Xuegong Yu

Grown-in defects in nitrogen-doped Czochralski silicon

Grown-in defects including oxygen precipitates and voids in nitrogen-doped Czochralski (NCZ) silicon have been investigated. It was found that the formation of grown-in oxygen precipitates in NCZ silicon can be divided into two stages. The large precipitates supposed to be enhanced by N2–V2–Ox complexes are generated around 1150 °C, while the small precipitates supposed to be enhanced by NmOn complexes are formed at 750 °C and below. Moreover, it was revealed that the oxygen precipitation behavior in the mixed-type NCZ silicon, which contains vacancy-type and interstitial-type defects distinguished by an OSF-ring in the oxidized wafer, is in sharp contrast to that in the mixed-type Czochralski (CZ) silicon, when subjected to one-step high temperature annealing (1050 °C/32 h) and two-step annealing (800 °C/4 h+1050 °C/16 h). On the other hand, it was found that, compared with CZ silicon, NCZ silicon has much denser crystal originated particles in smaller sizes, which were verified to have been annihilated ...

A 12%-efficient upgraded metallurgical grade silicon-organic heterojunction solar cell achieved by a self-purifying process.

Low-quality silicon such as upgraded metallurgical-grade (UMG) silicon promises to reduce the material requirements for high-performance cost-effective photovoltaics. So far, however, UMG silicon currently exhibits the short diffusion length and serious charge recombination associated with high impurity levels, which hinders the performance of solar cells. Here, we used a metal-assisted chemical etching (MACE) method to partially upgrade the UMG silicon surface. The silicon was etched into a nanostructured one by the MACE process, associated with removing impurities on the surface. Meanwhile, nanostructured forms of UMG silicon can benefit improved light harvesting with thin substrates, which can relax the requirement of material purity for high photovoltaic performance. In order to suppress the large surface recombination due to increased surface area of nanostructured UMG silicon, a post chemical treatment was used to decrease the surface area. A solution-processed conjugated polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was deposited on UMG silicon at low temperature (<150 °C) to form a heterojunction to avoid any impurity diffusion in the silicon substrate. By optimizing the thickness of silicon and suppressing the charge recombination at the interface between thin UMG silicon/PEDOT:PSS, we are able to achieve 12.0%-efficient organic-inorganic hybrid solar cells, which are higher than analogous UMG silicon devices. We show that the modified UMG silicon surface can increase the minority carrier lifetime because of reduced impurity and surface area. Our results suggest a design rule for an efficient silicon solar cell with low-quality silicon absorbers.

High Performance Nanostructured Silicon-Organic Quasi p-n Junction Solar Cells via Low-Temperature Deposited Hole and Electron Selective Layer

UNLABELLED Silicon-organic solar cells based on conjugated polymers such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT PSS) on n-type silicon (n-Si) attract wide interest because of their potential for cost-effectiveness and high-efficiency. However, a lower barrier height (Φb) and a shallow built in potential (Vbi) of Schottky junction between n-Si and PEDOT PSS hinders the power conversion efficiency (PCE) in comparison with those of traditional p-n junction. Here, a strong inversion layer was formed on n-Si surface by inserting a layer of 1, 4, 5, 8, 9, 11-hexaazatriphenylene hexacarbonitrile (HAT-CN), resulting in a quasi p-n junction. External quantum efficiency spectra, capacitance-voltage, transient photovoltage decay and minority charge carriers life mapping measurements indicated that a quasi p-n junction was built due to the strong inversion effect, resulting in a high Φb and Vbi. The quasi p-n junction located on the front surface region of silicon substrates improved the short wavelength light conversion into photocurrent. In addition, a derivative perylene diimide (PDIN) layer between rear side of silicon and aluminum cathodes was used to block the holes from flowing to cathodes. As a result, the device with PDIN layer also improved photoresponse at longer wavelength. A champion PCE of 14.14% was achieved for the nanostructured silicon-organic device by combining HAT-CN and PDIN layers. The low temperature and simple device structure with quasi p-n junction promises cost-effective high performance photovoltaic techniques.

The effect of germanium doping on oxygen donors in Czochralski-grown silicon

In this paper the effect of germanium doping on oxygen donors in Czochralski (CZ) silicon has been investigated. It is found that germanium suppresses the formation of thermal donors during annealing at 450??C, as a result of the reaction of Ge with point defects in CZ silicon. Meanwhile, it is clarified that germanium enhances the formation of new donors in CZ silicon, which is proposed to be a process associated with the nucleation enhancement of oxygen precipitation by germanium doping.

An 8.68% efficiency chemically-doped-free graphene-silicon solar cell using silver nanowires network buried contacts.

Graphene-silicon (Gr-Si) heterojunction solar cells have been recognized as one of the most low-cost candidates in photovoltaics due to its simple fabrication process. However, the high sheet resistance of chemical vapor deposited (CVD) Gr films is still the most important limiting factor for the improvement of the power conversion efficiency of Gr-Si solar cells, especially in the case of large device-active area. In this work, we have fabricated a novel transparent conductive film by hybriding a monolayer Gr film with silver nanowires (AgNWs) network soldered by the graphene oxide (GO) flakes. This Gr-AgNWs hybrid film exhibits low sheet resistance and larger direct-current to optical conductivity ratio, quite suitable for solar cell fabrication. An efficiency of 8.68% has been achieved for the Gr-AgNWs-Si solar cell, in which the AgNWs network acts as buried contacts. Meanwhile, the Gr-AgNWs-Si solar cells have much better stability than the chemically doped Gr-Si solar cells. These results show a new route for the fabrication of high efficient and stable Gr-Si solar cells.

