**5.2 Upconversion of perovskite photovoltaic cell**

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

#### **Figure 8.**

*(a) Photographic image, (b) schematic, (c) low-resolution, (d) high-resolution, and (e) cross-sectional SEM images of SHJ solar cells. (f) J* − *V characteristics of SHJ solar cells with various concentrations of GQDs. (g) EQE spectra of SHJ solar cells without and with 0.3 wt % of GQDs. Reprinted with permission from [116]. Copyright (2016) American Chemical Society.*

28 W cm−2 [128]. The sample (semiconductor plasmon-sensitized nanocomposites i.e. mCu2-xS@SiO2@ Er2O3), which changes the broadband infrared light to visible light by the localized surface plasmon resonance (LSPR) of Cu2-xS integrated with TiO2 paste [129]. This arrangement is used in PSCs as an electron extraction layer with an enhanced response of 800–1000 nm. With the dependency of the power density of the 980 nm extraction layer (>0.85 mAcm−2), the short circuit current density was noticed to rise 45 Wcm−2.
