Improvement in open circuit voltage of n‐ZnO/p‐Si solar cell by using amorphous‐ZnO at the interface

Several research groups are currently working on n‐ZnO/p‐Si heterojunction solar cell, and recently, Pietruszka et al [Sol. Energ. Mat. Sol. Cells 147 (2016) 164‐170] has reported the highest efficiency of 7.1% for this structure. The main challenge is to enhance the open circuit voltage up to theoretically predicted value of >0.6 V. This paper reports >20% improvement in open circuit voltage of n‐ZnO/p‐Si solar cell by depositing amorphous‐ZnO at the interface at room temperature that possibly improves the passivation and/or avoids oxide formation at the interface during ZnO deposition. Two other materials, aluminum nitride and amorphous‐Si, have also been used as buffer layers to evaluate their effect on suppression of interface states. Furthermore, additional advantage of ZnO as an antireflector has been experimentally verified for different thicknesses of ZnO film.     

CuOx/a‐Si:H heterojunction thin‐film solar cell with an n‐type µc‐Si:H depletion‐assisting layer

We showed that thin n‐type CuO_x films can be deposited by radio‐frequency magnetron reactive sputtering and demonstrated the fabrication of n‐CuO_x/intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) heterojunction solar cells (HSCs) for the first time. A highly n‐doped hydrogenated microcrystalline Si (n‐µc‐Si:H) layer was introduced as a depletion‐assisting layer to further improve the performance of n‐CuO_x/i‐a‐Si:H HSCs. An analysis of the external quantum efficiency and energy‐band diagram showed that the thin depletion‐assisting layer helped establish sufficient depletion and increased the built‐in potential in the n‐CuO_x layer. The fabricated HSC exhibited a high open‐circuit voltage of 0.715 V and an efficiency of 4.79%. Copyright © 2015 John Wiley & Sons, Ltd.     

Wide‐bandgap p‐type microcrystalline silicon oxycarbide using additional trimethylboron for silicon heterojunction solar cells

We report a new wide‐bandgap p‐type microcrystalline silicon oxycarbide (p‐μc‐SiO_xC_y:H) film prepared by plasma‐enhanced chemical vapor deposition. As an additional doping gas, trimethylboron was introduced into the standard processing gas‐mixture of silane, carbon dioxide, hydrogen, and diborane. With both trimethylboron and diborane as doping gases, the optical bandgap (E ₀₄) of the formed p‐μc‐SiO_xC_y:H film was 0.18 eV higher than that of reference microcrystalline silicon oxide (p‐μc‐SiO_x:H) processed with only diborane doping gas for the same levels of film thickness and electrical conductivity. To demonstrate the effectiveness of the developed p‐layer, we applied it as an emitter in silicon heterojunction solar cells, which delivered a markedly high open circuit voltage of 0.702 V and a power conversion efficiency of 18.9% based on a non‐textured flat wafer. Copyright © 2017 John Wiley & Sons, Ltd.     

Laser‐fired contact for n‐type crystalline Si solar cells

Laser‐fired contacts to n‐type crystalline silicon were developed by investigating novel metal stacks containing Antimony (Sb). Lasing conditions and the structure of metals stacks were optimized for lowest contact resistance and minimum surface damage. Specific contact resistance for firing different metal stacks through either silicon nitride or p‐type amorphous silicon was determined using two different models and test structures. Specific contact resistance values of 2–7 mΩcm² have been achieved. Recombination loss due to laser damage was consistent with an extracted local surface recombination velocity of ~20 000 cm/s, which is similar to values for laser‐fired base contact for p‐type crystalline silicon. Interdigitated back contact silicon heterojunction cells were fabricated with laser‐fired base contact and proof‐of‐concept efficiencies of 16.9% were achieved. This localized base contact technique will enable low cost back contact patterning and innovative designs for n‐type crystalline solar cell. Copyright © 2014 John Wiley & Sons, Ltd.     

