Organic Solar Cells

From Wikisym

An organic solar cell is a solar cell that the light-absorption layers are made from organic materials.

Contents 1 Materials 2 Advantages 3 Disadvantages 4 Fabrication 5 Working principle 6 Organic vs. Inorganic materials 7 Power conversion efficiency 8 Device structure design 9 Research topics 10 References

Materials

There are two types of organic materials which are often used for organic solar cells:

1. Small molecule, e.g., ZnPc, Me-Ptcdi, PCBM.
2. Conjugated polymer, e.g., MDMO-PPV, P3HT, PFB, CN-MEH-PPV, F8BT.

Advantages

Low cost synthesis and ease of forming of thin film

Disadvantages

Low efficiency compared to the inorganic solar cells

Fabrication

For small molecule evaporation is best choice, whereas polymer are mainly processed from solution technique, e.g. spin coating, screen printing, ink-jet pringting

Working principle

1. Photon absorption: photons are absorbed by active organic materials leading to formation of an excitation state, the electron-hole pair, (exciton)
2. Exciton diffusion: excitons move by diffusion process to a region where charge separation occurs.
3. Exciton dissociation: separation of electron and hole pairs. This is possible at sharp potential at the donor-acceptor interface. However, this process is not well understood.
4. Charge transport: the free charge carriers transport to the electrodes (holes to the anode and electrons to the cathode) with help of internal field.

Organic vs. Inorganic materials

	Inorganic	Organic
Electron-hole pair	free	tightly bound (0.1-1eV)
Charge carrier generation	throughout the cell	at the interface
Absorption coefficients		high (>105 cm-1)
Optical band gap	1.12 eV for Si	around 2 eV
Charge transfer at the interface	drift and back diffusion	charge are highly localized at the interface
Exciton diffusion		10-100 nm
Carrier mobility	100-104 cm2V-1s-1	1 cm2V-1s-1 for highly ordered structure
Mobility and temperature	temperature increase, mobility decrease	temperature increase, mobility increase
Voc comes from	built-in voltage	built-in voltage and the diffusional force

Power conversion efficiency

The photovoltaic power conversion efficiency depends on three factors:

1. Open circuit voltage (V_oc)
   • Limitation
     difference in two work function electrodes, and difference between the HOMO of the donor and the LUMO of the acceptor
   • Improvement
2. Short circuit current (I_sc)
   • Limitation
     The number of photocreated charges that are collected by the electrode. It is directly propotional to the amount of absorbed light, exciton dissociation efficiency and charge carrier mobility.
   • Improvement
3. Fill factor (FF)
   • Limitation
     Capability of the transport of charges carriers, possible recombination losses, ohmic contribution of the electrode and the contact
   • Improvement

Some of current PCE status:

11% - DSSC (electrolyte)

Michael Gratzel, Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells, Inorganic Chemistry 44, 6841-6851 (2005).

8.1% - DSSC (polymer)

R. Komiya, et. al., Highly efficient quasi-solid state dye-sensitized solar cell with ion conducting polymer electrolyte, J. Photochem. Photobio. A 164, 123 (2004).

4.2% - Bilayer HJ with EBL (ITO/CuPc/C60/BCP/Ag)

J. Xue, et. al., Appl. Phys. Lett. 84, 3013 (2004).

5.0% - Planar-mixed HJ with EBL (ITO/CuPc/CuPc:C60/C60/BCP/Ag)

J. Xue, et. al., Adv. Mater. 17, 66 (2005).

5.7% - 2PM-HJ using Tadem cell architecture

J. Xue, et. al., Appl. Phys. Lett. 86, 5757 (2005).

5.0% - Blend HJ (RR-P3HT:PCBM)

W.L. Ma, et. al., J. Adv. Funct. Mater. 15, 1617-1622 (2005).

3.8% - Blend HJ (P3HT:PCBM)

C. Waldauff, et. al., Material and device concepts for organic photovoltaics: toward comperitive efficencies, Thin Solid Films 451-452, 503 (2004).

2.6% - Nanocrystal-polymer blend HJ (CdSe nanorods:P3HT)

Baoquan Sun and Neil C. Greenham, Phys. Chem. Chem. Phys. 8, 3557-3560 (2006).

Device structure design

1. Dye-sensitized solar cell
   1. electrolyte as hole conductor
   2. non-electrolyte (polymer or inorganic solid) as hole conductor
2. Bilayer heterojunction
3. Blend heterojunction
   1. organic-organic composite
   2. organic-inorganic composite
4. Ordered bulk heterojunction e.g. diblock copolymer

   The problem with the disodered bulk heterojunction are that phase separation is too large and there are dead ends.

