**3. Two-dimensional cells with chains of quantum dots**

Let us now consider examples of possible planar photovoltaic cells containing onedimensional networks of quantum cells. The simplest such structure is shown in **Figure 4**. Quantum dots are embedded in a thin layer of optically active semiconductor, which form one-dimensional chains. The distance between the strings is so long that the interaction between dots from different strings can be ignored. This interaction is non-zero only for the adjacent dots in a given string. If the quantum dots within one chain are the same, then the interaction between the dots generates energy bands from discrete levels of quantum dots. The electrons and holes generated in a certain quantum dot can then freely flow to the appropriate electrode (collecting electrons or holes). This is possible because the states in the electron bands made of discrete dot levels are conductive. If both electrodes were the same, then there would be no such separation. Note that the condition is the formation of bands from discrete dot levels, however, these bands may be different within different chains. Thus, a more general version of the photovoltaic cell shown in **Figure 4** would be a cell in which each of the strings would be composed of other quantum dots (however, the same in the given chain).

However, even in this case, the same electrodes can be achieved by means of appropriate modulation of quantum dots. Besides, such a modified arrangement of dots should also strengthen the separation in the case of two different electrodes (as in **Figure 4**). Well, if quantum dots in a given chain will gradually change their sizes (in any direction), as shown schematically in **Figure 5**, then the energies of a given dot level will gradually increase for electrons and decrease for holes (or vice versa). In **Figure 5**, shown quantum dots are stretched in one direction but one can consider different direction. The change of dot parameters is so small that the shift of a given level is much smaller than the distance between the nearest discrete levels. Then the

#### **Figure 4.**

*Diagram of a planar photovoltaic cell with single-winding chains of the same quantum dots. Red- electron transporting electrode, green- hole transporting electrode.*

#### **Figure 5.**

*Diagram of a planar photovoltaic cell with single-axis chains of quantum dots, the parameters of which (e.g. sizes) gradually change. Red- electron transporting electrode, green- hole transporting electrode.*

electrons will go to the dot with a lower energy level (losing excess energy in thermalization processes), while the holes will be transported from the given dot to the dot with higher energy, which effectively leads to electron-hole separation. As before, a more general situation can be considered in which individual one-dimensional strings are different.
