Polymer/fullerene solar cells

This Hnear dependence has been confirmed by Halls et al. using the same bilayer structure but employing PPV as the electron donor [44]. The authors estimated the exciton diffusion length of PPV to be in the range of 6-8 nm from both the spectral response and the absolute efficiency [44]. Later Roman et al. demonstrated optical modeling to be a useful tool for the optimization of such bilayer solar cells, which in their case was based on a polythiophene derivative and Ceo [89]. The optical modeling was detailed by Petterson et al. [46]. 

Furthermore, Martens et al. have shown by AFM that the drying time is an important parameter for the size of the phase-separated structures. By introducing a hot air flow over a drying film, the drying time could be decreased and consequently the extent of phase separation was reduced [136]. 

Hoppe et al. studied MDMO-PPV PCBM solar cells for decoding the different nanophases within these MDMO-PPV PCBM blends cast from the two solvents (toluene and chlorobenzene) [55]. A large difference in the scale of phase separation could be identified as a major difference between toluene and chlorobenzene cast blends (see Fig. 23), but this could not directly explain the observed differences in photocurrent generation [55]. 

Use of high-resolution scanning electron microscopy (SEM) allowed the uncovering of a further substructure in these polymer-fullerene blends, besides some larger fullerene clusters (see Fig. 24) MDMO-PPV nanospheres representing a coiled polymer conformation were detected together with some solvent-dependent amount of PCBM fullerenes [55,60-62,137]. 

The commonly observed larger scale of phase separation of the toluene cast MDMO-PPV PCBM blends has generally been interpreted as the main reason for the reduced photocurrents in comparison to those of the chlorobenzene cast blends. It can then be expected that a lower charge carrier generation efficiency may result when exciton diffusion lengths of 10-20 nm are well exceeded by the PCBM cluster size (200-500 nm), since many exci-tons are generated within these clusters. Experimentally, it has been identified that indeed some unquenched photoexcitations give rise to residual PCBM photoluminescence in toluene cast blends, whereas in chlorobenzene cast 

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Table 1 Radiative and non-radiative voltage losses in polymer fullerene solar cells...

Muller JG, Lupton JM, Feldmann J, Lemmer U, Scharber MC, Sariciftci NS, Brabec CJ, Scherf U (2005) Ultrafast dynamics of charge carrier photogeneration and geminate recombination in conjugated polymer fullerene solar cells. Phys Rev B 72 195208... 

Vandewal K, Tvingstedt K, Gadisa A, Inganas O, Manca JV (2009) On the origin of the open-circuit voltage of polymer-fullerene solar cells. Nat Meter 8 904... 

Yang LQ, Zhou HX, You W (2010) Quantitatively analyzing the influence of side chains on photovoltaic properties of polymer-fullerene solar cells. J Phys Chem C 114 16793... 

The fill factor (FF), defined as Ima.xVma.x/IScVoc, is 0.25. The relatively low FF may be explained by recombination of charges at the ITO electrode. The present values of Isc and Voc are significantly enhanced in comparison with the device characteristics of a related Ceo-oligophenylenevinylene dyad [102] and quite similar to those previously reported for 7r-conjugated polymer/fullerene solar cells [115], although there has been considerable progress in energy conversion efficiencies of these devices recently [116]. 

Fig. 5.46. Typical I/V curves for an as-produced polymer-fullerene solar cell before and after sealing. Measurements were performed with a solar simulator (Steuer-nagel Solar Constant 575) at an irradiance level of 800 W/m2 and a cell temperature of 55°C. Measured data were corrected to the plotted AM 1.5 values using a calculated mismatch factor of 0.76...

Fig. 5.47. Temperature dependence of the principal photovoltaic parameters for a typical polymer-fullerene solar cell derived from outdoor measurements of its I/V curves. Plotted values of efficiency and Isc have been adjusted to the STC irradiance...

Fig. 5.48. Temperature dependence of normalized photovoltaic parameters for a typical polymer fullerene solar cell derived from indoor measurements of its I/V curves. The ordinate axis displays all parameters normalized to their measured values at 25°C, namely, Jsc 3.1 mA/cm2, Voc- 840 mV, FF 0.55, and 77 1.45%. Active cell area 7.5 mm2. Measurements were performed with a class A solar simulator (Spectrolab X-10). Measured data were corrected to their corresponding AM 1.5 values using a mismatch factor of 0.9...

In the previous section on the short-circuit current, it was demonstrated theoretically and experimentally that Isc in conjugated polymer-fullerene solar cells is controlled to a considerable extent by mobility of the majority charge carriers in the cell s active layer [158]. Moreover, activated behavior of charge carrier mobility in conjugated polymers is known to result in higher mobility at higher temperatures (for a review, see [159]). Accordingly,... 

