Organisers

• Wanda Andreoni (IBM Research Zurich)
• Claudia Felser (University of Mainz)
• Tanja Schilling (University of Mainz )

Supports

 CECAM

 Psi-k

 Mainz Graduate School in Materials Science

Description

Background

The possibility to convert sunlight into electrical power is becoming increasingly important as fossil fuels become less available and the environmental, political and physiological cost of our current energy sources becomes more apparent. Although the first solar cell dates back to more than 50 years ago, progress must still be made in solar photovoltaic technology to render sunlight a practical alternative. Although the ultimate improvement on performance will depend on device design and engineering, fundamental scientific issues if addressed properly could guide the choice of the component materials and help optimize their use.

Photovoltaics based on crystalline silicon have been predominant since the time of the production of the first (modern) solar cell in 1954. Starting in the 70s, thin film photovoltaics have been investigated especially based on amorphous silicon or chalcopyrites like CuInSe2 (CIS) CuGaSe2 (CGS) and CuIn1-xGaxSe2 (CIGS) [1]. Originally less efficient than silicon, they offered other good characteristics like cost and mass reduction. Also, in contrast to crystalline silicon, chalcopyrites are direct band gap semiconductors, and do not require high purity to function well. Currently, CIGS-based devices are in production. However their efficiency is at most ~20%. Moreover they are expensive. In particular Indium has become rare and needs to be replaced in the perspective of massive production. Also CdS, the typical material forming the buffer layer, will have to be replaced because of the toxic character of cadmium. While great effort is spent on the improvement of these inorganic thin films, novel solutions have appeared on the horizon for the light absorbers, namely quantum dots and organic semiconductors, as well as dye-sensitized (Gr�¤tzel) cells [2]. In principle, all offer interesting advantages including lower cost and higher flexibility.

Moreover the intermediate band (IB) concept - well known in the physics of semiconductors - has been introduced in the engineering of novel solar cells, which provides a mechanism to exceed the limiting efficiency of single gap solar cells. By IB one means an energy band located within the semiconductor band gap; this allows the absorption of two below-band gap energy photons to produce one electron-hole pair and eventually generate enhanced photocurrent without voltage degradation. This scheme can be realized with quantum dots in a host semiconductor or with deep level impurities [3].
It is not a surprise that each choice above implies different fundamental issues to be tackled also by theoreticians in their effort to understand and use their results to help optimize materials design. We restrict our interest to thin film PVs and quantum dots.

Theoretical and Computational Approach: State of the Art

I. Thin Film Inorganic PVs (CIS et al)

Current understanding of the fundamental physics underlying the behaviour of absorber layers of thin film PVs (polycrystalline, multinary materials) relies on the extensive studies pursued over the last 15 years or so by the group of Alex Zunger at the Institute of Solar Energy, Boulder (CO). Examples of issues investigated more recently are

• The role of defects

Se-Cu divacancy complex (V-Se-V-Cu) was identified as the source of light- and bias-induced metastabilities in Cu(In,Ga)Se2 (CIGS) based solar cells [4]. The theoretical approach used numerical simulations of equilibrium defect thermodynamics relying on defect formation energies calculated ab initio, and a model describing the transition dynamics after creation of a metastable non-equilibrium state.


• The role of grain boundaries

DFT-based calculations led to explain why polycrystalline CuInSe2 solar cells are superior to their crystalline counterpart [5]. GB holes are depressed because an energetic barrier exists for holes from grain interior to grain boundary, which is related to Cu vacancy surface reconstruction. These results were confirmed by more recent experiment on CIGS [6] and are obviously of critical importance for future design of superior polycrystalline devices.


• The beneficial role of sodium incorporation

The presence of small percentages of sodium in CIGS is known to improve conversion efficiencies, which has prompted huge effort in controlling growth and deposition processes (see e.g. [7]). Based on DFT calculations, the main relevant mechanism of Na in Cu-poor CIS was identified as the replacement of InCu by NaCu, until most of the compensating InCu donors are removed [8]. Moreover NaCu defects or NaInSe2 phases (also predicted to form for high Na concentration) at grain boundaries were proposed to lower the valence-band maximum - owing to the lack of Na d-electron states and thus create a barrier to the GBs for the holes (as above) [5].


