Characterization of tandem perovskite / organic solar cells

Characterization of tandem perovskite /organic solar cells

Do you know there are almost 96% publications reporting tandem OPV characterizations characterized their devices insufficiently or even incorrectly!?
The original article is as follows: 

To the Editor — The tandem solar cell approach is a promising concept for organic photovoltaics (OPVs). However, efficiency determination or, in general, electrical characterization of multi-junction solar cells is challenging. We performed a literature survey of publications reporting tandem OPV characterizations over the period January 2009 to September 2014 and came to the alarming conclusion that 96% of the published efficiencies were not measured according to the relevant standards (Table 1).

Analysis of the publication practice of tandem organic solar cell efficiency

Even most of the publications claiming to report record efficiencies characterize their devices insufficiently or even incorrectly. The main reason for this malpractice is probably that researchers lack access to adequate experimental equipment for correctly determining spectral mismatches. The absence of matched reference cells for OPVs makes this even more challenging. Some authors treating tandem organic solar cells exactly like single solar cells even appear to be unaware of the existing standards for measuring tandem solar cells. For the sake of the reliability of the entire OPV community, especially when considering the commercial potential of this technology, correct and comparable characterization of devices should be emphasized much more in the future1. Here, we attempt to draw attention to this problem by giving specific practical instructions for a correct measurement of tandem organic solar cells. The in-depth analysis underlying these recommendations can be found in the Supplementary Information, where, in the first part, we summarize the measurement procedures as outlined by the standards and associated literature. In the second part of the Supplementary Information the different characterization methods for organic tandem solar cells are applied to an example device. Based on these experiments, general rules to achieve correct characterization results for tandem organic solar cells have been formulated and are presented here. Usually, the sub-cells of a tandem solar cell are connected in series and there is no separate electrical access to the sub-cells. Therefore, two main challenges exist. First, the measurement of the spectral response or external quantum efficiency (EQE) — which is essential for the spectral correction when using a Sun simulator for current–voltage (J–V) measurements — is only possible with special measurement procedures2. Second, for the efficiency determination itself, a spectrally adjustable Sun simulator is mandatory to achieve correct results2.

General results for characterizing tandem organic solar cells.

• Estimate the bias light conditions for the spectral response (SR) measurement. Select the spectrum of the bias light according to the absorption spectrum of the absorber materials. If both sub-cells comprise identical absorber materials, proceed as described in ref. 10, where detailed optical simulations are carried out to decide on the bias light spectrum. Select the intensity of the bias light such that only the SR of the intended sub-cell is always being measured. (ref: 10. Timmreck, R., Leo, K. & Riede, M. Prog. Photovolt. Res. Appl. (2014).)

• Estimate the bias voltage conditions for the SR measurement based on the expected voltages of the sub-cells. As a first approximation, the open circuit voltages, VOC, of single solar cells incorporating the same absorber system can be used.

• Measure the SR of both sub-cells according to ASTM E2236 standard test methods2 using chopped monochromatic light in conjunction with the lock-in technique applying the determined bias voltage and bias light conditions. Additionally, measure the SR without bias light and check if it follows the lower envelope of the spectra measured with bias light.

• Based on the results of the SR measurement, determine the intensities of the two sources of a spectrally adjustable Sun simulator such that both sub-cells are mismatch corrected. Use either the iterative procedure of ASTM E2236 or one of the methods described in refs 11 and 12. Measure the J–V characteristics of the solar cell and determine the efficiency. As the effective solar cell area has a crucial influence on the efficiency, put emphasis on its determination.(refs: 11. Meusel, M., Adelhelm, R., Dimroth, F., Bett, A. W. & Warta, W. Prog. Photovolt. Res. Appl. 10, 243–255 (2002). 12. Moriarty, T., Jablonski, J. & Emery, K. Proc. 38th IEEE Photvolt. Spec. Conf. 1291–1295 (2012).)

In addition to this procedure, we strongly advise that in publications reporting tandem organic solar cell efficiencies, descriptions of the characterization procedures and set-ups used should be given to ensure reproducibility and comparability as well as an estimation of the accuracy of the published results.
Concerning SR and EQE measurements this should include at least: set-up description, aperture size, bias voltage used for each sub-cell as well as the bias illumination intensity and light sources used. Concerning J–V measurements this should include at least: a description of the set-up, aperture size, description of the solar simulator and, in the case of a single-source Sun simulator, the mismatch factor M of both sub-cells under the solar simulator used and a detailed analysis of the potential deviation of the efficiency from the standard-reporting conditions value. Furthermore, the spectra of all light sources used should be included in the supplementary information.

Cited from Characterization of tandem organic solar cells
Authors: Ronny Timmreck, Toni Meyer, Jan Gilot, Holger Seifert, Toni Mueller, Alice Furlan, Martijn M. Wienk, David Wynands, Jochen Hohl-Ebinger, Wilhelm Warta, René A. J. Janssen, Moritz Riede and Karl Leo
Nature Photonics volume9, pages478–479 (2015)