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.     

Linear and propeller-like fluoro-isoindigo based donor–acceptor small molecules for organic solar cells

Two donor–acceptor type fluoro-isoindigo based small molecule semiconductors are synthesized and their optical, electrochemical, thermal, and charge transport properties are investigated. The two molecular chromophores differ by their architecture, linear (M1) vs propeller-like (M2). Both molecules present a broad absorption in the visible range and a low optical HOMO–LUMO gap (∼1.6 eV). AFM images of solution-processed thin films show that the trigonal molecule M2 forms highly oriented fibrils after a few seconds of solvent vapor annealing. The materials are evaluated as electron donor components in bulk heterojunction organic solar cells using PC₆₁BM as the electron acceptor. The devices based on the propeller-like molecule M2 exhibit a high open-circuit voltage (around 1.0 V) and a power conversion efficiency of 2.23%.     

Merocyanines for vacuum-deposited small-molecule organic solar cells

We report here synthesis and photovoltaic properties of three merocyanines dyes (DPPT, DTPT, 1-NPPT) which are functionalized with electron withdrawing thiazolidenemalononitrile and electron rich diarylamine functionalities. It is found that structural feature of the diarylamino groups has a profound effect on the physical properties such as the absorption spectrum, oxidation potential, and HOMO/LUMO energy levels. The compound DTPT containing a better electron-donating ditolyl group, exhibits red-shifted absorption with relatively higher molar extinction coefficient, indicating its better light-harvesting ability. Hole mobility of these compounds is found to be strongly dependent on the various intermolecular interactions. Interestingly, single crystal structures reveal that the crystal packing motifs are rather closely related to the observed hole mobility in a trend of DPPT > DTPT > 1-NPPT. Vacuum-processed small-molecule organic solar cells were fabricated using the title merocyanines as p-type materials (donor) in combination with fullerene (C₆₀ or C₇₀) as n-type material (acceptor) with various device configurations. Among them, the DPPT-based devices outperform the devices based on DTPT and 1-NPPT. The power conversion efficiency (PCE) of DPPT-based device was improved from 1.55% of a BHJ device to 2.63% of a PMHJ device and 3.52% of a PMHJ device without the thin donor layer.     

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.     

Vinazene end-capped acceptor-donor-acceptor type small molecule for solution-processed organic solar cells

An acceptor-donor-acceptor (A-D-A) type molecule based on dioctyltertthiophene-benzo[1,2-b:4,5-b′]dithiophene-dioctyltertthiophene central donor and vinazene terminal acceptor was designed and synthesized for solution-processed small molecule bulk-heterojunction (BHJ) solar cells. The thermal and optochemical properties, BHJ morphology and solar cell performance were investigated. The BHJ morphology was systematically optimized by thermal annealing, solvent vapor annealing, and the use of solvent additives. Processed by a combination of thermal annealing and solvent vapor annealing treatments, V-BDT:PC₇₁BM device showed an optimized PCE of 3.73% with a V_OC of 0.89 V, an J_SC of 6.88 mA cm⁻² and a FF of 0.61.     

Oligothiophene-based small molecules with 3,3′-difluoro-2,2′-bithiophene central unit for solution-processed organic solar cells

Two new oligothiophene-based small molecules, namely DRCN6T-F and DRCN8T-F, with 3,3′-difluoro-2,2′-bithiophene as the central building block and 2-(1,1-dicyanomethylene)-rhodanine as end groups, were designed and synthesized. Compared to their non-fluorinated counterparts DRCN6T and DRCN8T, DRCN6T-F and DRCN8T-F exhibit enhanced intermolecular interactions and lower HOMO energy levels. However, PCEs of 2.26% and 5.07% were obtained for DRCN6T-F and DRCN8T-F based optimized devices, respectively, lower than those of non-fluorinated molecules DRCN6T and DRCN8T. The relatively poor performance for the DRCN6T-F and DRCN8T-F were mainly caused by their low short-circuit current densities, due to their unfavorable morphologies and low charge carrier mobilities.     

