Increase In Open-circuit Voltage Research Articles

A series of porphyrins as interfacial materials for inverted perovskite solar cells

Abstract Interface materials can improve the photovoltaic performance of the hybrid organic-inorganic perovskite solar cells (PSCs). Here, a series of porphyrin derivatives (metalloporphyrins: MTPPS4, M = Zn2+, Cu2+, Co2+, Ni2+; free base porphyrin: TPPS4) were developed to act as interfacial materials in PSCs. The introduction of porphyrin interlayers between PEDOT:PSS and perovskite layer in the inverted PSCs not only made energy level more matching but also improved the quality of the perovskite layer. Compared with the control device without an interlayer, the maximum power conversion efficiencies (PCE) of devices with the interlayers (in the order of ZnTPP4, CuTPP4, CoTPP4, NiTPP4 and TPP4) were increased by 23%, 25%, 21%, 17% and 13%, respectively. The highest PCE of 14.61% was based on the CuTPPS4-modified device. The porphyrins acted as the interlayers to promote carrier transport and reduce charge recombination at the interface, resulting in the increase of open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF).

Hexagonal Boron Nitride for Surface Passivation of Two-Dimensional van der Waals Heterojunction Solar Cells.

Two-dimensional (2D) semiconductors can be promising active materials for solar cells due to their advantageous electrical and optical properties, in addition to their ability to form high-quality van der Waals (vdW) heterojunctions using a simple process. Furthermore, the atomically thin nature of these 2D materials allows them to form lightweight and transparent thin-film solar cells. However, strategies appropriate for optimizing their properties have not been extensively studied yet. In this paper, we propose a method for reducing the electrical loss of 2D vdW solar cells by introducing hexagonal boron nitride (h-BN) as a surface passivation layer. This method allowed us to enhance the photovoltaic performance of a MoS2/WSe2 solar cell. In particular, we observed ∼74% improvement of the power conversion efficiency owing to a large increase in both short-circuit current and open-circuit voltage. Such a remarkable performance enhancement was due to the reduction of the recombination rate at the junction and surface of nonoverlapped semiconductor regions, which was confirmed via a time-resolved photoluminescence analysis. Furthermore, the h-BN top layer was found to improve the long-term stability of the tested 2D solar cell under ambient conditions. We observed the evolution of our MoS2/WSe2 solar cell for a month and found that h-BN passivation effectively suppressed its degradation speed. In particular, the degradation speed of the passivated cell was twice as low as that of a nonpassivated cell. This work reveals that h-BN can successfully suppress the electrical loss and degradation of 2D vdW heterojunction solar cells under ambient conditions.

Oxidative Passivation of Metal Halide Perovskites

Summary Metal halide perovskites have demonstrated extraordinary potential as materials for next-generation optoelectronics including photovoltaics and light-emitting diodes. Nevertheless, our understanding of this material is still far from complete. One remaining puzzle is the phenomenon of perovskite “photo-brightening”: the increase in photoluminescence during exposure to light in an ambient atmosphere. Here, we propose a comprehensive mechanism for the reactivity of the archetypal perovskite, MAPbI3, in ambient conditions. We establish the formation of lead-oxygen bonds by hydrogen peroxide as the key factor leading to perovskite photo-brightening. We demonstrate that hydrogen peroxide can be applied directly as an effective “post-treatment” to emulate the process and substantially improve photoluminescence quantum efficiencies. Finally, we show that the treatment can be incorporated into photovoltaic devices to give a 50 mV increase in open-circuit voltage, delivering high 19.2% steady-state power conversion efficiencies for inverted perovskite solar cells of the mixed halide, mixed cation perovskite FA0.83Cs0.17Pb(I0.9Br0.1)3.

