Perspective
Standardizing Perovskite Solar Modules beyond Cells Yue Hu,1 Yanmeng Chu,1 Qifei Wang,1 Zhihui Zhang,1 Yue Ming,1 Anyi Mei,1 Yaoguang Rong,1,* and Hongwei Han1,*
Thanks to the excellent optoelectronic properties, perovskite solar cells (PSCs) are considered a promising next-generation candidate for photovoltaics (PVs). Their high-power conversion efficiency and low solution-processed production cost hold promise for realizing solar grid parity at a low cost. Despite the great progress in cell efficiency and stability, most research efforts have focused on laboratory-scale, small-area PSCs. The key issues for successfully commercializing perovskite PVs depend on the efficiency, stability, and cost of PSC modules. Now, the efficiency of PSC modules lag far behind the laboratory-scale cells. We need to be concerned about achieving industrial-scale, large-area manufacturing with high throughput for practical application of this technique. This perspective urges the community to present the module performance and long-term stability data using standard methods and reliable measurement protocols, which can be recognized by conventional PV industries and compared with other PV techniques. Identifying the degradation mechanism along with collecting the statistical data under actual operating conditions should start now or in the very near future. Introduction Ever since the first photovoltaic solar cell was made around 1883, the devices have undergone several generations of changes.1 At this time, there is concurrent research into all varieties of photovoltaic (PV) devices, such as monocrystalline silicon wafers, thin-film solar cells as well as other new techniques, including organic photovoltaics, multi-junction solar cells, dye-sensitized solar cells, and so on. To facilitate a transition from laboratory-scale, small-area fabrication to industrial-scale, large-area manufacturing with high throughput, the PV technologies need solid demonstrations in four important fields, namely, high efficiency, low-cost, longterm operational stability, and scalability. The past few years have witnessed important progress in laboratory-scale perovskite solar cells (PSCs) with reported powerconversion-efficiencies (PCEs) over 20% from increasing number of groups around the world as well as improved stability of the perovskite composition and device architecture, enabling testing over thousands of hours under harsh conditions.1–5 In the meantime, more focus needs to be paid on the perovskite solar module development research.6 Standing on the shoulder of giants, the development of both PSCs and modules is more rapid than any other existing PV technologies as shown in Figure 1A, which compares the certified PV cell and module efficiency revolution of crystalline Si and CdTe. With an increase in the device area, the PCEs of all PV technologies tend to decrease. The so-called ‘‘ohmic loss’’ caused by high sheet resistance (around 10 Ohm/,) of existing transparent electrode is inevitable.7 With regard to PV techniques like Si and CdTe, the series resistance increases approximately linearly, making the PCE drop resulted by ‘‘ohmic loss’’ follow an inverse scaling law.8 When the device area increases by an order of magnitude, the PCE value decreases by 0.8%.9 However, presently the PSC modules do not
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Context & Scale Perovskite solar cells (PSCs) have attracted intensive research attention due to the advantages of low material cost and simple fabrication process. Now that the certified power conversion efficiency of the laboratory-scale PSC has reached 25.2% and the improvement in stability has enabled testing over thousands of hours, it is essential to facilitate a transition from laboratory-scale, small-area fabrication to industrial-scale, large-area manufacturing with high throughput. In this perspective, we urge the community to evaluate and present the cell and module performance data using standard methods and establish standard protocols for evaluating the longterm stability of perovskite solar cells, which will accelerate PSC’s transition from laboratory toys to real market game changers.
A
B
Figure 1. Development of Perovskite Solar Modules (A) The power conversion efficiency development of different PV technologies over the last few decades. (B) The evolution of perovskite solar modules based on different device architectures.
