Power Conversion Efficiency Of Solar Cells Research Articles

Frameworks with metal nodes improve organic solar cell power conversion efficiency

First principle calculations assess and optimize the physical and optoelectronic properties of a 2-D metal organic network.

Si-quantum-dot heterojunction solar cells with 16.2% efficiency achieved by employing doped-graphene transparent conductive electrodes

To overcome small- and indirect-bandgap nature of crystalline bulk Si, a lot of efforts have been made to utilize Si quantum dots (SQDs) in optoelectronic devices. By controlling the size of Si quantum dots (SQDs), it is possible to vary the energy bandgap based on quantum confinement effect, which can maximize the power-conversion efficiency (PCE) of solar cells due to the energy harvesting in a broader spectral range. Here, we first employ graphene transparent conductive electrodes (TCEs) for SQDs-based solar cells, showing a maximum PCE of 16.2%, much larger than ever achieved in bulk-Si solar cells with graphene TCEs. In this work, the graphene TCEs are doped with two kinds of materials such as AuCl3 and Ag nanowires for efficient collection of the carriers photo-induced in SQDs. The encapsulation of the doped-graphene TCE with another graphene layer prevents the doping elements from being desorbed or oxidized, thereby making the PCE higher, its doping dependence more evident, and the long-term performance more stable. The observed unique solar cell characteristics prove to be dominated by the trade-off effects between doping-induced variations of diode quality, transmittance/sheet resistance of graphene, energy barrier at the graphene TCE/SQDs interface, and reflectance.

Medium Bandgap D-A Type Photovoltaic Polymers Based on an Asymmetric Dithienopyran Donor and a Benzotriazole Acceptor.

Conjugated polymers based on the donor of an asymmetric 5H-dithieno[3,2-b:2′,3′-d]pyran (DTPa) and the acceptors of benzo[d][1,2,3]triazole (BTA) or di-fluorinated benzo[d][1,2,3]triazole (ffBTA) with thiophene as π-bridge were designed and synthesized. Two asymmetric-building-block-containing polymers (ABC-polymers) possess a strong and broad absorption in the range of 300–750 nm and medium optical bandgap of 1.73 and 1.77 eV for PDTPa-TBTA and PDTPa-TffBTA, respectively. Polymer solar cells using PDTPa-TBTA as donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor exhibited power conversion efficiencies (PCE) of 2.22% with a Voc of 0.58 V, a Jsc of 6.04 mA/cm2, and an FF of 63.41%. The introduction of fluorine substituents on the BTA unit evidently influenced the optical and photovoltaic properties. Interestingly, although the HOMO energy level indeed decreased, PDTPa-TffBTA showed a decreased Voc of 0.52 V in solar cells. Combined with an obviously enhanced Jsc of 10.23 mA/cm2, and an outstanding FF of 0.64, the PCE of solar cells based on PDTPa-TffBTA was improved by nearly 55%, reached 3.43%. Our results indicate that the BTA unit can be used to construct ABC polymers with a medium bandgap, and the introduction of fluorine on the BTA unit is also effective in improving the photovoltaic performance.

Novel thienoisoindigo-based dyes for near-infrared organic photovoltaics - A combination of theoretical and experimental study

Abstract A series of thienoisoindigo (TII)-based dyes (TII 1–4) using different electron-donating moieties (D) and π-linkers were synthesized. The experimental results show that the maximum absorption band of TII 1–4 was located at 611–749 nm. Moreover, their absorption onset was extended to 741–901 nm that covers the near infrared (NIR) region. The bulk-heterojunction solar cell device fabricated by using TII 3/PC61BM as the active layer exhibited a power conversion efficiency (PCE) of 0.82%, showing the potential of TII-based dyes for NIR organic photovoltaics (OPVs). In order to study the structural effect of dyes on the PCE of solar cells, we further designed four TII-based dyes (TII 5–8) and the photophysical and photovoltaic properties for TII 1–8 were investigated through theoretical calculations. Calculated results demonstrate that carbazole group, whose has strong electron-donating ability than D moieties in this study, leads to generate long lifetime of the first excited state and whose generate large short-circuit current (Jsc). Thiophene group, whose has weak resonance energy than other π-linkers, can improve the π delocalization effect throughout whole molecule and generate large open-circuit voltage (Voc). According to these results, TII 3 and TII 8 with the carbazole as D moiety and thiophene as π-linker should be useful in development of a new SM-OPV device.

