Reduction In Open-circuit Voltage Research Articles

Quantifying recombination and charge carrier extraction in halide perovskites via hyperspectral time-resolved photoluminescence imaging

Identifying sources of nonradiative recombination and quantifying charge carrier extraction in halide perovskite solar cells are important in further developing this thin-film technology. Steady-state and time-resolved photoluminescence (TRPL), in combination with analytical modeling, have emerged as non-destructive tools to achieve the desired results. However, the exact location of the recombination and charge carrier extraction losses in devices is often obscured by various competing processes when photoluminescence measurements are analyzed. Here, we show via absolute-photon-calibrated hyperspectral photoluminescence and TRPL imaging how surface passivation and inhomogeneities at interfaces impact the photoluminescence quantum yields and minority carrier lifetimes. Laser illumination from the perovskite and glass/TiO2 sides allows us to disentangle changes in surface recombination velocity from the charge carrier extraction at the electron transport layer. We find that charge extraction is spatially modulated due to an inhomogeneous mesoporous (mp)-TiO2 film thickness. Our results show that the mp-TiO2 layer is not fully optimized since the electronic properties are spatially modified, leading to lateral changes in quasi-Fermi-level splitting, minority carrier lifetime and, consequently, a reduction in open-circuit voltage.

Achieving better balance on the mechanical stability and conduction performance of sulfonated poly(ether ether ketone) proton exchange membranes through polydopamine/polyethyleneimine co-modified poly(vinylidene fluoride) nanofiber as support

In this work, the polyvinylidene fluoride (PVDF) electrospun fibers was applied as robust mechanical support and subsequently functionalized with polydopamine/polyethyleneimine (PDA/PEI) composite, then filled with sulfonated poly (ether ether ketone) (SPEEK) to obtain SPEEK-PDA/PEI@PVDF composite proton exchange membranes. The composite membranes achieved better balance on the mechanical stability and conduction performance, owing to enhanced interface interactions and compatibility between robust PVDF and conductive SPEEK. In wet conditions, the composite membrane exhibited excellent mechanically robust with tensile strength of 34 MPa and elongation at break of almost 200 %. Furthermore, the composite membranes also showed high structure and performance stability during the measurement of proton conductivity and fuel cell performances, it could hold stable proton conductivity of 48 mScm−1 (at 80 °C) and maximum power density of 58.9 mWcm−2. The durability test further confirmed the super operating stability of composite membrane, only 2.6 % reduction in open circuit voltage was observed during operated at 80 °C for 100 h.

Open circuit voltage reduction due to recombination at the heterojunction solar cell edge

Today to achieve high efficiency solar cells, the crystalline silicon (c-Si) heterojunction (HJ) represents one of the best available options. The key factor of its success is the high open circuit voltage (Voc) achievable, because of the excellent surface passivation due to the intrinsic amorphous silicon (a-Si:H) layers. During cells manufacturing, the a-Si:H film deposition is simultaneously performed on both c-Si wafer sides, and consequently also on the edges of the wafer. However, the wafer edges could result non-passivated in many cases such as the so-called shingling manufacturing route.Moreover, at laboratory level it is quite common to manufacture small area cells from larger wafers. When a silicon edge is left uncovered by cutting, a recombining region is created due to the silicon non-passivated surface. This, in principle, leads to a reduction in the Voc of the cell.Nevertheless, this is not experienced for large area cells cut in half but it is commonly observed that cutting silicon HJ edges results in a lowering in Voc. In this work we have analyzed the correlation between the Voc, the cell area and the recombining surface introduced by cutting the cell into a smaller one. We have monitored the Voc as a function of the cell area during time, and we also have investigated the possibility of a re-passivation of the cell edges by depositing a thick a-Si:H layer after masking the sun exposed cell surface, exploring different deposition conditions to avoid re-annealing of the existing a-Si:H layers.

