An Overview on Dye Sensitized Solar Cells[edit | edit source]

Dye Sensitized Solar Cells (DSC) in terms of low cost materials and manufacturing was introduced by Michael Gratzel in 1991.

O'regan, B., Gratzel, M.,A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature, Vol 353, 737-740, (1991)

TiO2 as a cheap material is used as a an electron conductor in the cell. The TiO2 layer is prepared from colloidal solution and to make the electric connection briefly sintered at 450 ̊C. The layer has the thickness of 10µm and the particle size of 15nm which gives the cell both larger surface area and higher roughness factor. This average size make the cell to absorb more dye and harvest more light. The applied dye is trimeric ruthenium complex which absorbs the photon. The absorption onset is about 750nm. The hole conductor to complete the circuit is the iodide/triiodide redox electrolyte Li+.

Nanocrystalline dye‐sensitized solar cells having maximum performance, Progress in Photovoltaics, Vol 15, 1-18, (2007)

In this contribution the development of DSC in matters of efficiencies and stabilities are considered. Some of the drawbacks of DSC: 1- Inefficient light absorption of the dye 2-Surface recombination 3- Limited Fill Factor(FF) due to series resistance losses, light intensity recombination,....

Concept development Three different design considered. A standard one with the dye on a high surface area nc-TiO2, A high surface transparent TiO2 with a light back scattering, TCO-less design that for the secondary electrode uses a porous metal layer.

Materials Nano Crystalline TiO2 is prepared from collodial solutions following by hydrothermal sol-gel procedures via either acidic or basic preparation. An experiments shows the base peptization results in a higher device open circuit voltage. See Acid versus base peptization of mesoporous nanocrystalline TiO2 films. Home-made TiO2 or P25(Degussa) are used.

scattering Layers To enhance the light absorption layers with particle sizes between 100-400 nm are used such as ZrO2, Different phase of TiO2. By applying the scatter layer the photocurrent is increased about 10%.

Conformal coatings of nanocrystalline electrodes Palomares, E., Clifford, J., Haque, S., Lutz, T., Durrant, J. Control of Charge Recombination Dynamics in Dye Sensitized Solar Cells by the Use of Conformally Deposited Metal Oxide Blocking Layers American Chemical Society, Vol 125, 475-482, (2003)

Other Materials

Other oxides has been investigated but so far TiO2 shows the best response. SnO2 has also been considered for its faster charge transport, but its conduction band energy is 0.4V more positive than TiO2.

TiO2 Microstructure and Fabrication in DSCs[edit | edit source]

In this part different microstructure and characterization of TiO2 that has been worked on so far is reviewed chronologically.

Almeida, P., Deelen, J., Catry, C., Sneyers, H., Pataki, T., Andriessen, R., Roost, c., Kroon, J.M. Microstructure characterization of titanium dioxide nanodispersions and thin films for dye-sensitized solar cell devices Applied Physics, Vol 79, 1819-1828, (2004)

TiO2 nano dispersions synthesized by calcination of the dried hydrolysis product of Titanium isopropoxide and the films were formed by doctorblade technique following by heat or pressure sintering. Depending on the film forming process different electrical responses is measured. The I-V characterizations show better performance of TiO2 heat sintered films. This is because the heat-sintered TiO2 films have a homogeneous structure as the SEM results indicate. The pressure-sintered films on the other hand have a heterogeneous structure with inclusions which might account for the poor electrical performance. It is also possible to associate these inclusions to the the existence of volatile compounds which are left in the pressure-sintered films.

Kiema, G.K., Colgan,M.J, Brett, M.J. Dye sensitized solar cells incorporating obliquely deposited titanium oxide layers, Solar Energy Materials and Solar Cells, Vol 85, 321-331, (2005)

The randomly porous structure of TiO2 produced by sol-gel methods have some disadvantages such as low conductivity and enhanced electron recombination. These problems are all due to the random micro-structure and different orientation of the nano-particles. The intention of this work is to produce porous films with directional order in order to have higher surface area with better electron conductivity. In this study porous TiO2 films are deposited by oblique electron beam evaporation in different angles following by annealing for 3 hours in air at 500o C. When the deposition angle increases the columns become more widely separated that leads to higher surface areas (60-75o C). In very high angle such as 85o C the columns become isolated so the Surface area decreases as the density of the number of columns decreases due to increases column spacing. The results show that for the deposition angles,(60-75o C), the short circuit current is higher than the sol-gel based solar cells. This is attributed to their columnar nature since this structure and connected pores provide more accessibility of the entire film surface to the electrolyte and make a more direct path for injected electrons. This directed pass makes the electron transit time faster and therefore reduces the chances for back electron recombination. On the other hand the result show lower fill factor for cells with obliquely deposition. This might be due to higher internal resistance of the films.

