Photo-induced Electron injection in CdSe quantum dots sensitized ZnO nanowires

Oral, Kaibo Zheng, presentation 021
Publication date: 1st April 2013

Semiconductor quantum dots (QDs) have been considered as promising light harvesting materials in photovoltaic applications during the last decades due to their large optical cross section, size related tunable band gap and possible multiple excition generation to break the bottleneck of Shockley−Queisser thermodynamic efficiency limit.^1-3 Recently, QD solar cells with various architectures, such as QD sensitized solar cells and QD bi-layer hybrid solar cells, have been investigated.^4-6  Photo-excited electrons in these solar cells are usually collected by large band gap semiconductors (e.g. metal oxides (MOs)) to yield the final photocurrent. Therefore, the electron injection from QDs to MOs would be the very first key step determining the conversion efficiency.  Recent reports have reveal electron transfer from quantum dots to metal oxide acceptors with the timescales from picoseconds up to nanoseconds.^7-9  However the detailed electron injection process could be more complicated considering the complexity of QD-MO systems induced by various materials selection and synthesis processes.

In this work, we studied the electron injection process in CdSe quantum dots sensitized ZnO nanowires.  In the first stage, a complementary investigation combined transient absorption (TA) spectroscopy and time-resolved terahertz (THz) spectroscopy has been carried out to clearly identify the electron injection from directly attached CdSe QDs to ZnO nanowires from other possible processes.  In CdSe-ZnO NWs system, this injection occurs within picoseconds timescale (3-12 ps) which exhibits both recovery of absorption bleach and increase of photoconductivity in TA and THz signal, respectively (Fig. 1a).  Besides the well known band energy alignment between CdSe QDs and MO, we also found that the nanostructure morphologies of MO acceptors play an important role in electron injection kinetics. Typically, CdSe-ZnO nanowires exhibit higher injection rates than CdSe-ZnO nanoparticles due to additional band-edge states. However, the low dielectric permittivity of nanowires decrease the driving force for injection (fig 1b).  In addition, we found that QDs which are not directly attached to the MO, probably due to the aggregation in the synthesis process can also transfer photoexcited electrons to MO to yield photocurrent. However, this process involves an initial inter-dot Förster energy transfer from the indirectly attached QDs to the dots with direct contact to MOs with a much slower timescale (~ 5 ns).(fig 1c)

All the above-mentioned results provide an extensive reference to build up moreefficientQDs sensitized solar cellsin the future applications.

Fig. 1 a) TA kinetics (cyan line) compared to THz kinetics (black line) of as-obtained CdSe quantum dots sensitized ZnO nanowires, b) Electron injection rate ket vs. driven force ¦¤G plots of CdSe quantum dots sensitized ZnO nanowires and nanoparticles. Experimental result of CdSe-ZnO NPs (green squares) was fitted by Macrus theory taking width of density of state tail ¦¤ of 54 meV (solid line). Dashed line refers to theoretical estimation of ket vs. ¦¤G relationship in CdSe-ZnO NWs using the same fitting parameter expect for the substitution of ¦¤ with 144 meV, c) main photo-induced processes in CdSe QDs sensitized ZnO NWs with directly and indirectly attached quantum dots.
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