Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs_1-xFA_xPbI₃ and FAPbI₃ reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

M. V. Kovalenko et al. Science 2017, 358, 745-750

L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

I. Lignos et al. ACS Nano 2018, DOI: 10.1021/acsnano.8b01122

F. Krieg et al. ACS Energy Letters 2018, 3, 641–646.

Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted

Perovskite solar cells have electrified the solar cell research community with astonishing performance and surprising material properties. Very efficient (>20 %) devices with perovskite layers of low defect density can be prepared by cheap and simple solution based processes at moderate temperatures (<150°C). For commercializing this technology, a stable and reliable operation has to be ensured. In perovskite solar cells, however, the output power is strongly influenced by the history of the device in terms of bias voltage (causing hysteresis) or illumination (known as light soaking effect). The underlying process is assumed to be the slow migration of ionic charges within the perovskite layer.

In my presentation I will demonstrate how scanning probe microscopy can help understanding these processes. Using Kelvin probe force microscopy, we were able to follow the vertical charge distribution in the active perovskite layer of an operating device. We observed the formation of a localized interfacial charge at the anode interface, which screened most of the electric field in the cell. The formation of this charge happened within 10 ms after applying a forward voltage to the device. After switching off the forward voltage, however, these interfacial charges were stable for over 500 ms and created a reverse electric field in the cell. This reverse electric field directly explains higher photocurrents during reverse bias scans by electric field-assisted charge carrier extraction. We thereby show that instead of the slow migration of mobile ions, the formation and the release of interfacial charges is the dominating factor for current-voltage hysteresis.

Recent research and progress in organic photovoltaic (OPV) repeatedly insist on the importance of the molecular organization of the compounds forming the active bulk-heterojunction (BHJ) blends. The morphology of the blend has been to tremendously affect both the charge transfer at the donor-acceptor interface and the carrier transport to the electrodes. And still, for each material combination, much remains to be understood to fully assess its specific and ultimate morphology. For this purpose, high resolution characterization methods are of primary interest to locally depict tthe photovoltaic process. Conductive Atomic Force Microscopy (C-AFM) and Kelvin Probe Force Microscopy (KPFM) have already proven to be of significant help to yield nanoscale two-dimensional mapping of electrical properties. C-AFM and related PeakForce TUNA emerged as powerful technique to electrically evidence phase separation in blends. An additional key feature lies in local I-V curve providing useful information about the charge transport mechanisms within the materials forming the blends. Quantitative measurements leading to local determination of hole mobility have already been reported. It appears that upon illumination the technique has shown to be sensitive to photocurrent. With photoconductive-AFM (pc-AFM), a dedicated external calibrated module has been recently introduced allowing full quantitative mapping of photovoltaic mechanisms.

In this lecture, we will carefully analyze, as a model sample, BHJ made of poly(3-hexylthiophene) (P3HT) and fullerene derivative (PCBM). While photocurrent is determined at 0 V DC bias, additional parameters including the open-voltage, the fill factor and the resistances can be obtained spanning the DC bias between the probe and the sample back electrode.(KPFM is also used in this work to delineate phase separation and potential variations at interfaces. Upon illumination, photovoltage can also be evidenced. With the additional external calibrated illumination module, mapping of photovoltage in BHJ blends can be obtained, opening the doors of local characterization of charge transfer at donor-acceptor interfaces, where crucial processes are occurring in photovoltaic devices.

In fine, we will also address the photovoltaic properties of  other systems such hybrid TiO₂ nanopillars : conjugated polymers.

The development of third generation photovoltaics relies on the use of materials displaying a heterogeneous morphology at the mesoscopic or nanoscopic scale. A universal problem consists in identifying the sources of carrier losses (due to chemical, structural and interfacial defects) by recombination of photo-generated charge carriers. In nano-phase segregated organic bulk heterojunction (BHJs) thin films, a number of questions remain open concerning the impact of the donor-acceptor phases and interfaces morphology on the photo-carrier dynamics. It is also crucial to assess the impact of grain boundaries, chemical impurities and other local defects on the photo-carrier recombination in polycrystalline films of hybrid organic-inorganic perovskites.

