Fabrication of monolithic photovoltaic arrays on crystalline silicon by wafer bonding and deep etching techniques

In this work, a novel technology to fabricate small (∼1 cm²) c‐Si photovoltaic mini‐modules is shown. This technology combines two main bulk micro‐machining techniques: fusion (or adhesive) bonding and anisotropic etching of silicon. Due to the fact that the photovoltaic cells are fabricated in the same wafer, it is mandatory to etch the whole substrate to ensure electrical isolation. Once the individual cells are bulk‐isolated they can be connected in series so as to scale up the output voltage of the mini‐array. A handling wafer is required to provide mechanical stability to the device wafer. Adhesive and fusion bonding are used to join the handling and the device wafer. First electrical results, under standard Air Mass 1·5 (AM 1·5) solar spectrum light (100 mW/cm²), using a 9‐cell series connected mini‐module fabricated by fusion bonding, leads to a total open‐circuit voltage of 4·11 V, a short‐circuit current of 2·45 mA, and a maximum delivered power of 3·8 mW for each mini‐module (1·4 cm²). A 16‐cell series‐connected mini‐module fabricated by adhesive bonding and wire bonding, yields an open‐circuit voltage of 7·45 V, a short‐circuit current of 390 µA, and maximum delivered power of 1·8 mW, with 1·1 cm² of mini‐module area. Copyright © 2005 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.     

A masked process for the industrial production of buried contact solar cells on multi‐crystalline silicon

This paper reports on the development of a masked process for the production of buried contact solar cells on multi‐crystalline silicon. The process results in high efficiencies, and only includes steps that would be feasible in an industrial environment. We report here on different mask candidates and on the importance of hydrogenation with the new process. Using the developed process, we produced 111 large area (12 × 12 cm²) cells and achieved an average cell efficiency of 16·2%. The best cell had an efficiency of 16·9%, a V_oc of 616 mV, a J_sc of 35·0 mA/cm² and a fill factor of 78·3%. Copyright © 2008 John Wiley & Sons, Ltd.     

Narrow‐bandgap CuIn3Te5 thin‐film solar cells

We propose CuIn₃Te₅ as a ternary semiconductor material for narrow‐bandgap thin‐film solar cells. Well‐developed CuIn₃Te₅ grains were obtained at a substrate temperature of 250 °C by single‐step co‐evaporation. The best solar cell that was fabricated using 4·0‐µm‐thick CuIn₃Te₅ layers grown at 250 °C yielded a total area efficiency of 6·92% (V_oc = 407 mV, J_sc = 33·1 mA/cm², and FF = 0·514). To clarify the loss in the device performance, the cell was compared with a standard CuInSe₂ reference cell. A band diagram of the CdS/CuIn₃Te₅ solar cell was also presented. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrical and photoelectric properties of nanostructures obtained by electroless etching of silicon

The electrical and photoelectric properties of nanostructures with porous silicon layers obtained by electroless etching of silicon have been investigated. It is found that the photoelectric and photovoltaic properties of these structures depend on their morphology and are determined by not only the properties of the modified layer, but also the presence of possible barriers in the layered porous silicon. The ratio of the photoconductivity to the dark conductivity reached 10²−5 × 10². An open-circuit voltage V _oc was detected that amounted to ∼250 mV at an incident light power close to AM-1 (∼100 mW/cm²). In this case, the density of short-circuit current I _sc was about 20 μA/cm².     

Solar cells with 11% efficiency on ribbon-growth-on-substrate (RGS) silicon

Solar cells have been prepared on Bayer ribbon-growth-on-substrate (RGS) crystalline silicon. This low-cost material contains a fair amount of impurities and crystal defects whose adverse effect on solar cell performance may be significantly reduced by gettering and bulk passivation treatments. This is demonstrated in solar cells by having a mechanical surface texturization, an aluminium gettering as well as a hydrogen passivation step which led to an open circuit voltage V_oc of 538 mV, a short circuit current density J_sc of 28·5 mA cm⁻², a fill factor of 72·4% and a confirmed record efficiency β of 11·1% (2×2 cm²). Strong improvements in the diffusion length could be observed after the hydrogen treatment. © 1998 John Wiley & Sons, Ltd.     

