Perovskite Tandem Solar Cell Technologies

With the increasing population worldwide, the consumption of fossil energy is grows to be enormous. As a result, the fossil energy is now facing a shortage situation. In addition, the environmental pollution is gradually becoming more severe. Therefore, it is urgent that human beings develop renewable and sustainable energy. Solar radiation is inexhaustible in its supply of energy for the earth. The solar radiation has a wide spectrum, spanning from visible range to near-infrared (NIR) range, which are roughly equal in proportion. Usually, a single-junction solar cell absorbs the solar light for a certain range. For example, a cadmium telluride (CdTe)-based solar cell and a fullerene-based organic solar cell only absorb the light in visible range; and a silicon solar cells absorbs both the visible light and the NIR light, but the absorption coefficient in the visible range is relatively low. To expand the absorption range (i.e., covering both the visible range and the NIR range of the solar spectrum) and increase the absorption efficiency (i.e., the absorption of more photons in a certain range), tandem solar cell technology (i.e., integrating two kinds of solar cells) emerges. Such integrated solar cells, formed by monolithically integrating two photoactive layers of perovskite and other photovoltaic materials with complementary absorption, provide a promising platform for further improving solar cell efficiency.

To show the advances of perovskite tandem solar cells, in this Special Issue, we highlight nine papers, including eight research papers and one review paper, in which different kinds of tandem solar cells are discussed, including perovskite/silicon, perovskite/copper indium gallium sulfide (CIGS), and perovskite/organic. Note that all-perovskite tandem solar cell (i.e., perovskite/perovskite) is also a branch of tandem solar cell, but it is beyond of this Special Issue. This Special Issue focus on perovskite/other photovoltaic material tandem solar cells because they possess much better stability compared with the perovskite/perovskite tandem solar cells. These nine dedicated papers present the newest progress in the field of perovskite tandem solar cells and provide readers in the photovoltaic community with very important knowledge.

Perovskite/silicon tandem solar cells are the most investigated tandem solar cells. Silicon is usually the bottom sub-cell, and perovskite is used as the top cell. The most important point for integrating the silicon and perovskite solar cells is the selection of the interconnection layer to adjust the current matching. In the following six research papers, the authors focused on different interconnection techniques for versatile types of cells, such as homojunction silicon solar cells, heterojunction silicon solar cells, and TOPCon silicon solar cells. In Ref. [1], entitled “Monolithic Perovskite-Carrier Selective Contact Silicon Tandem Solar Cells Using Molybdenum Oxide as a Hole Selective Layer”, Kim and Lee et al. reported a carrier selective layer using MoO_x instead of a p-n homojunction structure to produce an efficient monolithic perovskite/silicon tandem cell. The perovskite solar cell stacked on the bottom silicon cell showed ~16% efficiency as a whole cell, where MoO_x simultaneously acted as a passivation layer and a hole-collecting layer. The perovskite/silicon tandem solar cells with such simple carrier selective contact structures may shed light on other commercially valuable materials such as metal oxides, carbon-based materials, and organic polymers. In Ref. [2], entitled “Interlayer Microstructure Analysis of the Transition Zone in the Silicon/Perovskite Tandem Solar Cell”, Kulesza-Matlak et al. developed a porous layer of Si wires on the bottom silicon cell by using metal-assisted etching, which fulfilled a double role of the silicon solar cell texturing and the scaffolding for the perovskite absorber. The wires etched on polished or typically textured silicon significantly reduced the reflectance of solar radiation in the wavelength range of 300–1000 nm and therefore are beneficial to increasing the efficiency.

