Pyranometer design literature review

This is a literature page for project pyranometer in 2014 spring

Review

Quantum dot solar cells^[1]

A. J. Nozik, “Quantum dot solar cells,” Physica E: Low-dimensional Systems and Nanostructures, vol. 14, no. 1–2, pp. 115–120, Apr. 2002.

Background: Hot carrier and impact ionization Hot carrier @ solar cells: Electrons/holes receives photons with energy much higher than their band gap will carry kinetic energy create effective temperature much higher than lattice temperature (3000K of carrier @ 300K of lattice etc.). Primary loss for this:

• Heat through scattering (KN is transferred into band vibration?)
• Photon emission (electrons occupied holes to let out photons?). Approach to stop this loss:
• Stacked cascaded multiple p-n junctions to match multiple band of solar spectrum. This reduce carrier relaxation via photon emission. Efficiency increase to 66%.
• Reduce thermal relaxation – utilize hot carriers before their relaxation.
  • Enhanced photo-voltage: being extracted before cooling; Requirement: transportation of hot carriers faster compare to the cool down rate – which should be related to the material itself.
  • Enhanced photo-current: create second or more electron-hoe pair(s) by impact ionization (Auger effect: one electron hit a band create one e-h pair. What will happen to this electron?); Requirement: impact ionization rate faster compare to the cool down rate & e-h pair transport rate faster than cool rate?.
  • Quantization confinement will dramatically reduce hot carrier’s cooling rates. (So this confinement will also reduce the cool down rate in Auger process which is not desired?)
• What about some part of solar spectrum is absorbed to stimulate electron from En to En+1 or higher and the left spectrum will cause these electrons to emission from En+1 to En-1 to let out more photons with same lambda. Then one layer will be used to specifically to absorb these electrons.

Hot electrons and hot holes cool down rate are different because:

• Mass difference (Which one is faster?)
• Hot carrier cool rate depends on density of photo-generated hot carriers (How this makes diff?).

Predicted way for QDs: hot electrons with slowed cooling rate in QD --- Auger process, fast cool down hole due to mass & closer quantized space --- photo-currents. Bottleneck due to fast hole trap at the surface will slow cooling rate (7 ps at CdSe, InP QDs compare to 0.3 ps cooling rate without hole trap). This will prevent Auger process. Suggested Syetems:

• QD arrays in p-i-n cells. (Quasi-1D system)
  • Advantages:
    • Delocalized quantized 3-D miniband states might slow the carrier cooling rate to allow hot electron transportation and collection.
    • Impact ionization can occur, however not in the same time with hot electron transportation/collection.
  • Fabrication:
    • Colloidal
    • Epitaxial
  • Challenge:

Disorder of shape, surface state etc.

• QD sensitized DSSC (Dye-sensitized solar cell)
  • Advantage:
    • Tunability of optical properties with size of QDs.
    • Quantum yield might be greater than 1, meaning one photon might generate more than one electron-hole pairs.
  • Fabrication:

Absorption from QD colloidal.

• Quantum dots dispersed in organic semiconductor polymer matrices.

CdSe QDs are formed in hole conducting polymer. Structure of QDs affects the efficiency. Different layers can be applied to conduct electrons (TiO2) or holes (MEH-PPV). Challenge: electron-hole pair might recombine in surface of polymers. Refer to CdSe Nanocrystal Rods/Poly (3-hexylthiophene) Composite Photovoltaic Devices for rodlike CdSe QDs structure. Refer to Charge transfer in photovoltaics consisting of interpenetrating networks of conjugated polymer and TiO2 nanoparticles for multiple polymer structure.

CdSe Nanocrystal Rods/Poly (3-hexylthiophene) Composite Photovoltaic Devices^[2]

W. U. Huynh, X. Peng, and A. P. Alivisatos, “CdSe Nanocrystal Rods/Poly(3-hexylthiophene) Composite Photovoltaic Devices,” Adv. Mater., vol. 11, no. 11, pp. 923–927, Aug. 1999.

Important factors affect the performance of a PV device includes:

• Overlap of the absorption of solar spectrum;
• Efficiency of carrier separation and transportation;
• Enhanced charge separation occurs at interface between two materials with disparate electron affinities will transport in two materials and have low probability of recombination.

Paper demonstrated:

• 5 nm PbSe nanocrystal: 5% external quantum efficiency (EQE, electrons/incident-photons per-second; internal QE: electrons/absorbed-photons per-second. 0.25% power conversion efficiency. Poor behavior due to poor transportation.
• Elongated particles have a tendency, that increases with size, to form *chains of particles connected along the c-axis
• 4*7 nm PbSe nanocrystal after forming chain, can reach EQE as high as 4%
• 8*13 nm PbSe nanocrystal after forming chain, can reach EQE as high as 16%

Larger size of PbSe nanocrystal are hard to fabricated, lead to question of what influence of nanocrystal’s shape has.

Charge transfer in photovoltaics consisting of interpenetrating networks of conjugated polymer and TiO2 nanoparticles ^[3]

A. C. Arango, S. A. Carter, and P. J. Brock, “Charge transfer in photovoltaics consisting of interpenetrating networks of conjugated polymer and TiO2 nanoparticles,” Applied Physics Letters, vol. 74, no. 12, pp. 1698–1700, Mar. 1999.

Use of interpenetrating donor-acceptor heterojunctions and interpenetrating polymer network can substantially improve the photoconductivity. Layer fabrication: Opaque TiO2 layer is fabricated in thickness of 4-6 um, at particle size of 80 nm. Polymer is penetrated into TiO2 particle pores with size of 20 nm. Polymer is used as charge transporter. ITO-(Layer)-Calcium sandwich. For current flow:

• Calcium acts like electron transfer;
• ITO acts like hole transfer. Hole should transfer from ITO to TiO2 valence band;
• Hole transfer from TiO2 valence band to ITO. However, insulation property of TiO2 prevent upper two situation from happening. But, under illumination, TiO2 presents property of accepting and conducting photo-excited electrons.

Advantage: TiO2 has low conduction band, and has low saturation voltage (achieved by using stable and high-working function contact)

Reference