Organic-inorganic Solar Cells Research Articles

Hybrid Organic–Inorganic Solar Cells with Electrodeposited Al-Doped Zinc Oxide

Hybrid solar cells were fabricated using aluminum-doped zinc oxide (AZO) grown by electrochemical deposition from chloride electrolyte solutions with Al/Zn molar ratios of 0.5, 2.5, and 5.0%. The substrates were AZO- and ZnO-seeded ITO. Ordered nanorod structures with high optical transmittance were grown at 0.5% Al/Zn ratio while interconnected micron-sized flakes were grown at 2.5% and 5.0%. The estimated band gap energies increase for higher Al dopant content, showing Burstein-Moss effect. EDX analysis detected high aluminum content in the 5.0% samples suggesting that insulating aluminum oxide phases were formed thus causing reduced solar cell efficiencies. The highest power conversion efficiency of 1.71%, from the 0.5% sample grown on ZnO-seeded ITO, can be attributed to the presence of AZO nanorods which provide a large interfacial area and effective charge transport.

Lattice thermal conductivity of organic-inorganic hybrid perovskite CH3NH3PbI3

Great success has been achieved in improving the photovoltaic energy conversion efficiency of the organic-inorganic hybrid perovskite-based solar cells, but with very limited knowledge on the thermal transport in hybrid perovskites, which could affect the device lifetime and stability. Based on the potential field derived from the density functional theory calculations, we studied the lattice thermal conductivity of the hybrid halide perovskite CH3NH3PbI3 using equilibrium molecular dynamics simulations. Temperature-dependent thermal conductivity is reported from 160 K to 400 K, which covers the tetragonal phase (160–330 K) and the pseudocubic phase (>330 K). A very low thermal conductivity (0.59 W/m K) is found in the tetragonal phase at room temperature, whereas a much higher thermal conductivity is found in the pseudocubic phase (1.80 W/m K at 330 K). The low group velocity of acoustic phonons and the strong anharmonicity are found responsible for the relatively low thermal conductivity of the tetragonal CH3NH3PbI3.

Improvement of device performance by using zinc oxide in hybrid organic–inorganic solar cells

Zinc oxide (ZnO) nanopowder was applied to hybrid solar cells in combination with poly(3-hexylthiophene). Stability tests of the hybrid solar cell with or without encapsulation with glass and UV cut-off films were performed under 1 sun at 63 °C at a relative humidity of 50%. It was found that the sealed cell showed worse device performance in terms of the loss of the open-circuit voltage (Voc), whereas the unsealed cell exposed to air retained an almost constant Voc for more than 3 d under dark and atmospheric conditions. Placement in O2 atmosphere in the dark led to the recovery of Voc. Cation (Sn4+) doping into ZnO was performed, and the loss of Voc was effectively suppressed through the restraint of the supply of the excited electron from the valence band to the conduction band.

Ag–In–Zn–S quantum dots for hybrid organic–inorganic solar cells

Quantum dots of (AgIn)xZn2(1−x)S2 (x = 0.6, 0.8, and 1.0) capped by oleylamine were prepared and applied for hybrid organic–inorganic solar cells consisting of glass–indium–tin-oxide/ZnO/(AgIn)xZn2(1−x)S2/poly(3-hexylthiophene)/MoO3/Ag. The short-circuit current density (Jsc) and open-circuit voltage (Voc) of the hybrid solar cells were measured, and we found a low power conversion efficiency (PCE) below 0.1%. From the incident photon-to-current efficiency (IPCE) profiles of the hybrid devices, there is no marked photocurrent generation from 350 to 700 nm, which is ascribed to the absorption region of (AgIn)xZn2(1−x)S2. To improve the photovoltaic performance, ligand substitution from oleylamine to pyridine was performed. The PCE of the hybrid cell using the pyridine-capped (AgIn)xZn2(1−x)S2 was improved twofold in terms of both Jsc and Voc as compared with that of the oleylamine-capped one. In particular, from the IPCE measurements, a remarkable (more than doubled) enhancement of photocurrent generation from 400 to 450 nm was observed with the pyridine-substituted nanoparticles.

