2,2-Dicyanovinyl-end-capped oligothiophenes as electron acceptor in solution processed bulk-heterojunction organic solar cells

Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680 nm with a LUMO energy level around −3.80 eV, which is slightly higher than that of [6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM, E_LUMO = −4.01 eV), but lower than that of poly(3-hexylthiophene) (P3HT, E_LUMO = −2.91 eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)₂, whereas PTB7:DTSi(THDCV)₂ based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10⁻⁵ cm² V⁻¹ s⁻¹ by space charge limited current method, which is much lower than that of PC₆₁BM, and was considered as one of the reason for the low photovoltaic performance.     

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%).     

High open-circuit voltage polymer solar cells based on D–A copolymer of indacenodithiophene and fluorine-substituted benzotriazole

A medium band gap D–A copolymer of indacenodithiophene (IDT) and fluorinated dithienylbenzotriazole (FBTA), PIDT-FBTA, was synthesized for the application as donor material in polymer solar cells (PSCs). PIDT-FBTA showed deeper highest occupied molecular orbital (HOMO) energy level due to the strong electron-withdrawing difluorine substitution on benzotriazole acceptor unit in the D–A copolymer. The PSCs based on PIDT-FBTA:PC₇₀BM (1:3) exhibited a high V_oc of 0.90 V and a power conversion efficiency (PCE) of 3.62% under the illumination of AM 1.5G, 100 mW cm⁻². The device performance was further improved by methanol treatment with PCE increased to 4.90% and V_oc increased to 0.92 V.     

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%.     

Large-area organic solar cells by accelerated blade coating

Large-area photovoltaic devices have been fabricated using the blade coating technique. In this study, the use of accelerated blade motion in this technique significantly improved the thickness uniformity of blade-coated layers of polymer solar cells on an A4 glass substrate. Two types of active layers, P3HT:PC₆₁BM and POD2T-DTBT:PC₇₁BM, were studied. For the P3HT:PC₆₁BM film, a thickness of 221 ± 14 nm was realised in a 12 × 15 cm² active region with a coating blade acceleration of 8 mm/s². For the POD2T-DTBT:PC₇₁BM film, a thickness of 98 ± 6 nm was realised with a coating blade acceleration of 10 mm/s². Ten cells, each measuring 0.9 cm × 12 cm and monolithically fabricated, were connected in series, yielding a total active area of 108 cm². The power conversion efficiency of the resulting 10-cell module was 2.66% and 3.64% for P3HT:PC₆₁BM and POD2T-DTBT:PC₇₁BM, respectively. The blade coating technique involving the accelerated blade motion is therefore useful for fabricating low-cost large-area organic solar cells, and it may be a promising alternative for the commercialisation of organic solar cells.     

Development of a new diindenopyrazine–benzotriazole copolymer for multifunctional application in organic field-effect transistors,polymer solar cells and light-emitting diodes

A new donor–acceptor (D?A) copolymer (PIPY–DTBTA) containing 6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine donor and benzotriazole acceptor was synthesized and characterized for multifunctional applications in organic field-effect transistors (OFETs), polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs). The polymer exhibits high molecular weights, excellent film-forming ability, a deep HOMO energy level, and good solution processability. Solution-processed thin film OFETs based on this polymer revealed good p-type characteristic with a high hole mobility up to 0.0521 cm² V^?1 s^?1. Bulk-heterojunction PSCs comprising this polymer and PC₆₁BM gave a power conversion efficiency (PCE) of 0.77%. The single-layer PLEDs based on PIPY–DTBTA emitted a yellow–red light with a maximum brightness of 385 cd m^?2 at the turn-on voltage of 6 V.     

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42 eV in solution and in film, respectively, and a HOMO level of 5.09 eV. PDTDP and PC₇₁BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of J_SC and V_OC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μ_h and μ_e, which affects the J_SC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53 cm² V⁻¹ s⁻¹ at 250 °C and 0.36 cm² V⁻¹ s⁻¹ at 310 °C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1 cm² V⁻¹ s⁻¹ at V_d = −5 V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.     

