Photovoltaic cells based on pulsed electrochemically deposited SnS and photochemically deposited CdS and Cd1−xZnxS

CdS/SnS and Cd_1−xZn_xS/SnS solar cells were fabricated. SnS films were deposited by the pulsed electrochemical deposition method using an aqueous solution containing SnSO₄ and Na₂S₂O₃. CdS and Cd_1−xZn_xS window layers were deposited by using the photochemical deposition method using an aqueous solution containing CdSO₄, ZnSO₄ and Na₂S₂O₃. Both the techniques were simple, economical and advantageous for fabricating cheap solar cells. The fabricated cells showed rectification characteristics. The photovoltaic properties were measured under AM 1.5 illumination. The cells with the Cd_1−xZn_xS window layer show larger photocurrent than those with the CdS window layer.     

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.     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

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.     

Fluctuation model for p–n heterojunction solar cells

Influence of the roughness (microrelief) of an active interface in p–n junction solar cells (SC) on the photovoltage (the open-circuit voltage V_oc) has been studied. Nonuniformity of contact potential difference between p- and n-regions leads to barrier height fluctuation that are exponentially enhanced when dealing with barrier current. This results in some decrease of the V_oc value. Three theoretical models of averaging open-circuit voltage were used. Experimental results on p⁺-Al_xGa_1−xAs/p⁺-n-GaAs heterostructure SC with various microrelief, obtained by the anisotropic chemical etching, are compared with theoretical calculations.     

Bulk heterojunction organic photovoltaic based on polythiophene–polyelectrolyte carbon nanotube composites

It is shown that carbon nanotubes can be used to enhance carrier mobility for efficient removal of the charges in thin film polymer-conjugated/fullerene photovoltaic devices. The fabricated photovoltaic devices consist of poly(3-octylthiophene) (P3OT) polymer blended with undoped multiwalled carbon nanotubes (MWNTs) and carbon nanotubes doped with nitrogen (CNx-MWNTs). Nanophase formation and dispersion problems associated with the use of carbon nanotubes in polymer devices were addressed through the generation of functional groups and electrostatic attaching of the polyelectrolyte poly(dimethyldiallylamine) chloride (PDDA) in both MWNTs and CNx-MWNT systems. The resultant nanophase was highly dispersed allowing for excellent bulk heterojunction formation. Our results indicate that CNx-MWNTs enhance the efficiency of P3OT solar cells in comparison with MWNTs.     

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.     

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.     

Extremely thin absorber solar cells based on nanostructured semiconductors

Abstract

Extremely thin absorber (eta) solar cells aim to combine the advantages of using very thin, easily and cheaply produced absorber layers on nanostructured substrates with the stability of all-solid-state solar cells using inorganic absorber layers. The concept of using nanostructured substrates originated from the dye-sensitised solar cell, where having a very high surface area allows the use of very thin layers of dye while still absorbing sufficient sunlight. However, both the dye and liquid electrolyte used in these devices demonstrated poor stability, and efforts were made to replace them with very thin inorganic absorber layers and solid state hole collectors respectively. The combination of these concepts – a nanostructured substrate coated with a very thin inorganic absorber and completed with a solid state hole collector – is known as an eta solar cell. This review summarises the development of both the inorganic absorbers and solid state hole collectors in porous TiO₂ and ZnO nanorod based cells, focusing on the material properties and growth/deposition methods. Future possibilities for eta solar cells are discussed, including utilisation of a wider range of materials, synthesis methods and novel materials such as quantum dots to produce tuned band gap and multijunction solar cells.     

Degradation of transparent metal–insulator–semiconductor solar cells due to heating effects

All the output parameters of the metal–insulator–semiconductor solar cells are degraded after heating. Also the dark current and the non-ideality factor are increased with heating. A reduction in the built-in potential has been detected. The capacitance–voltage–frequency measurements indicate the presence of interface states. These states are heavily occupied by electrons. Heating will increase the density of these states and consequently reduce the barrier height and the overall cell efficiency.     

