**5.1 Downconversion of perovskite photovoltaic cell**

In the previous investigations, various techniques and methods have been adopted to improve the efficiency of solar cells, higher by the Shockley and Queisser limit (32%). The phenomenon of splitting, low energy photons by high energy photons (single) is known as downconversion (quantum cutting). An ample work has been done on downconversion for photovoltaic devices. This was performed

**277**

*Two-Dimensional Materials for Advanced Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94114*

(JSC) and fill factor (FF) of 37.47 mA/cm2

and UV photon harvesting [117–118].

**5.2 Upconversion of perovskite photovoltaic cell**

with lanthanide ions because of its excellent optical properties. Various experiments also proved the use of nanomaterials as down converters. If we chose the lanthanide ions and design of the solar cell, there will be a maximum benefit to using the down-conversion materials. The host materials should possess properties like low scattering, absorption strength, thermal and chemical strength, high transmittance, photo-stability, and excitation energy [109–110]. The necessary conditions to pick the lanthanide ion are good electrical and chemical stability and high emission lifetime. Downconversion Tb3+-Yb3+ has also been demonstrated in GdAl3(BO3)4 [111], GdBO3 [112], Y2O3 [113], CaF2 nanocrystals [114] and lanthanum borogermanate glass [115]. Tsai *et al.* [116] explored the graphene quantum dots (GQDs) as a

The photographic image and the cross-sectional schematic of the SHJ solar cell are shown in **Figure 8a-b**, respectively. **Figure 8c-d** shows the low-magnification and high-magnification top-view SEM images of the micro pyramids of SHJ solar cells. The device with 0.3 wt % of GQDs shows the highest short-circuit current

the highest PCE of 16.55% (**Figure 8f**). The external quantum efficiency (EQE) of the devices with 0.3 wt % and without GQDs and the EQE enhancement are shown in **Figure 8g**. The efficiency enhancement is due to the photon down conversion phenomenon of GQDs to make more photons absorbed in the depletion region for effective carrier separation, leading to the enhanced photovoltaic effect. Various down conversion materials were described and synthesized to enhance UV stability

The conversion (nonlinear optical process) in which minimum two low energy photons, present in the near-infrared region into high energy photon with the visible region known as upconversion [119–120]. The upconversion materials contain large bandgap, seem to be most favorable for solar cell applications. Various uses of upconversion materials are optical data storage, medical therapy, display technology, light-harvesting, temperature sensors, and solid-state lighting. Trupke *et al.* [121] theoretically investigated that if a perfect upconvertor is used with conventional single-junction bifacial solar cells (bandgap 2 eV), we can obtain PCE of 47.6% (non-concentrated sunlight) and 63.2% concentrated sunlight. Lanthanide based upconverters and organic upconverters are very common to improve the efficiency of photovoltaic devices. The elements lanthanum to lutetium are used as the upconverters. In addition, enhancement in the photocurrent has been achieved by the use of two commercial upconverters on both sides of Si solar cells (Pan *et al.*) [122]. The nano precursor upconversion materials Er3+/Yb3+ co-doped with TiO2 and LaF3 have been explored by Shan *et al.* [123]. Shang *et al*. [124] also explored the various techniques to enhance efficiency via upconversion. The utilization of light beyond the visible region is not possible by PSCs (CH3NH3PBI3), due to their intrinsic bandgap. The upconversion is a specific way, to harvest this regime and convert it in the visible regime, so that the PSCs IR response can be increased. Chen and co-workers reported that the efficiency will be increased if LiYF4: Yb3+, Er3+ single-crystal attached in the front part of PSCs [125]. Taking nano prisms NaYF4: Yb3+, Er3+, which is hydrothermally formed to the TiO2 layer in PSCs, the efficiency enhancement has been demonstrated by Roh *et al.* [126]. In another study, Wang *et al.* introduced efficiency increment via hydrothermally grown 3% Er3+ and 6% Yb3+ co-doped TiO2 nanorod in PSCs [127]. In 2016, He *et al.* integrated NaYF4:Yb3+, Er3+ nanoparticles as mesoporous electrode with PSCs (CH3NH3PbI3 based), this leads a short circuit current density 0.74 mAcm−2 excite with 980 nm laser with

and 72.51%, respectively, which leads to

down conversion in then-type Si heterojunction (SHJ) solar cells.

