Query: Maximizing tandem solar cell power extraction using a three-terminal design

Candidate papers:

abstract: Tandem or multijunction solar cells can greatly increase the efficiency of solar energy conversion by absorbing different energies of the incident solar illumination in semiconductors with different band gaps, which can operate more efficiently than a single absorber. Many different designs of tandem cells based on high efficiency top cells and Si bottom cells have been proposed, and there is ongoing debate as to whether the sub cells should be wired in series (to create a tandem device with two terminals) or operated independently (four terminals) An alternative cell configuration that combines some of the strengths of both is a three terminal device consisting of a top cell optically in series with a modified interdigitated back contact (IBC) Si cell featuring a conductive top contact. Such a configuration can enable improved energy yield while only requiring external wiring on the front and back of the solar cell stack. In this paper, we investigate the operation of three terminal tandems in detail using technology computer aided design (TCAD) device physics simulations. Using III V top cells as an example case, we show how the addition of a third terminal can deliver comparable power output to a four terminal device, and substantially more power than a two terminal device, while also enabling power injection and extraction between the two sub circuits under a variety of spectral conditions.
author_names: Emily L Warren

Michael G Deceglie

Michael Rienacker

Robby Peibst

Adele C Tamboli

Paul Stradins
title: Maximizing tandem solar cell power extraction using a three terminal design
year: 2018

abstract: We demonstrate an inverted metamorphic multijunction (IMM) photovoltaic cell comprising lattice mismatched 1.2 eV AlGaInAs and 1.0 eV GaInAs junctions optimized for high temperature thermophotovoltaic (TPV) applications. This device differs from traditional IMM solar cells because the mismatched junctions are grown at a single lattice constant. This architecture enables removal of the compositionally graded buffer that otherwise filters light from the junctions below and absorbs sub bandgap light via free carrier absorption. Sub bandgap absorption dramatically reduces the efficiency of TPV systems using high reflectivity cells to enable band edge spectrum filtering. Three components required development to enable this device: (1) a lattice mismatched 1.2 eV AlGaInAs junction, (2) a metamorphic contact layer grown after the graded buffer, and (3) a transparent tunnel junction that sits in front of the 1.0 eV GaInAs junction. Growth conditions that minimize oxygen defect incorporation maximize AlGaInAs cell quality, enabling a 0.41 V bandgap open circuit voltage offset at 22 mA/cm2 under AM1.5D. A mismatched GaInAs:Se layer is developed as a low resistance contact. Lastly, we develop a GaAsSb:C/GaInP:Se tunnel junction suitable for high power densities with more transparency than the GaAsSb:C/GaInAs:Se structure used in past IMM cells. We characterize the tandem device under a high intensity spectrum that approximates the emission from a 2150 degC blackbody radiator and deduce a projected ideal TPV efficiency of 39.9% at ~30% of the blackbody irradiance and 36% ideal TPV efficiency under the full 118 W/cm2 irradiance. Improvements to the back surface reflectivity and series resistance are expected to increase the ideal TPV efficiency well above 40%
author_names: Kevin L Schulte

Ryan M France

Daniel J Friedman

Alina LaPotin

Asegun Henry

Myles A Steiner
title: Inverted metamorphic AlGaInAs/GaInAs tandem thermophotovoltaic cell designed for thermal energy grid storage application
year: 2020