Activation of Carbon Porous Paper for Alkaline Alcoholic Fuel Cells

In this study, various treatment methods to increase the reactivity of carbon porous electrodes for alkaline alcoholic fuel cells were investigated with commercially available carbon papers to understand the characteristic electrochemical behaviors of the treated carbon electrodes and to find the best method to enhance the cell performance. Effects of thermal treatment, potassium hydroxide (KOH) treatment, N2 doping, and reaction-area control via a multi-layered structure were compared in the cell-based tests, and a huge improvement in the cell performance (i.e., 64% increase of open circuit voltage (OCV) and 320% increase of max power density) was found from the thermal-treated four-layered carbon porous electrode. The results were compared with those from platinum on carbon (Pt/C)-based cells, and a discussion on the direction of research in the future was conducted. The results of this study are expected to provide key guidelines for alcoholic fuel cell (AFC) developers to develop cost-effective AFC with a carbon electrode.

High Voc upon KF Post-Deposition Treatment for Ultrathin Single-Stage Coevaporated Cu(In, Ga)Se2 Solar Cells

A simplified Cu(In, Ga)Se2 (CIGS) solar cell structure based on a 500 nm thin CIGS layer is presented. The absorber layers are grown with a single-stage coevaporation process, and various KF post-deposition treatments (KF-PDT) are performed. The KF-PDT leads to an efficiency increase from 7% to 12%. For all cells an increase in open circuit voltage (Voc) and fill factor is measured, which is attributed to an improved pn junction. By changing the annealing conditions, an additional Voc increase is measured. This increase is attributed to the reduction of light-induced defects at the CIGS/CdS interface in addition to the improved pn junction. A reduction of defects is confirmed by reduced sub band gap emission in the photoluminescence spectra, an increased decay time, and increased quasi Fermi level splitting. With SCAPS the results are simulated, and it is concluded that after KF-PDT the Voc is limited to 640 mV due to recombination at the back contact. A higher Voc can then only be achieved by applying a ...

Pre- or post-TiCl4 treated TiO2 nano-array photoanode for QDSSC: Ti3+ self-doping, flat-band level and electron diffusion length

Of the photoanode in QDSSC, the TiCl4 solution treatment is widely used in mesoporous TiO2 surface modification. However, device performance amplification and electron transfer capability improvement of the two common pre- or post treatment are seldom yet. In this work, we explore the TiCl4 different treatments based on the typical TiO2 nanowire array photoanode for CdSxSe1-x QDSSC. In terms of the changes of electron transfer pathway, flat-band potential (Vfb) and charge carrier concentration (ND), we explored the inducement and increase of electron transfer capability and efficacy under different treatments. As for the influence of device performance, the more negative Vfb (−0.315 V, NHE) and higher ND (7.63 × 1019 cm−3) of PBS are beneficial to the increase of open-circuit voltage (Voc) and short-circuit current (Jsc). Therefore, the TiCl4 pre-treatment on photoanode modification (in PBS) is beneficial to reduce the resistances of charge transfer and recombination. The cell performance of PBS in CdSxSe1-x QD sensitization was improved significantly: the PCE was 7.54%, the Jsc was 18.93 mA/cm2, Voc was 0.78 V and fill factor (FF) was 51.1%.

Effects of Cu(In,Ga)3Se5 defect phase layer in Cu(In,Ga)Se2 thin film solar cells

Cu(In,Ga)3Se5 (135-CIGS) layers with various thicknesses were deposited on the surface of Cu(In,Ga)Se2 (112-CIGS) photon absorber in the fabrication of CIGS thin film solar cells. This significantly affects the shift of the optical band gap energy from 1.15 eV (112-CIGS) to 1.19 eV, with only 10 nm thick of 135-CIGS capping layer, leading to the increase in open-circuit voltage (Voc) of the solar cells. The optical transmission spectra show no sign of single 135-CIGS layer. The maximum Voc of 670 mV was obtained from 5 to 10 nm thick 135-CIGS capping layer on 112-CIGS compared to 646 mV of only 112-CIGS. The power conversion efficiencies of the devices covered with 135-CIGS with thickness of less than 80 nm are slightly lower than that of the uncovered 112-CIGS solar cells due to lower generated photocurrents. The solar cell parameters become dramatically deteriorate with thicker 135-CIGS capping layer. The XRD also shows the shift of the diffraction peak toward larger 2θ without peak broadening or splitting when the thickness of 135-CIGS is increased. The external quantum efficiency (EQE) measurements indicate the shift of absorption threshold towards shorter wavelength when the thickness of 135-CIGS is increased, which is consistent with the optical transmission measurements.