• The possibility of doping

Based on DFT calculations, the solubility of transition metal dopants, their site preferences, and electronic and magnetic properties as well were investigated (see e.g. [9]).

Examples of other DFT-based calculations:


• More on the role of sodium: a combined theoretical and experimental (XPS) study of Cu and Na compounds, which are expected to form at the Cu(In,Ga)Se2/In2S3 interface. In CuIn5S8, Na0.5Cu0.5In5S8 and NaIn5S8 compounds variations in the band gaps are confirmed as due to a shift of the top of the valence band with increasing sodium content [10].


• More on transition metal doping: the effects of cation substitution with Fe in CIS have been investigated on structural and electronic properties of ordered phases also with the aim of making predictions on the influence this replacement might have on photovoltaic conversion efficiencies. Simple analysis of the density-of-states leads to negative conclusions at least for the Fe-rich compounds [11].


• Intermediate band materials based on CGS: search for suitable candidates. Partial substitution of gallium by transition metal atoms (Ti, V, Cr, Mn) is found to induce the desired additional narrow bands in the band gap of the parent sulphide compound. Ti appears to provide the best characteristics, also concerning stability of the crystal structure [12].

II. Semiconductor Quantum Dots

QDs have the potential of greatly enhancing the photon conversion efficiency in two ways: (i) by inducing the formation of intermediate narrow bands in the band gap of the host semiconductor (IB effect) and (ii) by allowing production of multiple excitons from a single photon of sufficient energy. Cd(S,Se) and Pb(S,Se) are the mainly investigated compounds.
The high efficiency of QDs for the multiple exciton generation (MEG) has indeed been demonstrated especially for the lead chalcogenides, giving rise to conversion efficiencies >60% [13]. In a series of papers over the years, Zunger and his team have been provided clues to the mechanisms underlying direct carrier multiplication and quantitative predictions for the rates of such processes compared to competing (especially Auger) recombination processes [14]. In particular, the recently observed super-efficiency of PbSe QDs (>90%) [15] has provoked a debate amongst theorists on whether impact ionization were sufficient to explain multiexciton generation or exotic unforeseen quantum phenomena had to be invoked [16].
During the last few years, electronic excitations in semiconductor QDs, the generation of electron-hole pairs and their relaxation channels have been the focus of increasingly intense experimental, theoretical and computational studies. In particular, the methods used by Zunger et al. are based on the semiempirical nonlocal pseudopotential, lately combined with the configuration-interaction approach.

III. Computational Methods: a Perspective

• DFT-based calculations and beyond.

First-principles DFT-based calculations have so far been the leading theoretical approach to the study of the physical properties of the absorber components of real PV devices. It is conceivable that this will continue to be the case for a while, especially in combination with large-scale simulations (e.g. Car-Parrinello molecular dynamics) that allow to investigate directly the formation of defects in inhomogeneous materials, identify probable configurations (structure and composition) of their interfaces, and eventually tackle the intricate problems related to device fabrication (annealing, deposition, doping..). Also, the general current investigation of more accurate exchange-correlation functionals in DFT implementations, notably for the prediction of band gaps, as well as further development and application of TDDFT will have an impact in the study of photovoltaics in general. Especially the combination of the restricted open-shell Kohn-Sham (ROKS) theory and dynamical simulations could be of special interest for the investigation of the excited states of the QDs.


• Materials informatics.

