Organic Thin-film Solar Cells Research Articles

(Invited) Computational Analysis of the Excited States in the Film States of Non-Fullerene Acceptors

Non-fullerene acceptors are currently widely used as the acceptors of organic thin film solar cells due to their high efficiency. However, the molecular mechanism is still unclear. We have studied the excited states in the film states of non-fullerene acceptors together with experimental collaborators by using theoretical methods. In this presentation, we will present recent progress of our research. For example, we will talk about the excited states in the film states of ITIC. ITIC is a popular acceptor-donor-acceptor-type non-fullerene acceptor. Recently, our collaborator, Prof. Yamakata at Okayama University, Japan, found that the charge separation can take place in ITIC acceptor film even without donor molecules. We investigated this phenomenon by using MD simulations and quantum chemical calculations. MD simulations of the ITIC film showed that the acceptor parts of ITIC are stacked with each other. The dominant angle between two stacked ITIC was found to be ~180 degrees, making J-type dimer. On the other hand, it was also found that there are quite a few V-type dimers with the angles less than 90 degrees. Quantum chemical calculations revealed that the dipole moment in the excited state of J-type dimer is almost zero whereas that of V-type dimer is quite large, indicating strong charge-transfer excited state. Therefore, it is considered that the charge separation of ITIC film can take place in the V-type dimers. We also investigated the excited states in the film states of ITIC analogs. Recently, our collaborator, Prof. Ie at Osaka University, Japan, synthesized several ITIC analogs with different sidechains and found the different efficiency of solar cells. We theoretically investigated the excited states in the film sates of ITIC analogs as in the case of ITIC. It is found that the modification of side chains can change the distributions of angles between two stacked molecules and excited-state properties. These findings highlight the importance of controlling stacking structures in the film state for the efficiency of organic thin film solar cells.

Combined Charge Extraction by Linearly Increasing Voltage and Time-Resolved Microwave Conductivity to Reveal the Dynamic Charge Carrier Mobilities in Thin-Film Organic Solar Cells.

This article reports a purely experiment-based method to evaluate the time-dependent charge carrier mobilities in thin-film organic solar cells (OSCs) using simultaneous charge extraction by linearly increasing the voltage (CELIV) and time-resolved microwave conductivity (TRMC) measurements. This method enables the separate measurement of electron mobility (μe) and hole mobility (μh) in a metal-insulator-semiconductor (MIS) device. A slope-injection-restoration voltage profile for MIS-CELIV is also proposed to accurately determine the charge densities. The dynamic behavior of μe and μh is examined in five bulk heterojunction (BHJ) OSCs of polymer:fullerene (P3HT:PCBM and PffBT4T:PCBM) and polymer:nonfullerene acceptor (PM6:ITIC, PM6:IT4F, and PM6:Y6). While the former exhibits fast decays of μh and μe, the latter, in particular, PM6:IT4F and PM6:Y6, exhibits slow decays. Notably, the high-performing PM6:Y6 demonstrates both a balanced mobility (μe/μh) of 1.0-1.1 within 30 μs and relatively large CELIV-TRMC mobility values among the five BHJs. The results exhibit reasonable consistency with a high fill factor. The proposed new CELIV-TRMC technique offers a path toward a comprehensive understanding of dynamic mobility and its correlation with the OSC performance.

Nickel-doped silver nanoclusters as a mechanism to capture photons

Narrow width of optical absorption of conducting polymers and photons energy losses have been the challenges for fabricating highly efficient thin-film organic solar cell. Nickel-doped silver nanoclusters (Ni/Ag NCs) are employed here to capture more photons using polymers blend solar absorber medium to improve solar cell performances. The poly-3-hexylthiophene and (6-6)phenyl-C61-butyric acid methyl ester molecules blend were used a solar absorber layer in this investigation. The solar cells fabricated with NCs exhibited enhanced opt-electronic properties compared to the reference solar cell. Consequently, the experimental results suggest that the power conversion efficiency (PCE) has substantially increased with the incorporations of NCs in absorber layer, which is dependent on the concentrations of NCs in the medium. The maximum PCE achieved, in this work, is η\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\eta $$\\end{document} = 6.2% at 2% of NCs by weight, which has exhibited to the lowest energy losses compared to other doping levels. This improvement in PCE is attributed to the occurrence of local surface plasmon resonance effect due to the inclusion of Ni/Ag NCs in polymer matrix. The results provide valuable insights on the use of Ni/Ag NCs for efficient photons capture in thin-film polymers blend medium.