TOPcon route with quantum wells in GaInP/Si dual junction cell for efficiency enhancement

The GaInP/Si dual junction solar cell with Quantum Well and TOPCon technology has achieved 41.91 % efficiency in the simulation model. Quantum wells are proven to enhance the short circuit current without any significant degradation of open circuit voltage. The voltage preservation measure (for minimum reduction in open circuit voltage) is applied in design of Quantum well solar cell with the improvement in short circuit current, therefore the overall efficiency of dual junction cell is enhanced. In this work, the GaInP/Si dual junction solar cell is proposed by incorporating the InP quantum wells in the p-i-n region of GaInP top cell. Also, the TOPCon technology is applied at the rear side of the bottom c-Si solar cell. The quantum well increases the absorption of sub-bandgap photon energy. The buffer layer reduces the misfits between the lattice mismatched layers and tunnel oxide provides excellent passivation mechanism to increase the open circuit voltage. The ultrathin oxide layer provides extremely high electric field as described by the MIS tunneling theory. The electron and hole bound state energy and wavefunctions provides “miniband” formation and charge carrier trapping. The formation of type-A band structure in quantum well region and excellent electron mobility results in enhanced carrier transport. The GaInP/Si integrated model achieves efficiency of 33.30 % when the quantum wells are incorporated in the top cell. When the Combination of both QWs (in top cell) and TOPCon technology (in bottom cell) is applied, the model achieves 41.95 % efficiency. This model provides an excellent route toward achieving the 45 % efficiency limit of III-V/Si dual junction 2-terminal solar cell. The industry standard Silvaco TCAD platform is used for the calculation of the performance characteristics in AM1.5G environmental condition.

Influence of Acid-Doped Poly(ethylene imine) Interlayers on the Performance of Polymer:Nonfullerene Solar Cells.

This work reports that ultrathin polymeric films doped with organic acid molecules can act as an electron-transporting interfacial layer in polymer:nonfullerene solar cells. The polymeric interfacial layers, which consist of poly(ethylene imine) (PEI) doped with 3-hydroxypropane-1-sulfonic acid (HPSA) at various HPSA molar ratios, are introduced between transparent indium-tin oxide (ITO) electrodes and polymer:nonfullerene bulk heterojunction layers. The HPSA-doped PEI (PEI:HPSA) films are optically translucent in the wavelength range of ≈300-800nm, while the acidity of PEI solutions reached ≈pH = 7 at HPSA = 30 mol%. The power conversion efficiency of solar cells is improved by doping 20 mol% HPSA due to the increased short circuit current density without open circuit voltage reduction. The improvement in solar cell performances is attributed to an adequate control of HPSA doping ratios, which spares undoped amine units of PEI for making sufficient net dipole layers with ITO surfaces and makes permanent charges for high electrical conductivity in the layers. The surface morphology and doped states are characterized with atomic force microscopy and X-ray photoelectron spectroscopy.

Role of p-GaN layer thickness in the degradation of InGaN-GaN MQW solar cells under 405 nm laser excitation

GaN-based solar cells with InGaN multiple quantum wells (MQWs) are promising devices for application in space environment, concentrator solar systems, wireless power transmission and multi-junction solar cells. It is therefore important to understand their degradation kinetics when submitted to high-temperature and high-intensity stress. We submitted three samples of GaN-InGaN MQW solar cells with p-AlGaN electron-blocking-layer with different thickness of the p-GaN layer to constant power stress at 310 W/cm2, 175 °C for several hundred hours. The main degradation modes are a reduction of open-circuit voltage, short-circuit current, external quantum efficiency, power conversion efficiency and electroluminescence. In particular, we observed that a thinner p-GaN layer results in a stronger degradation observed on the cell operating parameters. The analysis of the dark I-V characteristics showed an increase in low-forward bias current and the analysis of electroluminescence showed a decrease in the electroluminescene emitted by the (forward biased) cell, as a consequence of stress. This work highlights that the cause of degradation is possibly related to a diffusion mechanism, which results in an increase of defect density in the active region. The impurities involved in the diffusion processes possibly originate from the p-side of the devices, therefore a thicker p-GaN layer reduces the amount of defects reaching the active region.

Polydopamine-coated graphene for supercapacitors with improved electrochemical performances and reduced self-discharge

