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Precise performance characterization of perovskite solar cells
Accepted Manuscript Precise performance characterization of perovskite solar cells Yoshihiro Hishikawa, Haruya Shimura, Takashi Ueda, Ayumi Sasaki, Yuki Ishii PII:
S1567-1739(16)30115-8
DOI:
10.1016/j.cap.2016.05.002
Reference:
CAP 4226
To appear in:
Current Applied Physics
Received Date: 20 February 2016 Revised Date:
19 April 2016
Accepted Date: 2 May 2016
Please cite this article as: Y. Hishikawa, H. Shimura, T. Ueda, A. Sasaki, Y. Ishii, Precise performance characterization of perovskite solar cells, Current Applied Physics (2016), doi: 10.1016/ j.cap.2016.05.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Precise Performance Characterization of Perovskite Solar Cells Yoshihiro HISHIKAWA, Haruya SHIMURA, Takashi UEDA, Ayumi SASAKI, and Yuki ISHII Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Japan
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ABSTRACT Experimental procedure for precisely characterizing the performance of perovskite solar cells such as the current-voltage (I-V) curves and the maximum output power Pmax is investigated, based on results of the samples with different structures. Special care is taken for the hysteresis effects. Confirming the hysteresis of the I-V curves in a wide range including the timescale of minutes is preferred for precise measurements, since the hysteresis remains for more than 10 min. in some samples. When the hysteresis is persistent even for a long sweep time, the stable Pmax can be determined by examining the temporal trend of the output current by fixing the bias voltage at near Vpm after forward and reverse bias histories.
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e-mail: [email protected]
possibly show different kinds of hysteresis [8]. Although the origin of the hysteresis effect has been recently extensively investigated [5]-[8], it has not been clearly identified at the present stage. When we look at other photovoltaic technologies, there have been many kinds of devices with some hysteresis or metastability features, which should be taken into account during their performance measurements. For example, crystalline silicon (c-Si) solar cells show significant hysteresis in their I-V curves when the sweep time is the order of milliseconds or shorter, due to the diffusion capacitance of p-n junction. Especially, recent amorphous silicon/c-Si heterojunction cells and backside contact cells show clear hysteresis even at sweep time of about 100 ms, which is due to much larger capacitance [11]-[13]. It is also well known that thin film devices such as amorphous silicon and CIGS solar cells show reversible changes in the I-V curve and Pmax, related to light soaking and thermal anneal in a typical timescale of hours to months, which is attributed to metastable defects and modification of interfaces [14-16]. It is an important aspect of PV performance characterization to take into account the hysteresis and reduce the measurement uncertainty [17]. It is noted that, although the present standards such as IEC and JIS mention that the measurement condition should be chosen considering the response time of the sample, the detailed procedure is not described [18][19]. The very slow temporal response and hysteresis effect, have been also observed for dye-sensitized solar cells (DSC) [20][22], whose measurement procedure is partly applicable to perovskite solar cells. However, there are more features to be considered for perovskite solar cells, which is a major issue of the present study. A standardized procedure to precisely determine the performance of perovskite solar cells has not been established so far. Major purpose of the present study is to experimentally investigate and confirm the procedure to determine their I-V curve and Pmax of different kinds of stateof-the-art perovskite solar cells, rather than to investigate the device physics underlying the hysteresis effect.
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Corresponding Author: Yoshihiro HISHIKAWA Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Central2, 1-1-1, Umezono, Tsukuba, 305-8568, Japan
1. Introduction
perovskite,
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performance,
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Keywords I-V curve, photovoltaic, characterization, hysteresis
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Highlights • Precise performance characterization of various perovskite solar cells. • Confirming the hysteresis effects of the I-V curves over wide conditions. • Estimating stable Pmax at a fixed voltage around the maximum operation voltage.
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Development of perovskite solar cells has attracted much attention in the photovoltaic academia and industry, owing to their recent fast improvement of performance. Their conversion efficiency η has risen to >15% and >20% for cells of about 1 cm2 and 0.1 cm2, respectively [1]-[4]. Here, η is defined byη = Pmax (E × S ) , where Pmax is the maximum output power of the cell, E is the irradiance of incident light, and S is the area of the device. Accurate evaluation of their performance, such as the η and current-voltage (I-V) curves are essential for the device development. Performance characterization of the perovskite solar cells is complex, because of their slow temporal response and existence of significant hysteresis in the I-V curve measurements. For example, a long I-V sweep time of the order of seconds and minutes are sometimes required to reduce the measurement error due to the hysteresis [5]-[10]. In addition, various kinds of structure and composition are currently investigated, which
2. Experiments
The I-V curve measurement was carried out at STC (standard test conditions; AM1.5G, 1 kW/m2, 25ºC) by using an ADC6246 DC Voltage/Current Generator with dualchannel source measurement units and a custom software under four-terminal configuration. The curves were measured at bias voltages between -0.05 V and Voc plus about 0.1 V, and comprised of 100 data points, unless otherwise specified.
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and SC under AM1.5G 1 kW/m2 illumination. When the bias voltage was switched from SC to OC, the Voc initially increased by about 5% within 1 min., followed by a gradual decrease of about 10% over 10 min.. The initial increase in Voc is consistent with the results of Fig. 1(a), where the Voc of the forward sweep is lower than that of the reverse sweep for relatively short sweep time of 10 s.
