Improving charge transport and suppressing charge recombination in small molecule ternary solar cells via incorporating Bis-PC71BM as a cascade material

The development of small molecule organic solar cells (SMOSCs) has attracted considerable attention and achieved comparable power conversion efficiency (PCE) with polymer solar cells. Here, we demonstrate a bulk heterojunction (BHJ) small molecular solar cell with PCE of 5.31% by incorporating Bisadduct of phenyl-C₇₁-butyric acid methyl ester (Bis-PC₇₁BM) as an additional acceptor material into the host binary blend of 2-[4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate (SQ-BP): [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM). The short circuit current (J_SC) and the fill factor (FF) of ternary SMOSCs are improved by decreasing the carrier recombination loss, increasing exciton dissociation and enhancing the carrier transport. The transient photovoltage (TPV) measurement indicates that the gradient HOMO energy alignment suppresses the charge recombination and leads to the increased charge carrier lifetime in ternary SMOSCs. As a result, the PCE of ternary devices with 5 wt% Bis-PC₇₁BM is about 20% greater than that of SQ-BP: PC₇₁BM based binary SMOSCs.     

Efficient ternary organic solar cells based on immiscible blends

Organic photovoltaic cells based on ternary blends of materials with complementary properties represent an approach to improve the photon-absorption and/or charge transport within the devices. However, the more complex nature of the ternary system, i.e. in diversity of materials' properties and morphological features, complicates the understanding of the processes behind such optimizations. Here, organic photovoltaic cells with wider absorption spectrum composed of two electron-donor polymers, F8T2, poly(9,9-dioctylfluorene-alt-bithiophene), and PTB7, poly([4,8-bis[(2′-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2′-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]), mixed with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) are investigated. We demonstrate an improvement of 25% in power conversion efficiency in comparison with the most efficient binary blend control devices. The active layers of these ternary cells exhibit gross phase separation, as determined by Atomic Force Microscopy (AFM) and Synchrotron-based Scanning Transmission X-ray Microscopy (STXM).     

Efficient bilayer heterojunction polymer solar cells with bumpy donor–acceptor interface formed by facile polymer blend

We demonstrated a facile method for the fabrication of bilayer polymer solar cells with a controlled heterojunction structure via simple polymer blends. The spontaneous phase separation of poly(3-hexylthiophene)/polyethylene glycol blends provides a bumpy electron-donor layer with characteristic circular depressions. The diameter and depth of the circular depressions can be controlled by varying the PEG content of the blend. The deposition of -phenyl-C61-butyric acid methyl ester as an electron-acceptor layer then creates an interpenetrating donor–acceptor interface for bilayer heterojunction polymer solar cells. The bumpy morphology of the interface results in a significant enhancement in the power conversion efficiency over that of the bilayer polymer solar cells with a typical planar interface, which is mainly due to an increase of photocurrent. An estimation of the field-dependent possibility of charge separation indicates that charge extraction is more efficient than charge recombination in the bilayer devices and the increase in the interfacial area of solar cells with a bumpy-interface leads to generate more electron-hole pairs at the interface.     

A two-dimension-conjugated small molecule for efficient ternary organic solar cells

Ternary organic solar cells (OSCs) are burgeoning as one of the effective strategies to achieve high power conversion efficiencies (PCEs) by incorporating a third component with a complementary absorption into the binary blends. In this study, we presented a new two-dimension-conjugated small molecule denoted by DR3TBDTTVT, which alone gave rise to a best PCE of 5.71% with acceptor PC₇₁BM as active layer. Given the complementary absorption with PTB7-Th, DR3TBDTTVT was doped into (PTB7-Th:PC₇₁BM)-based binary blends, and ternary OSCs were developed. The ternary OSCs with 10 wt% of DR3TBDTTVT displayed improved hole-mobility, reduced device resistance and better phase separation of active layer, thus leading to an impressive PCE of 7.77% with open-circuit voltage of 0.77 V, short-circuit density of 14.52 mA cm⁻² and fill factor of 70.3%. Ternary OSCs well make up for the light-harvesting insufficiency of binary OSCs, and this research provides a new material for the improvement of PCEs for single-junction OSCs.     