abstract: We report on a wide band gap polymer donor composed of benzo[1,2 b:4,5 b']dithiophene (BDT) and 3,4 difluorothiophene [2F]T) units (Eopt ~2.1 eV) and show that the fluorinated analog PBDT[2F]T performs significantly better than its non fluorinated counterpart PBDT[2H]T in BHJ solar cells with PC71BM. While control P3HTand PBDT[2H]T based devices yield PCEs of ca. 4% and 3% (Max. respectively, PBDT[2F]T based devices reach PCEs of ca. 7% combining a large VOC of ca. 0.9 V and short circuit current values (ca. 10.7 mA/cm) comparable to those of the best P3HT based control devices. In bulk heterojunction (BHJ) solar cells of p conjugated polymer donors and fullerene acceptors, such as phenyl C61butyric acid methyl ester or its C71 analog (PCBM) some important design rules govern the efficiency of the polymer donor. While conventional (single cell) BHJ devices composed of poly(3 hexylthiophene) (P3HT) and PC71BM or the indene C60 bisadduct ICBA can yield power conversion efficiencies (PCE) of ca. 4% and 6.5% respectively, lower bandgap systems that absorb visible light at longer wavelengths (550 800 nm) such as PBDTTPD, PTB7, and several other analogs, have been shown to reach PCEs 8% Among those, several low band gap systems substituted with fluorine F) atoms have been described as especially promising compared to their non fluorinated counterparts. In particular, F substituted benzothiadiazole, thieno[3,4 b]thiophene, benzo[1,2 b:4,5 b']dithiophene, quinoxaline, benzotriazole, isoindigo, and more recently thiophene motifs, have frequently been included in the backbone of low band gap polymer donors. It is worth noting, however, that the underlying reasons that may justify the importance of F substituted motifs in polymer donors remain a matter of some debate. A wide range of possible determining factors have been suggested, spanning i) improved polymer backbone planarity, resulting in higher carrier mobilities, 29 ii) more favorable orientation of the polymer aggregates relative to the device substrate, iii) improved molecular arrangement and orbital overlap at the donor/acceptor interface, iv) dipole driven charge separation, and v) lower lying HOMO levels that contribute to larger open circuit voltages (VOC) in BHJ devices. 18 While low band gap polymer donors are especially promising in singlecell BHJ devices with PCBM, and are commonly used in efficient tandem and triple junction solar cells, wide band gap analogs that can outperform P3HT in the high band gap cell of multi junction devices are required in order to continue improving upon currently reported PCEs (ca. 11.6% However, we note that only a few polymer systems combine a band gap wider than that of P3HT (Eopt ~1.9 eV) a low lying HOMO amenable to larger open circuit voltages (VOC) and comparably high PCEs in BHJ devices. In this contribution, we report on a wide band gap polymer donor composed of benzo[1,2 b:4,5 b']dithiophene (BDT) and 3,4 difluorothiophene [2F]T) motifs (Eopt ~2.1 eV) and show that the fluorinated analog poly(4,8 bis((2ethylhexyl)oxy)benzo[1,2 b:4,5 b']dithiophene 3,4 difluorothiophene) namely PBDT[2F]T (Chart 1) outperforms both P3HT (Eopt ~1.9 eV) and its non fluorinated counterpart PBDT[2H]T (described in earlier work) in BHJ solar cells with PC71BM. Chart 1. Structures of P3HT, and the Wide Band Gap PBDT[2X]T Polymers (with X H or F) Importantly, we point to the relevance of the [2F]T motifs unit practically unexplored to date in the design of efficient polymer donors used as alternatives to P3HT, and emphasize the stark differences in device characteristics between PBDT[2F]T and PBDT[2H]T in the BHJs with PC71BM. Our device analyses suggest notably improved charge separation and extraction in PBDT[2F]T based BHJ solar cells. The PBDT[2X]T polymers (with X H or F) were synthesized via a microwave assisted approach (150 degC in chlorobenzene (CB) for 1 h, ca. 190 W) in order to control polymer growth and molecular weight (MW) while minimizing reaction times. Both analogs were prepared following the same Pd mediated cross coupling polymerization conditions (cf. details in Supporting Information (SI) being found soluble, the polymers were purified using established methods, yielding batches of comparable MW (cf. SI, Table S1) Page 1 of 6 ACS Paragon Plus Environment Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 Figure 1. (a) Potential energy surfaces resulting from the rotation of the [2X]T motifs (X H or F) with respect to BDT; relative energies determined by DFT modeling at the B3LYP/6 31G(d,p) level. (b) Representations of the BDT[2F]T tetramer HOMO and LUMO as obtained at the B3LYP/631G(d,p) level of theory (see BDT[2H]T tetramer in SI; Fig. S1) Both frontier orbitals are well delocalized along the pconjugated backbone. It is worth noting that the 2 ethylhexyl (2EH) substituted BDT motifs provided sufficient solubility in