Germanium effect on oxygen precipitation in Czochralski silicon

The oxygen precipitation in germanium-doped Czochralski (GCZ) silicon has been investigated. After a prolonged annealing at 800 and 1000°C, it was found that the Ge-doping enhanced the formation of oxygen precipitates with a higher density and modified their morphology and size. Furthermore, the existing oxygen precipitates in the GCZ silicon were readily dissolved, because the Ge-doping facilitated the formation of smaller oxygen precipitates. Based on the facts, the mechanism for the enhancement effect of Ge-doping on oxygen precipitation has been preliminarily discussed.

High Efficiency Organic/Silicon-Nanowire Hybrid Solar Cells: Significance of Strong Inversion Layer

Organic/silicon nanowires (SiNWs) hybrid solar cells have recently been recognized as one of potentially low-cost candidates for photovoltaic application. Here, we have controllably prepared a series of uniform silicon nanowires (SiNWs) with various diameters on silicon substrate by metal-assisted chemical etching followed by thermal oxidization, and then fabricated the organic/SiNWs hybrid solar cells with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). It is found that the reflective index of SiNWs layer for sunlight depends on the filling ratio of SiNWs. Compared to the SiNWs with the lowest reflectivity (LR-SiNWs), the solar cell based on the SiNWs with low filling ratio (LF-SiNWs) has a higher open-circuit voltage and fill factor. The capacitance-voltage measurements have clarified that the built-in potential barrier at the LF-SiNWs/PEDOT:PSS interface is much larger than that at the LR-SiNWs/PEDOT one, which yields a strong inversion layer generating near the silicon surface. The formation of inversion layer can effectively suppress the carrier recombination, reducing the leakage current of solar cell, and meanwhile transfer the LF-SiNWs/PEDOT:PSS device into a p-n junction. As a result, a highest efficiency of 13.11% is achieved for the LF-SiNWs/PEDOT:PSS solar cell. These results pave a way to the fabrication of high efficiency organic/SiNWs hybrid solar cells.

Formation of pnp bipolar structure by thermal donors in nitrogen-containing p-type Czochralski silicon wafers

The carrier concentration profile in boron-doped p-type nitrogen-containing Czochralski silicon wafer subjected to a one-step high-temperature (1150 °C) annealing followed by a prolonged 450 °C annealing has been investigated by spreading resistance profile. It is found that the carrier concentration profile is characteristic of a pnp bipolar structure, while, that in the control wafer of p-type conventional Czochralki silicon subjected to the identical thermal treatment is just characteristic of a p-n junction. Moreover, it is suggested that only one-step annealing at high temperatures is an efficient method for intrinsic gettering of a nitrogen-containing Czochralski silicon wafer due to the outdiffusion of oxygen and nitrogen in the near-surface region and the nitrogen-enhanced oxygen precipitation in the bulk region.

Intrinsic gettering in germanium-doped Czochralski crystal silicon crystals

Abstract The intrinsic gettering (IG) of germanium-doped Czochralski (GCZ) silicon with different concentrations of germanium has been investigated in this paper. The conventional Czochralski (CZ) and the GCZ silicon samples were annealed using a one-step high temperature process followed by a sequence of low–high temperature annealing cycles. It was found that the good defect-free denude zones in the near surface of the GCZ silicon could be achieved using simply a one-step high temperature annealing process. Furthermore, the density of bulk microdefects as IG sites was higher than that in the CZ silicon, as a result of germanium enhancing oxygen precipitation during three-step annealing. Meanwhile, the experimental results showed that germanium also enhanced the out-diffusion of oxygen. Furthermore, it is believed that germanium doping can increase the ability of IG in CZ silicon wafers.

Enhanced performance and light soaking stability of planar perovskite solar cells using an amine-based fullerene interfacial modifier

Organic–inorganic lead halide perovskite solar cells (PSCs) with TiO2-based architectures have emerged for highly efficient photovoltaic conversion in recent years, while their serious light soaking instability limits their practical applications. Here, we have successfully introduced fullerene [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)ethyl)(methyl)-amino)-ethyl ester (PCBDAN) as an interfacial modifier for the TiO2 electron transport layer (ETL) in planar PSCs, which can significantly improve the photovoltaic conversion efficiency and light soaking stability of the devices. The quality of the perovskite film and electron extraction efficiency between the perovskite and ETL are both improved by introducing the PCBDAN interfacial layer. An improved power conversion efficiency (PCE) of 16.78% can be obtained for the device with PCBDAN under AM 1.5G illumination (100 mW cm−2). And the light soaking stability of the planar device is greatly improved after modification. This work provides a feasible way by interfacial modification for the realization of highly efficient devices without light-soaking degradation.