NSF Workshop: Emerging Opportunities of Nanoscience to Energy Conversion and Storage

 

Section 2: Nanophotonics and Photovoltaics

P. Braun, D. Carlson, V. Klimov, J. Michl, and A.J. Nozik

 

2.1 High-Efficiency Multiple Exciton Generation in Semiconductor Nanocrystals

 

Background

Solar cells represent a potentially very important source of carbon-free energy. However, in order to make them competitive with traditional energy sources, the cost-to-efficiency ratio of photovoltaics must be reduced appreciably [1]. Cost considerations have been a strong driver for the development of non-silicon photovoltaic devices that are based on, e.g., polymers (plastic cells) [2] and dye-synthesized porous metal oxides (GrŠtzel cells) [3]. Increases in efficiency have typically relied upon evolutionary improvements of material quality (for both Si and non-Si systems) and/or device engineering aspects including, e.g., the use of tandem architectures. However, other approaches can lead potentially to major increases in photovoltaic performance through the use of new principles for conversion of solar energy into electricity. One such approach involves the use of multiple exciton generation (MEG) from a single photon that can greatly increase the photocurrent of solar cells [3, 4].

In the MEG process, absorption of a single photon produces multiple electron-hole pairs (excitons) and hence the internal quantum efficiency (QE) for converting photons into charge carriers becomes greater than 100%. The  phenomenon of multiple electron-hole pair generation from single photons has been known in bulk semiconductors since the 1950s [5] and has been explained in terms of impact ionization [3-5]. However, because of restrictions imposed by energy and momentum conservation and extremely fast intraband relaxation that competes with impact ionization, carrier multiplication (CM) efficiencies in bulk materials are quite low, particularly in the range of energies that are relevant to solar-energy conversion. For example, in bulk Si p-n junctions, the quantum yield reaches only 130% at photon energies of 5 eV—an energy value that is outside of the solar spectrum (ref)

 

Carrier multiplication in nanocrystals: Proof-of-principle experiments

 

As was first suggested by Nozik [6], the CM efficiency might be enhanced in nanoscale semiconductor particles [semiconductor nanocrystals (NC)] due to reduced rates of intraband relaxation and enhanced Coulomb interactions. Experimentally, high-efficiency CM in NCs was first demonstrated by Schaller and Klimov [7] who utilized significant differences in the recombination dynamics of single excitons and multiexcitons in order to detect CM and quantify its efficiency. Single excitons decay via relatively slow radiative recombination (in CdSe and PbSe NCs, e.g., it occurs with time constants of tens and hundreds of nanoseconds, respectively [8 - 10]), while multiexcitons decay on a much faster, picosecond timescale because of Auger recombination [11-13]. As a result of this difference, the generation of multiexcitons can be detected via a fast decay component in NC carrier population dynamics, while the CM efficiency can be calculated from the ratio of the signal amplitude at short times after excitation (before the Auger decay of the multiexcitons occurs) to the amplitude of the slow single-exciton background (after recombination of the multiexcitons is completed) (Fig. 1, inset).  Using this approach the experiments in ref. 7 demonstrated that absorption of a single photon by a PbSe NC could produce up to 2.2 excitons, which translated into 220% internal QE. Nozik and co-workers confirmed these results in similar studies of PbSe NCs, in which they detected QEs up to 300% (3 excitons per absorbed photon) [14]. They also demonstrated high-efficiency CM in NCs of  two other lead-salt compounds, PbS and PbTe, and they found that the carrier multiplication process is essentially instantaneous (less than the instrument response time of their systems which was 250 fs); because of this the carrier multiplication effect was termed Òmultiple exciton generation (MEG)Ó to reflect the fact that the CM process is instantaneous and is not driven by the impact ionization mechanism present in bulk semiconductors.

 

Generality of MEG and MEG quantum efficiency limits

 

These first observations of highly efficient MEG in NCs raise questions regarding the generality of this phenomenon to other materials and the potential limits on photon-to-exciton conversion efficiency. One specific issue is: How important are the unique features of lead salts (such as mirror symmetry between the conduction and valence bands resulting from nearly identical electron and hole masses) for obtaining highly efficient CM?