Performance improvement of low bandgap polymer bulk heterojunction solar cells by incorporating P3HT

This work demonstrates a significant improvement of device performance by incorporating the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) into a low bandgap polymer poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b′]dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) host system, to form a ternary blend bulk heterojunction solar cell. The P3HT concentration was varied from 1 to 5 wt% in the host system. P3HT functions as a morphology control agent in this ternary system. A small weight percentage of P3HT can enhance the light absorption, polymer phase separation, exciton separation and charge carrier mobilities. These results are supported by UV–vis spectroscopy, X-ray diffraction, photoluminescence analysis and other characterisation methods. The highest average power conversion efficiency improvement of 10% was achieved by adding 1 wt% P3HT to the host system. This study reveals a promising way to achieve high efficiency solar cells using a low bandgap polymer.     

Morphology-dependent charge recombination through localized states in polymer/polymer blend solar cells

Polymer solar cells (PSCs) based on the PBDTTT-EFT (polymer donor):N2200 (polymer acceptor) blend have been fabricated with the active layer processed with various solvents including o-dichlorobenzene (o-DCB), chlorobenzene (CB), chloroform (CF), and CF:p-xylene (PX) mixed solvent. A highest power conversion efficiency (PCE) of 5.37% is achieved with the active layer processed with CF:PX mixed solvent. The dependence of short-circuit current density (J_SC) on incident light intensity indicates that the charge recombination is to some extent suppressed in the device with the active layer processed with the CF:PX mixed solvent and results in an enhanced J_SC. Morphology studies disclosed that the domains are preferential face-on orientation in the active layers processed with o-DCB, CB and CF, while it shows a combined face-on and edge-on orientation in CF:PX-processed film. The long-lived trap-assisted charge recombination originated from the active layer morphological variance has been focused on and investigated. And the nanosecond transient absorption experiment further demonstrated that the PBDTTT-EFT:N2200 film processed with o-DCB shows obvious long-lived trap-assisted charge recombination, while the trap-assisted charge recombination is effectively suppressed in the PBDTTT-EFT:N2200 film processed with CF:PX mixed solvent. This implies that deep localized traps correlated with the PBDTTT-EFT:N2200 morphology are reduced by using the CF:PX mixed solvent to process the active layer. In addition, the domains with a mixed orientation in the active layer may also enhance the three-dimensional charge transport and hence improve the charge-collection efficiency.     

Bifunctional-based small molecule as multifunctional additive for improved performance of perovskite solar cells

Hybrid perovskites have great potential as light-absorbing materials, while its degradation of poor crystallization and ion migration have been the major obstacle in perovskite solar cells (PSCs). Herein, the bifunctional-based small molecule dicyandiamide (DCD) was applied as additive in the PSCs. The amino and cyano group of DCD could effectively control crystallization and passivating grain defects, resulting in the high-quality perovskite films with large grain size. In addition, the adding of DCD increases the film conductivity of perovskite active layer, which is beneficial for charge transport in perovskite film. The DCD added PSCs shows an optimal power conversion efficiency (PCE) of 19.08% with negligible hysteresis. Furthermore, the long-term stability of PSCs is significantly enhanced. The results indicate that the device's integrative performance could be efficiently improved by the synergistic effect of amino and cyano functional groups, meaning that the addition of DCD into perovskite precursor could enable this optimization.     

Optical,electrical and mechanical properties of indium tin oxide on polyethylene terephthalate substrates: Application in bulk-heterojunction polymer solar cells

We investigated optical, electrical and mechanical properties of indium tin oxide (ITO) on flexible polyethylene terephthalate (PET) substrate, considering bulk-heterojunction (BHJ) polymer solar cells applications. Encapsulation of flexible solar cells with the architecture PET/ITO/PEDOT:PSS/P3HT:PCBM (or P3HT:PCBM:AZ-NDI-4)/Al was done by direct brush-painting with nail enamel. Active cell layer blends of [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with regioregular or regiorandom poly(3-hexylthiophene-2,5-diyl) (P3HT) were applied. Additionally for this role the mixture of regioregular P3HT:PCBM with naphthalene diimide–imine with four thiophene rings AZ-NDI-4 was tested. Obtained photovoltaic (PV) and optical (UV–vis) results of the flexible polymer solar cells were compared with the same architecture of devices on the glass/ITO substrate.     

Incorporation of diketopyrrolopyrrole dye to improve photovoltaic performance of P3HT:PC71BM based bulk heterojunction polymer solar cells

Ternary blend solar cells have been intensively studied in recent years to harvest more photons over the near-IR region. In this work, the effects of adding a diketopyrrolopyrrole dye (py-DPP) into a conventional P3HT:PC₇₁BM based bulk heterojunction photovoltaic cell are investigated. The near infrared absorption of the blend is enhanced by the doped py-DPP dye, leading to more than 20% increased power conversion efficiency compared to the P3HT:PC₇₁BM binary system. The highest efficiency of 4.05% is achieved for a P3HT:PC₇₁BM blend with 2.4 wt % of py-DPP.     

