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.     

Systematic optimization of low bandgap polymer/[6,6]-phenyl C70 butyric acid methyl ester blend photodiode via structural engineering

Synthetic approaches for optimizing polymer-based organic photodiodes (OPDs) by systematically analyzing the effects of the hole-blocking layer, the electron-blocking layer, and the thickness and morphology of the active layer with respect to the dark current and detectivity have been reported. PBDTT-DPP with a repeating alkylthienylbenzodithiophene (BDTT) and diketopyrrolopyrrole (DPP) units is used as a p-type polymer for achieving both broadband absorption and a high absorption coefficient in conjunction with n-type [6,6]-phenyl C₇₀ butyric acid methyl ester (PC₇₀BM) for constructing photoactive layers. Through systematic investigations of various interfacial layers, we found that the thickness of the active layer and the energy level of the hole/electron blocking layer were critical for minimizing the dark current of OPDs. By changing the deposition method of the PBDTT-DPP/PC₇₀BM blend and using post treatment, we discovered that the morphology of the active layer was directly related to the photocurrent of OPDs. Furthermore, we conducted a comparative study between a bulk heterojunction and a planar heterojunction (PHJ) to demonstrate the effectiveness of the PHJ for suppressing the dark current. Consequently, we realized a high detectivity of 5.3 × 10¹² Jones with an optimized device architecture and morphology. This work shows the importance of a synthetic approach for optimizing OPDs that requires both a high photocurrent and a low dark current in the reverse saturation regime.     

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%).     

Influence of molar mass ratio,annealing temperature and cathode buffer layer on power conversion efficiency of P3HT:PC71BM based organic bulk heterojunction solar cell

In bulk heterojunction (BHJ) solar cells, the molar mass ratio of donor-acceptor polymers, the annealing temperature (T_an) and the cathode buffer layer plays very consequential role in improving the power conversion efficiency (PCE) by tuning the film morphology and enhancing the charge carrier dynamics. A comprehensive understanding of each of these factors is essential in order to optimize the performance of organic solar cells (OSCs). Albeit there are several fundamental reports regarding these factors, an altogether meticulous correlation of these physical processes with experimental evidence of the photo active layer are required. In this work, we systematically analyzed the influence of different molar mass ratio, the annealing temperature (T_an) and the cathode buffer layer of rrP3HT:PC₇₁BM based BHJ solar cells and their corresponding photovoltaic performances were correlated carefully with their thin film growth structure and energy level diagram. The device having 1:0.8 molar mass ratio of rrP3HT:PC₇₁BM and T_an = 150 °C annealing temperature with Bathocuproine (BCP) as the cathode buffer layer having ITO/PEDOT:PSS/rrP3HT:PC₇₁BM (molar mass ratio = 1:0.8; (T_an = 150 °C)/BCP/Al) configuration showed the best device performance with PCE, ɳ = 4.79%, Jsc = 14.21 mA/cm², V_oc = 0.58 V and FF = 57.8%. This drastic variation in PCE of the device having BCP/Al as the cathode contact compared to the other device configurations is due to the coalesced effects of better hole-blocking capacity of BCP along with Al and better phase separation of the active blend layer at 150 °C annealing temperature. These results explicate the cumulate role of all these physical parameters and their combined contribution to the PCE amendment and overall device performance with rrP3HT:PC₇₁BM based organic BHJ solar cell.     

Synthesis,optical and electrochemical properties new donor–acceptor (D–A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b″] trithiophene donor and different acceptor units: Application as donor for photovoltaic devices

Two new conjugated D–A polymers P3 (PBTT-d-BTT) and P4 (PBTT-d-TPD) based on same benzo[1,2-b:3,4-b′:6,5-b″] trithiophene (BTT) donor and different acceptors monomers 5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b′:6,5-b″] trithiophene (d-BTT), and 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione (d-TPD) respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), ¹H NMR, UV–Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. Photovoltaic properties of the polymers were studied by using the polymers as donor and PC₇₁BM as acceptor with a weight ratio of polymer:PC₇₁BM 1:1, 1:2 and 1:2.5. The optimized photovoltaic device was fabricated with an active layer of a blend P3:PC₇₁BM and P4:PC₇₁BM with a blend ratio of 1:2 showed PCE 3.16% and 2.42%, respectively under illumination of AM 1.5 at 100 mW/cm² with solar simulator. The PCE of the device based on P3:PC₇₁BM processed with DIO/o-DCB has been further improved up to 4.64% with J_sc of 10.52 mA/cm² and FF of 0.58 attributed to the increase in crystalline nature of active layer and more balanced charge transport in the device, induced by DIO additive.     