both PBDT[2X]T analogs, and that PBDT[2H]T serves as a model polymer in this study (along with P3HT) Prior to examining the effect of swapping H for F in [2X]T motifs on the ionization, electronic, and optical properties of the PBDT[2X]T polymers, it is important to understand how F substitutions influence backbone geometry. Figure 1a shows the potential energy surfaces (PES) for twisting the [2X]T unit relative to the BDT motif; density functional theory (DFT) calculations at the B3LYP/6 31G(d,p) level (cf. details in SI) The PES plot pertaining to PBDT[2F]T reaches two minima, corresponding to the fully planar anti/0deg and syn/180deg conformations. The anti conformation is predicted to be only slightly more stable by 0.31 kcal mol (ca. 0.5 kT) at room temperature, suggesting that backbone planarization in PBDT[2F]T is achieved via statistical syn and anti conformations. On the other hand, the PES plot of PBDT[2H]T shows a shallow minimum at ca. 10deg and a higher energy local minimum at 150deg, with an energy difference of 0.92 kcal mol (ca. 1.5 kT) between the two conformations, indicating that ca. 25 [2H]T motifs out of 100 adopt the syn conformation at thermodynamic equilibrium (cf. SI) From these results, it is worth noting that the PBDT[2F]T backbone is expected to be slightly more planar than that of PBDT[2H]T result consistent with the higher binding energies induced by nontraditional intramolecular hydrogen bonding interactions. Overall, the significant energetic barrier of 2.5 3.5 kcal mol on going from anti to syn conformations parallels the idea that the backbones of both PBDT[2X]T polymers are expected to be rather coplanar (i.e. disfavoring "out of plane" conformations) The thin film UV Vis optical absorption spectra of P3HT and the PBDT[2X]T polymers (with X H or F) are Figure 2. (a) Superimposed, normalized UV Vis optical absorption spectra of P3HT and the wide band gap PBDT[2X]T systems (with X H or F) (neat films) (b) PESA estimated ionization potentials (IP, triangles) optical band gaps (Eopt, squares) estimated from the onset of the UV Vis absorption spectra (films) DFT calculated HOMO energy levels |HOMO| absolute value, stars) and HOMO LUMO gaps (HLgap, pentagons) for the polymers. superimposed in Figure 2a (normalized spectra) Figure S2 accounts for the relative thin film absorbance (ca. 40 nm) and solution absorption coefficients of the PBDT[2X]T polymers. Figure 2b provides the ionization potentials (IP) of the polymers measured by photoelectron spectroscopy in air (PESA) As seen from Figure 2a, the range of absorption of both PBDT[2H]T and PBDT[2F]T falls within that of P3HT (400 650 nm) with a slight apparent hypsochromic shift of the absorption onset (by ca. 50 nm compared to P3HT) The two derivatives have near identical optical gaps (Eopt) of 2.1 eV, estimated from the onset of their thin film absorption (Eopt(P3HT) 1.9 eV) However, as shown in Fig. 2b, the IP of PBDT[2F]T (5.29 eV) is significantly larger than that of its non fluorinated counterpart PBDT[2H]T (5.03 eV) and also markedly larger than that of P3HT (4.65 eV) Considering that the two PBDT[2X]T analogs have the same Eopt values, it can be inferred that the [2F]T motifs suppresses both the HOMO and LUMO of PBDT[2F]T comparably. In parallel, comparing the solution and thin film absorption data of PBDT[2H]T and PBDT[2F]T (see Fig. S2) the slight intensity variations of the higher wavelength absorption peak point to the presence of p aggregates in both solutions and films. The temperature dependent UV Vis spectroscopy data, shown in Figure S3, confirms the propensity of the two polymers to form p aggregates albeit aggregation in PBDT[2H]T can be more efficiently mitigated at elevated temperatures. Overall, the tendency of the PBDT[2X]T analogs to form paggregates, and the weak bathochromic shifts observed on moving from solution to film, are in agreement with the high degree of backbone coplanarity predicted by DFT (Fig. 1a) and suggest pronounced polymer ordering in solution. The DFT computed frontier orbitals of the BDT[2X]T tetramers shown in Figures 1b and S1 are well delocalized along the pconjugated backbones, correlating with predicted backbone planarity. Note that tetramers have been shown to reproduce the electronic and optical properties of analogous polymers appropriately. The DFT estimated HOMO energy for the tetramer of PBDT[2F]T 5.05 eV) is predicted to be 0.17 eV deeper than that of PBDT[2H]T 4.88 eV) relative values in line with the significant IP offset of 0.26 eV inferred earlier from the PESA measurements (Fig. 2b) In parallel, the comparable LUMO offset of 0.2 eV calculated for the tetramers makes the predicted HOMO LUMO gap consistent with the near identical Eopt values of 2.1 eV estimated from the thinfilm absorption of PBDT[2H]T and PBDT[2F]T (see Fig. 2b) a) b) a) b) Page 2 of 6 ACS Par
author_names: Jannic Wolf

