| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.hl.TZ2.20 |
| Level | HL | Paper | 3 | Time zone | TZ2 |
| Command term | Sketch | Question number | 20 | Adapted from | N/A |
Question
Semiconductors and light-sensitive dyes are used in photovoltaic cells.
Sketch graphs to show the general effect of increasing temperature on the electrical conductivity of semiconductors and metals on the axes below.
Explain the function of dyes in a dye-sensitized solar cell (DSSC).
Markscheme
Semiconductors:
increases [✔]
Metals:
decreases [✔]
Note: Accept any graph showing general increase for semiconductor.
Accept any graph showing general decrease for metal.
Accept a graph showing vertical section below transition temperature for a superconducting metal.
dye absorbs light [✔]
electrons from «excited» dye pass to TiO2/semiconductor/electrolyte/cell
OR
dye undergoes photo-oxidation [✔]
Examiners report
Several candidates managed one mark to show conductivity of semiconductors on increasing the temperature but were unable to show that generally conductivity decreases for metals when the temperature is increased.
The question on dye-sensitized solar cell invited mixed responses. While most candidates correctly stated that dyes absorb light but several failed to mention that electrons from the excited dye pass to TiO2/semiconductor.
Syllabus sections
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18M.3.hl.TZ2.18b.ii:
Outline why complex B absorbs light of longer wavelength than complex A.
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18M.3.hl.TZ2.18b.i:
State the feature of the molecules responsible for the absorption of light.
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18M.3.hl.TZ1.15c:
The structure of two dyes used in DSSCs are shown.
Predict, giving a reason, which dye will absorb light of longer wavelength.
- 18N.3.hl.TZ0.15d.ii: Outline the effect of sunlight on the dye in the solar cell.
- 18N.3.hl.TZ0.15d.iii: State the purpose of TiO2.
- 18N.3.hl.TZ0.15d.i: Identify the structural feature of the dye that allows the conversion of solar energy into...
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18N.3.hl.TZ0.15d.iv:
Deduce the reduction half-equation at the cathode.
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17M.3.hl.TZ2.18a.ii:
The structures of 11-cis-retinal and β-carotene are given in section 35 of the data booklet. Suggest a possible wavelength of light absorbed by each molecule using section 3 of the data booklet.
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17M.3.hl.TZ1.19b:
Both photosynthesis and the Grätzel cell use energy from sunlight to bring about reduction. Deduce an equation for the reduction reaction in the electrolyte of a Grätzel cell.
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17M.3.hl.TZ2.19a:
Contrast how absorption of photons and charge separation occur in each device.
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17N.3.hl.TZ0.19b:
The natural absorption of light by chlorophyll has been copied by those developing dye-sensitized solar cells (DSSCs). Outline how a DSSC works.
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19M.3.hl.TZ1.18a:
Some solar cells use photovoltaic semi-conductors. Compare, giving reasons, the electrical conductivity of metals and semi-conductors as temperature increases.
- 19N.3.hl.TZ0.20d(ii): Suggest an advantage of the DSSC over silicon-based photovoltaic cells.
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19M.3.hl.TZ1.18b:
Suggest one advantage of a dye-sensitized solar cell (DSSC) over a silicon based photovoltaic cell.
- 19N.3.hl.TZ0.20d(i): Outline the functions of the dye, TiO2 and the electrolyte in the operation of the...
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18M.3.hl.TZ1.15a:
Early photovoltaic cells were based on silicon containing traces of other elements. State the type of semiconductor produced by doping silicon with indium, In, giving a reason that refers to its electronic structure.
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19M.3.hl.TZ2.20b:
Explain the function of dyes in a dye-sensitized solar cell (DSSC).
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16N.3.hl.TZ0.21c:
Dye-sensitized solar cells (DSSC) convert solar energy into electrical energy.
(i) Describe how a DSSC converts sunlight into electrical energy.
(ii) Explain the role of the electrolyte solution containing iodide ions, I−, and triiodide ions, I3−, in the DSSC.
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17M.3.hl.TZ2.19b:
Suggest one advantage a DSSC has over a silicon based photovoltaic cell.
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18M.3.hl.TZ1.15b:
Dye-sensitized solar cells, DSSCs, use a dye to absorb the sunlight. State two advantages that DSSCs have over traditional silicon based photovoltaic cells.
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17M.3.hl.TZ1.19a:
Identify two ways in which the structure of the dye shown resembles the chlorophyll molecule. Use section 35 of the data booklet.
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18M.3.hl.TZ2.18a:
Draw the Lewis (electron dot) structure for an appropriate doping element in the box in the centre identifying the type of semiconductor formed.
- 20N.3.hl.TZ0.14b: Doping of silicon increases the conductivity in semiconductors. Explain how doping improves...
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20N.3.hl.TZ0.14a:
Doping of silicon increases the conductivity in semiconductors.
Describe the doping in p-type and n-type semiconductors.
