Solution-based emerging hybrid solar cells

Resume : Photovoltaic devices based on hybrid organic-inorganic perovskite absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 22%. In this talk we discuss the role of the interface in optimizing device performance as measured by both power conversion efficiency and stability. We present an examination of different perovskite active layers and interfacial electronic structure of these remarkable materials. Interface formation of the active layer with different carrier transport materials has direct implications for performance of the resulting devices. We present interface studies, which permit identification of charge transfer mechanisms across the interface with chemical specificity and insight into the requirements for realizing high performance devices. We will also discuss chances in the interface and change transport properties across different halide/hybrid perovskite absorber layers as well as different process approaches. Our findings from surface science approaches are combined with time resolved spectroscopy, structural studies and device level studies to validate impacts and demonstrate their technological relevance for these materials in the PV application.

Resume : Flexibility, compliance and weight will turn out to be key metrics for future electronic appliances and their power supplies. Imperceptible electronic wraps integrate nanometer thin film active components on sub-2-μm polymer foils and create devices unmatched in mechanical flexibility, stretchability and weight. Emerging applications like solar powered aviation or wearable electronics require photovoltaic technologies that are highly efficient, light-weight, low-cost, and stable during operation. Organolead halide perovskites constitute a highly promising class of materials, but suffer limited stability under ambient conditions without heavy and costly encapsulation. In this talk we introduce methods, materials and design strategies for ultrathin (3 µm), highly flexible perovskite solar cells with stabilized 12% efficiency and a record power-per-weight as high as 23 W/g. To facilitate air stable operation, we introduce a chromium oxide-chromium interlayer that effectively protects the metal top contacts from reactions with the perovskite. We show prolonged stable operation in the maximum power point in ambient air of non-encapsulated planar perovskite solar cells with gold, and low-cost copper and aluminium electrodes. The use of a transparent polymer electrode treated with dimethylsulphoxide (DMSO) as bottom layer allows the deposition - from solution at low temperature - of pinhole-free perovskite films at high yield on arbitrary substrates, including thin plastic foils. These ultra-lightweight solar cells are successfully used to power aviation models. Potential future applications include unmanned aerial vehicles - from airplanes to quad-copters and weather balloons - for environmental and industrial monitoring, rescue and emergency response, and tactical security applications. In a hybrid architecture laminated atop pre-strained elastomers, such solar cells are highly stretchable and conformable. We introduce surface structuring as a light-trapping microstructure for improved efficiency. Prolonged operational lifetime and stability towards water ingress by superhydrophobic coatings will be discussed. We acknowledge funding from the FWF Wittgenstein award and the ERC advanced investigators grant “Soft Map”. M. Kaltenbrunner et al., Nature Communications 3, 770 (2012) M. Kaltenbrunner et al., Nature 499, 458 (2013) M. Kaltenbrunner et al., Nature Materials 14, 1032-1039 (2015)

Resume : The power conversion efficiency (PCE) of perovskite solar cells based on organometal halides such as CH3NH3PbI3 and CH3NH3PbI3−xClx has skyrocketed from 3.8 to more than 20% in the past several years. Recent studies have shown that some factors underlying such efficient device performance are due to excellent light harvesting property, long charge carrier diffusion length, high charge carrier mobility, ultrafast charge generation and slow charge carrier recombination, and small exciton binding energy. However, most of perovskite solar cells exhibit poor reproducibility in the device performance with J–V hysteresis even though they are fabricated by the same fabrication procedures. This is probably because crystalline growth of perovskite materials cannot be well controlled for efficient and reproducible photovoltaic performances. Consequently, it has been difficult to discuss the relationship between the photovoltaic performance and device structure quantitatively. In this study, we fabricated planar heterojunction solar cells with a layer structure of FTO/d-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au in which dense CH3NH3PbI3 layers were prepared by fast deposition–crystallization (FDC) method. The average grain size of CH3NH3PbI3 was varied from 100 to 500 nm by changing the concentration of stock solutions. The perovskite layer obtained consists of monograins in the direction normal to the substrate. As a result, the CH3NH3PbI3 perovskite solar cells exhibiting 19% achieved for the largest grain size of ≈500 nm by optimizing the condition of device fabrication. Using these devices, we investigated the relationship between the open-circuit voltage (Voc) and the grain size of perovskites. Based on analysis of the intensity-dependent Voc by considering both direct and SRH recombinations, we found that the trap density (Nt) decreases with increasing grain size of perovskites, leading to increasing Voc. This enhancement is ascribed to the reduced trap-assisted SRH recombination. On the basis of the experimental data, we further discuss the potential for improvement in the device performance of perovskite solar cells if Nt of perovskite films was suppressed to the order of <10^13 cm−3 as reported for mm-scale perovskite crystals.

Resume : Formamidinium (FA) lead iodide/bromide perovskite thin films were synthetized in a single step from a solution containing a mixture of FAI, PbI2 and FABr, PbBr2, respectively. Then by mixing halide components in the desired proportions FAPbI3Br3-x perovskite thin films were also produced. Perovskite thin films were deposited from then mentioned solutions onto indium-tin oxide substrates by spin-coating method. X-ray diffraction analyses indicated the formation of a cubic phase Pm-3m in all composition range (0≤x≤ 3). Mixed formamidinium lead iodide/bromide perovskites showed a high absorbance in the UV–vis range. The band gap energy of mixed thin films was estimated from absorbance spectral measurements and it can be tuned by choosing the desired x ratio for the halides. It was found that the onset of the absorption edge for FAPbBr3-xIx (x ≤ 1) thin films is ranging between 1.47 to 2.2 eV. Photoluminescence emission energies for mixed halide perovskites presented intermediate values from 810.4 nm for FAPbI3 to 547.3 nm for FAPbBr3.