Synergism of molecular weight,crystallization and morphology of poly(3-butylthiophene) for photovoltaic applications

A serial of poly(3-butylthiophene) (P3BTs) with molecular weights (MWs) ranged from 7 kDa to 50 kDa is synthesized and characterized. The DSC studies show that the thermal property and crystallinity of P3BT increase with MWs within the range from 7 kDa to 25 kDa, then decrease with further higher MW. The characteristic morphologies of incontinuous crystalline nanofibrils, interconnected nanofibrillar networks, and relative large clusters associate to low, medium and high MW P3BTs, respectively, demonstrate the strong correlation between MW, crystallinity and morphology of P3BT. It is found that the P3BT could be re-considered as the promising candidate for applications in organic optoelectronics if synergism of the crystallinity and morphology could be precisely controlled via tuning molecular weight. The polymer solar cells (PSCs) device based on P3BT with medium MW achieves an attractive power conversion efficiency of 3.5%, which is, to the best of our knowledge, the record for P3BT/PC₆₁BM PSCs and comparable to the well-studied P3HT devices.     

D–A–Ar-type small molecules with enlarged π-system of phenanthrene at terminal for high-performance solution processed organic solar cells

Two novel D–A–Ar-type small molecules of TPA–DPP–P and TPA(DPP–P)₂ were synthesized and characterized, in which triphenylamine (TPA), diketopyrrolopyrrole (DPP) and phenanthrene (P) were used as the donor (D) core, acceptor (A) arm, and enlarged π-system of polycyclic arene (Ar) terminal. Their absorptive, electro-chemical, thermal, and photovoltaic properties were preliminary investigated. Significantly improved photophysical and photovoltaic performances were observed for both small molecules containing the planar P terminal in comparison with those for their parent D–A-type molecule of TPA–DPP. The highest power conversion efficiency (PCE) of 3.42% and a maximum short-circuit current density (J_sc) of 9.2 mA/cm² were obtained in the solution-processed TPA(DPP–P)₂-based solar cells using [6,6]-phenyl-C-71-butyric acid methyl ester (PC₇₁BM) as acceptor. The PCE and J_sc values are 8.76 and 4.97 times higher than those of the TPA–DPP-based cells, respectively. It indicates that appending the enlarged π-system of the planar P terminal and incorporating the DPP–P arm into D–A-type small molecule are efficient approaches to improve photophysical and photovoltaic performances for its resulting molecules.     

Synthesis,characterization, and photovoltaic performance of the polymers based on thiophene-2,5-bis((2-ethylhexyl)oxy) benzene-thiophene

Three novel conjugated copolymers based on thiophene-2,5-bis((2-ethylhexyl)oxy)benzene-thiophene (TBT) as electron-donating units, either isoindigo or both isoindigo and diketopyrrolopyrrole (DPP) as electron-withdrawing units have been designed and synthesized by Stille-coupling reaction. All the polymers exhibit high thermal stability, broad absorption in the range of 300–800 nm, and the low-lying energy level of highest occupied molecular orbits (HOMO) (−5.47 to −5.19 eV). After introduced with additional hexylthiophenes and further introduced with DPP units, the polymers PTBT-HTID and PTBT-HTID-DPP show smaller lamellar distance and π–π stacking distance, and the morphology of the corresponding photoactive layers possess more appropriate microphase separation and smaller domain size, which lead to high short circuit current densities (J_sc) and power conversion efficiency (PCE). The polymer photovoltaic devices based on PTBT-HTID-DPP/PC₆₁BM exhibit a high J_sc value of 11.13 mA cm⁻², a fill factor (FF) of 0.57, and the PCE of 4.2%.     

Detailed investigation of dependencies of photovoltaic performances of P3HT:PC61BM based solar cells on anodic work function modified by surface treatment of indium-tin-oxide electrode with benzenesulfonyl chloride derivatives

The photovoltaic (PV) characteristics of bulk-heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₁BM) were improved using indium-tin-oxide (ITO) anode electrodes modified chemically with CH₃O-, H-, Cl-, CF₃-, and NO₂-terminated benzenesulfonyl chlorides as a self-assembled monolayer (SAM). The ITO electrode surfaces were easily treated through the chemical modification of the reactive –SO₂Cl binding group, and the work function (WF) of the modified ITO was effectively changed depending on the permanent dipole moments introduced in the para-position of benzenesulfonyl chloride. We examined the correlation between the ITO WFs corrected by the change in the contact potential difference and the calculated dipole moments of the SAM models. Moreover, we examined the PV characteristics of the P3HT:PC₆₁BM based BHJ organic PV cells using the SAMs or poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-treated ITOs with different WFs lying within ±0.2 eV from the highest occupied molecular orbital (HOMO) level of P3HT. We found that the enhancement effect of the SAMs on the power-conversion efficiency (η_P) reached a maximum with Cl (η_P = 3.72%), and became larger than that of PEDOT:PSS (η_P = 3.62%). Two distinct J_sc dependencies, increasing and decreasing with the increasing WF of the anode ITO, were observed at higher and lower WFs than the HOMO level of the donor, respectively. Almost constant V_oc values (around 0.6 V) were observed with different SAM-modified ITOs, which suggested that Fermi level pinning was achieved by aligning the anode Fermi level and positive polaronic level of the donor polymer.     