Flexible thermoelectric cells fabricated by rubbing-in technology with rubber-carbon nanotubes/graphene composites

In this research, CNT and graphene-rubber composites based thermoelectric cells were fabricated by using the rubbing-in technology. However, the comparative analysis of both elastic thermoelectric cells was also carried out. The carbon nanotubes and graphene composites with rubber were used as an active layer of thermoelectric cells. The films were deposited over rubber substrate (length 7 mm and width 5 mm) and pressure of (7–12) g/cm2. The electric dependence of the cells i.e., voltage (open-circuited), current (short-circuited), resistance and Seebeck coefficient with respect to averaged temperature (with gradient = 10 °C) was measured in the temperature interval of 30 °C–55 °C. The CNT/Graphene-rubber composite based active layer properties were investigated and results showed that with an increment in average temperature, there is an increase in current and voltage as well. In case of short circuit current, the increase in CNT-rubber and graphene-rubber composites was 1.82 and 4.5 times while an increase in open circuit voltage was 1.75 and 6.5 times, respectively. It was also found, that the graphene-rubber composites cells had a higher thermoelectric coefficient as compared to CNT-rubber thermoelectric cells. These cells can be potentially utilized as flexible and vibrations free thermoelectric sensors for measurement of the gradient of temperature and elastic thermoelectric modules for low power applications.

Ultrathin, flexible and transparent graphene-based triboelectric nanogenerators for attachable curvature monitoring

Pursuits of wearable electronics include the features of flexible, self-powered, and even being ultrathin and transparent for a better fit on different curved surfaces in an imperceptible way. In this paper, by stacking two graphene-covered parylene films layer by layer and introducing serpentine structures in one parylene film as the spacer, ultraflexible triboelectric nanogenerators (TENGs) with a total thickness of 5.5 μm and a transmittance of 80% were achieved. Under the stimulation of vertical impacts at frequencies of 0.5 Hz, 1 Hz, 1.5 Hz, 2 Hz, and 2.5 Hz, an open-circuit (OC) voltage of 3 V was maintained in a TENG measuring 2 × 2 cm2 throughout all the cases, which indicated a good mechanical stability of the device. When the TENG was bent, there was an increase in OC voltage and short-circuit current in response to an increased curvature. Due to its ultrathin nature and being sensitive to curvature, the TENG was demonstrated to serve as a self-powered curvature sensor for movement monitoring when conformably attached on the finger joint.

Activating and optimizing evaporation-processed magnesium oxide passivating contact for silicon solar cells

Irrespective of the success on reduction of contact resistivity, lack of chemical passivation of evaporated metal oxides heavily hinders their applications as passivating contacts, such contacts can be an alternative route for high efficiency and cost effective silicon solar cells. Here, we demonstrate that electron beam evaporated magnesium oxide (MgOx) thin film can work as a promising electron-selective passivating contact for n-Si solar cells after a post-annealing treatment and an alumina-initiated atomic hydrogenation. 10 nm MgOx on n-Si provided a surface recombination velocity down to 14.9 cm/s while 1 nm MgOx showed a low contact resistivity of 14 mΩ cm2. Comprehensive characterizations revealed the formation of Si–O–Mg bonds and the activation of atomic hydrogens were the main reasons for such high-level passivation. A PERC-like dopant-free rear contact was formed by using the 1 nm-MgOx as electron-collector and the 10 nm-MgOx as passivating layer, the resultant solar cells achieved 27% increment in efficiency and 51 mV increase in open-circuit voltage in comparison with reference devices. The ways of improving passivation quality of MgOx and novel design of contact structure open up the possibility of using evaporation-processed metal oxides as effective and low-cost carrier-selective passivating contacts for n-Si photovoltaic devices.

Effect of Charge-Transfer State Energy on Charge Generation Efficiency via Singlet Fission in Pentacene-Fullerene Solar Cells.

