Porous One Dimensional Photonic Crystals. Enhanced Photovoltaic Performance. Dye Solar Cells. for. Doctoral Thesis presented by Silvia Colodrero

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2 Porous One Dimensional Photonic Crystals for Enhanced Photovoltaic Performance of Dye Solar Cells Doctoral Thesis presented by Silvia Colodrero Institute Of Materials Science Of Seville (CSIC-US) Department of Condensed Matter Physics Supervisor Dr. Hernán R. Míguez García Tutor Dr. Diego Gómez García Seville, June 2013

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4 To my family and friends

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6 Preface In the context of photovoltaics, dye solar cells (DSCs) constitute an interesting choice to solid state semiconductor devices since they are expected to meet not only relatively high efficiency values but also non conventional functionalities that could promote their valuable characteristics, namely their low production costs and transparency. The optimization of their conversion efficiency is therefore a key issue that will depend, in a first approximation, on how efficient dye molecules adsorbed on the surface of a wide band gap semiconductor collect incident radiation. Some of the light trapping strategies commonly employed within the DSC field are reviewed along Chapter 1. In that regard, the present research work proposes for the first time the use of one dimensional photonic crystal (1DPC) structures as a viable pathway to effectively enhance the photovoltaic performance of the device. In the last years, the synergy between optics and photovoltaics has pushed the potential of these versatile optical thin films, a new concept being developed. For this reason, the first practical realization of this approach has encountered a high impact in such fields and is presented in detail in Chapter 3 and Chapter 4. This approximation also looks forward to fulfil, to a large extent, the requirements needed for the application of semi transparent DSCs in building integrated photovoltaics (BIPV), which is nowadays one of the most important sectors in which commercialization and investments are expected to take place. On the other hand, the porous nature of the periodic nanostructures herein presented opens the way to further applications in different research fields, such as biological and chemical sensing, detection and recognition, light emission devices and others. Once again, colloidal chemistry offers a powerful tool in designing 1DPC structures of high optical quality and with different functionalities, as will be discussed in Chapter 2. From now on, new advances and challenging opportunities based on these preliminary results, maybe concerning more complex architectures, will come...and I really hope so!

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8 "The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead: his eyes are closed" Albert Einstein

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10 Table of Contents Chapter 1 Dye solar cells and light trapping strategies: an overview 1.1 Introduction Dye solar cells Generalities Photovoltaic characterization and DSC prospective applications 1.3 Light trapping strategies Diffuse light scattering approach Coherent light scattering approach Other light trapping approaches 1.4 Motivation and goals of this thesis References Chapter 2 Porous and nanoparticle based one dimensional photonic crystals 2.1 Introduction Porous 1DPC structures Generalities 2.3 Fabrication of nanoparticle based 1DPCs Preparation of the nanoparticle precursor suspensions Film deposition method Structural and optical characterization of nanoparticle based 1DPCs Fabrication of optical cavities in porous 1DPCs 2.4 Response of porous 1DPCs to different guest compound Response to liquid infiltration Response to vapour infiltration 2.5 Conclusions References Chapter 3 Integration of porous one dimensional photonic crystals in dye solar cells 3.1 Introduction

11 table of contents 3.2 Preparation and assembly of DSCs based on porous 1DPCs Fabrication of TiO2 films coupled to porous 1DPCs Counter electrodes Liquid electrolyte Cell assembly 3.3 Optical and photovoltaic characterization of DSCs based on porous 1DPCs Mechanism of light harvesting enhancement in DSCs coupled to porous 1DPCs DSCs coupled to porous 1DPCs: effect on both optical properties and conversion efficiency 3.4 Conclusions References Chapter 4 Optimization of porous one dimensional photonic crystals for dye solar cells 4.1 Introduction Preparation of nanoparticle based 1DPCs of enhanced porosity Fabrication of highly porous 1DPCs made of nanoparticles Structural and optical characterization of highly porous 1DPCs 4.3 Effect of highly porous 1DPCs on the performance of efficient transparent DSCs Preparation and assembly of efficient transparent DSCs Comparative performance of DSCs coupled to 1DPCs of enhanced porosity Performance of very thin electrodes coupled to 1DPCs of enhanced porosity 4.4 Interplay between conversion efficiency and optical properties of standard thin DSCs coupled to highly porous 1DPCs Preparation and assembly of standard transparent DSCs Standard DSCs coupled to 1DPCs of enhanced porosity: relation between photovoltaic performance and optical properties 4.5 Conclusions References General Conclusions Appendix 1 Experimental details Appendix 2 Optical properties of multilayer ensembles: analysis through the scalar wave approximation method

