[Show abstract][Hide abstract] ABSTRACT: Trap-assisted recombination is one of the main loss mechanisms in bulk-heterojunction (BHJ) solar cells. Our results suggest that introducing a near infrared (NIR) polymer into the 2,3-bis(3-(octyloxy)phenyl)quinoxaline (PIDTTQ):[6,6]-phenyl C70 butyric acid methyl ester (PC70BM) host system suppresses trap-assisted recombination in the binary blend, leading to a significant improvement of ≈60% in power conversion efficiency for ternary organic solar cells at low light intensity.
Advanced Energy Materials 09/2015; DOI:10.1002/aenm.201501527 · 16.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The synthesis of a series of A-π-D-π-A oligomers bearing coplanar electron-donating dithieno[3,2-b:2',3'-d]silole (DTS) units linked through bithiophene π-bridge with the electron-withdrawing alkyldicyanovinyl (alkyl-DCV) groups is described. This study demonstrates a systematic investigation of structure–property relationships in this type of oligomers and shows obvious benefits of alkyl-DCV groups as compared to commonly used DCV ones, in terms of elaboration of high performance organic solar cells (OSCs). Considerable efforts have been made to improve the power conversion efficiency (PCE) of oligomer-based OSCs by diverse strategies including fine-tuning of the oligomers’ properties via variation of their terminal and central alkyl chains, blend morphology control via solvent vapor annealing (SVA) treatment, and surface modification via interfacial engineering. These efforts allowed to achieve PCEs of up to 6.4% for DTS(Oct)2-(2T-DCV-Me)2 blended with PC70BM. Further morphological investigations demonstrated that the usage of SVA treatment indeed effectively results in increased absorption and ordering of the BHJ composite, with the only exception for the most soluble oligomer DTS(Oct)2-(2T-DCV-Hex)2. Besides, a detailed study analyzed the charge transport properties and recombination loss mechanisms for these oligomers. This study not only revealed the importance of integrated alkyl chain engineering on gaining morphological control for high performance OSCs, but also exhibits the clear correlation between molecular ordering and charge carrier mobility respective to carrier dynamics. These results outline a detailed strategy towards a rather complete characterization and optimization methodology for organic photovoltaics, thereby paving the way for researchers to easily find the performance parameters adapted for widespread applications.
Journal of Materials Chemistry A 09/2015; DOI:10.1039/C5TA06706E · 7.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The molecular weight of an electron donor-conjugated polymer is as essential as other well-known parameters in the chemical structure of the polymer, such as length and the nature of any side groups (alkyl chains) positioned on the polymeric backbone, as well as their placement, relative strength, the ratio of the donor and acceptor moieties in the backbone of donor–acceptor (D–A)-conjugated polymers, and the arrangement of their energy levels for organic photovoltaic performance. Finding the “optimal” molecular weight for a specific conjugated polymer is an important aspect for the development of novel photovoltaic polymers. Therefore, it is evident that the chemistry of functional conjugated polymers faces major challenges and materials have to adopt a broad range of specifications in order to be established for high photovoltaic performance. In this review, the approaches followed for enhancing the molecular weight of electron-donor polymers are presented in detail, as well as how this influences the optoelectronic properties, charge transport properties, structural conformation, morphology, and the photovoltaic performance of the active layer.
[Show abstract][Hide abstract] ABSTRACT: The production of high performance, solution-processed kesterite Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) solar cells typically relies on high-temperature crystallization processes in chalcogen-containing atmosphere and often on the use of environmentally harmful solvents, which could hinder the widespread adoption of this technology. We report a method for processing selenium free Cu2ZnSnS4 (CZTS) solar cells based on a short annealing step at temperatures as low as 350°C using a molecular based precursor, fully avoiding highly toxic solvents and high temperature sulfurization. We show that a simple device structure consisting of ITO/CZTS/CdS/Al and comprising an extremely thin absorber layer (~110 nm) achieves a current density of 8.6 mA/cm2. Over the course of 400 days under ambient conditions encapsulated devices retain close to 100% of their original efficiency. Using impedance spectroscopy and photo-induced charge carrier extraction by linearly increasing voltage (photo-CELIV), we demonstrate that reduced charge carrier mobility is one limiting parameter of low-temperature CZTS photovoltaics. These results may inform less energy demanding strategies for the production of CZTS optoelectronic layers compatible with large-scale processing techniques.