Kroon J. M., Wienk M. M., Verhees W. J. H. and Hummelen J. C. (2002), Accurate efficiency determination and stability studies of conjugated polymer/fullerene solar cells , Thin Solid Films 403, 223-228. 

Fig. 16 Linear dependence of the compensation voltage Vq (a), defined by the net photocurrent being zero, on the oxidation potential work function) of electrochemically doped PEDOT layers (b) in polymer-fullerene solar cells. (Reproduced with permission from [128], 2002, Wiley-VCH)...

Thermally activated PCBM diffusion and formation of crystalhne aggregates within blends with PPV derivatives were observed even at moderate temperatures [55,68,137]. In contrast, polythiophene based polymer-fullerene solar cells had an overall performance improvement upon thermal anneahng steps [171,172]. This improvement has been mainly correlated with an improved order in the film. This is especially true in the case of polythiophene, which is known to convert to a more ordered phase upon... 

Fig. 30 Photovoltaic properties of an MDMO-PPV based polymer-fullerene solar cell with an active area of 0.1 cm. a External quantum efficiency (EQE) of [70]PCBM MDMO-PPV cells, spin-coated from chlorobenzene (triangles) and ODCB (squares) and of [60]PCBM MDMO-PPV devices spin-coated from chlorobenzene (open circles) b current-voltage characteristics of [70]PCBM MDMO-PPV devices, spin-coated from ODCB in the dark (open circles) and under illumination (AM 1.5, 1000 W/m squares). The inset shows the I-V characteristics in a semilogarithmic plot. (Reproduced with permission from [170], 2003, Wiley-VCH)...

Roster LJA, Mihailetchi VD, Ramaker R, Blom PWM (2005) Light intensity dependence of open-circuit voltage of polymer fullerene solar cells. Appl Phys Lett 86 123509... 

Pasquier AD, Unalan HE, Kanwal A, Miller S, Chhowalla M (2005) Conducting and transparent single-wall carbon nanotube electrodes for polymer-fullerene solar cells. Appl Phys Lett 87 203511... 

D. C. Coffey, O.G. Reid, D.B. Rodovsky, G.P. Bartholomew, and D.S. Ginger, Mapping local photocurrents in polymer/fullerene solar cells with photoconductive atomic force microscopy. Nano Lett., 7, 738 (2007). 

M. Campoy-Quiles, T. Ferenczi, T. Agostinelli, P. G. Etchegoin, Y. Kim, T. D. Anthopoulos, P. N. Stavrinou, D. D. C. Bradley, J. Nelson, Morphology Evolution via Self-Organization and Lateral and Vertical Diffusion in Polymer Fullerene Solar Cell Blends. Nat. Mater. 2008, 7, 158-164. 

S.C. Price, et al.. Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells. Journal of the American Chemical Society, 2011. 133(12) p. 4625-4631. 

Katz EA, Feiiman D, Tuladhar SM, Kroon JM, Wienk MM, Fromherz T, Padinger F, Brabec CJ, Saiiciftci NS (2001) Temperature dependence for the photovolteiic device paiameters of polymer-fullerene solar cells under operating conditions. J Appl Phys 90 5343-5350... 

Widely studied for a wide range of applicatisolar cells, non-linear optics, resists, batteries, diodes, electroluminescent devices, chemical sensors. Most potential applicaticms arise due to the fact that PTs become highly conducting when doped to a metallic level. A fiuOTene substituted PT has achieved efficiencies of 7 % in polymer-fullerene solar cells [115]. 

The capability of drift-diffusion models can be increased by introducing localized states into the band gap. Recently, several studies have introduced single trap levels [83] as well as distributions of localized states in order to describe the results of transient and steady state experiments on polymer fullerene solar cells [54, 84-89]. Most of these models use a Shockley-Read-Hall type occupation statistics for the localized states, which we will discuss in more detail in Sect. 2.3 and the Appendix 2 before discussing some of the implications of this model in the case studies in Sect. 4.1. 

Yun, J. J., Peet, J., Cho, N. S., Bazan, G. C., Lee, S. J., and Moskovits, M. (2008) Insight into the Raman shifts and optical absorption changes upon annealing polymer/fullerene solar cells, Appl. Phys. Lett, 92,251912/1-3. 

Kim Y, Cook S, Kirkpatrick J, Nelson J, Durrant JR, Bradley DDC, Giles M, Heeney M, Hamilton R, McCulloch 1 (2007) Effect of the end group of regioregular poly (3-hexylthiophene) polymers on the performance of polymer/fullerene solar cells. J Phys ChemC 111 8137-8141... 

S. C. Price, A. C. Stuart, L. Q. Yang, H. X. Zhou and W. You, Fluorine Substituted Conjugated Polymer of Medium Bandgap Yields 7% Efficiency in Polymer-Fullerene Solar Cells,/ Am. Chem. Soc., 2011,133(20), 4625. 

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