Study on Lossless Perovskite/Silicon Tandem Solar Cells

Abstract

Excessive use of fossil fuel increases carbon dioxide emissions which causes global warming. Research into eco-friendly energy is trying to prevent global warming. The most reliable clean source of energy is solar, which utilizes an infinite source of energy. Silicon solar cells (Si SCs) currently dominate the solar cell (SC) industry with over 90% market share. However, owing to the theoretical limitation of the single-junction SC, the efficiency of the Si SC is limited to 29.43%. To overcome this limitation, various multi-junction SCs have been studied. Among these, tandem SCs based on Si SCs are the most likely candidates for commercialization. The most suitable candidates for the top cell are perovskite (PVK) SCs with adjustable band gaps. This is emerging as a next-generation SC. This dissertation focuses on lossless, monolithic PVK/Si tandem SCs. A p-Si homojunction SC, the current mainstream of the SC market, was used as the bottom cell, and a PVK top cell was used. First, device design through optical simulation was performed, and second, each single junction SC was optimized. Finally, a lossless tandem SC was implemented by applying tandemization technology through optimization of each part of the SC. In Chapter 2, we study the optical properties of a tandem SC with a top PVK SC based on a triple cation, and a p-Si homojunction bottom SC based on Al-back surface field (BSF), which is a tandem SC with commercialization potential. The current matching point was found by incorporating PVK thickness and band gap variables in the optical simulation. We found that when the band gap and thickness of the PVK were 1.65 eV and 500 nm, respectively, the current density of the tandem SC was 16.84 mA/cm2. We assumed the voltage deficit of each top and bottom cell to empirically predict the efficiency of the tandem SC; the fill factor (FF) was 80%, and the maximum efficiency was 23.71%. Factors causing optical loss that affect the current density of our tandem device are discussed based on the optical simulation. First, reflections occurred at the interface between the air and the device surface. Second, light was absorbed in the indium zin oxide (IZO) layer that was used as a transparent electrode. Finally, light in the short-wavelength range was absorbed in the phenyl-C61-butyric acid methyl ester (PCBM) that was used as the electron transport layer (ETL). Therefore, it is necessary to develop an anti-reflective coating (ARC) with a textured surface and to gradually control the refractive index from the air to the device to reduce the surface reflection. In Chapter 3, we present design rules of a single cell for lossless monolithic tandem SCs. A p–i–n PVK top cell and p-Si homojunction bottom cell with Al-BSF layers were fabricated using cost-effective solution processes. To optimize the PVK top cell, first, a wide-bandgap PVK was produced by controlling the composition of the PVK using cations and halides. Second, we identified the optimal hole-transport layer (HTL) by considering the energy level in the layer. Third, methylammonium chloride (MACl) was introduced to increase the grain size and quality of the PVK film to reduce recombination at grain boundaries. Fourth, n-butylammonium bromide (BABr) was introduced to form a 3D/2D PVK structure that minimized recombination caused by surface defects. Finally, an ETL layer was developed to minimize the light absorption of PCBM. This layer was evenly coated thinner than the existing PCBM by adding poly[(9,9-dioctyluorene)-2,7-diyl–alt–(4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole)-2’,2”-diyl] (F8TBT) to the PCBM. The optimized opaque PVK top cell exhibited a highest photon conversion efficiency (PCE) of 18.99% with a short circuit current density (JSC) of 20.21 mA/cm2, open circuit voltage (VOC) of 1.18 V, and FF of 79.63%. The optimized semi-transparent PVK top cell exhibited a highest PCE of 15.83% with a JSC of 17.13 mA/cm2, VOC of 1.17 V, and FF of 78.97%. To optimize the Al-BSF Si bottom cell, emitter doping using a rapid thermal annealing (RTA) process was optimized using a spin-on-dopant (SOD) process with a cost-effective solution. The absorption of long wavelength light was increased by introducing a textured structure on the back surface. Finally, a SiNx ARC, whose refractive index was adjusted to a value between the refractive index of the PVK and Si, was inserted between the PVK and the Si. After introducing SiNx ARC on the Si bottom cell, Si bottom cell increased the absorption of long wavelengths and increased VOC due to a passivation effect. The optimized Si bottom cell for the tandem SC exhibited a highest PCE of 13.17% with a JSC of 29.52 mA/cm2, VOC of 0.607 V, and FF of 73.48%. In Chapter 4, we develop a lossless tandem SC. First, we introduced a current-matching method for optimizing the current density of tandem SCs. Second, we optimized the sputtering temperature and pressure for applying an indium tin oxide (ITO) layer, used for recombination to drive the tandem SC. Third, we implemented a tandem SC based on an Al-BSF Si SC using an optical design that minimized light loss by applying graded refractive index matching technology. JSC, VOC, FF and efficiency of the champion device were 16.40 mA/cm2, 1.75 V, 80.23% and 23.02%, respectively. Fourth, long-term stability tests such as light soaking, thermal stress, and damp heat were carried out on cells with glass to glass encapsulation technology. In most of the long-term stability tests, the cells maintained more than 90% of the initial efficiency. Finally, a tandem SC based on a passivated emitter rear contact (PERC) Si SC was fabricated. Although graded refractive index matching technology was not applied, JSC, VOC, FF, and efficiencies of the champion device were 17.47 mA/cm2, 1.725V, 79.34%, and 23.91%, respectively. In PERC Si SCs, the long-wavelength absorption is superior to Al-BSF cells, hence there is a significant increase in efficiency. It is expected that more than 25% efficiency can be achieved in these cells when graded refractive index matching is used.