Symposium EN08 : Low-Cost Tandem Photovoltaic Cells

GaAsP / Si tandem cells represent a potential path to simultaneously achieve high efficiency and the requisite 25-year lifetime, both of which are necessary to attain low cost. However, threading dislocation densities (TDD) exceeding 10⁸ cm^-2 have historically limited the GaAsP top cell efficiency to <10%, which is too low to realize a tandem efficiency greater than that of Si alone. In this talk, I will discuss how we have lowered the TDD of GaAsP top cells on GaP/Si by controlling dislocation dynamics throughout the growth process to consistently achieve TDD values of 4-5×10⁶ cm^-2, slightly higher than in III-V metamorphic solar cells grown on GaAs. The lower TDD has enabled V_oc values of 1.16V for 1.68 eV GaAsP top cells grown on Si, while a well-designed anti-reflection coating has enabled an NREL-certified single-junction efficiency of 15.33%. Analysis of the spectral efficiency of these GaAsP top cells unambiguously shows that their performance is strong enough to allow tandem cells with higher efficiency than Si alone; the GaAsP top cells convert visible photons more efficiently than the best Si solar cells, a necessary precondition for a high-efficiency tandem. We have also demonstrated a GaAsP-filtered Si bottom cell that has undergone III-V growth with efficiency of ~5%. I will describe the materials and device design improvements that will be necessary to obtain efficiencies of 30% and provide an update on progress towards these goals.

The monolithic integration of high-performance III-V compound semiconductor materials and devices with low-cost, high-availability Si substrates remains a ‘holy grail’ of photovoltaics materials research, despite decades of effort. However, beyond merely a low-cost materials platform, III-V/Si multijunction solar cells utilizing active Si sub-cells—a “Si-plus” architecture—also hold the potential for high conversion efficiencies on par with pure III-V multijunction structures. One of the most promising approaches to this end is via direct heteroepitaxial GaP/Si integration and bandgap/lattice constant engineering via compositionally-graded GaAs_yP_1-y and/or Ga_1-xIn_xP alloys to reach III-V compositions that possess bandgaps ideally matched to the underlying Si. Substantial work by an international collaborative team led by The Ohio State University has, in recent years, delivered the first demonstrations of monolithic epitaxial GaAsP/Si dual-junction (2J) and GaInP/GaAsP/Si triple-junction (3J) solar cells. Continued effort toward optimization the 2J prototypes has brought ARC-projected efficiencies up to the point of competitions with conventional Si cells, with clear pathways for further improvement. This presentation will discuss recent progress in both epitaxial materials/process development and device development toward the production of high-performance GaAsP/Si 2J solar cells. Topics will include optimization and characterization of the GaP/Si integration process and GaAs_yP_1-y metamorphic grading, advanced Si bottom cell and GaAs_0.75P_0.25 top cell design and development, development and optimization of high-performance metamorphic tunnel junctions, and up-to-date results from fully-integrated tandem devices.

We report the development of GaInP/GaAs monolithic tandem solar cells grown by hydride vapor phase epitaxy (HVPE). HVPE is a route to reduced III-V growth costs because the technique takes advantage of low cost inputs (elemental metals and HCl) and high source utilization, and exhibits high growth rates (up to 5 µm/min). The tandem device consists of three main components: a 1.90 eV Ga_0.5In_0.5P top cell, a p-Ga_0.5In_0.5P/n-GaAs tunnel junction, and a 1.41 eV rear heterojunction GaAs cell. The open circuit voltage (V_OC) of the tandem is 2.41 V, indicating high material quality in both subcells, and voltage addition through the tunnel junction. Electroluminescence measurements indicate that the individual V_OC’s of the top and bottom cells are 1.40 and 1.01 V, respectively, at short circuit. This yields a band gap (E_G) voltage offset, W_OC = E_G/q-V_OC = 0.50 and 0.40 V, respectively, where q is the elementary charge. The W_OC of the top cell is higher in part because the structure contains an unpassivated front surface, and because the back surface field (BSF) consists of p+ GaInP rather than a higher- E_G heterobarrier. The top cell limits the current of this series-connected device for these reasons, with a short-circuit current density, J_SC, of 10.1 ± 0.2 mA/cm². The overall efficiency is 20.3 ± 0.4% (uncertified). We measured the device under concentration to look for signs of tunnel junction breakdown, which we did not observe up to ~1000x, which was the highest concentration tested. Importantly, our dynamic-HVPE reactor enables us to deposit this device, which requires multiple abrupt changes in composition and doping, in a short growth time. We discuss next steps to improve the current result, with a clear pathway towards 30% efficiency. The potential for terrestrial applications of this lower cost III-V technology is also discussed.

Efficiency of III–V solar cells has increased with the optimization of the device design [1], though the use of these cells is limited to space and high-concentration terrestrial systems owing to their high manufacturing cost using metal-organic vapor-phase epitaxy (MOVPE) growth. Therefore, it is crucial to develop low-cost, high-throughput growth techniques for implementing large-scale terrestrial modules operating at 1 sun. Hydride vapor-phase epitaxy (HVPE), in contrast to MOVPE, can reduce the cost due to a higher growth rate, use of cheaper group III metal sources, and the capability to grow crystals under low arsenic overpressure [2]. However, the properties of GaAs cells grown by HVPE are not understood well in contrast to those of MOVPE- and molecular beam epitaxy (MBE)-grown devices. In the present study, a comparison of HVPE and MBE growth for GaAs cells was studied. In HVPE, we fabricated the cell on p-GaAs(001) substrates in a custom-built, hot-wall reactor at atmospheric pressure. The quartz reactor tube was designed to have three chambers, two growth chambers and a preparation chamber, with a horizontal flow. Detailed reactor design and growth sequences have been described previously [3]. The temperature of the source and substrate region were set to 850 and 680 °C. For the growth of GaAs, the flow rates of HCl(Ga) and AsH₃ were 10 and 50 sccm, respectively, resulting in a growth rate of 14 μm/h. For the growth of InGaP, the flow rates of HCl(Ga), HCl(In), and PH₃ were 1.5, 20, and 50 sccm, respectively, resulting in a growth rate of 14 μm/h. The DMZn and H₂S were used for p- and n-type dopants. The cell consists of 200 nm-thick p-InGaP BSF / 2 μm-thick p-GaAs base / 100 nm thick n-GaAs emitter / 150 nm-thick n-InGaP window / 200 nm-thick n-GaAs contact layers. An identical cell structure was fabricated at 510 °C at a growth rate of 1 μm/h by MBE. A SiO₂/TiO₂ antireflection coating was deposited on the cell after the deposition of ohmic contacts. The cell size was 0.08 cm². The short-circuit current density, open-circuit voltage, fill factor, and efficiency was 23.59 mA/cm², 0.914 V, 0.768, and 16.56% for the HVPE-grown cell, which was found to be smaller than those of 26.81 mA/cm², 0.960 V, 0.848, and 21.82% for the MBE-grown cell. The anomarous extra inter layer was clearly observed at the heterointerface between GaAs base layer and InGaP BSF layer for HVPE-grown devices. As a result, recombination of photo-generated carriers was enhanced at the heterointerfaces, resulting in the reduction in the short-circuit current density. In addition, incorporation of iron atoms in wide range of the growth region for HVPE-grown cell, which may come from the reaction of HCl gas with the stainless-steel pipes, degraded its open-circuit voltage and fill factor. [1] F. Dimroth, et al., IEEE J. Photovolt. 6, 343 (2016). [2] J. Simon et al., J. Photovolt. 7, 157 (2017). [3] R. Oshima et al., Proceedings of IEEE PVSC 2017, Washington D.C.

Semiconducting nanowires have been recognized as promising materials for high-performance electronics and optics where optical and electrical properties can be tuned individually, where the nanowires due to excellent light absorbing properties [1] have been suggested for future high efficiency solar cells [2, 3]. Especially, the geometrical shape of the NWs offers excellent light absorption.
In order to further optimize the performance of NWPV, and integrate them on Si in a tandem junction configuration, nanowires with dimensions corresponding to optimal light harvesting capability are necessary. We developed nano imprint lithography for patterning of catalytic metal particles with a diameter of 200 nm in a hexagonal pitch of 500 nm, for which synthesis was redeveloped since the metal particles were found to move during annealing, destroying pattern fidelity before nucleation. We found that a pre anneal and nucleation step was necessary to keep the particles in place during high temperature annealing to remove surface oxides. We intend to transfer these grown nanowires to a Si platform (existing PV), either by direct growth on Si PV, or by nanowire peel off in polymer, followed by transfer and electrical contacting, or by aerotaxy and alignment for transfer to Si. The optimal band gap in combination with Si is about 1.7 eV, where we identify GaInP and GaAsP as materials for development of nanowire pn junctions by doping, the heart in a solar cell.
1. J. Wallentin et al. Science, 339, 1057 (2013)
2. N. Anttu et al., Phys. Rev. B 83, 165431 (2011)
3. J. Kupec et al., Opt. Express 18, 27589 (2010)
4. Åberg et al, IEEE J. of Photov, 6, 185 (2016)

In order for tandem-on-silicon solar cells to have impact, the top cell partner must be scalably manufacturable. This places constraints on both the overall cost per Watt at the cell level, and the fraction of that cost related to capital equipment expense (capex), which must be minimized for large-scale production. III-V compound semiconductors are promising tandem partners for silicon because of their ideal bandgaps, high radiative efficiencies and mature development. However, all previously proposed III-V tandem cells utilize epitaxial growth to define the cell and the high capital equipment and supply costs of epitaxy currently preclude the economic viability of these cells. In order to leverage the mature development of III-V devices without incurring the capex penalty of epitaxial growth, we have developed epitaxy-free processes for synthesis of GaAs and InP solar cells to serve as a tandem partner to silicon where III-V nanowire arrays are etched using liquid-phase metal assisted chemical or plasma etching process, and mechanically exfoliated from a bulk III-V crystal, enabling many cells to be generated from a bulk crystal source material. Electron and hole selective contacts and passivation materials are then grown using low-temperature, non-epitaxial techniques.

Because low cost, epitaxy-free designs do not permit use of standard epitaxially-grown III-V heterojunctions and window layers, we first developed a coupled optoelectronic model to understand the design space of the proposed solar cell using nontraditional heterojunction carrier-selective contacts. Using this model, we determined the optimal design to be a radial junction and discovered the absolute necessity of wide-bandgap carrier selective contacts due to the high density of photogenerated carriers throughout the nanowire. An optimized GaAs nanowire solar cell using TiO₂ and ZnTe heteropartners operating as a single junction is predicted to be 26.5% efficient with 1.04V V_oc and 28mA/cm² Jsc, and have a greater spectral efficiency than a typical silicon cell, recommending its use as a tandem partner.

We have also performed proof-of-principle experiments for each fabrication step of our epitaxy-free cell processes. Highlights to date for InP include developing a plasma etching recipe for defining InP nanowires and successful exfoliation and fabrication of complete nanowire cells from their substrates by embedding them in a polymer handle. In addition, a nanowire-on-wafer InP/TiO₂ heterojunction cell exhibits a 70% increase in photocurrent over its planar counterpart.
For GaAs cells, we have developed a room temperature and ambient pressure metal-assisted wet chemical etch for defining the nanowires, and have also exfoliated them from host wafer in a polymer handle. In addition, we have constructed ‘mini-cells’ demonstrating the non-epitaxial carrier selective contacts. A planar GaAs/CuI junction has demonstrated an open circuit voltage of V_oc = 750mV.

