Stacked organic solar cells based on pentacene and

In order to improve photon harvesting, two small molecule organic solar cells are placed in series on top of each other. These stacked cells need an efficient recombination center in between both cells. In this study we test vacuum deposited metal layers as recombination centers with pentacene and buckminsterfullerene (C₆₀) as donor and acceptor, respectively. S-shaped curves are visible in the I–V characteristics when using thin layers of aluminum, indicating a barrier for extraction inside the device. Thin metal layers of gold or silver result in an increased open-circuit voltage without the appearance of these S-shaped features.     

Organic donor, acceptor and buffer layers of small molecules prepared by OVPD technique for photovoltaics

Organic vapour-phase deposition (OVPD^®) is used for the growth of the organic solar cell component materials such as the donor copper phthalocyanine (CuPc), the acceptor fullerene C₆₀, and electron-conducting buffer layers of bathocuproine (BCP) on Si1 0 0 wafers and indium tin oxide (ITO) substrates on areas as large as 15×15 cm². By means of X-ray diffraction (XRD) analysis we show that under continuous operating conditions the source materials possess long-term stability. The CuPc, C₆₀ and BCP thin film morphology and structure are characterised using scanning electron microscopy and XRD analysis. We demonstrate CuPc thin films with a highly folded surface morphology suitable for the preparation of solar cells with an interpenetrating donor–acceptor interface. The XRD diffraction patterns of the CuPc and C₆₀ layers deposited under conditions appropriate for the preparation of organic solar cells show spectra typical for these materials. Mixed CuPc:C₆₀ layers with controlled constituent ratios and layer thickness are deposited for the preparation of organic solar cells. First ITO/CuPc:C₆₀/Al organic photovoltaic devices are prepared with an efficiency of 1% (conditions AM1.5).     

Enhanced photovoltaic performance of inverted organic solar cells with In-doped ZnO as an electron extraction layer

In the present work, a systematic study has been carried out to understand the effect of In doping on the various properties of the ZnO nanocrystalline thin films. In-doped ZnO nanocrystalline thin films with different indium concentrations (1.98%, 4.03%, 6.74%, 8.62% and 10.48% In) have been synthesized by sol–gel method. The grain size and surface roughness of the In-doped ZnO thin films are observed to be smaller than those of the ZnO thin films. 6.74% In-doped ZnO films with a low resistivity of 1.95 × 10⁻³ Ω cm and a high mobility of 2.19 cm² V⁻¹ S⁻¹ have been prepared under optimal deposition conditions. Inverted organic solar cells containing In-doped ZnO as an electron extraction layer with the structure indium tin oxide (ITO)/In-doped ZnO/poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT): [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM)/MoO₃/Al have been fabricated. The inverted organic solar cell with 6.74% In-doped ZnO exhibited a power conversion efficiency of 5.58%, which is the best efficiency reported so far for these type of solar cells. The device performance has been optimized by varying the indium doping concentration. The results clearly demonstrate that significant improvement in power conversion efficiency can be obtained by incorporating In into the ZnO films.     

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.     

Poly(3-hexylthiophene)/C60 heterojunction solar cells: Implication of morphology on performance and ambipolar charge collection

The performance of heterojunction organic solar cells is critically dependent on the morphology of the donor and acceptor components in the active film. We report results of photovoltaic devices consisting of bilayers and bulk heterojunctions using poly(3-hexylthiophene) (P3HT) and Buckminsterfullerene C₆₀. White light power efficiencies of η2.2% (bulk heterojunction) and 2.6% (bilayer) were measured after a thermal annealing step on completed devices. Optical and structural investigations on non-annealed bilayer thin films indicated a distinct porosity of the spin-coated polymer, which allows C₆₀ to penetrate the P3HT layer and to touch the anode. This resulted for these bilayer solar cells in the experimental observation that electrons were collected predominantly at the cathode after photo-excitation of P3HT, but predominantly at the anode after C₆₀ excitation. A morphological model to explain the ambipolar charge collection phenomenon is proposed.     

