Low Short-circuit Current Density Research Articles

Synthesis and Photovoltaic Performance of β-Amino-Substituted Porphyrin Derivatives.

New β-amino-substituted porphyrin derivatives bearing carboxy groups were synthesized and their performance as sensitizers in dye-sensitized solar cells (DSSC) was evaluated. The new compounds were obtained in good yields (63-74%) through nucleophilic aromatic substitution reactions with 3-sulfanyl- and 4-sulfanylbenzoic acids. Although the electrochemical studies indicated suitable HOMO and LUMO energy levels for use in DSSC, the devices fabricated with these compounds revealed a low power conversion efficiency (PCE) that is primarily due to the low open-circuit voltage (Voc) and short-circuit current density (Jsc) values.

Perovskite Colloidal Nanocrystal Solar Cells: Current Advances, Challenges, and Future Perspectives.

The power conversion efficiencies (PCEs) of polycrystalline perovskite (PVK) solar cells (SCs) (PC-PeSCs) have rapidly increased. However, PC-PeSCs are intrinsically unstable without encapsulation, and their efficiency drops during large-scale production; these problems hinder the commercial viability of PeSCs. Stability can be increased by using colloidal PVK nanocrystals (c-PeNCs), which have high surface strains, low defect density, and exceptional crystal quality. The use of c-PeNCs separates the crystallization process from the film formation process, which is preponderant in large-scale fabrication. Consequently, the use of c-PeNCs has substantial potential to overcome challenges encountered when fabricating PC-PeSCs. Research on colloidal nanocrystal-based PVK SCs (NC-PeSCs) has increased their PCEs to a level greater than those of other quantum-dot SCs, but has not reached the PCEs of PC-PeSCs; this inferiority significantly impedes widespread application of NC-PeSCs. This review first introduces the distinctive properties of c-PeNCs, then the strategies that have been used to achieve high-efficiency NC-PeSCs. Then it discusses in detail the persisting challenges in this domain. Specifically, the major challenges and solutions for NC-PeSCs related to low short-circuit current density Jsc are covered. Last, the article presents a perspective on future research directions and potential applications in the realm of NC-PeSCs.

Cu-Doped Sb2Se3 Thin-Film Solar Cells Based on Hybrid Pulsed Electron Deposition/Radio Frequency Magnetron Sputtering Growth Techniques

In recent years, research attention has increasingly focused on thin-film photovoltaics utilizing Sb2Se3 as an ideal absorber layer. This compound is favored due to its abundance, non-toxic nature, long-term stability, and the potential to employ various cost-effective and scalable vapor deposition (PVD) routes. On the other hand, improving passivation, surface treatment and p-type carrier concentration is essential for developing high-performance and commercially viable Sb2Se3 solar cells. In this study, Cu-doped Sb2Se3 solar devices were fabricated using two distinct PVD techniques, pulsed electron deposition (PED) and radio frequency magnetron sputtering (RFMS). Furthermore, 5%Cu:Sb2Se3 films grown via PED exhibited high open-circuit voltages (VOC) of around 400 mV but very low short-circuit current densities (JSC). Conversely, RFMS-grown Sb2Se3 films resulted in low VOC values of around 300 mV and higher JSC. To enhance the photocurrent, we employed strategies involving a thin NaF layer to introduce controlled local doping at the back interface and a bilayer p-doped region grown sequentially using PED and RFMS. The optimized Sb2Se3 bilayer solar cell achieved a maximum efficiency of 5.25%.

Investigation of hybrid SnSe/SnS bilayer absorber for application in solar cells

The vapor-transport deposition of SnS and SnSe can help achieve low-cost and scalable photovoltaic devices. However, the performance of SnS- and SnSe-based solar cells is limited by shunt losses and low open-circuit voltage. Single SnS absorber based solar cells exhibit a high open-circuit voltage and low short-circuit current density, whereas single SnSe based solar cells demonstrate higher short-circuit current and low open-circuit voltage. In this study, we introduce a hybrid bilayer absorber configuration combining SnSe and SnS and fabricate a solar cell device using this structure (Mo/SnSe/SnS/CdS/ZnO/AZO/Al). Specifically, we establish this bilayer absorber configuration and a heterojunction device using CdS as the buffer layer. The bilayer-absorber-based device exhibits a higher open-circuit voltage than SnS based devices, along with a short-circuit current density comparable to those of SnSe based device. The optimal device featuring the bilayer absorber exhibits an efficiency of 3.65%, representing a considerable improvement over single SnS (2.87%) and SnSe (1.4%) absorber-based solar cells.