In order to surpass the Shockley-Queisser efficiency limit multijunction solar cells have been designed and developed for many years now. However, apart from band-gap engineering there are still many issues to consider. For instance, fabricating monolithic multijunctions is still challenged by various limiting factors such as lattice mismatching, high fabrication cost, and the size/weight of the tandem designs which makes them unsuitable for many applications. One way to tackle these issue is epitaxial growth of vertically standing semiconductor nanowires on the –mismatched, yet band gap compatible- ultrathin substrate. This design has many advantages: Not only the wires can overcome the lattice mismatching problem thanks to their intrinsic strain relaxation properties, they also create a natural anti-reflection coating. Besides, the periodic design of the wires adds unique optical features that are of great interest for PV applications. In particular, two aspects have been the focus of this design to improve solar energy conversion: Waveguiding properties in each wire and the grating properties of the periodic structure of such wires.

In this work, we study light-matter interactions in GaAs-based nanowire arrays on ultrathin silicon films with the dual goal to obtain large absorption in the array and to improve light trapping in the bottom thin film cell. There are two functions for the top cell here. The first one is not to lose absorption efficiency in the cell by using a thinner layer of absorbing material (which reduces the cost, size and weight of the solar cell). To do so, the geometry of wires (the radius and height) has been optimized so that the incoming light is coupled to the waveguiding modes of the wires. In other words, the intrinsic waveguiding property of high refractive index wires will make them absorb the light with much larger absorption cross section with respect to the geometrical cross section. The second function of the top cell is to help the bottom cell to absorb more efficiently as well. The bottom cell is an ultrathin silicon slab which can be viewed as a 2D waveguide in which the electromagnetic field is bounded in one direction and is forced to propagate in the other two. By optimizing the grating geometry (the pitch of the wires) for the wavelengths close to the bandgap of silicon –where the absorption coefficient is very low- the transmitted light’s momentum is manipulated to match the momentum of the waveguiding modes of the slab.

To conclude, by optimizing the geometry of both each wire and the grating we are able to firstly couple the light into waveguiding modes of each wire and later couple the transmitted/scattered light into waveguiding modes of the ultrathin silicon layer underneath. By combining these 1D and 2D waveguiding properties a high efficiency ultrathin and flexible tandem cell is designed.

Combining perovskites with silicon is a particularly attractive option for producing cheap, high efficiency and high voltage solar cells. Perovskite-on-silicon tandem solar cells can potentially achieve over 30% tandem efficiency, which greatly surpasses the current record of 25.6% for the single silicon cell.

We demonstrate a 4-terminal tandem configuration in which the efficiency is as high as 26%, which is the highest efficiency that has been achieved with a potentially low cost process [1]. We also demonstrate a two-terminal monolithic tandem device with a perovskite top subcell and a high-temperature tolerant homojunction c-Si bottom subcell, with an efficiency of over 22% [2]. Several developments have been instrumental in achieving these high efficiencies. The first is development of a combined grid/transparent conducting oxide back contact for the top subcell of 4-terminal tandems, which allows over 80% of long wavelength light to pass through to the silicon solar cell underneath [3]. Incorporation of rubidium in a multi-cation perovskite improves both efficiency and stability [4]. Doping the compact TiO₂ layer with indium also improves conductivity and band alignment [5]. Finally we have also found that a combination of PMMA and PCBM can provide effective interface passivation for perovskite solar cells [6]. These results show the clear potential of perovskites in practical high efficiency tandem devices.

[1] The Duong et al, Rubidium Multication Perovskite with Optimized Bandgap for Perovskite-Silicon Tandem with over 26% Efficiency, Advanced Energy Materials, 7, 14, 1700228 (2017).
[2] Yiliang Wu et al., Monolithic perovskite/silicon-homojunction tandem solar cell with over 22% efficiency, Energy and Environmental Science, in press (2017).
[3] The Duong et al, Semi-transparent perovskite solar cell with sputtered front and rear electrodes for a four-terminal tandem, IEEE Journal of Photovoltaics 6 (3), 679-687 (2016).
[4] The Duong et al, Structural engineering using rubidium iodide as a dopant under excess lead iodide conditions for high efficiency and stable perovskites, Nano Energy, 30, 330 (2016).
[4] Jun Peng et al, Interface engineering of high performance perovskite solar cells and perovskite-silicon tandems, Advanced Energy Materials, 10.1002/aenm.201601768 (2016).
[6] Jun Peng et al, Interface passivation using ultrathin polymer-fullerene films for high-efficiency perovskite solar cells with negligible hysteresis, Energy and Environmental Science, DOI: 10.1039/C7EE01096F (2017).

As photovoltaic module manufacturing costs have plummeted, improving the power conversion efficiency has emerged as the best way to reduce the cost of installed solar power. Metal halide ABX₃ perovskite solar cells benefit from versatile processing - they have been demonstrated by roll-to-roll-compatible processes using both solution and vapor phase deposition. They can be compositionally tuned to have band gaps appropriate to both top and bottom subcells in a tandem. All-perovskite tandem solar cells, therefore, can surpass the Shockley-Queisser efficiency limit without requiring cost-prohibitive epitaxial growth methods needed for III-V solar cells.

We recently demonstrated the first monolithic perovskite tandems based on a tin lead perovskite as the bottom cell. With 50% tin and 50% lead at the B-site, a band gap of 1.27 eV is attained. A 1.8 eV band gap lead-based mixed halide perovskite is used as the top cell. A sputtered ITO interlayer functions as the recombination layer connecting the subcells and serves to protect the first cell from solvents used to deposit the second cell. This proof-of-concept device shows a stabilized efficiency of 17%. We present a roadmap for development of all-perovskite tandems to reach significantly higher efficiencies than single junction devices.

Perovskite tandems so far have been hampered by poor quantum efficiency in the infrared. Depositing high quality optically thick tin lead perovskite films has proved challenging. In addition, processing the second cell by methods that do not damage the first cell is necessary for high yield over large areas. There is room for improvement in open circuit voltage of the top cell - larger voltage losses have typically been observed for wider band gaps that are needed to match high-performing low gap perovskites.

Tin-lead perovskites for the bottom cell already attain excellent open circuit voltage relative to band gap. We show that there is further room for improvement - the photoluminescence quantum efficiency is far below typical values for lead perovskites, and improving this should yield voltage losses smaller than in the best crystalline-Si solar cells.

Optical modeling combined with device simulations fit to experimental JV curves shows that perovskite tandems can feasibly reach 32% efficiency. Energy yield modeling shows they can achieve over 42% improvement over a single junction silicon solar cell in both sunny and overcast climates. Cost models of thin-film solar manufacturing show that substrate costs outweigh materials costs for the absorber. Depositing two absorber stacks to make a tandem is not expected to add much cost. Perovskite tandems, therefore, show great promise for low-cost high-efficiency photovoltaics. This presentation aims to identify the potential of all-perovskite tandems and discuss strategies to tackle the challenges that must be overcome in order to realize this promise.

Perovskite tandem solar cells are highly desirable to realize power conversion efficiencies (PCEs) beyond the single junction Shockley-Queisser (SQ) limit. Previously we employed an integrated approach to successfully realize highly efficient, bandgap-matched perovskite-perovskite tandem solar cell with 18.5% PCE and high photovoltages (~2 V) reaching 80% of theoretical limit.¹ It is imperative to note that this performance of perovskite-perovskite tandem is limited by the photovoltage loss in individual subcells. Photovoltages (V_oc) of the best performing small bandgap (~1.2 eV) and large bandgap (~1.8 eV) perovskite solar cells (PVSCs) are currently limited to 88% and 80% of the SQ limit. Efforts to improve photovoltages of small and large bandgap PVSCs to higher values are critical for further advancement of perovskite tandems.

Herein, we improve the optoelectronic quality of small bandgap perovskites via defect passivation through incorporation of fluoroalkyl-substituted fullerene via a graded heterojunction structure. Remarkably high V_oc ~ 0.89 V was realized for corresponding small bandgap (~1.2 eV) PVSCs, which is ~92% of the SQ limit, comparable to the state-of-the-art inorganic technologies and is the best among PVSCs reported till date.² On the other hand, for large bandgap perovskites, through engineering material characteristics via composition of organic cation, we simultaneously improve optoelectronic quality and alleviate instabilities due to light-induced phase segregation. The resultant large bandgap (~1.8 eV) PVSCs had much improved V_oc around 1.3-1.35 eV, which corresponds to 85-87% of the SQ limit and is a significant enhancement with respect to the current state-of-the-art. The above realized improvements in photovoltages of small and large bandgap devices will be pivotal for unleashing the complete potential of perovskite-perovskite tandems and get past the PCE of single junction PVSCs.

References:
[1] Rajagopal, A. et al. Highly Efficient Perovskite-Perovskite Tandem Solar Cells Reaching 80% of the Theoretical Limit in Photovoltage. Adv. Mater. 29, 1702140 (2017).
[2] Rajagopal, A. et al. Defect Passivation via a Graded Fullerene Heterojunction in Low-Bandgap Pb–Sn Binary Perovskite Photovoltaics. ACS Energy Lett. 2531–2539 (2017).

Metal halide perovskite solar cells are rapidly approaching performances that can rival those of crystalline silicon. After only 5 years of intensive research, the record certified perovskite research solar cell efficiency is 22.1%, while the record certified multi-crystalline silicon cells are at 21.9%, which is the dominant commercially deployed PV technology. For the most advanced c-Si concepts, the last efficiency gains are being squeezed out, with efficiencies approaching 27%. Although improvements in perovskite solar cell efficiency can be expected over the next few years, single junction perovskite solar cells will always be limited to performances near or only slightly better than c-Si. Mainstream PV module manufacturing costs have continued to diminished so extensively over the last decade, that now the cost of the module amounts to less than half the overall solar PV installation. Most of the non-module costs, referred to as the balance of systems (BoS), scale with area of deployed PV rather than power generated. Therefore, increasing the overall power output of the module per unit area, i.e. efficiency, is the surest means to continue to drive down the overall cost installed PV generated electricity. Therefore, we need to develop a strategy and road map, which will lead perovskite solar cells to much higher efficiency than c-Si.

Here I will present a combination of both experimental and theoretical work on multi-junction perovskite solar cells. I will demonstrate how efficiencies significantly in excess of 30% will be achievable, and I will present experimental work progressing towards this goal.

Most modern solar cell architectures utilize electron/hole collecting contacts in order to efficiently collect photogenerated carriers. In the case of Si heterojunction cells, the carrier collecting contacts are heterojunctions stacks which block the collection of one carrier while optimizing the collection of the other. However, the resistive losses incurred by the heterojunction stacks are not been completely understood, as the description of transport requires further investigation. These resistive losses are detrimental to the overall fill factor (FF) of the solar cell.

The transmission line method (TLM) is often used to characterize the resistive losses of contacts by measuring the contact resistivity. This method allows us to analyze a simpler device structure whilst capturing all the representative physics of the contact stack. Previously, we conducted experiments using the TLM technique to measure the contact resistivity of a contact stack that consisted of n-type indium tin oxide (ITO), p-type amorphous silicon (a-Si(H):p) and intrinsic amorphous silicon (a-Si:(H):i) on top of an n-type crystalline silicon (c-Si:n) wafer [1]. In this work we perform numerical simulation using Silvaco ATLAS of the TLM method and calculate the contact resistivity for the ITO:n/a-Si(H):p/a-Si(H):i/c-Si:n contact stack in comparison to the experimentally extracted values. By recreating the TLM structure in simulations, we can explore the contributions of the different layers and interfaces that are present in the contact stack. This methodology can also be applied to study contact stacks comprised of different materials.

