 V) and probed at the peak of the electroabsorption signal at k¼ 758 nm, energy fluence
¼ 0.1 lJ cm�2.
061503-21 Teuscher et al. Struct. Dyn. 4, 061503 (2017)
hole transfer from photo-excited MAPbI3 to spiro-MeOTAD was investigated. It has been shown
that the hole transfer is essentially ultrafast occurring on a sub-80 fs time scale.114 This timescale
is faster than carrier thermalization and extraction of hot holes has been observed. Changing
from the small molecular hole transporter spiro-MeOTAD to polymeric hole transporter like
PTAA, P3HT, and PCPDTBT (Fig. 15) hole injection from the photo-excited perovskite is
slowed down and the hole transfer occurs on a time scale of a few nano-seconds from thermal-
ized states and is independent of excess excitation energy (Fig. 16).115 Consequently, it has been
shown that energy transfer from the photoexcited P3HT to MAPbI3 can occur, provided the exci-
tation in the P3HT is in close proximity to the P3HT j MAPbI3 interface. In this way, P3HT can
be used to sensitize MAPbI3 and excitations in the HTM are not lost for photocurrent genera-
tion.115 The differences in hole transfer rates between small molecular HTMs as spiro-MeOTAD
and polymeric HTMs as P3HT might be partially explained by poor physical contact in the case
of the polymers, given that the thermodynamic driving force for hole transfer to all investigated
HTMs is rather similar. Furthermore, the ultrafast hole transfer in spiro-MeOTAD suggests an
extraordinarily good coupling of its LUMO to the perovskite valence band.
VI. PERSPECTIVE AND FUTURE DIRECTIONS
Several ultrafast spectroscopy techniques have been developed with the aim of providing
the necessary tools to scrutinize charge carrier dynamics at donor-acceptor heterojunctions that
are central to third-generation photovoltaic and LED devices. Results obtained so far have been
quite successful but call for a deeper and more fundamental investigation of the dynamics of
free- and bound photo-generated carriers, particularly in OPV and lead halide perovskite
materials.
One of the outstanding issues regarding the charge carrier dynamics in photovoltaic materi-
als is the clear identification of the fundamental excitations at and below the band gap.37 The
foreseen approach by the Chergui group to address them will be to combine resonant inelastic
X-ray scattering (RIXS) studies with angle-resolved photoelectron spectroscopy (ARPES) and
ultrafast 2D UV spectroscopy. This unique combination of tools will also be used to investigate
the charge carrier dynamics, in particular that of holes, which have so far escaped observation.
As far as perovskites are concerned,110 of great interest is the understanding of spin-orbit cou-
pling and the critical role of local inversion-symmetry breaking fields, which leads to phenomena
such as Rashba splitting and possible spin polarization for future applications. Understanding the
FIG. 15. Simplified band alignment diagram for ETM j MAPbI3 j HTM double donor-acceptor heterojunctions. Curved
arrows represent the hole transfer processes taking place from the valence band maximum of the perovskite absorber to the
various investigated hole-transport materials.
061503-22 Teuscher et al. Struct. Dyn. 4, 061503 (2017)
interaction of molecules with the surface of materials is another subject of crucial importance
for energy-related applications. Time-resolved 2D UV and surface-selective ultrafast ARPES
will allow for probing adsorbates at the TiO2 (110) surface and perform orbital tomography
on their LUMOs, which represents the doorway to either electron injection or photocatalytic
processes.
The exact mechanism by which electrons and holes overcome trapping and fast recombina-
tion to yield free carriers in materials at the base of OPV and perovskite devices is still
debated.14 Increasing evidence points to the critical role of large polarons and hot charge trans-
fer excitons in assisting this process. The precise properties of incoherent excitonic and polar-
onic species, as well as trapped carrier populations will be the focus of future research efforts
of the Moser group.
Conventional experimental techniques, such as ultrafast transient absorption and broadband
fluorescence up-conversion, will be supplemented by direct quasi-particle probing using a com-
bination of time-resolved electroabsorption and ultra-broadband time-resolved terahertz spec-
troscopies. TREAS will be used to investigate the electron and hole drift mobilities in organic
photovoltaic devices and perovskite solar cells and provide the necessary insight into charge
trapping and detrapping processes and the possible implication of CTEs at grain bound-
aries.51,112 TRTS will allow to distinguish between free carriers and excitonic species involved
in the various recombination pathways. This technique will also enable the identification of
phonons involved in indirect transitions and in the formation of polarons through the direct
monitoring of the time-evolution of their resonant bands.
Pump-push-probe and pump-dump-probe spectroscopy techniques have been used to inves-
tigate the properties of excitons and charge-transfer states of conjugated polymers.116–118 A
real-time view of bound charge states formation and relaxation will be provided for dye-
sensitized, small molecule-based OPV, and perovskite solar cells systems using a comparable
approach. A time-resolved optical pump-IR push-terahertz probe spectroscopy (PPTPS) scheme
FIG. 16. (a) Transient absorption spectra of mesoporous TiO2 jMAPbI3 and mesoporous-TiO2 jMAPbI3 j spiro-MeOTAD
systems 1ps after excitation, as well as the corresponding difference spectrum. (b) Dynamics at 620 nm for mesoporous
TiO2 jMAPbI3 and mesoporous TiO2 jMAPbI3 j P3HT.
061503-23 Teuscher et al. Struct. Dyn. 4, 061503 (2017)
will be employed to scrutinize excitons, CTE, polaron, and trapped carrier dynamics in various
systems and conditions and monitor directly the time evolution of their binding energy.
Time-resolved terahertz spectroscopy provides key insights into quantum confinement phe-
nomena and carrier transport in nanomaterials. Conventional THz spectroscopy, however, is
restricted by the diffraction limit to measurements of the dielectric functions averaged over the
size, structure, orientation, and density of nanoparticles, crystal grains, or nanodomains constitut-
ing the material under study. Ultrafast THz nanoscopy combines femtosecond optical excitation
with sub-cycle broadband terahertz probing and scattering-type near-field scanning optical
microscopy (s-NSOM). This technique was recently shown to allow for imaging the complex
dielectric function of nanostructured materials in three dimensions (x, y, t) with 10 nm spatial-
and sub-100 fs time-resolution.119 A similar experimental approach will enable the characteriza-
tion of carrier dynamics in OPV systems and in hybrid perovskite thin film solar cells and
quantum-confined nanostructures for LED and laser applications. Important scientific challenges
will be to map the carrier density across the active film thickness and at heterojunctions, to deter-
mine local doping and space charge layers, to image the spatial distribution of trap states and
recombination centers and to monitor the ultrafast time evolution of these various properties.
ACKNOWLEDGMENTS
Financial support by the Swiss National Science Foundation (SNSF) and the National Center of
Competence in Research “Molecular Ultrafast Science and Technology” (NCCR MUST), a
research instrument of the SNSF, is gratefully acknowledged.
NOMENCLATURE
AFM Atomic-force microscopy
AM1.5 Solar irradiance at mid-latitudes, where the sunlight has to pass through 1.5
atmosphere thickness, corresponding to a solar zenith angle z ¼ 48.2
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