Matthew Stocks

A novel low-cost, high-efficiency micromachined silicon solar cell

This letter presents a new process for the fabrication of solar cells and modules from single crystal silicon wafers with substantially reduced silicon consumption and processing effort compared to conventional wafer-based cells. The technique of narrow trench etching in an alkaline solution is used to create a series of thin silicon strips extending vertically through the wafer. By turning the silicon strips on their side, a large increase in surface area is achieved. Individual cells fabricated using the new process have reached efficiencies up to 18.5% while a 575 cm/sup 2/ module incorporating a rear reflector and a cell surface coverage of 50% has displayed an efficiency of 12.3% under standard rating conditions. The technique has the potential to reduce silicon consumption by a factor of 10 compared to standard wafer-based silicon solar cells and, therefore, to dramatically reduce the dependence to the expensive silicon feedstock.

Texturing of polycrystalline silicon

Abstract A wet isotropic etching technique (“tubs”) has been developed for texturing polycrystalline silicon solar cells. Reflection losses are reduced by using total internal reflection from a glass encapsulant layer. The texture developed shows excellent reflection results, equivalent to microgrooves or inverted pyramids on encapsulated single crystal substrates. Light trapping properties are superior to these anisotropic etching techniques. The texturing technique developed is suitable for thin substrates where the superior light trapping properties will be most beneficial.

High minority carrier lifetime in phosphorus-gettered multicrystalline silicon

The electronic quality of multicrystalline material produced by directional solidification has been evaluated by means of photoconductance techniques. Very high minority carrier lifetimes, in the vicinity of 200 μs, have been measured in p-type 1.5 Ω cm material that had received a phosphorus diffusion gettering treatment. The measurements correspond to an effective lifetime averaged over an area of 3 cm2 that includes several grain boundaries and reflects the combined bulk, grain boundary and surface recombination mechanisms. The high lifetime (15 μs) also obtained in low resistivity 0.2 Ω cm wafers has allowed the fabrication of solar cells with an open-circuit voltage of 657 mV (AM1.5 G, 100 mW/cm2, 25 °C), probably the highest ever reported for multicrystalline silicon.

Recombination and trapping in multicrystalline silicon

Minority carrier recombination and trapping frequently coexist in multicrystalline silicon (mc-Si), with the latter effect obscuring both transient and steady-state measurements of the photoconductance. In this paper, the injection dependence of the measured lifetime is studied to gain insight into these physical mechanisms. A theoretical model for minority carrier trapping is shown to explain the anomalous dependence of the apparent lifetime with injection level and allow the evaluation of the density of trapping centers. The main causes for volume recombination in mc-Si, impurities and crystallographic defects, are separately investigated by means of cross-contamination and gettering experiments. Metallic impurities produce a dependence of the bulk minority carrier lifetime with injection level that follows the Shockley-Read-Hall recombination theory. Modeling of this dependence gives information on the fundamental electron and hole lifetimes, with the former typically being considerably smaller than the latter, in p-type silicon, Phosphorus gettering is used to remove most of the impurities and reveal the crystallographic limits on the lifetime, which can reach 600 /spl mu/s for 1.5 /spl Omega/cm mc-Si. Measurements of the lifetime at very high injection levels show evidence of the Auger recombination mechanism in mc-Si. Finally, the surface recombination velocity of the interface between mc-Si and thermally grown SiO/sub 2/ is measured and found to be as low as 70 cm/s for 1.5 /spl Omega/cm material after a forming gas anneal and 40 cm/s after an anneal. These high bulk lifetimes and excellent surface passivation prove that mc-Si can have an electronic quality similar to that of single-crystalline silicon.

