Characterization of Organic Solar Cells

1.5.1 J-V Characteristics

OSCs are typically characterized under 1000 W/m2 light of AM 1.5 solar spectrum [65]. The operation of a solar cell at difference bases is illustrated in Fig. 1.8.

At (i) reverse bias, the applied bias reinforces the built-in electric field, enhancing exciton dissociation and charge transport and results in a large photo­current. Drift current is dominant due to the presence of a strong electric field. When (ii) the applied bias is close to zero, mainly the built-in field exists in the device and the built-in field drives the carriers to the corresponding electrodes for collection. When the applied bias is increased in positive direction, the positive bias opposes the built-in field. As the resultant field inside the device reduces, drift current becomes smaller and the magnitude of current decreases. Eventually the field reaches a point where (iii) the applied field is equal to the built-in field. Around this point, diffusion current dominates the current, as the electric field is very small inside the device. When (iv) the external bias is further increased, the applied field is larger than the built-in field and the potential gradient in the device is reversed. As the barrier is now triangular, carrier injection occurs through the tunneling mechanism and positive current results.

When the applied bias and current are opposite in direction, power is outputted from the solar cell. The point where the magnitude of the product of J and V is maximum is the maximum power output point. A number of the parameters that are commonly used to evaluate solar cell performance are described below.

Подпись: Fig. 1.8 Operation of a solar cell at different biases: a large reverse bias; b small reverse bias; c positive bias, zero resultant internal field, and corresponding to open circuit condition; d positive bias, carrier injection

Short circuit current (Jsc). The short-circuit current is defined as the current at which the externally applied voltage is 0. Jsc represents the number of charge carriers that are generated and eventually collected at the electrodes at short circuit condition. Enhanced optical/electrical parameters such as a small band gap, high absorption coefficient, smaller phase separation, and high carrier mobility improve Jsc.

Open circuit voltage (Voc).The open circuit voltage defined as the voltage at which the current density output is 0. Voc has been reported to be mainly dependent on the work function difference of metal contacts. If an ohmic contact is formed at the electrodes, Voc is dependent on the HOMO-LUMO difference between the donor and the acceptor.

Подпись: FF Подпись: Jmpp Vmpp Jsc Voc Подпись: (1.2)

Fill Factor (FF). The fill factor defines the shape of the J-V curve and is defined as

where Jmpp and Vmpp are the current density and the voltage at the point of maximum output power respectively. As shown above, FF is the ratio between the maximum power output point and the maximum attainable power output, i. e., Jsc times Voc. FF represents dependence of current output on the internal field of the device and is quantified by the series resistance and shunt resistance. For instance, low carrier mobility will cause carriers to recombine before reaching a hetero­junction. In this case, increasing the external bias will sweep the carriers, which otherwise may recombine at lower field strength, to the heterojunction for disso­ciation, resulting in an increase in current output. This leads to a strong depen­dence of current on the applied bias, which is shown by a lower FF.

Power conversion efficiency (PCE). Finally, the PCE represents the efficiency of the solar cell and can be calculated as follows:

VocJscFF

PCE = ; (1.3)

P in

where Pin is the input power density.

Updated: July 31, 2015 — 9:32 am