Low-bandgap Solar Cells Research Articles

Improved Sequentially Processed Cu(In,Ga)(S,Se)2 by Ag Alloying

Alloying small quantities of silver into Cu(In,Ga)Se2 is shown to improve the efficiency for wide and low bandgap solar cells. Low bandgap industrial Cu(In,Ga)(S,Se)2 absorbers are studied, substituting less than 10% of the copper with silver, using absolute photoluminescence and cathodoluminescence spectroscopy. Silver improves the grain size and promotes the interdiffusion of Ga and In across the depth of the absorber, resulting in a smoother bandgap gradient. However, a certain lateral inhomogeneity is observed near the front and back sides. The nonradiative losses in the bare absorbers are reduced by up to 30 meV.

Synthesis of Fluorene Based Alternating Copolymers using Direct Arylation Polymerization

Many researches have been done to obtain a low band gap and high Polymeric solar cell (PSCs) polymer either by creating new polymer or revising reported polymers from previous studies. In present work, two new copolymers were synthesized through direct arylation polymerization to produce poly(9,9-didodecylfluorene-alt-benzo[c][1,2,5]thiadiazole (P1) and poly(9,9-didodecylfluorene-altthieno[ 3,2-b]thiophene) (P2). The P1 and P2 are donor-accepter copolymers. P1 and P2 were compared to investigate its suitability to be applied in PSCs. The polymers obtained were characterized using FT-IR, NMR and UV-Vis spectroscopy. P1 shows two adsorption bands at λmax1 = 243 nm and λmax2 = 320 nm, whereas P2 also shows two adsorption bands at λmax1 = 243 nm and λmax2 = 427 nm. The optical band gap was calculated, P1 enabled band gap of 3.88 eV while P2 showed band gap of 2.91 eV. This work could be provided an insight to design and synthesize more efficient fluorene-based copolymers as active layer of PSCs in due course.

Optics of Perovskite Solar Cell Front Contacts.

The front contact has a major impact on the electrical and optical properties of perovskite solar cells. The front contact is part of the junction of the solar cell and must provide lateral charge transport to the terminals and should allow for an efficient light incoupling, while having low optical losses. The complex requirements of the perovskite solar front contact will be described and the optics of the front contact will be investigated. It will be shown that the front contact has a distinct influence on the short-circuit current and energy conversion efficiency. Metal oxide films were investigated as potential front contacts. The incoupling of light in the solar cell is investigated by three-dimensional finite-difference time-domain optical simulations and optical measurements of experimentally realized self-textured zinc oxide films. The zinc oxide films were prepared by metal-organic chemical vapor deposition at low temperatures. Furthermore, the influence of free carrier absorption of metal oxide films on the optics of low bandgap and/or tandem solar cells is investigated. Guidelines are provided on how to choose the doping concentration and thickness of the metal oxide films. Finally, it will be shown that by selecting an optimal front contact design the short-circuit current and energy conversion efficiency can be increased by at least 15%.

Low band‐gap CuIn(S,Se)2 thin film solar cells using molecular ink with 9.5% efficiency (Phys. Status Solidi C 6/2017)

In order to achieve low band-gap thin film solar cells which can play an important role in tandem solar cells combined with perovskite top solar cells, the preparation of chalcopyrite absorbers is a major task. Wang et al. (article no. 1600169) report a non-vacuum simple solution coating approach for CuIn(S,Se)2 absorbers using an air stable molecular ink containing metal chlorides and thiourea in organic solvents. Careful balancing of ink concentration, number of precursor deposition cycles, and annealing temperature profile will transform the multilayer precursor into a sufficiently thick, continuous, large grained film as required for high quantum efficiency, and good red response of the cells. These optimizations aimed at high photo current have already led to an efficiency of 9.5% at a band-gap of 1.02 eV.