Singlet fission in pentacene creates two triplet excitons per absorbed photon. In a solar cell, each triplet can generate an electron–hole pair, and hence, external quantum efficiencies exceeding 100% have been reported for pentacene–fullerene solar cells. The energetics of this process are intriguing because the minimum photon energy loss, defined as the energy difference between the (triplet) exciton state and the open-circuit voltage, is less than 0.5 eV and distinctively smaller than that in most organic donor–acceptor solar cells. To investigate the energetics of this process, we analyze the effect of the energy of the lowest unoccupied molecular orbital (LUMO) for different fullerene derivatives. With the LUMO energy becoming less negative, the open-circuit voltage increases and charge generation decreases. For all but one of the fullerenes tested, the charge-transfer state energy is distinctively higher than the pentacene triplet energy, revealing that charge generation via singlet fission is actually endergonic. An elementary Marcus model for the rate of electron transfer provides a qualitative description of the experimental trends, in accordance with an endergonic charge transfer. Considering that charge generation from triplet states is endergonic, involvement of pentacene singlet states, either from direct photoexcitation or via triplet–triplet annihilation, cannot be excluded.

Rear point contact structures for performance enhancement of semi-transparent ultrathin Cu(In,Ga)Se2 solar cells

Semi-transparent ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells offer many promising applications but their efficiencies are still limited. In this work, SiO2 point contact structures were prepared using nanosphere lithography on Sn:In2O3 (ITO) substrate and ultrathin CIGSe solar cells were fabricated on top. It was discovered that the SiO2 point contact structure at the CIGSe/ITO interface behaves significantly different from at the CIGSe/Mo interface and does not improve the rear interface reflectivity or short-circuit current density. However, the SiO2 point contact structure brings pronounced electrical benefits, arising from the joint effect of reduced back recombination and induced field effect by internal charges. Semi-transparent ultrathin CIGSe solar cells with an absorber thickness of 380 nm exhibited a significant increase in open circuit voltage of 26 mV and fill factor of 9.4% (absolute). Consequently, the efficiency is improved by 23% (from absolute 6.8%–8.4%).

Photovoltaic and Physical Characteristics of Screen-Printed Monocrystalline Silicon Solar Cells with Laser Doping and Electroplated Copper

Photovoltaic and physical characteristics of screen-printed monocrystalline silicon solar cells (SPMSCs) were presented with electroplated copper (EPC) as the rear contact. The boron back surface field (B-BSF) formed by spin-on doping and laser doping (LD) was prepared as a seed layer for the EPC. The LD parameters, including the laser focus, laser power, laser speed, and laser line pitch, were investigated. Moreover, the effects of KOH etching on the surface properties after the LD process were explored. Furthermore, to enhance the adhesion between the B-BSF seed layer and EPC contact layer, a laser pinhole process was proposed. Finally, the EPC processes with various electroplating times were addressed. The results revealed that the mechanism of enhancements could be attributed to a continuous B-BSF seed layer and a reduction of series resistance, as well as an increase of open-circuit voltage and adhesion between the B-BSF seed layer and EPC contact layer.

The investigation of the unseen interrelationship of grain size, ionic defects, device physics and performance of perovskite solar cells

Controlling the phenomenological morphology effects on the performance of the perovskite solar cell (PSC) is a continuing concern due to its photo-physical complexity and the existing contrary reports. Distinguishing the effect of the formed electron and hole traps in the bulk and at surface/interfaces of the perovskite layer from their impact on the performance of the device can be beneficial in optimizing fabrication methods. Here, the transient AC and steady state DC measurements, and morphology characterizations confirm the variation of performance parameters with respect to grain boundaries growth. The device physics is uncovered with respect to the grain size (GS) of the perovskite layer employing the theoretical drift-diffusion framework incorporating the electronic and ionic contributions. The increase of open circuit voltage () for devices with large GS can be associated to the density of defect states. The findings here suggest a more pronounced role of interfaces in efficiency enhancement of the PSCs with the emphasis on the impact of the hole transport layer (HTL)/perovskite layer interface which is also found to be accountable to the difference between the device internal voltage and the terminal voltage and minimizing this difference can lead to an enhancement of approximately 100 mV in . Additionally, the electron traps in the bulk of the perovskite layer play a distinguishable role in the reduction of for the device with the smallest GS. The ionic defect density is also estimated. Considering our results and previous reports, the performance of the PSC is remarkably dependent on the method of fabrication and the associated perovskite conversion mechanism, and not necessarily on GS. The results are expected to deliver important guidelines for the development of more efficient PSCs by further enhancement of the towards its thermodynamic limit of 1.32V, via creating optimal interfaces.

