Reducing recombination losses in perovskite solar cells
Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
Final Report Abstract
In the RECOLPER project, we were able to broaden as well as deepen the experimental and theoretical understanding of recombination dynamics in perovskite-based photovoltaics. The project aimed for the overall identification and quantification of charge carrier recombination as well as ionic dynamics in devices under real operational conditions. Optical and electrical transient and steady state methods, as well as theoretical analysis and simulations were intensively employed, and we were able to build a device-modelling tool able to simulate all relevant aspects of complete perovskite photovoltaic devices. Our initial results clarified that the traditional analysis of electrical transient methods implemented on whole devices is not straightforward, and certainly associated with limitations. The inhomogeneous distribution of excess charge carriers in thinner devices renders the determination of both their density and lifetime heavily distorted. We described these features in detail for homemade perovskite cells and compared them to both commercial crystalline silicon devices as well as to organic solar cells. We demonstrated experimentally, with exemplary external capacitors, analytically as well as with drift-diffusion simulations under which conditions these limitations primarily occurs and also how to best avoid them. By increasing the illumination intensity as well as the thickness of co-evaporated devices, we could reduce these effects and confirm the influence of various recombination contributions. A reliable reconstruction of the steady state parameters of the j-V curve could finally be obtained from the transients measurements, described by recombination orders around and below 2. By exchanging constituents in the bulk absorber, defect-induced limitations as well as improvements were analyzed. Bismuth was proven to generate deep traps, substantially increasing the non-radiative recombination and limiting the performance of devices whereas Cesium and Rubidium was established to instead reduce the density of defects and improve the mobility, respectively. By a novel approach to determine the transient device capacitance under true open circuit conditions, thus not suffering from the ambiguity of previously needed equivalent circuit models, we were finally able to explain the origin of the unique open-circuit voltage decay behavior of perovskite solar cells. We developed a fast and smooth computational tool employing drift-diffusion simulations that accounts for both the recombination and motion of free charges, as well as for the simultaneous slower migration of mobile ionic charges. With only one global set of parameters, we could successfully reconstruct both steady-state, quasi-steady state as well as fully transient measurements, at a large range of intensities and temperatures. This allowed us to provide accurate spatial information of how both charges as well as ions are redistributing inside the perovskite film in fully operational devices. We identified further that one of the simplest methods to reduce recombination in planar P-i-N or N-i-P devices, was simple annealing and waiting in time. A considerable increase of charge carrier lifetimes, leading to radiative efficiencies exceeding 1% and accordingly increased open circuit voltages, could be identified upon crystal defect healing achieved either via mild heating to 320K or simply, allowing the device to rest for several days. We could show that the improvements was originating from a reduction of Shockley-Read-Hall recombination, and not surface recombination. We conclude that although perovskites are in literature frequently referred to as being defect tolerant, they still suffer not only from surface recombination, but certainly also to a non-negligible extent of defect mediated Shockley-Read-Hall recombination. Hysteresis free device are often to lesser degree restricted by surface recombination, but this loss mechanism should still be denoted a dominant limiter in the so far best perovskite devices. We emphasize nonetheless that the relative contribution from surface and bulk recombination is always strongly affected by both the chosen device thickness, the quality of the active layer film, and the charge selectiveness of the employed electrodes and accordingly that no general statement about one universal single dominant culprit should in fact be made.
Publications
- Doping Profile in Planar Hybrid Perovskite Solar Cells Identifying Mobile Ions. ACS Applied Energy Materials. 1, 5129 (2018)
Fisher et.al.
(See online at https://doi.org/10.1021/acsaem.8b01119) - Revisiting lifetimes from transient electrical characterization of thin film solar cells; a capacitive concern evaluated for silicon, organic and perovskite devices. Energy and Environmental Science. 11, 629 (2018)
Kiermasch et. al.
(See online at https://doi.org/10.1039/c7ee03155f) - Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States, Charge Transport, and Recombination. Advanced Energy Materials. 8, 1703057 (2018)
Hu et. al.
(See online at https://doi.org/10.1002/aenm.201703057) - Effects of Masking on Open-Circuit Voltage and Fill Factor in Solar Cells. Joule 3, 16 (2019)
Kiermasch et. al.
(See online at https://doi.org/10.1016/j.joule.2018.10.016) - How far does the defect tolerance of lead-halide perovskites range? The example of Bi impurities introducing efficient recombination centers. Journal of Materials Chemistry A. 7, 23838 (2019)
Yavari et. al.
(See online at https://doi.org/10.1039/c9ta01744e) - Theoretical Perspective of Transient Photovoltage and Charge Extraction Techniques. The Journal of Physical Chemistry C. 123, 14261 (2019)
Sandberg et. al.
(See online at https://doi.org/10.1021/acs.jpcc.9b03133) - Unravelling steady-state bulk recombination dynamics in thick efficient vacuum-deposited perovskite solar cells by transient methods. Journal of Materials Chemistry A. 7, 14712 (2019)
Kiermasch et. al.
(See online at https://doi.org/10.1039/c9ta04367e) - Temperature dependence of the spectral line-width of charge-transfer state emission in organic solar cells; static vs. dynamic disorder. Materials Horizon 7, 1888 (2020)
Tvingstedt et. al.
(See online at https://doi.org/10.1039/d0mh00385a) - Assigning ionic properties in perovskite solar cells; a unifying transient simulation/experimental study. Sustainable energies and fuels 5, 3578 (2021)
Fischer et. al.
(See online at https://doi.org/10.1039/d1se00369k) - Reduced Recombination Losses in Evaporated Perovskite Solar Cells by Post Fabrication Treatment. Solar RRL (2021)
Kiermasch et. al.
(See online at https://doi.org/10.1002/solr.202100400)
