Peiting Zheng

Magnesium Fluoride Electron-Selective Contacts for Crystalline Silicon Solar Cells

In this study, we present a novel application of thin magnesium fluoride films to form electron-selective contacts to n-type crystalline silicon (c-Si). This allows the demonstration of a 20.1%-efficient c-Si solar cell. The electron-selective contact is composed of deposited layers of amorphous silicon (∼6.5 nm), magnesium fluoride (∼1 nm), and aluminum (∼300 nm). X-ray photoelectron spectroscopy reveals a work function of 3.5 eV at the MgF2/Al interface, significantly lower than that of aluminum itself (∼4.2 eV), enabling an Ohmic contact between the aluminum electrode and n-type c-Si. The optimized contact structure exhibits a contact resistivity of ∼76 mΩ·cm(2), sufficiently low for a full-area contact to solar cells, together with a very low contact recombination current density of ∼10 fA/cm(2). We demonstrate that electrodes functionalized with thin magnesium fluoride films significantly improve the performance of silicon solar cells. The novel contacts can potentially be implemented also in organic optoelectronic devices, including photovoltaics, thin film transistors, or light emitting diodes.

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%.

Upgraded metallurgical-grade silicon solar cells with efficiency above 20%

We present solar cells fabricated with n-type Czochralski–silicon wafers grown with strongly compensated 100% upgraded metallurgical-grade feedstock, with efficiencies above 20%. The cells have a passivated boron-diffused front surface, and a rear locally phosphorus-diffused structure fabricated using an etch-back process. The local heavy phosphorus diffusion on the rear helps to maintain a high bulk lifetime in the substrates via phosphorus gettering, whilst also reducing recombination under the rear-side metal contacts. The independently measured results yield a peak efficiency of 20.9% for the best upgraded metallurgical-grade silicon cell and 21.9% for a control device made with electronic-grade float-zone silicon. The presence of boron-oxygen related defects in the cells is also investigated, and we confirm that these defects can be partially deactivated permanently by annealing under illumination.

A Contactless Method for Determining the Carrier Mobility Sum in Silicon Wafers

In this paper, we present a new method to determine the simultaneous injection and temperature dependence of the sum of the majority and minority carrier mobilities in silicon wafers. The technique is based on combining transient and quasi-steady-state photoconductance measurements. It does not require a full device structure or contacting but only adequate surface passivation. The mobility dependence on both carrier injection level and temperature, as measured on several test samples, is discussed and compared with well-known mobility models. The potential of this method to measure the impact of dopant concentration, compensation ratio, injection level, and temperature on the mobility is demonstrated.

Evidence for Vacancy-Related Recombination Active Defects in as-Grown N-Type Czochralski Silicon

A recombination of active defect in very high lifetime Czochralski grown n-type silicon wafers, which can be thermally deactivated at 150 °C, is described. In addition, the existence of a recently measured defect, which is deactivated at 350 °C, is confirmed. Both defects are found to significantly degrade the lifetime of millisecond-range Czochralski-grown n-type silicon wafers: a material widely used for high-efficiency solar cells. The observed deactivation temperature suggests that it may be caused by vacancy-phosphorus pairs. The deactivation temperature of the second defect is consistent with the presence of vacancy-oxygen (V-O) pairs.

21.1% UMG Silicon Solar Cells

We present n-type Czochralski-grown silicon solar cells made from 100% upgraded metallurgical grade silicon feedstock, with an independently certified peak efficiency of 21.1%. We look at the impact of net doping and minority carrier lifetime and mobility on the short-circuit current and the open-circuit voltage.

>20.5% Diamond Wire Sawn Multicrystalline Silicon Solar Cells With Maskless Inverted Pyramid Like Texturing

In this paper, we demonstrate novel texturing technologies developed for diamond wire sawn multicrystalline silicon wafers. SEM analysis shows that inverted pyramid like texture is produced on the wafer surface by a combination of reactive ion etching and metal-assisted chemical etching technologies. The inverted pyramid like textured surface shows remarkable antireflection performance over the other two types of texturing technologies. Screen-printed passivated emitter rear contact cells are fabricated to illustrate the effect of different texturing on cell performance. The cells with inverted pyramid like texture show the highest batch average efficiency of 20.51%, with the best efficiency of 20.69%. Detailed performance parameters and quantum efficiency for each type of cells are shown in the paper. Quantum efficiency simulations are also performed based on the reflectance and control parameters measured during the cell fabrication process. In addition, light-induced degradation of the cells is also analyzed.

Recombination sources in p-type high performance multicrystalline silicon

This paper presents a comprehensive assessment of the electronic properties of an industrially grown p-type high performance multicrystalline silicon ingot. Wafers from different positions of the ingot are analysed in terms of their material quality before and after phosphorus diffusion and hydrogenation, as well as their final cell performance. In addition to lifetime measurements, we apply a recently developed technique for imaging the recombination velocity of structural defects. Our results show that phosphorus gettering benefits the intra-grain regions but also activates the grain boundaries, resulting in a reduction in the average lifetimes. Hydrogenation can significantly improve the overall lifetimes, predominantly due to its ability to passivate grain boundaries. Dislocation clusters remain strongly recombination active after all processes. It is found that the final cell efficiency coincides with the varying material quality along the ingot. Wafers toward the ingot top are more influenced by carrier recombination at dislocation clusters, whereas wafers near the bottom are more affected by a combination of their lower intra-grain lifetimes and a greater density of recombination active grain boundaries.

Accuracy of Interstitial Iron Measurements on P-Type Multicrystalline Silicon Blocks by Quasi-Steady-State Photoconductance

A detailed knowledge of the distributions of carrier lifetimes, impurities, and crystal defects in silicon ingots is key for understanding and improving wafer quality, as well as solar cell processing steps. In this work, we have validated the use of the quasi-steady-state photoconductance method on p-type multicrystalline silicon blocks to determine the interstitial iron concentration. The extracted iron concentrations along a silicon block were compared with the interstitial iron concentrations measured on wafers from different heights of an adjacent block. The lifetime measurements were performed on the block before and after flashing to break the iron–boron pairs. The impact of nonuniform carrier profiles during the block measurements on the extraction of the Fe profiles is discussed and quantified based on simulations of the quasi-steady-state measurement conditions. The simulation results reveal a slight error in the extracted interstitial iron concentration along the block. However, this error is generally less than 20% for iron concentrations below 1011 cm–3, which is typical in the central region of an ingot, and in any case, can be corrected for based on the modeling results.

Low resistance TiO 2 -passivated calcium contacts to for crystalline silicon solar cells

It has recently been shown that low resistance Ohmic contact to lightly doped n-type crystalline silicon (c-Si) is possible by direct metallization via a thin layer of the low work function metal calcium (φ ~2.9 eV) and an overlying aluminium capping layer. Using this approach upper limit contact resistivities of <; 2 mΩcm2 can be realised on undiffused n-type surfaces. However, recombination at the Ca / Si interface limits the application of the Ca contact to very low contact fractions which leads to non-negligible resistive losses and an increase in device fabrication complexity. Here we show that the low resistance Ohmic contact of the Ca / Al structure is retained after the addition of a TiO2 interlayer, leading the way to the development of a passivated contact device utilizing TiO2 and Ca.