Dye Sensitized Solar Cells for Economically Viable ... - ACS Publications

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Dye Sensitized Solar Cells for Economically Viable Photovoltaic Systems Hyun Suk Jung and Jung-Kun Lee* Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Korea ABSTRACT: TiO2 nanoparticle-based dye sensitized solar cells (DSSCs) have attracted a significant level of scientific and technological interest for their potential as economically viable photovoltaic devices. While DSSCs have multiple benefits such as material abundance, a short energy payback period, constant power output, and compatibility with flexible applications, there are still several challenges that hold back large scale commercialization. Critical factors determining the future of DSSCs involve energy conversion efficiency, long-term stability, and production cost. Continuous advancement of their long-term stability suggests that state-of-the-art DSSCs will operate for over 20 years without a significant decrease in performance. Nevertheless, key questions remain in regards to energy conversion efficiency improvements and material cost reduction. In this Perspective, the present state of the field and the ongoing efforts to address the requirements of DSSCs are summarized with views on the future of DSSCs. between 20 and 50 °C is negligible.3 These features enable DSSCs to produce more power in outdoor applications than other solar cells, although their efficiencies are similar at room temperature. The performance of DSSCs increases at a lower light intensity (0.5 V) adversely affects Voc, and the photovoltage of DSSC using the I−/I3− redox couple is restrained to ∼0.8 V.32,33 Since redox couples play a key role in determining Voc, researchers have focused on finding alternative redox couples. Various halides, organic compounds, and transition metal systems have been tested as redox couples. However, most new redox couples have not outperformed I−/ I3−. Recently, Co(2+/3+) tris(bipyridine)-based redox electrolyte matching with donor−π−bridge−acceptor zinc porphyrin dye as a sensitizer (YD2-o-C8) significantly improved energy conversion efficiency up to 12.3% (Figure 2).34 This high efficiency is due to the retarded back electron transfer of photoelectrons to Co complexes, thereby increasing Voc and JSC to 0.94 V and 17.7 mA/cm2. The presence of the four octyloxy groups in the YD2-o-C8 dye suppresses the access of Co3+tris(bipyridyl) to the TiO2 surface and retards the carrier recombination rate. This result indicates that study on the dyes needs to be performed in conjunction with electrolyte and semiconductor film studies to match their electrochemical

Study on the dyes needs to be performed in conjunction with electrolyte and semiconductor film studies to match their electrochemical properties and maximize the energy conversion efficiency of DSSCs. Emerging Strategies in DSSCs. Levelized cost of energy (LCOE) is the fairest methodology to compare the sustainability of different energy supply technologies. In the case of a photovoltaic system, the LCOE is determined by several variables such as solar insolation, energy conversion efficiency, and system degradation rate.35 The detailed equation for LCOE has been reported in previous studies on energy economics.35−37 In addition to high energy conversion efficiency, a lower system degradation rate that is closely related to long-term stability of the devices is important in reducing the LCOE and making the photovoltaic system more economically viable. From the stand point of long-term stability and reliable operation, a problematic component of DSSCs is the liquid electrolyte. If the sealing of the device is not perfect, the liquid electrolyte gradually evaporates away and impurities such as water and oxygen molecules permeate into the cell. Therefore, the assembly of DSSCs containing liquid electrolyte is required to minimize solvent leakage/vaporization. To improve the stability of DSSCs, different kinds of electrolytes have been extensively studied to supersede the liquid-type electrolyte. A solid-state hole conductor is an ideal form of the electrolyte for commercialization of DSSCs, since this addresses the problem of conventional liquid electrolytes such as leakage and evaporation. Therefore, the compatibility of p-type inorganic semiconductors and organic hole conductors with DSSCs has been investigated in detail. At present, Spiro-OMeTAD (2,2(,7,7(-tetrakis-(N,N-dipmethoxyphenylamine)9,9′-spirobifluorene) is the most commonly used organic hole conductor for solid state dye sensitized solar cells (SDSSCs), due to its small molecular size and high solubility in organic media. In 1685

