Nick Bosco

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.

Determining Outdoor CPV Cell Temperature

An accurate method is needed for determining cell temperature when measuring CPV modules outdoors. It has been suggested that cell temperature can be calculated through a procedure that shutters sunlight to the cells while measuring the transients in open‐circuit voltage (Voc) and heat sink temperature. This paper documents application of this shutter procedure to multiple CPV modules at NREL. The challenges and limitations are presented along with an alternate approach to measuring CPV cell operating temperature.

Evaluation of high-temperature exposure of rack-mounted photovoltaic modules

Photovoltaic (PV) modules operate in an extreme environment and are exposed to radiation, humidity, and hot and cold thermal extremes. This paper focuses on polymeric-material degradation during PV-module operation at high ambient temperatures, high solar irradiance and low wind speed. The 2004 version of the IEC 61730 specification requires all polymeric materials used in a photovoltaic module to have a Relative Thermal Index (RTI) or Relative Thermal Endurance Index (RTE) at least 20°C greater than the maximum material temperature measured during the temperature test conducted at 40°C ambient. There is currently an international debate regarding this requirement. This paper explores the thermal exposure of photovoltaic modules in the field as a technical basis for this debate. For the hottest cities, the thermal exposure is found to be equivalent to aging at a constant temperature of 42–53°C, with maximum temperatures of 75°C.

CPV Cell Infant Mortality Study

Six hundred and fifty CPV cells were characterized before packaging and then after a four‐hour concentrated on‐sun exposure. An observed infant mortality failure rate was reproduced and attributed to epoxy die‐attach voiding at the corners of the cells. These voids increase the local thermal resistance allowing thermal runaway to occur under normal operating conditions in otherwise defect‐free cells. FEM simulations and experiments support this hypothesis. X‐ray transmission imaging of the affected assemblies was found incapable of detecting all suspect voids and therefore cannot be considered a reliable screening technique in the case of epoxy die‐attach.

Quantifying the thermal fatigue of CPV modules

A method is presented to quantify thermal fatigue in the CPV die‐attach from meteorological data. A comparative study between cities demonstrates a significant difference in the accumulated damage. These differences are most sensitive to the number of larger (ΔT) thermal cycles experienced for a location. High frequency data (<1/min) may be required to most accurately employ this method.

Reliability testing the die-attach of CPV cell assemblies

Results and progress are reported for a course of work to establish an efficient reliability test for the die-attach of CPV cell assemblies. Test vehicle design consists of a ∼1 cm2 multijunction cell attached to a substrate via several processes. A thermal cycling sequence is developed in a test-to-failure protocol. Methods of detecting a failed or failing joint are prerequisite for this work; therefore both in-situ and non-destructive methods, including infrared imaging techniques, are being explored as a method to quickly detect non-ideal or failing bonds.

III-V/Si wafer bonding using transparent, conductive oxide interlayers

We present a method for low temperature plasma-activated direct wafer bonding of III-V materials to Si using a transparent, conductive indium zinc oxide interlayer. The transparent, conductive oxide (TCO) layer provides excellent optical transmission as well as electrical conduction, suggesting suitability for Si/III-V hybrid devices including Si-based tandem solar cells. For bonding temperatures ranging from 100 °C to 350 °C, Ohmic behavior is observed in the sample stacks, with specific contact resistivity below 1 Ω cm2 for samples bonded at 200 °C. Optical absorption measurements show minimal parasitic light absorption, which is limited by the III-V interlayers necessary for Ohmic contact formation to TCOs. These results are promising for Ga0.5In0.5P/Si tandem solar cells operating at 1 sun or low concentration conditions.

An Infant Mortality Study of III–V Multijunction Concentrator Cells

Six hundred and forty III-V triple-junction solar cells were evaluated in this study. The cells were initially electrically and optically characterized prior to being packaged and placed on-sun for a short exposure. Following exposure, the cells were partitioned according to their performance change. An infant mortality rate of 0.5% was observed and attributed to preexisting voids in the die attach that promoted thermal runaway. All other cells that significantly degraded following exposure were initially measured with shunt currents >;100 mA at 1.5 V; therefore, a similar limit would serve as an appropriate screening current and only reduce yield by ~1.5% . While many cells both above and below this shunt current limit exhibited artifacts in their electroluminescence (EL) emission, it was not found to predict subsequent performance. The current investigation, however, focused on detecting a short-term degradation and did not evaluate how artifacts in the EL emission or a short-term change in shunt current may correlate with other wear out mechanisms.

The Influence of PV Module Materials and Design on Solder Joint Thermal Fatigue Durability

Finite element model (FEM) simulations have been performed to elucidate the effect of flat plate photovoltaic (PV) module materials and design on PbSn eutectic solder joint thermal fatigue durability. The statistical method of Latin Hypercube sampling was employed to investigate the sensitivity of simulated damage to each input variable. Variables of laminate material properties and their thicknesses were investigated. Using analysis of variance, we determined that the rate of solder fatigue was most sensitive to solder layer thickness, with copper ribbon and silicon thickness being the next two most sensitive variables. By simulating both accelerated thermal cycles (ATCs) and PV cell temperature histories through two characteristic days of service, we determined that the acceleration factor between the ATC and outdoor service was independent of the variables sampled in this study. This result implies that an ATC test will represent a similar time of outdoor exposure for a wide range of module designs. This is an encouraging result for the standard ATC that must be universally applied across all modules.

Evaluation of Dynamic Mechanical Loading as an accelerated test method for ribbon fatigue

Dynamic Mechanical Loading (DML) of photovoltaic modules is explored as a route to quickly fatigue copper interconnect ribbons. Results indicate that most of the interconnect ribbons may be strained through module mechanical loading to a level that will result in failure in a few hundred to thousands of cycles. Considering the speed at which DML may be applied, this translates into a few hours of testing. To evaluate the equivalence of DML to thermal cycling, parallel tests were conducted with thermal cycling.