We simulated the contact resistance for a contact stack with increasing a-Si(H):i layer thickness and obtained a good match with experiments when we explicitly model the ITO as a semiconductor (as opposed to a simple metal contact). For an increase in intrinsic layer thickness from 4 to 16 nm, we observed an increase in contact resistivity from 0.5 to 0.98 Ω cm². The contact resistance increased with a decrease in emitter doping as low dopings caused the emitter to deplete; hence increasing the resistance of the layer. Simulations treating the ITO as a metallic contact gave poor agreement with experiment in contrast. The treatment of ITO as a degenerate widebandgap semiconductor adds another layer of detail to the transport picture as the ITO:n/a-Si(H):p forms a reverse biased pn junction when the cell is forward biased. Our simulations indicate that tunneling is a dominant mechanism at ITO/emitter interface. Finally, we also explore the behavior of contact resistance under different temperatures and illuminations in comparison with experiment.

References :
[1] M. Leilaeioun, W. Weigland, P. Muralidharan, M. Boccard, D. Vasileska, S.M. Goodnick and Z. Holman, "TLM measurements varying the intrinsic a-Si:H layer thickness in silicon heterojunction solar cells", PVSC 2017.

Monocrystalline CdTe cells grown epitaxially from lattice-matched InSb substrates have recently been demonstrated with record V_ocs over 1.1 V and active-area efficiencies of 20%. These cells are attractive alternatives to III-V and perovskite materials for top cells in silicon-based tandems, but their bandgap—at 1.5 eV—is slightly too low for optimal pairing, particularly in a two-terminal configuration. Alloying with 13% Mg increases the bandgap to the desired 1.7 eV, and we have just submitted a manuscript detailing a 11.2%-efficient MgCdTe cell. The front-surface reflectance ranks amongst the largest power losses in this cell: 2.0 mA/cm² is available to be gained if it can be eliminated. Scattering light by texturing the surface is not an option, as the layers are thin and epitaxially grown on a single-crystal substrate, and thus multi-layer anti-reflection coatings are the most promising route to boost current.

This contribution investigates single- and double-layer anti-reflection coatings via both simulation and measurement. SiO₂, Al₂O₃, and MgF₂ were studied as candidate materials and optimal thicknesses were found using the transfer-matrix method. Fabricated single-layer MgF₂ coatings on a MgCdTe solar cell exhibited less than 3% reflectance across the spectral region of interest, boosting J_sc in this particular cell by over 2 mA/cm². When applied to hero 1.7-eV MgCdTe devices, the best anti-reflection coatings resulted in a 15.2%-efficient cell, which is a new world record for this material.

Wide bandgap Cd_1-xZn_xTe and Cd_1-xMg_xTe have drawn attention as top cells in tandem devices. These materials offer flexibility of tuning the band gap over a wide range by controlling the Zn (or Mg) concentration in CdTe with little alteration to its properties. Historically, CdS has been extensively used as a heterojunction partner for CdTe based devices. However, use of CdS as a window material has shown many drawbacks in these devices. Due to the small band gap, photons with higher energy wavelengths (> 2.4 eV) absorbed by CdS do not contribute to the photocurrent of the completed devices. Another drawback is that, CdS forms an energetic “cliff” in the conduction band at the interface between CdTe and CdS resulting in increased recombination, which reduces the open circuit voltage, fill factor, and overall device efficiency. Since the wider bandgaps of Cd_1-xZn_xTe and Cd_1-xMg_xTe are mostly formed due to an increase in energy of the conduction band relative to CdTe, this cliff with CdS will be even larger in these devices, indicating that a CdS hetero-partner for these devices can severely degrade the performance.
Recent work shows that appropriate band alignment at the interface between the window layer and the absorber results in significant improvements in device performance. While using wide band gap oxides to create this energetic “spike” at the window layer-absorber interface yields these advances, the interface between the transparent conducting oxide and window layer also needs to be optimized to allow barrier free electron flow to the front contact. In this study, we will use SCAPS 1D software to model the wider band gap Cd_1-xZn_xTe and Cd_1-xMg_xTe devices to determine the appropriate alignment between the absorber and the window layer. We will also investigate how the material properties of TCO and window layer will affect the front contact alignment to determine the optimized device structure for high efficiency Cd_1-xZn_xTe and Cd_1-xMg_xTe.

Polycrystalline gallium indium phosphide (Ga_xIn_1-xP or GaInP) alloy is a promising material system for the top junction of a silicon tandem solar cell, since it has a tunable direct bandgap from 1.35 eV to 2.2 eV. It offers an interesting alternative to the most commonly studied CIGS, CdTe and perovskite alloys for several reasons. Epitaxially grown GaInP alloys have achieved single junction 1-sun efficiencies of over 20%, proving their potential for high efficiencies and stability at room temperature. This is important, since other promising alloys with higher defect tolerance are still trying to overcome this challenge. The band structure of GaInP alloy system along with its various III-V alloys is also well studied for epitaxial growth. This knowledge would be extremely valuable for defect engineering.
This work focuses on optimizing the growth parameters of various polycrystalline GaInP alloys, quantifying the defect characteristics and eventually demonstrating the polycrystalline GaInP solar cells. GaInP alloys with several gallium compositions were grown at various substrate temperatures using molecular beam epitaxy (MBE). Epitaxial growth on polycrystalline substrates was carried out to isolate the effect of interface and surface from that of the grain boundaries. MBE was particularly used to limit impurity segregation at the grain boundaries. This is expected to help better associate composition and structure variation with the grain boundary characteristics. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the structural properties, and photoluminescence (PL) spectroscopy was used to assess the optical properties of the grown materials.
XRD was used to calculate the lattice parameters and also confirmed the zincblende crystal structure of the GaInP alloys. Grainsize of 0.5-1μm was characterized using SEM. It is observed that the grainsize and its distribution increases with temperature. Contrarily, the grainsize is found to reduce with higher gallium incorporation. Photoluminescence spectroscopy showed a higher peak intensity for the 10% gallium alloy compared to the 0, 5 and 50% gallium alloys. This result is promising since it suggests a better optoelectronic property of polycrystalline material with bandgap more than 1.43 eV. The discovery has reinforced the promise of this new family of polycrystalline material system to be considered for top junction of silicon tandem solar cells.
Once the growth conditions for various compositions are optimized, temperature and power dependent PL will be used to characterize the nature of recombination in the material system. Time resolved PL and admittance spectroscopy will be used to quantify the minority carrier decay lifetime and defect energy state density respectively. Surface potential across grain boundaries will be measured using Kelvin probe force microscopy. Finally, demonstration of polycrystalline GaInP solar cell will also be presented in the article.

Incorporating the two most widely used solar cell materials, II-VI/Si tandem cells based on CdTe and silicon have the potential to exhibit high efficiencies at a low cost. Silicon especially proves to be an attractive bottom cell material due to its ideal bandgap of 1.12 eV and already well-established industry. Unfortunately, during the deposition of CdTe and other II-VIs on silicon, the silicon lifetime can degrade. The degradation has also been seen in other groups depositing III-V materials on silicon, as well as across various deposition methods. The exact nature of this degradation and the extent to which it occurs is not yet fully understood.
To this end, we are studying the degradation mechanisms in the silicon on bare wafers, wafers with CdTe deposition, and wafers with IZO and CdTe deposition. A contaminant-free rapid thermal annealing system has been used to mimic in bare silicon the temperature profile during CdTe deposition, demonstrating no discernable degradation in the bare silicon between 350 and 600 C at 3 minute anneals. An HF-nitric-acetic clean was done for 5 minutes with a 10-minute rinse before annealing, as well as before 25 nm of a-Si deposition to measure lifetime. N-type silicon lifetimes ranged from 1 to 2.4 milliseconds, with non-annealed wafers all falling within the same range.
We will report on the degradation of silicon bulk lifetime with CdTe deposition as a function of temperature from 350 to 600 C. Higher temperatures typically result in greater silicon lifetime degradation, yet also may be required for the best CdTe, MgCdTe, and ZnCdTe top cells. The primary suspects in lifetime degradation are the metallic impurities Cd and Te, and, if present, Mg and Zn. The surface recombination velocity is passivated by removing the II-VI films and depositing a-Si:H to form a silicon heterojunction; the separation of surface and bulk recombination components will be reported. To test the extent that impurities have diffused into the bulk wafer, we will additionally etch off 5-40 microns of silicon on each side, and remeasure the silicon lifetime.
Secondary ion mass spectroscopy (SIMS) analyses will also be presented to quantify the concentration and depth of Cd, Te, and other impurities in the silicon wafer. Deep-level transient spectroscopy (DLTS) will give information on the energy level, distribution, and cross section of the recombination centers created by these defect atoms, aiding in their identification and the understanding of the lifetime degradation mechanism.
Finally, the introduction of the IZO layer between the silicon and cadmium telluride during deposition is expected to act as a diffusion barrier and help preserve the silicon lifetime. The extent of this benefit will be quantified using the characterization techniques listed above. The knowledge gained from this study will help in the ultimate goal of creating a high-efficiency, low-cost II-VI/Si tandem solar cell.

Cadmium telluride (CdTe) alloy solar cells have seen a rise of more than 5% in the absolute conversion efficiency in the past five years. This rise is mainly credited to the development of cadmium chloride post deposition treatment (PDT). This treatment has shown to passivate the grain boundaries and aid in surface reconstruction. Though PDT has been greatly successful for single junction CdTe cells, increase in the efficiencies of magnesium and zinc alloys of CdTe – which have a higher bandgap – is yet to be realized. Also, though the macroscopic effect of the post deposition treatment is consistent and well agreed upon, the effect it has on grain boundary structure and the mechanism of surface reconstruction is still an active area of debate and research.
In this work, we have focused on understanding the effect of various temperatures used for PDT on the grain boundary potential and conductance. Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), with a tip size of less than 25nm, is used to probe the local characteristics.
The KPFM and C-AFM studies carried out confirmed that PDT affects the electronic properties of the grain boundaries. At lower temperatures, it is observed that the potential at the grain boundaries is higher than that at the grain core. As PDT temperature increases, the potential difference between the grain boundary and the grain core is observed to increase and reach a maximum value. Increasing temperature further shows an abrupt decrease in the grain boundary potential to values lower than that at the grain core. The conductance of grain boundary appears to follow an exactly inverted behavior compared to the potential variation. It is observed that the tipping point at which the potential difference at the grain boundary is the maximum, is also the temperature at which we observe maximum rise in efficiency of the solar cell.
This study creates pathways to better understand the mechanism of increase in efficiency of CdTe solar cells due to the post deposition treatment. A similar study on magnesium and zinc alloys of CdTe and attempts to engineer potential at the grain boundaries will also be presented in the final article.

CdTe-based thin film solar cells occupy the largest market share among the commercially produced thin-film solar cells. CdTe has the advantages of a direct bandgap of 1.45 eV at room temperature and a large absorption coefficient. A II-VI/Si based Tandem solar cell shows great promise, as modeling indicates that efficiency greater than 30% in practical cells is possible, while at a comparable cost per watt with CdTe and Si single-junction alternatives.
In this study we focus on the temperature effects of the interlayers that connect the II-VI top cell and the Si bottom cell in tandem solar cells. CdTe is used as the top cell in these experiments. We analyze how the high temperature deposition of CdTe affects each interconnection layer separately, and integrated in the full tandem stack, in terms of their electrical, optical characteristics and the morphology. In the tandem structure under study, the TCO acts as a vertical conduction layer between the Si and CdTe layers, and ZnTe acts as a back-surface field for the CdTe cell. Specific contact resistance(ρ_c) and sheet resistance(R_s) of the TCO layers are measured through TLM patterning measurements. Transmittance, reflectance, absorptance, refractive index, and haze will be reported before and after the thermal treatments that simulate the II-VI top cell growth on Si, in a substrate configuration rather than the conventional superstrate CdTe cell structure. Optical properties of individual interlayers as well as of the stacked layer combination will be discussed. Test structures used for the analysis of the TCO interlayer were n+ diffused n-type and p-type Si wafers. ITO layers of 50 nm were deposited in a TLM pattern through a shadow mask, followed by Ag contacts to the ITO formed through the same mask, to measure ρ_c between the ITO and Si layers. The same process was followed for the deposition of the IZO layer. Two runs were made with different O₂ concentration in the DC sputtered TCO layers. The measured R_s for the ITO and IZO are 82 and 115 Ω/sq respectively. The samples were subjected to rapid thermal annealing at 450^○C for 3 minutes. After annealing, the sheet resistances measured were 41 and 61 Ω/sq for the ITO and IZO respectively. Additionally, it was observed that the contacts did not exhibit ohmic behaviour after the anneal, and that the TCOs deposited under higher O2 concentration are more resistive after the anneal.
A full study of the I-V characteristics at ZnTe/TCO and TCO/doped Si interfaces will be presented, including fluorine-doped tin oxide (FTO), and their evolution with temperature exposures typical for II-VI cell deposition. The transmittance and absorptance of optical stacks from these interlayer materials will also be discussed as a function of the II-VI growth temperature.
Temperature studies of the interlayers in monolithic II-VI/Si tandem cells will facilitate a better understanding of this promising low-cost, high-efficiency multijunction cell architecture.

