Hybrid Organic-inorganic Solar Cells Research Articles

(Invited) Organic-Inorganic Hybrid Solar Cells with Antimony Sulfide-Metal Composites

Organic-inorganic hybrid solar cells composed of glass/F-doped SnO2/TiO2/La-decorated or Zn-doped Sb2S3/ poly[(3-hexylthiophene)-2,5-diyl]/poly[3-(3-carboxypropyl)thiophene-2,5-diyl]/Au have been prepared and evaluated their photovoltaic performance. Using La-decorated Sb2S3 for hybrid solar cell resulted in 33% improvement of power conversion efficiency (PCE) as compared to that of the non-decorated Sb2S3 because of lowering the resistance at the interface, which was confirmed by the impedance analyses. It was found that the added La was phase-separated and accumulated between the layers of TiO2 and Sb2S3. While doping of Zn into Sb2S3 resulted in the enlargement of the crystallite size of Sb2S3 from 14 nm to 25 nm. Utilization of Zn-doped Sb2S3 for hybrid solar cell attained 57% enhancement of PCE. This result is ascribed to the effective lowering the bulk-resistance.

High-Performance Ultrathin Organic-Inorganic Hybrid Silicon Solar Cells via Solution-Processed Interface Modification.

Organic-inorganic hybrid solar cells based on n-type crystalline silicon and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) exhibited promising efficiency along with a low-cost fabrication process. In this work, ultrathin flexible silicon substrates, with a thickness as low as tens of micrometers, were employed to fabricate hybrid solar cells to reduce the use of silicon materials. To improve the light-trapping ability, nanostructures were built on the thin silicon substrates by a metal-assisted chemical etching method (MACE). However, nanostructured silicon resulted in a large amount of surface-defect states, causing detrimental charge recombination. Here, the surface was smoothed by solution-processed chemical treatment to reduce the surface/volume ratio of nanostructured silicon. Surface-charge recombination was dramatically suppressed after surface modification with a chemical, associated with improved minority charge-carrier lifetime. As a result, a power conversion efficiency of 9.1% was achieved in the flexible hybrid silicon solar cells, with a substrate thickness as low as ∼14 μm, indicating that interface engineering was essential to improve the hybrid junction quality and photovoltaic characteristics of the hybrid devices.

Embedded Metal Electrode for Organic-Inorganic Hybrid Nanowire Solar Cells.

We demonstrate here an embedded metal electrode for highly efficient organic-inorganic hybrid nanowire solar cells. The electrode proposed here is an effective alternative to the conventional bus and finger electrode which leads to a localized short circuit at a direct Si/metal contact and has a poor collection efficiency due to a nonoptimized electrode design. In our design, a Ag/SiO2 electrode is embedded into a Si substrate while being positioned between Si nanowire arrays underneath poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), facilitating suppressed recombination at the Si/Ag interface and notable improvements in the fabrication reproducibility. With an optimized microgrid electrode, our 1 cm2 hybrid solar cells exhibit a power conversion efficiency of up to 16.1% with an open-circuit voltage of 607 mV and a short circuit current density of 34.0 mA/cm2. This power conversion efficiency is more than twice as high as that of solar cells using a conventional electrode (8.0%). The microgrid electrode significantly minimizes the optical and electrical losses. This reproducibly yields a superior quantum efficiency of 99% at the main solar spectrum wavelength of 600 nm. In particular, our solar cells exhibit a significant increase in the fill factor of 78.3% compared to that of a conventional electrode (61.4%); this is because of the drastic reduction in the metal/contact resistance of the 1 μm-thick Ag electrode. Hence, the use of our embedded microgrid electrode in the construction of an ideal carrier collection path presents an opportunity in the development of highly efficient organic-inorganic hybrid solar cells.