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.     

Two new D–A conjugated polymers P(PTQD-Th) and P(PTQD-2Th) with same 9-(2-octyldodecyl)-8H-pyrrolo[3,4-b]bisthieno[2,3-f:3′,2′-h]quinoxaline-8,10(9H)-dione acceptor and different donor units for BHJ polymer solar cells application

We report the synthesis, characterization and photovoltaic properties of bulk heterojunction polymer solar cells of new donor–acceptor conjugated copolymers P(PTQD-Th) and P(PTQD-2Th) that incorporate same strong 9-(2-octyldodecyl)-8H-pyrrolo[3,4-b]bisthieno[2,3-f:3′,2′-h]quinoxaline-8,10(9H)-dione as strong acceptor and different weak thiophene (Th) and bi-thiophene (2Th) as donors, respectively. Both the copolymers showed suitable unoccupied lowest molecular orbital (LUMO) energy levels, compatible with the LUMO of PC₇₁BM for efficient electron transfer from copolymer to PC₇₁BM in the blended copolymer: PC₇₁BM thin films. Moreover the deeper highest occupied molecular orbital (HOMO) energy levels of both copolymers ensures the high open circuit voltage (V_oc) of the BHJ polymer solar cells. The optimized P(PTQD-Th):PC₇₁BM and P(PTQD-2Th):PC₇₁BM with weight ratio of 1:2 processed with chloroform solvent showed PCE of 3.65% and 3.96%, respectively. The higher value of J_sc for the device processed with P(PTQD-2Th):PC₇₁BM as compared to that for P(PTQD-Th):PC₇₁BM, attributed to narrower optical bandgap and broader absorption profile for P(PTQD-2Th) as compared to P(PTQD-Th). The PCE values of polymer solar cells were further improved (5.54% and 5.67% for P(PTQD-Th):PC₇₁BM and P(PTQD-2Th):PC₇₁BM, respectively) when small amounts of solvent additive, i.e. 1,8-diiodoctane (DIO) were used for the processing of active layers. The improved PCE has been attributed to both the enhanced values of short circuit current (J_sc) and fill factor (FF) due to the better nanomorphology and charge transport, induced by the high boiling point of solvent additive.     

Effect of blend layer thickness on the dielectric response of PCDTBT:PC71BM-based bulk heterojunction solar cells

Bulk heterojunction solar cells based on a blend of poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), were studied. The organic photoactive layers were spin coated onto a poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT-PSS) interfacial layer at speeds of 600 and 2000 rpm. The molecular structure of PCDTBT, PC₇₁BM, and the PCDTBT:PC₇₁BM blend was investigated using Fourier-transform IR (FTIR) spectroscopy, which confirmed the absence of interactions between the individual components of the composite. The dielectric properties of PCDTBT:PC₇₁BM-based solar cells were studied under illumination by means of impedance analysis. The dielectric constant, impedance, and ac conductance were analyzed as a function of frequency at different bias voltages close to the open circuit voltage (Voc). We found that the dielectric constant, dielectric loss, and conductance increased with increasing PCDTBT:PC₇₁BM thickness. Impedance spectroscopy analysis revealed decreases in charge recombination and the resistance of the whole device with increasing spin coating speed for the active layer. Moreover, an increase in recombination resistance for the solar cells was observed close to V_OC.     

A plasmonically enhanced polymer solar cell with gold–silica core–shell nanorods

We report the use of chemically synthesized gold (Au)–silica core–shell nanorods with the length of 92.5 ± 8.0 nm and diameter of 34.3 ± 4.0 nm for the efficiency enhancement of bulk heterojunction (BHJ) polymer solar cells. Silica coated Au nanorods were randomly blended into the BHJ layers of these solar cells. This architecture inhibits the carrier recombination at the metal/polymer interface and effectively exploits light absorption at the surface plasmon resonance wavelengths of the Au–silica nanorods. To match the two plasmon resonant peaks of the Au–silica nanorods, we employed a low bandgap polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) to construct a solar cell. The absorption spectrum of PCPDTBT:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) is relatively wide and matches the two plasmon resonance peaks of Au–silica nanorods, which leads to greater plasmonic effects. We also constructed the poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₀BM) cells for comparison. The absorption spectrum of P3HT:PC₆₀BM only overlaps one of the plasmon resonance peak of Au–silica nanorods. The efficiency of the P3HT:PC₆₀BM device incorporating optimized Au–silica nanorods is enhanced by 12.9% from 3.17% to 3.58%, which is due to the enhanced light absorption. Compared with the P3HT:PC₆₀BM device with Au–silica nanorods, the PCPDTBT:PC₇₀BM device with 1 wt% Au–silica nanorods concentration has a higher efficiency of 4.4% with an increase of 26%.     