Cu₂ZnSnS₄ (CZTS) is interesting for tandem solar cell application due to its bandgap of 1.5 eV and possibilities for further bandgap increase through alloying. In recent years significant efforts has been devoted to investigate the properties of the CZTS solar cell largely on the molybdenum (Mo) back contact resulting in a record efficiency of 10%¹. However, there have been very sparse studies made on the transparent back contacts (TBC), which are essential for both tandem and bifacial solar cell structures^2,3. Recent investigations have reported an efficiency of 2.9 % and 2.1 % for CZTS deposited on transparent ITO and FTO back contact respectively^2,4. The high temperatures involved during the sulphurization of the CZTS absorber results in the diffusion of atoms from the TBC into the CZTS layers thereby degrading the device performance⁵. The use of a barrier interlayer between the TBC/CZTS was shown to have a beneficial effect on the device performance³.
This work investigates the role of the molybdenum oxide (MoO_x) interfacial layer between the transparent FTO electrode and the CZTS absorber. Commercially available FTO glass, hereafter referred as FT1, with a sheet resistance of 13 Ω/o was used as the substrate. Additionally two FTO substrates, namely FM1 and FM2, were coated with an interfacial MoO_x layer with a thickness of 5 and 40 nm respectively. The CZTS precursors were sputtered using binary metal targets of CuS, ZnS, SnS in argon environment followed by annealing in sulphur (S) atmosphere at 580 °C to form the absorbers. Analysis using secondary ion mass spectroscopy (SIMS) shows an out diffusion of fluorine (F) atoms from the FTO into the CZTS absorber. Furthermore, the concentration of F atoms in the CZTS increased as a function of MoO_x thickness. From the current-voltage (IV) characteristics, it was observed that the samples FT1 and FM2 were shunted with no light-to-electricity conversion. However, the presence of thin MoO_x interlayer, sample FM1, had a beneficial effect on the device performance resulting in an efficiency of 3.5 %. Experiments are underway in order to optimize and understand how the annealing time and temperature of the sulphurization process effects the back contact/CZTS interface and its implications on the overall performance of the devices.
Reference
¹ M.A. Green, Y. Hishikawa, W. Warta, E.D. Dunlop, D.H. Levi, J. Hohl-Ebinger, and A.W.H. Ho-Baillie, Prog. Photovolt. Res. Appl. 25, 668 (2017).
² J. Ge, J. Chu, J. Jiang, Y. Yan, and P. Yang, ACS Appl. Mater. Interfaces 6, 21118 (2014).
³ M. Espindola-Rodriguez, D. Sylla, Y. Sánchez, F. Oliva, S. Grini, M. Neuschitzer, L. Vines, V. Izquierdo-Roca, E. Saucedo, and M. Placidi, ACS Sustain. Chem. Eng. (2017).
⁴ S. Mahajan, E. Stathatos, N. Huse, R. Birajdar, A. Kalarakis, and R. Sharma, Mater. Lett. 210, 92 (2018).
⁵ J. Ge, J. Chu, J. Jiang, Y. Yan, and P. Yang, ACS Sustain. Chem. Eng. 3, 3043 (2015).

Monolithic integrated thin film tandem solar cells consisting of a high bandgap perovskite top cell and a low bandgap thin film bottom cell are expected to reach higher power conversion efficiencies (PCEs) with lower manufacturing cost and environmental impacts than the market-dominated crystalline silicon photovoltaics. There have been several demonstrations of 4-terminal and 2-terminal perovskite tandem devices with CuInGaSe₂ (CIGS) or CuInSe₂(CIS). However, these devices employed the CH₃NH₃PbI₃ top cells with a bandgap of 1.55 eV, which is not optimal for the tandem configuration. Further advance will be enabled by tuning the bandgap and thickness of the perovskite absorber to maximize the photocurrent limited by the current match condition. Here, we systematically study the optical absorption and transmission of perovskite thin films with varying absorber layer thickness and band gap and demonstrate high efficiency bifacial transparent perovskite devices. Based on these results, we model the photocurrent generations in both perovskite and CIS subcells and estimate the performances of projected tandem devices by integrating the fabricated perovskites on an ideally functioning CIS device. Our results show that optimal bandgap for the perovskite layer is 1.7 to 1.8 eV, and the optimal thickness is 300 to 500 nm, depending on the bandgap of the perovskite film. With these configurations, PCEs above 20% could be achieved by monolithically integrated perovskite/CIS tandem solar cells.

The highest confirmed efficiency of single-junction solar cells under standard conditions (1-sun AM1.5G spectrum at 25 ^oC) has remained at 28.8 % for the past 5 years [1, 2]. Although improvements towards reaching the Shockley-Queisser limit (33.5 %) [3, 4] are possible with a single-junction structure, tandem/multi-junction solar cells have demonstrated a pathway to achieving higher efficiencies, e. g., the current 1-sun solar conversion efficiency record, 38.8 % was made by Spectrolab with a 5-junction bonded cell [5], and under concentrated illumination, 46.0 % was demonstrated by a 4-junction bonded cell [6]. However, the cell cost is a key factor for deployment to impact real-world energy production. Si-based tandems are good candidates for low-cost high-efficiency solar cells. They have achieved 31–36 % by mechanical stacking or wafer bonding techniques [2]. A monolithic approach is preferred in terms of cost and manufacturability, since it only requires a single substrate, and fewer growth and processing steps.

Dilute nitride GaNP can be grown lattice-matched to Si, and is a candidate absorber for making monolithic high-efficiency solar cells. We are theoretically and experimentally investigating the properties of GaNP relevant to photovoltaics. Here we report on the theoretical aspects of GaNP/Si tandems using a multi-scale approach starting with the electronic structure and phonon dispersion of GaNP using semi-empirical tight binding calculations. They are then fed into a Monte Carlo simulator to calculate the transport properties. The optical absorption is calculated from the electronic structure, which is poorly understood for the dilute nitrides. These calculated properties will later be input into a commercial device package (Silvaco ATLAS) to simulate the performance of the tandem structures.

[1] BM Kayes, H Nie, R Twist, SG Spruytte, F Reinhardt, IC Kizilyalli, GS Higashi, "27.6% Conversion efficiency, a new record for single-junction solar cells under 1 sun illumination", Proc. 37th IEEE Photovolt. Spec. Conf., Jun. 2011.
[2] MA Green, Y Hishikawa, W Warta, ED Dunlop, DH Levi, J Hohl-Ebinger, AWY Ho-Baillie, "Solar cell efficiency tables (version 50)", Prog. Photovolt. Res. Appl. 25, 2017.
[3] W Shockley, HJ Queisser, "Detailed balance limit of efficiency of p-n junction solar cells", J. Appl. Phys., Vol. 32, Mar. 1961.
[4] OD Miller, E Yablonovitch, SR Kurtz, "Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit", IEEE J. Photovolt., Vol. 2, No. 3, Jul. 2012.
[5] PT Chiu, DL Law, RL Woo, S Singer, D Bhusari, WD Hong, A Zakaria, JC Boisvert, S Mesropian, RR King, NH Karam., “35.8% space and 38.8% terrestrial 5J direct bonded cells”, Proc. 40th IEEE Photovolt. Spec. Conf., Jun. 2014.
[6] Press Release, Fraunhofer ISE, Dec. 2014 (https://www.ise.fraunhofer.de/en/press-media/press-releases/2014/new-world-record-for-solar-cell-efficiency-at-46-percent.html)

CdTe solar cell technology holds immense potential for performance improvement through enhanced voltage output. When a carrier concentration exceeding 5x10¹⁶ cm^-3 can be effected with adequate minority carrier lifetime (1-10 ns), open circuit voltages (V_OC) more than 1 V are possible, higher than competing thin film and multi-crystalline silicon solar cell technologies [1]. Consequently, in-situ substitutional p-type doping of CdTe solar cells processed using vapor transport deposition film growth technique was explored for P, As and Sb, and dopant densities exceeding 10¹⁶ cm^-3 were reported [2]. This process requires a post-deposition treatment (PDT) to electronically activate the dopant. We explore the process space by varying the time, temperature, and ambient atmosphere of the PDT and report the effect on V_OC, N_CV, device behavior.
In this work, a combination of conventional PDT techniques common to CdTe material system are explored for devices made with Sb-doped CdTe films. Devices prepared from doped films processed in identical deposition sequences are subject to a matrix of different annealing conditions with varying temperature, ambient and duration. The effect of the different treatment variables on doped devices will be evaluated by studying the doping efficiency, computed using capacitance-voltage (C-V) and secondary-ion mass spectroscopy (SIMS) measurements, and density of states from admittance spectroscopy measurements. Density of responding charge from C-V (N_CV) vs W at 0V DC bias follows the trend expected for an abrupt p-n junction with uniform doping, giving us good confidence that N_CV represents free carrier concentration.
N_CV in devices annealed in an air ambient exceeded the Ncv for devices annealed in CdCl₂ by more than an order of magnitude. These cells had the highest N_CV among all the devices in the anneal matrix, exceeding 1x10¹⁶ cm^-3. In general, we find that the presence of air during the PDT allows for higher doping and higher short circuit current, while CdCl₂ is required for high open circuit voltages. Nearly equivalent doping was achievable with or without the presence of CdCl₂. Open circuit voltages greater than 685 mV were measured on the devices treated with CdCl₂ and short circuit currents as high as 23 mA/cm² were obtained for those treated in an air ambient with CdCl₂ coating. It is important to note that no Cu passivation was used to passivate the carbon-ink back contact. Typically Cu passivation increases V_OC by 150-200mV. Further details of the anneal sequences performed, and the performance of the resulting devices will be discussed in the report.
[1] Brian McCandless, “CdTe Solar Cells: Processing Limits and Defect Chemistry Effects on Open Circuit Voltage,” Conf. Rec. 2013 MRS, MRS-13-1538-C13-04.R1, 2013.
[2] Brian McCandless et. al. “Enhancing p-type Doping in Polycrystalline CdTe Films”, IEEE Photovoltaics Specialists Conference, 2017.

We investigated synthetic strategies for the functionalization of Si(111) surfaces with organic species containing moieties were covalently bonded to silicon and ionically coupled to solar-relevant perovskite thin films. X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy characterized the bonding and packing of covalently bound molecules to the silicon surface, while microwave photoconductivity experiments quantifed the interfacial defect density due to surface oxides. Atomic force microscopy (AFM) quantified perovskite–silicon adhesion, and nonaqueous photoelectrochemistry explored solar-energy-conversion performance. Adhesion forces/interactions between the perovskite and the organic-functionalized films were comparable to the interaction between the perovskite and native-oxide silicon surface. Photoelectrochemistry of perovskite thin films on organic-functionalized n⁺-Si showed significantly higher V_oc than n⁺-Si with a native oxide when in contact with a nonaqueous ferrocene^+/0 redox couple. We discuss the present results in the context of utilizing molecular organic recognition to attach perovskites to silicon utilizing organic linkers so as to inexpensively modify silicon for future tandem-junction photovoltaics.