Enhancement of power conversion efficiency of Ag-substituted Cu2ZnSn(S,Se)4 solar cells via tuning Cu2+/(Cu++Cu2+) percentage in precursor solution

Although trace silver (Ag) substitution for Cu in Cu2ZnSn(S, Se)4 (CZTSSe) offers benefits in improving open-circuit voltage (VOC) of CZTSSe solar cells, its further development is still hindered by the fairly low short-circuit current density (JSC) owing to the increase of bandgap (Eg) of the absorber layer. Herein, a strategy is demonstrated to mitigate the poor JSC arising from the Ag substitution by tuning the Cu2+/(Cu++Cu2+) percentage in the (Cu, Ag)2ZnSnS4 (CAZTS) precursor solution. Depth characterization indicates that the appropriate Cu2+/(Cu++Cu2+) percentage in the CAZTS precursor solution increases the photogenerated charges separation and carrier transport processes for Ag-substituted CZTSSe (CAZTSSe) solar cells. Benefiting from this strategy, the power conversion efficiency (PCE) of 9.57% is achieved when the Cu2+/(Cu++Cu2+) is 50 at.%. The main factor for the improvement of PCE is due to the increase in photogenerated current density (JL) and decrease in shunt resistance (Rsh). It is demonstrated that the increase in JL results from a widening in the depletion layer width (Wd) of the p-n junction and an increase in carrier lifetime, induced by optimization of the Cu2+/(Cu++Cu2+) percentage. While the decrease in Rsh comes from the change of reaction pathways due to the change in Cu2+/(Cu++Cu2+) percentage.

Overcoming Charge Confinement in Perovskite Nanocrystal Solar Cells.

The small nanoparticle size and long-chain ligands in colloidal metal halide perovskite quantum dots (PeQDs) cause charge confinement, which impedes exciton dissociation and carrier extraction in PeQD solar cells, so they have low short-circuit current density Jsc , which impedes further increases in their power conversion efficiency (PCE). Here, a re-assembling process (RP) is developed for perovskite nanocrystalline (PeNC) films made of colloidal perovskite nanocrystals to increase Jsc in PeNC solar cells. The RP of PeNC films increases their crystallite size and eliminates long-chain ligands, and thereby overcomes the charge confinement in PeNC films. These changes facilitate exciton dissociation and increase carrier extraction in PeNC solar cells. By use of this method, the gradient-bandgap PeNC solar cells achieve a Jsc = 19.30mA cm-2 without compromising the photovoltage, and yield a high PCE of 16.46% with negligible hysteresis and good stability. This work provides a new strategy to process PeNC films and pave the way for high performance PeNC optoelectronic devices.

Photo-induced oxygen vacancy formation and anomalous photoconductivity in solution-processed CoFe2O4 thin films

Designing an optoelectronic device requires adequate information about the photo-response of the active material. Here, we have studied the optical and optoelectrical properties of the solution-processed inverse spinel structured CoFe2O4 (CFO) thin film and correlated it with the photovoltaic performance of CFO active material-based all-oxide solar cell. The optical absorption spectra of spin-coated CFO films show the existence of a direct bandgap of 2.64 eV with a maximum absorption coefficient >105/cm. A simple spin-coated CFO/TiO2p–n heterojunction shows an open circuit voltage of over 0.95 V under 1 sun illumination. However, photo-induced oxygen vacancy formation and the desorption of surface oxygen lead to additional electron generation and hole capture, respectively, in the p-type CFO, resulting in an anomalous photocurrent decay under white light illumination, resulting in a low short-circuit current density. This study provides a fundamental understanding of photo-carrier dynamics in solution-processed CFO thin films and apprehends the photophysics of designing spinel-ferrite-based optoelectronic devices.

