Acknowledgment is made to the Australian Research Council (LE130100146, DP160103008). JAG is supported via an Australian Postgraduate Award, and DRM by an ARC Future Fellowship (FT130100214). We thank Nikos Kopidakis for helpful discussions.

<ol>
	<li>Park, J., Reid, O. G., Blackburn, J. L., Rumbles, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photoinduced+spontaneous+free-carrier+generation+in+semiconducting+single-walled+carbon+nanotubes.">Photoinduced spontaneous free-carrier generation in semiconducting single-walled carbon nanotubes.</a> Nat. Comm. 6 (8809), (2015).</li><li>Dicker, G., de Haas, M. P., Siebbeles, L. D., Warman, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrodeless+time-resolved+microwave+conductivity+study+of+charge-carrier+photogeneration+in+regioregular+poly+(3-hexylthiophene)+thin+films.">Electrodeless time-resolved microwave conductivity study of charge-carrier photogeneration in regioregular poly (3-hexylthiophene) thin films.</a> Phys. Rev. B. 70 (4), 045203 (2004).</li><li>Oga, H., Saeki, A., Ogomi, Y., Hayase, S., Seki, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improved+understanding+of+the+electronic+and+energetic+landscapes+of+perovskite+solar+cells:+high+local+charge+carrier+mobility,+reduced+recombination,+and+extremely+shallow+traps.">Improved understanding of the electronic and energetic landscapes of perovskite solar cells: high local charge carrier mobility, reduced recombination, and extremely shallow traps.</a> J. Am. Chem. Soc. 136 (39), 13818-13825 (2014).</li><li>Ponseca, C. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Organometal+halide+perovskite+solar+cell+materials+rationalized:+ultrafast+charge+generation,+high+and+microsecond-long+balanced+mobilities,+and+slow+recombination.">Organometal halide perovskite solar cell materials rationalized: ultrafast charge generation, high and microsecond-long balanced mobilities, and slow recombination.</a> J. Am. Chem. Soc. 136 (14), 5189-5192 (2014).</li><li>Guse, J. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectral+dependence+of+direct+and+trap-mediated+recombination+processes+in+lead+halide+perovskites+using+time+resolved+microwave+conductivity.">Spectral dependence of direct and trap-mediated recombination processes in lead halide perovskites using time resolved microwave conductivity.</a> Phys. Chem. Chem. Phys. 18, 12043-12049 (2016).</li><li>Kunst, M., Abdallah, O., W&#252;nsch, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Passivation+of+silicon+by+silicon+nitride+films.">Passivation of silicon by silicon nitride films.</a> Solar energy materials and solar cells. 72 (1-4), 335-341 (2002).</li><li>Hutter, E. M., Eperon, G. E., Stranks, S. D., Savenije, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Charge+Carriers+in+Planar+and+Meso-Structured+Organic-Inorganic+Perovskites:+Mobilities,+Lifetimes+and+Concentrations+of+Trap+States.">Charge Carriers in Planar and Meso-Structured Organic-Inorganic Perovskites: Mobilities, Lifetimes and Concentrations of Trap States.</a> J. Phys. Chem. Lett. 6 (15), 3082-3090 (2015).</li><li>Cosme, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Lifetime+assessment+in+crystalline+silicon:+From+nanopatterned+wafer+to+ultra-thin+crystalline+films+for+solar+cells.">Lifetime assessment in crystalline silicon: From nanopatterned wafer to ultra-thin crystalline films for solar cells.</a> Solar Energy Materials and Solar Cells. 135, 93-98 (2015).</li><li>Katoh, R., Furube, A., Yamanaka, K. I., Morikawa, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Charge+separation+and+trapping+in+N-doped+TiO2+photocatalysts:+A+time-resolved+microwave+conductivity+study.">Charge separation and trapping in N-doped TiO2 photocatalysts: A time-resolved microwave conductivity study.