Perovskite solar cells fabricated using dicarboxylic fullerene derivatives

Perovskite solar cells were fabricated using a novel benzoic acid fullerene bis adduct material (BAFB). The BAFB material was found to be a promising material for use in perovskite hybrid organic–inorganic solar cells. The efficiency was reported to be 9.63% for perovskite solar cells.

Hybrid organic–inorganic solar cells based on bismuth iodide and 1,6-hexanediammonium dication

Heavy metal toxicity and device instability are prominent limitations in the push for commercialization of photovoltaics based on low-cost, solution-processed materials. Therefore, in this study a hybrid organic–inorganic material containing trivalent bismuth and dicationic 1,6-hexanediammonium was used as the photoactive layer in solution-processed photovoltaics.

Semiconducting organic-inorganic nanocomposites by intimately tethering conjugated polymers to inorganic tetrapods.

Semiconducting organic-inorganic nanocomposites were judiciously crafted by placing conjugated polymers in intimate contact with inorganic tetrapods via click reaction. CdSe tetrapods were first synthesized by inducing elongated arms from CdSe zincblende seeds through seed-mediated growth. The subsequent effective inorganic ligand treatment, followed by reacting with short bifunctional ligands, yielded azide-functionalized CdSe tetrapods (i.e., CdSe-N3). Finally, the ethynyl-terminated conjugated polymer poly(3-hexylthiophene) (i.e., P3HT-[triple bond, length as m-dash]) was tethered to CdSe-N3 tetrapods via a catalyst-free alkyne-azide cycloaddition, forming intimate semiconducting P3HT-CdSe tetrapod nanocomposites. Intriguingly, the intimate contact between P3HT and CdSe tetrapod was found to not only render the effective dispersion of CdSe tetrapods in the P3HT matrix, but also facilitate the efficient electronic interaction between these two semiconducting constituents. The successful anchoring of P3HT chains onto CdSe tetrapods was substantiated through Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy measurements. Moreover, the absorption and photoluminescence studies further corroborated the intimate tethering between P3HT and CdSe tetrapods. The effect of the type of bifunctional ligands (i.e., aryl vs. aliphatic ligands) and the size of tetrapods on the device performance of hybrid organic-inorganic solar cells was also scrutinized. Interestingly, P3HT-CdSe tetrapod nanocomposites produced via the use of an aryl bifunctional ligand (i.e., 4-azidobenzoic acid) exhibited an improved photovoltaic performance compared to that synthesized with their aliphatic ligand counterpart (i.e., 5-bromovaleric acid). Clearly, the optimal size of CdSe tetrapods ensuring the effective charge transport in conjunction with the good dispersion of CdSe tetrapods rendered an improved device performance. We envision that the click-reaction strategy enabled by capitalizing on two consecutive effective ligand exchanges (i.e., inorganic ligand treatment and subsequent bifunctional ligand exchange) to yield intimately connected organic-inorganic nanocomposites provides a unique platform for developing functional optoelectronic devices.

Comparing the performance of Organic-inorganic hybrid tandem multijunction solar cells of different organic bulk thicknesses

In this study, J-V curves of a-Si:H/PCPDTBT:PC70BM hybrid tandem solar cells were simulated using a modified drift-diffusion model, and the influence of the thickness of the organic blend layer was investigated. The results of the simulations were compared with experimental data from literature.It is shown that as the thickness of the blend layer increases, the fill factor and the voltage corresponding to maximum power point decrease whereas the maximum power point and the short circuit current density of solar cell increase up to thicknesses of 60 nm and 138 nm respectively. Finally, the modified organic solar cell was used as second sub-cell and the power conversion efficiency increased from 1.90% to 2.1% in simulation.