In this communication, we will present the state of the art and ongoing developments in local measurements of the photo-carrier dynamics in organic and hybrid solar cell materials by time-resolved Kelvin probe force microscopy. After introducing the basic concepts of time-resolved surface photo-voltage (trSPV) imaging by KPFM under frequency-modulated illumination^[1-3] ( FMI-KPFM), we will discuss several key issues and technical hints. What is the achievable lateral and temporal resolution? How shall we take into account photo-induced capacitive changes^[4] in the analysis of FMI-KPFM data? We will also introduce a new experimental methodology combining FMI-KPFM with surface potential transients imaging, and we will explain what its benefits are for a proper data analysis and for simultaneous investigations of “fast” and “slow” SPV dynamics.

All items will be discussed in the light of experimental results obtained on BHJs, hybrid perovskites thin films and single crystals. In the last case, we will moreover show that the surface photovoltage and crystal photostriction can be simultaneously investigated by implementing a specific experimental protocol. Last, we will explain how the comparison with model photovoltaic type-II interfaces based on 2D transition metal dichalcogenides heterojunctions^[5] shall help us in understanding the more complex case of BHJs.  

References

[1] M. Takihara, T. Takahashi, T. Ujihara, Appl. Phys. Lett., 2008, 93, 021902 (3pp).

[2] G. Shao, M. Glaz, M. Fei, H. Ju, D. Ginger, ACS Nano, 2014, 8, 10799-10807.

[3] P. A. Fernández Garrillo, Ł. Borowik, F. Caffy, R. Demadrille, B. Grévin, ACS Appl. Mater. Interfaces, 2016, 8, 31460–31468.

[4] Z. Schumacher, Y. Miyahara, A. Spielhofer, P. Grutter, Phys. Rev. Appl. 2016, 5, 044018 (6pp).

[5] Y. Almadori, D. Moerman, J. Llacer Martinez, Ph. Leclère, B. Grévin, accepted for publication in BJNANO

[6] Y. Almadori, N. Bendiab, B. Grévin, ACS Appl. Mater. Interfaces, 2018, 10, 1363-1373.

To understand the characteristics of fuel cell electrodes, a compound of platinum nanoparticle-covered mesoporous carbon und ionomer, they have to be examined at a nanometer scale. Their structure, porosity, and the properties of the inner surfaces at this scale determine their behavior. In particular, the lateral variation of surface energy and ionomer coverage has a major influence on water balance and transport resistance. To visualize this structure in two dimensional cuts SEM is being used. Combined with material-sensitive AFM measurements physical surface properties can be derived and modeled. In this presentation an approach to model the inner structure of a fuel cell electrode and the properties of inner surfaces is presented. The electrode is examined with material-sensitive AFM. Derived from deformation, adhesion and DMT-modulus mappings, an automated separation into the different phases is put into effect. The so constructed segmented image is rendered into 3D by its height information.

By ion milling further cuts of the same material are being done in layers of few micrometers. The AFM analysis of these series can be used to generate a larger 3D model of the electrode. A method to combine the series of layers to a 3D-model is suggested.

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Herein, we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity dependent current-voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space-charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. In addition, we elucidate the effects of oxygen and water surface adsorption on band alignment, interfacial charge transfer, and charge carrier transport by using complementary Kelvin probe measurements and photoconductive atomic force microscopy on this material. By observing variations in surface potential, we show that adsorbed oxygen acts as an electron trap state at the surface of bismuth vanadate, whereas adsorbed water results in formation of a dipole layer without inducing interfacial charge transfer. The apparent change of trap state density under dry or humid nitrogen, as well as under oxygen-rich atmosphere, proves that surface adsorbates influence charge carrier transport properties in the material. The finding that oxygen introduces electronically active states on the surface of bismuth vanadate may have important implications for understanding local functional characteristics of water splitting photoanodes and their effects on the macroscopic performance, and for devising strategies to passivate interfacial trap states, and elucidating important couplings between energetics and charge transport in reaction environments. 

