BENCHMARKING LIGHT AND ELEVATED TEMPERATURE INDUCED DEGRADATION (LETID) Matthias Pander*1, Marko Turek1, Jan Bauer1,2, Tabea Luka1, Christian Hagendorf1, Matthias Ebert1, Ralph Gottschalg1,2 1 Fraunhofer Center for Silicon Photovoltaics, Otto-Eissfeldt-Str. 12, 06120 Halle (Saale), Germany phone: +49-345-5589-5215, fax: +49-345-5589-5999,
[email protected] 2 Anhalt University of Applied Sciences, Bernburger Str. 55, D-06366 Köthen, Germany ABSTRACT: Light Induced Degradation (LID) is a well-known cell effect, impacting in the first hours of testing or first months of operation. All c-Si device types are affected, typically with less than 5% degradation. The known mechanisms (Boron Oxygen complex formation and Iron Boron pair dissociation) are identified during certification by IEC 61215:2016 standard stabilization procedures carried out at around 50°C. Unfortunately, there is a second degradation mode which is only apparent at higher temperatures. This Light and Elevated Temperature Induced Degradation (LeTID) is not identified during certification. Typically, there is no specific information commonly available for system planners to estimate the magnitude of the losses due to this effect. Testing at higher temperatures of PERC devices resulted in much higher degradation levels (up to 10 %), but slower degradation rates than BO. Therefore, this new type of degradation was termed Light and elevated Temperature Induced Degradation (LeTID). Initially LeTID was thought to only affect multi-PERC but recent studies have shown that mono-PERC may be affected equally. LeTID is not fully understood but can virtually be eliminated in production. Currently there is no generally accepted standard to test for LeTID degradation on cell and module level. Thus it is not possible to identify modules susceptible to LeTID. The question arises to what extent currently available modules contain LeTID-susceptible cells, and the magnitude of long-term yield losses. For this purpose, nine module types from different manufacturers were procured from different suppliers and subjected to a LeTID-specific test procedure. Power losses up to 6 % were found during the LeTID-Test. The three multi-Si modules tested show moderate degradation or are even stable. Some of the commercially available mono-Si PERC modules show high LeTID. Keywords: LeTID, LID, PERC, PV Module, Degradation
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INTRODUCTION
Passivated Emitter Rear Contact (PERC) Solar Cells are rapidly gaining market share [1]. This new technology does have many advantages over conventional Aluminum Back Surface Field (Al-BSF) devices, but it may have additional reliability issues. Reliability is typically being tested through the certification [2] at independent test houses. This standard includes tests for most known failure mechanisms and sets appropriate conditions to test for these. Most cells show a small amount of Light Induced Degradation (LID) in the initial weeks of operation. A test at 50oC and an irradiance of 1000W/m2 for a minimum of 2 times 5 kWh/m2 is included in the certification cycle to identify LID sensitive devices. It was found recently that PERC devices may be susceptible to another type of LID, which is apparent only at elevated temperatures. This is termed Light and elevated Temperature Induced Degradation (LeTID) [3]-[5]. PERC technology is more affected than Al-BSF [7]. Initially, LeTID was thought to only affect multi-PERC, as the grain boundaries in the multi-Si material may contribute to degradation ([5],[10]). However, it has been shown recently that mono-PERC can be affected equally [6]. LeTID is a cell effect typically associated with the bulk of the device. It correlates well with the current passing through the cell. The traditional LID mechanisms express themselves in the first hours of testing or first months of operation with power losses around 3 % (year 1 degradation). These known mechanisms are: Iron Boron (FeB) pair dissociation (Fe-B-LID) Boron Oxygen complex formation (B-O-LID) Susceptibility to these mechanisms is normally identified in type approval. Testing at higher temperatures for PERC devices has been shown to result in much higher degradation levels in some cases with significantly slow-
er degradation rates than B-O-LID but potentially of higher magnitude. Therefore, this new type was termed Light and elevated Temperature Induced Degradation (LeTID) [3]. Hanwha QCells linked LeTID to degradation seen in the field and thus demonstrated that LeTID is a relevant degradation mechanism in outdoor operation [8]. The observed degradation of >7% in one year in Nicosia (Cyprus) is critical for PV systems. As a temperature activated failure mode, LeTID develops faster in hot climates due to the increased operation temperatures. Results of outdoor data from Hanwha QCells have shown that labdegradation tests with 290 h at 75°C CID (Mpp mode) are comparable to one year outdoor exposure in Nicosia (Cyprus) [8]. The effect is nevertheless relevant even in temperate climates due to the long operation times power degradation might occur more slowly within 5 to 10 years. The root cause analysis of LeTID is inconclusive and ongoing. Some authors suggest the degradation to be caused by defect complexes of mobile hydrogen and an intrinsic crystal defect [11]. During these studies adequate testing procedures have been developed on cell level which supports testing conditions on module level. Since LeTID might lead to substantial yield loss the following questions shall be investigated and answered in this publication: 1. What are the factors influencing LID/LeTID degradation? 2. What are reliable conditions for testing LeTID on module level? 3. How susceptible are commercially available PERC modules to LeTID?