Design of an eco-friendly perovskite Au/NiO/FASnI3/ZnO0.25S0.75/FTO, device structure for solar cell applications using SCAPS-1D

Formamidinium Tin Iodide (FASnI3) based perovskite solar cells (PSC) have captured the interest of scientific community because of wide bandgap and good thermal stability, but limited power conversion efficiency (PCE) restricts their extensive adaptation. Here, we present the design of a FASnI3 active layer-based PSC with Nickel oxide (NiO) as Hole Transport Layer (HTL) & Zinc Oxy-Sulphide (ZnO0.25S0.75) as the Electron Transport Layer (ETL). The proposed Au/NiO/FASnI3/ZnO0.25S0.75/FTO, device structure is investigated using SCAPS-1D solar cell capacitance simulator. The performance parameters of proposed solar cells device structure are investigated with variations in active layer (FASnI3) thickness, defect density & doping concentration; and variations of NiO HTL & ZnO0.25S0.75 ETL thickness, electron affinity & doping concentration to obtain higher performance. Moreover, the influence of series & shunt resistance, and temperature on the performance parameters of proposed solar cell device structure is inspected. The proposed Au/NiO/FASnI3/ZnO0.25S0.75/FTO, device structure revealed excellent solar cell performance parameters such as low short-circuit current density (Jsc) of ∼28.35 mA/cm2, high Fill Factor (FF) of ∼89.64%, reasonable open-circuit voltage (Voc) of ∼1.2419 V, & admirable PCE of ∼31.57%, suitable for lead-free and eco-friendly solar cell device applications.

Highly efficient color-control technique for building-integrated photovoltaics using optical dielectric thin film

A color-control technique based on optical thin films on glass is investigated by means of numerical simulation and experiment, with a particular focus on building-integrated photovoltaic (BIPV) applications. We show that white-colored BIPV modules can be realized with a minimum optical loss by applying optical thin films that have multiple reflection peaks in a complementary color relationship, like the emission spectrum of white LEDs. The angular dependence of hue, which is an inherent drawback of optical thin film systems, is suppressed by modifying the design of the film to maintain the complementary color relationship of multiple reflection peaks. The simulated thin film design is confirmed by an experiment with a silicon solar cell, resulting in a white color and a low short-circuit current density loss (ΔJ SC) of 5.9%. These results indicate that our approach is a promising way to realize fascinating colors and a high energy yield simultaneously in BIPV modules.

Electrode sticker for electrode lamination process enables highly durable inverted organic solar cells

Organic solar cells (OSCs) have attracted great interest in recent years owing to such features as low cost, light weight, and flexibility. The top electrodes of OSCs are generally fabricated by vapor deposition on organic layers. Recently, electrode lamination has emerged as a cost-effective process for the fabrication of top electrodes of OSCs because it does not damage organic thin layers. However, problems abound, including the low short-circuit current density (JSC) and the poor device durability of the resulting OSCs, which are due to the weak contact between the laminated electrode films and the organic layers. Herein we report an electrode sticker that can be firmly attached to an organic layer by thermocompression to realize electrode sticker-laminated OSCs with good JSC and device durability. The electrode sticker-laminated OSCs showed high JSC that approached that of electrode-sublimated OSCs, and their PCE values remained unchanged and were 98% of the maximum value after continuous photo-irradiation for 100 h under ambient conditions. The electrode sticker can be used to fabricate low-cost OSCs with good performance and durability for practical applications.

Dimethylamine Exchanging of a New Hole Transport Material for Highly efficient CsPbI2Br solar cells

All inorganic α-CsPbI2Br films with a large open circuit voltage (V OC) and excellent thermal stability are promising for highly stable and efficient perovskite solar cells (PSCs). However, its device performance is limited by a low short circuit current density (J SC) and low V OC, which is strongly related to the energy misalignment and defects recombination at the perovskite/transport layer interfaces. In this work, we develop two Ni based polymers: bis (dithiobenzil) nickel (II) (BDTB) and bis(4-dimethylaminodithiobenzil) nickel (II) (BDMA), and use them as the hole transport layer for α-CsPbI2Br PSCs. As compared to BDTB based reference device (PCE ~3.2%), BDMA based device demonstrate a simultaneous increase on V OC, J SC and fill factor (FF), resulting in a remarkable power conversion efficiency (PCE) of 13.32 %. We attribute the improved device performance to a better interfacial alignment, the reduced defect recombination and improved charge extraction. Moreover, the BDMA based devices also exhibit an improved shelf-life stability than BDTB control device, which sustain over 80 % of PCE after 50 days in ambient air.

