Bilayer Solar Cells Research Articles

Crystallites of α‐Sexithiophene in Bilayer Small Molecule Organic Solar Cells Double Efficiency

Recent efforts in research and development of small molecule based organic solar cells have led to power conversion efficiencies exceeding 10%. Understanding the incorporated interfaces in these devices is an utterly important parameter for their improvement. Here we investigate the influence of α‐sexithiophene (α‐6T) nanostructures on the performance parameters of α‐6T/C60 inverted bilayer solar cells. By in situ controlled growth, crystalline α‐6T nanostructures are formed in the devices and a correlation between the morphology of the structures and the device performance is presented. Under certain, well‐defined circumstances, we observe an efficiency increase of around 100% when implementing crystalline nanostructures.

The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH3NH3PbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm−2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH3NH3PbI3 relative to its thickness. Furthermore, the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the CH3NH3PbI3/PC61BM interface is very efficient. With further increase in CH3NH3PbI3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solution-processable bilayer solar cells to date.

Solution-processed CdS transistors with high electron mobility

Solution-processed CdS field effect transistors (FETs) and solar cells are demonstrated via spin-coating and thermal annealing of soluble cadmium thiolate compounds. The synthesis is carried out in one simple step using cadmium oxide and tertiary alkane thiols. The cadmium thiolates are soluble in organic solvents such as chloroform and may be spin-coated, like organic semiconductors, to form thin films. The cadmium thiolate films decompose rapidly at 300 °C to yield semiconducting cadmium sulfide films. FETs are easily fabricated using these films and exhibit electron mobilities of up to 61 cm2 V−1 s−1, which compare favourably to FETs prepared from other solution-processed materials such as organic semiconductors, inorganic nanoparticles or chalcogenide films. Initial attempts to prepare hybrid bilayer solar cells were successfully realized by spin-coating a p-type semiconducting polymer layer on top of the n-type CdS film. These devices show significant photocurrent response from both the CdS and polymer layers, indicating that the CdS films are able to participate in photo-induced electron transfer from the polymer to the CdS layer as well as photo-induced hole transfer from CdS to the polymer layer.

Influence of the Excited-State Charge-Transfer Character on the Exciton Dissociation in Donor–Acceptor Copolymers

We synthesize a polytriphenylamine homopolymer and two donor–acceptor copolymers (D–A-copolymers) based on triphenylamine (TPA) as donor in combination with two different acceptor moieties to study the effect of the acceptor unit on the excited-state charge-transfer characteristics (CT-characteristics) and charge separation. The two acceptor moieties are a dicyanovinyl group in the side chain and a thieno[3,4-b]thiophene carboxylate in the main chain. Absorption and photoluminescence studies show new CT-bands for both of the D–A-copolymers. Field-dependent charge extraction studies in bilayer solar cells indicate a stronger CT-character for the copolymer in which the acceptor group is less conjugated with the copolymer backbone. The D–A-copolymer carrying the acceptor unit in the main chain exhibits smaller excitonic CT-character and good conjugation leading to less-bound electron–hole pairs and a better charge separation. This fundamental study gives insight into the interdependence of conjugation, charge carrier mobility, and solar cell performance for two different D–A-copolymers.

Understanding the Effect of Donor Layer Thickness and a MoO3 Hole Transport Layer on the Open-Circuit Voltage in Squaraine/C60 Bilayer Solar Cells

Small molecule organic solar cells are becoming increasingly efficient through improved molecular design. However, there is still much to be understood regarding device operation. Here we study bilayer solar cells employing a 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) donor and fullerene acceptor to probe the effect of donor layer thickness and a MoO3 electron transport layer on device performance. The thickness of SQ is seen to drastically affect the open-circuit voltage (VOC) and fill factor (FF), while the short circuit current is not altered significantly. The fact that the VOC of the bilayers with thin (6 nm) donor layers shows a strong dependence on the material and workfunction of the anode cannot be explained with a model for a perfect bilayer. Recombination of electrons from C60 at the anode contact has to be possible to understand the strong effect of the anode workfunction. Using numerical simulations and a simple two-diode model we show that the most likely interpretat...