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% @ 100 mW/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% (J_sc = 8.36 mA/cm², V_oc = 0.90 V and FF = 0.54) and η = 3.12% (J_sc = 6.78 mA/cm², V_oc = 0.92 V 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% (J_sc = 10.72 mA/cm², V_oc = 0.88 V 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.     

Improved efficiency of solution processed small molecules organic solar cells using thermal annealing

A solution processable A-D-A-D-A structure small molecule DCAEH5TBT using a BT unit as the core has been designed and synthesized for application in BHJ solar cells. The device employing DCAEH5TBT/PC₆₁BM as active layer shows PCE of 2.43% without any post treatment. After thermal annealing (150 °C, 10 min), the PCE of this molecule based device increased to 3.07%, with J_sc of 7.10 mA/cm², V_oc of 0.78 V and FF of 55.4%, which indicates that high performance of solution processed small molecule based solar cells can be achieved using thermal annealing by carefully design molecule structure.     

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.     

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.     

The dual localized surface plasmonic effects of gold nanodots and gold nanoparticles enhance the performance of bulk heterojunction polymer solar cells

In this study, we investigated the effects of plasmonic resonances induced by gold nanodots (Au NDs), thermally deposited on the active layer, and octahedral gold nanoparticles (Au NPs), incorporated within the hole transport layer, on the performance of bulk heterojunction polymer solar cells (PSCs) based on poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C₆₁butyric acid methyl ester (PC₆₁BM). Thermal deposition of 5.3-nm Au NDs between the active layer and the cathode in a P3HT:PC₆₁BM device resulted in the power conversion efficiency (PCE) of 4.6%—that is 15% greater than that (4.0%) for the P3HT:PC₆₁BM device without Au NDs. The Au NDs provided near-field enhancement through excitation of the localized surface plasmon resonance (LSPR), thereby enhancing the degree of light absorption.     

High‐Yield Synthesis and Electrochemical and Photovoltaic Properties of Indene‐C70 Bisadduct

[6, 6]‐Phenyl‐C₆₁‐butyric acid methyl ester (PC₆₀BM) is the widely used acceptor material in polymer solar cells (PSCs). Nevertheless, the low LUMO energy level and weak absorption in visible region are its two weak points. For enhancing the solar light harvest, the soluble C₇₀ derivative PC₇₀BM has been used as acceptor instead of PC₆₀BM in high efficiency PSCs in recent years. But, the LUMO level of PC₇₀BM is the same as that of PC₆₀BM, which is too low for the PSCs based on the polymer donors with higher HOMO level, such as poly (3‐hexylthiophene) (P3HT). Here, a new soluble C₇₀ derivative, indene‐C₇₀ bisadduct (IC₇₀BA), is synthesized with high yield of 58% by a one‐pot reaction of indene and C₇₀ at 180 °C for 72 h. The electrochemical properties and electronic energy levels of the fullerene derivatives are measured by cyclic voltammetry. The LUMO energy level of IC₇₀BA is 0.19 eV higher than that of PC₇₀BM. The PSC based on P3HT with IC₇₀BA as acceptor shows a higher V_oc of 0.84 V and higher power conversion efficiency (PCE) of 5.64%, while the PSC based on P3HT/PC₆₀BM and P3HT/PC₇₀BM displays V_oc of 0.59 V and 0.58 V, and PCE of 3.55% and 3.96%, respectively, under the illumination of AM1.5G, 100 mW cm^?2. The results indicate that IC₇₀BA is an excellent acceptor for the P3HT‐based PSCs and could be a promising new acceptor instead of PC₇₀BM for the high performance PSCs based on narrow bandgap conjugated polymer donor.     