Obtaining a higher Voc in HIT cells

We have achieved a very high conversion efficiency of 21·5% in HIT cells with a size of 100·3 cm². One of the most striking features of the HIT cell is its high open‐circuit voltage V_oc, in excess of 710 mV. This is due to the excellent surface passivation at the a‐Si/c‐Si heterointerface realized by Sanyo's successful technologies for fabricating high‐quality a‐Si films and solar cells with low plasma damage processes. We have studied ways to treat the surface to produce a good interface throughout our fabrication processes. We have also investigated the deposition conditions of a‐Si layers for optimizing the barrier height for the minority carriers in the heterojunction. Our approach for obtaining HIT cells with a high V_oc is reviewed here. Copyright © 2005 John Wiley & Sons, Ltd.     

High efficiency n-CdS/p-InP solar cells prepared by the close-spaced technique

Heterojunction solar cells have been made by epitaxial growth of CdS on p-type InP using the close-spaced technique. Good rectification and photovoltaic properties have been observed in the cells grown on the (110) face of InP. The characteristics of the most efficient cell are V_oc = 807 mV, the fill factor = 0.74 and the power conversion efficiency = 14.4% under the solar input of 77 mW/cm². The photovoltaic properties of the cells obtained in this experiment are better than those reported elsewhere, and this is attributed to the superiority of the growth system to those of others in points that the junction is formed at relatively high temperature in a short time.     

p‐CuO/n‐Si heterojunction solar cells with high open circuit voltage and photocurrent through interfacial engineering

Heterojunction solar cells of p‐type cupric oxide (CuO) and n‐type silicon (Si), p‐CuO/n‐Si, have been fabricated using conventional sputter and rapid thermal annealing techniques. Photovoltaic properties with an open‐circuit voltage (V_oc) of 380 mV, short circuit current (J_sc) of 1.2 mA/cm², and a photocurrent of 2.9 mA/cm² were observed for the solar cell annealed at 300 °C for 1 min. When the annealing duration was increased, the photocurrent increased, but the V_oc was found to reduce because of the degradation of interface quality. An improvement in the V_oc resulting to a record value of 509 mV and J_sc of 4 mA/cm² with a high photocurrent of ~12 mA/cm² was achieved through interface engineering and controlling the phase transformation of CuO film. X‐ray diffraction, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy analysis have been used to investigate the interface properties and crystal quality of sputter‐deposited CuO thin film. The improvement in V_oc is mainly due to the enhancement of crystal quality of CuO thin film and interface properties between p‐CuO and n‐Si substrate. The enhancement of photocurrent is found to be due to the reduction of carrier recombination rate as revealed by transient photovoltage spectroscopy analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Light Detection in Open‐Circuit Voltage Mode of Organic Photodetectors

Organic photodetectors (OPDs) are promising candidates for next‐generation light sensors as they combine unique material properties with high‐level performance in converting photons into electrical signals. However, low‐level light detection with OPD is often limited by device dark current. Here, the open‐circuit voltage (V_oc ) regime of OPDs is shown to be efficient for detecting low light signals (<100 µW cm^?2). It is established that the light‐dependence of V_oc exhibits two distinct regimes as function of irradiance: linear and logarithmic. Whereas the observed logarithmic regime is well understood in organic photovoltaic cells (OPVs), it is shown experimentally and theoretically that the linear regime is due to the non‐infinite shunt resistance of the OPD device. Overall, OPDs composed of rubrene and fullerene show photovoltage light sensitivity across nine orders of magnitude with a detection limit as low as 400 pW cm^?2. A photovoltage responsivity of 1.75 V m² W^?1 demonstrates highly efficient performance without the necessity to supress high dark current. This approach opens up new possibilities for resolving low light signals and provides simplified design rules for OPDs.     

Monolithic Si nanocrystal/crystalline Si tandem cells involving Si nanocrystals in SiC

Monolithic tandem cells involving a top cell with Si nanocrystals embedded in SiC (Si NC/SiC) and a c‐Si bottom cell have been prepared. Scanning electron microscopy shows that the intended cell architecture is achieved and that it survives the 1100 °C anneal required to form Si NCs. The cells exhibit mean open‐circuit voltages V_oc of 900–950 mV, demonstrating tandem cell functionality, with ≤580 mV arising from the c‐Si bottom cell and ≥320 mV arising from the Si NC/SiC top cell. The cells are successfully connected using a SiC/Si tunnelling recombination junction that results in very little voltage loss. The short‐circuit current densities j_sc are, at 0.8–0.9 mAcm⁻², rather low and found to be limited by current collection in the top cell. However, equivalent circuit simulations demonstrate that in current‐mismatched tandem cells such as the ones studied here, higher j_sc, when accompanied by decreased V_oc, can arise from shunts or breakdown in the limiting cell rather than improved current collection from the limiting cell. This indicates that V_oc is a better optimisation parameter than j_sc for tandem cells where the limiting cell exhibits poor junction characteristics. The high‐temperature‐stable cell architecture developed in this work, coupled with simulations highlighting potential pitfalls in tandem cell analysis, provides a suitable route for optimisation of Si NC layers for photovoltaics on a tandem cell device level. Copyright © 2016 John Wiley & Sons, Ltd.     