Synthesis of a Modified PC70BM and Its Application as an Electron Acceptor with Poly(3‐hexylthiophene) as an Electron Donor for Efficient Bulk Heterojunction Solar Cells

A simple and effective modification of phenyl‐C₇₀‐butyric acid methyl ester (PC₇₀BM) is carried out in a single step after which the material is used as electron acceptor for bulk heterojunction polymer solar cells (PSCs). The modified PC₇₀BM, namely CN‐PC₇₀BM, showed broader and stronger absorption in the visible region (350–550 nm) of the solar spectrum than PC₇₀BM because of the presence of a cyanovinylene 4‐nitrophenyl segment. The lowest unoccupied molecular energy level (LUMO) of CN‐PC₇₀BM is higher than that of PC₇₀BM by 0.15 eV. The PSC based on the blend (cast from tetrahydrofuran (THF) solution) consists of P3HT as the electron donor and CN‐PC₇₀BM as the electron acceptor and shows a power conversion efficiency (PCE) of 4.88%, which is higher than that of devices based on PC₇₀BM as the electron acceptor (3.23%). The higher PCE of the solar cell based on P3HT:CN‐PC₇₀BM is related to the increase in both the short circuit current (J_sc) and the open circuit voltage (V_oc). The increase in J_sc is related to the stronger light absorption of CN‐PC₇₀BM in the visible region of the solar spectrum as compared to that of PC₇₀BM. In other words, more excitons are generated in the bulk heterojunction (BHJ) active layer. On the other hand, the higher difference between the LUMO of CN‐PC₇₀BM and the HOMO of P3HT causes an enhancement in the V_oc. The addition of 2% (v/v) 1‐chloronapthalene (CN) to the THF solvent during film deposition results in an overall improvement of the PCE up to 5.83%. This improvement in PCE can be attributed to the enhanced crystallinity of the blend (particularly of P3HT) and more balanced charge transport in the device.     

Fluorine functionalized asymmetric indo [2,3-b]quinoxaline framework based D-A copolymer for fullerene polymer solar cells

Innovating molecular structure of copolymer donor materials is still one of the prominent approach to obtain high-performance polymer solar cells (PSCs). In this paper, two novel wide bandgap (WBG) copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric planar aromatic core indo [( Li et al., 2012; Wang et al., 2020) 2,32,3-b]quinoxaline (IQ) as acceptor unit through tuning side chains with fluorine (F) atom engineering and exemplary alkylthio-thienyl substituted benzodithiophene (BDTTS) donor group, are synthesized and finally employed as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). After blending with PC₇₁BM acceptor, the PBDTTS-DFIQ:PC₇₁BM blend film presented better efficient exciton dissociation and charge extraction, more balanced electron/hole mobility (μ_h/μ_e), and nice morphology in comparison with PBDTTS-IQ:PC₇₁BM blend film. Encouragingly, the PBDTTS-DFIQ:PC₇₁BM based PSCs exhibits a higher power conversion efficiency (PCE) of 7.4% than that of the device based on the PBDTTS-IQ:PC₇₁BM blend with a PCE of 4.96%, which thanks to an enhancement of open-circuit voltage (V_oc) of 0.84 V, short current density (J_sc) of 13.26 mA cm⁻² and fill factor (FF) of 66.00% simultaneously. These results demonstrate that this asymmetric IQ framework is a wonderful acceptor moiety to build light-harvesting copolymers for highly efficient PSCs.     