Structure-property relationship of D-A type copolymers based on thienylenevinylene for organic electronics

A series of D-A type conjugated polymers based on (E)-1,2-bis(3-dodecyllthiophen-2-yl)ethene (TV) as electron donor unit and with different repeating subunits, PTVBO8, PTVBT8, PTVTBO12, and PTVTBT12 were synthesized for use in organic field effect transistors and bulk heterojunction organic photovoltaics. Upon incorporation of alkoxy substituents in acceptor units, benzooxadiazole (BO) and benzothiadiazole (BT), polymer solubility improved and higher molecular weight polymers were obtained. In addition, all copolymers showed favorable thermal stability (T_d > 300 °C), and low band gap properties (1.49–1.67 eV). The thiophene-flanked TV-TBX copolymers, PTVTBO12 and PTVTBT12, exhibited higher molecular weight and superior device performance in both OFETs and OPVs compared with the TV-BX copolymers. The electronic energy levels of copolymers were strongly influenced by the nature of acceptor units, while optical band gaps and shape of molecular orientation of polymer chains were affected by the presence or absence of thiophene spacer. Charge carrier mobilities in TV-TBX copolymers were 1 order of magnitude greater than in TV-BX copolymers. OFETs based on a PTVTBT12 with TG/BC configuration displayed the highest hole mobility of 0.48 cm² V⁻¹ s⁻¹. The photovoltaic device containing a PTVTBO12:PC₇₁BM (1:2 w/w) blend system exhibited best performance with a V_oc of 0.56 V, a short-circuit current density (Jsc) of 13.1 mA cm⁻², a fill factor (FF) of 69%, and a power conversion efficiency (PCE) of 5.0%.     

Room temperature solution-processed electron transport layer for organic solar cells

We present a new recipe for a solution-processed titanium oxide (TiO_x) based electron transport layer at room temperature. Due to its high chemical compatibility with all types of organic blends (semi-crystalline or amorphous) and it is good adhesion to both surfaces of glass/ITO substrate and the active layer (blend), the buffer layer is suitable for use in organic solar cell devices with conventional, inverted or multi-junction structures. The main goal of this recipe is producing with easiness an repeatable and stable precursor that will leads to titanium oxide buffer layer each time with the same quality. Since the processing of the titanium oxide layer itself does not require any initial or additional treatment before and after the coating, and can even be carried in air as well as under protective atmosphere, our room temperature solution-processed electron transport layer is highly versatile and very promising for cost effective mass production of organic solar cells.     

Ternary organic solar cells based on non-fullerene acceptors: A review

Organic solar cells (OSCs) have reached their second golden age in recent two years with a boosted number of publications. Non-fullerene acceptor (NFA) materials have become a rising star in the field which are widely applied in organic solar cells because of their excellent optoelectronic properties, such as strong light-harvesting ability and tunable energy level. Unlike the low synthetic flexibility and high production cost of fullerene materials, NFAs exhibit flexible structures, and relatively low fabrication costs. Recently, the ternary strategy has become another hot research topic in the field, which introduces a third component into the binary host system for OSCs. The application of a ternary strategy can break the limits of light absorption brought by the host system, improve the morphology and energy level alignment for the active layer and thus improved the efficiency of organic solar cell devices. Benefiting from the advancement in both NFA and ternary strategy, the power conversion efficiency (PCE) of organic solar cell has exceeded over 17.5% to date. A comprehensive review of the recent progress in NFA based ternary OSCs (TOSCs) is needed in the field. Herein, this review mainly focuses on recent research on ternary organic solar cells using NFA materials during the last two years. Firstly, device physics and frequently used active materials in NFA based TOSCs are summarized and discussed. Then, the recent reported high-performance NFA based TOSCs are reviewed. Finally, the outlook and future research direction in the field are proposed. This review aims to provide an insight into NFA based TOSCs and help researchers to explore the full potential of OSCs.     