Improved Crystallization of Perovskite Films by Optimized Solvent Annealing for High Efficiency Solar Cell

Organic-inorganic halide perovskite-based thin film solar cells show excellent light-to-power conversion efficiency. The high performance for the devices requires the preparation of well-crystallized perovskite absorbers. In this paper, we used the postannealing process to treat the perovskite films under different solvent vapors and observed that the solvent vapors have a strong effect on the film growth. A model regarding the perovskite film growth was proposed as well. Intensive characterizations including scanning electron microscopy, electrochemical impedance spectroscopy, and admittance spectroscopy allowed us to attribute the improved performance to reduced recombination loss and defect density. Solar cell based on the DMSO-treated films delivered a power conversion efficiency of over 13% with negligible photocurrent hysteresis.

Junction formation and current transport mechanisms in hybrid n-Si/PEDOT:PSS solar cells.

We investigated hybrid inorganic-organic solar cells combining monocrystalline n-type silicon (n-Si) and a highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). The build-in potential, photo- and dark saturation current at this hybrid interface are monitored for varying n-Si doping concentrations. We corroborate that a high build-in potential forms at the hybrid junction leading to strong inversion of the n-Si surface. By extracting work function and valence band edge of the polymer from ultraviolet photoelectron spectroscopy, a band diagram of the hybrid n-Si/PEDOT:PSS heterojunction is presented. The current-voltage characteristics were analyzed using Schottky and abrupt pn-junction models. The magnitude as well as the dependence of dark saturation current on n-Si doping concentration proves that the transport is governed by diffusion of minority charge carriers in the n-Si and not by thermionic emission of majorities over a Schottky barrier. This leads to a comprehensive explanation of the high observed open-circuit voltages of up to 634 mV connected to high conversion efficiency of almost 14%, even for simple planar device structures without antireflection coating or optimized contacts. The presented work clearly shows that PEDOT:PSS forms a hybrid heterojunction with n-Si behaving similar to a conventional pn-junction and not, like commonly assumed, a Schottky junction.

μSR Study of Charge Carrier Motion in Active Layer P3HT:ZnO:PCBM Hybrid Solar Cells

The so-called hybrid (organic-inorganic) solar cell has been developed due to the combining advantage between organic material such as Poly(3-hexylthiohene)/P3HT and inorganic material such as zinc oxide (ZnO). By adding fullerene derivative (6,6-phenyl-C61-butyric acid methyl ester/PCBM) to the hybrid solar cell, the performance of solar cell is predicted to be increased. We have studied the charge carrier motion in blend of P3HT:ZnO:PCBM using LF-μSR with and without light irradiation to clarify the charge carrier motion in bulk ternary system of hybrid material. We found that charge carrier motion in P3HT:ZnO:PCBM changes from intra-chain to inter-chain diffusion. One-dimensional intra-chain diffusion is observed in the sample at low temperature below 15 K, while three-dimensional inter-chain is observed at high temperature above 25 K.

Optical, Structural and Morphological Properties of Ternary Thin Film Blend of P3HT:PCBM:ZnO Nanoparticles

Ternary blend film of conjugated polymer, fullerene and inorganic nanoparticles has intensively studied as active material for high power conversion efficiency (PCE) of hybrid organic-inorganic solar cells. The incorporation of two electron acceptor materials of organic fullerene and inorganic nanoparticles into hybrid with electron donor conjugated polymer is strongly believed can improve the PCE of solar cells by increasing exciton dissociation efficiency due to an increase of interface area between donor and acceptor materials where the positive and negative charges dissociated. We studied optical, structural and morphological properties of ternary thin film containing blend of conjugated polymer poly(3-hexylthiophene (P3HT):fullerene derivative PCBM:Zinc oxide nanoparticles (ZnO-NP) by measuring its optical absorption, crystal structure and film surface morphology. Zinc oxide nanoparticle was prepared by sol-gel method. It has optical absorption below 370 nm and average particle size 40 nm as shown by TEM picture. Ternary thin blend films of P3HT:PCBM:ZnO-NP were prepared by use of spin-coating method. The UV-Vis spectrum of thin film contains absorption peaks originated from contribution of P3HT at wavelengths 520 nm, 550 nm and 600 nm, from contribution of PCBM at 260 nm and 330 nm and from ZnO-NP at wavelengths below 370 nm which confirms that these three materials were well mixed in the films. Its XRD pattern also contains the peaks from each of these three-materials. In this report, we compare surface morphology of thin films of pure P3HT, pure ZnO-NP, blend of P3HT:PCBM, blend of P3HT:ZnO-NP and ternary blend of P3HT:PCBM:ZnO-NP.

