Organic solar cells polymer:fullerene devices

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... 

Xu et al. [78] have synthesized gold (Au)-silica nanorods with shell thickness of 0-10 nm incorporated into the bulk heterojunction of organic solar cell and observed that at optimal (1 wt. %) concentration, Au-silica nanorods with 5 nm shell thickness resulted in the highest power conversion efficiency of 8.29%. A 26% relative enhancement in PCE was observed for a low bandgap polymer fullerene bulk heterojunction (BHJ) device after incorporation of 1% weight ratio of Au-silica nanorods by Xu et al. [79]. A similar enhancement was found when Ag-silica NPs were introduced at the interface of the anode buffer layer and active layer of a polymer fullerene BHJ device by Choi et al. [80]. Yang et al. [81] have developed a novel hole transport layer (HTL) composed of ultrathin two-dimensional, molybdenum disulfide (M0S2) sheets decorated with 20 nm... 

More recently, intense research efforts have been devoted to developing [60] fullerene-polymer hybrids in which [60]fuUerene molecules are incorporated onto polymer main chains or side chains [23-25]. These [60]fullerene-polymer hybrids have proven to be innovative materials, functioning as organic solar cells, magnets, photoconductors, optical-limiting devices and semiconductors [26-28]. Synthetically, the 30 highly reactive double bonds of a [60]fullerene molecule limit the possible polymerization reactions [24, 28-31]. However, supramolecular methods of incorporating [60]fullerene onto a polymer main chain can overcome this Umitation. 

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. 

Various alternative acceptor components for organic BHJ solar cells have been tried in an attempt to improve cell performance. Fullerene derivatives such as C70 PCBM and Cg4 PCBM have been used in place of Ceo PCBM, because the lower molecular symmetry compared with Ceo PCBM enables stronger light absorption by the fullerene. The C70 PCBM derivatives were relatively successful, leading to 3% power conversion efficiency for devices made with MDMO-PPV polymer (Wienk et al, 2003). The Cg4 derivatives resulted in rather poor device efficiencies, attributed to the unfavourable film morphology resulting from the immiscibility of Cs4 derivatives with typical organic solvents (Kooistra et al, 2006). 

Fullerene/polymer organic bulk heterojunction solar cells have very simple architecture (Figure 47a). To make such devices, fullerene derivative and a conjugated polymer are dissolved together in an appropriate solvent. The resulting solution is deposited on a conducting substrate... 

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