Difluorochloromethane treated thin CdS buffer layers for improved CdTe solar cells

One of the major improvements for CdTe solar cells is the increase of the current density by enhancing the transparency of the buffer layer. CdS, with its 2.4 eV band gap, results in being opaque at low wavelength regions. For this reason, in order to gain in blue and UV light, extremely thin CdS has to be applied, if the buffer is not substituted with other materials. On the other hand, by thinning the CdS layer we create pinholes also due to the wide intermixing of CdTe and CdS layers and subsequently consumption of CdS.For this reason we have applied and compared different CdS post-deposition treatments in order to stabilize the layer and avoid excess intermixing. Treatments at temperatures above 500 °C in an argon and chlorine atmosphere on 80 nm thick CdS have led to the fabrication of more stable samples compared to untreated CdS, resulting in improved performance with a 10% increase in current density and a 5% increase in open circuit voltage and fill factor. Moreover, light transmission of the CdS treated layers from 300 to 450 nm is increased by about 10%.In this paper, a study of treated thin CdS with recrystallization treatments is presented; the layers have been analysed by means of X-ray photoelectron spectroscopy, X-ray diffraction and atomic force microscopy. Finished CdTe devices made on thin CdS are characterized in terms of current-voltage and quantum efficiency.

In Situ Measurement of Lithium-Ion Cell Internal Temperatures during Extreme Fast Charging

Here we report an investigation of Li-ion cell thermal behaviors during extreme fast charging by in situ measurement of its internal temperatures. An experimental 2 Ah LiNi0.6Co0.2Mn0.2O2/graphite pouch cell with embedded micro-thermocouples was developed and charged as fast as 7C at room temperature. With forced convection air cooling, the cell core temperature increased by 22.5°C in 5 minutes during 7C charging while it increased by less than 1.5°C during 1C charging. The difference between cell core temperature and surface temperature was up to 3.4°C during 7C charging while less than 0.2°C during 1C charging. We estimated heat generation of the cell and found that the average heat generation rate during 7C constant current charging was 34 times higher than that during 1C charging. The temperature gradient was smaller but the temperature increase was higher with natural air convection than those with forced convection. A temporary voltage drop phenomenon was observed during 7C charging with forced convection and 5C charging with natural convection, in similar SOC range from ∼22% to ∼40%. The phenomenon can be attributed to drop of cell resistance with rapid temperature rise and slow increase of open circuit voltage in the SOC range.

Improvement of the photovoltaic performance of Ag-alloyed Cu2ZnSn(S,Se)4-based solar cells by optimizing the selenization temperature

(Cu1-xAgx)2ZnSn(S,Se)4 films with Ag content of x = 0 (CZTSSe), 0.05 (CAZTSSe) and the corresponding solar cells were fabricated on soda-lime glass (SLG) substrates by sol-gel method and rapid selenization thermal processing (RTP). As Ag is alloyed into CZTSSe, Cu+ cation in CZTSSe films was replaced by Ag+ cation, leading to the improvement in crystal quality, the decrease of hole concentration and the increase of bandgap. We observed an increase in open-circuit voltage (VOC) and an accompanying rise in device efficiency from 4.48% to 5.94% as Ag is alloyed into CZTSSe. To further improve the efficiency, we optimized the selenization temperature of the CAZTSSe films. It is found that the optimized selenization temperature is 530 °C and the best efficiency of the corresponding CAZTSSe solar cell is 7.91%. The improvement of device performance with selenization temperature is mainly attributed to the optimization of device parameters such as reversion saturation current density (J0), diode ideal factor (A) and series resistance (RS), which are determined by crystal quality and grain size of CAZTSSe films as well as structure of the CAZTSSe/CdS and CAZTSSe/Mo interfaces.

