Photovoltaic performance and stability of CdTe/polymeric hybrid solar cells using a C60 buffer layer

The influence of C₆₀ as a buffer layer on photovoltaic performance and stability of bulk hetero-junction solar cells using a photoactive layer of CdTe nanocrystals and poly(p-phenylenevinylene) derivative has been investigated. The incident photon-to-current conversion efficiency of the sealed-cells with a C₆₀ buffer layer was about 70% of that of solar cells using a buffer layer of LiF. The introduction of C₆₀ buffer layer gave an improvement in the stability for a light-harvesting test. In a long-term thermal stability test, there was no difference between the solar cells using C₆₀ and LiF. An impedance analysis suggested that the degradation of the performance in the stability tests was associated with a decrease in the conductance of the cells.     

Improved performance of Cu(InGa)Se2 thin-film solar cells using evaporated Cd-free buffer layers

Polycrystalline Cu(InGa)Se₂ (CIGS) thin-film solar cells using evaporated In_xSe_y and ZnIn_xSe_y buffer layers are prepared. The purpose of this work is to replace the chemical bath deposited CdS buffer layer with a continuously evaporated buffer layer. In this study, a major effort is made to improve the performance of CIGS thin-film solar cells with these buffer layers. The relationship between the cell performance and the substrate temperature for these buffer layers is demonstrated. Even at the high substrate temperature of about 550°C for the buffer layer, efficiencies of more than 11% were obtained. Furthermore, the I−V characteristics of the cells using these buffer layers are compared with cells using CdS buffer layers fabricated by chemical bath deposition method. We have achieved relatively high efficiencies of over 15% using both the ZnIn_xSe_y and the CdS buffer layers.     

High-efficiency Cu(In,Ga)Se2 thin-film solar cells with a novel In(OH)3:Zn buffer layer

We propose the inclusion of a novel In(OH)₃:Zn²⁺ buffer layer for fabricating high-efficiency CIGS solar cells. This buffer layer was deposited using a solution consisting of ZnCl₂, InCl₃·4H₂O, and thiourea. The In(OH)₃:Zn²⁺ films showed high resistivities of 2.1×10⁸ Ω cm and transmittance of above 95% in the visible range. We expected two effects due to this new buffer layer: first is the formation of a passivation layer on the CIGS surface and the second is Zn-doping into CIGS layer, resulting in the formation of a buried junction. A cell efficiency of 14.0% (V_oc: 0.575 V, J_sc: 32.1 mA/cm², FF: 0.758) was achieved by using an In(OH)₃:Zn²⁺ buffer layer, without the light soaking effect.     

Experimental investigation of Cu(In1−x,Gax)Se2/Zn(O1−z,Sz) solar cell performance

In this study we investigate the performance of Cu(In_1−x,Ga_x)Se₂/Zn(O_1−z,S_z) solar cells by changing the gallium content of the absorber layer in steps from CuInSe₂ to CuGaSe₂ and at each step vary the sulfur content of the Zn(O,S) buffer layer. By incorporating more or less sulfur into the Zn(O,S) buffer layer it is possible to change its morphology and band gap energy. Surprisingly, the best solar cells with Zn(O,S) buffer layers in this study are found for close to or the same Zn(O,S) buffer layer composition for all absorber Ga compositions. In comparison to their CdS references the best solar cells with Zn(O,S) buffer layers have slightly lower open circuit voltage, V_oc, lower fill factor, FF, and higher short circuit current density, J_sc, which result in comparable or slightly lower conversion efficiencies. The exception to this trend is the CuGaSe₂ solar cells, where the best devices with Zn(O,S) have substantially lowered efficiency compared with the CdS reference, because of lower V_oc, FF and J_sc. X-ray photon spectroscopy and X-ray diffraction measurements show that the best Zn(O,S) buffer layers have similar properties independent of the Ga content. In addition, energy dispersive spectroscopy scans in a transmission electron microscope show evidence of lateral variations in the Zn(O,S) buffer layer composition at the absorber/buffer layer interface. Finally, a hypothesis based on the results of the buffer layer analysis is suggested in order to explain the solar cell parameters.     

Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells

Solar cells of CuInS₂/In₂S₃/ZnO type are studied as a function of the In₂S₃ buffer deposition conditions. In₂S₃ is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In³⁺. In parallel, variable amounts of In₂O₃ are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 °C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In₂O₃ content in the buffer. On the other hand, the film deposited at lower temperature has higher In₂O₃ content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film.     

