Planar Heterojunction Solar Cells Research Articles

Ultrathin Ta2O5 electron-selective contacts for high efficiency InP solar cells.

Heterojunction solar cells with transition-metal-oxide-based carrier-selective contacts have been gaining considerable research interest owing to their amenability to low-cost fabrication methods and elimination of parasitic absorption and complex semiconductor doping process. In this work, we propose tantalum oxide (Ta2O5) as a novel electron-selective contact layer for photo-generated carrier separation in InP solar cells. We confirm the electron-selective properties of Ta2O5 by investigating band energetics at the InP-Ta2O5 interface using X-ray photoelectron spectroscopy. Time-resolved photoluminescence and power dependent photoluminescence reveal that the Ta2O5 inter-layer also mitigates parasitic recombination at the InP/transparent conducting oxide interface. With an 8 nm Ta2O5 layer deposited using an atomic layer deposition (ALD) system, we demonstrate a planar InP solar cell with an open circuit voltage, Voc, of 822 mV, a short circuit current density, Jsc, of 30.1 mA cm-2, and a fill factor of 0.77, resulting in an overall device efficiency of 19.1%. The Voc is the highest reported value to date for an InP heterojunction solar cells with carrier-selective contacts. The proposed Ta2O5 material may be of interest not only for other solar cell architectures including perovskite cells and organic solar cells, but also across a wide range of optoelectronics applications including solid state emitting devices, photonic crystals, planar light wave circuits etc.

Passivation optimization and performance improvement of planar a-Si:H/c-Si heterojunction cells in perovskite/silicon tandem solar cells

Recently, the monolithic spin-coating perovskite/planar silicon heterojunction tandem solar cells with high performance have attracted attention mainly due to simple fabrication, low preparation cost and high efficiency, especially compared with fully textured multi-junction perovskite/silicon tandem device. As is well known, the excellent passivation of a-Si:H/c-Si interface is the key to achieving a high-efficiency planar silicon heterojunction solar cell, which further improves the performance of the corresponding tandem cell. Therefore, we investigate the elements affecting a-Si:H/c-Si interface passivation, including the c-Si surface treatment technique, a-Si:H passivation layer and P-type emitter layer and so on. In these experiments, we adjust the immersed time of diluent hydrofluoric acid and pre-deposited hydrogen plasma with different gas mixture flows. Also, the suitable deposition parameters of intrinsic a-Si:H passivation layer are regulated by varying hydrogen dilution and time, and variously slight silane content is embedded into i-a-Si:H /P-type (I/P) emitter interface by hydrogen-rich plasma treating which is for acquiring optimal experimental processing conditions to promote the chemical passivation. In addition, the p-a-Si:H and p-nc-Si:H are comparatively studied as buffer layers to further improve the I/P interface passivation by varying the hydrogen dilution in the gas mixture during deposition. It can be found that p-nc-Si:H buffer layer with high conductivity and wide bandgap can not only reduce the defect density at the I/P interface, but also increase the conductivity of P-type emitter, which further improves the field passivation effect. By the above- mentioned optimization, the highest minority carrier lifetime and implied open-circuit voltage (<i>iV</i><sub>oc</sub>) of the structure of P-type emitter/a-Si:H(i)/c-Si/a-Si:H(i)/N-type layer (inip) sample can respectively reach 2855 μs and 709 mV, which demonstrates authentically outstanding passivation performance. An efficiency of 18.76% can be obtained for the planar a-Si/c-Si heterojunction solar cell with a <i>V</i><sub>oc</sub> of 681.5 mV, which is 34.3 mV higher than that of the reference device. Regarding the optimized planar a-Si:H/c-Si heterojunction solar cell as the bottom cell, we also obtain an efficiency of 21.24% for perovskite/silicon heterojunction tandem solar cell with an open-circuit voltage of 1780 mV, which proves that the above strategies are very effective for improving the passivation optimization and performance of bottom cell in the tandem device.

Interfacial Engineering by Indium-Doped CdS for High Efficiency Solution Processed Sb2(S1- xSe x)3 Solar Cells.