A renewed interest in the role of grain boundaries in chalcopyrite-based thin-films solar cells is triggered by the highly efficient energy conversion obtained through the application of alkali post deposition treatments (PDT) [1,2]. Recent compositional studies indicate the presence of alkali metals at the Cu(In,Ga)Se₂ (CIGSe) absorber surface and also in the bulk, localized especially at the grain boundaries (GB) [3-5]. Assessing the impact of GB properties on the efficiency requires a throughout insight into the nanoscale structural, chemical, and electronic properties of CIGSe. In this contribution, Kelvin probe force microscopy (KPFM) is used to study the local electronic properties at GBs by spatially resolved imaging of the surface potential. From a statistical analysis we obtain the alkali-dependent (K, Rb, and Cs) potential variation across the GBs and compare the results with those obtained for samples subjected to an 8 minutes chemical bath deposition (CBD) of Zn(O,S). Different types of GBs are mainly found: majority neutral (the potential difference between GB and grain interior at the sides is negligible) and GBs with positive/upward potential increase. In average, however, a lower barrier potential and more homogenous potential variation is found for RbF. The early stage deposition of solution-grown Zn(O,S) buffer slightly reduce (in average) the potential difference at GBs in both cases.

[1] P. Jackson et al., Phys. Status Solidi RRL 9, 28 (2015); P. Jackson et al., Phys. Status Solidi RRL 10, 583 (2016).

[2] Press Release: Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency of 22.9%; http://www.solar-frontier.com/eng/news/2017/1220_press.html.

[3] P.-P. Choi, O. Cojocaru-Miredin, R. Wuerz, and D. Raabe, J. Appl. Phys. 110, 124513 (2011).

[4] D. Abou-Ras, B. Schaffer, M. Schaffer, S. S. Schmidt, R. Caballero, and T. Unold, Phys. Rev. Lett. 108, 075502 (2012).

[5] O. Cojocaru-Miredin, P.-P. Choi, D. Abou-Ras, S. Schmidt, R. Caballero, and D. Raabe, IEEE J. Photovoltaics 1, 207 (2011).

We applied peak force based conductive scanning force microscopy (cSFM) to investigate local conductance differences in TiO₂ anatase thin films. We found that the current perpendicular to the interface increased by two orders of magnitude after a UV-ozone (UVO) treatment. This increase in current is attributed to a reduction of oxygen vacancies at the surface after UVO-treatment. Cleaning, i.e. removal of hydrocarbons plays only a minor role.

Thin films of titanium dioxide (TiO₂) are applied in electronics, e.g. thin film transistors, anode materials for Lithium ion batteries, photoanodes for water oxidation and as hole blocking layer in perovskite solar cells (PSC). TiO₂ anatase films can have a lower conductance compared to electron blocking layers like P3HT, PCPDTBT and spiro-OMEOTAD [1]. Thus the conductivity of TiO₂ films is crucial for the electron charge transport and it can limit the solar cell power conversion efficiency (PCE). Therefore, an improved conductance perpendicular trough this layer is highly desirable [2]. Various cleaning procedures were reported for anatase TiO₂ thin films which result in an increase in PCE [3-7]. However, the cleaning mechanism and the local conductivity were not investigated in detail on a nanometer scale. We could show that the PCE of planar PSC can be increased by 2% to a maximum of 15.4% by a controlled UV-ozone (UVO) treatment. Finally, peak force based cSFM is stable for over 3 million singe force distance curves. This stability allows to compare cSFM current images quantitatively for differently treated samples.

References:

[1] T. Leijtens, et al., Adv. Mater. 2013, 25, 3227–3233.

[2] C. Liu, et al., ACS Appl. Mater. Interfaces 2015, 7, 1153–1159.

[3] L. Cojocaru, et al., Chem. Lett. 2015, 44, 674–676.

[4] F. Zhang, et al., Chem. Mater. 2016, 28, 802–812.

[5] X. Ma, et al., ChemPhysChem 2017, 1–9.

[6] W. Ke, et al., Nat. Commun. 2015, 6, 6700.

[7] I. S. Kim, et al., ACS Appl. Mater. Interfaces 2016, 8, 24310–24314.