seem to follow this trend yet due to the difficulty in uniform deposition of all functional layers on industrial-scale by solution process, including the perovskite lightabsorber layer, the electron-transport layer, and the hole-transport layer. Depending on the nature of the transporting materials and the sequence of layers, the PSCs can be divided into five categories, namely, the mesoscopic p-i-n structure, the mesoscopic n-i-p structure, the planar p-i-n structure, the planar n-i-p structure, and the triple mesoscopic structure.5 Mesoporous structures showed high stability10 and easy perovskite deposition with low hysteresis,11 while planar structures typically have a simpler architecture and allow low-temperature process.12,13 The one-step less and low-temperature fabrication process makes planar structures attractive for large-scale manufacture, but it is still under debate whether the mesoporous scaffold is essential to facilitate charge separation and achieve higher performance as well as better stability. The triple mesoscopic structure refers to a screen-printed mesoporous titania (TiO2), zirconia (ZrO2), and carbon triple-layer architecture.14,15 This structure shows potential in increasing industrial throughput by preparing the TiO2/ZrO2/carbon scaffolds and subsequently filling in perovskite absorbers, which differs from the traditional layer-bylayer fabrication methods.16,17 Efforts have been devoted to up-scale PSCs based on all structures, as shown in Figure 1B, which shows the evolution of the area of PSCs and PSC modules based on different architectures.18–29 Initially, most solution-processing steps to fabricate the PSC modules were identical to the small-area PSCs. For example, spin coating was used to deposit perovskite layer and thermal evaporation was used to deposit metal electrode. Later, various scalable deposition methods were developed to meet the needs for future industrial manufacturing, including spray coating,22 doctor blading,30 slot-die coating, screen printing,16 electrodeposition,31 vapor-assisted deposition,11 chemical vapor deposition,32 dip coating,33 soft cover deposition,34,35 roll-to-roll (R2R)-compatible method,36 and sheet-to-sheet (S2S) deposition.21 While all deposition methods have resulted in some promising results, the printing method is probably the simplest, most versatile, and reproducible way for high volume factory manufacturing.37 For example, by using industrial screen printing, researchers have demonstrated the possibility of producing organic PVs (OPVs) on the order of 1,000–100,000 m2 on a process line per day while producing the silicon single-junction PVs of the same area typically takes one year.38 PSC modules using printing techniques also demonstrate larger module areas.39 Despite the advances in both efficiencies and device areas, it is intractable to effectively compare the performance of different PSC modules or to compare PSC
1Michael
Gra¨tzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074 Hubei, PR China *Correspondence:
[email protected] (Y.R.),
[email protected] (H.H.) https://doi.org/10.1016/j.joule.2019.08.015
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modules with other existing PV technologies as no consensus has been reached yet in the field. In this perspective, we emphasize the importance of reporting PSC module performance using standard definitions that are recognized by conventional PV community. We also summarize the stability behaviors of PSCs referring to the standard of International Electrotechnical Commission (IEC) 61215 for traditional PV industry and discuss the future directions. Much attention should be paid to identify the degradation mechanism of PSC modules along with collecting the statistical data under actual operating conditions. Standardizing the Area Definition of PSC Modules In the early stage of development, all PSC devices with a substrate area R1.0 cm2 name themselves ‘‘large-area modules.’’ However, as the field develops, it is more accurate to call them ‘‘large lab cells.’’ The record efficiency of a silicon monocrystalline module is 24.4% with an area of 13,177 cm2 (designated area). The record efficiency of CdTe module is 18.6% with an area of 7,038.8 cm2 (designated area).40 To effectively compare the performance of PSC modules with all other commercially available PV modules, the first step is to follow the international designation of PV modules. In a recently updated Champion Module Efficiencies chart by the National Renewable Energy Laboratory (NREL), the module sizes are classified into four categories, namely mini-module, small module, standard module, and large module, in which the module size varies from 200 cm2 to over 14,000 cm2. However, high efficiency is the only factor that determines whether the cell will be included in the chart or not. In addition, the advances of emerging technologies with area smaller than 200 cm2 have been completely ignored. The more interesting definition is brought up by Martin A. Green in the Solar Cell Efficiency Tables, which has been updated for 54 versions since 1993. In his definition, the size of a mini-module is 10–200 cm2 (integrated by 4-10 unit cells with each unit cell > 1 cm2), the size of a sub-module area is 200–800 cm2 while the size of a module is >800 cm2. As we summarized in Figures 1 and 2A, the area of most reported