Metal Halide Perovskite Polycrystalline Films Exhibiting Properties of Single Crystals

Summary Metal halide perovskites are generating enormous excitement for use in solar cells and light-emission applications, but devices still show substantial non-radiative losses. Here, we show that by combining light and atmospheric treatments, we can increase the internal luminescence quantum efficiencies of polycrystalline perovskite films from 1% to 89%, with carrier lifetimes of 32 μ s and diffusion lengths of 77 μ m, comparable with perovskite single crystals. Remarkably, the surface recombination velocity of holes in the treated films is 0.4 cm/s, approaching the values for fully passivated crystalline silicon, which has the lowest values for any semiconductor to date. The enhancements translate to solar cell power-conversion efficiencies of 19.2%, with a near-instant rise to stabilized power output, consistent with suppression of ion migration. We propose a mechanism in which light creates superoxide species from oxygen that remove shallow surface states. The work reveals an industrially scalable post-treatment capable of producing state-of-the-art semiconducting films.

Colloidal quantum dot solar cell power conversion efficiency optimization using analysis of current‐voltage characteristics and electrode contact imaging by lock‐in carrierography

AbstractAlthough the power conversion efficiency (PCE) of colloidal quantum dot solar cells (CQDSCs) has increased sharply, researchers are struggling with the lack of comprehensive device efficiency optimization strategies, which retards significant progress in CQDSC improvement. This paper addresses this critical issue through analyzing the impact of colloidal quantum dot (CQD) carrier hopping mobility, bandgap energy, illumination intensity, and electrode/CQD interface on device performance to develop a guiding criterion for CQDSC PCE optimization. This general strategy has been used for the successful fabrication of high‐efficiency CQDSCs yielding certified PCEs as high as 11.28 %. A major experimental finding of this work is that the widely used constant photocurrent density (Jph) assumption is invalid as Jph is external‐voltage dependent due to the low carrier hopping mobility. Furthermore, the theoretical model developed herein predicts the nonmonotonic dependence of CQDSC PCE on carrier hopping mobility and bandgap energy, which were also demonstrated with the high‐efficiency CQDSCs. These results constitute a revision basis of the widespread belief that higher mobility and lower bandgap energy correspond to a higher CQDSC efficiency. Furthermore, electrode/CQD interface‐dependent surface recombination velocities were investigated in the framework of our abovementioned theoretical model using lock‐in carrierography, a contactless, large‐area frequency‐domain photocarrier diffusion‐wave imaging methodology that elucidated the carrier collection process at the electrodes through open‐circuit voltage distribution imaging. Lock‐in carrierography eliminates the limitations of today's widely used small‐spot (<0.1 cm2) testing methods which, however, raise questionable overall solar cell performance and stability estimations.

Tailor-Making Low-Cost Spiro[fluorene-9,9′-xanthene]-Based 3D Oligomers for Perovskite Solar Cells

The power-conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have increased rapidly from about 4% to 22% during the past few years. One of the major challenges for further improvement ...

Effect of Interfacial Molecular Orientation on Power Conversion Efficiency of Perovskite Solar Cells.

A wide variety of charge carrier dynamics, such as transport, separation, and extraction, occur at the interfaces of planar heterojunction solar cells. Such factors can affect the overall device performance. Therefore, understanding the buried interfacial molecular structure in various devices and the correlation between interfacial structure and function has become increasingly important. Current characterization techniques for thin films such as X-ray diffraction, cross section scanning electronmicroscopy, and UV-visible absorption spectroscopy are unable to provide the needed molecular structural information at buried interfaces. In this study, by controlling the structure of the hole transport layer (HTL) in a perovskite solar cell and applying a surface/interface-sensitive nonlinear vibrational spectroscopic technique (sum frequency generation vibrational spectroscopy (SFG)), we successfully probed the molecular structure at the buried interface and correlated its structural characteristics to solar cell performance. Here, an edge-on (normal to the interface) polythiophene (PT) interfacial molecular orientation at the buried perovskite (photoactive layer)/PT (HTL) interface showed more than two times the power conversion efficiency (PCE) of a lying down (tangential) PT interfacial orientation. The difference in interfacial molecular structure was achieved by altering the alkyl side chain length of the PT derivatives, where PT with a shorter alkyl side chain showed an edge-on interfacial orientation with a higher PCE than that of PT with a longer alkyl side chain. With similar band gap alignment and bulk structure within the PT layer, it is believed that the interfacial molecular structural variation (i.e., the orientation difference) of the various PT derivatives is the underlying cause of the difference in perovskite solar cell PCE.

