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Excitons and polarons in organic semiconductors
An exciton is a quasiparticle formed by an electron and an electron hole which are attracted to each other by electrostatic Coulomb force. It exists extensively in insulators, semiconductors and in some liquids.
An exciton can be formed by photo-induced excitation in semiconductors, which excites an electron to conduction band from valence band. In semiconductors with large dielectric constants and small energy gaps, the electric field screening tends to reduce the coulomb attraction force between electrons and holes. This screened Coulomb interaction together with the small effective mass of electron, the binding energy of excitons is typically small and on the scale of 0.01 eV. Thus the excitons can extend over many molecular units . This type of excitons are termed as Wannier-Mott exciton . Whereas in organic semiconductors, the dielectric constants are relatively lower (~ 3 to 4) as compared to their inorganic counterparts, leading to a larger screening length and thus a larger binding energy. The binding energy of excitons is typically in the order of 0.4 ~ 0.5 eV , with a variation range from 0.1 eV to > 1 eV [28,29]. In this case, the term Frenkel exciton is applied . Since the high binding energy, the Frenkel excitons usually reside on one molecule. Moreover, Frenkel excitons in organic semiconductors have a relatively short lifetime in the scale of nanoseconds and a short diffusion length (typically ~ 10 nm) [30,31]. If not dissociated, the excitons can relax to ground state by photon or phonon emission.
There are two kinds of excitons in organic semiconductors: singlet and triplet excitons. Singlet excitons have total spin S = 0. They are generated directly from photon absorption with spin conservation as they are in ground state. The triplet excitons have total spin S = 1 with three possible linear combinations of wavefunctions. They normally cannot be gained from direct photon absorption but can be converted from singlet by intersystem crossing (time scale ~ 10 ps) (Figure 1. 10). The triplet excitons have lower energy than singlet due to their larger electron-hole spatial overlap .
If the excitons are separated, for instance, by an energetic driving force originated from the difference in the electronic levels of the donor and acceptor materials in organic solar cells [33,34], a positive (hole) and a negative (electron) charge are generated. The presence of a hole or electron charge in organic semiconductors will lead to a local distortion of polymer chains; the coupling of charge and the induced structural relaxation is termed polarons. The presence of polarons creates two new states in the bandgap (since the lattice relaxation lowers the total energy) and thus introduces new absorption transitions (Figure 1. 6).
The complete photo-conversion process consists of six steps in organic solar cells. In bulk heterojunction organic solar cells these steps include: (i) absorption of photons, (ii) generation of excitons, (iii) diffusion of excitons, (iv) dissociation of excitons at the donor/acceptor interface, (v) charge transport to electrodes (holes to anode and electrons to cathode) and (vi) charge collection at the electrodes. These six steps are shown in Figure 1. 8. More details about the concept of bulk heterojunction organic solar cells will be given in Section 1.3.5 and 1.4.
Absorption of photons and generation of excitons
When sunlight reaches the earth, part of it is scattered by molecules, aerosols and dust particles in the atmosphere; other part of it is absorbed by different gases in the atmosphere, such as oxygen, O3, H2O, CO2. Hence, the intensity and spectrum of sunlight that reaches the Earth’s surface significantly depend on the length of the light path through the Earth’s atmosphere, whereas this length varies during the day time and also depends on latitude and time of year. In practice, the term Air Mass (AM), ratio of any actual sunlight path length to the minimum value (when the sun is directly overhead, i.e. at the zenith), is usually used to help characterize the spectral content and intensity of solar radiation after traveling through the Earth’s atmosphere. The AM value is given by Equation 1. 1 :
Hence, just outside the earth’s atmosphere, the solar energy intensity is about 1353 W/m2 , and the spectral distribution is referred to as Air Mass zero (AM 0) radiation spectrum. An AM 1.5 (θ = 48.2°) which has an intensity of 1000 W/m2 is usually used as standard spectrum to characterize the photovoltaics. The AM 0 and AM 1.5 spectra are shown in Figure 1. 9.
Figure 1. 7 Illustration of AM 0, AM 1.5 and AM X. Note that at the zenith, the sun is directly overhead and θ = 0; AM 0 is the solar spectrum just above the atmosphere at θ = 0; AM 1.5 is the solar spectrum with an incident angle of 48.2°from the zenith; X = 1/cosθ.