The Origin of the High Voltage in DPM12/P3HT Organic Solar Cells

Organic solar cells made using a blend of DPM12 and P3HT are studied. The results show that higher V_oc can be obtained when using DPM12 in comparison to the usual mono‐substituted PCBM electron acceptor. Moreover, better device performances are also registered when the cells are irradiated with sun‐simulated light of 10–50 mW cm^?2 intensity. Electrochemical and time‐resolved spectroscopic measurements are compared for both devices and a 100‐mV shift in the density of states (DOS) is observed for DPM12/P3HT devices with respect to PCBM/P3HT solar cells and slow polaron‐recombination dynamics are found for the DPM12/P3HT devices. These observations can be directly correlated with the observed increase in V_oc, which is in contrast with previous results that correlated the higher V_oc with different ideality factors obtained using dark‐diode measurements. The origin for the shift in the DOS can be correlated to the crystallinity of the blend that is influenced by the properties of the included fullerene.     

Evaluation of Zn?Sn?O buffer layers for CuIn0.5Ga0.5Se2 solar cells

Thin Zn Sn O films are evaluated as new buffer layer material for Cu(In,Ga)Se₂‐based solar cell devices. A maximum conversion efficiency of 13.8% (V_oc = 691 mV, J_sc(QE) = 27.9 mA/cm², and FF = 71.6%) is reached for a solar cell using the Zn Sn O buffer layer which is comparable to the efficiency of 13.5% (V_oc = 706 mV, J_sc(QE) = 26.3 mA/cm², and FF = 72.9%) for a cell using the standard reference CdS buffer layer. The open circuit voltage (V_oc) and the fill factor (FF) are found to increase with increasing tin content until an optimum in both parameters is reached for Sn/(Zn + Sn) values around 0.3–0.4. Copyright © 2010 John Wiley & Sons, Ltd.     

Photovoltaic effects in Cu2OCu solar cells grown by anodic oxidation

Thin Cu₂OCu photovoltaic cells grown by anodic oxidation of Cu in an alkaline solution at T = 86°C give an open-circuit voltage V_oc of up to 400 mV and a short-circuit current density J_sc of up to 0.6 mA/cm² under AM1 illumination. The cells display charge storage properties as well; that is, a small fraction of the solar energy can be stored and given back by the cells in the dark.     

Improved performance in ZnO/CdS/CuGaSe2 thin‐film solar cells

We report the growth and characterization of improved efficiency wide‐bandgap ZnO/CdS/CuGaSe₂ thin‐film solar cells. The CuGaSe₂ absorber thickness was intentionally decreased to better match depletion widths indicated by drive‐level capacitance profiling data. A total‐area efficiency of 9·5% was achieved with a fill factor of 70·8% and a V_oc of 910 mV. Published in 2003 by John Wiley & Sons, Ltd.     

Impact of silicon quantum dot super lattice and quantum well structure as intermediate layer on p‐i‐n silicon solar cells

The photovoltaic effect of the silicon (Si)/silicon carbide (SiC) quantum dot super lattice (QDSL) and multi‐quantum well (QW) strucutres is presented based on numerical simulation and experimental studies. The QDSL and QW structures act as an intermediate layer in a p‐i‐n Si solar cell. The QDSL consists of a stack of four 4‐nm Si nano disks and 2‐nm SiC barrier layers embedded in a SiC matrix fabricated with a top‐down etching process. The Si nano disks were observed with bright field‐scanning transmission electron microscopy. The simulation results based on the 3D finite element method confirmed that the quantum effect on the band structure for the QDSL and QW structures was different and had different effects on solar cell operation. The effect of vertical wave‐function coupling to form a miniband in the QDSL was observed based on the solar‐cell performance, showing a dramatic photovoltaic response in generating a high photocurrent density J_sc of 29.24 mA/cm², open circuit voltage V_oc of 0.51 V, fill factor FF of 0.74, and efficiency η of 11.07% with respect to a i‐QW solar cell with J_sc of 25.27 mA/cm², V_oc of 0.49 V, FF of 0.69, and η of 8.61% and an i‐Si solar cell with J_sc of 27.63 mA/cm², V_oc of 0.55 V, FF of 0.61, and η of 10.00%. A wide range of photo‐carrier transports by the QD arrays in the QDSL solar cell is possible in the internal quantum efficiency spectra with respect to the internal quantum efficiency of the i‐QW solar cell. Copyright © 2015 John Wiley & Sons, Ltd.     