Besides morphological characterization, atomic-force microscopy (AFM) based techniques can also successfully be employed to study electrical and optoelectronic properties on the nanometer scale via conductive atomic-force microscopy (C-AFM) and Kelvin Probe Force Microscopy [1]. This will be demonstrated for upright standing ZnO nanorods [2,3] radial junction Si solar cells [4]. With respect to photovoltaic applications, the operation of these techniques under simultaneous illumination with white or monochromatic light - which are called photoconductive AFM (PC-AFM) and photo-assisted KPFM (PA-KPFM) is demonstrated [3,4]. For crystalline needles composed of small organic semiconductor molecules grown on graphene, the light-induced  charge spreading is measured by electrostatic force microscopy.

Work has been performed in collaboration with A. Andreev, I. Beinik, A. Nevosad, A. Matković, M. Mirkowska, M. Kratzer, K. Gradwohl, A. Matković, (Leoben), Y. Kozyrev, S. Kondratenko  (Kiev), M. Müller, A. Hývl, A. Vetushka, M. Ledinský, A. Fejfar (Prague), and B. Vasić, R. Gajić (Belgrade).

[1] C. Teichert, I. Beinik, in “Scanning Probe Microscopy in Nanoscience and Nanotechnology”, Vol. 2, Edited by B. Bhushan, (ISBN 978-3-642-10496-1) (Springer-Verlag, Berlin, 2011), pp. 691-721.

[2] I. Beinik, et al., J. Appl. Phys. 110 (2011) 052005.

[3] I. Beinik, et al., Beilstein J. Nanotechnol. 4 (2013) 208.

[4] M. Müller, et al., Jap. J. Appl. Phys. 54 (2015) 08KA08.

Electrochemical strain microscopy (ESM) and its derivative methods are an essential set of tools to study ionic solid materials and to help understand the difficulties in the design of new materials for next generation battery systems and their aging behavior.

A variation of the standard ESM proposed by Balke et al. was developped. We studied the diffusion-migration behavior of Li ions in nanostructured silicon anodes. In standard ESM, the Li-ions in the volume underneath the AFM tip are excited by an alternating electrical field and cause a movement of the sample surface. Contrary to the standard ESM our approach applies a longer voltage step of several milliseconds with an AC voltage overlaid. This does not only lead to a vibration of the ions but also to a change in ion concentration in the vicinity of the AFM tip. The correlated volume expansion and the amplitude in sample height increase are extracted for every image point using Matlab. The Silicon anodes were cycled against Lithium. 

Electrodes before and after cycling were analysed. In the ESM images, phases with different diffusion-migration coefficients appear. High initial diffusion-migration coefficiants and Li-concentration are assumed to depend on the crystal orientation. The fresh electrode has areas with high Li-concentration and well separated areas with no signal, whereas in the aged sample the Li-concentration dropped significantly. In conclusion, the observed capacity loss can be explained by the loss of Lithium ions.

The properties of NiO, such as charge transport or optoelectronic characteristics, can be modified by functionalization with organic molecules. These kinds of organic/inorganic surfaces are of great interest, in particular, for the design of hybrid devices like dye sensitized solar cells [1]. However, a key parameter in the design of optimized interfaces is not only the choice of the compounds but also the properties of adsorption. Thus, fundamental studies of such hybrid systems at the nanoscale are desirable. So far, characterization of adsorbates at ambient temperature through spectroscopy techniques, such as x-ray photoelectron spectroscopy, has been limited to large agglomerates or self- assembled molecules. Recently, first studies of the adsorption properties of single molecules on NiO measured by force microscopy at low temperatures have been published [2]. This limit can be stretched to the level of individual adsorbates measured by means of non-contact atomic force microscopy at room temperature.

We investigated the deposition of a 2,2´-bipyridine based molecule, functionalized with carboxylic acid anchoring domains on a NiO(001) single crystal surface [3]. Depending on the coverage, single molecules, groups of adsorbates with random or recognizable shapes, or islands of closely packed molecules could be identified. Single molecules and self assemblies, as visible in the image on the right side, are resolved with submolecular resolution showing that they are lying flat on the surface with the 2,2´-bipyridine in a trans-conformation. Only in the close-packed form was a measurable charge transfer from the NiO to the molecular layer of 0.3 electrons per molecule observed independent on the molecular orientation of the islands.

[1] C. Wood et al., Phys. Chem. Chem. Phys. 18 (2016) 10727.
[2] A. Schwarz et al., J. Phys. Chem. C 117 (2013) 1105.
[3] S. Freund et al., submitted to ACS Nano (2017).

