Bilayer Heterojunction Solar Cells Research Articles

Solution‐Processed, Insoluble Thin Films through In Situ Chemical Polymerization of Semiconducting Native Polythiophene

The use of multilayer, solution‐processed organic thin film devices can benefit strongly from the controlled deposition of insoluble thin films. Here, a solution‐based in situ polymerization technique for semiconducting polythiophene thin films based on nonchlorinated solvents forming a range of large‐scale structured or homogeneous, insoluble thin films is systematically developed. The high‐quality nature of the resulting polythiophene thin films is demonstrated by successfully implementing the films in the active layer of bilayer heterojunction solar cells. It is possible to transfer this protocol to large‐scale processing without additional changes to the material system.

CH3NH3PbI3/GeSe bilayer heterojunction solar cell with high performance

Perovskite (CH3NH3PbI3) solar cells have made significant advances recently. In this paper, we propose a bilayer heterojunction solar cell comprised of a perovskite layer combining with a IV-VI group semiconductor layer, which can give a conversion efficiency even higher than the conventional perovskite solar cell. Such a scheme uses a property that the semiconductor layer with a direct band gap can be better in absorption of long wavelength light and is complementary to the perovskite layer. We studied the semiconducting layers such as GeSe, SnSe, GeS, and SnS, respectively, and found that GeSe is the best, where the optical absorption efficiency in the perovskite/GeSe solar cell is dramatically increased. It turns out that the short circuit current density is enhanced 100% and the power conversion efficiency is promoted 42.7% (to a high value of 23.77%) larger than that in a solar cell with only single perovskite layer. The power conversion efficiency can be further promoted so long as the fill factor and open-circuit voltage are improved. This strategy opens a new way on developing the solar cells with high performance and practical applications.

Rapid and green synthesis of complementary D-A small molecules for organic photovoltaics

Two push-pull molecules with close molecular structures have been synthesized through two green steps, direct heteroarylation and Knoevenagel condensation. The electronic properties and the basic bilayer heterojunction solar cells demonstrate that the two molecules are complementary in term of light absorption. Thus solar cells using a blend of the two molecules present higher power conversion efficiencies, reaching 3%, than the cells made from each compound alone.

Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics

The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.

Cyanine tandem and triple-junction solar cells

Ultrathin bilayer heterojunction solar cells using cyanine electron donors and electron acceptor C60 are used to fabricate monolithically stacked tandem and triple junction devices. Sub-cell stack sequences as well as C60 layer thicknesses are optimized by optical modeling and maximum efficiency is corroborated experimentally. The highest power conversion efficiency of 4.3% under full sun irradiation is achieved with a tandem cell where heptamethine and trimethine cyanine dyes are used in the front and back cell, respectively. The open circuit voltage matches the sum of the two respective open circuit voltages of the individual single junction solar cells within 3%. Triple junction cells using an additional sub-cell with a pentamethine cyanine suffer from electrical series resistance. At low light irradiation intensity, however, both triple and tandem solar cells reach power conversion efficiencies above 5% in agreement with the performance increase predicted from numerical simulation.

Interfacial Morphology and Effects on Device Performance of Organic Bilayer Heterojunction Solar Cells

The effects of interface roughness between donor and acceptor in a bilayer heterojunction solar cell were investigated on a polymer–polymer system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-alt-benzothiadiazole) (F8BT). Both polymers are known to reorganize into semicrystalline structures when heated above their glass-transition temperature. Here, the bilayers were thermally annealed below glass transition of the bulk polymers (≈140 °C) at temperatures of 90, 100, and 110 °C for time periods from 2 min up to 250 min. No change of crystallinity could be observed at those temperatures. However, X-ray reflectivity and device characteristics reveal a coherent trend upon heat treatment. In X-ray reflectivity investigations, an increasing interface roughness between the two polymers is observed as a function of temperature and annealing time, up to a value of 1 nm. Simultaneously, according bilayer devices show an up to 80% increase of power conversion efficiency (PCE) for short annealing periods at any of the mentioned temperatures. Together, this is in agreement with the expectations for enlargement of the interfacial area. However, for longer annealing times, a decrease of PCE is observed, despite the ongoing increase of interface roughness. The onset of decreasing PCE shifts to shorter durations the higher the annealing temperature. Both, X-ray reflectivity and device characteristics display a significant change at temperatures below the glass transition temperatures of P3HT and F8BT.

