Molecular engineering of Diketopyrrolopyrrole-based photosensitizer for solution processed small molecule bulk heterojunction solar cells

New symmetrical low band-gap small molecule materials, DPP-bis[ter-3HT-TPA] and DPP-bis[ter-3HT] as novel derivatives of Diketopyrrolopyrrole-thiophene with/without triphenylamine (TPA) end group have been synthesized and characterized. And the effects of TPA moiety were investigated. Compared to DPP-bis[ter-3HT], DPP-bis[ter-3HT-TPA] shows red-shifted absorption and significantly higher molar absorption coefficient. And the HOMO level of DPP-bis[ter-3HT-TPA] is elevated than DPP-bis[ter-3HT]. Moreover, DPP-bis[ter-3HT-TPA] exhibited one order higher hole mobility than DPP-bis[ter-3HT], suggesting that TPA contributes to a better hole mobility. The bulk-heterojunction photovoltaic devices with DPP-bis[ter-3HT-TPA] showed better efficiencies than DPP-bis[ter-3HT], showing the best power-conversion efficiency (PCE) of 1.5% (±0.12) under 100 mW/cm² with a short-circuit current (J_sc)=5.73 mA/cm², a fill factor (FF)=0.45, and an open-circuit voltage (V_oc)=0.59 mV.     

Benzoselenadiazole-core asymmetric D-A-A small molecule for solution processed bulk heterojunction organic solar cells

The present work is accounted on the designing of a new and efficient asymmetric organic chromophore, named 4-(3,5-bis [trifluoromethyl] phenyl)-7-(5′-hexyl-[2,2′-bithiophen]-5-yl)-benzo[c [1, 2, 5]selenadiazole, (RT-BSe-F), based on benzoselenadiazole central acceptor building blocks. The acceptor unit of 3,5-bis (trifluoromethyl) benzene and donor unit of alkyl bithiophene attached with benzoselenadiazole central unit showed large impacts on the optical and electrochemical properties. The reasonable optical band gap of ~2.02 eV and HOMO of −5.33 eV were obtained by RT-BSe-F chromophore due to a strong electron accepting nature of fluorine based compound. With 3,5-bis(trifluoromethyl) benzene unit, the absorption of RT-BSe-F chromophore was considerably increased to higher wavelength which might enhance the crystallinity of thin film with high hole mobility. RT-BSe-F chromophore was effectively applied to fabricate the solution-processed bulk-heterojunction organic solar cells (BHJ-OSCs) and attained a high power conversion efficiency (PCE) of ~3.75% accompanying high J_SC of ~12.56 mA cm⁻², FF of ~0.42 and V_OC of ~0.71 V. The obtained high PCE might be associated to a high surface energy of TiO₂ layer as buffer and the use of high mobility organic RT-BSe-F chromophore.     

Solution-processed bulk heterojunction organic solar cells with high polarity small molecule sensitizer

A new sensitizer molecule, HMBI (9,18-(di-2-hexyldecyl)-2,11-dimethoxy-9,18-dihydrobenzo[5,6]-s-indaceno[1,2-b]indeno[2,1-h]carbazol-6,15-dione), containing electron-donating carbazole and electron-accepting diketone units, has been synthesized for solution-processed bulk heterojunction organic solar cells. The HMBI material has good solubility in common organic solvents. Its HOMO and LUMO energy levels were found to be at 5.6 and 3.0 eV, respectively. It has absorption bands ranging from 300 to 500 nm. Dispersion of HMBI molecules in the P3HT/PCBM blend broadens the absorption bands over the spectral range of 350-500 nm. Uniform thin film devices doped with varying concentration of HMBI, incorporated within the P3HT/PCBM blend, were fabricated. The 3 wt% of HMBI doping produces an improvement in power-conversion efficiency (PCE) up to 11.5% compared with the reference P3HT/PCBM device. Efficient light harvesting caused by HMBI sensitizer molecules primarily yields increased carrier generation and short-circuit current. In addition, some morphological improvements in the P3HT/PCBM system may contribute to the generation of enhanced photocurrent.     