Asymptotic and numerical prediction of current-voltage curves for an organic bilayer solar cell under varying illumination and comparison to the Shockley equivalent circuit

In this study, a drift-diffusion model is used to derive the current-voltage curves of an organic bilayer solar cell consisting of slabs of electron acceptor and electron donor materials sandwiched together between current collectors. A simplified version of the standard drift-diffusion equations is employed in which minority carrier densities are neglected. This is justified by the large disparities in electron affinity and ionisation potential between the two materials. The resulting equations are solved (via both asymptotic and numerical techniques) in conjunction with (i) Ohmic boundary conditions on the contacts and (ii) an internal boundary condition, imposed on the interface between the two materials, that accounts for charge pair generation (resulting from the dissociation of excitons) and charge pair recombination. Current-voltage curves are calculated from the solution to this model as a function of the strength of the solar charge generation. In the physically relevant power generating regime, it is shown that these current-voltage curves are well-approximated by a Shockley equivalent circuit model. Furthermore, since our drift-diffusion model is predictive, it can be used to directly calculate equivalent circuit parameters from the material parameters of the device.

Semitransparent organic photovoltaics using a near-infrared absorbing cyanine dye

A selective near-infrared absorbing heptamethine cyanine dye (Cy7-P) electron donor was used for the fabrication of bilayer solar cells with the acceptor C60. Using a reflective metal top electrode, solar cell optimization resulted in an external power conversion efficiency (η) of 1.5% for layer thicknesses of ~20nm for Cy7-P and of ~40nm for C60, with using either PEDOT:PSS or MoO3 as anode and Alq3 as cathode buffer layers. Highly transparent devices were then fabricated by using silver/Alq3 cathodes. Average visible transmittance (450–670nm) values of, for example, 67.2% (η=0.7%), 62.1% (η=0.9%) or 50% (η=1%) were obtained for different thickness combinations of silver and Alq3. Optimized solar cells had a maximum transparency of 79.8% at 568nm. The operational stability under 1sun illumination was T80=30h, after which 80% of the initial cell performance was reached.

Controlling the Texture and Crystallinity of Evaporated Lead Phthalocyanine Thin Films for Near-Infrared Sensitive Solar Cells

To achieve organic solar cells with a broadened spectral absorption, we aim to promote the growth of the near-infrared (NIR)-active polymorph of lead phthalocyanine (PbPc) on a relevant electrode for solar cell applications. We studied the effect of different substrate modification layers on PbPc thin film structure as a function of thickness and deposition rate (rdep). We characterized crystallinity and orientation by grazing incidence X-ray diffraction (GIXD) and in situ X-ray reflectivity (XRR) and correlated these data to the performance of bilayer solar cells. When deposited onto a self-assembled monolayer (SAM) or a molybdenum oxide (MoO3) buffer layer, the crystallinity of the PbPc films improves with thickness. The transition from a partially crystalline layer close to the substrate to a more crystalline film with a higher content of the NIR-active phase is enhanced at low rdep, thereby leading to solar cells that exhibit a higher maximum in short circuit current density (JSC) for thinner donor layers. The insertion of a CuI layer induces the formation of strongly textured, crystalline PbPc layers with a vertically homogeneous structure. Solar cells based on these templated donor layers show a variation of JSC with thickness that is independent of rdep. Consequently, without decreasing rdep we could achieve JSC=10 mA/cm2, yielding a bilayer solar cell with a peak external quantum efficiency (EQE) of 35% at 900 nm, and an overall power conversion efficiency (PCE) of 2.9%.

Stability of bilayer trimethine cyanine dye/fullerene organic solar cells

The long-term (up to 100 days) stabilities of thin films of a trimethine cyanine dye (Cy3-P) and of Cy3-P/C60 bilayer solar cells with initial power conversion efficiencies of up to 3.6% in the regular and inverted device architecture were studied. For storage conditions under nitrogen at room temperature in the dark, Cy3-P films were stable and no decrease in performance was measured for inverted ITO/TiO2/C60/Cy3-P/MoO3/Ag solar cells. ITO/PEDOT:PSS and aluminum were found to be responsible for a slight inherent degradation of regular solar cells under inert storage conditions. Solar cells stressed at accelerated (80°C) full sun simulation showed a burn-in period of ~5 days with a performance loss of 75%. This initial performance decrease might be linked to the photopolymerization reaction of C60. For longer times, performances stabilized and arrived at 15% of the initial values after a test period of 17 days.

Enhanced Photovoltaic properties of P3HT/Cr–TiO2 bilayer film heterojunction solar cells

The simple bilayer solar cells, composed of poly (3-hexylthiophene-2,5-diyl) (P3HT) and chromium doped titanium dioxide (Cr–TiO2) nanocrystal films, were fabricated by sol–gel method. The performances of structurally identical polymer–inorganic heterojunction solar cells Cu/Cr–TiO2/P3HT/FTO/Glass with various Cr3+ doping concentration were systematically investigated and compared. The results indicate that the samples’ open-circuit voltage is remarkably enhanced from 0.42V to 0.99V, and short-circuit photocurrent increases at first and then reduces, with the increase of Cr3+ doping concentration. The enhancement mechanism of photovoltaic properties is discussed in detail.