(Invited) Impact of Indiumand Gallium Doping on the Photovoltaic Performance of CdSe Quantum Dot Hybrid Solar Cells

Colloidal CdSe quantum dots show great promise for fabrication of hybrid solar cells with enhanced power conversion efficiency, yet controlling the doping on the atomic length scale is challenging. Here, we demonstrate that gallium-, indium-, tin-doped CdSe quantum dots show significantly improved conductivity and charge carrier density, and also temperature dependent behavior. Furthermore, the doped CdSe hybrid solar cells greatly enhance photocurrent and photovoltage, in which the gallium doped CdSe quantum dots and P3HT bi-layer heterojunction solar cells leads to a maximum power conversion efficiency of 2.0% at elevated temperatures under AM 1.5 solar illumination. All the doped samples exhibit inverted temperature dependent power conversion of the photovoltaic cells, which could be effectively utilized in solar concentrators. The approach presented can be applied to a wide range of doped quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general tactic for improving the efficiency of quantum dot based solar cells.

(Invited) Impact of Indiumand Gallium Doping on the Photovoltaic Performance of CdSe Quantum Dot Hybrid Solar Cells

Colloidal CdSe quantum dots show great promise for fabrication of hybrid solar cells with enhanced power conversion efficiency, yet controlling the doping on the atomic length scale is challenging. Here, we demonstrate that gallium-, indium-, tin-doped CdSe quantum dots show significantly improved conductivity and charge carrier density, and also temperature dependent behavior. Furthermore, the doped CdSe hybrid solar cells greatly enhance photocurrent and photovoltage, in which the gallium doped CdSe quantum dots and P3HT bi-layer heterojunction solar cells leads to a maximum power conversion efficiency of 2.0% at elevated temperatures under AM 1.5 solar illumination. All the doped samples exhibit inverted temperature dependent power conversion of the photovoltaic cells, which could be effectively utilized in solar concentrators. The approach presented can be applied to a wide range of doped quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general tactic for improving the efficiency of quantum dot based solar cells.

ChemInform Abstract: Recent Advances of Non‐Fullerene, Small Molecular Acceptors for Solution Processed Bulk Heterojunction Solar Cells

AbstractReview: 145 refs.

High voltage and efficient bilayer heterojunction solar cells based on an organic–inorganic hybrid perovskite absorber with a low-cost flexible substrate

A low temperature (<100 °C), flexible solar cell based on an organic-inorganic hybrid CH3NH3PbI3 perovskite-fullerene planar heterojunction (PHJ) is successfully demonstrated. In this manuscript, we study the effects of energy level offset between a solar absorber (organic-inorganic hybrid CH3NH3PbI3 perovskite) and the selective contact materials on the photovoltaic behaviors of the planar organometallic perovskite-fullerene heterojunction solar cells. We find that the difference between the highest occupied molecular orbital (HOMO) level of CH3NH3PbI3 perovskite and the Fermi level of indium-tin-oxide (ITO) dominates the voltage output of the device. ITO films on glass or on the polyethylene terephthalate (PET) flexible substrate with different work functions are investigated to illustrate this phenomenon. The higher work function of the PET/ITO substrate decreases the energy loss of hole transfer from the HOMO of perovskite to ITO and minimizes the energy redundancy of the photovoltage output. The devices using the high work function ITO substrate as contact material show significant open-circuit voltage enhancement (920 mV), with the power conversion efficiency of 4.54%, and these types of extra-thin planar bilayer heterojunction solar cells have the potential advantages of low-cost and lightweight.

Recent advances of non-fullerene, small molecular acceptors for solution processed bulk heterojunction solar cells

Organic, planar, and electron deficient small molecules were utilized as acceptors in the first reported bilayer heterojunction solar cells, however, current state-of-the-art organic photovoltaic (OPV) cells utilize fullerene derivatives as acceptor molecules. Recently, intensive efforts have been directed towards the development and understanding of soluble, non-fullerene, organic small molecules to fabricate bulk heterojunction (BHJ) solar cells. These efforts have been aimed at overcoming the inherent limitations of fullerene compounds such as the limited spectral breadth, air instability, and the typically higher production costs of fullerenes. In this focused review, we have highlighted the most recent progress over the last couple of years towards developing n-type organic small molecules utilized in BHJ devices in order to provide insight towards improving the overall performance of OPVs.

Bilayer polymer/fullerene solar cells with a liquid crystal

In this paper the effect of 5-(10-undecenyloxy)-2-[[[4-hexylphenyl]imino]methyl]phenol liquid crystal addition into poly(3-hexylthiophene):fullerene containing solar cells were investigated. Based on current–voltage characteristics, we report on the efficiency enhancement for bilayer heterojunction solar cells using this liquid crystal.

