Enhanced Perovskite Solar Cells: new insights into defects impact on stability

by Virginia Greco

A study recently published in "Joule", coordinated by ICN2 Group Leader Dr Monica Lira-Cantú and Prof. Anders Hagfeldt (EPFL), demonstrates that the addition of the organic molecule H3pp to the halide perovskite layer increases the long-term stability of these solar cells, due to shallow point defect passivation.

Continuous efforts in photovoltaic energy technology research has led to encouraging results in terms of device efficiency, with single-junction halide Perovskite Solar Cells (PSC) achieving the record of >25% of certified Power Conversion Efficiency (PCE). While improving this value is certainly key to a wider application of this technology, stability over time is also crucial. There are a few factors, among which are phenomena occurring in the device due to the presence of crystal defects, which cause a rapid deterioration of the solar cell performance.

A study recently published in a paper in Joule, and featured on its cover, investigates the impact of defects on both efficiency and stability of PSCs and demonstrates the importance of shallow point defect passivation in halide perovskite to boost the long-term stability of the cells. This work was coordinated by Dr Monica Lira-Cantú, leader of the ICN2 Nanostructured Materials for Photovoltaic Energy group, and Prof. Anders Hagfeldt, Professor at the Ecole Polytechnique Federale de Lausanne (EPFL, Switzerland) and currently Vice-Chancellor of Uppsala University (Sweden). It involved several researchers from these two institutes, as well as from Eindhoven University of Technology (the Netherlands), the University of Cambridge (UK), the Max-Planck Institute for Polymer Research in Mainz (Germany), the Swiss Federal Laboratories for Materials Science and Technology of Dübendorf (Switzerland), the Institute of Microelectronics of Barcelona (IMB-CNM, Spain), and Linköping University (Sweden). The findings presented are the result of the extraordinary experimental work carried out by Dr Haibing Xie (ICN2) and Dr Zaiwei Wang (EPFL), the first authors of the scientific article.

Defects in the crystal structure of the solar cell can either boost charge transfer, by making more carriers available, or hinder it, by trapping or scattering the carriers. Their effects on the conductivity strongly depend on their location in the structure, at the surface, in the bulk, at the boundaries, or at the interfaces between different materials. The introduction of additives (organic and inorganic compounds) is a method that can be used to passivate defects and change the behaviour of the device. Understanding how deep and shallow defects influence solar cells performance, though, is often non-trivial, as is decoupling their effects on efficiency and stability respectively.

The authors of this work focused on gaining insight into defect effects and on improving the PSC performance by introducing the H3pp organic molecule into the halide perovskite layer. They fabricated complete solar cells based on a multi-cation perovskite absorber, with and without the H3pp molecule. While the two types of PSCs showed similar remarkable optoelectronic functioning, with a PCE value of about 21%, the one “enriched” with H3pp performed significantly better in terms of stability. In particular, the device with H3pp kept almost unchanged the initial efficiency after 1,000 hours of continuous illumination, working at the maximum power point. The reference cell (without H3pp), on the contrary, underwent a loss of more than 20% under the same conditions.

The effects of the introduction of H3pp into the halide perovskite absorber were extensively studied by means of several characterization techniques, as well as theoretical calculations. This allowed the researchers to demonstrate that the addition do not modify the density of deep defects –which explains the similar efficiency and the maximum voltage (V_oc) that can be provided by the two types of PSC– while causes the passivation of shallow defects through hydrogen bonding with the perovskite, which results in a strong enhancement of the solar cell long-term stability. This proves that, while the passivation of deep point defects is required to prevent device degradation, shallow point defects are responsible for the immobilization of ions (achieved through strong hydrogen bonds) that leads to stability.

These findings are unexpected because, in classical semiconductors like silicon, the effect of defects on efficiency and stability are usually interconnected. However, halide perovskites are “soft” ionic-electronic conductors with a strong ionic component, which now, thanks to the results of this work, can be associated to shallow point defects. Therefore, this research provides new relevant information about the impact of defects in halide perovskite solar cells that will certainly help disentangle the effects of distinct defects on the different properties of the devices.

Reference article:

Xie, Z. Wang, Z. Chen, C. Pereyra, M. Pols, K. Gałkowski, M. Anaya, S. Fu, X. Jia, P. Tang, D. J. Kubicki, A. Agarwalla, H.-S. Kim, D. Prochowicz, X. Borrisé, M. Bonn, C. Bao, Xiaoxiao Sun, S. M. Zakeeruddin, L. Emsley, J. Arbiol, F. Gao, F Fu, H. I. Wang, K.-J. Tielrooij, S. D. Stranks, S. Tao, M. Grätzel, A. Hagfeldt and M. Lira-Cantu, Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells. Joule, May 2021. DOI: 10.1016/j.joule.2021.04.003

The article is accessibe for free until 24 June 2021 at: https://www.sciencedirect.com/science/article/pii/S2542435121001550