Highly structured TiO2/In(OH)xSy/PbS/PEDOT:PSS for photovoltaic applications

A system of highly structured TiO₂/In(OH)_xS_y/PbS/PEDOT:PSS has been developed and investigated by photovoltage spectroscopy, X-ray photo- and Auger electron spectroscopies, electron microscopy, and photovoltaic response. TiO₂, In(OH)_xS_y, PbS, and PEDOT:PSS serve as electron conductor, buffer layer, absorber, and hole conductor, respectively. Both buffer and absorber layers were prepared by chemical bath deposition. The band gap of as-prepared In(OH)_xS_y varied between 2.4 and 3.5 eV depending on the pH-value of the solution. In addition, the band gap of the PbS could be widened to about 0.85 eV making the application as absorber for solar cells feasible. At present, corresponding solar cell devices reach short-circuit current densities of about 8 mA/cm² and open-circuit voltages of about 0.3 V.     

Organic photovoltaic cells based on TPBi as a cathode buffer layer

The performance of organic photovoltaic (OPV) cells based on copper phthalocyanine (CuPc)/C₆₀ heterojunction was investigated by focusing on the role of 1,3,5-tris(2-N-phenylbenzimidazolyl) benzene (TPBi) as a cathode buffer layer. The effect of the film thickness of TPBi layer on the electrical characteristics of the device was systematically studied. The interface between the acceptor and cathode was studied with the characterization of atomic force microscope. Optical field distribution inside the OPV cell was also simulated to gain insight into the mechanism responsible for TPBi used as an optical spacer. The results indicated that at an optimal film thickness, TPBi cathode buffer layer is essential to enhance device performance by forming improved interfacial contact without introducing more series resistance and current loss.     

Properties of amorphous silicon solar cells fabricated from SiH2Cl2

Chlorinated intrinsic amorphous silicon films [a-Si:H(Cl)] and solar cell i-layers were fabricated using electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) and SiH₂Cl₂ source gas. n–i–p solar cells deposited on ZnO–coated SnO₂ substrates had poor photovoltaic performances despite the good electronic properties measured on the a-Si:H(Cl) films. Improved open–circuit voltage (V_oc) of 0.84 V and fill factor (FF) of 54% were observed in n–i–p solar cells by providing an n/i buffer layer and by using Ga-doped ZnO coated glass substrates. However, the FF improvement was still rather poor, which is thought to originate from high interface recombination in the ECR deposited solar cells. The V_oc and the FF showed much stable feature against light soaking.     

The spray-ILGAR (ion layer gas reaction) method for the deposition of thin semiconductor layers: Process and applications for thin film solar cells

The spray Ion Layer Gas Reaction (ILGAR) process starts with ultrasonic nebulisation of the precursor solution, e.g. InCl₃/ethanol for our successful buffer material In₂S₃. In an aerosol assisted chemical vapour deposition (AACVD) type reaction an In(O,OH,Cl) film is deposited on a heated substrate and is subsequently converted to In₂S₃ by H₂S gas. The cycle of these steps is repeated until the required layer thickness is obtained. The robust and reproducible process allows a wide control of composition and morphology.Results of this “spray-ILGAR” method with respect to process, material properties and its application depositing the buffer layer in chalcopyrite solar cells are reviewed. New aspects such as the investigation of the complex chemical mechanism by mass spectrometry, the process acceleration by the addition of H₂S gas to the aerosol, the controlled deposition of ZnS nano-dot films and finally the latest achievements in process up-scaling are also included.Solar cells based on industrial Cu(In,Ga)(S,Se)₂ absorbers (Avancis GmbH) with a Spray-ILGAR In₂S₃ buffer reached 14.7% efficiency (certified) and 15.3% with a ZnS/In₂S₃ bi-layer buffer comparable to reference cells using standard CdS buffer layers deposited by chemical bath deposition (CBD).The quasi-dry, vacuum-free ILGAR method for In₂S₃ buffer layers is well suited for industrial in-line production and is capable of not only replacing the standard buffer material (the toxic CdS) but also the often slow CBD process. A tape coater for 10 cm wide steel tape was constructed. It was shown that In₂S₃ layers could be produced with an indium yield better than 30% and a linear production speed of 1m/min. A roll-to-roll pilot production line for electrochemically deposited Cu(In,Ga)Se₂ with ILGAR buffer is running in industry (CIS-Solartechnik, Hamburg). A 30x30 cm² prototype of an ILGAR in-line coater developed by Singulus and Helmholtz Zentrum Berlin is currently being optimised. First 30×30 cm² encapsulated modules achieved efficiencies up to 13.0% (CdS buffered reference 13.3%).     

Analysis of illumination-intensity-dependent j–V characteristics of ZnO/CdS/CuGaSe2 single crystal solar cells

Current–voltage characteristics of ZnO/CdS/CuGaSe₂ single crystal solar cells measured at room temperature are investigated depending on illumination intensity. The characteristics can be described using the two-diode model, indicating two current transport mechanisms acting in the cells. The first and dominant mechanism is recombination of carriers at the interface between CdS and CuGaSe₂. The second one is recombination in the depletion region, which has been found to have a small effect on the solar cell photovoltaic performance. Both the diode ideality factor and the saturation current density of the dominant diode increase under illumination. A model based on interface recombination can explain these results. This model allows the estimation of diffusion voltage, capture cross-section of holes at the interface and mobility of electrons in the CdS layer.     

