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.     

Analysis of internal quantum efficiency in double-graded bandgap solar cells including sub-bandgap absorption

State of the art ZnO/CdS/Cu(In,Ga)Se₂ (CIGS) solar cells use bandgap grading, requiring special tools for the analysis of the experimentally obtained characteristic curves. We develop an analytical model for the photon flux and internal quantum efficiency in double-graded bandgap solar cells, considering the effects of sub-bandgap absorption and grading-dependent carrier collection properties. The short-circuit photocurrent density is calculated as a function of carrier diffusion length and front/back bandgaps, establishing optimum design criteria under solar operation. Even for a diffusion length of only 0.5 μm in a 3-μm-thick absorber, and no contribution from the CdS layer, an optimum back bandgap of 1.35 eV is found, yielding short-circuit current densities of 36.0 (33.5) mA cm⁻² for a front bandgap of 1.05 (1.68) eV. Furthermore, simplifications to the model for specific energy ranges allow to extract the Urbach Energy E_U and the minimum bandgap E_g,min in the grading profile from experimental IQE curves. Finally, our model fits IQE measurements of 18% efficient CIGS solar cells, yielding values of E_U between 31 and 41 meV, minimum bandgaps E_g,min between 1.10 and 1.16 eV, and diffusion lengths close to 0.5 μm.     

Synthesis and photovoltaic properties of a low bandgap donor-acceptor alternating copolymer with benzothiadiazole unit

A new donor-acceptor alternating copolymer as the donor material of the active layer in polymer solar cells has been synthesized. The alternating structure consisted of dithieno[3,2-b:2′,3′-d]thiophene (DTT) donor unit and 5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole (BT) acceptor unit. Both units were confirmed by ¹H NMR and elemental analysis. Since the BT unit has long alkyoxyl side chains, the polymer was soluble in common organic solvents. Optoelectronic properties of the copolymer (PDTTBT) were investigated and observed by UV-vis, photoluminescence (PL) spectra, and cyclic voltammogram (CV). UV-vis spectrum exhibited a broad absorption band in the range of 300-750 nm and a low bandgap of 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of PDTTBT could be determined from the data of CV and UV-vis spectrum. Based on the ITO/PEDOT:PSS/PDTTBT:PCBM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100 mW/cm²) was 0.113%. It was found that PCE of 0.301% could be acquired under the annealing condition at 150 °C for 30 min. In addition, solar cells fabricated with the 1,8-octanedithiol (OT) additive in the mixture solvent or adding TiO_x optical spacer show efficiencies significantly improved over 15%.     

Synthesis and characterization of 2,1,3-benzothiadiazole-thieno[3,2-b]thiophene-based charge transferred-type polymers for photovoltaic application

New low-band-gap copolymers, including thieno[3,2-b]thiophene and 2,1,3-benzothiadiazole, were synthesized as photovoltaic materials. Thiophene was introduced to provide extended π-conjugation length and charge transfer properties. A band gap (E_g^op=1.62 eV, E_g^ec=1.51 eV) of this polymer was investigated through UV-vis spectroscopy and cyclic voltammetry. A bulk heterojunction structure of glass/indium tin oxide (ITO)/PEDOT:PSS/polymer-PCBM(1:3)/LiF/Al was fabricated for investigating photovoltaic properties. PC₇₁BM was used as an acceptor material, due to its increased absorption in the visible region, in comparison with PC₆₁BM. In this polymer, incident photon-to-current conversion efficiency (IPCE) was as high as 50%. Moreover, maximum power conversion efficiency (PCE) of up to 1.72% was achieved under AM 1.5 G conditions. It demonstrated relatively high V_OC (0.67 V) and J_SC (6.86 mA/cm²), while a low fill factor value (0.37) was obtained.     

Efficient bulk heterojunction solar cells with copolymers based on fluorene, dithienylbenzothiadiazole, and thiophene derivatives

Three copolymers, PFTpBt, PFbCNTpBt, and PFC6TpBt, which are based on dioctylfluorene, dithienylbenzothiadiazole, and thiophene derivatives with different functional groups, were synthesized via a Suzuki reaction. The copolymers were characterized by NMR, GPC, TGA, UV-vis absorption, and electrochemical cyclic voltammetry. Thermogravimetric analysis showed that the copolymers exhibited good thermal stability. The optical, electrochemical, photovoltaic properties, and hole mobility of the copolymers were investigated and discussed. The polymer solar cell based on PFTpBt had the best performance, with a power conversion efficiency (PCE) of 2.17% under the illumination of AM 1.5, 100 mW/cm².     