Improved power conversion efficiency of bulk heterojunction poly(3-hexylthiophene):PCBM photovoltaic devices using small molecule additive

We report that the power conversion efficiency (PCE) of the bulk heterojunction organic photovoltaic device based on poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) blend was improved by incorporating a small molecule SM having absorption band in the longer wavelength region. SM is a small molecule containing thienothiadiazole central unit with terminal cyanovinylene 4-nitrophenyl at both sides, which were connected to the central unit via a thiophene ring. The combination of SM with P3HT and PCBM allows not only a broad band absorption up to longer wavelength, but also tuning the inter-energy level leading to a higher short circuit current (J_sc) and open circuit voltage (V_oc). The device based on the as cast P3HT:PCBM:SM exhibits a PCE of 3.69%, which is higher than the device based on P3HT:PCBM and SM:PCBM blends. The overall PCE of the device based on thermally annealed blend is further improved up to 4.1%. The improvement of the PCE has been attributed to a better charge transport in the device, due to the increased crystallinity of the blend through thermal annealing.     

Bulk heterojunction organic photovoltaic devices based on low band gap small molecule BTD-TNP and perylene-anthracene diimide

Bulk heterojunction (BHJ) photovoltaic devices were fabricated from the blends of compounds BTD-TNP (electron donor) and P-A (electron acceptor) and characterized through current-voltage measurements under illumination. Compound BTD-TNP contains dithyenyl-benzothiadiazole central unit and cyanovinylene-p-nitrophenyl terminal moieties. Compound P-A is a symmetrical perylene-anthracene diimide with tert-butylphenoxy side groups at the 1,7-bay positions. Both the absorption spectra and the incident photon to the current conversion efficiency (IPCE) spectra of the device were extended up to 800 nm. A power conversion efficiency of 2.85% with short-circuit current density of 6.8 mA/cm², open-circuit voltage of 0.88 V and fill factor of 0.47 were obtained. It was found that the hole and electron mobility in the active layer were about 4.6×10⁻⁵ and 8.8×10⁻⁴ cm²/Vs, respectively, which indicates the fairly balanced charge transport in the device.     

Total charge amount as indicator for the degradation of small molecule organic solar cells

We show that the number of extracted charge carriers is a suitable measure to compare lifetime measurements on organic solar cells at different intensities. In detail, we used pin-structures with active layers containing a bulk heterojunction of Zincphthalocyanine (ZnPc) and C₆₀. Extended lifetime measurements under constant monochromatic or white illumination at defined temperatures of 50 °C or 90 °C are done. On the one hand, we show that the number of extracted charge carriers is important to determine the degree of degradation. On the other hand, our results show that the energy of irradiated photons is significant for accelerated measurements. This is an major advantage for the realisation of accelerated lifetime measurements. Additionally, we find that not single charge carriers, but excitons cause the degradation of the observed solar cells.     

Effect of organic salt doping on the performance of single layer bulk heterojunction organic solar cell

The effect of organic salt, tetrabutylammonium hexafluorophosphate (TBAPF₆) doping on the performance of single layer bulk heterojunction organic solar cell with ITO/MEHPPV:PCBM/Al structure was investigated where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. In contrast to the undoped device, the electric field-treated device doped with TBAPF₆ exhibited better solar cell performance under illumination with a halogen projector lamp at 100 mW/cm². The short circuit current density and the open circuit voltage of the doped device increased from 0.54 μA/cm² to 6.41 μA/cm² and from 0.24 V to 0.50 V, respectively as compared to those of the undoped device. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the active layer/electrode interfaces.     

