The properties of photovoltaic devices can be characterized by plotting the measured current density output J of the cell versus the voltage output V of the cell (J-V graph, Figure 17). In the dark, this J-V curve passes through the origin, since at that moment no current is flowing through the device and no potential is present. By exposing the photovoltaic device to light, the J-V curve shifts downwards. The most important characteristic parameters of photovoltaic devices can be found on this J-V curve.
Figure 17. Current-voltage (J-V) characteristics of a typical solar cell. Essential parameters determining the cell performance are shown.
a) Open-circuit voltage (VOC)
It is the maximum possible voltage across a photovoltaic device. This is the voltage across the cell, under sunlight, when no current is flowing through the device. The VOC in organic
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photovoltaics has been found to be dependent to a first approximation on the difference between the HOMO of the donor and the LUMO of the acceptor.140
b) Short-circuit current (ISC)
It is the current that flows through an illuminated solar cell when there is no external resistance i.e. when the electrodes are simply connected or short-circuited. ISC is the maximum current that a photovoltaic device is able to produce. Under an external load, the current will always be less than ISC. The short-circuit current depends on a number of factors, such as the area of the solar cell. To remove the dependence of the solar cell area, it is more common to list the short-circuit current density (JSC, in mA cm-2), rather than the short-circuit current (ISC, in mA).
c) Maximum power point (MPP)
This magnitude makes reference to the point on the J-V curve (Vm, Jm) at which the maximum power is produced. Power is the product of current J and voltage V. On the graph in Figure 17, it is represented by the rectangle formed between the point on the J-V illuminated graph and the axes where the area is maximum.
d) Fill factor (FF)
It is the ratio of its actual maximum power output to its theoretical power output, if current and voltage would be at their maxima, JSC and VOC, respectively. This is a very important property used to measure photovoltaic device performance. It is a measure of the
‘squareness’ of the J-V curve. FF can be written down as follows:
𝐹𝐹 = 𝐽𝐽𝑚×𝑉𝑚
𝑆𝐶×𝑉𝑂𝐶 (Eqn. 1)
It has been recently demonstrated that the competition between recombination and extraction of free charges ultimately determines the FF of organic solar cells.141
e) Power conversion efficiency (PCE or η)
It is the ratio of power output (Pout), to power input (Pin). PCE measures the amount of power produced by a photovoltaic device relative to the power available in the incident solar
140Qi, B.; Wang, J. J. Mater. Chem. 2012, 22, 24315.
141Bartesaghi, D.; del Carmen Pérez, I.; Kniepert, J.; Roland, S.; Turbiez, M.; Neher, D.; Koster, L. J. A. Nat.
Commun. 2015, 6, 7083.
Chapter I
49 radiation. Pin is the sum over all wavelengths, which usually has a value of 100 mW cm-2 when solar simulators are used. This is the most general way to define the efficiency of a photovoltaic device. PCE can be written down as follows:
𝑃𝐶𝐸 (𝜂) =𝑃𝑜𝑢𝑡
𝑃𝑖𝑛 × 100% = 𝐽𝑚×𝑉𝑚
𝑃𝑖𝑛 × 100% = 𝐽𝑆𝐶×𝑉𝑂𝐶×𝐹𝐹
𝑃𝑖𝑛 × 100% (Eqn. 2) PCE is one of the most important parameters to characterize solar cell performances. In order to compare results from various devices, regardless of the design and active material, photovoltaic cells are all subjected to the same standard test conditions. The cells are typically illuminated at a constant density of roughly 100 mW cm-2, which is defined as the standard “1 Sun” value, with a spectrum consistent to an air-mass global value of 1.5 (AM 1.5G), at a temperature of 25 °C. Air mass describes the spectrum of radiation and can be defined as the amount of atmosphere through which sunlight has to travel to reach the Earth’s surface. This is abbreviated as AM x, in which x is the inverse of the cosine of the zenith angle ofthe sun.
The above mentioned AM 1.5G conditions correspond to the spectrum and irradiance of sunlight incident with a zenith angle of 48.2° (Figure 18).
f) External quantum efficiency (EQE)
It is one of the two ways to measure the Incident photon-to-current efficiency (IPCE). It is calculated by the number of electrons extracted in an external circuit, divided by the number of incident photons at a certain wavelength under short-circuit conditions. EQE can be written down as follows: processes in OSCs, and it can be described as follows:
𝐼𝑃𝐶𝐸 = 𝜂𝑎𝑏𝑠(𝜆) × 𝜂𝑑𝑖𝑓𝑓(𝜆) × 𝜂𝑐𝑡(𝜆) × 𝜂𝑐𝑜𝑙𝑙(𝜆) (Eqn. 4) where ηabs is the photoabsorption efficiency, ηdiff is the exciton diffusion efficiency to the D/A interface, ηct is the charge transfer efficiency, ηcoll is the charge collection efficiency, and λ is the wavelength of interest.
The molecular orbital levels, the absorption coefficients, the morphology of the layers and the exciton diffusion lengths, are therefore the main factors that control the device performance.
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Research on these aspects during the last decade, based on rational design of newly optimized materials, has led to up of 11.0% PCE for single-junction OSCs.119
Figure 18. The air-mass value AM 0 equates to insolation at sea level with the Sun at its zenith. AM 1.0 represents sunlight with the Sun at zenith above the Earth’s atmosphere. AM 1.5 is the same, but with the Sun at an oblique angle of 48.2°, which simulates a longer optical path through the Earth’s atmosphere; AM 2.0 extends that oblique angle to 60.1°.