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Here, Voc is the open circuit voltage of the cell. Temporal response of the Voc was measured by an Agilent 34970A precision digital voltmeter under continuous solar simulator slight. The same equipment, combined with a shunt resistance of 0.1 ohm, was used for the temporal response measurement of the short circuit current Isc. The actual operation voltage of the solar cells during the Isc measurement was less than a few millivolts. A large area cell solar simulator (LACS; modified WACOM WXS-220S-20) was used as a light source. Its irradiance was precisely monitored and controlled by calibrated PV reference cells, taking into account the nonuniformity and spectral mismatch [17][23]. The spectral irradiance of the solar simulator was measured by a precision spectroradiometer Bunkoukeiki BSR-250D. The spectral response of the cells was measured by Bunkoukeiki CEP2003W. The temperature of the sample was controlled by attaching the sample to a metallic plate, whose temperature was regulated by a water bath circulator, and monitored by resistive temperature sensors (RTD; Pt100). The sensors were attached to the sample. A thermally conductive sheet was inserted between the sample and the metallic plate when necessary, in order to confirm good thermal contact between them. Other details are described elsewhere [17]. The following structures of perovskite solar cells were investigated; (A) glass/FTO/compactTiO2/ mesoporous TiO2:perovskite/ HTM (spiro-OMeTAD)/ Au, (B) glass/TiO2/meso-TiO2/perovskite/HTM/Ag, (C) glass/TCO/NiO/perovskite/PCBM/Tix(Nb)1-xO2/Ag. The size of the cells ranged between about 0.15 cm2 and 1 cm2. Their initial η ranged between about 8% and 16%. 3. Results and Discussion 3.1 Features of Perovskite solar cells relevant to performance characterization
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The I-V curves of a sample of type A structure, measured at various sweep time and sweep directions, are shown in Figs. 1(a) and 1(b) [10]. The curves measured by the forward and reverse sweeps show clear difference, which is a typical hysteresis of perovskite solar cells. Here, the forward and reverse sweep indicate that the bias voltage increases and decreases during the measurement, respectively. The hysteresis tends to become smaller at longer sweep time, or slower sweep speed. However, the difference of about 5% was observed even at a long sweep time of 2 min., as shown in Fig. (b). The hold time; i.e., the time to hold the bias voltage under illumination before the sweep, also affected the shape of the IV curves, as shown in the figures. These results qualitatively agree with previous studies [5]- [9]. In order to investigate the nature of their hysteresis effect, the temporal response of the cells at open circuit (OC) and short circuit (SC) conditions was examined. Fig. 2(a) shows the transient response of the same type of sample as Fig. 1, when the bias voltage was repeatedly switched between OC
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(b) Fig. 1. I-V characteristics of a Perovskite solar cell (type A) measured by various sweep conditions at sweep times of (a) 10 s and (b) 2 min..
The variation of device temperature throughout the measurement of Fig. 2 was within ±1ºC as measured by the RTD attached to the sample, indicating that the variation in Voc is not the temperature effect. When the bias voltage is switched from OC to SC, the Isc initially decreased by about 10% within 1 min., followed by a gradual increase by about 5%, also shown in Fig. 2(a). The variation of Isc qualitatively agrees with the result of Fig. 1(a), where a significant increase in Isc of the forward sweep is observed for short hold time of 0.1 s - 1.6 s. The initial increase in Voc and decrease in Isc within 1 min. qualitatively agrees with the observation of Tress et al. [6]. Results of Fig. 2(a) indicates that longer temporal response of the order of 10 min. should be considered for defining the stable performance. The gradual variation of Voc and Isc over 10 min. is also consistent with results of Fig. 1(b),
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3(a)]. However, clear hysteresis is observed at 15 s - 60 s [Fig. 3(b)-(d)], and persists at even much longer sweep time such as 600 s - 1200 s [Fig. 3(e), (f)]. Similar behavior of hysteresis is also reported in previous studies [6] [8], although the present study investigates wider timescale. Smaller Pmax is observed in Fig. 3(f) than Fig. 3(e), possibly due to slight degradation of the device. These results confirm that agreement of the forward and reverse I-V curves of perovskite solar cells does not always mean that they are "true" curves, as already discussed in previous studies [6][8]-[10]. Experimental confirmation of the hysteresis effect in a wide range of sweep time, e.g., in the order of second to minute, is recommendable for precisely determining the performance of state-of-the-art perovskite solar cells. This is in clear contrast to the hysteresis of c-Si, which is attributed to the diffusion capacitance, and always decreases for longer sweep time [11]-[13]. It is noted that the forward sweep has larger Pmax in Fig. 3 and smaller Pmax in Fig. 1, which is also an example of the dependence of the hysteresis on the structure and history of devices. In addition, the hysteresis of the sample in Fig. 3 was also dependent on the hold time, showing virtually no hysteresis at a short hold time (i.e., 1 s hold + 30 s sweep), although the graph is not shown here. Confirmation of stable performance at a fixed voltage are very useful for characterizing the devices with such complex hysteresis, as discussed below. (a)
(b)
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where the hysteresis effect is distinct even at a sweep time of 2 min.. The detailed correlation of the results of Figs. 1 and 2 is not simple, because those results are also affected by the instability, or degradation, of the sample. Corresponding results of a sample type B is shown in Fig. 2(b). The trend of Voc is similar to the sample in Fig. 1(a), whereas the trend of Isc is different, showing gradual decrease over 10 min.. This is a clear example that different types of hysteresis effects are observed depending on their structure. Since the relation among the device structure and the features of the perovskite solar cells such as the hysteresis is not clear at the present stage, experimental confirmation for each structure, or sometimes each sample, is necessary. It is noted that the data in Figs. 2(a) and 2(b) was integrated and acquired every 0.8 s and 2 s, respectively. Therefore, the temporal responses shorter than the intervals are possibly averaged out.
