Eugene Iwaniczko

Efficient heterojunction solar cells on p-type crystal silicon wafers

Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance.

GROWTH OF EPITAXIAL SILICON AT LOW TEMPERATURES USING HOT-WIRE CHEMICAL VAPOR DEPOSITION

We demonstrate epitaxial silicon growth of 8 A/s at temperatures as low as 195 °C, using hot-wire chemical vapor deposition. Characterization by transmission electron microscopy shows epitaxial layers of Si. We briefly discuss various aspects of the process parameter space. Finally, we consider differences in the chemical kinetics of this process when compared to other epitaxial deposition techniques.

Material quality requirements for efficient epitaxial film silicon solar cells

The performance of 2-μm-thick crystal silicon (c-Si) solar cells grown epitaxially on heavily doped wafer substrates is quantitatively linked to absorber dislocation density. We find that such thin devices have a high tolerance to bulk impurities compared to wafer-based cells. The minority carrier diffusion length is about half the dislocation spacing and must be roughly three times the absorber thickness for efficient carrier extraction. Together, modeling and experimental results provide design guidelines for film c-Si photovoltaic cells.

High-deposition rate a-Si:H n–i–p solar cells grown by HWCVD

Abstract We grow hydrogenated amorphous silicon (a-Si:H) solar cells in a device structure denoted as SS/n–i–p/ITO. We grow all the a-Si:H layers by hot-wire chemical vapor deposition (HWCVD) and the indium-tin-oxide (ITO) by reactive evaporation. We are able to grow HWCVD i-layer materials that maintain an AM1.5 photoconductivity-to-dark-conductivity ratio of 105 at deposition rates up to 130 A/s. We have put these high-deposition rate i-layer materials into SS/n–i–p/ITO devices and light-soaked them for ≥1000 h under AM1.5 conditions. We obtain stabilized solar cell efficiencies of 5.5% at 18 A/s, 4.8% at 35 A/s, 4.1% at 83 A/s and 3.8% at 127 A/s.

Monitoring and modeling silicon homoepitaxy breakdown with real-time spectroscopic ellipsometry

Real-time spectroscopic ellipsometry (RTSE) is used to monitor the breakdown of low-temperature homoepitaxial growth of silicon on silicon wafers in a hot-wire chemical-vapor deposition reactor. We develop and evaluate two optical models to interpret the RTSE data, revealing the progression of epitaxy and its eventual breakdown into amorphous silicon growth. Comparison of the RTSE analysis with cross-sectional transmission electron microscopy, ex situ variable-angle spectroscopic ellipsometry, and Raman spectroscopy measurements shows that RTSE provides accurate and fast quantitative feedback about the progression of epitaxy.

Heteroepitaxial film crystal silicon on Al2O3: new route to inexpensive crystal silicon photovoltaics

Crystal silicon (c-Si) film photovoltaics (PV) fabricated on inexpensive substrates could retain the desirable qualities of silicon wafer PV—including high efficiency and abundant environmentally-benign raw materials—at a fraction of the cost. We report two related advances toward film c-Si PV on inexpensive metal foils. First, we grow heteroepitaxial silicon solar cells on 2 kinds of single-crystal Al2O3 layers from silane gas, using the rapid and scalable hot-wire chemical vapor deposition technique. Second, we fabricate heteroepitaxial c-Si layers on large-grained, cube-textured NiW metal foils coated with Al2O3. In both experiments, the deposition temperature is held below 840 °C, compatible with low fabrication costs. The film c-Si solar cells are fabricated on both single-crystal sapphire wafer substrates and single-crystal γ-Al2O3-buffered SrTiO3 wafer substrates. We achieve ∼400 mV of open-circuit voltage despite crystallographic defects caused by lattice mismatch between the silicon and underlying substrate. With improved epitaxy and defect passivation, it is likely that the voltages can be improved further. On the inexpensive NiW metal foils, we grow MgO and γ-Al2O3 buffer layers before depositing silicon. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirm that the silicon layers are epitaxial and retain the ∼50 μm grain size and biaxial orientation of the foil substrate. With the addition of light-trapping, >15% film c-Si PV on metal foils is achievable.

