Si solid-state quantum dot-based materials for tandem solar cells
© Conibeer et al; licensee Springer. 2012
Received: 7 October 2010
Accepted: 21 March 2012
Published: 21 March 2012
The concept of third-generation photovoltaics is to significantly increase device efficiencies whilst still using thin-film processes and abundant non-toxic materials. A strong potential approach is to fabricate tandem cells using thin-film deposition that can optimise collection of energy in a series of cells with decreasing band gap stacked on top of each other. Quantum dot materials, in which Si quantum dots (QDs) are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin-film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approximately 2-nm diameter QDs, these result in an effective band gap of 1.8 eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. A modified modulation doping model for the doping mechanisms in these materials is discussed which relies on doping of a sub-oxide region around the Si QDs.
An increase in efficiency can be achieved through the use of multiple energy levels in a third-generation photovoltaic device. One configuration which increases the voltage output from the device is the tandem or multi-junction cell, in which cells with increasing band gap are stacked on top of each other such that each cell absorbs a different part of the solar spectrum with a narrower range and hence more optimally for each absorbed photon. The sum of the output from these cells can boost the overall efficiency. In practice, it is easiest to configure the cells such as to have the same current through each cell in an in-series or current-matched two-terminal device.
Incorporation of P or B during growth results in n- or p-type materials, respectively, and allows growth of rectifying junctions, which have photovoltaic properties. Si QD devices have been demonstrated by some of the current authors with VOC up to 490 mV [4–7]. However, further optimisation of these materials and devices such as to increase VOC and improve the presently very low current densities (tens of microamperes per square centimetre) requires a better understanding of the exact mechanisms of doping.
Fabrication of Si QD nanostructures
Structures are grown using magnetron sputtering on quartz substrates. Alternating layers of a Si-rich dielectric and a stoichiometric layer of dielectric are deposited. Layer thicknesses are kept constant within one structure, although the stoichiometric dielectric may well have a different thickness to the Si-rich layer. Layer thicknesses are varied from sample to sample between 2 and 7 nm. The number of bi-layers is also varied but is typically between 20 and 40. The dielectric used is usually SiO2, although Si3N4 and SiC are also used. This layer growth technique for SiO2 was borrowed from  and adapted for photovoltaics by .
Precipitation of excess Si from Si-rich dielectrics in SiN x and SiC x follows a similar crystallisation reaction as Si precipitates from the amorphous matrix.
Fluctuations in spacing and size of the dots around their mean values can be investigated using similar calculations. Using this approach, it is also found that the calculated Bloch mobilities do not depend strongly on variability in the dot position around a mean position, Δd, but do depend strongly on variation in the dot size within the QD material . This is an important result for engineering a real thin-film structure, because although it is necessary to have a reasonably uniform QD size and to minimise the mean spacing between QDs, d, to give high mobilities for a given matrix, the variation around this mean value, Δd, is less critical. Hence, transport between dots can be significantly increased by using alternative matrices with a lower barrier height, ΔE.
Optical absorption properties of Si nanostructures
This suggests the presence of a weakly absorbing background surrounding the crystalline Si QDs. The small dependence of absorption of this region on the Si content suggests it is a sub-stoichiometric oxide with only a very small amount of O. The identification of this sub-oxide region is important for the possible modulation doping of this region by incorporated P or B atoms, as discussed below in 'Doping mechanisms for Si QD superlattice structures' section, as a theory to explain doping behaviour.
Further evidence for the presence of such a sub-oxide region is given by a study by Daldosso et al. for very similar Si nanocrystals material in SiO2 . In this work, experimental data on X-ray absorption and energy-filtered TEM clearly show the presence of amorphous Si sub-stoichiometric oxide regions around Si nanocrsystals, with these regions extending for at least 1 nm. They further use ab initio methods to model the energy levels associated with an interface between nanocrystals and a stoichiometric SiO2 matrix which has an influence on absorption and light emission properties. This is very consistent with the sub-oxide region proposed around Si nanocrystals in the current work.
