Enhanced absorption in tandem solar cells by applying hydrogenated In2O3 as electrode

To realize the high efficiency potential of perovskite/chalcopyrite tandem solar cells in modules, hydrogenated In2O3 (IO:H) as electrode is investigated. IO:H with an electron mobility of 100 cm 2 ·V -1 ·s -1 is demonstrated. Compared to the conventional Sn doped In2O3 (ITO), IO:H exhibits a decreased electron concentration and leads to almost no subbandgap absorption up to the wavelength of 1200 nm. Without a trade-off between transparency and lateral resistance in the IO:H electrode, the tandem cell keeps increasing in efficiency as the IO:H thickness increases and efficiencies above 22% are calculated. In contrast, the cells with ITO as electrode perform much worse due to the severe parasitic absorption in ITO. This indicates that IO:H has the potential to lead to high efficiencies, which is otherwise constrained by the parasitic absorption in conventional transparent conductive oxide (TCO) electrode for tandem solar cells in modules. Despite great progress achieved in photovoltaics in the last decades, energy from photovoltaics is still less competitive compared to the conventional fossil energy. Therefore, it is desirable to further improve the efficiencies of solar cells at low cost. Tandem solar cells are a concept exceeding the Shockley-Queisser efficiency limit of 30 % without light concentration 1 . Related work has been intensively done based on various combinations of two single-junction solar cells 2–8 . However, efficiencies beyond 20 % were not yet experimentally reported among thin-film tandem solar cells. The underlying challenges mainly lie in 9, 10 : a) there is lack of high performing high-bandgap solar cells on the top; b) sub-bandgap transparency from the top cell is poor due to parasitic absorption, which inhibits the realization of high efficiencies of the bottom cell. The recent emergence of organic–inorganic CH3NH3PbI3 perovskite solar cells may provide a way out of this tandem stalemate 11-13 . Organic–inorganic CH3NH3PbI3 perovskite solar cells have achieved an efficiency above 20% up to now 14 . CH3NH3PbI3 is the absorber and has a relatively large bandgap of 1.6 eV 15 . It also has a high absorption coefficient, exhibits a steep 2 absorption edge and little sub-bandgap absorption 16 . All these features render the perovskite solar cell an appealing candidate for the top cell in tandem architecture. In the last two years, the perovskite/CIGSe (Si) tandem solar cells in both monolithic (two-terminal) and mechanically stacked (four-terminal) architectures have been intensively investigated and efficiencies beyond 30% have been theoretically predicted 9,17–22 . One of the assumptions made is excellent sub-bandgap transparency from the perovskite cell. Perovskite solar cells typically have a TCO and an Au electrode. To realize this, transparent conductive oxides (TCOs) were recommended for replacing the typical Au electrode in mechanically stacked architecture 19 . For a monolithic tandem structure, only the top TCO electrode is required for the top perovskite cell, also see Fig.2 (a). We note that the conventional TCOs (eg. In2O3:Sn ITO; SnO2:F FTO; ZnO:Al AZO) inherently suffer from transparency loss, which lowers the illumination for the bottom cell. For lab-scale cells in small size, the TCO can be relatively thin since the current collection can be assisted by metal grids on top of the TCO 23 . The resulting parasitic absorption in TCO is moderate. However, when the solar cell is up-scaled to module size, the modules are generally scribed into multiple strips of cells which are monolithically connected without metal grids. A much thicker top TCO electrode is required to minimize the resistive loss due to the lateral transportation of current through the TCO 23 . The resulting parasitic absorption from TCO is expected to be quite serious and the potential for high efficiency can be thus restrained. The conductivity of TCOs is determined by carrier concentration as well as carrier mobility. In conventional TCOs, a higher free carrier concentration indicates a better conductivity but at the cost of sacrificing transparency. To improve transparency without compromising conductivity, a high-mobility TCO is therefore desirable. Therefore, in this contribution, we will investigate the opto-electronic properties of high-mobility TCO. Using optical simulations, we then evaluate how much of optical benefit can be gained by applying a high-mobility TCO compared to a conventional one as the thick electrode in a module. Table 1: Comparison of electrical parameters of ITO and an IO:H layers Electron concentration N (cm) Electron mobility μ (cm/(V x s)) Resistivity ρ (Ω x cm) ITO 6.7 x 10 20 29 3.2 x 10 IO:H 1.5 x 10 20 100 4.2 x 10