(b) Fig. 2 Temporal response of the Voc (solid symbols) and Isc (open symbols) of (a) the same perovskite solar cells as in Fig. 1 and (b) a perovskite solar cell with type B structure [10], where the bias voltage is repeatedly switched between OC and SC under continuous illumination.
The I-V curves of a type C sample, measured at various sweep time, direction and hold time, are shown in Figs. 3(a)3(f). The hysteresis effect of this sample appears to be negligibly small at a relatively short sweep time of 3 s [Fig.
Fig. 3 I-V curves of a perovskite solar cell with structure C, measured by various sweep conditions [10].
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Fig. 4. Temporal trend of the output current of the same perovskite solar cell as Fig. 3, when the bias voltage is abruptly switched from the OC (black line) and SC (red line) to Vpm and fixed for about 1,000 s [10].
Another examples are illustrated in Figs. 5 and 6. Here, the bias voltage was fixed to Vpm in the middle of the forward and reverse sweep [10]. The difference in Pmax of the forward and reverse I-V curves was about 5% at a sweep time of 600 s (Fig. 5), which was similar to the sample of Fig. 3. The stable Pmax, determined by fixing the bias voltage at Vpm, was close to the
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Current (mA)
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Precise determination of Pmax is a major interest in the performance characterization. When the forward and reverse IV curves agree at a wide range of sweep conditions, the measurement result can be defined as the "true" I-V curve, in a sense that they are stable values, and not the artifact of the hysteresis effect. However, additional procedure is necessary, if the hysteresis persists even at very long sweep time, which is sometimes the case with state-of-the-art perovskite solar cells, as shown in Figs. 1 and 3. In that case, the stable Pmax can be examined by measuring the temporal trend of the output current at a fixed voltage near the maximum operation voltage Vpm. The procedure was previously proposed for the characterization of DSC [21], and also discussed for perovskite solar cells [8]-[10]. Fig. 4 is an example of the same sample as in Fig. 3. The black and red lines show the variation of the output current, after the bias voltage is abruptly switched from SC and OC, respectively, to the Vpm, determined from the I-V curve of the reverse I-V curve of Fig. 3(e). Each line approaches to nearly the same value and stabilizes at about 16.0 mA in 1,000 s, which is also nearly the same as its maximum operation current in Fig. 3(e). Therefore, the stable Pmax can be calculated as the product of the output voltage and current in Fig. 4, and the stable I-V curve is estimated to be the same as the reverse I-V curve in the sample of Fig. 4. Confirming the agreement of both lines in Fig. 4 is preferable, since the hysteresis of perovskite solar cells, such as shown in Fig. 3, are more complicated than that of DSC.
Pmax of the forward sweep. It is noted that the hysteresis and Pmax are often affected by measurement history and instability. For example, Fig. 7(a) shows the result of the same sample as Fig. 6 by the same conditions on the following day, and Fig. 7(b) is the results where the bias voltage was abruptly changed to Vpm from OC and SC. Figs. 6, 7(a) and 7(b) showed stable output currents of 18.5 mA ± 0.1 mA, 17.8 mA ± 0.1 mA, and 18.1 mA ± 0.1 mA, respectively, in the timescale of 600 s to 1,000 s. This indicates that a stable Pmax within the timescale can be defined by the present procedure for each figure. Relative variation of about 4% in the output current among Figs. 6, 7(a) and 7(b) is interpreted as the instability or degradation of the device, in regard to the time and the history of bias voltage. Fig. 8 shows similar results for the same sample as Fig. 1. The performance is lower than in Fig. 1, possibly due to degradation caused by many I-V curves measurements between the experiments of Fig. 1 and Fig. 8. The output current after a forward sweep is stable at about 1.08 mA, which seems to well define the stable output current. However, the current after a reverse sweep is smaller by about 3% (~1.05 mA), and tends to gradually decrease even after 600 s. Although the decrease is possibly attributed to the degradation of device, clear experimental distinction between the degradation and hysteresis effects is difficult.
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3.2 Determination of stable I-V curve and Pmax
Fig. 5 IV curves of a Perovskite solar cell (Type C structure) at different sweep directions. The hold time and sweep time is 60 s and 600 s, respectively.
Fig. 8 Temporal variation of output current of a sample with the same type as in Fig. 1, when the bias voltage is fixed at Vpm in the middle of a forward sweep (red line) and reverse sweep (black line).
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Fig. 6 Temporal variation of output current of the same sample as in Fig. 5, when the bias voltage is fixed at Vpm in the middle of a forward sweep (red line) and reverse sweep (black line).
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Another experimental aspect of the hysteresis is the effect of the start voltage on the measured I-V curve [6] [9]. Results of the same sample as Figs. 5-7 are shown in Fig. 9. The start voltage mainly affects the Isc of the forward sweep results as shown in Fig. 9(a), and mainly affects the Voc of the reverse sweep results, as shown in Fig. 9(b). Although the variation of Pmax, Voc, and Isc due to this effect was not large, i.e. about ±0.5%, in the conditions of the present study, it is not negligible in precise performance measurements. Therefore, the start voltage, end voltage, hold time and sweep time should be specified for each I-V curve measurements. The effect will possibly become larger when the sweep time and hold time are shorter.
OC to Vpm
(b) Fig. 7 Temporal variation of the output current of the same sample as in Fig. 6 on the following day at a fixed bias voltage near Vpm, (a) when the voltage was swept in advance, and (b) when the voltage was abruptly changed from OC and SC.
