









































Abstract: As energy consumption increases across the globe, current energy supplies such as fossil fuel, natural gas, 
and coal are rapidly diminishing. Our current solar cell technologies are reaching all time high efficiencies, but our 
infrastructure and cost are still preventing expansion. Even though only 0.1% of our primary energy comes from 
sunlight (Raval and Gupta, 2015), colloidal quantum dot solar cells have properties that can be tuned and utilized 
to enhance energy conversion. This review article explores past research and analyzes the benefits and drawbacks 
of various synthesis, fabrication, and cell structures of colloidal quantum dots and their utilization in solar cells. In 
addition, further research on properties of CQDs, such as shape and multiple exciton generation, are discussed. 

Aisthesis      Volume 9,  201836

Photovoltaic Properties and Solar Cell Applications of 
Colloidal Quantum Dots

by Jackson Nash

Introduction
One attractive option to help achieve high 

efficiency and affordable energy are quantum dot 
(QD) solar cells. QD solar cells have the ability to 
enhance light absorption, not only in visible light, 
but also the infrared light range. Because of this, they 
serve as an appealing alternative solar cell material. 
Due to their unique properties, QDs can be used for 
a wide variety of applications, including displays, 
sensors, batteries, and cancer treatment.

This review will focus on colloidal quantum dot 
(CQD) solar cells and their benefits. The quantum 
confinement effect will be discussed to demonstrate 
the photovoltaic benefits provided by the physics of 
CQDs. Additionally, synthesis, physical application 
methods, and solar cell structure will be reviewed  to 
highlight benefits and disadvantages of each. Finally, 
the possibility of multiple exciton generation will be 
reviewed to illustrate promising characteristics of 
CQD solar cells. 
      
Quantum Dots

QDs are very tiny semiconductors that are only 
nanometers in size. To put this into perspective, 
these particles are 10,000 times smaller than a 
human hair. Scientists have found that by reducing 
a particle to this level, its properties—that typically 
remain constant—can change drastically. The 
properties of QDs can be adjusted by their size, 
shape, and material. One characteristic property that 
is easily visualized is the photoactive nature of QDs. 

A QD absorbs light and then re-emits light in pure 
colors when excited. The size of the dot determines 
the color of the light, such that smaller dots emit a 
higher energy light than larger dots. The size of the 
dots can be controlled through various synthesis 
methods and can be formed to produce nearly any 
color in the visible spectrum.    

The ability to adjust the size and shape of QDs is 
a desirable characteristic (Carey et al., 2015; Kedera 
et al., 2008; Zhao & Rosei, 2017). The adjustable 
properties of QDs are entirely dependent upon their 
extremely small size. This can be described by the 
quantum confinement effect, which is a result of 
reducing a particle down to the nanometer scale. 
When a particle becomes this small, the energy 
level transitions from continuous to discrete energy 
levels. The idea of quantum confinement was first 
investigated in the early 1970s with Molecular Beam 
Epitaxy. Guyot-Sionnest (2008) states that “Quantum 
dots are the extension of quantum confinement 
ideas to three dimensions” (p. 778). This property 
plays an important role in the production of solar 
cell technology, as quantum confinement allows 
fine-tuning of specific optical properties such as 
absorption wavelength (Guyot-Sionnest, 2008; 
Zhao & Rosei, 2017) and bandgap (Xin, 2014). In 
comparison to other solar cells, QD solar cells have 
unique benefits and a bright future. Their potential 
for multiple exciton generation and narrow band-
gap puts QD solar cells at a prospect of very high 
energy absorption and efficiency.



