ITO-free flexible organic solar cells with printed current collecting grids

The presence of a transparent conductive electrode such as indium tin oxide (ITO) limits the reliability and cost price of organic photovoltaic devices as it is brittle and expensive. Moreover, the relative high sheet resistance of an ITO electrode on flexible substrates limits the maximum width of a single cell. We have developed an alternative ITO-free transparent anode, based on solution processed high conductive PEDOT:PSS in combination with a printed current collecting grid. The screen printed silver grid demonstrates a typical sheet resistance of 1 Ω/□ with 6.4-8% surface coverage. The efficiency of a flexible device with an active area of 4 cm² with such a grid is much higher than a similar device based on ITO. Furthermore, as this composite anode is solution-processed, it is a step forward towards low-cost large area processing.     

ITO free MoO3/Au/MoO3 structures using Al2O3 as protective barrier between MoO3 and PEDOT:PSS in organic solar cells

A MoO₃/Au/MoO₃ structure with a protective barrier Al₂O₃ was developed to suppress the reactions between MoO₃ and the PEDOT:PSS film in organic solar cells (OSCs). Though the maximum optical transmittance of this structure was 66% at 550 nm wavelength, the power conversion efficiency of a MoO₃/Au/MoO₃/Al₂O₃/PEDOT:PSS based OSCs was 2.77%, comparable to the 2.89% of an ITO-based OSCs. The introduction of a very thin Al₂O₃ layer between the MoO₃ and the acidic PEDOT:PSS film effectively protected the MoO₃ from the acidic and water dispersed PEDOT:PSS film, increasing the J_sc, V_oc and FF of the structure above those of the MoO₃/Au/MoO₃/PEDOT:PSS structure. The Al₂O₃ (1 nm) introduced to the MoO₃/Au/MoO₃ structure improved J_sc because it suppressed the reactions between MoO₃ and PEDOT:PSS and lowered the work function of the PEDOT:PSS film. The MoO₃/Au/MoO₃/Al₂O₃ electrode was shown to be a promising replacement of ITO for use in flexible optoelectronic devices.     

ITO-free inverted polymer solar cells using a GZO cathode modified by ZnO

A gallium-doped ZnO (GZO) layer was investigated and compared with a conventional indium-tin-oxide (ITO) layer for use as a cathode in an inverted polymer solar cell based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) bulk heterojunctions (BHJ). By modifying the GZO cathode with a ZnO thin layer, a high power conversion efficiency (3.4%) comparable to that of an inverted solar cell employing the same P3HT:PCBM BHJ photoactive layer with a conventional ITO/ZnO cathode was achieved. This result indicates that GZO is a transparent electrode material that can potentially be used to replace high-cost ITO.     

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).     

Ultra-thin metal layer passivation of the transparent conductive anode in organic solar cells

Efficiency of organic solar cells shows a strong improvement when the transparent conductive anode (indium tin oxide—ITO, aluminium-doped zinc oxide—AZO, fluorine-doped tin oxide—FTO), is covered with an ultra-thin metallic film. It is shown that the best results are achieved with a gold film (0.5 nm). The efficiency of the solar cells using AZO or FTO is improved up to one order of magnitude, while in the case of ITO it is at least 50%. It is shown that if the matching between the work function of the anode and the highest occupied molecular orbital (HOMO) of the organic electron donor is the most important factor limiting the hole transfer efficiency, others factors such as transparent conductive oxide (TCO) surface roughness and adhesion of the organic layer are also key factors.     

Photocarrier generation in organic thin-film solar cells with an organic heterojunction

Photocurrent–voltage characteristics for organic solar cells with a heterojunction formed between copper phthalocyanine and a perylene derivative (or C₆₀) were studied. The photocurrent was observed under both reverse and forward biases. From the analysis of the photocurrent action spectra, the origin of the reverse photocurrent was attributed to the excitons formed in both the organic layers, whereas that of the forward photocurrent was attributed to the excitons formed in the perylene derivative (or C₆₀) layer. The photocurrent density under reverse bias increased at higher temperatures, suggesting that the charge recombination possibility was lowered at higher temperatures. On the basis of the time responses of the photocurrents observed after pulsed photoirradiation, the charge separation and transport processes are discussed.     

Determination of photo-active region in organic thin film solar cells with an organic heterojunction

The photo-active region in the solar cells consisting of Cu-phthalocyanine (CuPc) and perylene-derivative (PV) layers was determined by using exciton blocking layers (EBLs) inserted in these layers. The photocurrent density was low when the EBL was placed near the CuPc/PV interface. With the increase of the distance between the EBL and the CuPc/PV interface, the photocurrent increased. However, when the distance reached a certain value, it leveled off owing to the limited diffusion length of excitons. From the analysis of the relationship between the position of EBL and the photocurrent density, the photo-active regions in the CuPc and PV layers were estimated to be 8 and 12 nm thick from the interface, respectively.     

