Peter Hacke

System voltage potential-induced degradation mechanisms in PV modules and methods for test

Over the past decade, degradation and power loss have been observed in PV modules resulting from the stress exerted by system voltage bias. This is due in part to qualification tests and standards that do not adequately evaluate for the durability of modules to the long-term effects of high voltage bias experienced in fielded arrays. High voltage can lead to module degradation by multiple mechanisms. The extent of the voltage bias degradation is linked to the leakage current or coulombs passed from the silicon active layer through the encapsulant and glass to the grounded module frame, which can be experimentally determined; however, competing processes make the effect non-linear and history-dependent. Appropriate testing methods and stress levels are described that demonstrate module durability to system voltage potential-induced degradation (PID) mechanisms. This information, along with outdoor testing that is in progress, is used to estimate the acceleration factors needed to evaluate the durability of modules to system voltage stress. Na-rich precipitates are observed on the cell surface after stressing the module to induce PID in damp heat with negative bias applied to the active layer.

Testing and analysis for lifetime prediction of crystalline silicon PV modules undergoing degradation by system voltage stress

Acceleration factors are calculated for crystalline silicon photovoltaic modules under system voltage stress by comparing the module power during degradation outdoors with that in accelerated testing at three temperatures and 85% relative humidity. A lognormal analysis is applied to the accelerated lifetime test data, considering failure at 80% of the initial module power. Activation energy of 0.73 eV for the rate of failure is determined for the chamber testing at constant relative humidity, and the probability of module failure at an arbitrary temperature is predicted. To obtain statistical data for multiple modules over the course of degradation in situ of the test chamber, dark I–V measurements are obtained and transformed using superposition, which is found to be well suited for rapid and quantitative evaluation of potential-induced degradation. It is determined that shunt resistance measurements alone do not represent the extent of power degradation. This is explained with a two-diode model analysis that shows an increasing second diode recombination current and ideality factor as the degradation in module power progresses. Failure modes of the modules stressed outdoors are examined and compared with those stressed in accelerated tests.

Potential-induced degradation in photovoltaic modules: a critical review

Potential-induced degradation (PID) has received considerable attention in recent years due to its detrimental impact on photovoltaic (PV) module performance under field conditions. Both crystalline silicon (c-Si) and thin-film PV modules are susceptible to PID. While extensive studies have already been conducted in this area, the understanding of the PID phenomena is still incomplete and it remains a major problem in the PV industry. Herein, a critical review of the available literature is given to serve as a one-stop source for understanding the current status of PID research. This paper also aims to provide an overview of future research paths to address PID-related issues. This paper consists of three parts. In the first part, the modelling of leakage current paths in the module package is discussed. The PID mechanisms in both c-Si and thin-film PV modules are also comprehensively reviewed. The second part summarizes various test methods to evaluate PV modules for PID. The last part focuses on studies related to PID in the omnipresent p-type c-Si PV modules. The dependence of temperature, humidity and voltage on the progression of PID is examined. Preventive measures against PID at the cell, module and system levels are illustrated. Moreover, PID recovery in standard p-type c-Si PV modules is also studied. Most of the findings from p-type c-Si PV modules are also applicable to other PV module technologies.

Test-to-Failure of crystalline silicon modules

Accelerated lifetime testing of five crystalline silicon module designs was carried out according to the Terrestrial Photovoltaic Module Accelerated Test-to-Failure Protocol. This protocol compares the reliability of various module constructions on a quantitative basis. The modules under test are subdivided into three accelerated lifetime testing paths: 85°C/85% relative humidity with system bias, thermal cycling between −40°C and 85°C, and a path that alternates between damp heat and thermal cycling. The most severe stressor is damp heat with system bias applied to simulate the voltages that modules experience when connected in an array. Positive 600 V applied to the active layer with respect to the grounded module frame accelerates corrosion of the silver grid fingers and degrades the silicon nitride antireflective coating on the cells. Dark I–V curve fitting indicates increased series resistance and saturation current around the maximum power point; however, an improvement in junction recombination characteristics is obtained. Severe shunt paths and cell-metallization interface failures are seen developing in the silicon cells as determined by electroluminescence, thermal imaging, and I–V curves in the case of negative 600 V bias applied to the active layer. Ability to withstand electrolytic corrosion, moisture ingress, and ion drift under system voltage bias are differentiated according to module design. The results are discussed in light of relevance to field failures.

Application of a boron source diffusion barrier for the fabrication of back contact silicon solar cells

Interdigitated back contact and emitter wrap-through solar cells were fabricated using a diffusion barrier to achieve selective phosphorus diffusion for the patterning of the base and the emitter regions on the cell rear. The addition of boron to the diffusion barrier for emitter formation in the underlying n-type base and for the formation of a back surface field in the case of a p-type base was further examined. Boron was successfully incorporated into the n-type Si for the creation of rear p/sup +/ emitters in an interdigitated back contact cell. An order of magnitude improvement in the surface recombination velocity to 10/sup 3/ cm/s could be achieved with a p/sup +/ surface field applied to the base of p-type wafers. Incorporating this technology, best multi-crystalline emitter wrap-through cell performance could be gained with a 1k-2k /spl Omega///spl square/ surface field; however, the characteristics were rapidly dominated by increased saturation current as the surface field layer concentration was increased.

