Paula C.P. Bronsveld

Diode breakdown related to recombination active defects in block-cast multicrystalline silicon solar cells

Solar cells in modules are reverse biased when they are shaded. This can lead to diode breakdown and eventually to the occurrence of hot spots, which may, in the extreme case, destroy the module by thermal degradation. We observed at least three different types of diode breakdown in multicrystalline silicon solar cells. One of them is found to be related to the recombination activity of defects. This type is indicated by a slow increase in the reverse current with reverse bias and a relatively low breakdown voltage around −10 V. The local breakdown voltage depends significantly on the level of contamination of the material. When the solar cell is reverse biased, the breakdown sites emit bright light which shows a broad spectral distribution in the visible range with a maximum at 700 nm.

Metal–Insulator–Semiconductor Nanowire Network Solar Cells

Metal-insulator-semiconductor (MIS) junctions provide the charge separating properties of Schottky junctions while circumventing the direct and detrimental contact of the metal with the semiconductor. A passivating and tunnel dielectric is used as a separation layer to reduce carrier recombination and remove Fermi level pinning. When applied to solar cells, these junctions result in two main advantages over traditional p-n-junction solar cells: a highly simplified fabrication process and excellent passivation properties and hence high open-circuit voltages. However, one major drawback of metal-insulator-semiconductor solar cells is that a continuous metal layer is needed to form a junction at the surface of the silicon, which decreases the optical transmittance and hence short-circuit current density. The decrease of transmittance with increasing metal coverage, however, can be overcome by nanoscale structures. Nanowire networks exhibit precisely the properties that are required for MIS solar cells: closely spaced and conductive metal wires to induce an inversion layer for homogeneous charge carrier extraction and simultaneously a high optical transparency. We experimentally demonstrate the nanowire MIS concept by using it to make silicon solar cells with a measured energy conversion efficiency of 7% (∼11% after correction), an effective open-circuit voltage (Voc) of 560 mV and estimated short-circuit current density (Jsc) of 33 mA/cm(2). Furthermore, we show that the metal nanowire network can serve additionally as an etch mask to pattern inverted nanopyramids, decreasing the reflectivity substantially from 36% to ∼4%. Our extensive analysis points out a path toward nanowire based MIS solar cells that exhibit both high Voc and Jsc values.

Crystalline silicon interconnected strips (XIS): Introduction to a new, integrated device and module concept

A new device concept for high efficiency, low cost, wafer based silicon solar cells is introduced. To significantly lower the costs of Si photovoltaics, high efficiencies and large reductions of metals and silicon costs are required. To enable this, the device architecture was adapted into low current devices by applying thin silicon strips, to which a special high efficiency back-contact heterojunction cell design was applied. Standard industrial production processes can be used for our fully integrated cell and module design, with a cost reduction potential below 0.5 €/Wp. First devices have been realized demonstrating the principle of a series connected back contact hybrid silicon heterojunction module concept.

XIS: A low-current, high-voltage back-junction back-contact photovoltaic device

In this paper we present a low-current, highvoltage back-junction back-contact integrated photovoltaic concept and experimental results of such a device, consisting of strip cells: narrow solar cells instead of conventional square cells. The concept is demonstrated by the successful transformation of a completely finished IBC cell into a XIS (Crystalline Silicon Interconnected Strips) device, leading to a Voc of 8.5 V for a series connection of 14 strip cells. For cell separation, different grooving methods were evaluated with respect to their effect on the quality of the groove surface. The effect of the surface passivation in the grooves, which is regarded as a critical parameter, on the XIS device was simulated to gain a better understanding of the processing requirements.

Highly conductive Ag nanowire hybrid electrodes improve silicon heterojunction solar cells

Replacing ITO with a hybrid nanowire-based electrode is shown to enhance efficiency in front-contacted silicon heterojunction (SHJ) solar cells. Electrode performance was tested on practical scale (4.0 cm2) planar SHJ cells using substrate conformal imprint lithography (SCIL) for nanowire fabrication. The NW hybrid electrodes exhibit anomalous transmission and a 10-fold improvement in sheet conductance relative to the standard ITO, enabling larger finger spacings without compromising fill factor. By replacing >70% of ITO with transparent SiNx we simultaneously reduce parasitic optical absorption and reflection losses. Combined, these effects increase the short circuit current density by 1.4 mA cm-2, yielding an absolute increase in cell efficiency of 2.2%.

Moly-poly solar cell: Industrial application of metal-oxide passivating contacts with a starting efficiency of 18.1%

We present large-area “moly-poly” cells, with a front side MoOx/a-Si:H(i) passivating contact and a rear-side poly-Si/SiOx stack, and we have demonstrated that MoOx based c-Si solar cell technology can be scaled to industrial wafer size. Excellent surface passivation was achieved using MoOx and poly-Si, leading to implied Voc values above 700 mV, and a final cell Voc of 687 mV. However, some care needs to be taken to avoid parasitic optical losses in the infra-red (IR) spectral range due to free-carrier absorption (FCA). These losses were investigated by comparing poly-Si layers of different thicknesses, deposited by low-pressure or plasma-enhanced chemical vapor deposition (LPCVD or PECVD), at the rear side of moly-poly cells. We found that ultra-thin PECVD layers are most suitable for solar cell applications due to a very good trade-off between surface passivation and reduced FCA. Based on this result, a 18.1% efficient 9.2×9.2 cm2 molypoly cell was made, which is the highest reported efficiency so far for moly-poly cells. Finally, we present a preliminary study of the parasitic IR losses in the MoOx layer itself, when deposited on either a-Si:H or SiOx passivation layers

Electron beam evaporated molybdenum oxide as hole-selective contact in 6-inch c-Si heterojunction solar cells

Electron beam (E-beam) deposited molybdenum oxide (MoOx) has been investigated for its potential to replace p-type hydrogenated amorphous silicon (a-Si:H) in Si heterojunction (SHJ) solar cells. Excellent passivation was achieved for our best MoOx/c-Si junction based device, reaching an average implied Voc (iVoc) of 734 mV on textured, commercially available 6-inch Cz wafers. This confirms the compatibility of MoOx as a hole selective layer with industrial SHJ cell processing. A hole barrier was, however, observed for our MoOx-based solar cells due to inefficient hole extraction. The formation of this hole barrier can be related to annealing of MoOx and the presence of a native oxide grown on the intrinsic a-Si:H interface layer below. Pre-annealing, followed by an HF treatment on the a-Si:H(i) layer prior to MoOx deposition, proved to be useful to mitigate the formed barrier, while making it more stable under standard SHJ annealing conditions.

Controlling P and B diffusion during polysilicon formation

High quality passivating contacts can be realized by using the combination of a thin interfacial oxide (SiOx) and doped polysilicon (polySi). Recombination losses are minimized by providing very good passivation between the thin hydrogenated oxide and the cSi, a high field effect by the highly doped polySi [1-2], combined with the low level penetration of dopants in the wafer [2-3]. To realize this low level in-diffusion of dopants, several interacting options are evaluated in this work: the quality of the thin oxide layer (growth method), combined with a diffusion blocking method (nitridation), doping concentration levels in the polySi and temperature of diffusion. It is shown that for Phosphorus (P)-doped polySi, in-diffusion can be reduced by adding an i-layer in between the oxide and the highly doped polySi, lowering the overall doping level in the system slightly. For Boron (B)-doped polySi, in-diffusion can be blocked by nitridation of the SiO2 layer