An improved circuit model for polymer solar cells

Authenticity of conventional circuit model, to interpret the characteristics of polymer solar cells (PSCs) is examined. Conventional circuit model is found to be quite limited, and various assumptions used there are not valid for PSCs. By understanding the nature of photovoltaic characteristics, through detailed investigations, we developed an improved circuit model, which explains correctly the behavior of PSCs under different environmental conditions. Investigations are carried out on the solar cells, made of the blend of regioregular poly(3‐hexylethiophene) (P3HT) and phenyl [6,6] C₆₁ butyric acid methyl ester (PCBM). The model is developed by treating both the dark and illuminated characteristics separately, even the characteristics were dealt with separately in reverse and forward biases. The formulated equivalent circuit model helps us in explaining many other important features, observed in the characteristics of PSCs. Copyright © 2013 John Wiley & Sons, Ltd.     

Acceptor-rich bulk heterojunction polymer solar cells with balanced charge mobilities

In this work, we reported efficient polymer solar cells with balanced hole/electron mobilities tuned by the acceptor content in bulk heterojunction blend films. The photovoltaic cells were fabricated with two new wide band-gap D-A polymers PBDDIDT and PBDDIDTT as the donor material. The molecular conformations of new polymers are carefully evaluated by theoretical calculations. The results of photovoltaic studies show that two devices reach their optimal conditions with rich PC₇₁BM content up to 80% in blend films, which is uncommon with most of reported PSCs. The as-cast devices based on PBDDIDT and PBDDIDTT reveal good photovoltaic performance with PCE of 7.04% and 6.40%, respectively. The influence of PC₇₁BM content on photovoltaic properties is further detailed studied by photoluminescence emission spectra, charge mobilities and heterojunction morphology. The results exhibit that more efficient charge transport between donor and acceptor occurs in rich PC₇₁BM blend films. Meanwhile, the hole and electron mobilities are simultaneously enhanced and afford a good balance in rich PC₇₁BM blend films (D/A, 1:4) which is critical for the improvement of current density and fill factors.     

Recent advances and prospects of asymmetric non-fullerene small molecule acceptors for polymer solar cells

Recently,polymer solar cells developed very fast due to the application of non-fullerence acceptors.Substituting asym-metric small molecules for symmetric small molecule acceptors in the photoactive layer is a strategy to improve the perform-ance of polymer solar cells.The asymmetric design of the molecule is very beneficial for exciton dissociation and charge trans-port and will also fine-tune the molecular energy level to adjust the open-circuit voltage (Voc) further.The influence on the ab-sorption range and absorption intensity will cause the short-circuit current density (Jsc) to change,resulting in higher device per-formance.The effect on molecular aggregation and molecular stacking of asymmetric structures can directly change the micro-scopic morphology,phase separation size,and the active layer's crystallinity.Very recently,thanks to the ingenious design of act-ive layer materials and the optimization of devices,asymmetric non-fullerene polymer solar cells (A-NF-PSCs) have achieved re-markable development.In this review,we have summarized the latest developments in asymmetric small molecule acceptors(A-NF-SMAs) with the acceptor-donor-acceptor (A-D-A) and/or acceptor-donor-acceptor-donor-acceptor (A-D-A-D-A) struc-tures,and the advantages of asymmetric small molecules are explored from the aspects of charge transport,molecular energy level and active layer accumulation morphology.     

Small molecule ternary solar cell with two synergistic electron acceptors for enhanced photovoltaic performance

In recent years, tremendous progresses have been achieved for solution processed organic solar cells (OSCs). The strategy of adding a third component to fabricate ternary solar cells has emerged as an effective method to enhance the power conversion efficiency (PCE) of devices. Furthermore, small molecules feature as lower viscosity and excellent repeatability which facilitate the effective morphology control during fabrication process for enhanced photovoltaic performance. Herein, we report a series of ternary solar cells based on a liquid crystal molecule BTR and two electron acceptors of PC₇₁BM and Y6. These molecules show complementary absorption to broaden spectra coverage and form energy levels cascade for efficient charge transfer. Meanwhile, thanks to the improved molecular packing and formed efficient charge transport network in the ternary blend film, the optimal ternary device possesses the improved charge dynamics and suppressed charge recombination. Thus, ternary solar cells deliver the highest PCE of 11.82% with simultaneously enhanced parameters of J_SC, V_OC and FF. This finding further illustrates the important roles of synergistic effect of fullerenes and non-fullerene acceptors in fabricating highly efficient ternary solar cells.     

