Towards the Next Generation of Ultra-low-cost Photovoltaics Using Nanocrystal Inks Figure 1: Nanocrystal Inks AB Nanocrystal Ink Formulation
Chemical methods have been developed to synthesise nanocrystals of many different materials suitable for PV devices. The nanocrystals are made using a process called ‘arrested precipitation’. Chemical reactants are decomposed in a solvent in the presence of ‘capping ligands’ that bond to the nanocrystal surface, as illustrated in Figure 1. The capping ligands are an integral part of the nanocrystal formulation, enabling good dispersion in solvents by preventing aggregation. This is important for uniform film deposition and device fabrication.
C D
Many research teams have now demonstrated that it is indeed possible to print inorganic layers of light-absorbing semiconductors from nanocrystal inks and make functioning solar cells. Nanocrystals of cadmium- and lead-based chalcogenides (i.e. CdS, CdSe, CdTe, PbS, PbSe and PbTe) have been incorporated into PV devices with
Inorganic core
Organic capping ligand
A: Copper indium gallium selenide (CIGS) nanocrystal ink. B: A large-area scanning electron micrograph of a nanocrystal film deposited by spray-coating the ink. C: Transmission electron micrograph of CIGS nanocrystals. D: Illustration of a nanocrystal, depicting the inorganic crystalline core coated by the organic capping ligand layer that stabilises the nanocrystals.
Figure 2: Nanocrystal-based Photovoltaics
The cost of solar power depends roughly on the ratio of the efficiency to the manufacturing and installation costs, making these technologies competitive with silicon.
reasonable efficiencies (between 2 and 5%).11–15 Cu2S solar cells made
from a nanocrystal ink have also been manufactured, with efficiencies of up to 1.6%.16
Finally, nanocrystals of relatively complicated
composition have also been synthesised, including ternary and quaternary compounds.17–20 interesting have been CIGS17,18
Light
Transparent top electrode Zinc oxide
Cadmium sulphide CIGS nanoparticles
Back contact electrode Substrate
4 6
2 0
-2 -4 -6 -8
-10 -0.4 -0.2 0.0 Dark 0.2 Voltage (V) Light
Top panel: Solar cells made by spray-coating copper indium gallium selenide (CIGS) nanocrystal inks on various substrates: on glass with either molybdenum (top left) or gold back contacts (top middle), and on plastic (Kapton; top right). Middle panel: Schematic showing the layer structure of a photovoltaic (PV) device; in this case, light is absorbed by the CIGS layer and electron-hole separation leads to photogenerated power. Bottom panel: An example of the device characteristics obtained from a PV device made with a
CuInSe2 nanocrystal ink. The open circuit voltage (Voc) and fill factor (FF) of the device are similar to those of commercially available CIGS cells; however, the short circuit current density (Jsc) is only 25% of the expected value for an absorber material with this bandgap, resulting in the low device efficiency.
0.4 0.6
Voc Jsc
= 476mV
= 8.3mA/cm2 FF = 0.488 η = 1.9%
Of these materials, some of the most and copper zinc tin sulphide (CZTS),19,20
as these are proven solar cell materials and do not face the same negative environmental implications as widespread Cd and Pb incorporation into solar cells.
Copper Indium Gallium Selenide Nanocrystal Inks To date, the highest device efficiency achieved by a PV device fabricated by nanocrystal ink deposition is just over 10%, reported by Hillhouse and Agrawal for CIGS.21
This work provides an
important and encouraging benchmark for PV devices made using nanocrystal inks, and has demonstrated that nanocrystal inks can indeed provide commercially viable efficiencies. However, there is a catch: these efficiencies were achieved by annealing the films at rather extreme temperatures, exceeding 500°C, under Se atmosphere. Such extreme processing conditions make it impossible to fabricate devices by a roll-to-roll process or on cheap plastic substrates. Nonetheless, CIGS is a particularly interesting semiconductor for a nanocrystal-ink-based approach to solar cell fabrication.
Single-junction CIGS cells fabricated using state-of-the-art high-temperature vacuum deposition processes have achieved device efficiencies of nearly 20%.22
These devices are made with
polycrystalline films, so these very high efficiencies are rather remarkable. Theoretical calculations and experimental analyses have shown that polycrystalline CIGS films can behave in a similar manner
26 MODERN ENERGY REVIEW – VOLUME 2 ISSUE 2
Current density (mA/cm2
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