12 porous 1dpcs for enhanced photovoltaic performance of dscs List of Contributions Resumen en Español Agradecimientos

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14 Dye Solar Cells and Light Trapping Strategies: An Overview C h a p t e r Introduction Development of new technologies focused on an efficient harnessing of energy from renewable resources emerged in the late seventies due to both the growing energetic demand and the depletion of fossil fuels, which nowadays still supply the major part of all energy consumed worldwide [1,2]. This trend was also reinforced by considering the ecological implications of the current energy systems, such as global warming caused by greenhouse emissions, reason why the search for clean energy sources has sped up in the last years. Among them, solar energy arises as one of the most promising alternatives, the major challenge to overcome being the fabrication of devices capable of capturing as much light intensity as possible with considerable reduction in their production cost and with high long term stability [3-8]. Fortunately, a huge effort in this direction has pushed photovoltaics to become an actual competitive solution that is expected to substantially contribute to global electricity generation in the near future [9]. This chapter will give a general prospect on emerging photovoltaic devices, in particular on those that mimic natural photosynthesis and are referred to as dye solar cells (DSCs). The research field concerning this new generation of solar devices has received a great deal of attention since its inception, which has been largely motivated by the possibility of achieving reasonably good efficiency solar cells by means of using non toxic and inexpensive materials through simple processing methods. Even though DSCs can be designed to offer some attractive features, such as transparency and flexibility, which might facilitate their entry in the market, the main disadvantage such devices are still facing up is the significantly lower efficiency when compared to conventional p-n junction 13

15 chapter 1 solar cells. Herein, both the operating mechanism and the main characteristics of DSCs will be briefly described. Also, evaluation of the different strategies proposed in the last years to enhance their performance will be carried out, special interest being put in those related to the optical design of the cell. The motivation and the aim of this thesis will be presented at the end of this chapter. 1.2 Dye Solar Cells The photovoltaic field, conventionally dominated by crystalline silicon solid state junction cells, has undergone abysmal changes over the last years and is being now challenged by the emergence of new devices based on technologies that involve the use of lesser quantities of cheaper and less refined input materials [3-6,8]. The indirect band gap for silicon, responsible for relatively low optical absorption over the near infrared spectral region, constitutes one of the main factors that impede the reduction in the optimum thickness required to fabricate efficient solar cells and, therefore, the lowering in the production costs. So, current research is being devoted, firstly, to the search for more suitable materials in terms of a good compromise between efficient harnessing of solar radiation and lower production costs due to savings in both materials and processing and, secondly, to the development of light trapping tools that could help to further improve absorption, thus compensating the effect of using thin film technologies [10-14]. Within this context, potentially useful alternatives exploited in recent times for thin film solar cell applications are those concerning amorphous silicon, ternary compound semiconductors, organic/inorganic hybrid or purely organic compounds that can display p or n-type semiconducting properties. Among them, DSCs are foreseen as promising candidates and will be at the focus of our attention along this thesis since, after recent progress, they are expected to meet not only relatively high efficiency values but also non conventional functionalities that could also promote their valuable characteristics, such as low production costs, transparency and good performance at low light intensities and under different incidence angles [15] Generalities DSCs are photovoltaic devices that separate the optical absorption and charge separation processes by the combination of a sensitizer and a wide band gap semiconductor, such as titanium dioxide (TiO 2 ). This novel concept of solar device arose as a result of two significant observations. First, it was found that TiO 2 from photoelectrochemical cells could split water with a small bias voltage when exposed to light [16]. However, the conversion efficiency was low due to the large band gap for TiO 2 (~3.2 ev for anatase), which makes it transparent for visible light. Next achievement involved an absorption range extension of the system into the visible region by means of dye sensitization of the semiconductor electrode [17,18]. Charge transport was realized by 14

16 dscs and light trapping strategies injection of electrons from photoexcited dye molecules into the conduction band of the semiconductor oxide on one side and by ion flux within a redox couple based liquid electrolyte that covered the whole system on the other side. Since only dye molecules directly attached to the semiconductor surface were able to efficiently inject charge carriers into the semiconductor network, light absorption was low and limited when employed flat surfaces of the semiconductor electrode. This inconvenience was solved by introducing mesoporous layers made of TiO 2 nanocrystals (around 20 nm in size), which enormously increased the amount of adsorbed dye due to their huge active surface area, and yielded laboratory efficiencies of 7%, as reported by B. O Regan and M. Grätzel in a pioneer work in 1991 [19-21]. Figure 1 shows a common scheme of a liquid electrolyte based DSC. It usually consists of a layer made of a few micrometers of TiO 2 nanocrystals, which have been sintered together to allow electronic conduction to take place, that is deposited onto a conductive transparent substrate, typically fluorinated doped tin oxide (FTO). A monolayer of dye molecules, generally based on a ruthenium polypyridyl complex, is then attached to the surface of such nanocrystals. The whole ensemble is finally soaked with a liquid electrolyte, often containing the redox pair iodide/tri-iodide, which is also in contact with a colloidal platinum catalyst coated counter electrode. The liquid electrolyte plays a crucial role in the device functioning since the electron donator species (I - ) that compose it are responsible for reducing the oxidized dye molecules generated after light absorption, whereas the electron acceptor ones (I 3 - ) are reduced back on the counter electrode, the electrical circuit being completed via electron migration through the external load. Figure 1. (a) Illustrative scheme and (b) energetics of the operation mechanism that governs the functioning of a typical liquid electrolyte based DSC. 15