Exploring electronic and optical properties of CH3NH3GeI3 perovskite: Insights from the first principles

The three phases for MAGeI3 in different temperatures play an important role in hybrid organic-inorganic perovskites solar cells. Up to now, their intrinsic nature still lacks systematic study. In this work, we established the crystal structures of MAGeI3 (MA+=CH3NH3+) with orthorhombic, tetragonal and cubic phases. The electronic and optical properties of MAGeI3 with three crystal structures including band gaps, density of states (DOS), absorption spectrum and dielectric function are calculated using the density functional theory (DFT). The calculated results reveal that MAGeI3 pervoskites show substantial stability, a similar band gap, high carrier mobility and outstanding optical properties compared with MAPbI3. The electronic properties are computed by GGA+U, demonstrating that the band gaps of MAGeI3 for orthorhombic, tetragonal and cubic phases are 1.60eV, 1.58eV and 1.54eV, respectively. The calculated results are generally in accord with available reported experimental data. Since the band gaps of MAGeI3 are appropriate, the absorption region covers almost the entire visible zone. Our work not only uncover screening new materials for solar cells, but also further understand the inherent properties of MAGeI3.

(Invited) Organic-Inorganic Hybrid Solar Cells with Antimony Sulfide-Metal Composites

Since antimony sulfide (Sb2S3) has much attraction for photovoltaics in terms of appropriate bandgap, high absorption coefficient, low toxicity, and abundance, it has been recently applied for organic-inorganic hybrid solar cells, which has been attained the power conversion efficiency (PCE) of 7.5%.[1] We also have prepared Sb2S3-based solar cells with TiO2 or ZnO nanoparticles for electron transporting layer in addition to poly(3-hexylthiophene)-2,5-diyl (P3HT)/ (3, 4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS), zinc phthalocyanine (ZnPc), or MoO3 for hole transporting layer and compared their photovoltaic performance. Among these hybrid solar cells, it was found that the combination with ZnPc and TiO2 has the highest durability with keeping the relative PCE of 90% after the stability test under 1 sun at 63 °C at a relative humidity of 50% for 1,500 h.[2] For further improvement of the photovoltaic performance, we newly added La or Zn into the precursor of Sb2S3 to prepare Sb2S3–metal composites and applied them for organic-inorganic hybrid solar cells. Remarkable enhancement of the open-circuit voltage for glass-FTO/TiO2/Sb2S3 + La/P3HT/poly[(3-carboxybutyl)thiophene-2,5-diyl] (P3CT)/Au was observed after the addition of La to Sb2S3 and 33% improvement of PCE was attained. Impedance analyses of the hybrid solar cells with or without La revealed that the resistance at the interface was effectively lowered through the addition of La though the bulk-resistance had not been changed. It was found that the added La was phase-separated and accumulated between the layers of TiO2 and SbS3 through the TEM-EDX observations of the cross-sectional views of the La-added device. In this context, La was not doped into Sb2S3 but inserted into the interface of TiO2 and SbS3. On the contrary, addition of Zn into the Sb2S3 precursor resulted in the enlargement of the crystalline size of the Sb2S3 from 14 nm to 25 nm, which was estimated from XRD peaks. No phase-separation was confirmed by TEM observations of the cross-sectional views. Therefore, it was concluded that the Zn was doped into Sb2S3. By using the Zn-doped Sb2S3, remarkable increment of the current density was observed in addition to relatively high fill factor and 57% improvement of PCE. These results are ascribed to the effective lowering the bulk-resistance, which was confirmed by the impedance analyses. [1] J. A. Chang, J. H. Rhee, S. H. Im, Y. H. Lee, H. J. Kim, S. I. Seok, M. K. Nazeeruddin, and M. Graetzel, Nano Lett., 2010, 10, 2609–2612. [2] A. Hayakawa, M. Yukawa, and T. Sagawa, ECS Trans., ahead of print.