Photovoltaic Properties and Solar Cell Applications of Colloidal Quantum Dots

Aisthesis      Volume 9,  201837

Literature Review
Synthesis - Colloidal Quantum Dots 

The synthesis technique used to develop QDs is 
crucial to obtain specific desired properties. Colloidal 
quantum dots refer to dots that are made in a colloid, 
or mixture, where the particles are suspended 
throughout another substance. In recent years, 
CQDs have gained a lot of attention and research, 
especially pertaining to solar cell technology. In 
2008, Guyot-Sionnest claimed that in the last five 
years (2004-2008), there have been five times more 
scientific publications on CQDs than the previous 
twenty years combined. Two key elements of CQDs 
are their high monodispersity and small size; this is 
important because properties that increase percent 
conversion of solar cells are size dependent. There 
are many different synthesis methods to produce 
CQDs. Four of the most common methods are 
solvothermal synthesis, aqueous media synthesis, 
ultrasonic synthesis, and hot injection synthesis. 

Solvothermal Synthesis
Solvothermal synthesis uses a sealed vessel 

and an organic solvent as the reaction medium. 
One key advantage of this method is that it is heat 
promoted. Using heat increases the reaction and 
enhances the solubility of the reagents. Combining 
an appropriate organic solvent and a temperature 
higher than the normal solvent boiling point can be 
rewarding. This method allows raw materials which 
are poorly reactive to be utilized, such as Sulfur 
(S), Selenium (Se), and Tellurium (Te) (Barcelo et 
al., 2014). One disadvantage of this method is the 
use of toxic surfactants and organic solvents that 
require temperatures up to 300ºC to decompose the 
precursor species (Zhao & Rosei, 2017). This is an 
environmental concern which must be considered 
when choosing between different synthesis methods.  

Aqueous Synthesis
In contrast to the solvothermal method, 

synthesizing CQDs with aqueous media is 
considered an environmentally conscious choice 
due to the low synthetic temperature, which is 
less than 100ºC. This application, however, has 
consequences in performance. It exhibits a lower 
photoluminescence quantum yield (PLQY) (Zhao & 
Rosei, 2017), which is the ratio of photons absorbed 

to the photons emitted. A precipitation reaction is 
utilized to collect the CQDs formed by this method. 
Thiols are added as stabilizing agents and play 
an important role by preventing uncontrollable 
growth as they actively absorb on the surface of the 
nanocrystal (Barcelo et al., 2014). The components 
of thiol-capping molecules allow the ability to obtain 
CQDs with a functional shell (Barcelo et al., 2014; 
Zhao & Rosei, 2017). While the other synthesis 
methods are typically capped with an inert organic 
molecule, obtaining a functional shell allows more 
adjustment to specific properties and characteristics, 
such as solubility in water.  

Ultrasonic Synthesis
The ultrasonic method is a low temperature 

synthesis which contributes to a much lower cost 
to produce QDs. Chen et al. (2017) used ultrasonic 
synthesis and its ligand-assisted re-precipitation to 
provide up to 70% PLQY at room temperature. This 
is an excellent improvement for low temperature 
production, in comparison to the aqueous method. 
The ultrasonic method can also produce high-
quality dots that absorb a wide range of colors from 
blue to near infrared wavelengths. Other advantages 
include reduced particle size and the ability to 
produce different shapes (Chen, 2017), including 
spheres, rods, wires, and rings. Some researchers 
suggest that controlling the shape of the CQDs can 
influence their quantum confinement (Carey et al., 
2015), absorption properties (Barcelo et al., 2014), 
and optical transitions of energy (Zhao & Rosei, 
2017). However, Bagher (2016) states that there is 
insufficient evidence to support any effect that shape 
has on optical properties. By continuing research 
on shape and determining how to fine tune these 
characteristics, CQD solar cells could potentially 
reach record efficiencies.

Hot Injection Synthesis 
One of the most discussed and widely used 

methods in forming CQDs is hot injection synthesis. 
By mixing highly reactive precursors at a high 
temperature, a rapid nucleation is followed by slow 
growth. High temperature causes a burst nucleation 
that produces monodisperse nuclei that will continue 
to grow homogeneously. Such nucleation is typically 
controlled by temperature, supersaturation, and 



Photovoltaic Properties and Solar Cell Applications of Colloidal Quantum Dots

Aisthesis      Volume 9,  201838

interfacial tension (Carey et al., 2015). One side effect 
of this method is the high surface-to-volume ratio, 
which can be compensated by surface-passivating 
reagents or ligands (Carey et al., 2015; Kedera et al., 
2008), which is similar to the ultrasonic method. 
With hot injection synthesis, products can be formed 
with 5-10% standard deviation from the average size 
without post-preparative size fractionation (Reiss, 
2008).