17 chapter 1 In contrast to silicon devices, charge separation process in DSCs is primarily driven by the energy levels of the different species at the TiO 2 /dye/electrolyte interface, any electric field gradient in the TiO 2 electrode being screened out due to the high concentration of mobile ions employed in the liquid electrolyte [22-24]. As schematized in Figure 1, electron injection requires the dye excited state to be more reducing than the TiO 2 conduction band, whereas regeneration of the dye ground state by the redox couple requires the dye cation to be more oxidizing than the redox pair I - - /I 3 [25,26]. Consequently, the voltage output of the device is given by the splitting between the TiO 2 Fermi level and the chemical potential of the redox electrolyte, the former being related with the density of both injected electrons and charge traps in the band gap of TiO 2. Under illumination conditions, the density of electrons injected into the semiconductor conduction band increases, thus raising the Fermi level towards the conduction band edge and generating a photovoltage in the external circuit [27,28]. On the other hand, charge transport processes within the cell are considered to be diffusive and are driven by concentration gradients generated in the device, which make electrons to move towards - the working electrode and I 3 ions towards the counter electrode [27,29]. Although there also exist loss pathways of photogenerated electrons due to interfacial charge recombination processes that might limit the quantum efficiency of charge separation and collection processes, they are estimated to be much slower than the electron diffusion across the TiO 2 film [30,31]. At this point, it is important to mention that one of the most critical recombination processes takes place between electrons injected into the semiconductor and the I - 3 species (also known as hole carriers in these systems) present in the surrounding liquid electrolyte. Since kinetics of the system plays a key factor determining the main characteristic parameters of the device, extensive research has been addressed to give further insights on the influence the former can have on the different mechanisms involved in the solar cell functioning and largely motivated to achieve optimized efficiency devices [26,32-34] Photovoltaic characterization and DSC prospective applications Since light harvesting and charge transport are well separated processes in this type of solar cells, and taking the abovementioned energetic requirements into account, a vast amount of options is opened up for the absorber material [35-38]. Figure 2 displays the optical absorption spectrum corresponding to a typical N719 sensitized transparent nanocrystalline TiO 2 film. Such sensitizer has been established as a standard dye commonly employed in the DSC field due to its relatively good optical absorption and chemical stability properties [39], electron injection from the excited state of the dye to the conduction band of the semiconductor occurring with a quantum yield near 100%. For the sake of comparison, the spectral distribution of power density for AM 1.5 solar radiation is also plotted in the same graph. From it, the main drawback when most of ruthenium based sensitizers are employed can be appreciated and concerns the extremely low optical absorption on the red region of the visible spectrum. As expected, efficient solar radiation 16

18 dscs and light trapping strategies harnessing by the dye molecules adsorbed onto the semiconductor oxide will be intimately linked to effective solar to electric energy conversion by DSCs. The latter can be experimentally checked by measuring the current voltage characteristics (also known as IV curves) of the solar device when connected to an external load, as it is illustrated in Figure 2. The relation between photocurrent and voltage is obtained by varying the resistance of the outer circuit, I SC being attained when the resistance of the outer circuit is zero (thus voltage is zero) and V OC when the resistance reaches its maximum value (thus photocurrent is zero). The power output of the device is finally calculated as the product of I and V, the maximum value of this curve being commonly referred to as the maximum power output (P max ) of the device. Pictures showing several prototypes of transparent, coloured and flexible DSCs are also presented in Figure 2. Figure 2. (a) Comparison between the spectral irradiance of AM 1.5 sunlight and the absorption spectrum corresponding to a N719 sensitized TiO 2 film. (b) Characteristic IV curve measured for a DSC and photovoltaic parameters determining the overall conversion efficiency of the device (I SC, V OC, ff). (c-e) Prototypes of DSCs displaying the main features they can be designed with: colour control, transparency and flexibility (extracted from Oxford Photovoltaics, Dyesol and G24i web sites). Based on these considerations, it is clear that approaches aimed for improving or extending toward the red the optical absorption properties of DSCs will be needed to attain optimized performance devices. Actual research is now being divided in two main branches that directly deal with this key issue. The first one concerns the development of newly designed dyes, which display extraordinarily high molecular extinction coefficients 17