perovskite cells is still distributed in the range of 10–100 cm2, which should be classified as ‘‘mini-modules.’’40–46 However, in the literature they are usually roughly regarded as modules, which result in difficulties in comparing the results. Here, we urge the society to follow the standard definition of modules for more transparent reporting. The first certified PSC module was fabricated by Toshiba (Japan) and measured by Advanced Industrial Science and Technology (AIST) in April 2018.6 A 802 cm2 PSC module was achieved by applying meniscus printing and controlling the process by adjusting the perovskite crystal growth condition during the printing process. The module displayed an impressive certified PCE of 11.6% with an open-circuit voltage (VOC) of 23.79 V, a short-circuit current (ISC) of 0.577 A, and a fill factor (FF) of 0.68. They also certified a 703 cm2 sub-module with PCE of 11.7%.47 Recently, Microquanta (China) updated the record of PSC sub-module with 11.98% efficiency in 300 cm2 area.6 WonderSolar (China) reported the largest PSC module of 3,600 cm2 in September 2018.5 Based on screen printing technique and triple mesoscopic scaffold, they also launched a 110-m2 perovskite PV system.5 These achievements demonstrate the potential of scaling-up PSCs to be comparable with other thin-film PV technologies and are an important milestone in the development of PSC modules. The PSC modules are normally separated into smaller-area series interconnected unit cells to achieve a better performance. Thus, a PSC module can be divided into two areas, as shown in Figure 2B. The ‘‘active area’’ is the area where light absorption and PV energy conversion occurs, which can be calculated by the overlap area of each functional layer. Correspondingly, the interconnecting areas are called
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200 cm 2
~10 cm 2
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Area (cm2)
1000 800 600 400 200 0 Mini-modules
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Active area
Designated area
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Dead area
Dead area
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Figure 2. Standardizing the Area Definition of PSC Modules (A) The designation of PV modules and their corresponding sizes. (Reproduced from 41–46 with permission, and Courtesy of Microquanta Semiconductor, Solliance Solar Research, and WonderSolar). (B) The definition of areas used for PSC module performance measurement.
‘‘ dead area’’ and do not generate any electricity. Techniques used to achieve the series interconnection include mask-guided methods (e.g., doctor blading, screen printing, vapor deposition, or spray coating), pattern-capable printing methods (e.g., slot-die printing or inkjet printing), and mechanical or laser scribing. Currently, active area efficiencies are mostly used to describe a PSC mini-module performance in order to present a more decent PCE value. However, for practical applications, the power output delivered by a PV module with certain dimensions is the key parameter that the consumers care about. Thus, definitions used in reporting the module efficiency only includes total area, aperture area, and designated illumination area, while the active area has never been recognized.40 As shown in Figure 2B,40 these defined areas are all larger than the active area, making the PCE values on these areas smaller than those on the active area. A designated illumination area is the active area plus the interconnection areas between them. An aperture area includes all essential components of a module such as the active materials and all the contacting components. A total area is the total projected area of a module that equals to the substrate area and includes the frame. At this stage of development, it is necessary to clearly present and discuss the used area in the reports and the PCE over aperture area or total area is preferred. If a PCE over an active area is reported, the geometrical fill factor (GFF), which is defined as the ratio of the active area in the module to the aperture area or total area, should also be reported at the same time. Solliance Solar Research sets a good example in reporting the module efficiency. They reported a module fabricated on 6 3 6 inch substrates with an aperture area of 168.75 cm2 (12.5 3 13.5 cm) and a GFF of 90%. The PCE of this module is 10% over the aperture area and 11.1% over the active area.21
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Standardizing the Evaluation of PSC Modules Accurately determining the PCE of solar cells is essential to compare the performance among different laboratories and established technologies. The performance measurement of mature PV technologies such as silicon solar cells, CIGS, and CdTe follow the IEC TC82, which can typically be obtained with rapid current-voltage (J-V) scans under pulsed solar illumination. However, it is found that PSCs do not respond fast enough under pulsed solar illumination and a steadystate simulator is normally used. AAA class solar simulators designated by IEC standard 60904-9 are preferred because they can meet the requirements of a well-matched spectrum, high-quality uniformity, and both short- and long-term temporal uniformity of light. A calibrated reference diode should be used to set the light intensity for one-sun