Current-Voltage Measurement

The current-voltage measurement, short IV-measurement, is the standard measurement for characterising solar cells. A linear voltage sweep, in our case usually between ±1V, is applied to the OSC and the resulting current I in the circuit is measured. In order to allow comparison between dierent solar cells the current is usually scaled to J, the current density being an intensive property. The measurement is carried out both without and with illumination. In the dark, the IV-curve shows a diodic behaviour; under illumination, extra charge carriers are generated in the solar cell and the

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60 CHAPTER 4. MEASUREMENT METHODS AND AUTOMATION

Figure 4.1: Typical dark (black) and illuminated (red) IV-curves of a solar cell with a focus on the forth quadrant. The important parameters are indicated:

the open circuit voltageV_oc, the short circuit current density J_sc, the voltage V_mpp and the current density J_mpp at the maximum power point (mpp). The ll factor F F is the ratio of the area of the lled square, i.e. the maximum extractable power P_mpp = V_mppJ_mpp, to that of the empty square (V_ocJ_sc).

From these parameters and the incoming illumination intensityP_ill the power conversion eciency η is calculated as given in equation 4.2.

dark IV-curve is shifted in the negative direction. Exemplar IV-curves for both cases are shown in gure 4.1.

Under illumination, the key gures of a solar cell can be determined directly from the IV-curve. The open circuit voltage Voc and short circuit current densityJsc1 are the intersections of the illuminated IV-curve with theV-axis andJ-axis respectively. In the fourth quadrant the electric powerPel=V J is negative and this power can be extracted from the solar cell. The maximum powerPmppgenerated by the solar cell is at the maximum power point (mpp), the point on the curve where the absolute value |Pel| reaches its maximum in the fourth quadrant. The corresponding voltage and current density is denoted asVmpp and Jmpp respectively. The ratio between the real maximum power Pmpp and product of VocJsc is called ll factor F F.

F F = V_mpp J_mpp

V_oc J_sc (4.1)

Finally, the power conversion eciency η is the ratio betweenP_mpp and the

1Strictly speaking,Jsc is negative, but following convention its absolute value will be

quoted throughout this thesis.

4.1. MEASUREMENT METHODS 61 intensity of the incident light P_ill.

η= P_mpp

P_ill =F FV_oc J_sc

P_ill (4.2)

This fraction of the incident light intensity is converted into electrical power and is available to external loads.

The basic measurement setup is shown in gure4.2. A Keithley 2400 Source-Meter, controlled by a LabView program, is used for generating the voltage sweep and measuring the resulting current through the OSC. The OSC is connected in a four wire conguration to avoid parasitic serial resistance of the cables and the interconnects on the substrate. When measured in dark, the OSCs are covered to ensure absolute darkness. For illuminated IV-curves, two dierent light sources are available. They are employed in two dierent setups, which will be briey described below.

LabView Program Source−Meter

N −filled Glovebox₂

Sun Simulator

Organic Solar Cell

Figure 4.2: The basic IV-measurement conguration is depicted. A LabView controlled Keithley 2400 Source-Meter is used to record the IV-curve of the OSC in a four wire conguration. There are two light sources available in dierent setups for measuring IV-curves. In the one shown, the illumination of the OSC is provided by a sun simulator. Its light is transmitted into the glovebox through a borosilicate window. The second setup, which is used for IV-measurements, is mainly used for the spectral response measurement (section 4.1.2) and thus shown in gure 4.5.

Sun Simulator

The sun simulator (SolarCelltest 575 from K. H. Steuernagel Lichttechnik GmbH) generates a spectrum close to the standard AM1.5g solar spectrum with a homogeneous illumination on an area of about 10×10cm². The sim-ulated sun light is coupled into the glovebox from below through a borosili-cate glass window. Before recording the IV-curve on the sun simulator, the

62 CHAPTER 4. MEASUREMENT METHODS AND AUTOMATION illumination intensity is measured with a calibrated solar cell (LOTSDSC5, Fraunhofer ISE Callab) and corrected for the spectral mismatch (see sec-tion 4.1.2) such that the OSC generates the J_sc which it would have when exposed to an intensity of 1000W/m² with the spectrum of the sun. While on the sun simulator, all OSCs are cooled by a fan. The temperature for the OSCs has been measured with a thermocouple and stabilised below 35^◦ C after ve minutes. IV-measurements were then started after the substrate temperatures had equilibrated. Although this is not according to the stan-dard reporting conditions (SRC²), it allows reliable and comparable testing.