 

 

 


 

Text Box: Figure 1. CM QEs for CdSe (circles) and PbSe (squares) NCs as a function of excitation photon energy divided by Eg. Arrows mark onsets of CM. Dashed lines are linear fits with a slope of ca. 110% per Eg. Inset: Illustration of the method for calculations of QE from transient absorption traces (Δα is carrier induced absorption change). At long time after excitation (longer than the Auger decay constant), all multiexcitons recombine to produce single excitons and, therefore, exciton multiplicity is one, which provides a calibration scale bar for calculating multiplicity immediately after excitation. From ref. 15.

 

 

 

 


 

Text Box: Figure 2. ÒIdealÓ QEs derived from energy conservation (black bars) and experimental QEs measured for PbSe NCs (blue solid squares) as a function of pump-photon energy, hω, normalized by energy gap. Experimental QEs approach the limit determined by energy conservation at hω = 7.8Eg, for which QE Å 700%. The blue line is a linear fit to experimental data in the range of energies >3Eg; it indicates that complete filling of the lowest 1S quantized state (8 electron-hole pairs) produced by a single photon is expected at hω Å 9Eg.

 


This issue has been recently addressed by Klimov and co-workers in a comparative study of CdSe and PbSe NCs [15]. These two types of NCs have distinctly different electronic structures and carrier relaxation behavior. However, despite these differences, both materials show comparable MEG efficiencies for similar excess energies above the MEG threshold (Fig. 1), which is indicative of the generality of this phenomenon to quantum-confined, semiconductor NCs. Further, it is observed that CdSe NCs have a lower MEG activation threshold than PbSe NCs: ~2.5Eg vs. ~2.9Eg (Eg is the NC energy gap). This result can be explained in terms of

simple carrier effective-mass arguments and, specifically, by the difference in the distribution of the photon excess energy (the energy in the excess of the energy gap) between the conduction and the valence band [15]. The spectral dependence of the MEG efficiency is almost linear above the MEG threshold with a slope of ca. 110% per energy gap, which is nearly identical for both CdSe and PbSe NCs (Fig. 1). In addition to this fast growth, the remarkable result of these measurements was the observation that in the case of PbSe NCs, QE approaches 700% (7 excitons per single absorbed photon) at photon energy of 7.8Eg, which corresponds to the ultimate limit allowed by energy conservation for this excitation wavelength (Fig. 2).       

 

MEG mechanism

 CM in bulk materials has been traditionally explained in terms of impact ionization, which is the inverse of Auger recombination. However, the first experiments on carrier multiplication in NCs [7, 14] as well as more recent studies in Ref. 16 indicate a significant disparity between time constants of Auger recombination and MEG in nanocrystalline materials. While the Auger decay is characterized by tens-to-hundreds of picosecond time scales, MEG occurs with much faster time constants. Specifically, direct measurements of the multiexciton population build-up indicate that the MEG time constant is shorter than 50-to-200 fs [16], strongly suggesting that it is an instantaneous process. 

To explain very fast generation of multiexcitons in the MEG process, Nozik, Efros, and co-workers proposed a Òcoherent superpositionÓ model, in which the combined single exciton/multiexciton system is photo-excited through its single-exciton component and then experiences coherent oscillations between various resonant exciton and multiexciton states [14]. This mechanism predicts an oscillatory buildup of the multiexciton population if the dephasing of the combined single-exciton/multiexciton wavefunction occurs primarily via its multiexcitonic component. 

An alternative model for high-efficiency CM was recently proposed in Ref. 16. It explains this effect in terms of direct (instantaneous) photo-generation of multiexcitons via virtual single-exciton states. This process also relies on confinement-enhanced Coulomb coupling between single excitons and multiexcitons and also takes advantage of a large spectral density of high-energy single- and multiexciton resonances in nano-sized semiconductor crystals. In this model, CM is a second-order process, which can be described in the framework of second-order perturbation theory. This model produces tens-of-percent CM efficiencies assuming the presence of only a single virtual exciton resonance; and these values rapidly increase in the case of the multiple resonances that exist in real NC systems. An interesting feature of this model is that high-energy, single excitons are involved in the CM process as ÒvirtualÓ but not ÒrealÓ states, which may explain why NC ionization (i.e., ejection of carriers from NCs) does not occur despite the use of high pump photon energies.