Energy materials provide a large multitude interfaces with inhomogeneously distributed reaction rates often directly dictating their functional properties.¹ Local characterization for deriving structure reactivity relationships can be achieved by scanning electrochemical microscopy (SECM) but poses a set of challenges:

Composite materials make preparation of smooth uniform surfaces impossible. SECM studies have to deal with rough and even porous electrodes as in the case of dye-sensitized solar cells² and gas-diffusion electrodes for fuel cells and lithium-oxygen batteries.³

Shielding reactive surfaces from a typical laboratory environment may be mandatory.¹ Selective passivation has been studied negative electrodes in lithium-ion batteries (pyrolytic graphite,⁴ graphite composite⁵ and lithium).⁶

SECM can be used to distinguish between different parallel reaction pathways as has been demonstrated for gas diffusion electrodes.

Entirely new opportunities are enabled by liquid-liquid interfaces which can be chemically polarized to support light driven reaction. Photochemically active materials can be arranged and regenerated at these soft interfaces.⁷

Important contributions of my PhD students and Postdocs H. Bülter, P. Schwager, E. dos Santos Sardinha, S. Scarabina, I. Schmidt, I. Plettenberg, S. Rastgar and cooperation partners F. Peters, D. Fensker, J. Schwenzel (IFAM), M. Stenard, M. Wilkening (TU Graz) are gratefully acknowledged. Funding: DFG, State of Lower Saxony, Humboldt Foundation and the Conselho Nacional Brazil.

1     Bülter, Schwager, Fenske, Wittstock, Electrochim. Acta, 2016, 199, 366.

2     Shen, Nonomura, Schlettwein, Zhao, Wittstock, Chem. Eur. J., 2006, 12, 5832; Tefashe, Nonomura, Vlachopoulos, Hagfeldt, Wittstock, J. Phys. Chem. C, 2012, 116, 4316; Ellis, Schmidt, Hagfeldt, Wittstock, Boschloo; J. Phys. Chem. C 2015, 119, 21775; Schmidt, Plettenberg, Kimmich, Ellis, Witt, Dosche, Wittstock, Electrochim. Acta, 2016, 222, 735.

3     Schwager, Dongmo, Fenske, Wittstock, Phys. Chem. Chem. Phys., 2016, 18, 10774; Schulte, Liu, Plettenberg, Kuhri, Lüke, Lehnert, Wittstock; J. Electrochemcial Soc. 2017, 164, F873.

4     Bülter, Peters, Wittstock, Energy Technol. 2016, 4, 1486.

5     Bülter, Peters, Schwenzel Wittstock, Angew. Chem., Int. Ed., 2014, 53, 10531–10535; Schwager, Bülter, Plettenberg, Wittstock; Energy Technol. 2016, 4, 1472.

6     Bülter, Peters, Schwenzel, Wittstock; J. Electrochem. Soc. 2015, 162, A7024.

7     Rastgar, Pilarski Wittstock, Chem. Commun., 2016, 52, 11382–11385; Rastgar, Wittstock, J. Phys. Chem. C 2017, 121, 25961.

Solution processed graphene oxide (GO), has gained increased interest as optoelectronic device platform due  to its ability to act as amphiphilic macromolecule, favoring  the formation  of  complexes with conjugated polymers, that can be used in  improved optoelectronic devices. Recently, charge transfer has been demonstrated in complexes of poly(3-hexylthiophene nanoparticles (P3HT _NPs)  and GO sheets (P3HT_NPs – GO hybrids) prepared by self-assembly in-situ reprecipitation [1]. In this work, Kelvin Probe Force Microscopy (KPFM) is used to investigate the P3HT_NPs–GO nanohybrids formation. By mapping the local surface potential (SP) in darkness, the polymer chain aggregrate structure is resolved. In addition, surface photovoltage (SPV) measurements on individual nanoparticles under super-band-gap illumination shed light on the photoinduced charge generation and charge recombination mechanisms at the different interfaces. This is an important step to understand the rather overall functionality thin film layers composed of individual nanoscale objects and for further optimizing the optoelectrical performance of thin film devices.

