Andrew Blakers

High-efficiency silicon solar cells

Silicon solar cells are described which operate at energy conversion efficiencies independently measured at 18.7 percent under standard terrestrial test conditions (AM1.5, 100 mW/cm2, 28°C). These are apparently the most efficient silicon cells fabricated to date. The high-efficiency results from a combination of high open-circuit voltage due to the careful attention paid to the passivation of the top surface of the cell; high fill factor due to the high open-circuit voltage and low parasitic resistance losses; and high short-circuit current density due to the use of shallow diffusions, a low grid coverage, and an optimized double layer antireflection coating.

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

Approximately half the cost of a finished crystalline silicon solar module is due to the silicon itself. Combining this fact with a high-efficiency potential makes thin-film crystalline silicon solar cells a growing research area. This paper, written in two parts, aims to outline world-wide research on this topic. The subject has been divided into techniques which use native substrates and techniques which use foreign substrates. Light trapping, vapour- and liquid-phase deposition techniques, cell fabrication and some general considerations are also discussed with reference to thin-film cells.

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.

3-D Simulation of Interdigitated-Back-Contact Silicon Solar Cells With Quokka Including Perimeter Losses

An interdigitated-back-contact (IBC) version of Quokka, a recently developed free and fast solar cell simulation program, is presented. It is capable of simulating IBC unit cells with a variety of interdigitated contact and diffusion patterns in both 2-D and 3-D. The program is evaluated by comparing simulated and experimental current-voltage (I-V) curves of high-efficiency IBC solar cells. The simulations include the perimeter effects of edges and busbars by simulating the inner unit cell in 3-D, and accounting for the edges and busbars by 2-D unit cell approximations. The simulation agrees well with the experiment under 1-sun conditions with different aperture areas. Furthermore, simulations of the inner unit cell are successfully validated against Sentaurus Device, for both the I-V curve and detailed free energy losses at maximum power point. The results evidence the validity of the quasi-neutral and conductive-boundary approximations employed by Quokka for fast simulation of IBC solar cells.

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%.

Sliver solar cells: high efficiency, low cost PV technology

Sliver cells are thin, single-crystal silicon solar cells fabricated using standard fabrication technology. Sliver modules, composed of several thousand individual Sliver cells, can be efficient, low-cost, bifacial, transparent, flexible, shadow tolerant, and lightweight. Compared with current PV technology, mature Sliver technology will need 10% of the pure silicon and fewer than 5% of the wafer starts per MW of factory output. This paper deals with two distinct challenges related to Sliver cell and Sliver module production: providing a mature and robust Sliver cell fabrication method which produces a high yield of highly efficient Sliver cells, and which is suitable for transfer to industry; and, handling, electrically interconnecting, and encapsulating billions of sliver cells at low cost. Sliver cells with efficiencies of 20% have been fabricated at ANU using a reliable, optimised processing sequence, while low-cost encapsulation methods have been demonstrated using a submodule technique.

Investigation of reactive ion etching of dielectrics and Si in CHF3∕O2 or CHF3∕Ar for photovoltaic applications

Using a combination of etch rate, photoconductance, and deep level transient spectroscopy (DLTS) measurements, the authors have investigated the use of reactive ion etching (RIE) of dielectrics and Si in CHF3∕O2 and CHF3∕Ar plasmas for photovoltaic applications. The radio frequency power (rf-power) and gas flow rate dependencies have shown that the addition of either O2 or Ar to CHF3 can be used effectively to change the etch selectivity between SiO2 and Si3N4. The effective carrier lifetime of samples degraded upon exposure to a CHF3-based plasma, reflecting the introduction of recombination centers in the near-surface region. The extent of minority carrier lifetime degradation was similar in both types of plasmas, suggesting that the same defects were responsible for the increased recombination. However, the rf-power dependence of lifetime degradation in n- and p-type Si was different. Moreover, the lifetime degradation did not exhibit a linear rf-power dependence, suggesting that primary defects were n...

Modelling the PERC structure for industrial quality silicon

The passivated emitter and rear cell (PERC) structure has significant efficiency advantages over the conventional 100% metallised back contact structure for industrial quality multicrystalline silicon. For this material the PERC structure also has only a slightly lower efficiency potential than a more complex structure with rear local diffusions. The PERC structure has previously only been modelled for high efficiency applications. In this work, an optimisation of the PERC structure was performed over a range of wafer resistivities and material qualities. It was shown that the PERC structure has a broad optimum in back contact design, allowing flexibility in manufacturing. There was little difference between a stripe structure and a dot structure of the same contact fraction provided the back contact spacing was optimised in each case. It was shown that contact resistance was negligible in PERC cells compared to spreading resistance for optimised back contact spacings. It was also demonstrated that an analytical expression due to Cox and Strack provided a good approximation for spreading resistance in thick PERC cells, but underestimated the spreading resistance in thinner PERC cells.