Federico Cruciani

Abdulrahman El Labban

Pierre M Beaujuge
title: Wide Band Gap 3,4 Difluorothiophene Based Polymer with 7% Solar Cell Efficiency: An Alternative to P3HT
year: 2015

abstract: This paper presents the proposed power distribution system designed for the power management of the Puerto Rico CubeSat. Power is provided to the CubeSat using Gallium Arsenide (GaAs) solar panels mounted on each side of the satellite. It is vital to maximize the available electrical energy obtained from the minimal solar cell area available. SEPIC topology performs the Maximum Power Point Tracking (MPPT) to extract the utmost available power from the solar panels. The Buck converter is used to provide voltage regulation for the CubeSat payloads. The power distribution for the CubeSat is designed for higher efficiency, utilizing high performance GaAs solar panels and DC/DC converters in combination with a MPPT technique to improve the power extraction and further increase efficiency.
author_names: Rachid Darbali-Zamora

Nicols Cobo-Yepes

John E Salazar-Duque

Eduardo I Ortiz-Rivera

Amilcar A Rincon-Charris
title: Buck Converter and SEPIC Based Electronic Power Supply Design with MPPT and Voltage Regulation for Small Satellite Applications
year: 2017

abstract: Exploitation of green energy sources is essential to diminish the deterioration of our environment. The energy harvesting, represents an alternative to achieve greater range in electric and hybrid vehicles. An energy management strategy (EMS) must be optimized to obtain the best benefits in such vehicles, which is not a trivial task. If harvesting or energy recovery devices are added, the EMS becomes a dual purpose algorithm: minimizing fuel consumption and maximizing energy harvest through maximum power point tracking (MPPT) controllers. Known studies consider separate EMS, one for traction and another for regenerative braking, without considering harvest devices such as solar panels, regenerative suspension, thermal generators, among others. Furthermore, the electronic power converters used, are not designed to handle such unequal power levels. In this article, an electronic platform to include multiple energy harvesting devices in a fuel cell hybrid electric vehicle, was presented together with a multiple MPPT EMS. The EMS is easily implementable, and considers quasiconstant cell energy extraction and filtering of current transients to the battery bank ensuring the longevity of the devices. A new mathematical model of the platform, a closed loop stability analysis, and numerical and Hardware in the Loop (HIL) validations were presented. Some experimental validation results were also provided.
author_names: Juan-Gerardo Parada-Salado

Luis-Fernando Gaona-Cardenas

Martin A Rodriguez-Licea

Francisco Javier Perez-Pinal
title: Harvesting in electric vehicles: combining multiple power tracking and fuel cells
year: 2020

abstract: This paper presents the second Electric Power Supply (EPS) prototype designed for supplying power to the Space Plasma Ionic Charge Analyzer (SPICA) CubeSat. In order to power the CubeSat with higher efficiency, Gallium Arsenide (GaAs) solar panels are mounted on each side of the cube, converting solar power into electrical energy. Solar cell area limitations require maximizing the available electrical energy generated. To extract the utmost available power from the solar panels, a Maximum Power Point Tracking (MPPT) method that uses the optimal duty ratio of a DC/DC converter is implemented. The DC/DC converter in combination with high performance GaAs solar panels and a MPPT technique will not only improve the power extraction but will also further increase efficiency.
author_names: Rachid Darbali-Zamora

Daniel A Merced-Cirino

Jose R Rivera-Alamo

Eduardo I Ortiz-Rivera

Amilcar A Rincon-Charris
title: Design and thermal testing of a Power Supply prototype for the Space Plasma Ionic Charge Analyzer (SPICA) CubeSat
venue: 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC)
year: 2015

abstract: Polymer solar cells may be applied in portable electronic devices, where light intensity and spectral distribution of the illuminating source can be very different compared to outdoor applications. As the power output of solar cells depends on temperature, light intensity and spectrum, the design of the module must be optimized for the specific illumination conditions in the different applications. The interconnection area between cells in a module must be as narrow as possible to maximize the active area, also called geometrical fill factor, of the module. Laser scribing has the potential to realize this. The optimal width of the interconnection zone depends both on technological limitations, e.g. laser scribe width and the minimal distance between scribes, and electrical limitations like resistive losses. The latter depends on the generated current in the cell and thus also on illumination intensity. Besides that, also the type of junction, i.e. a single or tandem junction, will influence the optimal geometry. In this paper a calculation model is presented that can be used for electrical modeling of polymer cells and modules in order to optimize the performance for the specific illumination conditions.
author_names: A R Burgers

Jan M Kroon

Lenneke H Slooff

Sjoerd Veenstra

Evert Eugene Bende
title: Parameter study for polymer solar modules based on various cell lengths and light intensities
year: 2013