Revealing the effect of donor/acceptor intermolecular arrangement on organic solar cells performance based on two-dimensional conjugated small molecule as electron donor

A series of solution processed organic solar cells (OSCs) were fabricated with a two-dimensional conjugated small molecule SMPV1 as electron donor and fullerene derivatives PC₇₁BM or ICBA as electron acceptor. The champion power conversion efficiency (PCE) of OSCs arrives to 7.05% for the cells with PC₇₁BM as electron acceptor. A relatively large open circuit voltage (V_OC) of 1.15 V is obtained from cells using ICBA as electron acceptor with an acceptable PCE of 2.54%. The fill factor (FF) of OSCs is 72% or 61% for the cells with PC₇₁BM or ICBA as electron acceptor, which is relatively high value for small molecule OSCs. The relatively low performance of OSCs with ICBA as electron acceptor indicates that ICBA cannot play positive role in photoelectric conversion processes, which is very similar to the phenomenon observed from the OSCs with high efficient narrow band gap polymers other than P3HT as electron donor, the underlying reason is still in debate. The SMPV1 has strong self-assemble ability to form an ordered two dimensional lamellar structure, which provides an effective platform to investigate the effect of electron acceptor chemical structure on the performance of OSCs. Experimental results exhibit that ICBA molecules may prefer to vertical cross-intercalation among side chains of SMPV1, PC₇₁BM molecules may have better miscibility with SMPV1 in the active layer. The different donor/acceptor (D/A) intermolecular arrangement strongly influences photon harvesting, exciton dissociation and charge carrier transport, which may provide a new sight on performance improvement of OSCs by adjusting D/A intermolecular arrangements.     

High open-circuit voltage of the solution-processed organic solar cells based on benzothiadiazole–triphenylamine small molecules incorporating π-linkage

Two novel small molecular photovoltaic (PV) materials, BDPTBT and BDATBT were designed and synthesized, consisting of 5,6-bis-(octyloxy)benzo[c][1,2,5]thiadiazole (DOBT) as electron-withdrawing core (A), and triphenylamine (TPA) as electron-donating side group (D). Moreover, the benzene and ethynylbenzene as π-linkage were introduced to form donor–π-acceptor–π-donor (D–π-A–π-D) typed molecular structures, respectively. To fully investigate the linkage effect of a series of small molecules, two reference compounds BDCTBT and BDETBT were also studied systematically, consisting of 2-phenylacrylonitrile and styrene as π-linkage, respectively. As a result, the π-linkage units, benzene, styrene, ethynylbenzene and 2-phenylacrylonitrile played an important role in modifying molecular structure and improving PV performance. Bulk heterojunction (BHJ) solar cells based on BDPTBT/PC₆₁BM and BDATBT/PC₆₁BM yielded the power conversion efficiencies (PCEs) of 2.99% and 2.03%, respectively. Notably, BDATBT based device showed a high open-circuit voltage (Voc) of 1.03 V. Compared to the results we have reported previously, the reference devices based on BDCTBT/PC₆₁BM and BDETBT/PC₆₁BM with the optimized weight ratio showed dramatically enhanced PCEs of 4.84% and 3.40%, respectively, and BDCTBT based device showed a high Voc of 1.08 V. To our knowledge, the Voc of 1.08 V is the highest voltage reported to date for devices prepared from solution-processed small-molecule-donor materials, and the PCE of 4.84% is the highest efficiency reported so far for D–A–D-typed benzothiadiazole (BT)–TPA based solution-processed small molecules PV devices.     

Effect of thermal annealing on the electrical properties of P3HT:PC70BM nanocomposites

We studied the electrical properties of organic photovoltaic (OPV) devices based on poly (3-hexylthiophene) and fullerene derivative [6, 6]-Phenyl-C₇₀-butyric acid methyl ester nanocomposite (P3HT:PC₇₀BM) as a function of the annealing temperature. Thermal annealing enables crystallization of the polymer and diffusion of the PC₇₀BM molecules. Diode parameters, such as the barrier height ϕ_b and the ideality factor n were calculated. They were found to be depend strongly on the annealing temperature. This dependence is attributed to surface states, inhomogeneity in the material and series resistance. Best OPV devices had a short circuit current density of 3.35 mA/cm², an open circuit voltage of 0.68 V, a fill factor of 0.45, and a power conversion efficiency of 2.2%, by applying a thermal annealing temperature of 150 °C for 10 min.