19 chapter 1 and better optical absorption on the red [40-47]. Although considerable advances have been made in this direction, the engineering of a single sensitizer that absorbs efficiently over the entire visible continues being a very difficult task, which is linked to a large extent to essential structural requirements it must fulfil to work properly [47-50]. For this reason, combination of several dyes displaying complementary absorption spectra to both enhance and broaden the spectral response of DSCs is seen as a promising approach to obtain panchromatic systems, that is, those that are sensitive to light of all colours within the visible spectrum. Among them, alternatives based on either the co-sensitization of nanocrystalline semiconductor oxide films or the use of Förster resonance energy transfer between different chromophores, one of them commonly dissolved inside the liquid electrolyte and other attached to the semiconductor, have been also explored in recent years [35,38,51,52]. Nonetheless, further optimization of these systems is still needed to surpass the record reached by using the most efficient single dye approaches. The second way to enhance the capability of DSCs to harvest incident light infers the implementation of light trapping assemblies, which could increase the residence time of photons of a certain wavelength range within the active layer, thus improving its optical absorption on that spectral range. This attempt could be in principle implemented without affecting the extremely delicate sensibility of charge separation and recombination dynamics within the solar cell whenever proper flow of charges is not hindered. Fundamental aspects of some of the strategies that make use of this last approach will be treated in next section. Following the above mentioned approximations, the capability of DSCs as incident light harvesters, which will be given by their absorptance (A) or light harvesting efficiency (LHE), will undergo changes that will affect their spectral photoelectric response, commonly referred to as incident photon to collected electron (IPCE) efficiency or external quantum efficiency (EQE). This is defined as the number of generated electrons measured as photocurrent in the external circuit divided by the number of incident photons that strikes the cell at a certain wavelength, and will depend upon the intrinsic characteristics of the device as follows: IPCE LHE (1), where and are, respectively, the yield of electron injection from the excited state of the dye to the conduction band of the semiconductor oxide and the charge collection efficiency. The photocurrent density produced by the device at short circuit conditions, J SC, can be calculated by integrating the IPCE expression weighted by the solar radiation spectrum: J SC= qipce( )F( )d (2), where q is the electron charge and F( ) is the ratio between the solar spectral irradiance and the photon energy. 18

20 dscs and light trapping strategies The overall conversion efficiency (η) of the device will be finally determined by the characteristic parameters extracted from the IV curve and by the incident light intensity (P in ) that impinges the cell [53,54]: J V ff SC OC = (3) Pin, where ff, denoted as the fill factor of the cell, can be calculated through the following equation: ff P max JSCV (4) OC Figure 3 depicts the increase in conversion efficiency undergone over the last years for this new generation of photovoltaic devices when compared to other thin film technologies. As it can be observed, a huge progress due to improvements in both performance and stability of DSCs has become this technology competitive for practical uses within the photovoltaic field. Figure 3. Increase in conversion efficiency (%) undergone over the last years for different thin film technologies, among them, those related to DSCs (extracted from Improved long term stability makes the latter competitive with other existing alternatives. It is important to note that most of the interest devoted to this new generation of solar devices has been envisioned primarily for building integrated photovoltaic (BIPV) applications, since DSCs can be designed as indoor colourful decorative elements, roof walls or lightweight flexible modules with the added value of transparency, property that can be tailored as a function of the spectral range in which optical absorption by the sensitizer takes place. Their low cost and ease of production by means of scalable and semi automated manufacturing processes, such as roll to roll, should also benefit large scale applications [55-57]. Figure 4 shows some of the potential applications they can be 19