Simultaneous inclusion of quantum dots in multi-functional layers of thin film organic solar cells

The role of quantum dot (QD) decoration in the hole transport buffer layer and the photoactive medium on the photovoltaic parameters of thin film organic solar cells (TFOSCs) was investigated. A cadmium–tellurium-based QD was synthesized successfully and embedded in two of the functional layers of a TFOSC to improve its overall power conversion efficiency. The experimentally determined optimum concentration of the QD was maintained in the interfacial layer to investigate the effect of QD concentration in the active layer. The observed increased short-circuit current density (Jsc) and open circuit voltage (Voc) are attributable to the enhanced energy level tuning, broadened optical absorption, and charge transport process facilitated by the integration of QDs inside the media. Moreover, an improved device efficiency was obtained when the solvent additive was introduced into the bulk heterojunction photoactive layer films to facilitate QD dispersion and increase the interpenetrating network of the active layer blend that reduces the occurrence of trap sites, which, in turn, limits the Auger recombination rates. The QD-doped TFOSCs catalyzed with solvent additives displayed an enhanced overall photovoltaic parameter, which is quite appreciable in comparison with that of the pristine devices.

Efficiency enhancement of the CIGSSe/Perovskite inorganic and organic thin-film solar cell

Efficiency enhancement of the CIGSSe/Perovskite inorganic and organic thin-film solar cell

Optimization and Efficiency Enhancement of Modified Polymer Solar Cells.

In this study, an organic bulk heterojunction (BHJ) solar cell with a spiro OMeTAD as a hole transport layer (HTL) and a PDINO as an electron transport layer (ETL) was simulated through the one-dimensional solar capacitance simulator (SCAPS-1D) software to examine the performance of this type of organic polymer thin-film solar cell. As an active layer, a blend of polymer donor PBDB-T and non-fullerene acceptor ITIC-OE was used. Numerical simulation was performed by varying the thickness of the HTL and the active layer. Firstly, the HTL layer thickness was optimized to 50 nm; after that, the active-layer thickness was varied up to 80 nm. The results of these simulations demonstrated that the HTL thickness has rather little impact on efficiency while the active-layer thickness improves efficiency significantly. The temperature effect on the performance of the solar cells was considered by simulations performed for temperatures from 300 to 400 K; the efficiency of the solar cell decreased with increasing temperature. Generally, polymer films are usually full of traps and defects; the density of the defect (Nt) value was also introduced to the simulation, and it was confirmed that with the increase in defect density (Nt), the efficiency of the solar cell decreases. After thickness, temperature and defect density optimization, a reflective coating was also applied to the cell. It turned out that by introducing the reflective coating to the back side of the solar cell, the efficiency increased by 2.5%. Additionally, the positive effects of HTL and ETL doping on the efficiency of this type of solar cells were demonstrated.

An empirical analysis of ZnO/Ag nanocomposites thin film organic solar cell with optical and structural properties

For organic solar cells (OSCs), a novel polymer donor (PMMA) and a new small molecule acceptor (Ag NPs) were devised and produced. The effects of the donor and acceptor on the absorption spectra, molecular energy levels, and charge mobilities were thoroughly investigated. The results showed that the PMMA/ZnO/Ag NPs OSCs exhibited a high power conversion efficiency of 5.7%, which is attributed to the complementary absorption spectra, matched energy levels, and efficient charge transport. These findings suggest that PMMA and Ag NPs are promising materials for the development of high-performance OSCs. The PMMA-based OSC device demonstrated with a thickness of 200–300 nm achieving an efficiency of over 5.7%, indicating the promise of photovoltaic OSCs in practical applications. The findings were used to regulate nanomorphology and ultimately improve solar cell performance.