[Show abstract][Hide abstract] ABSTRACT: With the aim of fully utilizing the low processing temperatures of perovskite solar cells, significant progress in replacing high temperature processed TiO2 by various low-temperature solution processed electron transporting layers (LT-ETLs) was recently reported. Here, recent progress in the development of LT-ETLs for regular planar structure perovskite solar cells, which is essential for achieving high efficiency in parallel to avoiding hysteresis, is reviewed. In addition, the application of a novel hysteresis-free LT-ETLs for regular planar perovskite solar cells in our laboratory is briefly discussed. By incorporating a low temperature processed WOx nanoparticular layer in combination with a mixed fullerene functionalized self-assembled monolayers (SAMs), a regular, planar structure, and hysteresis-free perovskite solar cell with a maximum efficiency of almost 15% can be fabricated.
Advanced Energy Materials 09/2015; DOI:10.1002/aenm.201501056 · 16.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nonradiative recombination reduces the open-circuit voltage relative to its theoretical limit and leads to reduced luminescence emission at a given excitation. Therefore, it is possible to correlate changes in luminescence emission with changes in open-circuit voltage and in the charge carrier lifetime. Here we use luminescence studies combined with transient photovoltage and differential charging analyses to study the effect of polymer fractionation in indacenoedithiophene-co-benzothiadiazole (IDTBT):fullerene solar cells. In this system, polymer fractionation increases electroluminescence emission at the same injection current and reduces nonradiative recombination. High-molecular-weight and fractionated IDTBT polymers exhibit higher carrier lifetime–mobility product compared to that of their nonfractionated analogues, resulting in improved solar cell performance.
The Journal of Physical Chemistry C 08/2015; DOI:10.1021/acs.jpcc.5b05709 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The power conversion efficiencies (PCEs) of the state-of-the-art organic tandem solar cells are steadily improved in the range of 10-12%, which can be mainly attributed to the design and development of highly efficient absorbers with complementary absorption spectra. However, the impressive record efficiencies are only achieved for devices spin-coated in an inert atmosphere, which does not directly contribute to commercialization of the organic photovoltaic technology. Here, we perform a systematic study of PTB7-Th based single-junction solar cells fabricated under various conditions. The relatively low photovoltaic performance and poor environmental stability of the air-processed devices are successfully recovered by a post-treatment with alcohol-based solvents. The effect of solvent treatment is valid for both regular and inverted device architecture. Tandem devices fabricated by doctor-blading in air achieve a high PCE of 10.03% along with an unprecedented high FF of 76.6%.
Energy & Environmental Science 08/2015; DOI:10.1039/C5EE02145F · 20.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Understanding the degradation and failure mechanisms of organic photovoltaic devices is a key requirement for this technology to mature toward a reliable product. Here, a report on accelerated temperature and moisture long-term stability testing (>20 000 h) of inverted and glass-encapsulated poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester solar cells is presented. The degradation kinetics using the Arrhenius model is analyzed and the activation energy for the diffusion of water is measured to be ≈43 kJ mol−1. Through comparison of electroluminescence imaging, lock-in thermography, and photoluminescence mapping, the device performance is correlated with the loss of effective cell area and it is shown that the reaction of water at the hole extraction/active layer interface is likely to be the dominant cause for long-term device failure. The diffusion of water through the packaged solar cell using classical diffusion theory is described here. Based on an analytical solution of a simple diffusion model, the diffusion coefficient is estimated to be 4 × 10−12 m2 s−1 and a shelf life of 100 000 h is anticipated at 65 °C/85% RH using a 9.3 cm wide protective adhesive rim. The findings of this study may inform strategies for predicting lifetimes of organic solar cells and modules based on local in situ tracking of moisture-induced device performance loss using IR imaging.
