Symposium FB
Towards Next Generation Solar Cells: Emerging Materials, Phenomena and Device Architectures


Session FB-1 - Thin-film photovoltaics

FB-1.1  Silicon thin films and multi-junction Si solar cells

FB-1.1:IL01  Optimal Atomic Structure of Amorphous Silicon Obtained from Density Functional Theory Calculations
P. PEDERSEN1, L. PIZZAGALLI2, H. JONSSON1, 1Faculty of Physical Sciences and Science Institute, University of Iceland, Reykjavík, Iceland 2Dept. of Physics and Mechanics of Materials, Institut P’, CNRS-Université de Poitiers UPR 3346, SP2MI, Futuroscope Chasseneuil Cedex, France

Atomic structure of amorphous silicon consistent with several reported experimental measurements has been obtained from annealing simulations using electron density functional theory calculations and a systematic removal of weakly bound atoms. The excess energy and density with respect to the crystal are well reproduced in addition to radial distribution function, angular distribution functions, and vibrational density of states. No atom in the optimal configuration is locally in a crystalline environment as deduced by ring analysis and common neighbor analysis, but coordination defects are present at a level of 1%–2%. The simulated samples provide structural models of this archetypal disordered covalent material without preconceived notion of the atomic ordering or fitting to experimental data.

FB-1.1:IL02  Atomic Layer Deposited Nanolayers to Enhance Silicon Photovoltaics
E. KESSELS, B. MACCO, Department of Applied Physics, Eindhoven University of Technology, Eindhoven, Netherlands

In recent years, it has been realized that the unique features of atomic layer deposition (ALD) can be employed to face processing challenges for various types of solar cells. With this, ALD for photovoltaics (ALD4PV) has attracted great interest in academic and industrial research and it has even been introduced in high-volume manufacturing. In this presentation, the status of the use of ALD nanolayers in silicon photovoltaics will be reviewed and their future prospects will be discussed. The presentation will focus particularly on the application of ALD oxides (Al2O3 and beyond) for the passivation of surfaces of high-efficiency silicon solar cells and the preparation of highly transparent conductive oxides (doped ZnO films and In2O3:H). Furthermore, the upcoming field of passivated contact solar cells will be addressed as well as the use of ALD-prepared nanolayers in tandem solar cells combining silicon and perovskite solar cells.

FB-1.2   CIGS (and related compounds) and CdTe solar cells

FB-1.2:IL02  New Disruptive Design of CIGS(e) Solar Cells Based on Advanced Surface Techniques Structures and Layers used in Silicon Solar Technology
B. VERMANG et al., University of Hasselt and Imec, Diepenbeek, Belgium

The aim of the Uniting PV project is to revolutionize the design of thin film solar cells through implementation of advanced three-dimensional silicon solar cell concepts. This novel design consists of surface passivation layers and light management methods integrated into ultra-thin thin-film solar cells. An overview of surface passivation layer schemes for thin film solar cells will be presented, from its fundamentals to tangible solar cell applications, and their perspective.
This work has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement n° 715027).

FB-1.2:IL03  Combination of Heat-light Soaking and Light Soaking for Performance Improvement of Cu(In,Ga)(S,Se)2 Solar Cell
JAKAPAN CHANTANA1, TAKUYA KATO2, HIROKI SUGIMOTO2, TAKASHI MINEMOTO1, 1Department of Electrical and Electronic Engineering, Ritsumeikan University, Shiga, Japan; 2Atsugi Research Center, Solar Frontier K. K., Atsugi, Kanagawa, Japan

Cu(In,Ga)(S,Se)2 (CIGSSe)-based solar cell was fabricated by replacing CdS/ZnO buffer-window layers with (Cd,Zn)S/(Zn,Mg)O buffer-window layers for increasing short-circuit current density because band-gap energies of (Cd,Zn)S and (Zn,Mg)O are larger than those of CdS and ZnO, respectively. Moreover, introduction of approximately 10% Mg into ZnO:Al to form (Zn,Mg)O:Al as transparent conductive oxide (TCO) layer in the solar cell leads to optimize minimum conduction band difference between TCO and CIGSSe absorber, resulting in the increase in conversion efficiency (η) to 20%. Next, heat-light soaking (HLS) at 110 C under illumination (AM1.5 G) and then light soaking (LS) under illumination (AM1.5 G), called HLS+LS process, were conducted successively on the finished solar cell to further increase η. It is revealed that HLS mainly gives rise to the increase in open-circuit voltage (VOC), while LS primarily yields the enhancement of fill factor, thereby enhancing the η to 21.1%. In addition, individual recombination rates at the buffer/absorber interface, in the space-charge region, and in the quasi-neutral region of the solar cell were determined from the temperature and illumination dependence of the VOC and investigated before and after the HLS+LS process in detail.

