Efficiency and Open Circuit Voltage

The presence of a step introduces two significant changes. First, in contrast to a smooth surface which does not exhibit unsaturated bonds in the direction perpendicular to the surface, strong covalent bonds are exposed to the electrolyte at the edge of a step. These are likely to chemisorb species from the environment and introduce surface states within the band gap (29). These are recognized recombination sites and shunts in solar cells, known to reduce fill factors, current collection efficiencies and open circuit voltages (30, 31). 

In this chapter, the fundamentals of classical thermodynamics as it applies to the study of fuel cells is introduced. Although the reader is assumed to have a background in basic thermodynamics, this chapter includes a review of the physical meaning of several parameters used frequently in electrochemistry and how calculations of their values can be made. This chapter concludes by applying the thermodynamic concepts presented to determine the maximum expected thermodynamic efficiency and open-circuit voltage expected for a fuel cell at a given condition. 

Fig. 4.8. Efficiency rj, open-circuit voltage Voc, and short-circuit current density jsc as a function of the band gap a of a 2-band system illuminated by blackbody radiation at 5 800 K with an incident energy current density of 1.39 kW/m2...

Fig. 41 short circuit current (A), power conversion efficiency (B), fill factor (C), and open circuit voltage (D) of a number of P3HT PCBM devices using different weight fractions (x-axis) as compared to the unfractionated sample. Clearly the larger weight fractions yield better values for all parameters investigated. (Reprinted with permission from [193], 2005, American Chemical Society)... 

PPV polymers 249 containing bis(phenylimino)acenaphthalene rhenium(I) complexes were prepared by Heck coupling, as in Scheme 4.52.139 The polymers had molecular weights of 9200-17,200 Da (Mn, GPC) and were characterized by a number of techniques. A multilayer photovoltaic device incorporating polymer 249 (x = 0.3, y = 0.7) showed a low conversion efficiency ( 0.06%) with short circuit current and open circuit voltage of 380 pA cm 2 and 0.81 V, respectively.140... 

Table 3. Peak stack power, energy efficiency, energy density and open circuit voltages of a 28-cell DMFC stack with 22 cm electrode area operated at 60 "C using air as oxidant and methanol as fuel with different concentrations.

Spitzer et al found that the limiting efficiency was very sensitive to the magnitude of Sg "As shown in Fig. 14, the maximum efficiency achievable in a device with Sg infinite would be about 20% for a device having a thickness of about 7ym, and about 17% for a device having a thickness of 1.5ym. Both short circuit current and open circuit voltage decrease with thickness if Sg is high. 

There is experimental consensus on the most important parameters of singlelayer polymer photovoltaic devices, the short circuit current / , the open circuit voltage V c, and the filling factor FF. From these parameters the efficiencies of PPV based devices were typically calculated to be around 0.1% under monochromatic low light intensities. Efforts to extend the classical semiconductor picture of... 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

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