Bulk heterojunction polymer additives

As a synthetic strategy, simple and versatile reactive blending will continue to play a pivotal role in the development of newer materials. For example, the blending technique is being used to produce bulk heterojunction polymer solar cells (polymer/fullerene) and to develop electrically conductive polymer blends using electrically conductive fillers and additives (Huang and Kipouras 2012). 

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. 

The first realizations of polymer-polymer bulk heterojunction solar cells were independently reported in the mid-1990s by Yu and Heeger as well as by Halls et al. [28,30]. These solar cells were prepared from blends of two poly(para-phenylenevinylene) (PPV) derivatives the well-known MEH-PPV (poly[2-methoxy-5-(2 -ethylhexyloxy)-l,4-phenylenevinylene]) was used as donor component, while cyano-PPV (CN-PPV) served as acceptor component (identical to MEH-PPV with an additional cyano (- CN) substitution at the vinylene group). The blends showed increased photocurrent and power conversion efficiency (20-100 times) when compared to the respective single component solar cells. 

Jung, J.W., Jo, J.W., Chueh, C.-C., Liu, F, Jo, W.H., Russell, T.P., Jen, A.K., 2015. Eluoro-substituted n-type conjugated polymers for additive-free all-polymer bulk heterojunction solar cells with high power... 

M. S. Su, C. Y. Kuo, M. C. Yuan, U. S. Jeng, C. J. Su and K. H. Wei, Improving Device Efficiency of Polymer/Fullerene Bulk Heterojunction Solar Cells Through Enhanced Crystallinity and Reduced Grain Boundaries Induced by Solvent Additives, Adv. Mater., 2011, 23(29), 3315-3319. 

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)...

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