Multiscale Study of Plasmonic Scattering and Light Trapping Effect in Silicon Nanowire Array Solar Cells.

Nanometallic structures that support surface plasmons provide new ways to confine light at deep-subwavelength scales. The effect of light scattering in nanowire array solar cells is studied by a multiscale approach combining classical electromagnetic (EM) and quantum mechanical simulations. A photovoltaic device is constructed by integrating a silicon nanowire array with a plasmonic silver nanosphere. The light scatterings by plasmonic element and nanowire array are obtained via classical EM simulations, while current-voltage characteristics and optical properties of the nanowire cells are evaluated quantum mechanically. We found that the power conversion efficiency (PCE) of photovoltaic device is substantially improved due to the local field enhancement of the plasmonic effect and light trapping by the nanowire array. In addition, we showed that there exists an optimal nanowire number density in terms of optical confinement and solar cell PCE.

Contributions of poly(3-hexylthiophene) nanowires to alteration of vertical inhomogeneity of bulk-heterojunction active layers and improvements of light-harvesting and power-conversion efficiency of organic solar cells

We report a 47.1% efficiency improvement resulting from the combination of device-architecture modification and inclusion of poly(3-hexylthiophene-2,5-diyl) nanowires (P3HT NWs) in bulk heterojunction (BHJ) active layers (ALs). Modelling of ellipsometry spectra shows substantial changes in inhomogeneity and optical constants of BHJ ALs with P3HT-NW inclusion. Furthermore, finite-difference time-domain simulation results based on actual device structures with inhomogeneous AL models indicate that enhanced light harvesting is a main contributing factor to efficiency improvement. On the contrary, P3HT-NW inclusion has no significant effect on charge carrier collection, other than suppressing occurrence of cul-de-sac in hole-transport pathways and unfavourable indene-C60 bisadduct domains near top anodes.

Cu2ZnSnS4 thin film solar cells grown by fast thermal evaporation and thermal treatment

Cu2ZnSnS4 thin films have been produced via rapid thermal evaporation of off-stoichiometric kesterite powder followed by annealing in an Ar atmosphere. Different heating rates were applied during the thermal treatments. The chemical composition and structural properties of the deposited layers as well as the distribution of the elements through the kesterite thin film have been investigated. The initial growth of a SnS secondary phase during evaporation led to the formation of this secondary phase next to the Mo back contact. Solar cell power conversion efficiencies were limited to values about 3% due to this secondary phase. Furthermore, an increased open circuit voltage was demonstrated by using a Zn(O,S) buffer layer.

Ultrafast Photogenerated Hole Extraction/Transport Behavior in a CH3NH3PbI3/Carbon Nanocomposite and Its Application in a Metal‐Electrode‐Free Solar Cell

Aligned and flexible electrospun carbon nanomaterials are used to synthesize carbon/perovskite nanocomposites. The free-electron diffusion length in the CH3 NH3 PbI3 phase of the CH3 NH3 PbI3 /carbon nanocomposite is almost twice that of bare CH3 NH3 PbI3 , and nearly 95 % of the photogenerated free holes can be injected from the CH3 NH3 PbI3 phase into the carbon nanomaterial. The exciton binding energy of the composite is estimated to be 23 meV by utilizing temperature-dependent optical absorption spectroscopy. The calculated free carriers increase with increasing total photoexcitation density, and this broadens the potential of this material for a broad range of optoelectronics applications. A metal-electrode-free perovskite solar cell (power conversion efficiency: 13.0 %) is fabricated with this perovskite/carbon composite, which shows great potential for the fabrication of efficient, large-scale, low-cost, and metal-electrode-free perovskite solar cells.

