Solution-Phase Hybrid Passivation for Efficient Infrared-Band Gap Quantum Dot Solar Cells

Abstract

The broad tunability of the energy band gap through size control makes colloidal quantum dots (QDs) promising for the development of photovoltaic devices. Large-size lead sulfide (PbS) QDs, exhibiting a narrow energy band gap, are particularly interesting as they can be used to augment perovskite and c-Si solar cells due to their complementary NIR absorption. However, their complex surface chemistry makes them difficult to process for the development of solar cells. The shape of the QDs transformed from octahedron to cuboctahedron as their size increases, a phenomenon guided by surface energy minimization. As a result, the surface properties change drastically for large-size QDs, which exhibit nonpolar (200) facets and polar (111) facets, as opposed to only (111) facets in small-size QDs. Recent advancements in solution-phase surface passivation strategies, used for the development of high-performance solar cells using the small size and wide band gap QDs, failed to translate a similar enhancement in the case of large-size and narrow band gap QDs. Here, we report a hybrid passivation strategy for large-size and narrow band gap QDs to passivate both ( 111) and ( 200) facets, respectively, using inorganic lead triiodide (PbI 3 − ) and organic 3-chloro-1-propanethiol (CPT). By employing charge balance calculation, we identified the desired narrow band gap for QDs to complement the perovskite and c-Si absorption. The distinct choice of the organic ligand CPT enhances the colloidal stability of QDs in the solution phase and improves surface passivation to stop QD fusion in solid films. Photophysical properties show narrower excitonic and emission peaks and a reduction in the Stokes shift. Hybrid passivation leads to a 94% increase in the power conversion efficiency of solar cells and a 74% increase in the external quantum efficiency at the excitonic peak.

Crack-Free Conjugated PbS Quantum Dot–Hole Transport Layers for Solar Cells

“…Capacitance–voltage measurement is used to determine the carrier depletion width ( W ) of solar cells (Figure e). Depletion with W is determined using the equation W = ε 0 ε r A / C , where ε 0 is the permittivity of air, ε r the relative permittivity of PbS QDs (considered as 43), A the active area, and C the capacitance. Solar cells based on ammonium-salt-treated HTLs show a higher depletion width than 8W-MeOH.…”

“…For development of the active layer, the QDs were processed using the earlier-reported method . A total of 172 mg of PbI 2 (0.075M), 45 mg of PbBr 2 (0.025M), and 72 mg of NH 4 I (0.1 M) were dissolved in 5 mL of DMF, and 10 μL of CPT (0.02M) was added to it to prepare the ligand stock solution.…”

“…The bottom ETL was prepared by depositing ZnO nanocrystals on top of precleaned indium–tin oxide (ITO) and heated at 250 °C, as reported elsewhere . CPT–PbS QDs from butylamine were then coated on top of ZnO.…”

“…For development of the active layer, the QDs were processed using the earlier-reported method. 28 A total of 172 mg of PbI 2 (0.075M), 45 mg of PbBr 2 (0.025M), and 72 mg of NH 4 I (0.1 M) were dissolved in 5 mL of DMF, and 10 μL of CPT (0.02M) was added to it to prepare the ligand stock solution. In a typical ligand-exchange process, 5 mL of 14 mg/mL PbS QDs in octane was added to 5 mL of the ligand stock solution in a centrifuge tube and vortexed vigorously for 2 min.…”

“…The device architecture of a QD solar cell consists of a transparent electrode, an n-type electron transport window layer (ETL), a light-absorbing active QD layer, a hole transport layer (HTL), and a metal electrode. , Several excellent advancements have been made to improve the ETL − and the active QD layer ,,,,− in the past. The development of a HTL to improve the performance of QD solar cells has gained significant research attention lately. − Several materials have been investigated for development of the HTL, including organic semiconductors, − metal oxides, − 2D semiconductors, , and p-type QDs. ,,,,, HTLs built using surface-functionalized p-doped QDs yield the highest photovoltaic performance to date. , …”