Optimizing Multi-Junction Structures

In perovskite/silicon tandem structures (and consequently in any multi-junction structures), it is necessary to study not just the overall I-V curve. Instead, every sub-cell needs to be monitored individually to subsequently determine the optimum current match and overall external quantum efficiency (EQE). The best practice methodology to apply is called spectrometric characterization (Meusel et. al. 2002).

A spectrometric characterization consists of a subsequent measurement of the solar cell characteristics for a series of spectral irradiation variations. The simulator spectrum is varied between a” blue-rich” spectrum to a “red rich” spectrum (see fig. 2). The total irradiation intensity (in Watt/m2) of the light source is kept stable during this measurement. The “blue-rich” spectrum shifts the bottom sub-cell (c-Si) into current limit while keeping the top sub- cell (perovskite) saturated. Consequently, the “red-rich” spectrum shifts the top sub-cell to limit the overall current while the c-Si sub-cell is kept saturated. This technique allows to determine the current matching behavior at short circuit current (Isc) and MPP (Impp). Selecting suitable spectral irradiation also allows to maximize either STC efficiency or to optimize energy yield for specific sites and their specific environmental conditions.

While this methodology has been around for about 20 years, the right tooling was not always at hand. Spectrometric characterization requires a sun simulator that allows controlled spectral variations and long irradiation times. In the past this was achieved with complicated and cumbersome adjustments combination of Xenon and halogen light sources. While this might have been a somewhat sufficient auxiliary crutch in long-term monothematic research projects, it has no use in industry-scale production with varying conditions and cell types.

To establish spectrometric characterization in production lines and dynamic R&D environments, advanced solar simulators are a must-have. They need to feature flexible and stable light sources to enable the detailed measurement of a cell's behavior under different light conditions (fig. 2). They also need to allow for preconditioning, tracking of the maximum power point and accurate I-V measurements of perovskite/silicon tandem solar cells. All in one tool. And all easy as pie.

Results of the spectrometric characterization of a PST cell achieved in one measurement utilizing an LED-powered solar simulator. Isc, Voc, FF and Pmpp are plotted versus the ratio of used irradiance spectrum and the AM1.5g spectrum. Small spectra quotients (ratio < 1) indicate a “blue-rich” spectrum, while large values (ratio > 1) indicate a “red-rich “spectrum. The results show an optimized utilization of blue-rich light by the measured PST cell. This level of detail allows for a perfect matching of solar cells on module level (Mette et al. 2021).

The Story Continues

Even though there are still some uncertainties regarding industry-scale production of perovskite/silicon tandem cells, the basic principles of the manufacturing processes are well known in semiconductor and photovoltaics production, including wet processes like slot die, spin coating, doctoral blade, atomic layer deposition (ALD) and physical vapor deposition (PVD).

Overall, those developments increase the energy yield and improve the value-for-money ratio for both PV manufacturers and PV users. PST cells truly offer a chance to boost the energy transition process and to achieve net zero carbon emissions faster. 2024 has seen groundbreaking advances in preparing the industrial launch of Perovskite technology, demonstrated, for example, by Fraunhofer ISE and Oxford PV in January.

References:

M. Meusel, R. Adelhelm, F. Dimroth, A. W. Bett, and W. Warta, “Spectral mismatch correction and spectrometric characterization of monolithic III-V multi-junction solar cells,” Prog. Photovoltaics Res. Appl., vol. 10, no. 4, pp. 243–255, 2002.

B. Mette, E. K?hnen et al.: Advanced LED solar simulator: flexible and fast characterization tool for research and industrialization of perovskite/silicon tandem solar cells, 38th EUPVSEC proceedings, 2021, p. 490-495