21 chapter 1 implemented for. Indeed, the latest improvements have made possible commercialization of initial products aimed towards portable applications, such as transportable chargers, solar bags and wireless solar keyboards. On the basis of this success, new challenging opportunities are opened up for the DSC industry with the purpose of being finally incorporated into much bigger installations. Figure 4. Sectors in which DSC technology can find applications as functional and decorative elements. Some of the companies involved in the development and commercialization of DSCs for some of these applications are: Sony Global, Dyesol, Logitech, G24i, Solaronix, Polysolar. 1.3 Light trapping strategies As commented before, thin film photovoltaic technologies are governed by the general trend of reducing the cost of solar electricity. To achieve this goal, thin photoactive layers need to be used, which consequently leads to a poor optical absorption of the device. Also, the thickness of such layer cannot be enlarged at will without affecting adversely some of the characteristic parameters of the cell, reaching mass transport limitations in the electrolyte or reducing the photovoltage due to an increased recombination rate of injected electrons [24,32,58,59]. Although this inconvenience can be solved in part by employing recently developed dyes of high molecular extinction coefficient, light trapping strategies are also proposed as useful tools to compensate that incomplete light harnessing and to achieve optimal cell efficiencies. Whereas some of these structures have been largely employed in other types of thin film solar cells [11,60-66], their implementation in DSCs has involved a great challenge and high expectations are now being put on them. In this section, some of the most promising strategies proposed to enhance the LHE of such devices will be discussed, going from more conventional approaches based on diffuse light scattering to more original ones in which light trapping due to photonic effects occurs. 20

22 dscs and light trapping strategies Diffuse light scattering approach Diffuse light scattering by large TiO 2 particles was first theoretically examined by A. Usami in 1997, who proposed a DSC structure in which those were implemented in the cell as a backscattering layer for a more effective absorption of incident solar energy [67]. Later, J. Ferber and J. Luther demonstrated through theoretical calculations that significant enhancement of optical absorption could be also expected with a suitable mixture of small and large particles, which resulted in both a large surface area for the dye molecules to be adsorbed and an effective light scattering [68]. Although the strength of the scattering effect by a single particle can be precisely described according to the Mie theory and will depend on features such as its size and refractive index in a first approximation [69,70], simulation of multiple scattering caused by a film in which particles are in contact with each other becomes a more complex task. For this reason, numerous research works have addressed since then a more accurate description of multiple light scattering from large particles when incorporated in DSCs under different configurations, which have also allowed evaluating its potential for an efficient light collection [71-74]. Schemes representing the main optical designs of the cell by considering the abovementioned approximations are displayed in Figure 5. When photons collide with larger particles they are strongly scatter, thus leading to an increase in the optical path length of the incident light within the absorbing film. Figure 5. (a-c) Schemes showing different trajectories travelled by the incident light that strikes a DSC depending on its optical design. The standard model consists of a semitransparent electrode, whereas the one based on diffuse scattering involves the incorporation of larger particles inside the nanocrystalline film or deposited as an additional layer on top of it (extracted from reference [78]). By comparing both theory and experiments, it was concluded that enhanced LHE for the cell could be obtained when larger particles were incorporated as a consequence of the increase in the matter radiation interaction time of the incident light within the nanocrystalline film, which might be eventually traduced into an improved photocurrent density [72,75-77]. This enhancement was preferentially located at the spectral range of longer wavelengths, for which the dye extinction coefficient was also smaller. Although it has been demonstrated that the double layer approach can provide larger reinforcements of the optical absorption than the one in which a mixture of small and larger particles is employed [72,76,78], certain critical aspects for the abovementioned configurations need to be considered when optimum performance devices are looked for. Apart from an appropriate balance between surface area and light scattering, which is crucial to obtain 21

23 chapter 1 the best efficiency values, a full analytical description of how the different light scattering designs might electrically and optically affect the complete systems under operating conditions is also of major relevance. From the experimental point of view, it is noticeable to remark that practically all DSCs exhibiting the highest efficiency values reported until now at laboratory scale incorporate an additional porous backscattering film made of larger particles, which are deposited onto the photoactive layer to reflect any non absorbed light back into the film [79-81]. To do so, packings of particles having sub-micrometer size, typically above 300 nm, made of transition metal oxides of high refractive index, namely TiO 2, are proposed as effective light scattering centers. In this regard, the dependence of the light scattering effect on particle size has been also analyzed in recent times [82,83]. Figure 6 shows a comparison between the IPCE values and the current voltage characteristics obtained when a scattering overlayer is deposited on a semitransparent film of a DSC with respect to those attained for a reference one in which no additional layer has been incorporated. It can be clearly observed that better IPCE efficiencies are reached over the entire visible range in the former case, which is traduced into an improved efficiency for that cell. SEM pictures of the different kinds of particles commonly employed to form both the transparent active and the diffuse scattering layers are also presented in Figure 6. Figure 6. (a,b) Effect the addition of a backscattering layer deposited on top of a nanocrystalline working electrode of around 7 μm in thickness has on the DSC performance (red line). For the sake of comparison, results corresponding to a reference cell in which no scattering layer has been incorporated are also plotted (black line). (c,d) SEM pictures of different kinds of TiO 2 particles commonly employed to form both the transparent active and the diffuse scattering layers (graphs and pictures extracted from reference [82]). 22