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3. Spectral response
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(b) Fig. 9. Effects of the start voltage of the (a) forward sweep and (b) reverse sweep I-V curve measurements in the same sample as Fig. 5. The hold time and sweep time is 60 s and 600 s, respectively.
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The spectral response measurements are also important for precise performance measurements. It is usually expressed by dimensionless "quantum efficiency (QE)" or "IPCE", defined as the number of output electron per incident photon in device research, whereas it is expressed as "spectral responsivity" in A/W, defined as the output current per incident energy in metrology. Chopped monochromatic light is usually used as the probe light, and white bias light of variable irradiance is used for confirming the linearity of the spectral response. Typical example of the quantum efficiency, measured at various chopper frequency of the monochromatic light and irradiance of the white bias light is shown in Fig. 10(a). Although the measured absolute value was dependent on those parameters, the normalized spectra were nearly constant throughout the measurement conditions of the present study, as shown in Fig. 10(b). This is in contrast to the measurements of DSC, where the chopper frequency and the bias light clearly affected the relative spectral response [21]. The spectra shown in Fig. 10(a) are raw data, and their absolute values are obviously affected by measurement artifacts, which are dependent on conditions such as the chopper frequency and the existence of bias light, as well as the slow response of the device. Although their details are not investigated here, their impact on the uncertainty of the performance measurement is estimated to be negligible in the case of sample type C under the solar simulator of the present study, since only the relative spectral response is relevant to the spectral mismatch factor [23]. Further study is required to confirm the impact for various kinds of perovskite solar cells under various spectra of incident light.
(b) Fig. 10 Quantum efficiency of a perovskite solar cells (sample type C) at various white bias light levels and chopping frequencies of the monochromatic light; (a) raw data and (b) normalized data [10].
4. Conclusion
Main features of the performance measurements of perovskite solar cells include the slow temporal response, significant hysteresis effect, various device structures, and possible instability of the device. It is important to define their performance such as the I-V curve and Pmax, which are stable and not the artifact of those features. The present results indicate that the following procedure is appropriate. • I-V curves are measured at a wide range of sweep time, e.g., in the order of second to minute, for confirming the perspective of hysteresis. It may persist even at a long sweep time of 10 min. for some devices. Measurement conditions such as the start voltage, end voltage, sweep time, sweep direction, and the hold time before the sweep are also relevant. • If the I-V curves of the forward and reverse sweeps agree within acceptable range at a long-enough sweep time, they can be regarded as the stable performance. Although the criteria for the sweep time is not established at the present stage, the present results indicates that a few second is too short, and the order of minute is preferable. Confirmation by a wide range of sweep time is strongly recommended.
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Acknowledgements
This work was supported by NEDO under METI. The authors are grateful to L. Han, and T. Umeyama for supplying the perovskite samples, and also for useful discussion. References
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[1] A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, “Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells”, 2009, J. Am. Chem. Soc., 131, 6050-6051 [2] T. Miyasaka, "Perovskite Photovoltaics: Rare Functions of Organo Lead Halide in Solar Cells and Optoelectronic Devices", Chem. Lett. 44 (2015) 829-830 [3] Y. Wu, I. Ashraful, Y. Xudong, Q. Chuanjiang, J. LIU, Z. Kun, W. PENG, L. Han, "Retarding the crystallization of PbI2 for highly reproducible planar-structured perovskite solar cells via sequential deposition" Energy & Environmental Science (2014) [7] 2934-2938 [4] M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, “Solar cell efficiency tables (version 46)”, 2015, Prog. Photovolt. Res. Appl, 23, 805-812 [5] H. S Kim, N. G. Park, "Parameters Affecting I-V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer", J Phys Chem Lett. 2014 Sep 4;5(17):2927-34 [6] W. Tress, N. Marinova, T. Moehl, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Gratzel, " Understanding the rate-dependent J– V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field", Energy & Environmental Science (2015) [8] 995-1004 [7] L. Cojocaru, S. Uchida, P. V. V. Jayaweera, S. Kaneko, J. Nakazaki, T. Kubo, H. Segawa, "Origin of the Hysteresis in I-V Curves for Planar Structure Perovskite Solar Cells Rationalized with a Surface Boundary Induced Capacitance Model", Chem Lett. 2015, 44(12), 1750-1752
[8] H. J. Snaith, A. Abate, J. M. Ball, G. E. Eperon, T. Leijtens, N. K. Noel, S. D. Stranks, J. T. Wang, K. Wojciechowski, and W. Zhang, “Anomalous Hysteresis in Perovskite Solar Cells”, J. Phys. Chem. Lett. 2014, 5, 1511−1515 [9] J. A. Christians, J. S. Manser, P. V. Kamat, " Best Practices in Perovskite Solar Cell Efficiency Measurements. Avoiding the Error of Making Bad Cells Look Good", J. Phys. Chem. Lett., (2015) 6 (5), 852–857 [10] Y. Hishikawa, H. Shimura, T. Ueda, A. Sasaki, and Y. Ishii, " Performance Characterization of Perovskite Solar Cells ", Proceedings of JSPS JWEA Joint Conference (2015), Miyazaki, pp49-52 (in Japanese) [11] . Friesen and H. A. Ossenbrinck "Capacitance effects in highefficiency cells " Solar Energy Materials and Solar Cells 48 (1997) 77-83 [12] J. Metzdorf, A. Meier, S. Winter et al. Proceedings of the 12th EUPVSEC, Amsterdam (1994) 496-499 [13] Y. Hishikawa, "Precise Performance Measurement of HighEfficiency Crystalline Silicon Solar Cells", Conference Record of the 4th World