David Berney Needleman

Engineering Solutions and Root-Cause Analysis for Light-Induced Degradation in p -Type Multicrystalline Silicon PERC Modules

We identify two engineering solutions to mitigate light-induced degradation (LID) in p-type multicrystalline silicon passivated emitter and rear cells, including modification of metallization firing temperature and wafer quality. Lifetime measurements on etched-back samples confirm that LID has a strong bulk component. Spatially resolved lifetime maps indicate that the defects responsible for LID are dispersed ubiquitously across the wafer. Reversibility of LID upon low-temperature annealing suggests a low-activation-energy barrier inconsistent with precipitated impurity dissolution. Lifetime spectroscopy of the LID-affected state reveals an asymmetry of electron and hole capture cross sections of ~28.5, consistent with a deep-level donor point defect (e.g., interstitial Ti, interstitial Mo, substitutional W), charged nanoprecipitate, or charged structural defect, such as a dislocation. Finally, we explain two possible root causes of this LID, including 1) a point-defect complex involving a hydrogen atom and a deep-level donor and 2) configurational change of a point-defect complex involving fast-diffusing impurities.

Lifetime Spectroscopy Investigation of Light-Induced Degradation in p-type Multicrystalline Silicon PERC

When untreated, light-induced degradation (LID) of p-type multicrystalline silicon (mc-Si)-based passivated emitter and rear cell (PERC) modules can reduce power output by up to 10% relative during sun-soaking under open-circuit conditions. Identifying the root cause of this form of LID has been the subject of several recent investigations. Lifetime spectroscopy analysis, including both injection and temperature dependencies (IDLS and TIDLS), may offer insight into the root-cause defect(s). In this paper, to illustrate the root-case defect identification method, we apply room-temperature IDLS to intentionally Cr-contaminated mc-Si. Then, we apply this technique to the p-type mc-Si that exhibits LID in PERC devices, and we provide further insights by analyzing qualitatively the injection-dependent lifetime as a function of temperature. We quantify the sensitivity of the capture cross-section ratio to variations in the measured lifetime curve and in the surface recombination. We find that the responsible defect most likely has an energy level between 0.3 and 0.7 eV above the valence band and a capture cross-section ratio between 26 and 36. Additionally, we calculate the concentrations of several candidate impurities that may cause the degradation.

Evolution of LeTID Defects in p-Type Multicrystalline Silicon During Degradation and Regeneration

While progress has been made in developing engineering solutions and understanding light- and elevated temperature-induced degradation (LeTID) in p-type multicrystalline silicon (mc-Si), open questions remain regarding the root cause of LeTID. Previously, lifetime spectroscopy of multicrystalline silicon (mc-Si) passivated emitter and rear cell semifabricates in the unaffected and the degraded states enabled identification of the effective recombination parameters of the responsible defect. To gain further insight into the root cause of LeTID, in this paper, we measure the injection-dependent lifetime throughout degradation and regeneration and perform lifetime spectroscopy at several time points. Our analysis indicates that the change in lifetime during most of the process can be described by a corresponding change in the concentration of a single responsible defect. We also explore further exposure to light and temperature after nearly complete regeneration and a subsequent dark anneal to demonstrate that the behavior is no longer consistent with LeTID and the same defect is not detected by lifetime spectroscopy at maximum degradation. We consider our results in the context of the proposed hypotheses for LeTID and conclude that both hydrogenation and precipitate dissolution during firing are consistent with our results.

Three-Dimensional TCAD Modeling of Grain Boundaries in High-Efficiency Silicon Solar Cells

Grain boundaries degrade the performance of solar cells based on polycrystalline materials; yet, device models rarely accurately account for all their effects or their spatially localized nature. Here, we use 3-D technology computer-aided design (TCAD) simulations to determine the effect of grain boundaries on the performance of silicon solar cells. We find that grain boundary-limited performance of multicrystalline silicon can approach that of monocrystalline silicon in high-efficiency devices. We identify higher carrier injection as a design feature that improves defect tolerance by reducing grain boundary charging, shrinking the depletion region surrounding the grain boundary.

Sensitivity Analysis of Optical Metrics for Spectral Splitting Photovoltaic Systems: A Case Study

Spectral splitting of sunlight to increase photovoltaic (PV) efficiency beyond the Shockley-Queisser limit has gained interest in recent years. Sensitivity analysis can be a useful tool for system designers to determine how much deviation from ideal conditions can be tolerated for different optical parameters. Understanding the origin of these sensitivities can offer insight into materials and device design. We employ 2-D TCAD simulations to analyze the sensitivity of system performance to two optical parameters: spectral fidelity (the fraction of photons directed to the intended material) and the spatial uniformity of illumination intensity. We analyze a system using crystalline silicon (Si) and cuprous oxide (Cu2O) as absorbers. We find that the spectral fidelity of the light directed to the Si cell has to be greater than 90% for the system to outperform a high-efficiency single-junction Si device. Varying the fidelity of the light directed to the Cu2O cell from 55% to 90% changes system efficiency by less than 10% relative. In some cases, increasing the fidelity of this light reduces system efficiency. We find no significant impact of spatial variation on length scales from 600 μm to 4.8 mm in devices with emitter sheet resistance less than 500 Ω/□.

Economically sustainable scaling of photovoltaics to meet climate targets

To meet climate goals, photovoltaics (PV) deployment will have to grow rapidly over the next fifteen years. We identify two barriers to this growth: scale-up of manufacturing capacity and the cost of PV module production. We explore several technoeconomic approaches to overcoming these barriers and identify deep reductions in the capital intensity (capex) of PV module manufacturing and large increases in module efficiency as the most promising routes to rapid deployment. Given the lag inherent in rolling out new technology, we explore an approach where growth is fueled by debt or subsidies in the short-term and technological advances in the medium term. Finally, we analyze the current capex structure of crystalline silicon PV module manufacturing to identify potential savings.

Thin absorbers for defect-tolerant solar cell design

Thin silicon wafers provide a pathway to lower cost and lower capital intensity PV module manufacturing. They can also produce higher-efficiency devices with less expensive feedstock and crystallization processes because they require shorter diffusion lengths and operate at higher carrier injection. Through simulation, we show that thin Si wafers can be incorporated into high-efficiency cells with greater defect tolerance than thick wafers. Experimentally, we demonstrate the importance of excellent surface passivation to realizing the efficiency potential of thin silicon solar cells and show that such passivation can be achieved in silicon/amorphous silicon heterojunction devices.

Characterizing and evaluating the impact of dislocations and grain boundaries on silicon solar cells

High efficiency and low-cost, low-capex silicon substrates are necessary for the PV industry to grow to meet climate-driven deployment targets. The efficiency gap between the best devices using low-cost, low-capex substrates and monocrystalline silicon produced by the Czochralski method (CZ-Si) have shrunk recently. Here, we present numerical device simulations that show that current crystal growth, phosphorus diffusion gettering, and hydrogen passivation can produce low-cost, low-capex silicon with an efficiency potential well over 20%. We further show that incorporating these materials into higher efficiency architectures that operate at higher injection is likely to further improve their performance.

Toward defining tolerances for structural defects in silicon through 2D and 3D device simulations

We present a Technology Computer Aided Design (TCAD) implementation of literature models for the recombination activity of extended structural defects in crystalline silicon (c-Si) using the software package Synopsys Sentaurus Device. The model is validated using literature and experimental electron beam-induced current (EBIC) data. This implementation could potentially be used to determine tolerances for these defects in various materials and device architectures.