FB-1.2:IL04  Sprayed Non-doped and Ga-doped ZnO Films for CuInGaSe2 Solar Cells
KENJI YOSHINO, University of Miyazaki, Miyazaki, Japan

Low cost processes are very important problems for solar cell devices. The spray method is one of non-vacuum processes and is respected as low cost method. In our previous works, high quality transparency ZnO films were successfully grown at 100 °C by a conventional atmospheric spray pyrolysis using diethylzinc (DEZ) based solution [1]. The DEZ was diluted with diisopropyl ether to control its reactivity to air and water. Moreover, the growth of Ga-doped ZnO (GZO) /glass films was carried out by spray pyrolysis at 150 °C. The samples had an average optical transmittance of more than 80% and were strongly a-axis orientated. The sheet resistivity of 30 Ω/sq. could be obtained. The GZO films were successfully grown on ZnO/CdS/CIGS/Mo/glass by spray pyrolysis. After covering clean SLG substrates with back electrodes of Mo films by sputtering, CIGS films were deposited using physical vapor deposition. Buffer layers of CdS films were prepared by chemical vapor deposition. Buffer layers of ZnO films (≈ 20 nm) were also prepared by RF sputtering method. The efficiency of the obtained device was 10.3%. The short-circuit current density (Jsc) of 34.3 mA/cm2, open circuit voltage (Voc) of 0.50 V and fill factor (FF) of 0.60 are obtained.
[1] K. Yoshino et al, Jpn. J. Appl. Phys. 50, 040207.

FB-1.2:IL05  Interfaces in CdTe Thin-film Solar Cells
B.G. MENDIS, A.A. TAYLOR, Durham University, Durham, UK; J.D. MAJOR, K. DUROSE, University of Liverpool, UK

CdTe thin-film solar cells have seen remarkable progress in recent years, despite the large lattice mismatch between CdS and CdTe and the polycrystalline nature of CdTe. Sulphur diffusion takes place at the CdS-CdTe interface during fabrication, which is thought to benefit performance since it relieves the lattice mismatch. In order to produce high efficiency devices a chlorine activation step is also required. Chlorine segregates to the grain boundaries during activation, thereby passivating them. Here we show how electron microscopy can be applied to extract important information about interfaces in CdTe, such as CdTe grain boundaries and the CdS-CdTe junction. The structure, chemistry and electrical activity are determined at the nano-scale. Cathodoluminescence is used to measure the carrier lifetime and recombination velocity of individual grain boundaries. Furthermore experimental measurements of sulphur diffusion is used to model the device properties. Contrary to popular belief it is shown that sulphur diffusion is actually harmful to device performance. The underlying reason is that the misfit dislocation density is conserved, but their re-arrangement within the space charge region during diffusion leads to an overall increase in carrier recombination.

FB-1.2:IL06  Low-cost Thin Film Solar Cells for BIPV Applications
E. GILIOLI, IMEM-CNR, Parma, Italy

The Low-Temperature Pulsed Electron Deposition (LT-PED) is a novel single-stage deposition technology developed @ IMEM-CNR for the simple and cheap growth of Cu(In,Ga)Se2 (CIGS)-based thin film solar cells, with no need for post deposition treatments like selenization or high temperature annealing. The very low deposition temperature is one of the key features; by exploiting the high-energy species generated by the PED rapid material ablation, 17% efficiency solar cells on conventional soda lime glass substrate are obtained at deposition temperature of 250°C, significantly lower than the alternative techniques, enabling the use of a large number of substrates, including flexible polymeric and thermo-labile ones. Indeed, this represents a great opportunity in the field of the Building Integrated Photovoltaic (BIPV). In particular, the LT-PED enables the fabrication of bifacial solar cells (BFCS), where CIGS absorber is directly deposited onto transparent back contact, preventing the formation/diffusion of unwanted phases at the interface. The promising results of CIGS-based BFSCs on different transparent conductive oxides (TCO) exceeding the record values reported in literature, are presented.

