Open-circuit voltage up to 1.07 V for solution processed small molecule based organic solar cells

With the goal of increasing the open-circuit voltage, two new solution-processable A–D–A structure small molecule donor materials, named DCAO3TF and DCAO3TCz, using two weak electron-donating units, fluorene and carbazole as the central block have been designed and synthesized for photovoltaic applications. While bulk heterojunction photovoltaic devices based on DCAO3TF:PC₆₁BM and DCAO3TCz:PC₆₁BM as the active layers exhibit moderate power conversion efficiencies of 2.38% and 3.63%, respectively, devices based on DCAO3TF:PC₆₁BM do exhibit an impressively high open-circuit voltage (V_oc) up to 1.07 V, which is one of the highest V_oc in organic solar cells based on donor:PCBM blend films.     

Influence of the density of states on the open-circuit voltage in small-molecule solar cells

Open-circuit voltages are strongly dependent on the density-of-states in solar cells based on disordered semiconductors. In this work, organic solar cells based on tetraphenyldibenzoperiflanthene and fullerene C₇₀ with a bilayer structure were fabricated to investigate the variation in the density-of-states with the substrate temperature during deposition of the donor. The maximum open circuit voltage was reached at a substrate temperature of 60 °C. Organic thin-film transistors were also fabricated to study their electrical properties, such as the mobility and the density-of-states. Finally, an organic solar cell with p–i–n structure was fabricated at the optimized substrate temperature, and a power conversion efficiency of almost 4% was obtained.     

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.     

Vacuum deposited ternary mixture organic solar cells

Ternary mixtures of photo-active organic materials are an intuitive approach to achieve enhanced photocurrent in organic solar cells (OSCs). In this work, we study ternary mixtures of vacuum deposited small molecules, complementing the recent surge of interest in solution processed ternary OSCs. The mixed layer composition is systematically varied to study all possible film configurations, and the resulting OSCs are successful in harvesting photocurrent from all three components to grant broad spectral photoresponse. However, the performance of the ternary OSC is generally less than the binary OSC, largely due to reduced fill factors. By examining ternary OSC transient photocurrents and multi-donor planar heterojunction devices, we demonstrate that the ternary OSC is strongly affected by the energy levels of its constituent materials, with small differences in the two donor materials’ highest occupied molecular orbitals degrading hole transport. The results stress the importance of fine molecular engineering for ternary OSCs, and further hint that the enhancements commonly observed in solution processed ternary OSCs may in part be associated with morphological variations that are not present in vacuum deposited OSCs. The research verifies that, by designing small molecules with specific energy levels, ternary OSCs provide an alternative pathway to low cost, high efficiency photovoltaics in lieu of more complicated device architectures.     

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.     

Light dependent open-circuit voltage of organic bulk heterojunction solar cells in the presence of surface recombination

An analytical model for the light intensity dependence of open circuit voltage V_oc in the presence of bimolecular, trap assisted and surface recombination mechanisms was proposed. The model quantitatively explains reported experimental deviations from the bimolecular and bimolecular/trap assisted recombination models.V_oc was found to be the most sensitive photoelectric parameter to surface recombination. The relative effect of surface recombination on V_oc increases with the increase of trap density as well as V_oc becomes more sensitive to the presence of deep traps due to surface recombination. In the presence of surface recombination slope V_oc vs. light intensity A does not reach value of 2.0 kT/q even at very high density of traps.Also a possible misinterpretation of the experimental V_oc vs. light intensity dependences in the presence of trap assisted and surface recombination was outlined.     

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.     

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.     

All thermal-evaporated surface plasmon enhanced organic solar cells by Au nanoparticles

Au nanoparticles (NPs) are fabricated on indium-tin-oxide substrates by a thermal evaporation method and incorporated to an efficient small molecule organic solar cell (OSC). This renders an all thermal evaporated surface plasmon enhanced OSC. The optimized device shows a power conversion efficiency of 3.40%, which is 14% higher than that of the reference device without Au NPs. The improvement is mainly contributed to the increased short-circuit current which resulted from the enhanced light harvesting due to localized surface plasmon resonance of Au NPs and the increased conductivity of the device.     

Tail states recombination limit of the open circuit voltage in bulk heterojunction organic solar cells

An analytical theory is presented for bimolecular recombination through tail states and open circuit voltage in bulk heterojunction organic solar cells. It is developed rigorously using the hopping transport and the drift diffusion theory. Based on the proposed model, a variety of temperature, energy disorders of the material and illumination intensity dependencies of the open circuit voltage can be well described. Good agreement between the calculation and recent experimental data is found.     

Improving the performance of radial n-i-p junction Si nanowire solar cells by catalyst residue removal

It is hoped that silicon nanowire (SiNW)-based solar cells will provide the basis for a new generation of photovoltaics. However, metal-catalyzed SiNWs contain metal residues (such as indium) which may degrade the performance of solar cells. In this study, we prepared SiNW solar cells by plasma-enhanced chemical vapor deposition using indium as the catalyst to grow the SiNWs. The SiNWs were treated with hydrochloric acid to reduce the indium contamination at different concentrations, C_HCl (1–5%). We found the decreasing the indium contamination improved the performance of the solar cells at optimum C_opHCl. However, the performance of the solar cells decreased when C_HCl exceeded C_opHCl. This was attributed to the variation in the conduction-band offset ΔE_c between the n type amorphous silicon layer (E_c n-a-Si) and the n type crystalline silicon nanowires (E_c n-c-SiNWs). Finally, a conversion efficiency (Eff) improvement from 2.11% to 6.18% was obtained with the optimized C_HCl.     