Spatial Localization of Excitons and Charge Carriers in Hybrid Perovskite Thin Films

The fundamental photophysics underlying the remarkably high-power conversion efficiency of organic-inorganic hybrid perovskite-based solar cells has been increasingly studied using complementary spectroscopic techniques. However, the spatially heterogeneous polycrystalline morphology of the photoactive layers owing to the presence of distinct crystalline grains has been generally neglected in optical measurements; therefore, the reported results are typically averaged over hundreds or even thousands of such grains. Here we apply femtosecond transient absorption microscopy to spatially and temporally probe ultrafast electronic excited-state dynamics in pristine methylammonium lead tri-iodide (CH3NH3PbI3) thin films and composite structures. We found that the electronic excited-state relaxation kinetics are extremely sensitive to the sample location probed, which was manifested by position-dependent decay time scales and transient signals. Analysis of transient absorption kinetics acquired at distinct spatial positions enabled us to identify contributions of excitons and free charge carriers.

A generic concept to overcome bandgap limitations for designing highly efficient multi-junction photovoltaic cells.

The multi-junction concept is the most relevant approach to overcome the Shockley–Queisser limit for single-junction photovoltaic cells. The record efficiencies of several types of solar technologies are held by series-connected tandem configurations. However, the stringent current-matching criterion presents primarily a material challenge and permanently requires developing and processing novel semiconductors with desired bandgaps and thicknesses. Here we report a generic concept to alleviate this limitation. By integrating series- and parallel-interconnections into a triple-junction configuration, we find significantly relaxed material selection and current-matching constraints. To illustrate the versatile applicability of the proposed triple-junction concept, organic and organic-inorganic hybrid triple-junction solar cells are constructed by printing methods. High fill factors up to 68% without resistive losses are achieved for both organic and hybrid triple-junction devices. Series/parallel triple-junction cells with organic, as well as perovskite-based subcells may become a key technology to further advance the efficiency roadmap of the existing photovoltaic technologies.

Flexible Graphene Electrode-Based Organic Photovoltaics with Record-High Efficiency

Flexible organic and hybrid organic-inorganic solar cells require both the flexible photoactive media and flexible electrodes with good conductivity and transparency. Graphene has been proposed as a promising flexible electrode due to its mechanical and chemical robustness, excellent electrical and optical properties, and potentially low-cost processing. We have designed graphene electrode-based flexible polymer solar cells with thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) by resolving the issues occurring at the interface between graphene and charge transporting semiconducting materials. We demonstrate highly efficient flexible devices with power conversion efficiencies of 6.1% (anode) and 7.1% (cathode) achieved via thermal treatment of molybdenum trioxide (MoO3) electron blocking layer, direct deposition of ZnO electron transporting layer on graphene. The advances in this work demonstrate graphene has great potential as a promising transparent electrode in a variety of flexible optoelectronic devices.

Solution processed TiO2/BiFeO3/poly(3-hexylthiophene) solar cells

We report the fabrication of solar cells by spin-coating and annealing sol–gel layers of TiO2 and BiFeO3 on indium oxide coated glass substrates. A layer of poly(3-hexylthiophene) was spin coated onto the inorganic layers to form hybrid organic–inorganic solar cells. X-ray powder diffraction revealed the BiFeO3 layers are polycrystalline with rhombohedral texture and average crystallite size about 20nm. Scanning electron microscopy images showed the films are granular and microporous. Irradiation with visible light resulted in a photocurrent of 0.05mAcm−2 and an open circuit voltage of −0.46V.