Filter-Free Narrowband Photomultiplication-Type Planar Heterojunction Organic Photodetectors

Filter-free narrowband photomultiplication-type planar heterojunction (PHJ) organic photodetectors (PM-PHOPDs) are first realized by employing a thick front donor layer and an ultrathin PC₇₁BM layer. The thick front donor layer is employed as an optical field adjusting (OFA) layer. The sequentially coated PC₇₁BM will diffuse slightly into OFA layer, which works as interfacial electron traps to capture photogenerated electrons for assisting hole tunneling injection. The P3HT/PC₇₁BM-based PM-PHOPDs exhibit narrowband response with full-width of half-maximum of 32 nm and external quantum efficiency (EQE) of 1700% at 650 nm under −20 V bias. Due to the enhanced hole transport and reduced charge recombination in PHJ compared to those in bulk heterojunction (BHJ), the EQE of P3HT/PC₇₁BM-based narrowband PM-PHOPDs is twice as P3HT:PC₇₁BM BHJ-based narrowband PM-OPDs under the same bias. The response peak of PM-PHOPDs is adjusted from 650 to 695 or 745 nm by incorporating SMPV1 or DRCN5T in OFA layers due to the red-shifted absorption edge. The EQEs of 3600% at 695 nm and 870% at 745 nm are obtained for P3HT:SMPV1 and P3HT:DRCN5T-based PM-PHOPDs under −20 V bias, respectively. This work provides a smart strategy to achieve narrowband PM-OPDs by designing different OFA layers.     

Improving the nanoscale morphology and processibility for PCDTBT-based polymer solar cells via solvent mixtures

The effect of solvent mixtures on the morphologies of poly[N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) blend films is investigated. 1,2,4-Trichlorobenzene (TCB) which is a good solvent for PCDTBT is selected to mix with chloroform (CF), chlorobenzene (CB) and o-dichlorobenzene (oDCB) for tuning the morphology of the PCDTBT:PC₇₁BM blend. It is found that formation of nanoscale phase separation with a fibrillar PCDTBT nanostructure of PCDTBT:PC₇₁BM blend which is favorable for exciton separation and charge carrier transport is strongly dependent on the solubility parameters of the solvent mixtures. A clearly defined nanoscale phase separation of the PCDTBT:PC₇₁BM blend can be obtained with TCB:CF mixture. The resulted morphology is similar to that produced with sole DCB solvent that is currently the best solvent for PCDTBT:PC₇₁BM blend solar cells. Moreover, the TCB:CF mixture demonstrates better solubility and processibility for PCDTBT:PC₇₁BM blend and allows us to prepare thick active layer that is required in large-area roll-to-roll process. The polymer solar cells with 250 nm- thick active layer are fabricated by using TCB:CF solvent mixture and the power conversion efficiency of the devices reaches 6.45%. A highest short-circuit current of 13.6 mA/cm² is achieved due to enhanced optical absorption of thick active layer.     

Solvent–vapor induced morphology reconstruction for efficient PCDTBT based polymer solar cells

This paper reports polymer solar cells with a 7% power conversion efficiency (PCE) based on bulk heterojunction (BHJ) composites of the alternating co-polymer, poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), and the fullerene derivative [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM). As confirmed by transmission electron microscopy, solvent–vapor annealing (SVA) of the thin (70 nm) BHJ photoactive layer by exposure to chloroform vapor, for a short period of time (30 s) after deposition, leads to reconstructed nanoscale morphology of donor/acceptor domains, well-dispersed fullerene phase and effective photo-absorption of BHJ. Consequently, SVA-reconstructed devices with a PCDTBT:PC₇₁BM blend ratio of 1:5 (wt%) exhibit ～50% improvement in PCE, with short-circuit current J_sc = 15.65 mA/cm², open-circuit voltage V_oc = 0.87 V, and PCE = 7.03%, in comparison to those of the 1:4 (wt%) blends with SVA treatment.     

Synthesis and characterization of a wide bandgap polymer based on a weak donor-weak acceptor structure for dual applications in organic solar cells and organic photodetectors

We synthesized a novel wide bandgap polymer, PDTFBT, forming a weak donor (WD)-weak acceptor (WA) structure for use in organic photodetectors (OPDs) and organic solar cells (OSCs). The fluorination in the D unit and the alkoxy substitution in the A unit induced WD and WA properties, respectively. The WD-WA structure of PDTFBT effectively broadened the bandgap compared to typical D-A structures, and the S-F and S-O dipole-dipole interactions induces a highly planar backbone structure with excellent π-π stacking in the vertical direction. In OPDs, conformationally less disordered PDTFBT polymer retained the constant responsivity and significantly improved the detectivity of PDTFBT:PC₇₁BM devices even with a thick active layer of 470 nm, contrary to the variation in the responsivity of P3HT:PC₆₁BM devices depending on the thickness. In OSCs, the deep HOMO energy level (−5.57 eV) of PDTFBT led to high Voc of 0.92 V in PDTFBT:PC₇₁BM devices, which was 0.3 eV higher than that of P3HT:PC₆₁BM devices (0.62 V), resulting in 1.8-fold enhanced power conversion efficiency. We demonstrated that the WD-WA structure with S-F and S-O interactions is highly promising strategy to make wide bandgap polymers for organic photodetectors and for the bottom cell of tandem architecture.     