When sunlight reaches the Earth’s surface and strikes a solar cell, part of it will be scattered, reflected, and absorbed by the encapsulation materials and electrodes. Part of the remaining light, successfully reaching the active layer, will be absorbed by the active layer.
The efficiency of photon absorption depends on the thickness and absorption coefficient of the active materials. The absorption coefficient is determined by the components of the active layer. For poly(3-hexylthiophene) (P3HT) with a bandgap of about 2.1 eV, its absorption can cover the visible spectrum up to 660 nm (Figure 1. 9). Considering the absorption coefficient of a layer composing of a blend of P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to be about 105 cm-1, ∼300 nm thick layers absorb most of the light , which is much thinner than silicon solar cells. But due to their modest charge transport properties (e.g., short exciton diffusion length ~ 4 nm to 10 nm ), the optimal thickness of active layer for P3HT:PCBM blend is between 100 nm and 200 nm [38–40]. Developing new active materials such as low bandgap polymers (e.g., PTB7) is currently a promising approach to improve light absorption efficiency in organic solar cells.
When a photon is absorbed, it will promote an electron from the ground state S0 into higher lying unoccupied levels, such as S1 and Sn>1, creating a singlet exciton. The singlet excitons in higher levels (Sn>1) then relax back to S1 through internal conversion by phonon emission. The excitons in S1 can also relax to the ground S0 state either by radiative (via photon emission and this phenomenon is termed fluorescence) or non-radiative (such as via phonon emission) recombination (Figure 1. 10).
Singlet excitons may convert to triplet excitons through intersystem crossing. Triplet excitons are most likely to relax to ground state through phonon emission. Triplet excitons can also decay to ground state radiatively but with a much longer lifetime (μs – ms) than singlet recombination. This triplet radiative process leads to photon emission termed phosphorescence.
Diffusion of excitons
As shown in Figure 1. 11, we use Förster and Dexter mechanisms to interpret the exciton diffusion process, respectively [40,42]. According the Förster mechanism, it involves the long-range electrostatic coupling between the excitation transition dipoles located at the initial and final sites. In the case of triplet excitons, they diffuse following a short-range exchange (Dexter-type) mechanism relying on the orbital overlap between adjacent sites. As a result, singlet excitons diffuse more rapidly than triplets. But as mentioned in Section 1.3.3, the lifetime of singlet excitons is on the scale of nanosecond, while triplet excitons have a rather longer lifetime (μs – ms). The actual diffusion efficiency of singlets and triplets to the Donor/Acceptor (D/A) interface depends on the actual device system .
Dissociation of excitons
When the excitons reach the donor/acceptor (D/A) interface, if they do not recombine, they may be separated at this interface by an energetic driving force originated from the difference in the electronic levels of the donor and acceptor materials [33,34]. An exciton is a quasi-particle formed by a Coulomb-bound electron-hole pair which is initially located on one molecule. The dissociation of excitons at the D/A interface are generally considered to involve two steps [40,43–45] (Figure 1. 12). First, the exciton dissociates into a charge transfer (CT) state (D+/A-). In this state, the hole sits in the donor molecule and electron travels to the acceptor molecule by hopping. The next step is that the CT exciton separates into charge-separated (CS) states and finally free charge carriers. The final energy of CS state, Efinal = ID + EA (ID is the ionization potential of donor, EA is the electron affinity of the acceptor and Efinal is the energy level of completely unbound hole-electron pair (Figure 1. 12)) can be approximately estimated from the energy difference between the LUMO of the acceptor and HOMO of donor . That is to say, the dissociation occurs when the energy of exciton (energy difference of donor LUMO and donor HOMO) is larger than Efinal . CT excitons could recombine to the ground state, termed geminate recombination. The recombination of CS carriers is termed non-geminate recombination .
The mechanism of CT excitons is still under debate. A new excitation mechanism of CT excitons was proposed recently [44,47]. According to this new mechanism an excited CT exciton (also named hot CT excitons, with excess energy than the CT exciton lying in the lowest CT state) could be directly formed by photon absorption (Figure 1. 13).
Transport of charge carriers
Once excitons are separated into free charge carriers, electrons will transport to cathode and holes to anode. Otherwise, non-geminate recombination will occur. Various recombination processes are involved during charge transport: (1) radiative recombination by photon emission; (2) non-radiative recombination by phonon emission; (3) recombination at the sub-bandgap trap states induced by the impurities or defects in the material (Shockley-Read-Hall recombination) .