Concepts for external light trapping and its utilization in colored and image displaying photovoltaic modules

AbstractThe reflection of incident sunlight prevents photovoltaic modules from reaching their full energy conversion potential. Recently, we demonstrated significant absorption enhancement in various solar cells by external light trapping, using 3D‐printed and milled light traps. In order to facilitate direct module integration, we introduce an external light trapping design concept tackling the reflection issues at the module level. In this module design, a lens array on the module cover glass funnels the incident sunlight through small apertures in a reflective coating at the backside of the cover glass. This adapted cover glass can be applied on a conventional module design. The reflector and the solar cells together form an optical cavity, in which most reflected light is recycled by directing the reflected light back to the solar cell. A unique feature of this light trapping module is its capability to simultaneously recycle the broadband reflection from the metal front grid, the front interfaces, the reflective backside of the cell, and the white back sheet. Moreover, it allows separate integration and contacting of low and high bandgap solar cells for highly efficient hybrid tandem modules that have no current matching related losses. We show five module designs for improved light management combined with additional features such as color and image display. We discuss the optimal harvesting of the diffuse and direct component of the sunlight. The outlook of highly efficient, colored, and even image displaying modules makes this technology an interesting candidate for a new class of photovoltaic modules. Copyright © 2017 John Wiley & Sons, Ltd.

Back reflectors with periodic gratings for light trapping in a-SiGe:H solar cells

In the present study, we introduce a new method for preparing textured reflectors for thin film solar cells: this novel design of textured surfaces increases light trapping in tandem solar cells. The structure under study comprises periodic nanoislands fabricated using self-assembly silica spheres, which are applicable to large area fabrication with a low cost. First we focus this study on the fabrication of textured back reflector and their optical properties. The periodic nanoislands increase the diffuse reflectivity of the back reflector. The optical results are compared to industrial solutions currently used in tandem solar cells. Optical simulation was also performed in order to design the best diffusive textured back reflectors. In a second time we applied this textured back reflector on a-SiGe:H low band gap solar cells. This cell was only selected in order to be a reference for demonstrating the efficiency of textured back reflector in bottom cell (low band gap) of tandem structures. External Quantum Efficiency (EQE) and electrical characteristics (J(V) experiments under AM1.5 illumination) are measured on a-SiGe:H solar cells with an optimized textured back reflector and a standard flat silver reflector. An improvement of 10% in current density with this low cost solution compared to standard silver back reflector is obtained. Moreover, the other electrical parameters are not damaged by applying this process of texturation which leads to a 15% enhancement of the solar cell efficiency.

Novel fluoride-substituted donor/acceptor polymers containing benzodithiophene and quinoxaline units for use in low–band gap solar cells

In this study we used Stille polycondensation to synthesize a series of low–band gap fluoride-substituted conjugated polymers containing electron-donating benzodithiophene (BDT) and electron-accepting quinoxaline (Qx) units, with the latter presenting either two or four fluoride atoms. These donor/acceptor polymers exhibited a broad absorption range (from 300 to 750nm), with band gap energies of less than 1.20eV (measured using cyclic voltammetry). Because of their strongly electron-donating thiophene units, the donor/acceptor polymers featuring the more highly fluorinated structures possessed the lowest band gaps (as low as 1.14eV) and, thus, induced the greatest photovoltaic performance among these tested polymers. The highest power conversion efficiency (2.13%) was obtained from the polymer solar cell featuring an active layer of the polymer PTF2 blended with PC70BM (1:1.5, w/w), with a short-circuit current density, open circuit voltage, and fill factor of 8.7mAcm−2, 0.72V, and 0.34, respectively.

