1 Potential-induced degradation of n-type crystalline silicon photovoltaic modules K. Ohdaira 1, S. Yamaguchi 1, C. Yamamoto 2, K. Hara 2, and A. Masuda 2 1 Japan Advanced Institute of Science and Technology 2 National Institute of Advanced Industrial Science and Technology Acknowledgment This work is supported by NEDO.
Outline 2 Importance of PID research for n-type PV modules PID of front-emitter c-si PV modules PID of rear-emitter c-si PV modules PID of silicon heterojunction PV modules
n-type c-si PV modules 3 High conversion efficiency Increasing market share Insufficient understanding for the mechanism of their potential-induced degradation (PID) Cells with n-type wafers Front emitter (FE) Rear emitter (RE) Si heterojunction (SHJ) Back contact (BC)
PID of n-type FE and RE cell modules 4 Bifacial cells Sunlight p + n + Encapsulant Backsheet p + side up Front-emitter (FE) module n + side up Rear-emitter (RE) module
Experimental procedures 5 PID-stress experiment Grounded Conventional tempered cover glass Cell (one-cell module) Conditions EVA PVF/PET/PVF 1000 or 2000 V 85 C No intentional humidity stress (<2%RH)
Outline 6 Importance of PID research for n-type PV modules PID of front-emitter c-si PV modules PID of rear-emitter c-si PV modules PID of silicon heterojunction PV modules
PID of FE c-si PV modules 7 Characterized by reductions in V oc and J sc Due to enhanced surface recombination of minority carriers K. Hara et al., SOLMAT 140, 361 (2015).
Proposed PID mechanism 8 Surface polarization effect 1. Leakage current induced by negative bias 2. Positive fixed charges in SiN x 3. Enhancement in surface recombination due to the fixed charges in the SiN x K. Hara et al., SOLMAT 140, 361 (2015).
Detailed observation of PID for FE c-si modules 9 1000 V S. Yamaguchi et al., APEX (in press) Rapid reduction and following saturation in J sc and V oc Limited number of fixed charges?
Voltage dependence 10 Constant saturated P max independent of applied voltage Limited positive fixed charge density Possible origin: K + centers in SiN x S. Yamaguchi et al., APEX (in press) For more details P-06 Yamaguchi (JAIST)
Outline 11 Importance of PID research for n-type PV modules PID of front-emitter c-si PV modules PID of rear-emitter c-si PV modules PID of silicon heterojunction PV modules
PID of rear-emitter PV modules J V 12 1000 V V oc reduction Saturation Slight J sc and FF reduction S. Yamaguchi et al., SOLMAT 151, 113 (2016).
PID of rear-emitter PV modules EQE 13 1000 V EQE reduction in short-wavelength region Enhancement in surface recombination Reduction in V oc S. Yamaguchi et al., SOLMAT 151, 113 (2016).
Mechanism of PID for p-type modules 14 Na accumulation from outside (e.g. cover glass) Decoration of stacking faults in c-si by Na (serious shunting in p-type c-si PV modules) J V of p-type c-si PV modules V. Naumann et al. Sol. Energy Mater. Sol. Cells 120 (2014) 383. Current density (ma/cm 2 ) 40 20 0-20 -40 After PID Initial -0.4-0.2 0.0 0.2 Voltage (V) 0.4 0.6
Mechanism of PID for n-type RE modules 15 Na-decorated stacking faults Recombination centers in n-type RE modules Enhanced surface recombination, V oc reduction Consistent with the saturation behavior Front side Rear side
16 Observation of enhanced surface recombination Lifetime (µ-pcd) Minority carrier lifetime[µs] 200 150 100 50 1000 V 0 0 20 40 Time[h] 60 80 100 For more details P-14 Nishikawa (JAIST)
Outline 17 Importance of PID research for n-type PV modules PID of front-emitter c-si PV modules PID of rear-emitter c-si PV modules PID of silicon heterojunction PV modules
PID of SHJ modules V oc and FF 18 Open-circuit voltage (V oc ) Fill factor (FF) 1.1 1.0 1000 V 1.1 1.0 1000 V Normalized V oc 0.9 0.8 0.7 2000 V +2000 V (Recovery) Normalized FF 0.9 0.8 0.7 2000 V +2000 V (Recovery) 0.6 0 5 10 15 20 25 30 PID-stress duration (day) 35 0.6 0 5 10 15 20 25 30 PID-stress duration (day) 35 No reduction in V oc and FF
PID of SHJ modules Dark I V 19 Dark I V 0.1 Current (A) 0.0-0.1-0.2-0.3-0.4 1000 V Initial 7 days 13 days -0.5-5 -4-3 -2-1 0 1 2 Voltage (V) No variation in I V curves No degradation in p n junction
PID of SHJ modules I sc and P max 20 Short-circuit current (I sc ) Maximum power (P max ) 1.1 1.1 Normalized I sc 1.0 0.9 0.8 0.7 0.6 0 5 1000 V 2000 V 10 15 20 +2000 V (Recovery) 25 PID-stress duration (day) 30 35 Normalized P max 1.0 0.9 0.8 0.7 0.6 0 2000 V 1000 V +2000 V (Recovery) 5 10 15 20 25 30 PID-stress duration (day) 35 Decrease in I sc with PID-stress duration P max reduction simply governed by I sc No significant recovery
PID of SHJ modules EQE 21 EQE spectra Initial 1000 V 13 days (Center) 13 days (Edge) Reduction in EQE in entire wavelength region More reduction in edge parts
PID of SHJ modules EL 22 Initial 1000 V, 32 days Initial 2000 V, 21 days Possible PID mechanism Optical loss (TCO and/or EVA) Reaction with Na? Ununiform EL and EQE: Property distribution of TCO films?
PID of SHJ modules Ionomer 23 Cover glass SHJ cell EVA Ionomer 1.1 Normalized P max 1.0 0.9 0.8 0.7 2000 V Suppression of PID by the usage of ionomer encapsulant 0.6 0 5 10 15 20 25 30 PID-stress dutation (day) For more details about the PID of SHJ modules P-15 Yamamoto (AIST)
Performance stability 24 SHJ p-type SHJ Higher stability than conventional p-type c-si PV modules S. Yamaguchi et al.: Jpn. J. Appl. Phys. 54 (2015) 08KC13. K. Hara et al.: Sol. Energy Mater. Sol. Cells 140 (2015) 361.
Summary 25 PID of n-type c-si PV modules Front-emitter c-si PV modules Rapid reduction in J sc and V oc Enhanced surface recombination of minority carriers Positive fixed charges in SiN x Rear-emitter c-si PV modules Reduction mainly in V oc Enhanced surface recombination of minority carriers Na accumulation into c-si SHJ PV modules Reduction only in J sc Optical loss Performance stability of n-type c-si PV modules Higher than conventional p-type c-si PV modules