Two-dimensional conjugated copolymers composed of diketopyrrolopyrrole,thiophene, and thiophene with side chains for binary and ternary polymer solar cells

Two new two-dimensional conjugated copolymers (named r-PTTDPP50 and r-PTTDPP75) consisting of a diketopyrrolopyrrole (DPP) derivative, thiophene with a conjugated side chain, and 2,5-bis(trimethylstannyl)thiophene were designed and synthesized via Stille cross-coupling reactions for use in bulk heterojunction (BHJ) polymer solar cells (PSCs); the feed-in ratios were varied to obtain the copolymers. It was found that the content of DPP units in the copolymer main chain significantly affected the molecular weight, absorption range, electronic energy level, and morphology of thin films of the copolymers. In the thin-film state, both copolymers exhibited a broad absorption band with two obvious peaks and a vibronic shoulder, as well as an absorption edge for wavelengths of up to 1000 nm. The vibronic shoulder in the absorption spectrum of r-PTTDPP75 was more intense than that in the spectrum of r-PTTDPP50, owing to the presence of a greater number of coplanar DPP units in the former. Electrochemical measurements indicated that the highest occupied molecular orbital (HOMO) energy levels for r-PTTDPP50 and r-PTTDPP75 were −5.16 and −5.19 eV, respectively, while their lowest unoccupied molecular orbital (LUMO) energy levels were −3.89 and −3.99 eV, respectively. On increasing the number of electron-deficient DPP segments in r-PTTDPP75, the LUMO energy level was lowered. Further, its HOMO energy level was also affected. BHJ PSCs composed of the electron-donor copolymers blended with an electron acceptor, namely [6,6],-phenyl-C61-butyric acid methyl ester (PC₆₁BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), in 1:2 wt ratio were fabricated and characterized. The power conversion efficiency (PCE) of the r-PTTDPP50/PC₇₁BM-based (w/w = 1:2) PSC reached 2.32% for an open-circuit voltage of 0.632 V, short-circuit current of 9.81 mA/cm², and fill factor of 37.4%, under the illumination of AM 1.5G (100 mW/cm²). Ternary blend BHJ solar cells formed by doping r-PTTDPP50 into the common binary blend of P3HT and PC₆₁BM were also investigated. The optimized r-PTTDPP50:P3HT:PC₆₁BM device exhibited a PCE of 3.85%, which was significantly higher than that of the P3HT:PC₆₁BM device (2.97%).     

Porphyrin small molecules containing furan- and selenophene-substituted diketopyrrolopyrrole for bulk heterojunction organic solar cells

Two porphyrin small molecules substituted with either furan- or selenophene-linked diketopyrrolopyrrole units are designed and synthesized. The impacts of the O and Se chalcogen atoms in the linking 5-membered ring in these new donor materials on the performance of organic solar cells are discussed and contrasted with the previously described thiophene analogue. The heavy atoms broaden the absorption and narrow the bandgap by tuning the energy levels regularly. Furthermore, the selenophene containing analogue shows better miscibility and smaller phase separation with [6,6]-phenyl C₆₁-butyric acid methyl ester (PC61BM) than the furan analogue. The optimized organic solar cells based on the furan and the selenophene containing species are achieved in the presence of pyridine and 1,8-diiodooctane additives with power conversion efficiencies of 4.3% and 5.8%, respectively, under simulated AM 1.5 illumination (100 mW cm⁻²).     

Investigation of VOPcPhO as an acceptor material for bulk heterojunction solar cells

In this study, we have successfully demonstrated a new system of donor–acceptor blend for bulk heterojunction solar cells of poly(3-hexylthiophene) (P3HT) by using vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (VOPcPhO) as acceptor material. A broad absorption over the whole visible range (450–750 nm) is achieved. Utilizing this blend system in solar cell fabrication, ITO/PEDOT:PSS/P3HT:VOPcPhO/Al solar cells have been fabricated and characterized in open air. A maximum power conversation efficiency up to 1.09% has been recorded. To confirm the charge transport, the electron and hole mobility of VoPcPhO has been measured. The results show that the VoPcPhO has bipolar transport and can act as an electron as well as hole transporting material. The electron mobility is comparable with hole mobility.     