Development of DA-type polymers with phthalimide derivatives as electron withdrawing units and a promising strategy for the enhancement of photovoltaic properties

We report on two push-pull type polymer semiconductors involving phthalimide derivatives as electron withdrawing units. The solubility and energy level of phthalimide could be easily controlled by introducing various functional groups in its nitrogen site. Additionally, the V_OC value of polymer semiconductor materials with phthalimide as an electron withdrawing unit could be efficiently enhanced because of the low HOMO energy level of phthalimide. Nevertheless, there are just a few of studies regarding the use of phthalimide in OPVs. In this study, we synthesized two photovoltaic polymer materials based on phthalimide with high V_OC value, PFTPT and PCTPT. Between the two polymers, PCTPT/PC₇₁BM-based photovoltaic cell afforded the best PCE value of 1.4% (V_OC=0.94 V, J_SC=4.41 mA/cm², FF=0.33) under 100 mW/cm² irradiation. In addition, a promising strategy for the development of high performance photovoltaic polymers with phthalimide derivatives as electron withdrawing units was investigated.     

Efficiency enhancement of polymer solar cells by incorporating a self-assembled layer of silver nanodisks

We report the efficiency enhancement of polymer solar cells by incorporating a silver nanodisks' self-assembled layer, which was grown on the indium tin oxide (ITO) surface by the electrostatic interaction between the silver particles and modified ITO. Polymer solar cells with a structure of ITO (with silver nanodisks)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevious P VP AI 4083)/poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC₆₁BM)/LiF/Al exhibited an open circuit voltage (V_OC) of 0.61±0.01 V, short-circuit current density (J_SC) of 9.24±0.09 mA/cm², a fill factor (FF) of 0.60±0.01, and power conversion efficiency (PCE) of 3.46±0.07% under one sun of simulated air mass 1.5 global (AM1.5G) irradiation (100 mW/cm²). The PCE was increased from 2.72±0.08% of the devices without silver nanodisks to 3.46±0.07%, mainly from the improved photocurrent density as a result of the excited localized surface plasmon resonance (LSPR) induced by the silver nanodisks.     

All-inorganic CdSe/PbSe nanoparticle solar cells

We report on solar cells consisting of a sintered active bilayer of CdSe and PbSe nanoparticles in the structure ITO/CdSe/interlayer/PbSe/Al, where an interlayer of LiF or Al₂O₃ was found necessary to prevent low shunt resistance from suppressing the photovoltaic behavior. We fabricated unoptimized solar cells with a short-circuit current of 6 mA/cm², an open-circuit voltage of 0.18 V, and a fill factor of 41%. External quantum efficiency spectra revealed that photons from the infrared portion of the spectrum were not collected, suggesting that the low bandgap PbSe film did not contribute to the photocurrent of the structure despite exhibiting photoconductivity. Other measurements, however, showed that the PbSe film was indeed necessary to produce a photovoltage and transport electrons. Through sintering, the nanoparticle films acquired bandgaps similar to those of the corresponding bulk materials and became more conductive. Because the PbSe films were found to be considerably more conductive than the CdSe films, we suggest that the PbSe layer is effectively behaving like a low conductivity electrical contact; therefore, this solar cell architecture does not follow typical type-II heterojunction donor/acceptor models used to describe organic polymer solar cells.     

The detailed analysis of auger effect on the efficiency of intermediate band solar cells

The effect of Auger mechanism on the efficiency of intermediate band solar cell (IBSC) has been investigated using detailed balance equations. Four types of the IBSCs, cells (a), (b), (c) and (d) including optical transitions and low (below bandgap)/high (above bandgap) threshold Auger generation/recombination between three bands have been analyzed. The effect of carrier multiplication probability, σ, on the variation of efficiency due to the bandgap, position of intermediate band energy level, the amount of overlap between equal and nonequal absorption coefficients and the sun concentration have been studied in detail. Since the Auger mechanism was more effective at lower bandgaps, it has been found out that the efficiency rised and the optimum bandgap providing the maximum efficiency moved towards the lower bandgaps, for three types of the cells, as multiplication probability increased. The highest efficiency of 72.9% has been obtained for new cell (c) at E_G=1.42 eV, if both optical transitions and low/high threshold Auger processes were taken into account. This value was higher than the maximum efficiency of 63.2% obtained for E_G=1.95 eV without Auger effect (cell (d)). Under maximum overlap condition, cell (b) exhibited the highest efficiency for the case of equal absorption coefficients and efficiency was highest in cell (c) for the case of nonequal absorption coefficients. Although the increment of sun concentration improved the efficiencies of all the cells, cell (b) had shown the highest efficiency below 200 sun concentration.     