abstract: The ongoing interest in organic and perovskite solar cells comes from the forecasted excellent potential for lowcost solar electricity generation that these technologies offer. Organic and perovskite solar cells can be fabricated from abundant materials with high throughput compatible processes on flexible substrates and promise near future large scale electricity production as well as novel and alternative applications in buildings, portable electronics or in tandem thin film devices. Research activities on perovskite solar cells have been exploding over the last years and a simple title search including 'perovskite solar cell' in the web of science delivers well over 1500 hits for the time January October 2018 alone! Likewise, organic solar cells are still in the spotlight and recent advances with ternary blends and non fullerene acceptors allow now for power conversion efficiencies exceeding 10% almost as a matter of routine. In this Focus Issue of Science and Technology of Advanced Materials, we present a collection of excellent research and review articles on recent milestones in materials development, device architectures and advanced characterization techniques that highlight the great progress that is being made in both photovoltaic technologies. The efficiency of ternary organic solar cells depends on the developing nanostructured network of donor and acceptor phases during film formation. In a typical ternary system for photovoltaic applications, a third sensitizing component is added to an underlying high performing binary system. However, finding the ideal three component composition by trial and error is a cumbersome process. The research paper of Makha and Heier et al. [1] demonstrates a general approach to understanding the behaviour of organic ternary blend solar cells from simple thermodynamic principles. The authors show that phase diagrams are a fundamental tool to better understand and control the morphology of ternary blend, thereby optimizing the solar cell performance in a rational way. Newman and Tsoi et al. [2] carried out a detailed study on the burn in period of a high performing solution processed organic small molecule solar cell under illumination. Device performance was followed as a function of prior solvent vapour annealing time, controlling the crystallization of the donor phase. Results from grazing incident X ray diffraction, UVvis absorbance, Raman and photoluminescence experiments indicate a correlation between the increase in cell stability and the degree of crystallinity of the donor; however, the degradation originates not directly from the crystallinity changes but correlates with changes in molecular conformation. Two articles are dealing with 'photon management' in organic and perovskite solar cells. Zhang and Toudert [3] review recent works reporting on optical management approaches for maximizing the light absorption and reducing transmission and reflection losses in perovskite solar cells. These include the design of optical cavities, the incorporation of plasmonic or dielectric nanostructures, the use of antireflection coatings and the structuration of the internal layers. This review points out that the performance of a solar cell is not only determined by the proper materials selection and fabrication method, but that optical optimization can substantially contribute to efficiency enhancement. The article of Pascual San Jose and Campoy Quiles et al. [4] considers that organic materials and perovskites are interesting for building integrated applications, where the colour and aesthetic appeal of a solar panel plays an important role. The authors implemented a theoretical methodology to assess colour tuning in polymer based solar cells. They compare quantitatively interference effects, binaries with different donors and acceptors, and ternary systems where the third component is either active or a simple dye. The concept can be applied to perovskite solar cells as well but the colour tuning capability of perovskites is smaller than for organic materials. The methodology allows also addressing the inverse problem, in which the materials and geometry that produce a targeted colour are obtained by a fitting routine.
author_names: Roland Hany

Hong Lin

Fernando A Castro
title: Focus issue on organic and hybrid photovoltaics
venue: Science and technology of advanced materials
year: 2019

