Natrium Doping Pushes the Efficiency of Carbon-Based CsPbI3 Perovskite Solar Cells to 10.7

摘要

class="head no_bottom_margin" id="sec1title">IntroductionOrganic-inorganic hybrid perovskite solar cells (PSCs) have attracted tremendous attention for their rapidly rising power conversion efficiency (PCE), currently reaching over 23%, achieved by solution-based techniques (, , , ). However, the organic ions in the organic-inorganic hybrid perovskites can easily escape from lattice under thermal stress, which restricts their long-term practical application (, , , ). In this regard, inorganic perovskites appear to be highly promising light absorbers owing to their good thermal stability (, ). Among various inorganic perovskites, CsPbI3 perovskite is the most suitable one because of its appropriate band gap (1.73 eV) for photovoltaic (PV) applications (, , , , ).Other than the potential problems of perovskites as an active layer material in PSCs, the adoption of organic hole transport materials (HTMs) may also limit the stability of PSCs because they are commonly thermally unstable and susceptible to ion migration and metal electrode corrosion eventually causing device degradation (href="#bib29" rid="bib29" class=" bibr popnode">Swarnkar et al., 2016, href="#bib10" rid="bib10" class=" bibr popnode">Eperon et al., 2015, href="#bib22" rid="bib22" class=" bibr popnode">Luo et al., 2016, href="#bib21" rid="bib21" class=" bibr popnode">Liang et al., 2017b). To tackle this issue, proposal has been put forward to replace the organic HTM and the metal electrode with a carbon electrode, which for the past few years has proved to be an effective approach (href="#bib7" rid="bib7" class=" bibr popnode">Chen and Yang, 2017). Some works on carbon-based HTM-free PSCs (C-PSCs) have employed CsPbI3 as a light absorber, and in our own work, a PCE of 9.5% has been achieved previously (href="#bib37" rid="bib37" class=" bibr popnode">Xiang et al., 2018). However, such a PCE is far lower than those achieved by the C-PSCs based on organic-inorganic hybrid perovskites. According to the previous works, the low PCE can be mainly attributable to the low obtainable open-circuit voltage (Voc) (href="#bib3" rid="bib3" class=" bibr popnode">Ball et al., 2013, href="#bib13" rid="bib13" class=" bibr popnode">Han et al., 2015, href="#bib40" rid="bib40" class=" bibr popnode">Zhang et al., 2017, href="#bib20" rid="bib20" class=" bibr popnode">Liang et al., 2017a), i.e., below 0.8 V. The low Voc should originate from the low grain quality of CsPbI3 film or the mismatched energy band levels at charge collection interface (href="#bib1" rid="bib1" class=" bibr popnode">Ahmad et al., 2017, href="#bib10" rid="bib10" class=" bibr popnode">Eperon et al., 2015, href="#bib41" rid="bib41" class=" bibr popnode">Zhang et al., 2018, href="#bib6" rid="bib6" class=" bibr popnode">Chen et al., 2018, href="#bib28" rid="bib28" class=" bibr popnode">Sanehira et al., 2017).Herein, we address the above issues by doping the CsPbI3 inorganic perovskite at A site with Na element. It is found that the Na doping not only improved the morphology of CsPbI3 film but also significantly enhanced the grain quality, thus reducing the defect density. Furthermore, the Na doping offers a good handle to adjust the energy band levels of CsPbI3 film, raising the built-in potential and Voc. As a result, C-PSCs based on the Na-doped CsPbI3 film achieved a PCE as high as 10.7% with a Voc of 0.92 V, which are considerably higher than those obtained by the C-PSCs based on pure CsPbI3 film (PCE = 8.6%, Voc = 0.77 V). Besides, the non-encapsulated C-PSCs based on the Na-doped CsPbI3 film exhibit almost no PCE degradation after 70 days of storage in a dry air atmosphere.The whole procedure of C-PSC fabrication, including CsPbI3 deposition, was conducted in a dry air atmosphere (humidity∼10%–20%). The CsPbI3 films were deposited on TiO₂ mesoporous scaffolds via a one-step spin-coating method with the precursor solution containing ∼1 M