Sb2(S1- xSe x)3 alloy material is a kind of encouraging material for realistically apposite solar cell because it benefits from high absorption coefficient, suitable bandgap, superior stability, and plentiful elemental storage. Interfacial engineering is vital for effective charge carrier transport in solar cells, which could upgrade the photoelectric conversion efficiency (PCE). Herein, as an interlayer, indium-doped CdS thin film fabricated by chemical bath deposition is found to remarkably enhance the photovoltaic performance of Sb2(S1- xSe x)3 solar cells. Mechanistic investigations show that the interlayer can both optically and electrically optimize the device quality. With that a PCE of 6.63% is obtained, which is the highest efficiency among the planar heterojunction solar cells and slightly higher than the reported record efficiency of mesoscopic Sb2(S1- xSe x)3-sensitized solar cells. This research provides an efficient interfacial engineering for high performance Sb2(S1- xSe x)3 solar cells.

HPbI3 as a Bifunctional Additive for Morphology Control and Grain Boundary Passivation toward Efficient Planar Perovskite Solar Cells.

One of the key aspects contributing to the rapid development of perovskite solar cells is to prepare high-quality perovskite films via morphology control and interface engineering. Here, we demonstrate that the additive HPbI3 works effectively on both morphology control and grain boundary passivation of CH3NH3PbI3- xCl x thin films. By inducing HPbI3 to the crystal transformation process, high-quality perovskite films consisting of micro-sized grains with boundaries passivated by PbI2 can be readily produced. The perovskite film obtained with HPbI3 as the additive achieves a much longer carrier lifetime compared to the pristine perovskite film without the additive. Under the optimal HPbI3 amount (5.0%), the average power conversion efficiency of the planar-heterojunction solar cells is increased by ∼24% to 17.42% from 14.09% for the device without the additive, and the champion efficiency reaches 18.59%. The devices without any encapsulation show impressive shelf stability, retaining more than 85% of the initial efficiency after being stored in ambient environment for over 40 days.

Efficient composition tuning via cation exchange and improved reproducibility of photovoltaic performance in FAxMA1-xPbI3 planar heterojunction solar cells fabricated by a two-step dynamic spin-coating process

We synthesized uniform FAxMA1-xPbI3 perovskite films with a single α phase by a two-step process combined with a dynamically dispensed spin-coating technique. It uses the continuous dropping of precursor solutions with a constant CH3NH3I (MAI)/HC(NH2)2I (FAI) concentration enabling the kinetically controlled grain growth. Dynamic coating cycles are also changed to facilitate a cation-exchange process, control the degree of the mutual inter-mixing between formamidinium lead triiodide (FAPbI3) and methylammonium lead triiodide (MAPbI3) and examine the formation process and properties of the mixed perovskite films formed under the excess MA/FA cation environment, which has not been clarified so far. Notably, without additional solvent washing steps, FAxMA1-xPbI3 films are adjustable in composition, pinhole-free, and have various grain sizes depending on the coating cycles. Perovskite solar cells (PSCs) synthesized from FAxMA1-xPbI3 films with two cycles of the dynamic spin coating have achieved a maximum power conversion efficiency (PCE) of 18.50% with an average PCE of 17.06 ± 0.42%, which shows much-improved performance as well as reproducibility compared with 14.79 ± 1.82% obtained from the conventional static spin-coating method. In addition, we first found mixed FAxMA1-xPbI3 perovskites synthesized under an excess cation environment containing local stoichiometric inhomogeneities as well as excess residual cations (C˭N and NH3+) acting as recombination traps, which is supported by the calculation of trap densities.

Phosphotungstic Acid Regulated Chemical Bath Deposition of Sb2S3 for High‐Efficiency Planar Heterojunction Solar Cell

AbstractChemical bath deposition is a convenient method for preparing Sb2S3 film which is considered as a promising photovoltaic material. However, impurity phases such as Sb2O3, SbOCl, Sb2(SO4)3 and Sb(OH)3 are inevitably generated in this solution process. Herein, we demonstrate that phosphotungstic acid in the precursor solution is able to suppress the formation of oxide and oxyhalide by‐products, leading to considerably optimized composition and crystallinity for solar cell applications. With this method, a remarkable improvement of power conversion efficiency from 1.91 % to 4.61 % is obtained, which is also the highest efficiency in planar heterojunction solar cells by solution processed Sb2S3 absorber layer.