24 dscs and light trapping strategies In recent years, the fabrication of novel hierarchical pore structures, which consist of a kind of large spherical aggregates formed by primary highly crystallized nanoparticles, have received significant attention in the field of DSCs since they can be designed to provide both large specific surface area and light scattering effects at the same time [84-89]. These uniform sub-micrometer sized TiO 2 beads with tunable inner pore sizes could be synthesized through a combination of sol gel and hydrothermal processes. Besides the existence of internal pores inside the spheres, large interstitial voids were also generated between them when those were employed as base material to create the photoactive layer of a DSC [84-87]. Although in a first approximation this strategy could be beneficial to achieve efficient electrolyte diffusion while keeping high dye loading, it could sometimes result in a poorer electrical contact due to the small contact area between neighboring spheres [86]. In spite of this drawback, better efficiency values were usually reached when compared to all nanocrystal made electrodes as a consequence of the superior light scattering properties for the aggregate architectures. Other attempts have explored not only the use of such mesoporous spheres to form a typical scattering layer on top of a nanocrystalline film [88] but also the mixture of both to give rise to the active photoelectrode [89]. Figure 7 summarizes some of the most common ways through which diffuse scattering can be introduced into a DSC, as well as some SEM images corresponding to the abovementioned mesoporous TiO 2 spheres at different stages of their fabrication process. Figure 7. (Top scheme) Illustration of different alternatives for light scattering centers to be introduced into DSCs. Light scattering and dye loading can be simultaneously promoted when these particles are designed as aggregate structures (extracted from reference [89]). (a-c) SEM images corresponding to novel hierarchical pore structures proposed as efficient scattering centers (extracted from reference [85]). 23

25 chapter 1 Under certain experimental conditions, efficiency improvements up to 20% have been reported for DSCs displaying some of the diffuse scattering configurations abovementioned when compared to those made of nanocrystalline electrodes of comparable thickness. Also, since multiple scattering caused by disordered particles is not spectrally selective in the regime usually employed in DSCs, a general consequence when these strategies are employed to increase the optical path length of the incident light within the device is the loss of transparency of the cells, which make them useless for potential applications such as photovoltaic windows Coherent light scattering approach Dielectric materials comprised of periodic arrays of submicroscale elements, instead of random type structures like those explained before, have been also suggested as promising alternatives to enhance the optical absorption of thin film solar cells [90-92]. They are referred to as photonic crystals and were first theoretically envisioned by E. Yablonovitch and S. John in 1987 as viable pathways to manipulate light propagation [93,94]. In such materials, periodic modulation of the refractive index along one, two or three spatial dimensions could give rise to ranges of forbidden frequencies, known as photonic band gap, through which light could not propagate [95,96]. Depending on their dimensionality, these materials are classified as one, two or three dimensional photonic crystals (1DPC, 2DPC and 3DPC, respectively). Light of wavelength ranges falling within the photonic band gap will be diffracted or reflected back, therefore leading to brilliant colours of structural origin similar to those observed in nature, which can be exemplified by the iridescent colours of opals, butterfly wings and others. The first practical realization in which a photonic crystal structure was incorporated in a DSC was reported by S. Nishimura et al. in 2003 [97]. In this case, a particular kind of 3DPC, denoted as inverse opal, was coupled to a standard TiO 2 nanocrystalline electrode. The former structure was composed of air voids periodically arranged in a semiconductor oxide matrix, in this case TiO 2, thus allowing proper electrolyte diffusion to take place. By doing so, the photocurrent generated across the visible spectrum was increased by around 26% when compared to a standard reference cell, although the origin of this enhancement remained unclear. A couple of years later, a thorough theoretical analysis reported by A. Mihi and H. Míguez provided insight into the origin of this improvement. After considering different configurations in which the photonic crystal could be implemented into the solar device and evaluating the corresponding simulated optical absorption spectra, they demonstrated that the photocurrent improvement could be mainly attributed to the optical coupling of the absorbing nanocrystalline TiO 2 film and the photonic crystal [98]. This coupling resulted in multiple resonant modes partially confined within the absorbing layer at wavelengths lying within the photonic band gap of the periodic structure, which would give rise to longer matter radiation interaction times and thus to higher probability of optical absorption on such spectral range [99]. Although further experimental results convincingly confirmed the proposed model, it was also accepted a

