Tandem Solar Cell Devices Research Articles

Solution processed cathode and interconnecting layer of silver nanowires in an efficient inverted tandem organic solar cells

Due to the potential of achieving high conductivity and high transparency, low-cost, large-area and flexibility for versatile applications as organic solar cells, silver nanowires (AgNWs) have attracted great attention as a good alternative to conventional ITO. However, design and production of conducting transparent film and interconnecting layers plays a very important role to achieve a high device performance. We discuss the benefit of the AgNWs inserted in the organic tandem solar cell devices as transparent electrode (TE) and as interconnecting layer (ICL) between the two subcells having actives layers with complementary absorption and made of poly(3-hexylthiophene):indene-C60 bisadduct and polythieno [3,4-b]thiophene/benzodithiophene: phenyl-C71-butyric acid methyl ester. ZnO nanoparticles have been used as interfacial layer in the ICL and lead to a decrease of the AgNWs roughness. The tandem cells on AgNWs show a good efficiency (PCE=9.23%), a short circuit current density of 11.23mA/cm2 close to the lower short circuit current of the two solar subcells, and an open circuit voltage (Voc=1.47V) matching the sum of the two subcells Voc. This suggests that the ICL provides a good carrier injection through the interfacial layer and an efficient recombination of the current coming from each subcells. Moreover, optical modeling is done in order to calculate the optimized active layers thicknesses and short term stability in open air evaluated.

Present Status of Thin Film Solar Cells Using Textured Surfaces: A Brief Review

This is a brief review on light trapping in Si based thin film solar cells with textured surfaces and transparent conducting oxide front electrodes. The light trapping scheme appears to be essential in improving device efficiency over 10%. As light absorption in a thin film solar cells is not sufficient, light trapping becomes necessary to be effectively implemented with a textured surface. Surface texturing helps in the light trapping, and thereby raises short circuit current density and its efficiency. Such a scheme can be adapted to single junction as well as tandem solar cell, amorphous or micro-crystalline devices. A tandem cell is expected to have superior performance in comparison to a single junction cell and random surface textures appears to be preferable to a periodic structures.

Numerical simulations: Toward the design of 27.6% efficient four-terminal semi-transparent perovskite/SiC passivated rear contact silicon tandem solar cell

In this work, a novel four-terminal perovskite/SiC-based rear contact silicon tandem solar cell device has been proposed and simulated to achieve 27.6% power conversion efficiency (PCE) under single AM1.5 illumination. 20.9% efficient semitransparent perovskite top subcell has been used for perovskite/silicon tandem architecture. The tandem structure of perovskite-silicon solar cells is a promising method to achieve efficient solar energy conversion at low cost. In the four-terminal tandem configuration, the cells are connected independently and hence avoids the need for current matching between top and bottom subcell, thus giving greater design flexibility. The simulation analysis shows, PCE of 27.6% and 22.4% with 300 μm and 10 μm thick rear contact Si bottom subcell, respectively. This is a substantial improvement comparing to transparent perovskite solar cell and c-Si solar cell operated individually. The impact of perovskite layer thickness, monomolecular, bimolecular, and trimolecular recombination have also been obtained on the performance of perovskite top subcell. Reported PCEs of 27.6% and 22.4% are 1.25 times and 1.42 times higher as compared to experimentally available efficiencies of 22.1% and 15.7% in 300 μm and 10 μm thick stand-alone silicon solar cell devices, respectively. The presence of SiC significantly suppressed the interface recombination in bottom silicon subcell. Detailed realistic technology computer aided design (TCAD) analysis has been performed to predict the behaviour of the device.