FB-1.2:IL07  Electronic and Chemical Structure of Interfaces in CIGS and CdTe Thin-film Solar Cells
C. HESKE, Institute for Photon Science and Synchrotron Radiation (IPS) and Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany; Department of Chemistry and Biochemistry, University of Nevada, Las Vegas (UNLV), Las Vegas, NV, USA

Among the various technologies for thin-film solar cells, Cu(In,Ga)(S,Se)2- (CIGSSe-) and CdTe-based systems rank prominently, having reached conversion efficiencies well above 20%. Nevertheless, much remains to be understood and optimized, in particular when alternative buffer materials (i.e., materials that deviate from the traditional materials in CdTe- or CIGSSe-based devices) are to be employed. Using electron and soft x-ray spectroscopic methods, it is possible to unravel (some of) the secrets of the alternative buffer/absorber interfaces. This includes lab-based x-ray and UV photoelectron spectroscopy, inverse photoelectron spectroscopy, and x-ray-excited Auger electron spectroscopy. These techniques are complemented by soft x-ray emission and absorption spectroscopy using high-brilliance synchrotron radiation. While the electron-based techniques are very surface-sensitive, the two synchrotron-based techniques are photon-in-photon-out techniques that probe the bulk region near the surface. In the talk, experiments to gain insights into alternative buffer/absorber interfaces will be presented, in particular in view of band alignment and intermixing behavior, and the impact of these properties on the performance of corresponding solar cell devices will be discussed.

FB-1.2:L08  Optimization of Pulsed Laser Deposition Parameters for the Growth of High-quality CuIn1-xGaxSe2 Thin Films
CH. NICOLAOU1, A. ZACHARIA2, G. ITSKOS2, J. GIAPINTZAKIS1, 1Department of Mechanical and Manufacturing Engineering, University of Cyprus, Nicosia, Cyprus; 2Experimental Condensed Matter Physics Lab, Department of Physics, University of Cyprus, Nicosia, Cyprus
CuIn1-xGaxSe2-based thin film solar cells have attracted considerable attention over the last decades due to the promising potential of high efficiency (record~22.6%) and low-cost photovolatic applications. CuIn1-xGaxSe2 quaternary compound is a p-type, direct semiconductor with tunable energy gap and superior absorptivity of ~10^5cm^-1 in the visible spectrum. Many deposition methods have been utilized to grow CIGS thin films; however, only few works on the pulsed laser deposition (PLD) method can be found in the literature. In this presentation, the growth of CuIn0.7Ga0.3Se2 films on bare and Mo coated- soda-lime glass (SLG) using PLD will be discussed. The influence of the growth conditions of CIGS films has been investigated through a complete characterization of structure, composition and morphology. Also, electrical and optical measurements of CIGS films have been performed in order to study the transport and optical properties of the films and the Mo/CIGS interface. In addition, Na-free substrates have been used to investigate the effect of Na diffusion on CIGS films’ electrical properties. Finally, the optimum PLD growth conditions for CIGS films will be presented based on an overall assessment of the film and interface characteristics.

FB-1.3   Kesterite and other novel materials/concepts for inorganic thin film PV

FB-1.3:IL01  Development of ZnSnP2 Solar Cells: A Novel Absorber Material

We report on solar cells with II-IV-V2 type chalcopyrite compound ZnSnP2 (ZTP), which is a novel absorber material. The cells with the structure of Al/AZO/ZnO/(Zn,Cd)S/ZTP/Cu were prepared using ZTP bulk crystals obtained by flux method. The buffer and back electrode materials were modified compared to conventional CIGS solar cells. An ohmic contact could not be obtained using a conventional electrode Mo, and (Zn,Cd)S was adopted from the viewpoint of the band alignment between buffer materials and ZTP. Finally, we achieved the conversion efficiency of 3.4 % for ZTP solar cells, where Jsc, Voc and FF were 8.2 mA/cm2, 0.452 V and 0.533. This result is the world record for ZTP solar cells, however, the loss of Voc is serious considering the bandgap of ZTP (~1.6 eV). We thus tried to improve the offset of conduction band minimum (CBM) between buffer materials and ZTP. It was clarified that ZnS and In2S3 were suitable for ZTP solar cells based on the analysis by XPS. In particular, Voc over 0.68 V could be achieved in the cells with In2S3 although the efficiency was lower because of low Jsc.