D-A-D-A-D push pull organic small molecules based on 5,10-dihydroindolo[3,2-b]indole (DINI) central core donor for solution processed bulk heterojunction solar cells

Two D-A-D-A-D small molecules based on same 5,10-dihydroindolo [3,2-b]indole central donor core and different benzothiadiazole (BT) and fluorine substituted BT (FBT) acceptor units, denoted as p-DINI-(BTTh₃)₂ (1) and p-DINI-(FBTTTh₃)₂ (2), respectively were synthesized and their optical and electrochemical properties were investigated. These molecules were applied as donor along with PC₇₁BM as electron acceptor for the fabrication of solution processed bulk heterojunction organic solar cells. The solar cells prepared from the optimized active blended layer (1:2) cast from dichlorobenzene (DCB) showed overall power conversion efficiency (PCE) of 2.02% and 2.70% for 1 and 2, respectively as donor. The higher PCE of 2 as compared to 1 is attributed to the higher hole mobility and broader IPCE spectra. In order to improve the PCE we have employed a two step treatment of active layer i.e. solvent vapor annealing after thermal annealing (SVA-TA) and the PCE has been enhanced up to 4.14% and 5.27% for optimized 1:PC₇₁BM and 2:PC₇₁BM active layers, respectively. The improvement in the PCE has been resulted from the improvement in the balanced charge transport and better crystallinity of the donor in the blended active layer.     

A new dithienosilole-based oligothiophene with methyldicyanovinyl groups for high performance solution-processed organic solar cells

A new linear dithienosilole-based oligothiophene end-capped with methyl and electron-withdrawing dicyanovinyl groups, DTS(Oct)₂-(2T-DCV-Me)₂, was prepared in good yield. This oligomer exhibited broad absorption spectra in bulk down to the near-IR region with the optical edge at 900 nm, resulting in an initially high power conversion efficiency of 5.44% in solution-processed organic solar cells using PC₇₁BM as an acceptor.     

Triphenylamine modified bis-diketopyrrolopyrrole molecular donor materials with extended conjugation for bulk heterojunction solar cells

Two new solution-processable enlarged π-conjugated donor–acceptor (D–A) organic small molecules consisting of dialkoxysubstituted benzo[1,2-b:4,5-b′]dithiophene (BDT) or dioctyltertthiophene (3T) as the central donor units, diketopyrrolopyrrole (DPP) as the acceptor unit and triphenylamine (TPA) as the terminal conjugated segment, TPA–DPP–BDT and TPA–DPP–3T, were designed and synthesized. Both small molecules possess broad absorption ranging from 300 to 800 nm with an optical band at approximately 1.50 eV and relatively low HOMO energy levels from −5.12 to approximately −5.16 eV. Expectedly, the UV–Vis absorption onset (810 nm) of TPA–DPP–BDT is largely red-shifted (60 nm) relative to that (750 nm) of previously reported BDT(TDPP)₂, which consists of BDT and DPP units. Unlike most of the TPA based molecules, strong molecular aggregation was observed in the solid state for both small molecules. In addition, atomic force microscopy (AFM) and X-ray diffraction (XRD) investigations indicated that TPA–DPP–3T and TPA–DPP–BDT exhibit good miscibility with fullerene derivatives. The organic solar cells based on TPA–DPP–BDT/PC₆₁BM(1:1) demonstrated power conversion efficiencies as high as 4.04% with a short-circuit current density (J_sc) of 11.40 mA cm⁻² and a fill factor (FF) of 53.2% when the active layer of the cell was annealed at 130 °C for 10 min.     

Power conversion efficiency enhancement of organic solar cells by addition of gold nanostars,nanorods, and nanospheres

The effects of gold (Au) nanoparticles (NPs) with different morphologies (star, rod, sphere) incorporated into buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), of polymer-based organic solar cells (OSCs) were investigated. Solar cells having gold nanoparticles exhibited significant improvement in device efficiency relative to the reference device. The observed improvement is most likely due to the surface plasmon and enhanced light reflection and scattering properties of the Au NPs. The power conversion efficiency (PCE) is increased ca. 29% with Au nanostars, ca. 14% with Au nanorods and 11% with Au nanospheres compared to the device with no Au NP (reference device). Au nanostars provide the strongest contribution to the efficiency among all NP morphologies studied as they have large size, sharp features, and strongest localized surface plasmon resonance effect associate with their morphology.     