Organic solar cells with 2-Thenylmercaptan/AU self-assembly film as buffer layer

The interface between an electrode and the organic active layer is an important factor in organic solar cells (OSCs) that influences the power conversion efficiency (PCE). In this report, a buffer layer of 2-thenylmercaptan/Au self-assembly film is introduced into OSCs as a substitute for the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) layer. The electrode/active layer interface is meliorated by Au-S coordinate bond of self assembly after applying this buffer layer. The series resistance reduces from 20 Ω cm² in a device based on PEDOT:PSS to 10.2 Ω cm². Correspondingly, the fill factor (FF) increases from 0.50 to 0.64. Moreover, due to the dipole of this self-assembled layer, the open circuit voltage (V_oc) also increases slightly from 0.54 V to 0.56 V and the PCE reaches 2.5%.     

Improved the efficiency of small molecule organic solar cell by double anode buffer layers

Small molecule organic solar cell with an optimized hybrid planar-mixed molecular heterojunction (PM-HJ) structure of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) doped with 4 wt% sorbitol/ pentacene (2 nm)/ copper phthalocyanine (CuPc) (10 nm)/ CuPc: C₆₀ mixed (20 nm)/ fullerene (C₆₀) (20 nm)/ bathocuproine (BCP) (10 nm)/Al was fabricated. PEDOT: PSS layer doped with 4 wt% sorbitol and pentacene layer were used as interlayers between the ITO anode and CuPc layer to help the hole transport. And then the short-circuit current (J_sc) of solar cell was enhanced by inserting both the PEDOT: PSS (4 wt% sorbitol) and the pentacene, resulting in a 400% enhancement in power conversion efficiency (PCE). The maximum PCE of 3.9% was obtained under 1sun standard AM1.5G solar illumination of 100 mW/cm².     

Air-stable triarylamine-based amorphous polymer as donor material for bulk-heterojunction organic solar cells

A new air-stable triarylamine-based amorphous polymer, TSP-T11, which consists of thiophene and triarylamine units, can be successfully utilized to fabricate bulk-heterojunction organic photovoltaics (OPVs) using PC₆₀BM or PC₇₀BM as acceptor materials. The highest level of performance of OPVs optimized at TSP-T11:PC₇₀BM (weight ratios of 1:4) with thicknesses of 68 nm exhibited an open circuit voltage (V_oc) of 0.75 V, a short circuit current (J_sc) of 8.03 mA cm⁻², and a power-conversion efficiency (PCE) of 2.22% under simulated air mass 1.5 solar irradiation at 100 mW cm⁻². Although TSP-T11 has a lower hole mobility (1.5×10⁻⁴ cm² V⁻¹ s⁻¹) than P3HT, the use of amorphous film of TSP-T11 as a donor material for OPVs offers advantages over the use of polycrystalline film of P3HT in terms of its air-stability and pinhole-free homogeneous morphology.     

Patternable solution process for fabrication of flexible polymer solar cells using PDMS

PH 500, a highly conducting poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), is a typical conducting polymer anode material used in organic electric devices. However, it has the disadvantages of low conductivity and poor surface roughness and requires a patterning method for the electrode through including the laser and plasma. In this paper, therefore, the conducting polymer ink for a transparent anode was formulated by adding dimethyl sulfoxide (DMSO) and BYK-333 as the surfactant to enhance the conductivity and surface roughness. The conducting polymer anode was patterned through the application of a new patterning method that used polydimethylsiloxane (PDMS) on a flexible substrate. In addition, a photoactive layer was formed by applying the new patterning method to the conventional brush painting method in which patterning had previously been impossible. The resulting material was compared with the device fabricated by the spin coating method. The fabricated flexible polymer solar cells (PSCs) exhibited short-circuit current density (J_sc), open-circuit voltage (V_oc), fill factor (FF) and power conversion efficiency (PCE) values of 4.2 mA/cm², 0.878 V, 26.5% and 0.98%, respectively, which represented an efficiency improvement of 38% over those fabricated by the spin coating method. Meanwhile, the J_sc value was increased when the series resistance (R_s) decreased to 150 Ω cm².     