Xenon Lamp

In the second setup the illumination is provided by a 1000W Xenon-lamp with a stabilised power supply (both by company Müller GmbH). This setup has a dual use. It can be used for IV-curves at dierent illumination in-tensities and the spectral response, which will be described in section 4.1.2, can be measured there. The light of the Xe-lamp is coupled into the glove-box through an optical quartz glass bre cable (custom made by Schölly Fiberoptic GmbH), with a high transmission into the UV range. The light beam is homogenised before being focused onto one OSC at a time. The area of the light spot is approximately 1×1cm², which is still larger than the used photovoltaic active area. Variations in the illumination intensity are achieved by inserting neutral density lters into the light beam in front of the measured solar cell, allowing intensities between about 2 and 0.001 suns.

Lower intensities are in principle possible, but then the setup requires a bet-ter shielding against stray light. The lamp intensity is measured in parallel with a monitor diode to compensate for any lamp drift. The intensity at the OSC is then calculated using the transmission of the lter and both the intensity and spectrum of the lamp. This setup is described in more detail in the section on spectral response measurements (see gure4.5), because most components are only necessary there. For IV-measurements all components of the setup are set such that the white light of the Xe-lamp is transmitted directly and not being modulated.

2The standard reporting conditions state an illumination of 1000W/m² with the

stan-dard spectrum AM1.5g and the solar cell at 25^◦C.

4.1. MEASUREMENT METHODS 63 Measurement Uncertainties

The main uncertainties in the IV-curve and its derived parameters arise from measuring the illumination intensity and from assessing the photovoltaic ac-tive area. The measurement accuracy of the Keithley source-meter is not limiting the accuracy of the IV-measurement.

The main problem in assessing the illumination intensity an OSC sees is due to the fact that the spectra of the used lamps dier from the standard reference spectrum AM1.5g for which eciencies should be reported. Mea-suring and adjusting the intensity of the lamp is done with calibrated silicon solar cells. However, both the reference cell and the measured OSC have a dierent spectral response (see section 4.1.2). Without any further correc-tion this dierence leads to a wrong calculacorrec-tion of the light intensity seen by the OSC. If the spectral response of both the reference solar cell and the measured solar cell are known, this source of error can be corrected by intro-ducing a mismatch or spectral correction factor [63]. Calculating the spectral response and the mismatch factor is described in section4.1.2.

The homogeneity of the illuminated area is the second critical issue with regard to the illumination intensity. For the setup with the Xe-lamp, the ho-mogeneity has been measured to be better than 4% across the photovoltaic active area of the used OSCs. Since the light spot of the Xe-lamp is always focussed the same way on an OSC, there are no dierences in illumination between dierent OSCs. On the sun simulator, the variations in illumina-tion intensity have been measured with a calibrated reference silicon solar cell (LOTSDSC5) and amount at 1 sun to less than 5% on the≈10×10cm² area used for measurements. Here, the variations have a bigger inuence on the measurements, because the intensity can vary from OSC to OSC, depending on where on the sun simulator it is being measured. This source of uncer-tainty will be corrected in future by mapping out the spatial inhomogeneities and considering them during the calculations of J and η.

The last major source of uncertainty is given by measuring the photovoltaic active area of the OSC. The active area is given by the overlap between anode and cathode as shown in gure4.3. The active area is underestimated when cross-talk, e.g. ITO shorts between dierent OSCs, leads to an extra contribution in current or overestimated, if the pattern of top and bottom electrode does not match. The active area of the OSCs in this work is in the

64 CHAPTER 4. MEASUREMENT METHODS AND AUTOMATION (c)

(a) (b) (d)

Electrode 1 Electrode 2

Active Cell Area (electrode overlap)

Figure 4.3: Denition of the photovoltaic active area of an OSC. Generally, in plane conduction can be neglected and thus the active area of an OSC is dened as overlap between the two electrodes as shown in (a) (see also sections 3.2.4 and 4.2.1). Depending on the tolerances of the pattern, a certain misalignment is allowed without changing the active area as shown in (b). In the examples shown in (c) and (d) the misalignment is beyond the tolerances and the area loss amounts to ≈ −9% and ≈ −25% respectively.

order of a few mm² and uncertainties in the area can cause errors in the area of 10% or more. Both cases, however, can be reduced by a careful inspection of substrates.