Solution-processed annealing-free ZnO nanoparticles for stable inverted organic solar cells

We report the development and application of high-quality zinc oxide nanoparticles (ZnO NPs) processed in air for stable inverted bulk heterojunction solar cells as an electron extraction layer (EEL). The ZnO NPs (average size ∼11 nm) were dispersed in chloroform and stabilized by propylamine (PA). We demonstrated that the ZnO NP dispersion with 4 vol.% of PA as stabilizer can be used in air directly and remains clear up to one month after preparation. Our inverted solar cells consisted of a blade-coated 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) (1: 4 by weight) active layer sandwiched between a ZnO electron extraction layer and a MoO₃/Ag anode. All solar cells with ZnO films fabricated in air using PA-stabilized ZnO dispersions prepared within a time window of one month exhibited power conversion efficiencies (PCE) above 4%. In contrast, if the ZnO film was prepared in air using regular un-stabilized ZnO NP dispersion, the PCE would drop to 0.2% due to poor film quality. More interestingly, X-ray photoelectron spectroscopy and nuclear magnetic resonance measurements indicated that the PA ligands were not covalently bonded to ZnO NPs and did not exist in the deposited ZnO films. The spin-cast ZnO thin films (without any thermal treatment) are insoluble in organic solvents and can be directly used as an EEL in solar cells. This feature is beneficial for fabricating organic solar cells on flexible polymer substrates. More importantly, our non-encapsulated inverted solar cells are highly stable with their PCEs remaining unchanged after being stored in air for 50 days.     

Synthesis and photophysical properties of semiconductor molecules D1-A-D2-A-D1-type structure based on derivatives of quinoxaline and dithienosilole for organics solar cells

A novel small molecule with D1-A-D2-A-D1 structure denoted as DTS(QxHT₂)₂ based on quinoxaline acceptor and dithienosilone donor units was synthesized and its optical and electrochemical properties were investigated. The thin film of DTS(QxHT₂)₂ showed a broad absorption profile covering the solar spectrum from 350 nm to 780 nm with an optical bandgap of 1.63 eV. The energy levels estimated from the cyclic voltammetry indicate that this small molecule is suitable as donor along with PC₇₁BM as acceptor for the fabrication solution processed bulk heterojunction solar cells for efficient exciton dissociation and high open circuit voltage. The organic solar cells based on optimized DTS(QxHT2)2:PC₇₁BM active layers processed with chloroform and DIO/CF showed overall power conversion efficiency of 3.16% and 6.30%, respectively. The higher power conversion efficiency of the solar cell based on the DIO/CF processed active layer is attributed to enhanced short circuit photocurrent and fill factor may be related to better phase separation between donor and acceptor in the active layer and more balanced charge transport, induced by the solvent additive. The power conversion efficiency of the organic solar cell was further improved up to 7.81% based on active layer processed with solvent additive, using CuSCN as hole transport layer instead of PEDOT:PSS and mainly attributed to increased fill factor and open circuit voltage due the formation of better Ohmic contact between the active layer and the CuSCN layer.     

Acenaphtho[1,2-b]quinoxaline diimides derivative as a potential small molecule non-fullerene acceptor for organic solar cells

We designed and synthesized a small molecule acenaphtho[1,2-b]quinoxaline diimide derivative AQI-T2 as an electron-accepting material for non-fullerene organic solar cells. This molecule exhibits a relatively broad absorption band from 300 to 650 nm, with a moderately low-lying lowest unoccupied molecular orbital energy level of −3.64 eV. Non-fullerene organic solar cells with conventional structure using PTB7-Th as the electron donor and AQI-T2 as the electron acceptor exhibited moderate photovoltaic performances. The best performance was attained from the pristine device, which showed a power conversion efficiency of 0.77% with a relatively high open-circuit voltage of 0.86 V, a short circuit current of 2.04 mA cm⁻² and a fill factor of 43.98%. These results indicated that this n-type molecule can be a promising electron-accepting material for non-fullerene organic solar cells.     