P3HT/PCBM bulk heterojunction solar cells: Relation between morphology and electro-optical characteristics

The performance of organic solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by blend composition and thermal annealing conditions. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) diffraction measurements show that in the considered blends, ordering of P3HT plays a key role in understanding the PV-performance. It is demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. Annealing however improves the crystallinity, explaining the observed spectral broadening and is also resulting in a higher mobility of the holes in P3HT.     

Bulk heterojunction organic solar cells utilizing 1,4,8,11,15,18,22,25-octahexylphthalocyanine

Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH₂ and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH₂ and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO₃ hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH₂ and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO₃ structure, and a high V_oc of 1.27 V has been achieved.     

Tetrabutyl-tetraphenyl-diindenoperylene derivatives as alternative green donor in bulk heterojunction organic solar cells

We present the material 2,3,10,11-tetrabutyl-1,4,9,12-tetraphenyl-diindeno[1,2,3-cd:1′,2′,3′-lm] perylene (Bu4-Ph4-DIP) as alternative green donor for bulk heterojunction small molecule organic solar cells (SMOSC). It is shown that Bu4-Ph4-DIP exhibits suitable absorption characteristics to be a potential material to fill the absorption gap between the commonly used standard absorbers ZnPc and C₆₀.Devices with bulk heterojunctions of Bu4-Ph4-DIP:C₆₀ display very high open circuit voltages of 0.99 V, high fill factors of up to 57%, and experiments yield promising efficiencies of η>2%. Such green-blue absorbing SMOSC are characterized by current voltage and external quantum efficiency measurements, and material properties are studied. It is shown that the devices are responsive to substrate heating, and that different donor-acceptor mixing ratios can increase device performance. Possible influences of mixing ratio and heating on device morphology and electrical properties are discussed.     

Long-lived bulk heterojunction solar cells fabricated with photo-oxidation resistant polymer

We report the fabrication of long-lived polymer solar cells using a new donor-acceptor type alternating copolymer, poly(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-α]indene-2,7-diyl)-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole (PININE-DTBT) in bulk heterojunction composites with the fullerene derivative [6,6]-phenyl C₇₀-butyricacidmethyl ester (PC₇₀BM). The PININE-DTBT:PC₇₀BM solar cells exhibit an extended device lifetime (as compared with other polymer systems) with a reasonable power conversion efficiency of ∼2.7% under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW/cm². The long-lived feature of the devices originates from the photo-oxidation resistant backbone unit and the deep HOMO (highest occupied molecular orbital) level of PININE-DTBT.     

Effect of concentration gradients in ZnPc:C60 bulk heterojunction organic solar cells

A concentration gradient in a mixed absorber layer with increasing content of donor (acceptor) towards the hole (electron) collecting contact could improve the charge carrier collection in bulk heterojunction organic solar cells. We study p-i-metal type solar cells where the gradient in a 45 nm thick ZnPc:C₆₀ absorber layer is introduced by varying the deposition rate during co-evaporation. It is shown that the observed increase in the performance is mainly caused by a better energy level alignment and reduced recombination at the p-side. A significant influence on charge carrier transport is not observed. However, regions with a concentration of less than 20% of one component do not fully contribute to the photocurrent. Voltage dependent external quantum efficiency data are used to identify the photoactive regions.     

Organic solar cells based on the spin-coated blend films of TPA-th-TPA and PCBM

In this paper, the optical and photo-induced electron transfer properties of a new conjugated molecule, 4,7-bis{5′-[4″,4″-N,N-diphenylamino-styryl]thiphen-2′-yl} -benzo[1,2,5-thiadiazole] (simplified as TPA-th-TPA), were investigated. Using TPA-th-TPA as a photoactive layer, organic solar cells with three different architectures were fabricated by spin-coating method. The photosensitive layers of these architectures comprise pure TPA-th-TPA layer, heterojunction of bi-layered TPA-th-TPA and C₆₀, and bulk-heterojunction of TPA-th-TPA and [6, 6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend. Furthermore, towards the bulk-heterojunction devices, the effect of the cathode materials (Mg, Ca, LiF/Al, Ba) on the performance of the devices was studied. The power conversion efficiency reached 0.26% for the device based on the blend of TPA-th-TPA and PCBM with Ba/Al as the cathode.     

