Solar cells optical losses

Fig. 8.11. Monolithic series-connection of silicon thin film solar modules by a sequence of deposition and laser scribing steps. The current flow is indicated by the dotted arrow. For calculation of resistive and optical losses the active cell width ioa and dead area width Wd are needed...

The function Q(X) is illustrated in Fig. 10.4. The atmospheric solar spectrum peaks at 5000 A, drops rapidly below 4000 A but extends far into the infra-red. There are collection losses in the solar cells at both ends of the visible spectrum. Most of the loss is at long wavelength where the collection efficiency is imavoidably reduced by the optical absorption edge and by the limited thickness of the cell. On the short wavelength side of the spectrum there are collection losses due to... 

Transparent polymer solar cells (i.e., polymer solar cells with transparent electrodes) can be easily fabricated based on inverted architecture and have important application in tandem architectures as well. We can form transparent solar cells by replacing the Al top electrode with 12 nm Au in the inverted structure. The J-V curves for this transparent polymer solar cell, with light incident from ITO and Au side, are shown in Figure 11.17. The difference between the two J-V curves is due to the partial loss by the reflection and absorption at the semitransparent Au electrode. To provide sufficient electrical conductance, Au layer thickness has to be sufficient and the optical loss at Au electrode becomes significant. However, the inverted solar cell structure has the V2O5 layer which is not only transparent but also provides effective protection to the polymer layer. A transparent conductive oxides electrode, such as ITO, can therefore be deposited without compromising device performance. 

In order to uncover the photophysical processes and the loss mechanism in polymeric solar cells, two main approaches can be distinguished electrical measurements and time-resolved optical spectroscopy. Electrical measurements have the fundamental disadvantage that they lack time and spatial resolution to probe the processes that occur directly after exciton ionization. To investigate these dynamics, time-resolved spectroscopy is a much more promising approach, because ultrafast laser systems allow observing these proeesses directly with subpicosecond time resolution. 

Lee S, Lee E (2007) Characterization of nanoporous silicon layer to reduce the optical losses of crystalline silicon solar cells. J Nanosci Nanotech 7 3713-3716 Lipinski M, Panek P, Bielanska E et al (2000) Influence of porous silicon on parameters of silicon solar cells. Opto Electron Rev 8 418-420... 

The N3 and N719 dyes (Table 38.1) show the highest incident photon-to-current conversion efficiency (IPCE) as compared with other dyes. When the optical properties of the dyes are taking into consideration, there are two quantum efficiencies (QEs), that is, an external quantum efficiency (EQE) and an internal quantum efficiency (IQE) [43]. EQE includes the effect of optical losses by transmission and reflection, while IQE refers to the efficiency of the photons that are not reflected or transmitted out of the cell [43]. From the reflection and transmission of a solar cell, the EQE curve can be corrected to obtain the internal quantum efficiency curve [43]. IPCE is related to EQE and therefore IPCE depends on the absorption of light as well as the collection of charges. 

The super-efficient, super thin cells described in the preceding section can of course stand alone as the building blocks of solar arrays. However, they can be used as elements in tandem cell systems. There are three principle ways in which they can be assembled to form such systems. In the first method, illustrated in Fig. 2, the cells are placed one over the other but they are electrically isolated so that each bandgap cell is connected to a separate load. Optical losses at the interfaces can be reduced by using a split spectrum system like that shown schematically in Fig. 16. [23] Here mirror M reflects that portion of sunlight which can be used by cell C, the largest E cell in this three cell tandem system. The dichroic mirror M ... 

Optical coatings The losses of optical devices can be reducedby coating them with an aerogel of (matched) low refractive index (Hrubesh and Poco, 1995 Hru-besh, 1998) in this way, more sunlight can reach the active surface of a solar cell and, in fiber optics, light collection at the fiber entrance and signal propagation efficiency can be improved. 

In Sect. 2.1 we discussed the many applications of optical modelling for organic solar cells. The case study we chose to present as one application of optical modelling of organic solar cells is the analysis of parasitic absorption losses in the different layers of the solar cell stack. One example is shown in Fig. 8. We calculated the absorptance of each layer in a layer stack glass/ITO (160 nm)/poly(3,4-ethylene-dioxythiophene) poly(styrenesulfonate) (PEDOT PSS) (30 nm)/poly[(4,40-bis (2-ethylhexyl)dithieno [3,2-h 20,30- 7 silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,l,3-benzothiadiazole)-5,50-diyl] (SiPCPDTBT) [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (100 nm)/Ca (20 nm)/Al(100 nm). The transfer matrix formalism... 

Moreover, we can use the calculated absorptance of the active layer to determine the internal quantum efficiency (IQE) of a device and thereby quantify the electronic losses relative to the optical losses [112-114]. The IQE is the ratio of the measured external quantum efficiency to the absorptance of the active layer of the solar cell. However, because absorptance of the active layer alone cannot easily be measured, the most effective way to determine IQE is to calculate the absorptance of the active layer using the experimentally determined complex refractive indices of all layers (obtained using ellipsometry and sometimes photothermal deflection spectroscopy [230]). 

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