Graphene-based supercapacitors have attracted significant attention owning to the high specific surface area and electrical conductivity of graphene with the potential to deliver superior energy and power performances than other supercapacitor materials. However, graphene-based supercapacitors inevitably have self-discharge behavior like any other supercapacitors, resulting in rapid voltage drop and limiting their applications for long-term energy storage. Reported methods for mitigating the self-discharge of graphene-based supercapacitors are often associated with deteriorated performances such as decreased specific capacitances or rate performances. Here we report that self-polymerized dopamine on the surface of graphene can reduce the self-discharge of supercapacitors while achieving improved specific capacitance and rate performance. Specifically, a 40% increase in specific capacitance (150.8 vs. 108.0 F g–1 at 0.1 A g–1), much improved capacitance retention at high currents (80% vs. 55% at 10 A g–1), and a 37% reduction in open circuit voltage (OCV) decay rate (0.33 V vs. 0.52 V in 12 h) were attained simultaneously after PDA-coating on graphene. Analysis of the open circuit potential attenuation of both positive and negative electrodes of the supercapacitors suggests that the coating of PDA suppressed the self-discharge caused by diffusion-controlled faradaic reaction process. This can be attributed to the catechol and amine functional groups of PDA which not only offered additional pseudocapacitance when the electrodes were charged but also served as trapping sites for dissolved impurity ions and oxygen molecules so that the reactions of these species on graphene surface were impeded during open circuit test, leading to both improved energy storage performances and reduced self-discharge rate.

Origin of open-circuit voltage reduction in high-mobility perovskite solar cells

Despite the rapid progress made in solar cell researches based on organic–inorganic hybrid perovskites, a fundamental understanding of their operation principles is still limited. This problem is associated with a lack of parameterization and modeling tools that capture complete cell behaviors. This article provides a new insight into the charge-carrier mobility in perovskite solar cells using experimentally calibrated numerical simulations. Although increasing the mobility substantially improves the short-circuit current, a simultaneous decrease in the open-circuit voltage is observed, eventually inducing efficiency roll-off in the high-mobility regime. The increased bending of potential profiles (electrode regions) and decreased electric field (central region) due to carrier diffusion were found to be the key mechanisms behind this behavior, thus providing a theoretical guideline for material and device engineering with the goal of optimum cell performance.

Experimental analysis of internal leakage current using a 100 cm2 class planar solid oxide fuel cell

This work studies the effect of the electronic conductivity of a commercially used yttria-stabilised zirconia (YSZ) electrolyte on solid oxide fuel cell performance. For the first time, an experimental method is used to measure the electronic conductivity of YSZ in commercially manufactured and operated single cells. An inert-gas step addition (ISA) method is employed to measure the electronic conductivity in the form of voltage changes at the open-circuit state. Since the electronic conductivity allows oxide ion transfer and water generation at the anode, a reduction in open-circuit voltage (EOCV) occurs due to the increased water partial pressure. The step increase in the anode flow rate in the ISA method reduces water partial pressure and raises EOCV. The EOCV change is then converted into internal leakage currents and electronic resistivities. The YSZ's electronic resistivity obtained in this work is comparable to the results obtained using the Hebb-Wagner polarisation technique and the oxygen permeation method.

Effect of pinholes in electrolyte on re‐oxidation tolerance of anode‐supported solid oxide fuel cells

AbstractThe effects of through‐thickness pinholes in an electrolyte on re‐oxidation and mechanical damage of anode‐supported solid oxide fuel cells are discussed in this paper. Pinholes with a dimension greater than approximately 10 μm were detected by dye penetrant inspection using half‐cells fabricated through a co‐sintering method. In single cells with such pinholes, the leakage of oxygen occurred through the through‐thickness pinholes under open‐circuit voltage (OCV) conditions when the fuel supply was discontinued. The oxygen leakage caused the oxidation of Ni in the anode, forming semicircular re‐oxidized regions on the electrolyte side of the anode substrate. The oxygen permeation fluxes in the electrolyte, which were deduced from the measured reduction in OCV during the operation of fuel shut‐off, are shown to correlate well with the time elapsed from the shut‐off of the fuel gas supply to the onset of mechanical damage monitored by acoustic emissions. The correlation indicates that the oxidation rate of the anode substrate is accelerated due to the presence of pinholes in the electrolyte, which can result in mechanical damages.

Investigation on Open-Circuit Voltage of an Efficiency-Boosting Solar Cell Technique Featuring V-Configuration

The V-Shaped Module (VSM) solar cell technology, which breaks the traditional concept of solar cell system, has been proven to enhance power conversion efficiency of some solar cells and has offered opportunities to increase generation power densities in area-limited applications. Compared to a planar cell system, the VSM has an additional opportunity to absorb photons and taps the potential of solar cells. In this study, the VSM, the proposed common technique enhancing efficiencies of various solar cells, was investigated by using commercially available multi-crystalline silicon solar cells. The VSM technique enables the efficiencies of the multi-crystalline silicon cells to increase from 13.4% to 20.2%, giving an efficiency boost of 51%. Though the efficiency of the cells increases, the open-circuit voltage of the cells decreases owing to the VSM technique. Furthermore, the obvious reduction in open-circuit voltage in the VSM was found and the phenomenon is explained for the first time.

