Solid-state dye-sensitized solar cells

Figure 2.12: Schematic illustration of the electronic processes in an ssDSSC. The electronic processes are similar to those shown in figure 2.11, except that the positive charge carriers are transported in the organic HTMs via hopping transport.

Although DSSCs have reached a high power conversion efficiency (P CE) of over 12 %, there remain many problems related to liquid-electrolyte leakage and electrode corrosion.

Alternatively, solid-state hole transport materials (ssHTMs) have been developed to re-place liquid electrolytes. In this way, the device is termed as solid-state dye-sensitized solar cell (ssDSSC). A schematic illustration of the working mechanism during device operation is shown in figure 2.12. The electronic processes such as dye photoexcitation, electron injection and dye regeneration are the same as shown in figure 2.11, except that the positive charge carriers are transported in the organic HTMs via hopping transport to the counter electrode.

Photoexcitation and electron injection

Dye molecules adsorbed at the surface of titania photoanodes are an essential part for current generation. The incoming light is absorbed by dye molecules, and thereby pho-toexcitation of the sensitizers occurs resulting in the generation of an electron-hole pair, known as an exciton. However, this reaction only occurs when the energy of the incident photons is equal or larger than the band gap of the dye.

E_photon = hc

ν ≥∆E_gap (2.18)

As titania has a high electron affinity, the electrons are injected to the conduction band of titania with the aid of an inner electrical field built by the interface of titania/dye/ssHTM.

The electron transfer from the dye to titania can be regarded as a tunneling process described by Gerischer [54]. In Gerischer’s model, the energy levels of the ground and excited states of the dye are assumed to have a Gaussian distribution due to thermal fluctuations. For electron injection, it is necessary that the energy states of the excited dye and the conduction band (CB) of titania are overlapping. An optimum electron injection occurs in the case of a complete overlap. In principle, the holes can transfer from the valence band (VB) of titania to the ground state of the dye during the electron injection. However, this process is unfavorable since it would increase the possibility for the recombination of separated charges. Therefore, in the ideal case lies the ground state energy of the dye within the band gap of titania, i.e. does not overlap with the VB of titania.

Up to now, there are many dyes used as sensitizers. Basically, they can mainly be divided into metal complexes and metal-free sensitizers according to structure. Both types are based on three main components: an electron donor moiety (D), a π bridge (π) and an electron acceptor moiety (A). The dyes typically have molecular structures of D–π–A or D–A-π–A. The A parts are located in the vicinity of titania, whereas the D parts are situated close to ssHTMs, i.e. far from the titania surface. In this way, rapid electron injection and dye regeneration is facilitated. Moreover, the recombination of the

oxidized dye with the injected electrons is inhibited to a certain degree due to a specific distance between the titania surface and D parts. The metal complexes includes either polypyridyl complexes with metal ions such as Ru, Os, Pt, Re, Cu and Fe, or porphyrins and phthalocyanines complexes with Zn, Ru, Ti, Si, Fe, Hf and Zr [55–64]. The latter class contains functional groups of coumarin, heteroanthracene, perylene or indole [65–68].

The sensitizers used in the present thesis are indoline dyes D149 and D205. The detailed description of both dyes is given in section 4.1.

Charge carrier transport

After exciton dissociation, electrons are transported through mesoporous titania to the transparent electrode. The electron mobility in mesoporous titania films is of several or-ders of magnitude lower than in a single anatase crystals. The existing sub-band-gap states in the TiO2are significantly influencing the transport rate. Considering electrons that can only be transferred into the conduction band of titania, the probability of electrons present in the conduction band dominates the overall electron mobility. The probability depends on the processes of trapping and detrapping of electrons from the sub-band-gap states, which implies that the exact location of the trap states is of high importance. Kopidakis et al. have reported that the transport-limiting traps are mainly located on the titania surface rather than in the volume of TiO2 nanoparticles or at the particle boundaries [69].

This appears to indicate that the traps are proportional to the roughness and porosity of a given titania film. As known, titania films with a high surface-to-volume ratio are beneficial for dye loading. Therefore, the electron mobility in titania and dye adsorption on the titania surface are in contradictory existence. This explains the observation by Yang et al., who demonstrated that a TiO2 nanoparticle size of about 25 nm gives rise to a higher power conversion efficiency of solar cells than smaller nanoparticles [70].

The hole transport on the other side happens from the ssHTM to the counter electrode after the injection of electrons. To be a good performing ssHTM in ssDSSCs, on the one hand, the upper edge of the VB of the ssHTM should be located above the ground state of the dye with respect to the energy level. On the other hand, ssHTM should have a good infiltration into mesoporous titania film to form a decent interface area of titania/dye/ssHTM. In the present thesis, spiro-OMeTAD and P3HT are used as organic ssHTMs. These two ssHTMs are described in detail in section 4.1. Positive charge carriers are transported either through spiro-OMeTAD or P3HT via polaron hopping, as explained in section 2.1.2. The mobility of positive charge carriers in the solid films of pristine spiro-MeOTAD is about 2 × 10⁻⁴ cm² V⁻¹s⁻¹, while in the case of the spiro-MeOTAD inside TiO2 network this value is one order of magnitude less, about 4× 10⁻⁵ cm² V⁻¹s⁻¹ [71,

72]. Many techniques have been developed to increase the mobility of positive charge carriers. The ionic additives Li-TFSI has been found to enhance the positive charge carrier mobility remarkably up to one order of magnitude with a molar ratio of Li salt to spiro-OMeTAD ranging from 0.1 to 0.2 [73]. Chemical p-doping is another strategy to increase the conductivity of OMeTAD. Burschka et al. have reported that the spiro-OMeTAD conductivity increased from 4.4 ×10⁻⁵ S cm⁻¹ to 5.3 ×10⁻⁴ S cm⁻¹ with 1.0

% p-type dopant [74]. The mobility of positive charge carriers in P3HT is closely related to its crystallinity, orientation and regioregularity. Amorphous regions of P3HT usually hamper the hopping processes, which leads to a low charge carrier mobility of less than 10⁻⁵ cm² V⁻¹s⁻¹. However, the ordered crystalline domains give a much higher charge carrier mobility. With respect to the orientation, the face-on geometry offers a higher mobility than the edge-on geometry by more than a factor of 100 [75,76]. Regarding the regioregularity, the mobility is only 2 × 10⁻⁴ cm² V⁻¹s⁻¹ with P3HT regioregularity of 81 %, while this value jumps to 0.05 - 0.1 cm² V⁻¹s⁻¹ in the crystalline P3HT with 96 % regioregularity [16].

Due to the relatively low charge mobility through organic ssHTMs (compared to liquid electrolytes) and the difficulty for organic ssHTMs being infiltrated into the mesoporous TiO2, an optimum active layer thickness is estimated to be around 2 µm [77,78].