FB-1.3:IL02  Development of Antimony Selenide Solar Cells by a Scalable Deposition Route
L.J. PHILLIPS1, P.J. YATES1, H. SHIEL1, O.S. HUTTER1, M. BIRKETT1, S. MARIOTTI1, C. SAVORY2, K. DUROSE1, D.O. SCANLON2, T.D. VEAL1, J.D. MAJOR1, 1Department of Physics, University of Liverpool, UK; 2Department of Chemistry, University College London, UK

Antimony selenide solar cells are an emerging thin-film technology of growing interest. They benefit from a direct ~1.17eV bandgap, containing no scarce materials, have a simple phase chemistry and an interesting 1D nanoribbon grain structure. Despite the first respectable device efficiency being reported as recently as 2014 and the relative paucity of research, they have already reached efficiencies of over 6%. This work will report on the development of a new close space sublimation (CSS) route to Sb2Se3 fabrication, capable of producing devices of >5.5.% efficiency and with VOC values in excess of current champion devices. We will discuss the distinct materials and device differences between CSS cells and thermally evaporated equivalents, as well as the relative merits and difficulties associated with the CSS technique. This will include analysis of electrically active defects within the material, the role the 1D crystal structure plays in film growth and how the form of the Sb2Se3 films may be varied from nanowires to compact thin films by careful control of deposition conditions. We will demonstrate that this new deposition route offers huge potential to aid the development of this technology and will signpost the key challenges than need to be addressed to further improve the device performance.

FB-1.3:IL03  Kesterite Thin Tilm Solar Cells Prepared by Chemical Route
SHIGERU IKEDA1, THI HIEP NGUYEN2, TAKASHI HARADA2, 1Department of Chemistry, Konan University, Kobe, Japan, 2Research Center of Solar Energy Chemistry, Osaka University, Toyonaka, Japan

In this study, sulfide-based Kesterite CZTS thin films were fabricated by a spray pyrolysis method. X-ray diffraction patterns of resulting films showed typical diffraction peaks assignable to the CZTS crystal. Moreover, morphological analyses of these fabricated films by SEM indicated that the fabricated films had grain size as large as the thickness of the film, i.e., well grown CZTS grains were successfully obtained by using the facile spray-based process. Regarding solar cell properties, we found that compositions of CZTS films were significant, as expected from various literature results. Compared to stoichiometric ratios (Cu/Sn = 2; Cu/Zn = 2), films composed of Cu-poor/Sn-rich compositions were advantageous to obtain high conversion efficiency. Current best efficiency achieved was more than 8.5%. We also found that incorporation of a certain amount of silver into the CZTS was beneficial to improve film qualities as well as solar cell properties. The best result was obtained by using a device based on the Ag-incorporated film with Ag / (Ag + Cu) ratio of ca. 0.02; the efficiency values in the present system were found to depend strongly on film morphologies (e.g., presence of voids and pinholes), defect disparities, and band alignments toward the CdS layer.

FB-1.3:L04  Silicon Heterojunction with Organic Thin Layer (HOT) Solar Cells
HAJIME SHIRAI, RYO ISHIKAWA, TAKUYA MIURA, Saitama University, Saitama, Japan

Recently, n-type crystalline-Si(c-Si) heterojunction solar cells employing highly conductive and hole-transporting polymer poly(3,4-ethyenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been extensively studied, because it works as a photovoltaic device without using p-n junction and a TCO layer. We named this type solar cell as “HOT” (c-Si heterojunction with organic thin layers) solar cell. Promising PCE of this front junction “HOT” solar cell has been reported to 13–15% by adjusting a type of solvent, thickness of PEDOT:PSS layer, and resistivity of silicon wafer despite of no use of light management such as texturing and antireflection (AR) layer. For further efficient organic/c-Si junction solar cells using the solution process, it necessitates the decrease of contact resistance of a cathode. Soluble Ba(OH)2 or BaCl2 salt has been attempted as an electron-injection layer in organic field-effect transistors (TFTs) and polymer solar cells as a hole blocking layer. In this paper, we demonstrate the potential of Ba(OH)2 as a hole blocking layer for efficient PEDOT:PSS/c-Si junction solar cells including solar cell module.