abstract: Hole transport material optimization is an important step toward maximizing the efficiency of perovskite solar cells. Here, we investigate the role of one hole transport material property, the ionization potential, on the performance of perovskite solar cells. We employ a device architecture that allows us to systematically tune the ionization potential while avoiding any impact to other device parameters, and we find that for a wide range of ionization potentials the photovoltaic performance is minimally affected. This finding relaxes the requirement for the development of hole transport materials with particular ionization potentials, allowing for the optimization of hole transport materials that can improve performance in differing ways such as through increased stability or decreased parasitic absorption. In a brief moment in the history of solar cells, perovskite solar cells have risen from a humble improvement of dyesensitized solar cells to highly studied and specialized devices that have more in common with thin film solar cell technologies than with their dye predecessors. Through advances in materials processing, chemistry, and device engineering, power conversion efficiencies for devices made with hybrid perovskites have quickly climbed above 20% However, despite all of these advances, some components of the perovskite solar cell have remained relatively unimproved. Notably, six years after their original implementation, most perovskite solar cells still use spiro OMeTAD as the default hole transport material (HTM) which is surprising given the tremendous interest in developing novel HTMs for improved perovskite solar cell efficiency and stability. This encourages an investigation into the specific design parameters that make some HTMs better than others for high efficiency perovskite solar cells, with the objective of identifying a betterperforming HTM than spiro OMeTAD. One significant HTM design parameter is the ionization potential (IP) There are many groups that attribute improvements in the photovoltage of perovskite solar cells to increasing the IP of the HTM. Several researchers, including Polander et al. and Kulbak et al. have varied HTMs and see improvements in VOC for iodideand bromide containing perovskites, respectively, that correlate with increasing IP. These studies have strongly encouraged the design and implementation of new HTMs with IPs that more closely match the MAPbI3 valence band near 5.4 eV. However, as of yet, none of these new HTMs have been incorporated into devices with higher VOC than champion devices made with spiro OMeTAD (which has a lower IP, between 5.0 and 5.2 eV depending upon doping) Additionally, a recent report from Abate et al. showed similar efficiency for two devices made with HTMs of significantly different IPs. These results call into question whether tuning the IP is indeed necessary for improved performance. To better understand the role of the HTM and how to optimize it, our work undertakes a systematic investigation of the effects of varying HTM IP on perovskite solar cell performance. A complication in determining the role of the HTM IP on device performance is that changing the HTM often affects multiple facets of the solar cell, all of which can have a significant impact on photovoltaic performance. When the perovskite is deposited on top of the HTM in an "inverted" architecture, the HTM serves as a substrate and significantly affects the perovskite morphology and grain size in addition to acting as a selective contact. For HTMs deposited on top of the perovskite, solution processing can result in additional complications such as variations in the HTM film uniformity that may affect the shunting of the devices. Solventannealing or surface passivation of the perovskite from HTM additives can obscure the effects of changing the IP of the HTM. Additionally, HTMs may have varying selectivity (as determined by the semiconductor band gap) which should affect performance. In this study, we construct a device architecture that allows us to tune the IP of the HTM while minimally impacting any other device parameter to elucidate the true impact of the IP on the performance of CH3NH3PbI3 perovskite solar cells. We evaporate a series of HTMs on top of Received: July 12, 2016 Accepted: August 15, 2016 Published: August 15, 2016 Leter http:/pubs.acs.org/journal/aelccp (c) 2016 American Chemical Society 556 DOI: 10.1021/acsenergylett.6b00270 ACS Energy Lett. 2016, 1, 556 560 the perovskite, and in doing so, we find similar photovoltages for devices made with HTMs of widely varying IPs. These results suggest that the IP should not be the principal design constraint when developing novel HTMs, but instead, properties such as minimized parasitic absorption, beneficialprocessing, and effects on perovskite morphology should be optimized to maximize perovskite device efficiency. The device architecture for this study, depicted in Figure 1, was optimized for reproducibility