M.Zukalová, A.Zukal, L.Kavan, M.K.Nazeeruddin, P.Liska, M.Grätzel Organized Mesoporous TiO2 Films Exhibiting Greatly Enhanced Performance in Dye-Sensitized Solar Cells Nano Letters, Vol 5, 1789–1792, (2005)

In this work thicker mesoporous TiO2 are fabricated in order to achieve higher efficiency by increasing the roughness factor (increasing light harvesting). The procedure of making organized mesoporous TiO2 films are as the following. A solution of HCL and titanium etoxide was prepared under strong stirring (Solution 1). A solution of block copolymer Pluronic was disolved in 1-butanol and added to solution 1. The films were prepared by dip coating following by aging and calcinatin (the precise procedure is described in the article). For thicker films with 2 or 3 layers the same procedure is repeated once or twice. The 3 layer mesoporous TiO2 films has the largest RF. the porous structure has not changed much during the dip-coating process. Solar cell test has been done on 3 laye fims and on annonorganzed mesoporous TiO2 film with the same conditions (Type of the dye, the electrolyte,..). The solar-conversion efficiency of DSC with organized mesoporous film (1µm thick)was larger about 50% of the efficiency of 1µm thick film composed of randomly nanoparticles which is all thank to the huge large surface area.

A. Agrios, I. Cesar, P. Comte, M. K. Nazeeruddin, M. Grätzel Nanostructured Composite Films for Dye-Sensitized Solar Cells by Electrostatic Layer-by-Layer Deposition Chemistry of Materials, Vol 18, 5395–5397, (2006)

In this article some of the possibilities of fabrication of nano-composite thin film for dye sensitized solar cells by Elastic layer by layer (ELBL) method is studied. TiO2 films made by this method for DSC were used for the first time by He et al. TiO2 nano particle films for example can be fabricated by dipping a glass slide in a cationic polymer solution and then in an anionic TiO2 solution or the other way around, anionic polymer, cationic TiO2 and it continues till to get to the desired thickness. The charge of TiO2 is balanced by pH. He et al. showed the results are better with the first solution. Dye-sensitized Solar Cell Fabricated by Electrostatic Layer-by-Layer Assembly of Amphoteric TiO2 Nanoparticles Langmuir, Vol 19, 2116-2174, (2003)

The films were fabricated with the same sequences as He et al. and effects of scattering layers and different treatment investigated. A film made by coating 50 polymer-TiO2 cycles with the thickness of 1.5 µm after sintering showed very good performance. Another film(4 µm) fabricated by 50 cycle layer of 250-400nm particles on top of the latter film. The solar cell made of it, in comparison with the first one showed double the current while having a similar VOC and FF.

S. Ito, T. N. Murakami1, P. Comte1, P. Liska1, C. Grätzel1, M. K. Nazeeruddin1, M. Grätzel Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10% Thin Solid Films, Vol 516, 4613-4619, (2008)

In this paper some new methods are applied to the fabrication of TiO2 films in DSCs to achieve higher efficiency. These methods include pre-treatment of the working photoelectrode by TiCl4, screen printing for TiO2 (nano and sub-micron crystalline) layers, variation in the thickness of the transparent nano-crystalline TiO2, using light scattering layer and applying an anti-reflecting film to the surface. By the use of these layers the the incident photon to electricity conversion efficiency is improved. A detailed procedure for this cell fabrication is available in the article. Surface contamination such as iron oxides increase the back electron recombination. In order to prevent it, in the experiment all the equipment for TiO2 preparation were plastic or glass. TiCl4 treatment used to remove the iron contamination before and after the screen printing of porous TiO2 films. The other advantages of this treatment are 1- On the FTO substrate: improving the bonding between FTO substrate and TiO2 layer, blocking the electron recombination 2- the second treatment: increase the surface roughness factor and necking of TiO2 particles. At the end V-I results show this treatment suppress the dark current. The advantages of the light scattering layer is not only a photon trapper but also corporate in photovoltaic generation.

D. Kuang, J. Brillet†, P. Chen†, M. Takata§, S. Uchida, H. Miura, K. Sumiokas, S. M. Zakeeruddin, M. Gratzel Application of Highly Ordered TiO2 Nanotube Arrays in Flexible Dye-Sensitized Solar Cells American Chemical Society, Vol 2, 1113–1116

DSCs are mostly based on randomly nano-crystalline TiO2 film produced by sol-gel method. There has been many efforts to develop ordered or one dimensional TiO2 films in order to improve the electron transport. In this article for the first time ordered TiO2 nano-tubes array used as photoanode for ionic electrolyte in dye sensitized solar cells. the nano tubes were prepared by anodization of Ti foils and then immersed in TiCl4 solution and rinsed following by sintering at 500o ̊C for 3 hours.

