Improved Eco-Friendly Photovoltaics Based on Stabilized AgBiS₂ Nanocrystal Inks

Abstract

AgBiS2 nanocrystals (NCs) have emerged as attractive absorbers in eco-friendly photovoltaics because of their nontoxic components and high absorption coefficient. Native long-chain ligands of AgBiS2 NCs should be replaced with short-chain ligands for their photovoltaics; however, conventional approaches have been performed using solid-state ligand exchange (SSLE), resulting in inhomogeneous NC aggregation, broad bandtail, large trap density, and resultantly low open-circuit voltage (V OC) in devices. Herein, we first report that long-chain ligands of AgBiS2 NCs are replaced with halometallate-based short ligands via solution-phase ligand exchange (SPLE). AgI and BiI3 are used as halometallate sources, and we find that colloidally stable, highly concentrated AgBiS2 NC inks in polar solvents are prepared via SPLE using AgI-based halometallates, enabling one-step-deposition suitable for roll-to-roll process. This leads to higher degree of ligand exchange, sharper bandtail, lower trap density, and resultantly higher V OC in devices compared to conventional SSLE. We also first demonstrate that the photovoltaic performance can be improved by introducing ethanedithiol-exchanged AgBiS2 NCs on SPLE-prepared AgBiS2 NC solids because of favorable band alignment and extended depletion width. Thus, this enables improving device performance up to a power conversion efficiency of 4.08% with the highest V OC of 0.55 V among the AgBiS2 NC photovoltaics reported so far.

Environmentally Friendly AgBiS2 Nanocrystal Inks for Efficient Solar Cells Employing Green Solvent Processing

“…The performance reported here is significantly higher than previously reported ink based devices, indicating the superiority of our MPA based solution-phase ligand-exchange method. [7] The J sc value was also estimated from external quantum efficiency (EQE) spectra (Figure 3e), which showed an integrated current density of 24.7 mA cm −1 , [2] in good agreement with the J sc from J-V curve measurements. After storage in ambient air under an average humidity of 30% for one month without encapsulation, the high performance device retained more than 95% of the original efficiency, which confirms the robustness of our nanocrystal inks.…”

“…[6] Currently, the most widely used solvents in solution-phase ligand-exchange procedures are formamide (FA), N,N-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO). [3,[7][8][9] However, these polar solvents are highly toxic, raising major concerns on overall environmental impact. [10] Moreover, occupational safety must be taken into account for operators in manufacturing or scientists in the lab, who may come into direct contact with the ink or inhale solvent fumes during fabrication.…”

“…[14][15][16] Solution-phase ligand-exchange method has been recently introduced into AgBiS 2 NCs, yet their performance has been lower than their solid-state ligand-exchanged counterparts. [7,17] Here, we report a facile solution-phase ligand-exchange method for the production of AgBiS 2 inks employing 3-mercaptopropionic acid (MPA) and methanol (MeOH) as the ligand and solvent pair. The resultant MPA capped nanocrystals can be easily dissolved in water, rendering the final ink used for device fabrication, completely environmentally friendly.…”

Oxygen-induced degradation in AgBiS2 nanocrystal solar cells

“…13 Similarly, attention has been devoted to optimising the ligand exchange procedure, which can be performed either in a layer-by-layer fashion 12,14 or via a liquid phase exchange. 15,16 Alternatively, it has been shown that the ligand exchange step can be omitted by directly synthesising the nanocrystals with the final ligands. 17 While these efforts are very important for further improving the performance of AgBiS 2 PVs, very little is known about the stability of these devices.…”

Mixed AgBiS2 nanocrystals for photovoltaics and photodetectors

“…In the past few years there has been a remarkable progress in the synthesis and application of colloidal AgBiS 2 nanocrystals as the photoactive material in solar cell devices. 7,11,12,[17][18][19][20][21][22][23]24 The low carrier diffusion length and the non-negligible recombination pathways have been identified as the main limiting factors for photocurrent generation, especially when using thick active layers. 7,12 Therefore, the record solar cell efficiencies reported so far typically limit the optimal AgBiS 2 layer thickness at around 35 nm.…”