Advanced Energy Materials 08/2015; DOI:10.1002/aenm.201501065 · 16.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We developed an optical model for simulation and optimization of luminescent down-shifting (LDS) layers for photovoltaics. These layers consist of micron-sized phosphor particles embedded in a polymer binder. The model is based on ray tracing and employs an effective approach to scattering and photoluminescence modelling. Experimental verification of the model shows that the model accurately takes all the structural parameters and material properties of the LDS layers into account, including the layer thickness, phosphor particle volume concentration, and phosphor particle size distribution. Finally, using the verified model, complete organic solar cells on glass substrate covered with the LDS layers are simulated. Simulations reveal that an optimized LDS layer can result in more than 6% larger short-circuit current of the solar cell.
[Show abstract][Hide abstract] ABSTRACT: The photovoltaic performance and optoelectronic properties of a donor–acceptor copolymer are reported based on indacenodithienothiophene (IDTT) and 2,3-bis(3-(octyloxy)phenyl)quinoxaline moieties (PIDTTQ) as a function of the number-average molecular weight (Mn). Current–voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo-CELIV) reveal improved charge generation and charge transport properties in these high band gap systems with increasing Mn, while polymers with low molecular weight suffer from diminished charge carrier extraction because of low mobility–lifetime (μτ) product. By combining Fourier-transform photocurrent spectroscopy (FTPS) with electroluminscence spectroscopy, it is demonstrate that increasing Mn reduces the nonradiative recombination losses. Solar cells based on PIDTTQ with Mn = 58 kD feature a power conversion efficiency of 6.0% and a charge carrier mobility of 2.1 × 10−4 cm2 V−1 s−1 when doctor bladed in air, without the need for thermal treatment. This study exhibits the strong correlations between polymer fractionation and its optoelectronics characteristics, which informs the polymer design rules toward highly efficient organic solar cells.
[Show abstract][Hide abstract] ABSTRACT: We have modified the well-known Scharber model for the efficiency limits of organic solar cells for the case of semitransparent devices, which are considered as one of the most promising applications for this photovoltaic (PV) technology. We introduce a modified external quantum efficiency (EQE) relation and derive the transparency from the convolution of the calculated transmittance spectrum with the human eye sensitivity. We find that the design guidelines for the optimum band gap of the absorber material have to be carefully adapted for the desired transparency, especially for transparency values exceeding 40 %. In addition, we examine the relationship between efficiency, transmitted color, and band gap and conclude that high efficiencies can be achieved for a wide range of colors. Our model shows good agreement with experimental values from the literature and can be easily adapted to different applications (e.g., greenhouses) by using the appropriate spectrum.
Energy Technology 07/2015; DOI:10.1002/ente.201500131 · 2.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is crucial to control the lowest unoccupied molecular orbital (LUMO) of electron accepting materials for producing efficient charge transfer in bulk heterojunction (BHJ) solar cells. Due to their high LUMO level, soluble bis-adducts of C60 are of high interest for improving the Voc in BHJ solar cells. In this work, we have developed a novel bis-4-propylpentyl [6,6] methanofullerene bis-adduct, NCBA, using a alkyl solubilizing group. The optoelectronic, electrochemical and photovoltaic properties of this bis-product are investigated. NCBA is successfully applied as the electron acceptor with poly(3-hexylthiophene) (P3HT) in BHJ solar cell showing a high Voc of 0.73 V.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Building integrated semitransparent thin-film solar cells is a strategy for future eco-friendly power generation. Organic photovoltaics in combination with dielectric mirrors (DMs) are a potential candidate as they promise high efficiencies in parallel to the possibility to adjust the color and thus the transparency of the whole device. A fully solution processed and printable DM with an easily adjustable reflection maximum is presented that can be facilely attached to solar cells. The DM is optimized via optical simulations to the particular needs of the device with regard to photocurrent enhancement. The excellent agreement between experimental and theoretical results confirms the high optical quality of the printed layers with respect to homogeneity and surface roughness. The used inks are organic–inorganic nanocomposites with a large refractive index contrast of ≈0.7. The short-circuit current is enhanced by up to ≈24% for a semitransparent polymer solar cell.
