**9. Metallic nanocluster contacts for high-effective photovoltaic devices**

High efficiency of solar energy conversion is a main challenge of many fields in novel nanotechnologies. Various nanostructures have been proposed early (Pillai et al., 2007; Hun et al., 2007; Johnson et al., 2007; Slaoui & Collins, 2007). However, every active element cannot function without electrodes. Thus, the problem of performing effective contacts is of particular interest.

The unique room-temperature electrical characteristics of the porous metallic nanoclusterbased structures deposited by the wet chemical technology on conventional silicon-based solar cells were described in (Laptev & Khlyap, 2008). We have analyzed the current-voltage characteristics of Cu-Ag-metallic nanocluster contact stripes and we have registered for the first time dark currents in metallic structures. Morphological investigations (Laptev & Khlyap, Kozar et al., 2010) demonstrated that copper particles are smaller than 0.1 μm and smaller than the pore diameter in silver.

Electrical measurements were carried out for the nanoclustered Ag/Co-contact stripe (Fig.18, inset) and a metal-insulator-semiconductor (MIS) structure formed by the silicon substrate, SiNx cove layer, and the nanocluster stripe. Fig. 18 plots experimental roomtemperature current-voltage characteristics (IVC) for both cases.

As is seen, the nanocluster metallic contact stripe (function 3 in Fig. 18) demonstrates a current-voltage dependence typical for metals. The MIS-structure (functions 1 and 2 in Fig. 18) shows the IVC with a weak asymmetry at a very low applied voltage; as the external electric field increases, the observed current-voltage dependence transforms in a typical "metallic" IVC. More detailed numerical analysis was carried out under re-building the experimental IVCs in a double-log scale.

The current in the silver contact with copper clusters while illuminating the solar cell is caused by the generation of charge carriers in the semiconductor part of the silicon wafer. The number of charge carriers generated in the *p–n* junction is two orders of magnitude larger than the number of charge carriers in copper clusters since the light current is so

The copper deposition onto silver does not lead to the formation of a silver–copper solid solution. The contact of the crystal structures gives rise to an electric potential difference.

However, the contact of the copper and silver crystal structures causes compression of the

We consider that the charge carrier generation in the dark by copper clusters in the contact strip as a component of the solar cell is caused by the deformation of the strip. It is known (Albert & Chudnovsky, 2008), that deformation of metal cluster structures can induce hightemperature superconductivity. Therefore, it is necessary to investigate the behavior of the

Solar energy conversion is widely used in electric power generation. Its efficiency in domestic and industrial plants depends on the quality of components (Slaoui A & Collins, 2007). Discovered in this work, the dark current in the silver contact on the illuminated side of a silicon solar cell generates electricity in amount of up to 5% of the rated value in the absence of sunlight. Therefore, the efficiency of solar energy conversion plants with copper– silver contacts is higher even at the same efficiency of the semiconductor part of the solar

**9. Metallic nanocluster contacts for high-effective photovoltaic devices** 

High efficiency of solar energy conversion is a main challenge of many fields in novel nanotechnologies. Various nanostructures have been proposed early (Pillai et al., 2007; Hun et al., 2007; Johnson et al., 2007; Slaoui & Collins, 2007). However, every active element cannot function without electrodes. Thus, the problem of performing effective contacts is of

The unique room-temperature electrical characteristics of the porous metallic nanoclusterbased structures deposited by the wet chemical technology on conventional silicon-based solar cells were described in (Laptev & Khlyap, 2008). We have analyzed the current-voltage characteristics of Cu-Ag-metallic nanocluster contact stripes and we have registered for the first time dark currents in metallic structures. Morphological investigations (Laptev & Khlyap, Kozar et al., 2010) demonstrated that copper particles are smaller than 0.1 μm and

Electrical measurements were carried out for the nanoclustered Ag/Co-contact stripe (Fig.18, inset) and a metal-insulator-semiconductor (MIS) structure formed by the silicon substrate, SiNx cove layer, and the nanocluster stripe. Fig. 18 plots experimental room-

As is seen, the nanocluster metallic contact stripe (function 3 in Fig. 18) demonstrates a current-voltage dependence typical for metals. The MIS-structure (functions 1 and 2 in Fig. 18) shows the IVC with a weak asymmetry at a very low applied voltage; as the external electric field increases, the observed current-voltage dependence transforms in a typical "metallic" IVC. More detailed numerical analysis was carried out under re-building the

metal strip and can decrease the metal work function of copper clusters.

larger than the dark current (Fig. 17).

studied samples in a magnetic field.

smaller than the pore diameter in silver.

experimental IVCs in a double-log scale.

temperature current-voltage characteristics (IVC) for both cases.

cell.

particular interest.

