Principle and working of hybrid solar cells

One of the methods to build hybrid SCs is Bulk Hetrojunction (BHJ) SCs, composed of two semiconductors. Excitons created upon photoexcitation are separated into free charge carriers at interfaces between two semiconductors in a composite thin film such as a conjugated polymer and fullerene mixtures. One of these materials of an HJ obviously must be an absorber. The other may be an absorber, too, or it may be a window material; i. e., a wider-gap semiconductor that contributes little or nothing to light absorption but is used to create the HJ and to support carrier transport. Window materials collect holes and electrons, which function as majority-carrier transport layers, and can separate the absorber material from deleterious recombination at contacts. The interface they form with the absorber is also used for exciton dissociation in cells where absorption is by exciton formation. Absorber and window materials may be inorganic semiconductors, organic semiconductors, or mixtures (Fonash, 2010, as cited in Khalili et al. 2010; Sohrabi et al. 2011). For applying HJ structure (HJS) for hybrid SCs, the blends of inorganic nanocrystals with semiconductive polymers as a photovoltaic layer should be employed.

Schematically, the HJ hybrid SCs consist of at least four distinct layers, excluding the substrate, which may be glass. These our layers are anode, cathode, hole transport layer and active layer. Induim tin oxide (ITO) is a popular anodic material due to its transparency and glass substrate coated with ITO is commercially available. A layer of the conductive polymer mixture PEDOT:PSS may be applied between anode and the active layer. The PEDOT:PSS layer serves several functions. It not only serves as a hole transporter and exciton blocker, but it also smoothens out the ITO surface, seals the active layer from oxygen, and keeps the anode material from diffusing into the active layer, which can lead to unwanted trap sites. Next, on the top of the PEDOT:PSS, layer deposited is the active layer. The active layer is responsible for light absorption, exciton generation/ dissociation and charge carrier diffusion (Chandrasekaran et al., 2010). The so-called two materials are inserted in active layer namely donor and acceptor. Polymers are the common donors whereas nanoparticles act as common acceptors. On the top of active layer is cathode, typically made of Al, Ca, Ag and Au (Chandrasekaran et al., 2010).

BHJ hybrid SCs attracts much interest due to these features:

a. HJs allow the use of semiconductors that can only be doped either n-type or p-type and yet have attractive properties which may conclude their absorption length, cost, and environmental impact. The existence of concentration gradient of the n-type nanoparticles within the p-type polymer matrix may allow optimization of the topology of the HJ network.

b. HJs allow the exploitation of effective forces.

c. HJs of window-absorber type can be used to form structures that shield carriers from top-surface or back-surface recombination sinks (Fonash, 2010).

d. The affinity steps at HJ interfaces can be used to dissociate excitons into free electrons and holes.

e. HJs can also permit open-circuit voltages that can be larger than the built-in electrostatic potential.

Fig. 1. Structure of HJ hybrid SCs

f. Inorganic semiconductor materials can have high absorption coefficients and photoconductivity as many organic semiconductor materials (Gunes & 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 size- dependent 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 layer, and not at least by the size and shape of the nanocrystals.

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 also the timescale during which the module works efficiently in a daytime of use and the cost the module itself requires. At this point, a-Si solar cell comes with its advantages of broader timescale and lower cost (Wu et al., 2005, as cited in Goetzberger et al., 2003).