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Figure 3. (a) Temperature dependence of electrical conductivity (s, filled squares) and Seebeck coefficient (S, filled circles) of CsSnI3. (b) Energy levels of the components of the CsSnI3 solid-state solar cell. The valence band maximum (VBM; orange) and the conduction band minimum (CBM; blue) are represented in eV, along with the energy difference between the edges. The ground (HOMO; orange) and excited states (LUMO; blue) of N719 dye is also shown. Reproduced with permission from ref 47. Copyright 2012 Nature Publishing Group.

increase melting temperature and hole mobility. For instance, when the alkyl chain in imidazolium salts is replaced by an ester group, dimers of conductor molecules are formed and threedimensional (3-D) ionic channels of iodides are created.41 These changes facilitate charge transfer along the polyiodide chain and increase conductivity of the solid electrolyte. The second research direction is to develop a new kind of polymer electrolyte. One example is an organic ionic plastic crystal that is in the intermediate stage between solid crystals and liquids. Different ionic plastic crystals such as N-methyl-N-ethylpyrrolidinium dicyanamide [C2mpyr] [N(CN)2] and 1-ethyl1-methyl pyrrolidinium iodide (P12I) have been tested as the electrolyte of SDSSCs.42 Their efficiency is higher than 5%, which is similar to the efficiency of SDSSCs using SpiroOMeTAD. In addition, these ionic plastic crystals allow for the stable operation of SDSSCs even at 80 °C. The third research direction is to gelate the liquid electrolyte by adding low molecular weight polymers or oxide nanoparticles. Recently, cyclohexanecarboxylicacid-[4-(3-octadecylureido)phenyl]amide was used as the gelator of the 3-methoxypropionitrile (MPN)based liquid electrolyte.43 This electrolyte is not in a complete solid state, but in a quasi-solid state. The efficiency of quasisolid dye sensitized solar cells (QSDSSCs) with the new molecular weight gelator is 9.1%, which is close to that of the liquid electrolyte DSSCs. As an alternative to polymer-based solid electrolytes, inorganic p-type semiconductors including CuSCN, CuI, NiO, CuAlO2, and CsSnI3 have been also investigated. CuSCN is a typical inorganic solid electrolyte that has been explored for DSSCs. It is relatively stable and provides reasonable energy conversion efficiency (∼2%).44 However, the low electric conductivity of CuSCN (∼10−2 S/m) prevents further improvement of SDSSCs, which motivates subsequent research on the doping effect. Cl− in the thiocynate site and triethylamine coordinated Cu(I) in the cuprous site are found to increase the electric conductivity to ∼1 S/m by increasing the carrier concentration. However, the best efficiency of SDSSCs using CuSCN in the literature is still 3.4% at AM 1.5.45 CuI-based SDSSCs exhibit better energy conversion efficiency (3−6%) than those that are CuSCN-based.46 However, CuI is quickly photodegenerated to Cu2O or CuO at the CuI/TiO2 interface. Very recently, Chung et al. reported a new type of p-

addition, the redox potential of Spiro-OMeTAD is more positive than that of I−/I3− couple, which is beneficial to increasing the open circuit voltage (Voc) of SDSSCs. When Spiro-OMeTAD is combined with a novel high extinction organic dye using 4,4′-didodecyl-4H-cyclopenta[2,1-b:3,4-b′] dithiophene as a spacer between donor and acceptor groups, the efficiency of SDSSCs is as high as 6.08%.38 Compared with the liquid electrolyte, the solid hole conductors have lower charge carrier mobility. In SpiroOMeTAD, the hole mobility is only 10−4 cm2/(V s), which is much smaller than the charge carrier diffusivity in the liquid electrolyte. The low carrier mobility increases the probability of carrier recombination during the transport process and reduces the photocurrent density. Although certain p-type organic semiconductors such as PEDOT/PSS have better electrical conductivity (10−3 to 500 S/cm), the size of PEDOT/PSS in a secondary or tertiary structure is too large to pass through the mesopores of TiO2 nanoparticle films. This partial filling of the pores is caused by the small pore size of the mesoporous films. The unfilled portion of the mesoporous photoelectrode and the low carrier mobility of the solid electrolyte are the source for electron−hole recombination and parasitic current in SDSSCs where the charge transport is controlled via a trap-limited diffusion. Although the large molecules clearly have a problem filling the mesopores, the pore filling capability of the small molecules such as Spiro-OMeTAD is controversial. Melas-Kyriazi et al. reported that it is difficult to fully fill the pores of the thick photoelectrode with Spiro-OMeTAD, although it is well dissolved in the organic media.39 Their depth profiling analysis shows that the filling fraction of the 2.5 um thick mesoporous film is 60−65%. As the film becomes thicker than 3 um, the pore filling fraction and energy conversion efficiency of SDSSCs decreases. However, Docampo et al. claimed that the pore filling fraction is 80% in 2-um-thick mesoporous films and 60% in the 5-um-thick mesoporous film.40 Their conclusion is that the fast carrier recombination at the interface of the hole conductor limits the energy conversion efficiency of SDSSCs. New p-type conductors are extensively studied to solve problems of existing solid electrolytes. First, the molecular structure of imidazole-based ionic liquids is modified to 1686