 

Current challenges

 

One current challenge for practical applications of MEG is the extraction of charges from NCs on time scales that are faster than Auger recombination and other non-radiative and radiaitive channels. While the kinetics and the mechanism of charge separation in NC systems are not well understood, several researchers have demonstrated that charge separation at the interface between NCs and, e.g., surface adsorbed electron acceptors or porous TiO2 is highly efficient and occurs with sub-picosecond to picosecond time constants, which are significantly faster than those of Auger and radiative decay.

Other challenges are associated with poor electronic conductivity of NC solids, photo-corrosion of NCs, and the toxicity of materials that so far have been utilized in studies of MEG. However, most of these problems are not conceptual but rather are associated with material quality issues and design aspects of NC-based photovoltaic structures.

 

1.         ÒBasic Research Needs for Solar Energy UtilizationÓ, Report of the U.S. DOE/Basic Energy Sciences Workshop, April 18-21, 2005; available online at: http://www.sc.doe.gov/bes/reports/list.html

2.         G. Yu, J. Gao, J. C. Himmelen, F. Wudl, and A. J. Heeger, Science 270, 1789 (1995).

3.         M. GrŠtzel, Nature 414, 338 (2001).

4.         P. T. Landsberg, H. Nussbaumer, and G. J. Willeke,  Appl. Phys. 74, 1451 (1993).

5.         S. Kolodinski, J. H. Werner, T. Wittchen, and H. J. Queisser, Appl. Phys. Lett. 63, 2405 (1993).

 6.        S. Koc, Czech J. Phys. B 7, 91 (1957).

7.         A. J. Nozik,  Physica E 14, 115 (2002).

8.         R. D. Schaller, V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004).

9.         S. A. Crooker, T. Barrick, J. A. Hollingsworth, and V. I. Klimov, Appl. Phys. Lett. 82, 2793 (2003).

10.       B. L. Wehrenberg, C. J. Wang, and P. Guyot-Sionnest, J. Phys. Chem. B 106, 10634 (2002).

11.       H. Du, C. Chen, R. Krishnan, T. D. Krauss, J. M. Harbold, F. W. Wise, M. G. Thomas, and J. Silcox, Nano Lett. 2, 1321 (2002).

12.       V. I. Klimov, A. A. Mikhailovsky, D. W. McBranch, C. A. Leatherdale, and M. G. Bawendi, Science 287, 1011 (2000).

13.       R. D. Schaller, M. A. Petruska, and V. I. Klimov, J. Phys. Chem. B 107, 13765 (2003).

14.       V. I. Klimov, J. Phys. Chem. B 104, 6112 (2000).

15.       R. Ellingson, M. C. Beard, J. C. Johnson, P. Yu, O. I. Micic, A. J. Nozik, A. Shabaev, and A. L. Efros, Nano Lett. 5, 865-71 (2005).

16.       R. D. Schaller, M. A. Petruska, and V. I. Klimov, Appl. Phys. Lett. 87 (December 12, 2005).

17.       R. D. Schaller, V. M. Agranovich, and V. I. Klimov, Nature Phys. 1, 189 (2005).

 

 

2.2       Plasmonics and Photonic Crystals for Photon Management

 

Increased light  absorption

Photonic crystals have the potential to strongly modulate, focus, and concentrate light in ways that might increase the efficiency of PV devices which may reduce the need for solar tracking and increase the efficiency of PV structures.  Importantly, there are a number of rapid and low cost fabrication routes for photonic crystals increasing the possibility of photonic crystals for large area PV devices.

 

The field intensity within a 3-D photonic crystal is highly nonuniform, and strongly dependent on the details of the structure (Figure 3).  It may be possible to use the dependence of these field intensities to spatially modulate the absorption of light.  We can imagine that through appropriate theory and simulation, it may be possible to design a 3-D structure that can modulate, focus, and enhance the absorption of light in a fashion that would increase the efficiency of a PV device.

 

Figure 3.  Lower left, simulated field intensity map for light of a specific wavelength and incident direction inside a selenium three-dimensional inverse opal structure.  Background, SEM micrograph of the photonic crystal.
 

