Enhancing Built‐In Electric Field and Defect Passivation through Gradient Doping in Inverted CsPbI₂Br Perovskite Solar Cells

Abstract

Organic/inorganic hybrid perovskite solar cells (PeSCs) have been of great interest due to their easy fabrication and high power conversion efficiency (PCE) exceeding 25%. [1-4] Nevertheless, hybrid perovskites (PVKs) suffer from low thermal stability due to the volatile nature of organic cations (i.e., methylamine), [5] which limits the commercial applications of hybrid PeSCs. [6,7] Thus, inorganic cations such as Cs þ are used to replace organic cations, [8] yielding all-inorganic PVKs with considerably improved thermal stability. [9,10] Among various Cs-based inorganic PVKs, dual-halide PVKs of CsPbI 2 Br feature a suitable bandgap (E g) of around 1.9 eV and reasonable phase stability, [11] which make it a promising photoabsorber. To date, champion PCEs of 17.03% and 15.92% have been achieved for n-i-p-type (normal) and p-in (inverted)-type PeSCs, respectively, both based on CsPbI 2 Br. [12-18] Nevertheless, such PCE values are still lower than those of hybrid PeSCs. Further improvement is highly demanded for all-inorganic PeSCs. It has been well recognized that solution-processed inorganic PVK films, especially those prepared at a low temperature, are highly defective, [19] which usually result in large energy loss in PeSCs. For example, the highest reported open-circuit voltages (V OC) are 1.40 and 1.27 V for normal and inverted CsPbI 2 Br PeSCs, [15,16] respectively, both much lower than the E g (%1.9 eV) of PVK. Such large energy loss might be attributed to the severe defect-induced nonradiative recombination. [20,21] In addition, high-density interfacial defects also lead to poor contact between the PVK and electrode, which increases device series resistance and reduces short-circuit current density (J SC) and fill factor (FF). [21] To minimize the bulk and interfacial defects associated with inorganic PVKs, a variety of passivation strategies have been used, [21-28] including additive engineering and post-treatment. Nevertheless, most strategies focus on the passivation of either bulk or interfacial defects rather than both. On the other hand, considerable efforts have been devoted to maximizing the built-in electric field (BEF) of PeSCs, which facilitates carrier separation/transport and hence contributes to the V OC of the device. As the BEF is primarily determined by the work function difference between the two electrodes or transport layers (i.e., electron transport layer [ETL] and hole transport layer [HTL]), [29] BEF enhancement still remains

High‐Polarizability Organic Ferroelectric Materials Doping for Enhancing the Built‐In Electric Field of Perovskite Solar Cells Realizing Efficiency over 24%

“…It may act as a carrier recombination center, thus leading to unfavorable nonradiative recombination. [58] In addition, the iodine ion migration in the Target device was also alleviated under operational conditions ascribed to the low-defect density PVK-PVDF:DH film (Figure S25, Supporting Information), which would produce less iodine vacancy and increase the activation energy of ion migration, [10,[59][60][61] thus improving the long-term operational stability. Based on these results, we hypothesize that the improved operational stability of the 2-Control and Target devices originated from the reduced PbI 2 residue and the increased perovskite quality, which significantly suppressed the PbI 2 -induced photodegradation of the perovskite and unfavorable ion migration.…”

“…Commonly, the incident light is absorbed by the perovskite photoactive layer, generating free electrons and holes owing to the weak exciton binding energy; [ 9 ] these charge carriers are then extracted to the corresponding electrodes under the driving force of the built‐in electric field (BEF) of the pero‐SCs. [ 10 ] In this process, the electrons or holes are likely to be captured or trapped by the defects through Coulombic interactions, resulting in nonradiative recombination, which causes unexpected energy losses in the pero‐SCs. [ 11 ] Because of the polycrystalline nature of the solution‐processed perovskite film, it is difficult to suppress the defects, even via crystal growth regulation, [ 12 ] carrier‐concentration manipulation, [ 13 ] and defect passivation strategies.…”

Managing Defects Density and Interfacial Strain via Underlayer Engineering for Inverted CsPbI2Br Perovskite Solar Cells with All‐Layer Dopant‐Free

“…[33,51] The Spiro/TS-CuPc HTL was deposited by successively spin-coating an aqueous solution containing 6 mg mL −1 TS-CuPc on the Spiro-OMeTAD layer under different conditions, followed by a thermal annealing at 150 °C for 10 min (Figure S16, Supporting Information).To prepare perovskite layer, a 1.1 m precursor solution (1 mL DMSO/DMF mixture with v/v of 9/1) containing CsBr (235 mg) and PbI 2 (507 mg) was spin-coated on the HTL at a speed of 4000 rpm for 30 s, followed by a two-step thermal annealing process (45 °C for ≈30 s and then 140 °C for 2 min, see Figure S17, Supporting Information). [52,53] The bilayer ETL was deposited by spin-coating the Nano-Eu 2 O 3 solution (0.1 mg in 1 mL ethanol) [37] at a speed of 4000 rpm for 40 s, followed by spin-coating the PC 61 BM solution (20 mg in 1 mL o-DCB) at a speed of 2000 rpm for 40 s. Next, a buffer layer was prepared by spin-coating the Bphen solution (0.5 mg in 1 mL ethanol) at a speed of 2000 rpm for 40 s. Finally, Ag electrodes with thickness of 80-100 nm were vacuum deposited under 1 × 10 −4 Torr using a shadow mask. The device area was defined as 4 mm 2 .…”

Minimizing the Voltage Loss in Hole‐Conductor‐Free Printable Mesoscopic Perovskite Solar Cells

“…[24] For the planar inverted CsPbI 2 Br PSC, the strategy to reinforce BEF also works by gradient doping of 2,2′-bis(trifluoromethyl)-[1,1′biphenyl]-4,4′-diamine iodine. [25] Notably, besides the molecular doping, the observation of self-doping in perovskite has been reported, that is, the precursor stoichiometry could lead…”