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Figure 4. (a) Configuration of solar cells containing silver nanoparticles (NPs) and dye. (b) IPCE of the solar cell with dye only, with silver NPs only, and with dye and silver NPs.48 Reproduced with permission from ref 48. Copyright 2009 American Chemical Society.

determined by the refractive index of the metal and the surrounding dielectrics, it is difficult to control the plasmonic frequency of metal nanoparticles. Therefore, several groups have changed the shape of the nanostructure metals and shifted the surface plasmon frequency within the absorption spectrum of DSSCs. Ding et al. explored the effect of the Ag nanoshell, and Chang et al. investigated the role of Au nanorods.51,52 Both groups proved that a change in the shape of nanostructured metals enhances the photon-electron conversion of DSSCs, which is more pronounced at the modified plasmonic frequency of the nanoshell or nanorods. The third research direction is based on the surface plasmon polariton that is confined near the metal-dielectric interface. When the two-dimensional (2-D) plasmonic layer of nanoscale Ag domes is used as the backreflector of SDSSCs, the short circuit current of the devices with high extinction dyes is increased by more than 10%.53 Although the nanostructured metals have been found to improve the performance of DSSCs, Choi et al. raise a question on the mechanism of that enhancement.54 They have argued that stored electrons in the metal nanoparticles can change the Fermi energy level of the semiconductor−metal nanocomposites. Consequently, the open circuit voltage and the energy conversion efficiency can be increased. This implies that the nanostructured metals may play two different roles in enhancing the performance of DSSCs. In inorganic semiconductor nanoparticles, the energy level in the conduction band and the valence band becomes discrete due to the quantum confinement. This has led to extensive research on nanoparticle-based photovoltaics that can potentially solve the problems of silicon-based solar cells. The initial inorganic nanosensitizer solar cells used inorganic nanoparticles coated on mesoporous TiO2 films.55 The network of TiO2 or ZnO nanoparticles works as the electron acceptor, which collects electrons from the inorganic sensitizers. The energy conversion efficiency of the early solar cells was lower than 1%, due to the fast carrier recombination at the inorganic nanoparticle sensitizers. To improve the carrier injection process from the inorganic nanomaterials, the photoelectrode structure and the sensitizing materials have been modified. For example, 1-D nanowires and nanotubes were sensitized by inorganic nanoparticles. Leschkies et al. showed that the extremely thin PbSe layer on ZnO nanowires absorbs most of the incoming solar light, and the electrons go through the ZnO nanowires.56 The power output of their device was significantly improved, but the efficiency is still about 2% when illuminated with 100 mW/cm2 light. This was attributed to recombination at the CdSe/ZnO interface. Therefore, several following studies on inorganic nanosensitizers have been aimed at minimizing the electron−hole recombination near the sensitizing component. A TiO2/CdS/

