A group of researchers has recently published a paper in the journal ACS Energy Letters that demonstrated the feasibility of using luminescence imaging to identify macroscopic heterogeneities in perovskite solar cells.
Study: Visualizing Macroscopic Inhomogeneities in Perovskite Solar Cells. Image Credit: luchschenF/Shutterstock.com
Multi-terawatt scale photovoltaic (PV) deployment is a crucial requirement for achieving a zero-carbon economy. However, the insufficient power conversion efficiency (PCE) of PV modules is increasing the price of PV-generated electricity, which is restricting their adoption in real-world applications and necessitating the development of highly efficient module-scale PVs.
Hybrid perovskite PVs have gained considerable attention as efficient module-scale PVs due to the rapid increase in their PCE to more than 25.5% in the past decade. The rise was attributed to the high-quality optoelectronic properties of hybrid perovskites and the use of an extensive range of PV module characterization methodologies, which reduced the dependence on time-consuming trial and error experimentation.
A good solar cell is fundamentally considered a good light-emitting diode as the trap states capture the excited charge carriers and facilitate recombination that does not lead to photon emission. Thus, the photoluminescence quantum efficiency (PLQE) can serve as an excellent indicator of the PV device or material quality.
Although significant advancements were made in PCEs, extremely efficient perovskite solar cells are still limited to small active areas of less than one cm2. Even the standard perovskite solar cells have demonstrated regions of both poor and high luminescence efficiencies.
The processing techniques utilized for metal halide perovskites introduce large fluctuations locally at both long length scales of more than 10 μm and short length scales of less than 100 nm. Thus, understanding and overcoming this extensive spatial heterogeneity are crucial for manufacturing perovskite PV to the module scale with large active areas, ranging from 250 to 20000 cm2, with high efficiencies of more than 23%.
However, conventional mapping techniques, such as light-beam-induced current (LBIC) and hyperspectral luminescence imaging, used to assess heterogeneities require specialized equipment, are time-consuming, and can operate only at microscopic length scales.
Luminescence measurements have emerged as a robust contactless probe of loss mechanisms. Spontaneous emission depends on physical quantities such as the quasi-Fermi level splitting (QFLS), traps, density of states, and carrier densities. Thus, several internal processes and properties can be obtained from luminescence.
In this study, researchers used a sensitive optical camera to image the operational perovskite solar cell luminescence under varying electrical biases and optical irradiances. The spatial maps of several crucial devices and optoelectronic parameters, such as ideality factor, were derived using the principle of detailed balance.
Researchers experimentally evaluated the feasibility of using the ratio of short-circuit PLQE to open-circuit PLQE as an indicator of the effectiveness of the generated charge collection in the perovskite solar cells and used this parameter to create spatially resolved current loss maps across centimeter-scale images.
The spatially resolved maps of different parameters, such as the ideality factor AND QFLS, were used to identify the correlations between the parameters using the data obtained from single sample measurements.
The inhomogeneities introduced when seven most commonly used charge transport layers, including [6,6] phenyl-C61-butyric acid methyl ester (PCBM), poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine (poly-TPD), poly[bis(4-phenyl)-(2,4,6-trimethylphenyl)amine] (PTAA), nickel oxide (NiOx), 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD), C60, and tin oxide nanoparticles (SnO2 NP), were processed on a partial device stack were visualized.
The inhomogeneities were described using quantitative metrics, which can help in ranking the processing routes and materials by their suitability for upscaling. The luminescence measurements were combined with the detailed balance calculations to generate maps of the parameters directly related to PV performance losses, including open circuit QFLS for output circuit voltage (VOC) loss, quality of charge collection for short circuit current density (JSC) loss, and the ideality factor that encodes the information about the major recombination mechanism in the material.
The samples used in the study were composed of a perovskite active layer and transport layers. Indium tin oxide/fluorine-doped tin oxide (ITO/FTO) glass was used as the base substrate for full devices, and the gold contacts were evaporated after the deposition of all layers. In half stacks, the bottom contacting transport layers were deposited in a transparent conductive oxide/transport layer/perovskite layer configuration and the top contacting layers were deposited in a glass/perovskite/transport layer configuration.
The heterogeneities over entire perovskite cell areas were visualized by producing maps of different device parameters using the simple luminescence imaging method. Luminescence imaging was fast compared to conventional mapping methods such as LBIC and generated large amounts of data. The relationship between the QFLS and ideality factor was validated using the data. The perovskite cells demonstrated significant spatial heterogeneities in charge collection and VOC at the 0.1-1 mm length scale.
The feasibility of using the ratio of PLQEs at open circuit and short circuit as a suitable current loss indicator in perovskite solar cells was demonstrated successfully, which allowed the visualization of JSC loss over the active area and acted as a useful complement to established VOC mapping methods.
The quantitative metrics effectively described the inhomogeneity introduced by the charge transport layers in the partial device stack, enabling the quantification of nonradiative recombination introduced by each charge transport layer on a macroscopic scale.
The top contacting the transport layers were the dominant source of heterogeneities in the multilayer material stack. For instance, spiro-OMeTAD led to a substantial rise in heterogeneity when it was processed on half-stacks. In contrast to top contacting layers, bottom contacting layers do not lead to any considerable increase in heterogeneity. This opposing behavior of top and bottom contacting layers was determined as the key factor responsible for the distinctive limitations and advantages of p-i-n and n-i-p structures.
Taken together, the findings of this study demonstrated the effectiveness of luminescence imaging in assessing the heterogeneities of perovskite solar cells in a fast and simple manner on a macroscopic scale, which can accelerate the development of large-area, highly efficient modules, specifically through high-throughput experimentation.
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Mahesh, S., Dasgupta, A., Zhou, S. et al. Visualizing Macroscopic Inhomogeneities in Perovskite Solar Cells. ACS Energy Letters 2022. https://pubs.acs.org/doi/10.1021/acsenergylett.2c01094