If charges do not recombine, they will travel in the form of polarons (Section 1.3.3) in conjugated organic materials. Polarons moves intra- or inter-molecular by hopping. Compared to the high mobility (on the scale of 102 – 103 cm2 V-1 s-1) in inorganic semiconductors, the mobilities in organic semiconductors are quite low (typically, 10-6-10-3 cm2 V-1 s-1) [40,48]. This modest mobility is owing to the weak electronic couplings, large electron-vibration couplings and disorder packing in polymers. As a result, the polymer morphology and crystallinity affect the transport mobilities significantly [40,48]. If the polymer is well crystallized, the mobility can reach over 1 cm2 V-1 s-1 [40,48].
Extraction of charges
Once arriving at the electrodes, polarons may be extracted into the external circuit. Metal conductors form two kinds of contacts with semiconductors: ohmic or Schottky contact, determined by the work function of the metal ΦM, work function (ΦSC) and electronic affinity (χSC) of the semiconductors . Here, the work function ΦM is the minimum energy needed to remove an electron to vacuum from the Fermi level of the metal; ΦSC is the minimum energy needed to remove an electron to vacuum from the Fermi level of the semiconductor; the electronic affinity χSC is the energy required to promote an electron to the vacuum energy level from the bottom of the conduction band. The formation conditions of these two contacts are shown in Figure 1. 14.
Architectures of organic solar cells
General architecture of organic solar cells
− The substrate can be typically a thin glass or a flexible and transparent substrate such as Polyethylene terephthalate (PET).
− The anode should be transparent since it locates before the active layer. Typically it is indium tin oxide (ITO) or fluorine doped tin oxide (FTO). ITO has bandgap of 3.7 eV, thus most photons can travel through it. For visible light the transmittance of commercial ITO is higher than 80% . The back electrode does not need to be transparent and is typically a layer of aluminum or silver or gold.
− Generally speaking, a hole extraction layer (HEL) is inserted between the anode and the active layer. This layer is also thin and highly transparent. It typically has a work function very close to the ionization potential of the donor component in the active layer to facilitate hole extraction. The most common material for HEL is Poly(3,4-ethylenedioxythiophene):Polystyrene sulfonate (PEDOT:PSS). Similarly, there is an electron extraction layer (EEL) inserted between the active layer and cathode. Calcium and lithium fluoride are frequently used for this layer.
− The active layer is deposited onto the HEL layer. It is the layer responsible to absorb photons and it is the main part of the solar cell. It normally consists of a donor material and an acceptor material.
Depending on the variations within the active layer, there are four common architectures which will be described below.
Single layer cells
The active layer consists of just one semiconductor material. This layer forms a Schottky contact with an electrode. Due to the energy band bending of the Schottky contact excitons generated in the active layer will be spitted into electrons and holes and transported to the corresponding contact electrode. The drawback in such device structure lies in the serious charge recombination during transport.
Planar heterojunction cells
This structure is typically composed of two layers: one from the donor material and the other from the acceptor material. The excitons are transported to the donor/acceptor interface and then dissociated there. Solar cells with this device structure is limited by the small D/A interface since only a limited portion of excitons can reach the D/A interface and dissociate there.
Bulk heterojunction cells
In this structure, the donor and acceptor materials are well mixed in solution first. They are then deposited onto a substrate resulting in the formation of a number of small domains allowing a large interfacial area (Figure 1. 8). Due to the enlarged interfacial area, more excitons can reach the donor/acceptor (D/A) interface in organic solar cells. The morphology of active layer therefore affects the charge separation significantly. We used this architecture in this thesis and more details will give in the following Section 1.4.