Glass/ITO/In(O,S)/CuIn(S,Se)2 solar cell with conductive polymer window layer

Hybrid solar cells based on the combination of conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS) and inorganic semiconductor CuIn(S,Se)₂ (CISSe) were investigated. The CuInSe₂ (CISe) absorber layers were electrodeposited on ITO covered glasses from aqueous solutions with various ratios of elements. The ITO/In(O,S)/CISSe photovoltaic (PV) junctions were prepared by the sulfurization of ITO/CISe precursors at 450 °C in the H₂S atmosphere.The PEDOT–PSS layer of p-type is considered an alternative to the traditional window top layer on the CISSe absorber layer in the cell structure. The polymer deposition was performed by help of the spin-casting technique. PV properties of the prepared ITO/In(O,S)/CISSe and ITO/In(O,S)/CISSe/PEDOT–PSS structures were studied, with emphasis on the role of conductive polymer layer in the cell structure.     

A high-performance counter electrode based on poly(3,4-alkylenedioxythiophene) for dye-sensitized solar cells

A poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et₂) counter electrode prepared by electrochemical polymerization on a fluorine-doped tin oxide (FTO) glass substrate was incorporated in a platinum-free dye-sensitized solar cell (DSSC). The surface roughness and I⁻/I₃⁻ redox reaction behaviors based on PProDOT-Et₂, poly(3,4-propylenedioxythiophene) (PProDOT), poly(3,4-ethylenedioxythiophene) (PEDOT), and sputtered-Pt electrodes were characterized, and their performances as counter electrodes in DSSCs were compared. Cells fabricated with a PProDOT-Et₂ counter electrode showed a higher conversion efficiency of 7.88% compared to cells fabricated with PEDOT (3.93%), PProDOT (7.08%), and sputtered-Pt (7.77%) electrodes. This enhancement was attributed to increases in the effective surface area and good catalytic properties for I₃⁻ reduction. In terms of the film thickness effect, the fill factor was strongly dependent on the deposition charge capacity of the PProDOT-Et₂ layer, but the aggregation of PProDOT-Et₂ in thicker layers (>80 mC cm⁻²) resulted in decreases in J_SC and the cell conversion efficiency. The charge transfer resistances (R_ct1) of the PProDOT-Et₂ counter electrodes had the lowest value of ∼18 Ω at a deposition charge capacity of 40 mC cm⁻². These results indicate that films with high conductivity, high active surface area, and good catalytic properties for I₃⁻ reduction can potentially be used as the counter electrode in a high-performance DSSC.     

Using modified poly(3,4-ethylene dioxythiophene): Poly(styrene sulfonate) film as a counter electrode in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs), assembling with nano-crystalline TiO₂ adsorbed cis-Ru(dcb)₂(NCS)₂ dye (known as N3) using polar solvent-treated poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) coating on a conductive glass (fluorine-doped tin oxide, FTO) as a counter electrode, were studied. The conductivity of a bare PEDOT:PSS film was only 2±0.05 S/cm. However, the conductivities of PEDOT:PSS films treated with dimethyl sulfoxide (DMSO), N,N-dimethyl acetamide (DMAc), N,N-dimethyl formamide (DMF), and dichloromethane (DMC) reached 85±15, 45±10, 36±7, and 20±6 S/cm, respectively. In addition, carbon blacks (0.02, 0.1, 0.5, 1.0, 2.0 wt% with respect to PEDOT:PSS aqueous solution) were added into the DMSO-treated PEDOT:PSS solution (denoted as DMSO-PEDOT:PSS) to enhance the conductivity. Atomic force microscopy (AFM) images of PEDOT:PSS and various DMSO-PEDOT:PSS films coated on the FTO glasses were examined. The topographical images reveal that the increased surface roughness is responsible for the enhanced electrochemical property of the DMSO-PEDOT:PSS films. AC impedance technique was also employed to analyze the kinetics at the electrolyte/counter electrode interface. The DSSC using carbon black (0.1 wt%)-modified DMSO-PEDOT:PSS conductive coating as a counter electrode reached a cell efficiency of 5.81% under 100 mW/cm². This efficiency is higher than a DSSC using Pt as a counter electrode (5.66%).     

