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Electron relaxation in the CdSe quantum dot - ZnO composite: prospects for photovoltaic applications

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ABSTRACT

Quantum dot (QD)-metal oxide composite forms a “heart” of the QD-sensitized solar cells. It maintains light absorption and electron-hole separation in the system and has been therefore extensively studied. The interest is largely driven by a vision of harvesting the hot carrier energy before it is lost via relaxation. Despite of importance of the process, very little is known about the carrier relaxation in the QD-metal oxide composites. In order to fill this gap of knowledge we carry out a systematic study of initial electron dynamics in different CdSe QD systems. Our data reveal that QD attachment to ZnO induces a speeding-up of transient absorption onset. Detailed analysis of the onset proves that the changes are caused by an additional relaxation channel dependent on the identity of the QD-ZnO linker molecule. The faster relaxation represents an important factor for hot carrier energy harvesting, whose efficiency can be influenced by almost 50%.

No MeSH data available.


Transient absorption signal fitting showing two distinct spectral regions of onset rates.(a) Dynamics of TA onset for QDs in solution at various probe wavelengths with 3.18 eV (390 nm) excitation. Each kinetics (symbols) is fitted by an exponential rise convoluted with a response function (thick lines) – see text for details. Response-function-limited rise (dashed line) is added for comparison. (b) Onset rates resulting from the fits (symbols), compared to absorption spectrum (solid black line), PL spectrum (solid blue line) and TA spectrum at 2 ps delay (gray dashed line).
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f3: Transient absorption signal fitting showing two distinct spectral regions of onset rates.(a) Dynamics of TA onset for QDs in solution at various probe wavelengths with 3.18 eV (390 nm) excitation. Each kinetics (symbols) is fitted by an exponential rise convoluted with a response function (thick lines) – see text for details. Response-function-limited rise (dashed line) is added for comparison. (b) Onset rates resulting from the fits (symbols), compared to absorption spectrum (solid black line), PL spectrum (solid blue line) and TA spectrum at 2 ps delay (gray dashed line).

Mentions: We will now focus closely on the initial TA signal. As we discussed before, TA dynamics of CdSe QDs consist of two dominating contributions: bleach of absorption due to state filling and shift of absorption due to the so-called biexciton shift. Whereas the bleach is present only after the electron relaxes into the lowest state, the biexcitonic shift is an immediate response of the QD. To take into account both components each curve was fitted by an exponential rise (rate kR) combined with a consequent decay (rate kD), which account for the dominating bleach. The second immediate component is included without any exponential rise: Abl and Aim denote the amplitudes of the dominant bleach and immediate component, respectively. The function in equation (1) was convoluted with Gaussian response function , where the response distribution width (FWHM 120–160 fs) was set from measurements of Kerr effect in a thin quartz plate. The fit is an approximation assuming that a single relaxation time can describe the electron relaxation. We demonstrate in SI that this approximation can be used for our system. This approach is also supported by the fact that we obtained a very good agreement between the onset data and the fit (see Figure 3a).


Electron relaxation in the CdSe quantum dot - ZnO composite: prospects for photovoltaic applications
Transient absorption signal fitting showing two distinct spectral regions of onset rates.(a) Dynamics of TA onset for QDs in solution at various probe wavelengths with 3.18 eV (390 nm) excitation. Each kinetics (symbols) is fitted by an exponential rise convoluted with a response function (thick lines) – see text for details. Response-function-limited rise (dashed line) is added for comparison. (b) Onset rates resulting from the fits (symbols), compared to absorption spectrum (solid black line), PL spectrum (solid blue line) and TA spectrum at 2 ps delay (gray dashed line).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5384232&req=5

f3: Transient absorption signal fitting showing two distinct spectral regions of onset rates.(a) Dynamics of TA onset for QDs in solution at various probe wavelengths with 3.18 eV (390 nm) excitation. Each kinetics (symbols) is fitted by an exponential rise convoluted with a response function (thick lines) – see text for details. Response-function-limited rise (dashed line) is added for comparison. (b) Onset rates resulting from the fits (symbols), compared to absorption spectrum (solid black line), PL spectrum (solid blue line) and TA spectrum at 2 ps delay (gray dashed line).
Mentions: We will now focus closely on the initial TA signal. As we discussed before, TA dynamics of CdSe QDs consist of two dominating contributions: bleach of absorption due to state filling and shift of absorption due to the so-called biexciton shift. Whereas the bleach is present only after the electron relaxes into the lowest state, the biexcitonic shift is an immediate response of the QD. To take into account both components each curve was fitted by an exponential rise (rate kR) combined with a consequent decay (rate kD), which account for the dominating bleach. The second immediate component is included without any exponential rise: Abl and Aim denote the amplitudes of the dominant bleach and immediate component, respectively. The function in equation (1) was convoluted with Gaussian response function , where the response distribution width (FWHM 120–160 fs) was set from measurements of Kerr effect in a thin quartz plate. The fit is an approximation assuming that a single relaxation time can describe the electron relaxation. We demonstrate in SI that this approximation can be used for our system. This approach is also supported by the fact that we obtained a very good agreement between the onset data and the fit (see Figure 3a).

View Article: PubMed Central - PubMed

ABSTRACT

Quantum dot (QD)-metal oxide composite forms a “heart” of the QD-sensitized solar cells. It maintains light absorption and electron-hole separation in the system and has been therefore extensively studied. The interest is largely driven by a vision of harvesting the hot carrier energy before it is lost via relaxation. Despite of importance of the process, very little is known about the carrier relaxation in the QD-metal oxide composites. In order to fill this gap of knowledge we carry out a systematic study of initial electron dynamics in different CdSe QD systems. Our data reveal that QD attachment to ZnO induces a speeding-up of transient absorption onset. Detailed analysis of the onset proves that the changes are caused by an additional relaxation channel dependent on the identity of the QD-ZnO linker molecule. The faster relaxation represents an important factor for hot carrier energy harvesting, whose efficiency can be influenced by almost 50%.

No MeSH data available.