Resume : During my lecture I will present our latest results on the characterization of different type of devices that include MAPI as light harvesting material using advanced photo-induced time resolved techniques1-4. Using PICE (Photo-induced charge extraction), PIT-PV (Photo-induced Transient PhotoVoltage) and other techniques, we have been able to distinguish between capacitive electronic charge, and a larger amount of charge due to the intrinsic properties of the perovskite material. Moreover, the results allow us to compare different materials, used as hole transport materials (HTM), and the relationship between their HOMO and LUMO energy levels, the solar cell efficiency and the charge losses due to interfacial charge recombination processes occurring at the device under illumination. These techniques and the measurements carried out are key to understand the device function and improve further the efficiency and stability on perovskite MAPI based solar cells.

Resume : In the past few years halide perovskite based solar cells have shown an unprecedentedly rapid development in terms of photoconversion efficiency (PCE). While the high PCEs and potentially low production costs are important assets, a number of challenges and open questions still need to be addressed. One of these is the anomalous dependence of the current-voltage response on the bias scan direction, rate and range [1]. The origin of the dynamic hysteresis is still under debate and has been attributed to different phenomena, such as ferroelectric effects, trapping and de-trapping of the charges and ion migration [2]. We introduce a dynamic electrical model in order to investigate the hysteresis effects in the I-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of the measured I-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. Furthermore, the dynamic hysteresis is measured and analyzed with respect to changing the bias scan rate, the obtained results matching also other experimental reported data [3]. Additional analysis is performed by taking into account single and multiple relaxation times. [1] Bo Chen, Mengjin Yang, Shashank Priya and Kai Zhu, J. Phys. Chem. Lett. 7, 905 (2016) ; [2] S. van Reenen, M. Kemerink, and H. J. Snaith, J. Phys. Chem. Lett. 6, 3808 (2015) ; [3] W. Tress, N. Marinova, T. Moehl, S. M. Zakeeruddin, M. K. Nazeeruddin and M. Gratzel, Energy Environ. Sci. 8, 995 (2015).

Resume : Organometal halide based perovskite are emerging materials for wide range of optoelectronic applications which includes high efficiency solar cells, color pure LEDs and optical pumped lasers. Despite this a lot needs to be understood regarding the underlying charge transport which required fabrication of field effect transistors. Here, we report the demonstration of a high performance field effect transistor fabricated from iodide perovskite material at room temperature. The devices exhibit clean saturation behavior with electron field effect mobility > 3 cm2/Vs and current modulation in the range of 10^6 – 10^7 which are till date the best performance achieved with these class of materials. This high performance is attributed to a combination of novel film fabrication technique and device engineering strategies. Detailed understanding of the observed band-like transport phenomenon is developed by tuning the different sources of dynamic and static disorder prevalent in the system. These finding are expected to pave way for developing next generation electronic application from perovskite materials.

Resume : < p align="justify">Bismuth(III) halide perovskites are currently investigated as alternative solution-processable absorber materials for photovoltaic applications to circumvent the toxicity and stability issue of lead halide perovskites. In addition to the low toxicity and ambient stability, bismuth(III) halide perovskites are due to their optical bandgaps in the range of 2.0-2.4 eV promising absorber materials in particular for multi-junction solar cells.< sup>[1,2]< /sup> < p> < p align="justify">The focus of this contribution is on alternative hybrid organic-inorganic bismuth(III) halide perovskites with organic methylammonium (CH< sub>3< /sub>NH< sub>3< /sub>< sup>+< /sup>) as A-site cation, inorganic bismuth (Bi< sup>3+< /sup>) as B-site cation and halide counter ions (I< sup>-< /sup>, Cl< sup>-< /sup>) incorporated on the X-site of the perovskite structure. The perovskite absorber materials (CH< sub>3< /sub>NH< sub>3< /sub>)< sub>3< /sub>Bi< sub>2< /sub>I< sub>9< /sub> and (CH< sub>3< /sub>NH< sub>3< /sub>)< sub>3< /sub>Bi< sub>2< /sub>I< sub>9-x< /sub>Cl< sub>x< /sub> were prepared in a one-step, solution-based processing route at low temperatures and were evaluated with regard to their photovoltaic performance in planar (compact TiO< sub>2< /sub>) and meso-structured (compact TiO< sub>2< /sub>/mesoporous TiO< sub>2< /sub>) perovskite solar cells. < p> < p align="justify">In addition, the influence of fullerene-based passivation layers on the characteristic solar cell parameters as well as hysteresis effects was investigated. Therefore, interlayers of [6,6]-phenyl-C< sub>71< /sub>-butyric acid methyl ester ([70]PCBM) and indene-C< sub>60< /sub> bisadduct (ICBA) were introduced between the electron-transport and the perovskite layers.< sup>[3]< /sup> The bismuth(III) halide perovskites were characterized optically (UV/VIS spectroscopy, light microscopy), structurally (X-ray diffraction), and electronically with regard to their performance as absorber materials in perovskite solar cells. < p> < p> References < p> < p align="justify">[1] Eckhardt et al., Chem. Commun. 2016, 52, 3058-3060.< p>[2] Park et al., Adv. Mater. 2015, 27, 6806-6813.< p>[3] Ip et al., Appl. Phys. Lett. 2015, 106 (14), 143902.< /p>

Resume : In times of very high electricity consumption, scientists try to improve amount of electricity produced from all kinds of energy sources. In photovoltaics, to gain the low-cost and non-toxic materials that will absorb light in a wide range of wavelength is one of greatest importance. An example of such a material could be MnSnS3. This compound has not been synthetized yet, but some attempts were made, and the first samples are being tested already. There are indications that MnSnS3 will show semiconductor and ferromagnetic properties. These give some hope of using this material for improving efficiency of solar cells.