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INFLUENING FACTORS FOR LID/LETID
A recombination active defect is formed especially in PERC solar cells at elevated temperatures (≥50°𝐶) under illumination. This reduces Voc, Isc, FF and efficiency 𝜂. The maximum degradation is typically reached after 300 to 400 h at 75°C. Going to even higher test temperatures accelerates the degradation [11]. However, new recombination active defect types might also appear that are unrelated to LeTID [9]. Light beam induced current (LBIC) images exhibit nearly homogeneous degradation of the whole cell area, hence it is assumed that the underlying LeTID defect is a volume defect [10],[11]. It is assumed that impurities in the Si material influences LeTID, however, at the present time no final mechanism is known. In mc-Si PERC cells non-active grain boundaries show a slower or weaker degradation [9]. There is even a thickness dependence of the LeTID effect shown by Bredemeier et al. [12]. The two latter findings are only two pieces of the puzzle to reveal the origins of the LeTID effect. It seems that hydrogens plays a role as well as other elements [11],[12],[13]. Pretreatment of PERC solar cells in the dark with different heat budgets, e.g. during firing processes or intentionally dark anneal, changes the LeTID effect significantly for mono-Si and mc-Si as well [9],[11],[14],[15]. The PERC cells investigated for instance in [13] exhibit a stronger degradation towards a dark anneal of about 175°C, and a lower LeTID effect was seen by increasing the dark anneal temperature further. Screening the literature above it turned out that common LeTID tests conditions are 75°C and illumination intensities from approx. 0.5 to 1 sun respectively and operating conditions with equivalent currents in the dark (so called current induced degradation - CID, see e.g. [16]) trigger LeTID reliable if the cells are prone to this effect. B-O-LID as well as LeTID is driven by injected charge carriers. Thus, the degradation can either be activated by illumination or application of an external current (CID). Different equivalent operational modes are described in literature for testing LeTID: LID Voc mode ≅ carrier injection with an Iscequivalent injection level LID MPP mode ≅ carrier injection with a reduced injection level of around (Isc – Impp) ~ 0.5 …1A Higher injection accelerates degradation, but the maximum degradation is only reached in MPP mode [3]. After reaching the maximum degradation regeneration is observed. This can be close to the initial level.
