Heavy-metal-free quantum dots (QDs) have emerged as promising alternatives to traditional lead-based QDs in solar cell applications due to their comparable optoelectronic properties and reduced environmental toxicity. Among the most studied candidates are indium phosphide (InP), copper indium sulfide (CuInS2), and silver bismuth sulfide (AgBiS2), each offering distinct advantages and challenges in synthesis, stability, and device performance. This article examines their properties, compares them with lead-based QDs, and evaluates their commercialization potential.

InP QDs exhibit tunable bandgaps ranging from 1.35 eV to 2.4 eV, covering visible to near-infrared wavelengths, making them suitable for broad-spectrum light absorption. Their excitonic properties are comparable to those of lead sulfide (PbS) and lead selenide (PbSe) QDs, with high extinction coefficients and quantum yields exceeding 70% in optimized conditions. However, InP QDs face challenges in achieving narrow size distributions due to the high reactivity of phosphorus precursors, which can lead to broad photoluminescence peaks. Surface passivation with zinc sulfide (ZnS) shells has been shown to improve photoluminescence quantum yield and stability against oxidation, but this adds complexity to synthesis.

CuInS2 QDs offer a bulk bandgap of approximately 1.5 eV, aligning well with the solar spectrum. Their composition-tunable optoelectronic properties allow for adjustments in stoichiometry to optimize carrier dynamics. Unlike InP, CuInS2 QDs exhibit intrinsic defect tolerance due to their chalcopyrite structure, reducing non-radiative recombination losses. Their extinction coefficients are lower than those of Pb-based QDs, but their broad absorption profiles compensate for this limitation. A key advantage is their relatively straightforward synthesis via hot-injection or solvothermal methods, though control over copper and indium precursor reactivity remains a challenge. Stability under prolonged illumination is moderate, with encapsulation strategies required to prevent sulfur loss.

AgBiS2 QDs are a newer class of heavy-metal-free QDs with ultra-small sizes and strong light absorption coefficients exceeding 10^5 cm^-1, rivaling Pb-based QDs. Their bandgap of around 1.2 eV to 1.8 eV is ideal for near-infrared harvesting. AgBiS2 QDs demonstrate exceptional air stability without additional shelling, attributed to their robust ionic lattice. However, synthesis reproducibility is hindered by the sensitivity of silver and bismuth precursor ratios, which affect crystallinity and defect formation. Their defect-tolerant nature results in moderate charge carrier mobilities, but their high absorption compensates for this in solar cell applications.

When comparing optoelectronic properties, InP and AgBiS2 QDs approach the performance of Pb-based QDs in terms of absorption and tunability, while CuInS2 excels in defect tolerance. Lead-based QDs still hold an edge in charge carrier mobility and ease of bandgap tuning, but heavy-metal-free alternatives are closing the gap. Stability assessments reveal that AgBiS2 outperforms InP and CuInS2 in ambient conditions, while InP requires shelling for comparable durability. CuInS2 shows intermediate stability but benefits from simpler encapsulation requirements.

In solar cell architectures, InP QD-based devices have achieved power conversion efficiencies (PCEs) exceeding 15%, approaching the performance of Pb-based QD solar cells. CuInS2 devices lag slightly, with PCEs around 12%, due to lower carrier mobilities, but their defect tolerance enables better open-circuit voltages. AgBiS2 solar cells have demonstrated PCEs up to 9%, with potential for improvement through interface engineering. While these values are below the highest recorded for Pb-based QDs (over 18%), the non-toxic nature of these materials makes them attractive for scalable deployment.

Synthesis challenges for heavy-metal-free QDs include precursor reactivity control, stoichiometric precision, and surface defect management. InP synthesis often requires hazardous phosphorus precursors, while CuInS2 and AgBiS2 demand strict control over cation ratios. Scalable production methods such as continuous-flow reactors and mechanochemical synthesis are being explored to address these issues. Purification steps also add complexity, as residual ligands and byproducts can degrade device performance.

Commercialization potential is promising but hinges on overcoming material costs and synthesis scalability. Indium and silver are expensive, raising concerns about resource availability, whereas bismuth is more abundant. Efforts to reduce precursor waste and develop earth-abundant alternatives are ongoing. Regulatory advantages favor heavy-metal-free QDs, particularly in consumer electronics and building-integrated photovoltaics where lead restrictions apply.

In conclusion, heavy-metal-free QDs such as InP, CuInS2, and AgBiS2 present viable alternatives to lead-based QDs for solar cells, with competitive optoelectronic properties and improved environmental profiles. While challenges in synthesis and cost remain, their performance metrics and stability advancements position them as strong candidates for sustainable photovoltaic technologies. Continued research into scalable synthesis and interface engineering will be critical for their widespread adoption.