Numerical simulation of photocurrent generation in bilayer organic solar cells: Comparison of master equation and kinetic Monte Carlo approaches

Numerical approaches can provide useful information about the microscopic processes underlying photocurrent generation in organic solar cells (OSCs). Among them, the Kinetic Monte Carlo (KMC) method is conceptually the simplest, but computationally the most intensive. A less demanding alternative is potentially represented by so-called Master Equation (ME) approaches, where the equations describing particle dynamics rely on the mean-field approximation and their solution is attained numerically, rather than stochastically. The description of charge separation dynamics, the treatment of electrostatic interactions and numerical stability are some of the key issues which have prevented the application of these methods to OSC modelling, despite of their successes in the study of charge transport in disordered system. Here we describe a three-dimensional ME approach to photocurrent generation in OSCs which attempts to deal with these issues. The reliability of the proposed method is tested against reference KMC simulations on bilayer heterojunction solar cells. Comparison of the current-voltage curves shows that the model well approximates the exact result for most devices. The largest deviations in current densities are mainly due to the adoption of the mean-field approximation for electrostatic interactions. The presence of deep traps, in devices characterized by strong energy disorder, may also affect result quality. Comparison of the simulation times reveals that the ME algorithm runs, on the average, one order of magnitude faster than KMC.

Bilayer and bulk heterojunction solar cells with functional poly(2,2′-bithiophene) films electrochemically deposited from aqueous emulsion

Ultra thin films of poly(2,2′-bithiophene) with thickness of 11–33nm were electrochemically deposited on poly(3,4-ethylenedioxithiophene):poly(styrene sulfonic acid) modified indium tin oxide substrates using a new approach for electrochemical synthesis. Ultrasonic emulsification of aqueous perchloric acid and 2,2′-bithiophene monomer is an effective method to disperse this water insoluble monomer in micro droplets that improved the contact with the electrode. Rigorous control of the poly(2,2′-bithiophene) thickness was achieved by controlling the potential applied and the charge during the deposition. We studied thin films solar cells based on poly(2,2′-bithiophene) polymer as bilayer with C60, resulting a power conversion efficiency of 0.3%, and as buffer layer in bulk heterojunction solar cells. We show that poly(2,2′-bithiophene) films influenced positively the open circuit voltage and increased about 1.6 times the short circuit current, increasing the efficiency from 1.3% up to 2.9% from devices without buffer layer or with 11nm of poly(2,2′-bithiophene), respectively.

A Paradigmatic Change: Linking Fullerenes to Electron Acceptors

The potential of Lu(3)N@C(80) and its analogues as electron acceptors in the areas of photovoltaics and artificial photosynthesis is tremendous. To this date, their electron-donating properties have never been explored, despite the facile oxidations that they reveal when compared to those of C(60). Herein, we report on the synthesis and physicochemical studies of a covalently linked Lu(3)N@C(80)-perylenebisimide (PDI) conjugate, in which PDI acts as the light harvester and the electron acceptor. Most important is the unambiguous evidence--in terms of spectroscopy and kinetics--that corroborates a photoinduced electron transfer evolving from the ground state of Lu(3)N@C(80) to the singlet excited state of PDI. In stark contrast, the photoreactivity of a C(60)-PDI conjugate is exclusively governed by a cascade of energy-transfer processes. Also, the electron-donating property of the Lu(3)N@C(80) moiety was confirmed through constructing and testing a bilayer heterojunction solar cell device with a PDI and Lu(3)N@C(80) derivative as electron acceptor and electron donor, respectively. In particular, a positive photovoltage of 0.46 V and a negative short circuit current density of 0.38 mA are observed with PDI/Ca as anode and ITO/Lu(3)N@C(80) as cathode. Although the devices were not optimized, the sign of the V(OC) and the flow direction of J(SC) clearly underline the unique oxidative role of Lu(3)N@C(80) within electron donor-acceptor conjugates toward the construction of novel optoelectronic devices.