The need for improved or novel materials is a common feature of today technologies, and with it the need for combined research in materials screening, growth and characterization. Given the unmanageably large number of a-priori possibilities, structural chemistry and modem computer-aided design can play a unique role in screening potentially important candidates, just as bioinformatics does in the drug discovery process. In particular, virtual screening could complement already existing related experimental effort and/or foster and guide it.
As materials informatics, we intend
Combinatorial theoretical chemistry, as counterpart of analogous experimental procedures. Combinatorial approaches have been used by experimentalists for the search of thin film PVs based on non-traditional semiconductors, thus producing combinatorial libraries and specific screening methodologies [see e.g. Ref. 17].
- Diagrammatic approaches, which use large databases of empirical, semi-empirical and also quantum parameters on known materials to narrow the search space for potentially beneficial new materials and for the prediction of even more specific information such as stoichiometry, crystal structure and other properties [18].
- The Inverse Band Structure (IBS) method, which is aimed at identifying atomic structures with given desired electronic characteristics. Combined with genetic algorithms for configuration sampling, the IBS scheme has already been applied successfully to determine binary alloys with target physical properties [19].


• Monte Carlo and classical molecular dynamics simulations.

Traditionally these methods are useful for the study of materials processing. For example, they could be used in conjunction with layer-by-layer assembly, which is often employed for exploratory research [see Ref 20]. Special attention will have to be devoted to the selection of the interatomic potentials and possibly more accurate potentials than those currently available will have to be devised.

Scientific Objectives

Scope of the Workshop
The purpose of this workshop is to settle the state of the art of computational methods (modelling and simulations as well as combinatorial/data mining, diagrammatic approaches and band engineering) in this field and especially clarify the relevant open issues for which the aid of theory and computations can be important or even crucial. The discussion is expected to lead to new ideas and new ways of merging different approaches thus fostering new collaborations and efforts focused towards the design of novel or improved materials or growth/deposition processes. For this reason, we intend to gather experts in theory and computations and also include several experimentalists and experts from industry.
We wish to focus on inorganic thin films PVs and quantum dots. As for organic PVs we may wish to have an overview of the state-of-the-art from one/two world-wide experts.

Format
Workshop 3 days at CECAM central location.
Preferred dates:
3 days preferably in late spring 2008.
Budget
The exact budget will depend on the number of participants. We aim at about 30 participants. We are applying simultaneously for 15K Euro from CECAM and 8K Euro from Psi-K.

References

[1] see e.g.
A. Goetzberger, C. Hebling and H-W Schock, Photovoltaic materials, history, status and outlook,
Mat. Sci. and Eng. R40, 1-46 (2003)

[2] see e.g.
K. Kalyanasundaram and M. Graetzel, Applications of Functionalized Transition Metal Complexes in Photonic and Optoelectronic devices,
Coord. Chem. Rev., 77, 347- 414 (1998)

[3] see e.g.
A. Luque and A. Mart, Prog. in Photov, Res.and Appl. 9, 73-86 (2001);
A. Mart, N. Lopez, E. Antoln, E. Cinovas, C. Stanley, C. Farmer, L. Cuadra and A. Luque,
Novel semiconductor solar cell structures: The quantum dot intermediate band solar cell,
511-512, 638-644 (2006)

[4] Lany, S. and Zunger, A., Light- and bias-induced metastabilities in Cu(In,Ga)Se-2 based solar cells caused by the (V-Se-V-Cu) vacancy complex,
J. Appl. Phys. 100, 113725 (2006).

[5] C. Person and A. Zunger, Anomalous Grain Boundary Physics in Polycrystalline CuInSe2: The Existence of a Hole Barrier,
Phys. Rev. Lett. 91, 266401 (2003).

[6] Hetzer, M. J., Strzhemechny, Y. M., Gao, M., Goss, S., Contreras, M. A., Zunger, A., Brillson, L. J.,On microscopic compositional and electrostatic properties of grain boundaries in polycrystalline CuIn1-xGaxSe2,
J. Vac. Sci.Tech. B24, 1739 (2006).

[7] D. Rudmann, G. Bilger, M. Kaelin, F. -J. Haug, H. Zogg and A. N. Tiwari, Effects of NaF coevaporation on structural properties of Cu(In,Ga)Se2 thin films,
Thin Solid Films 431-432, 37-40 (2003) and references therein.

[8] S.-H. Wei, S. B. Zhang, and A. Zunger‚ Effects of Na on the electrical and structural properties of CuInSe2,
J. Appl. Phys. 85, 7214ƒ¯‚‚7218, 1999.

