Preparation of (n) a-Si: H/(p) c-Si heterojunction solar cells

Heterojunction solar cells have been manufactured by depositing n-type a-Si: H on p-type 1–2Ω cm CZ single crystalline silicon substrates. Although our cell structure is very simple - neither a BSF nor a surface texturing is used - a conversion efficiency of 13.1% has been achieved on an area of 1 cm². In this paper the technology is described and the dependence of the solar cell parameters on the properties of the n-type a-Si: H layer is discussed. It is shown that this cell type exhibits no degradation under light exposure.     

Comparison of multicrystalline silicon surfaces after wet chemical etching and hydrogen plasma treatment: application to heterojunction solar cells

The modifications of the surface and subsurface properties of p-type multicrystalline silicon (mc-Si) after wet chemical etching and hydrogen plasma treatment were investigated. A simple heterojunction (HJ) solar cell structure consisting of front grids/ITO/(n)a-Si:H/(p)mc-Si/Al was used for investigating the conversion efficiency. It is found that the optimized wet chemical etching and cleaning processes as a last technological step before the deposition of the a-Si:H emitter are more favorable to HJ solar cells fabrication than the hydrogenation. Solar cells on p-type mc-Si were prepared without high-efficiency features (point contacts, back surface field). They exhibited efficiencies up to 13% for a cell area of 1 cm² and 12% for a cell area of 39 cm².     

Dark I–V–T measurements and characteristics of (n) a-Si/(p) c-Si heterojunction solar cells

Heterojunction solar cells have been manufactured by depositing n-type a-Si:H on p-type 1–2 Ω cm Cz single-crystalline silicon substrates. An efficiency of 14.2% has been obtained for 1 cm² solar cells by using a simple (Al/(p) c-Si/(n) a-Si:H/ITO/metal grid) structure. With an additional surface texturing, we have reached an efficiency of 15.3% for 1 cm² solar cells. We have investigated the dark IV-curves in order to contribute to a better understanding of the basis of solar cells.     

Solution processable quinacridone based materials as acceptor for organic heterojunction solar cells

Two series of novel quinacridone (QA) based materials that combined a strong absorption over a broad range in visible region with good electrical characteristics, which were used as the new electron-accepting materials for organic solar cells, are explored. Unique cyclic compounds 1-6 are synthesized by incorporating electron withdrawing groups (CN, COOH) at carbonyl position of alkyl substituted quinacridones, which lead to the compounds possessing the characteristics of solution-processed and being suitable for photovoltaic applications. Heterojunction solar cells with simple device configuration using these soluble materials as acceptor and effective donor poly (3-hexyl thiophene) (P3HT) were fabricated. The maximum power conversion efficiency (PCE) achieved in the solar cell based on compound 5 is 0.42% under simulated AM 1.5 solar irradiation with J_sc=1.80 mA cm⁻², V_oc=0.50 V and FF=47%. Although the aimed devices just exhibit moderate PCE, our results clearly suggest that the new-type electron-accepting materials different from fullerene have great potential as acceptor in heterojunction solar cell due to many advantages of the QA derivatives such as relatively inexpensive, good electrochemical stability and could be readily modified.     

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.     

Optimal optical generation profiles in a-Si:H p–i–n solar cells

The dependence of the maximum power output P_M and short-circuit current J_SC on the form (relative variation with position) of the optical generation rate profile in an a-Si:H p–i–n solar cell has been investigated computationally. It was found that there was an optimal form for the generation profile, and that P_M increased from 4.64 to 5.29 mWcm⁻², an increase of about 14%, when this optimal generation profile was used in the simulation. Optimal doping of the i-layer of the cell with phosphorous led to a P_M of 5.60 mWcm⁻², and when the optimal generation profile for this P-profiled cell was found, it yielded a P_M of 7.86 mWcm⁻², an increase of about 40%. This suggests that the combination of P-profiling and optimal generation could lead to significant improvements in cell performance. Moreover, it was found that for both cells the form of the optimal generation profile could be associated with the position of the peak in the external quantum efficiency, obtained from the spectral response. The possibility of using band-gap grading to achieve an optimal generation rate profile has been suggested.     

Laser fired contacts applied to the rear surface of heterojunction silicon solar cells

In this work, we fabricate heterojunction silicon solar cells on p-type substrates whose rear surface configuration is based on dielectric passivation and laser fired contacts (LFC cells). This is an alternative to boron-doped amorphous silicon film, with which we also fabricate solar cells for direct comparison (HJ cells). As substrates, 3.5 and 0.8 Ω cm p-type double-side polished FZ c-Si wafers are used. Regarding surface passivation for highly doped substrates, LFC configuration has some advantage due to the higher difficulty in creating an efficient amorphous back surface field. Additionally, those substrates are also more advantageous in terms of carrier injection when the rear surface is locally contacted. Thus LFC cells made on 0.8 Ω cm substrates reach V_oc values up to 680 mV, in the same range as that of their HJ cell counterpart, with better FF demonstrating that LFC configuration is a feasible alternative for highly doped substrates. Focusing on the impact of the distance between rear contacts on cell performance, we found a trade-off between open circuit voltage V_oc and fill factor FF. Finally electroluminescence characterization and the dependence of V_oc on pitch, modeled by Fischer's equation, indicate that the depassivated area due to the laser processing of the contacts is bigger than the contacted area.     