Effective interfaces in silicon heterojunction solar cells

Thin hydrogenated amorphous silicon (a-Si:H) layers deposited by hot-wire chemical vapor deposition (HWCVD) are investigated for use in silicon heterojunction (SHJ) solar cells on p-type crystalline silicon wafers. A requirement for excellent emitter quality is minimization of interface recombination. Best results necessitate immediate a-Si:H deposition and an abrupt and flat interface to the c-Si substrate. We obtain a record planar HJ efficiency of 16.9% with a high V/sub oc/ of 652 mV on p-type float-zone (FZ) silicon substrates with HWCVD a-Si:H(n) emitters and screen-printed Al-BSF contacts. H pretreatment by HWCVD is beneficial when limited to a very short period prior to emitter deposition.

Amorphous silicon films and solar cells deposited by HWCVD at ultra-high deposition rates

Abstract The deposition conditions for hydrogenated amorphous silicon, deposited by hot wire chemical vapor deposition, are linked to the film structure as we increase deposition rates (Rd) to >100 A/s. At low Rd ( 100 A/s), optimum films are deposited under silane depletion conditions as high as 75–80%, and all structural properties except for the SAXS results once again indicate a compact material. We relate changes in the film electronic structure (Urbach edge) with increasing Rd to the increase in the SAXS signals, and note the invariance of the saturated defect density versus Rd, discussing reasons why these microvoids do not play a role in the Staebler–Wronski effect for these films. Finally, we present device results over the whole range of Rd that we have studied and suggest why, at high Rd, device quality films can be deposited at such high silane depletions.

Development of a hot-wire chemical vapor deposition n-type emitter on p-type crystalline Si-based solar cells

Abstract We have developed a p-type, crystalline Si-based solar cell using hot-wire chemical vapor deposition (HWCVD) n-type microcrystalline Si to form an n-p junction (emitter). The CVD process was rapid and a low substrate temperature was used. The p-type Czochralski (CZ) c-Si wafer has a thickness of 400 μm and has a thermally diffused Al back-field contact. Before forming the n-p junction, the front surface of the p-type c-Si was cleaned using a diluted HF solution to remove the native oxides. The n-type emitter was formed at 220 °C by depositing 50 A a-Si:H and then a 100 A μc-Si n-layer. The total deposition time to form the emitter was less than 1 min. The top contact of the device is a lithograph defined and isolated 1×1 cm 2 and 780 A indium tin oxides (ITO) with metal fingers on top. Our best solar cell conversion efficiency is 13.3% with V oc of 0.58 V, FF of 0.773, and J sc of 29.86 mA cm −2 under one-sun condition. Quantum efficiency (QE) measurement on this solar cell shows over 90% in the region between 540 and 780 nm, but poor response in the blue and deep red. We find that the ITO top contact that acts as an antireflection layer increases the QE in the middle region. To improve the device efficiency further, J sc needs to be increased. Better emitter and light trapping will be developed in future work. The cell shows no degradation after 1000 h of standard light soaking.

Well Passivated a-Si:H Back Contacts for Double-Heterojunction Silicon Solar Cells

We have developed hydrogenated amorphous silicon (a-Si:H) back contacts to both p-and n-type silicon wafers, and employed them in double-heterojunction solar cells. These contacts are deposited entirely at low temperature (<250degC) and replace the standard diffused or alloyed back-surface-field contacts used in single-heterojunction (front-emitter only) cells. High-quality back contacts require excellent surface passivation, indicated by a low surface recombination velocity of minority-carriers (S) or a high open-circuit voltage (Voc). The back contact must also provide good conduction for majority carriers to the external circuit, as indicated by a high light I-V fill factor. We use hot-wire chemical vapor deposition (HWCVD) to grow a-Si:H layers for both the front emitters and back contacts. Our improved a-Si:H back contacts contribute to our recent achievement of a confirmed 18.2% efficiency in double-heterojunction silicon solar cells on p-type textured silicon wafers [1]