Mechanistic Understanding of Polarization‐Type Potential‐Induced Degradation in Crystalline‐Silicon Photovoltaic Cell Modules

Abstract

Potential‐induced degradation (PID) has been identified as a central reliability issue of photovoltaic (PV) cell modules. Several types of PID depend on the cell structure. Among those types, polarization‐type PID, which is characterized by reductions in short‐circuit current density (JSC) and open‐circuit voltage (VOC), is the fastest PID mode. Additionally, polarization‐type PID occurs readily at room temperature or at markedly low magnitudes of electric potential difference. Therefore, polarization‐type PID is a severe difficulty affecting silicon PV modules. Recently, degradation behavior, preventive measures, and mechanism have been investigated. As described herein, mechanistic aspects of polarization‐type PID are specifically examined and details of a recently proposed model involving a charge accumulation process at K centers in SiNx dielectric layers: the K‐center model are discussed. The K‐center model consistently explains previously reported results of experimentation, which indicates the validity of this model. Discussions presented herein are expected to improve the mechanistic understanding of polarization‐type PID in the PV community and to stimulate further discussions and verifications of the model.

Second-stage potential-induced degradation of n-type front-emitter crystalline silicon photovoltaic modules and its recovery

“…J sc and V oc decrease simultaneously in the first 120 s. This is typical behavior of the first-stage PID, PID-p, in n-FE c-Si PV modules, due to the accumulation of positive charges in SiN x and the resulting acceleration of surface recombination. 22,23,30) The modules then show different type of PID characterized by a reduction in FF alone after the PID test for 12 h. Note that the degradation of FF is not by the formation of shunting paths but by the increase in recombination current, current through the carrier recombination in the depletion layer of a p + -n junction, as confirmed in the negative bias region of Fig. 1.…”

“…The voltage-independent saturated J sc and V oc values originate from the existence of the upper limit of positive charges, in which all the K center are converted to K + centers. 22,23,30) The third-stage PID, V oc and FF reductions due to the formation of Na-based domes, is seen after the PID test at voltages of −1000 and −600 V for ∼10 2 h. The duration needed for the emergence of the third-stage PID and its degree depend on applied voltage, and the degradations are not clearly seen in the case of PID tests at voltages of −200 and −50 V. The formation of the Na-based domes requires a quantity of Na ions, and the Na ions may come from the cover glass of the modules. The voltage dependence of the third-stage PID is reasonable because more Na ions may reach the cells at higher voltage, while the domes may not be formed if the amount of Na is insufficient in the case of lower voltage.…”

“…[5][6][7][8][9][10][11][12] On the other hand, detailed studies on the PID for the n-type c-Si PV modules are insufficient and need to be investigated furthermore. [13][14][15][16][17][18][19] We have thus far investigated the PID of n-type c-Si PV modules 20) with cells such as rear-emitter, 21) front-emitter (FE), [22][23][24][25][26][27][28][29][30][31] interdigitated back-contact, 32) and Si heterojunction structures. [33][34][35][36][37] Of those, n-type FE (n-FE) c-Si PV modules have been investigated in the most detail.…”

“…18) In the n-FE c-Si PV modules, positive fixed charges are accumulated in the surface SiN x by extracting electrons from K centers by a negative bias stress. 22,23,30) This accelerates the recombination of minority carriers, electrons, on the surface of the p + emitter, resulting in simultaneous reductions in short-circuit current density (J sc ) and open-circuit voltage (V oc ). The second-stage PID of n-FE c-Si PV modules is characterized by a reduction in fill factor (FF) alone.…”

Delay of the potential-induced degradation of n-type crystalline silicon photovoltaic modules by the prior application of reverse bias

“…We have previously proposed that the origin of the charges in SiN x is K centers. 14,20,26) K centers are Si dangling bonds backbonded to three N atoms in SiN x and can be negativelycharged (K − ), neutral (K 0 ), and positively-charged (K + ) by extracting or adding electrons. [27][28][29] By the PID stress, i.e.…”