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Measured and modelled JV characteristics of crystalline silicon cells below one sun intensity have been investigated. First, the JV characteristics were measured between 3 and 1000 W/m 2 at 6 light levels for 41 industrially produced mono-and multi-crystalline cells from 8 manufacturers, and at 29 intensity levels for a single multi-crystalline silicon between 0.01 and 1000 W/m 2. Based on this experimental data, the accuracy of the following four modelling approaches was evaluated: (1) empirical fill factor expressions, (2) a purely empirical function, (3) the one-diode model and (4) the two-diode model. Results show that the fill factor expressions and the empirical function fail at low light intensities, but a new empirical equation that gives accurate fits could be derived. The accuracy of both diode models are very high. However, the accuracy depends considerably on the used diode model parameter sets. While comparing different methods to determine diode model parameter sets, the two-diode model is found to be preferred in principle: particularly its capability in accurately modelling V OC and efficiency with one and the same parameter set makes the two-diode model superior. The simulated energy yields of the 41 commercial cells as a function of irradiance intensity suggest unbiased shunt resistances larger than about 10 kO cm 2 may help to avoid low energy yields of cells used under predominantly low light intensities. Such cells with diode currents not larger than about 10 À9 A/cm 2 are excellent candidates for Product Integrated PV (PIPV) appliances.

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The bandgap of solid C 60 is found to be 2.3±0.1 eV . The on-site molecular C 60 Coulomb interaction (U) as determined from the KVV C 60 Auger spectrum is found to be 1.6 ±.2 eV . This value of U is shown to be consistent with Frenkel type molecular excitons in the 1.5–2 eV range. These results lead us to suggest that doped C 60 should be considered as a highly correlated system with U/W comparable to that in high T c cuprates. The Auger spectroscpoy results are consistent with a rather long range Coulomb interaction on a single bucky ball indicating that a Hubbard model is not suitable to describe the electronic structure of a C 60 molecule.

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