measurement. With the steady-state simulator, we have to take into account the temperature raising and light-soaking effect on the performance of PSCs during the testing process.48 Depending on the architectures and material systems, PSCs can be very sensitive to light-soaking processes and some PSCs degrade upon atmosphere, humidity, temperature, bias, and illumination.49–52 In addition, the J-V curves of PSCs may show hysteretic behaviors, which yield unreliable or even irreproducible PCE results, as shown in Figure 3A.53 To bring up a reliable and transferable methodology that can correctly reflect the device’s true performance, various measurement protocols have been proposed and updated during the last few years. Snaith et al. have introduced the concept of ‘‘stabilized power output’’ by measuring the current continuously at a fixed bias until stabilization occurs (Figure 3B).54 Though this method has been widely used to confirm the stabilized PCE of PSCs, the measured value should always be below the true value of PCE since the bias is selected by assuming the maximum power point (MPP) according to ‘‘unstabilized’’ J-V curves. MPP tracking method could solve this issue by providing a dynamic bias and constantly approach the MPP of PSCs (Figure 3C).55 Nevertheless, a J-V curve that contains the information of VOC, JSC, and FF is still needed for PSCs. Recently, accredited testing laboratories as well as an increasing number of research groups56 have started to adapt a measurement technique called ‘‘stabilized J-V’’ or ‘‘steady-state J-V.’’ As shown in Figure 3D, the J-V scans are first performed both in the forward and reverse directions at a rapid scan rate. According to the results, each bias voltage is applied and held until the measured current has stabilized. By fitting the stabilized current values, a stabilized J-V is obtained. So far, the same testing protocols are used for PSC laboratory cells and modules. As long as the testing area is properly and accurately determined, no particular requirements are needed for PSC modules. Particularly, to avoid erroneously reporting the device performance, the device can be sent to an accredited laboratory that meets the ISO/IEC standard 17025:2017 requirements for certification. The accredited testing laboratories include NREL, the Commonwealth Scientific and Industrial Research Organization (CSIRO), AIST, the Fraunhofer Institute for Solar Energy Systems (ISE), and Sandia National Laboratories (Sandi).4 Typically, the test environment like the temperature, atmosphere, and humidity need to be provided. The J-V plots should be supplied in both forward and backward directions while the detailed information about the scan conditions like the scan speed, dwell times, or any pre-conditioning protocols should be provided at the same time. Previously, the accredited testing laboratories required that the difference between the J-V curves measured in different scan directions and scan rates should be below a specific value. In another word, the device should present no or quite tiny hysteresis effect. However, later it was found that hysteresis free J-V curves guaranteed no stabilized PCE. To provide a certification of the device, now, these laboratories usually require
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B
C
D
Figure 3. Measurements Toward Accurately Determining the PCE of PSCs (A) J-V measurement of PSCs. (B) Stabilized output measurement of PSCs. (C) Maximum power point tracking measurement of PSCs. (D) Stabilized J-V curve fitting of PSCs.
that the MPP tracking value of PCE should be stable for at least 300 s. Thus, this evolution of requirements not only aims to accurately determine the PCE of PSCs but also shows concerns to the stability of the PSCs. Standardizing the Aging Measurements of PSC Modules Besides the efficiency, the lifetime is another critical parameter for all types of solar cells, since the long-term operations under outdoor conditions determine how much power they can finally generate. For mature PV technologies, international standards such as ‘‘IEC 61215 Terrestrial PV modules—design qualification and type approval’’ have been established and released. The standards call for severe environmental (e.g., temperature, humidity, and irradiation), electrical, and mechanical stress tests in order to qualify the entry of these modules in the marketplace. No international standards for PSCs have been established yet, so the researchers are exploring the stability behavior of PSCs following the methodology developed in IEC 61215. As for now, the laboratory-scale PSCs have obtained abundant indoor stability results under various conditions, such as continuous illumination, heat stress, high humidity, and thermal cycles, as shown in Figure 4.57,58 Particularly, Solaronix has reported that triple mesoscopic printable PSCs enabled stable performance under continuous illumination for 10,000 h,10 for which the irradiation equals 10 years of sunlight in most of Europe. M. McGehee’s group employed ethylene vinyl acetate (EVA) and polyolefin (POE) films to encapsulate
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Figure 4. The Full Test Flow for Design Qualification and Type Approval of PV Modules The laboratory-scale PSCs already passed tests are highlighted in orange. The aging testing should also be focused on the modules.