Table of contents :
Introduction and Background Knowledge
1.2. State-of-art of photovoltaics
1.2.1. History and development of photovoltaics
1.2.2. Development of organic solar cells
1.3. Basic knowledge of organic solar cells
1.3.1. Organic semiconductors and their applications in solar cells
1.3.2. The electronic structure of conjugated polymers
1.3.3. Excitons and polarons in organic semiconductors
1.3.4. Photo-conversion process
18.104.22.168. Absorption of photons and generation of excitons
22.214.171.124. Diffusion of excitons
126.96.36.199. Dissociation of excitons
188.8.131.52. Transport of charge carriers
184.108.40.206. Extraction of charges
1.3.5. Architectures of organic solar cells
220.127.116.11. General architecture of organic solar cells
18.104.22.168. Single layer cells
22.214.171.124. Planar heterojunction cells
126.96.36.199. Bulk heterojunction cells
188.8.131.52. Tandem solar cells
1.4. Organic bulk heterojunction solar cells using polymers and small molecules
1.4.1. Hole extraction layer: PEDOT:PSS
1.4.2. Bulk heterojunction
184.108.40.206. Donor material: P3HT
220.127.116.11. Acceptor material: PCBM
18.104.22.168. P3HT:PCBM film
1.4.3. Photovoltaic Characteristics of organic solar cells
22.214.171.124. Current-voltage response and efficiency
126.96.36.199. Performance-limiting factors
1.5. Plasmonic organic solar cells
188.8.131.52. Bulk plasmons
184.108.40.206. Surface plasmons on planar metal-dielectric interfaces
220.127.116.11. Localized surface plasmons in metallic nanoparticles
1.5.2. Organic solar cells utilizing localized surface plasmons
18.104.22.168. Mechanisms of light absorption enhanced by localized surface plasmons
22.214.171.124. Plasmonic organic solar cells
Experimental methods and techniques
2.2. Film and photovoltaic device preparation
2.2.1. General preparation technique for thin films: Spin-coating
2.2.2. Preparation of films and photovoltaic device samples
126.96.36.199. General preparation procedure for films on glass substrate or ITO-coated substrate and for photovoltaic devices
188.8.131.52. Preparation of sample PEDOT:PSS films and OSCs for optimization of PEDOT:PSS layer by post-deposition thermal annealin
184.108.40.206. Preparation of sample PEDOT:PSS films and OSCs for optimization of PEDOT:PSS layer by adding glycerol
220.127.116.11. Sample preparation for optimization of photoactive layer
18.104.22.168. Sample preparation for plasmonic OSCs using Ag NPSMs in PEDOT:PSS layer 53
22.214.171.124. Sample preparation for plasmonic OSCs using Ag NPSMs and glycerol in PEDOT:PSS layer
2.3. Characterization Methods
2.3.1. Characterization technique for Ag nanoparticles and solutions
126.96.36.199. UV-visible absorption for solutions
188.8.131.52. X-ray Diffraction
184.108.40.206. Transmission electron microscopy
2.3.2. Characterization techniques for films
220.127.116.11. Thickness determination
18.104.22.168. Integrating sphere photometer for films
22.214.171.124. Atomic Force Microscopy and conductive Atomic Force Microscopy
126.96.36.199. Four point probe measurement
2.3.3. Characterization techniques for solar cells
188.8.131.52. Current-Voltage Characterization
184.108.40.206. External quantum efficiency
Structural, optical and electrical properties of PEDOT:PSS thin films doped with silver nanoprisms
3.2. Ag NPSM synthesis and characterizations
3.2.1. Ag NPSMs synthesis
3.2.2. Characterizations of Ag NPSMs
3.3. Hybrid PEDOT:PSS-Ag NPSM solutions and films
3.3.1. Preparation of hybrid PEDOT:PSS-Ag NPSM solutions and films
3.3.2. Characterizations of hybrid PEDOT:PSS films
220.127.116.11. Bidirectional Reflectance Distribution Function (BRDF)
18.104.22.168. Surface profile and electrical conductivity
Plasmonic organic solar cells using silver nanoprisms
4.1. Optimization of regular P3HT:PCBM solar cells
4.1.2. Optimization of PEDOT:PSS layer
22.214.171.124. Thermal annealing of PEDOT:PSS films
126.96.36.199. Glycerol modified PEDOT:PSS
4.1.3. Optimization of photoactive layer
188.8.131.52. Thermal annealing
184.108.40.206. Solvent annealing and additive for photoactive layer
4.2. Studies of plasmonic solar cells composing silver nanoprisms in PEDOT:PSS
4.2.2. plasmonic solar cells composing silver nanoprisms in PEDOT:PSS
220.127.116.11. Characterization and discussion
4.2.3. Plasmonic OSCs composing Ag NPSMs and glycerol in PEDOT:PSS layer 100
18.104.22.168. Characterization and discussion