</a> J. Phys. Chem. Lett. 1 (22), 3261-3265 (2010).</li><li>Colbeau-Justin, C., Valenzuela, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Time-resolved+microwave+conductivity+(TRMC)+a+useful+characterization+tool+for+charge+carrier+transfer+in+photocatalysis:+a+short+review.">Time-resolved microwave conductivity (TRMC) a useful characterization tool for charge carrier transfer in photocatalysis: a short review.</a> Revista mexicana de f&#237;sica. 59 (3), 191-200 (2013).</li><li>Luna, A. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synergetic+effect+of+Ni+and+Au+nanoparticles+synthesized+on+titania+particles+for+efficient+photocatalytic+hydrogen+production.">Synergetic effect of Ni and Au nanoparticles synthesized on titania particles for efficient photocatalytic hydrogen production.</a> Applied Catalysis B: Environmental. 191, 18-28 (2016).</li><li>Ferguson, A. J., Kopidakis, N., Shaheen, S. E., Rumbles, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quenching+of+excitons+by+holes+in+poly+(3-hexylthiophene)+films.">Quenching of excitons by holes in poly (3-hexylthiophene) films.</a> J. Phys. Chem. C. 112 (26), 9865-9871 (2008).</li><li>Ferguson, A. J., Kopidakis, N., Shaheen, S. E., Rumbles, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dark+carriers,+trapping,+and+activation+control+of+carrier+recombination+in+neat+P3HT+and+P3HT:+PCBM+blends.">Dark carriers, trapping, and activation control of carrier recombination in neat P3HT and P3HT: PCBM blends.</a> J. Phys. Chem. C. 115 (46), 23134-23148 (2011).</li><li>Savenije, T. J., Ferguson, A. J., Kopidakis, N., Rumbles, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Revealing+the+Dynamics+of+Charge+Carriers+in+Polymer:fullerene+Blends+Using+Photoinduced+Time-Resolved+Microwave+Conductivity.">Revealing the Dynamics of Charge Carriers in Polymer:fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity.</a> J. Phys. Chem. C. 117 (46), 24085-24103 (2013).</li><li>Xiao, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient,+high+yield+perovskite+photovoltaic+devices+grown+by+interdiffusion+of+solution-processed+precursor+stacking+layers.">Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers.</a> Energy Environ. Sci. 7 (8), 2619-2623 (2014).</li><li>Infelta, P. P., De Haas, M. P., Warman, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+study+of+the+transient+conductivity+of+pulse+irradiated+dielectric+liquids+on+a+nanosecond+timescale+using+microwaves.">The study of the transient conductivity of pulse irradiated dielectric liquids on a nanosecond timescale using microwaves.</a> Radiat. Phys. Chem. 10 (5-6), 353-365 (1977).</li><li>Saeki, A., Seki, S., Sunagawa, T., Ushida, K., Tagawa, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Charge-carrier+dynamics+in+polythiophene+films+studied+by+in-situ+measurement+of+flash-photolysis+time-resolved+microwave+conductivity+(FP-TRMC)+and+transient+optical+spectroscopy+(TOS).">Charge-carrier dynamics in polythiophene films studied by in-situ measurement of flash-photolysis time-resolved microwave conductivity (FP-TRMC) and transient optical spectroscopy (TOS).</a> Philosophical Magazine. 86 (9), 1261-1276 (2006).</li><li>Choi, W., Miyakai, T., Sakurai, T., Saeki, A., Yokoyama, M., Seki, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Non-contact,+non-destructive,+quantitative+probing+of+interfacial+trap+sites+for+charge+carrier+transport+at+semiconductor-insulator+boundary.">Non-contact, non-destructive, quantitative probing of interfacial trap sites for charge carrier transport at semiconductor-insulator boundary.</a> Appl. Phys. Lett. 105 (3), 033302 (2014).</li></ol>

The authors have nothing to disclose.