Formation of vertical concentration gradients in poly(3-hexylthiophene-2,5-diyl): Phenyl-C61-butyric acid methyl ester-graded bilayer solar cells

In the present work, we demonstrate that graded bilayer solar cells provide a very interesting alternative to the bulk heterojunction active layers commonly used in organic photovoltaic cells. One of the main advantages of this type of active layers is the possibility to optimize independently both donor and acceptor layers. Using various process methods, we obtain active layers that demonstrate a donor–acceptor vertical concentration gradient. These devices exhibit not only a high fill factor but also a remarkable increase in open-circuit voltage (Voc). In order to understand the influence of the film morphology over the device parameters, we provide a complete study using energy-dispersive x-ray spectroscopy elemental mapping of the device cross sections, showing evidence that ideal donor–acceptor concentration gradient are required to obtain high fill factors. Furthermore, we use a simple equivalent electrical model to extrapolate device parameters such as reverse saturation current for a clearer understanding of the origin of the Voc increase.

Influence of crystalline titanium oxide layer smoothness on the performance of inverted organic bilayer solar cells

Due to the small exciton diffusion length in organic materials, the donor-acceptor heterointerface in simple bilayer solar cells must be placed in close proximity to the bottom electrode. This makes great demands on the planarity of the base layer, since a non-uniform topography can cause adverse shorting through overlying layers. We fabricated indium tin oxide (ITO)/titanium oxide (TiOx)/fullerene (C60)/cyanine dye/molybdenum oxide (MoO3)/silver (Ag) solar cells with TiOx layers deposited via sputtering, coated from a nanoparticle suspension or prepared via a sol-gel process. A power conversion efficiency of 3.7% was measured when using a smooth sol-gel derived TiOx film.

Bilayer and bulk heterojunction solar cells with functional poly(2,2′-bithiophene) films electrochemically deposited from aqueous emulsion

Ultra thin films of poly(2,2′-bithiophene) with thickness of 11–33nm were electrochemically deposited on poly(3,4-ethylenedioxithiophene):poly(styrene sulfonic acid) modified indium tin oxide substrates using a new approach for electrochemical synthesis. Ultrasonic emulsification of aqueous perchloric acid and 2,2′-bithiophene monomer is an effective method to disperse this water insoluble monomer in micro droplets that improved the contact with the electrode. Rigorous control of the poly(2,2′-bithiophene) thickness was achieved by controlling the potential applied and the charge during the deposition. We studied thin films solar cells based on poly(2,2′-bithiophene) polymer as bilayer with C60, resulting a power conversion efficiency of 0.3%, and as buffer layer in bulk heterojunction solar cells. We show that poly(2,2′-bithiophene) films influenced positively the open circuit voltage and increased about 1.6 times the short circuit current, increasing the efficiency from 1.3% up to 2.9% from devices without buffer layer or with 11nm of poly(2,2′-bithiophene), respectively.

Influence of the cyanine counter anions on a bi-layer solar cell performance

ABSTRACTWe present normal and inverted solution processed bi-layer solar cells using cationic cyanine dyes as the electron donor and a fullerene as the electron acceptor. The cells exhibit high open circuit voltages up to 1 volt showing the optimal alignment of donor and acceptor energy levels. We demonstrate the large effect that cyanine dye counter ions can have on the energetics of the solar cells and how the S-shaped current density vs. voltage (J-V) curves can be avoided.

Organic solar cells featuring nanobowl structures

Although organic solar cells having a bilayer configuration provide continuous conducting pathways for carrier transport to the requisite electrodes, their efficiencies have remained low because of the short exciton diffusion lengths of organic semiconductors. In this paper, we describe unique spatial organic solar cells featuring nanobowl array structures that capture more light and generate more power than planar organic solar cells. To construct bilayer solar cells, we used electrochemical deposition (with polystyrene beads as the template) to fabricate poly(3,4-ethylenedioxythiophene) nanobowl arrays, functioning as the hole extraction layer, on indium tin oxide substrates and then deposited copper phthalocyanine and a fullerene (C60 or C70) to function as the active layer, onto the nanobowl arrays. By implementing this spatial structure, we could control the active layer's thickness, such that it would be suitable for exciton dissociation, while maintaining a high absorption of incident light (by increasing the absorber volume without decreasing the area of the donor–acceptor interface, such that the light path in the active layer was increased) and ensuring high exciton dissociation efficiency (by enlarging the donor–acceptor interface). Relative to an equally thick planar control active layer the photocurrent generated by such bilayer solar cells increased by approximately 90%.