Session FB-2 - III-V Solar Cells

FB-2:IL01  High Efficiency Multi-junction Solar Cells for Concentrator Photovoltaics
G. TIMÒ, N. ARMANI, G. ABAGNALE, RSE, Piacenza, Italy

The success of concentrator photovoltaic (CPV) technology is due to the utilization of ultra-high efficient solar cells coupled with optical concentration. This technology is still in a pulsing development phase, in particular concerning the solar devices. For high concentration system ( > 300 X) a technology jump from three junction devices to four junction ones is near to be accomplished. In order to get this jump, two main technological paths are under development: the realization of “monolithic devices”, in which different materials are deposited on top of each other, exploiting a common substrate and the realization of “stacked” or “wafer bonded” devices, where single or tandem devices are deposited on different substrates and then are joined removing one of the substrate. With this contribution the different technological approaches for realizing high efficiency Multi-junction solar cells will be first introduced and subsequently the presentation will be focalized on the frontier development of MJ to be realized by combining III-V compounds with IV elements. This solution expands the band gap engineering possibilities for getting high efficiency devices and, at the same time, allows keeping down the cost.

FB-2:IL02  III-V Compound Semiconductor Nano-epitaxial Structures for High-efficiency Multi-junction Solar Cells
MASAKAZU SUGIYAMA, Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Japan

Epitaxial III-V compound semiconductors have contributed to the highest conversion efficiency owing to its potential to minimize non-radiative carrier recombination (heat dissipation) and its adjustable bandgap which is mandatory for multi-junction structure. Recently, nanostructure such as quantum wells and dots are introduced to such III-V layers for solar cells. Among several potential applications, multiple quantum wells (MQWs) can be used to adjust the absorption edge wavelength of a cell, improving the current matching among subcells in multi-junction cells. Normally, adjustment of the absorption edge imposes the change in the lattice constant of a semiconductor, making it difficult to find materials with the same lattice constant for epitaxial stacking and yet varied absorption edges to achieve current matching. Strain-balanced quantum wells can mitigate such a constraint between absorption edge and lattice constant. However, they impose significant bottleneck of carrier transport and elaborate design on their structure is indispensable such as thin barriers for tunneling carrier transport. Coupled with the improvement in metalorganic vapor-phase epitaxy (MOVPE), MQWs can function as a “quasi-bulk” to improve current matching and the efficiency of multi-junction cells.

FB-2:IL03  Efficient Solar Cells and Water Reduction with Nanowires
D. VAN DAM, YINGCHAO CUI, A. STANDING, S. ASSALI, LU GAO, M.A. VERHEIJEN, P.H.L. NOTTEN, J.E.M. HAVERKORT, E. HENSEN, E.P.A.M. BAKKERS, Eindhoven University of Technology, Eindhoven, The Netherlands

Semiconducting nanowires have several appealing advantages for the fabrication of efficient solar cells and for the reduction of water. In this talk I will discuss recent results from our group on nanowire solar cells. By optimizing the in- and outcoupling of light we can enhance both the short circuit current and the open circuit potential with respect to bulk geometry, reaching an efficiency of 17.8%. Similar structures can be used for water reduction. By tuning the dopant profile, carrier recombination can be suppressed resulting in unprecedented photocathodic power-saved efficiency of 15.8%.

Session FB-3 - Organic, Dye Sensitised and Nanoparticle Photovoltaics

FB-3:IL01  Design of Molecular Donor and Acceptor Materials for Organic Solar Cells
P. BLANCHARD, MOLTECH-Anjou, UMR CNRS 6200, University of Angers, Angers, France