Doping and device fabrication for QD nanostructures
Doping in these multilayer QD structures is achieved by incorporation of either phosphorous pentoxide for n-type doping or boron for p-type doping into the Si-rich material by co-sputtering during growth of the appropriate layers. To obtain a p-n junction, several layers (typically 20) doped with P2O5 are followed by several more (again typically 20) doped with B, sometimes with a few undoped layers in-between. The sample is then annealed at approximately 1,100°C to form Si QDs and to activate these dopants. Typically, hydrogenation is then performed in a cold-wall vacuum system at 600-625°C for 15 min.
p-n junction devices, fabricated in this way, exhibit photovoltaic properties . Current-voltage (I-V) measurements in the dark and under 1-sun illumination indicate a good rectifying junction and generation of an open circuit voltage, VOC, up to 492 mV . The high sheet resistance of the deposited layers, in conjunction with the insulating quartz substrates, causes an unavoidable very high series resistance in the devices. The high resistance severely limits both the short circuit current and the fill factor of the cells, particularly under illumination. It also makes it necessary to include effects due to in-plane current flow in the analysis of the measured electrical characteristics .
Doping mechanisms for Si QD superlattice structures
As discussed in "Doping and device fabrication for QD nanostructures" section, there is significant evidence for doping on incorporation of B and P in Si QD nanostructures from resistivity data and from the formation of rectifying p-n junctions which behave as photovoltaic devices. However, the mechanisms for this doping are not clear. Direct doping of the QDs by impurity atoms is extremely unlikely as the perfectly crystalline nanocrystals segregate any impurity atoms during growth, as shown in ab initio calculations [16, 17]. Experimentally, this is supported by data on the free electron density in Si nanocrystals using electron paramagnetic resonance (EPR)  which show that at least 95% of P atoms are inactive and segregated to the surface of Si nanocrystals, such that their contribution to doping is at least an order of magnitude lower than their atomic concentration.
Another possibility for doping is that the matrix material, rather than the QDs, is doped by the impurity atoms and that this then provides free carriers which are captured by the QDs. This 'modulation doping' mechanism is commonly used in III-V nanostructures. But in III-Vs, the barrier height is typically only a few 100 meV and hence ionisation of dopant atoms is relatively easy . In the dielectric matrices used in the Si QD work (particularly for the SiO2 barriers), the barrier height is several electronvolts. Under these circumstances, there would be no ionisation of dopant atoms at room temperature and hence modulation doping could only occur by direct tunnelling of carriers from dopants in SiO2 less than 1 nm from the QDs.
This explanation remains a hypothesis at present. More evidence on the exact positioning of dopant atoms is required from techniques such as atom probe tomography together with direct evidence of doping from EPR.
Quantum confinement in Si QD nanostructures is proposed to engineer an increase in the effective band gap and hence produce materials suitable for upper cell elements in a thin-film tandem cell. Si QD materials grown by sputtering alternating Si-rich oxide and stoichiometric SiO2 layers, with a subsequent anneal to precipitate a layered growth of QDs, have been fabricated in previous work. Absorption data on these show a strong absorption region with absorption dependent on Si content and a significant weak absorption tail attributed to sub-oxide regions surrounding QDs due to incomplete diffusion to the growing QDs. Dramatic decreases in resistivity have been demonstrated on incorporation of P or B dopants, and rectifying p-n junctions have been fabricated which give a VOC up to 490 mV. However, the mechanism of doping in these materials can neither be direct doping of the QDs nor modulation doping of a stoichiometric SiO2 matrix. A modified modulation doping mechanism is proposed in which P or B atoms dope the sub-oxide region shown to surround QDs, which because of its reduced band gap has dopant levels shallow enough to be ionised and hence provide free carriers to the QDs.
This work is supported by the Australian Research Council (ARC) via its Centres of Excellence scheme. The authors also acknowledge the support of the Global Climate and Energy Project (GCEP), administered by Stanford University, for helping to fund this work. The authors also thank all members of the Third Generation Group at the ARC Photovoltaics Centre of Excellence for their contributions to this project.
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