Bilayer heterojunction solar cells

Roncali et al. prepared tetrahedral oligothienylsilane derivatives 5.44 and 5.45 by reaction of lithiated terthiophenes with SiCU (Chart 1.72) [486]. In comparison with the linear parent terthiophenes, the tetrahedral structures gave a red shift of 17-19 nm in the absorption spectra. These materials were implemented as the donor component in bilayer heterojunction solar cells, showing a significant increase in performance (jj < 0. 20 %) compared with cells including only the terthiophene branches (t] = 0.04 %) as active material. In bulk-heterojunction solar cells with PCBM as acceptor, 5.45 showed an efficiency of 0.3 %, which is fairly low compared with standard P3HT-PCBM solar cells [195, 196, 487-490]. 

The open circuit voltage (Vqc) of bilayer heterojunction solar cells is governed by the energetics at the D/A interface rather than that involving the electrodes, as pointed out already by Tang in his seminal paper [26]. Conversely, bulk heterojunc-... 

Cuiffi J, Benanti T, Nam WJ, Fonash S (2010) Modeling of bulk and bilayer organic heterojunction solar cells. Appl Phys Lett 96 143307... 

This view of Voc generation is additionally supported by the fact that the values of the temperature coefficient dUoc/dT = -(1.40-1.65) mVK-1 for the cells under the present study (with bilayer LiF/Al and ITO/PEDOT contacts) coincide with those for polymer/fullerene bulk heterojunction solar cells of the previous generation (with the same components of the active layer but without LiF and PEDOT contact layers) [156]. In this picture, the temperature dependence of Voc is directly correlated with the temperature dependence of the quasi-Fermi levels of the components of the active layer under illumination, i.e., of the polymer and the fullerene. Therefore, the temperature dependence of Voc over a wide range, and in particular V),c(0 K), are essential parameters for understanding bulk hetero junction solar cells. 

In two-component charge transfer systems, such as in the bulk-heterojunction solar cells presented here, deviations of the Vgc from the results of pristine single layer or bilayer devices are expected for two reasons first, some part of the available difference in electrochemical energy is used internally by the charge transfer to a lower energetic position on the electron acceptor second, the rela-... 

Q. Sun, L. Dai, X. Xiaoli, L. Li, Q. Li, Bilayer- and bulk-heterojunction solar cells using liquid crystalline porphyrins as donors by solution processing. Appl. Phys. Lett. 91, 253505... 

Another method that was successfully applied for the formation of a nano-structured bulk heterojunction organic solar cell is nanoimprint lithography (NIL). In direct comparison with a flat bilayer organic solar cell design, the nanostructured version exhibited nearly a double increase in solar PCE [119]. 

Sun, Q., Park, K., Dai, L. Liquid crystalline polymers for efficient bilayer-bulk-heterojunction solar cells. J. Phys. Chem. C 113(18), 7892-7897 (2009)... 

It is the purpose of this chapter to introduce photoinduced charge transfer phenomena in bulk heterojunction composites, i.e., blends of conjugated polymers and fullerenes. Phenomena found in other organic solar cells such as pristine fullerene cells [11,12], dye sensitised liquid electrolyte [13] or solid state polymer electrolyte cells [14], pure dye cells [15,16] or small molecule cells [17], mostly based on heterojunctions between phthalocyanines and perylenes [18] or other bilayer systems will not be discussed here, but in the corresponding chapters of this book. 

There are four different types of organic or organic/inorganic hybrid solar cells single-layer, bilayer, bulk heterojunction (BHJ), and dye-sensitized. The basic operation of each device type is described below. 

Dye-Sensitized Cells Dye-sensitized solar cells (DSSCs) are slightly more complex than bilayer and bulk heterojunction cells, but as was the case for bilayer cells, the increase in device complexity reduces the number of functions that must be performed by each of the materials. A schematic drawing of a dye-sensitized solar cell is shown in Fig. 8.8. A layer of sintered, interconnected TiC>2 nanoparticles, which serves as the electron transport material (ETM), is coated by a thin layer of light absorbing dye. The remaining pores in the dye-coated TiC>2 layer are then filled with a... 

As shown in Figure 5.9, randomly mixed polymer-inorganic BHJ and ordered heterojunction (OHJ) are two common structures to prepare hybrid solar cells [44]. Similar to polymer-fullerene BHJ solar cells, polymer-inorganic hybrid BHJ solar cells (Figure 5.9a) can overcome the limitations of bilayer devices having small donor-acceptor interfacial area with inefficient exciton dissociation. NCRs can be surface modified to render them soluble in organic solvents (such as chloroform, toluene and chlorobenzene) to facilitate solution processability. Various surface... 

Figure 18.1 Structural configurations of typical solar cell made on glass substrate with transparent indium tin oxide (ITO) electrode. The active layer can typically have three configurations (1) single organic layer, (2) bilayer (double-layer) heterojunction between donor and acceptor and (3) bulk heterojunction...

Figure 6.19 Working principle of a bilayer heterojunction organic solar cell.

With the development of organic semicondnctors which support electron or hole transport (in analogy of p- and n-type inorganic semiconductors) bilayer type p-n heterojunction devices have been constructed. In the case when organic p- and n-type materials are deposited consecutively in two layers we get lateral heterojunction devices which mimic classical p-n junction junction solar cells based on silicon. The first such cell was designed by Tang and pnb-lished in 1986 (Fignre 3)." Copper phthalocyanine was utilized as electron donor (p-type) material, whereas pery-lene derivative was nsed as electron acceptor connterpart (n-type material). 

A special type of solar cells as a hybrid combination of planar bilayer and bulk heterojunction devices has been... 

There are two main types of PSCs including bilayer heterojunction and bulk-heterojunction [7]. Bulk-heterojunction PSCs are more attractive due to their high surface area junction that increases conversion efficiency. This type of polymer solar cell consists of Glass, ITO, PEDOT PSS, active layer, calciiun and aluminum in which conjugated polymer are used as active layer [8]. The organic solar cells with maximum conversion efficiency about 6% still are at the begiiming of development and have a long... 

In particular, the introduction of fullerene showed its ability as electron acceptor for performance polymer solar cells. The single-layer device based on conducting polymer was showing the 0.001-0.1 % PCE [22, 23]. Since the discovery of ultrafast photoinduced electron transfer from MDMO-PPV to fullerene, the bilayer cell of conducting polymer/C6o was investigated [24, 25]. The heterojunction device showed the rectification ratios on the order of 10", while energy conversion efficiency was not high. 

The first organic tandem solar cell was published in 1990 by Hiramoto et al, who employed two identical subcells composed of bilayers of a metal-free phthalocyanine and a perylene tetracarboxylic derivative with an ultrathin Au layer to interconnect the two subcells to achieve an almost doubling of the voltage. More than a decade later, in 2002, Forrest et reported on two, three, and five stacked heterojunction cells consisting of copper phthalocyanine (CuPc) as a donor and perylenetetracarboxylic bis-benzimidazole (PCTBI) as an acceptor. Ultrathin ( 5 A) layers of Ag clusters were placed between the heterojunctions to interconnect the subcells. Similar results were described by Tsutsui et In 2004, Leo and Pfeifer et intro-... 

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