perovskite-silicon tandem cells and obtained stable devices under high temperature (85 C) and thermal cycle test ( 40 C–85 C).59,60 Even under 85 C and continuous illumination at MPP, PSCs can maintain 95% of the initial performance after a 500-h test.61 Considering the large variation of stability testing methods employed by different laboratories around the world, it is urgent to establish a standard stability test protocol and investigate the degradation modes of PSCs. At the same time, the aging test should not only focus on laboratory cells but also transfer to mini-modules and modules, which at least satisfy the damp-heat aging (85 C and 85%, 1,000 h), thermal cycling ( 40 C–85 C, 200 cycles), light-soaking aging (MPP at 1 sun and 50 C G 10 C), and UV pre-conditioning (15 kWh m 2 at 60 C G 5 C) tests. To meet future large-scale applications, stricter standards such as higher temperature should be encouraged. At module level, additional problems introduced by encapsulation and connection should be considered. Light-induced degradation (LID),62 potential-induced degradation (PID),63 partial shade stress,64 and mechanical shock tests should be involved. Notably, the ion drift between the adjacent series-connected unit cells might exist and should be investigated. When it finally comes to PSC module products and systems, failures triggered by other external factors including j-box and/or string, glass breakage, loose frame, diode failure, cell interconnect breakage, glass antireflection degradation, encapsulation material degradation, delamination, and corrosion of interconnects should also be considered.65
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At last but not least, we would like to point out that it is still too early to predict the lifetimes of PSCs. The 25-year warranty of traditional silicon solar cells is obtained through decades of field data collection and failure analysis of degraded modules, not by performing the accelerated aging tests.66 For a long time during the initial stage of the development of silicon solar cells, the degradation rate in publications is higher than the required value for the warranty, even when the warranty is only 5–10 years around 1990. Only after 2005, there have been studies that meet or exceed a ‘‘cashable’’ warranty. Thus, for PSCs, the urgent focus is to establish a basic standard for PSCs to survive under general practical conditions. When we collect enough data to identify the degradation mechanism and modes, we can further improve the stability accordingly. At this time, most of the stability results are obtained by PSC laboratory cells, not mini-modules or modules. It seems particular device design and material system have been quite stable for meeting the requirements of IEC 61215, but whether they can work in mini-modules or modules is still uncertain, since the influence from the defects in adjacent unit cells is ignored. Normally, we do not need to enlarge the device area to over 800 cm2 to fabricate a module in a rush. Instead, the performance at the area of 10–100 cm2 also plays a significant role in evaluating the stability of PSCs. In the next step, the aging behaviors of unit cells and integrated mini-modules should be compared and analyzed. Conclusions The dimensions and efficiencies of the PSC modules are both increasing rapidly in recent years. This can be attributed to a combination of progresses in the development of module architectures, the design and synthesis of new material compositions, and the process engineering aiming at production scale. Especially, certain architecture-material-processing combinations have shown great potential to be scaled up with long lifetime. Researchers need to clarify and standardize the area definition, evaluation, and aging measurements of PSC modules through further in-depth study. The modules need to be classified properly according to the international definition and the testing area of the PSC modules should be clarified in the reports. MPP tracking value of PCE should be used to obtain a true and stabilized PCE value. Finally, the aging test should not only focus on laboratory cells but also transfer to mini-modules and modules. Device encapsulation and MPP tracking at higher temperature beyond IEC61215 should be encouraged. The statistical data collections of PSC modules under actual working conditions and identification of the working and degradation mechanism of PSC modules should be started now or in the very near future.
ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (grant no. 21702069 and 91733301), the Fundamental Research Funds for the Central Universities, the Science and Technology Department of Hubei Province (no. 2017AAA190), the 111 Project (no. B07038), the Program for HUST Academic Frontier Youth Team (2016QYTD06), and the Double first-class research funding of China-EU Institute for Clean and Renewable Energy (no. ICARE-RP2018-SOLAR-001 and ICARE-RP-2018-SOLAR-002). We acknowledge a constructive review process, including the suggestions of one referee who kindly drew our attention to the important matter of interconnect fabrication methods and critical thinking of IPCE. We thank Prof. Martin A. Green for an inspiring discussion on the area definition of PSC mini-modules, which is not officially claimed by the Solar Cell Efficiency Tables.
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