Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

Abstract

Electron transfer and subsequent charge separation across donor-acceptor heterojunctions remain the most important areas of study in the field of third-generation photovoltaics. 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 mechanisms 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.

Unveiling Ultrafast Carrier Extraction in Highly Efficient 2D/3D Bilayer Perovskite Solar Cells

“…A subband positive absorption feature was also observed in the low photon energy region of 760 to 830 nm (Figure b), which can be attributed to the phenomena of electroabsorption (EA). EA is a Stark effect due to the local electric field induced by the correlated charge pair dipoles in the heterostructures. , The time-resolved TAS revealed that the lifetime of this short-lived EA signal at 810 nm is only 440 fs (Figure c); only this signal is short-lived compared to other features that persist on longer time scales. These charge pair dipoles are direct evidence of the enhancement of the built-in voltage due to the localized electric field inside the 2D/3D bilayer perovskite heterostructures. , A redshift is also noted in the PB peak due to the renormalization occurring due to EA phenomena following excitation of the 2D perovskite above its bandgap (Figure S5).…”

“…EA is a Stark effect due to the local electric field induced by the correlated charge pair dipoles in the heterostructures. , The time-resolved TAS revealed that the lifetime of this short-lived EA signal at 810 nm is only 440 fs (Figure c); only this signal is short-lived compared to other features that persist on longer time scales. These charge pair dipoles are direct evidence of the enhancement of the built-in voltage due to the localized electric field inside the 2D/3D bilayer perovskite heterostructures. , A redshift is also noted in the PB peak due to the renormalization occurring due to EA phenomena following excitation of the 2D perovskite above its bandgap (Figure S5). The PB feature displayed an enhancement, whereas the broad absorption below 650 nm and the EA above 750 indicated decay in the early time spectrum (Figure b).…”

Ultrafast photoinduced energy and charge transfer

“…61,62 It also plays a crucial role in charge separation in organic photovoltaics (Voss et al, DOI: 10.1039/ c8fd00210j). 63 ET at the molecular/solid interface is also crucial in the function of photocatalytic reactions, 12,64,65 molecular electronics, 66 photo-electrolysis, 67 and photography. 68,69 In many of these systems, the ET is extremely fast (few femtoseconds) [70][71][72][73] such that it cannot be described by a simple rate description, and quantum-chemical methods are needed, oen with time-domain modelling.…”

Quantum Battles in Attoscience -- Tunnelling

“…But with the advent of attosecond science [4] the question "Does tunnelling take time, and if yes, how much?" has gained a lot of new interest, since electron dynamics often include quantum tunnelling portions, be that in biological processes such as photosynthesis [5] or charge transport in semiconductors [6], tunnelling ionisation as the first step for high-order harmonic generation (HHG) spectroscopy [7], photoelectron holography [8], laser induced electron diffraction (LIED) [9] or many more.…”