While the TRMC technique can offer a wealth of information about photoinduced charge carrier dynamics, this is an indirect measurement of conductivity, and therefore care needs to be taken when interpreting results. The TRMC technique measures total mobility, and cannot be used to distinguish between electron and hole mobilities. The underlying assumption that conductivity is proportional to change in reflected power holds only when that change is small (&#60; 5%)16. Furthermore, if the shift in r...

Flash-photolysis time-resolved microwave conductivity (FP-TRMC) monitors dynamics of photo-excited charge carriers on the ns-&#181;s timescale, making it an ideal tool for investigating charge carrier recombination processes. Understanding the decay mechanisms of photo-induced charge carriers in thin film semiconductors is of key importance in a range of applications, including photovoltaic device optimization. The induced carrier lifetimes are often functions of induced carrier density, excitation wavelength, mobility, trap density and trapping rate. This paper demonstrates the versatility of the Time Resolved Microwave Conductivity (TRMC) technique for investigating a wide range of carrier dynamic dependencies (intensity, wavelength, microwave frequency) and their interpretations.
Photogenerated charges can modify to both the real and the imaginary parts of the dielectric constant of a material, depending on their mobility and degree of confinement/localization1. The conductivity of a material <img alt="Equation" src="/files/ftp_upload/55232/55232eq1.jpg" /> is proportional to its complex dielectric constant
<img alt="Equation" src="/files/ftp_upload/55232/55232eq2.jpg" />
where <img alt="Equation" src="/files/ftp_upload/55232/55232eq3.jpg" /> is the frequency of a microwave electric field, <img alt="Equation" src="/files/ftp_upload/55232/55232eq4.jpg" /> and <img alt="Equation" src="/files/ftp_upload/55232/55232eq5.jpg" /> are the real and imaginary parts of the dielectric constant. Thus, the real part of the conductivity is related to the imaginary part of the dielectric constant, and can be mapped onto microwave absorption, while the imaginary part of the conductivity (subsequently referred to as polarization) is related to a shift in the resonance frequency of the microwave field1.
TRMC offers several advantages over other techniques. For instance, DC photoconductivity measurements suffer from a range of complications arising from contacting the material with electrodes. Enhanced recombination at the electrode/material interface, back injection of charges through this interface, as well as enhanced dissociation of excitons and geminate pairs due to the applied voltage2 all lead to distortions in the measured carrier mobilities and lifetimes. In contrast, TRMC is an electrodeless technique which measures the intrinsic mobility of the carriers without distortions due to charge transfer across contacts.
A significant advantage of using microwave power as a probe for carrier dynamics is that, as well as monitoring the decay lifetimes of charge carriers, decay mechanisms/pathways can also be investigated.
TRMC can be used to determine the total mobility3 and lifetime4 of induced charge carriers. These parameters can subsequently be used to distinguish between direct and trap-mediated recombination mechanisms3,5. The dependence of these two separate decay pathways can be quantitatively analyzed as a function of carrier density3,5 and excitation energy/wavelength5. The localization/confinement of induced carriers can be investigated by comparing the decay of the conductivity vs polarizability5 (imaginary vs real part of dielectric constant).
Additionally, and perhaps most importantly, TRMC can be used to characterize trap states which act as charge carrier decay pathways. Surface traps, for example, can be distinguished from bulk traps by comparing passivated vs unpassivated samples6. Sub-bandgap states can be directly investigated using sub-bandgap excitation energies5. Trap densities can be deduced by fitting TRMC data7.
Due to the versatility of this technique, TRMC has been applied to study a wide range of materials including: traditional thin film semiconductors such as silicon6,8 and TiO29,10, nanoparticles11, nanotubes1, organic semiconductors12, material blends13,14, and hybrid photovoltaic materials3,4,5.
In order to obtain quantitative information using TRMC, it is crucial to be able to accurately determine the number of absorbed photons for a given optical excitation. Since methods for quantifying absorption of thin films, nanoparticles, solutions and opaque samples differ, the sample preparation and calibration techniques presented here are designed specifically for thin film samples. However, the TRMC measurement protocol presented is very general.