Resume : Solution processed hybrid inorganic-organic materials (e.g metal chalgodenide and hybrid perovskites) are generating huge interest for application in range of optoelectronic and electronic devices such as solar cells, light emitting diodes and transistors. In this presentation, I will start by talking about some of our recent work on hybrid perovskites and in particular, our work aimed at identifying the key factors affecting the stability of hybrid perovskite solar cells. In recent years, spectacular advances have been made in the power conversion efficiency of perovskite solar cells with recent reports of devices exhibiting PCE’s over 20%. However, the relatively low stability of this technology still remains a concern. A number of factors have been shown to influence the stability of perovskite materials; these include water, UV and temperature. In this talk I will consider the affect of light and oxygen on the stability of perovskite materials and devices [1, 2]. I will first consider the effect of light and oxygen on the absorption properties of the films, device performance / stability and photo-induced charge separation yields. I will then go onto discuss the mechanism of how light and oxygen causes degradation of the perovskite absorber [2]. I will then go onto outline two strategies that can be used to reduce the severity of light and oxygen degradation in perovskite materials and devices, namely (i) the use of electron extraction layers within the deices structure [3] and (ii) via structural modification of perovskite absorber itself.  In the final part of the talk will conclude by discussing some of the lessons that can be learnt from the hybrid perovskite technology to the design of more efficient and stable metal chalogenide (e.g. tin(II) sulfide) solar cells. References: [1] O’Mahony et al. J. Mater. Chem. A. 2015, 3,7219 –7223. [2] Aristidou et al. Angew. Chemie. Int. Ed. 2015, 54,8208 –8212   [3] Bryant et. al. Energy Environ. Sci., 2016, 9, 1655-1660

Resume : Organic-Inorganic Hybrid Perovskites have not only turned out to be technically very promising as absorber layers in high-efficiency large-scale photovoltaics but also provide a number of challenges on the level of basic research. From our point of view, three central questions still have to be answered: - How can the layers be stabilized against chemical decomposition ? In this contribution we will discuss detailed investigations on the hydrolysis characteristics of methylammonium lead halides CH3NH3Pb(I1-xBrx)3 in the dark and under illumination [1]. Different degradation mechanisms were characterized. Bromide can successfully suppress the transformation of the perovskite to the monohydrate, presumably caused by stronger hydrogen bonding interactions with the organic cation. Under illumination in humid air, however, a rather rapid decomposition of the perovskites was still observed. - Can the toxic element Pb be replaced ? Tin is one of the chemical elements discussed to replace lead. We have prepared layers of methylammonium tin iodide (CH3NH3SnI3) in a two-step process by spin-coating a solution of methylammonium iodide (MAI) onto SnI2 prepared by physical vapor deposition [2]. Full surface coverage of the substrate was reached, the films consisted of 200 nm large crystalline domains and the films turned out to be remarkably stable even after exposure to air. - What is the origin of electrical hysteresis often observed in layers ? CH3NH3PbI¬3 and CH3NH3Pb(I¬0.95Br0.05)3 were prepared as thin films via different established deposition techniques also on microstructured gold electrode arrays on Si/SiO2 wafers to characterize the hysteresis of I-V characteristics in the dark and under illumination [3]. Persistent changes in the polarization of the perovskite films was observed following positive or negative poling. At higher bias voltages additional inverted hysteresis loops were measured pointing at a decrease in barrier width at the blocking perovskite/metal contact by migrating iodide ions leading to characteristics of two diodes operated back-to-back. - Implications of these different results for use of the materials for next-generation solar cell devices are discussed. [1]. R. Ruess, F. Benfer, F. Böcher, M. Stumpp and D. Schlettwein, ChemPhysChem, Article first published online: 3 MAR 2016 DOI: 10.1002/cphc.201501168. [2]. M. Weiss, J. Horn, C. Richter, D. Schlettwein, Phys. Status Solidi A, 213, 975-981 (2016). [3]. M. Stumpp, R. Ruess, J. Horn, J. Tinz, C. Richter, D. Schlettwein, Phys. Status Solidi A, 2016, 213, 38–45.

Resume : Besides hybrid perovskite-based solar cells, which have attracted much attention in recent years, also the interest in hybrid polymer/nanocrystal photovoltaic technologies has increased markedly. Polymer/nanocrystal solar cells have power conversion efficiencies now reaching 5.5% [1] and are promising due to their ability to incorporate the attractive qualities of both material classes, conjugated polymers and inorganic nanocrystals in one device. These hybrid solar cells can take advantage of the high absorption coefficients and easy solution-based processability of organic materials as well as the superior electronic properties and chemical stability of inorganic semiconductors.
The conventional approach to prepare polymer/nanocrystal hybrid absorber films is to blend a conjugated polymer with semiconducting nanocrystals, which are stabilized by capping agents. Besides the advantage that nanocrystals with well-defined sizes and shapes can be used, the main drawback of this approach is that the capping ligands can partly remain in the absorber layers, where they limit the efficient charge separation and transport in the final device. Consequently, in the last years, we have targeted our research on the in situ preparation of nanocrystals directly in the conjugated polymer film leading to ligand-free hybrid interfaces. Using metal xanthates as precursors, hybrid solar cells based on various metal sulfides and conjugated polymers have already been studied.[2-5]
Key issues to further develop these in situ prepared hybrid solar cells are an improved control of materials synthesis, a good control over nanomorphology formation as well as contacting the absorber layer with suitable selective electrode materials including proper interfacial layers. In this study, we therefore focused on electrode materials and the influence of device architecture (regular and inverted geometry) and different interlayers on the performance and lifetime of polymer/nanocrystal hybrid solar cells. The absorber layers of the investigated solar cells consist of the low band gap conjugated polymer PCDTBT and in situ prepared copper indium sulfide (CIS) nanocrystals.[6] In regular device architecture, the interlayers PEDOT:PSS, V₂O₅ and MoO₃ were investigated as hole extraction layers between absorber layer and anode. Here, the best performance was obtained in solar cells with PEDOT:PSS interlayer. Preparing PCDTBT/CIS hybrid solar cells in inverted device architecture unveiled interesting results. In the device architecture glass/ITO/TiO₂/PCDTBT-CIS/PEDOT:PSS/Ag, open circuit voltages up to 550 mV combined with high photocurrents could be obtained. Achieving such high photovoltages with this absorber material was so far only possible by using low work function metal electrodes like aluminum, which resulted in poor device stability. Using MoO₃ as interlayer instead of PEDOT:PSS in inverted solar cells, led to similar photocurrents compared to devices with PEDOT:PSS interlayer, however, the observed photovoltages were significantly lower. In the inverted devices with PEDOT:PSS interlayer, only the fill factor is lower compared to solar cells in regular geometry, which is limiting the PCE of the inverted solar cells to 2.1% up to now. However, solar cells in inverted architecture having a PEDOT:PSS interlayer and an additional CIS layer show good stability and maintain their high open circuit voltages over hundreds of hours of continuous illumination.
[1] Z. Liu et al., Adv. Mater., 2013, 25, 5772.
[2] T. Rath et al., Hybrid Mater., 2014, 1, 15.
[3] A. J. MacLachlan et al., Adv. Funct. Mater., 2015, 25, 409.
[4] T. Rath et al., Adv. Energy Mater., 2011, 1, 1046.
[5] N. Bansal et al., Adv. Energy Mater., 2013, 3, 986.
[6] S. Dunst et al., Synth. Met., 2016, doi:10.1016/j.synthmet.2016.04.003.