Dicyanovinyl substituted oligothiophenes: Thermal stability, mobility measurements, and performance in photovoltaic devices

A series of dicyanovinyl-oligothiophenes are investigated concerning their thermal stability, absorption in thin film, and hole mobility. Due to very high extinction coefficients, these materials are interesting for application as donor in solar cells. The quinquethiophene DCV₂-5T, which shows a hole mobility of 2.2×10^-5 cm²/Vs, is used as donor material in a flat heterojunction organic small molecule solar cells. Despite a very thin donor layer of only 6 nm, these devices exhibit in a planar heterojunction with 15 nm C₆₀ an efficiency of up to 2.8% with a fill factor of up to 58%, a short circuit current density of 5.2 mA/cm², an open circuit voltage of 1.03 V, and an external quantum efficiency of 30% in the green spectral range.     

Environment-friendly energy from all-carbon solar cells based on fullerene-C60

An efficient single layer organic solar cell based on plain buckminsterfulerence (C₆₀) has been fabricated. By inserting a very thin N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine layer between the indium tin oxide and single C₆₀ active layer, a short-circuit current of 1.98 mA/cm² and an open-circuit voltage of 0.52 V are obtained under 100 mW/cm² AM1.5G simulated illumination. The highest power conversion efficiency of 0.414% based on plain C₆₀ is thus demonstrated, which is the first step to realize an environment-friendly energy source.     

Solid-phase crystallized Si films on glass substrates for thin film solar cells

We investigate the potential of solid-phase crystallized Si films on glass for use in polycrystalline Si thin film solar cells. Low-pressure chemical vapour deposition serves to form amorphous Si films on borosilicate, SiO₂-coated borosilicate, aluminosilicate glass and fused silica substrates. The films are crystallized at temperatures of around 600°C. Using transmission electron microscopy we determine the grain size in the crystallized films. The average grain size strongly depends on the substrate type, increases with the deposition rate of the amorphous film and is independent of the film thickness. The grain size distribution in our films is log-normal. Films crystallized on SiO₂-coated borosilicate glass have an average grain size up to 2.3 μm, while the area weighted average grain size peaks at 4 μm. Since thin crystalline Si solar cells only require a film thickness of several micron, our films seem to be suitable for application to such devices.     

Influence of Alq3/Au cathode on stability and efficiency of a layered organic solar cell in air

A relatively long lifetime organic solar cell, containing a thin tris-8-hydroxy-quinolato aluminum (Alq₃) layer under an Au cathode, is described. Half-lifetime of the cell in the darkness is over 7 weeks and 30% of the initial power conversion efficiency (η) is obtained after 18 weeks. This represents a substantial increase in lifetime compared to that of the unencapsulated Al-cathode cells. Efficiency is enhanced by 60 times compared to a cell that does not contain Alq₃, being 0.60%. The proposed role of Alq₃/Au cathode in this cell is discussed in detail.     

Electrical characterization of Al, Ag and In contacts on CuInS2 thin films deposited by spray pyrolysis

The specific contact resistivity (ρ_C) for aluminum (Al), silver (Ag) and indium (In) metallic contacts on CuInS₂ thin films was determined from I-V measurements, with the purpose of having the most appropriate ohmic contact for TCO/CdS/CuInS₂ solar cells; ρ_C was measured using the transmission line method (TLM) for the metallic contacts evaporated on CuInS₂ thin films deposited by spray pyrolysis with ratios x=[Cu]/[In]=1.0, 1.1, 1.3 and 1.5 in the spray solution. The results show that In contacts have the lowest ρ_C values for CuInS₂ samples grown with x=1.5. The minimum ρ_C was 0.26 Ω cm² for the In contacts. This value, although not very low, will allow the fabrication of CuInS₂ solar cells with a small series resistance.     

Device simulation and modeling of microcrystalline silicon solar cells

Device modeling for p–i–n junction basis thin film microcrystalline Si solar cells has been examined with a simple model of columnar grain structure utilizing two-dimensional device simulator. The simulation results of solar cell characteristics show that open-circuit voltage (V_oc) and fill factor considerably depend on structural parameters such as grain size and acceptor doping in intrinsic layer, while short-circuit current density (J_sc) is comparatively stable by built-in electric field in the i-layer. It is also found that conversion efficiency of more than 16% could be expected with 1 μm grain size and well-passivated condition with 10 μm thick i-layer and optical confinement.     