Interfacial engineering for high efficiency solution processed Sb2Se3 solar cells

As a binary semiconductor, Sb2Se3 possesses appropriate band gap, high absorption coefficient and remarkable air and moisture stability, promising for light absorption material in a practical solar cell. This study demonstrates that interfacial enginnering of TiO2/Sb2Se3 by CdS is able to considerably enhance the photovoltaic performance in planar heterojunction solar cells. Mechanistic investigations show that the optimized energy alignment upon the introduction interfacial layer is responsible for the improved device performance. With this approach, Sb2Se3 based solar cell delivers a power conversion efficiency of 3.9% with a high current density of 27.2 mA cm−2, which is the highest efficiency in solution processed Sb2Se3 solar cells. This research provides basic understanding regarding the interfacial engineering of Sb2Se3 solar cell and practical strategy for improving the device efficiency.

Stabilization of α-CsPbI3 in Ambient Room Temperature Conditions by Incorporating Eu into CsPbI3

Although inorganic perovskite, CsPbI3, shows superior thermal stability over organic–inorganic hybrid perovskites, stabilization of the photoactive black phase (α-CsPbI3) of CsPbI3 perovskite at room temperature and in ambient conditions has remained a challenge. Herein, we present a method of stabilizing the α-CsPbI3 at lower annealing temperature (85 °C) by incorporation of Eu3+ (EuCl3) into CsPbI3, which prevents the black to the yellow phase (δ-CsPbI3) transformation in ambient air (room temperature) for a reasonably long time (>30 days). Photovoltaic performance of this Eu-stabilized α-CsPbI3, as assessed in planar heterojunction solar cells (FTO/TiO2/CsPbI3:xEu/spiro-OMeTAD/Au), shows a power conversion efficiency above 6% on backward scan (stabilized power output above 4%) for CsPbI3:xEu cells with 5–6 mol % of Eu, while CsPbI3 without Eu, as expected, shows no photovoltaic property at all. However, as the cell stability was found to be affected by composition of organic hole transport material (HT...

Electrode Design to Overcome Substrate Transparency Limitations for Highly Efficient 1 cm2 Mesoscopic Perovskite Solar Cells

Summary Fluorine-doped tin oxide glass substrate is typically used for state-of-the-art perovskite solar cells (PSCs). However, indium-doped tin oxide (ITO) is better due to higher transparency for a given conductivity, although it has lower tolerance to high-temperature processes required for the compact and mesoporous TiO2 layers. Here we overcome this challenge by developing and utilizing a new electrode design. We successfully demonstrate high-efficiency mesoscopic PSCs on annealed ITO substrates showing improved photocurrent without sacrificing fill factor. After further optimizations of cell geometry and substrate conductivity guided by simulation, a certified 19.63% efficiency is achieved on 1 cm2 for ITO-based mesoscopic PSC, which is the highest among PSCs prepared by gas quenching. This work is useful for providing design principles and methods for optimizing cell geometry, metal electrode design, and substrate conductivity requirements for large-area PSCs.

Highly Conducting Hybrid Silver-Nanowire-Embedded Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) for High-Efficiency Planar Silicon/Organic Heterojunction Solar Cells.

Embedding nanowires, such as silver nanowires (AgNWs), in a transparent conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to enhance its conductivity is technologically important for improving the performances of devices comprising transparent conductive layers. Addition of nanowires in the highly conducting form of cosolvent (ethylene glycol) or mixed-cosolvent (ethylene glycol and methanol) modified PEDOT:PSS could change the nanowire structure and significantly alter the conductivity. Here, we report a simple method to embed AgNWs in PEDOT:PSS efficiently to improve its conductivity. By incorporating nanowires in the mixed cosolvent matrix prior to addition into PEDOT:PSS, this method preserves the structure of the nanowires while enabling conductivity enhancement. In contrast, the addition of AgNWs into cosolvent-premodified PEDOT:PSS leads to breaking of nanowires and conductivity impediment. The hybrid films with efficiently embedded AgNWs and mixed-cosolvent-modified PEDOT:PSS show a sheet resistance of 104 Ω/□, which is among the lowest ever reported for the as-deposited films, with conductivity enhancement of 33% relative to that of mixed-cosolvent-modified PEDOT:PSS. The resulting planar heterojunction solar cell (HSC) based on AgNW-embedded PEDOT:PSS exhibits a power conversion efficiency of greater than 15%. This demonstrates the importance of reducing sheet resistance by integrating nanowires into the PEDOT:PSS matrix as effective charge-transfer conduits interconnecting the highly conducting quinoid chains. The present approach to efficiently embed AgNWs in PEDOT:PSS could be readily extended to other nanowires or nanoparticles for improving the performance of PEDOT:PSS for applications in not just HSCs but indeed other electronic devices that require both transparent and highly conductive layers.