Optimization of n-doping in n-type a-SI : H/p-type textured c-Si heterojunction for photovoltaic applications

In this paper is investigated an heterostructure based on p-doped textured wafers of crystalline silicon on which we deposited a buffer of lightly n-doped amorphous layer and an n⁺-doped layer. In particular, the effect of n-doping of amorphous silicon on the photovoltaic characteristics of the heterojunctions is studied. Starting from an extensive analysis of the doping efficiency of phosphine in microdoped materials we fabricated several devices varying the PH₃/SiH₄ ratio in the PECVD system. An optimum value of this ratio is found at 10⁻², corresponding to the maximum of the photovoltaic efficiency of 11.5%.     

Simulation studies on photovoltaic response of ultrathin CuSb(S/Se)2 ternary compound semiconductors absorber-based single junction solar cells

Copper-based ternary CuSb(S/Se)₂ compound semiconductors are showing promise for ultrathin photovoltaic devices. The high absorption coefficient of these semiconductors makes them suitable for very thin absorber, where maximum absorption can be achieved in a photovoltaic device with only nanometers thick CuSb(S/Se)₂ based thin films. The device structure under consideration consists of AZO/i-ZnO/n-CdS/absorber layer/back contact, as the constituent material layers. The device structure is simulated using one dimensional solar cell capacitance simulator (SCAPS 1D) under one sun illumination and considering flat band approximation for the back contact and CuSb(S/Se)₂ interface. The optimized single junction device efficiencies are approximately 14% and approximately 10.18% with CuSbS₂ and CuSbSe₂ absorbers, respectively. Further, the impact of various material parameters such as thickness, acceptor concentration of bulk absorber layer, donor concentration of CdS buffer layer, and defects present at bulk absorber layer and at the buffer/absorber interface is discussed in correlation with the photovoltaic performance of the considered devices. The bandgap of CuSb(S/Se)₂ reduces linearly with Se alloying, and their impact on device performance is quantified in terms of capacitance voltage (CV), capacitance frequency (Cf), and impedance spectra of the photovoltaic device.     

Hybrid inverted organic photovoltaic cells based on nanoporous TiO2 films and organic small molecules

Solar cells based on nanoporous TiO₂ films with an inverted structure of indium tin oxide (ITO)/TiO₂/copper phthalocyanine (CuPc):fullerene (C₆₀)/CuPc/poly(3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/Au were fabricated. The best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO₂ (30 nm)/nanoporous TiO₂ (130 nm)/C₆₀:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The device using the nanoporous TiO₂ films has better photovoltaic properties compared to those using dense TiO₂ films. Higher photovoltaic performances were obtained by introducing a coevaporated layer of C₆₀:CuPc between TiO₂ and CuPc. The stability of inverted structure was better than that of the normal device, which gives a promising way for fabrication of solar cells with improved stability.     

In-situ Kelvin probe and ellipsometry study of the doping of a-Si : H and a-SiC : H layers:: Correlation with solar cell parameters

To underscore the effects of boron doping on the photovoltaic characteristics of a-Si : H based p–i–n solar cells we used the in-situ Kelvin probe technique to determine the evolution of the contact potential at the SnO₂/p interface in two series of solar cells: one with a p(a-Si : H) layer and another with a p(a-SiC : H) layer plus a buffer a-SiC : H layer. In each series, the flow of diborane was varied between 0.5 and 7 sccm. The in-situ Kelvin probe measurements on thick layers indicate that the doping efficiency is higher in a-Si : H than in a-SiC : H layers. In the case of cells with a p(a-Si : H) layer we observe an optimum in V_oc as a function of the diborane flow rate. Below this optimum the increase in V_oc is correlated to the increase of the contact potential measured by the Kelvin probe. Above the optimum value of the diborane flow, the further increase of the contact potential contrasts with the decrease of V_oc. Moreover, we found that V_oc of the cells with a p(a-SiC : H) is independent of the diborane flow rate. This unexpected behaviour is interpreted in terms of the diffusion of impurities and changes in the p-layer induced by boron atoms; it is supported by secondary-ion mass spectrometry and in-situ spectroscopic ellipsometry measurements.     

