Hao-Chih Yuan

Efficient black silicon solar cell with a density-graded nanoporous surface: Optical properties, performance limitations, and design rules

We study optical effects and factors limiting performance of our confirmed 16.8% efficiency “black silicon” solar cells. The cells incorporate density-graded nanoporous surface layers made by a one-step nanoparticle-catalyzed etch and reflect less than 3% of the solar spectrum, with no conventional antireflection coating. The cells are limited by recombination in the nanoporous layer which decreases short-wavelength spectral response. The optimum density-graded layer depth is then a compromise between reflectance reduction and recombination loss. Finally, we propose universal design rules for high-efficiency solar cells based on density-graded surfaces.

Multi-scale surface texture to improve blue response of nanoporous black silicon solar cells

We characterize the optical and carrier-collection physics of multi-scale textured p-type black Si solar cells with conversion efficiency of 17.1%. The multi-scale texture is achieved by combining density-graded nanoporous layer made by metal-assisted etching with micron-scale pyramid texture. We found that (1) reducing the thickness of nanostructured Si layer improves the short-wavelength spectral response and (2) multi-scale texture permits thinning of the nanostructured layer while maintaining low surface reflection. We have reduced the nanostructured layer thickness by 60% while retaining a solar-spectrum-averaged black Si reflectance of less than 2%. Spectral response at 450 nm has improved from 57% to 71%.

Nanoporous black silicon photocathode for H2 production by photoelectrochemical water splitting

Nanostructured Si eliminates several critical problems with Si photocathodes and dramatically improves a photoelectrochemical (PEC) reaction important to water-splitting. Our nanostructured black Si photocathodes improve the H2 production by providing (1) near-ideal anti-reflection that enables the absorption of most incident light and its conversion to photogenerated electrons and (2) extremely high surface area in direct contact with water that reduces the overpotential needed for the PEC hydrogen half-reaction. Application of these advances would significantly improve the solar H2 conversion efficiency of an ideal tandem PEC system. Finally, the nanostructured Si surface facilitates bubble evolution and therefore reduces the need for surfactants in the electrolyte.

Crystal silicon heterojunction solar cells by hot-wire CVD

Hot-wire chemical vapor deposition (HWCVD) is a promising technique for fabricating Silicon heterojunction (SHJ) solar cells. In this paper we describe our efforts to increase the open circuit voltage (Voc) while improving the efficiency of these devices. On p-type c-Si float-zone wafers, we used a double heterojunction structure with an amorphous n/i contact to the top surface and an i/p contact to the back surface to obtain an open circuit voltage (Voc) of 679 mV in a 0.9 cm2 cell with an independently confirmed efficiency of 19.1%. This is the best reported performance for a cell of this configuration. We also made progress on p-type CZ wafers and achieved 18.7% independently confirmed efficiency with little degradation under prolong illumination. Our best Voc for a p-type SHJ cell is 0.688 V, which is close to the 691 mV we achieved for SHJ cells on n-type c-Si wafers.

Nanoimprinting for diffractive light trapping in solar cells

The authors investigate the light-trapping efficiency of nanoimprinted ceramic grating reflectors for crystal silicon photovoltaic cells. Using 25 μm silicon wafers as a model system and hemispherical reflection measurements, they demonstrate a 4%–6% increase in AM 1.5 solar-photon absorption for one-dimensional square and sinusoidal gratings compared to flat reflectors. The extrapolated increase in a short-circuit current for a 2 μm thick silicon film cell due to diffractive light trapping is 20%.The authors investigate the light-trapping efficiency of nanoimprinted ceramic grating reflectors for crystal silicon photovoltaic cells. Using 25 μm silicon wafers as a model system and hemispherical reflection measurements, they demonstrate a 4%–6% increase in AM 1.5 solar-photon absorption for one-dimensional square and sinusoidal gratings compared to flat reflectors. The extrapolated increase in a short-circuit current for a 2 μm thick silicon film cell due to diffractive light trapping is 20%.

