Vincenzo LaSalvia

Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

Interdigitated Back Passivated Contact (IBPC) Solar Cells Formed by Ion Implantation

We describe work toward an interdigitated back passivated contact (IBPC) solar cell formed by patterned ionimplanted passivated contacts. Formation of electron and hole passivated contacts to n-type Cz wafers using a thin SiO₂ layer and ion-implanted amorphous silicon (a-Si) is described. P and B were ion implanted into intrinsic a-Si films, forming symmetric and IBPC test structures. The recombination parameter J₀, as measured by a Sinton lifetime tester after thermal annealing, was J₀ ~ 2.4 fA/cm² for Si:P and J₀ ~ 10 fA/cm² for Si:B contacts. The contact resistivity for the passivated contacts was found to be 0.46 Ω·cm² for the n-type contact and 0.04 Ω·cm² for the p-type contact. The IBPC solar cell test structure gave 1-sun V_oc values of 682 mV and pFF = 80%. The benefits of the ion-implanted IBPC cell structure are discussed.

Biaxially-textured photovoltaic film crystal silicon on ion beam assisted deposition CaF2 seed layers on glass

We grow biaxially textured heteroepitaxial crystal silicon (c-Si) films on display glass as a low-cost photovoltaic material. We first fabricate textured CaF2 seed layers using ion-beam assisted deposition, then coat the CaF2 with a thin, evaporated epitaxial Ge buffer and finally deposit heteroepitaxial silicon on the Ge. The silicon is grown by hot-wire chemical vapor deposition, a high-rate, scalable epitaxy technology. Electron and X-ray diffraction confirm the biaxial texture of the CaF2 and epitaxial growth of the subsequent layers. Transmission electron microscopy reveals columnar silicon grains about 500 nm across. We fabricate a proof-of-concept epitaxial film c-Si solar cell with an open circuit voltage of 375 mV that is limited by minority carrier lifetime.

Implementation of tunneling pasivated contacts into industrially relevant n-Cz Si solar cells

We identify bottlenecks, and propose solutions, to implement a B-diffused front emitter and a backside pc-Si/SiO2 pasivated tunneling contact into high efficiency n-Cz Si cells in an industrially relevant way. We apply an O-precipitate dissolution treatment to make n-Cz wafers immune to bulk lifetime process degradation, enabling robust, passivated B front emitters with J0 <; 20fA/cm2. Adding ultralow recombination n+ pc-Si/SiO2 back contacts enables pre-metallized cells with iVoc=720 mV and J0=8.6 fA/cm2. However, metallization significantly degrades performance of these contacts due to pinholes and possibly, grain boundary diffusion of primary metal and source contaminates such as Cu. An intermediate, doped a-Si:H capping layer is found to significantly block the harmful metal penetration into pc-Si.

Low temperature Si/SiO x /pc-Si passivated contacts to n-type Si solar cells

We describe the design, fabrication, and results of low-recombination, passivated contacts to n-type silicon utilizing thin SiO_x, and plasma enhanced chemical vapor deposited doped polycrystalline-silicon (pc-Si) layers. A low-temperature silicon dioxide layer is grown on both surfaces of an n-type CZ wafer to a thickness of <;20 Å. Next, a thin layer of P-doped plasma enhanced chemical vapor deposited amorphous silicon (n/a-Si:H) is deposited on top of the SiO_x. The layers are annealed to crystallize the a-Si:H and diffuse H to the Si/SiO_x interface, after which a metal contacting layer is deposited over the conducting pc-Si layer. The contacts are characterized by measuring the recombination current parameter of the full-area contact (J_o,contact) to quantify the passivation quality, and the specific contact resistivity (ρ_contact). The Si/SiO_x/pc-Si contact has an excellent J_o,contact = 30 fA/cm² and a good ρ_contact = 29.5 mOhm-cm². Separate processing conditions lowered J_o,contact to 12 fA/cm². However, the final metallization can substantially degrade this contact and has to be carefully engineered. This contact could be easily incorporated into modern, high-efficiency solar cell designs, benefiting performance and yet simplifying processing by lowering the temperature and growth on only one side of the wafer.

