Semiconductor electrodes photoemission

Semiconductor electrodes exhibit electron photoemission into the solution, like metal electrodes, but in addition they exhibit further photoelectrochemical effects due to excitation of the electrode under illumination. The first observations in this area were made toward the middle of the twentieth century. At the end of the 1940s,... 

A quantum yield of about 0.07 electrons emitted into the solution per absorbed photon was found. Interestingly, this value exceeds, by several orders of magnitude, the yields encountered in photoemission experiments with compact semiconductor electrodes [53]. This result indicates that the particle size may be important and indeed it was found that the e absorption occurs only with small particles (nm range) and the absorption coefficient increased with decreasing par-... 

As early as 1839, Becquerel had detected electric currents when one of two immersed electrodes in dilute acid solutions was illuminated. These effects were found to depend on the pH and they were larger when the violet part of the optical spectrum was used. Since then, considerable research has been devoted to the study of such processes. An outline of the historical developments of this field is available in a monograph, together with details of the major theoretical and experimental studies of photoemission at metallic and semiconductor electrodes. This chapter, therefore, will be restricted in scope... 

As relatively less is known concerning electrical double layers at solid and semiconductor electrodes than at liquid mercury electrodes, it is not surprising that the majority of photoemission double-layer studies have been made at the latter. A detailed and clear account of the double layer itself has been given by Mohilner. The photoemission influences broadly may be classified as primary if the electron emission step is affected directly, or secondary if subsequent reactions of the solvated electron with homogeneous acceptors in solution and/or the electrode are modified. 

The theoretical developments in the above areas were influenced, to a considerable extent, by concepts borrowed from semiconductor physics and the physics of surfaces. Other fields of photoelectrochemistry of semiconductors were affected to a greater degree by progress achieved in the study of metal electrodes. Here we mean photoemission of electrons from semiconductors into solutions and electroreflection at a semiconductor-electrolyte interface. 

Photoelectrochemistry — In principle, any process in which photon absorption is followed by some electrochemical process is termed photo electro chemical, but the term has come to have a rather restricted usage, partly to avoid confusion with photoemission (q.v.). The critical requirements for normal photo electro chemical activity is that the electrode itself should be a semiconductor that the electrolyte should have a concentration substantially exceeding the density of -> charge carriers in the semiconductor and that the semiconductor should be reverse biased with respect to the solution. To follow this in detail, the differences in potential distribution at the metal-electrolyte and semiconductor-electrolyte interfaces need to be understood, and these are shown in Fig. 1, which illustrates the situation for an n-type semiconductor under positive bias. 

OSITs based on pentacene thin films have been fabricated on TTO formed on glass substrafes. It is well known that the work function of TTO is con-frolled by the method used to clean its surface. OSITs were fabricated based on pentacene thin films wifh a high-work function ITO of 5.3 eV and a low-work function of 4.2 eV. The effect of the work function of ITO on the static characteristics of fhe OSITs was investigated using I-V measurements and ultraviolet photoemission spectroscopy (UPS) [34]. These results provided an important clue, in that the characteristics of the OSITs were strongly associated with the work function of ITO used as a source electrode. In general, the hole injection barrier at the organic semiconductor/metal interface is influenced by the work function of fhe metal. 

If the electrode is a semiconductor, or if it forms a semiconducting oxide layer (e.g., Fe or Pb), light absorption may be directly monitored by photocurrent spectroscopy. Such photocurrents arise from minority carriers and are much larger than photoemission effects at metallic electrodes. 

More recently, photoemission studies have been extended to polarized light excitation of single-crystal substrates of known epitaxy. This approach is definitely rewarding because it provides an additional parameter to delineate the roles of volume and surface contributions, as well as clarify the basic internal excitation mechanism. We are not aware of any semiconductor/electrolyte studies using these techniques, but their application will prove extremely helpful in resolving the various contributions to the net photocurrents and scattering effects. There is, clearly, a need for better experimentation and theoretical development for the photoemission phenomena into condensed media from each class of electrode. 

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