Enhancing Light harvesting in DSCs[edit | edit source]

As a part of the DSC structure a sensitizer is adsorbed on the surface of the semiconductor and plays the role of light harvesting. The excited sensitizer injects its electron to the conduction band of the semiconductor. The electron passes the circuit from the semiconductor to the back contact and then via the the external load to the counter electrode and there after it reduces the redox mediator which it regenerates the oxidized sensitizer so the circuit is completed. To get higher efficiency in DSCs it is important to have a larger Incident Photon to Current Conversion efficiency(IPCE). Ideally all photons below a threshold wavelength about 920nm should be harvested and and converted into electric current. the problem is that many of the applicable sensitizers don't cover or have low absorption of the whole visible spectrum specially at the red region. The other issue to be considered the directionality between the sensitizer and the semiconductor in order to provide an efficient electron transfer from the excited dye to the conduction band of the semiconductor.

J.H.Yum, E.Baranoff, S.Wenger, Md. K. Nazeeruddin and M.Grätzel panchromaric engineering for dye sensitized solar cells Energy& environmenttal Science, Vol 4, 842, (2011)

This is both a very good review and perspective of achieving panchromatic response in DSCs.

1- Single Dye: N3 and N719, two of the complexes of Ru(ll) played a major role in advancing of DSCs. One of their problem is the the lack of absorption in the red region. To overcome this inefficiency:

1.1-Panchromatic Ru(11) complex; N749 (Black dye) is synthesized; another Ru(11) complex which ruthenium center is coordinated to a monoprotonated tricarboxyterpyridine ligand and three thiocyanate ligands. Therefore better harvesting of the red region of visible spectrum is achieved. In comparison with N719its response extends 100nm into the infra-red region. Conversion efficiency 0f 11.1% has been achieved using the N749.

1.2- Panchromatic ruthenium free organic dyes; There has been lots of research on metal-free dyes to replace Ru complexes. But so far the results achieved yet not overcome the responses from Ru(11) complexes such as N719.

1.3- semiconductors; Interests has been drawn towards semiconductors such as utilisation of Quantum Dots instead of dye molecules. their advantages include: 1) High light harvesting 2) Tunable band gap over a wide range 3) Large intrinsic dipole moment. For example PbS has a broad light harvesting ability because of its small band gap. But the combination of the most efficient redox couple and low band gap semiconducting materials is not compatible.

2- Multiple Dyes: One way to cover the visible spectrum is to take advantage of multiple dyes, each absorbing strongly a part of the spectrum.

2.1- Co-sensitization with red light absorbing dyes; The ruthenium based dyes and organic dyes lack a strong absorption around the red region. by combining other sensitizer with stronger absorption in the red region a more efficient IPCE would be obtained. But there are some problems such as limited number of sites on TiO2 surface to which dye molecules attach or unfavourable reaction of molecules which all end up in a lower IPCE. There is a successful combination of the black dye and an organic dye (D131)with the efficiency of 11% presented by Sony research team. These two dyes have different adsorption sites.

2.2- Layered co-sensitization; A good absorption of light by 2 layered dye achieved by Hayes et al. The sensitizer consists of two layers of black dye and N3 produced in pressurized CO2 condition. Park et al. made a triple layered dye based on chromatographic principle of the black dye, P5, N719 which results in a stronger light absorption. These methods show a promising path for capturing the whole spectrum for DSCs.

3- Tandem cells: In this configuration different sub cells with different band gap energy are connected optically and electrically in a series with the order of higher to lower energy band gap. In this way the efficiency of solar cell can go further than the Shockley-Queisser limit of 30%.

3.1 DSC/Cu(In,Ga)Se2; Tandem configuration of DSC/CIGS were constructed mechanically stacking these two cells in a series. The disadvantages of this configuration include reflection losses at the stack interface and absorption losses of low energy photons in the conducting glass of the top cell. In this article they report a monolithic integration which exclude those problems; the top cell is deposited on the bottom one which ends with using just two electrical contacts.

3.2 p- Type semiconductor; In this configuration similar to tandem DSC/CIGS, two sells are used. One is a sensitized n-type semiconductor and a p type semiconductor as a photocathode. Applying different type of dyes on these semiconductor is a promising way to cover a broader spectrum. In addition the Voc is increased in this structure Voc=Voc(p-type)+Voc(n-type). So far an efficiency of 2.4% is reported.

4- Energy down conversion by energy relay dye

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T.Yamaguchi,Y.Uchida,Sh.Agatsuma,H.Arakawa Series-connected tandem dye-sensitized solar cell for improving efficiency to more than 10% Solar Energy Materials and Solar Cells, Vol 93, 733, (2009)

In this article Tandem DSCs are constructed in order to achieve higher efficiencies. In this configuration the top cell should provide a high voltage and the bottom cell should have a longer wave length absorption area. Some different combination of dyes as for top and bottom cells were examined. The best result was achieved by N719 for the top cell and black dye for the bottom cell. The applied photo anode in this study was nanoparticle-TiO2 paste. Efficiency of 10.4% were obtained. With this structure both Voc and IPCE of the tandem cell were higher than the individual cells. The author suggested 3 methods for improvement;1) Expanding and separating the absorption region for each cell, 2) Applying a higher voltage to the top cell, 3) Transparent counter electrode.

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