Resume : Development of Semiconducting Polymers

Resume : Nanoparticles of oxide semiconductors play crucial roles in mesoscopic solar cells such as dye-sensitized solar cells (DSSCs), quantum dot solar cells and perovskite solar cells. Not only the absolute size of the particles but also their shape and preferentially exposed crystal facets are expected to cause drastic changes in their performance, because it serves as the interface for carrier extraction and also for recombination. Also expected is the influence of the chemical stability of the light absorber (dye, Q-dot, perovskite) to be deposited on the oxide electron extracting layer. While titanium dioxide is widely used, zinc oxide (ZnO) is an attractive alternative due to its high electron mobility and flexibility in its structure control. We have succeeded in synthesis of size and structure controlled ZnO nanoparticles by rapid hydrothermal crystallization employing microwave radiation and structure directing agents (SDAs). Here, the synthetic methods, structural characterizations and their properties in DSSC applications are to be reported. Rapid microwave radiation for only 30 mins to aqueous solution containing Zn(CH3COO)2 and structure directing agents (SDAs) such as triethanolamine (TEOA) and 1,2,4,5-benzenetetracarboxylic acid (BTCA) with controlled pH by addition of KOH resulted in formation of differently shaped ZnO. The precipitates were centrifuged, washed and dried in oven. Well crystallized ZnO but with totally different morphology was obtained by using TEOA or BTCA. Use of TEOA resulted in nanoparticles with hexagonal and bi-pyramidal (HBP) shape in about 50 to 100 nm size. Combination of the observation techniques of transmission electron microscope (TEM), selected area electron diffraction (SAED) revealed that the axis connecting two tips of HBP corresponds to the c-axis of ZnO. While the rectangular sides in 6-fold symmetry obviously correspond to (100), the triangular facets could possibly be (101), (102) or (103). The angle between ridges and the horizontal line of HBP fell in the range between 38 and 41˚ from the scanning electron microscope (SEM) images, so that the triangle is most likely (102) with its occupancy to the total surface area of ca. 84%. On the other hand, the use of BTCA resulted in formation of nanosheet (NS) ZnO. TEM and SAED lattice images of NS identified that the mainly exposed facets are of the (100) planes of ZnO. Surface area and dye (D149) adsorption capability were checked for the synthesized ZnO in HBP and NS shapes in comparison with a commercial ZnO (MZ). Such difference was found for the area occupied a D149 molecule as 0.68 (commercial), 0.62 (HBP) and 1.2 (NS) nm2 indicating the dye adsorption stability on (102) ZnO crystal facets is greater than that on (100). Besides, the synthesized ZnO shows some interesting properties applying to DSSCs. The cells employing (100) dominated NS ZnO achieved about 30mV higher Voc than MZ and 40mV higher than that of HBP when using D149. The higher surface concentration of the dye on (102) resulted in a higher IPCE as compared to the other samples and Jsc is 2 mA/cm2 higher than that on (100) , despite of the reduced Voc and FF to limit the conversion efficiency. Their differences in charge extract properties are also studied and discussed.

Resume : Organo-lead halide perovskite solar cells can be prepared in a cheap an energy-efficient way and show high power conversion efficiencies already exceeding 20%. Not only high power conversion efficiencies, but also solution processability at low temperature make perovskite solar cells a very attractive method for solar energy conversion. The morphology of the perovskite films is one of the most critical issues to realize high efficient perovskite solar cells. Crucial process parameters to control the morphology of the perovskite films are the heating rate, annealing temperature and time.< sup>[1]< /sup> They have also a great effect on the structural, electronic and optical properties of the perovskite material.< sup>[1, 2, 3]< /sup> In order to investigate the different annealing conditions and coating techniques, solar cells were built on glass substrates coated with patterned indium tin oxide (ITO) films and have a configuration glass/ ITO/ c-TiO< Sub>x< /Sub>/ mp-TiO< Sub>x< /Sub>/ CH< Sub>3< /Sub>NH< Sub>3< /Sub>PbI< Sub>3< /Sub>/ Spiro-OMeTAD/ Au. In the first approach, perovskite films were deposited by doctor blading using precursor solutions in different dilutions. For further experiments, a dilution of 1:4 was chosen because solar cells prepared with this concentration revealed the best power conversion efficiency (PCE), highest I< Sub>SC< /Sub> and highest V< Sub>OC< /Sub>. In the second approach, spin coating was used for the preparation of the perovskite absorber layers. In the annealing temperature experiments, the perovskite films were annealed at 70, 80, 90 and 100°C with a one-step method and from 70 to 100°C with a slow annealing multi-step method as well as by gradual heating at different rates. It was evident that the solar cells annealed at 90°C had the best performance. Also the open circuit voltage and the fill factor showed the highest values for perovskite films prepared at this temperature. The I< Sub>SC< /Sub> is the only parameter which was highest at an annealing temperature of 100°C and decreased when lower annealing temperatures have been applied. For the determination of the solar cell parameters, current voltage curves were measured and optical properties of the layers were characterized using UV/Vis spectroscopy. Characteristics like roughness, crystallinity, chemical composition, purity and morphology of the prepared perovskite films were investigated by surface profilometry, X-ray diffraction and scanning electron microscopy. < p> References: < BR> [1] Eperon GE et al., Adv. Funct. Mater., 24 (2014), pp. 151?157 < BR> [2] Dualeh A et al., Adv. Funct. Mater., 24 (2014), pp. 3250?3258 < BR> [3] Tripathi B et al., Sol. Energy Mater. Sol. Cells, 132 (2015), pp. 615?622