Estimation of the potential for increasing the efficiency of GaInP/GaAs/Ge multi-junction solar cells by using quantum-size objects

A study of GaAs solar cells (SCs) with various quantum-sized objects has been carried out. In0.2Ga0.8As quantum wells, In0.4Ga0.6As quantum well-dots (QWDs), In0.8Ga0.2As and InAs quantum dots have been considered. The increment in photocurrent and the reduction of open-circuit voltage have been calculated using dark IV and spectral characteristics. An analytical model for estimating the efficiency of triple-junction SCs with quantum objects has been described. The calculations have shown that the optimized QWDs have a potential for increasing the efficiency of GaInP/GaAs/Ge SC from 29.8% to 30.8% (AM0, 1 sun) and from 41.6% to 43.1% (AM1.5D, 360 suns).

The light-trapping effect in various textured cover glass for enhancing the current density in silicon heterojunction solar cells

Light trapping by front surface texturing is considered an important mechanism since it can reduce the reflectance on the surface, enhance the photo-generated charge carriers and hence the performance of solar cells. Since the silicon (p, i, n) layers are deposited after surface texturing that may induce various defects leading to a reduction in open-circuit voltage (Voc) and fill factor (FF). In this study, textured glass surfaces and silicon heterojunction solar cells (HJSCs) were prepared separately. The textured glass surface was optically coupled with the index matching solution (IMS) at the front surface of HJSCs for an enhancement in current density. Three wet chemical textured glass surfaces named Etch 1, Etch 2 and Etch 3 were employed based on various optical properties. It was observed during the current density–voltage (J–V) measurements that Voc and FF of the device did not change. Reference HJSC coupled with flat glass showed J–V characteristic as Voc=726.3 mV, FF=76.62%, Jsc=37.9 mA/cm2, η=21.09% without IMS. The Jsc of HJSCs coupled with textured glass surface can be further enhanced by using IMS due to better light trapping. HJSCs coupled with textured glass (Etch 2) surface showed the highest Jsc of 38.4 mA/cm2 with an efficiency of 21.4% without IMS whereas it showed an efficiency of 22.41% with Jsc of 40.2 mA/cm2 using IMS. The trends in the external quantum efficiency of the HJSCs indicate that the Jsc can be improved if a suitable index matching solution and glass material are used.

Limitations and Characterization of Energy Storage Devices for Harvesting Applications

This paper aims to study the limitations and performances of the main energy storage devices commonly used in energy harvesting applications, namely super-capacitors (SC) and lithium polymer (LiPo) batteries. The self-discharge phenomenon is the main limitation to the employment of SCs to store energy for a long time, thus reducing efficiency and autonomy of the energy harvesting system. Therefore, the analysis of self-discharge trends was carried out for three different models of commercial SCs, describing the phenomenon in terms of self-discharge rate and internal resistance. In addition, physical interpretations concerning the self-discharge mechanism based on the experimental data are provided, thus explaining the two super-imposed phenomena featured by distinct time constants. Afterwards, the dependence of self-discharge phenomenon from the charging time duration (namely, SCs charged at 5 V and then kept under charge for one or five hours) was analyzed; by comparing the voltage drop during the self-discharge process, a self-discharge reduction for longer charging durations was obtained and the physical interpretation provided (at best −6.8% after 24 h and −13.4% after 120 h). Finally, self-discharge trends of two commercial 380 mAh LiPo batteries (model LW 752035) were acquired and analyzed; the obtained results show an open circuit voltage reduction of only 0.59% in the first 24 h and just 1.43% after 124 h.

Partial coverage methylammonium lead bromide films for solar cell application

Hybrid perovskite compounds as methylammonium lead halides are currently been investigated for different optoelectronic applications. CH3NH3PbBr3 is a promising member because of its electronic structure and large bandgap that gives rise to a high open circuit voltage. Although higher performances can result by using sophisticated processing, one-step methods are particularly interesting because of their easy implementation. Solar cells based on CH3NH3PbBr3 are fabricated using one-step spin coating deposition. Remarkable effect on the morphological, structural, optical and electrical properties is observed by changing the molar ratio of the precursors. Dual film morphology, with micrometer-size CH3NH3PbBr3 crystals embedded in a mesoporous TiO2 background, explains minimal shunting paths and open-circuit voltage reduction. Full perovskite coverage of TiO2 background is not necessary for proper operation.