<table><tbody><tr><td>Hellmanex III detergent</td><td>Sigma Aldrich&lt;br/&gt; www.sigmaaldrich.com/catalog/product/sial/z805939?lang=en&amp;amp;region=AU</td><td>Z805939</td><td>Corrosive and toxic. See SDS.</td></tr><tr><td>Lead(II) iodide (99%)</td><td>Sigma Aldrich&lt;br/&gt; www.sigmaaldrich.com/catalog/product/aldrich/211168?lang=en&amp;amp;region=AU</td><td>211168</td><td>Toxic. See SDS.</td></tr><tr><td>Anhydrous dimethylformamide (99.8%)</td><td>Sigma Aldrich&lt;br/&gt; www.sigmaaldrich.com/catalog/product/sial/227056?lang=en&amp;amp;region=AU</td><td>227056</td><td>Toxic. See SDS.</td></tr><tr><td>Anhydrous dimethylsulfoxide (99.9%)</td><td>Sigma Aldrich&lt;br/&gt; www.sigmaaldrich.com/catalog/product/sial/276855?lang=en&amp;amp;region=AU</td><td>276855</td><td>Toxic. See SDS.</td></tr><tr><td>Anhydrous 2-Propanol (99.5%)</td><td>Sigma Aldrich&lt;br/&gt; www.sigmaaldrich.com/catalog/product/sial/278475?lang=en&amp;amp;region=AU&amp;amp;gclid=&lt;br/&gt; COnlgPaw780CFQZvvAod17EA4Q</td><td>278475</td><td></td></tr><tr><td>Methylammonium iodide</td><td>Dyesol&lt;br/&gt; www.dyesol.com/products/dsc-materials/perovskite-precursors/methylammonium-iodide.html</td><td>MS101000</td><td>Also sold by Sigma Aldrich</td></tr><tr><td>Poly(methyl methacrylate)</td><td>Sigma Aldrich</td><td>445746</td><td></td></tr><tr><td>Anhydrous chlorobenzene (99.8%)</td><td>Sigma Aldrich&lt;br/&gt; www.sigmaaldrich.com/catalog/product/aldrich/445746?lang=en&amp;amp;region=AU</td><td>284513</td><td>Toxic. See SDS.</td></tr><tr><td>&lt;strong&gt;Equipment&lt;/strong&gt;</td><td>&lt;strong&gt;Company&lt;/strong&gt;</td><td>&lt;strong&gt;Model&lt;/strong&gt;</td><td>&lt;strong&gt;Comments/Description&lt;/strong&gt;</td></tr><tr><td>UV-VIS-NIR spectrophotometer</td><td>Perkin-Elmer&amp;nbsp;</td><td>Lambda 900</td><td></td></tr><tr><td>Profilometer</td><td>Veeco</td><td>Dektak 150</td><td></td></tr><tr><td>Vector Network Analyzer</td><td>Keysight&lt;br/&gt; www.keysight.com/en/pdx-x201927-pn-N9918A/fieldfox-handheld-microwave-analyzer-265-ghz?cc=US&amp;amp;lc=eng</td><td>Fieldfox N9918A</td><td></td></tr><tr><td>Tunable wavelength laser</td><td>Opotek&lt;br/&gt; www.opotek.com/product/opolette-355</td><td>Opolette 355</td><td></td></tr><tr><td>Neutral density filters</td><td>Thorlabs&lt;br/&gt; www.thorlabs.hk/newgrouppage9.cfm?objectgroup_id=3193</td><td>NUK01</td><td></td></tr><tr><td>Power meter</td><td>Thorlabs&lt;br/&gt; www.thorlabs.com/thorproduct.cfm?partnumber=PM100D</td><td>PM100D</td><td></td></tr><tr><td>Power sensor</td><td>Thorlabs&lt;br/&gt; www.thorlabs.com/thorproduct.cfm?partnumber=S401C</td><td>S401C</td><td></td></tr><tr><td>Cavity</td><td>Custom built</td><td></td><td>The cavity used in for this experiment was designed and built in-house.</td></tr></tbody></table>

recombination dynamics in thin-film photovoltaic materials via time-resolved microwave conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

&#160;The representative results presented here were obtained from a 250 nm CH3NH3PbI3 thin film sample.
The dynamics of the conductivity <img alt="Equation" src="/files/ftp_upload/55232/55232eq54.jpg" /> can be related to the dynamics of the charge carriers <img alt="Equation" src="/files/ftp_upload/55232/55232eq55.jpg" /> via
<img alt...