Resume : Heterojunction solar cells consisting of a wide band gap n-type semiconductor and a p-type nanocrystal film as absorber have shown remarkable improvements in performance in the last decade. However, this progress is limited to merely two materials, PbS and PbSe, while solar cells based on other materials, as for example copper-based compounds show lower power conversion efficiencies. Much less effort has been made to gain a deeper understanding of factors limiting the performance of these material systems. Here, we present a detailed study on the photocurrent loss mechanisms in nanocrystalline CuInS2/ZnO heterojunction solar cells. For this purpose, we combined steady-state characterization methods using different illumination conditions with transient photocurrent and photovoltage measurements. We demonstrate the presence of two different loss mechanisms: An extraction barrier at the CuInS2/ZnO interface, which can be reduced upon illumination with UV light, as well as significant trap-assisted recombination in the CuInS2 layer. The presented results confirm the potential of heterojunctions made from CuInS2 and ZnO nanocrystals in solution producible solar cells, but also clearly highlight the importance of a substantial reduction of sub-band gap states to improve the performance.

Resume : A growing interest of two-dimensional (2D) layered nanostructures has been observed due to their peculiar nanoscale phenomena and promising future applications. Particularly gallium (II) sulfide (GaS) nanoplates show appealing features as novel materials for electronics. Single crystals of gallium(II) sulfide were obtained by chemical transport method. We present a method of obtaining single GaS crystals size up to 2,5x2x0,5mm useful for a single-layer exfoliation. The crystals were prepared from elements with amounts of iodine in quartz ampoules closed under vacuum and heated in a tube furnace at 750°C for 10 to 30 days. The obtained crystals and layers were characterized by X-ray, TEM and Raman studies.

Resume : ZnO has various applications in photocatalysis, transparent electrode, sensors and solar cells. In this study, ZnO nanocrystals are synthesized via a microwave (MW)-assisted hydrothermal reaction employing benzene carboxylic acids (BCs) as structure directing agents (SDAs). Evolution of unique nano-sheet ZnO (NS ZnO) has been discovered and conditions for its structure control has been studied. Typically, an aqueous mixture of 0.1 M Zn (CH3COO) 2 0.1 M BC, such as 1, 2, 4, 5-benzenetetracarboxylic acid (BTCA), trimesic acid (TMA), trimellitic acid (TMLA), phthalic acid (PA), isophthalic acid (IPA) and terephthalic acid (TPA) whose pH was adjusted to ca. 13 by addition of KOH was put into a pressure-withstanding vessels and subjected to a 2.45 GHz MW radiation to promote reaction at 150°C for 30 min. The precipitates were washed, dried and analyzed by XRD, SEM and TEM. While NS ZnO of a few hundred nm width and a few tens of nm thickness, and with predominant exposure of (100) facets were obtained by addition of BTCA, TMA, TMLA, PA and IPA, a drastically different result was obtained with TPA. Crystalline nanoparticles seemingly that of Zn/TPA metal-organic framework (MOF) was formed. Due to insertion of TPA into the ZnO matrix, its reaction yield was in the range of 130% and showed a significant exothermic weight loss around 450°C in the TG-DTA analysis. Studies about the properties of NS ZnOs as well as Zn/IPA MOF are also under way.

Resume : Doping of cobalt ions into zinc oxide yields magnetic semiconducting properties to be a promising material in spintronic devices. Also, introduction of new bands arising from the d-orbital of the cobalt ion leads to a deep coloration of ZnO in the visible range, suggesting its possible application as a light absorber in solar cells. In this study, we have attempted microwave (MW)-assisted hydrothermal synthesis of oxide nanoparticles of zinc, cobalt and their mixtures. Aqueous solutions of 0.1 M ZnCl2 and 0.1 M CoCl2 were mixed at various ratios to which an appropriate amount of NaOH was added to achieve pH ca. 12. The precursor solution was put into a pressure-withstanding vessel and subjected to a 2.45 GHz MW radiation to promote reaction at 160°C for 30 min. The resultant precipitates were centrifugally separated, washed with water and ethanol, and dried at 100°C for overnight. While crystalline white nanoparticles of ZnO were obtained from ZnCl2 solution, those of black Co(OH)2 were obtained from CoCl2. For the mixed solutions, addition of CoCl2 up to ca. 30% still retained crystalline ZnO structure, although the powder turned into dark-green to black. Further addition of CoCl2 resulted in phase-separated co-precipitation of ZnO and Co(OH)2. It is likely that Co ion can be doped into ZnO as long as the Co concentration is kept small. Fabrication of thin film electrodes by printing of these nanoparticles and their photoelectrochemical studies are under way.