Hydrogenated amorphous si deposition for high efficiency a-Si/c-Si heterojunction solar cells

We study the differences in hydrogenated amorphous Si (a-Si:H) depositions between Hot-Wire Chemical Vapor Deposition (HWCVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD) for high efficiency a-Si/c-Si heterojunction (HJ) solar cells. In HWCVD, process gases such as silane decompose from the high-temperature hot filament. The resulting deposition is thought to be gentle due to the lack of ion bombardment that may cause damage to c-Si surface. In PECVD, process gases decompose from a high frequency electric field and ion bombardment is expected during the a-Si:H deposition. We found that the initial minority carrier lifetime of a-Si:H passivated high-quality n-type wafer was higher (about a ms) with the HWCVD process, and the final minority carrier lifetime (after 250°C annealing) was higher (over a few ms) with the PECVD process. These findings suggest that the damage from the ion bombarding in PECVD is not as detrimental as we expected; or if there is damage, it can be repaired by the annealing. We also speculate that the lack of further increase of the lifetime after annealing with HWCVD intrinsic a-Si:H layer can be related to the direct substrate heating from the hot filament during the deposition. A high substrate temperature will promote epi-Si growth and drive hydrogen out of the a-Si/c-Si interface to decrease the quality of surface passivation. To reduce the heating effect, a shutter and a low filament temperature are preferred. With the optimized process, we were able to fabricate HJ solar cells with high open circuit voltage of 714 mV and efficiency greater than 19% on an un-textured n-type wafer using the PECVD process, and independently confirm best efficiency of 19.7% on textured n-type wafer with the HWCVD process.

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.

Microstructure and surface chemistry of nanoporous “black silicon” for photovoltaics

We report our detailed investigation of the microstructure and surface chemistry of nanoporous black Si layers using transmission electron microscopy techniques. We find that the one-step nanoparticle-catalyzed liquid etch creates deep conical nanovoids. The cones provide the density-graded surface that suppresses reflection. The surface of the as-etched nanoporous black Si is an amorphous Si suboxide (SiOx) produced by the strongly oxidizing nanocatalyzed etch. The oxygen concentration decreases monotonically away from the nanovoid surface. This suboxide tends to be thinner near the cone tip than nearer to the wafer surface. The c-Si/suboxide interface is rough at the nanometer scale. Diffraction contrast reveals a high density of point defects in the c-Si near the c-Si/suboxide interface. These features account for the poor blue response of as-etched black Si solar cells. The passivation treatment is seen to convert the Si suboxide quite completely into SiO2 with a smooth c-Si/SiO2 interface. These changes are essential to achieve our 16.8%-efficient solar cells.

Efficient black silicon solar cells with nanoporous anti-reflection made in a single-step liquid etch

We fabricated black silicon solar cells with conversion efficiency of 16.8% on p-type single crystal Si wafers with a conventional diffused emitter and Al back-surface field (BSF). We replaced the anti-reflection coating step with a single 3-minute ‘black-silicon’ etch of the bare wafer before processing. The nanoporous black-silicon layer, about 300-nm thick is produced in a 3-minute single-step liquid etch based upon catalysis by Au nano-particles formed in a solution containing HF and H2O2. Solar cell reflectance is well below 5% at incident wavelengths from 350 to 1000 nm. We present reflectance versus time data during this simple single-step etching. We also characterize cell performance and find that recombination in the black silicon surface layer must still be reduced.

Photoconductive decay lifetime and Suns-V oc diagnostics of efficient heterojunction solar cells

Minority carrier lifetime and Suns-Voc measurements are well-accepted methods for characterization of solar cell devices. We use these methods, with an instrument from Sinton Consulting, as we fabricate and optimize state-of-the-art all hot-wire chemical vapor deposition (HWCVD) silicon heterojunction (SHJ) devices. For double-sided SHJ devices, lifetime measurements were performed immediately after hydrogenated amorphous silicon (a-Si:H) deposition of the front emitter and back base contacts on a Silicon wafer, and also after indium tin oxide (ITO) deposition of transparent conducting oxide contacts. We report results of minority carrier lifetime measurements for double-sided p-type Si heterojunction devices and compare Suns-Voc results to Light I–V measurements on 1-cm2 solar cell devices measured on an AM1.5 calibrated XT-10 solar simulator.