Conventional solar cells

Although conventional solar cells based on silicon are produced from abundant raw materials, the high-temperature fabrication routes to single-crystal and polycrystalline silicon are energy intensive and expensive. The search for alternative solar cells has therefore focused on thin films composed of amorphous silicon and on other semiconductor heterojunction cells (e.g., cadmium telluride and copper indium... 

It is not yet clear if the DSSC will become the dominant type of solar cell in the future, but its ability to achieve a high photoconversion efficiency by a mechanism that is fundamentally different from conventional solar cells makes it a perfect system for exploring those features that are indispensable to all solar cells. It is now clear, for example, that a p-n junction is not a necessary require-... 

A quantitative analysis of dark currents is a standard tool for characterizing conventional solar cells. However, this is not appropriate for DSSCs or most... 

The cathodic dark currents measured under negative bias in a DSSC (Fig. 4) are related to the recombination reactions (4) and (5) in the illuminated cells. However, the relation between the dark current and reaction (4), especially, is difficult to quantify. In conventional solar cells, dark current measurements provide quantitative information about the photorecombination processes [34] because (1) the number of photogenerated charge carriers is only a small perturbation on the dark carrier density and (2) the current flows along the same pathway in the light and in the dark. Neither of these conditions hold for a DSSC. 

Because this is similar to the dark J-V curves of conventional solar cells, it has led to suggestions that the current in DSSCs is controlled by a p-n electrical junction. This was used as one justification for modeling DSSCs as p-n junctions (Sec. V.D). However, such J-V data can be fit just as well to the Butler-Volmer equation, a mainstay of electrochemistry [48], as to the diode equation. The... 

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. 

Low production cost. The production procedure of DSSC is relatively simple and the materials for DSSC are relatively inexpensive. Fabrication costs will, therefore, be less than that of conventional solar cells. For example, the estimated production cost, 0.60 US /Wp for a DSSC with 10% efficiency, is quite competitive with those of conventional solar cells [11,12]. 

Nanocrystals and nanowires are utilized in a new generation of solar collectors (a nanometer is one billionth of a meter). In conventional solar cells, at the P-N junction one photon splits one electron from its "hole companion" as it travels to the electron-capturing electrode. If solar collectors are made of semiconducting nanocrystals that disperse the light, according to TU Delft s professor Laurens Siebbeles, an avalanche effect results and one photon can release two or three electrons, because this effect maximizes photon absorption while minimizing electron-hole recombination. This effect of the photon-scattering nanoparticles substantially increases cell efficiency. 

Unlike room temperature solar PV and photoelectrochemical electrolysis, the hybrid approach utilizes energy of the full solar spectrum, leading to substantially higher solar energy efficiencies. The IR radiation is energetically insufficient to drive conventional solar cells, and this solar radiation is normally discarded (by reflectance or as re radiated heat.) On the other hand, in the hybrid approach, as seen in Fig. 2... 

Conventional solar cells are built from inorganic silicon-like materials. Efficiency of such solar cells is high, but they originate from expensive materials and special techniques are required for their processing. Recently hybrid and photoelectrochemical solar cells [54] have been cost effective alternatives for conventional silicon solar cells. The correspondence between the photon absorption and charge separation events is the point of differentiation between the photovoltaic effect in a semiconductor junction, and that in a photon-induced generation of a chemical potential in natural systems, i.e. photosynthesis. In the latter, and this is very simple but highly relevant in the context of artificial photosynthetic systems, the point in space at which the... 

Figure 9.13 Schematic of perovskite solar cells (a) DSSC architecture using CH NH Pbl perovskite (b) conventional solar cell architecture (c) tandem silicon/ perovskite architecture...

In a conventional solar cell, R results from the recombination of injected minority carriers. The dark current is rectifying and can be approximated by the form... 

At larger forward bias, the net photocurrent drops because of increased electron recombination. In a conventional solar cell, the process responsible for the recombination current is normally the same in light and dark, and it is common to make the approximation that J V) = Jsc — - darkC )-Although this may be good for semiconductor p-n junction devices, it is not reliable for dye-sensitized photovoltaic devices. Ionization of the sensitizer opens up a new path for recombination in light, which is not active in dark (Fig. 11). There is some evidence [69] that the reverse current is larger under illumination than in the dark. 

The use of flexible solar panels on clothing is also the subject of a three nation European development project called H-Alpha Solar whose partners include Akzo Nobel, which has established a pilot plant to produce rolls of silicon cells which are 40 cm wide. Whilst of a similar construction to a conventional solar cell, this type is only 1 pm thick as a result of depositing polymorphous silicon at high pressures and temperatures. 

In the type of dye-sensitized solar cell, the dye and the semiconductor perform the processes of light absorption and charge separation, respectively, unlike the case of p-n junction type solar cells where the semiconductor performs both processes. This sensitization process minimizes losses due to recombination at the semicon-ductor/electrolyte interface (reaction 4). Since the electron transport in photoelectrochemical (PEC) cells is purely due to kinetic effects surfaces with structural defects do not therefore cause recombination and can be used in contrast to conventional solar cells [3]. That is the electron injection occurs anywhere in the cell and the time for electron recombination has been studied by flash photolysis and has been found to be very much longer than for electron injection [6]. 

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