 

 

It is possible that simple photonic crystals will not be sufficient for modulating light.  It may be necessary to imbed features within the photonic crystal to focus light to specific regions.  Such features can be directly written within a photonic band gap crystal using a direct multiphoton writing [2].  Simple z-bend waveguide structures have been formed via this approach (Figure 4), and more complex structures designed explicitly for a PV device could also be formed.  Success in this area will require close coupling between theory and experiment.

 

Figure 4.  Waveguide structure written through multiphoton polymerization within a 3D self-assembled photonic crystal.
 
 

 

 

 

 

 

 

 

 

 

 


[1] P. V. Braun, R. W. Zehner, C. A. White, M. K. Weldon, C. Kloc, S. S. Patel and P. Wiltzius, Adv. Mater. 2001, 13, 721.

[2] W. Lee, S. A. Pruzinsky, P. V. Braun, Adv. Mater. 2002, 14, 271.

 

New Transparent Electrodes

 

Presently, there are no materials that satisfy both the transparency and conductivity requirements that are needed for electrodes in a parallel multijunction organic photovoltaic cell. Transparent electrodes based on Indium Tin Oxide (ITO) thin films have been widely used in single junction OPV cells because the transparency window of ITO is well matched to the solar spectrum. However, the DC conductivity in ITO is at least two orders of magnitude lower than that of a metal such as Ag and its sheet-resistance contributes significantly to losses, especially in large-area high-efficiency cells. One possibility is to replace the ITO electrode with optically transparent nanopatterned metal films. This will have the following benefits:

¤  The lower sheet-resistance will reduce losses in large-area OPV cells.

¤  Replacement of the brittle ITO metal-oxide will allow OPV cells to withstand larger deformations associated with substrate bending, leading to more rugged cells.

¤  Since the ITO electrode accounts for as much as 50% of the cost of an OPV cell, its replacement by a thin metal film will result in substantial cost savings.

¤  In parallel multijunction OPV cells where electrodes are required in between adjacent organic subcells, sputtered ITO electrodes are of limited use since the sputtering process inflicts damage on the underlying organic cells, resulting in dramatically decreased efficiencies. Metal deposition, on the other hand, is well suited for the fabrication of high efficiency cells.

 

It was recently discovered that metal films patterned with arrays of nanoscale holes transmit light with efficiencies much higher than expected from purely geometric considerations [T. W. Ebbessen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, Nature, 391, 667, 1998]. Since nanoscale holes have little effect on the in-plane DC conduction, such films retain their low sheet-resistance, resulting in a unique combination of optical transparency and metallic sheet-resistance. While transmissivities of only a few percent were demonstrated until now [F. I. Baida, and D. Van Labeke, Physical Review B, 62, 16100, 2003], simulations have indicated vastly improved structures that transmit up to 80% of the incident radiation. [H. Shin, P. Catrysse, and S. Fan, Physical Review B, 72 085436, 2005] We believe there are important opportunities in exploiting nano-structured metal films for PV applications and especially for parallel-connected multijunction OPV cells.

 

 

 

2.3       Singlet Fission

 

            Singlet fission in a molecular chromophore would produce two triplet excitons in two neighboring chromophores from a single exciton generated by the absorption of a single photon of sufficient energy by a molecule, and is a molecular analog of multiple exciton generation (MEG) by photon absorption in a semiconductor.  Each triplet exciton would then produce an electron-hole pair, doubling the photocurrent.  In principle, the two chromophores on which the triplet excitations are created could have different structures, in which case the usual term would not be singlet fission but quantum cutting.

            The use of singlet fission or quantum cutting  represents a possible disruptive dye-sensitized solar cell technology that would surpass the 32 % power conversion efficiency restriction imposed on a dye-sensitized Graetzel cell by the Shockley-Queisser limit and would replace it by a 47% theoretical efficiency limit for a singlet-fission sensitizer used in tandem with an ordinary sensitizer (quantum dots or dyes) that would absorb photons of energies intermediate between the lowest triplet and singlet states of the sensitizer (Fig. 5). For electricity production, the best choice of excitation energies would be about 1.1 eV for the triplet and 2.2 eV for the singlet, and for solar hydrogen splitting, the numbers would be somewhat different, depending on the specific design used.  The forbidden nature of singlet-triplet absorption would permit photons of intermediate 1.1 - 2.2 eV energies to pass the singlet-fission sensitizer zone and proceed to be absorbed by the ordinary sensitizer.  Transparency in a large intermediate region endows molecules with an advantage over semiconductors, in which the singlet and triplet levels are nearly degenerate, such that there hardly are any photons of intermediate energies.