Highly‐efficient Cd‐free CuInS2 thin‐film solar cells and mini‐modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

Recent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (E_g = 3·6–3·8 eV) than the conventional buffer material CdS (E_g = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS₂ (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm² in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm² (total area: 22·5 mA/cm²). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm² substrates have been prepared and already reach an efficiency (active area: 17·2 cm²) of above 8%. Copyright © 2006 John Wiley & Sons, Ltd.     

13·7%-Efficient Zn(Se,OH)x/Cu(In,Ga)(SSe)2 thin-film solar cell

Cu(In,Ga)Se₂ (CIGS) and related semiconducting compounds have demonstrated their high potential for high-efficiency thin-film solar cells. The highest efficiency for CIGS-based thin-film solar cells has been achieved with CdS buffer layers prepared by a solution growth method known as chemical bath deposition (CBD). With the aim of developing Cd-free chalcopyrite-based thin-film solar cells, Zn(Se,OH)_x buffer layers were deposited by CBD on polycrystalline Cu(In,Ga)(S,Se)₂ (CIGSS). A total-area conversion efficiency of 13·7% was certified by the Frauenhofer Institute for Solar Energy Systems. The CIGSS absorber was fabricated by Siemens Solar Industries (California). For device optimization, the thickness and good surface coverage were controlled by XPS–UPS photoemission spectroscopy. A Zn(Se,OH)_x thickness below 7 nm has been found to be optimum for achieving a homogeneous and compact buffer film on CIGSS, with open-circuit photovoltage V_oc=535 mV, fill factor FF=70·76% and a high short-circuit photocurrent density J_sc=36·1 mA cm⁻². Copyright © 1998 John Wiley & Sons, Ltd.     

Research on ZnO/Si heterojunction solar cells

We put forward an n-ZnO/p-Si heterojunction solar cell model based on AFORS-HET simulations and provide experimental support in this article. ZnO:B (B-doped ZnO) thin films deposited by metal-organic chemical vapor deposition (MOCVD) are planned to act as electrical emitter layer on p-type c-Si substrate for photovoltaic applications. We investigate the effects of thickness, buffer layer, ZnO:B affinity and work function of electrodes on performances of solar cells through computer simulations using AFORS-HET software package. The energy conversion efficiency of the ZnO:B(n)/ZnO/c-Si(p) solar cell can achieve 17.16% (V_oc: 675.8 mV, J_sc:30.24 mA/cm², FF: 83.96%) via simulation. On a basis of optimized conditions in simulation, we carry out some experiments, which testify that the ZnO buffer layer of 20 nm contributes to improving performances of solar cells. The influences of growth temperature, thickness and diborane (B₂H₆) flow rates are also discussed. We achieve an appropriate condition for the fabrication of the solar cells using the MOCVD technique. The obtained conversion efficiency reaches 2.82% (V_oc: 294.4 mV, J_sc: 26.108 mA/cm², FF: 36.66%).     

Flexible CdTe solar cells on polymer films

Lightweight and flexible CdTe/CdS solar cells on polyimide films have been developed in a ‘superstrate configuration’ where the light is absorbed in CdTe after passing through the polyimide substrate. The average optical transmission of the approximately 10‐μm‐thin spin‐coated polyimide substrate layer is more than ∼75% for wavelengths above 550 nm. RF magnetron sputtering was used to grow transparent conducting ZnO:Al layers on polyimide films. CdTe/CdS layers were grown by evaporation of compounds, and a CdCl₂ annealing treatment was applied for the recrystallization and junction activation. Solar cells of 8·6% efficiency with V_oc = 763 mV, I_sc = 20·3 mA/cm² and FF = 55·7% were obtained. Copyright © 2001 John Wiley & Sons, Ltd.