Vacuum-deposited interconnection layers for tandem solar cells

Two different types of vacuum-deposited interconnection layers (ICLs) were investigated for tandem solar cells: (1) a pure metal oxide and (2) an organic matrix doped with conductive dopants. The optical and electrical properties of these ICLs were systematically studied and compared. Taking the characteristics of ICLs into consideration, optical design methodology for balancing the photocurrent of each sub-cell in the tandem cell is presented. According to the design, highly efficient small-molecule tandem solar cells with power conversion efficiencies up to 7.3%–7.4% were experimentally demonstrated in both devices utilizing pure metal oxide and organic matrix ICLs.     

Cooperative plasmon enhanced organic solar cells with thermal coevaporated Au and Ag nanoparticles

Cooperative plasmon enhanced small molecule organic solar cells are demonstrated based on thermal coevaporated Au and Ag nanoparticles (NPs). The optimized device with an appropriate molar ratio of Au:Ag NPs shows a power conversion efficiency of 3.32%, which is 22.5% higher than that of the reference device without any NPs. The improvement is mainly contributed to the increased short-circuit current which resulted from the enhanced light harvesting due to localized surface plasmon resonance of Au:Ag NPs and the increased conductivity of the device. Besides, factors that determining the performance of the Au:Ag NPs cooperative plasmon enhance organic solar cells are investigated, and it finds that the thickness of MoO₃ buffer layer plays a crucial role. Owing to the different diameter of the thermal evaporated Au and Ag NPs, a suitable MoO₃ buffer layer is required to afford a large electromagnetic enhancement and to avoid significant exciton quenching by the NPs.     

Electroabsorption studies of organic p-i-n solar cells: Increase of the built-in voltage by higher doping concentration in the hole transport layer

The built-in voltage in solar cells has a significant influence on the extraction of photogenerated charge carriers. For small molecule organic solar cells based on the p-i-n structure, we investigate the dependence of the built-in voltage on the work function of both the hole transport layer and the electrode material. The model system investigated here consists of a planar heterojunction with N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as donor and Buckminster Fullerene (C₆₀) as acceptor material. A higher concentration of the dopant C₆₀F₃₆ in the hole transport layer induces a shift of the work function towards the transport level. The resulting increase of the built-in voltage is studied using electroabsorption spectroscopy, measuring the change in absorption (Stark effect) caused by an externally applied electric field. An evaluation of these electroabsorption spectra as a function of the applied DC voltage enables the direct measurement of the built-in voltage. It is also shown that an increased built-in voltage does lead to a larger short-circuit current as well as a larger fill factor.     

The substituents on the intermediate electron-deficient groups in small molecular acceptors result appropriate morphologies for organic solar cells

The small molecule acceptors with the structure of acceptor-donor acceptor' donor-acceptor (A−DA'D−A) boost the power conversion efficiency of organic solar cells. Compared with multifused DA'D core with an electron-deficient unit at the center, the unfused core emerges low cost and high yield. Herein, we designed and synthesized three small molecule acceptors named as TR2F-IC4F, BTOC8-IC4F and, BTOC6C8-IC4F, with an unfused core containing one benzothiadiazole (BT) or triazole (TR) unit and two cyclopentadithiophene (CPDT) units. The nearly planar geometry is realized through noncovalent F⋯H, F⋯S and S⋯O interactions. These acceptors have broad near-infrared absorption. For devices with BTOC8-IC4F, TR2F-IC4F and BTOC6C8-IC4F, the highest PCEs are only 5.81%, 5.89% and 7.55%, respectively. For devices with BTOC6C8-IC4F, the PCEs are further improved to 10.01% for ternary cells, which add PC₇₁BM into devices. In this work, we found with the increase of the substituents on the intermediate electron-donating groups, the planarity of the molecules become weaken, but can obtain more compatible with the donor molecules, which could result in the different film-forming properties and morphologies in blending films. This work provides a typical example of introducing unfused core into small molecule acceptors, which is important to design new solution-processable small molecule acceptors in the future.     

Theoretical investigation of voltage effect on magnetoresistance in an organic small molecule device

We theoretically study the voltage effect on organic magnetoresistance (OMAR) in a weak disordered small molecule device on the basis of the quantum dynamics. It is found that with the increase of the voltage, the OMAR effect is reduced. The results show a good agreement with the experimental data. In addition, the carrier density effect on OMAR has also been discussed.     

A small molecule with selenophene as the central block for high performance solution-processed organic solar cells

A solution-processable A–D–A structure small molecule donor material called DRCN7T-Se with selenophene as the central block was synthesized. Conventional bulk-heterojunction solar cell devices based on DRCN7T-Se and PC₇₁BM were optimized by thermal annealing and an excellent power conversion efficiency of 8.30% was achieved under AM 1.5G irradiation (100 mW cm⁻²).     

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.     

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.     

Stability of diketopyrrolopyrrole small-molecule inverted organic solar cells

Small-molecule DPP(TBFu)₂-based inverted organic solar cells were fabricated and their stability investigated. The effects of thermal annealing and solvent annealing on device performance and stability were compared. To increase the stability, mix-PCBM (PC₆₁BM and its C₇₀ analogue), which is reported to give higher device stability, was also included. Solvent-annealed devices showed the highest power conversion efficiency (PCE) of 4.62%, whereas thermally annealed devices showed a PCE of 3.94%. After the aging process, which involved thermal stress and exposure to air, thermally annealed and mix-PCBM devices retained a PCE of 3%, whereas solvent-annealed devices had a much lower PCE of 1.7%. Therefore, our results show that in the long-term stability perspective, thermal annealing is better than solvent annealing, and mix-PCBM is better than PC₆₁BM in the case of DPP(TBFu)₂. We fabricated small-molecule inverted organic solar cells that retain their performance in air for 3 weeks without encapsulation.