4.1 Screen output

\(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} SimSS}}\) shows its progress by listing the voltage and current (\(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} Vint}}\), \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} Jext}}\)) ¹ it has found, including an estimate of the error on the current (see section 2.9). If the simulation at some voltage fails to converge, then the user is warned and the current is not shown. If \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} SimSS}}\) suspects that the user is simulating a solar cell under illumination, then it will try to obtain and display the main PV parameters (fill-factor, short-circuit current density, etc.). These parameters are calculated based on the external current and voltage, see section 2.8. So, if there is a finite series resistance (\(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} R\_ series}}\)), and \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} Vint}}\) = 0 is one of the simulated voltages, then \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} SimSS}}\) will not necessarily have simulated the short-circuit condition (\(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} Vext}}\) = 0) and will, therefore, use interpolation to estimate the short-circuit current density. The error margins that \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} SimSS}}\) shows are purely based on interpolation errors and their propagation: they do not take into account any inaccuracies due to grid spacing, or tolerances of the iterative solvers.

If an external current-voltage curve is used, ² then \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} SimSS}}\) will also calculate and show the PV parameters for the external curve so the two can be compared: both the simulated and the experimental parameters are shown as well as their differences. Additionally, \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} SimSS}}\) will calculate a fit error such that simulated and experimental current-voltage curves can be compared directly: this error compares the curves, not just the main PV parameters as defined by parameter \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} fit\_ mode}}\). Note, this comparison is also done if the simulated device is not a solar cell. The fit error is defined as the area between the simulated and experimental current-voltage curves normalised with the maximum spanned area, as illustrated in Fig. 4.1.

\includegraphics[width=15cm]{figs/fit_error.png} — Figure 4.1 An illustration of the definition of the fit error. The fit error is calculated by dividing the difference area between the simulated and experimental curves by the normalisation area.

If \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} fit\_ mode}}\) is ’lin’ then the fit error \(\epsilon \) is calculated as

\begin{equation} \epsilon =\frac{1}{\left(J_{\rm max}-J_{\rm min}\right)\left(V_{\rm max}-V_{\rm min}\right)} \int _{V_{\rm min}}^{V_{\rm max}} \left|J_{\rm exp} - J_{\rm sim}\right|\mathrm{d}V. \end{equation}

In the case of ‘log’ we only sum over currents for which the experimental and simulated values have the same sign, and we calculate the fit error as

\begin{equation} \epsilon =\frac{1}{\ln \left(\frac{J_{\rm max}}{J_{\rm min}}\right)\left(V_{\rm max}-V_{\rm min}\right)} \int _{V_{\rm min}}^{V_{\rm max}} \ln \left(\frac{J_{\rm exp}}{J_{\rm sim}}\right)\mathrm{d}V. \end{equation}

Note, we only use voltages (\(V\)) that converged.

\(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} ZimT}}\) shows its progress as it reads which times, voltages, and generation rates should be simulated. As there can be very many such time steps, this output (on screen) is shown only every \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} outputRatio}}\) instances. \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} ZimT}}\) shows the external current and voltage, see section 2.8, including an error estimate of the current. If one such combination of time, voltage, and generation rate fails to converge, then \(\mathtt{\require{color}{\color[rgb]{0.000000000000000,0.500000000000000,0.500000000000000} ZimT}}\) switches to red font to warn the user.