Comparison of TiO 2 Nanoporous Films in Hybrid Organic-inorganic Solar Cells

Abstract Hybrid metal-oxide: polymer photovoltaic devices offer a promising cheap alternative to conventional photovoltaics and include the advantages of both the inorganic metal-oxide and organic polymer. The polymer functions as the light absorbing and the hole transporting material, while the metal-oxide functions as the electron acceptor and electron transporting material. A main approach to fabricating this type of solar cell involves forming a connected metal-oxide semiconductor layer by sintering metal-oxide nanoparticles to form a randomly arranged nanostructure, and then including the polymer. However several factors limit the performance of this type of solar cell and the need to optimise the structure and the materials of the device has been highlighted in many studies. This work investigates nanoporous TiO2:P3HT solar cells and compares devices fabricated from a commonly employed TiO2 nanoparticle paste (DSL-18NRT) to devices fabricated from a larger particle novel TiO2 nanoparticle paste (DSL-30NRD). Particle size and the resulting pore size of the TiO2 nanostructure is an important factor in improving device performance, as the difficulty of polymer infiltration into the nanostructure is one of the major limiting factors of the performance in this type of hybrid solar cell. With the larger particle paste it was possible to increase the device thickness without compromising device parameters. Multilayer structures, including interface modifying layers such as Z907 dye molecule were investigated for the larger particle photovoltaic device. With interface modification we obtained 0.53% power conversion efficiency in devices fabricated from these larger nanoparticles. With thickness optimisation we obtained a relatively high JSC of 1.46 mA/cm2 for standard TiO2:P3HT devices.

Zero-dimensional methylammonium iodo bismuthate solar cells and synergistic interactions with silicon nanocrystals.

Organometal trihalide perovskite solar cells have attracted monumental attention in recent years. Today's best devices, based on a three-dimensional perovskite structure of corner-sharing PbI6 octahedra, are unstable, toxic, and display hysteresis in current-voltage measurements. We present zero-dimensional organic-inorganic hybrid solar cells based on methylammonium iodo bismuthate (CH3NH3)3(Bi2I9) (MABI) comprising a Bi2I9 bioctahedra and observe very low hysteresis for scan rates in the broad range of 150 mV s-1 to 1500 mV s-1 without any interfacial layer engineering. We confirm good stability for devices produced and stored in open air without humidity control. The MABI structure can also accommodate silicon nanocrystals, leading to an enhancement in the short-circuit current. Through the material MABI, we demonstrate a promising alternative to the organometal trihalide perovskite class and present a model material for future composite third-generation photovoltaics.

Formation of zinc oxide nanostructures by wet oxidation of vacuum deposited Zn thin film

Zinc oxide (ZnO) nanostructures with various morphologies (pencil-like nanorods, nanotubes, and lotus-like structures) have been successfully formed by simple oxidation of vacuum deposited Zn thin film on glass/ITO substrates in hot water at 90 °C. The morphology evolved from pencil-like nanorods to nanotubes to lotus-like structures with prolonged oxidation of Zn thin film for 6–24 h. The change in morphology of ZnO is attributed to the combined effects of electrochemical reactions in the solution, morphology and structure of Zn thin film, and wurtzite structure of resulting ZnO. A hybrid organic–inorganic solar cell following an inverted bulk heterojunction configuration was fabricated based on the lotus-like ZnO structures. The solar cell achieved a power conversion efficiency of up to 1.18%, which demonstrates the applicability of the technique for photovoltaic applications.

Photovoltaic performance of TCVA-InSe hybrid solar cells based on nanostructure films

Hybrid organic-inorganic solar cells of the configuration ITO/InSe/TCVA/Au have been fabricated using thermal evaporation technique. The electrical transport mechanisms of the manufactured cell were studied by performing the capacitance-voltage (C-V) and the current density-voltage (J-V) estimations. The TCVA/InSe cell displayed rectifying behavior with a rectification ratio of 157 at ±1V. At the lower voltages, the dark current density, J, is ruled by the thermionic emission instrument. While a space charge-limited-conduction mechanism was operated at a higher voltage. The fabricated cell demonstrates a significant improvement in the short circuit photocurrent density and the fill factor by including InSe layer, leading to nearly a nine-fold improvement in the conversion efficiency. The temperature dependent on photovoltaic parameters was also investigated in the temperature range of 303–373K. It found that the power conversion efficiency was increased with the increasing temperature.

