Bulk hetero junction cells

The performance of both types of composites in OPVs is still modest as compared to semiconductor-based solar cells. Improvement in the layer morphology and development of low-bandgap organic materials can lead to higher power conversion efficiencies. An estimation of the maximum expectable efficiency of bulk hetero-junction cells predicts that 10% for solar cells will be reached in the next few years, while practical values of 20-25% would be the limits of the best devices, still to be realized [92]. However, even with low but acceptable rj 5%, mass production of low-cost cells will be a nice solution for renewable energy sources. The main and difficult to solve issue is the stability of the devices, which should attain industrial commercialization requirements. Stable operation for more than 4000 h has been reported for P3HT PCBM bilayer structure cells and can be further improved [93]. 

From the previous results it is reasonable to propose the following device model for a bulk hetero junction solar cell (Fig. 5.17) ... 

The use of low bandgap polymers (ER < 1.8 eV) to extend the spectral sensitivity of bulk heterojunction solar cells is a real solution to this problem. These polymers can either substitute one of the two components in the bulk hetero junction (if their transport properties match) or they can be mixed into the blend. Such a three-component layer, comprising semiconductors with different bandgaps in a single layer, can be visualized as a variation of a tandem cell in which only the current and not the voltage can be added up. 

The interpenetrating network in bulk hetero junction solar cells [9] helps to overcome the limitations of bilayer systems [25,95] with low mobility materials. However, less is known about the nanometer morphology of an interpenetrating network or the optimum density of donor/acceptor interfacial... 

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. 

Polythiophenes and MEH-PPV have been used as hole-transporting layers to replace the electrolyte in cells with TiC>2 electrodes (Gratzel cells). Bulk hetero-junction devices have been made by the hydrolysis in air of titanium(IV) wopropoxide, co-deposited in MDMO-PPV spin-coated films, to form TiC>2 nanoparticles (van Hal et al., 2003). While reasonable peak performance under monochromatic illumination has been reported, the performance as a solar cell does not match that of the photo-electrochemical cells. 

Martens T, Beelen Z, D Haen J, Munters T, Goris L, Manca J, D Olieslaeger M, Vanderzande D, Schepper LD, Andriessen R (2003) Morphology of MDMO-PPV PCBM bulk hetero-junction organic solar cells studied by AFM, KFM and TEM. In Kafafi Z H, Fichou D (eds) Organic photovoltaics III. SPIE Proc 4801 40... 

Munters T, Martens T, Goris L, Vrindts V, Manca J, Lutsen L, Ceunick WD, Vanderzande D, Schepper LD, Gelan J, Sariciftci NS, Brabec CJ (2002) A comparison between state-of-the-art gilch and sulphinyl synthesised MDMO-PPV/PCBM bulk hetero-junction solar cells. Thin Solid Films 403-404 247... 

Yang, F., Shtein, M., and Forrest, S.R., Controlled growth of a molecular bulk hetero-junction photovoltaic cell. Nature Materials, 2005a. 4 37. 

T. Martens, et al, Disclosure of the nanostructure of MDMO-PPV PCBM bulk hetero-junction organic solar cells by a combination of SPM and TEM. Synthetic Metals, 2003. 138(1-2) p. 243-247. 

Riedel, L, J. Parisi, V. Dyakonov, L. Lutsen, D. Vanderzande, and J.C. Hummelen. 2004. Effect of temperature and illumination on the electrical characteristics of polymer-fullerene bulk-hetero-junction solar cells. Adv Pune Mater 14 38. 

Beiley, Z.M., Hoke, E.T., Noriega, R., Dacuna, J., Burkhard, G.F., Bartelt, ).A., Salleo, A., Toney, M.F., and McGehee, M.D. (2011) Morphology-dependent trap formation in high performance polymer bulk hetero junction solar cells. Adv. Energy Mater., 1,954-962. 

Sharma GD, Zervaki GE, Angaridis PA et al (2014) Triazine-bridged porphyrin triad as electron donor for solution-processed bulk hetero-junction organic solar cells. J Phys Chem C 118(ll) 5968-5977... 

Semiconductor structures that develop space charge layers and contact potentials, like films of proper thickness, films with applied external bias, homo- and hetero-(nano)junctions, permit significant suppression of bulk recombination processes and, potentially, allow high quantum yields. Spatial separation of electron and holes also allows the separation of cathodic and anodic processes in a photoelec-trochemical cell (eventually at the micro and nano level), minimizing surface re-... 

---