Vacuum-free processed transparent inverted organic solar cells with spray-coated PEDOT:PSS anode

Spray coating is a high throughput coating technique that is scalable and adaptable for organic photovoltaic manufacturing. To ensure uniform coating of the organic layers, the wettability, surface tension and boiling points of the solvents have to be optimized. Here, we used microscopic videos to understand the dynamics of the spray coating process. By optimizing the wettability and drying time of the PEDOT:PSS suspension on a hydrophobic surface, we attained a spray coated transparent anode without compromising on device performance. We further applied this vacuum-free process to a near infrared absorber to achieve a transparent organic solar cell with close to 60% transparency.     

Annealing dependent performance of organic bulk-heterojunction solar cells: A theoretical perspective

Organic photovoltaic (OPV) technology promises a relatively inexpensive option for the solar energy conversion, provided its efficiency increases beyond the current level (∼7-8%) along with significant improvement in operational lifetime. To achieve this high efficiency/reliability, a systematic theoretical approach is required to optimize the underlying device fabrication process. In this article, we use an anneal-time dependent process-device co-simulation framework (the phase-field model for phase separation coupled with the self-consistent drift-diffusion transport for free carriers) to explore the effects of the process conditions (e.g., annealing temperature, anneal duration) on the performance of organic solar cells. Our results explain experimentally observed annealing effects on the solar cell performance, namely, (i) peak of the short circuit current, (ii) insensitivity of the open circuit voltage, (iii) low fill factor, etc., that would otherwise be deemed anomalous from the perspective of conventional solar cells. As such, this work offers a detailed analysis of the effects of annealing on OPV morphology and its electrical performance. This work also provides a theoretical framework for the optimization of process conditions, which might eventually lead to higher efficiency/reliability of the organic photovoltaic technology.     

Spray-coated organic solar cells with large-area of 12.25 cm

This study evaluated the possibility of utilizing a spray-coating process for large-area organic solar cells (OSCs) combined with a metal electrode geometry. The effects of the cell area in spray-coated OSCs were investigated systematically by introducing a metal sub-electrode and grid-electrode to realize large-area cells of up to 12.25 cm². The series resistance could be reduced significantly by inserting a metal grid-electrode into the indium tin oxide (ITO) anode, yielding a power conversion efficiency of 2.11% at a cell area of 12.25 cm² and 2.49% at an effective photocurrent generated area of 11.23 cm² under AM.1.5 simulated illumination. This is comparable to the 3.13% obtained in the cell produced by spray-coating at a cell area of 0.38 cm².     

Fabrication and characterization of 4-tricyanovinyl-N,N-diethylaniline/p-silicon hybrid organic–inorganic solar cells

Hybrid organic–inorganic solar cell was fabricated by thin film of 4-tricyanovinyl-N,N-diethylaniline deposited on p-Si substrates. The capacitance–voltage characteristics indicated that the junction is of abrupt nature. The dark forward current density-voltage characteristics indicated a tunneling conduction at relatively low voltages followed by a space-charge-limited-conduction mechanism at relatively high voltages. Under illumination, the cell exhibits photovoltaic characteristics with an open-circuit voltage of 0.70 V, a short-circuit current density of 9.15 mA cm⁻², and a power conversion efficiency of 3.10%. The effect of γ-rays irradiation (100 kGy absorbed dose) on the characteristics of the cell was also investigated. The fill factor and the power conversion efficiency decrease by 20.9% and 39% of the original value, respectively.     

High-voltage (1.8 V) tandem solar cell system using a GaAs/AlXGa(1−X)As graded solar cell and dye-sensitised solar cells with organic dyes having different absorption spectra

Stacked multijunction (tandem) solar cells have been prepared by mechanically stacking dye-sensitised solar cells (DSCs) and a GaAs/Al_XGa_(1−X)As graded solar cell (GGC) as the top and bottom cells, respectively. Three organic dyes with different absorption spectra (D131, D102 and D205) were used in the DSCs, in order to match the photocurrent density between the DSC and the GGC. Tuning the absorption range of the DSC by choosing an appropriate dye, increased the overall photovoltaic conversion efficiency due to the optimal utilisation of the solar spectrum in the individual cells. The open circuit photovoltages (V_OC) of the GGC and the DSC with D131 were 1.11 V and 0.76 V, respectively, resulting in a V_OC of 1.85 V and a photovoltaic conversion efficiency of 7.63% for the tandem cell. Although the overall conversion efficiency has not exceeded that of the GGC (7.66%), these tandem cells provide adequate V_OC values for water splitting applications.     