A Poly-(3-Hexylthiophene) (P3HT)/[6,6]-Phenyl-C61-Butyric Acid Methyl Ester (PCBM) Bilayer Organic Solar Cell Fabricated by Airbrush Spray Deposition

Airbrush spray deposition is applied to fabricate a bilayer heterojunction solar cell based on P3HT/PCBM. This solar cell device shows an open-circuit voltage of 0.36 V, a short circuit current density of 6.76 mA/cm2, a conversion efficiency of 0.74%, and a fill factor of 30.4%. The results demonstrate that airbrush spray deposition is an effective method to fabricate multilayer or other complex polymer-based organic solar cells. Although spin-coated bulk heterojunction devices have better performance than the airbrushed ones, the airbrush is indeed feasible as a low-cost yet simple process. It is noteworthy that such preliminary results of the airbrush spray solar cell is unoptimized and thus its performance can be further improved with the development of this technology. Furthermore, this method itself has huge potential as it can be used for other polymer-based organic thin film devices.

Coarse-grained kinetic modelling of bilayer heterojunction organic solar cells

The on-lattice kinetic Monte Carlo (KMC) method provides a powerful tool to simulate the J–V properties of organic solar cells. However, the computational cost associated with charge injection may limits its applicability. In the attempt to overcome this limitation, we describe in this paper a coarse-grained numerical approach to photocurrent generation in bilayer heterojunction solar cells. Starting from the KMC algorithm, a self-consistent numerical procedure is proposed to find the steady-state solutions of the kinetic equations describing particle dynamics in one dimension across the layer thickness. Our model incorporates the generation, transport and recombinations of charge carriers, excitons, and electron/hole pairs, whose introduction is required to correctly describe interfacial phenomena at the coarse-grained level. A continuum model of the electrostatic interactions among charge carriers is proposed and used to compute their hopping rates during the simulation. The model is used to investigate the J–V properties of Cathode/PCBM/P3HT/PEDOT:PSS/ITO bilayer devices, showing that Fermi level pinning at the Cathode/PCBM interface must be invoked to accurately model the experimental behavior. From the fitting to the experimental J–V data, we conclude the short-circuit current density to be mainly associated with a high exciton diffusion length. The analogies and differences between our model and existing KMC and drift–diffusion approaches are also discussed.

Effect of the Temperature of Annealing on the Performance of Fluorene and Bithiophene Copolymer in Bilayer Solar Cells

ABSTRACTWe have investigated the effect of temperature annealing on bilayer heterojunction solar cells based on poly[9,9’-hexyl-fluorene-alt-bithiophene] as active layer. Film morphology for different temperature annealing was probed by atomic force microscopy (AFM) and the values of roughness range from 0.59 up to 2.15 nm. The best photovoltaic performance was found for devices with active layer annealed at 200°C with power conversion efficiency (η) of 2.8 % while devices without annealing presented only 0.4%. This performance enhancement is attributed to the reduction of traps and increased hole mobility after the thermal annealing.

Bilayer heterojunction polymer solar cells using unsubstituted polythiophene via oxidative chemical vapor deposition

We demonstrate the use of a vacuum-based, vapor phase technique for the deposition of a donor polymer for use in polymer solar cells. Unsubstituted polythiophene (PT), which is insoluble and infusible and thus typically difficult to process, is easily prepared by oxidative chemical vapor deposition (oCVD). The oCVD process results in a conductive PT film that is heavily doped with FeCl3, which is used as the oxidizing agent. A post-deposition methanol rinse sufficiently dedopes the film and removes spent oxidant, leaving semiconducting PT with an optical bandgap close to 2eV. Drastic changes in the film color, absorption spectra, and film composition confirm the dedoping process. The resulting semiconducting PT is then applied as an electron donor in bilayer heterojunction solar cells with a thermally evaporated C60 electron acceptor layer, resulting in power conversion efficiencies up to 0.8%. The absorption edge of the PT at ∼620nm closely matches the edge present in the external quantum efficiency spectra, indicating that the oCVD PT contributes to the photocurrent of the devices. This demonstrates that the oCVD technique can be used in the processing and design of polymer active layers for polymer solar cells and hybrid small molecular organic solar cells without solubility, temperature, or substrate considerations.

Hole mobility effect in the efficiency of bilayer heterojunction polymer/C60 photovoltaic cells

We report here bilayer heterojunction solar cells fabricated by using poly[9,9′-hexyl-fluorene-alt-bithiophene] (LaPPS43) polymer as active layer. The power conversion efficiency (η) displays a sevenfold increase upon annealing at 200 °C, reaching the value of 2.8%. This result is comparable to the highest η reported so far for bulk heterojunction solar cells using the same polymer. Simulation, absorbance spectra, and current versus voltage results indicate that the π–π stacking in solid state is enhanced after annealing with a reduction in traps and thus reflecting in higher hole mobility.