Long-term operational lifetime and degradation analysis of P3HT:PCBM photovoltaic cells

We study the degradation of photovoltaic cells with poly(3-hexyl thiophene) (P3HT) and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) blends under long-term continuous illumination as well as in shelf-life conditions, both in inert N₂ atmosphere. Degradation of the illuminated solar cells mainly occurs by a rapid decrease of the fill factor (FF) after 300 h, while short-circuit current and open-circuit voltage follow a linear decay after the initial burn-in. The sudden drop of the FF is correlated with an increase of the series resistance and proves irreversible upon annealing. Electrical measurements indicate that it stems from reduced charge extraction due to the photodegradation of the organic-electrode interfaces. Furthermore, as the external quantum efficiency (EQE) spectrum is evenly lowered over the entire wavelength range, we could exclude major changes in the blend morphology or significant changes to optical properties of the active layer. Introducing a thin C₆₀ layer leads to complete suppression of the FF decay over 1000 h, further proving that interface degradation dominates. Interestingly, similar improved lifetime over 1000 h was achieved by separate substitution of MoO₃ for PEDOT:PSS.     

Production of large-area polymer solar cells by industrial silk screen printing, lifetime considerations and lamination with polyethyleneterephthalate

The possibility of making large area (100 cm²) polymer solar cells based on the conjugated polymer poly 1,4-(2-methoxy-5-ethylhexyloxy)phenylenevinylene (MEH-PPV) was demonstrated. Devices were prepared by etching an electrode pattern on ITO covered polyethyleneterephthalate (PET) substrates. A pattern of conducting silver epoxy allowing for electrical contacts to the device was silk screen printed and hardened. Subsequently a pattern of MEH-PPV was silk screen printed in registry with the ITO electrode pattern on top of the substrate. Final evaporation of an aluminum electrode or sublimation of a Buckminsterfullerene (C₆₀) layer followed by an aluminum electrode completed the device. The typical efficiency of the prototype devices consisting of three solar cells in series were 0.0046% (under AM1.5 conditions) with open-circuit voltages (V_oc) of 0.73 V and short-circuit currents (I_sc) of 20 μA cm⁻². The half-life based on I_sc in air for the devices were 63 h. The cells were laminated in a 125 μm PET encasement. Lamination had a negative effect on the lifetime.We demonstrate the feasibility of industrial production of large area solar cells (1 m²) by silk screen printing and envisage the possibility of production volumes 10000 m² h⁻¹ at a cost that is on the order of 100 times lower than that of the established monocrystalline silicon solar cells in terms of materials cost.     

Photoelectron spectroscopy and atomic force microscopy study of 1,2-dicyano-methanofullerene C60(CN)2 thin film for photovoltaic applications

1, 2-dicyano-methanofullerene (C₆₀(CN)₂) is a soluble fullerene derivative that has been reported to have stronger electron affinity than parent C₆₀. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) experiments were carried out on C₆₀(CN)₂ thin films spin coated on heavily doped n-type Si substrate. UPS spectra enabled the determination of the vacuum shift at the fullerene derivative/Si interface and the onset of the highest occupied molecular orbital (HOMO). From the UV-vis absorption spectra of C₆₀(CN)₂ thin films spin coated on quartz substrates, the optical band gap (E_g) and the onset of absorption were determined. These measurements allowed the determination of the lowest occupied molecular orbital (LUMO) position. The morphology of the deposited film was probed by AFM and reveals non-uniformity of the thin film. Open circuit voltage (V_oc) measurements on P3HT/C₆₀(CN)₂ based organic solar cell device are compared to the commonly used P3HT/PCBM device.     

Application of rapid thermal processing on SiNx thin film to solar cells

Rapid thermal processing (RTP) of SiN _x thin films from PECVD with low temperature was investigated. A special processing condition of this technique which could greatly increase the minority lifetime was found in the experiments. The processing mechanism and the application of the technique to silicon solar cells fabrication were discussed. A main achievement is an increase of the minority lifetime in silicon wafer with SiN _x thin film by about 200% after the RTP was reached. PC-1Dsimulation results exhibit an enhancement of the efficiency of the solar cell by 0.42% coming from the minority lifetime improvement. The same experiment was also conducted with P-diffusion silicon wafers, but the increment of minority lifetime is just about 55%. It could be expected to improve the solar cell efficiency if it would be used in silicon solar cells fabrication with the combination of laser firing contact technique. __________ Translated from Journal of Shanghai Jiaotong University, 2008, 42(1): 152–155 [译自: 上海交通大学学报]     

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.     