Semitransparent Perovskite Solar Cell With Sputtered Front and Rear Electrodes for a Four-Terminal Tandem

A tandem configuration of perovskite and silicon solar cells is a promising way to achieve high-efficiency solar energy conversion at low cost. Four-terminal tandems, in which each cell is connected independently, avoid the need for current matching between the top and bottom cells, giving greater design flexibility. In a four-terminal tandem, the perovskite top cell requires two transparent contacts. Through detailed analysis of electrical and optical power losses, we identify optimum contact parameters and outline directions for the development of future transparent contacts for tandem cells. A semitransparent perovskite cell is fabricated with steady-state efficiency exceeding 12% and broadband near infrared transmittance of >80% using optimized sputtered indium tin oxide front and rear contacts. Our semitransparent cell exhibits much less hysteresis than opaque reference cells. A four-terminal perovskite on silicon tandem efficiency of more than 20% is achieved, and we identify clear pathways to exceed the current single silicon cell record of 25.6%.

Theoretical comparison of conventional and multilayer thin silicon solar cells

Thin solar cells based on low-quality silicon are assessed for a range of possible material parameter values and device structures. Device thickness is freely optimized for maximum efficiency for a range of doping densities and numbers of junctions, le ading to results differing markedly from previous investigations. Modelling of conventional and multilayer structures in this paper indicates little difference in efficiency potential on low-lifetime ( 15%) are possible given adequate light trapping. Conventional structures (single and double junction cells) are superior if excellent light trapping is assumed. Thicker multilayer structures are advantageous in the case of poor light trapping or surface passivation. In an optimized cell in low-quality silicon, increasing the number of junctions allows a high current to be maintained, but at the cost of a reduced voltage and fill factor caused by increased junction recombination. Formidable pra ctical difficulties are likely to be encountered to realize the theoretical performances discussed.

Multicrystalline silicon solar cells with low rear surface recombination

Improvements in the manufacture of multicrystalline silicon (mc-Si) and processing induced impurity gettering have enabled the demonstration of diffusion lengths in mc-Si much greater than the substrate thickness. High recombination velocities at the rear surface, rather than bulk recombination, can then limit cell efficiency. The traditional n+/p/p/sup +/ cell structure (produced with aluminium alloying) is therefore less suitable for high lifetime material due to high effective rear surface recombination velocities. Rear surface recombination can be reduced by reducing the rear metal contact area and passivating most of the rear with thermal oxides. Record open circuit voltages (654 mV) and high efficiencies (18.2%) are demonstrated with 4 cm/sup 2/ cells on 0.5 /spl Omega/cm Eurosil substrates. Cells with local boron diffusions under the rear contacts also demonstrate high efficiencies.

High-efficiency multicrystalline silicon solar cells by liquid phase-epitaxy

Thin-film silicon cells produced on crystalline silicon substrates have the potential to achieve high cell efficiencies at low cost. We have used a modified liquid-phase epitaxy growth process to produce very smooth, high-quality silicon films on multicrystalline silicon substrates. Photoconductivity decay measurements indicate that the minority carrier lifetimes in these layers are at least 10 μs, sufficient to achieve cell efficiencies in excess of 16%. This efficiency potential is confirmed in small area cells, which have displayed efficiencies up to 15.4%. Further improvements up to 17% efficiency are possible in the short term, even without the introduction of any light-trapping schemes into the device structure.

Lift-off of silicon epitaxial layers for solar cell applications

We have developed a technique which allows the fabrication of single crystalline layers of silicon of arbitrary size and shape and with a thickness ranging from less than 50 to greater than 100 /spl mu/m. The films are grown by liquid phase epitaxy (LPE) on single crystal silicon substrates which have been patterned with a suitable masking layer material such as SiO/sub 2/. Detachment of the layers proceeds by etching through the regions where the epitaxial layer is attached to the substrate. In contrast to the technique utilised for the epitaxial lift-off of III-V compounds, this approach does not require an extremely selective etchant which etches a buffer layer while not attacking the epitaxial layer. The substrate can be re-used many times.

Metal-assisted chemical etching for very high aspect ratio grooves in n-type silicon wafers

Metal-assisted chemical etching (MACE) is an inexpensive, simple method for etching silicon structures, including the etching of high aspect ratio grooves. We improve on the best reported results in this area by etching grooves with aspect ratios of 65 (vertical depths 650 µm) in n-type silicon. The grooves maintain an excellent degree of verticality and show minimal width variation. We elucidate some limiting factors and demonstrate the effect of silicon surface roughness on the groove etching.