Poly(o-phenylenediamine) thin film for organic solar cell applications

Poly(ortho phenylenediamine) (PoPDA) microrods with ladder-type structure are synthesized in acidic medium (pH = 0.5) at room temperature based on a developed and facile method. The molecular structure, thermal stability, and surface morphology of the microrods-shaped PoPDA were described by different characterization techniques comprising FTIR, TGA, 1HNMR, and SEM. Optimized thin films of PoPDA were successfully fabricated by thermal deposition method. The thin film was characterized by XRD and AFM. Au/PoPDA/p-Si/Al heterojunction diodes have been successfully fabricated. Dark current–voltage and capacitance–voltage characteristics were applied to get the particular parameters of the devices. Photovoltaic features of Au/PoPDA/p-Si/Al heterojunction at various intensities of white light illumination are studied. Under optimized conditions, the fabricated planar heterojunction solar cells utilizing a 95 ± 10 nm of PoPDA film exhibited a power conversion efficiency (PCE) of ~ 5.64% (JSC = 41.78 A/m2, VOC = 0.36 V, and FF = 28.11%). The relatively small FF (at room temperature with IP) and Rs of the devices designate that by increasing of the absolute temperature, the photovoltaic performance of the solar cells can be improved.

Aqueous-Solution-Based Approach Towards Carbon-Free Sb2 S3 Films for High Efficiency Solar Cells.

Sb2 S3 is a new kind of photovoltaic material that is promising for practical application in solar cells owing to its suitable bandgap, earth-abundant elements, and excellent stability. Here, we report on an aqueous-solution-based approach for the synthesis of Sb2 S3 films from easily accessible Sb2 O3 as antimony source. In this reaction, 3-mercaptopropionic acid was applied as both solvent and sulfur precursor, aqueous ammonia was employed as a solvent. After simple annealing at a temperature as low as 270 °C, the spin-coated precursor solution can generate compact, flat, uniform, and well-crystallized Sb2 S3 film. Mechanistic study showed that the formation of Sb-complex with ammonium carboxylates leads to the successful dissolution of Sb2 O3 powder. A suitable annealing process was able to generate carbon-free Sb2 S3 films. Planar heterojunction solar cell based on the as-prepared Sb2 S3 film delivered a power conversion efficiency of 5.57 %, which is the highest efficiency of solution-processed planar heterojunction Sb2 S3 solar cells and a high value in all kinds of Sb2 S3 solar cells. This research provides a convenient approach for the fabrication of device-quality Sb2 S3 films, and highlights solution processing of carbon-free metal chalcogenide thin films as a suitable process for application in optoelectronic devices.

Highly reproducible perovskite solar cells via controlling the morphologies of the perovskite thin films by the solution-processed two-step method

Organic–inorganic halide perovskites are one of the most attractive materials for the next generation solar cells. The PCE has rapidly increased to more than 22% using different configurations and techniques and further developments are predicted. However, perovskite solar cells suffer from fabrication reproducibility mainly due to difficulty in controlling the morphology of the perovskite films themselves. In this paper we present a low temperature solution-processed two-step deposition method to fabricate CH3NH3PbI3 perovskites. This method offers a simple route with great potential in fabricating reproducible perovskite solar cells. In the present work, we demonstrate that the morphology of the perovskite thin films is highly determined by the concentration of Methylammonium iodide (MAI) as well as the reaction time between MAI and PbI2. High-performance solar cells have been reproducibly achieved with a highest PCE of 15.01% for PCBM-based planar heterojunction solar cells.