In Ref. [3], entitled “Efficient n-i-p Monolithic Perovskite/Silicon Tandem Solar Cells with Tin Oxide via a Chemical Bath Deposition Method”, Noh and Kang et al. reported the fabrication of low-temperature-processed SnO₂ via chemical bath deposition and its usage in a homojunction silicon solar cell. By controlling the reaction time, a tandem efficiency of ~17% was achieved. This study shows that tandem implementation is possible through the chemical bath deposition method and demonstrates the potential of this method in commercial applications to textured silicon surfaces with large areas. In Ref. [4], entitled “Monolithic Perovskite/Silicon-Heterojunction Tandem Solar Cells with Nanocrystalline Si/SiO_x Tunnel Junction”, Mercaldo et al., investigated monolithic perovskite (Cs_0.05FA_0.8MA_0.15PbI_2.5Br_0.5)/silicon-heterojunction tandem solar cells with a p/n nanocrystalline Si/SiO₂ recombination junction for improving infrared light management. In particular, ~1.8 V of open circuit voltage is reached. This work avoids the use of an expensive ITO interlayer and provides a simple fabrication process of the tunnel junction for a perovskite/silicon tandem solar cell.

In Ref. [5], entitled “Amorphous Silicon Thin Film Deposition for Poly-Si/SiO₂ Contact Cells to Minimize Parasitic Absorption in the Near-Infrared Region”, Kang and Lee et al. proposed a thin amorphous silicon layer to reduce parasitic absorption in the NIR region in TOPCon solar cells, which are the key emerging silicon photovoltaic technology in the commercial silicon photovoltaic market. The thin amorphous silicon layer can crystalize to polysilicon, and the thickness of poly-Si can dramatically affect the efficiency of perovskite/silicon tandem solar cells. Therefore, a thin poly-Si layer with good quality is important for utilizing the light in the long-wavelength range. In Ref. [6], entitled “Potential of NiO_x/Nickel Silicide/n⁺ Poly-Si Contact for Perovskite/TOPCon Tandem Solar Cells”, Kim and Lee et al. applied nickel silicide as an interlayer for perovskite/silicon tandem solar cell, where nickel silicide was formed by nickel diffusion from thermally evaporated NiO_x to the n⁺ poly-Si layer during the deposition and annealing process. This structure is advantageous for electrical connection between the perovskite top cell and Topcon bottom silicon cell as compared to the NiO_x/transparent conductive oxide/n⁺ poly-Si structure showing Schottky contact. The work shows the potential of a NiO_x/nickel silicide/n⁺ poly-Si structure as a perovskite/silicon tandem solar cell interlayer.

Besides the perovskite/silicon tandem solar cells, perovskite/CIGS solar cells also show great potential because CIGS has the main advantage of feasible band gap tuning. In Ref. [7], entitled “Investigation of Electron Transport Material-Free Perovskite/CIGS Tandem Solar Cell”, Salah et al. used SCAPS (i.e., a solar cell capacitance simulator) to simulate a perovskite/CIGS tandem cell. They indicate that the tandem cell with electron-transport-layer free perovskite structure delivers a power conversion efficiency as high as ~35%. Another integration technology for perovskite solar cells is integration with organic solar cells. Perovskite/organic integrated tandem solar cells show great potential because organic photovoltaic technology has the distinct advantages of being flexible, lightweight, and low-cost, in addition to its high power conversion efficiency (certified efficiency: >18%; lab-based efficiency: >19%) [8]. In Ref. [9], entitled “Recent Advances and Challenges toward Efficient Perovskite/Organic Integrated Solar Cells”, Hong and Lee et al. reviewed the recent progress in perovskite/organic integrated solar cells, including operational mechanism and structural development and remaining challenges to achieving efficient devices.

Overall, tandem solar cells can greatly increase power conversion efficiency by absorbing more solar light, thus decreasing the overall cost. Therefore, it is a vital technology for future energy harvesting. These aforementioned authors provided important insights on versatile perovskite/photoactive integrated tandem solar cell technologies, such as how to design high-efficiency tandem solar cells, how to make perovskite tandem solar cells by combining different photovoltaic materials, how to further increase the power conversion efficiency, and how to lower the cost. Their contributions will certainly accelerate the commercialization of perovskite technology. The editor team herein thanks all the contributions of the authors and hopes this Special Issue draws the attention of the researchers both in the research field and the industrial community.