In the field of crystalline silicon solar cells, great efforts are being devoted to the development of selective contacts in search of a fully low-temperature and dopant-free fabrication process ...compatible with high photovoltaic conversion efficiencies. For high-efficiency devices, selective contacts have to simultaneously combine high conductivity with excellent passivating properties. With this objective, a thin passivating extra layer of a-Si:H or SiO 2 is usually introduced between the conducting layer and the silicon substrate. In this work, we present an interdigitated back-contacted (IBC) silicon based solar cell that avoids the use of either thermal SiO 2 or a-Si:H interlayers achieving a dopant-free, ITO-free and very low thermal budget fabrication process. In this work, we propose a new electron transport layer using ultrathin Al 2 O 3 /TiO 2 stacks deposited by atomic layer deposition at 100 °C covered with a thermally evaporated Mg capping film. A specific contact resistance of 2.5 mΩ cm 2 has been measured together with surface recombination velocities below 40 cm s −1 . This electron-selective contact is combined with a thermally evaporated V 2 O x -based hole selective contact to form the rear scheme of an IBC structure with a 3 × 3 cm 2 active area as a proof-of-concept resulting in efficiencies beyond 19%. This approach sheds light on potential technological simplification and cost reduction in crystalline silicon solar cells.
Heterojunction solar cells based on molybdenum sub-oxide (MoOx) deposited on n-type crystalline silicon have been fabricated. The hole selective character of MoOx is explained by its high ...workfunction, which causes a strong band bending in the Si substrate. This bending pushes the surface into inversion. In addition, the sub-stoichiometry of the evaporated MoOx layers leads to a high density of states within the bandgap. This is crucial for charge transport. The J-V electrical characteristics at several temperatures were analysed to elucidate the dominant charge transport mechanisms of this heterojunction structure. We have identified two different transport mechanisms. At low bias voltage, transport is dominated by hole tunnelling through the MoOx gap states. At higher voltage the behaviour is similar to a Schottky junction with a high barrier value, due to the high MoOx work function. These results provide a better understanding of the hole selective character of MoOx/n-type silicon heterocontacts, which is key to further improve this new kind of solar cells.
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•Transport mechanisms in a silicon solar cell with MOOX hole-selective contact have been studied.•Conversion efficiencies were among the highest reported for this structure without any additional passivation layer.•A tunnellingcharge-carrier transport is clearly resolved by analysing the electrical J-V characteristics.•Multitunneling capture emissionbetween the MOOX gap states is proposed as the transport process in the MOOX layer.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
In this work we study conjugated polyelectrolyte (CPE) films based on polyamidoamine (PAMAM) dendrimers of generations G1 and G3. These fractal macromolecules are compared to branched ...polyethylenimine (b-PEI) polymer using methanol as the solvent. All of these materials present a high density of amino groups, which protonated by methoxide counter-anions create strong dipolar interfaces. The vacuum level shift associated to these films on n-type silicon was 0.93 eV for b-PEI, 0.72 eV for PAMAM G1 and 1.07 eV for PAMAM G3. These surface potentials were enough to overcome Fermi level pinning, which is a typical limitation of aluminium contacts on n-type silicon. A specific contact resistance as low as 20 mΩ·cm2 was achieved with PAMAM G3, in agreement with the higher surface potential of this material. Good electron transport properties were also obtained for the other materials. Proof-of-concept silicon solar cells combining vanadium oxide as a hole-selective contact with these new electron transport layers have been fabricated and compared. The solar cell with PAMAM G3 surpassed 15% conversion efficiency with an overall increase of all the photovoltaic parameters. The performance of these devices correlates with compositional and nanostructural studies of the different CPE films. Particularly, a figure-of-merit (V σ) for CPE films that considers the number of protonated amino groups per macromolecule has been introduced. The fractal geometry of dendrimers leads to a geometric increase in the number of amino groups per generation. Thus, investigation of dendrimer macromolecules seems a very good strategy to design CPE films with enhanced charge-carrier selectivity.
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IJS, KILJ, NUK, PNG, UL, UM
•We report the first Germanium PV cell formed by a MoOx/n-Ge heterojunction.•Photocurrent density is 44.8 mA/cm2, comparable to that of conventional Ge PV cells.•Open circuit voltage is 138 mV, lower ...than that of conventional Ge PV cells.•Tunneling mechanisms through the MoOx/Ge interface may explain the small Voc.