Organoboron donor-π-acceptor chromophores for small-molecule organic solar cells

We introduce the use of facilely synthesizable and low-bandgap boron chromophores as donors in planar heterojunction solar cells. We show that simple changes in the compositional properties of these molecules can improve the performance of the devices. A simultaneous grafting of NO2 acceptor and N(Et2) donor groups into the molecule core causes an increase in efficiency of almost 50%. Such enhanced efficiency is mainly due to a higher photocurrent. The origin of this phenomenon is investigated.

V2O5 as Hole Transporting Material for Efficient All Inorganic Sb2S3 Solar Cells.

This research demonstrates that V2O5 is able to serve as hole transporting material to substitute organic transporting materials for Sb2S3 solar cells, offering all inorganic solar cells. The V2O5 thin film is prepared by thermal decomposition of spin-coated vanadium(V) triisopropoxide oxide solution. Mechanistic investigation shows that heat treatment of V2O5 layer has crucial influence on the power conversion efficiency of device. Low temperature annealing is unable to remove the organic molecules that increases the charge transfer resistance, while high temperature treatment leads to the increase of work function of V2O5 that blocks hole transporting from Sb2S3 to V2O5. Electrochemical and compositional characterizations show that the interfacial contact of V2O5/Sb2S3 can be essentially improved with appropriate annealing. The optimized power conversion efficiency of device based on Sb2S3/V2O5 heterojunction reaches 4.8%, which is the highest power conversion efficiency in full inorganic Sb2S3-based solar cells with planar heterojunction solar cells. Furthermore, the employment of V2O5 as hole transporting material leads to significant improvement in moisture stability compared with the device based organic hole transporting material. Our research provides a material choice for the development of full inorganic solar cells based on Sb2S3, Sb2(S,Se)3, and Sb2Se3.

Optimizing film morphology and crystal orientation of perovskite for efficient planar-heterojunction solar cells by slowing crystallization process

Abstract The morphology of perovskite film plays a key role in determining photoabsorption, carrier transport and ambient stability in perovskite solar cells. However, due to the fast crystallization kinetics, the perovskite film fabricated by solution-processing technique usually suffers from low crystallinity and poor orientation. Here, both the morphology and the ambient stability of perovskite films were improved through regulating crystallization rate and enhancing intermolecular interaction by adding α, ω-diamino poly (ethylene glycol) (PEG-NH2) into precursor solution (mixture of CH3NH3I and PbCl2). The PEG-NH2 and Pb2+ tended to form a new chelated compound Pb2+-PEG-NH2. The stable multidentate chelation between Pb2+ and PEG-NH2 retarded the crystal growth rate to half of the one of the film without PEG-NH2, which is determined by the in situ UV–vis absorption spectra and in situ two-dimensional detection capability. The slow crystallization process is beneficial for realizing an uniform perovskite film with enhanced crystallinity almost 3 times of the pristine film, and increased crystal size from smaller than 100 nm to larger than 300 nm. In addition, the content of perovskite crystals adopted (h 0 0) orientation increased from 9.59% to 20.70% due to the flexible PEG-NH2 chains. Moreover, the ambient stability of perovskite films was also enhanced because the Pb-halogen bond in perovskite crystals became stronger as well as the film became defect-free. As a result, the device fabricated with PEG-NH2 attained a high efficiency of 15.6% with remarkably prolonged ambient stability without encapsulation. This work demonstrates that introducing the polymer that can coordinate to Pb2+ is a feasible way to simultaneously enhance both efficiency and stability of perovskite solar cells.