In addition to size control, the optical properties 
can be tuned based on the choice of ligand attached. 
Controlling the size of CQDs is relatively easy because 
it is a simple time dependent reaction. The time of 
the reaction and the size of the CQD are correlated—
as time increases, size increases. When using hot 
injection synthesis, controlling the size of the dot 
also allows the bandgap and optical absorption to 
be easily tuned. The ability to adjust these properties 
increases percent conversion of sunlight into energy. 

Batch vs. Continuous Synthesis
Traditionally, CQDs are synthesized in a 

laboratory setting using benchtop glassware that 
produce high percent yields but can only be produced 
in small batches. Small batch synthesis works well 
because it is much easier to control the temperature 
and monitor the progress of the reactions. When 
trying to produce CQDs with large batch synthesis, 
the batch-to-batch products lack consistency and 
fine control over their properties (Jeong et al., 
2016). Most importantly, when these production 
procedures are scaled-up, the quality of the CQDs 
are compromised. This limits the commercialization 
of QD-based devices (Jeong et al., 2016). Therefore, 
an alternative production method must be employed 
to promote large scale production. Microfluidic 
reactors are one solution to this problem, offering 
complete automation of the process as well as 
excellent consistency. In comparison to the small 
batch reactions, the microfluidic reactors also can be 
tuned to control the reaction time and flow rates of 
precursor solutions (Jeong et al., 2016). This enables 
quality CQDs to be produced continuously, which 
can be applied to create solar cells. In 2016, with 
the use of microfluidic reactors and their ability to 
control flow rates, Jeong et al. were able to tune the 
spectral range and PLQY of CQDs. They were also 

able to increase the efficiency of the solar cells by 19% 
when using the enhanced QDs from the microfluidic 
reactors (Jeong et al., 2016).

Fabrication for Solar Cell Application
Processing CQDs for solar cell application 

requires two steps, depositing solution and solvent 
evaporation. After a small volume of the solution is 
deposited onto a substrate, the solvent evaporates, 
leaving behind immobile QDs (Carey et al., 2015; 
Kedera et al., 2008). Various methods have been 
used to apply the CQDs to a substrate, which allows 
them to be used within solar cells. 

Spin Coating 
The method of spin coating requires a rotating 

vacuum stage upon which the substrate is mounted. 
Once the CQD solution is deposited, the stage is 
rotated at speeds between 1000-5000 rpm. The 
spinning spreads the solution and shearing occurs to 
produce an even layer of deposited CQDs. Dispersing 
the CQDs via spin coating creates consistency and 
high-quality film morphology (Carey et al., 2015). 
Past research suggests that this application method 
can be successfully utilized. However, this method 
is primarily used for research and testing solar cell 
properties. The inability to be used in scaled-up or 
continuous large-scale fabrication prevents spin 
coating from becoming an industrial application 
method. Also, spin coating wastes much of the 
QD solution and is a sensitive procedure where 
spin speed, humidity, and temperature must be 
considered and observed (Carey et al., 2015). 

Dip Coating
Much like spin coating, dip coating is a process 

that is not compatible with continuous processing, 
but it has potential to be scaled-up. Dip coating 
involves dipping the substrate directly into the CQD 
solution. This process is typically automated to help 
maintain a consistent dip and removal rate. Despite 
its scalability, dip coating does not produce uniform 
films and is sensitive to changes in processing 
conditions. Another hindrance of dip coating is 
the necessity of using a lower boiling point solvent 
to ensure correct deposition of the liquid onto the 
substrate (Carey et al., 2015).  