Efficiency enhancement of organic solar cells by inserting PbS quantum dots film as the infrared absorption layer

Organic materials show great potentials in high-efficiency organic solar cells, however, few organic materials can efficiently utilize the near infrared light due to their relative large band-gap. Here, we demonstrate quantum dots-based hybrid solar cells by inserting PbS nanocomposite film as the infrared absorption layer, resulting in a significant enhancement in short-circuit current. After appropriate ligands exchange, the device with PbS absorption layer showed an improved power conversion efficiency (PCE) of 3.08% under AM 1.5 solar illumination, amounting to 91% enhancement over that of the pure organic multilayer structure solar cell indium-tin oxide (ITO)/poly(3-hexylthiophene) (P3HT)/ polymer-[6,6]-phenyl-C61-butyric acidmethyl ester (PCBM)/Ca/Al. Therefore, this approach can be applied to a wide range of quantum dots and polymer hybrids for its solution-processing, thereby offering a general scheme to improve the efficiency of organic–inorganic hybrid solar cells and it provides a promising way to make high-efficiency solar cells.

Vertically aligned ZnCdS nanowire arrays/P3HT heterojunctions for solar cell applications

Nowadays, solid-state inorganic-organic hybrid solar cells based on one-dimensional (1D) inorganic semiconducting nanostructures and organic polymers are believed to offer convincing solutions for the realm of next generation solar cells. In this regard, 1D ZnCdS nanowire (NW) arrays were fabricated on transparent conducting substrates through a catalyst free co-evaporation method and their wurtzite structural characteristics, 1D morphological layout and valence state/composition were studied in detail using X-ray diffraction, high-resolution electron microscopy and X-ray photoelectron spectroscopy, respectively. The existence of deep level traps and optical band gap of ZnCdS NWs were additionally studied using room-temperature cathodoluminescence and UV–vis absorbance measurements. The inorganic-organic hybrid cells were then fabricated using these NWs via spin coating poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) based polymers. The morphological dissemination of the polymer deposits on NWs were also studied individually by electron microscopy. The solar cell (J-V) characteristics of the fabricated architectures were investigated at room-temperature and as a function of temperature and different intensities of incident light irradiation. The trap energy of the devices was noted to decrease from 68.1 to 40.7eV, suggesting the active role of trap sites that could have originated from the surface defects and other structural disorders across the hybrid heterostructures.

Ion-exchange synthesis and improved photovoltaic performance of CdS/Ag2S heterostructures for inorganic-organic hybrid solar cells

A facile ultrasound-assisted ion exchange route was developed for the synthesis of CdS/Ag2S heterojunctions by ion exchange between the nanostructured CdS film and [Ag(NH3)2]+ under ultrasonication. The CdS/Ag2S heterojunction film was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis DRS spectroscopy, photoelectrochemical measurements, and the transient photovoltage (TPV) technique. CdSAg2S heterojunctions exhibit a dense morphology, enhanced visible light absorption and stronger photocurrent response than the pure CdS films. Poly(3-hexylthiophene) (P3HT) was then spin coated into the CdS/Ag2S framework. Hybrid solar cells constructed with FTO/CdS/Ag2S/P3HT/Au display relatively higher power conversion efficiency than FTO/CdS/P3HT/Au.

Hybrid Solar Cell with TiO2 Film: BBOT Polymer and Copper Phthalocyanine as Sensitizer

An organic-inorganic hybrid solar cell was fabricated using Titanium dioxide (TiO2): 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (BBOT) film and Copper Phthalocyanine (CuPc) as a sensitizer. BBOT was used in photodetector in other reported research works, but as per best of our knowledge, it was not implemented in solar cells till date. The blend of TiO2: BBOT blend was used to fabricate the film on ITO-coated glass and further a thin layer of CuPc was coated on the film. This was acted as photoanode and another ITO coated glass with a platinum coating was used as a counter electrode (cathode). An optimal blend of acetonitrile (solvent) (50-100%), 1,3-dimethylimidazolium iodide (10-25%), iodine (2.5-10%) and lithium iodide, pyridine derivative and thiocyanate was used as electrolytes in the hybrid solar cell. The different structural, optical and electrical characteristics were measured. The Hybrid solar cell showed a maximum conversion efficiency of 6.51%.

Induced Infiltration of Hole-Transporting Polymer into Photocatalyst for Staunch Polymer-Metal Oxide Hybrid Solar Cells.

