Managing measurement data

6.4.7 The QE-panel

This panel allows to analyse the QE simulations. Specifically, the meaning of QE in SCAPS is external quantum efficiency QE: the number of electrons leaving the cell (ass current) divided by the total number of photons incident on the cell.

On the horizontal axis one can display either the wavelength or the photon energy of the monochrome light.

On the vertical axis one has a wider choice:

Figure 6.8 Ordinate choices on the QE-panel

The fourth option (spectral response SR) is a recent addition (august 2017) to serve users that want to simulate photodetectors. SR is defined as the short circuit current divided by the total incident light power, and thus is in A/W (this is also mA/cm² divided by mW/cm², should you insist). Note that SR = QE/h when SR is in A/W, QE dimensionless and h in eV. The four last options are sometimes used to derive a band gap value from QE measurements or simulations.

The QE is calculated by comparing the current at the workingpoint conditions and the current when adding an additional amount of monochromatic photons. This number of photons can be set on the numerical panel, Figure 6.9. When both currents are large compared to their difference, it is possible that your computer makes a numerical error in the subtraction leading to a non-physical QE which is negative or larger than 100 %.

Figure 6.9 Setting the number of photons for the QE simulation in the Numerical Panel.

In SCAPS 3.3.02, version august 2015, the defaults values displayed in standard units of the SCAPS panels (thus: cm, mA, mW-based), and were changed to Pphot= 0.1 mW/cm², corresponding to 0.001 sun, and to 310¹⁸ /cm²s, that is for ( = 620 nm, h = 2 eV) photons, also roughly corresponding to 0.001 sun. These default values could be representative for actual QE measurements with a monochromator set-up.

6.5 Managing measurement data

SCAPS provides facilities to compare simulation results with measurement data. Measurement data can be loaded by clicking the -button, which redirects to the ‘Manage measurements panel’, Figure 6.10.

6.5.1 The Manage measurements panel

This panel shows a list of the measurement files which are currently loaded. Additional measurements can be added, measurements can be removed or replaced and the order of the list can be changed. By (un)checking items in the list you can control which measurements have to be displayed in the graphs. When selecting an item in the list, a summary of its properties are displayed on top of the panel. This allows to check whether you have loaded the measurement you intended and whether the working point conditions of the file have been read correctly.

Figure 6.10 Manage measurements panel

6.5.2 Structure of a measurement file

Any ASCII-text file can be read as a measurement file. In particular, any of the simulation results which have been saved by SCAPS can be read as a measurement file. So, if you feel unsure whether your file will be read correctly, you can try to start from a SCAPS-results file.

The file extension indicates which kind of measurement it contains. The allowed extensions are ‘.iv’, ‘.cv’,

‘.cf’ and ‘.qe’.

A measurement file is read until the first line which can be interpreted as measurement data is found. This is a line starting with at least two numeric values. All lines above are considered to be part of the header (which might be mere comments or contain information about the working point conditions) All lines below the first data line are considered to contain measurement data. As a result, any line which can not be interpreted as data which occurs below the first data line will be discarded and can thus not be interpreted as working point conditions. Hence, when more than one measurement is contained in a file only the working point conditions of the first measurement will be read, even though the data points of all measurements will be loaded!

6.5.2.1 The workingpoint conditions

SCAPS will try to interpret the information in the header of the file as the working point conditions of the file.

When any of these words is found at the beginning of a line, the numerical value which follows is saved.

None of these words is case-sensitive. An overview of the code-words used is listed below.

Table 6.1 Workingpoint code words

code-word(s) unit Default value remarks

area; cell area; cellarea; cm² 1.0 See below

6.5 Managing measurement data 69

 The area is used to scale the current, capacitance and conductance values to an area of 1 cm².

 Alternative units for the area are supported. Adding any of the words [“cm2”, “cm^2”, “cm²”]; [“mm2”,

“mm^2”, “mm²”] or [“m2”, “m^2”, “m²”] to the area statement will change the unit of the area read to cm²; mm² or m². If none of these words are found the unit of the area is cm².

 In a similar way as for the area, different units for the incident power are supported: mW/cm2;

mW/cm^2; mW/cm²; W/m2; W/m^2; W/m²; mW/m2; mW/m^2; mW/m²; W/cm2; W/cm^2; W/cm².

 The temperature and bias voltage are only used to calculate admittance spectra

 The incident power is used to calculate the efficiency.

 When the incident power is zero (or when the code-word dark has been found) the I-V parameters (thus:

, Jsc, Voc and FF) will not be calculated.

6.5.2.2 The measurement data

When a first data line is found, all subsequent lines will be interpreted as data lines. Up to five numeric values will be read. The meaning of these numbers varies according to the measurement type as listed below.

Table 6.2 Data in the columns of a measurement file

measurement #1 #2 #3 #4 #5

 The depletion width W and the apparent doping density Napparent can be read from the measurement-file, but they can also be calculated from C and V by SCAPS. This choice is governed by a switch button on the capacitance analysis panel.

The default units for the measurement data are given in the table. It is however possible to change them. In order to do this, a line starting with the word “units” has to be added in the header. Next to this code-word a list of properties with their unit can be specified.

Examples: units: I: A V: mV

Units: C: pF V: V G: mS

A list of property-names which can be changed for every measurement type are given in the Table below:

I-V V: I: J:

C-V V: C: G: W: N:

C-f f: C: G:

QE lambda: QE:

Pay attention! The semicolon ‘:’ is obligatory.