High efficiency cadmium free Cu(In,Ga)Se2 thin film solar cells terminated by an electrodeposited front contact

The possibility to reach up to 14.7% efficiency with Cu(In,Ga)Se₂ (CIGS) solar cell, using a cadmium free buffer layer (indium sulphide:In₂S₃) and an electrodeposited front contact (chloride doped ZnO:ZnO:Cl) is demonstrated in this article. This is the first time that costly gas phase deposition processes for ZnO, by high vacuum sputtering, can be replaced by an efficient low cost atmospheric technology, representing an important breakthrough in further cost reduction for photovoltaic application. In addition, the compatibility with cadmium free buffer layers brings this new approach at the cutting edge of strategic evolution of the CIGS technology. In this study the influences of the In₂S₃ buffer layer thickness, the presence of an intrinsic ZnO layer and a soft annealing treatment are studied. It is shown that the growth behavior of the electrodeposited ZnO:Cl is controlled by nucleation phenomena on different surfaces, with a unique morphology on indium sulphide. Finally the best performances have been achieved with a cell annealed at 150 °C under atmospheric conditions containing a very thin In₂S₃ layer (15 nm) but without intrinsic ZnO (CIGS/In₂S₃/ZnO:Cl).     

Spectral response of a-Si:H p–i–n solar cells

The spectral response of a typical thin-film a-Si:H p–i–n solar cell has been investigated using the simulation RAUPV2. The peak in the external quantum efficiency has been observed to shift towards the violet part of the spectrum on decreasing the cell thickness. Moreover, the height of the peak increases as cell thickness is decreased. This is correlated with an enhancement in cell performance for thinner cells, due to a general increase in the drift field within the cell. The external quantum efficiency of a cell with an optimal concentration of phosphorous in the intrinsic layer has also been investigated. The external quantum efficiency for this cell is similar to that of the thinner cell, and is associated with the enhancement of the drift field near the p/i interface that is brought about by the phosphorous doping of the intrinsic layer. However, the integrated recombination for the thinner cell and the phosphorous-profiled cell differ significantly at long wavelengths, despite their similarity at shorter wavelengths. This effect is due to the weakening of the drift field near the n/i interface in the phosphorous-profiled cell.     

Unsymmetrical alkoxy zinc phthalocyanine for sensitization of nanocrystalline TiO2 films

A new photosensitizer, unsymmetrical alkoxy zinc phthalocyanine based on ‘push–pull’ concept, has been synthesized and fully characterized by CHN, MALDI-TOF, UV–Vis, fluorescence spectroscopies and cyclic voltammetry. The new phthalocyanine photosensitizer has eight alkoxy and two carboxyl groups that act as electron releasing (push) and withdrawing (pull), respectively. Moreover, the alkoxy groups increase the solubility of the new photosensitizer in common organic solvents, and the two carboxyl groups serve to graft on to nanocrystalline TiO₂. The new photosensitizer was tested in dye-sensitized solar cells and its performance was compared with PCH001.     

Efficient hybrid bulk heterojunction solar cells based on phenylenevinylene copolymer, perylene bisimide and TiO2

A soluble alternating phenylenevinylene copolymer P containing a side anthracene, which was attached to the thiophene ring via a vinylene bridge, was synthesized by Heck coupling. The copolymer had relatively low glass transition temperature (61 °C) and decomposed above 400 °C. The absorption maximum of P was located at 387–402 nm with an optical band gap of 2.32 eV. The emission spectra of P indicated that an intramolecular energy transfer from the side anthracene to the main chain took place via the vinylene bridge. In addition, a new symmetrical compound A based on perylene–anthracene was synthesized and used as electron acceptor in the device. Photovoltaic devices were fabricated using a blend of copolymer P as donor and compound A as acceptor, as photoactive layer film sandwiched between indium tin oxide-coated glass and Al electrodes. This device showed a power conversion efficiency of 0.72%. However, when TiO₂ nanoparticles were incorporated on the pristine P:A blend, the power conversion efficiency of the device was enhanced up to 1.32%, which is attributed to the enhanced photoinduced excitons due to the increase of the interfacial area and improved charge carrier mobility. The power conversion efficiency of the P:A:TiO₂ based photovoltaic device was further improved up to 2.64%, when the hybrid composite was treated with a mixture of Li salt and 4-tert-butylpyridine, which is attributed to the reduction in the recombination of charge carriers.     