            Although attractive in principle, the notion of using singlet fission for improving the efficiency of excitonic solar cells is totally unproven.  Singlet fission has been observed accidentally on half-a-dozen organic chromophores, mostly in crystals or polymers, and it is not obvious how to design molecules that undergo the process efficiently.  The inverse process, triplet-triplet annihilation, is much better known, and tends to be efficient and exothermic.  It generates a single excited species starting with two, and seems to occur whenever diffusion brings two triplet molecules into close proximity.

Software: Microsoft OfficeFigure 5.  Energy level diagram of nanocrystalline solar cell based on a singlet fission chromophore (C1) in optical series but parallel current flow with a conventional dye or semiconductor chromophore (C2).  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The requirements that would need to be met in order for a singlet-splitting sensitizer to be efficient are severe.  Its lowest excited singlet state needs to undergo singlet fission faster than any other process that might compete, such as fluorescence, internal conversion into the ground state, intersystem crossing into the triplet state, or injection of an electron into the conduction band of the semiconductor on which is adsorbed.  In its turn, electron injection by the triplet state must be faster than any other competing process, such as intersystem crossing into the ground state or triplet-triplet annihilation.  Furthermore, the singlet-fission sensitizer will have to meet the usual conditions that are already well recognized for any ordinary excitonic solar cell sensitizer, such as a high absorption coefficient in the ground state, good adsorption on the semiconductor, fast kinetics of hole transfer from oxidized dye to a relay, slow recombination, and stability upon irradiation.

            Some of the structural requirements for singlet-fission sensitizers are recognized relatively easily.  On an absolute energy scale, the T1 state has to be positioned just above the bottom of the conduction band of the semiconductor, to permit rapid electron injection, and this can be fine-tuned by introduction of suitable substituents.  Intersystem crossing rate can be reduced by minimizing spin-orbit coupling, and it will be best to avoid heavy atoms in the molecular design.  Internal conversion to the ground state will be slowed down by molecular rigidity in the adsorbed state, and internal rotors and low-frequency modes would be best avoided.  Fast electron injection will be favored by strong coupling to states of the semiconductor, and it is likely that chemisorption will be preferable to physisorption, and that the triplet state should be of intramolecular charge-transfer nature.  Injection from the initially excited singlet state could be discouraged if it is of locally excited nature or if it is spatially separated from the semiconductor surface by suitable nanoengineering.

            Avoidance of triplet-triplet annihilation process is of particular concern, given that is normally very fast, and that the two triplets are created in immediate vicinity.  The coupling of two triplets can in principle produce an overall singlet, a triplet, or a quintet.  Three of the possibly competing processes are unlikely to represent serious problems.  The formation of the ground state singlets of the two chromophores, as well as the formation of one ground state singlet and one triplet, would be highly exothermic and likely to be slow due to the energy gap law.  The formation of a ground state singlet and a quintet is likely to be strongly endothermic and negligible.  However, two of the processes are a real concern.  They are the formation of a singlet ground state plus an excited singlet state and the formation of a ground state plus an upper triplet state.  It would appear best to make sure that they are both sufficiently endothermic and therefore very slow at room temperature.  This would lead to a highly unusual requirement for the excitation energies from the ground state: E(T2) $ E(S1) > 2E(T1).  This is much more severe than the above identified condition E(S1) $ 2E(T1) alone.