Currently, the key issues in the solar cell market are reducing price and improving efficiency of the device. The crystalline silicon solar cell, which account for more than 90% of the total solar cell market, are already limited in terms of the high efficiency, reaching the theoretical limit. Also, in case of Al-BSF p-type silicon solar cells, it has an excellent price of 0.2$/Wp. There is a tandem solar cell as a way to improve the limited cell efficiency of a standard silicon cells to a more high efficiency. By joining the top cell with high bandgap and the bottom cell with low bandgap, the high efficiency can be achieved. In particular, perovskite solar cell, which has reached the highest efficiency of 22.1% in recent year, is an ideal material for tandemization with silicon solar cells because it is easy to control the bandgap by adjusting the chemical composition of perovskite. According to reported simulation data, the highest theoretical efficiency of perovskite/silicon tandem solar cell is expected to be over 30%.
Thanks to these advantages, many structures of perovskite/silicon tandem solar cells have been actively studied. Some results for 2-terminal, 4-terminal tandem solar cells already have been reported. However, the current matching and junction techniques of the upper and lower devices have not yet been optimized. For monolithic tandem solar cell fabricating the perovskite on top of the silicon bottom cell, the current matching and tunneling junction are crucial points. On the other hand, mechanically 4-terminal tandem solar cells can be made without the current matching.
In this study, we report a mechanically 2-terminal perovskite/silicon tandem solar cells. The low-cost Al-BSF p-type silicon solar cell with TCO was used for better light absorption. In case of the top devices, a transparent perovskite solar cell was used minimized the light loss. Through the mechanical junction of the upper and lower solar cell, the current flowed well to the interface, and high Voc, which is over 1.5V, was obtained. It is expected that the independent optimization of the upper and lower cells contributes to enhancement of the cell efficiency and it has a good place in the future tandem solar cell market.

Crystalline silicon (c-Si) solar cells account for more than 90% of the total solar cell market, and 90% of silicon solar cells are standard structure (Al BSF) solar cells based on p-type wafers. Standard structure Solar cell is accepted as a limit of efficiency by 20% due to limitation of cell structure. High efficiency cells such as PERX (PERC, PERT, PERL), Interdigitated Back Contact (IBC) and HIT (Heterojunction with Intrinsic Thin layer) have been developed and commercialized to overcome the limitations of these standard structure solar cells. To improve the efficiency of solar cells, there is one strategy to improve the efficiency of the device. By joining the perovskite solar cell and the low-cost silicon solar cell, the high efficency over 30% can be achieved.
Here, we present a new design for a monolithic perovskite/silicon-homojunction tandems solar cell. This work opens up new possibilites in a silicon / perovskite tandem solar cell technology to improve the cell efficiency of a standard structure solar cell to a high efficiency cell level without replacing the standard structure solar cell production infrastructure. For monolithic tandem solar cell fabricating the perovskite on top of the silicon bottom cell, the current matching and tunneling junction are crucial points. By the current matching and optimization of the tunneling junction, we fabricated high efficiency monolithic tandem solar cells.

Contact resistivity is a crucial parameter in continuing to drive the improvement of solar cell device performance. This is especially true in tandem cells where such resistive losses can manifest themselves in both top and bottom cells. Currently, crystalline silicon has emerged as the primary bottom cell used in most tandem configurations due to its high performance, low cost, and reliability. An established method for measuring the resitivity associated with contacts to silicon is the transmission line measurement (TLM) technique. In this work, TLM structures were fabricated to ascertain the properties of two important contacts to silicon bottom cells – intrinsic hydgrodenated amorphous silicon (a-Si:H(i))/ p-type hydrogenated amorphous silicion (a-Si:H(p)/indium tin oxide (ITO)/silver (Ag) stack (a-Si:H(p)/ITO/Ag) and a-Si:H(i,p)/aluminum. a-Si:H(p)/ITO/Ag contacts are commonly used silicon heterojunction solar cells

For the a-Si:H(i,p)/ITO/Ag stack we vary the a-Si:H(i) layer thickness, a-Si:H(p) doping and thickness, and oxygen partial pressure during ITO sputtering. By increasing the a-Si:H(i) thickness from 4 to 16 nm there is a marked increase in contact resistivity from 0.48 to 1.91 Ωcm². Similarly, we find that by increasing the oxygen partial pressure from 0.14 to 0.85 mTorr the contact resistivity increases from 0.1 to 2.75 Ωcm². The doping of the a-Si:H(p) show a unique trend in that increasing the trimethylborane flow from 9 to 18 sccm during PECVD results in an initial decrease in contact resitivity from 0.26 to 0.24 Ωcm² while further increases up to 100 sccm results in contact resistivity values up to 1.24 Ωcm². Interestingly, the contact resistivity does not vary for a-Si:H(p) layer thicknesses greater than 3 nm. The contact resistivity remains constant at 0.26 Ωcm². For a thinner layer of 3 nm the contact resistivity is 0.99 Ωcm².
The doped amorphous silicon thickness was also varied in the aluminum contacts and annealed between 150-240 °C. It was found that contact resistivities as low as 1x10^-3 and 4x10^-3 Ωxcm² could be obtained for n- and p-type samples respectively. However, it was also observed that annealing at high temperatures counter-dopes the n-type samples leading to the formation of a rectifying junction and prohibitively high resistivity values. The same high thermal loads in the p-type samples exhibited a large decrease in contact resistivity due to the aluminum spiking to the wafer and thus making direct contact at the expense of passivation, which is ultimately undesired for solar cells. Thus, these studies have shown the promise of such contacts for application in silicon bottom cells and provided insight into the window of process parameters for their fabrication. Such techniques will continue to be used for parameter/process optimization and extended to other contact structures.

Perovskite PV materials are unique polycrystalline semiconductors that can be deposited on surfaces using low-temperature solution-processes to achieve high efficiencies with high open circuit voltage (Voc) over 1.2 V. The bandgap of the perovskite absorbers can be tuned by controlling the halide (Cl, Br, and I) and cationic (MAI, FAI, Cs, etc.) contents in the crystal. These features make it one of the best materials for the top layer of tandem PV applications. To realize highly efficient and cost effective tandem PV devices, we have developed wide bandgap perovskite cells using low-temperature process (<100°C) as the top cell of perovskite-silicon tandem applications. Here, we developed efficient wide bandgap perovskite based device using highly crystalline fullerene derivatives with long alkyl chains as electron transport layer (ETL). The device with C60-fused N-methylpyrrolidine-meta-dodecyl phenyl (C60MC12) as ETL demonstrated an enhanced efficiency of 16.7 % with VOC of 1.24 V. This was achieved by mitigating the recombination loss through the use of the highly crystalline C60MC12 film compared to amorphous PCBM layer. We also made the entire cell semi-transparent to the bottom c-Si cell by using the ITO electrodes for the top and bottom layers of the perovskite cell. The semi-transparent cells with NiOx hole transport layer achieved over 12% efficiencies, showing no performance degradation over 4000 hours of continuous operation under 1 sun illumination at MPPT. Finally, preliminary studies demonstrated over 16% efficiencies for the perovskite-silicon monolithic two-terminal tandem devices.

Development of high-efficiency solar cell modules and new application fields are very important for further development of photovoltaics (PV) and creation of new clean energy infra structure based on PV. Especially, technologies for making III-V compounds and silicon solar cells are currently the most mature among other materials because of their efficiency potential. As a result, combining these two materials is a promising way to further improve the efficiencies of state-of-the-art solar cells. Therefore, the Toyota Technological Institute recently launched significant research efforts in III-V on Si technologies based on its existing resources in both silicon and III-V technologies, ranging from crystal growth, material characterization, cell design, cell prototyping and module and system integration including automobile applications. Especially, III-V/Si tandem solar cells are expected to have great potential of various applications because of high efficiency with efficiencies of more than 35 % under 1-sun AM1.5 G, light weight and low cost potential.
This paper overviews efficiency potential and recent activities of various Si tandem solar cells such as III-V/Si, II-VI/Si, chalcopyrite/Si, perovskite/Si and nanowire/Si tandem solar cells. Our analytical results for high-efficiency potential of various Si tandem solar cells analyzed by considering non-radiative recombination losses in Si tandem solar cells and materials show efficiency potentisal of more than 42% with 3-junction Si tandem solar cells and 35% with 2-junction Si tandem solar cells. Recent results for our 28.2 % efficiency and Sharp’s 33% mechanically stacked InGaP/GaAs/Si 3-junction solar cell are also presented. Approaches on automobile application by using III-V/Si tandem, partial concentration and static low concentration and so forth is also presented. The authours will also present our next targets such as 1) development of high-efficiency Si tandem solar cell modules with an efficiency of more than 30% under 1-sun, 2) improvements in large-area, high yield and highly reliable fabrication process, 3) development of application fields such as automobile applications.

In order to improve substantially on the efficiency of c-Si solar cells, we have been developing stacked GaInP/Si tandem cells, recently attaining efficiencies above 32% in four-terminal configuration. However, while four-terminal devices allow maximum power extraction and easy integration of two separately processed sub-cells, highly doped layers or grids are required above and below each sub-cell, which leads to optical losses and contributes to processing complexity and thus cost. By rewiring a four-terminal (4T) device in two- and three-terminal (2T, 3T) configuration, we demonstrate that while a 2T configuration is less efficient, a 3T configuration can yield exactly the same overall efficiency as four terminals. This finding has been confirmed through Sentaurus simulations and, with respect to the Si cell, by experimental investigations on isolated 3T Si bottom cells. The 3T configuration used consists of two circuits that share a terminal on the back of the Si cell: a conventional, 2T tandem circuit, and a Si IBC (interdigitated back contact) circuit.
We subsequently prepared a fully integrated three-terminal device. The cells were integrated by bonding an inverted GaInP cell stack onto a Si IBC cell with a full-area front surface field on a planar front-side using a transparent conductive adhesive. Both cells were coated with an optimized ITO layer before bonding to improve optical coupling between the cells. The GaInP stack is subsequently processed on top of the Si IBC cell (contact formation, mesa isolation). We show that the conductive adhesive, which consists of conductive spheres embedded in epoxy, does not result in appreciable series resistance losses while maintaining acceptable transparency and protecting the Si cell during GaInP processing. Furthermore, a voltage-voltage mapping approach to study the full operating space of the cell with its two interacting circuits is presented.
In a 2T measurement (keeping the IBC circuit open), we achieved V_oc=2.09V, J_sc=13.45 mA/cm², FF=85.6%, and an efficiency of 24.1%, which increased to a 3T efficiency of 25.2% when power was collected in both circuits. This demonstrates the added benefit of a third terminal, and based on PVLighthouse simulations it is expected that when using textured Si cells, this benefit will be more pronounced, and higher 3T tandem cell efficiencies will be achieved.

Monolithic perovskite/silicon tandem solar cells show great promise for the further efficiency enhancement of the current silicon photovoltaic. This work reports large area (>10cm²) 2-terminal perovskite/silicon that uses homo-junction silicon solar cell as the bottom cell and a low temperature processed (≤150 degC) CH₃NH₃PbI₃ perovskite solar cell as the top cell. A power conversion efficiency (PCE) of 21.0% under reverse-scan is achieved with V_oc of 1.68 V, J_SC of 16mA/cm² and FF of 78% on a 4 cm² device yielding a stabilized efficiency at 20.5%. On a even larger area of 16 cm², a stabilized efficiency at 17.1 % is achieved due to 10% absolute drop in FF. To our knowledge, these are the most efficient perovskite/silicon-homojunction tandem solar cells larger than 1cm² reported. Further efficiency improvement will be reported as a rear-textured Si cell is used instead of a planar Si cell (used in above denonsrations) and FF is further improved. Details of the low temperature perovsktie cell process sequence and of the transport mechanism across the perovsktie/Si interface will be presented. This work is relevant to the commercialisation of efficient large-area perovskite/silicon homojunction tandem solar cells.

