*3.1.2 Short circuit current density JSC*

The short circuit current density is the photoelectrical current that can be observed when the OPVD is irradiated with light and a direct contact between both contacts forces the potential difference to be 0 V. Practically you produce a short circuit of the photoelectrical cell.

It has been demonstrated that the current density observed in these conditions is related to the interaction between the electron donor and the ABL as well as the light absorption by the donor material. In this sense a molecule with a large absorption rate over large parts of the electromagnetic spectrum contributes to increase the photoelectrical yield. Anyhow you also need a large efficiency for passing positive charges to the anode through the buffer layer. Increase in efficiency will lead directly to higher short circuit currents and as consequence to higher photoelectrical yields. As a third factor also the electrical resistance of the organic substrate is important here as resistance and electrical current in a DC device are directly related by ohms law.

#### *3.1.3 Form factor (or fill factor) ff*

The form factor of a solar cell is defined as the relation of the measured maximum electrical power that is provided by the cell and the product of JSC and VOC. Thus you can use the form factor as an indicator of how close the electrical behavior of the cell is to an ideal device.

In the figure you can see Jmp and Vmp as current and voltage delivered by the cell at maximum power. The product of both will give directly the maximum power that can be obtained from the device.

#### *3.1.4 Photovolatic yield η*

## 4 Photonatic yield  $\eta$ 

$$\eta\_{\text{(\%)}} = \frac{I\_{\text{SC}}[mAcm^{-2}]\,\upsilon\_{\text{OC}}\,\text{[V]}\,\text{ff}}{I\_0[m\,\text{Wc}\,\text{m}^{-2}]} \tag{1}$$

The photoelectrical yield *η* is defined as the relation between the maximum electrical power delivered from the photoelectrical device calculated as the product of short circuit current JSC, open circuit voltage VOC and form factor *ff* divided by the power it receives in form of light energy *I*0 by the formula:

As one can see from an electrical point of view JSC, VOC and *I*0 have to be optimized together in order to obtain high yields. As we have seen before they are related to the chemical nature of the substrates that are used to build up the cell. This explains why until now improvements of the cell yields always implied the design of optimized molecules. The improvement has to imply the donor and acceptor molecules as well as the material employed in the ABL as only an optimized hole extraction work will lead to a high short circuit current. Thus an intelligent cell design will take into account that:

• The electron donor interacts with the anode in that way that allows to extract the highest possible percentage of positive charges possible.


On a molecular level these needed photophysical and electrochemical properties can be achieved designing molecules with a high degree of order and preferably π-stacking interactions that bear functional groups able to optimize the interaction with the anode material and herewith the energy transference. The functional group design has to take into account the affinities that exist towards the elements involved in the ABLs. When copper is used in ABL—construction good candidates will be thio- and nitrogen(-III) containing groups as copper is known for his high affinity towards this kind of moieties. For molybdenum-(VI) also oxygen groups could be important as the small size of the element in this oxidation state fits well to the small orbitals of oxygen.