High performance polymer electrolytes based on main and side chain pyridine aromatic polyethers for high and medium temperature proton exchange membrane fuel cells

Novel aromatic polyether type copolymers bearing side chain polar pyridine rings as well as combination of main and side chain pyridine units have been evaluated as potential polymer electrolytes for proton exchange membrane fuel cells (PEMFCs). The advanced chemical and physicochemical properties of these new polymers with their high oxidative stability, mechanical integrity and high glass transition temperatures (T_g's up to 270 °C) and decomposition temperatures (T_d's up to 480 °C) make them promising candidates for high and medium temperature proton exchange membranes in fuel cells. These copolymers exhibit adequate proton conductivities up to 0.08 S cm⁻¹ even at moderate phosphoric acid doping levels. An optimized terpolymer chemical structure has been developed, which has been effectively tested as high temperature phosphoric acid imbibed polymer electrolyte. MEA prepared out of the novel terpolymer chemical structure is approaching state of the art fuel cell operating performance (135 mW cm⁻² with electrical efficiency 45%) at high temperatures (150-180 °C) despite the low phosphoric acid content (<200 wt%) and the low platinum loading (ca. 0.7 mg cm⁻²). Durability tests were performed affording stable performance for more than 1000 h.     

Dye-sensitized, nano-porous TiO2 solar cell with poly(acrylonitrile): MgI2 plasticized electrolyte

Dye-sensitized solar cells are promising candidates as supplementary power sources; the dominance in the photovoltaic field of inorganic solid-state junction devices is in fact now being challenged by the third generation of solar cells based on dye-sensitized, nano-porous photo-electrodes and polymer electrolytes. Polymer electrolytes are actually very favorable for photo-electrochemical solar cells and in this study poly(acrylonitrile)-MgI₂ based complexes are used. As ambient temperature conductivity of poly(acrylonitrile)-salt complexes are in general low, a conductivity enhancement is attained by blending with the plasticizers ethylene carbonate and propylene carbonate. At 20 °C the optimum ionic conductivity of 1.9 × 10⁻³ S cm⁻¹ is obtained for the (PAN)₁₀(MgI₂)_n(I₂)_n/10(EC)₂₀(PC)₂₀ electrolyte where n = 1.5. The predominantly ionic nature of the electrolyte is seen from the DC polarization data. Differential scanning calorimetric thermograms of electrolyte samples with different MgI₂ concentrations were studied and glass transition temperatures were determined. Further, in this study, a dye-sensitized solar cell structure was fabricated with the configuration Glass/FTO/TiO₂/Dye/Electrolyte/Pt/FTO/Glass and an overall energy conversion efficiency of 2.5% was achieved under solar irradiation of 600 W m⁻². The I-V characteristics curves revealed that the short-circuit current, open-circuit voltage and fill factor of the cell are 3.87 mA, 659 mV and 59.0%, respectively.     

Photovoltaic properties of poly(benzothiadiazole-thiophene-co-bithiophene) as donor in polymer solar cells

We report the photovoltaic properties of a D-A copolymer, poly(benzothiadiazole-thiophene-co-bithiophene) (PBTTbT), containing the donor (D) unit of oligothiophene with a hexyl side chain and the acceptor (A) unit of 2,1,3-benzothiadiazole (BT) with a methyl side chain. The geometry, electronic and absorption spectroscopic properties of bithiophene-benzothiadiazole-thiophene monomer (M1) of the polymer were investigated theoretically by the density functional theory (DFT) method for deep understanding the relationship of the structure and properties of the polymer. Polymer solar cells (PSCs) were fabricated with PBTTbT as an electron donor blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM) as an electron acceptor. The power conversion efficiency (PCE) of PSC is 0.87% for an optimized PBTTbT:PC₇₀BM weight ratio of 1:3, under the illumination of AM 1.5, 100 mW/cm². With the additive of 1% 1,8-dioctanedithiol and thermal annealing at 130 °C for 15 min, the PCE of the device was improved to 1.98%. The efficiency improvement of the device was ascribed to a better morphology of the PBTTbT:PC₇₀BM active layer with the additive and thermal annealing.     

Two-dimensional regioregular polythiophenes with conjugated side chains for use in organic solar cells

We have prepared two two-dimensional polythiophenes (2D-PTs; P1 and P2) possessing alkyl-thiophene side chains by Stille coupling reactions. Optical measurements indicate that the bandgaps of P1 and P2 being 1.98 and 1.77 eV, respectively. P2 displayed a red-shift in its absorption spectrum because of the longer length of its conjugated side chains. Desirable highest occupied molecular orbital (HUMO) and lowest unoccupied molecular orbital (LUMO) energy levels were obtained from electrochemical studies, which suggested that these systems would exhibit high open-circuit voltages when blended with fullerene as electron acceptors. The hole mobility (thin film transistor (TFT) measurement) of P1 and P2 are 3.5×10⁻⁴ and 4.6×10⁻³ cm² V⁻¹ s⁻¹, respectively. A power conversion efficiency of 2.5% is obtained under simulated solar illumination (AM 1.5G, 100 mW cm⁻²) from a polymer solar cell comprising an active layer containing 25 wt% P1 and 75 wt% [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM).     