Effects of processing additives on nanoscale phase separation,crystallization and photovoltaic performance of solar cells based on mesogenic phthalocyanine

A study on the effects of processing additives on the nanoscale phase separation, crystallization, and photovoltaic performance of bulk heterojunction (BHJ) thin films made of 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) via spin-casting for photovoltaic applications is reported. By incorporating various solvents as processing additives to a volume of a few percent, the separation of donor and acceptor phases in C6PcH₂:PCBM thin films, which discussed by taking the photoluminescence quenching, Davydov splitting at the Q-band of the absorbance spectra and the surface nanomorphology into consideration, is improved, and the crystallinity of the discotic C6PcH₂ molecules with hexagonal structures is reinforced. Photovoltaic cells with the optimum phase-separated BHJ materials and high crystallinity of the discotic C6PcH₂ molecules are demonstrated to have a power conversion efficiency of 4.2%.     

Synthesis and characterization of triphenylamine flanked thiazole-based small molecules for high performance solution processed organic solar cells

Two new small molecules, 5,5-bis(2-triphenylamino-3-decylthiophen-2-yl)-2,2-bithiazole (M1) and 2,5-bis(2-triphenylamino-3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole (M2) based on an electron-donor triphenylamine unit and electron-acceptor thiophene-thiazolothiazole or thiophene-bithiazole units were synthesized by a palladium(0)-catalyzed Suzuki coupling reaction and examined as donor materials for application in organic solar cells. The small molecules had an absorption band in the range of 300-560 nm, with an optical band gap of 2.22 and 2.25 for M1 and M2, respectively_. As determined by cyclic voltammetry, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of M1 were −5.27 eV and −3.05 eV, respectively, which were 0.05 eV and 0.02 eV greater than that of M2. Photovoltaic properties of the small molecules were investigated by constructing bulk-heterojunction organic solar cell (OSC) devices using M1 and M2 as donors and fullerene derivatives, 6,6-phenyl-C61-butyric acid methyl ester (PC₆₁BM) and 6,6-phenyl-C71-butyric acid methyl ester (PC₇₁BM) as acceptors with the device architecture ITO/PEDOT:PSS/M1 or M2:PCBM/LiF/Al. The effect of the small molecule/fullerene weight ratio, active layer thickness, and processing solvent were carefully investigated to improve the performance of the OSCs. Under AM 1.5 G 100 mW/cm² illumination, the optimized OSC device with M1 and PC₇₁BM at a weight ratio of 1:3 delivered a power conversion efficiency (PCE) of 1.30%, with a short circuit current of 4.63 mA/cm², an open circuit voltage of 0.97 V, and a fill factor of 0.29. In contrast, M2 produced a better performance under identical device conditions. A PCE as high as 2.39% was recorded, with a short circuit current of 6.49 mA/cm², an open circuit voltage of 0.94 V, and a fill factor of 0.39.     

Temperature and light dependent diode current in high-efficiency solution-processed small-molecule solar cells

This paper reports on the detail analysis of the DC electrical and photoelectrical properties of the high-efficient (η = 8.01% under standard 100 mW/cm² AM1.5 illumination) small molecule bulk heterojunction (SM BHJ) solar cells p-DTS(FBTTh₂)₂/PC₇₀BM. In this SM BHJ solar cell, the dark diode current is determined by the multistep tunnel-recombination via interface states at low forward bias (V < 0.65 V) and the interface state assisted thermionic emission at high forward bias (V > 0.65 V). The effect of illumination on the diode current was also quantitatively investigated. It was observed a reduced Shockley–Read–Hall recombination via interface states at large forward bias (from the maximum power point to the open-circuit conditions). The expression of the load I–V characteristic of the illuminated high-efficient SM BHJ solar cells was derived in the presence of the light dependent series and shunt resistance.     

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.