**4. Conclusions**

**Figure-6: Concentrated solar radiation incident on a 1-cm<sup>2</sup>**

**N-period lattice-matched GaAs/Ge superlattice** 

**study.**

*P X N*

*in*

*δη* <sup>=</sup> 1.323(*<sup>N</sup>* )

[12])

( ln ) 1.323( ) *<sup>V</sup> <sup>V</sup> <sup>X</sup>*

*oc t*

covering most of the SL region are under study.

194 Solar Cells - New Approaches and Reviews

**matched superlattice (SL) in its intrinsic region. Total efficiency of the hybrid cell is the sum of bulk cell efficiency and excess efficiency. Implanted SLs in the bulk are feasible based on routine MBE techniques [15]. Details of the optical system and the spot covering most of the SL region are under** 

**Figure 6.** Concentrated solar radiation incident on a 1-cm2 GaAs p-i-n cell with a GaAs/Ge lattice-matched superlattice (SL) in its intrinsic region. Total efficiency of the hybrid cell is the sum of bulk cell efficiency η and excess efficiency. Implanted SLs in the bulk are feasible based on routine MBE techniques [15]. Details of the optical system and the spot

**ARC** 

As seen from Figure 6, a superlattice can become the source for excess carriers in the conduction band if light can be concentrated on it inside the pin cell. This is an immediate advantage: the SL

As seen from Figure 6, a superlattice can become the source for excess carriers in the conduction band if light can be concentrated on it inside the pin cell. This is an immediate advantage: the SL region generates excess carriers and causes efficiency increase δη as seen in expression (18):

*Pin <sup>X</sup>* (*Voc* <sup>+</sup> *Vt*ln*<sup>X</sup>* ); (No losses due to excess carrier scattering are assumed, see also

region generates excess carriers and causes efficiency increase as seen in expression (18):

 **1eV photons @ X suns** 

**Optics for X suns** 

; (No losses due to excess carrier scattering are assumed, see also [12])

Figure-6 presents the main concept: a lattice-matched superlattice in the middle of the bulk GaAs i-layer of the control cell and illuminated at X suns improves cell performance through thermionic escape of photo-electrons from individual quantum wells. In our analysis we

 **GaAs p-i-n cell with a GaAs/Ge lattice-**

**N**

**I**

**P**

**Low-doped mid region**

A different type of multijunction cell structure is proposed in this chapter. The multijunc‐ tion term refers to a proposed lattice-matched superlattice, grown in the bulk of the intrinsic region of a p-i-n cell; current high efficiency cells are basically designed as a top-cell/bottomcell tandem arrangement, on GaAs or Ge. These cells are in-series connected via tunnel junctions and the whole structure is illuminated at high solar irradiance [13, 14, and 16]. We demonstrate the case for higher efficiencies achievable in GaAs p-i-n solar cells through embedded lattice-matched superlattices in the intrinsic region. Our model is based on thermionically escaping carriers from individual quantum wells of the superlattice in the intrinsic region of the p-i-n control cell. The total carrier concentration in quantum wells is the sum of (a) photo-excited carriers and of (b) carriers that occupy eigen-energy levels under dark. The total population in each quantum well is dominated by photo-carries at highly concentrated incident solar light. The latter has been the major approximation in our model, namely, the photocarrier population is much greater than the bulk and quantum well populations respectively in each and every quantum trap in the cell. We then proposed a p-i-n device that includes a superlattice structure in the middle of its intrinsic region, a superlattice layer composed of GaAs/Ge units (the low gap material is in essence the quantum well: Ge). The advantages of such a proposal are (a) concentrated light on the cell (X=400 suns) produces excess thermionically escaping carriers (b) these carriers may overcome the potential barriers of the superlattice region and contribute excess photocurrent that depends strongly on concentration level X and on the number of superlattice (SL) periods N and (c) the SL region is only a small fraction of the total device length. Our results stand in good agreement with efficiency improvements of standard designs of tandem/multijunction cells. We simulate a GaAs control/reference device hosting a superlattice embedded in its intrinsic region that can generate appreciable currents at 400 suns. Specifically, a 26.26% efficient all-GaAs control solar cell (X=400) increases its efficiency to 30.21% when a 50-period GaAs/Ge-superlattice is grown in its intrinsic region. Excess collection efficiency depends on the period number of the superlattice and the concentra‐ tion level. We claim that 50% efficiency levels are feasible for p(GaAs)-i(GaAs/Ge-SL)- GaAs cells with either 330 periods at 400 suns or 160 period under 100 suns. The sequence of steps for high cell performance is outlined in the figure below:

The sequence shown above describes the steps undertaken in this chapter. The control cell is the primary choice that provides the fundamental efficiency of the cell. The superlattice unit, which is a small fraction of the mid-region of the control cell, can be implanted in the cell as a lattice-matched layer. Generalizing we show that cell efficiency may increase by means of a superlattice implanted in the mid of a p-i-n GaAs cell according to the formula:

$$\eta(\%) = 26.26 + \left[ (N)(\mathcal{W}) \frac{1.323(V\_{OC} + V\_T \ln(X))}{P\_{\text{all}}\sqrt{X}} \right]; X \ge 100 \text{sums}$$

Where N is the superlattice number of periods, W is the probability of excess carriers being collected [12], Pin is the incident solar power, VT is the thermal voltage. The proposed design is ideal for concentrated photovoltaics (CPV): small size cells (therefore reduced material costs) and low-cost optics. superlattice and the concentration level. We claim that 50% efficiency levels are feasible for p(GaAs)-i(GaAs/Ge-SL)-GaAs cells with either 330 periods at 400 suns or 160 period under 100 suns. The sequence of steps for high cell performance is outlined in the figure below:

The sequence shown above describes the steps undertaken in this chapter. The control cell is the

*<sup>V</sup> <sup>V</sup> <sup>X</sup> <sup>N</sup> <sup>W</sup> in*

*OC <sup>T</sup>* ; <sup>100</sup> 1.323( ln( )) (%) 26.26 ( )( )

*<sup>X</sup> suns <sup>P</sup> <sup>X</sup>*

 

**Figure 7: four steps for hybrid cell simulation Figure 7.** Four steps for hybrid cell simulation

#### primary choice that provides the fundamental efficiency of the cell. The superlattice unit, which is a small fraction of the mid-region of the control cell, can be implanted in the cell as a lattice-**Author details**

#### matched layer. Generalizing we show that cell efficiency may increase by means of a superlattice A.C. Varonides

implanted in the mid of a p-i-n GaAs cell according to the formula: Physics & Electrical Engineering Dept. University of Scranton, USA