Highly and quickly stabilized p–i–n/p–i–n-type protocrystalline silicon multilayer tandem solar cells

We have developed p–i–n/p–i–n-type protocrystalline silicon (pc-Si:H) multilayer tandem solar cells. The purpose of this work is to make a thin film silicon solar cell with low degradation by combining the virtues of a pc-Si:H multilayer and tandem structure. The usefulness of the pc-Si:H multilayer as a low degradation top and bottom cell was confirmed when we achieved a low degradation ratio of 10.0%. Notably, this tandem cell stabilized rapidly, within 1 h. Nanocrystalline silicon (nc-Si) grains embedded in a pc-Si:H multilayer were detected with the aid of a planer transmission electron microscope. The isolated nc-Si grains may suppress the photocreation of dangling bonds due to non-radiative recombination in an a-Si:H matrix. Because of these embedded nc-Si grains, the pc-Si:H multilayer has a fast and high light-induced metastability.     

A review on PEDOT‐based counter electrodes for dye‐sensitized solar cells

The dye‐sensitized solar cell (DSSC) is a promising alternative for the Si solar cell due to its low‐cost and easy fabrication. As a novel conductive polymer, poly(3,4‐ethylenedioxythiophene) (PEDOT) has attracted much attention for DSSCs. In this review article, the progress of PEDOT‐based counter electrodes for DSSCs is presented. First, the properties and structure of PEDOT are briefly described, and its feasibility as a DSSC counter electrode is demonstrated. Then, the effect of various treatments on the electrical conductivity and catalytic activity of PEDOT as well as its stability is examined. Furthermore, efficient and low‐cost composite counter electrodes consisting of PEDOT and other materials are deeply discussed. Finally, an outlook for PEDOT counter electrodes is provided. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Hybrid solar cells based on tetrapod nanocrystals: The effects of compositions and type II heterojunction on hybrid solar cell performance

We synthesized oleic acid capped tetrapod nanocrystals of CdSe, CdTe and type II heterostructured CdTe/CdSe to investigate the effects of nanocrystal compositions and type II heterojunction on the photovoltaic properties of hybrid solar cells. The hybrid solar cell based on the blend of CdSe tetrapod nanocrystals and P3HT with a weight ratio of 6:1 showed the maximum power conversion efficiency of 1.03% under AM 1.5 G condition, and the maximum incident photon to current conversion efficiency of the solar cell was 43% at 415 nm. Although CdTe and CdTe/CdSe tetrapod nanocrystals showed relatively poor performance, the power conversion efficiency and the short circuit current density of the hybrid solar cell based on type II heterostructured CdTe/CdSe tetrapod nanocrystals was 4.4 and 3.9 times higher than that of the solar cell based on CdTe tetrapod nanocrystals, respectively. These results can be explained by the effects of nanocrystal compositions and type II heterojunction on the photovoltaic properties of hybrid solar cells.     

Hybrid inverted bulk heterojunction solar cells with nanoimprinted TiO2 nanopores

Photovoltaic devices with highly ordered nanoporous titanium dioxide (titania; TiO₂) were fabricated to improve the photovoltaic performances by increasing TiO₂ interface area. The nanoimprinting lithography technique with polymethyl methacrylate (PMMA) mold was used to form titania nanopores. The solar cell with poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) active layer on nanoporous titania showed higher power conversion efficiency (PCE) of 1.49% than on flat titania of 1.18%. The improved efficiency using nanoporous titania is interpreted with the enhanced-charge separation and collection by increasing the interface area between TiO₂ and active layer.     

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.