Efficient bulk heterojunction solar cells using an alternating phenylenevinylene copolymer with dithenyl(thienothiadiazole) segments as donor and PCBM or modified PCBM as acceptor

A novel low band gap alternating phenylenevinylene copolymer, P, with dithenyl (thienothiadiazole) segments was synthesized by Heck coupling. It was soluble in common organic solvents, showed broad absorption curve with long-wavelength absorption maximum at 580–598 nm and optical band gap of 1.74 eV, which is comparable with the electrochemical band gap of about 1.80 eV. P (electron donor) was blended with PCBM or modified PCBM i.e. F (electron acceptor) to fabricate bulk heterojunction (BHJ) solar cells. The power conversion efficiency (PCE) of the devices based on P:PCBM and P:F cast from 1.2-dichlorobenzene (DCB) was found to be 1.40% and 2.32%, respectively. The higher value of PCE for the device with P:F as compared to P:PCBM is attributed to the increase in both short circuit current (J_sc) and open circuit voltage (V_oc). The increase in the J_sc is due to the stronger light absorption of F in visible region as compared to PCBM, i.e. more exciton generation in the blend. On the other hand, the higher difference between the LUMO levels of P and F, as compared to P and PCBM is responsible for the enhancement in the V_oc. A maximum overall PCE of 4.20% was obtained for the BHJ polymer cell based on the active layer (P:F) deposited from mixed solvents 1-chloronapthalene/1,2-dichlorobenzene (CN/DCB) and subsequent thermal annealing at 120 °C. This improvement in the PCE has been attributed to the enhanced crystallinity of the blend and more balanced charge transport in the device due to the thermal treatment.     

Efficient organic tandem cell combining a solid state dye-sensitized and a vacuum deposited bulk heterojunction solar cell

In this letter, we report on an efficient organic tandem solar cell combining a solid state dye-sensitized with a ZnPc/C₆₀-based, vacuum deposited bulk heterojunction solar cell. Due to an effective serial connection of both subcells and to the complementary absorption of the dyes used, a power conversion efficiency of η_p=(6.0±0.1)% was achieved under simulated AM 1.5 illumination. The device parameters are , and FF=(54±1)%.     

Solvent additives for tuning the photovoltaic properties of polymer-fullerene solar cells

We use solvent additives as a simple method to tune the photovoltaic performance of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojuncton solar cells. 1,2-dichlorobenzene (oDCB) was used as the reference solvent; chlorobenzene (CB) and 1,2,3,4-tetrahydronaphthalene (THN) were used as additives to influence film formation. An increase in the short circuit current and the power conversion efficiency of solar cells with blends cast from mixed solvents was observed. Blends prepared with THN, the highest boiling point solvent, resulted in the best device performance, while blends prepared with the pure reference solvent resulted in the lowest photocurrent. In-plane investigations of the morphology using transmission electron microscopy (TEM) revealed improved phase segregation for blends prepared with mixed solvents, and increased crystallinity in the P3HT phase is demonstrated using atomic force microscopy (AFM) coupled with Kelvin probe force microscopy (KPFM). Optical modeling reveals that the increase in the photocurrent is not due to changes in the optical properties of the blends. Electrical characterization reveals that the electron mobilities decrease slightly in blends cast from mixed solvents, corresponding to a decrease in the fill factor and an increase in P3HT crystallinity observed at the surface of the blend. The increase in the photovoltaic performance is discussed in terms of increased charge separation at the donor-acceptor interface due to increased ordering in the P3HT phase induced by the solvent additives.     