type inorganic semiconductors, CsSnI3, which successfully addresses major problems of the solid electrolyte for highly efficient SDSSCs.47 As shown in Figure 3, the band edges of CsSnI3 allow for hole transfer from the photoexcited dye to CsSnI3. Hole mobility of CsSnI3 is mh = 585 cm2/(V s), which is 106 times larger than that of Sprio-OMeTAD. When F is doped into CsSnI3, the hole concentration increases, and the energy conversion efficiency of SDSSCs reaches 10.2% without a mask on the device. This recent article indicates that inorganic SDSSCs can be comparable to state-of-the-art liquid electrolyte DSSCs. When the size of the metal medium is smaller than the mean free path of the electrons, the collective oscillation of the electrons is bound to the surface of the nanostructured metals, and the plasma frequency is quantized. This is known as surface plasmons. When both the energy and momentum of the incident light match those of the surface plasmons, the extinction coefficient of the plasmonic nanostructures can be significantly increased. The unique optical properties of the surface plasmons have been used to improve the energy conversion efficiency in DSSCs. Standridge, et al. used atomic layer deposition (ALD) to conformally coat arrays of silver nanoparticles with a very thin layer of TiO2 and studied the correlation between photocurrent density and TiO2 layer thickness.48 Their results, shown in Figure 4, demonstrated that dye molecules in close proximity to silver nanoparticles produce more electrons than those on bare TiO2. However, these thin film type plasmonic structures are not suitable for DSSCs using mesoporous TiO2 films, since the near-field of the surface plasmons decay dramatically a few hundred nanometers away from the metal component. To solve this problem, the plasmonic particles were directly added to mesoporous TiO2 films by mixing noble metal (i.e., Au and Ag) nanoparticles and TiO2 nanoparticles. Recent studies on photoelectrodes with embedded metal nanoparticles are classified into three groups. One research direction is dedicated to preventing the erosion of metal nanoparticles and the trapping of photogenerated carriers, which can occur by adding bare nanoparticles into DSSCs. For this purpose, Au or Ag nanoparticles were coated with the dielectric layer such as SiO2 and TiO2 or were placed inside TiO2 hollow spheres.49,50 The experimental results show that the surface plasmons can promote the light absorption of DSSCs and increase their energy conversion efficiency by 30− 100%. It is noted that the plasmon-assisted carrier generation becomes dominant over the plasmons’ oscillation damping when photogenerated electrons are transferred from the dye to the TiO2 within 10 fs. The second research direction of plasmonic DSSCs is to tune the plasmonic frequency of the nanostructured metals. Since the resonance frequency is mainly 1687

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Figure 5. (a) Ultraviolet to visible (UV−vis) absorbance spectra of the photoactive layer in the solar cell (mesoporous oxide; perovskite absorber; spiro-OMeTAD) sealed between two sheets of glass in nitrogen and exposed to simulated AM1.5 sunlight (Inset: extracted optical density at 500 nm as a function of time. (b) Current density−voltage characteristics at AM1.5 condition for Al2O3-based cells, one cell exhibiting high efficiency (red solid trace with crosses) and one exhibiting VOC > 1.1 V (red dashed line with crosses); for a perovskite-sensitized TiO2 solar cell (black trace with circles); and for a planarjunction diode with structure FTO/compact TiO2/ CH3NH3PbI2Cl/spiro-OMeTAD/Ag (purple trace with squares). Reproduced with permission from ref 59. Copyright 2012 American Association for the Advancement of Science.

period, long-term stability, and production cost. DSSCs have advantages in terms of raw material abundance and energy payback period. TiO2 is a very abundant material, and the expected consumption of Ru for the DSSC dye is only a few % of annual usage. Also, the energy payback period of DSSCs is about 1 year, which is shorter than that of Si solar cells (>3 years). On-going studies on the electrolyte, the sensitizing material, and the hermetic sealing technique, indicate that the marginal degradation of the device performance is expected after 20 years of operation. However, the energy conversion efficiency of DSSCs may be only slowly improved. The energy conversion efficiency of the commercially available module is not expected to be much higher than 10%. The lower efficiency of the solar cells means that the balance of system (BOS) is increased. Compared with the 10% efficiency module, the 15% efficiency module and the 25% efficiency module reduce the installation costs by 20% and 35%, respectively.61 This leads to the conclusion that the module price of DSSCs should be close to $0.6/Wp to compete with current Si PV modules (∼$0.8/Wp) and eventually reach $0.35/Wp to achieve wide grid parity. In the U.S. Department of Energy roadmap, the target price and efficiency of the solar cell modules for the grid parity is $0.5/Wp and 25% by 2020.62 Since the current material cost of solar cells is slightly less than half of the module price, the material cost of economically viable DSSCs with 10% module efficiency must range from $15/m2 to $30/m2. In a recent cost analysis of DSSCs, however, the material cost production is projected to be less than $50/m2, based on 100 000 m2 production of 10% efficiency module per year by 2020. This indicates that electricity generation by DSSCs will be more costly than by Si solar cells in 2020. Therefore, efforts to reduce the amount and cost of materials required to fabricate DSSCs are imperative for the widespread use of DSSCs. For this purpose, intensive research is ongoing for all components of DSSCs. Dye and TiO2 nanoparticles take up more than 50% of the production cost, which is attributed to complex synthesis methods.3,63 Ti is an abundant and cheap material, and even the price of Ru is 5−10% of the dye cost on the assumption that the annual production of DSSCs is 7 MWp. Time-consuming synthesis of high purity dye molecules and hydrothermal growth of highly crystalline TiO2 nanoparticles are responsible for the increase in production cost. While