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.     

Light soaking effect in a-Si:H based n–i–p and p–i–n solar cells

Hydrogenated amorphous silicon solar cells have been realised in both a p–i–n configuration on a Corning glass substrate as well as in a n–i–p configuration on stainless-steel substrate. The performance degradation of the two kinds of cell under solar illumination has been examined for a 140 h period. During degradation, the two devices were kept under load in the maximum power condition that is normally used in a solar plant. The performance of the Corning glass deposited device exhibited a higher rate of degradation with respect to the other cell. A discussion on the possible reasons for this behaviour is given.     

High-rate microcrystalline silicon deposition for p–i–n junction solar cells

We have developed a high-rate plasma process based on high-pressure and silane-depletion glow discharge for highly efficient microcrystalline silicon (μc-Si:H) p–i–n junction solar cells. Under high-rate conditions (2–3 nm/s), we find that the deposition pressure becomes the dominant parameter in determining solar-cell performance. With increasing deposition pressure from 4 to 7–9 Torr, short-circuit current increases by 50% due to a remarkable improvement in quantum efficiencies at the visible and near infrared. As a result, the maximum efficiency of 9.13% has been achieved at an i-layer deposition rate of 2.3 nm/s. We attribute the improved performance of high-pressure-grown μc-Si:H solar cells to the structural evolution toward denser grain arrangement that prevents post-oxidation of grain boundaries.     

Optical spectroscopy study of nc-Si-based p–i–n solar cells

In the present study we analyzed nanocrystalline silicon (nc-Si)-based p–i–n thin film structures (SiC/nc-Si/n-doped amorphous Si) on glass produced by radio-frequency plasma-enhanced chemical vapor deposition. The crystallinity of the nc-Si layer was modified by varying the deposition conditions ([SiH₄]/[H₂] ratio in the plasma and radio-frequency power). Structural properties of the samples (crystalline fraction and crystal size distribution) were inferred by Raman spectroscopy. Different optical spectroscopy methods were combined for the determination of the optical constants in different spectral ranges: spectrophotometry, ellipsometry and photothermal deflection spectroscopy. Characterization results evidence that the optical properties of the nc-Si layers are strongly connected with the layer structural properties. Thus, the correlation between density of defects, Urbach energy, band-gap and line-shape of dielectric function critical points with the crystalline properties of the films is established.     

Electrodeposition of p–i–n type CuInSe2 multilayers for photovoltaic applications

Copper indium diselenide polycrystalline thin films of p-, i- and n-type electrical conductivity were grown using a one-step electrodeposition process in a single bath. The bulk structure and the stoichiometry of the layers were determined using X-ray diffraction and X-ray fluorescence. The material composition was correlated with the electrical conductivity type variation, detected by the photoelectrochemical cell. Atomic force microscopy analysis showed copper-rich films deposited at low cathodic potentials (0.6 V vs Ag/AgCl) are of spherical and granular morphology and the grain sizes were 0.3–0.5 μm, while stoichiometric CIS films deposited at 1.0 V vs Ag/AgCl have grain sizes of 0.1–0.4 μm. The initial studies of optoelectronic properties (V_oc, J_sc and FF) of the four-layer solar cell devices (glass/FTO/n-CdS/n-CIS/i-CIS/p-CIS/Au) are presented.     

Investigation of non-aqueous electrodeposited CdS/Cd1−xZnxTe heterojunction solar cells

Polycrystalline Cd_1−xZn_xTe solar cells with efficiency of 8.3% were grown by cathodic electrodeposition on glass/ITO/CdS substrates using non-aqueous ethylene glycol bath. The deposit is characterised versus the process conditions by XRD and found to possess a preferred (1 1 1) orientation on Sb doping in the electroplating bath. The surface morphology of the deposit is studied using atomic force microscope. The average RMS roughness for the ternary film was higher than that for the binary CdTe. Optical properties of the films were carried out to study the band gap and calculation of molar concentration ‘x’. The effects of Sb doping in CdS/Cd_1−xZn_xTe heterojunctions have been studied. The short circuit current density (c) was found to improve and series resistance (R_s) reduced drastically upon Sb doping. This improvement in J_sc is attributed to an increase in quantum efficiency. The evaluation of solar cell parameters was also carried out using the current–voltage characteristics in dark and illumination. The best results were obtained when 2×10⁻³ M ZnCl₂ along with antimony were present in the deposition bath. Under AM 1.5 conditions the open circuit voltage, short circuit current density, and fill factor of our best cell were V_oc=600 mV, J_sc=26.66 mA/cm², FF=0.42 and efficiency, η=8.3%. The carrier concentration and built-in potential of Cd_1−xZn_xTe calculated from Mott–Schottky plot was 2.72×10¹⁷ cm⁻³ and 1.02 eV.     

