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

Abstract

Colloidal quantum dots (QDs) benefit from solution-phase processing and band-gap tuning for their application in solar cell development. Today's QD solar cells rely on solid-state ligand exchange (SLE) to replace bulky oleic acid (OA) ligands with small 1,2-ethanedithiol (EDT) ligands to develop a conducting hole transport layer (HTL). High volume contraction in EDT conjugated QD films, however, leads to crack and porosity in the HTL, which is a major cause of concern for the device reproducibility and large-area solar cell development. We show that partial removal of the OA ligands in the solution phase reduces the volume contraction in solid films, thereby allowing the growth of crack-free QD films in the SLE process. The cleaning of QDs by repeated precipitation and redispersion using a protic methanol (MeOH) solvent helps with partial removal of the OA ligands, but it is detrimental to the electronic properties of QDs. We develop a one-step solution-phase partial ligand-exchange process using ammonium salts, which enable partial replacement of the OA ligands and passivation of the QD surface. Introduction of the facile partial ligand-exchange process eliminates the need for tedious and wasteful multiple cleaning steps with MeOH, while improving the photophysical properties of QDs. The advancement in QD processing helps to build crackfree, smooth, and conjugated QD films for their deployment as HTLs in solar cell development. Partial ligand exchange with NH 4 SCN leads to a 1.5 times increase in p doping and mobility over multiple MeOH-cleaned PbS QD films. HTLs developed using NH 4 SCN QDs show an improved photovoltaic performance to attain a 10.5% power conversion efficiency. Improvement in the depletion width and hole collection efficiency leads to a superior photovoltaic performance, as confirmed from experimental studies and one-dimensional solar cell capacitance simulation.

Thiocyanate‐ and Thiol‐Functionalized p‐Doped Quantum Dot Colloids for the Development of Bulk Homojunction Solar Cells

“…We performed SCAPS simulation to determine the active QD layer's optimum electron and hole mobilities for PCE maximization. [50,51] The PCE of solar cells as a function of electron mobility (within the range 10 À4 to 1 cm 2 V À1 s À1 ) is shown in Figure 5b, determined for different hole mobilities. The SCAPS parameters used for the simulation study are shown in Table S2, Supporting Information.…”

“…This prompts us to determine the electron and hole mobility of the n-PbS and p-PbS layer using the space charge limited conduction (SCLC) method (Figure S11 a&b, Supporting Information). [10,51] The electron mobility of n-PbS (1.06 Â 10 À3 cm 2 V À1 s À1 ) is almost nine times higher than p-PbS (1.22 Â 10 À4 cm 2 V À1 s À1 ). However, the hole mobility of n-PbS (6.85 Â 10 À4 cm 2 V À1 s À1 ) is four times lower than p-PbS (2.63 Â 10 À3 cm 2 V À1 s À1 ).…”

Toward efficient hybrid solar cells comprising quantum dots and organic materials: progress, strategies, and perspectives

“…73,92,93 Nevertheless, EDT ligands have introduced some negative issues, e.g., notorious pungent odor, pinhole stemming from their strong reactivity, passivation destruction of the active layer, and tedious solid-state ligand exchange (SSLE). [94][95][96] In this regard, Sargent's group has contributed considerably to address this challenge. Their recent work replaced conventional EDT ligands with nontoxic malonic acid (MA), which has nearly no impact on the bottom layer, due to the moderate reactivity (Fig.…”

Reducing the Open-Circuit Voltage Loss of PbS Quantum Dot Solar Cells via Hybrid Ligand Exchange Treatment

“…Furthermore, due to the strong reactivity of EDT in removing the initial long-chain oleic acid (OA) from the surface of PbS-CQDs, film volume contraction can lead to cracking and the formation of more interface defects. 31,32 Therefore, the PbS-EDT HTL leads to significant V OC losses in PbS-CQDSCs, underscoring the urgent need to search for alternative ligand exchange strategies.…”