The broader timescale merit of a-Si solar cell arises from its high absorption of light with wavelength around 300-800nm, no matter if it is scattered or not, and no matter if it is weak or blazing. The Sc-, Mc – and a-Si solar cells, therefore, reinforce each other in performances, which could be exploited to construct a hybrid PV system with lower cost in view of the well balanced set of system performance (Wu et al., 2005). The last efficiencies reported for c-Si, Mc-Si and a-Si are approximately 25%, 20% and 10%, respectively (Green et al., 2011). The newest configuration for hybrid SCs is dye-sensitized SC developed by O’Reagan and Graetzel in 1991.This class of cell has reached efficiencies of over 11%. The basic structure of a dye-sensitized SC involves a transparent (wide-band-gap) n-type semiconductor configured optimally in a nanoscale network of columns, touching nanoparticles, or coral­like protrusions. The surface area of the network is covered everywhere with a monolayer of a dye or a coating of quantum dots, which functions as the dye (Fonash, 2010). A monolayer of dye on a flat surface can only harvest a negligibly small fraction of incoming light. In this case it is useful to enlarge this interface between the semiconductor oxide and the dye. As mentioned above, it is achieved by introducing a nanoparticle based electrode construction which enhances the photoactive interface by orders of magnitude (Gratzel, 2004). The dye sensitizer is the absorber. An electrolyte is then used to permeate the resulting coated network structure to set up a conduit between the dye and the anode. The dye absorbs light, producing excitons, which dissociate at the dye-semiconductor interface, resulting in photogenerated electrons for the semiconductor and oxidized dye molecules that must be reduced and thereby regenerated by the electrolyte (Fonash, 2010).

A dye-sensitized SC of Graetzel type comprises of several different materials such as nanoporous TiO2 electrodes, organic or inorganic dyes, inorganic salts and metallic catalysts (Gratzel, 2004, 2005, as cited in Nogueira et al., 2004; Mohammadpour et al., 2010) which will be discussed more in next section. The dye sensitization of the large band-gap semiconductor electrodes is achieved by covering the internal surfaces of porous TiO2 electrode with special dye molecules which absorb the incoming photons (Halme, 2002). Alternatives to the liquid electrolyte in dye-sensitized SCs are a polymer gel electrolyte or solid state dye-sensitized SCs which can contain organic hole conductor materials, inorganic p-type semiconductors or conjugated polymers (Fonash, 2010). The impetus for this effort is the increased practicality of an all-solid-state device and the avoidance of chemical irreversibility originating from ionic discharging and the formation of active species (Fonash, 2010, as cited in Tennakone et al., 2000). And also this method avoids problems such as leakage of liquid electrolyte. In solid state dye-sensitized SCs, the sensitizer dye is regenerated by the electron donation from the hole conductor (Wang et al., 2006). The hole conductor must be able to transfer holes from the sensitizing dye after the dye has injected electrons into the TiO2. Furthermore, hole conductors have to be deposited within the porous nanocrystalline (nc) layer penetrating into the pores of the nanoparticle and finally it must be transparent in the visible spectrum, or, if it absorbs light, it must be as efficient in electron injection as the dye. (Gunes & Sariciftci, 2008)

Fig. 3. Schematic of solid state dye-sensitized SC

Other quasi-dye sensitized SCs are nanoparticle sensitized SCs and extremely thin absorber (ETA) SCs. Nanoparticle sensitized SCs are prepared by replacing the dye with inorganic nanoparticles or quantum dots. They can be adsorbed from a colloidal quantum dot solution (Zaban et al., 1998; as cited in Gunes et al., 2006; Guenes et al., 2007) or produced in situ (Liu & Kamat, 1993; as cited in Hoyer & Konenkamp, 1995). Inorganic nanocrystals instead of organic dyes could imply tunability of the band-gap and thereby the absorption range. Nanocrystals have large extinction coefficients due to quantum confinement and intrinsic dipole moments, leading to rapid charge separation and are relatively stable inorganic materials (Alivisatos, 1996). To embed the particles into porous TiO2 films and to use those modified layers as light converting electrodes, the incorporated nanoparticles need to be much smaller than the pore sizes of the nanoporous TiO2 electrodes (Shen, 2004).

ETA SCs are conceptually close to the solid state dye-sensitized solar cells. In the ETA SCs, an extremely thin layer of a semiconductor such as CuInS2 or CdTe or CuSCN replaces the dye in TiO2 based SCs. The ETA SCs has the advantage of enhanced light harvesting due to the surface enlargement and multiple scattering. Similar to the solid state dye sensitized Scs, the operation of the ETA SC is also based on a heterojunction with an extremely large interface (Nanu et al., 2005).