as well as minimization of undesirable effects of the HTM deposition. A 15 nm layer of phenyl C61 butyric acid methyl ester (PCBM) was deposited on top of TiO2 to act as a reproducible and low hysteresis electron transport layer (ETL) A planar layer of CH3NH3PbI3 (MAPI3) was deposited from a 3:1 solution of methylammonium iodide and lead acetate as described elsewhere, resulting in a uniform and continuous perovskite layer as confirmed by SEM (Figure 1a) In addition to spiroOMeTAD, five organic small molecules were chosen with a range of IPs for use as HTMs for these devices (Supporting Information (SI) Figure 1) These molecules were chosen for their range in IP, their wide band gap to ensure selectivity (SI Figure 4) their demonstrated processing by thermal evaporation, and their previous use as contact layers in perovskiteor organic based devices. Measured IPs of the selected small molecules range from 5.0 to 5.5 eV, meaning that the highest occupied molecular orbital (HOMO) of these various molecules will either encourage or be a barrier to charge extraction depending upon the molecule. All HTMs except for spiro OMeTAD were deposited as a 15 nm thick conformal layer via thermal evaporation, removing the complexities of solution processing. To increase the conductivity of these films, 5 nm of MoO3 was evaporated onto the HTMs, leveraging the remote doping effect recently demonstrated by Sargent et al. Completed devices were tested to determine the effect of the HTM IP on the photovoltaic performance (Figure 2 and Table 1) The majority of HTMs employed in this study (meoTPD, spiroTTB, spiroTAD, a NPD, and spiro OMeTAD) showed comparable efficiency, with champion power conversion efficiencies of ~14% (SI Table 1) and minimal current voltage hysteresis for these evaporated HTMs (SI Figure 2) While these device efficiencies are significantly lower than those of record perovskite devices employing titania scaffolds and more complicated perovskite chemistries, they are comparable to the best device efficiencies for solar cells using the leadacetate deposition method that was chosen by this study for its reproducibility. Of all of the devices, only those made with TPD performed significantly differently. The uniqueness of TPD can be explained by the formation of an extraction barrier at the perovskite/HTM interface, as clearly indicated by the sharp skink JV characteristic and the restoration of the photocurrent at large reverse bias. This behavior is unsurprising given that the IP of TPD was the largest of the HTMs (as determined by photoemission spectroscopy in air) at approximately 5.4 eV (SI Figure 3) For devices with IPs within the band gap of MAPbI3, the variation in performance is minimal. All devices have similar photocurrents, suggesting that a substantial energetic offset between the valence band of the perovskite and HTM is unnecessary for current collection. The open circuit voltage, which is most often predicted to increase with deeper IP, only exhibits improvement on the order of tens of mV. When replacing MeO TPD with spiro TTB, the HTM IP is shifted by 150 mV, but the VOC is only improved by 40 mV. As the HTM Figure 1. Cross sectional SEM of the perovskite solar cell used in this study (a) and the associated device schematic (b) Architecture was employed for all evaporated HTMs with varying IPs as determined by photoemission spectroscopy in air (PESA) (c) Solid lines under HTMs show HOMO values measured by PESA, while boxes represent the range of HOMO values previously reported for these materials. Figure 2. Current voltage characteristics of champion perovskite devices made with alternative HTMs taken at 0.025 V/s from forward to reverse bias. ACS Energy Letters Letter DOI: 10.1021/acsenergylett.6b00270 ACS Energy Lett. 2016, 1, 556 560 557 IP is further deepened, no additional increases in VOC are observed. Ultimately, the VOC begins to decrease as the HOMO approaches the perovskite band edge, with a NPD devices having comparable VOC to those made with meoTPD, despite a 250 meV change in IP. Analyzing the variation in VOC with IP, it is clear that the overall response of the photovoltage of perovskite solar cells is flat; for a wide range of IPs, the VOC of the perovskite is practically unaffected. Beyond the IP, the similarity in doping density for the evaporated HTMs used in this study suggests that the VOC of these perovskite devices is independent of the HTM work function as well (as estimated by conductivity data in SI Table 2) According to the Fermi level approximation for nondegenerate semiconductors, by assuming a similar density of states, we can relate a constant carrier density to a fixed energetic offset between the HTM valence band and HTM work function; therefore, the work function of our evaporated HTMs should shift in step with the IP. The constant VOC reported here stands in sharp contrast to trends previously observed for dye sensitized solar cells, wh
author_names: Rebecca A Belisle

