Time resolved charge extraction techniques

Transient photovoltage decay (PVD) measurements allow the time resolved analysis of charge carrier recombination in photovoltaic devices under working conditions [167, 168]: A white light source is used to bias the photovoltaic device generating a constant open circuit potential. Pulsed laser light is used to excite additional charge carriers in the device resulting in an increase in device potential (as is further explained in Section 2.4.2). After the laser stroke charge recombination results in a decay of the potential back to open ciruit conditions which is monitored using a digital oscilloscope. Figure 3.4 shows the assembly of the components including exemplary decay curves. Typical time scales for the voltage decay are in the order of multiple microseconds.

Sample irradiation is accomplished using a LOT-Oriel LS0106 solar simulator equipped with an AM 1.5g solar spectrum lter yielding an illumination intensity of approxi- mately 50 mW/cm2. The illumination is superimposed with a pulsed laser (532 nm,

Figure 3.4: Transient photovoltage/photocurrent decay measurement setup. The schematic drawing shows the main components of the setup and an exemplary trajectory recorded.

pulse frequency10 Hz, pulse energy 1−10µJ, pulse duration≈4 ns). The changes in

the potential between top and bottom contact of the photovoltaic devices caused by the additional laser pulses are monitored using a digital oscilloscope (Tekscope DPO7254)

with a termination resistance of1 MΩ. Data obtained is commonly averaged over 100

transient decay events.

The decay process can be described using a bi-exponential function,

VBias= Afaste−kfastt+ Aslowe−kslowt (3.3) with decay rateskfastandkslowdiering by about one order of magnitude and respective constantsAf ast andAslow. Investigations at dierent laser pulse intensities have shown

thatkfast strongly depends on the perturbing light intensity and does not reveal blend specic properties [169]. The slow decay rate instead gives insight to the material specic non-geminate recombination present in the photovoltaic devices: For solar cells under working conditions an e−1 lifetime of charges can be estimated from the decay

rate as

3.2 Experimental characterization techniques

Transient photocurrent decay (PCD)

Similar to PVD measurements for photocurrent decay (PCD) analysis solar cells are excited by pulsed laser irradiation. Time dependent current characteristics are recorded at an potential drop over the50 Ω termination of the oscilloscope yielding quasi-short

circuit conditions.

The amount of additionally generated charges induced by the laser pulse can be esti- mated at∆Q= ∆I·∆tfrom the transient data obtained. Furthermore, using a combi-

nation of PVD and PCD measurements insight to the chemical capacitanceCchemmay

be obtained [167]:

Cchem = ∆Q

∆V. (3.5)

Transient photocurrents were calculated from the potential drop over the 50 Ω termi-

nation resistance of the oscilloscope. The RC constant of the system was estimated to beτ =RC ≈150 nsassuming a geometrical capacitance ofCgeo≈3 nF. Photocurrent

decay signals depend strongly on the laser intensity used. Furthermore, photocurrent generation induced by layer excitation and white light background illumination vary for dierent solar cells, especially in the case of non equal short circuit currents (ISC).

For the experimental studies presented herein two types of PCD measurements were conducted: In the rst case the laser energy is kept constant resulting in the same perturbation intensity but in dierent photocurrent peak values. For the second type of PCD experiments the laser energy was adjusted each device to yield similar peak pho- tocurrents for all samples. For all experiments the laser excitation energy was chosen to be low in order to minimize non-linear eects.

Charge carrier mobility analysis and the Photo-CELIV method

Improvement of charge carrier mobility µ in organic semiconducting lms is of great

importance for the development of more ecient OPV devices [58]. Several techniques are known to analyze both electron and hole mobility in organic thin lms: Field eect transistors allow the determination ofµ parallel to the substrate plane [12, 20]. Time

of ight measurements give insight to the mobility obtained for organic lms in the sandwich geometry [170]. However, this technique is limited to application in rather thick organic layers (necessity of strong photon absorption within a fraction of the sample thickness - commonlyd≥1µm).

Photo-CELIV is an analysis technique that allows to determine the transport properties

Figure 3.5: Photo-CELIV setup and signal analysis. a) A schematic representation of the com- ponents.b) Laser pulse, voltage sweep and current response of the photovoltaic device shown versus time axis. The shaded area (A1) reects the initial capacitative response of the device. Addi- tional charges are extracted upon voltage sweep after laser excitation (A2). The inset c) shows a magnication of the extraction signal after a laser excitation.

directly on photovoltaic devices: The solar cell is exposed to a short laser pulse prior to a voltage sweep allowing for the collection of photo-generated charges [171]. This charge extraction technique allows to investigate charge carrier mobility and gives valuable information on the recombination kinetics of the device [172].

A linear voltage sweep is applied in reverse bias to the photovoltaic devices [173] and charges generated by the laser light can be extracted at a dened delay time tdel. In

Figure 3.5 both assembly of the components and the typical device response to laser pulse and voltage sweep are shown. In the steady state condition a compensation voltage is applied to the device using the waveform generator which prevents charges from exiting the device via the external contacts. The laser light generates free charge carriers in the device that may either recombine or become extracted through the subsequent voltage sweep. Using the characteristic parameters as shown in Figure 3.5 c) one can determine the mobility of the active layer of a photovoltaic device [171]:

µ ≈ 2d 2 3At2 max h 1 + 0.36_i∆₍₀₎ii (3.6)

Furthermore, it is possible to calculate the charge carrier density in the device under investigation with respect to the laser delay time. As such, the recombination kinetics of the active layer can be analyzed based on the experimental data.

3.2 Experimental characterization techniques