Current and efficiency improvement for a GaAsP/SiGe on Si tandem solar cell device achieved by light trapping techniques

A GaAsP/SiGe tandem solar cell on Si substrate has been further fabricated using light trapping techniques, such as texturing and adding a back surface reflector, and thinning the Si substrate. This is of importance to increase the Jsc of the Si0.18Ge0.82 bottom cell in this tandem system since bottom cell is current limiting. The Jsc of the bottom cell has been increased by relative 7.4%. This current improvement leads to a predicted efficiency of near 21% for this tandem device, with an absolute efficiency improvement of 0.3% over previous results without light trapping processes. The current of the bottom cell can be further improved by optimizing the bottom cell structure and texturing process, further thinning the Si substrate and increasing Ge concentration. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Graphene-Based Bulk-Heterojunction Solar Cells: A Review.

The current highest power-conversion efficiencies found for different types of solar cell devices range from 20% to 46%, depending on the nature of the photovoltaic materials used and device configuration. Graphene has emerged as an important organic photovoltaic material for photoenergy conversion, where graphene can be used as a transparent electrode, active interfacial layer, electron transport layer, hole transport layer, or electron/hole separation layer in fabricating solar cell devices. This review article briefly discusses some recent advances made in different types of photovoltaic materials, and then summarizes the current status of graphene-based bulk-heterojunction (BHJ) solar cells, including graphene-containing perovskite and tandem solar cell devices. Power-conversion efficiencies currently exceed 10% for heteroatom-doped multilayer graphene-based BHJ solar cells and 15.6% for graphene-containing perovskite-based solar cells. The role of graphene layer thickness, bending, thermal annealing, passivation, heteroatom doping, perovskite materials, and tandem solar cell structure on the photovoltaic performance of graphene-based solar cells is discussed. Besides aiming for high power-conversion efficiency, factors such as long-term environmental stability and degradation, and the cost-effectiveness of graphene-based solar cells for large-scale commercial production are challenging tasks.

Multistep deposition of Cu2Si(S,Se)3 and Cu2ZnSiSe4high band gap absorber materials for thin film solar cells

Cu2ZnSi(S,Se)4 and Cu2Si(S,Se)3 are potential materials to obtain cost effective high band gap absorbers for tandem thin film solar cell devices. A method to synthesize Cu2SiS3, Cu2SiSe3and Cu2ZnSiSe4thin film absorbers is proposed. This method is based on a multistep process, using sequential deposition and annealing processes. X‐ray diffraction analysis performed on the final thin films have confirmed the presence of the Cu2Si(S,Se)3 and Cu2ZnSiSe4phases. Scanning electron microscopy images revealed the formation of polycrystalline layers with grains size up to 1 µm. The band gap of the ternary Cu2SiSe3 and Cu2SiS3, and quaternary Cu2ZnSiSe4 based thin films as determined from optical and photoluminescence measurements are found to be close to their theoretical values. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Theoretical study of optical properties of anti phase domains in GaP

III-V/Si heterostructures are currently investigated for silicon photonics and solar energy conversion. In particular, dilute nitride alloy GaAsPN grown on a GaP/Si platform exhibits lattice match with Si and an optimal band gap configuration for tandem solar cell devices. However, monolithic “coherent” growth of the GaP thin layer on Si suffers from the nucleation of extended structural defects, which can hamper device operation as well as the GaP/Si interface level and through their propagation inside the overall heterostructure. However, the effect of such structural defects on optical and transport properties is actually not well understood in details. In this letter, we investigate the anti phase domains defect (also called inversion domains) by means of ab initio calculations giving insights into the alteration of optical and transport properties of GaP due to the defective GaP/Si interface.

Toward Tandem Photovoltaic Devices Employing Nanoarray Graphene-Based Sheets

Graphene quantum dots (GQDs) are promising photonic materials for light harvesting. However, only low photoelectron conversion efficiency can be generated in single-junction graphene-based solar cells when isolated GQDs with the edge bonding defects are used as semiconductors. To address this issue, a four-junction GQD-based tandem solar cell with high theoretical conversion efficiency was proposed in this paper. Instead of isolated GQDs, nanoarray GQDs embedded in hexagonal host materials, such as graphane or boron nitride, was adopted as the photoactive layer. Utilizing our universal thermodynamic approach to the gap openings in low-dimensional graphene, nanoarray armchair-interfaced GQDs embedded in graphane to achieve the maximal diameter of confined GQDs are found preferential for fabricating tandem solar cell devices. Besides these, the separation between GQDs and the thickness of GQD-based sheets were determined. This contribution is of benefit to the application of graphene for solar cell devices.