Pratham Jain

Rohit Prasanna

T Leijtens

Michael D McGehee
title: Minimal E ff ect of the Hole Transport Material Ionization Potential on the Open Circuit Voltage of Perovskite Solar Cells
year: 2016

abstract: Photon Enhanced Thermionic Emission (PETE) is a newly proposed form of solar energy harvesting which relies upon a combination of quantum and thermal processes to generate electricity. Unlike standard solar cells which rapidly lose efficiency at elevated temperatures, PETE is designed to operate at temperatures commensurate with thermal solar devices. In concentrated solar power designs the PETE module will receive the direct incident light, convert a fraction to electricity and transfer its waste heat to a solar thermal cycle. Theoretical efficiencies of combined PETE/solar thermal cycles reach above 50% and may provide a route to affordable renewable energy on the utility scale. The key challenges to this technology are developing higher efficiency cathodes that are stable at elevated temperatures, requiring both theoretical and experimental analysis of new materials. Over the last year we have demonstrated experimental results which match extremely well to the predicted physics for the PETE process, validating our initial concept. These experiments allow us to follow the behavior of a photoexcited electron, and prove that the electrons emitted from the PETE process come from a thermalized population, and also demonstrate a thermally boosted output voltage of up to 0.5 eV. These proof of concept devices based on GaN provide the first example of a combined thermionic and photo excited device. The device performance highlights two key challenges for improving the PETE device efficiency: carrier recombination and hightemperature material stability. In addition, we are constructing a new surface preparation and characterization chamber designed specially for PETE that can provide detailed measurements of material performance under anticipated operating conditions. Introduction Solar harvesting technology usually takes one of two forms; the 'quantum' approach using the large per photon energy as in photovoltaic (PV) cells, or the 'thermal' approach using solar radiation as the heat source for a classical heat engine. Quantum processes boast high theoretical efficiencies but suffer in practice from a limited spectral energy collection window, whereas thermal processes have inherently lower efficiency limits but take advantage of energy throughout the entire solar spectrum. Simple combinations of the two fail because PV cells rapidly lose efficiency at elevated temperatures, while heat engines rapidly lose efficiency at low temperatures. As a result, these two approaches remain disjointed. Photon Enhanced Thermionic Emission (PETE) is a method of solar harvesting that uses the quantum nature of solar photons along with thermal energy to generate electricity at temperatures compatible with solar thermal engines. The method relies on a semiconductor cathode separated by vacuum gap from an anode (Figure 1a) Incident photons excite electrons into the cathode's conduction band. If these carriers reach the surface before recombination, they can emit into vacuum and be collected by the anode, generating a voltage. Physical separation of the anode and cathode allows a large temperature differential, reducing the thermally generated reverse current which limits PV cells at high temperatures.[1] Thus, unlike other proposed "hot electron" devices, [25] the PETE process is still efficient for fully thermalized electrons, providing a more realistic path to hot electron harvesting. Figure 1: The PETE process. a, Energy diagram of the PETE process. Photoexcitation increases the conduction band population, leading to larger thermionic currents and allowing the device to harvest both photon and heat energy. b, One possible implementation of a parallel plate PETE converter. Photons impinge on a nanostructured cathode and excite electrons, which then emit into vacuum and are collected by an anode. Unused heat from the PETE cycle is used to drive a thermal engine. The possibility for efficient operation at elevated temperatures makes PETE an ideal candidate for high concentration solar applications, including use as a "topping cycle" for a solar thermal generator, wherein the PETE device would harvest a fraction of incident solar energy and deliver the rest as heat to a backing solar thermal engine (Figure 1b, 2) Even a PETE module with modest 20% efficiency in tandem with a 30% efficient Stirling solar thermal engine could boast a remarkable total system efficiency of 44% Background In a photovoltaic cell, solar photons with energies above the semiconductor's band gap excite electrons into the conduction band, which diffuse to electrodes and generate current. In high performance solar cells, charge separation and collection are very efficient. However, the quantum approach of PV cells places intrinsic limitations on single junction conversion efficiency. Photon energy in excess of the band gap is lost as heat, known as thermalization loss, and sub bandgap photons are not absorbed at all, known as absorption loss. In silicon solar cells, thermalization and absorption losses account for approximately 50% of the incident solar energy the majority of the total energy loss. In principle, these losses could be reclaimed by using this waste heat from the PV cell to power a secondary thermal cycle. Combinations of PV and thermal engines are predicted to have efficiencies >60% yet fail in practice because PV cells rapidly lose efficiency at elevated temperatures, while heat engines rapidly lose efficiency at low temperatures. Thermionic energy converters (TEC) are less well known heat engines which directly convert heat into electricity. A simple thermionic converter consists of a hot cathode and cooler anode separated by a vacuum gap. In the TEC cathode, a fraction of the electrons have sufficient thermal energy to overcome the material's work function and escape into vacuum, generating current between the two electrodes. The thermionic current density is dictated by the cathode work function and temperature according to the Richardson Dushman (RD) equation: C C kT C C e T A J 2 where C is the cathode work function, TC the temperature, and C A is the material specific Richardson constant. Thermionic converters were first proposed and fabricated in the 1950s, with experimental conversion efficiencies eventually reaching 10 15% Both NASA and the Soviet space program funded the development of TECs for deep space missions and other applications requiring high power autonomous generators, but the technology was never commercialized. Thermionic conversion's main challenges relate to the very high temperatures and substantial current densities required for efficient operation. Photon Enhanced Thermionic Emission (PETE) combines photovoltaic and thermionic effects into a single physical process to take advantage of both the high perquanta energy of photons, and the available thermal energy due to thermalization and absorption losses. A PETE device has the same vacuum gap parallel plate architecture as Figure 2: Energy flow for a tandem PETE/solar thermal cycle. Waste heat from the PETE device is used to power a conventional thermal engine, boosting the overall efficiency. a TEC, except with a p type semiconductor as the cathode (Fig. 1) PETE occurs in a simple three step process: First, electrons in the PETE cathode are excited by solar radiation into the conduction band. Secondly, they rapidly thermalize within the conduction band to the equilibrium thermal distribution according to the material's temperature and diffuse throughout the cathode. Finally, electrons that encounter the surface with energies greater than the electron affinity can emit directly into vacuum and are collected at the anode, generating current (Fig. 1a) Each emitted electron thus harvests photon energy to overcome the material bandgap, and also thermal energy to overcome the material's electron affinity. The total voltage produced can therefore be higher than for a photovoltaic of the same bandgap due to this 'thermal boost, thus more completely utilizing the solar spectrum. Figure 3: Theoretical PETE efficiency. a, PETE efficiency for the AM1.5 direct+circumsolar spectrum as a function of band gap. Cathode temperature and electron affinity are chosen to maximize overall efficiency. Anode temperature is 227degC to minimize reverse current. In the inset, the power output at 1000x is shown to be due to roughly equal contributions from thermal (kh) and photon (Eg) energy. b, Power conversion efficiency increases with due to larger voltage but requires higher temperatures. Power output decreases almost linearly with temperature after current saturation in the PETE regime due to the increasing cathode Fermi level. Concentration is 1000x. c, J V curves for PETE devices with the same electron affinities in (b) showing the flat band condition for =0.4eV The temperatures are those at which the efficiencies in (b) are maximized.
author_names: Nicholas A Melosh

Stanford Simes

Zhi-Xun Shen

Jared W Schwede

Daniel P Riley

Samuel Rosenthal

Vijay K Narasimhan

Xiaofei Ye

Kunal Sahasrabddhe

Nazinin Devani
title: Photon Enhanced Thermionic Emission for Solar Energy Harvesting Final Report to the Global Climate and Energy Project
year: 2013