Watch this Scientific Journal Video about Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity at JoVE.com

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

recombination-dynamics-thin-film-photovoltaic-materials-via-time

Universidad de Cartagena UDEC

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JoVE Journal

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11.6K Views.  ARC Centre of Excellence in Exciton Science. A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

Video: Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young's modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC61BM system correspond well with literature data. The P3HT:PC61BM blend separates onto PC61BM-rich and P3HT-rich domains. Mechanical properties of PC61BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution.

Organic photovoltaic  (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale  inhomogeneity of OPV materials affects performance ...

Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system.&#160; Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization.&#160; The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted.&#160; The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism.&#160; We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques.&#160;&#160; This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.

The technique of femtosecond four-wave mixing is described, including spectrally-resolved and time-resolved configurations. We illustrate the utility of this technique for the investigation of crucial physical properties in the III-V diluted magnetic semiconductors, afforded by its nonlinearity and high temporal resolution.

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, ...

Using Neutron Spin Echo Resolved Grazing Incidence Scattering to Investigate Organic Solar Cell Materials

The spin echo resolved grazing incidence scattering (SERGIS) technique has been used to probe the length-scales associated with irregularly shaped crystallites. Neutrons are passed through two well defined regions of magnetic field; one before and one after the sample. The two magnetic field regions have opposite polarity and are tuned such that neutrons travelling through both regions, without being perturbed, will undergo the same number of precessions in opposing directions. In this case the neutron precession in the second arm is said to &#34;echo&#34; the first, and the original polarization of the beam is preserved. If the neutron interacts with a sample and scatters elastically the path through the second arm is not the same as the first and the original polarization is not recovered. Depolarization of the neutron beam is a highly sensitive probe at very small angles (&#60;50&#160;&#956;rad) but still allows a high intensity, divergent beam to be used. The decrease in polarization of the beam reflected from the sample as compared to that from the reference sample can be directly related to structure within the sample.
In comparison to scattering observed in neutron reflection measurements the SERGIS signals are often weak and are unlikely to be observed if the in-plane structures within the sample under investigation are dilute, disordered, small in size and polydisperse or the neutron scattering contrast is low. Therefore, good results will most likely be obtained using the SERGIS technique if the sample being measured consist of thin films on a flat substrate and contain scattering features that contains a high density of moderately sized features (30 nm to 5 &#181;m) which scatter neutrons strongly or the features are arranged on a lattice. An advantage of the SERGIS technique is that it can probe structures in the plane of the sample.

Progress has been made in utilizing spin echo resolved grazing incidence scattering (SERGIS) as a neutron scattering technique to probe the length-scales in irregular samples. Crystallites of [6,6]-phenyl-C61-butyric acid methyl ester have been probed using the SERGIS technique and the results confirmed by optical and atomic force microscopy.

The spin echo resolved grazing incidence scattering (SERGIS) technique has been used to probe the length-scales associated with irregularly shaped ...

Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). In this article we will describe the methods for constructing and characterizing macroscopic disordered photonic structures using microwaves. The microwave regime offers the most convenient experimental sample size to build and test PBG media. Easily manipulated dielectric lattice components extend flexibility in building various 2D structures on top of pre-printed plastic templates. Once built, the structures could be quickly modified with point and line defects to make freeform waveguides and filters. Testing is done using a widely available Vector Network Analyzer and pairs of microwave horn antennas. Due to the scale invariance property of electromagnetic fields, the results we obtained in the microwave region can be directly applied to infrared and optical regions. Our approach is simple but delivers exciting new insight into the nature of light and disordered matter interaction.