π-Conjugated molecules containing electron-donating (e-D) and electron-accepting (e-A) building-blocks are a focus of considerable interest as donor as well as acceptor materials in organic photovoltaics (OPV).1,2  When used as donor materials in Bulk Heterojunction Organic Solar Cells in combination with PC71BM as acceptor, these types of molecules have shown Power Conversion Efficiencies (PCEs) up to 11 %.3  On the other hand, non-fullerene acceptor materials3 based on π-conjugated molecules combining e-D and e-A building-blocks, can now surpass the photovoltaic performance of fullerene derivatives giving rise to PCEs as high as 13 %.4  However, these π-extended molecules of sophisticated design are often prepared in several synthetic steps increasing their cost and limiting their potential for large scale production.5  In this context, this presentation will show that more simple and accessible small π-conjugated molecules can be used either as donor or acceptor material for OPV.
1. a) Mishra, A. et al. Angew. Chem. Int. Ed. 2012, 51, 2020. b) Roncali, J. et al. Adv. Mater. 2014, 26, 3821. 2. Chen, W. et al. J. Mater. Chem. C 2017, 5, 1275. 3. Deng, D. et al. Nat. Commun. 2016, 7, 13740. 4. Zhao, W. et al. J. Am. Chem. Soc. 2017, 139, 7148. 5. Po, R. et al. J. Mater. Chem. C 2016, 4, 3677.

FB-3:L03  Improve the Photo-stability by Controling the Chemical Structures of Photoactive Materials for Polymer Solar Cells
VU VAN DOAN1, 2, QUOC VIET HOANG1, 2, RASOOL SHAFKET1, 2, CHANG EUN SONG1, 2, WON SUK SHIN1, 2, 1Energy Materials Research Center, Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon, South Korea; 2Department of Advanced Materials and Chemical Engineering, University of Science and Technology (UST), Daejeon, South Korea

To commercialize the photoactive materials for polymer solar cells many conditions should be satisfied such as cell efficiency should be well conveyed to module scale, process condition should be applicable to large area coating, and stability of the device should be confirmed at sun light-exposed conditions. So many photoactive materials with high power conversion efficiency were reported, but most of them are not adequate for these requirements. On these requirements, obtain the satisfactory photo-stability and reduce the burn-in loss during exposed on sun light are the biggest challenges to go to the market. Lost more than 40% of their initial efficiency in just 24 hours after exposing to 1 sun condition is common for polymer solar cell devices. This burn-in loss does not come from the decomposition of the photoactive materials but rather caused by the electro- and physical-properties of photoactive materials. This presentation will include our approach to overcome these hurdles through designing and synthesizing new photoactive materials.

FB-3:L06  Development of New Narrow Bandgap π-Conjugated Small Molecules for Organic Solar Cells
SEIICHI FURUKAWA, H. KOMIYAMA, T. YASUDA, Kyushu University, Fukuoka, Japan

Organic solar cells (OSCs) based on π-conjugated molecules as an electron donor and fullerene derivatives as an electron acceptor have paid much attention over the past decade. Recently, high power conversion efficiencies (PCEs) have been reported for some polymers possessing fluorine groups as substituents, and several explanations for the substituent effect have been proposed. In this research, we designed and synthesized a series of narrow-bandgap π-conjugated small molecules based on benzodithiophene central units functionalized with several types of halogenated end groups including fluorine and chlorine as donor materials for systematically studying their structure–property relationship in OSCs. The terminal halogenation induced dipole–dipole interaction between the molecules, resulting in better molecular packing and improved carrier mobility. With such proper molecular engineering, the bulk heterojunction OSCs based on the halogenated molecules as donor materials and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor material demonstrated enhanced fill factors compared to that obtained with a non-halogenated one. Among this series of donor materials, the monochloro-substituted donor material gave high fill factors of up to 67%, and achieved PCEs of 6.5%.

Session FB-4 - Multiple Energy Level Devices

FB-4:IL01  Two Step Photon Absorption in III-V Solar Cells
V. TASCO, A. PASSASEO, CNR-Nanotec, Nanotechnology Institute, Campus Ecotekne, Lecce, Italy; A. CRETI', M. LOMASCOLO, IMM-CNR Institute for Microelectronic and Microsystems, Campus Ecotekne, Lecce, Italy

For practical demonstration of Intermediate Band Solar Cells(IBSC) operation, a critical issue is the thermodynamic requirement for the Two Step-Two Photon Absorption(TS-TPA), involving two sub-bandgap distinct photons[1]. For IBSCs based on III-V quantum dots(QD), clear evidence of photon absorption promoting electrons transition from the valence band to the intermediate band can be easily provided by quantum efficiency measurements. On the other hand carrier thermal escape, field assisted tunneling, fast radiative recombination and other dynamics compete with the second photon absorption, hindering the achievement of high conversion efficiency IBSCs. In this contribution, we will discuss the mutual competition between hole and electron carrier generation/escape mechanisms and TS-TPA in state-of-art IBSC based on InAs/GaAs QDs[2], highlighting the importance of charge spatial separation to preserve the TS-TPA efficiency up to room temperature. Moreover we will show how, by exploiting nanoscale engineering of the band structure, also involving 2D confinement states[3], additional inter-level filling processes are enabled, enhancing the IBSC design flexibility.
[1] Adv Mater 22, 160(2010). [2] Appl Phys Lett 108, 063901 (2016). [3] Sol. Energy Mater. Sol. Cells 171, 142–147(2017).