Hexa-peri-hexabenzocoronene and diketopyrrolopyrrole based D-A conjugated copolymers for organic field effect transistor and polymer solar cells

Hexa-peri-hexabenzocoronene (HBC) is a disc-shaped conjugated molecule with strong π-π stacking property, high intrinsic charge mobility and good self-assembly property. But for a long time, the organic photovoltaic (OPV) solar cells based on HBC small organic molecules demonstrated low power conversion efficiencies (PCEs). In this study, a series of polymers named as PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT were designed and synthesized through copolymerization of HBC with bulky mesityl substituents and strong electron-withdrawing diketopyrrolopyrrole (DPP) with different alkyl side chains and various π-bridges. Introduction of DPP unit into the HBC derivatives broadened the absorption spectra and lowered the band gaps. Bulky mesityl substituents attached to periphery of HBC prevented polymers from self-aggregating into too large domain size in the blend films of photovoltaic devices. The different π-bridges have significant effect on the structure conformation of the polymers. The polymer PHBCDPPDT with bithiophene π-bridges demonstrated the broadest absorption for its extensive π-conjugation and more coplanar conformation compared with the thiophene π-bridge one. PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT gave field-effect hole mobilities of 1.35 × 10⁻³, 2.31 × 10⁻⁴, 2.79 × 10⁻⁴ and 8.60 × 10⁻³ cm² V⁻¹ s⁻¹, respectively. The solar cells based on these polymers displayed PCEs of 2.12%, 2.85%, 1.89% and 2.74%. To our knowledge, 2.85% is the highest PCE for the HBC-based photovoltaic materials till now.     

Improved mobility and lifetime of carrier for highly efficient ternary polymer solar cells based on TIPS-pentacene in PTB7: PC71BM

Highly efficient ternary polymer solar cells (T-PSCs) realized by the improved mobility and lifetime of carrier in PTB7: PC₇₁BM: TIPS-pentacene blends were fabricated. By adjusting the weight ratios of third component TIPS-pentacene in the binary PTB7: PC₇₁BM blends, we found that the short circuit current and fill factor (FF) were simultaneously enhanced, resulting in a maximum power conversion efficiency (PCE) of 8.09% with 21.3% improvement. The improved photovoltaic performance of T-PSC was mainly due to the enhanced light absorption, energy level cascading, optimized blend morphology, and increased hole mobility. It was also found that the incorporation of TIPS-pentacene increased the average hole lifetime, ensuring efficient hole transport and collection with suppressed bimolecular recombination, contributing to the photocurrent. Additionally, the low thickness dependent row-off of FF indicates TIPS-pentacene is a promising third component for the realization of thick film T-PSC. The improved PCEs were obtained as well for other ternary donor: acceptor: TIPS-pentacene systems, demonstrating that the incorporation of TIPS-pentacene is a wide practicable methodology for the development of highly efficient T-PSCs.     

Thermally stable high performance non-fullerene polymer solar cells with low energy loss by using ladder-type small molecule acceptors

Two ladder-type small molecule acceptors IDT-BT-R and IDT-BT-R-CN are utilized in non-fullerene polymer solar cells by pairing with PTB7-Th as donor polymer, in which PTB7-Th:IDT-BT-R solar cells achieve high performance up to 8.3% with high voltage of 1.02 V and low energy loss of 0.59 eV. Thermal annealing triggered local rearrangement of ladder-type molecules in n-type phases of bulk heterojunction films, increased their absorption abilities and electron transport properties, therefore resulting in improved short-circuit current densities (Jscs) and fill factors (FFs), in contrast to fullerene-based solar cells which suffered from extensive aggregation of fullerenes upon annealing. The outstanding thermal stability, high performance and low energy loss demonstrated here show great potential of non-fullerene polymer solar cells.     

High open circuit voltage in efficient thiophene-based small molecule solution processed organic solar cells

We have synthesized and fully characterized an oligothiophene small organic molecule for its use as electron donor moiety in solution processed bulk-heterojunction organic solar cells. Our results show that device solvent annealing process of the conjugated oligothiophene molecule leads to a light-to-energy conversion efficiency of 3.75% under standard illumination conditions. The solar cell presents open-circuit voltage and fill factors as high as 1.01 V and 63.05% respectively, which are among the highest values obtained for small molecule solution processed organic solar cells.