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Near-Infrared Semiconductor Plasmonic Nanocrystals for Enhanced Photovoltaics

Final Report Summary - NIRPLANA (Near-Infrared Semiconductor Plasmonic Nanocrystals for Enhanced Photovoltaics)

1. Project objectives
The NIRPLANA project aimed at the fabrication of a near-infrared (NIR) photovoltaic cell with enhanced performance through incorporation of plasmonic nanocrystals (NCs). These allow strong light scattering in the device and thus enhanced absorption in the active layer. Two crucial steps needed to be taken to achieve our goals. First, we needed to further develop the synthesis of NIR excitonic and plasmonic NCs. Second, novel strategies were developed to fabricate NC-based thin film photovoltaic cells. The device performance was evaluated for different thin film configurations, in order to quantify the conversion efficiency. As an MC project, strong focus lied on the training of the fellow to prepare him for an career as independent researcher/team leader.

2. Work performed
The most promising plasmonic material that we synthesized was copper-deficient Cu(2-x)S. Their localized surface plasmon resonance (LSPR) could be strongly enhanced by further reducing the copper-content, to the point of synthesizing covellite CuS. We also achieved a control over the shape of the nanocrystals, preparing flat nanodisks of varying thickness and diameter. Another potential candidate for photovoltaics was Cu2Te, which we synthesized in the form of thin nanodisks. However, these NCs turned out to be very resistant against photo-oxidation, hence almost no copper vacancies could be created and only a weak LSPR was observed.
NIR Qdot materials form the absorber layer in the solar cells. We optimized the synthesis of PbS spherical NCs toward a band gap of 1.0-1.4 eV, equivalent to an NC diameter of about 2-3 nm. We also developed CdTe quantum disks. CdTe is a material that is already considered for photovoltaic applications; here we synthesized them from Cu2Te nanodisks via cation exchange. Finally, as a potential alternative material, we also synthesized CdSe nanosheets with lateral dimensions of 5-50 nm and a thickness of 1.5-2 nm. In contrast with spherical nanocrystals, these might be better suited for electrical transport because of the larger lateral size, yet further work is required to reduce the band gap and achieve NIR absorption.

For efficient charge transport throughout the solar cell, we need thin films of NCs with intimate interparticle contact. This was achieved by replacing the long organic molecules attached to the NC surface during synthesis by shorter ligands or even by atomic moieties. We successfully obtained S-, MPA- and silane- capped NCs soluble in polar solvents. An investigation of the ligand exchange efficiency using nuclear magnetic resonance spectroscopy for a particular case of CIGS NCs revealed that, although the NCs were soluble in polar solvents, the exchange from oleylamine to S-ligands was only 45% complete.
Deposition of NCs capped with short-chain ligands remained challenging however. As an alternative method to prepare thin films of NCs in close contact, we first deposited a thin NC layer by the layer-by-layer dipcoating technique, followed by a treatment which removes the original ligands from the NC surface. The PbS NC thin films capped with S-ligands were deposited on interdigitated gold contacts, where they showed a strongly improved conductivity. This was key to obtain solar cells based on PbS NCs.
Next to direct electron and hole charge transport, NC thin films often exhibit Förster resonant energy transfer. This can play an important role in NC-based solar cells, as it can be used to funnel energy efficiently throughout the device. We investigated the effect of the band-edge fine structure on the energy transfer process using a Qdot-bilayer, fabricated by sequential deposition of two Qdot monolayers. Results showed that the fine structure strongly affects the energy transfer rate between Qdots, while leaving the energy transfer efficiency unaltered.

Finally, solar cell stacks were fabricated using solution-processable techniques for both the hole-blocking and the absorber layer. The layout consisted of an ITO bottom contact, followed by a titania hole-blocking layer, the PbS absorber layer and finally a small gold top contact fabricated by masked sputtering. Importantly, all steps were carried out in air to evaluate the processability of the PbS NCs. Initial devices showed almost no photocurrent, yet after optimization of the stack fabrication we were able to observed a current in the mA-range. This corresponded to a device efficiency of about 0.5% when measured with a solar simulator under AM1.5 conditions.

3. Scientific results and potential impact
The main outcome of the project can be summarized as follows. We developed different plasmonic nanocrystal materials, with covellite CuS NCs as the most promising candidate for photovoltaics. These have a large LSPR amplitude around a wavelength of 1 um and the control over both thickness and diameter allows us to tune the LSPR. We also synthesized three different materials suitable as absorber material in the NC-based solar cells. The band-edge absorption of the PbS NCs showed a spectral match with the LSPR of the CuS nanocrystals, while for the CdTe a plasmonic NC should still be developed. The CdSe nanosheets have a band in the visible spectrum, thus not directly suitable for PV applications, but the control over the lateral dimensions could prove beneficial for other systems as well. We developed procedures to remove the long-chained organic ligands from the surface of the NCs in solution. In addition, we have determined a suitable method to prepare Qdot thin films with inorganic (S-) ligands, to be integrated into Qdot-based solar cells. A procedure was developed to prepare PbS NC-based solar cells in air. After optimization, we reached about 0.5% conversion efficiency. Most importantly, these solar cells are operated under ambient conditions.
On a longer term, the results achieved can inspire new directions in nanocrystal synthesis, processing and applications in solar energy harvesting. Especially the latter is highly desirable, considering the current demand for energy and the limited fossil resources that are available to the different societies around the globe.