Transparent electrode with ZnO nanoparticles in tandem organic solar cells

The transparent inter-electrodes with the p/n heterojunction consisting of the solution-processible ZnO nanoparticles as the n-type and the conventional hole injection layers (MoO₃ or PEDOT:PSS) as the p-type materials are studied for developing tandem organic solar cells employing different band gap active materials (i.e., P3HT:PCBM blend layer for larger band gap material in the bottom cell and ZnPc/C₆₀ bilayer for smaller band gap material in the top cell). For the ZnO/PEDOT:PSS inter-electrode, the V_OC corresponding to the sum of V_OC’s of the top and bottom unit cells is obtained, denoting that the two unit cells are successfully connected in series. For the ZnO/MoO₃ inter-electrode, the open-circuit voltage (V_OC) of the tandem cell is smaller than the sum of V_OC’s of the top and bottom unit cells, but it can be increased by inserting a very thin Al layer (∼3 nm) between ZnO and MoO₃ (ZnO/Al/MoO₃) as the recombination center for carriers.     

Highly reflective nanotextured sputtered silver back reflector for flexible high-efficiency n-i-p thin-film silicon solar cells

High reflectivity is essential when a metal is used as back contact and reflector in thin-film silicon solar cells. We show that thermal annealing at 150 °C improves the reflectivity of silver films deposited by sputtering at room temperature on nanotextured substrates. The annealing provokes two interlinked effects: rearrangement of the silver layer with a modification of its morphology and an increase of up to 42% in the grain size of the polycrystalline film for the preferential orientation as measured by X-ray diffraction. The main consequence of these two mechanisms is a large increase in the reflectivity of silver when measured in air. This reflectivity increase is also noticeable in devices: amorphous silicon thin-film solar cells grown on annealed silver films yield higher internal and external quantum efficiencies compared to cells grown on as-deposited silver. The morphology modification smoothes down the substrate, which is revealed by a clear increase of the open-circuit voltage and fill factor of the cells grown on top. An amorphous silicon cell with a 200 nm nominally thick i-layer fabricated on a flexible plastic substrate yielded an initial efficiency close to 10% with 15.9 mA/cm² of short-circuit current using highly reflective annealed textured silver. We also propose, for industrial purpose, the sputtering of thin silver layer (120 nm) under moderate substrate temperature (∼150 °C) to increase the layer reflectivity, which avoids lengthening of the back reflector fabrication.     

Solar cells from thin silicon layers on Al2O3

The surface of the applied Al₂O₃ ceramic substrate consists of small crystals with a maximum grain size of 5 μm. A 40 μm thick layer deposited on this surface shows polycrystalline quality and grain sizes in the order of 10 μm. Silicon layers of different thickness and doping have been deposited on Al₂O₃ substrates to make thin film silicon solar cells. These structures have been processed to solar cells in a two mask process. From measurements of the spectral response a diffusion length of 8 μm can be extracted.     

Microstructures and photovoltaic properties of C60 based solar cells with copper oxides,CuInS2, phthalocyanines,porphyrin, PVK,nanodiamond, germanium and exciton diffusion blocking layers

Abstract

Organic solar cells have a potential for use in lightweight, flexible, inexpensive and large scale solar cells. However, significant improvements of photovoltaic efficiencies are mandatory for use in future solar power plants. One of the improvements is donor–acceptor proximity in the devices, which are called bulk heterojunction solar cells. Bulk heterojunction is an efficient method to generate free charge carriers, and the charge transfer is possible at the semiconductor interface. The purpose of the present work is to fabricate and characterise C₆₀ based solar cells with copper oxides, CuInS₂, phthalocyanines, porphyrin, poly-vinylcarbazole, nanodiamond, germanium and exciton diffusion blocking layers. In the present work, C₆₀ and fullerenol [C₆₀(OH)_10–12] were used for n-type semiconductors, and metal copper oxides, metal phthalocyanine derivative, porphyrin and poly-vinylcarbazole were used for p-type semiconductors. In addition, nanodiamond and germanium based molecules were added into the active layers of the solar cells. The novel aspect of the research is to investigate the relation between properties and microstructures of the solar cells using transmission electron microscopy, X-ray diffraction and electronic structure calculation. The impact of the research concerns the study of organic solar cells by means of microstructural analysis, property measurements and theoretical calculations.