26 dscs and light trapping strategies certain contribution to the photocurrent enhancement due to diffuse scattering phenomena [ ]. Figure 8 shows the experimental results obtained in reference [97] in comparison to those theoretically calculated in reference [98]. It can be seen the similarity between the spectral variation of the photogenerated current and the optical absorptance calculated for both DSCs, the first one in which the absorbing film is coupled to the photonic crystal structure and the other one having a standard and non periodically structured electrode. Photographs corresponding to natural biomaterials, which are composed of micro and nanostructures in the microscale, displaying structural colour are also presented in Figure 8. SEM image of an artificial opal employed as colloidal crystal template for the fabrication of air void periodically arranged structures is illustrated in this figure together with those ones representing the final inverse opal structure. Figure 8. (a,b) Photographs displaying iridescent colour of opals and butterfly wings as a result of the arrangement of their components at the microscale and SEM image of an artificial opal made of sub-micrometer sized spheres that resembles those structures present in nature. (c,d) SEM images corresponding to inverse opals made of TiO 2 (extracted from reference [102]). (e,f) Spectral variation of both the measured photocurrent and the calculated optical absorptance, as reported in references [97,98], for a DSC in which the absorbing film is coupled to an inverse opal film (red line) and a reference cell fabricated using a standard and non periodically structured electrode (black line). Thus, by making use of this approach, optical absorption enhancement of a DSC could be selectively tuned over the visible range by appropriately selecting the back reflecting properties of the periodic structure, which could be done by a simple modification of the lattice parameter of the periodic arrangement. This effect was analyzed, both theoretically and experimentally, by A. Mihi et al., who concluded that it was also possible to pile up several inverse opal films with different lattice parameters to obtain an amplification of optical absorption over a wider spectral range [102,103]. Another attempt to integrate such optically active layers was performed onto copolymer derived mesoporous TiO 2 films, which were deposited as the supporting active layers of the solar device [104]. It provided both a close contact and a smooth interface between the mesoporous TiO 2 and the 25

27 chapter 1 photonic crystal layers, as well as direct pore and electrical contact between them. Despite the noticeable interest arisen by this kind of systems, the need of employing long fabrication steps have limited their practical use in DSCs. Also, since thick layers are usually required for the inverse opals to attain high reflectance intensities, it could also lead to deleterious effects on the cell performance under real operation conditions. Based on photovoltaic elements commonly employed for other solar cell technologies, an original idea was conceived by means of the development of a wavelength selective photonic crystal solar concentrator with the aim of improving the output power of DSCs [105]. In general, those elements are designed to collect light over large areas and to focus it onto smaller areas of solar cells through the use of large and expensive mirrors that track the sun. The authors of this work proposed a novel 3DPC based concentrator made of latex spheres, which could be fabricated to present different photonic band gaps depending on the lattice parameter of the periodic structure, deposited onto a concave watch glass with the idea of making light of different wavelength ranges to converge on the solar device. Figure 9 shows a scheme of the photovoltaic system containing a photonic crystal concentrator, as well as a series of photographs of the different colours observed for solar concentrators constructed using several sphere diameters. When the photonic band gap of the periodic structure was tailored to roughly overlap the spectral range of maximum absorption of the dye (around 530 nm for a N719 sensitized film), the largest increase in photocurrent was obtained. Figure 9. (a,b) Scheme that illustrates a photovoltaic system based on a 3DPC concentrator and a set of photographs showing different colours reflected by the system (extracted from reference [105]). 26

28 dscs and light trapping strategies These results also open the way to alternatives in which the photonic crystal is not directly attached to the nanocrystalline electrode of a DSC and that become interesting choices to fully exploit the optical properties of these materials without negatively affecting the solar cell functioning. In that regard, recently reported methods that involve the fabrication of 3DPC films of high optical quality, which can be easily transferred to arbitrary substrates for an enhanced light trapping, have been also tested [106]. Apart from the performance improvements these latter structures can give rise to, they present the added advantage of allowing for a precise selection of the spectral range at which optical absorption is going to occur, thus leading to both control over the aspect and the semi transparency of the cell. As commented before, even greater enhancement factors might be expected by broadening the wavelength range reflected by the photonic structure Other light trapping approaches Engineering of suitable metallic structures at the nanoscale has become the field of plasmonics a rapidly expanding new alternative for light trapping in thin film photovoltaics [14,107,108]. In general, those approaches are supported by surface plasmons, which are optically induced collective oscillations of free electrons occurring at the interface between a metal and a dielectric. The attractiveness of plasmon resonances is that they can effectively confine the optical excitation in a nanoscale volume, and thus mediate strong optical interactions within this volume. Among them, the use of nanometer sized metallic particles, typically composed of silver or gold, has been recently proposed as a viable strategy to increase the optical absorption of DSCs by taking advantage of the strong local field enhancement effects occurring in the vicinity of such particles when embedded within the active layer [ ]. Although initial attempts included bare metallic particles under different configurations within the bulk of the active layer, they were demonstrated to be corroded by the liquid electrolyte or act as charge recombination centres, thus preventing the beneficial effects of plasmonics [111,112]. Subsequent works have focused on providing solutions to this problem through the incorporation of core shell metal oxide nanoparticles, which are expected to show better thermal and chemical stability during functioning and also to inhibit possible quenching phenomena [ ]. Figure 10 shows a general scheme of the light trapping mechanism that takes place by the excitation of localized surface plasmons in metal nanoparticles embedded in a semiconductor. As it can be observed, the incorporation of such nanoparticles gives rise to an intense localization of the electric field close to their surface, which is employed to enhance the optical absorption and, hence, the photogenerated current by the solar device. 27

