Doping n-type

A photovoltaic cell is basically a semiconductor diode consisting of a junction similar to the junction of a transistor. An electrical potential is formed by n-type doping on one side and p-type on the other. Under the impact of light (photons), such as in sunlight, electrons move from the p side, across the junction to the n side, and, through electrical contacts, can be drawn as a usable current (Fig. 15.4). 

For undoped a-Si H the (Tauc) energy gap is around 1.6-1.7 eV, and the density of states at the Fermi level is typically lO eV cm , less than one dangling bond defect per 10 Si atoms. The Fermi level in n-type doped a-Si H moves from midgap to approximately 0.15 eV from the conduction band edge, and in / -type material to approximately 0.3 eV from the valence band edge [32, 86]. 

The purpose of the n-type doped a-Si H layer is to prevent injection of charge from the substrate into the photoconductor. Thus it serves as a blocking layer. Injection of surface charge into the photoconductor is prevented by the surface... 

Figure 13 shows the irreversible conversion of a nonconjugated poly (p-phenylene pentadienylene) to a lithiun-doped conjugated derivative which has a semiconducting level of conductivity (0.1 to 1.0 S/cm) (29). Obviously, the neutral conjugated derivative of poly (p-phenylene pentadienylene) can then be reversibly generated from the n-type doped material by electrochemical undoping or by p-type compensation. A very similar synthetic method for the conversion of poly(acetylene-co-1,3-butadiene) to polyacetylene has been reported (30), Figure 14. This synthesis of polyacetylene from a nonconjugated precursor polymer containing isolated CH2 units in an otherwise conjugated chain is to be contrasted with the early approach of Marvel et al (6) in which an all-sp3 carbon chain was employed.

When both donors and acceptors are present, compensation results, whereby the electrons supplied by the donor are given to the acceptor. Thus, the free carrier concentration can be considerably reduced below that expected from introducing a known donor or acceptor if the opposite type of dopant is unintentional. For example, semi-insulating (SI) InP (used as a substrate for epitaxial growth) can be made by incorporating low levels of Fe3+ as a deep acceptor (reduced to Fe2+) to compensate for unintentional n-type doping in the sample [19]. 

Fig. 9. Time decay of the occupied band tail density n Bx, measured by the voltage pulse charge sweepout technique, for various temperatures. The n-type doped a-Si.H sample was first annealed at 210°C for 10 min. and then cooled to the indicated temperatures (Street et al., 1988).

Fig. 21. Deuterium concentration profiles, obtained by SIMS, for n-type doped a-Si H (10 4[PH3]/[SiH4]) with columnar microstructure. The bottom curve is the profile for the as grown sample, while the middle and top profiles (vertical scale offset) are obtained after annealing at 240°C for 35 min. and 24 hours, respectively (Street and Tsai. 1988).

The speed of p- and n-type doping and that of p-n junction formation depend on the ionic conductivity of the solid electrolyte. Because of the generally nonpolar characteristics of luminescent polymers like PPV, and the polar characteristics of solid electrolytes, the two components within the electroactive layer will phase separate. Thus, the speed of the electrochemical doping and the local densities of electrochemically generated p- and n-type carriers will depend on the diffusion of the counterions from the electrolyte into the luminescent semiconducting polymer. As a result, the response time and the characteristic performance of the LEC device will highly depend on the ionic conductivity of the solid electrolyte and the morphology and microstructure of the composite. 

Electrical cells based on semiconductors that produce electricity from sunlight and deliver the electrical energy to an external load are known as photovoltaic cells. At present most commercial solar cells consist of silicon doped with small levels of controlled impurity elements, which increase the conductivity because either the CB is partly filled with electrons (n-type doping) or the VB is partly filled with holes (p-type doping). The electrons have, on average, a potential energy known as the Fermi level, which is just below that of the CB in n-type semiconductors and just above that of the VB in p-type semiconductors (Figure 11.2). 

The figure on the inner front cover of this book can be used to convert between doping density, carrier mobility and resistivity p for p- or n-type doped silicon substrates. One of the major contaminants in silicon is oxygen. Its concentration depends on the crystal growth method. It is low in FZ material and high (about 1018 cm-3) in Czochralski (CZ) material. 

A sufficiently anodic bias and the availability of holes are the two necessary conditions for the dissolution of silicon aqueous HF. In this case the Si dissolution rate is proportional to the current density divided by the dissolution valence. In all other cases silicon is passivated in HF this is the case under OCP, or under cathodic conditions, or under anodic conditions if the sample is moderately n-type doped and kept in the dark. If an oxidizing agent like HN03 is added silicon will already dissolve at OCP, but the dissolution rate remains bias dependent. If an anodic bias is applied the dissolution rate will be enhanced, whereas a cathodic bias effectively decreases the rate of dissolution. 

For doping-dependent anodic etch stops in HF, a general hierarchy of dissolution is observed [La5] illuminated n-doped and n+-doped areas are most easily dissolved, followed by p+-doped areas. Next likely to be dissolved are p-type areas. Moderately n-type doped areas kept in the dark are least likely to be etched. This hierarchy corresponds to the potential shift of the I-V curve in the regime of PS formation [Gal, Zh5]. 

For n-type doping densities below 1017 cm-3 and an anodization bias above 10 V, avalanche breakdown becomes relevant. The interface morphology generated in this regime is very complex and shows large etch pits, macropores and mesopores. The formation of this structure is not understood in detail. A hypothetical model will be discussed in Section 8.5. 

Fig. 8.7 SEM micrographs of the interface between bulk and meso PS for n-type doped (100) silicon electrodes anodized galvanostatically in 6% aqueous HF. After [Le23].

Fig. 8.8 SEM micrograph of the interface be- galvanostatically in ethanoic HF (5xl018cm 3, tween bulk and meso PS for an n-type doped 50 mA cm-2). After [Le23]. electrode of (111) crystal orientation anodized...

Fig. 9.10 SEM micrographs showing the dependence of macropore morphology on n-type doping density (2.5% HF, 5 mA cm 2, 2 V).

Fig. 18 Doping mechanisms for molecular p-type doping (top) and for n-type doping (bottom). P-type (n-type) doping is achieved when the molecular dopant acts as acceptor (donor). After [107]...

Attempts to dope organic semiconductors have been made very early in the field, motivated by the prospect of possibly reaching metallic conductivities [108, 109]. These synthetic metals, however, have not been realized. While p-type doping could be obtained, for example, with iodine gases for poly-p-phenylene vinylene (PPV) derivatives, and n-type doping was demonstrated with sodium for a cyano-derivative of PPV, the doping levels obtained were not stable with time. The dopant molecules readily diffused into the organic semiconductor, yet also out of it. Due to the lack of stability, these approaches were not suitable for commercial applications. 

Nollau A, Pfeiffer M, Fritz T, Leo K (2000) Controlled n-type doping of a molecular organic semiconductor naphthalenetetracarboxylic dianhydride (NTCDA) doped with bis (ethylenedithio)-tetrathiafulvalene (BEDT-TTF). J Appl Phys 87 4340... 

Chan CK, Amy F, Zhang Q, Barlow S, Marder S, Kahn A (2006) N-Type doping of an electron-transport material by controlled gas-phase incorporation of cobaltocene. Chem Phys Lett 431 67... 

Werner A, Li F, Harada K, Pfeiffer M, Fritz T, Leo K, Machill S (2004) n-Type doping of organic thin films using cationic dyes. Adv Funct Mater 14 255... 

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