Photovoltaic solar cells using poly(3,3-didodecylquaterthiophene)

The fabrication and the optimization of photovoltaic solar cells based on poly(3,3-didodecylquaterthiophene) (12-PQT) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) are reported. On the one hand, the effect of the blend composition shows the necessity to increase the amount of fullerene derivative compared to conventional poly(3-hexylthiophene)-based systems. On the second hand, thermal annealing of devices is optimized and discussed. Overall, in this work, the highest power conversion efficiencies are shown in the range of 0.3–0.4% which represents a lower value than that reported with poly(3-hexylthiophene). Results are discussed in terms of charge carrier mobility and phase segregation in this bicontinuous donor-acceptor network.     

Polymer photovoltaic devices using highly conductive poly(3,4-ethylenedioxythiophene-methanol) electrode

In this work, modified poly(3,4-ethylenedioxythiophene) (PEDOT) was used as an anode in polymer photovoltaic devices (PVDs) based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₀-butyric acid methyl ester (PCBM). We synthesized poly(3,4-ethylenedioxythiophene methanol) (PEDTM) with a transmittance of 87% (at 510 nm) and a conductivity of 700 S/cm. PEDTM was applied in photovoltaic devices as a hole transporting layer on indium-tin oxide (ITO) electrode as well as a direct anode layer. PVDs with PEDTM as hole transporting layers on ITO showed a very high short-circuit density of 14.87 mA/cm² and power conversion efficiency of 2.67% under an illumination of AM 1.5 G (100 mW/cm²). In addition, we also fabricated ITO-free PVDs using PEDTM as an anode, which exhibited a performance of 0.61% with a result of J_sc of 4.48 mA/cm², V_oc of 0.51 V, and FF of 27%.     

The effect of solvent induced crystallinity of polymer layer on poly(3-hexylthiophene)/C70 bilayer solar cells

A bilayer polymer solar cell is demonstrated with the device configuration ITO/PEDOT:PSS/poly(3-hexylthiophene)/C₇₀/Al. In this article, we highlight the importance of polymer surface morphology, its crystallinity and mobility on device output parameters. The solvent used for polymer processing plays a major role in deciding these parameters and it was observed that high boiling point solvents are desirable for achieving large surface roughness of the polymer layer, which in turn provide more interface area in the bilayer device structure. Due to the increased interface area for exciton dissociation, these bilayer devices resulted in a maximum power conversion efficiency of 3.65% under one sun radiation.     

Improved high temperature stability of organic solar cells using a phosphine oxide type cathode modification layer

The high temperature stability of the organic solar cells (OSCs) was improved using a phosphine oxide based organic cathode modification layer instead of LiF. The OSC modified with the phosphine oxide cathode modification layer showed constant device performances after a thermal treatment at 90 °C, while the LiF based OSC exhibited degraded device performance after the thermal treatment. The use of the phosphine oxide based interlayer was effective to improve the high temperature stability of the OSC.     

Enhanced quantum yield in porphyrin/electron-donor double-layer solar cells

When the layer of 3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene (MC(COOH)) is inserted into the Au/Zntpyp interface in Al/Zntpyp/Au sandwich-type solar cell (Zntpyp: 5,10,15,20-tetra(3-pyridyl)porphyrinatozinc), the photovoltaic properties are remarkably improved. For the Al/Zntpyp(thickness 10 nm)/MC(COOH)(20 nm)/Au cell, a short-circuit photocurrent (J_sc) of 0.93 μ Acm⁻², open-circuit photovoltage (V_oc) of 0.71 V, fill factor (ff) of 0.41, and energy conversion yield (η) of 3.6% are obtained when illuminated at the Al/Zntpyp interface with 455 nm monochromatic light of 7.5 μ Wcm⁻² intensity. A rapid electron-transfer from the donor MC(COOH) to photogenerated holes in Zntpyp suppresses the charge recombination of the photogenerated carriers. The energetically well-arranged valence band levels eventually enhance the η value about 9 times compared with the Al/Zntpyp/Au cell. Further the Al/HD(9 nm)/MC(COOH)(20 nm)/Au cell using a longer-lived sensitizer (HD) instead of Zntpyp gives a J_sc value of 2.36 μ Acm⁻², V_oc value of 0.69 V, ff value of 0.34, and η value of 4.8% when illuminated with 445 nm monochromatic light of 11.7 μ Wcm⁻² intensity at the Al/HD interface, where HD represents a heterodimer consisting of 5,10,15-tri(4-chlorophenyl)-20-(3-pyridyl)porphyrin(H₂pyp₃p(Cl)) and 5,10,15,20-tetra(2,5-dimethoxyphenyl)porphyrinatozinc(Zntpp(OMe)₂).