Solar cells, modeling short-circuit current

Table 1.1 Solar cell performance parameters including the parameters of the equivalent-circuit model, short-circuit current density Jsc, open-circuit voltage Vqc, fill factor FF and solar-to-electrical energy conversion eiSciency rj at lOOmW cm illumination (AM 1.5).

Figure 13 Dependence on 10-MeV-proton and 1-MeV-electron fluence of the remaining factor of short circuit current (Isc) for Si-BSFR solar cells. Circles and squares represent the results for samples irradiated with 10-MeV protons and 1-MeV electrons, respectively. Broken lines represent the fitting results based on a model mentioned in the text.

Short-Circuit Current. Key parameters for efficient charge collection by plastic solar cells are the hole and electron mobilities of the interpenetrating networks and the lifetime of the carriers within this network. While the lifetime of the carriers in the bulk heterojunction blends has already been discussed as a peculiarity of the interpenetrating network, the mobility of the individual components is a true material parameter. The interplay between network quality and mobility and their impact on the short-circuit current will be discussed by means of a simple model in this section. 

The ratio of the short circuit current to the fully saturated current, as argued, gives directly the ratio of the number of coulomb-pairs versus free carriers at short circuit, which is estimated to be only 60% in the studied MDMO-PPV/PCBM samples. More recently, the model has been expanded to describe the performance of bulk heterojunction solar cells depending on the PCBM concentration. In these recent models, the mobility of electrons and holes determined in independent experiments has been incorporated [103]. 

Mitzi et al. have reported an optical-design approach which resulted in improvement of the short-circuit current and power conversion efficiency of CZTSSe solar cells. They have optimized the thickness of the upper-device layers in a way that allows maximum transmission into the CZTSSe absorber. This design approach was based on optical modeling of idealized planar devices with a semi-empirical approach for treating the impact of surface roughness. Effectiveness of the new device architecture based on thinner CdS and TCO layers was experimentally demonstrated by fabricating solar device with overall power conversion efficiency of 12.0% a new record in CZTSSe based devices. 

Moet et al. found by modeling the photocurrent that the use of 1,8-octanedithiol can prevent recombination-limited photocurrent in PCPDTBTPCeiBM solar cells [86]. Modeling showed that the decay rate of bound electron-hole pairs is reduced by additive addition resulting in dissociation probability of 70% at short-circuit current... 

To obtain a better understanding of the effect of the mobility on the performance of a solar cell, a simplified model is introduced to provide an analytical description of the dependence of the short-circuit on the material parameters of the semiconductor for thin film bulk heterojunction solar cells. The following assumptions are suggested to give separate descriptions of the field current and diffusion current ... 

A simple model of band location in polymer solar cells is shown in Fig. 6. The term Fermi level ( f) is used as synonym for the chemical potential of the electrons. Since this level, even as a virtual one, is defined only for an equilibrium situation in the dark, the term quasi-Fermi level is used to describe the situation under illumination. On illumination, and if no current flows, the light-induced carriers split the quasi-Fermi levels, where the electron Ep, E-p,n, is essentially that in the polymer, while the hole p, p,p, is in the semiconductor NCs. If the system is short-circuited, there will be a gradual variation in Ep, and Ep n across the absorber (blend layer) [57]. 

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