**1. Introduction**

396 Solar Cells – New Aspects and Solutions

Zel'dovich Ya.B. & Syunyaev R.A*.* (1972). Shock wave structure in the radiation spectrum

85, ISSN: 0038-5646.

during bose condensation of photons, *Soviet Physics - JETP*, Vol. 35, No. 1, pp. 81-

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, hybrid SCs can be a cheap alternative for conventional SCs.

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

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 Solar Cells Based on Silicon 399

Light

f. Inorganic semiconductor materials can have high absorption coefficients and photoconductivity as many organic semiconductor materials (Günes & Sariciftci, 2008). Typically, inorganic semiconductors in macroscopic dimensions, irrespective of their size, will absorb all electromagnetic radiation with energy greater than the bandgap. However, if the particles become smaller than that of the exciton in the bulk semiconductor (typically about 10 nm), their electronic structure has changed. The electronic properties of such small particles will depend not only on the material of which they are composed, but also on their size, the so-called quantum confinement effect (Arici et al., 2004, as cited in Weller, 1993; Steigerwald & Brus, 1990; Alivisatos, 1996; Empedocles & Bawendi, 1999; Murphy & Coffer, 2002; Movla et al. 2010a). The lowest energy of optical transition, among others, will increase significantly due to the quantum confinement with decreasing size of the inorganic clusters. Since the energy levels of the polymers can be tuned by chemical modification of the backbone chain and the energy levels of the nanoparticles can be tuned through the sizedependent quantum confinement effects, blends of the two materials offer the possibility of tailoring optimal conditions for a solar cell, including energy gain from charge transfer for the efficient charge separation and the spectral range of the absorbing light (Arici et al., 2004). Therefore, in order to obtain hybrid polymer SCs with high current and fill factor, both electron and hole mobilities must be optimized and most importantly balanced (Chandrasekaran et al., 2010). However, diffusion of nanoparticles into the polymer matrix takes place with the penetration depth controlled by temperature, swelling of the polymer

**+ \_** 

Transparent layer Anode Hole transport layer Active layer

Cathode

Another module of HJ hybrid SCs consists of crystalline and amorphous silicon-based SCs which is the main discussion in this chapter. The present PV market is dominated by three kinds of Si-based solar cells, that is, single-, multi-crystalline or amorphous Si-based solar cells (for short, marked hereafter as Sc-Si, Mc-Si and a-Si solar cells, respectively). The conventional PV system in general uses Sc-Si or Mc-Si solar cell module as the element for solar energy conversion, which have comparatively higher conversion efficiency. However, it is not only the module efficiency that decides whether a PV system is cost effective but

layer, and not at least by the size and shape of the nanocrystals.

Fig. 1. Structure of HJ hybrid SCs

module and second, HJ SCs based on single crystalline, amorphous and microcrystalline Si and SCs in dye-sensitized configuration. Afterward, material characterization of these kinds of SCs will be investigated. Precisely, Crystalline Si thin film SCs and later amorphous and microcrystalline Si SCs and the recent works are discussed.
