**5.3. P-i-n structure solar cell**

The advances in multiple quantum well solar cells [29], see Figure 14, and remarkable efficiency improvement of such p-i--n structure configurations due efficient management of photogenerated carries have encouraged researchers to replace the III-V intrinsic region of semi‐ conductors quantum wells structures with quantum dots.

**Figure 14.** Multiple quantum well solar cell band structure. From [29].

face.Moreover, as illustrated in Figure 12-a, photoelectrons are firstly generated near the ITO (ohmic contact) and need to pass through long pathwayto the metallic electrode. Hence, many of the photoelectrons are encountering high recombination probability.Also on the onset of photovoltaic action the hole current injection to metallic contact is not reduce because the barrier to hole injection become less effectiveas shown in Figure 12-b. Gao J. et. al. [27] reported a method to eliminate such disadvantage by inserting a layer of a transition metal oxide (TMO) between the quantum dot (QD) layer and metal electrode. An n-Type transition metal oxide such asmolybdenum oxide (MoOx) and vanadium oxide (V2Ox) work as a hole extraction layer in PbS quantum dot solar cells resulted in power conversion efficiency =4.4%. The formation of dipole at the interface of MoOx and PbS enhances the band binding and hence allowing

Kilimov and his coworkers [28] demonstrated the feasibility of hybrid PV devices thatcombine advantages of mature silicon fabrication technologies with the uniqueelectronic properties of semiconductor QD (see Figure 13).The hybrid silicon/PbS QD film (Nanocrystals) solar cells show external quantum efficiencies of 7%at infrared energies and 50% in the visible and a

The advances in multiple quantum well solar cells [29], see Figure 14, and remarkable efficiency improvement of such p-i--n structure configurations due efficient management of photogenerated carries have encouraged researchers to replace the III-V intrinsic region of semi‐

efficient hole extraction from the PbS film valance band by the MoOx.

**5.2. Hybrid silicon/QD film (NCs) solar cells**

**Figure 13.** Hybrid silicon/PbS QD film solar cells. From [28].

316 Solar Cells - New Approaches and Reviews

power conversion efficiency ofup to 0.9%.

conductors quantum wells structures with quantum dots.

**5.3. P-i-n structure solar cell**

Nozik and his coworkers [11,13] suggested a general p-i-n junction configuration for incorpo‐ rating array of QDs into solar cells. QDs array forms the active medium of the cell (the intrinsic region). In this arrangement quantum size effects should not beeradicated due to cell archi‐ tecture, excitons must be separated prior to Auger recombination, and upon absorption of photonsexcitons ought to be separated into free-charge carriers and transported to appropriate electrodes. Figure 15, shows quantum dot layers forming the i-region of a solar cell. The QDs are electronically coupled to each other to sustain electron and hole transport. Also, mini-bands are formed and hence could facilitate intermediate band formation [30]. Intermediate energy band is energy absorbing band which can accept electron excitations from the valance band and then allow transitions tothe conduction band. In quantum dots solar cells with intermedi‐ ate bands photon of energies lower than the bandgap are basically absorbed and high energy photons produce hot-carries (see Figure 15). Collecting charge-carries while they are hot enhances cell voltage. On the other hand, photocurrent enhancement could be achieved by getting more from the hot-carriers via inverse Auger recombination (impact ionization) leading to multiple exciton generation MEG. One needs to bear in mind electron tunneling mechanism between quantum dots.

Investigation done by Nozik group found that [31] by comparing the performance of GaAs base solar cell to one incorporating InAs/GaAs QDs intrinsic region sandwiched between *p* and *n* GaAs emitters (grown by MBE system in the Stranski–Krastanov growth mod), the cell exhibits an extended response for photon energies lower than the GaAs bandgap. The contribution to the total current of the cell came as a result of harvesting below-bandgapenergy photons is minute (1%).

**Figure 15.** Possible p-i-n QD arrangements for solar cells.From [11].
