Development of novel imaging techniques for the identification of loss mechanisms in tandem solar cells
Final Report Abstract
Through the course of the project, we were successful in employing several methods to introduce defects into both single junction and tandem OPV devices and could reproducibly manufacture samples with well-defined defects in any desired layer. First and foremost, fs-laser ablation was used for the introduction of defects, but also other methods (e.g. inkjet-printing) were proven to be suitable for the creation of artificial defects. Establishing this framework for the creation of artificial defects allowed us to produce samples with defects in different layers of the device. Before moving on to more complicated tandem systems, a single junction system with a P3HT:PCBM active layer, one of the most widely used and studied organic bulk-heterojunction systems, was thoroughly investigated. Cells with defects in both of the interfaces, the electron transporting layer and the hole transporting layer, and the bulk active layer were produced. The cells were then tested with several complementary imaging methods (EL imaging, PL imaging and DLIT) and electrically characterized by recording JV-curves of both the defected devices and the reference devices. We could clearly discriminate between the defects in the interface layer and defects in the bulk active layer by only using imaging methods, meaning that a statement on both the lateral and the vertical position of a defect can be made by only recording several images of a sample. It was also shown that depending on the defected layer, the effect on the photovoltaic performance of a sample can be dramatically different, with defects in the bulk active layer being much more detrimental to device performance than defects in the interface layers. Similar experiments with higher performance materials showed that the results obtained for this layer stack with a P3HT:PCBM active layer may be also valid for other systems, however more tests for different materials would be needed to make a general statement for all OPV material systems and device geometries. The method of imaging samples with known calibration defects and then imaging such samples to learn the unique response of defects in each layer is however universal and can be applied to every OPV system and also other thin-film solar cell technologies such as CIGS, thin-film Si or perovskite solar cells. Experiments on tandem cells expanded on the results achieved for single-junction cells. We were successful in introducing defects into every layer of tandem devices based on a P3HT:PCBM active layer as the bottom subcell and a pDPP5T-2:PCBM active layer for the top subcell. It became clear that active layer defects in tandem devices show a different response and affect electrical parameters differently than active layer defects in single-junction cells. The use of PL imaging with two different excitation wavelengths, each only providing excitation for one of the active layers, allowed us to clearly state which of the two subcells was affected by the defect. Defects were also introduced into the recombination layer, a layer that is only needed in tandem devices and is critical to their performance. It was shown that recombination layer defects can have different effects depending on which part of the layer is affected. A powerful method for discerning which part of the recombination layer is affected is the use of EL imaging with spectral filters. We showed that for a sample with suitable EL emission wavelengths for each subcell and careful choice of optical filters the EL emission of each individual subcell can be recorded. This can be used to clearly determine which part of the recombination layer is affected by a defect in a certain sample. Applying the results of these experiments we could show that for a sample with lower performance due to a naturally occurring defect the affected layer could be unequivocally determined along with the lateral position of the defect. The method can be applied to any tandem solar cell system, OPV and otherwise, in order to determine the vertical and lateral position of manufacturing defects. Due to the non-destructive nature of the imaging methods it is very suitable for use in a production line. The method only requires that the response of defects in each imaging method is established once by using artificial defects. After performing this calibration routine for the desired tandem system, the vertical and lateral position of a manufacturing defect can be determined with non-destructive measures for every device with the same material and geometry. The principles demonstrated during this work have been extensively tested using small-area cells, however a central goal for further developments should be the application of the acquired information to the large area, roll-to-roll processed OPV modules. Our chair in conjunction with the Bavarian Center for Applied Energy Research (ZAE), and the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (HI ERN), is one of the leading innovators when it comes to upscaling high-efficiency OPV devices based on novel materials from small laboratory scale cells to large-area modules. In cooperation with the South China University of Technology (SCUT), OPV modules with a world record efficiency of 12.6 % PCE have been produced by researchers from our chair at FAU, the ZAE, and HI ERN. Since such modules are of considerable interest for large-scale production, imaging experiments for quality control have been conducted using our imaging setup. We were successful in imaging modules with areas of up to 22 x 22 cm, proving that the setup can be easily adapted to the requirements of the industry. With the built imaging setup, work going beyond the scope of this project is also possible and has already been performed, showing the great versatility of the setup and imaging techniques in general. We have visualized improvements in interface recombination in perovskite solar cells with novel interface materials using DLIT while PL imaging has been used to visualize non-radiative losses in perovskite solar cells. Also, defect detection in fields other than photovoltaics is possible, for example, defects in Ag-sinter die attach layers have been reliably detected performing DLIT and ILIT in our imaging setup.
Publications
- Non-destructive imaging of defects in Ag-sinter die attach layers – A comparative study including X-ray, Scanning Acoustic Microscopy and Thermography, Microelectron. Reliab. 2018, 88– 90, 365.
P. Dreher, R. Schmidt, A. Vetter, J. Hepp, A. Karl, C. J. Brabec
(See online at https://doi.org/10.1016/j.microrel.2018.07.121) - Adv. Mater. 2019, 31, 1
Y. Hou, C. Xie, V. V. Radmilovic, B. Puscher, M. Wu, T. Heumüller, A. Karl, N. Li, X. Tang, W. Meng, S. Chen, A. Osvet, D. Guldi, E. Spiecker, V. R. Radmilović, C. J. Brabec
(See online at https://doi.org/10.1002/adma.201806516) - Discriminating bulk versus interface shunts in organic solar cells by advanced imaging techniques. Prog. Photovoltaics Res. Appl. 2019, 27, 460
A. Karl, A. Osvet, A. Vetter, P. Maisch, N. Li, H. J. Egelhaaf, C. J. Brabec
(See online at https://doi.org/10.1002/pip.3121) - Visualizing and Suppressing Nonradiative Losses in High Open-Circuit Voltage n-i-p-Type CsPbI3 Perovskite Solar Cells, ACS Energy Lett. 2020, 271
W. Meng, Y. Hou, A. Karl, E. Gu, X. Tang, A. Osvet, K. Zhang, Y. Zhao, X. Du, J. Garcia Cerrillo, N. Li, C. J. Brabec
(See online at https://doi.org/10.1021/acsenergylett.9b02604)
