Charge Transfer Processes at Quantum Well Electrodes MQW, SQW

3 Charge Transfer Processes at Quantum Well Electrodes (MQW, SQW) 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into solution. The most interesting question here is whether an electron transfer from higher quantized levels to the oxidized species of the redox system can occur, as illustrated in Fig. 9.31. In order to accomplish such a hot electron transfer, the rate of electron transfer must be competitive with the rate of electron relaxation. It has been shown that quantization can slow down the carrier cooling dynamics and make hot carrier transfer competitive with carrier cooling. 

Time-resolved measurements of electron transfer times for quantum well photoelectrodes which can be compared with hot electron relaxation times, have not yet been reported. Only some excitation spectra, i.e. photocurrent vs. photon energy for MQWs and single quantum wells (SQWs), have been published so far [2]. In both cases, the photocurrent spectra show distinct structures corresponding to transitions between the hole and electron wells as shown for SQW electrodes in Fig. 9.32. The 

The electrons excited into the different levels within in the single well, could be transferred to an acceptor molecule in the electrolyte either by thermionic emission across the outer barrier layer Otherm) or tunneling through it (Jiun) (F g- 9.33). The photocurrent spectrum does not give any information about whether a hot electron was transferred. The observed structure in these spectra could in principle be caused simply by quantized absorption followed by a complete hot carrier relaxation and electron transfer from the lowest quantum level. 

The photoeletrochemical behavior has been extensively studied by Hodes [35, 36]. Surprisingly, the corresponding current-potential curve measured with such an electrode (CdSe) in contact with an electrolyte containing, for example, /Sp as a redox system, showed a typical diode characteristic (Fig. 9.34). On the other hand, when a gold layer was deposited on the CdSe nanocrystalline film instead of making a contact 

---