CdSe multilayer structure was found to form a stepwise bandedge level and facilitate the cascade-like extraction of electrons and holes, leading to an energy conversion efficiency close to 4%.57 In Sb2S3-sensitized TiO2 solar cells, the screening effect of the nanostructured film and the strong interfacial interactions of Sb2S3 with the p-type solid electrolyte have been found to suppress the carrier recombination and increase the fill factor.58 When the surface area of the TiO2 film was maximized and the functional group of the solid electrolyte was chelated to Sb2O3, Jsc and the energy conversion efficiency of the solar cell becomes larger than 15 mA/cm2 and 6% at AM 1.5. A remaining issue of solar cells with inorganic nanosensitizers is how to increase the open circuit voltage. For this purpose, Lee et al. implemented mesoporous Al2O3 film to replace the electronically disordered, low-mobility n-type TiO2.59 In their “meso-superstructured solar cell” shown in Figure 5, the layered perovskites of organometal halides (CH3NH3PbI3) and spiroOMeTAD are sequentially coated on the mesoporous Al2O3 film as the light absorber and the p-type conductor. Since the insulating Al2O3 provides only a scaffold function to the solar cells, the devices operate as two-component hybrid solar cells instead of sensitized solar cells. The modified structure leads to low fundamental carrier loss, large open circuit voltage (∼1.1 eV), and very high energy conversion efficiency (∼10.9%). Kim et al. have also shown that the perovskite-type nanomaterial, (CH3NH3)PbI3, is a promising light absorber for high performance solar cells.60 In their recent report on SDSSCs, a combined use of (CH3NH3)PbI3 and SpiroOMeTAD led to a high short circuit current (>17 mA/cm2), large open circuit voltage (0.888 V), and remarkable energy conversion efficiency (9.7%) at AM 1.5 condition. These results indicate that perovskite-structured inorganic sensitizers may offer a breakthrough to DSSCs.

Perovskite-structured inorganic sensitizers may offer a breakthrough to DSSCs. Perspectives on DSSC Commercialization. Critical factors determining the commercialization of DSSC are energy conversion efficiency, raw material abundance, energy payback 1688