Improvement of bifacial heterojunction silicon solar cells with light-trapping asymmetrical bifacial structures

Silicon heterojunction (SHJ) solar cell technology has recently made great progress in mass production. In particular, the average power conversion efficiency (PCE) of mass-produced solar cells has reached nearly 24%. However, the low short-circuit current density (Jsc), a drawback of the SHJ solar cells, has become a bottleneck limiting its further efficiency breakthrough. Therefore, an effective light trapping method is urgently needed for mass-produced SHJ solar cells. In this work, an innovative stacked mask method is developed and combined with a two-step texturing (TST) method to prepare micro-pyramid structures on the front and back side of SHJ solar cells with different pyramid bottom angles, respectively. This asymmetrical structure fabricated through stacked mask method significantly improves the light trapping effect without negative influence on the surface passivation effect. The optical and electrical properties of the SHJ solar cells with this asymmetrical structure under the front side illumination are discussed in detail. It is concluded that the smaller the bottom angle of the backside pyramid, the thinner the amorphous silicon passivation layer is required and the better the surface passivation quality is obtained. The external quantum efficiency (EQE) results demonstrate that the best bottom angle of backside pyramids corresponding to the optimum light trapping effect for the front side illumination is about 16°, which is consistent with the simulation results of the ray tracer. As for the optimal asymmetric structure, the Jsc of the mass-produced SHJ solar cell is improved by 0.36 mA/cm2 and the PCE is increased over 24%.

An Efficiency of 16.46% and a T80 Lifetime of Over 4000 h for the PM6:Y6 Inverted Organic Solar Cells Enabled by Surface Acid Treatment of the Zinc Oxide Electron Transporting Layer.

For the inverted organic solar cells (OSCs), the interface contacts between the ZnO electron transporting layer and the organic active layer play an important role in the device performance and stability. Since the solution-processed ZnO surface always contains some base or zinc salt contaminants, we explored how the surface pH conditions influence the performance and stability of the nonfullerene acceptor (NFA) cells. A tight relationship between the surface pH condition and the device performance and stability was established. Specifically, device performance and stability were improved by treating the ZnO films with acid solutions but worsened after base treatment. The large number of hydroxyl groups on the surface of the solution-processed ZnO films was proved to be the main reason for the surface pH condition-related performance, which caused oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindered the photogenerated charge transfer and collection, and consequently resulted in low short-circuit current density and power conversion efficiency (PCE). The surface -OH groups also boosted the photocatalytic activity and led to fast degradation of the nonfullerene acceptor. Removal of the surface -OH groups can alleviate such problems. Different acid solutions, ZrAcac, 2-phenylethylmercaptan (PET), and glutamic acid (GC), were used to treat the ZnO films, and PET treatment was the most effective treatment for performance improvement. An efficiency of 16.46% was achieved for the PM6:Y6 cells and the long-term stability under continuous illumination conditions was significantly improved with a T80 lifetime of over 4000 h (4410 h), showing the excellent long-term stability of this heterojunction solar cell. Our understanding of the surface pH condition-related device performance and stability would guide the development of a feasible method for solving the interface problems in OSCs. We also provide a practical strategy to modify ZnO with acid solutions for high-performance and stable NFA OSCs.

On interface recombination, series resistance, and absorber diffusion length in BiI3 solar cells

Bismuth triiodide is a lead-free direct wide-bandgap solution-processable semiconductor that could be an alternative to lead-based perovskites in tandem or multijunction solar cells. However, the power conversion efficiency of single-junction BiI3 solar cells remains low. Here, we determine the main loss mechanisms of BiI3 solar cells in both n-i-p and p-i-n architectures. Overall, p-i-n devices have higher power conversion efficiency than that of n-i-p. It is found that n-i-p devices have higher (and significant) non-radiative recombination at the interface of the BiI3/transport layer, resulting in a lower open-circuit voltage than p-i-n devices. Moreover, the high series resistance (>70 Ω cm2) and a low average electron–hole diffusion length (∼60 nm) contributes to the low short-circuit current density (<5 mA/cm2) and fill factor (<40%) in all devices. In addition, interface recombination also reduces short-circuit current density. Finally, we demonstrate that lithium doping of BiI3 can increase the diffusion length of BiI3 to improve the performance of BiI3 solar cells. Solar cells with the configuration ITO/NiOx/Li:BiI3/PC61BM/bis-C60/LiF/Ag obtain a power conversion efficiency of 1.3% under AM 1.5 G illumination. The deep understanding of the main loss mechanisms of this work paves the way for future optimization of BiI3 solar cells.