Influence of vibronic interaction of charge transfer excitons in PTB7/BTA-based nonfullerene organic solar cells.

We studied photoinduced charge transfer (CT) states and their dissociation processes at the donor/acceptor (D/A) interface of PTB7/BTAx (x = 1 and 3) nonfullerene organic thin-film solar cells using density functional theory (DFT) and time-dependent DFT calculations. We focused on the CT distances and electron coupling in the CT state generated by photoexcitation and the Huang-Rhys (HR) factors that describe the nonadiabatic processes associated with vibronic interactions. The PTB7/BTA3 system with a large short-circuit current density (JSC) exhibited a large charge CT distance and electronic coupling. Contrastingly, the PTB7/BTA1 system with a low JSC has a large HR factor because of the low-wavenumber vibrational modes in the CT state of the D/A complex and is prone to nonadiabatic relaxation to the ground state. Systematic theoretical analysis of the excitonic states in the D/A complex has provided insight into the control of CT exciton dynamics, namely JSC and electron-hole recombination.

Development of solar radiation measurement sensor with added value to promote wide-area solar power prediction technology

In this study, we investigated a scenario where the prediction technology for the total output of distributed photovoltaic power generation over a wide area could be more efficiently disseminated by sharing the costs with society, rather than burdening only specific companies with the costs. To achieve this goal, we not only focused on measuring the amount of solar radiation but also incorporated additional parameters into the solar radiation sensor, which is essential for improving the accuracy of the prediction technology. The fundamental configuration of the developed sensor consists of an organic thin film solar cell (OPV), and the output is utilized to transmit the output data externally. Utilizing the surplus power from the OPV, we examined and produced three types of sensors: 1) a sensor with a CO2 measurement function, 2) a sensor with a mist spray function, and 3) a sensor with a photography function using a drive recorder. As a result, all sensors met the predetermined functions and target specifications.

Reducing Energy Loss in Polymer Solar Cell through Optimization of Novel Metal Nanocomposite

Ag/CuPO4 nanocomposite was successfully synthesized using wet chemistry for potential application as a mechanism to suppress charge recombination and harvest more photons in thin-film organic solar cells (TFOSCs). The morphological and optical properties of the nanocomposite were studied using high-resolution electron microscopy and UV–vis spectroscopy. The Ag/CuPO4 nanocomposite exhibited a semispherical geometry which is suitable for the occurrence of localized surface plasmon resonance (LSPR) and light scattering in a dielectric medium. Conventional device architecture is employed that uses a solar absorber composed of a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend with and without the nanocomposite. As a result, significant changes in the device performances were observed by the incorporation of the metal nanocomposite, where the experimental evidence suggested that the doped active layer outperformed the undoped ones. Consequently, the best power conversion efficiency (PCE) grew by nearly 95% compared to the reference cell. Furthermore, the measured photocurrents are found to be dependent on the concentration of the nanocomposite in the absorber layer. In this article, the effects of metal nanocomposite are discussed in terms of the phenomena of local surface plasmon resonance and far-field scattering in the photoactive medium of TFOSCs.

Stepped metal gratings as an efficient way to design a broadband absorption efficiency and overcome recombination degradation in an organic solar cell

The Stepped stopped Groove Metal nano-grating (SSGMG) and Stepped Through Groove Metal nano-grating (STGMG) with a stepped hole transport layer (HTL) and a coating layer, is investigated as a novel method to obtain high absorption efficiency in a thin film organic solar cell. Enhancement of the electric field inside the gratings due to the near field and far-field coupling of wedge plasmon polaritons would lead to the improvement of the absorption efficiency of the solar cell. The proposed SSGMG model, with a 40 nm thickness of the photoactive layer, shows an absorption efficiency of 73.73% of the incident light in a wavelength range from 350 nm to 800 nm. the results show that the SSGMG model with an effective thickness of 40 nm has improved the absorption efficiency of the thickness-equivalent planar model (without coating layer) up to 133% of its initial value. Moreover, the effect of the incident angle (θ) and polarization angle (α) on the absorption efficiency was evaluated. We have found that SSGMG would lead to better absorption efficiency than STGMG because of its advantages over unpolarized light absorption. Excitation of surface plasmon polaritons inside the photo-active layer would help to reduce the recombination degradation as a result of the reduced thickness of the active layer as well as the enhanced mobility of charge. The designed structures can be used to overcome recombination degradation which is the intrinsic limitation of organic materials.