Influence of order-disorder in Cu2ZnSnS4 powders on the performance of monograin layer solar cells

In this study the influence of low-temperature structural ordering in absorber material to the performance of Cu2ZnSnS4 (CZTS) monograin layer (MGL) solar cells was investigated. The shift of peak position in low temperature and room temperature photoluminescence measurements was used to estimate the Cu-Zn disorder level in CZTS monograin powders. The degree of Cu-Zn ordering was changed by using different cooling times after annealing CZTS powders in S atmosphere at 740°C and by additional annealing at temperatures below critical temperature (Tc~260°C) for different time periods. Current-voltage measurements revealed that the low-temperature thermal treatments resulting in increased ordering of CZTS absorber material had significant impact on the MGL solar cell characteristics, mostly, the values of open circuit voltage (Voc) were increased. The highest Voc of 784mV was measured for the solar cell based on the powder that was cooled from 740°C to room temperature for 5min followed by annealing at 150°C for 24h. Optimal conditions for low-temperature annealing to improve CZTS MGL solar cells' performance were found to be 4h at 150°C. These improvements resulted in CZTS MGL solar cell power conversion efficiency of 9.1% (active area).

Research Update: Strategies for improving the stability of perovskite solar cells

The power-conversion efficiency of perovskite solar cells has soared up to 22.1% earlier this year. Within merely five years, the perovskite solar cell can now compete on efficiency with inorganic thin-film technologies, making it the most promising of the new, emerging photovoltaic solar cell technologies. The next grand challenge is now the aspect of stability. The hydrophilicity and volatility of the organic methylammonium makes the work-horse material methylammonium lead iodide vulnerable to degradation through humidity and heat. Additionally, ultraviolet radiation and oxygen constitute stressors which can deteriorate the device performance. There are two fundamental strategies to increasing the device stability: developing protective layers around the vulnerable perovskite absorber and developing a more resilient perovskite absorber. The most important reports in literature are summarized and analyzed here, letting us conclude that any long-term stability, on par with that of inorganic thin-film technologies, is only possible with a more resilient perovskite incorporated in a highly protective device design.

10.6% Certified Colloidal Quantum Dot Solar Cells via Solvent-Polarity-Engineered Halide Passivation.

Colloidal quantum dot (CQD) solar cells are solution-processed photovoltaics with broad spectral absorption tunability. Major advances in their efficiency have been made via improved CQD surface passivation and device architectures with enhanced charge carrier collection. Herein, we demonstrate a new strategy to improve further the passivation of CQDs starting from the solution phase. A cosolvent system is employed to tune the solvent polarity in order to achieve the solvation of methylammonium iodide (MAI) and the dispersion of hydrophobic PbS CQDs simultaneously in a homogeneous phase, otherwise not achieved in a single solvent. This process enables MAI to access the CQDs to confer improved passivation. This, in turn, allows for efficient charge extraction from a thicker photoactive layer device, leading to a certified solar cell power conversion efficiency of 10.6%, a new certified record in CQD photovoltaics.

Highly Efficient LiYF4:Yb(3+), Er(3+) Upconversion Single Crystal under Solar Cell Spectrum Excitation and Photovoltaic Application.

Luminescent upconversion is a promising way to harvest near-infrared (NIR) sunlight and transforms it into visible light that can be directly absorbed by active materials of solar cells and improve their power conversion efficiency (PCE). However, it is still a great challenge to effectively improve the PCE of solar cells with the assistance of upconversion. In this work, we demonstrate the application of the transparent LiYF4:Yb(3+), Er(3+) single crystal as an independent luminescent upconverter to improve the PCE of perovskite solar cells. The LiYF4:Yb(3+), Er(3+) single crystal is prepared by an improved Bridgman method, and its internal quantum efficiency approached to 5.72% under 6.2 W cm(-2) 980 nm excitation. The power-dependent upconversion luminescence indicated that under the excitation of simulated sunlight the (4)F(9/2)-(4)I(15/2) red emission originally results from the cooperation of a 1540 nm photon and a 980 nm photon. Furthermore, when the single crystal is placed in front of the perovskite solar cells, the PCE is enhanced by 7.9% under the irradiation of simulated sunlight by 7-8 solar constants. This work implies the upconverter not only can serve as proof of principle for improving PCE of solar cells but also is helpful to practical application.