High Efficiency (15.8%) All-Polymer Solar Cells Enabled by a Regioregular Narrow Bandgap Polymer Acceptor.

Despite the significant progresses made in all-polymer solar cells (all-PSCs) recently, the relatively low short-circuit current density (Jsc) and large energy loss are still quite difficult to overcome for further development. To address these challenges, we developed a new class of narrow-bandgap polymer acceptors incorporating a benzotriazole (BTz)-core fused-ring segment, named the PZT series. Compared to the commonly used benzothiadiazole (BT)-containing polymer PYT, the less electron-deficient BTz renders PZT derivatives with significantly red-shifted optical absorption and up-shifted energy levels, leading to simultaneously improved Jsc and open-circuit voltage in the resultant all-PSCs. More importantly, a regioregular PZT (PZT-γ) has been developed to achieve higher regiospecificity for avoiding the formation of isomers during polymerization. Benefiting from the more extended absorption, better backbone ordering, and more optimal blend morphology with donor component, PZT-γ-based all-PSCs exhibit a record-high power conversion efficiency of 15.8% with a greatly enhanced Jsc of 24.7 mA/cm2 and a low energy loss of 0.51 eV.

Fluorinated benzotriazole-modified random terpolymer acceptors towards less thickness-sensitive all-polymer solar cells

Naphthalene diimide (NDI)-based n-type polymers have been reported as the most widely employed polymer acceptors so far. However, all-polymer solar cells (all-PSCs) based on NDI-typed acceptors commonly suffer from low short-circuit current density (Jsc) due to the deficient absorption coefficients of polymer acceptors and unsatisfactory morphologies. In this work, based on the most widely used copolymer acceptor N2200, we designed terpolymer acceptors named PNDI-BTzx (x = 5 and 10) by randomly embedding a certain amount of fluorinated benzotriazole (2f-BTz) into N2200 backbone. The enhanced absorption coefficients, relatively weakened polymer crystallinities and improved polymer miscibility were obtained from terpolymer acceptors compared to N2200. Synchronously benefited from improved absorption and optimized morphology, all-PSCs based on PNDI-BTz5 demonstrated enhanced photovoltaic performance with power conversion efficiency (PCE) and Jsc of 6.72% and 12.54 mA cm−2 than 5.67% and 10.80 mA cm−2 from N2200-control. In addition, satisfactory reproducibility was obtained with comparable performance among different batches of PNDI-BTzx (x = 5 and 10). And more interestingly, all-PSCs based on PNDI-BTz5 possessed better thickness tolerance than N2200. These results offered useful guidelines to realize reproducible-performance and thick-film practical all-polymer photovoltaic applications through designing random terpolymer acceptors.

Reactive ion etched, self-aligned, selective area poly-Si/SiO2 passivated contacts

Front/back poly-Si/SiO2 contact devices suffer from low short-circuit current density, Jsc, due to parasitic optical absorption in the front poly-Si layer. Thin poly-Si (~20 nm) allows for high Jsc but is not compatible with screen-printed fire-through contacts. We therefore study the effects of post-deposition etching of a thick poly-Si (200 nm) front layer by reactive ion etching (RIE) using the metal grid lines as a self-aligned mask. We show that passivation is maintained in the device during RIE and that Jsc is increased by a gain in the blue quantum efficiency response. However, our specific etching parameters cause non-uniform etching of the poly-Si leading to premature loss of passivation without optimal gain in Jsc. Etched, unpassivated layers can be re-passivated with a H-containing dielectric layer leading to a gain in Jsc, open circuit voltage,Voc, Fill-Factor, FF, and efficiency.

