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Best Practices in Perovskite Solar Cell Efficiency Measurements. Avoiding the Error of Making Bad Cells Look Good
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erovskite solar cells employing hybrid organic−inorganic halide perovskites (e.g., CH3NH3PbI3) have taken the photovoltaic community by storm. In the short time since being deemed its own class of emerging photovoltaic technologies by the National Renewable Energy Laboratory (October, 2013), the certified record efficiency of perovskite solar cells has increased nearly 50%, from 14.1 to 20.1% (http://www.nrel.gov/ncpv/). In addition, several groups have reported reproducible efficiencies in excess of 16%.1,2 These devices show great promise for commercial applications as they combine low-cost fabrication techniques with earthabundant materials yet still deliver efficiencies rivaling traditional photovoltaic technologies. The possibility of using them in building facades or as a top cell in a tandem perovskite−Si architecture only increases their desirability. However, there is currently a dire need in the field for increased care on the part of authors in reporting their photovoltaic performance and on the side of reviewers and the scientific community at large in discriminating and evaluating reported results.3 Our hope is that this Viewpoint brings to light some of the issues pertaining to perovskite solar cells and provides the field with best practices for measuring and reporting perovskite solar cell performance.
vital to mitigate the potential harm caused by unintentional misrepresentation of solar cell performance, which can undermine the credibility of perovskite photovoltaics in the minds of both the scientific community and the general public. Several recent articles discuss in detail general issues and tips for accurately measuring photovoltaic efficiency;6−9 therefore, we will only discuss such topics in passing as they pertain particularly to perovskite solar cells. However, it should be stressed that these guidelines must also be followed in order to obtain accurate measurements. In this Viewpoint, we will focus specifically on issues that we have observed firsthand in our laboratory related to the accurate measuring of perovskite solar cells. J−V Curves and the Fallacies. For all photovoltaic devices, the only real figure of merit is the steady-state maximum power output under continuous illumination. That is, the voltage output of any commercially deployed PV module will be optimized so that the product of V times I is maximized. Therefore, it is this value that must be optimized in research situations as well. While this point seems trivial, it is crucial to keep in mind during photovoltaic testing. In the past, J−V curves were recorded by introducing known resistances (loads) into the circuit (see Scheme 1) and measuring the
Our hope is that this Viewpoint brings to light some of the issues pertaining to perovskite solar cells and provides the field with best practices for measuring and reporting perovskite solar cell performance.
Scheme 1. Circuit Diagram to Evaluate the J−V Curve of Solar Cell by Operating the Cell at Different Load Resistance
Issues arise primarily due to the well-known hysteresis, which has been observed in current density−voltage (J−V) curves taken of perovskite solar cells.4,5 As with other types of photovoltaics, the J−V curves measured for perovskite solar cells can be heavily dependent on both scan direction and scan rate. Surprisingly, however, this hysteresis is not only due to a capacitive effect in the film as it can sometimes be observed at very slow scan rates. In addition, the details of solar cell construction have a great effect on the measured J− V hysteresis, which is complicated by the vast array of different perovskite materials, film deposition techniques, and device architectures. In short, virtually no two perovskite solar cells behave exactly the same with respect to this hysteresis. The ambiguity that hysteresis causes in measuring the efficiency of such solar cells dictates that even greater care must be taken than for other systems in evaluating and reporting the photovoltaic parameters. This discussion is especially timely considering the vast influx of new researchers into the field of hybrid perovskites and its exceptionally rapid pace of advancement. An open discussion of these issues is © 2015 American Chemical Society
steady-state current (or voltage drop) across each load. Consequently, all of the values in these curves, including the maximum power point, represented the steady-state performance of the device. Modern laboratories today employ a constant-voltage generator in a two-electrode measurement to control the potential between the working and counter electrodes. Thus, the current−voltage curves that are ubiquitous in PV research for solar cell performance characterization are generally obtained by sweeping the potential difference between the working and counter electrodes while monitoring the current response. This method has gained widespread favor because of its relative ease and efficacy. Nevertheless, problems arise when the potential sweep applied by the potentiostat is faster than the Published: March 5, 2015 852
DOI: 10.1021/acs.jpclett.5b00289 J. Phys. Chem. Lett. 2015, 6, 852−857
The Journal of Physical Chemistry Letters
Viewpoint
when scanning from short-circuit to open-circuit (forward bias) due to hysteresis, there is little change in the measured efficiency when scanning from open-circuit to short-circuit (reverse bias) measurements. J−V Curves with Forward versus Reverse Scans. It is clear from these previous results that in addition to the scan rate, the directionality of the J−V sweep can further influence the anomalous hysteresis effect. To demonstrate this point, J−V scans of a higher-efficiency perovskite solar cell employing a mesoporous TiO2 scaffold and spiro-OMeTAD hole conductor are displayed in Figure 2. The rate of the voltage sweep was varied in both forward (JSC → VOC, Figure 2A) and reverse (VOC → JSC, Figure 2B) scans. It is immediately apparent that the two scan directions (forward versus reverse scans) yield vastly different power conversion efficiencies almost exclusively due to large deviation in fill factor. The fill factor in this case is inversely proportional to the rate of the forward voltage sweep. However, in the reverse scan, the fill factor is relatively insensitive to the scan rate. This phenomenon has been found to be a function of the mesoporous TiO2 scaffold thickness as well as perovskite crystallite size, with the most dramatic hysteresis observed in planar devices including a compact TiO2 blocking layer.5 Considering the results presented in Figure 1 in which the scan rate had the most pronounced effect on the short-circuit current, varying the design of the active layer can change not only the degree of hysteresis but also the extent to which each individual parameter (i.e., JSC, VOC, fill factor) is impacted by scan rate and direction. When comparing the forward and reverse J−V scans with the steady-state current density near the maximum power point (Figure 2C), neither J−V experiment (conducted with a modest voltage sweep rate of 25 mV s−1) correlates precisely with the steady-state value (blue diamond). Rather, the true output of the device lies between the two scans and is nearly the average of the two curves. This same relationship between forward, reverse, and steady-state values has recently been reported for the highest certified efficiency perovskite solar cell in the literature.1 It is also important to note that forward J−V scans most closely correspond to the steady-state efficiency at slower scan rates (longer delay times) in this architecture. In addition to scan rate and direction, the preconditioning of the solar cell before the J−V scan can also have a marked effect on the resultant power curve (Figure 3). Specifically, whether the solar cell is illuminated or not, if it is electrically biased, and the time that it is held at the specific conditions before the J−V scan commences all have varied effects on the measured photovoltaic performance. There exists significant debate in the literature regarding the exact reasons for the observed behaviors, with several mechanisms possible.10 Nevertheless, it appears clear that electrically biasing the device far past VOC before beginning the J−V scan can, in many cases, result in significant measurement error and should therefore be avoided. Consequently, we recommend starting forward bias to short-circuit J−V measurements from near the cell’s steady-state open-circuit condition (for example,