Potassium fluoride postdeposition treatment with etching step on both Cu-rich and Cu-poor CuInSe2 thin film solar cells

Recent progress in the power conversion efficiency of $\mathrm{Cu}(\mathrm{In},\mathrm{Ga})\mathrm{S}{\mathrm{e}}_{2}$ thin film solar cells has been achieved by an alkali postdeposition treatment. This treatment has been shown to change the surface composition and structure as well as the bulk properties. To investigate the relative importance of those two effects we study the impact of the treatment on Cu-rich and Cu-poor $\mathrm{CuInS}{\mathrm{e}}_{2}$, which show a different influence of interface recombination without the treatment. We develop a potassium postdeposition treatment that can be applied to Cu-rich material, where an additional etching step is necessary. The same postdeposition treatment with etching step is applied to Cu-poor material. In both cases we observe an increase of the power conversion efficiency and open circuit voltage. Comparing the increase in open circuit voltage to the increase in quasi-Fermi level splitting indicates that the improvement in Cu-poor solar cells is mostly due to changes in the bulk, whereas in Cu-rich solar cells both the bulk and the interface are improved. The improvement of the interface is corroborated by temperature dependent current-voltage characteristics, which show that the dominating recombination path in Cu-rich solar cells moves from the interface to the bulk after treatment and by admittance spectroscopy, which shows that the treatment removes a 200 meV deep defect. Photoluminescence spectroscopy shows that even in Cu-rich material the alkali treatment creates a Cu-poor surface, which in this case cannot be created by diffusion of Cu into the bulk, but is grown during the treatment.

Enhanced Heterojunction Interface Quality To Achieve 9.3% Efficient Cd-Free Cu2ZnSnS4 Solar Cells Using Atomic Layer Deposition ZnSnO Buffer Layer

Kesterite Cu2ZnSnS4 (CZTS) photovoltaics have been comprehensively investigated in the past decades but are still hampered by a relatively large open circuit voltage (Voc) deficit, which is correlated to bulk defects in CZTS and interface recombination. Heterojunction interface management is of critical importance to tackle the interface recombination. In this work, we use atomic layer deposition (ALD) to synthesize a wide range of Zn1–xSnxO (ZTO, 0 ≤ x ≤ 1) films for application as a buffer layer in CZTS solar cells. A favorable band alignment is achieved using a 10 nm Zn0.77Sn0.23O buffer layer that enabled an impressive 10% increase in open circuit voltage of the CZTS solar cell. The microstructure and chemical nature of the CZTS/ZTO interface are carefully studied and the presence of an ultrathin Zn(S, O) tunnel layer is demonstrated. The decreased interfacial defects stemming from the minor lattice mismatch at the CZTS/Zn(S,O)/ZTO heterointerface in combination with the passivation provided by a high...

Overcoming the trade-off between exciton dissociation and charge recombination in organic photovoltaic cells

The electron donor-acceptor (D-A) interface is an essential component for realizing efficient exciton dissociation and charge generation in organic photovoltaic cells (OPVs). It can also however enable rapid charge recombination due to the close spatial proximity of electrons and holes. To frustrate recombination losses, attempts have been made to separate charge carriers by introducing an insulating blocking interlayer at the D-A interface. It is challenging to realize increased efficiency using this approach as the relative similarity of interlayer optical and transport energy gaps may also frustrate exciton harvesting and charge generation. To overcome this trade-off, the interlayer must block charge carriers while continuing to permit exciton migration to the dissociating interface. In this work, we demonstrate this configuration in archetypical copper phthalocyanine (CuPc)-C60 planar OPVs containing a rubrene interlayer to frustrate charge recombination. Critically, the similarity in triplet exciton energy levels between rubrene and CuPc allows the interlayer to be permeable to excitons. Devices containing an interlayer show a reduction in the charge transfer state binding energy and non-geminate recombination rate with increasing interlayer thickness. For thin interlayers, geminate recombination is also suppressed. Thus, devices containing an exciton permeable interlayer show a simultaneous increase in open-circuit voltage, short-circuit current, and power efficiency.