Water and oxygen induced degradation of small molecule organic solar cells

Small molecule organic solar cells were studied with respect to water and oxygen induced degradation by mapping the spatial distribution of reaction products in order to elucidate the degradation patterns and failure mechanisms. The active layers consist of a 30 nm bulk heterojunction formed by the donor material zinc-phthalocyanine (ZnPc) and the acceptor material Buckminsterfullerene (C₆₀) followed by 30 nm C₆₀ for additional absorption. The active layers are sandwiched between 6 nm 4,7-diphenyl-1,10-phenanthroline (Bphen) and 30 nm N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine p-doped with C₆₀F₃₆ (MeO-TPD:C₆₀F₃₆), which acted as hole transporting layer. Indium-tin-oxide (ITO) and aluminum served as hole and electron collecting electrode, respectively. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) in conjunction with isotopic labeling using H₂¹⁸O and ¹⁸O₂ provided information on where and to what extent the atmosphere had reacted with the device. A comparison was made between the use of a humid (oxygen free) atmosphere, a dry oxygen atmosphere, and a dry (oxygen free) nitrogen atmosphere during testing of devices that were kept in the dark and devices that were subjected to illumination under simulated sunlight. It was found that water significantly causes the device to degrade. The two most significant degradation mechanisms are diffusion of water through the aluminum electrode resulting in massive formation of aluminum oxide at the BPhen/Al interface, and diffusion of water into the ZnPc:C₆₀ layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime.     

Influence of n-type chemical doping layer on the performance of organic photovoltaic solar cells

We report the performance improvement of organic solar cell by addition of an n-type chemical doping layer in organic bulk heterojunction device. The power conversion efficiency (PCE) of P3HT and PCBM-71 based polymer solar cells increases by adding a mixture of TCNQ (7,7,8,8-tetracyanoquinodimethane) and LCV (Leucocrystal violet) between active layer and cathode electrode. The PCE of the cell increases by 14% compared to the control cell with Al-only cathode electrode. The device with an organic n-doped layer shows the J_SC of 8.88 mA/cm², V_OC of 0.51 V, FF of 60.1%, and thus the PCE of 2.72% under AM1.5 illumination of 100 mW/cm².     

Electroabsorption studies of phthalocyanine/perylene solar cells

We report electroabsorption (EA) studies of electric fields in bilayer molecular organic solar cells made from zinc phthalocyanine (ZnPc) and a methyl substituted perylene pigment (MPP). We have detected an electric field at the metal/organic interface which is sensitive to the external DC bias. The interface field has a different spectral signature from that of the bulk of the two layers, which we attribute to interface species such as charge transfer-induced dipoles. The electric field is proportional to the applied bias in devices containing only ZnPc or MPP, but rectifying behavior is observed in the bilayer solar cell.     

New amorphous small molecules—Synthesis, characterization and their application in bulk heterojunction solar cells

We successfully synthesized a series of novel solution processible small molecules (2TAPM, 4TAPM and 2BTAPM) consisting of electron-accepting unit (2-pyran-4-ylidenemalononitrile) (PM) and electron-donating unit (Triphenylamine and different thiophene units). Differential scanning calorimetry (DSC) measurement indicates that these small molecules are amorphous. UV-vis absorption spectra show that the combination of PM with moieties having gradually increased electron-donating ability results in an enhanced intramolecular charge transfer (ICT) transition, leading to an extension of the absorption spectral range and a reduction of the band gap of the molecules. Both cyclic voltammetry measurement and theoretical calculations show that the highest occupied molecular orbital (HOMO) energy levels of the molecules could be fine-tuned by changing the electron-donating ability of the electron-donating moieties. The bulk heterojunction (BHJ) photovoltaic devices with a structure of ITO/PEDOT:PSS/small molecules:PC₇₁BM/LiF/Al were fabricated by using the small molecules as donors and (6,6)-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as acceptor. Power conversion efficiencies of 1.76% and 2.47% were achieved for the photovoltaic devices based on 2TAPM:PC₇₁BM and 4TAPM:PC₇₁BM under simulated air mass 1.5 global irradiation (100 mW/cm²), respectively.