Low temperature deposition of high open-circuit voltage (>1.0 V) p-i-n type amorphous silicon solar cells

P-i-n type hydrogenated amorphous silicon (a-Si:H) solar cells were deposited by the radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) process at a low substrate temperature of 125 °C, which is compatible with low-cost poly (ethylene terephthalate) (PET) plastic substrates. Wide band gap (E_opt>1.88 eV) intrinsic a-Si:H films were achieved before the onset of the microcrystalline regime by changing the hydrogen dilution ratios. On the other hand, the structural, optical and electrical properties of p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) window layers have been optimized at 125 °C. High quality p-a-SiC:H film with high optical band gap (E₀₄=2.02 eV) and high conductivity (σ_d=1.0×10⁻⁷ S/cm) was deposited at ‘low-power regime’ under low silane flow rates and high H₂ dilution conditions. With the combination of wide band gap p-a-SiC:H window layers and intrinsic a-Si:H layers, a high V_oc of 1.01 V (efficiency=5.51%, FF=0.72, J_sc=7.58 mA/cm²) was obtained for single junction a-Si:H p-i-n solar cell at a low temperature of 125 °C. Finally, flexible a-Si:H solar cell on PET substrate with efficiency of 4.60% (V_oc=0.98 V, FF=0.69, J_sc=6.82 mA/cm²) was obtained.     

Dye sensitized solar cells on paper substrates

This article reports for the first time in the literature, a dye sensitized solar cells with 1.21% efficiency (V_oc=0.56 V, J_sc=6.70 mA/cm² and F.F.=0.33) on paper substrates. The current dye sensitized solar cell technology is based on fluorine doped SnO₂ (FTO) coated glass substrates. The problem with the glass substrate is its rigidity and heavy weight. Making DSSCs on paper opens the door for both photovoltaic and paper industries. The potential of using mature paper making and coating technologies will greatly reduce the current PV cost. Paper substrate based DSSCs not only offer the advantages of flexibility, portability and lightweight but also provide the opportunities for easy implantation to textile. In this study, a low temperature process is developed to coat uniform nickel on paper substrate as the metal contact to replace the traditional expensive FTO. The Ni paper showed excellent conductivity of 8-10 Ω/□. It is found that the control of metal oxide electrode morphology is critical to solar cell performance. The TiO₂ film has the tendency to crack on Ni coated paper, which resulted in the shunt of the device and no solar cell efficiency was obtained. ZnO film on the other hand had good morphology tolerance on Ni coated paper and yielded solar cell efficiency of 1.21% (V_oc=0.56 V, J_sc=6.70 mA/cm² and F.F.=0.33) under AM 1.5 (activation area is 0.16 cm²). The control sample of ZnO solar cell on FTO glasses has the efficiency of 2.66% (V_oc=0.64 V, J_sc=9.97 mA/cm² and F.F.=0.42).     

Copolymer derived from dihexyl-2H-benzimidazole and carbazole for organic photovoltaic cells

A new accepter unit, dihexyl-2H-benzimidazole, was prepared and utilized for the synthesis of the conjugated polymers containing electron donor-acceptor pair for OPVs. Dihexyl-2H-benzimidazole unit was designed to substitute the BT unit of PCDTBT. A new series of copolymers with carbazole as the electron-rich unit and dihexyl-2H-benzimidazole as the electron-deficient unit are synthesized. To obtain absorption in the longer wavelength region, bithiophene units without any alkyl group are incorporated as one of the monomers, which may result in low solubility of the polymers. In dihexyl-2H-benzimidazole, sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon, while keeping the 1,2-quinoid form, to improve the solubility of the polymers. The spectra of the solid films show absorption bands with maximum peaks at 446-457 nm and the absorption onsets at 527-539 nm, corresponding to bandgaps of 2.30-2.35 eV. Under white light illumination (AM 1.5 G, 100 mW/cm²), the devices with PCBBTHBIs:PCBM layers showed open-circuit voltages (V_OC) of 0.13-0.23 V, short-circuit current densities (J_SC) of 3.52-5.69 mA/cm², and fill factors (FF) of 0.36-0.40, giving power-conversion efficiencies of 0.21-0.47%.