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temperature, due to the volatility of the cheap solvent-based electrolytes. In the following section, recent studies to reduce the cost of DSSCs by changing the conventional cell structure or replacing expensive constituents are summarized. During the last two decades, most of the DSSCs have been fabricated in the form of a sandwich that employs 2-D flat transparent conducting oxides (TCO) such as Sn-doped indium oxide (ITO) and F-dope tin oxide. However, this structure has limitations on fabricating flexible DSSCs and reducing material costs due to the employment of TCO. Although flexible DSSCs have been realized by using polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) films coated with TCO, the TCO films on organic substrates can be easily cracked after bending or stretching, subsequently deteriorating their electrical conductivity. Recently, newly structured DSSCs including (1) Ti foil-, (2) metal wire- and (3) metal mesh-type DSSCs have been exploited to overcome these aforementioned problems. DSSCs built on Ti foil electrodes have garnered a considerable amount of attention, because of their lower price and easy fabrication process. A major strength of Ti foil over FTO is that Ti foil is compatible to the low-cost fabrication method such as a roll-to-roll mass production. From the raw material cost, Ti foil and TCO glass are comparable. It is expected that the price of 0.2 mm thick Ti foil is as expensive as TCO glass (∼$10/square meter).65,66 In Ti-foil DSSCs, TiO2 films were deposited onto Ti-foil instead of FTO glasses. Binder burn-out and thermal annealing processes can be applicable in this type of the photoelectrode. Back illumination, i.e., light going through a backside Pt counter electrode, is required to operate this type of DSSC because the Ti foil is not transparent. Ito et al. reported 7.2% efficiency flexible DSSCs based on Ti foil photoelectrodes.66 Metal wire-type DSSCs use metal wires coated with TiO2 or ZnO active layers and counter electrode wires.67,68 However, the wire-type DSSCs have shown a fairly low energy conversion efficiency, because of the very small surface area for the dye adsorption. This problem can be avoided by growing nanowire or nanotubes arrays on the metal wires. Fan et al. reported wire-shaped flexible dye-sensitized solar cells (WSF-DSSCs) with a simple helical twisting structure formed by two fiber-like electrodes.67 They observed that the short circuit current of the devices was in direct proportion to the wire length. The 5 cm long WSF-DSSC performed relatively poorly, i.e., the Voc, Isc, and fill factor were 610 mV, 0.06 mA, and 0.38, respectively. Due to the easy synthesis method, ZnO nanowire arrays were grown on the metal wires to increase surface area. However, when the wire-type DSSCs were fabricated on the ZnO nanowire-coated metal wires, the energy conversion efficiency of the device did not exhibit a high performance. This was attributed to the fact that the inherent surface instability of ZnO prevents the monolayer coating of the dye molecules. Recently, a remarkably improved efficiency of 7% was achieved by employing a Ti fiber photoelectrode based on TiO2 nanotube arrays, which significantly boosted energy conversion efficiency of the wire-type DSSCs as shown in Figure 6a.68 Techniques used for high efficiency wire-type DSSCs can be applied to other substrates such as woven curtains, tents, and bags. This indicates that DSSCs can be built on nontraditional substrates that are widely used in day-to-day life. An understanding of metal wire-based DSSCs allows for the emergence of new DSSC structures built on the metal mesh

several companies are developing alternative synthesis routes to easy ligand exchange, fast dye purification, and simple crystallization of uniform TiO2 nanoparticles, breakthroughs have not been found yet. It is expected that the cost reduction of dye and TiO2 will be connected to the production volume and the extinction coefficient of the dyes. An 80% reduction of the dye cost is projected when the production volume of the dye is increased 100-fold. In addition, a small increase in the extinction coefficient of the dye will easily decrease the thickness of the dye-coated TiO2 film by 50%. The next most expensive component is glass panels. Since the cost of the transparent conducting oxide-coated glass is about $10/m2 and that of the Pt-coated glass is about $6/m2, the removal of even one glass piece will bring the material cost of DSSCs close to the generally targeted value. In addition, the replacement of the glass is required to utilize DSSCs in mobile applications. Therefore, intensive research is being conducted to change the traditional structure of DSSCs. Conductor materials connecting individual cells are also challenging, since the cost of Ag used as the conductor material is about 15% of the DSSCs. A more significant problem is that the price of Ag is continuously increasing. This is a universal issue in the solar cell industry. Consequently, several companies are working to reduce the amount of Ag in metal conductors or substitute Ag with cheap metals such as Cu and Ni. Although the complete replacement of Ag is ideal in terms of the cost, cheap metal conductors suffer from high electric resistivity and easy degradation. To prevent degradation, the cheap metal lines need to be fired in the inert ambience and encapsulated quickly after the firing process. The cost of the electrolyte is not problematic when only the solventbased electrolyte is used. In contrast, the ionic liquid or the solid hole conductor is still costly, and further work is required.

Efforts to reduce the amount and cost of materials required to fabricate DSSCs are imperative for the widespread use of DSSCs. If research into decreasing the amount of the materials per DSSCs module is not successful, there is a chance that DSSCs will be used in niche applications such as flexible solar cells or compete with amorphous Si solar cells for the low-end market. At present, a promising strategy for the DSSC industry is to modify existing products and add extra value rather than to build a photovoltaic power plant. A very recent market research study forecasts that the global market for DSSCs will grow very slowly to $290 million by 2023 and the indoor and mobile electronic sections will command half of that market.64 DSSCs in the initial commercialization stage are aimed at chargers, solar bags, and wireless solar keyboards. Later, BIPVs and automobile PVs are expected to be important applications of DSSCs. In these areas, the high temperature stability of the devices becomes critical, since DSSCs need to be installed in glass windows, building walls, and steel roofs. The temperature of DSSCs in contact with and 5 cm away from the outside of building walls was increased to 80 and 60 °C, respectively. Hence, DSSCs for BIPVs and automobile PVs are required to pass an accelerated aging test under harsh environments (temperature of 85 °C and humidity of 85%) to prove device reliability. It is noteworthy that expensive ionic liquid-based electrolytes must be used for DSSCs operated at high 1689

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Figure 6. (a) Actual optical photo of flexible fiber-shaped cell. Reproduced with permission from ref 68. Copyright 2012 Royal Society of Chemistry. (b) Low-magnification SEM images of TiO2 nanotube mesh anode; the inset figure is the surface of nanotube array. Reproduced from ref 70. Copyright 2010 american Chemical Society. (c) Schematic diagram of a 3D DSSC and the photovoltaic parameters of the 3D DSSC illuminated in different angles of the incident sunlight. The inset shows the schematic diagram of the 3D DSSC under illumination in different angles. Reproduced with permisison from ref 71. Copyright 2010 American Institute of Physics. (d) Design of TCO-free highly bendable DSSC and the cross-sectional view shown in the circle. Reproduced with permisison from ref 72. Copyright 2012 Royal Society of Chemistry.

efficiency of 2%, which was maintained even under bending of the device until the radius of curvature reached 2 cm. Pt-coated conducting glass has been used as a counter electrode in DSSCs due to its high electronic conductivity and catalytic activity. A Pt-loaded counter electrode fabricated by thermal deposition of Pt chloride showed fairly low charge transfer resistances of less than 1 Ω cm2. The only detriment to using Pt is that it is a noble metal. Therefore, cheap carbon and organic materials have been studied as alternative materials. Counter electrodes based on carbon nanopowders have been tested. Their charge transfer resistance can be decreased to 0.74 Ω cm2, which is comparable to that of the Pt electrode.73 Recently, new carbon materials such as carbon nanotubes and graphene have been introduced. Han et al. fabricated poly(styrene-4-sodiumsulfonate) (PSSNa)-grafted multiwall carbon nanotube (MWCNT-g-PSSNa) films using an electrostatic spray (e-spray) technique.74 With this approach, an energy conversion efficiency of more than 7% was achieved along with a charge transfer resistance of 1.52 Ω cm2 at a thickness of 0.31 μm. Graphene supported by Pt nanoparticles has been also exploited as the counter electrode material of DSSCs.75 Its charge transfer resistance was 2.36 Ω cm2, which is not much different than that of other counter electrodes. In addition, poly(3,4-ethylenedioxythiophene) (PEDOT) doped with toluenesulfonate anions also showed an excellent charge transfer resistance of 0.95 Ω cm2.76 Overall energy conversion efficiencies for DSSCs employing these cheap counter electro-

that is the network structure of the metal wires. Flexible stainless steel meshes coated with TiO2 nanoparticles were studied initially. The surface modification of steel meshes by coating ZnO nanowire and Zn2SnO4 nanowire arrays has also been explored.69 Replacement of the metal mesh by Ti mesh has significantly improved the performance of the mesh-type DSSCs, since Ti mesh is more compatible with TiO 2 nanotubes. Rustomji et al. reported an energy conversion efficiency of 5% by employing the TiO2 nanotube arrays in the Ti mesh (Figure 6b).70 Wang et al. built 3-D DSSCs on doubledeck cylindrical Ti mesh substrates.71 The photoanode is the Ti mesh that has been anodized to form a TiO2 nanotube layer, and the counter electrode is the Ti mesh that is platinized through electrodeposition. The advantage of the 3-D DSSCs is that the efficiency of the solar cells does not depend on the incident solar beam angle, due to its axial symmetrical structure as seen in Figure 6c. The optimized energy conversion efficiency was 5.5% under standard AM 1.5 conditions. In general, the mesh-type DSSCs have employed Pt nanolayer or TCO-coated substrates, which are unfavorable for achieving low cost and high flexibility. Recently, a glass paper that contains the electrolyte was utilized as a supporting material for a new type of flexible DSSCs.72 TCO-free and highly bendable DSSCs were constructed on a pair of the metal mesh and the glass paper, which is depicted schematically in Figure 6d. This flexible device was inspired by the traditional Korean door structure and showed an energy conversion 1690

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des is slightly lower than that of Pt-counter-electrode-based DSSCs. Recently, a DSSC employing a nanostructured PEDOT counter electrode demonstrated an efficiency of 8.3%, which is the highest efficiency so far achieved by PEDOT counter electrodes.77 Other materials that have been examined to replace Pt are sulfide and carbide compounds such as CoS, Mo2C, and WC. CoS coated on ITO/PEN exhibited good charge transfer resistance of 1.8 Ω cm2 with good catalytic properties. Therefore, the CoS nanorod counter electrode increased the efficiency of the DSSC to 7.7%.78 Mo2C and WC with ordered mesoporous carbon also demonstrated a large exchange current density on the electrode surfaces, which confirms that these alternative electrodes catalyze the reduction of triiodide to iodide in the electrolyte.79 While the efficiency of DSSCs using alternative counter electrodes approximates that of conventional DSSCs, their long-term stability and temperature-dependence are not well characterized at this time. This information is essential to evaluating the true potential of the alternative counter electrode materials. This Perspective reviews the recent progress with DSSCs and prospects for their commercialization. Due to their unique physical characteristics, the economical and environmentally friendly fabrication process, the short energy payback period, and the material abundance, DSSCs have the potential to compete with other thin film solar cell technology. Given this recent progress, it is expected that DSSC modules will soon be commercially available for the niche market where aesthetic aspects or the mechanical flexibility of the device is important. Although the electrical, optical, and chemical properties of DSSCs have been improved significantly, the widespread use of DSSCs still requires further optimization of device performance. Moreover, efforts to reduce the cost of the DSSCs modules should continue. To make DSSCs economically viable, their module price needs to be lower than at least $0.6/Wp. This challenge requires researchers to improve the energy conversion efficiency of the module by broadening the light absorption spectrum and decreasing the energy loss of carriers through a transport process. Another way to lower the cost is to reduce the consumption of raw materials by redesigning the structure of conventional DSSCs, which is also the subject of ongoing research carried out in both academia and industry around the world.

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quality of his research is validated by more than 130 publications in refereed journals. He also holds 10 patents on the dielectric and optical applications of functional materials. Hyun Suk Jung is an associate professor in the School of Advanced Materials Science & Engineering at Sungkyunkwan University (SKKU). He received his BS, MS, and Ph.D. degrees in materials science & engineering from Seoul National University (SNU), in 1997, 1999, and 2004, respectively. He joined Los Alamos National Laboratory (LANL) as a director’s postdoctoral fellow in 2005. He began working for Kookmin University (KMU) in 2006, and joined Sungkyunkwan University in 2011. He has published over 90 peerreviewed papers regarding the synthesis of inorganic nanomaterials and dye-sensitized solar cells. He presently researches flexible solar cells and inorganic sensitized solar cells. http://home.skku.edu/ ∼hjung/

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ACKNOWLEDGMENTS Dr. Lee acknowledges the financial support from the National Science Foundation (Grant No. DMR-0847319 and CBET1235979). A portion of Dr. Jung’s research was supported by the National Research Foundation (2012M3A6A7054864 and 2012M3A7B4049986).

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Jung-Kun Lee is an associate professor in the Department of Mechanical Engineering and Materials Science at the University of Pittsburgh. He received his Ph.D. degree from the Department of Materials Science and Engineering at Seoul National University, Korea in 2000. Then, he won the highly competitive Director’s Postdoctoral Fellowship of Los Alamos National Laboratory (LANL) in 2001. Later, he was promoted to a technical staff member at LANL. After over 5 years of service at LANL, he joined the University of Pittsburgh in 2007. His major research topics include sophisticated processing and characterization of nanostructured materials and electronic materials for photovoltaic and information technology. The scientific 1691

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