*Two-Dimensional Materials for Advanced Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94114*

*Solar Cells - Theory, Materials and Recent Advances*

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such as CH3NH3

**Figure 7.**

+ and Cs+

*from [98]. Copyright (2018) American Chemical Society.*

flakes via the method of micromechanical exfoliation.

**5. Conversion treatment for photovoltaic cells**

**5.1 Downconversion of perovskite photovoltaic cell**

. B represents Pb2+ and Sn2+, divalent metal cation, while X

describes the halides. Various values of small n provide us strict 2D structure (n = 1), quasi-2D structure (n = 2–5), conventional 3D structure (n = ∞), and represents the number of metal halide, monolayer sheets [100]. These 2DRP excellently perform thermal stability, humidity stability, and structure stability [101–106]. Giorgi *et al.* [98] showed a lateral and top view of the nanosheets Ruddlesden–Popper organic– inorganic halide perovskites (NS-RPPs) optimized structure in **Figure 7a-b**. Also, lateral and top view of the quantum-well Ruddlesden–Popper organic–inorganic halide perovskites (QW-RPPs) structures in **Figure 7c-d**. The solution base synthesis, colloidal base method, liquid, and vapor-based epitaxy, exfoliation method, and single crystal growth are the well-known growing technique to fabricate 2DRP perovskites [107]. Niu *et al.* [108] prepared mono and few layers (C6H9C2H4NH3)2 PbI4

*(a) Lateral view and (b) top view of the n = 2 sheet (NS-RPP) optimized structure. Same (c, d) for the bulk QW (QW-RPP). [gray: Pb; purple: I; Brown: C; light blue: N; white: H atoms]. Reprinted with permission* 

In the previous investigations, various techniques and methods have been adopted to improve the efficiency of solar cells, higher by the Shockley and Queisser limit (32%). The phenomenon of splitting, low energy photons by high energy photons (single) is known as downconversion (quantum cutting). An ample work has been done on downconversion for photovoltaic devices. This was performed

with lanthanide ions because of its excellent optical properties. Various experiments also proved the use of nanomaterials as down converters. If we chose the lanthanide ions and design of the solar cell, there will be a maximum benefit to using the down-conversion materials. The host materials should possess properties like low scattering, absorption strength, thermal and chemical strength, high transmittance, photo-stability, and excitation energy [109–110]. The necessary conditions to pick the lanthanide ion are good electrical and chemical stability and high emission lifetime. Downconversion Tb3+-Yb3+ has also been demonstrated in GdAl3(BO3)4 [111], GdBO3 [112], Y2O3 [113], CaF2 nanocrystals [114] and lanthanum borogermanate glass [115]. Tsai *et al.* [116] explored the graphene quantum dots (GQDs) as a down conversion in then-type Si heterojunction (SHJ) solar cells.

The photographic image and the cross-sectional schematic of the SHJ solar cell are shown in **Figure 8a-b**, respectively. **Figure 8c-d** shows the low-magnification and high-magnification top-view SEM images of the micro pyramids of SHJ solar cells. The device with 0.3 wt % of GQDs shows the highest short-circuit current (JSC) and fill factor (FF) of 37.47 mA/cm2 and 72.51%, respectively, which leads to the highest PCE of 16.55% (**Figure 8f**). The external quantum efficiency (EQE) of the devices with 0.3 wt % and without GQDs and the EQE enhancement are shown in **Figure 8g**. The efficiency enhancement is due to the photon down conversion phenomenon of GQDs to make more photons absorbed in the depletion region for effective carrier separation, leading to the enhanced photovoltaic effect. Various down conversion materials were described and synthesized to enhance UV stability and UV photon harvesting [117–118].