Organic-inorganic lead halide perovskites have emerged as one of the most promising photovoltaic materials. Most of these solar cells are prepared via solution-processing and record efficiencies (>20%) have been obtained on (mesoscopic) metal oxides. Vacuum deposited perovskite devices would have the advantage of being compatible with temperature sensitive substrates, allowing for conformal coatings and for the straightforward implementation into tandem solar cells. Our group has developed fully vacuum deposited perovskite solar cells by depositing the perovskite absorber in between small molecular weight organic semiconductors. These fully vacuum deposited solar cells allow to fine tune the device properties by controlling at will the individual layer thicknesses and composition for specific applications including tandem configurations. We will present efficient single junction and monolithic tandem solar cells based on two perovskite absorbers with different and complementary bandgaps. Avenues to further increase the device performance by using mixed cation/halide perovskites prepared via multiple source vacuum deposition will also be presented.

Perovskite - Silcon tandem cells have been suggested as a pormising concept to extend the ITR-PV roadmap beyond current efficiency and costs. We have developed interfaces and electrodes for transparent perovskite cells, allowing to solution process cells with a record high transparency in the IR while maintaining an efficiency of over 17 %. 4T with about 26.7% and 2T tandem cells with about 22.5 % are realized with different types of silicon architectures, among them PERL, IBC and HIT cells. The advantages of the various concepts are discussed in terms of temperature dependence, irradiation and stray light behavior but also in terms of LCOE for renewable power production. It is the detailed LCOE analysis which reveals the economical challenges for 4T and outlines concepts for the design of a cost competitive perovskite technology.

Crystalline silicon cells will soon reach their practical efficiency limit, requiring disruptive approaches to improve further their performance and lower the cost of photovoltaic electricity. One of the most promising approaches to overcome this limit involves stacking several absorber materials with different band gaps to form a tandem cell. For example, an established technology such as crystalline silicon can be combined with a low-cost, wide band gap top cell. With a tunable band gap, low material costs, and a high performance of up to 22.1%, perovskite solar cells are highly promising candidates for tandem solar cells with an efficiency potential of >30% when combined with a silicon bottom cell.

Based on a near-infrared-transparent perovskite cell processed at low temperatures, we developed perovskite/silicon tandem cells in two configurations: mechanically-stacked 4-terminal tandems and monolithic 2-terminal tandems. Both configurations offer advantages and present challenges, either in complex system integration for the 4-terminal configuration or demanding manufacturing for monolithic tandems. We present how these issues can be tackled to reach a high performance with both configurations. By using a high-band gap perovskite top cell, we were able to reach a 4-terminal perovskite/silicon tandem cell measurement efficiency of 25.6%. In addition, we developed a fully integrated 4-terminal tandem with a size of >1 cm², in contrast to the typically reported 4-terminal tandem cells, which combine sub cells with different active areas. Moreover, we demonstrate a recombination junction for monolithic tandem cells based on nanocrystalline silicon to mitigate optical losses. When employed in perovskite/silicon heterojunction tandem cells with a planar front side, this junction was found to increase the bottom cell photocurrent by >1 mA/cm² when compared to tandem cells with a transparent conductive oxide recombination layer. In combination with a cesium-based perovskite top cell, this led to tandem cell efficiencies of up to 22.7% obtained from current-voltage measurements and steady-state efficiencies of up to 22.0% during maximum power point tracking. Thanks to its low lateral conductivity, the nanocrystalline silicon recombination junction enables the upscaling of monolithic perovskite/silicon heterojunction tandem cells, resulting in a 13 cm² monolithic tandem cell with a steady-state efficiency of 18%. We found that these tandem cells were significantly limited by parasitic absorption in the transport layers and therefore developed perovskite cells with alternative, more transparent materials for the transport layer. Finally, we optically compare the 4-terminal and 2-terminal tandem devices and provide guidelines for future research directions, including strategies to further reduce parasitic absorption.

To exploit the theoretical limit of c-Si and the associated Shockley-Queisser efficiency, one needs a material that can be deposited inexpensively on c-Si and has an efficient bandgap match with c-Si in a tandem cell arrangement to harvest a broader spectrum of the sun’s energy. To match the bandgap of c-Si, a material with bandgap of ~1.6-1.8 eV is needed to produce ~20 mA/cm² under air mass (AM) 1.5 spectrum. These requirements can be met by using the recently developed Organic-Inorganic Halide Perovskite (OIHP) materials. In this talk, I will introduce the challenges and progresses in developing highly efficient and photostable wide bandgap perovskite solar cells with a low temperature solution process, and the integration of them onto silicon cells.

The wide, tunable bandgap of metal halide perovskites holds the promise of boosting the efficiency of silicon by employing them as the wide gap absorber in tandem solar cells on silicon. This offers a pathway to surpassing fundamental efficiency limits on single-junction solar cells by extracting a portion of photogenerated carriers at a higher voltage and thus enabling the realization of the next generation of low cost tandem photovoltaic cells. Perovskite silicon tandems have recently achieved record efficiencies of 23.6% for monolithically integrated¹ and 26.4% for mechanically stacked configurations². Additionally, recent developments in the stability and efficiency of low bandgap tin-based perovskites have drawn great attention for making highly efficient and potentially low cost perovskite-perovskite tandems³.
However, while the bandgap of perovskites can be easily and continuously tuned between 1.5 and 2.3eV by the substitution of bromide for iodide, open circuit voltages have not increased linearly with bandgap, largely negating the benefit of bandgap tuning. One major cause for this saturation in open circuit voltage is photoinduced halide segregation, referred to as the Hoke effect⁴, where halides segregate into lower bandgap I-rich and higher bandgap Br-rich regions upon illumination. Photogenerated carrier are funneled towards these lower bandgap regions, which act as recombination centers, resulting is a loss in open circuit voltage.
In this work, we explore two strategies to mitigate halide segregation in wide bandgap perovskites and target bandgaps of 1.68eV and 1.75eV due to their relevance in tandems. First, we characterize the FA/Cs and I/Br compositional space and find that higher open circuit voltages are achieved and photostability is improved when the band gap is attained by raising the Cs fraction rather than relying on the Br fraction. Second, we find that halide segregation is largely mediated by surface trap states and the passivation of surfaces can reduce the rate of halide segregation. Combining these two insights, we fabricate monolithically integrated perovskite/silicon tandems with high and stable open circuit voltages, indicating sufficient suppression of halide segregation. Our optimized 1.68eV bandgap perovskite enables a matched current density of 19mA/cm² to realize >25% efficient monolithic perovskite tandems.

1. Bush, K. A. et al. 23.6%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2, 17009 (2017).
2. Duong, T. et al. Rubidium Multication Perovskite with Optimized Bandgap for Perovskite Silicon Tandem with over 26% Efficiency. Adv. Energy Mater. 1700228, 1–11 (2017).
3. Eperon, G. E. et al. Perovskite-perovskite tandem photovoltaics with ideal bandgaps. Science. 9717, 1–9 (2016).
4. Hoke, E. T. et al. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci. 6, 613–617 (2015).

Perovskite solar cells have gone through rapid improvement in their power conversion efficiency (PCE) in the last few years reaching up to 22.7% [1]. Due to superior optical properties (abrupt band-edge, IR transparency) and their easy bandgap tunability to wide energy ranges, perovskites are attractive candidates for top-cells in tandem applications [2].
We have previously demonstrated a 23.6% efficient monolithic, two-terminal perovskite/silicon tandem using Cs_0.17FA_0.83Pb(Br_0.17I_0.83)₃ (CsFA) perovskite top-cell [3]. Though a world-record device, the PCE is still below the 30% that are achievable with two junctions in two-terminal devices with silicon as a bottom sub-cell [2]. Improving the PCE of the world-record device requires a detailed and systematic optical loss analysis that includes an analysis of parasitic absorption in layers other than perovskite and silicon in tandems, optimization of top and bottom cell thicknesses to achieve current matching, reduction of reflection losses etc. To perform this analysis, the first step is to accurately determine the optical constants of CsFA perovskite that are not available in the literature. We use multi-angle spectroscopic ellipsometry and spectrophotometry to uniquely determine the optical constants of two CsFA perovskites having different bandgaps of 1.61 and 1.68 eV, achieved by varying the Cs/Br ratio as 17/17 and 25/20 respectively. This method is verified on a common absorber, CH₃NH₃PbI₃. Furthermore, we simulate the absorption and reflection of single junction perovskite cells with the obtained refractive indices and compare them to the measured values of real cells which shows excellent agreement. Our results reveal that the biggest losses in single-junction perovskite cells are front surface reflectance followed by parasitic absorption in the electron contact (C₆₀) and front TCO. We further conclude that the front surface reflectance is very sensitive to the thicknesses of the top TCO, electron contact and perovskite absorber due to changes in the thin film interference pattern, hence the need for their optimization.
Further analysis will include the simulation of complete two-terminal perovskite/silicon tandems resulting in an understanding of various optical losses and approaches to minimize them.

[1] NREL Efficiency Chart (https://www.nrel.gov/pv/assets/images/efficiency-chart.png).
[2] Z.J. Yu, M. Leilaeioun, Z. Holman, Selecting tandem partners for silicon solar cells, Nature Energy, 1 (2016) 16137.
[3] K.A. Bush, A.F. Palmstrom, J.Y. Zhengshan, M. Boccard, R. Cheacharoen, J.P. Mailoa, D.P. McMeekin, R.L. Hoye, C.D. Bailie, T. Leijtens, 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability, Nature Energy, 2 (2017) 17009.

Two conflicting characteristics compete where the commercial viability of one-sun tandem solar cell is concerned: On the one hand, tandem solar cells have fundamentally higher efficiencies than single junction solar cells. On the other hand, tandem solar cells are intrinsically more complex than single junction solar cells and require more fabrication steps, which makes them more expensive. Only if the benefit from the additionally generated energy outweighs the higher fabrication cost can tandem solar cells be successful.
The question under which conditions the last sentence is true is a multilayered one. The value of efficiency is highest when considering the integrated PV system, which makes it necessary to explore system level aspects. These aspects include, among others, economic considerations as well as topics from material science; for example degradation.
In this work we attempt to provide a comprehensive overview about what is needed to make tandem solar cells economically successful. This overview includes a review of recent result in the field of tandem solar cell cost modelling, as well as some of our own result on this topic. Central to our findings is the concept of a “marriage of equals”, which states that the sub cells in a tandem should be similar, as well as enable high efficiencies. We will take a look at different material pairings, including perovskite on silicon, III-V on silicon and thin-film on thin-film tandems and investigate how these tandems compare to single junction solar cells in various types of systems and various locations.
We find that under the right circumstances one-sun tandem solar cells can outperform single junction solar cells economically. Yet more than techno-economic considerations are needed to make these types of solar cell a wide-spread reality. As an outlook we hope to offer a perspective of future opportunities for this technology.