Nanostructuring Mixed-Dimensional Perovskites: A Route Toward Tunable, Efficient Photovoltaics.

2D perovskites is one of the proposed strategies to enhance the moisture resistance, since the larger organic cations can act as a natural barrier. Nevertheless, 2D perovskites hinder the charge transport in certain directions, reducing the solar cell power conversion efficiency. A nanostructured mixed-dimensionality approach is presented to overcome the charge transport limitation, obtaining power conversion efficiencies over 9%.

Photophysics of Hybrid Lead Halide Perovskites: The Role of Microstructure.

Since the first reports on high efficiency, solution processed solar cells based on hybrid lead halide perovskites, there has been an explosion of activities on these materials. Researchers with interests spanning the full range from conventional inorganic to emerging organic and hybrid optoelectronic technologies have been contributing to the prolific research output. This has led to solar cell power conversion efficiencies now exceeding 20% and the demonstration of proofs of concept for electroluminescent and lasing devices. Hybrid perovskites can be self-assembled by a simple chemical deposition of the constituent units, with the possibility of integrating the useful properties of organic and inorganic compounds at the molecular scale within a single crystalline material, thus enabling a fine-tuning of the electronic properties. Tellingly, the fundamental properties of these materials may make us think of a new, solution processable, GaAs-like semiconductor. While this can be true to a first approximation, hybrid perovskites are intrinsically complex materials, where the presence of various types of interactions and structural disorder may strongly affect their properties. In particular, a clear understanding and control of the relative interactions between the organic and inorganic moieties is of paramount importance to properly disentangle their innate physics. In this Account we review our recent studies which aim to clarify the relationship between structural and electronic properties from a molecular to mesoscopic level. First we identify the markers for local disorder at the molecular level by using Raman spectroscopy as a probe. Then, we exploit such a tool to explore the role of microstructure on the absorption and luminescence properties of the semiconductor. Finally we address the controversy surrounding electron-hole interactions and excitonic effects. We show that in hybrid lead-halide perovskites dielectric screening also depends on the local microstructure of the hybrid crystals and not only on its chemical composition. This leads to the possibility of band gap engineering and the consequent control of the elementary photoexcitation dynamics that determine the perovskites' performances in different optoelectronic devices.

Ultrathin flexible planar crystalline-silicon/polymer hybrid solar cell with 5.68% efficiency by effective passivation

Ultrathin silicon based solar cells provide a viable way to reduce the material usage and diversify their applications. However, complex light-trapping structures are always needed to be fabricated to enhance light absorption, which will lead to exacerbation of carrier collection and expensive fabrication cost. Here, we report very simple planar flexible crystalline silicon-polymer hybrid solar cell with thickness about 18μm, whose power conversion efficiency (PCE) reaches 5.68%. By introducing the amorphous silicon layer to passivate the Silicon/Polymer interface in our device, with accuracy control of the thickness of 2nm to balance the passivation effect and the deterioration of internal electric field, the short current density reaches 83.0% of the theoretical limit. Additionally, we found that the average PCE of solar cells passivated by such technology is 5.8% and 7.1% enhanced compared with those without passivation (H-terminated) and passivated by native oxide approaches. The simple device structure provided in this study has great practicability, and the passivation processes can be duplicated for other silicon based photovoltaic devices.

NO2-induced performance enhancement of PEDOT:PSS/Si hybrid solar cells with a high efficiency of 13.44.

PSS) as an effective hole collecting layer, high-efficiency PSS/n-Si hybrid solar cells are fabricated and a power conversion efficiency (PCE) of 12.13% is obtained. After being treated by HNO3 vapor, the PCE value of PSS/n-Si hybrid solar cells enhances from 12.13% to 13.44%. NO2 gas molecules in HNO3 vapor play a significant doping role in raising the PCE of solar cells. As the doping level raises, NO2 would get more electrons from PEDOT and lead to the increase of hole carriers in the films, which improves the polymer film's conductivity and PCE of solar cells.