Small molecule-based tandem solar cells with solution-processed and vacuum-processed photoactive layers

A tandem solar cell device whose sub-cells are fabricated exclusively from small molecules (SMs) through both solution-processed and vacuum-processed deposition techniques is described. The front sub-cell's active layer consists of a bulk heterojunction (BHJ) DPP(TBFu)2:PC70BM device while the back cell has a typical bilayer structure employing a 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) donor and a C60 acceptor.

Impact of a Metal–Organic Vapor Phase Epitaxy Environment on Silicon Substrates for III–V-on-Si Multijunction Solar Cells

With the final goal of integrating III–V materials to silicon for tandem solar cells, the influence of the metal–organic vapor phase epitaxy (MOVPE) environment on the minority carrier properties of silicon wafers has been evaluated. These properties will essentially determine the photovoltaic performance of the bottom cell in a III–V-on-Si tandem solar cell device. A comparison of the base minority carrier lifetimes obtained for different thermal processes carried out in a MOVPE reactor on Czochralski silicon wafers has been carried out. The effect of the formation of the emitter by phosphorus diffusion has also been evaluated.

Si solid-state quantum dot-based materials for tandem solar cells

The concept of third-generation photovoltaics is to significantly increase device efficiencies whilst still using thin-film processes and abundant non-toxic materials. A strong potential approach is to fabricate tandem cells using thin-film deposition that can optimise collection of energy in a series of cells with decreasing band gap stacked on top of each other. Quantum dot materials, in which Si quantum dots (QDs) are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin-film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approximately 2-nm diameter QDs, these result in an effective band gap of 1.8 eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. A modified modulation doping model for the doping mechanisms in these materials is discussed which relies on doping of a sub-oxide region around the Si QDs.

Si and Other Group IV Quantum Dot Based Materials for Tandem Solar Cells

Quantum dot materials, in which Si QDs are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approx. 2nm diameter QDs these result in an effective band gap of 1.8eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open-circuit voltage of almost 500mV. However, the doping behaviour of P and B in these QD materials is not well understood. In addition P and B have large but opposite effects on QD crystallisation, with P (B) doped material forming larger (smaller) QDs than for undoped material. Alternative materials for quantum dots are Ge and Sn. These allow lower processing temperatures to be used, more compatible with underlying layers. Alternative matrices to SiO2 such as SiNx or SiC offer higher tunnelling probability and hence lower resistance. These alternative matrix materials can also be used as hetero interlayers to improve the transport in the growth direction whilst maintaining quantum confinement. Group IV alloys can also be used to modify band gap. GeC in particular looks to have useful band gap and sputtering properties. Such alloy materials could be used in hetero-junction or homojunction devices in combination with SiQD based materials to fabricate all thin film tandem cells.

Structural, electrical, and optical properties of as-grown and heat treated ultra-thin SnS films

The composition, structural, electrical, and optical properties of as-grown and heat treated tin-mono-sulfide (SnS) ultra-thin films have been studied. The ultra-thin SnS films were prepared on glass substrates by thermal resistive evaporation technique. All the SnS films contained nanocrystallites and exhibited p-type conductivity with a low Hall-mobility, < 50 cm 2/Vs. All these films are highly tin rich in nature and exhibited orthorhombic crystal structure. As compared to other films, the SnS films annealed at 300 °C showed a low electrical resistivity of ~ 36 Ωcm with an optical band gap of ~ 1.98 eV. The observed electrical and optical properties of all the films are discussed based on their composition and structural parameters. These nanocrystalline ultra-thin SnS films could be expected as a buffer layer for the development of tandem solar cell devices due to their low-resistivity and high absorbability with an optimum band gap.