FB-4:IL02  Recent Advances in Intermediate Band Solar Cells
A. MARTÌ, J. VILLA, E. ANTOLÍN, P.G. LINARES, C.TABLERO, A.LUQUE, Instituto de Energía Solar, Universidad Politécnica de Madrid, Madrid, Spain; I. RAMIRO, ICFO-Institut de Ciències Fotòniques, Barcelona, Spain; E. LOPEZ, Fraunhofer-Institut für Solare Energiesysteme ISE Freiburg, Germany

Intermediate band solar cells (IBSCs) promise photovoltaic conversion efficiency above that of single gap solar cells thanks to the absorption of below-bandgap energy photons. The principles of operation of these solar cells (absorption of below-bandgap energy photons and output voltage exceeding photon energy/electron charge) have been demonstrated in quantum dot (QD) systems but their actual impact in increasing the solar cell efficiency is minimal. Recently, and surprisingly, the operation of QD-IBSCs based on hole confinement (instead of electron confinement) have also been demonstrated.  In this presentation we will review our present understanding of the operation of these cells in an attempt to stimulate discussion about what are the next steps that in research should be taken. Discussion will not be restricted to QD systems but to other systems suggested to implement IBSCs such as, for example, those based on the insertion of impurities or those exploiting the band anti-crossing mechanism. A review of some of the most recent proposals for implementing these solar cells will also be included.

Session FB-5 - Excited State Enhanced Solar Cells

FB-5:IL01  Photothermoelectric Energy Harvesting and Light Detection in Heterostructure Nanowires
H. LINKE, NanoLund, Lund University, Lund, Sweden

In photo-thermoelectrics, incoming energy from light is converted into heat and then into electricity using the thermoelectric effect. A key advantage of using this effect is the possibility to directly harvest heat from photo-excited, hot carriers, thereby eliminating the phonon-mediated heat leaks that typically limit the efficiency of traditional thermoelectrics. Indeed, the carrier temperature of photogenerated carriers in nanowires has been observed to be substantially higher than that of the lattice, allowing for high, local temperature differentials. We demonstrate this approach by using heterostructure InAs/InP nanowires, employing an InP barrier as a thermionic energy filter. Under illumination of the full wire we demonstrate a phototermoelectric effect with high open-circuit voltage, consistent with a large carrier temperature difference across the thermionic barrier, and novel functionality in photodetection. We will also discuss these observations in terms of hot-carrier photovoltaics.
Limpert et al., Nano Lett., 17 (7), pp 4055–4060 (2017) Limpert et al., Nanotechnology 28 434001 (2017)

FB-5:IL02  Nanowires for Tandem Junction Solar Cells
M.T. BORGSTRÖM, Solid State Physics, Lund University, Lund, Sweden

Semiconducting nanowires have been recognized as promising materials for high-performance electronics and optics where optical and electrical properties can be tuned individually, where the nanowires due to excellent light absorbing properties [1] have been suggested for future high efficiency solar cells [2, 3]. Especially, the geometrical shape of the NWs offers excellent light absorption. In order to further optimize the performance of NWPV, and integrate them on Si in a tandem junction configuration, nanowires with dimensions corresponding to optimal light harvesting capability are necessary. We developed nano imprint lithography for patterning of catalytic metal particles with a diameter of 200 nm in a hexagonal pitch of 500 nm, for which synthesis was redeveloped since the metal particles were found to move during annealing, destroying pattern fidelity before nucleation. We found that a pre anneal and nucleation step was necessary to keep the particles in place during high temperature annealing to remove surface oxides. We intend to transfer these grown nanowires to a Si platform either by direct growth on Si PV, or by nanowire peel off in polymer, followed by transfer and electrical contacting, or by aerotaxy and alignment for transfer to Si.

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