Functionalized Graphene as an Electron‐Cascade Acceptor for Air‐Processed Organic Ternary Solar Cells

Functionalized graphene nanoflakes (GNFs) are used as an electron‐cascade acceptor material in air‐processed organic ternary bulk heterojunction solar cells. The functionalization is realized via the attachment of the ethylenedinitrobenzoyl (EDNB) molecule to the GNFs. Simulation and experimental results show that such nanoscale modification greatly influences the density of states near the Fermi level. Consequently, the GNF‐EDNB blend presents favorable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels to function as a bridge structure between the poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and the [6,6]‐phenyl‐C71‐butyric‐acid‐methyl‐ester (PC₇₁BM). The improved exciton dissociation and charge transport are associated with the better energy level alignment of the ternary blend and the high electrical conductivity of the GNFs, which act as additional electron transport channels within the photoactive layer. The resulting PCDTBT/GNF‐EDNB/PC₇₁BM ternary organic solar cells, fabricated entirely under ambient conditions, exhibit an average power conversion efficiency enhancement of ≈18% as compared with the binary blend PCDTBT/PC₇₁BM.     

An asymmetric small molecule based on thieno[2,3-f]benzofuran for efficient organic solar cells

A new asymmetric small molecule, named R3T-TBFO, with 4,8-bis(2-ethylhexyloxy)-substituted thieno[2,3-f]benzofuran (TBF) as central donor block, has been synthesized and used as donor material in organic solar cells (OSCs). With thermal annealing (TA) and solvent vapor annealing (SVA) treatment, the blend of R3T-TBFO/PC₇₁BM shows a higher hole mobility of 1.37 × 10⁻⁴ cm² V⁻¹ s⁻¹ and a more balanced charge mobilities. Using a structure of ITO/PEDOT:PSS/R3T-TBFO:PC₇₁BM/ZrAcac/Al, the device with TA treatment delivered a moderate power conversion efficiency (PCE) of 5.63%, while device after TA + SVA treatment showed a preferable PCE of 6.32% with a high fill factor (FF) of 0.72.     

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.     

Enhancing efficiency for additive–free blade–coated small–molecule solar cells by thermal annealing

Blade coating was successfully applied to realise high-efficiency small-molecule organic solar cells (OSCs) with a solution-processed active layer comprising a small organic molecule DR3TBDTT with a benzo[1,2–b:4,5–b′]dithiophene (BDT) unit as the central building block as the donor and [6,6]–phenyl–C71–butyric acid methyl ester (PC₇₁BM) as the acceptor. Using chloroform as the solvent, a DR3TBDTT/PC₇₁BM blend active layer without an additive was effectively formed through blade coating. The power conversion efficiency (PCE) of small organic molecule solar cells was enhanced by 3.7 times through thermal annealing at 100 °C. This method produces OSCs with a high PCE of up to 6.69%, with an open circuit voltage (V_oc) of 0.97 V, a short-circuit current density (J_sc) of 12.60 mA/cm², and a fill factor (FF) of 0.55.     

Donor–acceptor–acceptor–donor small molecules for solution processed bulk heterojunction solar cells

We report on the optical and electrochemical characterization (experimental and theoretical) of two donor substituted benzothiadiazole with different cyano based acceptor π-linkers, tetracyanobutadiene (TCBD) SM1 and dicyanoquinomethane (DCNQ) SM2, and explore them as the donor component for solution processed bulk heterojunction organic solar cells, along with PC₇₁BM as the electron acceptor. The solution bulk heterojunction (BHJ) solar cells based on dichloromethane (DCM) processed active layer with SM1 and SM2 as donor and PC₇₁BM as acceptor achieve power conversion efficiency (PCE) of 2.76% and 3.61%, respectively. The solar cells based on these two small molecules exhibit good Voc, which is attributed to their deep HOMO energy level. The higher PCE of the device based on SM2 compared to SM1 is attributed to the its small bandgap, broader absorption profile and enhanced hole mobility. Additionally, the PCE of the SM2:PC₇₁BM based solar cells processed with 1-chloronaphthalene CN (3 v%)/DCM is further improved reaching upto 4.86%. This increase in PCE has been attributed to the improved nanoscale morphology and more balanced charge transport in the device, due to the solvent additive.