Resume : Solution processible lead halide perovskite solar cells have achieved strikingly high efficiencies over 20%. Carrier selective charge extraction layers play crucial roles in such devices, as TiO2 and spiro-OMeTAD are typically used for electron and hole, respectively, in the perovskite cells. Not only for the perovskite cells but also all other thin film p-i-n solar cells, these contact layers need to be optimized in terms of their energy levels, carrier mobility and chemical stability. The method of film fabrication should also be versatile and tunable for various kinds of thin film solar cells, facile for structure control and scaling up. In this context, electrochemical processing could bear advantages. We have developed a versatile method to electrodeposit thin films of poly-oxometallates (POM) of Ti(IV), Al(III), Si(IV), Nb(V) and Ta(V), hybridized with benzoquinone (Q) by anodic oxidation of hydroquinone (H2Q). When alkoxides of these metals are hydrolyzed with bulky tetra-alkylammonium hydroxides (R4NOH), they produce clear alkaline colloidal solution of layered POM stabilized by insertion of R4N . Oxidation of H2Q to Q releases proton to exchange with R4N , leading to an aggregation of the colloid to deposit (POM)(Q) hybrids. Annealing of these films result in formation of corresponding metal oxides. In case of Ti(IV), relatively compact and flat anatase TiO2 was obtained. Performance of such metal oxide layers is to be tested by employing perovskite as light absorber.

Resume : Inorganic / organic layered hybrid compound of [DAMS ]Cu5I6 has been synthesized by Cariati et al. (DAMS : trans-4-(4-dimethylaminostyryl)-1-methylpyridinium) [1]. Due to a high ordering of the stilbazolium chromophore in a J-aggregate form, it gives a rise to SHG (second harmonic generation). In this study, we have attempted electrochemical self-assembly of CuSCN-DAST hybrid thin films by addition of DAST (4-N,N-dimethylamino-4’-N’-methylstilbazolium tosylate) to the bath for cathodic electrodeposition of CuSCN, namely, methanolic solution of 2.5 mM Cu(ClO4)2, 2.5 mM LiSCN and 0.1 M LiClO4. Cathodic electrolysis at 0.2 V (Ag/AgCl) at an FTO coated glass RDE (500 rpm) for 10 min at room temperature lead to a deposition of homogeneous red-colored CuSCN-DAST hybrid thin films. Increase of DAST concentration in the bath up to 250 μM resulted in a systematic increase of DAST concentration in the film, increase of the film thickness for about 30%, as well as significant change of the film morphology, despite of the unchanged current during the electrodeposition. The absorption peak of the hybrid film is clearly red-shifted compared to that of the DAST solution, indicating certain ordering of the stilbazolium. Detailed characterization for the crystalline structure, composition, as well as applications for SHG and solar cells are under way. [1] E. Cariati, et al., Adv. Mater., 13, 1665-1668 (2001).

Resume : Cd1-xZnxS films suitable for solar cell photovoltaic devices have been produced by chemical bath deposition (CBD) on to indium thin oxide (ITO) coated glass substrates using an aqueous solutions for solar cell photovoltaic application. Cadmium sulfide, zinc sulfide and thiourea were used as sources of Cd+2, Zn+2, and of S-2, respectively. Triethenolamine was used as complexing agent to control the Cd+2 and Zn+2 ions concentrations and ammonia to adjust the pH 10±5 of the solution. The temperature of the bath was kept at 70 °C. The as deposited films are well adherent, homogeneous and free from pinholes. The incorporation of Zn in CdS was found to be dependent on the annealing temperature and The structure of the films as observed by X-ray diffraction was polycrystalline with hexagonal structure and (100), (101), (102) and (110) as main peaks. Energy dispersive spectroscopy (EDS) shows non-stoichiometric films due to a deficit of sulfur by increasing Zn content. The strong absorption edge shifts towards the lower wavelength region and hence the band gap of the films increases as the Zn content increases. The values of the absorption edge are found to shift towards the shorter wavelengths with Zn content and hence the direct bandgap energy varies from (Eg ~ 2.42 eV) for the CdS film and (Eg ~ 3.30 eV) for the ZnS film. Cd1-xZnxS thin films can be useful as buffer and window layers in Cu(In,Ga)Se2 and Cadmium telluride (CdTe) thin films solar cells due to ability to tune the band gap through the Zn/Cd ratio present in the chemical bath.

Resume : This study investigates all-solution-processed CuInS2 (CIS) solar cells with the superstrate architecture: FTO/TiO2/In2S3/CuInS2/Carbon. The hole-blocking TiO2 and buffer In2S3 layers are fabricated by spray pyrolysis prior to the spin-coating of the CIS precursor ink. A conductive carbon paste is deposited by doctor blade method afterward. The impact of Indium Sulfide layer deposition conditions on cell performance is studied. The In2S3 layer is made by spray pyrolysis using an aqueous solution of indium chloride tetrahydrate and thiourea with the molar ratio of [1]:[6]. Spray rates of 2ml/min and 4ml/min and the substrate temperature of 250°C, 350°C and 450°C are examined. XRD measurement shows improvement in crystallinity – in the desired beta phase – for deposition temperate of 350°C compared to that of 250°C. The best cell performance is gained with the buffer layer sprayed at 350°C with the rate of 4ml/min with the efficiency of 4.1%. With the substrate temperature of 450°C, cell efficiency is not improved due to the phase transition of In2S3 at 420°C and potential diffusion of Copper atoms in the interface between In2S3 buffer and the CIS absorber layer. To avoid the diffusion, the spray pyrolysed In2S3 layer is subjected to chemical treatment with Na2S bath. The chemical treatment improves the solar cell efficiency from 0.36% and 0.6% to 0.72% and 1% for the deposition rate of 2 and 4 ml/min respectively.