Conference on Photovoltaic Energy Conversion (2006) Waikoloa, 1279-1282 [14] S.Fujikake, H. Ota, M. Ohsawa, et al., Light-induced recovery of a-Si solar cells , Solar Energy Materials and Solar Cells 34(1994) 449-454 [15] T. Kobayashi, H. Yamaguchi and T. Nakada, “Effects of combined heat and light soaking on device performance of Cu(In,Ga)Se2 solar cells with ZnS(O,OH) buffer layer”, Prog. Photovolt: Res. Appl. 2014; 22:115–121. [16] M. Gostein, L. Dunn, "Light soaking effects on photovoltaic modules: Overview and literature review ", 37th IEEE Photovoltaic Specialists Conference, Seattle, (2011) 3126-3131 [17] Y. Hishikawa "Traceable Performance Characterization of Stateof-the-Art PV Devices" Proceedings of the 27th EUPVSEC, Frankfurt, (2012), pp 2954-2960 [18] IEC 60904-1:2006, "Photovoltaic devices - Part 1: Measurement of photovoltaic current-voltage characteristics" [19] JIS C8934:2005, "Measuring method of output power for amorphous solar cells" [20] N. Koide and L. Han, ”Measuring methods of cell performance of dye-sensitized solar cells”, Rev. Sci. Instrum. 75-9 (2004) 2828-2831 [21] Y. Hishikawa, “Characterization of the Performance of DyeSensitized Solar Cells”, 2006, RENEWABLE ENERGY 2006 Proceedings, 184-188 [22] OITDA-PV01-2009, "Evaluation method of performance for dye-sensitized solar devices" (in Japanese) [23] IEC 60904-7: 2008 "Photovoltaic devices - Part 7: Computation of the spectral mismatch correction for measurements of photovoltaic devices "
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the stable Pmax can be determined by examining the temporal trend of the output current by fixing the bias voltage at near Vpm after forward and reverse bias histories. Although repeated measurements under a wide range of conditions is necessary for precise measurements, repeated measurements at long sweep time may also lead to the problem of instability or degradation of the device. Optimization and updating of the measurement procedure is preferable, by confirming the measurement procedure by various state-of-theart perovskite devices.
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• When the hysteresis is persistent even for a long sweep time,
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Highlights •Precise performance characterization of various perovskite solar cells. •Confirming the hysteresis effects of the I-V curves over wide conditions.
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•Estimating stable Pmax at a fixed voltage around the maximum operation voltage.
S1567-1739(16)30115-8
DOI:
10.1016/j.cap.2016.05.002
Reference:
CAP 4226
To appear in:
Current Applied Physics
Received Date: 20 February 2016 Revised Date:
19 April 2016
Accepted Date: 2 May 2016
Please cite this article as: Y. Hishikawa, H. Shimura, T. Ueda, A. Sasaki, Y. Ishii, Precise performance characterization of perovskite solar cells, Current Applied Physics (2016), doi: 10.1016/ j.cap.2016.05.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Precise Performance Characterization of Perovskite Solar Cells Yoshihiro HISHIKAWA, Haruya SHIMURA, Takashi UEDA, Ayumi SASAKI, and Yuki ISHII Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Japan
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ABSTRACT Experimental procedure for precisely characterizing the performance of perovskite solar cells such as the current-voltage (I-V) curves and the maximum output power Pmax is investigated, based on results of the samples with different structures. Special care is taken for the hysteresis effects. Confirming the hysteresis of the I-V curves in a wide range including the timescale of minutes is preferred for precise measurements, since the hysteresis remains for more than 10 min. in some samples. When the hysteresis is persistent even for a long sweep time, the stable Pmax can be determined by examining the temporal trend of the output current by fixing the bias voltage at near Vpm after forward and reverse bias histories.
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e-mail: [email protected]
possibly show different kinds of hysteresis [8]. Although the origin of the hysteresis effect has been recently extensively investigated [5]-[8], it has not been clearly identified at the present stage. When we look at other photovoltaic technologies, there have been many kinds of devices with some hysteresis or metastability features, which should be taken into account during their performance measurements. For example, crystalline silicon (c-Si) solar cells show significant hysteresis in their I-V curves when the sweep time is the order of milliseconds or shorter, due to the diffusion capacitance of p-n junction. Especially, recent amorphous silicon/c-Si heterojunction cells and backside contact cells show clear hysteresis even at sweep time of about 100 ms, which is due to much larger capacitance [11]-[13]. It is also well known that thin film devices such as amorphous silicon and CIGS solar cells show reversible changes in the I-V curve and Pmax, related to light soaking and thermal anneal in a typical timescale of hours to months, which is attributed to metastable defects and modification of interfaces [14-16]. It is an important aspect of PV performance characterization to take into account the hysteresis and reduce the measurement uncertainty [17]. It is noted that, although the present standards such as IEC and JIS mention that the measurement condition should be chosen considering the response time of the sample, the detailed procedure is not described [18][19]. The very slow temporal response and hysteresis effect, have been also observed for dye-sensitized solar cells (DSC) [20][22], whose measurement procedure is partly applicable to perovskite solar cells. However, there are more features to be considered for perovskite solar cells, which is a major issue of the present study. A standardized procedure to precisely determine the performance of perovskite solar cells has not been established so far. Major purpose of the present study is to experimentally investigate and confirm the procedure to determine their I-V curve and Pmax of different kinds of stateof-the-art perovskite solar cells, rather than to investigate the device physics underlying the hysteresis effect.