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3.1 Sampling Module samples were obtained anonymously from various wholesalers. An initial quality check was performed for all modules and includes visual inspection, EL imaging and STC measurements. The aim was to ensure that no modules are tested with defects potentially causing additional performance reduction due to thermal stress (e.g. ‘normal cell cracking’). STC power had to be within specifications. The final test batch included nine module types with two modules each. For rough classification we divide the test batch into: 6 mono-Si module types 3 multi-Si module types 3.2 Testing Procedure Based on the review in section 2, the test conditions for the modules have been chosen. Since the known LID defects may impact the results of the study a preconditioning was defined to distinguish between B-O-LID, FeB-LID and LeTID. For B-O-LID the module temperature is controlled at 25°C and current near ISC is injected. These conditions ensure that only B-O-LID is activated. To control the condition of FeB, a dark storage of 10 h was carried out before each intermediate power measurement. For temperature control, the module temperature was measured at 4 representative locations (see Figure 1), which were identified in a pre-test. After preconditioning the actual LeTID test procedure is carried out. The conditions had to consider that too low temperature may address different dominant processes or won’t activate LeTID. Similarly, slow degradation rates lead to long test times and consequently high costs. The lower temperature limit is roughly 50°C in this case. Too high temperatures may shift annealing and degradation processes, resulting in unrealistically low degradation. This may lead to conditions where maximum degradation is not observed. Additionally, LeTID-unrelated defects may be caused when module materials do not withstand the temperatures. This leads to an upper temperature limit of 85°C for standard modules. The injection level has to be close to field conditions, what normally means MPP-Tracking. Based on these considerations the following conditions are common in the community: Internal carrier injection: 1000 W/m², MPPTracking, 75°C in Solar Simulator (LID – MPP mode) External carrier injection: I = 1 A, 75°C in dark climate chamber (CID – MPP mode) These tests have been shown to be equivalent and trigger the same degradation mechanism. In this work, external carrier injection is used as suggested by [6]. The testing scheme is summarized in Figure 2. 4
Figure 1: Climate chamber setup
MODULE BENCHMARK
RESULTS AND DISCUSSION
The preconditioning was ended after 168 h, with retesting after 24 and 72 h, since the power change was below 0.3 % for the last interval. Temperature control was quite challenging due to joule heating with 9 A current injection during the B-O-LID testing. It was possible to realize a mean temperature of 25 °C with a fluctuation
range of ±6 K. Total power loss for preconditioning was less than 1 %, meaning that the tested modules contain stable or stabilized cells regarding B-O-LID.
Figure 4: EL images (0.85 A, 15s) of manufacturer Mono-1, M1 (left: initial, right: after LeTID) Figure 2: Testing scheme for LeTID module benchmark
In the LeTID test two module types had negligible power loss, one multi-Si as well as one mono-Si (Figure 3). The multi-Si devices in the current sample lost less than 2 % of power. Three mono-Si modules had substantial losses of more than 3 % up to 6 %. Another finding is that devices of the same manufacturer can scatter, but most are close to each other. This may happen if cell quality is not sufficiently controlled and LeTID susceptible and stable cells are present in the cell batch. Multi Multi-1 M1 Multi-1 M2 Multi-2 M1 Multi-2 M2 Multi-3 M1 Multi-3 M2
1 0
PMPP [%]
-1 -2
Mono
-3 -4 -5 -6 -7 0 100 200 300 400 500 600 700
time [h]
Figure 5: EL images (0.85 A, 15s) of manufacturer Mono-6, M1 (left: initial, right: after LeTID)
Mono-1 M1 Mono-1 M2 Mono-2 M1 Mono-2 M2 Mono-3 M1 Mono-3 M2 Mono-4 M1 Mono-4 M2 Mono-5 M1 Mono-5 M2 Mono-6 M1 Mono-6 M2
Figure 3: Measured power degradation during LeTID test
EL imaging is useful to qualitatively (in first order approximation) identify the degradation state of cells in the modules. As can be seen from Figure 4, there are also degraded cells in the stable modules, but in total the individual degradation is not enough to affect module power. For the module with the highest degradation almost every cell is affected by LeTID (Figure 5) and leads to the significant power loss. In contrast, module 2 from the same manufacturer has fewer degraded cells and consequently less power loss. Due to the dependency on individual cell degradation the module parameters degrade in different ways (Figure 7). Some modules show reduction mainly in short circuit current ISC (~ 1 - 2.7 %). Modules with highest power losses have also substantial losses in VOC. The FF varies from improving to degrading, which can be an artifact of interconnection and changing mismatch. Larger FF degradation was only observed for mono-Si.
Figure 6: EL images (0.85 A, 15s) of manufacturer Mono-6, M2 (left: initial, right: after LeTID)
Figure 8 shows the comparison of the measured I-Vcurves at the end of the different test stages. The module shows only slight changes between after preconditioning. After the LeTID test ISC as well as VOC degraded substantially and also the FF decreases. 5
CONCLUSION
It was verified that the chosen LeTID test conditions could reliably differentiate modules prone to this degradation mechanism. The preconditioning for B-O-LID resulted in minor degradation (