[Show abstract][Hide abstract] ABSTRACT: A new ultra low band gap (LBG) α,β-unsubstituted BODIPY-based conjugated polymer has been synthesized by conventional cross coupling polymerization techniques (Stille cross coupling) for the first time. The polymer exhibits a panchromatic absorption spectrum ranging from 300 nm to 1100 nm and an optical band gap (Egopt) of 1.15 eV, suitable for near infrared (NIR) organic photovoltaic applications as electron donor. Preliminary power conversion efficiency (PCE) of 1.1 % in polymer:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) 1:3 weight ratio bulk heterojunction (BHJ) solar cells has been achieved, demonstrating very interesting and promising photovoltaic characteristics, such as good fill factor (FF) and open circuit voltage (Voc). These results showing that by the proper chemical design, new α,β-unsubstituted BODIPY-based NIR copolymers can be developed in the future with suitable energy levels matching those of PC71BM towards more efficient NIR organic photovoltaics (OPVs).
Journal of Materials Chemistry A 07/2015; DOI:10.1039/C5TA04229A · 7.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Synthesis of novel acceptor-donor-acceptor oligomers with electron-withdrawing alkyldicyanovinyl groups linked through an oligothiophene π-bridge with either dithienosilole or cyclopentadithiophene electron donor units is described. Changing the bridgehead atom from carbon to silicon in the central donor unit leads to a significant change in optical, thermal and structural properties of the oligomers. In addition, elongation of the oligothiophene π-bridge in the oligomers increases energies of HOMO and LUMO levels and leads to an unexpected hypsochromic shift of their absorption spectrum, because extension of the conjugation length cannot compensate a decrease of the intramolecular charge transfer between the dithienosilole and dicyanovinyl units. Although these minor changes in the chemical structures have a pronounced impact on the morphologies of their blends with PC70BM, the optimized solution-processed organic solar cells based on these small molecules demonstrate similar power conversion efficiencies
Dyes and Pigments 07/2015; 122:212-223. DOI:10.1016/j.dyepig.2015.06.026 · 3.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A smart photovoltaic window is designed and constructed by solution-processing a layer of thermochromic VO2 nanoparticles on top of a semitransparent organic solar cell. The prepared smart window not only produces electricity using the visible part of the solar spectrum but also saves energy via intelligently modulating the amount of NIR radiation passing through the device in response to ambient temperature.
[Show abstract][Hide abstract] ABSTRACT: Length of the terminal alkyl chains at dicyanovinyl (DCV) groups of two dithienosilole (DTS) containing small molecules (DTS(Oct)2-(2T-DCV-Me)2 and DTS(Oct)2-(2T-DCV-Hex)2 ) is investigated to evaluate how this affects the molecular solubility and blend morphology as well as their performance in bulk heterojunction organic solar cells (OSCs). While the DTS(Oct)2-(2T-DCV-Me)2 (a solubility of 5 mg mL−1) system exhibits both high short circuit current density (J sc) and high fill factor, the DTS(Oct)2-(2T-DCV-Hex)2 (a solubility of 24 mg mL−1) system in contrast suffers from a poor blend morphology as examined by atomic force morphology and grazing incidence X-ray scattering measurements, which limit the photovoltaic properties. The charge generation, transport, and recombination dynamics associated with the limited device performance are investigated for both systems. Nongeminate recombination losses in DTS(Oct)2-(2T-DCV-Hex)2 system are demonstrated to be significant by combining space charge limited current analysis and light intensity dependence of current–voltage characteristics in combination with photogenerated charge carrier extraction by linearly increasing voltage and transient photovoltage measurements. DTS(Oct)2-(2T-DCV-Me)2 in contrast performs nearly ideal with no evidence of nongeminate recombination, space charge effects, or mobility limitation. These results demonstrate the importance of alkyl chain engineering for solution-processed OSCs based on small molecules as an essential design tool to overcome transport limitations
Advanced Energy Materials 06/2015; DOI:10.1002/aenm.201500386 · 16.15 Impact Factor