Our representative results include the first experimental demonstration of the existence of a complete and isotropic PBG in a two-dimensional (2D) hyperuniform disordered dielectric structure. Additionally we demonstrate experimentally the ability of this novel photonic structure to guide electromagnetic waves (EM) through freeform waveguides of arbitrary shape.

Disordered structures offer new mechanisms for forming photonic bandgaps and unprecedented freedom in functional-defect designs. To circumvent the computational challenges of disordered systems, we construct modular macroscopic samples of the new class of PBG materials and use microwaves to characterize their scale-invariant photonic properties, in an easy and inexpensive manner.

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications of semiconductor materials. However, it is still under debate how the defect structure determines the band structure, and therefore, the recombination behavior of electron-hole pairs responsible for the optical and electrical properties of the extended defects. The present paper is a survey of procedures for the spatially resolved investigation of structural and of physical properties of extended defects in semiconductor materials with a scanning electron microscope (SEM). Representative examples are given for crystalline silicon. The luminescence behavior of extended defects can be investigated by cathodoluminescence (CL) measurements. They are particularly valuable because spectrally and spatially resolved information can be obtained simultaneously. For silicon, with an indirect electronic band structure, CL measurements should be carried out at low temperatures down to 5 K due to the low fraction of radiative recombination processes in comparison to non-radiative transitions at room temperature. For the study of the electrical properties of extended defects, the electron beam induced current (EBIC) technique can be applied. The EBIC image reflects the local distribution of defects due to the increased charge-carrier recombination in their vicinity. The procedure for EBIC investigations is described for measurements at room temperature and at low temperatures. Internal strain fields arising from extended defects can be determined quantitatively by cross-correlation electron backscatter diffraction (ccEBSD). This method is challenging because of the necessary preparation of the sample surface and because of the quality of the diffraction patterns which are recorded during the mapping of the sample. The spatial resolution of the three experimental techniques is compared.

The optical, electrical, and structural properties of dislocations and of grain boundaries in semiconductor materials can be determined by experiments performed in a scanning electron microscope. Electron microscopy has been used to investigate cathodoluminescence, electron beam induced current, and diffraction of backscattered electrons.

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH3NH3PbI3 perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

We present an extensive study on the effects of different fabrication methods for organic/inorganic perovskite thin films by comparing crystal structures, density of states, energy levels, and ultimately the solar cell performance.

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to ...

Chemistry

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Here, we present the protocols of differential-detection analyses of time-resolved infrared vibrational spectroscopy and electron diffraction which enable observations of the deformations of local structures around photoexcited molecules in a columnar liquid crystal, giving an atomic perspective on the relationship between the structure and the dynamics of this photoactive material.

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational ...

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method

This work presents a protocol employing the microwave photoconductivity decay (&#956;-PCD) for measurement of the carrier lifetime in semiconductor materials, especially SiC. In principle, excess carriers in the semiconductor generated via excitation recombine with time and, subsequently, return to the equilibrium state. The time constant of this recombination is known as the carrier lifetime, an important parameter in semiconductor materials and devices that requires a noncontact and nondestructive measurement ideally achieved by the &#956;-PCD. During irradiation of a sample, a part of the microwave is reflected by the semiconductor sample. Microwave reflectance depends on the sample conductivity, which is attributed to the carriers. Therefore, the time decay of excess carriers can be observed through detection of the reflected microwave intensity, whose decay curve can be analyzed for estimation of the carrier lifetime. Results confirm the suitability of the &#956;-PCD protocol in measuring the carrier lifetime in semiconductor materials and devices.

As one of the important physical parameters in semiconductors, carrier lifetime is measured herein via a protocol employing the microwave photoconductivity decay method.

This work presents a protocol employing the microwave photoconductivity decay (&#956;-PCD) for measurement of the carrier lifetime in semiconductor ...