Efficient silicon heterojunction solar cells based on p‐ and n‐type substrates processed at temperatures < 220°C

We present the optimization and characterization of heterojunction solar cells consisting of an amorphous silicon emitter, a single crystalline absorber and an amorphous silicon rear side which causes the formation of a back surface field (a‐Si:H/c‐Si/a‐Si:H). The solar cells were processed at temperatures <220°C. An optimum of the gas phase doping concentration of the a‐Si:H layers was found. For high gas phase doping concentrations, recombination via defects located at or nearby the interface leads to a decrease in solar cell efficiency. We achieved efficiencies >17% on p‐type c‐Si absorbers and >17·5% on n‐type absorbers. In contrast to the approach of Sanyo, no additional intrinsic a‐Si:H layers between the substrate and the doped a‐Si:H layers were inserted. Copyright © 2006 John Wiley & Sons, Ltd.     

Tail states recombination limit of the open circuit voltage in bulk heterojunction organic solar cells

An analytical theory is presented for bimolecular recombination through tail states and open circuit voltage in bulk heterojunction organic solar cells. It is developed rigorously using the hopping transport and the drift diffusion theory. Based on the proposed model, a variety of temperature, energy disorders of the material and illumination intensity dependencies of the open circuit voltage can be well described. Good agreement between the calculation and recent experimental data is found.     

Efficient electron-blocking layer-free planar heterojunction perovskite solar cells with a high open-circuit voltage

Perovskite solar cells (PSCs) with a simple device structure are particularly attractive due to their low cost and convenient fabrication process. Herein, highly efficient, electron-blocking layer (EBL)-free planar heterojunction (PHJ) PSCs with a structure of ITO/CH₃NH₃PbI₃/PCBM/Al were fabricated via low-temperature, solution-processed method. The power conversion efficiency (PCE) of over 11% was achieved in EBL-free PHJ-PSCs, which is closed to the value of PSC devices with the PEDOT:PSS as the EBL. It is impressed that the open-circuit voltage (V_oc) up to 1.06 V, an average value of 1.0 V for 43 devices, was obtained in EBL-free PHJ-PSCs. The electrochemical impedance spectroscopy (EIS) results suggested that the high PCE and V_oc are attributed to the relatively large recombination resistance and low contact resistance in EBL-free PHJ-PSCs. The solution-processed, EBL-free PHJ structure paves a boulevard for fabricating high-efficiency and low-cost PSCs.     

High‐efficiency black silicon interdigitated back contacted solar cells on p‐type and n‐type c‐Si substrates

This work demonstrates the high potential of Al₂O₃ passivated black silicon in high‐efficiency interdigitated back contacted (IBC) solar cells by reducing surface reflectance without jeopardizing surface passivation. Very low reflectance values, below 0.7% in the 300–1000 nm wavelength range, together with striking surface recombination velocities values of 17 and 5 cm/s on p‐type and n‐type crystalline silicon substrates, respectively, are reached. The simultaneous fulfillment of requirements, low reflectance and low surface recombination, paves the way for the fabrication of high‐efficiency IBC Si solar cells using black silicon at their front surface. Outstanding photovoltaic efficiencies over 22% have been achieved both in p‐type and n‐type 9‐cm² cells. 3D simulations suggest that efficiencies of up to 24% can be obtained in the future with minor modifications in the baseline fabrication process. Copyright © 2015 John Wiley & Sons, Ltd.     

High‐rate deposition of a‐Si:H thin layers for high‐performance silicon heterojunction solar cells