Resume : Herein we report the application of chalcopyrite, p-type semiconductor CuInS2 (CIS) in nanofibrous structure for the reduction of CO2 to CO with faradaic efficiency of 70-80%. Initially the synthesis of CuInS2nanofibers was carried by adaptable electrospinning technique. In order to reduce the imperfection in the crystalline fiber polyacrylonitrile (PAN) was selected as template polymer [1]. Afterwards the desired chemical structure of nanofibers was achieved through sulfurization process. Making continuous CuInS2nanofibers on the cathode surface by electrospinning method brings the advantages of being economical, environmentally safe and versatile. The obtained nanofibers have well uniform size and diameter according to the Scanning Electron Microscope (SEM). An improvement of faradaic efficiency was achieved with the catalytic active CuInS2 in nanofibrous structure as compared to the solution processed CuInS2. The CuInS2nanofiber electrodes prepared by the electrospinning technique, show four times higher faradaic efficiency as compared to the solution prepared films. Electrode nanostructure and less contamination as compared to solution processed films and having a better defined chemical composition play a role in this improvement. Fabrication and application of nanofibrous materials through electrospinning technique might be of interest for other electrocatalytic systems for CO2 reduction as well [2]. [1] Ozel F.; Kus, M.; Yar, A.; Arkan, E.; Yigit, M. Z; Aljabour, A.; Büyükcelebi, S.; Tozlu, C.; Ersoz, Materials Letters. 2015, 140, 23–26 [2]Ozel F.; Journal of alloys and compounds. 2016, 657, 157-162

Resume : Development of high efficiency solar cells using a cost effective way is of great importance to achieve low generation cost and expansion of application areas. In this respect, colloidal quantum dots (CQDs) are promising absorbers and carrier transporter for the solar cells. This is because of high degree of freedom for controlling absorption band positions so as to cover a wide range of solar spectrum, and because CQDs are compatible with solution-based technology. Recently, depleted heterojunction solar cells made up of ZnO electron transporter and CQD have been steadily increasing its performance. However, short carrier diffusion length of CQD films limits the film thickness, namely, light harvesting efficiency in the near IR region. To avoid this problem, we then have focused on solar cells composed of PbS CQD/ZnO nanowire structures (NW-type), aiming at efficient carrier transport and light absorption in the near infrared region. We developed air-stable and high efficiency solar cells yielding a power conversion efficiency of 7% and a maximum EQE of 60% in the near-IR region (at 1020 nm) and over 80% in the visible region [1-3], and reported that NW-type solar cells gave a carrier diffusion length of over 1000 nm [4]. Although the implementation of spatially separated carrier pathways was verified effective for fabricating high efficiency CQD-based solar cells, there are much room to increase solar cell performance such as Voc and FF. In this presentation, we discuss the strategy for further increase in the performance by focusing on surface engineering of PbS CQDs and ZnO NWs. 1) J. Phys. Chem. Lett., 4, 2455 (2013); 2) Phys. Stat. Solidi (rrl), 8, 961 (2014); 3) ACS Nano, 9, 4172(2015); and 4) J. Phys. Chem. C, 119, 27265 (2015).

Resume : Plasmonic enhancements for solution-processed organic, inorganic, and hybrid photovoltaic technologies have been explored as a light trapping technique to help improve absorption in the transport-limited films. Despite optimistic predictions, experimental performance enhancements based on surface plasmonics have been limited. Here, we overview an effective medium model designed to predict realistically achievable photocurrent enhancements based on embedded plasmonic nanoparticles in solution-processed thin film solar cells. This model is used to critically evaluate the prospects for realizing plasmonic enhancements in practical devices including perovskite, organic, and colloidal quantum dot solar cells. Parameters such as nanoparticle size, shape, material type, and concentration are considered, and strategies for achieving a balance between optical field enhancement in the photovoltaic film verses parasitic absorption in the metal nanoparticles are quantified. We find that further plasmonic gains may be possible in organic photovoltaic cells and discuss specific experimental implementations.

Resume : Currently used solar materials are limited to harvest light from the visible and near-infrared portions of the solar spectrum. A full 30% of solar power is in the infrared spectral region and is thus effectively wasted by conventional solar cell technologies. Colloidal quantum dots are an innovative class of low-cost liquid-based materials for converting light into electricity. What makes them so appealing is that their optical properties are strongly dependent on their physical size: different-sized quantum dots will operate upon different parts of the solar spectrum with a range that extends from the visible all the way into the infrared. This unique feature can be applied to improve the solar power conversion efficiencies of mainstream commercial technologies such as silicon by integrating the infrared colloidal quantum dot solar cells in a hybrid configuration with the conventional solar technology. Alternatively, true tandem operation can also be realized for example with perovskite solar cells. We will present the recent results in the development of colloidal quantum dot solar cells which can harvest infrared solar radiation in an efficient and cost-effective way.

Resume : In the feedback mode of SECM an added redox mediator is converted under diffusion controlled conditions at a microelectrode (ME) probe. For the analysis of a DSC the reduced form of the redox electrolyte is formed in this process. If the ME is brought to a distance of a few ME radii to an illuminated DSC anode, the dye regeneration process at the photoanode also regenerates the original for of the mediator. This additional source of mediator is sensitively detected as a current increase at the ME. Two experimental approaches have been used for the analysis of photoanodes of dye sensitized solar cells (DSC) by scanning electrochemical microscopy (SECM). The first is the recording of an approach curve during which the flux of redox mediator reaching the sample is systematically varied. The current can be compared to numerical simulation, from which an effective pseudo-first order rate constant keff is accessible. By measuring such curves under with different light intensity and mediator concentration, the change of keff with experimental parameters can be used to deduce more details of the regeneration process. Recent progress in correlating these values to the performance of entire DSCs is reviewed. Another approach aims for resolving lateral heterogeneities of DSC performance. Despite the fact that SECM MEs are larger than the used nanaostructure and the strong light scattering in photoanodes, steady progress in this direction will be reviewed.