Low-temperature preparation of dense (Gd,Ce)O2−δ–Gd2O3 composite buffer layer by aerosol deposition for YSZ electrolyte-based SOFC

The (Gd_0.1Ce_0.9)O_2−δ (GDC)–Gd₂O₃ composite buffer layer was fabricated on yttria stabilized zirconia (YSZ) electrolyte by aerosol deposition for usage as diffusion barrier layer between YSZ and (La_0.6Sr_0.4)(Co_0.2Fe_0.8)O_3−δ (LSCF)–GDC composite cathode. The deposited composite buffer layer was quite dense in nature and effectively prevented the formation of SrZrO₃ and La₂Zr₂O₇ interlayer with low conductivity at the interfaces. The cell's I–V performance was enhanced with an increase in the GDC content in the composite buffer layer. The cell containing composite buffer layer showed maximum power density of up to 1.74 W/cm² at 750 °C, which was ∼30% higher than that of the cell containing GDC buffer layer prepared using conventional process.     

Spray CdS-polyacetylene thin films photovoltaic heterojunctions

We made thin films photovoltaic cells by a direct polymerisation of (CH)_x onto a sprayed CdS layer. Electrical and optical characteristics of this device were measured, both with undoped (CH)_x (p−n heterojunctions) and heavily doped (CH_x (Schottky diode). In spite of its low efficiency such a junction would lead, after improvement, to very cheap photovoltaic cells.     

Interface engineering in chalcopyrite thin film solar devices

Successful interface engineering requires compositional and electronic material characterization as a prerequisite for understanding and intentionally generating interfaces in photovoltaic devices. The paper gives an overview with several examples, all referring to Cu(In,Ga)(S,Se)₂ (“CIGSSe”)-based solar cells, with an emphasis on characterization using highly specialized methods, such as elastic recoil detection analysis, X-ray emission spectroscopy and photoelectron spectroscopy using synchrotron and ultraviolet light for excitation, inverse photoemission spectroscopy and Kelvin probe force microscopy. First, the determination of the depth profile of the band gap energy E_g in the absorber layer is demonstrated. The modification of E_g towards both interfaces is discussed in terms of beneficial electronic effects. Next, the interface between absorber and buffer layers with alternative and promising non-toxic materials is considered. Between CIGSSe and a ZnSe buffer deposited by the metalorganic chemical vapor deposition (MOCVD) method a buried ZnS interface was found. For a Zn(O,OH) buffer processed with an ion layer gas reaction (ILGAR) the correlation of surface composition, valence band maximum and efficiency of the resulting solar cell is shown. In addition, another approach is considered where a ZnMgO window layer is sputtered directly on the absorber omitting any buffer layer. The determination of the potential distribution at the ZnMgO/CIGSSe interface supports the understanding of this new and simpler way to get good cell performances even without any buffer. Finally, monolithically integrated solar modules without encapsulation were investigated before and after accelerated aging tests and changes at the interconnects were identified.     

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).     

Fabrication of microcrystalline silicon solar cells on a SnO2 coated substrate using seed layer insertion

We fabricated hydrogenated microcrystalline silicon (μc-Si:H) solar cells on SnO₂ coated glass using a seed layer insertion technique. Since rich hydrogen atoms from the μc-Si:H deposition process degrade the SnO₂ layer, we applied p-type hydrogenated amorphous silicon (p-a-Si:H) as a window layer. To grow the μc-Si:H layer on the p-a-Si:H window layer, we developed a seed layer insertion method. We inserted the seed layer between the p-a-Si:H layer and intrinsic bulk μc-Si:H. This seed layer consists of a thin hydrogen diluted silicon buffer layer and a naturally hydrogen profiled layer. We compared the characteristics of solar cells with and without the seed layer. When the seed layer was not applied, the fabricated cell showed the characteristics of a-Si:H solar cell whose spectral response was in a range of 400-800 nm. Using the seed layer, we achieved a μc-Si:H solar cell with performance of V_oc=0.535 V, J_sc=16.0 mA/cm², FF=0.667, and conversion efficiency=5.7% without any back reflector. The spectral response was in the range of 400-1100 nm. Also, the fabricated device has little substrate dependence, because a-Si:H has weaker substrate selectivity than μc-Si:H.     

Replacement of the CBD–CdS buffer and the sputtered i-ZnO layer by an ILGAR-ZnO WEL: optimization of the WEL deposition

As shown earlier the window extension layer (WEL) concept for thin film solar cells based on chalcopyrites results in device performances exceeding those of corresponding chemical bath deposited cadmium sulfide (CBD–CdS) buffered reference cells. The WEL concept is extended and it will be demonstrated, that now a single WEL successfully replaces both, the conventional buffer and the intrinsic part of the window bi-layer usually deposited by sputtering. Thus, one part of the window is deposited directly onto the absorber by a soft process called ion layer gas reaction (ILGAR). The optimization of ILGAR-ZnO WELs on Cu(In,Ga)(S,Se)₂ absorbers with respect to the efficiencies of the completed solar cells is presented. This effort results in ‘total area’ efficiencies of 14.5% (best cell) which are comparable to those of devices with CBD–CdS buffer (14.7%—best cell) without any antireflecting coating.