In this paper, we describe a technique for high‐quality interface passivation of n‐type crystalline silicon wafers through the growth of hydrogenated amorphous Si (a‐Si:H) thin layers using conventional plasma‐enhanced chemical vapor deposition. We investigated the onset of crystallization of the a‐Si:H layers at various deposition rates and its effect on the surface passivation properties. Epitaxial growth occurred, even at a low substrate temperature of 90 °C, when the deposition rate was as low as 0·5 Å/s; amorphous growth occurred at temperatures up to 150 °C at a higher deposition rate of 4·2 Å/s. After optimizing the intrinsic a‐Si:H layer deposition conditions and then subjecting the sample to post‐annealing treatment, we achieved a very low surface recombination velocity (7·6 cm/s) for a double‐sided intrinsic a‐Si:H coating on an n‐type crystalline silicon wafer. Under the optimized conditions, we achieved an untextured heterojunction cell efficiency of 16·7%, with a high open‐circuit voltage (694 mV) on an n‐type float‐zone Si substrate. On a textured wafer, the cell efficiency was further enhanced to 19·6%. Copyright © 2011 John Wiley & Sons, Ltd.     

An investigation of photocurrent loss due to reflectance and absorption in CdTe/CdS heterojunction solar cells

An attempt is made to understand, quantify, and reduce the reflectance and photocurrent loss in CdTe solar cells. Model calculations are performed to determine the optimum thicknesses of CdS and SnO₂ films and anti-reflection (AR) coating on glass that can minimize the reflectance and enhance the performance of CdTe/CdS/SnO₂/glass solar cells. Photocurrent loss due to absorption in CdS films is also calculated as a function of CdS thickness. It was found that the current loss due to reflectance ando absorption is more sensitive to the CdS film when its thickness falls below 1500Å. Model calculations show that reducing the CdS thickness from 1500 to 600Å increases short-circuit current density ( J_sc) by 3 mA/cm² because of reduced reflectance as well as absorption. Further decrease in CdS thickness below 600Å increases reflectance but results in higher J_sc, because current gain due to reduced absorption in thin CdS offsets the current loss due to higher reflectance. Model calculations also indicate that J_sc is not sensitive to SnO₂ thickness above 4000Å. Finally, an optimum thickness for single layer MgF₂ AR coating on glass was calculated to be 1100Å, which should provide an additional increase of 0.7 mA/cm² in J_sc. Some of these results are also experimentally validated in this paper.     

New explicit current/voltage equation for p‐i‐n solar cells including interface potential drops and drift/diffusion transport

Analytical modeling of p‐i‐n solar cells constitutes a practical tool to extract material and device parameters from fits to experimental data, and to establish optimization criteria. This paper proposes a model for p‐i‐n solar cells based on a new approximation, which estimates the electric field taking into account interface potential drops at the intrinsic‐to‐doped interfaces. This leads to a closed‐form current/voltage equation that shows very good agreement with device simulations, revealing that the inclusion of the interface potential drops constitutes a major correction to the classical uniform‐field approach. Furthermore, the model is able to fit experimental current/voltage curves of efficient nanocrystalline Si and microcrystalline Si p‐i‐n solar cells under illumination and in the dark, obtaining material parameters such as mobility‐lifetime product, built‐in voltage, or surface recombination velocity. Copyright © 2012 John Wiley & Sons, Ltd.     

Designing novel thin film polycrystalline solar cells for high efficiency: sandwich CIGS and heterojunction perovskite

Heterojunction and sandwich architectures are two new-type structures with great potential for solar cells. Specifically, the heterojunction structure possesses the advantages of efficient charge separation but suffers from band offset and large interface recombination; the sandwich configuration is favorable for transferring carriers but requires complex fabrication process. Here, we have designed two thin-film polycrystalline solar cells with novel structures:sandwich CIGS and heterojunction perovskite, referring to the advantages of the architectures of sandwich perovskite (standard) and heterojunction CIGS (standard) solar cells, respectively. A reliable simulation software wxAMPS is used to investigate their inherent characteristics with variation of the thickness and doping density of absorber layer. The results reveal that sandwich CIGS solar cell is able to exhibit an optimized efficiency of 20.7%, which is much higher than the standard heterojunction CIGS structure (18.48%). The heterojunction perovskite solar cell can be more efficient employing thick and doped perovskite films (16.9%) than these typically utilizing thin and weak-doping/intrinsic perovskite films (9.6%). This concept of structure modulation proves to be useful and can be applicable for other solar cells.     

Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (V_OC) due to the surface passivation and heterojunction contacts, and high short circuit current density (J_SC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence V_OC and J_SC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since J_SC and V_OC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

Heterojunction solar cells with 23% efficiency on n‐type epitaxial kerfless silicon wafers

We present a heterojunction (HJ) solar cell on n‐type epitaxially grown kerfless crystalline‐silicon with an in‐house‐measured conversion efficiency of 23%. The total cell area is 243.4 cm². The cell has a short‐circuit current density of 39.6 mA cm⁻², an open‐circuit voltage of 725 mV, and a fill factor of 0.799. The effect of stacking faults (SFs) is examined by current density (J) mapping measurements as well as by spectral response mapping. The J mapping images show that the localized lower J regions of the HJ solar cells are associated with recombination sites originating from SFs, independent of whether SFs are formed on the emitter or absorber side. The solar cell results and our analysis suggest that epitaxially grown wafers based on kerfless technology could be an alternative for low‐cost industrial production of Si HJ solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Simulation and fabrication of heterojunction silicon solar cells from numerical computer and hot‐wire CVD

In this paper, we will present a Pc1D numerical simulation for heterojunction (HJ) silicon solar cells, and discuss their possibilities and limitations. By means of modeling and numerical computer simulation, the influence of emitter‐layer/intrinsic‐layer/crystalline‐Si heterostructures with different thickness and crystallinity on the solar cell performance is investigated and compared with hot wire chemical vapor deposition (HWCVD) experimental results. A new technique for characterization of n‐type microcrystalline silicon (n‐µc‐Si)/intrinsic amorphous silicon (i‐a‐Si)/crystalline silicon (c‐Si) heterojunction solar cells from Pc1D is developed. Results of numerical modeling as well as experimental data obtained using HWCVD on µc‐Si (n)/a‐Si (i)/c‐Si (p) heterojunction are presented. This work improves the understanding of HJ solar cells to derive arguments for design optimization. Some simulated parameters of solar cells were obtained: the best results for J_sc = 39·4 mA/cm², V_oc = 0·64 V, FF = 83%, and η = 21% have been achieved. After optimizing the deposition parameters of the n‐layer and the H₂ pretreatment of solar cell, the single‐side HJ solar cells with J_sc = 34·6 mA/cm², V_oc = 0·615 V, FF = 71%, and an efficiency of 15·2% have been achieved. The double‐side HJ solar cell with J_sc = 34·8 mA/cm², V_oc = 0·645 V, FF = 73%, and an efficiency of 16·4% has been fabricated. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental and simulated analysis of front versus all‐back‐contact silicon heterojunction solar cells: effect of interface and doped a‐Si:H layer defects

Front silicon heterojunction and interdigitated all‐back‐contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high efficiency and low cost because of their good surface passivation, heterojunction contacts, and low temperature fabrication processes. The performance of both heterojunction device structures depends on the interface between the crystalline silicon (c‐Si) and intrinsic amorphous silicon [(i)a‐Si:H] layer, and the defects in doped a‐Si:H emitter or base contact layers. In this paper, effective minority carrier lifetimes of c‐Si using symmetric passivation structures were measured and analyzed using an extended Shockley–Read–Hall formalism to determine the input interface parameters needed for a successful 2D simulation of fabricated baseline solar cells. Subsequently, the performance of front silicon heterojunction and IBC‐SHJ devices was simulated to determine the influence of defects at the (i)a‐Si:H/c‐Si interface and in the doped a‐Si:H layers. For the baseline device parameters, the difference between the two device configurations is caused by the emitter/base contact gap recombination and the back surface geometry of IBC‐SHJ solar cell. This work provides a guide to the optimization of both types of SHJ device performance, predicting an IBC‐SHJ solar cell efficiency of 25% for realistic material parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: Thin‐film free‐standing monocrystalline si solar cells with heterojunction emitter

We propose a novel approach to thin‐film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS‐HJ). High crystallographic quality mono‐Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free‐standing mono‐Si (FMS) films were processed into solar cells by creating a‐a‐Si/c‐Si heterojunction. In our preliminary experiments a thin‐film FMS‐HJ solar cell with 9.6% efficiency was realized in a 20‐μμm‐thin active layer. Copyright © 2005 John Wiley & Sons, Ltd.     

