P-type conduction

Extensive structural, optical, and electronic studies on the chalcopyrite semiconductors have been stimulated by the promising photovoltaic and photoelectrochem-ical properties of the copper-indium diselenide, CuInSe2, having a direct gap of about 1.0 eV, viz. close to optimal for terrestrial photovoltaics, and a high absorption coefficient which exceeds 10 cm . The physical properties of this and the other compounds of the family can be modulated to some extent by a slight deviation from stoichiometry. Thus, both anion and cation deficiencies may be tolerated, inducing, respectively, n- and p-type conductivities a p-type behavior would associate to either selenium excess or copper deficiency. 

Cathodic photo-currents observed during the deposition of CdTe have been interpreted as evidence for p-type conductivity [90]. Verbrugge and Tobias [91] showed... 

Recendy, ID quantum dots of gallium selenide with average diameter 8-10 nm, connected in the form of chains of average length 50-60 nm, were synthesized on rro substrates by cathodic electrodeposition from acidic aqueous solutions of gallium(III) nitrate and selenious acid [186], The structural analysis from XRD patterns revealed the formation of Ga2Se3/GaSe composition. The films were found to be photoactive in aqueous sodium thiosulfate solution and showed p-type conductivity. 

The interest in the synthesis and properties of delafossite structured compounds that have the general formula of ABO2 have grown due to their p-type conductivity and optical transparency. The application of ultrasound for the synthesis of ternary oxide AgMC>2 (M = Fe, Ga) has been investigated by Nagarajan and Tomar [44]. Above materials were obtained in crystalline form within 40-60 min of sonication. 

A. Lewenstam, J. Bobacka, and A. Ivaska, Mechanism of ionic and redox sensitivity of p-type conducting polymers. 1. Theory. J. Electroanal. Chem. 368, 23—31 (1994). 

All of the films grown showed p-type conduction regardless of the reactor type, Cu/In ratio, or morphology. It was reported that S-rich material shows p-type conduction61,62 and that the Cu-on-In antisite (Cuin) in Cu-rich films is expected to be the major acceptor for p-type conduction.47 The bulk resistivities ranged from O.lQ-cm to 30Q-cm Cu-rich films generally had lower resistivity than In-rich films. 

Urban, J. I Talapin, D. V. Shevchenko, E. V. Kagan, C. R. Murray, C. B. 2007. Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films. Nature Mater. 6 115-121. 

FIGURE 1.24 p-type conductivity of Gd02Ce08O19 with 2% Co sintered at 900, 1100, and 1500°C. The dashed line is for a Gdo CeogOj 9 sample sintered without addition of Co [109],... 

The conductivity of ZnO, ITO, and Sn02 can be controlled across an extremely wide range such that they can behave as insulators, semiconductors, or metal-like materials. However, these materials are all n-type electrical conductors in nature. Their applications for optoelectronics are rather restricted. The lack of p-type conducting TCOs prevent fabrication of p-n junction composed from transparent oxide semiconductors [2], The fabrication of highly conducting p-type TCOs is, indeed, still a challenge. 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

One would expect the presence of trapped electrons in the oxidized samples to give rise to n-type conductivity, conduction possibly taking place by jump migration of the odd electron in the lattice of Cr + ions in a somewhat similar manner to the mechanism discussed by Heikes 174) for the migration of Ni + holes in lithia-doped NiO. The observed p-type conductivity of chromia in an oxygen atmosphere is presumably due to electron holes in a solid which is predominantly CraOs for the low concentrations of chromia-on-alumina where the 7-phase resonance intensity is maximum, the chromium is predominantly in the d-6 valence state 167). 

For the same reason, there are no grounds for seeking a relation between the catalytic activity of semiconductors and their conductivity type (n- or p-type conductivity) when dealing with semiconductors of different chemical nature. 

For heavily doped n-type semiconductors, the flat band is nearly coincident with the conduction band, while for heavily doped p-type semiconductors the flat band lies very close to the valence band edge. A necessary thermodynamic condition for the photoproduction of hydrogen and oxygen is that the p-type conduction band must be at or above the H7H2 half cell potential, while n-type valence band must lie below the 02/0H half cell potential. 

Recently, intense interest has been paid on doped poly(acetylene) because its film 68) showed markedly high conductivity on doping69 70 71 and the n- and p-type conductivities are depending on the dopants. The confirmation of p-n junction formation with the p- and n-type -fCH T, has roused great expectations to produce a polymer film solar cell.72a)... 

The earliest of these studies was on PbS. PbS can have either p- or n-type conductivity, although CD PbS is usually p-type. Based on the belief that the p-type conductivity may be due to alkali metal cations from the deposition solution, an alkali metal—free deposition, using lead acetate, thiourea, and hydrazine hydrate was used [33]. While initially n-type, the film converted to p-type in air. Attempts to stabilize the p-type material by adding trivalent cations to the deposition solution were unsuccessful. However, deposition of the PbS on a trivalent metal, such as Al, did stabilize the n-PbS, at least for a time. In this way, p-n junctions were made (the PbS close to the trivalent metal was n-type, while the rest of the film was p-type). Photovoltages up to 100 mV were obtained from these junctions at room temperature and almost 300 mV at low temperatures (90 K). 

Extrinsic Semiconductors. Impurity levels can be either donor levels near the empty zone (normal or n-type), or acceptor levels near the filled band (abnormal or p-type). Conductivity in n-type conductors will be due to electrons in the empty band donated by the impurity levels, and in p-type conductors, to positive holes in the previously filled band, arising from the transition of electrons to the impurity acceptor levels. 

For p-type conductivity one can expect that only neutral impurities, whose ionization potential is less than the potential of the molecules actively taking part in the charge transfer, will act as traps. Moreover the impurities with high ionization potential are not essential for charge transfer due to the added activation energy needed for holes to be transported to these localized states often. These proposals were confirmed for lexan films with IPC and TPA [291]. So the influence of the outside molecule on the charge transfer can be predicted knowing the relevant ionization potential and electron affinity. 

In the Ag-doped PbS (acceptor doping), the transition of electron to hole conduction is shifted to lower Ps., and the region of p-type conduction extends to a lower Ps.- In region (III), the concentration of carrier p ( = [Agpb]) is weakly dependent on P. ... 

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