Characterization and evaluation of Nafion HP JP as proton exchange membrane: transport properties, nanostructure, morphology, and cell performance

In this study, a novel membrane (Nafion HP JP) was studied and investigated intensively for use as a proton exchange membrane for fuel cell. A standard membrane, Nafion NRE212, was also studied under the same conditions for comparison. Fourier transform infrared spectroscopy, thermogravimetric analysis, wide-angle x-ray diffraction, atomic force microscope (AFM), proton conductivity, and mechanical test were used to study the structure and properties of the membranes. In addition, methanol permeability was measured as a function of methanol concentration. Nafion HP JP membrane exhibited a lower methanol permeability over the concentration range studied. Furthermore, Nafion HP JP has Young’s modulus of 88.3 MPa and tensile strength of 13.2 MPa which are higher than those of Nafion NRE212 (39.85 and 9.5 MPa, respectively). AFM data confirmed the higher value of water uptake for Nafion HP JP membrane. Free volume distribution data measured using positron annihilation lifetime (PAL) showed that the novel membrane has a smaller free volume hole size than that of Nafion NRE212. Moreover, the cell performance of the single cell test using the investigated membrane, hydrogen as fuel and oxygen as oxidant, was investigated at different temperatures and humidities. The power density delivered using Nafion HP JP is higher than that from Nafion NRE212 especially at 50 °C and 30% relative humidity. In addition, the membrane durability of both samples was performed for about 220 h and Nafion HP JP has an open circuit voltage reduction rate of 0.136 mV/h that nearly half that for Nafion NRE212 membrane. The optimal concentration for direct methanol fuel cell DMFC operation using both membranes was found to be 2 M which totally agrees with methanol permeability and PAL data. The obtained results reflect that Nafion HP JP shows great potential for fuel cell applications.

Degradation Mechanism in Cu(In,Ga)Se2 Material and Solar Cells Due to Moisture and Heat Treatment of the Absorber Layer

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In,Ga)Se2 films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities (NT) for both the EV + 0.65 eV and EV + 0.98 eV traps increase after water exposure, while the majority carrier concentration (NA) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage (VOC) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both NA and NT yield most of the change in VOC . A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.

Use of InGaAs/GaSb Quantum Ratchet in p-i-n GaAs Solar Cell for Voltage Preservation and Higher Conversion Efficiency

The concept of intermediate band solar cell (IBSC) is evolved through the effective use of the below bandgap photons, so as to improve upon the existing Shockley–Queisser limit for single-junction solar cells (SCs). But there are many challenges with the IBSC, for which they are unable to achieve the theoretical maximum. One of the most important constraints is the increase in recombination rate (both radiative and nonradiative), which results in a significant reduction in open-circuit voltage. In this paper, an antimonide-based quantum photon ratchet SC is proposed, where an InGaAs/GaSb quantum ratchet band is introduced into the IBSC. It reduces the transmission losses effectively by decoupling the intermediate band (IB) from the valence band, thereby increasing the lifetime of charge carriers in the IB state leading to increase in the conversion efficiency over IBSC.

Perfluorinated composite membranes with organic antioxidants for chemically durable fuel cells

A perfluorinated composite membrane incorporating an organic antioxidant was prepared to improve the chemical durability of a polymer electrolyte fuel cell. Bipyridine and benzoquinone (BQ) were added as organic antioxidants, and the oxidative stability of the composite electrolyte membranes containing them was evaluated via accelerated aging tests, i.e., an ex situ Fenton's test and in situ open-circuit voltage (OCV)-holding test. The effects of the antioxidants in the Fenton's test were verified using scanning electron microscopy, fluorine ion detection, Fourier-transform infrared spectroscopy, and tensile strength measurements. Moreover, a commercial membrane, NRE211, and a composite membrane, Nafion-BQ, were incorporated into a membrane electrode assembly (MEA) that then underwent an OCV-holding test for 500 h. The change in the MEA state over time was analyzed using cyclic voltammetry and linear sweep voltammetry. While NRE 211 showed a significant reduction in OCV from 300 h, Nafion-BQ maintained a stable OCV for 500 h. Hence, the organic antioxidants added to the electrolyte membrane effectively improved the oxidative stability of the proton exchange membrane fuel cell without sacrificing the ionic conductivity of the perfluorinated electrolyte membrane.

Charge carrier-selective contacts for nanowire solar cells

Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends.