perovskite solar cells: rising, last advances, and future ... · perovskite solar cells: rising,...

Chemistry of Ecomaterials Perovskite Solar Cells

† José M. Campiña, PhD; Dpt. Química e Bioquímica, Universidade do Porto (Portugal) [email protected] 1

MINI-REVIEW/PERSPECTIVE

Perovskite Solar Cells: Rising, Last Advances, and Future Perspectives

José M. Campiña †

The progress made by emerging photovoltaic technologies in the last year has been outstanding. Important steps towards the realization of silicon-free solid-state solar cells with a real potential for commercialization were taken. In particular, a number of milestones have been achieved in the development of hybrid mesoscopic and thin-film solar cells based on the use of nanocrystals of organometal halide perovskites as the light absorbers. Under this approach, the power conversion efficiency (PCE) has been boosted from values around 6-8% (hold by metal chalcogenide solar cells) to over the 19%. Such a performance is now very close to the 25% of crystalline silicon solar cells, the leading commercial technology. But the most intriguing is that these breakthroughs have been achieved in devices entirely fabricated in the solid state, which, so far, had shown worse energy conversion abilities than their counterparts based on liquid electrolytes like dye-sensitized solar cells.

The Dye-Sensitized Solar Cell (DSSC)

DSSCs are typically based on a mesoscopic layer of semiconducting metal oxides such as TiO2 or ZnO. Due to their wide bandgap (3-4 eV), these materials absorb mainly in the UV and lack the ability to exploit other important portions of the solar spectrum, such as the visible (Vis) and the near infrared (NIR), for the production of electricity.

However, if they are properly sensitized with organometallic dyes, this radiation can be also captured to some extent. For these purposes, dyes are usually immobilized or anchored on the surface of the metal oxide following different methodologies of assembly. Once a dye is excited under illumination, electron-hole pairs (the so-called excitons) have to be spatially separated.

In DSSCs, electrons are injected into a collecting electrode (or photoanode) consisting of the mesoscopic metal oxide layer deposited onto a transparent conducting glass. On the other hand, the holes are used to oxidize a redox probe in solution which is, later, reduced in the cathode to close the circuit. Due to the fact that the metal oxide particles employed are in the nanoscale and that the dye solutions used for their sensitization are in the mM range, these devices hold a great potential for manufacturing at a very low cost.

Nevertheless, a number of issues typically affect the performance and the prospects of commercialization of DSSCs. In first place, organic dyes can cover the visible part of the spectrum but they exhibit an insufficient light

harvesting capability in the NIR. Other important problems are the corrosion of certain components or the leakage of toxic liquids derived from the use of an electrolyte in liquid state. But beyond these limitations, the most important obstacle to commercialization comes from the low stability of organic dyes upon continue solar irradiation.

Scheme 1 Schematic illustration of a generic dye-sensitized solar cell (DSSC). The violet dots represent dye molecules. Source: Wikipedia Commons, Original Work Author: M. R. Jones.


J. M. Campiña, Chem. Ecomat. 2014, 1, pp. 1-6. 2

Chalcogenide Quantum Dot Solar Cells (QDSCs)

More durable inorganic light absorbers were pointed as an obvious alternative to organic dyes. In this respect, nanocrystals of metal chalcogenides have been intensely investigated in the last decade as suitable sensitizers in hybrid mesoscopic solar cells (HMSCs). These ultratiny nanocrystals exhibit quantum confinement and are commonly referred to as Quantum Dots (QDs).

Through the control of its crystal size (something that has become feasible in the last decade thanks to the great progress in the development of chemical routes for size controlled synthesis of nanoparticles) one can tune the energy gap between the conducting and valence band edges.

Figure 1. QDs can be engineered with different sizes and band energy gaps (i.e. with different colors) as it is shown in the picture. Source: www.teachengineering.com

As the bandgap usually determines the minimum wavelength that can be absorbed, access to the infrared spectrum has been possible through the synthesis of tiny chalcogenide nanoparticles of about 3-5 nm. Indeed, the use of these inorganic nanodots involves additional advantages such as large optical absorption coefficients, large dipole moments for facilitated exciton separation, and the possibility of carrier multiplication by hot photons [1].

A recurrent strategy employed to enlarge the range of absorbed light, while simultaneously keeping a good electron injection, has consisted in the co-sensitization of metal oxides with two types of chalcogenide QDs: a first one of large band gap (poor NIR absorption) but good electron injection (for instance, CdS) and a second type with lower bandgap (improved absorption range) but poorer injection as CdSe.

Co-sensitization has yielded improved results both for liquid (with a PCE=5.4% being reported for a TiO2 cell co-sensitized with CdSe & CdS and polysulfide electrolyte [2]) and solid-state designs (with maximum CPE of 4% [3]) of QD-sensitized solar cells (QDSSCs). Antimony sulfide (Sb2S3) QDs present a bandgap in the order of that of CdSe nanocrystals (about 1.7 eV) but a LUMO edge above the one for CdS (-3.7 vs -3.9 eV) both in one single material.

As a consequence, it has made an impact in liquid (maximum PCE about 3% [4]) and solid-state HMSCs (with efficiencies above the 6% [5]). Obviously, there is a wide range of chemical and thermal treatments that can influence the output parameters of the cells to a certain

extent. Some of them seek to improve the organization at the different interfaces and the stacking of aromatic hole conductors through mild thermal treatments.

Others look for an enlargement of carrier life by casting ionic liquids rich in Li+ cations and other species. In this respect, the passivation of metal oxide and QD surface with a thin capping layer of ZnS or SiO2 has been demonstrated to restrict recombinative pathways involving electrons trapped in surface traps [6].

Very recently, it has been demonstrated that the absolute energy of HOMO and LUMO levels of solution-processed chalcogenide QDs can be tailored through the control of the capping ligand used in their stabilization. In this context, the absolute energy levels of PbS QDs can rise up to 0.9 eV depending on the ligand used to this end [7]. This finding has enabled to maximize electron injection through wise band alignment engineering approaches.

In fact, a new record efficiency of 8.55% has been set for a thin-film QD solar cell based on a ZnO electron conductor and a couple of layers of PbS QDs capped with tetrabutylammonium iodide (TBAI) and 1,2-ethanedithiol (EDT) [8]. Surface capping with iodide ligands has also shown to improve performance by limiting the surface oxidation of QDs exposed to air [9].

Recent theoretical calculations for Sb2Se3 QD cells (wich present a very small bandgap of 1.0-1.2 eV) have concluded that they would double the current output of Sb2S3 cells. This line of research is currently under development and its main current challenge consists of finding suitable methods for the synthesis and deposition of these QDs within mesoscopic metal oxide matrices.

Organometal Halide Perovskite Solar Cells (PSCs)

Despite all this progress, the efficiency of QD-based HMSCs has remained below the 7% until very recently. A key step forward to overtake these limits was inadvertedly given in 2009 by Tsutomu Miyasaka and his team after the introduction of a novel type of organometallic halide nanocrystal with perovskite structure [10]. These materials are easily prepared and processed from solution, present high extinction coefficients, and intense visible to near-infrared absorptivity.

In the work of Misayaka, a mesoscopic layer of TiO2 was sensitized with nanocrystals of methylammonium lead bromide and iodide perovskites with formula CH3NH3PbBr3 (bandgap: 2.3 eV and LUMO: -3.3 eV) and CH3NH3PbI3 (bandgap: 1.5 eV and LUMO: -3.9 eV). For these purposes, they used an easy solution-based processing methodology consisting of the deposition of a solid layer containing CH3NH3Br (or CH3NH3I) and PbBr2 (or PbI2) by spin coating and its further annealing under mild conditions.



These cells exhibited identical limitations to those based on metal chalcogenide QDs with PCEs falling in the range 3-4%. In fact, co-sensitization and the application of capping treatments improved these results thanks to the restriction of exciton recombinative pathways (very recently, direct prove on the entrapment of electrons in defect-rich surface of TiO2 particles has been presented by means of picosecond X-ray absorption spectroscopy [11]). In this respect, through the passivation of the TiO2 film with Pb(NO3)2 (before the deposition of perovskite nanocrystals), an increase of PCE to values about the 7% was reported [12].

Perovskite solar cells (PSCs) have been recently classified as a new type of solar cell and their performance definitely exploded in 2013 with the achievement of a series of milestones that have quickly placed this technology in a leading position amongst the emerging generations of photovoltaic devices made of inorganic light harvesters.

But, before reaching this point, a major advancement was introduced by Park and Grätzel still in 2012 [13]. Through the reduction of the thickness of the mesoscopic skeleton under the micron (to about 600 nm) and the nearly complete extraction of the holes using a typical solid-state hole transporting material (HTM) such as Spiro-OMeTAD, the PCE raised to 9.7%.

Perovskites have exhibited the ability to convert the absorbed photons into collected charge with efficiency quite close the 100%. But, beyond their excellent light-harvesting and conversion properties, perovskite nanocrystals such as CH3NH3PbI2Cl are excellent ambipolar materials which can transport both holes and electrons [14].

In fact, they work so good transporting electrons that, as demonstrated by Henry Snaith and co-workers, the TiO2 mesoscopic layer can be replaced by an insulating layer of Al2O3 (bandgap ranging 7-9 eV), and the electron transporting rate is still 10 times superior than in their TiO2-sensitized counterparts (see Figure 2) [15].

In this “meso-superstuctured” device (which broke the barrier of 10% efficiency), the metal oxide plays the solely role of scaffold or supporting material for the nanocrystalline perovskite film. This strategy not only improved cell current due to enhanced electron transport but also boosted the open circuit voltage of the cell as the LUMO edge of TiO2 does not limit this parameter anymore.

The Ultrafast Rise of Perovskite Photovoltaics

In March 2013 Sang Il Seok and his co-workers published results in Nano Letters demonstrating how through an optimization of the halide ratio in mixed CH3NH3Pb(I1−xBrx)3 perovskite nanocrystals, highly efficient solid-state solar cells can be obtained with PCE=12.3% [16]. A smart engineering of the bandgap was achieved by sweeping the composition of

Figure 2. Mechanisms of electron transport in metal oxides modified with perovskites under different configurations. (Left) Electron injection into the LUMO level of TiO2 sensitized with perovskite nanodots. (Right) Charge transport through an extremely thin perovskite layer formed onto an insulating Al2O3 scaffold (meso-superstructured cell).

the perovskites from the Br-free to I-free nanocrystals so that almost the entire visible spectrum was covered and the best absorber in the series was identified.

Just one month later, Michael Grätzel and his collaborators will set an impressive new PCE record of 15% (the certified value was finally 14.1%) [17]. This work, which was published in Nature, will for sure change the future prospects of solid-state HMSCs. Despite using a methylammonium lead iodide perovskite (CH3NH3PbI3) like in the previous works, high efficiency came from the application of a sequential deposition of the perovskite precursors under a configuration where the mesopores of the oxide film are filled with a pure perovskite instead of the HTM (what has been referred to as a “pillared structure” configuration, see Figure 3).

This in contrast with the one step chemical bath deposition, CBD, employed before for the preparation of QD-sensitized or “meso-superstructured” PSCs. The sequential procedure proposed in this work permitted to surpass the limited pore-filling of HTMs and a better control over the morphology and size of the individual grains, thus, explaining the boost in efficiency.

Following this paper, the design of PSCs continued its fast evolution and a third breakthrough was presented by H. Snaith and his group in September 2013 [18]. The manuscript worked out the implications of the Al2O3-perovskite cell concept to obtain a metal oxide-free planar heterojunction cell with PCE=15.4%.

High efficiency was achieved after removing the mesoscopic oxide layer. The cell only consisted of a thin film of a high-purity perovskite (prepared by vapor deposition) as the absorbing layer and an HTM layer deposited ontop. Figure 3 (reproduced with permission from reference [19]) illustrates the evolution in the structural design of PSCs and its correlation with the increase in PCEs.

This vertiginous progress caused the designation of PSC technology as one of the top ten biggest scientific breakthroughs of 2013 by the journal Science [20]. However, new record-breaking achievements are still



Figure 3. Structural evolution of PSCs under different configurations and their correlation with the power conversion efficiency obtained (PCE). The authors and references of the corresponding works are indicated below. Adapted with permission from reference 17 (Nam-Gyu Park et al, J. Phys. Chem. C 2014, 118 (11), 5615–5625) .Copyright © 2014 American Chemical Society

popping up in 2014. According to the research cell efficiency record chart provided by the National Renewable Research Laboratory (NREL), from February to May 2014 the team of Sang Il Seok at KRICT has raised the efficiencies of PSCs two times to 16.2% and 17.9% [21]. These impressive advancements were possible thanks to the use of a new solvent-engineering technology for the formation of extremely uniform and dense perovskite films.

A couple of months back, April 2014, Y. Yang from the University of California Los Angeles announced in the meeting of the Materials Research Society the last boost given to the performance of PSCs to the current record PCE of 19.3% [22]. This impressive result temporarily closes (to the best of our knowledge) the sequence of ultrafast advancements followed by PSCs in the last year and a half.

In his talk, Yang did not give further details beyond claiming the development of perovskite growth methods targeting to limit the amount of defects in the crystals. Despite the result has to be still verified, this is a very exciting finding if one considers that commercial single crystal silicon cells have exhibited maximum efficiencies around the 25%. In this sense, an interesting combination of single crystal silicon cells with perovskite layers deposited onto it, has led very recently to a high PCE of 32% [22].

Significance and Future Challenges

The implications of Snaith´s work are deep. What this team has demonstrated is basically that the use of a mesoscopic electron transporting phase and further nanostructuring of the photoanodes is no longer necessary for the purposes of efficient exciton separation and charge transport in PSCs. Despite of creating a favorable energy gradient for electron-hole separation, the mesoscopic layer is also an important source of impedance for HMSCs due to the large interfacial area and the charge transfer barriers that electrons must

overcome in their way to the collecting glass electrode (along a distance of 0.5-1 micron).

Then, further enhancements seem more likely to come from the development of novel methods to obtain highly crystalline and pure perovskite films with very low density of defects so that charge transport can be optimized. The replacement of thick metal oxide layers by thinner supports for the perovskite light absorbers, may induce an important decrease in manufacturing costs.

But, more importantly, the fabrication of high performance planar thin-film cells (which can be deposited over flexible substrates for portable applications, etc) becomes feasible for the first time. Far beyond, Snaith´s work evokes the thought that even the supporting Al2O3 layer is fully dispensable what should result in further reductions of fabrication costs (as high temperature annealing treatments are no longer required).

The same conclusion can be draw from a more recent work by Y. Ogomi et al who deposited a monolayer of HOCO-R-NH3+I- onto a mesoscopic TiO2 layer for better anchoring of the perovskite film [23]. Apparently, metal oxides may be replaced in future designs by solution-processed organic layers applied at room temperature which is a more viable solution to low-cost photovoltaics.

In fact, before the end of 2013, M. Grätzel and H. Bolink (from the University of Valencia, Spain) confirmed these perspectives. A thin film planar PSC with a PCE of 12% was reported [24]. In this device, a high purity perovskite phase (deposited by sublimation in a high-vacuum chamber) was sandwiched between two thin films of organic p/n-semiconductors.

Similar PSCs consisting of a flat and thick CH3NH3PbI3 film and a thin PCBM film have been fabricated more recently through solution processing at low temperature by the team of Seok. Through the optimized PCBM thickness and insertion of a LiF



interlayer, a PCE=14.1% and a very high FF= 78.3% have been achieved [25].

All these findings seem to indicate that the hegemony of the DSSC, and their solid-state derivatives, is seriously questioned by PSC technology. However, not everything is hype around the PSCs. These have to face yet a series of challenges to be economically viable. The first one is common to other QD HMSCs: the lack of a cheap, efficient, and stable p-semiconducting material (to transport the holes to the collecting cathode) that can be processed in air and at low temperatures.

Almost every high performing QD-HMSC or PSC is based on 2,2´,7,7´-tetrakis-(N,N-dip- methoxyphenylamine) 9,9´-spirobifluorene HTM (an expensive aromatic molecular conductor with good p-p stacking abilities and high conductivity often called spiro-OMeTAD, see Figure 4) or conducting polymers which need to be processed under vacuum or inert atmospheres to avoid their oxidation.

Indeed, the long-term stability of organic materials under continuous irradiation is well known to be poor. In this sense, the works of Snaith [15, 18] and Grätzel [14, 24] allow to anticipate the future appearance of research on ultrathin planar PSCs with perovskites playing the roles of electron and hole transporters and featuring high efficiency.

M. G. Kanatzidis et al (from Northwestern University in Evanston, Illinois) reported in 2013 on a hybrid lead-tin methylamine iodide perovskite, CH3NH3Sn1–xPbxI3 [26]. The results provided solid evidence for the semiconducting nature of this family of compounds which behaved as p- or n-type depending on the Pb:Sn ratio. Sn compounds showed great tendency toward oxidation which causes the materials to be doped with Sn4+ and, thus, more prone to behave as p-type semiconductors.

These results open a window to a possible preparation of all-perovskite thin film cells in a near

Figure 4. Structure of Spiro OMe TAD, an aromatic molecule typically used as a hole transporting material in photovoltaic and other solid-state devices.

future (under p-i-n or n-i-p configurations). In addition, taking Pb out of PSCs is also important due to environmental reasons. In this respect, organolead halide compounds may pose important threats as these minerals readily dissolve in water (or even in humid air) what causes that the rooftops of houses equipped with PSC panels may become a potential source of environmental contamination by leakage of Pb.

S. Ghanavi from Uppsala University explored the possibilities of PSCs made of pure organolead (CH3NH3PbI3) and pure organotin perovskites (CH3NH3SnI3) playing the roles of light absorbers and hole transporting materials in TiO2-HMSCs [27]. This is an interesting work to judge the prospects of development of an all-perovskite solar cell. A broader absorption of light in the UV-Vis-NIR range was observed for the cells incorporating lead perovskite absorbers compared to those with the tin perovskite. As a consequence, much poorer PCEs were obtained for the latter (<2%).

Photo-induced absorption (PIA) measurements were also conducted to investigate charge separation and hole transfer occurring when both perovskites worked either as the light absorbers or the hole conductors. Agreeing with the observations of Kanatzidis [26], the results showed that tin perovskite is a better hole conductor (p-type semiconductor) than lead perovskite which works better as light absorber and electron conductor (as n-type semiconductor).

More recent studies have confirmed this view and have presented substantial improvements in the overall PCE of these cells. Just as recent as May 2014, two new contributions have appeared in this regard. The groups of Snaith and Kanatzidis have taken a similar approach (likely inspired by the previous work of the latter on hybrid lead-tin perovskites) to substitute Pb with Sn (a metal that shares a similar electronic structure but significantly less toxic) in the complex crystalline structure of perovskites

The first of these papers was posted online on 1 May in the Energy & Environmental Science journal (E&ES) and reported on the preparation of greener lead-free PSCs purely made of tin-perovskites achieving a maximum efficiency of 6.4% [28]. In an article posted only three days later in Nature Photonics, similar cells with PCEs up to 5.73% were described by Kanatzidis et al [29].

In conclusion, it appears that most of the ingredients for the future realization of planar thin-film all-perovskite solar cells (APSCs) are already in the pot. APSCs hold the ultimate promise of high efficiency at the lowest costs because of: (a) all the materials are processed in solution (and potentially through printing technologies), (b) they do not required the use of expensive and unstable organic HTMs, thick mesoscopic metal oxides, and/or the application of high temperature annealing processes, and (c) given that they can be applied as thin films, flexible photovoltaic devices for wearable items could be easily fabricated.



Although the big headlines have been for the huge breakthroughs achieved in terms of conversion efficiencies, PSCs have also showed to improve other output parameters such as the filling factor (FF) and the supplied voltage. It has been already commented above on the potential gains in terms of voltage coming from the removal of the mesoscopic metal oxide layer (cell voltage is usually limited by the magnitude of the pseudo-Fermi level of electrons in the anode).

These possibilities have been demonstrated in a very recent paper published online on 16 May 2014 by E&ES. In this work, Seok´s team introduced a new TiO2-PSC characterized with a low PCE of 6.2% but able to develop a high voltage of 1.4 V and featuring a very high FF=79% [30]. This voltage doubles the maximum theoretical voltage achievable in TiO2-DSSCs and is higher than the one typically supplied by a single standard AAA battery. The combination of a high-energy LUMO level in CH3NH3PbBr3 and a deep-energy HOMO in the HTM has been claimed as the main reason explaining such a high photovoltage.

Conclusions

PSC technology has followed a ramping development since the introduction of the first cells based on nanocrystalline organometal halide perovskites as light absorbers in 2009. Only 4 years later, this technology appears like the most promising amongst the new generations of photovoltaic devices. The PCE of PSCs has increased by about 5 times since and their designs have been considerably refined and simplified (in respect to previous concepts like the one represented by the DSSC) so that the fabrication of highly efficient and light thin-film photovoltaic devices at low cost becomes feasible for the first time. The fast progress of this field, allow us to envision further advancements based on improvement of the methodologies for perovskite deposition and the possible emergence, in a near future, of all-perovskite thin-film PSCs with full capabilities to compete (or even overtake) crystalline silicon cells in a commercial sense.

References

[1] O. E. Semonin, J. M. Luther, S. Choi, H. Y. Chen, J. Gao, A. J. Nozik, M. C. Beard, Science 2011, 334 1530-1533. [2] P. K. Santra, P. V. Kamat, J. Am. Chem. Soc. 2012, 134, 2508–2511. [3] L.Etgar, D. Yanover, R. K. Čapek, R. Vaxenburg, Z. Xue, B. Liu, M. K. Nazeeruddin, E. Lifshitz, M. Grätzel, Adv. Funct. Mater. 2013, 23, 2736–2741. [4] S. H. Im, H-J. Kim, J. H. Rhee, C-S. Lim, S. I. Seok, Energy Environ. Sci. 2011, 4, 2799-2802. [5] J. A. Chang, S. H. Im, Y. H. Lee, H-J. Kim, C-S. Lim, J. H. Heo, S. I. Seok, Nano Lett. 2012, 12, 1863–1867. [6] N. Guijarro, J. M. Campiña, Q. Shen, T. Toyoda, T. Lana-Villarreal, R. Gómez, Phys. Chem. Chem. Phys. 2011, 13, 12024-12032.