Photovoltaic Properties and Solar Cell Applications of Colloidal Quantum Dots

Aisthesis      Volume 9,  201839

Spray Coating  
Contrary to both spin and dip coating, spray 

coating has shown to be successful with scale-up 
manufacturing (Carey et al., 2015; Kramer et al., 
2014; Lin, 2014). Spray coating applies the colloidal 
solution through a nozzle to spray the solution onto 
the substrate. An air-injection mechanism is typically 
added to aid in prevention of particles accumulating 
in the nozzle (Lin, 2014). This method can apply 
ultrathin layers of CQDs that are almost a monolayer 
(Kramer et al., 2014). Spray coating can also be 
used to cover almost any surface, shape, and size, 
including curved surfaces. Kramer et al. (2014) found 
that their spray coating method, termed “sprayLD,” 
could achieve superior performance when applied 
to a flexed surface. With the ability to apply CQDs 
to any shape and surface, the cost of production can 
be minimized by reducing the cost of the solar cells 
themselves. Solar cells can be created using flexible 
and economical materials and can be easily coated 
with the CQDs. Another benefit to this method is 
the simplicity in layering the CQDs. The layers can 
be sprayed directly on top of previous layers. Kramer 
et al. (2014) achieved a 33% enhancement in percent 
conversion efficiency when layering was applied 
via spray coating. One disadvantage to this method 
is the need to be considerate of the viscosity of the 
CQD solution.

Solar Cell Structure
Cell structure for solar cells is an important 

consideration when developing CQD solar cells. 
Advances have been made to the performance of solar 
cells by improving the design of solar cell structure. 
This section will explore two of the architectural and 
device concepts discussed throughout the research 
of CQD solar cells.  

Schottky Cells
The Schottky CQD solar cell was developed 

after the first thin film CQD solar cell employed 
CQDs for absorption and charge transport. This 
elementary structure utilizes an ultrathin film of 
CQDs sandwiched between two contacts (Barcelo 
et al., 2014). Taking advantage of a work function 
difference between the top metal contact and 
conductive oxide, this model generates a built-in 
field which produces a photocurrent through the 

sandwich layers. The charge transfer between the 
metal contact and CQDs generates band bending 
between themselves and Schottky metal, which 
makes the contact selective (Barcelo et al., 2014). 
Advantages of this model include its simplicity and 
its ease of fabrication (Carey et al., 2015; Barcelo et 
al., 2014). The cells have limited number of interfaces 
and impressive photocurrents which have led to 
their use in testing different properties of CQD films. 
These properties include size-tuned band positions, 
drift, depletion, and diffusion. These simple models 
also come with their drawbacks. Fermi level pinning 
at the metal-QD connection creates an upper bound 
on open-circuit voltage (Barcelo et al., 2014), which is 
relatively low considering the CQD band gap (Carey 
et al., 2015). Finally, these cells require the non-
rectifying side of the junction to be illuminated due 
to the low electrical characteristics of the Schottky 
design. 

Heterojunction Cells
The depleted heterojunction cell was developed 

specifically to overcome the limitations of the 
previously discussed Schottky cell. Taking inspiration 
from another solar cell known as a sensitized solar 
cell, the heterojunction design uses a nanostructured 
wide band gap oxide that is supported on conducting 
glass (Carey et al., 2015; Barcelo et al., 2014). The 
CQDs are then multilayered, followed by a metal 
oxide which is deposited on top. Finishing the layers 
is a back reflective contact constructed with a deep 
work functioning metal such as gold (Barcelo et al., 
2014), followed by silver or aluminum (Carey et al., 
2015). These cells are then used to develop the bulk 
heterojunction cell. Creating a bulk heterojunction 
cell helped combat the limitations of planar device 
structures. A three-dimensional interpenetrating 
network ensures that no photogenerated exciton is 
farther than one diffusion length away from a charge 
separating interface. The bulk concept also fixes a 
problem with minority carrier transport of CQD solar 
cells (Carey et al., 2015). The electron accepting phase 
is now substituted, and the CQDs act as both the hole 
transporting phase and the light absorber (Barcelo 
et al., 2014). The wide band gap semiconductor and 
CQD film form an interpenetrating layer. This allows 
the depletion region to be extended and more CQD 
material to improve abortion and carrier collection. 



