Heterovalent Substitution in Mixed Halide Perovskite Quantum Dots for Improved and Stable Photovoltaic Performance

Abstract

One of the strategies to improve the device performance of metal-halide perovskite solar cells is to alter the electronic and optical properties of the perovskite lattice by metal ion doping. In this work, aliovalent doping of silver at the lead site of CsPbBr1.5I1.5 quantum dots (QDs) shows striking prospects in improving the photovoltaic (PV) device performance. Lattice doping could be achieved only up to ∼3.5 atom % Ag+ with respect to Pb2+ which has significant impact on the electronic and optical properties of the QDs. The band gap could be reduced from 2.15 eV for pristine QDs to 2.12 eV with 3.5 atom % Ag-doping, beyond which midband gap states are created along with the formation of a secondary AgI phase with higher Ag+ substitution. Density functional theory (DFT) shows enhanced optical absorption coefficient and p-type character of the Ag-doped QDs. With the QDs having 3.5 atom % Ag, a significant ∼20% enhancement in photoconversion efficiency (PCE) is observed up to 9.67 ± 0.12% due to the reduction of surface and intrinsic defects, decrease in nonradiative recombination leading to an increase in carrier lifetime, and increase in charge transfer. Although in general all-inorganic perovskite (AIPSK) QDs are attractive because of their higher crystallinity, better optical property, and easy solution processability, the solution stability of AIPSK QDs is not the same as that in PV devices. After 3.5 atom % Ag-doping, our PV devices show extremely promising ambient stability for 575 h (24 days) with less than 5% drop in PCE in comparison to 12% drop for pristine QD devices.

Colloidal Quantum Dot Solar Cells: Progressive Deposition Techniques and Future Prospects on Large‐Area Fabrication

“…This may explain the continued reliance on spin coating to the near exclusion of all other coating methods as shown in Table 3. [128][129][130][131][132][133][134][135][136][137][138][139][140][141][142][143][144][145] However, it should be noted that, compared to PQD-based solar cells, there are many reports of deposition techniques other than spin-coating for PbS/PbSe CQD-based solar cells, as demonstrated in Figure 7. The liquid phase ligand exchange process has been well-established for metal chalcogenide based CQDs, which allows the whole QD absorber layer to be produced by a single-step deposition using colloidal short-chain-ligandcapped QD inks without any requirement of post-deposition ligand exchange.…”

Perovskite Quantum Dots in Solar Cells

“…As a consequence, the charge recombination loss in PQDSCs is lessened (Figure 5e ) and strikingly, Zn:CsPbI 3 PQDSCs provided a high PCE exceeding 16%, which is one of the highest efficiency of pure PQDSCs up to now (Figure 5f ). [ 50 ] Furthermore, since Ag + possesses an ionic radius of (115 pm), closer to Pb 2+ (119 pm), Ghosh et al incorporated Ag + into CsPbBr 1.5 I 1.5 PQDs [ 52 ] and deemed it will not perturb the crystal structure significantly. [ 82 ] With the PQDs having 3.5 atom % Ag + , a significant ≈20% enhancement in PCE and ambient stability are observed for the reason that the reduction of surface and intrinsic defects, and the decrement in nonradiative recombination leads to an increased carrier lifetime in parallel with an enhanced charge transfer process.…”

“…CsPbI 3 PQDs 13.4% - [22] CsPbI 3 PQDs 14.1% 15 days (ambient condition) > 90% of initial PCE [42] CsPbI 3 PQDs 15.2% - [43] CsPbI 3 PQDs 13.7% 10 days (ambient condition) 87% of initial PCE [44] CsPbI 3 PQDs 14.3% 7 days (ambient condition) 95% of initial PCE [45] FAPbI 3 PQDs 12.7% - [46] CsPbI 3 PQDs 15.1% - [47] CsPbI 3 PQDs 14.9% - [48] Additive engineering CsPbI 3 PQDs 13.1% 7 days (ambient condition) 68% of initial PCE [49] CsPbI 3 PQDs 16.1% 10 days (ambient condition) 85% of initial PCE [50] CsPbI 3 PQDs 14.8% 50 h (ambient condition) > 95% of initial PCE [51] CsPbBr 1.5 I 1.5 PQDs 9.7% 24 days (ambient condition) > 95% of initial PCE [52] CsPbI 3 PQDs 12.2% >90 days (<20% relative humidity, room temperature) 85% of initial PCE [53] CsPbI 3 PQDs 11.6% 1 month (N 2 atmosphere) > 98% of initial PCE [54] CsPbI 3 PQDs 15.1% (rigid) 12.3% (flexible) - [55] CsPbI 3 PQDs 16.2% 30 days ≈ 83% of initial PCE [56] CsPbI 3 PQDs 12.3% - [57] Hybrid composition engineering CsPbBr 1.5 I 1.5 PQDs 7.9% 35 h (constant exposure to air) 88% of initial PCE [58] CsPbBr 0.6 I 2.4 PQDs 12.3% 15 days (ambient condition) 87% of initial PCE [58] Cs X FA 1− X PbI 3 PQDs 16.1% 1000 h (ambient condition) 96% of initial PCE [59] Cs x FA 1− x PbI 3 PQDs 17.4% - [29] Cs 1− x FA x PbI 3 PQDs 16.6% 600 h (continuous illumination) 94% of initial PCE [60] CsPbBrI 2 PQDs 5.3% - [61] CsSn 0.6 Pb 0.4 I 3 PQDs 2.9% - [62] formation energy, limiting its usage as photo absorbers in solar cells. In contrast, although Cs-and FA-based perovskite bulk crystals are metastable at room temperature, their stability is improved upon reducing the crystal size from bulks to quantum dots, rendering their application widespread in photovoltaics.…”

All-inorganic perovskite quantum dots as light-harvesting, interfacial, and light-converting layers toward solar cells

“…Interestingly, monovalent Ag + was also reported to substitute Pb 2+ in CsPbBr 1.5 I 1.5 QDs. 65 Ghosh et al presented that Ag + had closer ionic radius (115 pm) to Pb 2+ (119 pm) and partial substitution of Pb 2+ by Ag + did not perturb the crystal structure signicantly. The substitution of Pb 2+ with 3.5% Ag + could reduce the intrinsic defect states and carrier recombination, thereby increasing the lifetime of the charge carriers.…”

“…In the aspect of A-site regulation, Cs-salt doping (e.g., with Cs acetate) of CsPbI 3 QD lms were shown to enhance the SC stability in air by lling the Cs vacancies at the QD surface. 89 Aliovalent doping of silver ions 65 at the lead site of CsPbBr 1.5 I 1.5 QDs could improve the PV device stability. Ag doping is highly crucial for the reduction of surface and intrinsic defects, and decreasing non-radiative recombination.…”