Solution-processed bi-layer polythiophene–fullerene organic solar cells

An ionic polymer based on a polythiophene backbone with appended imidazolium moieties was successfully implemented as a donor material in fully solution-processed efficient bi-layer solar cells prepared by the low impact meniscus coating technique. High fill factors and device reproducibility were obtained, even for ultrathin polymer layers, indicating excellent film formation properties and good compatibility with solution processing techniques. The possibility of smooth counter ion exchange, allowing solubility modification and efficiency tuning, enables exploration of new functionalities and other device architectures.

Efficient truxenone-based acceptors for organic photovoltaics

Two electron-deficient truxenone derivatives are designed and synthesised for organic photovoltaic applications. The promise of this class of compounds as acceptor materials is illustrated by the fabrication of efficient bilayer solar cells with a subphthalocyanine (SubPc) donor, clearly outperforming reference cells with a soluble fullerene derivative as the acceptor.

Efficient, Cyanine Dye Based Bilayer Solar Cells

AbstractSimple bilayer solar cells, using commercially available cationic cyanine dyes as donors and evaporated C60 layer as an acceptor are prepared. Cyanine dyes with absorption maxima of 578, 615 and 697 nm having either perchlorate or hexafluorophosphate counter‐ions are evaluated. The perchlorate dye leads to cells with S‐shape current‐voltage curves; only the dyes with the hexafluorophosphate counter‐ions lead to efficient solar cells. When the wide bandgap dyes are employed, S‐shape current‐voltage curves are obtained when the conductive polymer PEDOT:PSS is used as hole transport layer. Substitution of PEDOT:PSS with MoO3 leads to cells with more rectangular current–voltage curves and high fill factors. Additionally, the cells using the MoO3 layer for hole extraction lead to high open circuit voltages of 0.9 V. In the case that a low bandgap hexafluorophosphate dye is used with the HOMO above that of the PEDOT:PSS the cell performance is independent on the type of hole transport layer employed. Using this approach, bilayer solar cells are obtained with power efficiencies ranging from 1.8 to 2.9% depending on the particular dye employed. These are impressive numbers for bilayer solar cell that are partially solution processed in ambient conditions.

Analysis of a device model for organic pseudo-bilayer solar cells

Polymer solar cells fabricated through sequential-solution-processing of donor and acceptor materials consist of partially intermixed heterojunctions and are therefore termed as organic “pseudo-bilayer” solar cells. A steady-state model for such organic pseudo-bilayer solar cells, which generalizes the bulk-heterojunction and the bilayer solar cell models, is derived based on a donor-blend-acceptor structure for the active layer. In short, the model considers intrinsic solar cell physics such as optical generation, charge carrier transport, generation, and recombination, as well as the space charge effect. Parameter fitting and model validation are carried out with current-voltage measurements from in-house fabricated pseudo-bilayer solar cells. The model shows that even with a large exciton diffusion length, a pure large-molecule bilayer cell is unable to produce the photocurrents seen in the measurements. This confirms the experimental observation that an intermixed region, instead of a well-defined interface, exists between the donor and acceptor phases in pseudo-bilayer solar cells. In addition, the effect of blend region thickness on device performance is explored. Local features, such as the electric field and charge carrier concentrations, are also discussed.

Study of polymer/ZnO nanostructure interfaces by Kelvin probe force microscopy

ZnO nanostructured films including nano-ridge and nanorod morphologies were used as electron acceptors/transporters and poly(3-hexylthiophene) (P3HT) as light absorber and electron donor in organic–inorganic hybrid solar cells. Bilayer solar cells with ZnO nano-ridges and ordered heterojunction solar cells with ZnO nanorods were each fabricated and characterized. ZnO nanorod/P3HT ordered heterojunction devices exhibited higher efficiency (0.31%) than the bilayer cells (0.094%) because the photocurrent was increased, attributed to the larger P3HT/ZnO interface area. Surface potential (SP) of P3HT/ZnO nano-ridges and P3HT/ZnO nanorods was studied by Kelvin probe force microscopy (KPFM). The SP difference between P3HT and ZnO nano-ridges was ∼0.10V in the dark, but increased to ∼0.25V under illumination. In the ZnO nanorod/P3HT films, ZnO nanorods showed lower SP with darker color, while P3HT exhibited higher SP with lighter color. In both nano-ridge/P3HT and nanorod/P3HT films, the SP difference demonstrated that electron transfer is energetically favorable from P3HT to ZnO.