[7] P. R. Brown, D. Kim, R. R. Lunt, N. Zhao, M. G. Bawendi, J. C. Grossman, V. Bulović, ACS Nano 2014, Article ASAP, DOI: 10.1021/nn500897c [8] C-H. M. Chuang, P. R. Brown, V. Bulović, M. G. Bawendi, Nat. Mater. 2014, Published Online, DOI: 10.1038/nmat3984 [9] Z. Ning, O. Voznyy, J. Pan, S. Hoogland, V. Adinolfi, J. Xu, M. Li, A. R. Kirmani, J-P. Sun, J. Minor, K. W. Kemp, H. Dong, L. Rollny, A. Labelle, G. Carey, B. Sutherland, I. Hill, A. Amassian, H. Liu, J. Tang, O. M. Bakr, E. H. Sargent, Nat. Mater. 2014, Published Online, DOI: 10.1038/nmat4007 [10] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050-6051. [11] M. H. Rittmann-Frank, C. J. Milne, J. Rittmann, M. Reinhard, T. J. Penfold, M. Chergui, Angew. Chem. Int. Ed. 2014, Published Online 12 May, DOI:10.1002/anie.201310522. [12] J-H. Im, C-R. Lee, J-W. Lee, S-W. Park, N-G. Park, Nanoscale 2011, 3, 4088-4093. [13] H-S. Kim, C-R. Lee, J-H. Im, K-B. Lee, T. Moehl, A. Marchioro, S-J. Moon, R. Humphry-Baker, J-H. Yum, J. E. Moser, M. Grätzel, N-G. Park, Sci Rep. 2012, 2: 591. [14] L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K. Nazeeruddin, M. Grätzel, J. Am. Chem. Soc. 2012, 134, 17396-17399. [15] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643-647. [16] J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Nano Lett. 2013, 13, 1764–1769. [17] J. Burschka, N. Pellet, S-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grätzel, Nature 2013, 499, 316-319. [18] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395-398. [19] H-S. Kim, S. H. Im, N-G. Park, J. Phys. Chem. C 2014, 118, 5615–5625. [20] Science News, Breakthrough of the Year, Newcomer Juices Up the Race to Harness Sunlight. Science 2013, 342, 1438−1439. [21] Research Cell Efficiency Records. NREL (http://www.nrel.gov/ncpv/). Accessed on June 17, 2014. [22] R. F. Service, Science 2014, 344, 458. [23] Y. Ogomi, A. Morita, S. Tsukamoto, T. Saitho, Q. Shen, T. Toyoda, K. Yoshino, S. S. Pandey, T. Ma, S. Hayase, J. Phys. Chem. C 2014 Article ASAP, DOI: 10.1021/jp412627n. [24] O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas, M. Graetzel, Mohammad K. Nazeeruddin, H. J. Bolink, Nature Photon. 2014, 8, 128-132. [25] J. W. Seo, S. Park, Y. C. Kim, N. J. Jeon, J. H. Noh, S. C. Yoon, S. I. Seok, Energy Environ. Sci. 2014, Accepted Manuscript, DOI: 10.1039/C4EE01216J. [26] C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem. 2013, 52, 9019–9038. [27] S. Ghanavi, Master Thesis 2013, Uppsala University (http://uu.diva-portal.org/smash/get/diva2:642174/FULLTEXT04.pdf). [28] N. K. Noel, S. D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G. E. Eperon, S. K. Pathak, M. B. Johnston, a. petrozza, L. Herz, H. Snaith, Energy Environ. Sci. 2014, Accepted Manuscript, DOI: 10.1039/C4EE01076K. [29] F. Hao, C. C. Stoumpos, D. H. Cao, R. P. H. Chang, M. G. Kanatzidis, Nature Photon. 2014, 8, 489-494. [30] S. Ryu, J. H. Noh, N. J. Jeon, Y. C. Kim, W. S. Yang, J. Seo, S. I. Seok, Energy Environ. Sci. 2014, Accepted Manuscript, DOI: 10.1039/ C4EE00762J.

perovskite solar cells: rising, last advances, and future ... · perovskite solar cells: rising,...

Documents