• The electrodes
  • Anode: ITO, work function = 4.8 - 5.0 eV
  • Cathode: Al, work function = 4.0 - 4.3 eV

Research topics

Photon absorption

• Band gap
  • Longer conjugation length, the lower energy band gap.
  • The dyes exhibit broad light absorption profiles.
  • The nanocrystal can be tailored to wide variety of optical band gap.
  • Quantum confinement can enhance the oscillator strength for absorption.
  • C60 moiety is replaced by a less symmetrical fullerene, such as C70, which results in dramatically increase in light absorption. [12]

• Device architecture
  • Tandem cell architecture is introduced to broad possible span of solar spectrum. Cells with efficiency of 6.5% are possible. [9]

Exciton diffusion

• Diffusion length
  • Substituting acceptor material PTCBI with C60, which has a longer exciton diffusion length (77 Å), improves the efficiency by factor of 2. [11]

Exciton dissociation

• Charge transfer at the interface
  • Transfer of electron from dye to the nanocrystals is very fast (femto-pico seconds).
  • Mobile electrolyte ions in DSSC can rapidly rearrange around the photogenerated charge pairs, neutralizing the Coulomb attraction between them, thereby slowing their recombination rate. Crystallization is undesirable, as it prevents close contact between the hole transport material and the highly corrugated surface of TiO2 film. Formation of an intimate contact between the two materials is necessary.

• Passivation techniques
  • Various passivation techniques have been investigated to minimize recombination at the oxide/electrolyte interface. [8]
  • Insulating surfactant coating on nanocrystals makes them soluble in organic solvents and are readily miscible with conjugated polymers, but it inhibit the electronic interaction with each other and surrounding polymer. Therefore, electroactive surfactant is need. [13, 14]

Electrode contact

• Metal electrode
  • An increase in the workfunction of the metal top electrode leads to a reduction of the open-circuit voltage, short-circuit current, and power conversion efficiency of organic bulk-heterojunction solar cells [17, 18]
  • The cell series resistance is ultimately limited by the ITO anode resistance, and its resistance to the adjacent donor layer. [26]

• Intermediate layer
  • Transparent, organic exciton blocking layer (EBL) is inserted between the photoactive region and the metal cathode. Inserting such a layer prevents damage (form of deep trap levels) due to cathode evaporation and eliminates parasitic exciton quenching at the electron-acceptor/cathode interface. [10]
  • PEDOT:PSS layer smooths out the rough ITO surface and increases the work function of positive electrode.

Carrier transport

• Mobility
  • Poole-Frenkel dependence, carrier mobility follow an exponential dependence on the square root of electric field [4]
  • High carrier mobilities have been measured in crystalline organic material. A preferential orientation leads to an anisotropic charge transport. [7]
  • Some side groups can induce self-organization lamellar stacking, which improves their conductivity. [3]
  • The electron mobility in films of nanocrystals, and in nanocrystal-polymer blends, is limited by nanocrystal-nanocrystal hopping. Nanorod [19] and branched tetrapod nanocrystals [15] provide a transport path across the blend film.

• Transport model
  • Gaussian disorder model (GDM) was used to model charge transport (hopping) in disordered organic material. [5, 6]
  • Quantum calculation of transfer integral to determine the carrier mobility [1, 2]
  • Modeling electrical transport in blend heterojucntion organic solar cells [16]
  • A numerical model for the I-V characteristic of bilayer polymer PV devices, including drift and diffusion currents, injection and extrantion at the electrodes, and the effects of space charge on the electric field within the device. [27]

Morphology

• Control
  • Learning how to control exactly where along the scale from amorphous to crystalline the structure of the material lies is important.
  • Large aggregation of nanocrystal can be reduced by mixing pyridine (washing surfactant agent) and choloform as solvents for the cosolution of polymer and nanocrystal. [22]
  • Plasticizer increases the miscibility of PT/C60. [23]
  • A thermal annealing step, above the Tg of the polymer, is necessary to increase its crystallinity in order to improve its electrical properties, but thermal annealing also drives the phase segregation of P3HT and PCBM. Phase segregation destroys the BHJ, thereby reducing device performance. Optimal thermal annealing conditions are required. [24] A recent discovery of conditions to attain a thermal stable interpenetrating network of P3HT and PCBM. [25]

• Ordered
  • Photovoltaic cells made from conjugated polymers infiltrated into mesoporous titania. [20]

• Modelling
  • Use computer simulation to predict the morphologies of diblock copolymers and their photovoltaic behavior. They have shown that ordered morphologies, with the aid of electric-field-induced alignment, give much higher power conversion efficiency than the planar and the disordered. [21]
  • Flory-Huggins theory predicts the mixing of polymer-polymer blend.

Stability

• Temperature
  • Spiro-MeOTAD, organic hole conductor, which has high thermal stability in glassy state, assuring stability under ambient conditions for years.

• Moisture
  • Packaging is important for practical organic semiconductor device, due to sensitivity to oxygen and water vapor.
  • Device sealing is a problem of the solid/liquid DSSC. Therefore, many studies have been done on solid- or quasi-solid hole transport materials.

References

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