[1] E. Istif, J. Hernández-Ferrer, E. P. Urriolabeitia, A. Stergiou, N. Tagmatarchis, G. Fratta, M. J. Large, A. B. Dalton, A. M. Benito, and W. K. Maser, “Conjugated Polymer Nanoparticle–Graphene Oxide Charge-Transfer Complexes”, Adv. Funct. Mater., 1707548, 2018.

The III-V nanowire (NW) technology platform has reached a level of advancement that allows atomic scale control of crystal structure and surface morphology as well as flexible device integration. In particular, controlled axial stacking of Wurtzite(Wz) and Zincblende(Zb) crystal phases is uniquely possible in the NWs. We explore how this can be used to affect electronic, optical and surface chemistry with atomic scale precision opening up for 1D, 2D and 3D structures with designed local properties.

We previously demonstrated atomically resolved Scanning Tunneling Microscopy/Spectroscopy (STM/S) on a wide variety of these III-V NWs and on operational NW devices[1-4]. We now study atomic scale crystal phase changes, their impact on local electronic properties and demonstrating atomic resolution STM during device operation[5-7]. We explore the surface alloying of Sb into GaAs NWs with controlled axial stacking of Wz and Zb crystal phases[5] demonstrating a simple processing-free route to compositional control at the monolayer level. Using 5K STM/S we measure local density of states of Zb crystal segments in Wz InAs NWs down to the smallest possible atomic scale crystal change[6]. The general Zb electronic structure is preserved locally in even the smallest possible segments and signatures of confined states are found.  We demonstrate a novel device platform allowing STM/S with atomic scale resolution across a III-V NW device simultaneously with full electrical operation and high temperature processing in reactive gases[7].

Using 5-15 femtosecond laser pulses combined with PhotoEmission Electron Microscopy (PEEM) we explore local dynamic response of carriers in the Wz and Zb crystal phases down to a few femtoseconds temporally and a few tens of nanometer spatially. We demonstrate that spatial control of multiphoton electron excitations is possible in semiconductor NWs by changing the crystal phase, orientation, and light polarization[8]. The control and understanding of multiphoton excitations could be used in the design of optoelectronic components that use hot electrons or photoelectrons for functionality.

[1] E. Hilner etal., Nano Lett., 8 (2008) 3978; M. Hjort et al., ACS Nano 6 (2012) 9679

[2] M. Hjort etal., Nano Lett., 13 (2013) 4492; M. Hjort et al., ACS Nano, 8 (2014) 12346

[3] J.L. Webb, etal Nano Lett. 15 (2015) 4865

[4] O. Persson etal., Nano Lett. 15 (2015) 3684

[5] M. Hjort etal Nano Lett., 17 (2017) 3634

[6] J.V. Knutsson etal ACS Nano, 11 (2017) 10519

[7] J.L. Webb etal, Sci.  Rep. 7 (2017) 12790

[8] E. Mårsell etal, Nano Lett. 18 (2018) 907

Our constantly increasing society’s need for energy has triggered a pressing need for the development of new materials for renewable sources. Concerning materials for energy harvesting, the most promising approaches for high-performance and low-cost photovoltaics rely in inhomogeneous compounds, such as perovskites and polycrystalline thin films (e.g. CIGS and CdTe). Thus, resolving their electrical and optical behavior at the nanoscale is imperative to advance their understanding. In this talk, I will share our scientific findings to image and quantify the local voltage response of nano- and mesoscale inhomogeneities in perovskites [1,2], CIGS [3], and CdTe through a variant of KPFM and NSOM [4-6]. By submitting the samples to illumination and humidity treatments under controlled conditions, we map the dynamic physical behavior of MAPI and triple-cation perovskites.

References:

[1] J. M. Howard et al. J. Phys Chem Letters, in press (2018)

[2] J. L. Garrett et al. Nano Letters 17, 2554 (2017).

[3] E. M. Tennyson et al. ACS Energy Letters 1, 899 (2016).

[4] E. M. Tennyson et al. ACS Energy Letters, 2, 2761 (2017). Invited Review

[5] E. M. Tennyson et al. ACS Energy Letters 2, 1825 (2017). Invited Perspective

[6] E. M. Tennyson et al. Advanced Energy Materials 5, 1501142 (2015).