Computational optimization and optical analysis of thin-film organic solar cells for high efficiency

Computational optimization and optical analysis of thin-film organic solar cells for high efficiency

Synergistic contribution of potassium sulfide doped with silver nanoparticles on the performance of thin film organic solar cells

Potassium sulfide doped with silver (K2S:Ag) nanoparticles were used in bulk heterojunction organic solar cells to assist in harvesting more photons using an absorber layer composed of poly (3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester blend (P3HT:PCBM). The optical and electrical properties of the organic-molecule-blended thin photoactive films were investigated using optical spectrometers and a computer-interfaced source meter coupled with a solar simulator. The structure and composition of the nanoparticles were examined by high resolution scanning and transmission electron microscopies. The measured photovoltaic parameters indicated that solar cell performance was dependent on the concentration of K2S:Ag nanoparticles in the P3HT:PCBM blended film. Consequently, enhanced photocurrent was recorded from the devices with a short circuit current as high as 15.8 mA cm−2, fill factor of 57.2%, and power conversion efficiency of 5.12%, representing a 15% and 97% growth in fill factor and power conversion efficiency, respectively, compared to the reference device. The results were attributed to the occurrence of localized surface plasmon resonance and the light scattering effect of K2S:Ag in the photoactive medium.

Silver decorated magnesium doped photoactive layer for improved collection of photo‐generated current in polymer solar cell

AbstractSilver doped magnesium (Ag:Mg) bimetallic nanoparticles (BMNPs) were successfully synthesized using wet chemical processing. The collection of enhanced photocurrents is possible through metal nanoparticles in the photoactive layer of a thin‐film organic solar cell (TFOSC). This investigation employed poly‐3‐hexylthiophene(P3HT) and [6‐6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) polymer blend solar absorbers in a conventional device structure. The solar cell performances were found to depend on the concentration of Ag:Mg BMNPs in the photoactive medium. Consequently, significant device performance was recorded for the solar cells containing Ag:Mg BMNPs at all doping levels compared to the un‐doped devices. Therefore, the highest power conversion efficiency (PCE) of 4.11% was achieved at a 1.5 wt% doping level with a high fill factor of 56% compared to the reference cell. The performance improvement in PCE constitutes a 79% improvement, which is much higher than undoped solar cells. This result was attributed to the occurrence of the localized surface plasmon resonance effect (LSPR), which is favorable for boosting the optical absorption and charge transport processes in TFOSC.

The roles of acceptors in the thermal-degradation of P3HT based organic solar cells

The effects of acceptors on thermal degradation of thin film organic solar cells (OSCs) were investigated under the controlled environment. The popular donor polymer molecule, poly(3-hexylthiophene) (P3HT) is used in the preparation of bulk-heterojunction (BHJ) organic solar absorber medium. The evidences of degradation were observed from the changes of the optical and morphological properties of the polymer film. The parameters gathered from various characterization methods suggest that film degradation is indeed dependent on the nature of acceptor molecules used in the polymer matrix. Interestingly, the PC60BM acceptor blend solar absorber films are found to be more stable than ITIC based polymer blend. These are evident on the measured device parameters such as Jsc and Voc values that contribute to poor performance in P3HT:ITIC-based devices. It has been noted that a 50% drop in open-circuit voltage was recorded in ITIC based solar cell within an hour of thermal degradation. The FTIR result revealed that the production of -OH groups as a result of oxidation leads to the creation of trap states that hinder conductivity by decreasing short-circuit current.

Effect of an ambient environment on light-induced degradation of organic solar cells based on a benzodithiophene–quinoxaline copolymer in air

Degradation of polymer molecules is one of the main factors that reduces the lifetime of thin film organic solar cells.

