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Organic-Inorganic Hybrid Heterojunctions in Extremely Thin Absorber Solar Cells Based on Arrays of Parallel Cylindrical Nanochannels

Periodic Reporting for period 1 - HYBRICYL (Organic-Inorganic Hybrid Heterojunctions in Extremely Thin Absorber Solar Cells Based on Arrays of Parallel Cylindrical Nanochannels)

Reporting period: 2018-07-01 to 2020-06-30

• What is the problem/issue being addressed?

Innovation and cutting-edge research in energy are necessary to reach sustainability and competitiveness in the energy sector and to revert the effects of the climate change that we are facing. In December 2019, it was presented the “European Green Deal” with the objective to reach a climate-neutral European continent by 2050, and renewable energies will play a crucial role to reach these objectives.
To meet this projection, renewable energy systems must increase their efficiencies, reduce costs, and be integrated into applications to optimize their contribution to the electrical supply. Therefore, renewable energies and, in particular, solar energy conversion systems present a field of technology and research of utmost importance for EU’s energy and climate change policies.

• Why is it important for society?

We are facing a climate and environmental crisis that eventually would have a direct effect on society. The actions taken now will decide the direction of the change that we are facing.
Research on new materials and technologies that offers avenues to benefit from energy resources with CO2-free emissions is contributing to an energetic and economical model that is sustainable which will have a positive reflection into the society.

• What are the overall objectives?

The HYBRICYL project is dedicated to the systematic study and understanding of the factors that limit the efficiency of PV devices from an innovative and scalable approach based on low-cost and low-toxicity materials structured in coaxial structures. This work intends to provide the PV community with new tools to optimize geometrical parameters for more cost-efficient solar energy conversion devices.
HYBRICYL project has been focused on the development and study of the so-called, “extremely thin absorber” solar cells.

A material system is formed by three different semiconductors with different properties and functions. These semiconductors form a p-i-n heterojunction, where the intrinsic semiconductor has the function of absorbing light. In this case, our intrinsic semiconductor is Sb2S3, with a bandgap of 1.7 eV and an absorption coefficient of >10^4 cm-1 which allows for high light absorption with very thin layers. The n-type semiconductor has the function of conducting electrons (electron transport material, ETM) and the most generally used material is TiO2. The p-type semiconductor has the function of conduct holes (hole transport material, HTM).

The most widely used methods to deposit Sb2S3 are solution-based, which brings the formation of oxides within the material. The presence of trap states for charge carriers has been attributed to the incorporation of oxygen to the Sb2S3. On the other hand, Sb2S3-sensitized solar cells are based on mesoporous layers, composed of TiO2 nanocrystals with no defined pathways for the charge carriers.
In order to improve the performance of the Sb2S3 based solar cells, it is necessary to work with a material that is pure in a configuration with pathways that allow for an efficient charge extraction.

HYBRICYL has adopted two approaches to benefit in a more effective manner from the optical and electrical properties of Sb2S3:
- Growth of Sb2S3 via atomic layer deposition (ALD), which provides a highly pure material.
- Nanostructured ETM layers with well-defined pathways for charge carriers.
However, a chemical incompatibility of the interfaces of the phase-pure materials TiO2 and Sb2S3 leads to a dewetting effect of the light absorber.

The first part of the research developed in HYBRICYL focused on planar solar cells. We studied the influence of oxygen incorporation into Sb2S3 and we further exploit ALD to obtain oxygen-free Sb2S3 and to generate an ultrathin (1.5 nm) ZnS interfacial layer that solves the dewetting issue and passivates defect states at the TiO2/Sb2S3 interface, resulting in an improvement of the efficiencies of the solar cells. These results have been published at ACS Applied Energy Materials with open-access: 10.1021/acsaem.9b01721 and presented an oral contribution at the E-MRS Spring Meeting 2019.
Following this research line, I am currently working on an in-depth study of the ZnS interlayer. ZnS has a dual behavior as surface defects passivation layer and as an energy barrier for charge carriers. Therefore, an optimization of the ZnS interlayer is necessary. In this work, the electrical and optoelectronic response of the interface with different ZnS thicknesses (0.2-1.5 nm) are being thoroughly studied. The manuscript of this work will be submitted soon, and the results have been presented at the Virtual Chalcogenides PV Conference 2020 as an oral contribution.

In the next stage of the project, we introduce a TiO2 NT-based solid-state heterojunction solar cells with ALD-grown Sb2S3 as absorber material. In this highly controlled geometry, one can experimentally thin down the light-absorbing layer and optimize the total light absorption accordingly by elongating the NT as needed. This kind of cell offers two experimental parameters fully decoupled from each other (the pore length and absorber layer thickness) and represents the missing link between the classical planar and mesoporous cell architectures. These results are currently under revision in a peer-reviewed journal. Additionally, this has been presented at the ALD/ALE 2020 Virtual Meeting as an oral contribution.
Sb2S3 layers in thin-film photovoltaics suffer limitations based on the deposition technique that relays on processes based on solution-based synthesis which are associated with oxygen incorporation. On the other hand, chemical incompatibility of high purity Sb2S3 with TiO2 also results in dewetting issues at the interface promoting recombination processes that hinder the performance of the solar cells.

HYBRICYL makes use of atomic layer deposition to overcome these limitations.

ALD has the advantage of generating highly pure Sb2S3 layers and this technique is also used in this project to deposit an ultrathin interfacial layer that avoids dewetting issues. The control over the thickness of this interfacial layer with ALD allows for the deposition of 0.2 - 1.5 nm thick ZnS. Additionally, this interfacial layer passivates defect states, reducing the recombination processes at the interface with TiO2. A comprehensive and systematic study of the physical processes at the interface reveals the influence of oxygen incorporation and that the charge recombination processes at the Sb2S3/TiO2 interface are one order of magnitude slower than those a the Sb2S3/HTM interface.

One logical route to follow is the fabrication of nanostructured heterojunctions to minimize transport and recombination processes. HYBRICYL has developed the first Sb2S3 sensitized solar cells based on a nanotubular scaffold. This system is the first Sb2S3-sensitized solar cell on a nanotubular platform. TiO2 nanotubes are used to fabricate a solid-state heterojunction solar cell where Sb2S3 is deposited via ALD. These systems feature well-defined transport pathways for the charge carriers generated at the light absorber from the heterojunction to the electrodes. The combination of TiO2 nanotubes and ALD of Sb2S3 allows for a highly controlled geometry, being possible to decouple light absorption from diffusion lengths of the charge carriers. This represents the missing link between classical planar and mesoporous architectures.
Cross section and energy level diagram of a planar Sb2S3-based photovoltaic device