             Meeting these requirements by molecular design encounters problems whose solution requires fundamental knowledge that is presently not available.  The leading two that are new and specific to singlet-fission sensitizers are (i) how to adjust the energy levels into a highly unusual arrangement, (ii) how to optimize the degree of coupling between chromophores to maximize the rate of singlet fission.  Perhaps a sensitizer molecule that meets these in addition to all the other requirements cannot be found.  However, the potential payoff is so high that it seems advisable to invest some effort into the fundamental research needed.  A search for a solution to problem (i) will have to start with the use of molecular quantum theory to identify structural classes likely to contain molecules with the desired arrangement of energy levels, continue to numerical calculations for a scan of a large number of representatives, identification of the most promising candidates for synthesis, higher level calculations for these candidates, their synthesis, and experimental determination of the positions of their T1, T2, and S1 levels.  A search for a solution to problem (ii) will involve the development of a detailed theory of the singlet fission (and triplet-triplet annihilation) process that will identify the optimal molecular structure for connecting two chromophores, synthesis of dimers or higher oligomers, and a search for evidence of singlet exciton fission and a measurement of its efficiency.  Then, after it is understood how to design good sensitizers for singlet fission, one will need to be concerned with the other issues identified above, such as adjusting absolute level positions, optimizing the electron injection speed for the triplet and minimizing it for the singlet, etc.

 

2.4       Hole Conductor Relay in Dye-Senitized Solar Cells

 

            The iodide ion relay currently used in the best performing excitonic solar cells is more strongly reducing than it needs to be and as a result, the voltage produced by the cell is more than half a volt lower than it could be.  This accounts for a large fraction of the difference between the Shockley-Queisser limit and the 11% efficiency actually attained.  Much effort has gone into a search for a relay with a better positioned redox potential, but none have been found.  The very fast hole-transfer kinetics of the iodide ion, and the conversion of the resulting iodine atom into the negatively charged triiodide ion, which discourages recombination, are advantages that are very hard to beat.

            Still, the effort ought to be continued.  For engineering reasons, there would be considerable advantages in replacing a liquid solution with a solid hole-conducting polymer, and rapid kinetics for hole transfer from the sensitizer to the polymer could perhaps be secured by a covalent link between them.  The discovery of new polymers with high hole conductivity whose redox potential can be adjusted to a desired level should be a high priority.

 

 

2.5       PV Industry Perspective

 

In the first few decades after the first silicon solar cell was made at Bell Labs in 1954, the photovoltaic (PV) industry grew at a rate of about 18% annually due largely to the use of PV systems in remote locations (such as telecommunication relay sites).  However, in the last six years, the market growth has increased to ~ 35% per year due to the rapid growth of grid-connected PV systems, which in turn has been largely driven by government-supported programs in Japan and Germany.  Silicon technology has dominated the PV industry since its inception, and in 2005 about 65% of all solar cells were made from polycrystalline (or multicrystalline) silicon, ~ 24% from monocrystalline silicon and ~ 4% from ribbon silicon.  While conversion efficiencies as high as 24.7% have been obtained in the laboratory for silicon solar cells, the best efficiencies for commercial PV modules are in the range of 17 – 18% (the efficiency limit for a silicon solar cell is ~ 29%).  A number of companies are commercializing solar cells based on other materials such as amorphous silicon, microcrystalline silicon, cadmium telluride, copper-indium-gallium-diselenide (CIGS), gallium arsenide (and related compounds) and dye-sensitized (Graetzel) solar cells.  Thin film CIGS solar cells have been fabricated with conversion efficiencies as high as 19.5% while efficiencies as high as 39% have been demonstrated for GaInP/Ga(In)As/Ge triple-junction cells operating at a concentration of 236 suns.  Thin film solar cells are being used in consumer products and in some building-integrated applications, while PV concentrator systems are being tested in grid-connected arrays located in high solar insolation areas.  However, most industry experts believe that crystalline silicon PV technology will continue to dominate the terrestrial market for at least the next decade.  It is likely that by 2020, silicon PV module efficiencies will exceed 20% and module prices will fall below $1/Wp. 

 

Photovoltaics has the potential to become a major energy source within the next several decades, but technology development must be accelerated in order for PV to provide a viable alternative to fossil fuels (which are being rapidly depleted) and to have a significant impact as soon as possible on carbon dioxide emissions and climate change .  Nanotechnology is an attractive option that may lead to new low-cost, high performance solar cells that could displace crystalline silicon solar cells in the next decade, and thus lead to a more rapid deployment of large-scale PV systems and the widespread availability of low-cost, clean PV electricity.