Extrinsic Movable Ions in MAPbI₃ Modulate Energy Band Alignment in Perovskite Solar Cells

Abstract

Both activation energy calculation [12][13][14][15] and experimental results [11,16,17] suggested that I − and MA + are movable ions in perovskite. There are negative concerns on the ionic movement in perovskite solar cells: one is the hysteresis problem which brings difficulty to characterize the device efficiency accurately. The shape of I-V curve is influenced by the voltage scan direction/rate/range, [7,18,19] which correlates with the arrangement of ions under bias. The other concern is that the ion migration which induces phase segregation in perovskite might impair the stability of the devices. [11,20] However, more and more evidences point out the importance of ion movement to achieve high-performance PSCs. Ionic movement leads to self-doping of perovskite materials and ions at the interface effectively setup a built-in field to encourage the charge transport. [6,[20][21][22] The other positive viewpoint is that ions at the interface can modulate the band bending near the interface, forming an Ohm contact to facilitate charge injection. [9,17] Based on the two-sides of the ionic movement, there are tremendous investigations to eliminate the hysteresis of PSCs caused by ions; [14,23] however, very little attention is paid on utilizing the ionic movement to improve the PSC performance, let alone to diminish the hysteresis. [24] Intrinsic ions movement always relates with the MA + and I − vacancies and lattice distortion, while extrinsic ions from the external ions might not induce such trap states, which remains a great challenge to explore.In this work, external Li + /I − is introduced by adding LiI in perovskite. Li is the 3rd element in periodic table, whose size and weight are far less than CH 3 NH 3 + . Therefore, Li + is supposed to be easily movable in perovskite together with I − . The enhanced ionic conductivity and tunable energy band alignment by extrinsic ions are clearly presented. The designed extrinsic ion doping is proved to be benign for both the energy band alignment and crystalline lattice. Results and DiscussionX-ray diffraction (XRD) patterns of the LiI doped samples are indicated in Figure 1a. All peaks are indexed to the conventional MAPbI 3 perovskite diffraction peaks with Miller indices Ionic movement is considered awful in perovskite solar cells (PSCs) for relating with the hysteresis and long-term instability. However, the positive role of ions to enhance the energy band bending for high performance PSC is always overlooked, let alone reducing the hysteresis. In this work, LiI is doped in CH 3 NH 3 PbI 3 . It is observed that the aggregation of Li + /I − tunes the energy level of the perovskite and induces n/p doping in CH 3 NH 3 PbI 3 , which makes charge extraction quite efficient from perovskite to both NiO and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) layer. Therefore, in NiO/LiI doped perovskite/PCBM solar cells, Li + and I − modulate the interface energy band alignment to facilitate the electron/hole transport and reduce the interface energy loss. On the other hand,...

Energy Level Alignment at Interfaces in Metal Halide Perovskite Solar Cells

“…While ion migration is believed to be detrimental to the efficiency and the stability of PSCs, beneficial energy level alignment for devices can also be achieved by doping. Han and co‐workers induced extrinsic Li + and I − in MAPbI 3 as ionic dopant to enhance efficiency and reduce hysteresis . The surface potential measurement by KPFM (as shown in the subpanel (i) of Figure b) indicates that the extrinsic ions can modulate the energy level of MAPbI 3 , and therefore facilitate the electron/hole transport and reduce the interface energy loss.…”

A Biopolymer Heparin Sodium Interlayer Anchoring TiO2 and MAPbI3 Enhances Trap Passivation and Device Stability in Perovskite Solar Cells

“…As an affirmation of the XPS results, we performed DFT calculations to examine the surface adsorption properties. For simplicity, we divided the polymeric HS into two repeat units A and B with the four different binding groups (see Figure a, denoted as A1: COO − , A2: OSO 3 − ; B1: CH 2 OSO 3 − , B2: NHSO 3 − ) and concerned all the possible adsorption structures on the (110) surfaces of MAPbI 3 and TiO 2 films which were chosen based on the XRD results (see Figure a,b). Specifically, units A and B tend to bind with single Pb atom ( d OPb of ≈2.4 Å) at the MAPbI 3 surface and to the two adjacent titanium atoms ( d OTi of ≈2.0 Å) at the TiO 2 surface.…”

Perovskite/c‐Si tandem solar cells with realistic inverted architecture: Achieving high efficiency by optical optimization

“…We then directly acquired the reflectance and the absorptance in top layers from the frequency‐domain transmission monitor and the P abs analysis group, respectively. The electron extraction was effective and sufficient because of appropriate work function resulting in forming an accumulation layer between ITO and PC 60 BM, so we assumed internal quantum efficiency of unity in the simulated materials. We can thus obtain the short‐circuit current density ( J sc ) by integrating the photon flux of the AM 1.5G solar spectrum with the corresponding absorptance.…”