29 chapter 1 Figure 10. (a,b) Scheme that simulates light trapping by the excitation of localized surface plasmons in metal nanoparticles embedded in a semiconductor matrix and, as an example, the near field calculated close to the surface of a 25 nm diameter Au particle embedded in a medium with refractive index of 1.5 (extracted from reference [14]). (c-d) Improvement in photocurrent attained for a DSC made of a thin active layer after incorporating core shell metal oxide (Ag@TiO 2 ) nanoparticles and the corresponding TEM micrograph (extracted from reference [113]). Finally, innovative hybrid structures that integrate optical fibers and nanowire (NW) arrays for achieving three dimensional DSCs have been recently developed [115,116]. NWs are grown normal to the optical fiber surface, so when light illuminates the fiber from the axial direction, multiple opportunities for light harnessing at the interfaces are created due to its internal reflection within the fiber. In comparison to the case of light illumination normal to the fiber, the internal axial illumination enhances the energy conversion efficiency of a rectangular fiber based hybrid structure by a factor of up to six for the same device. This strategy allows not only enhancing the surface area for the dye loading but also promoting rapid charge collection due to the use of aligned NW arrays. Although the cross section of the optical fiber can be cylindrical or rectangular, better results are obtained for the configuration that makes use of the second one. Figure 11 displays the design corresponding to this sophisticated ensemble, as well as the different results attained for the two different configurations employed. 28

30 dscs and light trapping strategies Figure 11. (a,b) Design and principle of a DSC based on a three dimensional arrangement of NWs that are grown vertically on the optical fiber surface. (c) IV curves measured for a DSC like the one mentioned before when it is oriented normal or parallel to the fiber axis. These figures have been extracted from reference [115]. 1.4 Motivation and goals of this thesis At the beginning of this research work, the light trapping strategies that had been employed the most within the field of DSCs to improve solar radiation harnessing involved the integration of diffuse scattering or opal based overlayers on top of the absorbing electrode. Keeping in mind the potential applications of such devices, it seems to be reasonable that those alternatives allowing a better solar cell performance while preserving their main features, i.e. transparency, would be of major practical interest. In the same way, they should be easily integrable and compatible with the processing method of DSCs. Among them, photonic structures have shown to be excellent candidates to tailor the spectral range in which amplification of optical absorption takes place, and hence the appearance and/or the transparency of the device. However, there is still a long way to go in terms of achieving an optimized efficiency device based on such kind of structures, which is mainly a consequence of the poor capability as back reflectors for the majority of photonic crystals implemented until now. Therefore, the search for more robust and highly reflecting ensembles, which would satisfy all the above mentioned requirements, might help to find a solution facing that state. At this point, the motivation of this work was to find a fast and reliable method to build highly reflecting 1DPCs, in which a periodic modulation of the refractive index could be created by alternating different types of nanoparticles and by controlling the level of porosity of each layer, with the aim of being used as efficient back reflectors for an enhanced light trapping when implemented in DSCs. The goals of the thesis herein 29

31 chapter 1 presented, entitled Porous One Dimensional Photonic Crystals for Enhanced Photovoltaic Performance of Dye Solar Cells, concern: The development of an experimental procedure that allows integrating nanoparticles of different type, mainly SiO 2 and TiO 2, to build highly reflecting multilayer stacks or 1DPCs with interconnected porosity. It also implies an exhaustive characterization of the structural and optical properties of the materials finally obtained. The main outcomes resulting from this study will be presented in Chapter 2. The integration of the so built nanoparticle based multilayers having Bragg reflector properties into standard DSCs. Later evaluation of the optical and photovoltaic properties of the 1DPC based solar devices is also required to propose a suitable design for the periodic ensemble. These issues will be discussed in detail in Chapter 3. A conscious optimization of the open pore network for the highly reflecting periodic ensembles that allows achieving optimized performance solar devices. The interplay between structural colour, transparency and efficiency in standard DSCs coupled to such 1DPCs of enhanced porosity operating at different visible wavelength ranges is also analyzed for future applications in BIPV. The most outstanding conclusions extracted from this study will be presented in Chapter 4. 30

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