Impacts of Post-metallisation Processes on the Electrical and Photovoltaic Properties of Si Quantum Dot Solar Cells
© The Author(s) 2010
Received: 14 June 2010
Accepted: 15 July 2010
Published: 1 August 2010
As an important step towards the realisation of silicon-based tandem solar cells using silicon quantum dots embedded in a silicon dioxide (SiO2) matrix, single-junction silicon quantum dot (Si QD) solar cells on quartz substrates have been fabricated. The total thickness of the solar cell material is 420 nm. The cells contain 4 nm diameter Si quantum dots. The impacts of post-metallisation treatments such as phosphoric acid (H3PO4) etching, nitrogen (N2) gas anneal and forming gas (Ar: H2) anneal on the cells’ electrical and photovoltaic properties are investigated. The Si QD solar cells studied in this work have achieved an open circuit voltage of 410 mV after various processes. Parameters extracted from dark I–V, light I–V and circular transfer length measurement (CTLM) suggest limiting mechanism in the Si QD solar cell operation and possible approaches for further improvement.
The concept of a tandem solar cell has been well developed as a method of improving solar cell efficiency. In a tandem cell, solar cells of different band gaps are stacked on top of one another. The cell with the highest band gap is placed on the top, while the cell with the lowest band gap is positioned at the bottom of the tandem stack. Each cell absorbs the light it can most effectively convert, with the rest passing through to the underlying cells . The highest efficiency cells to date are tandem cells made using single crystal III-V materials. These materials are grown by very expensive epitaxial techniques.
As an encouraging step towards the realisation of silicon-based tandem solar cells using silicon quantum dots embedded in a silicon dioxide (SiO2) matrix, single-junction silicon quantum dot (Si QD) solar cells on quartz substrates have been fabricated.
We also demonstrate that post-metallisation treatments such as phosphoric acid (H3PO4) etching and forming gas (Ar: H2) anneal significantly impact solar cell performance. So far, our best single-junction Si QD solar cell has achieved 490 mV Voc[6, 7] (In this paper, samples with Voc up to 410 mV are studied). Our medium-term goal is to demonstrate Voc over 700 mV on single-junction Si QD solar cells. As this would be close to the V oc record  of single-junction mono-crystalline silicon solar cells, in a thin film solar cell it would be a clear demonstration that the electronic band gap of the nanostructured material is enhanced due to the quantum confinement effect. At present, the emphasis is on increasing Voc and the devices are very unoptimised for absorption and collection. Hence, the very low currents currently obtained are not a concern.
Fabrication of Single-Junction Silicon Quantum Dot Solar Cell on Quartz Substrate
Alternating layers of a 2-nm silicon dioxide (SiO2) followed by a 4-nm silicon-rich oxide (SRO) are deposited on a quartz substrate using magnetron co-sputtering of Si and quartz (SiO2) targets . Either a phosphorous pentoxide for n-type doping or boron for p-type doping is incorporated into the Si-rich material during sputtering of appropriate layers, to obtain a p–n junction after annealing. The sample is then annealed at ~1100°C to form Si QDs and to activate these dopants. Hydrogenation was then performed in a cold-wall vacuum system featuring an inductively coupled remote plasma source (Advanced Energy), using a glass substrate temperature of 600–625°C for 15 min [10, 11].
Removal of localised Aluminium shunts
Effects of Nitrogen Gas Anneal and Forming Gas Anneal
Dark and Illuminated I–V Characteristics
Another sample metallised with the aligned photo-lithography method was subjected to an initial N2 anneal at 250°C followed by three consecutive forming gas anneals (250, 300 and 350°C). The duration of each annealing step was 20 min. Dark and illuminated (1-sun) I–V data were measured before and after each annealing step.
It has been noted that a N2 gas anneal at 250°C has a very limited influence on the I–V characteristics, while a forming gas anneal at the same temperature is able to alter the electrical properties (Fig. 6). With increasing forming gas annealing temperature, there is a clear change in both dark and light I–V curves. Information about the parasitic resistances (Rs and Rsh) is extracted from the dark I–V curve. Voc and Isc are obtained from the light I–V data. Details about the calculation of Rs are discussed in later sections.
Contact and Sheet Resistances
It can be seen from the data that the 250°C N2 gas anneal has a negative impact on the cell’s contact resistance, while the forming gas anneals improve the contact. The change of sheet resistance is negligibly small when annealed in N2 ambient. However, annealing in forming gas is able to reduce Rsheet by approximately three times. The contact resistance is small in comparison with the semiconductor sheet resistance, as shown in Fig. 8. Therefore, the reduction of series resistance is largely due to the reduction of the material’s resistivity.
The implication of the results is that the H2 in the forming gas is responsible for the improvement of the cell material. Hydrogen atoms are able to passivate the interfaces of the Si nanocrystals  and hence to reduce trap density and facilitate better carrier transport.
Extraction of Series Resistance and Apparent Ideality Factor (n)
Special attention has been paid to the analysis of the series resistance (Rs) of the cell. Instead of simply calculating the slope of the dark I–V curve at the high voltage region, Rs is obtained according to the following.
To obtain Rs from the dark I V data, it is convenient to plot dV/ dI against 1/I (See Eq. (5)). The plot appears to be a linear relationship. The intercept of the line with the y-axis gives Rs (Rs results are shown in Fig. 6b), while the slope of the line equals to nkT/q. Thus, the ideality factor n = slope/VT, where VT = kT/q is the thermal voltage.
In this work, we have fabricated single-junction Si QD solar cells on quartz substrates, as an important step to realise an ‘all-silicon’ tandem solar cell.
The impacts of post-metallisation treatments such as phosphoric acid (H3PO4) etching, nitrogen (N2) gas anneal and forming gas (Ar: H2) anneal on the cells’ electrical and photovoltaic properties have been studied. The Si QD solar cells investigated in this work have achieved an open circuit voltage of 410 mV after various processes.
Parameters extracted from dark I–V, light I–V and circular transfer length measurement (CTLM) suggest that the performance of the solar cell is strongly limited by poor carrier transport. This limiting factor can be partly eliminated by forming gas annealing.
Other possible solutions include reduction of the barrier height and thickness of the quantum mechanical tunnelling barrier, modification of the composition of the cell’s absorber material, improved Si QD growth, an improved device structure such asusing a transparent conducting contact (e.g. ITO) or a conductive substrate to avoid current crowding.
The authors gratefully thank all members of the Third Generation Group at the ARC Photovoltaics Centre of Excellence for their contributions to this project. This work is supported by the Australian Research Council (ARC) via its Centers 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.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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