Passivation of n‐type emitter and p‐type base in solar cells via oxygen terminated silicon nanoparticles

Various measurements and experiments are performed to establish the mechanism of passivation on emitter and base of conventionally manufactured solar cell with p‐type base. The surface coatings on the emitter are removed. The bare surface is then coated with silicon (Si) nanoparticles (NPs) with oxygen termination. It shows an increase in the cell efficiency up to 14% over bare surface of solar cell. The NPs show enhancement in light scattering from the surface, but shows an increase in the recombination lifetime indicating an improved passivation. When back contact is partially removed, the coating on bare back side ( p‐type) of the solar cell also improves the cell efficiency. This is also attributable to the increased recombination lifetime from the measurements. Same NPs are seen to degrade the surface of n and p‐type Si wafers. This apparently contradictory behaviour is explained by studying and comparing the emitter (n‐type) surface of the solar cell with that of n‐type Si wafer and the back surface ( p‐type) with that of p‐type Si wafer. The emitter surface is distinctly different from the n‐type wafer because of the shallow p–n junction causing the surface depletion. Back surface has aluminium (Al) metal trace, which plays an important role in forming complexes with the oxygen‐terminated Si NPs (Si–O NPs). With these studies, it is observed that increase in the efficiency can potentially reduce the thermal budget in solar cell preparation. Copyright © 2013 John Wiley & Sons, Ltd.     

Open‐circuit voltage recovery in type II GaSb/GaAs quantum ring solar cells under high concentration

We report on the open‐circuit voltage recovery in GaSb quantum ring (QR) solar cells under high solar concentration up to 2500 suns. The detailed behaviour of type II GaSb/GaAs QR solar cells under solar concentration, using different temperatures and light illumination conditions, is analysed through optical and electrical measurements. Although enhancement of the short‐circuit current was observed because of sub‐bandgap photon absorption in the QR, the thermionic emission rate of holes was found to be insufficient for ideal operation. The direct excitation of electron–hole pairs into QRs has revealed that the accumulation of holes is one of the causes of the open‐circuit voltage (V_OC) degradation. However, using concentrated light up to 2500 suns, the GaSb QR cell showed much quicker V_OC recovery rate than a GaAs control cell. Copyright © 2015 John Wiley & Sons, Ltd.     

Bifacial concentrator Ag‐free crystalline n‐type Si solar cell

We report results obtained using an innovative approach for the fabrication of bifacial low‐concentrator thin Ag‐free n‐type Cz‐Si (Czochralski silicon) solar cells based on an indium tin oxide/(p⁺nn⁺)Cz‐Si/indium fluorine oxide structure. The (p⁺nn⁺)Cz‐Si structure was produced by boron and phosphorus diffusion from B‐ and P‐containing glasses deposited on the opposite sides of n‐type Cz‐Si wafers, followed by an etch‐back step. Transparent conducting oxide (TCO) films, acting as antireflection electrodes, were deposited by ultrasonic spray pyrolysis on both sides. A copper wire contact pattern was attached by low‐temperature (160°C) lamination simultaneously to the front and rear transparent conducting oxide layers as well as to the interconnecting ribbons located outside the structure. The shadowing from the contacts was ~4%. The resulting solar cells, 25 × 25 mm² in dimensions, showed front/rear efficiencies of 17.6–17.9%/16.7–17.0%, respectively, at one to three suns (bifaciality of ~95%). Even at one‐sun front illumination and 20–50% one‐sun rear illumination, such a cell will generate energy approaching that produced by a monofacial solar cell of 21–26% efficiency. Copyright © 2014 John Wiley & Sons, Ltd.