Synthesis of Co-Electrospun Lead Selenide Nanostructures within Anatase Titania Nanotubes for Advanced Photovoltaics

Inorganic nano-scale heterostructures have many advantages over hybrid organic-inorganic dye-sensitized solar cells (DSSC or Grätzel cells), including their resistance to photo-bleaching, thermal stability, large specific surface areas, and general robustness. This study presents a first-of-its-kind low-cost all-inorganic lead selenide-anatase titania (PbSe/TiO2) nanotube heterostructure material for photovoltaic applications. Herein, PbSe nanostructures have been co-electrospun within a hollow TiO2 nanotube with high connectivity for highly efficient charge carrier flow and electron-hole pair separation. This material has been characterized by transmission electron microscopy (TEM), electron diffraction, energy dispersive X-ray spectroscopy (EDX) to show the morphology and material composition of the synthesized nanocomposite. Photovoltaic characterization has shown this newly synthesized proof-of-concept material can easily produce a photocurrent under solar illumination, and, with further refinement, could reveal a new direction in photovoltaic materials.

Novel planar heterostructure perovskite solar cells with CdS nanorods array as electron transport layer

Hybrid organic–inorganic solar cells with planar heterostructure based on hydrothermal-synthesized CdS nanorods (CdS NRs) array and organometal trihalide perovskite CH3NH3PbI3 have been fabricated and characterized. Thermal annealing and Ultraviolet Ozone (UV–Ozone) treatment were applied to modify the surface of the CdS NRs, in which UV–Ozone treatment was proved to be more effective in improving the morphology of perovskite layer, as well as the charge transfer between CH3NH3PbI3 and CdS NRs. Devices structured as ITO/CdS NRs array/CH3NH3PbI3/2,29,7,79-Tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,99-spirobifluorene (spiro-MeOTAD)/MoO3/Ag produced short-circuit current density of 18.77mA/cm2, open-circuit voltage of 0.89V, and fill factor of 49.9%, leading to a best power conversion efficiency of 8.36% in the case of 10min UV–Ozone treatment.

The Significance of Ion Conduction in a Hybrid Organic-Inorganic Lead-Iodide-Based Perovskite Photosensitizer.

The success of perovskite solar cells has sparked enormous excitement in the photovoltaic community not only because of unexpectedly high efficiencies but also because of the future potential ascribed to such crystalline absorber materials. Far from being exhaustively studied in terms of solid-state properties, these materials surprised by anomalies such as a huge apparent low-frequency dielectric constant and pronounced hysteretic current-voltage behavior. Here we show that methylammonium (but also formamidinium) iodoplumbates are mixed conductors with a large fraction of ion conduction because of iodine ions. In particular, we measure and model the stoichiometric polarization caused by the mixed conduction and demonstrate that the above anomalies can be explained by the build-up of stoichiometric gradients as a consequence of ion blocking interfaces. These findings provide insight into electrical charge transport in the hybrid organic-inorganic lead halide solar cells as well as into new possibilities of improving the photovoltaic performance by controlling the ionic disorder.

Plasmonic nanoparticle incorporation into inverted hybrid organic–inorganic solar cells

To enhance solar harvesting in hybrid organic–inorganic photovoltaic devices we positioned plasmonic gold nanostructures in titania-conjugated polymer devices. Two plasmonic structures were judiciously designed and studied. In the first, agglomerates of gold nanoparticles, ∼150nm in diameter, were positioned as scattering clusters at the active layer/ITO electrode interface. In the second approach, 5nm isolated Au particles were suspended inside the hybrid active layer to induce localized plasmonic field enhancement. The location of the particles, size and distribution in the film was confirmed using scanning and transmission electron microscopy and X-ray scattering. The optical absorption measurements confirmed that both plasmonic structures enhance light harvesting, and photovoltaic devices utilizing each of the approaches showed improved photocurrent generation. Finite difference time domain simulations have corroborated that scattering-based and near-field absorption enhancements occur in the active layer, in good agreement with the measured absorption. Finally, combining both approaches to fabricate a hybrid photovoltaic device with both plasmonic nanostructures, i.e. Au NPs both in and on the active layer, resulted in a twofold increase in photocurrent generation.