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Corresponding Author: Yoshihiro HISHIKAWA Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Central2, 1-1-1, Umezono, Tsukuba, 305-8568, Japan
1. Introduction
perovskite,
cells,
performance,
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Keywords I-V curve, photovoltaic, characterization, hysteresis
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Highlights • Precise performance characterization of various perovskite solar cells. • Confirming the hysteresis effects of the I-V curves over wide conditions. • Estimating stable Pmax at a fixed voltage around the maximum operation voltage.
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Development of perovskite solar cells has attracted much attention in the photovoltaic academia and industry, owing to their recent fast improvement of performance. Their conversion efficiency η has risen to >15% and >20% for cells of about 1 cm2 and 0.1 cm2, respectively [1]-[4]. Here, η is defined byη = Pmax (E × S ) , where Pmax is the maximum output power of the cell, E is the irradiance of incident light, and S is the area of the device. Accurate evaluation of their performance, such as the η and current-voltage (I-V) curves are essential for the device development. Performance characterization of the perovskite solar cells is complex, because of their slow temporal response and existence of significant hysteresis in the I-V curve measurements. For example, a long I-V sweep time of the order of seconds and minutes are sometimes required to reduce the measurement error due to the hysteresis [5]-[10]. In addition, various kinds of structure and composition are currently investigated, which
2. Experiments
The I-V curve measurement was carried out at STC (standard test conditions; AM1.5G, 1 kW/m2, 25ºC) by using an ADC6246 DC Voltage/Current Generator with dualchannel source measurement units and a custom software under four-terminal configuration. The curves were measured at bias voltages between -0.05 V and Voc plus about 0.1 V, and comprised of 100 data points, unless otherwise specified.
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and SC under AM1.5G 1 kW/m2 illumination. When the bias voltage was switched from SC to OC, the Voc initially increased by about 5% within 1 min., followed by a gradual decrease of about 10% over 10 min.. The initial increase in Voc is consistent with the results of Fig. 1(a), where the Voc of the forward sweep is lower than that of the reverse sweep for relatively short sweep time of 10 s.
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Here, Voc is the open circuit voltage of the cell. Temporal response of the Voc was measured by an Agilent 34970A precision digital voltmeter under continuous solar simulator slight. The same equipment, combined with a shunt resistance of 0.1 ohm, was used for the temporal response measurement of the short circuit current Isc. The actual operation voltage of the solar cells during the Isc measurement was less than a few millivolts. A large area cell solar simulator (LACS; modified WACOM WXS-220S-20) was used as a light source. Its irradiance was precisely monitored and controlled by calibrated PV reference cells, taking into account the nonuniformity and spectral mismatch [17][23]. The spectral irradiance of the solar simulator was measured by a precision spectroradiometer Bunkoukeiki BSR-250D. The spectral response of the cells was measured by Bunkoukeiki CEP2003W. The temperature of the sample was controlled by attaching the sample to a metallic plate, whose temperature was regulated by a water bath circulator, and monitored by resistive temperature sensors (RTD; Pt100). The sensors were attached to the sample. A thermally conductive sheet was inserted between the sample and the metallic plate when necessary, in order to confirm good thermal contact between them. Other details are described elsewhere [17]. The following structures of perovskite solar cells were investigated; (A) glass/FTO/compactTiO2/ mesoporous TiO2:perovskite/ HTM (spiro-OMeTAD)/ Au, (B) glass/TiO2/meso-TiO2/perovskite/HTM/Ag, (C) glass/TCO/NiO/perovskite/PCBM/Tix(Nb)1-xO2/Ag. The size of the cells ranged between about 0.15 cm2 and 1 cm2. Their initial η ranged between about 8% and 16%. 3. Results and Discussion 3.1 Features of Perovskite solar cells relevant to performance characterization
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The I-V curves of a sample of type A structure, measured at various sweep time and sweep directions, are shown in Figs. 1(a) and 1(b) [10]. The curves measured by the forward and reverse sweeps show clear difference, which is a typical hysteresis of perovskite solar cells. Here, the forward and reverse sweep indicate that the bias voltage increases and decreases during the measurement, respectively. The hysteresis tends to become smaller at longer sweep time, or slower sweep speed. However, the difference of about 5% was observed even at a long sweep time of 2 min., as shown in Fig. (b). The hold time; i.e., the time to hold the bias voltage under illumination before the sweep, also affected the shape of the IV curves, as shown in the figures. These results qualitatively agree with previous studies [5]- [9]. In order to investigate the nature of their hysteresis effect, the temporal response of the cells at open circuit (OC) and short circuit (SC) conditions was examined. Fig. 2(a) shows the transient response of the same type of sample as Fig. 1, when the bias voltage was repeatedly switched between OC
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(b) Fig. 1. I-V characteristics of a Perovskite solar cell (type A) measured by various sweep conditions at sweep times of (a) 10 s and (b) 2 min..