Investigation of halogen-free solvents towards high-performance additive-free non-fullerene organic solar cells

Abstract Environmental sustainability and low toxicity of processing are vital for next-generation organic solar cells (OSCs). Herein, two halogen-free solvents carbon disulfide (CS2) and ortho-xylene (o-XY) were used to illustrate the promising potential in additive-free non-fullerene OSCs with PM7 as the donor and IT-4F and IT-4Cl as the acceptors. The CS2-processed pristine films show higher crystallinity than those processed by o-XY and the corresponding blend films exhibit higher and more balanced charge mobilities. Relative to o-XY as the solvent, the CS2-processed blend films display lower absorption coefficients. As such, the CS2-processed devices show higher open-circuit voltage and fill factor, but with lower short-circuit current density if compared with those processed by o-XY. Specially, the PM7:IT-4Cl based device processed by CS2 solvent can obtain the highest power conversion efficiency (PCE) of 12.52% for conventional device, which is the highest PCE value in CS2 casted single-junction OSCs so far. Furthermore, a high PCE of 12.31% for PM7:IT-4Cl is also achieved in inverted structure. This work implies that it is of great potential for the halogen-free and non-aromatic green solvent towards mass production.

Polymer/Fullerene Blend Solar Cells with Cadmium Sulfide Thin Film as an Alternative Hole-Blocking Layer.

In this work, chemical bath-deposited cadmium sulfide (CdS) thin films were employed as an alternative hole-blocking layer for inverted poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. CdS films were deposited by chemical bath deposition and their thicknesses were successfully controlled by tailoring the deposition time. The influence of the CdS layer thickness on the performance of P3HT:PCBM solar cells was systematically studied. The short circuit current densities and power conversion efficiencies of P3HT:PCBM solar cells strongly increased until the thickness of the CdS layer was increased to ~70 nm. This was attributed to the suppression of the interfacial charge recombination by the CdS layer, which is consistent with the lower dark current found with the increased CdS layer thickness. A further increase of the CdS layer thickness resulted in a lower short circuit current density due to strong absorption of the CdS layer as evidenced by UV-Vis optical studies. Both the fill factor and open circuit voltage of the solar cells with a CdS layer thickness less than ~50 nm were comparatively lower, and this could be attributed to the effect of pin holes in the CdS film, which reduces the series resistance and increases the charge recombination. Under AM 1.5 illumination (100 mW/cm2) conditions, the optimized PCBM:P3HT solar cells with a chemical bath deposited a CdS layer of thickness 70 nm and showed 50% power conversion efficiency enhancement, in comparison with similar solar cells with optimized dense TiO2 of 50 nm thickness prepared by spray pyrolysis.

Micro-scale current path distributions of Zn1-xMgxO-coated SnO2:F transparent electrodes prepared by sol-gel and sputtering methods in perovskite solar cells

Methylammonium lead iodide perovskite solar cells with Zn1-xMgxO buffers (hole-blocking layers) were fabricated to optimize conduction band offset (CBO) of buffer/perovskite absorber interface. The Zn1-xMgxO films were prepared by sol-gel and sputtering methods. It is disclosed that the Zn1-xMgxO buffers with increasing [Mg]/([Mg] + [Zn]) from 0 (pure ZnO) to 0.15 prepared by the sol-gel method lead to the enhancement of conversion efficiency (η) from 10.7 to 15.1%, attributed to the improvement of the CBO. However, the Zn1-xMgxO buffers with enhancing [Mg]/([Mg] + [Zn]) from 0 to 0.10 deposited by the sputtering method yield the decrease in η from 6.6 to 5.6%. The lower η in the case of the sputtering method is mainly caused by the lower short-circuit current density (JSC). According to micro-scale current path distributions of ZnO-coated SnO2:F (FTO) substrates, local current easily flows in case of ZnO-coated FTO sample by sol-gel method. On the other hand, the local current hardly flows especially at peak regions of sample surface in case of ZnO-coated FTO sample by sputtering method. This is because ZnO (or Zn1-xMgxO with [Mg]/([Mg] + [Zn]) of 0) by sputtering has higher resistance than that by sol-gel method and tends to be formed on peak regions of the FTO surface. Consequently, the low JSC of the perovskite solar cells with the buffer by sputtering method is observed.