Time-resolved photoluminescence imaging reveals material modifications of laser scribes in perovskite solar cells
Christof Schultz a, Andreas Bartelt a, Antje Neubauer b, Cornelia Junghans b, Marko Jost c, Lukas Kegelmann c, Rutger Schlatmann a c, Steve Albrecht c, Bert Stegemann a
a HTW Berlin – University of Applied Sciences, Wilhelminenhofstraße, 75, Berlin, Germany
b Becker & Hickl GmbH
c Helmholtz-Center Berlin for Materials and Energy
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2020 May 12th - 14th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Andreas Bartelt, 145
Publication date: 6th February 2020

The fabrication of solar modules from metal-halide perovskites (MHP) requires monolithic series interconnection which is achieved by P1 – P3 patterning of the different solar cell layers. Laser patterning is preferred due to its contactless processing, high-reproducibility and accuracy. During the so-called P2 laser patterning step MHP material is removed and <100 µm thin grooves are formed within the absorber layer, enabling a connection between the front and the back contact layers. Complete MHP removal normally means low contact resistance and improved solar cell performance. However, unwanted laser-induced material modifications may increase the series resistance. MHP debris such as PbI2 can easily be formed within the trenches mostly due to the thermal sensitivity of the MHP layer, reducing the conductivity between front and back electrodes. Also, laser-induced modifications of the underlying front contact and the surrounding absorber material might occur during the laser ablation processes. This may induce recombination centers which reduce the charge carrier lifetimes. Therefore, high quality laser patterning of MHP solar cells with little remaining residuals on top of a pristine front contact is challenging [1,2].

Here we present a time-resolved photoluminescence imaging (TRPI) analysis of laser-scribed spin-coated perovskite solar cells consisting of ITO/PTAA/MHP/PCBM/BCP [3]. Photoluminescence images of spectral regions corresponding to MHP and PbI2 were recorded for different scribing laser fluences, pulse durations and wavelengths. Optical microscopy, scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX) helped to further inspect the scribes. We find that laser-induced debris formation and modifications of the surrounding materials depend on laser pulse lengths and wavelengths. With nanosecond laser pulses, trenches can be scribed with complete removal of the perovskite layers, however residuals remain in the scribe consisting of PbI2. In going towards shorter laser pulses, the amount of PbI2 is reduced to almost PbI2-free P2 laser patterning. This is consistent with our EDX results which show the successful removal of Pb-containing materials.

Laser-induced MHP modifications at the scribe edges were found to impose changes in the photo-induced charge carrier lifetimes. Nanosecond laser ablation shortened the lifetimes in the scribe edges, indicative of enhanced charge carrier recombination. Ultrashort laser patterning on the other hand produced longer lifetimes and enhanced photoluminescence yields at the scribe edges. This finding is connected to an ablation process which locally removes the electron-accepting PCBM-layer while reducing the laser-degradation of the underlying MHP material. In cross experiments on samples without PCBM layer we found similarly high photoluminescence yields and long lifetimes, indicative of an absent charge transfer to the electron acceptor.

On the basis of the optimized patterning parameters, mini-modules are being prepared and investigated. Preliminary results show that the ohmic losses connected to the series interconnection can be reduced to a minimum, even for single-pass processing, resulting in improved mini-modules of up to 16 % efficiency.

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