In past decades many materials, capable of converting solar energy into usable electrical energy, have been developed in order to increase the efficiency of energy conversion. Several of those materials are already mature enough to have made it to the mass market including monocrystalline and polycrystalline silicon, as well as several thin-film materials such as Cadmium Telluride (CdTe) and Copper Indium Gallium Arsenide (CIGS). The efficiency of these materials is still increasing and in some cases approaching the Shockley-Queisser limit. However, real-life outdoor illumination and climate conditions lead to performance of many solar cells that is significantly below the performance under standard test conditions (AM1.5G, 1 kW/m², 25^oC). These differences arise due to varying solar spectrum, light intensity, and temperature which all have an influence on the solar cell performance. We developed a model for modelling the efficiency of a perovskite/Si tandem solar cell under realistic conditions.^[1] Here we present an extension of this model for CdTe and CIGS photovoltaic materials and validate the model with recorded data from our solar field in Amsterdam, NL. We show that the measured performances of the solar field are in good agreement with the modeled ones. We further use our modelling tool to predict real-life efficiencies of many photovoltaic materials and predict the important optimizing parameters in order to increase realistic performance.

[1] Futscher, M. H., & Ehrler, B. (2017). Modeling the Performance Limitations and Prospects of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions. ACS Energy Letters, 2(9), 2089-2095.

Perovskite solar cells have entered the research field of photovoltaics by storm, already reaching efficiencies close to highly optimized silicon solar cells. Coupling perovskite and silicon solar cells in a tandem configuration has the potential to considerably out-perform conventional solar cells. Under standard test conditions, perovskite/silicon tandem solar cells already outperform the silicon single-junction solar cell alone. Under realistic conditions, however, tandem solar cells made from current record cells are hardly more efficient than the silicon solar cell alone. We model the performance of realistic perovskite/silicon tandem solar cells under real-world climate conditions, by incorporating parasitic cell resistances, nonradiative recombination, and optical losses into the detailed-balance limit. We show quantitatively that, when optimizing these parameters in the perovskite top cell, perovskite/silicon tandem solar cells could reach efficiencies above 38% under realistic conditions, even while leaving the silicon cell untouched. Despite the rapid efficiency increase of perovskite solar cells, our results emphasize the need for a concerted effort in material development, careful device design, and light management strategies, all necessary to further increase the efficiency of perovskite cells, and develop highly efficient perovskite/silicon tandem solar cells.

Many different designs of tandem cells based on high efficiency III-V top cells and Si bottom cells have been proposed, and there is ongoing to debate as to whether the sub-cells should be wired in series (to create a tandem device with two terminals) or operated independently (four terminal). In this talk we will discuss the design and operating principles of three-terminal (3T) tandem cells fabricated by combining a III-V (GaInP or GaAs) top-cell with a 3T Si bottom cell that has a conductive front contact in addition to two interdigitated back contacts (IBCs). We have developed a two dimensional TCAD device physics model to study the behavior of Si and full tandem solar cells operated in a 3T configuration.

Our simulations show that this type of 3T tandems has the potential to provide a robust operating mechanism to efficiently capture the solar spectrum without the need to current match sub-cells (as in monolithic 2-terminal tandem) or fabricate complicated metal grids/interconnects between cells (as in 4T stacked tandem). Moreover, the module integration is relatively straightforward provided the top cell’s operating voltage is about 2x that of the bottom cell, as in the case of GaInP/Si tandem. Under AM1.5G illumination, we predict that 3T GaInP/Si tandems can operate at efficiencies >32%. We will discuss how adding a third terminal to a Si device complicates analysis of the current-voltage behavior of a solar cell and provide a framework for analyzing power production from a 3T device. Fabrication and testing of 3T tandems using a transparent conductive adhesive (TCA) approach will also be discussed.

The further evolution of PV power generation is an important issue of global energy policy. According to ‘NEDO PV Challenges’ [1], the target of PV electricity at 2030 is to achieve both efficiency of >25% and power generation cost of <0.1 $/kWh. Against this background, heterogeneous multi-junction (MJ) solar cell is one of the strategies to realize this target. The mechanical stacking, as fabrication method of MJ solar cells, enables high efficiency and low-cost because of flexible and most appropriate combination of different cells. In here, GaAs//Si and GaAs//CuInGaSe MJ solar cell are the most realistic combination. GaAs-based top cell essentially has high efficiency. Si and CuInGaSe bottom cells have high sensitivity for long wavelength region and hold the promise of low cost bottom cell. According to our theoretical prediction, these cells have a potential to reach more than 30% efficiency.
In this paper, we show the way to practical use of GaAs//Si and GaAs//CuInGaSe MJ solar cells with mechanical stacking method. Our key technology is a powerful bonding method using Pd nano-particle alignment, which is named “Smart Stack” technology [2]-[4]. Using this method, InGaP/GaAs//Si and InGaP/GaAs//CuInGaSe 3-junction solar cells were fabricated. It was revealed that the total efficiency at 1 sun (AM1.5g) was 25.1% and 24.2% for GaAs//Si and GaAs//CuInGaSe solar cell, respectively. And also, we examined the low concentration performance for each solar cell. Low concentration system contributes to the expansion of the theoretical limit of conversion efficiency and the cost reduction of high expensive GaAs cell. As a result, we obtained the maximum efficiency of 23.7% at 8.0 suns and 25.3% at 5.7 suns for GaAs//Si and GaAs//CuInGaSe solar cell, respectively. This performance is acceptable level for low concentration system below 10 suns. In addition, we have developed the technology for practical application. As the mass productive mounting method, we have developed “Individual Transport Technology”, namely Epitaxial Lift-off (ELO) GaAs top cells prepared using Supercritical Rinse & Dryer equipment are transported and bonded to Si bottom cell individually and freely. Using this method, the large area of solar cell enables becoming it. On the other hand, we attempted the initial reliability verification with accelerated aging test and thermal cycling test. For GaAs//Si MJ solar cells, long-term stability was confirmed. Appling the general lifetime estimation method with Arrhenius plot, the life time was estimated to be more than 1.5×10⁵ hours at 60°C. These results strongly suggested the high potential of MJ solar cells with Smart Stack technology for practical use. Reference: [1] www.nedo.go.jp/news/press/AA5_100318.html. [2] H. Mizuno, et. al., Appl. Phys. Lett., 101, 191111 (2012). [3]H. Mizuno, et. al., Appl. Phys. Express, 10, 072301(2017). [4] K. Makita, et. al., 33rd EU PVSEC, 1AO.3.1 (2017).

We have fabricated a 100 cm² prototype two-junction tandem photovoltaic consisting of a luminescent solar concentrator (LSC) top cell featuring quantum luminophor concentrators and InGaP microcells that is combined with a commercially-available Si passivated emitter rear contact (PERC) cell. Our LSC employs ultrahigh luminescence efficiency CdSe/CdS quantum dot (QD) luminophores, absorbing light in 300-500 nm wavelength range and re-emitting luminescence radiation at 635 nm, a wavelength which is nearly ideally matched to the bandgap of InGaP microcells formed by epitaxial growth, liftoff and patterning. We form an LSC planar optical waveguide by dispersing QDs throughout a poly(laurylmethacrylate) (PLMA) waveguide layer along with an array of InGaP micro-cells, tuned to the photoluminescence emission profile. This planar LSC architecture of InGaP micro-cells achieved concentrator factors of 40-100x. The microcell architecture allows for scaling of the LSC waveguide to conventional module size and enables greater optical collection efficiencies than for traditional waveguide edge-lining cell geometries, and InGaP cells are thus cost-effective since they comprise only 1-3% of the module aperture area. We surround the LSC structure with wavelength-selective, top and bottom notch filters in order to minimize photon escape cone loss, which enables high LSC concentration factors to be achieved. We have integrated this layered LSC with a Si PERC cell for use as a 4-terminal tandem prototype. With this optimized architecture, our module captures and utilizes short wavelength light in InGaP with higher achievable cell voltage and power conversion and long wavelength light, thus resulting in efficiencies far beyond that of a stand-alone Si cell. Using Monte Carlo ray-tracing simulations, we have explored the radiative limit of such a tandem LSC-on-Si module architecture, showing an overall power conversion efficiency increase from 20% for a stand-alone Si cell under direct normal incidence, to 30.8% for the LSC-on-Si under solar irradiance with a 40% direct/50% diffuse ratio, typical of average US continental solar irradiance. Results of outdoor on-sun testing of our module will be presented in the talk. In addition to our computational analysis of efficiency and tandem module fabrication, we have also developed a thorough techno-economic (TEA) for this LSC-on-Si architecture. With our current module design, we estimate fabrication costs as low at $80/m², indicating that an LSC-on-Si tandem module is not only efficient under highly diffuse solar irradiance conditions, but also holds potential for mass market distribution.

The tunable bandgap, high efficiencies, and inexpensive fabrication techniques of perovskites coupled with the high performance and reliability of silicon create an intriguing partnership for a tandem solar cell. In such two-terminal, series-connected, monolithic cells, indium tin oxide (ITO) is commonly used as a transparent electrode due to its high conductivity and transparency to broadband wavelengths. However, sputtering ITO onto the soft, organic perovskite and electron transport layers induces degradation, leading to poor device performance. This work describes the development of an ITO nanoparticle buffer layer deposited via spray deposition to mitigate such damage. ITO has not been previously utilized in this capacity but is a promising candidate material due to its matching work function with the adjacent electrode. Similarly to perovskites, crystalline silicon wafers passivated with hydrogenated amorphous silicon experiences sputter damage, which results in severe carrier lifetime reduction. Consequently, silicon represents a convenient model system for development of these buffer layers intended for subsequent use on perovskites. Films of ITO nanoparticles deposited onto high-lifetime silicon wafers exhibited 40% absolute higher lifetime compared to bare wafers without the buffer layer following ITO sputtering. Additionally, these films demonstrated nearly identical transmittance to glass in the visible range and >80% transmittance at 1200nm. The continuity of these films has been confirmed by scanning electron microscopy and they exhibit a high degree of uniformity across areas typical of perovskite cell. During deposition, the solvent containing the ITO nanoparticles is completely evaporated and has no interaction with the substrate, a crucial step in limiting degradation. Before deposition, the nanoparticles are synthesized using solution chemistry, allowing flexibility of ITO particle diameter and ligand properties. The film porosity and thickness can also be tuned thereby providing further opportunities for process optimization. Future work includes additional optical and electrical characterization as well as full integration into perovskite-silicon tandems with subsequent characterization.

To identify new promising wide-band gap (1.6-2.0 eV) photabsorbers to be used in tandem solar cells, we have computationally screened 705 compounds with the ABS₃ formula (A,B = metals; S = sulfur). Within the final list of 15 theoretically promising candidates, we have synthesized and characterized three novel ABS₃ compounds in polycrystalline, thin film form by means of a two-step process. The first is sputter deposition of metallic (AB) or oxide (ABO₃) precursors; the second is sulfurization of such precursors in H₂S gas.
In this contribution, we will show that LaYS₃ is a particularly attractive wide-band gap photoabsorber. Experimentally, it features a direct band gap of 2.0 eV, an intense photoluminescence signal, and a relatively small offset (0.1 eV) between its band gap energy and its photoluminescence peak.
Our first attempts at fabricating a simple single-junction LaYS₃ solar cell will be presented. However, in line with the scope of this session, we will especially focus on the integration of ABS₃ materials, and LaYS₃ in particular, with a silicon bottom cell. This poses several challenges, among which the high-temperature process required to form high-quality LaYS₃ and its negative effect on the properties of Si. Another issue is the selection of an appropriate recombination layer between the Si and the LaYS₃ cell, which should also serve as a barrier to interdiffusion unless the two cells are mechanically stacked.
Finally, we will show that other novel ABS₃ materials, while more difficult to synthesize, may also be promising for tandem solar cells. Some of those compounds contain more earth abundant elements than La and Y, and can be grown at lower temperatures.

