Absorption polymer heterojunctions

The generation of photoexcited species at a particular position in the film structure has been shown in (6.19) and (6.20) to be proportional to the product of the modulus squared of the electric field, the refractive index, and the absorption coefficient. The optical electric field is strongly influenced by the mirror electrode. In order to illustrate the difference between single (ITO/polymer/Al) and bilayer (ITO/polymer/Ceo/Al) devices, hypothetical distributions of the optical field inside the device are indicated by the gray dashed line in Fig. 6.1. Simulation of a bilayer diode (Fig. 6.1b) clearly demonstrates that geometries may now be chosen to optimize the device, by moving the dissociation region from the node at the metal contact to the heterojunction. Since the exciton dissociation in bilayer devices occurs near the interface of the photoactive materials with distinct electroaffinity values, the boundary condition imposed by the mirror electrode can be used to maximize the optical electric field E 2 at this interface [17]. 

The use of Pt-acetylides containing phosphine ligands was extended further by the Schanze group in 2006 [84, 85], In one contribution, they incorporated platinum-acetylide polymers into photovoltaic devices which demonstrate good device efficiency. Transient absorption studies provide definitive evidence for photoinduced electron transfer from the Pt-acetylide to PCBM by the temporal evolution of the TA spectrum, observing the formation of the PCBM radical anion at 1,050 nm. The same system was eventually demonstrated to operate as a bulk heterojunction photovoltaic device [84],... 

Figure 7.10 Tandem solar cell structure for polymer blend solar cells, based on the design demonstrated by Hadipour et al. (2006). In this all-solution-processed device, the top cell consists of a polymer PCBM bulk heterojunction with an absorption maximum of 550 nm and preferentially absorbs short-wavelength light, while the bottom cell is made from a bulk heterojunction of PCBM with a red-absortring polymer and absorbs longer-wavelength light. The composite gold-PEDOT PSS internal layer connects the two cells in...

Hoppe H, Arnold N, Meissner D, Sariciftci NS (2003) Modeling the optical absorption within conjugated polymer/fullerene-based bulk-heterojunction organic solar cells. Sol Energy Mater Sol Cells 80 105... 

Since protection of electrodes against corrosion in the photoelectrolysis cells is a question of vital importance, many attempts have been made to use protective films of different nature (metals, conductive polymers, or stable semiconductors, eg., oxides). Of these, semiconductive films are less effective since they often cause deterioration in the characteristics of the electrode to be protected (laying aside heterojunction photoelectrodes specially formed with semiconducting layers of different nature [42]). When metals are used as continuous protecting film (and not catalytical "islands" discussed above), a Schottky barrier is formed at the metal/semiconductor interface. The other interface, i.e., metal/electrolyte solution is as if connected in series to the former and is feeded with photocurrent produced in the Schottky diode upon illuminating the semiconductor (through the metal film). So, the structure under discussion is but a combination of the "solar cell" and "electrolyzer" within the photoelectrode Unfortunately, light is partly lost due to absorption by the metal film. 

Figure 13.7 Exciton diffusion to the heterojunction. Experimental (symbols) and modeled (lines) diffusion-limited exciton populations are compared. The experimental data show the exciton population recovered from femtosecond transient absorption measurements of charge generation in PFB F8BT polymer blends. The modeled data are a fit using a modified Fokker-Planck equation, with (dashed line) and without (solid line) the drift component close to the interface. The inset shows a cartoon of the diffusion (D ) in the bulk of the domain and the additional drift towards the heterojunction (DV in the interface region. (Reprinted with permission from Physical Review Letters, Probing the morphology and energy landscape of blends of conjugated polymers with sub-10 nm resolution by S. Westenhoff, I. A. Howard and R. H. Friend, Physical Review Letters, 101, art.no. 016102. Copyright (2008) American Physical Society)...

At the early development of polymer solar cells, a planar p-n junction structure represented the mainstream in mimicking conventional silicon-based solar cells. However, the obtained devices demonstrated poor photovoltaic performances due to the long distance between the exciton and junction interface and insufficient light absorption due to the thin light absorber. It was not until 1995 that the dilemma was overcome with the discovery of a novel bulk heterojunction in which donor and acceptor form interpenetrated phases. Poly[2-methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene] was blended with Ceo or its derivatives to form the bulk heterojunction. A much improved power conversion efficiency of 2.9% was thus achieved under the illumination of 20 mW/cm. (Yu et al., 1995). The emergence of the donor/acceptor bulk-heterojunction structure had boosted the photovoltaic performances of polymer solar cells. Currently, a maximal power conversion efficiency of 10.6% had been reported on the basis of synthesizing appropriate polymer materials and designing a tandem structure (You et al., 2013). The detailed discussions are provided in Chapter 5. 

Andersson and coworkers have prepared solar cells based on blends of poly(2,7-(9-(2 -ethylhexyl)-9-hexyl-fluorene)-fl/t-5,5-(4, 7 -di-2-thienyl-2, l, 3 -benzothiadiazole) (223) and PCBM [416]. The polymer shows a Amax (545 nm) with a broad optical absorption in the visible spectrum and an efficiency of 2.2% has been measured under simulated solar light. The same group has also reported the synthesis of low bandgap polymers 200 (1 = 1.25 eV) and 224 (1 = 1.46 eV) which have been blended with a soluble pyrazolino[70]fiillerene and PCBM, respectively, to form bulk heterojunction solar cells of PCE of 0.7% [417] and 0.9% [418]. Incorporation of an electron-delident silole moiety in a polyfluorene chain affords an alternating conjugated copolymer (225) with an optical bandgap of 2.08 eV. A solar cell based on a mixture 1 4 of 225 and PCBM exhibits 2.01% of PCE [419]. 

A polymer based on EDOT containing a perylenetetracarboxylic diimide unit has been prepared by electropolymerization of 250 [443]. The related absorption spectrum covers the visible range and extends up to 850 nm. Similarly, PV devices based on dyads 251 in which oligo(3-hexylthiophene)s are covalently linked to perylenemonoimide have been recently reported [444]. Preliminary results based on bulk heterojunction devices consisting of ITO/PEDOT-PSS/251 PCBM (l 4)/LiF/Al showed an open circuit voltage of 0.94 V and efficiencies of 0.33% (251, n=l) and 0.48% (251, =3) under standard test conditions (AM 1.5G, 1000 W m ). 

Hoppe, H., N. Arnold, D. Meissner, and N.S. Sariciftci. 2004. Modeling of optical absorption in conjugated polymer/fullerene bulk heterojunction plastic solar cells. Thin Sol Films 451-452 589. 

They reported that these copolymers showed broad absorption curves with long-wavelength absorption maximum around 620 nm and optical band of 1.68 and 1.64 eV for both polymers. Both polymers were studied for photovoltaic response in bulk heterojunction solar cells. They observed an overall power conversion efficiency of 3.15 and 2.60% for the cast polymers. Further improvement led up to 4.06 and 3.35% for the devices based on thermally annealed materials. 

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