Small Solar Cells Research Articles

Efficient Disinfection of Tap and Surface Water with Single High Power 285 nm LED and Square Quartz Tube

A small water disinfection system based on the combination of a strong single 25 mW LED with a wavelength of 285 nm and a short quartz tube with an outer rectangular cross section is presented. For the disinfection tests clear tap water and slightly turbid and yellow pond water are contaminated with high concentrations of Escherichia coli bacteria. These water samples are exposed to the germicidal 285 nm LED radiation while they flow through the quartz tube. The portion of surviving germs is determined by membrane filtration for different water qualities and flow rates. For clear tap water the bacteria concentration can be reduced by at least three orders of magnitude up to flow rates of about 20 L/h. In pond water the maximum flow rate for such a reduction is less than 3 L/h. These high disinfection capabilities and the small size of this system, allow its integration in medical systems for point of use disinfection or even its application in the Third World for decentralized water disinfection powered by small solar cells, because this disinfection capacity should be sufficient for small groups or families.

Synergistic effects of solvent and polymer additives on solar cell performance and stability of small molecule bulk heterojunction solar cells

The combination of solvent and polymer additives synergistically enhances solar cell performance and the stability of small molecule bulk heterojunction solar cells.

A portable electrospinning apparatus based on a small solar cell and a hand generator: design, performance and application.

Electrospinning (e-spinning) devices and electrospun (e-spun) ultrathin fibers have shown promising applications in various fields. However, the poor portability of conventional e-spinning devices limits some potential applications especially in the case without a plug (electricity supply). Consequently, great efforts have been made to modify e-spinning setups with good portability. In this article, a solar cell and a hand generator-powered portable e-spinning (SHPE) setup with good flexibility is introduced, which can be used outdoors without a plug. The SHPE device shows good spinning efficacy both in solution and melt e-spinning processes for a wide range of polymers. Moreover, the designed SHPE apparatus demonstrates potential application in wound dressing by in situ e-spinning fibers onto human skin directly.

Low band gap diketopyrrolopyrrole-based small molecule bulk heterojunction solar cells: influence of terminal side chain on morphology and photovoltaic performance

Two low band gap small molecules based on DPP with different terminal side chains were synthesized. They show similar physical properties but different photovoltaic property.

A Highly Efficient Ultralow Photovoltaic Power Harvesting System With MPPT for Internet of Things Smart Nodes

Implementing a monolithic highly efficient ultralow photovoltaic (PV) power harvesting system is pivotal for smart nodes of Internet of things (IOT) networks. This paper proposes a fully integrated harvesting system in 0.18-μm CMOS technology. Utilizing a small commercial solar cell of 2.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the proposed system can provide 0-29 μW of power, which is much higher than the commonly used passive radio-frequency identification devices in IOT application. The hill-climbing maximum power point tracking algorithm is developed in an energy-efficient manner to tune the input impedance of the system and guarantee adaptive maximum power transfer under wide illumination conditions. The detailed impedance tuning approach is implemented with a capacitor value modulation to eliminate the quiescent power consumption as well as to achieve a higher efficiency than the traditional pulse-frequency modulation scheme. A supercapacitor is utilized for buffering, energy storing, and filtering purposes, which enables more functions of the IOT smart nodes such as active sensing and system-on-chip (SOC) signal processing. The output voltage ranges between 3.0 and 3.5 V for different device loads, such as sensors, SOC, or wireless transceivers. The measured results confirm that this PV harvesting system achieves both ultralow operation capability under 20 μW and a selfsustaining efficiency of 89%.

Understanding Open‐Circuit Voltage Loss through the Density of States in Organic Bulk Heterojunction Solar Cells

The field of organic photovoltaics has recently produced highly efficient single‐junction cells with power conversion efficiency &gt;10%, yet the open‐circuit voltage (VOC) remains relatively low in many high performing systems. An accurate picture of the density of states (DOS) in working solar cells is crucial to understanding the sources of voltage loss, but remains difficult to obtain experimentally. Here, the tail of the DOS is characterized in a number of small molecule bulk heterojunction solar cells from the charge density dependence of VOC, and is directly compared to the disorder present within donor and acceptor components as measured by Kelvin probe. Using these DOS distributions, the total energy loss relative to the charge transfer state energy (ECT)—ranging from ≈0.5 to 0.7 eV—is divided into contributions from energetic disorder and from charge recombination, and the extent to which these factors limit the VOC is assessed.

High-Efficiency Small Molecule-Based Bulk-Heterojunction Solar Cells Enhanced by Additive Annealing.