Great effort has been placed into improving the performance of single-junction solar cells in recent years. Cells featuring the three most notable materials—Si, GaAs and CdTe—have sat at efficiencies of 25.6%, 28.8%, and 22.1% respectively for several years, sitting well below the Schockley-Queisser limit. Further improvements in efficiency are constrained by high manufacturing costs and/or poor material quality. Tandem cells are an excellent way to circumvent the challenges of improving single-junction device efficiencies. The proposed design of a II-VI (1.7 eV)/Si (1.1 eV) tandem solar cell featuring a Mg_xCd_1-xTe top cell (x~13% Mg mol fraction) have the potential to reach an efficiency greater than 30% at low manufacturing cost.
This talk will summarize the latest study of structural and optical properties of CdTe/Mg_xCd_1-xTe and Mg_0.13Cd0_.87Te/Mg_yCd_1-yTe (y > 0.24) double-heterostructures, which are grown by molecular beam epitaxy. The MgCdTe barrier layers provide excellent passivation to the CdTe absorber, resulting in a longest carrier lifetime of 3.6 µs in undoped CdTe/Mg_xCd_1-xTe double heterostructures and the lowest interface recommendation velocity 1.2 cm/s. Also the Mg_0.13Cd0_.87Te/Mg_yCd_1-yTe double heterostructures revealed a long carrier lifetime of 560 ns, indicating excellent material quality.
Solar cells made of these double-heterostructures with 1- to 1.5-µm-thick n-type CdTe absorbers, and passivated hole-selective p-type a-SiC_y:H contacts have achieved a highest V_oc of 1.11 V and a maximum total-area efficiency of 18.5% and active-area efficiency of 20.3% measured under AM1.5G illumination. Using the similar device structure designs and measurement conditions, Mg_0.13Cd0_.87Te/Mg_yCd_1-yTe double-heterostructure solar cells have shown a device with a V_oc of 1.12 V and an efficiency greater of 15.2%.
References:
X.-H. Zhao, S. Liu, C. M. Campbell, Y. Zhao, M. B. Lassise, and Y.-H. Zhang, Ultralow interface recombination velocity (~1 cm/s) at CdTe/Mg_xCd_1-xTe heterointerface, IEEE Journal of Photovoltaics 7, 913 – 918 (2017).
Y. Zhao, X.-H. Zhao, Y.-H. Zhang, Radiative recombination dominated monocrystalline CdTe/MgCdTe double-heterostructures, IEEE J. of Photovoltaics 7, 690-694 (2017).
J. J. Becker, M. Boccard, C. M. Campbell, Y. Zhao, M. Lassise, Z. C. Holman, and Y.-H. Zhang, Loss analysis of monocrystalline CdTe solar cells with 20% active-area efficiency, IEEE J. of Photovoltaics 7, 900 – 905 (2017).

Solar cells based on chalcogenide compound semiconductors, such as chalcopyrites, kesterites, and perovskites offer numerous inherent advantages of thin film deposition technologies for potentially low cost production of large area solar modules on flexible foils as well rigid glass substrates. These compounds allow band gap engineering over a wide composition range and they are attractive for development of multi-junction solar cells, especially important is the combination of perovskite as top cell and Cu(In,Ga)Se2 as bottom cell. The talk would provide a review of the developments of all thin film tandem solar cells with a large part covering the progress and the state of the art of high efficiency tandem solar cells and mini-modules on glass and flexible foils in different configurations.

Cd_1-xMg_xTe (CMT) is a wide bandgap semiconductor which is optimal for use with silicon (Si) bottom cells in a tandem configuration for photovoltaic (PV) applications. This is due to its tunable bandgap with Mg content and high transmission of sub-bandgap photons. Additionally, it takes advantage of CdTe PV manufacturing techniques with proven scalability. By pairing the existing production capabilities of Si PV with the scalable production that CdTe manufacturing offers, devices of greater efficiency than that of a single junction device can be produced while maintaining or reducing cost of power. However, fabrication of high efficiency CMT devices necessary for this application has not yet been achieved. This is often attributed to issues in post-deposition CdCl₂ processing of the CMT films. It has been shown that CdCl₂ promotes recrystallization and grain growth of CdTe, as well as acting as a source of chlorine for diffusion along the grain boundaries. These effects are responsible for eliminating detrimental defects, growing large grains, and allowing grain boundaries to assist in current collection. The combination of these effects is attributed with bringing 1% efficient CdTe devices up to 13% or greater. When CdCl₂ processes are applied to CMT, the increase in device performance is not achieved. It has been reported that the process causes loss of Mg, degrading the optical and electrical properties which had made it ideal for use in tandem PV. It has been postulated that MgCl₂ passivation processes will produce CMT devices of higher performance without removing Mg from the film. It is thought that the loss reaction will be limited by removing the additional Cd of CdCl₂ needed to form CdTe during Mg loss. Devices that have undergone both processes are characterized using current density-voltage, quantum efficiency, cross-sectional transmission electron microscopy (TEM), and cross-sectional energy dispersive spectroscopy (EDS) measurements. This work shows that CdCl₂ treatments on CMT slightly improved device efficiency, but optically degraded the CMT film. Additionally, it is shown that a large numbers of voids developed after processing and there was loss of Mg from the grain boundaries and close to the junction. It was found that MgCl₂ passivation did not optically degrade the film, but it has not yielded the desired device performance.

For a top cell absorber in a two junction tandem solar cell, the numerical simulations indicate that the band gap of 1.72 eV is required for the current matching with the bottom cell. The band gap of Cd-Zn-Te alloy can be altered to 1.72 eV based on the concentration of different elements and is a good candidate for the fabrication of low cost tandem solar cells. The devices fabricated from as-deposited polycrystalline films of the Cd-Zn-Te alloy require a post deposition chloride treatment to improve the absorber quality. From the literature, it is known that CdCl₂ treatment is an essential post deposition process to improve the performance of devices fabricated with polycrystalline CdTe and the treatment can be implemented on devices with Cd-Zn-Te. It is also known that a buffer layer of Cd-Se-Te in between the emitter and CdTe absorber improves the carrier lifetimes. During the post deposition treatment on Cd-Zn-Te, CdCl₂ reacts with the bulk and forms volatile ZnCl₂ compound. Due to the loss, the alloy reduces to a lower band gap material which is not suitable for the top cell in a two junction tandem solar cell.
In this study, a polycrystalline Cd-Se-Te layer was incorporated in between the emitter and polycrystalline Cd-Zn-Te absorber. To prevent Zn loss, an Al₂O₃ layer was deposited on the back surface of Cd-Zn-Te before the CdCl₂ treatment. The composition of CdSe_0.2Te_0.8 and Cd_0.6Zn_0.4Te was kept constant. The thickness of Cd-Se-Te layer was varied from 0 nm to 300 nm to investigate the effect on the device performance. The thickness of Cd-Zn-Te with a band gap of 1.72 eV and the CdCl₂ treatment were the same for all the samples. After the CdCl₂ treatment, Al₂O₃ was etched from the back surface of Cd-Zn-Te. The devices were fabricated after copper doping of Cd-Zn-Te and application of electrodes.
In the external quantum efficiency (EQE) graphs, Cd-Zn-Te without Cd-Se-Te layer retained a sharp band edge at 1.72 eV at zero voltage bias conditions. The retention of the band edge at 1.72 eV indicated that Al₂O₃ acts as a barrier to prevent zinc loss during the CdCl₂ treatment. The band edge shift towards the longer wavelength regions with increasing thickness of Cd-Se-Te layer in between the emitter and Cd-Zn-Te. In all the samples, the band edge remained unchanged at the forward and reverse voltage bias conditions in the EQE measurements. The optical transmissions measurements conducted before fabricating the devices were in good agreement with the EQE graphs. The time resolved photoluminescence measurement on 300 nm thick Cd-Se-Te in between the emitter and Cd-Zn-Te showed Tau₁ and Tau₂ values of 1.04 ns and 3.46 ns. To understand the structural and compositional changes in Cd-Se-Te/Cd-Zn-Te, X-ray diffraction and cross-section viewed under transmission electron microscope were used.

By adding Zn or Mg to CdTe, the band gap increases from 1.5 eV for pure CdTe to > 2 eV. This, coupled with the fact that CdTe can be produced at low cost, suggests that CdZnTe or CdMgTe could be an ideal top cell for mass produced tandem devices. The addition of the Zn or Mg into the CdTe is through direct substitution for the Cd. As a result, the change in band gap is due to an increase in the conduction band energy level, leaving the valence band close to that of CdTe. Because of the low valence band energy of CdTe, the options for back contacts have been limited. This issue will be exacerbated by requiring the back contact to be near infrared (NIR) transparent when the CdZnTe or CdMgTe is incorporated into a tandem device. Recently we fabricated a low barrier, transparent back contact to CdTe. This was accomplished by treating a CdCl₂ activated CdTe sample with methylammonium iodide (CH₃NH₃I, MAI) solution followed with a mild thermal treatment to produce a layer of Te at the back surface. An indium tin oxide (ITO) overlay was applied to the Te layer as the transparent back contact. Improved open circuit voltage (V_OC) of this device over the standard Cu/Au back contacted device was achieved. The champion device efficiency from the MAI treated CdTe with the ITO back electrode was 12.2% with V_OC of 823 mV, short circuit current (J_SC) of 21.4 mA/cm², and fill factor (FF) of 69.3%. The best performing device from the standard Cu/Au back contacted devices showed V_OC, J_SC, FF, and power conversion efficiency (PCE) of 811 mV, 21.3 mA/cm², 75.6%, and 13.0%, respectively. Note that the FF was slightly lower due to the reduced sheet resistance of the ITO compared to Au. In monolithically integrated tandem devices, this will not be an issue as lateral conductivity is unnecessary. Further, the NIR transmittance reduction due to the MAI treatment was only ~6%. Previous work has shown that the interaction between MAI and Zn is similar to that of MAI and Cd – the Zn²⁺ or Cd²⁺ ions react with MA⁺ and I^- ions, resulting Zn- or Cd-based perovskites. Due to this reason, Cd was selectively remove from a CdTe surface by reacting a MAI thin film with the CdTe surface. The formed Cd-based perovskites can easily rinse out leaving a Te behind. Since the Zn²⁺ reacts with MAI similar to the Cd²⁺, this process should work for CdZnTe films as well. Furthermore, Te layers formed through the MAI process will be compared to deposited Te layers.

We are developing copper indium diselenide (CuInSe₂, CIS) thin films as the absorber layer for the bottom cell of a monolithically grown perovskite/CuInSe₂ tandem solar cell. Although the perovskite top cell behaves as nearly defect-free, CIS shows defect-dominated behavior which severely limits carrier lifetime, and therefore also limits efficiency. We report here on studies of the composition-dependent defect optical emission as studied at low temperatures for copper-poor CIS thin films. The goal of the project is to develop a correlation between defect emission and device performance. Achieving this will also require understanding and control of surfaces and interfaces. In support of these efforts, we are working to correlate PL measurements with spectroscopic ellipsometry (SE) studies of the electronic and crystalline structure. In this report, low temperature photoluminescence (PL) measurement has been applied to study native and impurity defects in several compositions of Cu-poor (i.e. Cu/In <1) CIS films deposited on soda-lime glass via a co-evaporation process. The compositions of the films studied to date range from Cu/In = 0.47 to Cu/In = 0.80. Laser excitation power dependence of PL emission from these CIS films indicates the existence of Donor-Acceptor-Pair (DAP) recombination, with the DAP peak energy varying by +/- 10 meV near 0.85 eV depending on Cu/In ratio. We also observed defect emission at ~0.78 eV on some CuInSe₂ films which has not been explored to date. Temperature dependent PL studies will be used to calculate the donor and acceptor binding energies contributing to DAP recombination, and the activation energies of other observed point defect states. Lastly, we report on initial results of correlating PL results with composition and results of spectroscopic ellipsometry (SE) modeling and analysis to understand the relationship between the complex optical constants and the observed radiative recombination.