Colloidal quantum dots being analyzed by X-ray beam

Unlock Solar Power's Potential: The Chemistry of Quantum Dots

Avery Sinclair in Science & Nature August 2025 • 4 min read.

"Dive into the Nano-World: Discover how surface and interface chemistry in colloidal quantum dots are revolutionizing solar applications"

Imagine a world powered by the sun, where energy is abundant, clean, and accessible to everyone. This vision edges closer to reality every day thanks to cutting-edge research in materials science, specifically the study of colloidal quantum dots (CQDs). These tiny semiconductors, smaller than a virus, possess unique optical properties that can revolutionize solar energy harvesting.

CQDs offer exciting possibilities. Their band gap can be tuned, meaning they can be optimized to absorb different parts of the solar spectrum. Moreover, they're capable of efficient multiple exciton generation, which translates to producing more electricity from a single photon of sunlight. Yet, these wonder materials have their challenges. Imperfect surfaces with dangling bonds lead to trap states that hinder performance.

To unlock the true potential of CQDs, scientists are developing innovative surface treatments to passivate these trap states. The record power conversion efficiency (PCE) of CQD-based solar cells has climbed to 13.4%, demonstrating significant progress. As CQDs have a high surface-area-to-volume ratio, many of their properties depend on surface chemistry making characterization of surfaces critical to our understanding of the properties of CQDs.

The Magic of Surface Chemistry: How XPS Helps Us Understand CQDs

Colloidal quantum dots being analyzed by X-ray beam

The key to optimizing CQDs lies in understanding and controlling their surface chemistry. This is where X-ray Photoelectron Spectroscopy (XPS) comes in. XPS is a surface-sensitive technique that provides detailed information about the elemental composition and chemical states of the CQD surface. With synchrotron radiation-excited depth-profiling XPS, scientists can analyze the CQD structure at the atomic scale.

Think of it like peeling back the layers of an onion, each layer revealing secrets about the CQD's composition and chemical environment. By carefully analyzing the emitted photoelectrons, researchers can identify oxidation processes, quantify surface composition after different treatments, and ultimately, gain insights into how these treatments affect air stability and device performance.

Here's how XPS helps in the development of efficient CQD solar cells:

Identifying Oxidation: XPS can detect and quantify oxidation products on the CQD surface, which can lead to performance degradation.
Optimizing Surface Treatments: By analyzing the surface composition after different treatments, researchers can identify the most effective passivation strategies.
Measuring Composition: XPS helps to measure and control the topmost surface layer of a CQD and nanocrystalline films.
Improving Air Stability: Understanding the surface chemistry allows scientists to develop CQDs that are less susceptible to oxidation and maintain their performance over time.

Many passivation strategies have been developed for CQDs. Long-chain organic ligands are used in synthesis to control the growth size and passivate some of the surface traps, creating a large potential barrier around the CQDs. However long ligands hinder charge transport out of CQDs as carriers have to tunnel through the large insulating barrier around the CQDs. Even when short chain organic ligands are used, their shape and size does not allow them to bind to all the dangling bonds on the CQD surface. For complete coverage, atomic ligands such as halide ions are needed. Using this technique, the record power conversion efficiency of CQD solar cells jumped from 4.4% to 7.4% in 2011-12.

The Future is Bright: Continued Advances in CQD Research

As CQD technologies rapidly improve towards commercial standards, surface characterization will continue to play a large role in our understanding and in development of new devices. Studies of detailed surface characterization and electronic structure combined with measurements of carrier dynamics will be critical to a full understanding of these materials. With continued research and development, CQD solar cells promise a future where clean, efficient energy is within everyone's reach.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1002/tcr.201800085, Alternate LINK

Title: Surface And Interface Chemistry In Colloidal Quantum Dots For Solar Applications Studied By X‐Ray Photoelectron Spectroscopy

Subject: Materials Chemistry

Journal: The Chemical Record

Publisher: Wiley

Authors: Pip C. J. Clark, Wendy R. Flavell

Published: 2018-11-02

Everything You Need To Know

What are colloidal quantum dots, and why are they promising for solar energy applications?

Colloidal quantum dots, or CQDs, are tiny semiconductor particles with unique optical properties that make them ideal for solar energy harvesting. One key advantage is their tunable band gap, which allows them to be optimized to absorb different parts of the solar spectrum. Additionally, CQDs can efficiently generate multiple excitons, increasing electricity production from a single photon. However, imperfections on their surfaces, like dangling bonds, can hinder their performance by creating trap states.

How does X-ray Photoelectron Spectroscopy (XPS) help in understanding the surface chemistry of colloidal quantum dots?

X-ray Photoelectron Spectroscopy, or XPS, is a surface-sensitive technique used to analyze the elemental composition and chemical states of CQD surfaces. It works by identifying oxidation processes and quantifying surface composition, providing insights into how different surface treatments affect air stability and device performance. By using synchrotron radiation-excited depth-profiling XPS, scientists can analyze the CQD structure at the atomic scale, similar to peeling back layers to reveal its composition and chemical environment.

What are the different passivation strategies used for colloidal quantum dots, and why are they important?

Passivation strategies for CQDs involve treating their surfaces to eliminate imperfections, like dangling bonds, that lead to trap states. Initially, long-chain organic ligands were used to control growth size and passivate some surface traps; however, these long ligands hinder charge transport. Atomic ligands, like halide ions, are now preferred for complete coverage of dangling bonds, leading to significant improvements in power conversion efficiency.

How does oxidation affect the performance of colloidal quantum dots, and how is X-ray Photoelectron Spectroscopy (XPS) used to address this issue?

Oxidation can significantly degrade the performance of CQDs in solar cells. X-ray Photoelectron Spectroscopy, or XPS, helps detect and quantify oxidation products on the CQD surface. Understanding the surface chemistry allows scientists to develop CQDs that are less susceptible to oxidation, maintaining their performance over time and improving air stability. Optimized surface treatments identified through XPS analysis can mitigate the negative effects of oxidation.

What future advancements are expected in the research and development of colloidal quantum dot (CQD) solar cells?

Future research on CQDs will likely focus on detailed surface characterization and electronic structure studies combined with measurements of carrier dynamics. Surface characterization of CQDs will involve ongoing refinement of techniques like X-ray Photoelectron Spectroscopy, or XPS, to better understand and control surface chemistry. Advances in these areas are crucial for developing new devices and fully realizing the potential of CQD solar cells for clean, efficient energy.