Photovoltaic Properties and Solar Cell Applications of Colloidal Quantum Dots

Aisthesis      Volume 9,  201840

However, this design introduces the drawback of bio-
molecular recombination, which can be attributed to 
the increased interfacial area. 

To prevent the back recombination, the need 
to manage the CQD metal oxide conduction band 
also lowers the device’s built-in voltage. Important 
considerations for this cell type include the need 
to prevent shunting in highly structured devices 
(Carey et al., 2015). So far, these cells have seen very 
low conversion rates. They have the potential to be 
efficient with the tuning of the CQDs previously 
discussed, but currently optimization of the solar 
cell design is in its infancy. In particular, the lack of 
pathways for the transport of charge need immediate 
attention (Barcelo et al., 2014). Continued research 
on the surface quality of CQDs will also play an 
important role in producing greater efficiency in 
bulk heterojunction solar cells. 

Multiple Exciton Generation 
Multiple Exciton Generation (MEG) is one 

property of CQDs with the most promising potential 
to increase solar cell efficiency. In a traditional solar 
cell, one photon from the sun’s light excites one 
electron across the band gap of the semiconductor, 
where any additional energy in excess of the bandgap 
is lost as heat. Within conventional solar cells, up to 
47% of the solar energy can be lost to heat (McElroy 
et al., 2014), making heat the largest source of energy 
loss. MEG is the ability for a CQD to take the excess 
energy and produce additional electron hole pairs 
which in turn will increase the efficiency of the solar 
cell. Carey et al. (2015) found MEG has been shown 
to double the efficiency of isolated quantum dots 
compared to the bulk material. This illustrates that 
MEG may have the potential to greatly increase solar 
cell efficiency. 

One difficulty working with MEG is how 
quickly the excess charges that are created disappear. 
However, Xiong et al. (2018) has indicated the 
possibility to extract the additional charges well 
within the multi-exciton’s lifetime. The potential of 
MEG has not been fully utilized within CQD solar 
cells. This is because the mechanism of MEG is not 
fully understood. To help achieve the full potential 
of MEG, McElroy et al. (2014) suggests controlling 
structure, composition, and size could be beneficial. 
Furthermore, analyzing occurrence, dynamics, and 

size will help with understanding the mechanism 
of MEG. Optimizing these characteristics could 
drastically increase the percent conversion of CQD 
solar cells. Schaller et al.’s (2006) experiment clearly 
demonstrates the potential to maximize solar cell 
efficiency using MEG. Schaller et al. (2006) were 
able to generate seven excitons from a single photon. 
This corresponds to an energy loss around 10%, 
where traditionally 90% of the photon energy would 
be dissipated as heat. The provides a photon-to-
exciton conversion efficiency of up to 700%, which 
exhibits the potential to extremely enhance solar cell 
technologies. 

Conclusions and Future Study
The photovoltaic properties of quantum dots have 

shown their potential to be improved and produce 
high quality and efficient solar cells. With the energy 
challenge presented to this generation, quantum 
dots will likely find their place in providing the 
world with renewable energy. The ability for colloidal 
synthesis to provide monodisperse and smaller QDs 
at a flexible and lower cost is promising a wide range 
of applications in many fields of technology. If the 
issues with solar cell structure can be reduced, CQD 
solar cells will be able to rely on their fine tuning to 
expand their percent conversion efficiency. By using 
lower cost synthesis methods, solar cell cost could be 
reduced to expand the infrastructure of solar capture 
worldwide. In addition, further research on shape 
and MEG could provide uncontested benefits and 
maximize overall efficiency. CQDs remain a relevant 
and promising opportunity to grow within the solar 
cell industry. 

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