Materials Data-science Approach for Comprehensive Understanding of Organic Thin Film Solar Cells

Materials Data-science Approach for Comprehensive Understanding of Organic Thin Film Solar Cells

Silver doped ZnS core-shell nanocomposites to promote photon harvesting in polymer cells

Silver doped zinc sulfide (ZnS:Ag) metal nanoparticles (NPs) were successfully synthesized and used as a dopant in thin-film organic solar cells (TFOSCs). Core-shell structured ZnS:Ag nano-particle is expected to improve photon harvesting in solar absorber medium. Polymers blend composed of poly-(3-hexylthiophene) (P3HT): [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) were used as absorber medium with and without ZnS:Ag metal NPs. The concentrations of the metal NPs were varied to investigate the effect of doping level in the generation photocurrent. Significant device performances were recorded with short-circuit current (Jsc) of 14.30 mAcm−2, fill factor (FF) of 52.63%, and PCE performance of 4.50% was achieved at 3% concentration by weight. Such an improvement is likely attributed to the presence of localized surface plasmonic resonance (LSPR) effect at the metal/polymer interfaces, which plays an essential role in augmenting the charge transport processes and augmenting the optical absorption. The results further indicate that PCE value has enhanced dramatically by 98% compared to the pristine reference cell. The finding of our investigation will be deliberated based on the measured optoelectronic and morphological properties of newly synthesized ZnS:Ag metal NPs and device performances.

(Invited) Photoabsorption and Excitation Energy Transfer in Fluorinated Non-Fullerene Acceptors for Organic Solar Cells

Recently, it was reported that OSCs based on non-fullerene acceptors have shown high photovoltaic performance with over 18% efficiency, being more than the power conversion efficiencies (PCEs) of 13% in OSCs based on fullerene. We considered theoretically the effects of fluorine atoms on the conversion efficiency of organic thin-film solar cells from the electronic analysis of donor/acceptor (D/A) complex interactions for NTz-Teh-FA and FNTz-Teh-FA using DFT. It was confirmed that it differs depending on the side-chain length, and it is necessary to consider the side-chain length. Because the longer the side-chain length, the lighter absorption and increased charge transfer. In the fluorinated acceptor, the polarity around the O atoms at both ends of the main chain was weakened, the twist angle of the molecular structure was reduced, and the light absorption at 600–700 nm was increased in the absorption spectrum. Therefore, it is considered that the fluorinated nonfullerene-type acceptor FNTz-Teh-FA, which has a long side chain, has the best light absorption and charge transfer. Excitation Energy Transfer (EET) calculations show that the Coulomb coupling contributes significantly. In addition, it was confirmed that the fluorinated complex had a higher EET. Therefore, it is considered that the charge separation efficiency is improved. Thus, it is considered that the short-circuit current density (JSC) is improved by improving the internal quantum efficiency (IQE), including the element of the charge separation efficiency.

Electric Field as A Novel Switch to Enhance Optical Absorption Spectra of Defect Blue Phosphorene Thin-films

Improvement of the optical absorption coefficient is essential to enhance the light conversion efficiency of thin-film organic solar cells. Here we report the use of an external electric field as a novel switch to improve the optical absorption capacity of two-dimensional defect blue phosphorene (BlueP) systems. Using the density functional theory with van der Waals functionals, we investigate the structural, electronic, magnetic and optical absorption properties of the pristine, single-vacancy (SV) BlueP thin films, and a BlueP system absorbing a Vanadium adatom. We demonstrate that a SV BlueP layer would exhibit half-metallic and its absorption spectrum under an electric field parallel to the material plane is significantly enhanced in the ultra-violet region. More interestingly, when a Vanadium transition metal is absorbed on a pristine BlueP, the applied electric field perpendicular to the BlueP plane not only doubles the optical absorption coefficient, but also switches ON/OFF the magnetic moments of this system. The prominent red shift of the absorption spectra towards the visible light range under selected polarized directions paves a novel way to engineer solar cell devices with BlueP materials.