Highly conductive ZnO films with high near infrared transparency

AbstractWe present an approach for deposition of highly conductive nominally undoped ZnO films that are suitable for the n‐type window of low band gap solar cells. We demonstrate that low‐voltage radio frequency (RF) biasing of growing ZnO films during their deposition by non‐reactive sputtering makes them as conductive as when doped by aluminium (ρ≤1·10−3Ω cm). The films prepared with additional RF biasing possess lower free‐carrier concentration and higher free‐carrier mobility than Al‐doped ZnO (AZO) films of the same resistivity, which results in a substantially higher transparency in the near infrared region (NIR). Furthermore, these films exhibit good ambient stability and lower high‐temperature stability than the AZO films of the same thickness. We also present the characteristics of Cu(InGa)Se2, CuInSe2 and Cu2ZnSnSe4‐based solar cells prepared with the transparent window bilayer formed of the isolating and conductive ZnO films and compare them to their counterparts with a standard ZnO/AZO bilayer. We show that the solar cells with nominally undoped ZnO as their transparent conductive oxide layer exhibit an improved quantum efficiency for λ > 900 nm, which leads to a higher short circuit current density JSC. This aspect is specifically beneficial in preparation of the Cu2ZnSnSe4 solar cells with band gap down to 0.85 eV; our champion device reached a JSC of nearly 39 mAcm−2, an open circuit voltage of 378mV, and a power conversion efficiency of 8.4 %. Copyright © 2015 John Wiley & Sons, Ltd.

Investigation of Radical and Cationic Cross‐Linking in High‐Efficiency, Low Band Gap Solar Cell Polymers

Dithienogermole‐co‐thieno[3,4‐c]pyrroledione (DTG‐TPD) polymers incorporating chemically cross‐linkable sidechains are reported and their properties compared to a parent polymer with simple octyl sidechains. Two cross‐linking groups and mechanisms are investigated, UV‐promoted radical cross‐linking of an alkyl bromide cross‐linker and acid‐promoted cationic cross‐linking of an oxetane cross‐linker. It is found that random copolymers with a 20% incorporation of the cross‐linker demonstrate a higher performance in bulk heterojunction solar cells than the parent polymer, while 100% cross‐linker incorporation results in deterioration in device efficiency. The use of 1,8‐diiodooctane (DIO) as a processing additive improves as‐cast solar cell performance, but is found to have a significant deleterious impact on solar cell efficiency after UV exposure. The instability to UV can be overcome by the use of an alternative additive, 1‐chloronapthalene, which also promotes high device efficiency. Cross‐linking of the polymer is investigated in the presence and absence of fullerene highlighting significant differences in behavior. Intractable films cannot be obtained by radical cross‐linking in the presence of fullerene, whereas cationic cross‐linking is successful.

Design, fabrication and analysis of germanium: silicon solar cell in a multi-junction concentrator system

A Ge:Si solar cell under a silicon solar cell can lead to as much as a 5.5% absolute efficiency gain for a multi-junction solar module at 30× concentration. This work demonstrated short circuit current densities that were 93% of the model prediction and open circuit voltages that were 92% of the model predictions for 88% Ge content, Ge:Si solar cells below Si at 30 suns. Silicon solar cells can absorb few photons in the wavelength range above 1150nm due to the effect of the absorption coefficient. One possible method to enhance the absorption of long wavelength photons is to apply a Ge solar cell below Si. However, this method is industrially impractical due to the high cost of Ge substrates. In this work, a low cost Ge:Si solar cell grown on silicon with strong long wavelength light sensitivity will be demonstrated. This work starts with an all epitaxial growth design, analyzes the performance limits, examines the trade-offs between solar cell performance, Ge composition and material quality and concludes with the pathways to higher efficiency. The high quality Ge:Si layers with Ge content above 85% were achieved on Si substrates using reduced pressure chemical vapor deposition (RPCVD) technology. Three high Ge content Ge:Si solar cells were designed, fabricated and analyzed. The encouraging results experimentally prove that low cost Ge:Si solar cells grown on Si can have high performance below Si. This has been achieved as a direct result of low dislocation density step graded Ge:Si buffers developed in this research. In this paper, the pathway to achieve low cost and high efficiency Ge:Si low band gap solar cells grown on silicon is described.