Resume : Lead-chalcogenide colloidal quantum dots represent a promising absorber class for photovoltaic cells. The ease of processing from solution and the optoelectronic properties result in an emerging technology. However, fabricating efficient photovoltaic devices using quantum dot materials has often been limited by non-radiative recombination. In this work the shows a pathway to suppress non-radiative losses resulting in an effective improvement of the open circuit voltage. This is performed by shifting the excitonic peak towards the infrared and by using an amine-based route to protect the surface from building defect state while processing. Thus our results show the pathway to a scenario with less non-radiative losses in a quantum dot solar cells .

Resume : This work is focused on the synthesis of novel Cu(In1–xGax)(Se1–yTey)2 (CIGST) nanostructures via hydrazine–free solution method and utilization as absorbing layer for the thin film solar cells. A preferential orientation along the [112] direction without secondary phases, an enhanced crystal quality, a better electrical conductivity, and a morphological transformation from nanoparticles to nanosheets were determined with increasing (Se+Te)/Se ratio by XRD, Raman, Van der Pauw, SEM/EDX, and AFM results. Furthermore, the UV–Vis–NIR spectra indicated that band gap of chalcopyrite compounds can be tuned by changing the (In+Ga)/In and (Se+Te)/Se ratio in the range of 1.15–1.57 eV and 1.32–1.73 eV, respectively. By optimizing the device architecture, a high power conversion efficiency (PCE) of 11.37% with Voc of 0.67 V, Jsc of 26.78 mAcm–2, and FF of 63.36% has been achieved for Cu(In0.50Ga0.50)(Se0.50Te0.50)2 based cell. This efficiency is the best record among the chalcopyrite based thin film solar cells in which each layers are performed by solution processable method. Besides, all the solar cells exhibited a quantum efficiency over 75% in the visible region. Consequently, low–cost Te has a favourable contribution on the photovoltaic performance of the solar cells due to the its high photoconductivity, and recombination reduction potential seen at grain boundaries around the Mo back contact. Our results not only demonstrate a method of producing all layers of thin film solar cells in a hydrazine–free solution processable route, but also provide important scientific insights that will facilitate further improvement of the low cost chalcopyrite based solar cells. Keywords: Cu(In1–xGax)(Se1–yTey)2 (CIGST), Thin Film Solar Cells, Sol–Gel Method. Acknowledgment Authors would like to thank the European Cooperation in Science and Technology through COST Action MP1302 Nanospectroscopy and the Scientific and Technological Research Council of Turkey (TUBITAK Grant number 112T981) for the financial support of this research.

Resume : Electron-deficient tetrazine-based building blocks have been already successfully applied as strong acceptors in donor-acceptor copolymers.[1] One limiting factor, however, is their poor solubility, which requires to combine them with electron-rich comonomers with large solubilising alkyl chains. As benzothiadiazole (BTD), one of the most successful electron-deficient units to date also lacks in solubility, 2,1,3-benzothiadiazole-5,6-dicarboxylic imide has been developed to improve solubility and processability. Copolymers with this acceptor unit led to excellent efficiencies of over 8% without requiring processing additives or additional processing steps such as thermal annealing.[2] This highly soluble building block also allows variation of the alkyl chains on both electron-rich and electron-deficient components to tailor the alkyl chain substitution pattern, which has a profound influence on blend morphology and consequently device performance.[3] In this contribution, we will discuss the preparation of novel acceptor monomers (6-alkylpyrrolo[3,4-d]pyridazine-5,7-diones, PPD) via inverse-electron-demand Diels-Alder reactions of tetrazines and N-alkyl substituted maleimides.[4] By this synthetic approach, alkyl chains and flanking aromatic units can be easily varied to study structure-property relationships. The PPD monomers are obtained in good yield and purity. Furthermore, conjugated polymers were derived from Pd-catalyzed Stille-type polymerizations with (4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethylstannane) (BDT) and (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethylstannane) (oBDT). The obtained copolymers (BDT-PPD and oBDT-PPD) show good solubility and film forming properties. Besides testing their performance in combination with PCBM in organic solar cells, we also focused on evaluating their properties in polymer/nanocrystal hybrid solar cells. Therefore, inorganic nanocrystals (e.g. copper indium sulfide) have been synthesized in situ in films of these novel polymers using metal xanthates to obtain hybrid bulk heterojunction absorber layers. [1] Z. Li et al., J. Am. Chem. Soc., 2010, 132, 13160-13161. [2] C. B. Nielsen et al., Adv. Mater., 2015, 27, 948-953. [3] I. McCulloch et al., Acc. Chem. Res., 2012, 45, 714-722. [4] Q. Ye et al., Org. Lett., 2014, 16, 6386-6389.

Resume : A stringent limitation in many optical devices, such as solar cells and light emitting diodes, is the intrinsic need for a transparent electrode. Uniting relevant aspects, indium tin oxide (ITO) is often the material of choice with present alternatives being in particular found in conductive polymers. In this work, we present the use of PEDOT:sulphate films prepared by chemical vapour deposition (CVD) as a transparent electrode in optical devices. The application of PEDOT:sulphate is especially advantageous as it is directly coated on top of the substrate, intrinsically doped and combines characteristics such as high conductivity (4000 S/cm), high degree of order, and high purity. Additionally, PEDOT:sulphate is in its application water-free which makes it a promising candidate for devices with moisture-sensitive components such as perovskites. Preliminary results of PEDOT:sulphate electrodes integrated in optical devices such as bulk heterojunction solar cells (glass/Cr/Au/PEDOT:sulphate/P3HT:PCBM/LiF/Al) and organic light emitting diodes (glass/Cr/Au/PEDOT:sulphate/MH-PPV/LiF/Al) will be shown.