For efficient solar cells based on organic semiconductors, a good mixture of photoactive materials in the bulk heterojunction on the length scale of several tens of nanometers is an important requirement to prevent exciton recombination. Herein, we demonstrate that nanoporous titanium dioxide inverse opal structures fabricated using a self-assembled monolayer method and with enhanced infiltration of electron-donating polymers is an efficient electron-extracting layer, which enhances the photovoltaic performance. A calcination process generates an inverse opal structure of titanium dioxide (<70 nm of pore diameters) providing three-dimensional (3D) electron transport pathways. Hole-transporting polymers was successfully infiltrated into the pores of the surface-modified titanium dioxide under vacuum conditions at 200 °C. The resulting geometry expands the interfacial area between hole- and electron-transport materials, increasing the thickness of the active layer. The controlled polymer-coating process over titanium dioxide materials enhanced photocurrent of the solar cell device. Density functional theory calculations show improved interfacial adhesion between the self-assembled monolayer-modified surface and polymer molecules, supporting the experimental result of enhanced polymer infiltration into the voids. These results suggest that the 3D inverse opal structure of the surface-modified titanium dioxide can serve as a favorable electron-extracting layer in further enhancing optoelectronic performance based on organic or organic-inorganic hybrid solar cell.

(Invited) Intramolecular Exchanging Crystallization for Formamidinium Lead Iodide Perovskite Solar Cells

Inorganic-organic hybrid solar cells using perovskite metal-halides such as methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3) have great potential for future photovoltaic systems in terms of high efficiency and low-cost. MAPbI3 had opened the field of perovskite solar cells, but FAPbI3 is recently attracting considerable attention for high efficiency exceeding 20 % and superior durability of the perovskite solar cell. However, FAPbI3 had been limited to utilize as a photovoltaic material due to its polymorph of non-perovskite phase and difficulty to make high quality film in solution process. Recently we reported that perovskite phase of FAPbI3 can be stabilized by compositional engineering of FAPbI3 with incorporation of MAPbBr3. In addition, we showed that intramolecular exchanging process successfully offers a way to make high quality FAPbI3 based perovskite film using dimethyl sulfoxide (DMSO) as a mediator to control crystallization of perovskite. The exchange of FAI with DMSO in pre-deposited PbI2(DMSO) film leads to crystallize into perovskite film with small volume expansion, preferred crystallographic orientation, and large grains, yielding to high power conversion efficiency exceeding 20 %. In this talk, FAPbI3 based perovskite solar cells will be discussed in terms of the compositional engineering and intramolecular exchanging process.

Hydrothermal synthesis of alpha- and beta-HgS nanostructures

We synthesized HgS nanostructures by the hydrothermal method in order to use them as electron acceptors in hybrid organic-inorganic solar cells. We employed different mercury sources (HgO and Hg(CH3COO)2) and polyvinylpyrrolidone (PVP) or hexadecanethiol (HDT) as stabilizing/capping agent for controlling size, crystallinity, morphology and stability of the obtained nanostructures. We also used thiourea as sulfur source, and a temperature of 180°C during 6h. Synthesized nanostructures were characterized by powder X-Ray Diffraction, Diffuse Reflectance Infrared Fourier Transform and Transmission Electron Microscopy. When PVP acts as stabilizing agent, the mercury source has influence on the size -but not in morphology- of the beta-HgS obtained nansostructures. HDT has control over nanostructures’ size and depending on the relation Hg:HDT, we obtained a mixture of alpha and beta HgS which can be advantageous in the application in solar cells, due their absorption in different spectral regions. The smallest nanostructures obtained have a mean diameter of 20nm when using HDT as capping agent. Also, we deposited the aforementioned nanostructures onto flat glass substrates by the spin coating technique as a first approach of an active layer of a solar cell. The depositions were characterized by atomic force microscopy. We obtained smaller particle deposition and higher particle density -but a lower area coverage (5%) – in samples with HDT as capping agent. This work presents promising results on nanostructures for future application on hybrid solar cells. Further efforts will be focused on the deposition of organic-inorganic layers.

On the Thermal and Thermodynamic (In)Stability of Methylammonium Lead Halide Perovskites.

