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.