The variation of device temperature throughout the measurement of Fig. 2 was within ±1ºC as measured by the RTD attached to the sample, indicating that the variation in Voc is not the temperature effect. When the bias voltage is switched from OC to SC, the Isc initially decreased by about 10% within 1 min., followed by a gradual increase by about 5%, also shown in Fig. 2(a). The variation of Isc qualitatively agrees with the result of Fig. 1(a), where a significant increase in Isc of the forward sweep is observed for short hold time of 0.1 s - 1.6 s. The initial increase in Voc and decrease in Isc within 1 min. qualitatively agrees with the observation of Tress et al. [6]. Results of Fig. 2(a) indicates that longer temporal response of the order of 10 min. should be considered for defining the stable performance. The gradual variation of Voc and Isc over 10 min. is also consistent with results of Fig. 1(b),
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3(a)]. However, clear hysteresis is observed at 15 s - 60 s [Fig. 3(b)-(d)], and persists at even much longer sweep time such as 600 s - 1200 s [Fig. 3(e), (f)]. Similar behavior of hysteresis is also reported in previous studies [6] [8], although the present study investigates wider timescale. Smaller Pmax is observed in Fig. 3(f) than Fig. 3(e), possibly due to slight degradation of the device. These results confirm that agreement of the forward and reverse I-V curves of perovskite solar cells does not always mean that they are "true" curves, as already discussed in previous studies [6][8]-[10]. Experimental confirmation of the hysteresis effect in a wide range of sweep time, e.g., in the order of second to minute, is recommendable for precisely determining the performance of state-of-the-art perovskite solar cells. This is in clear contrast to the hysteresis of c-Si, which is attributed to the diffusion capacitance, and always decreases for longer sweep time [11]-[13]. It is noted that the forward sweep has larger Pmax in Fig. 3 and smaller Pmax in Fig. 1, which is also an example of the dependence of the hysteresis on the structure and history of devices. In addition, the hysteresis of the sample in Fig. 3 was also dependent on the hold time, showing virtually no hysteresis at a short hold time (i.e., 1 s hold + 30 s sweep), although the graph is not shown here. Confirmation of stable performance at a fixed voltage are very useful for characterizing the devices with such complex hysteresis, as discussed below. (a)
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where the hysteresis effect is distinct even at a sweep time of 2 min.. The detailed correlation of the results of Figs. 1 and 2 is not simple, because those results are also affected by the instability, or degradation, of the sample. Corresponding results of a sample type B is shown in Fig. 2(b). The trend of Voc is similar to the sample in Fig. 1(a), whereas the trend of Isc is different, showing gradual decrease over 10 min.. This is a clear example that different types of hysteresis effects are observed depending on their structure. Since the relation among the device structure and the features of the perovskite solar cells such as the hysteresis is not clear at the present stage, experimental confirmation for each structure, or sometimes each sample, is necessary. It is noted that the data in Figs. 2(a) and 2(b) was integrated and acquired every 0.8 s and 2 s, respectively. Therefore, the temporal responses shorter than the intervals are possibly averaged out.
(b) Fig. 2 Temporal response of the Voc (solid symbols) and Isc (open symbols) of (a) the same perovskite solar cells as in Fig. 1 and (b) a perovskite solar cell with type B structure [10], where the bias voltage is repeatedly switched between OC and SC under continuous illumination.
The I-V curves of a type C sample, measured at various sweep time, direction and hold time, are shown in Figs. 3(a)3(f). The hysteresis effect of this sample appears to be negligibly small at a relatively short sweep time of 3 s [Fig.
Fig. 3 I-V curves of a perovskite solar cell with structure C, measured by various sweep conditions [10].
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Fig. 4. Temporal trend of the output current of the same perovskite solar cell as Fig. 3, when the bias voltage is abruptly switched from the OC (black line) and SC (red line) to Vpm and fixed for about 1,000 s [10].
Another examples are illustrated in Figs. 5 and 6. Here, the bias voltage was fixed to Vpm in the middle of the forward and reverse sweep [10]. The difference in Pmax of the forward and reverse I-V curves was about 5% at a sweep time of 600 s (Fig. 5), which was similar to the sample of Fig. 3. The stable Pmax, determined by fixing the bias voltage at Vpm, was close to the
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Current (mA)
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Precise determination of Pmax is a major interest in the performance characterization. When the forward and reverse IV curves agree at a wide range of sweep conditions, the measurement result can be defined as the "true" I-V curve, in a sense that they are stable values, and not the artifact of the hysteresis effect. However, additional procedure is necessary, if the hysteresis persists even at very long sweep time, which is sometimes the case with state-of-the-art perovskite solar cells, as shown in Figs. 1 and 3. In that case, the stable Pmax can be examined by measuring the temporal trend of the output current at a fixed voltage near the maximum operation voltage Vpm. The procedure was previously proposed for the characterization of DSC [21], and also discussed for perovskite solar cells [8]-[10]. Fig. 4 is an example of the same sample as in Fig. 3. The black and red lines show the variation of the output current, after the bias voltage is abruptly switched from SC and OC, respectively, to the Vpm, determined from the I-V curve of the reverse I-V curve of Fig. 3(e). Each line approaches to nearly the same value and stabilizes at about 16.0 mA in 1,000 s, which is also nearly the same as its maximum operation current in Fig. 3(e). Therefore, the stable Pmax can be calculated as the product of the output voltage and current in Fig. 4, and the stable I-V curve is estimated to be the same as the reverse I-V curve in the sample of Fig. 4. Confirming the agreement of both lines in Fig. 4 is preferable, since the hysteresis of perovskite solar cells, such as shown in Fig. 3, are more complicated than that of DSC.
Pmax of the forward sweep. It is noted that the hysteresis and Pmax are often affected by measurement history and instability. For example, Fig. 7(a) shows the result of the same sample as Fig. 6 by the same conditions on the following day, and Fig. 7(b) is the results where the bias voltage was abruptly changed to Vpm from OC and SC. Figs. 6, 7(a) and 7(b) showed stable output currents of 18.5 mA ± 0.1 mA, 17.8 mA ± 0.1 mA, and 18.1 mA ± 0.1 mA, respectively, in the timescale of 600 s to 1,000 s. This indicates that a stable Pmax within the timescale can be defined by the present procedure for each figure. Relative variation of about 4% in the output current among Figs. 6, 7(a) and 7(b) is interpreted as the instability or degradation of the device, in regard to the time and the history of bias voltage. Fig. 8 shows similar results for the same sample as Fig. 1. The performance is lower than in Fig. 1, possibly due to degradation caused by many I-V curves measurements between the experiments of Fig. 1 and Fig. 8. The output current after a forward sweep is stable at about 1.08 mA, which seems to well define the stable output current. However, the current after a reverse sweep is smaller by about 3% (~1.05 mA), and tends to gradually decrease even after 600 s. Although the decrease is possibly attributed to the degradation of device, clear experimental distinction between the degradation and hysteresis effects is difficult.