The interest of the scientific community on methylammonium lead halide perovskites (MAPbX3, X = Cl, Br, I) for hybrid organic-inorganic solar cells has grown exponentially since the first report in 2009. This fact is clearly justified by the very high efficiencies attainable (reaching 20% in lab scale devices) at a fraction of the cost of conventional photovoltaics. However, many problems must be solved before a market introduction of these devices can be envisaged. Perhaps the most important to be addressed is the lack of information regarding the thermal and thermodynamic stability of the materials towards decomposition, which are intrinsic properties of them and which can seriously limit or even exclude their use in real devices. In this work we present and discuss the results we obtained using non-ambient X-ray diffraction, Knudsen effusion-mass spectrometry (KEMS) and Knudsen effusion mass loss (KEML) techniques on MAPbCl3, MAPbBr3 and MAPbI3. The measurements demonstrate that all the materials decompose to the corresponding solid lead (II) halide and gaseous methylamine and hydrogen halide, and the decomposition is well detectable even at moderate temperatures (~60 °C). Our results suggest that these materials may be problematic for long term operation of solar devices.

Glass Texturing Affects Optical Properties of Perovskite Solar Cells: Comparison Study between Mesoscopic and Planar Structure

Glass texturing is an effective method for changing the surface morphology to improve light trapping. In this paper, the technique is applied to hybrid inorganic-organic solar cells (perovskite solar cell) to enhance light trapping and current density. We optimized wet chemical etched fluorine doped tin oxide (FTO) coated glass substrates using a simple treatment then compared the mesoscopic and planar structures. Thus, we present the implication study of the wet etching process with a random pattern surface that enables punch light scattering and ideally balanced the optical and electrical properties of the perovskite solar cell. As a result, we achieved short-circuit current density for 26.5 mA/cm2 and PCE for 15.2 %.

Time-resolved photocurrent of an organic-inorganic hybrid solar cell based on Sb2S3

In this study, the optical and the electrical properties of a hybrid solar cell (SC) based on a zinc oxide (ZnO)/antimony trisulfide (Sb2S3)/poly(3-hexylthiophene) (P3HT) heterojunction were investigated. ZnO and Sb2S3 films were grown using atomic layer deposition (ALD). Photoluminescence (PL) spectra show band-to-band emission of the Sb2S3 layer at around 724 nm at room temperature. The room-temperature current decay behaviors were investigated based on the time-resolved photocurrent (TRPC). With increasing intensity (P) of the excitation pulse laser, the current decay time gradually decreased due to an enhancement of the carrier recombination. Optically biased TRPC was demonstrated with energies above and below the band gap of Sb2S3 to investigate the effect of the background carrier concentration during carrier transport. We found three carrier exhaustion processes during carrier transport due to the recombination behaviors at given carrier injection levels. At high injection level, the current decay times were also affected by the hole diffusion rate in the P3HT h-conducting layer.

Interface-Dependent Ion Migration/Accumulation Controls Hysteresis in MAPbI3 Solar Cells

Hysteresis in the current-voltage characteristics of hybrid organic-inorganic perovskite-based solar cells is one of the fundamental aspects of these cells that we do not understand well. One possible cause, suggested for the hysteresis, is polarization of the perovskite layer under applied voltage and illumination bias, due to ion migration within the perovskite. To study this problem systemically current-voltage characteristics of both regular (light incident through the electron conducting contact) and so-called inverted (light incident through the hole conducting contact) perovskite cells were studied at different temperatures and scan rates. We explain our results by assuming that the effects of scan rate and temperature on hysteresis are strongly correlated to ion migration within the device, with the rate-determining step being ion migration at/across the interfaces of the perovskite layer with the contact materials. By correlating between the scan rate with the measurement temperature we show that the inverted and regular cells operate in different hysteresis regimes, with different activation energies of 0.28+-0.04 eV and 0.59+-0.09 eV, respectively. We suggest that the differences, observed between the two architectures are due to different rates of ion migration close to the interfaces, and conclude that the diffusion coefficient of migrating ions in the inverted cells is 3 orders of magnitude higher than in the regular cells, leading to different accumulation rates of ions near the interfaces. Analysis of VOC as a function of temperature shows that the main recombination mechanism is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar to what is the case for other thin film inorganic solar cells.