Solar cells charge generation

Organic semiconductor photovoltaic cells share many characteristics with both DSSCs and conventional cells. Charge generation occurs almost exclusively by interfacial exciton dissociation, as in DSSCs, but, in contrast, OPV cells usually contain no mobile electrolyte and thus rely on Vcharge separation. OPV cells may have planar interfaces, like conventional PV cells, or highly structured interfaces, like DSSCs. They provide a conceptual and experimental bridge between DSSCs and conventional solar cells. 

Figure 13.4 Charge generation in PCBMipolyhtiophene solar cells. Charge-transfer state generation, separation, and recombination rates for P3HT PCBM are shown. (Reprinted with permission from Journal of the American Chemical Society, Charge Carrier Formation in Polythiophene/ Fullerene Blend Films Studied by Transient Absorption Spectroscopy by H. Ohkita, S. Cook, Y. Astuti etal., 130, 10,3030-3042. Copyright (2008) American Chemical Society)...

Similar to organic solar cells, photocurrent generation is a multistep process in NC-polymer hybrid bulk heterojunction solar cells, as demonstrated in Figure 13.7. Briefly, when a photon is absorbed by the absorbing material, electrons are exited from the valance band (VB) to the conduction band (CB) to form excitons. The excitons diffuse to the donor/acceptor interface where charge transfer occurs, leading to the dissociation of the excitons into free electrons and holes. Driven by the... 

In lateral junction organic solar cells charge separation occurs at the interface between the layers of p- and n-type materials. It is reasonable to assume that improved interactions between donor and acceptor materials at the interface should facilitate charge separation and improve photocurrent generation in the system. 

The chapter is organized as follows the second section will discuss the photophysics of conjugated polymer/fullerene composites as a standard model for a charge-generating layer in plastic solar cells. Pristine polymer devices will be discussed in the third section while bilayer and interpenetrating network devices are presented in Sections 4 and 5. Section 6 contains some remarks on large area plastic solar cells and Section 7 conclusions. 

Through exothermic dissociation of a neutral excited state in molecule by electron transfer to an adjacent molecule. This process leads to the generation of geminately bound electron-hole pairs as precursors of free positive and negative charges in an organic solar cell. 

Fig. 4 Schematic illustration of the processes leading to photocurrent generation in organic solar cells, (a) Photon absorption in Step 1 leads to excitons that may diffuse in Step 2 to the donor/ acceptor (D/A) interface. Quenching of the exciton at the D/A interface in Step 3 leads to formation of the charge-transfer (CT) state. Note that processes analogous to Steps 1-3 may also occur in the acceptor material, (b) Charge separation in Step 4 leads to free polarons that are transported through the organic layers and collected at the electrodes in Steps 5 and 6, respectively, (c) The equilibria involved in Steps 1-4- strongly influence device efficiency...

Howard lA, Laquai F (2010) Optical probes of charge generation and recombination in bulk heterojunction organic solar cells. Macromol Chem Phys 211 2063... 

In order to obtain high conversion efficiencies, optimization of the short-circuit photocurrent (z sc) and open-circuit potential (Voc) of the solar cell is essential. The conduction band of the TiO is known to have a Nernstian dependence on pH [13,18], The fully protonated sensitizer (22), upon adsorption, transfers most of its protons to the TiO surface, charging it positively. The electric field associated with the surface dipole generated in this fashion enhances the adsorption of the anionic ruthenium complex and assists electron injection from the excited state of the sensitizer in the titania conduction band, favoring high photocurrents (18-19 inA/cm ). However, the open-circuit potential (0.65 V) is lower due to the positive shift of the conduction-band edge induced by the surface protonation. 

In photo-catalytic and solar energy conversion devices, the absorption of a photon results in the generation of electrons and holes, which, upon separation, can provide an electric potential or trigger chemistry. The efficiency of these devices is frequently determined by the transport of the charges following photo-generation. In particular, for TiC>2-based dye-sensitized solar cells, it has been demonstrated that the efficiency is limited by electron transport through TiC>2 nanoparticles [1]. 

Electrons and holes generated in the i-layer by incident light are driven to the n- and p-layer by the internal electric field, respectively. The material quality of the intrinsic layer and the strength and distribution of the electric field are responsible for the charge carrier collection and mainly determine the electrical solar cell performance. Defects affect the charge carrier collection in two different ways On the one hand they act as recombination centers, and on the other hand their charge state modifies the electric field distribution in the i-layer. 

As already stated, for a charge current to flow through the solar cell, electrons and holes must move in different directions. By convention, for the forward current, holes and electrons must move from the external circuit through the contacts into the solar cell, where they disappear by recombination. For the reverse current, electrons and holes must move out of the solar cell, where they were generated. In order to achieve this, the contacts themselves or the material in front of the contacts should have the properties of a semi-permeable membrane. The active volume of the solar cell, in which generation and recombination determine the electrical current, is between the membranes. The membranes must be close enough to the place where the electron-hole pairs are generated, so that they can be reached within the lifetime of the carriers. 

In many cases, local details are not important and an overall balance of generation and recombination, of extraction and injection of charge carriers gives the correct results for electrical and energy currents originating from a solar cell, as we know from extensive experience. 

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