Unconventional anode interlayer universally improving solar cell efficiency

Organic photovoltaic cells typically employ an interlayer sandwiched between the active donor or acceptor materials and the respective electrodes. Conventionally, the employed anode interlayer (AIL) adjacent to the anode has a wide optical band gap and a significantly higher lowest unoccupied molecular orbital (LUMO) energy level, compared to the adjunct electron donor material, such as boron subphthalocyanine chloride (SubPc) studied here. In this report, we synthesized three novel AIL materials, NP-β-PCN, NPAPMLI and NPAPMLI, having narrow optical band gaps but matching the LUMO energy levels of SubPc in a planar heterojunction solar cell of ITO/SubPc/C60/BCP/Al. Upon insertion of an ultrathin (2 nm) AIL, the device power conversion efficiency is increased from 3.98% to as high as 4.92%, mainly due to the significant increase of short-circuit current density (JSC) from 5.97 to 7.11–7.65 mA cm−2. From the detailed morphological and photophysical studies, we have demonstrated that the employed unconventional materials of AIL are effective in exciton (of SubPc) blocking, thereby enhancing exciton diffusion towards the charge-separating interface of SubPc/C60 and hence JSC. Since these AIL materials all have a LUMO energy level very close to that of SubPc, the study reported here clarifies that the electron blocking is not a necessary property of an AIL material.

GPVDM simulation of layer thickness effect on power conversion efficiency of CH3NH3PbI3 based planar heterojunction solar cell

Perovskite-based solar cell technologies have been a very attractive area of research in recent years. Organic-inorganic perovskite materials are in an increased evolution in power conversion efficiency. Inorganic materials have been tested at the laboratory level but their power conversion efficiency is still limited. In this paper, we used the GPVDM software to study the effect of some parameters on power conversion efficiency in a planar heterojunction solar cell based on CH3NH3PbI3 as an absorbing layer. The modifications were made by considering layers of perovskite without defects. The results show that the efficiency of the power conversion can be improved by adjusting layer thickness; in our case power conversion efficiency was increased from 9.96 % to 12.9 %.

Alcohol soluble porphyrin for the cathode buffer layers of fullerene/perovskite planar heterojunction solar cells

Abstract A π-conjugated ionized porphyrin 5,15-bis(3-acetenyl-pyridine ethyl ammonium bromide)-10,20-bis(5-(2-butyloctyl)-thienyl)-porphyrin zinc (II) (m-PYBrZnPor) is developed as cathode buffer layers (CBLs) to modify the interfaces between Ag cathode and PCBM ([6,6]-phenyl C61-butyric acid methyl ester) of fullerene/perovskite (CH3NH3PbI3) planar heterojunction solar cells (PSCs). m-PYBrZnPor is soluble in ethanol, and therefore can be spin-coated on the PCBM layer conveniently via solution processing as CBLs without corroding the under-layers of PCBM when we fabricate the devices. Furthermore, this simple modification of the cathode/PCBM interfaces by m-PYBrZnPor can remarkably enhance the power conversion efficiency (PCE) up to 17.5% with simultaneously enhanced open circuit voltage, short circuit current and fill factor values even compared with those of the devices with bathocuproine (BCP) CBLs, which are contributed by the suppressed leakage current, reduced recombination losses and reduced work function induced by the porphyrin CBLs.

Large-grained Sb2S3 thin films with Sn-doping by chemical bath deposition for planar heterojunction solar cells

Herein, the growth of large-grained and compact Sb2S3 thin films with good electrical properties by Sn doping using a chemical bath deposition (CBD) and annealing approach is detailed. Sn-doped Sb2S3 thin films were prepared using the CBD method with SbCl3, SnCl2.2H2O, and Na2S2O3 as source materials, and ethylenediamine tetraacetic acid (EDTA) as the complexing agent at 40 ° C for 3 h followed by annealing at 250 °C for 30 min under Ar ambience. Un-doped Sb2S3 films exhibited an orthorhombic crystal structure with lattice parameters of a= 1.142 nm, b= 0.381 nm, and c= 1.124 nm, crystalline grain sizes of 100 nm, a direct optical band gap of 1.70 eV, p-type electrical conductivity with high electrical resistivity, and low hole mobility. With Sn doping, a significant increase in the grain size of the films from 6 to >10μm was observed with increasing Sn content from 1.0 to 5.5 at% followed by a decrease in the grain size. The direct optical band gap of the films was 1.71–1.72 eV. By varying Sn at%, the electrical resistivity of the films decreased, and hole mobility increased from 117 to 205 cm2 V−1 s−1 up to 5.5 at% and decreased to 166 cm2 V−1 s−1 at 7.2 at%. With the addition of 1.0–5.5 at% Sn in the Sb2S3 films, the grain growth and electrical properties of the films were drastically enhanced, which is beneficial for the fabrication of planar heterojunction solar cells.