Ensuring quality of PV modules

Photovoltaic (PV) customers need to have confidence in the PV modules they purchase. Currently, no test can quantify a modules lifetime with confidence, but stress tests are routinely used to differentiate PV product designs. We suggest that the industry would be strengthened by using the wisdom of the community to develop a single set of tests that will help customers quantify confidence in PV products. This paper evaluates the need for quality assurance (QA) standards and suggests a path for creating these. Two types of standards are needed: 1) QA of the module design and 2) QA of the manufacturing process.

Towards a manufacturable back-contact emitter-wrap-through silicon solar cell

The emitter-wrap-through (EWT) silicon solar cell is an all back-contact cell design that features a high efficiency while using solar-grade silicon materials. This paper reports on progress towards a low-cost manufacturable back-contact cell using the EWT design. We first show that the EWT cell is particularly well suited for thin (<200 /spl mu/m) silicon solar cells. In particular, the EWT cell structure is inherently able to obtain high efficiencies (17% range) from low-quality materials by using thinner substrates and due to the double-sided carrier collection afforded by the n/sup +/pn/sup +/ structure. Secondly, we show that the manufacturing process is largely similar to conventional solar cells. Detailed estimates of manufacturing costs find that the EWT module could potentially reduce costs by one-half compared to current technology using front-contacted cells with screen-printed metallizations. Finally, we report on progress on several key processing steps required for a low-cost EWT cell design and process.

Sodium Accumulation at Potential-Induced Degradation Shunted Areas in Polycrystalline Silicon Modules

We investigated potential-induced degradation (PID) in silicon mini-modules that were subjected to accelerated stressing to induce PID conditions. Shunted areas on the cells were identified with photoluminescence and dark lock-in thermography (DLIT) imaging. The identical shunted areas were then analyzed via time-of-flight secondary-ion mass spectrometry (TOF-SIMS) imaging, 3-D tomography, and high-resolution transmission electron microscopy. The TOF-SIMS imaging indicates a high concentration of sodium in the shunted areas, and 3-D tomography reveals that the sodium extends more than 2 μm from the surface below shunted regions. Transmission electron microscopy investigation reveals that a stacking fault is present at an area identified as shunted by DLIT imaging. After the removal of surface sodium, tomography reveals persistent sodium present around the junction depth of 300 nm and a drastic difference in sodium content at the junction when comparing shunted and nonshunted regions.

Busbarless emitter wrap-through solar cells and modules

Back-contact crystalline-silicon photovoltaic solar cells and modules offer a number of advantages including the elimination of grid shadowing losses, reduced cost by use of thinner silicon substrates, simpler module assembly, and improved aesthetics. While the existing method for interconnecting and stringing edge-connected back contact cells is acceptably straight-forward and reliable, there are further gains to be exploited when you have both contact polarities on one side of the cell. In this work, we produce ‘busbarless’ emitter wrap-through (EWT) solar cells that use about 65% less gridline Ag metallization mass compared to the edge tab design. Further, series resistance power losses are reduced by extraction of current from more places on the cell rear leading to a fill factor improvement of about 6% (relative) on the module level. Series resistance and current-generation losses associated with large rear bondpads and bus bars are eliminated. Use of thin Si wafers is enabled because of the reduced Ag metallization mass and by interconnection with conductive adhesives leading to reduced bow. The busbarless cell design interconnected with conductive adhesives passes International Electrotechnical Commission damp heat and thermal cycling tests.

Acceleration factor determination for potential-induced degradation in crystalline silicon PV modules

Potential-induced degradation in conventional p-type silicon-based photovoltaic solar cell modules is described as a failure mechanism involving positive ion migration, understood to be primarily Na+, drifting from the glass to the cells in negative-voltage arrays. Acceleration factors for this mechanism are determined for silicon photovoltaic modules by comparing the module power during degradation outdoors to that in accelerated testing at three temperatures and 85% relative humidity. A lognormal analysis is applied to the accelerated lifetime test data considering failure at 80% of the initial module power. Activation energy of 0.73 eV for the rate of failure is determined for the chamber testing at the constant relative humidity, and the probability of module failure at an arbitrary temperature is predicted. Estimation of module power in-situ in the environmental chamber is achieved using dark I-V measurements transformed by superposition. By this means, the power of the degrading module can be semi-continuously determined so that statistical data for multiple modules undergoing potential-induced degradation can be easily and accurately obtained.