Very thin, thermally evaporated MoOx (x < 3) layer has been used as transparent hole-selective contact on an n-type Germanium substrate to effectively demonstrate PV conversion capability. The fabricated MoOx/Ge heterojunction PV cell shows a photocurrent density of 44.8 mA/cm2 under AM1.5G illumination, which is comparable to that of conventional Ge PV cells. However, a low open-circuit voltage of 138 mV is obtained, which might be explained by the presence of tunnelling mechanisms through the MoOx/Ge interface. To our knowledge, this is the first demonstration of a hole-selective contact made of transition metal oxide on an n-type semiconductor different from c-Si. Thus, this work may have important implications toward the development of new device architectures, such as novel low-cost Ge PV cells with possible applications in multijunction solar cells and thermophotovoltaics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Abstract
According to intermediate band (IB) theory, it is possible to increase the efficiency of a solar cell by boosting its ability to absorb low-energy photons. In this study, we used a ...hyperdoped semiconductor approach for this theory to create a proof of concept of different silicon-based IB solar cells. Preliminary results show an increase in the external quantum efficiency (EQE) in the silicon sub-bandgap region. This result points to sub-bandgap absorption in silicon having not only a direct application in solar cells but also in other areas such as infrared photodetectors. To establish the transport mechanisms in the hyperdoped semiconductors within a solar cell, we measured the
J
–
V
characteristic at different temperatures. We carried out the measurements in both dark and illuminated conditions. To explain the behavior of the measurements, we proposed a new model with three elements for the IB solar cell. This model is similar to the classic two-diodes solar cell model but it is necessary to include a new limiting current element in series with one of the diodes. The proposed model is also compatible with an impurity band formation within silicon bandgap. At high temperatures, the distance between the IB and the n-type amorphous silicon conduction band is close enough and both bands are contacted. As the temperature decreases, the distance between the bands increases and therefore this process becomes more limiting.
•LIFT technique is investigated to improve heterojunction HJ solar cells.•Doped silicon films are adequate precursors for LIFT application in HJ cells.•LIFT leads to a reduction of the series ...resistance of a-Si HJ diodes.•LIFT allows the improvement of the front contact resistance in a-Si HJ solar cells.
In this work the Laser Induced Forward Transfer (LIFT) technique is investigated to create n-doped regions on p-type c-Si substrates. The precursor source of LIFT consisted in a phosphorous-doped hydrogenated amorphous silicon layer grown by Plasma Enhanced Chemical Vapor Deposition (PECVD) onto a transparent substrate. Transfer of the doping atoms occurs when a sequence of laser pulses impinging onto the doped layer propels the material toward the substrate. The laser irradiation not only transfers the doping material but also produces a local heating that promotes its diffusion into the substrate. The laser employed was a 1064nm, lamp-pumped system, working at pulse durations of 100 and 400ns. In order to obtain a good electrical performance a comprehensive optimization of the applied laser fluency and number of pulses was carried out. Subsequently, arrays of n+p local junctions were created by LIFT and the resulting J–V curves demonstrated the formation of good quality n+ regions. These structures were finally incorporated to enhance the front contact in conventional silicon heterojunction solar cells leading to an improvement of conversion efficiency.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Structural and optical characterization of copper phthalocyanine thin film thermally deposited at different substrate temperatures was the aim of this work. The morphology of the films shows strong ...dependence on temperature, as can be observed by atomic force microscopy and x-ray diffraction spectroscopy, specifically in the grain size and features of the grains. The increase in the crystal phase with substrate temperature is shown by x-ray diffractometry. Optical absorption coefficient measured by photothermal deflection spectroscopy and optical transmittance reveal a weak dependence on the substrate temperature. Besides, the electro-optical response measured by the external quantum efficiency of Schottky ITO/CuPc/Al diodes shows an optimized response for samples deposited at a substrate temperature of 60 deg C, in correspondence to the I-V diode characteristics.
•LIFT with a high viscosity silver paste for the metallization of SHJ cells.•Simple non-contact thermal curing step.•Electrical characterization of deposited lines of high viscosity silver ...paste.•Electrical behavior of cells similar to ref cell metalized by thermal evaporation.
Laser-Induced Forward Transfer (LIFT) is a very versatile technique, allowing the selective transfer of a wide range of materials with no contact and high accuracy. This work includes the analysis of heterojunction silicon solar cells with the frontal grid deposited by LIFT, and the electric characterization of the deposited lines.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this article, we study the effect of the inversion charge ( Q inv ) in a solar cell based on the hole-selective characteristic of substoichiometric molybdenum oxide (MoO x ) and vanadium oxide (VO ...x ) deposited directly on n-type silicon. We measure the capacitance-voltage ( C - V ) curves of the solar cells at different frequencies and explain the results taking into account the variation of the space charge and the existence of Q inv in the c-Si inverted region. The high-frequency capacitance measurements follow the Schottky metal-semiconductor theory, pointing to a low inversion charge influence in these measurements. However, for frequencies lower than 20 kHz, an increase in the capacitance is observed, which we relate to the contribution of the inversion charge. In addition, applying the metal-semiconductor theory to the high-frequency measurements, we have obtained the built-in voltage potential and show new evidence about the nature of the conduction process in this structure. This article provides a better understanding of the transition metal oxide/n-type crystalline silicon heterocontact.
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