Direct Spectroscopic Evidence of Ultrafast Electron Transfer from a Low Band Gap Polymer to CdSe Quantum Dots in Hybrid Photovoltaic Thin Films

Ultrafast transient absorption spectroscopy is used to study charge transfer dynamics in hybrid films composed of the low band gap polymer PCPDTBT and CdSe quantum dots capped with <i>tert</i>-butylthiol ligands. By selectively exciting the polymer, a spectral signature for electrons on the quantum dots appears on ultrafast time scales (≲ 65 fs), which indicates ultrafast electron transfer. From this time scale, the coupling between the polymer chains and the quantum dots is estimated to be <i>J</i> ≳ 17 meV. The reduced quantum dot acceptors exhibit an unambiguous spectral bleach signature, whose amplitude allows for the first direct calculation of the absolute electron transfer yield in a hybrid solar cell (82 ± 5%). We also show that a limitation of the hybrid system is rapid and measurable geminate recombination due to the small separation of the initial charge pair. The fast recombination is consistent with the internal quantum efficiency of the corresponding solar cell. We therefore have identified and quantified a main loss mechanism in this type of third generation solar cell

Chalcogenol Ligand Toolbox for CdSe Nanocrystals and Their Influence on Exciton Relaxation Pathways

We have employed a simple modular approach to install small chalcogenol ligands on the surface of CdSe nanocrystals. This versatile modification strategy provides access to thiol, selenol, and tellurol ligand sets <i>via</i> the <i>in situ</i> reduction of R₂E₂ (R = ^<i>t</i>Bu, Bn, Ph; E = S, Se, Te) by diphenylphosphine (Ph₂PH). The ligand exchange chemistry was analyzed by solution NMR spectroscopy, which reveals that reduction of the R₂E₂ precursors by Ph₂PH directly yields active chalcogenol ligands that subsequently bind to the surface of the CdSe nanocrystals. Thermogravimetric analysis, FT-IR spectroscopy, and energy dispersive X-ray spectroscopy provide further evidence for chalcogenol addition to the CdSe surface with a concomitant reduction in overall organic content from the displacement of native ligands. Time-resolved and low temperature photoluminescence measurements showed that all of the phenylchalcogenol ligands rapidly quench the photoluminescence by hole localization onto the ligand. Selenol and tellurol ligands exhibit a larger driving force for hole transfer than thiol ligands and therefore quench the photoluminescence more efficiently. The hole transfer process could lead to engineering long-lived, partially separated excited states

Quantifying Charge Recombination in Solar Cells Based on Donor–Acceptor P3HT Analogues

The creation of semi-random donor–acceptor analogues of poly­(3-hexylthiophene) (P3HT) yields polymers that exhibit pan-chromatic absorption spectra extending into the near-infrared. Despite this extended absorption however, different semi-random polymers exhibit markedly different photovoltaic performance when blended as a bulk-heterojunction with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). To understand the physical origin of these differences, we performed transient absorption (TA) measurements and device characterization of blends of two representative semi-random polymers, poly­(3-hexylthiophene-thiophene-thienopyrazine) (P3HTT-TP-10%) and poly­(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), with PCBM. Although both polymers absorb strongly throughout the visible and near-infrared, devices based on P3HTT-DPP-10%:PCBM exhibit a power conversion efficiency of ∼6%, while films consisting of P3HTT-TP-10%:PCBM blends display values under 1%. TA experiments reveal that polarons generated upon excitation of a P3HTT-TP-10%:PCBM blend undergo a high degree of geminate recombination (survival percentage, ϕ_S ∼45%) independent of excitation wavelength, explaining its lower efficiency. In contrast, P3HTT-DPP-10%:PCBM blends show excitation wavelength-dependent polaron recombination dynamics. While excitation of the polymer in the visible region leads to less geminate recombination (ϕ_S ∼65%) compared to P3HTT-TP-10%:PCBM, this loss process is ∼1.5 times more deleterious following near-infrared (NIR) excitation. Despite this observation, a significant fraction (ϕ_S ∼ 45%) of the charges formed following NIR excitation escape recombination, partly explaining the high performance of P3HTT-DPP-10%:PCBM devices

Controlling the Trap State Landscape of Colloidal CdSe Nanocrystals with Cadmium Halide Ligands

We developed a simple and robust colloidal route for the installation of CdX₂ (X = Cl, Br, I) ligands on the surface of CdSe nanocrystals, which effectively displace the native ligands and form stable suspensions. After colloidal ligand exchange, these nanocrystals can be easily solution cast into nanocrystal films. Photoelectrochemical measurements on solution-cast nanocrystal films reveal a striking influence of surface cadmium halide on photocurrent response, with mildly annealed, CdCl₂-treated CdSe nanocrystals showing the greatest enhancement in photocurrent to above band gap illumination. The strong dependence of photoresponse on surface halide is thought to result from ligand-induced changes in the electronic structure of the nanocrystal samples. We arrive at this conclusion using a combination of ultrafast transient absorption, time-resolved photoluminescence, and surface photovoltage spectroscopies, which are being applied together for the first time to investigate nanocrystal trap states. From these measurements, we establish a trend for ligand-related sub-band gap states that accounts for electron and hole trapping at the nanocrystal surface. The nature of the electron and hole traps in the nanocrystal films are dependent on the thermal history of the sample as well as the specific halide surface treatment employed. After subjecting the nanocrystal films to mild thermal annealing, we find evidence that suggests a drastic reduction in electron trap states. Additionally, depending on the surface halide treatment employed, the energy of the hole trap states varies, with CdCl₂ treatment resulting in energetically shallow hole trap states, and CdBr₂ and CdI₂ treatments leading to much deeper hole traps. Thus, judicious choice of cadmium halide surface treatment can be used to manipulate the trap state landscape of these ligand exchanged CdSe nanocrystals