Solvent additive processing is important in optimizing an active layer's morphology and thus improving the performance of organic solar cells (OSCs). In this study, we find that how 1,8-diiodooctane (DIO) additive is removed plays a critical role in determining the film morphology of the bulk heterojunction OSCs in inverted structure based on a porphyrin small molecule. Different from the cases reported for polymer-based OSCs in conventional structures, the inverted OSCs upon the quick removal of the additive either by quick vacuuming or methanol washing exhibit poorer performance. In contrast, the devices after keeping the active layers in ambient pressure with additive dwelling for about 1 h (namely, additive annealing) show an enhanced power conversion efficiency up to 7.78% with a large short circuit current of 19.25 mA/cm(2), which are among the best in small molecule-based solar cells. The detailed morphology analyses using UV-vis absorption spectroscopy, grazing incidence X-ray diffraction, resonant soft X-ray scattering, and atomic force microscopy demonstrate that the active layer shows smaller-sized phase separation but improved structure order upon additive annealing. On the contrary, the quick removal of the additive either by quick vacuuming or methanol washing keeps the active layers in an earlier stage of large scaled phase separation.

Peculiarity of Two Thermodynamically-Stable Morphologies and Their Impact on the Efficiency of Small Molecule Bulk Heterojunction Solar Cells.

Structural characteristics of the active layers in organic photovoltaic (OPV) devices play a critical role in charge generation, separation and transport. Here we report on morphology and structural control of p-DTS(FBTTh2)2:PC71BM films by means of thermal annealing and 1,8-diiodooctane (DIO) solvent additive processing, and correlate it to the device performance. By combining surface imaging with nanoscale depth-sensitive neutron reflectometry (NR) and X-ray diffraction, three-dimensional morphologies of the films are reconstituted with information extending length scales from nanometers to microns. DIO promotes the formation of a well-mixed donor-acceptor vertical phase morphology with a large population of small p-DTS(FBTTh2)2 nanocrystals arranged in an elongated domain network of the film, thereby enhancing the device performance. In contrast, films without DIO exhibit three-sublayer vertical phase morphology with phase separation in agglomerated domains. Our findings are supported by thermodynamic description based on the Flory-Huggins theory with quantitative evaluation of pairwise interaction parameters that explain the morphological changes resulting from thermal and solvent treatments. Our study reveals that vertical phase morphology of small-molecule based OPVs is significantly different from polymer-based systems. The significant enhancement of morphology and information obtained from theoretical modeling may aid in developing an optimized morphology to enhance device performance for OPVs.

Efficient solution processed D1-A-D2-A-D1 small molecules bulk heterojunction solar cells based on alkoxy triphenylamine and benzo[1,2-b:4,5-b′]thiophene units

Two molecules denoted as VC96 and VC97 have been synthesized for efficient (η=6.13% @ 100mW/cm2 sun-simulated light) small molecule solution processed organic solar cells. These molecules have been designed with the D1-A-D2-A-D1 structure bearing different central donor unit, same benzothiadiazole (BT) as π-acceptor and end capping triphenylamine. Moreover, the optical and electrochemical properties (both experimental and theoretical) of these molecules have been systematically investigated. The solar cells prepared from VC96:PC71BM and VC97:PC71BM (1:2) processed from CF (chloroform) exhibit a PCE (power conversion efficiency) of η=4.06% (Jsc=8.36mA/cm2, Voc=0.90V and FF=0.54) and η=3.12% (Jsc=6.78mA/cm2, Voc=0.92V and FF=0.50), respectively. The higher PCE of the device with VC96 as compared to VC97 is demonstrated to be due to the higher hole mobility and broader IPCE spectra. The devices based on VC96:PC71BM and VC97:PC71BM processed with solvent additive (3 v% DIO, 1,8-diiodooctane) showed PCE of η=5.44% and η=4.72%, respectively. The PCE device of optimized VC96:PC71BM processed with DIO/CF (thermal annealed) has been improved up to 6.13% (Jsc=10.72mA/cm2, Voc=0.88V and FF=0.61). The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive plus the thermal annealing.

Effects of Alkyl Terminal Chains on Morphology, Charge Generation, Transport, and Recombination Mechanisms in Solution‐Processed Small Molecule Bulk Heterojunction Solar Cells

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.