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3.2 Determination of stable I-V curve and Pmax
Fig. 5 IV curves of a Perovskite solar cell (Type C structure) at different sweep directions. The hold time and sweep time is 60 s and 600 s, respectively.
Fig. 8 Temporal variation of output current of a sample with the same type as in Fig. 1, when the bias voltage is fixed at Vpm in the middle of a forward sweep (red line) and reverse sweep (black line).
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Fig. 6 Temporal variation of output current of the same sample as in Fig. 5, when the bias voltage is fixed at Vpm in the middle of a forward sweep (red line) and reverse sweep (black line).
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Another experimental aspect of the hysteresis is the effect of the start voltage on the measured I-V curve [6] [9]. Results of the same sample as Figs. 5-7 are shown in Fig. 9. The start voltage mainly affects the Isc of the forward sweep results as shown in Fig. 9(a), and mainly affects the Voc of the reverse sweep results, as shown in Fig. 9(b). Although the variation of Pmax, Voc, and Isc due to this effect was not large, i.e. about ±0.5%, in the conditions of the present study, it is not negligible in precise performance measurements. Therefore, the start voltage, end voltage, hold time and sweep time should be specified for each I-V curve measurements. The effect will possibly become larger when the sweep time and hold time are shorter.
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(b) Fig. 7 Temporal variation of the output current of the same sample as in Fig. 6 on the following day at a fixed bias voltage near Vpm, (a) when the voltage was swept in advance, and (b) when the voltage was abruptly changed from OC and SC.
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3. Spectral response
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(b) Fig. 9. Effects of the start voltage of the (a) forward sweep and (b) reverse sweep I-V curve measurements in the same sample as Fig. 5. The hold time and sweep time is 60 s and 600 s, respectively.
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The spectral response measurements are also important for precise performance measurements. It is usually expressed by dimensionless "quantum efficiency (QE)" or "IPCE", defined as the number of output electron per incident photon in device research, whereas it is expressed as "spectral responsivity" in A/W, defined as the output current per incident energy in metrology. Chopped monochromatic light is usually used as the probe light, and white bias light of variable irradiance is used for confirming the linearity of the spectral response. Typical example of the quantum efficiency, measured at various chopper frequency of the monochromatic light and irradiance of the white bias light is shown in Fig. 10(a). Although the measured absolute value was dependent on those parameters, the normalized spectra were nearly constant throughout the measurement conditions of the present study, as shown in Fig. 10(b). This is in contrast to the measurements of DSC, where the chopper frequency and the bias light clearly affected the relative spectral response [21]. The spectra shown in Fig. 10(a) are raw data, and their absolute values are obviously affected by measurement artifacts, which are dependent on conditions such as the chopper frequency and the existence of bias light, as well as the slow response of the device. Although their details are not investigated here, their impact on the uncertainty of the performance measurement is estimated to be negligible in the case of sample type C under the solar simulator of the present study, since only the relative spectral response is relevant to the spectral mismatch factor [23]. Further study is required to confirm the impact for various kinds of perovskite solar cells under various spectra of incident light.
(b) Fig. 10 Quantum efficiency of a perovskite solar cells (sample type C) at various white bias light levels and chopping frequencies of the monochromatic light; (a) raw data and (b) normalized data [10].
4. Conclusion
Main features of the performance measurements of perovskite solar cells include the slow temporal response, significant hysteresis effect, various device structures, and possible instability of the device. It is important to define their performance such as the I-V curve and Pmax, which are stable and not the artifact of those features. The present results indicate that the following procedure is appropriate. • I-V curves are measured at a wide range of sweep time, e.g., in the order of second to minute, for confirming the perspective of hysteresis. It may persist even at a long sweep time of 10 min. for some devices. Measurement conditions such as the start voltage, end voltage, sweep time, sweep direction, and the hold time before the sweep are also relevant. • If the I-V curves of the forward and reverse sweeps agree within acceptable range at a long-enough sweep time, they can be regarded as the stable performance. Although the criteria for the sweep time is not established at the present stage, the present results indicates that a few second is too short, and the order of minute is preferable. Confirmation by a wide range of sweep time is strongly recommended.
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Acknowledgements
This work was supported by NEDO under METI. The authors are grateful to L. Han, and T. Umeyama for supplying the perovskite samples, and also for useful discussion. References
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the stable Pmax can be determined by examining the temporal trend of the output current by fixing the bias voltage at near Vpm after forward and reverse bias histories. Although repeated measurements under a wide range of conditions is necessary for precise measurements, repeated measurements at long sweep time may also lead to the problem of instability or degradation of the device. Optimization and updating of the measurement procedure is preferable, by confirming the measurement procedure by various state-of-theart perovskite devices.
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• When the hysteresis is persistent even for a long sweep time,
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Highlights •Precise performance characterization of various perovskite solar cells. •Confirming the hysteresis effects of the I-V curves over wide conditions.
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•Estimating stable Pmax at a fixed voltage around the maximum operation voltage.