[9] Y.J. Zhao and A. Zunger, Site preference for Mn substitution in spintronic CuMX2 chalcopyrite semiconductors,
Phys. Rev. B, 69, 75208 (2004).

[10] Lafond, A.; Guillot-Deudon, C.; Harel, S.; Mokrani, A.; Barreau, N.; Gall, S.; Kessler, J., Structural study and electronic band structure investigations of the solid solution NaxCu1-xIn5S8 and its impact on the Cu(In,Ga)Se2/In2S3 interface of solar cells,
Thin Solid Films 515, 6020 (2007).

[11] J.-M. Raulot, C. Domain, and J.-F. Guillemoles, Fe-doped CuInSe2: An ab initio study of magnetic defects in a photovoltaic material,
Phys. Rev. B71, 035203 (2005)

[12] Palacios, P.; Sanchez, K.; Wahnon, P.; Conesa, J.C., Characterization by ab initio calculations of an intermediate band material based on chalcopyrite semiconductors substituted by several transition metals,
J. Solar Energy Engineering 129, 314-18 (2007).

[13] R. Schaller and V. Klimov, High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion,
Phys. Rev. Lett. 92, 186601 (2004);
R.J. Ellingson, M.C. Beard, J.C. Johnson, P. Yu, O.I. Micic, A.J. Nozik, A. Shabaev, and A.L. Efros, Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots,
Nano Lett. 5, 865 (2005);
R.D. Schaller, M.A. Petruska and V.I. Klimov,
Appl. Phys. Lett. 87, 253102 (2005)

[14] see e.g.
M. Califano, A. Zunger and A. Franceschetti, Efficient Inverse Auger Recombination at Threshold in CdSe Nanocrystals,
NanoLetters, 4, (2004);
Direct Carrier Multiplication due to Inverse Auger Scattering in CdSe Quantum Dots,
Appl. Phys. Lett., 84, 2409 (2004);
M. Califano, A. Franceschetti and A. Zunger, Lifetime and polarization of the radiative decay of excitons, biexcitons and trions in CdSe nanocrystal quantum dots,
Phys. Rev. B 75, 115401 (2007)

[15] Richard D. Schaller, Milan Sykora, Jeffrey M. Pietryga, and Victor I. Klimov, Seven Excitons at a Cost of One: Redefining the Limits for Conversion Efficiency of Photons into Charge Carriers,
Nano Lett. 6, 424, 2006

[16] see e.g.
A. Franceschetti, J.M. An, and A. Zunger, Impact ionization can explain carrier multiplication in PbSe quantum dots,
NanoLetters 6, 2191 (2006);
G. Allan and C. Delerue, Role of impact ionization in multiple exciton generation in PbSe nanocrystals,
Phys. Rev. B 73, 205423-205427 (2006) ; and references therein.

[17] Haber, J.A.; Gerein, N.J.; Hatchard, T.D.; Versavel, M.Y., Combinatorial discovery of new thin film photovoltaics
Photovoltaic Specialists Conference, IEEE 31, 155-158 (2005).

[18] J. Daams and P. Villars, Atomic environments in relation to compound prediction
Engineering Applications of Artificial Intelligence 13, 507-511 (2000);
P. Villars, K. Brandenburg, M. Berndt, S. LeClair, A. Jackson, Y. -H. Pao, B. Igelnik, M. Oxley, B. Bakshi, P. Chen and S. Iwata, Interplay of large materials databases, semi-empirical methods, neuro-computing and first principle calculations for ternary compound former/non-former prediction,
Engineering Applications of Artificial Intelligence 13, 497-503 (2000).

[19] Dudiy, S. V. and Zunger, A., Searching for alloy configurations with target physical properties: Impurity design via a genetic algorithm inverse band structure approach,
Phys. Rev. Lett. 97, 046401 (2006) and references therein.

[20] Katsuhiko, A.; Hill, J.P.; Ji Qingmin, Layer-by-layer assembly as a versatile bottom-up nanofabrication technique for exploratory research and realistic application,
PCCP 9, 2319-40 (2007)