Design, fabrication, and analysis of transparent silicon solar cells for multi‐junction assemblies

ABSTRACTTransparent silicon solar cells can lead to an increased efficiency of silicon‐based multi‐junction assemblies by transmitting near and below band gap energy light for conversion in a low band gap solar cell. This analysis shows that the maximum efficiency gain for a low band gap solar cell beneath silicon at a concentration of 50 suns is 5.8%, based on ideal absorption and conversion of the photons. This work analyzes the trade‐offs between increased near band edge absorption in the silicon and silicon solar cell transparency. Application of these results to real cases including a germanium bottom solar cell is analyzed, leading to a range of cases with increased system efficiency. Non‐ideal surfaces and real silicon and germanium solar cell device performance are presented. The range of practical system gains may be as low as 2.2 – 1% absolute when compared with the efficiency of a light‐trapped silicon solar cell for 1‐sun operation, based on this work. Copyright © 2012 John Wiley & Sons, Ltd.

A study of optical properties enhancement in low-bandgap polymer solar cells with embedded PEDOT:PSS gratings

Typically, most low bandgap materials have low absorption with wavelength at around 500nm. Besides, the restrictions of active layer thickness of thin film polymer solar cells (PSCs) make the devices reduce to absorb light in long wavelength region (around 700nm). As absorption would be a joint effect of material band properties and optical structures, well-designed light-trapping strategies for these low-bandgap PSCs will be more useful to further enhance efficiencies. We investigate the change of optical properties and device performances of organic solar cells based on our newly synthesized low-bandgap material with embedded poly-(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) PEDOT:PSS grating in the photoactive bulk heterojunction layer. Our results show that the PEDOT:PSS grating with a period of 320nm and depth of 40nm makes the light absorption improved in specific regions of the solar spectrum, especially the weak absorption region of our bulk heterojunction material near 500nm and the red/near-infrared region at around 700nm. The incident photon-to-electron conversion efficiency (IPCE) also improves with corresponding enhancement peaks. The physical understanding of the absorption enhancement will be investigated and described through our theoretical study. The power conversion efficiency is improved due to the enhancement of short circuit current. The work demonstrates the absorption enhancement of low bandgap solar cells using appropriate grating structures and provides the physical understanding of the absorption enhancement for improving the performances of organic solar cells.

Changes in the structural and electrical properties of vacuum post-annealed tungsten- and titanium-doped indium oxide films deposited by radio frequency magnetron sputtering

Tungsten- and titanium-doped indium oxide (IWO and ITiO) films were deposited at room temperature by radio frequency (RF) magnetron sputtering, and vacuum post-annealing was used to improve the electron mobility. With increasing deposition power, the as deposited films showed an increasingly crystalline nature. Compared with ITiO films, IWO films showed crystallinity at lower RF power. IWO films are partially crystallized at 10 W deposition power and become nearly fully crystalline at 20 W. ITiO films are fully crystalline only at 75 W. For this reason, film thickness has a greater impact on the electrical properties of IWO films than ITiO films. Vacuum post-annealing is more effective in improving electron mobility for amorphous than for (partially) crystalline IWO and ITiO films. Changes in the electrical properties of ITiO films can be better controlled as a function of annealing temperature than those of IWO films. Finally, post annealed 308 nm-thick IWO and 325 nm-thick ITiO films have approximately 80% transmittance in visible and near infrared wavelengths (up to 1100 nm), while their sheet resistances decrease to 9.3 and 10 Ω/□, and their electron mobilities are 51 cm 2V − 1 s − 1 and 50 cm 2V − 1 s − 1 , respectively, making them suitable for use as Transparent Conductive Oxide layers of low bandgap solar cells.

