Confirming the role of optimal crystallinity in high efficient CH3NH3PbI3 perovskite solar cells by photo and electroluminescence imaging

Martin C. Schubert^a, Welmoed Veurman^a, Andreas Hinsch ^a, Friedemann D. Heinz^a, Simone Mastroianni^a, Uli Würfel^a, Nam-Gyu Park^c, Jeong-Hyeok Im^c, Michael Grätzel^d

^aFraunhofer Institute for Solar Energy Systems ISE, Germany, Heidenhofstraße, 2, Freiburg im Breisgau, Germany

^bUniversity of Freiburg, Freiburg Materials Research Center (FMF), Stefan-Meier-Straße 21, Freiburg, 79104, Germany

^cSchool of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon 440-746

^dLaboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, Station 6, CH-1015 Lausanne, Lausanne, Switzerland

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Simone Mastroianni, presentation 232
Publication date: 5th February 2015

Electroluminescence (EL) and photoluminescence (PL) imaging are valuable and fast inspection tools for PV technologies to unveil the reason for reduced performance by locating recombination centers, cell defects and non-homogeneous areas with power loss. Spatially resolved EL, PL and and µLBIC (Light Beam Induced Current) measurements are carried out on TCO-TiO₂_compact-TiO₂_mesoporous-CH₃NH₃PbI₃-SpiroMeOTAD-Au cells as function of the methylammonium Iodide (MAI) concentration in the spin coating solution (0.032 M, 0.044 M and 0.063 M). High efficient cells (16.9 %, 0.044 M MAI) with 300-500 nm perovskite crystallites in size are compared to less efficient cells (9.2 %, 0.032 M MAI) with ample size distribution of the perovskite crystallites ranging from 200 nm to 2 µm¹. Here, high resolution PL and EL imaging are performed as a function of the applied bias to follow the emission properties of photo-generated (PL) and electrically induced charges in the perovskite absorber (EL). Through these techniques charge extraction and non-radiative recombination processes are investigated, which we show to be closely connected with the perovskite crystal morphology and the electronic properties of the contacting layers. Figure 1 reports PL, EL and µLBIC maps performed at different operating points on the 16.9 % efficient cell. The PL imaging comparison (I_SC-V_OC) shows quenched PL values at I_SC and bright large areas emerging under V_OC condition, which is a sign that photogenerated charges are efficiently separated and collected externally. The good uniformity observed in the EL map at 0.85 V and the high values recorded at 1.3 V forward bias explain that non-radiative recombination are minimized, thus contributing to the high V_OC generated by the cell (1 V). A clear different behavior will be shown on the less efficient cells. Furthermore, a clear stripe characteristic and non-homogeneous spots caused by the manufacturing process can be recognized in Figure 1. Hence, it can be asserted that these methods also emerge to be relevant for a systematic and sensitive investigation of the emission and electrical properties towards the optimization of cell performance. Lastly, µPL maps with 1 µm resolution are performed on incomplete TCO-TiO₂_(compact)-TiO₂_(mesoporous)-CH₃NH₃PbI₃devices to study local PL intensity and the spectral position of single crystals on the capping layer. It is found that the maximization of PL emission and spatial uniformity at the crystal level largely influences the final performance of the perovskite solar cell. The achievement of optimal perovskite crystallinity and uniformity is paramount for high-efficient cells with minimized performance losses.
Photoluminescence images (at ISC and VOC operating points), electroluminescence images (at 0.8 V and 1.3 V operating points) and µLBIC map (Light Beam Induced Current at ISC) measured on the cell with 0.044 M CH3NH3I concentration. Cell active area is 0.5 cm2. EL and PL measurement unit is CCD cts.
1. J.H. Im, I.H. Jang, N. Pellet, M. Grätzel and N.G. Park. Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nature nanotechnology, 2014, 9, 927–932

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