Some challenges in making accurate and reproducible measurements of minority carrier lifetime in high-quality Si wafers

Measurement of the minority carrier lifetime (τ) of high-quality wafers (having bulk minority carrier lifetime, τb > few milliseconds) requires surface passivation with very low surface recombination velocity, typically <; 1cm/s. Furthermore, for mapping large (e.g., 156 x156 mm) wafers, the passivation must also be stable and uniform over the entire wafer surfaces. These are very demanding requirements and it is a common experience that they are very difficult to achieve. Yet, they are necessary for performing defect analyses of the current N-type wafers. To understand the problems associated with these measurements, we have studied effect of wafer preparation (cleaning procedures, handling) and the passivation characteristics (stability, sensitivity to light, thickness of the passivation medium required for stable passivation) for many commonly used passivation media-iodine-ethanol (IE), quinhydrone-methanol (QHM), aluminum oxide (Al2O3), amorphous-silicon (a-Si), and silicon dioxide (SiO2). Here, we will discuss main factors that influence the accuracy and repeatability of lifetime measurements.

Bulk defect generation during B-diffusion and oxidation of CZ wafers: Mechanism for degrading solar cell performance

We describe results of our experimental study to investigate the effect of B diffusion and drive-in/oxidation on minority carrier lifetime of the wafer. We have observed that B diffusion generates stacking faults that can be attributed to injection of Si interstitials into the wafer by formation of a boron rich layer at the wafer surface. These Si interstitials are also believed to enhance interactions between the native point defects and impurities (such as O, Fe) in the wafers during subsequent processing leading to the development of swirl patterns. Spatial variation of the lifetime degradation follows the point defect interactions and impurity segregation/precipitation. Lifetime can be partially recovered by Phosphorous (P) gettering. The overall effect on the cell performance due to Si interstitial generation, impurity/point defect interactions, and P-gettering is briefly discussed.

600 mV epitaxial crystal silicon solar cells grown on seeded glass

We report progress made at the National Renewable Energy Laboratory (NREL) on crystal silicon solar cells fabricated by epitaxially thickening thin silicon seed layers on glass using hot-wire chemical vapor deposition. Four micron thick devices grown on single-crystal silicon layer transfer seeds on glass achieved open circuit voltages (Voc) over 600 mV and efficiencies over 10%. Other devices were grown on laser crystallized mixed phase solidification (MPS) seeds on glass and e-beam crystallized (EBC) a-Si on SiC coated glass seeds. We discuss the material quality of the various devices on seeds and summarize the prospects for the seed and epitaxy PV approach.

Utilization of Tabula Rasa to stabilize bulk lifetimes in n-Cz silicon for high-performance solar cell processing

We investigate a high temperature, high cooling-rate anneal Tabula Rasa (TR) and report its implications on n-type Czochralski-grown silicon (n-Cz Si) for photovoltaic fabrication. Tabula Rasa aims at dissolving and homogenizing oxygen precipitate nuclei that can grow during the cell process steps and degrade the cell performance due to their high internal gettering and recombination activity. The Tabula Rasa thermal treatment is performed in a clean tube furnace with cooling rates >100°C/s. We characterize the bulk lifetime by Sinton lifetime and photoluminescence mapping just after Tabula Rasa, and after the subsequent cell processing. After TR, the bulk lifetime surprisingly degrades to <; 0.1ms, only to recover to values equal or higher than the initial non-treated wafer (several ms), after typical high temperature cell process steps. Those include boron diffusion and oxidation; phosphorus diffusion/oxidation; ambient annealing at 850°C; and crystallization annealing of tunneling-passivating contacts (doped polycrystalline silicon on 1.5 nm thermal oxide). The drastic lifetime improvement during high temperature cell processing is attributed to improved external gettering of metal impurities and annealing of intrinsic point defects. Time and injection dependent lifetime spectroscopy further reveals the mechanisms of lifetime improvement after Tabula Rasa treatment. Additionally, we report the efficacy of Tabula Rasa on n-type Cz-Si wafers and its dependence on oxygen concentration, correlated to position within the ingot.

Boosting the efficiency of III-V/Si tandem solar cells

We have developed Si-based tandem solar cells with a certified 1-sun efficiency of 29.8% (AM1.5g). The four-terminal tandem devices consist of 1.8 eV rear-heterojunction GaInP top cells and silicon heterojunction bottom cells. The two subcells were fabricated independently in two different labs and merged using an optically transparent, electrically insulating epoxy. Work is ongoing to further improve the performance of each subcell and to push the tandem cell efficiency to > 30%.