Tihu Wang

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.

Atomic structure and electronic properties of c-Si∕a-Si:H heterointerfaces

The atomic structure and electronic properties of crystalline-amorphous interfaces in silicon heterojunction solar cells are investigated by high-resolution transmission electron microscopy, atomic-resolution Z-contrast imaging, and electron energy-loss spectroscopy. With these combined techniques, we directly observe abrupt and flat transition from crystalline Si to hydrogenated amorphous Si at the interface of Si heterojunction solar cells. We find that high-quality hydrogenated amorphous Si layers can be grown abruptly by hot-wire chemical vapor deposition on 200°C (100) Si substrates after a two-step pretreatment of the substrate, comprised of exposure to hot-wire decomposed H2-diluted NH3 followed by atomic H etching.

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.

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]

High Open-Circuit Voltage in Silicon Heterojunction Solar Cells

High open-circuit voltage (V{sub oc}) silicon heterojunction (SHJ) solar cells are fabricated in double-heterojunction a-Si:H/c-Si/a-Si:H structures using low temperature (< 225 C) hydrogenated amorphous silicon (a-Si:H) contacts deposited by hot-wire chemical vapor deposition (HWCVD). On p-type c-Si float-zone wafers, we used an amorphous n/i contact to the top surface and an i/p contact to the back surface to obtain a V{sub oc} of 667 mV in a 1 cm{sup 2} cell with an efficiency of 18.2%. This is the best reported p-type SHJ voltage. In our labs, it improves over the 652 mV cell obtained with a front amorphous n/i heterojunction emitter and a high-temperature alloyed Al back-surface-field contact. On n-type c-Si float-zone wafers, we used an a-Si:H (p/i) front emitter and an a-Si:H (i/n) back contact to achieve a V{sub oc} of 691 mV on 1 cm{sup 2} cell. Though not as high as the 730 mV reported by Sanyo on n-wafers, this is the highest reported V{sub oc} for SHJ c-Si cells processed by the HWCVD technique. We found that effective c-Si surface cleaning and a double-heterojunction are keys to obtaining high Voc. Transmission electron microscopy reveals that high V{sub oc} cells require an abrupt interface from c-Simorexa0» to a-Si:H. If the transition from the base wafer to the a-Si:H incorporates either microcrystalline or epitaxial Si at c-Si interface, a low V{sub oc} will result. Lifetime measurement shows that the back-surface-recombination velocity (BSRV) can be reduced to {approx}15 cm/s through a-Si:H passivation. Amorphous silicon heterojunction layers on crystalline wafers thus combine low-surface recombination velocity with excellent carrier extraction.«xa0less

Silicon Heterojunction Solar Cell Characterization and Optimization using in Situ and Ex Situ Spectroscopic Ellipsometry

We use in-situ and ex-situ spectroscopic ellipsometry to characterize the optical, electronic, and structural properties of individual layers and completed silicon heterojunction devices. The combination of in-situ measurements during thin film deposition with ex-situ measurements of completed devices allows us to understand both the growth dynamics of the materials and the effects of each processing step on material properties. In-situ ellipsometry measurements enable us to map out how the optical properties change with deposition conditions, pointing the way towards reducing the absorption loss and increasing device efficiency. We use the measured optical properties and thickness of the i-, n-, and p-layers in optical device modeling to determine how the material properties affect device performance. Our best solar energy conversion efficiencies are 16.9% for a non-textured, single-sided device with an aluminum back surface field contact on a p-type float zone silicon wafer, and 17.8% for a textured double-sided device on a p-type float zone silicon wafer

Materials and Interface Optimization of Heterojunction Silicon (HIT) Solar Cells Using in-situ Real-Time Spectroscopic Ellipsometry

We have applied real-time spectroscopic ellipsometry (RTSE) as both an in-situ diagnostic and post-growth analysis tool for hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction with intrinsic thin-layer (HIT) solar cells grown by hot-wire chemical vapor deposition. RTSE enables precise thickness control of the 5 to 25 nm layers used in these devices, as well as monitoring crystallinity and surface roughness in real time. Utilizing RTSE feedback, but without extensive optimization, we have achieved a photovoltaic energy conversion efficiency of 14.1% on an Al-backed p-type Czochralski c-Si wafer coated with thin i and n layers on the front. Open-circuit voltages above 620 mV indicate effective passivation of the c-Si surface by the a-Si:H intrinsic layer. Lifetime measurements using resonant coupled photoconductive decay indicate that surface recombination velocities can approach 1 cm/s. RTSE and transmission electron microscopy show that the intrinsic a-Si:H i-layers grow as a mixture of amorphous and nano-crystalline silicon.

High-throughput approaches to optimization of crystal silicon surface passivation and heterojunction solar cells

We use a high-throughput (combinatorial) hot-wire chemical vapor deposition system to passivate the crystal silicon surface and to grow heterojunction silicon solar cells. We study the effectiveness of crystal surface treatments by atomic H or/and NH/sub x/ radicals, followed by the growth of thin hydrogenated amorphous silicon (a-Si:H) films. Treatment and layer properties such as times, thicknesses and gas mixtures can be continuously graded, creating a two-dimensional sample with each variable varying in one direction. This results in high-throughput optimization of the processes. Effective carrier lifetime is measured by photoconductive decay to evaluate the effectiveness of the surface passivation by surface treatments. The effective carrier lifetime increases from about 5 /spl mu/s without passivation to about 24 /spl mu/s with an optimized surface treatment and thickness a-Si:H on single-sided c-Si. Transmission electron microscopy reveals that a-Si:H, a mixed phase, or epitaxial growth of thin-film Si depending upon the surface treatment. Improvement in effective carrier lifetime correlates to with an immediate a-Si:H growth on c-Si, rather than a mixed phase and epitaxial Si growth. We have obtained an efficiency of 13.4% on a non-textured single-sided heterojunction solar cell on a p-type CZ-Si processed with optimized surface treatment.