This is insufficient for generation of current carriers.

Fig. 18. Room-temperature current-voltage characteristics of the investigated structures <8see text above): functions 1 and 2 are "forward" and "reverse" currents of the MISstructure (contacts 1-2), and the function 3 is a IVC for the contacts 1-3.

Fig. 19 illustrates a double-log IVCs for the investigated structure. The numerical analysis has shown that both "forward" and 'reverse" currents can be described by the function

I = f(Va)m,

where I is the experimental current (registered under the forward or reverse direction of the applied electric field), and Va is an applied voltage. The exponential factor *m* changes from 1.7 for the "forward" current at Va up to 50 mV and then decreases down to ~1.0 as the applied bias increases up to 400 mV; for the "reverse" current the factor *m* is almost constant (~1.0) in the all range of the external electric field.

Thus, these experimental current-voltage characteristics (we have to remember that the investigated structure is a metallic cluster-based quasi-nanowire!) can be described according to the theory (Sze & Ng, 2007) as follows: the first section of forward current

*I* = *T*tunAel(4/9*L*2)(2e/m\*)1/2(*V*a)3/2

(ballistic mode) and the second one as

$$I = T\_{\rm tum} \mathbf{A}\_{\rm el} (2 \varepsilon \mathbf{v}\_s / L^2) V\_{\rm u}$$

and the reverse current is

$$I = T\_{\rm tum} \mathbf{A}\_{\rm el} (2 \varepsilon \mathbf{v}\_{\rm s} / L^2) V\_{\rm a}$$

(velocity saturation mode). Here Ttun is a tunneling transparency coefficient of the potential barrier formed by the ultrathin native oxide films, Ael and L are the electrical

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**18** 

*Iran* 

**Hybrid Solar Cells Based on Silicon** 

*3Faculty of Humanities and Social Sciences, University of Tabriz* 

Arash Nikniazi1, Mohammad Soltanpour3 and Khadije Khalili2

*2Research Institute for Applied Physics and Astronomy (RIAPA), University of Tabriz* 

Human need for renewable energy resources leads to invention of renewable energy sources such as Solar Cells (SCs). Historically, the first SCs were built from inorganic materials. Although the efficiency of such conventional solar cells is high, very expensive materials and energy intensive processing techniques are required. In comparison with the conventional scheme, the hybrid Si-based SC system has advantages such as; (1) Higher charging current and longer timescale, which make the hybrid system have improved performances and be able to full-charge a storage battery with larger capacity during a daytime so as to power the load for a longer time; (2) much more cost effective, which makes the cost for the hybrid PV system reduced by at least 15%(Wu et al., 2005). Thus,

One type of hybrid SCs is a combination of both organic and inorganic materials which combines the unique properties of inorganic semiconductors with the film forming properties of conjugated polymers. Organic materials are inexpensive, easily processable, enabling lightweight devices and their functionality can be tailored by molecular design and chemical synthesis. On the other hand, inorganic semiconductors can be manufactured as nanoparticles and inorganic semiconductor nanoparticles offer the advantage of having high absorption coefficients, size tenability and stability. By varying the size of nanoparticles the bandgap can be tuned therefore the absorption range can be tailored (Günes & Sariciftci, 2008). These kinds of hybrid SCs based on organic-inorganic materials are fabricated by using different concepts such as solid state dye-sensitized SCs and hybrid SCs using Bulk Heterojunction (BHJ) concept such as TiOx(Hal et al., 2003), ZnO (Beek et al., 2006), CdSe (Alivisatos, 1996; Huynh et al.,

Another generation of hybrid SCs are silicon-based modules due to the direct bandgap and high efficiency of Si. This system includes SC module consisting of crystalline and amorphous silicon-based SCs. The methods for enhancing the efficiencies in these types of hybrid SCs such as applying textured structures for front and back contacts as well as implementing an intermediate reflecting layer (IRL) between the individual cells of the tandem will be discussed (Meillaud et al., 2011). This chapter brings out an overview of principle and working of hybrid SCs consisting of HJ SCs which is itself devided into two groups, first organic-inorganic

hybrid SCs can be a cheap alternative for conventional SCs.

2002), Cds (Greenham et al., 1996), PbS (McDonald et al.,2005), and CuInS2.

**1. Introduction** 

Hossein Movla1, Foozieh Sohrabi1,

*1Faculty of Physics, University of Tabriz* 

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