N,N-Dimethylformamide (DMF)·HI·PbI₂, (1-x) M CsI, and x M NaI, followed by heating at 200°C to obtain black CsPbI₃ perovskite films.X-ray diffraction (XRD) patterns of the perovskite films with different Na doping concentrations are shown in href="/pmc/articles/PMC6503139/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figure 1A. As depicted in href="/pmc/articles/PMC6503139/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figures 1A and 1B, with Na doping content increasing, the (100) peak shifts to a higher 2θ, correlating to the lattice contraction. Therefore Na ions should have partially replaced Cs ions in the CsPbI₃ lattice because Na atoms (1.02Å) are significantly smaller than Cs atoms (1.67Å). Moreover, as depicted in href="#mmc1" rid="mmc1" class=" supplementary-material">Figure S1, the films after storage for 7 days show almost the same XRD patterns as the corresponding as-prepared films, suggesting high phase stability. Secondary ion mass spectroscopy was further employed to study the composition difference between CsPbI₃ and Cs_0.95Na_0.05PbI₃ films. The depth profiling in href="/pmc/articles/PMC6503139/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figures 1C and href="#mmc1" rid="mmc1" class=" supplementary-material">S2 shows that Na ions have been doped into the CsPbI₃ film, which distribute throughout the whole film. As a side note, Na atom concentration in the capping layer is slightly lower than that in the mesoporous layer. This could be interpreted as follows. The tolerance factor of CsPbI₃ perovskite is small (t = 0.81), and it would be further lowered after the substitution of Cs with Na atoms, which would reduce the structural stability. To make the structure stable, Na atoms might thermodynamically tend to stay in the mesoporous TiO₂ scaffold, because the nanopores in the scaffold could confine grain dimensions to nanosize (href="#bib8" rid="bib8" class=" bibr popnode">Choi et al., 2013) and there may be lattice strain at the CsPbI₃/TiO₂ interface, which both would help to stabilize the perovskite structure. Similar phenomenon has been reported for organic-inorganic perovskites when the ions with a large size mismatch degree were used as dopants (href="#bib27" rid="bib27" class=" bibr popnode">Qiao et al., 2018). To identify the chemical states of CsPbI₃ and Cs_0.95Na_0.05PbI₃ films, X-ray photoelectron spectra (XPS) have also been recorded. The high-resolution XPS spectra for various elements (Na 1s, Cs 3d, Pb 4f and I 3d) are shown in href="/pmc/articles/PMC6503139/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figures 1D and href="#mmc1" rid="mmc1" class=" supplementary-material">S3. Notably, a uniform shift in the peak positions of Cs 3d and Pb 4f to a lower binding energy is observed for Cs_0.95Na_0.05PbI₃ film, whereas the peak position of I 3d shifts to a higher binding energy. These shifts should be correlated to the Na doping because the smaller Na atoms would cause the volume contraction of the BX₆ (B = Pb or Mn; X = I or Br) octahedral and hence lead to the changes in chemical bonding (href="#bib44" rid="bib44" class=" bibr popnode">Zou et al., 2017, href="#bib19" rid="bib19" class=" bibr popnode">Li et al., 2016).href="/pmc/articles/PMC6503139/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">class="inline_block ts_canvas" href="/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=6503139_gr1.jpg" target="tileshopwindow">target="object" href="/pmc/articles/PMC6503139/figure/fig1/?report=objectonly">Open in a separate windowclass="figpopup" href="/pmc/articles/PMC6503139/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">Figure 1Composition Characterizations of Cs_1-xNa_xPbI₃ (0 ≤ x ≤ 0.1) Perovskite Films(A) XRD patterns.(B) Magnification of the (100) peak to show the peak shifts in the Na-doped film.(C) Secondary ion mass spectroscopy depth profile for Cs, Pb, I, and Na elements of CsPbI₃ and Cs_0.95Na_0.05PbI₃ films.(D) High-resolution XPS spectra of CsPbI₃ and Cs_0.95Na_0.05PbI₃ films.