Insight into loss processes in perovskite solar cells enables efficiency improvements
The analysed perovskite cell has a surface of 1 cm2. Credit. Uni Potsdam © Uni Potsdam
With additional layers between the perovskite semiconductor and the hole- and electron-transportlayers (red and blue lines), the team at the University of Potsdam was able to further increase efficiency of the perovskite cell. Bild. Uni Potsdam © Uni Potsdam
In perovskite solar cells, charge carriers are mainly lost through recombination occurring at interface defect sites. In contrast, recombination at defect sites within the perovskite layer does not limit the performance of the solar cells at present. Teams from the University of Potsdam and the Helmholtz-Zentrum Berlin (HZB) were able to reach this interesting conclusion through extremely accurate quantitative measurements on 1 cm2 perovskite cells using photoluminescence. Their results contribute to improving perovskite solar cells and have now been published in Nature Energy.
Even solar cells made of a perfect miracle material would never be able to convert 100 % of sunlight to electrical energy. This is because the theoretical maximum achievable power is limited by the position of the energy bands of the electrons, and by unavoidable radiation of photons (the thermodynamic or Shockley-Queisser limit). Maximum power conversion efficiency for silicon is about 33 %, for example. But even this value will never actually be reached. This is due to defects of various kinds causing the loss of some of the charge carriers released by sunlight. In order to approach the maximum value, it is therefore necessary to investigate the various defects in solar cells and determine which ones lead to losses and how.
Organometallic perovskite absorber layers are regarded as a particularly exciting new material class for solar cells – in just ten years, their efficiency has increased from three per cent to over twenty per cent, an amazing success story. Now a team headed by Prof. Dr. Dieter Neher at the University of Potsdam and Dr. Thomas Unold at HZB has succeeded in identifying the decisive loss processes in perovskite solar cells that limit the efficiency.
At certain defects in the crystal lattice of the perovskite layer, charge carriers (i.e. electrons and “holes“) that have just been released by sunlight can recombine again and thus be lost. But whether these defects were preferentially located within the perovskite layer, or instead at the interface between the perovskite layer and the transport layer was unclear until now.
To determine this, the scientists employed photoluminescence techniques with high precision, spatial and temporal resolution. Using laser light, they excited the square-centimetre-sized perovskite layer and detected where and when the material emitted light in response to the excitation. “This measurement method at our lab is so precise, we can determine the exact number of photons that have been emitted”, explains Unold. And not only that, the energy of the emitted photons was precisely recorded and analyzed as well using a hyperspectral CCD camera.
“In this way, we were able to calculate the losses at every point of the cell and thereby determine that the most harmful defects are located at the interfaces between the perovskite absorber layer and the chargetransport layers“, reports Unold. This is important information for further improving perovskite solar cells, for instance by means of intermediate layers that have a positive effect or through modified fabrication methods.
With the help of these findings, the group led by Prof. Dr. Dieter Neher and Dr. Martin Stolterfoht at the University of Potsdam has succeeded in reducing interfacial recombination and thus increasing the efficiency of 1cm2sized perovskite solar cells to well over 20 %.
Published in Nature Energy (2018):Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells; Martin Stolterfoht, Christian M. Wolff, José A. Márquez, Shanshan Zhang,Charles J. Hages,Daniel Rothhardt, Steve Albrecht, Paul L. Burn, Paul Meredith, Thomas Unold and Dieter Neher
Doi:10.1038/s41560-018-0219-8
- Nanosilver as an electrocatalyst for CO₂ reductionVia electrolysis, CO2 can be reduced to CO, a raw material for further chemical products such as fuels. Within the GreenQuest Project, an internation team led by HZB chemist Prashanth Menezes has now systematically investigated catalyst layers made of silver nanoparticles, varying both the size of the particles and their density. The best yield was achieved with nanoparticles with diameters of around 10 nm, which were loosely distributed. Furthermore, they demonstrated how the economic efficiency of the electrochemical cell can be enhanced by integrating an additional chemical reaction at the anode, enabling the simultaneous production of a valuable formic acid, hydrogen, and CO in one device.
- Spin waves inside a nano-oscillator imaged for the first timeFor the first time, researchers have directly imaged the magnetisation dynamics inside a spin Hall nano-oscillator — a nanoscale device that converts direct current into tunable microwave signals and is a promising building block for energy-efficient wireless communication and brain-inspired computing. A Swedish–German team led by the University of Gothenburg and Helmholtz-Zentrum Berlin (HZB) achieved this using time-resolved scanning transmission X-ray microscopy at the MAXYMUS instrument at BESSY II. The results, now published in Advanced Materials, reveal spin-wave features that had escaped previous, indirect measurement techniques.
- Joint power instead of duplicate structures:Berlin’s research community is further advancing its research excellence by establishing a high-performance, cross-institutional infrastructure for data and AI. With a joint agreement signed on 18 September 2026, the Berlin University Alliance (BUA), the Helmholtz-Zentrum Berlin (HZB) and the Zuse Institute Berlin (ZIB) are paving the way for a joint data science and AI centre in Berlin-Dahlem and Adlershof.