New D-A-D-A-D push–pull organic semiconductors with different benzo[1,2-b:4, 5-b′] dithiophene cores for solution processed bulk heterojunction solar cells

The synthesis and characterization of two D–A–D–A–D push–pull organic semiconductors based on either bis- (2-ethylhexyloxy) benzo [1,2-b:4,5-b′] dithiophene or bis- (triisopropylsilylethynyl) benzodithiophene electron rich core are described. The optical and electrochemical properties of these compounds indicate that these materials can absorb sunlight over a wide spectral range from 350 to 700 nm and exhibit suitable energy levels for efficient exciton dissociation. These small molecules were used as donor coupled with phenyl-C71-butyric acid methyl ester acceptor for the fabrication of solution processed small molecule bulk heterojunction solar cells. The rigid and -conjugation of the silylacetylene substituted analog π-extended compared to the hexyloxy substituted analog facilitates intermolecular packing interactions of small π–π charge transfer and intermolecular molecule which induce a deep HOMO level, producing a high open circuit voltage of ∼0.98 V. The achieved PCE was 5.69% when the active layer hexyloxy analog: phenyl-C71-butyric acid methyl ester was processed with optimized 1-chloronaphathalene additive.

3D-printed concentrator arrays for external light trapping on thin film solar cells

After our recent demonstration of a 3D-printed external light trap on a small solar cell, we now consider its potential for large solar panels. An external light trap consists of a parabolic concentrator and a spacer that redirects the photons that are reflected by the solar cell back towards the solar cell. These retro-reflections enable higher absorptance and improved power conversion efficiency. Scaling a single external light trap such that it covers a large solar panel has disadvantages in terms of height and cost of the external light trap. These disadvantages can be overcome by deploying an array of concentrators as the top part of the external light trap. We present an optimization study of concentrator arrays for external light trapping. We fabricated 3D-printed external light traps with a square, hexagonal and circular compound parabolic concentrator to test their suitability for concentrator arrays. The 3D-printed traps were placed on top of an organic solar cell which resulted in a significant enhancement of the external quantum efficiency. The required transmittance of these concentrator arrays is calculated as a function of the parameters of both the concentrator and the solar cell. We compare the theoretical and experimentally determined optical performance of the different concentrators. Finally, the prospects of external light trapping are analyzed and we give guidelines for improvements of the external light trap design.

The prediction of the morphology and PCE of small molecular organic solar cells

The predicted morphology, domain size, PCE (power conversion efficiency) of Small Molecular Organic Solar Cells.

Stable inverted small molecular organic solar cells using a p-doped optical spacer.

We report inverted small molecular organic solar cells using a doped window layer as an optical spacer. The optical spacer was used to shift the optical field distribution inside the active layers, generating more charge carriers from sunlight. In this report, N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) was doped with 2,2-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ), a p-type dopant material. P-doped MeO-TPD was adopted as an optical spacer because it has a large energy band gap, and its conductivity can be increased by several orders of magnitude through a doping process. As a result, a power conversion efficiency of 4.15% was achieved with the doped window layer of optimized thickness. Lastly, we present significantly improved stability of the inverted devices with the MeO-TPD layer.

Thiophene-based push–pull chromophores for small molecule organic solar cells (SMOSCs)

A concise review on small molecules organic solar cells based on π-conjugated thiophene scaffolds.

Cyano-substitution on the end-capping group: facile access toward asymmetrical squaraine showing strong dipole–dipole interactions as a high performance small molecular organic solar cells material

Cyano-substituted asymmetrical squaraines for organic solar cells with a high PCE was synthesized.

A–π–D–π–A based porphyrin for solution processed small molecule bulk heterojunction solar cells

Device AVC117:PC71BM (THF cast), device BVC117:PC71BM (pyridine/THF) and device CVC117:PC71BM (pyridine/THF) with thermal annealing.

Efficient small molecular ternary solar cells by synergistically optimized photon harvesting and phase separation

The champion PCE (7.40%) of optimized ternary solar cells is much higher than the PCEs sum (6.98%) of binary solar cells.

Morphology versus Vertical Phase Segregation in Solvent Annealed Small Molecule Bulk Heterojunction Organic Solar Cells

The deep study of solvent annealed small molecules bulk heterojunction organic solar cells based on DPP(TBFu)2 : PC60BM blend is carried out. To reveal the reason of the solvent annealing advantage over the thermal one, capacitance-voltage measurements were applied. It was found that controlling the vertical phase segregation in the solar cells a high fullerene population in the vicinity of the cathode could be achieved. This results in increase of the shunt resistance of the cell, thus improving the light harvesting efficiency.

Influence of thermal and solvent annealing on the morphology and photovoltaic performance of solution processed, D–A–D type small molecule-based bulk heterojunction solar cells

Four new small molecules CSDPP9–CSDPP12 were obtained with appended electron donating units in the molecular terminals of a DPP core. On solvent and thermal annealing, for the CSDPP11:PC71BM blend, the BHJ device displayed a PCE of 5.47%.