Ultrafast Charge Carrier Dynamics in Extremely Thin Absorber (ETA) Solar Cells Consisting of CdSe-Coated ZnO Nanowires

Abstract

The extremely thin absorber (ETA) solar cell architecture can enable higher efficiencies than planar cells for absorbers that have low carrier lifetimes or mobilities. Efficient charge separation requires that interfacial electron and hole transfer proceed much faster than the recombination lifetime of photoexcited carriers. In this work, transient absorption spectroscopy was employed to measure these ultrafast photophysical processes in CdSe-coated ZnO nanowire ETA cells and model planar films. At low pump fluences, carrier lifetime was controlled by Shockley–Read–Hall and surface recombination. Annealing the electrodeposited CdSe films increased the lifetime 50-fold, to >1 ns as measured by transient absorption spectroscopy, which correlated to improved ETA cell performance. Interfacial electron transfer from the CdSe coating into the ZnO nanowires occurred 3 orders of magnitude faster, within 1 ps, independent of the presence or absence of an interfacial CdS buffer layer. Interfacial hole transfer to the ferri­(o)­cyanide redox couple could not be directly measured, but photodegradation of the semiconductor surface in the presence of electrolyte under prolonged light exposure resulted in faster dynamic response due to higher densities of surface electron trap states. This study provides a framework for understanding photophysics and improving performance of nanostructured, semiconductor-sensitized solar cells through a combination of device measurements and ultrafast probes.

Electrochemical Deposition of Conformal Semiconductor Layers in Nanoporous Oxides for Sensitized Photoelectrodes

“…This model can also explain the conformal coating of CdSe on the nanoporous ZnO film reported by other groups. 15,25 Indeed, ZnO and TiO 2 are known to have very similar conduction band levels, as well as valence band positions and band gap values, 26 which results in similar band alignments with CdSe.…”

“…However, for the traditional SSC structure, a conformal coating of the inorganic sensitizer is required to ensure the open pore structure. A couple of papers , reported the electrochemical deposition of the conformal CdSe or CdTe layer on the surface of ZnO nanowire arrays having average diameters/pores of hundred nanometers where the pores are interconnected to each other. On the other hand, there has been no report on the electrochemical deposition of conformal semiconductor layers on the electrically conducting nanoporous network with isolated pores less than 100 nm.…”

Dynamics of Photoexcited Carriers in Polycrystalline PbS and at PbS/ZnO Heterojunctions: Influence of Grain Boundaries and Interfaces

“…With energy in meV range, THz pulses are uniquely suited to probe free carrier absorption, and their short duration allows the investigation of dynamics of photoexcited carriers with sub-picosecond time resolution. − Polycrystalline PbS films on quartz and on ZnO were excited by ∼100 fs duration, 800 nm pulses from a regeneratively amplified 1 kHz repetition rate Ti:sapphire laser at normal incidence as illustrated in Figure a. The sample was placed behind a 1.5 mm aperture in the center of a ∼3 mm excitation spot to ensure uniform illumination.…”

Critical Coupling of Visible Light Extends Hot-Electron Lifetimes for H2O2 Synthesis

“…The Al film provides excellent visible light reflection at significantly lower material costs than other common MIM mirrors, such as Auas of this writing, the cost of Al is ∼25 000 times cheaper than that of Au. , With our device, we measured a broad 7.4× increase in absorption across the visible spectrum (400–650 nm) through the implementation of the Al back reflector, with a 5-fold enhancement at the absorption maximum (530 nm) attributed to the localized surface plasmon resonance (LSPR) of the Au nanostructures. Furthermore, pump–probe spectroscopy revealed the ultrafast dynamics of charge carrier generation and electron transfer. , We measured the accumulation of electrons in the ZnO conduction band upon visible (450 nm < λ < 700 nm) photoexcitation of Au and showed a nearly 5-fold improvement in the injection efficiency in Al-backed devices compared to those without Al. We also observed a significant extension of the electron–phonon coupling time in Al-backed samples over a range of pump fluences, suggesting that some of the Al-induced improvement in the hot-electron collection may be attributed to higher effective electron temperatures and slower thermalization.…”