Theory and simulation of photogeneration and transport in Si-SiO x superlattice absorbers

Si-SiOx superlattices are among the candidates that have been proposed as high band gap absorber material in all-Si tandem solar cell devices. Owing to the large potential barriers for photoexited charge carriers, transport in these devices is restricted to quantum-confined superlattice states. As a consequence of the finite number of wells and large built-in fields, the electronic spectrum can deviate considerably from the minibands of a regular superlattice. In this article, a quantum-kinetic theory based on the non-equilibrium Green's function formalism for an effective mass Hamiltonian is used for investigating photogeneration and transport in such devices for arbitrary geometry and operating conditions. By including the coupling of electrons to both photons and phonons, the theory is able to provide a microscopic picture of indirect generation, carrier relaxation, and inter-well transport mechanisms beyond the ballistic regime.

Intermediate Layers in Tandem Organic Solar Cells

AbstractTandem structures can boost the efficiency of organic solar cell to more than 15%, compared to the 10% limit of single layer bulk heterojunction devices. Design and fabricating of intermediate layers plays a very important role to achieve high device performance. This article will review the main experimental progresses of tandem organic solar cells, and focus on the intermediate layers (charge recombination layers) in both thermal evaporated and solution processed organic tandem solar cell devices.

Current status of the thermo-catalytic (hot-wire) CVD of thin silicon films for photovoltaic applications

Thermo-catalytic (TC) or hot-wire (HW) chemical vapor deposition (CVD) is a promising technique for growing amorphous and microcrystalline silicon films with improved stability and high rates. In this paper we report on the photovoltaic (PV) applications of thin silicon films deposited by this method. After a short review of the history of PV applications of TCCVD, from the beginning in 1993, the main part of the paper deals with our research and development of films and interfaces needed for the fabrication of different solar cell structures entirely by TCCVD. So far, our highest conversion efficiency is η=10.2% for a pin structure, containing only an intrinsic a-Si:H film deposited by TCCVD. Depositing the whole pin structure entirely by TCCVD, we have obtained η=8.8% until now. After development of a tunnel junction, the first tandem solar cell device, a stacked pin–pin structure has been recently produced showing η=7%. First attempts have been made for large-area deposition. In a 30×30 cm2 batch system, a-Si:H films with device grade photoelectronical properties and high thickness uniformity can be produced at a high rate of 6 Å/s. When incorporating i-layers from this system into pin solar cells, a conversion efficiency of η=(6.4±0.8)% was obtained on an area of 20×20 cm2. Finally, we report on silicon wafer-based solar cells, where a-Si, μc-Si and epi-Si film emitters were deposited by TCCVD showing conversion efficiencies up to η=15.2%.

Current transport in amorphous silicon n/p junctions and their application as ‘‘tunnel’’ junctions in tandem solar cells

Current transport in a-Si based n/p (‘‘tunnel’’) junctions is investigated using current-voltage- temperature and quantum efficiency measurements. Currents are nearly ohmic and temperature independent under typical solar cell operating conditions. Incorporating a thin a-Si(B) p+ layer between the n and p layers and replacing either a-Si layer with a microcrystalline layer improves the device by reducing the resistance and increasing the recombination. Light soaking improves the devices slightly. These results are consistent with a recently proposed recombination-tunneling model. Incorporating improved interconnect junctions in tandem solar cell devices improved the initial and stabilized performance.

Integrated tandem solar cells

The performance of integrated tandem solar cell (ITSC) devices made from pn homojunctions has been investigated. The intrinsic electrical interconnection present in an ITSC device places important restrictions on the choice of the energy gaps of the semiconductors to be used. Total conversion efficiencies are calculated and they are compared both with single solar cells and with independent tandem solar cell systems. It is concluded that ITSC devices are attractive solar energy converters provided that proper care is taken in the choice of materials used.