Electrical properties of vacuum-annealed titanium-doped indium oxide films

Titanium-doped indium oxide (ITiO) films were deposited on Corning glass 2000 substrates at room temperature by radio frequency magnetron sputtering followed by vacuum post-annealing. With increasing deposition power, the as-deposited films showed an increasingly crystalline nature. As-deposited amorphous ITiO films obtained at 20 W began to crystallize at the annealing temperature of 155 °C. Although there was no significant change in the crystalline structure of the films, electron mobility improved gradually with further increase in the annealing temperature. After post-annealing at 580 °C, the highest electron mobility of 50 cm 2 V −1 s −1 was obtained. Compared with the amorphous ITiO films, the ITiO films with a certain degree of crystallinity obtained at high deposition power were less affected by the vacuum annealing. Their electron mobility also improved due to post-annealing, but the increase was insignificant. After post-annealing, the optical transmission of the 325 nm-thick ITiO films showed approximately 80% at wavelengths ranging from 530 to 1100 nm, while the sheet resistance decreased to 10 Ω/sq. This makes them suitable for use as transparent conductive oxide layers of low bandgap solar cells.

Role of balanced charge carrier transport in low band gap polymer:Fullerene bulk heterojunction solar cells

AbstractLowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylenevinylene] and 6,6‐phenyl C61‐butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Thermocleavable Low Band Gap Polymers and Solar Cells Therefrom with Remarkable Stability toward Oxygen

Thermocleavable esters of low band gap monomers and polymers based on diphenyldithienylthienopyrazine were prepared by incorporating carboxylic acid functionalities into the system. A series of different ester groups were prepared and the temperature of elimination of the ester group was studied. The lowest temperatures of elimination obtained were in the range 220−240 °C for tertiary esters giving the free acid. The highest temperatures of elimination were found for primary esters that also lead to decomposition of the molecule. Only the tertiary esters offer a good degree of control over the chemistry in the thermocleaved product. The photovoltaic performance of the polymers prepared was tested under simulated sunlight (1000 W m−2, AM1.5G, 72 °C) and the best power conversion efficiency that could be reached for devices with an active area of 3 cm2 was up to 0.4% in an ITO/PEDOT/polymer−PCBM/aluminum device geometry. The best performing polymer material was subjected to lifetime studies in four differen...

Deposition of Optimal a-Si:H and a-SiGe:H by HWCVD Using the Same Filament Temperature and Substrate Temperature

ABSTRACTThe incorporation of high Ge content a-SiGe:H into a low bandgap solar cell device commonly involves the use of bandgap (Ge) profiling. In previous work using the hot wire (HWCVD) technique, device quality low bandgap (ETauc= 1.25eV) a-SiGe:H films were deposited at low Tsub(∼200-250°C) filament operating at Tfil∼ 1750-1800°C. However, higher bandgap films containing little or no Ge and deposited under the same low temperature (Tsub,Tfil) conditions were of decidedly inferior quality to those deposited using higher temperatures (Tfil∼2000°C,Tsub∼360°C) solar cell with an efficiency ∼ 5.85% was fabricated using these materials, it was clear that our best ‘end point’ materials alloying at this low Tfilseverely limits film reproducibility and filament lifetime. This work explores deposition of device quality low bandgap a-SiGe:H and (high bandgap) a-Si:H, both at the same low Tsub, using a tantalum (Ta) filament operating at low Tfil. Film material properties are presented.

Thin film tandem solar cells based on CuInSe 2

The CuInSe 2/(CdZn)S heterojunction is the best developed low bandgap solar cell for use in a two-junction tandem device. The potential performance of large area terrestrial systems based on this junction is reviewed. A monolithic tandem cell in which the high bandgap cell is a (CdHg) Te/CdS heterojunction deposited onto a CuInSe 2/(CdZn)S cell is being developed and progress with this system is described.

Low-bandgap (1.15 eV) InGaP/InGaAs solar cell

The possibility of using In0.18Ga0.82As p-n junctions as a low-bandgap (1.15 eV) solar cell has been investigated. With a lattice-matched p+ In0.6Ga0.4P window layer, internal collection efficiency close to 100% has been demonstrated.