Influence of the intrinsic layer characteristics on a-Si:H p–i–n solar cell performance analysed by means of a computer simulation

In this paper a set of one-dimensional simulations of a-Si:H p–i–n junctions under different illumination conditions and with different intrinsic layer are presented. The simulation program ASCA permits the analysis of the internal electrical behaviour of the cell allowing a comparison among the different internal configurations determined by a change in the input set. Results about the internal electric configuration will be presented and discussed outlining their influence on the current tension characteristic curve. Considerations about the drift–diffusion and the generation–recombination balance distributions, outlined by the simulation, can be used to explain the correlation between the basic device output, the i-layer characteristics (thickness and DOS), the incident radiation intensity and photon energy.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

Boron-doped zinc oxide layers grown by metal-organic CVD for silicon heterojunction solar cells applications

In this work, we focus on ZnO:B layers as an alternative TCO for application on a-Si:H/c-Si heterojunction solar cells. First, the optimization of the material has been done in terms of optical and electrical properties. We have also studied the behaviour of ZnO:B against ageing. Finally, complete heterojunction solar cells have been fabricated with different back-side TCO configurations to understand the ageing mechanisms and to deeply study the influence of degradation on the solar cells parameters. Stable efficiencies up to 16.6% on polished n-type c-Si were obtained on 25 cm² heterojunction solar cells fabricated at low temperatures.     

Au–Cu/p–CdTe/n–CdO/glass-type solar cells

The heterostructure design proposed by us for the photovoltaic (PV) solar cell is: Au–Cu/p–CdTe:Sb/n–CdO:F/glass. The CdO:F films were grown by the sol–gel method, in conditions in order to get low resistivity 4.5×10⁻⁴ Ω-cm and an optical transmission higher than 85%. The CdTe:Sb films were prepared by means of the RF sputtering technique, in conditions to get resistivity value around 10⁶ Ω-cm, high crystalline quality and higher grain size. The Au–Cu contacts were thermally evaporated. For the study of PV-heterostructure a systematic variation of the preparation parameters were carried out. The parameters involved in the manufacture of the cell, in order to look for the highest efficiency were: (A) For the deposit of the p-CdTe:Sb films, a low argon pressure of 2.5 m Torr and high substrate temperature of 450 °C. The CdTe:Sb film thickness was varied in the interval 4.5–11 μm. (B) For the activation of the heterostructure: (i) The treatment temperature in vacuum, after the CdTe is deposited, was varied in the 350–550 °C range and (ii) the treatment temperature in Ar atmosphere, after the heterostructure is dipped in CdCl₂ solution, was studied in the 400–510 °C range. (C) Optimization of the Cu–Au contact with the adequate Cu-film thickness. The highest energy conversion efficiency (η) value was 5.48%. This work reports a systematic study of the parameters involved in the solar cell manufacture, for the search of a better value of η.     

Rie-texturing of multicrystalline silicon solar cells

We developed a maskless plasma texturing technique for multicrystalline silicon cells using reactive ion etching that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. Internal quantum efficiencies as high as those on planar cells have been obtained, boosting cell currents and efficiencies by up to 7% on evaporated metal and 4% on screen-printed cells.     

Recombination rates in heterojunction silicon solar cells analyzed by impedance spectroscopy at forward bias and under illumination

Impedance spectroscopy (at forward bias and under illumination) of solar cells comprised thin hydrogenated amorphous silicon (a-Si:H) films deposited on crystalline silicon (c-Si) wafers was analyzed in terms of ac equivalent circuits. Shockley–Read–Hall recombination at states on the device interfaces governs the cell dynamic response. Recombination process was modeled by means of simple RC circuits which allow to determine the capture rate of electrons and holes. Carrier lifetime is found to be stated by the electron capture time τ_SRH≈τ_n, and it results in the range of 300 μs. The Al-annealed back contact was regarded as the dominating recombination interface.     

Evaluation of bis-dicyanovinyl short-chain conjugated systems as donor materials for organic solar cells

Three conjugated compounds based on carbazole, cyclopentadithiophene and dithienopyrrole substituted by branched alkyl chains have been synthesized by the Knoevenagel condensation of malonodinitrile with the appropriate dicarboxaldehyde. Electronic properties of the target compounds have been analyzed in solution by UV-vis absorption spectroscopy and cyclic voltammetry and their potentialities as a donor material in donor-acceptor heterojunction solar cells have been evaluated in bilayer devices involving both solution processed or vacuum deposited donor layers and thermally evaporated fullerene C₆₀ as an acceptor.