Reproducible Solar Cells Research Articles

Photovoltaic performance of mixed cation K0.005MA0.995PbI3‐based perovskite solar module

AbstractHybrid halide perovskites (HHPs) are attracting large attention due to their excellent photovoltaic characteristics. MAPbI3 (MA = CH3NH3) based solar cells are widely studied both on experimental and theoretical fronts. Alkali metal ion doping in the absorbing layer material (MAPbI3) of HHP solar cells is helpful for enhancing the photovoltaic performance and stability of the complete solar module. In present work, a complete perovskite solar module with K‐doped MAPbI3 absorbing layer has been simulated numerically. The effect of various parameters such as thickness, defect density, series, and shunt resistances on device performance has been studied. After optimizing these parameters, a device with the maximum possible efficiency (20.69%) has been reported. The reproducibility of solar cells before and after optimization has also been compared with respect to the variation in carrier mobilities in absorber layer material. Higher reproducibility of the optimized device has been obtained as compared to the pristine device. This present work contains adequate information to run an experimental trial for designing solar module containing K‐doped MAPbI3 absorbing layer.

NiN-Passivated NiO Hole-Transport Layer Improves Halide Perovskite-Based Solar Cell.

The interfaces between inorganic selective contacts and halide perovskites (HaPs) are possibly the greatest challenge for making stable and reproducible solar cells with these materials. NiOx, an attractive hole-transport layer as it fits the electronic structure of HaPs, is highly stable and can be produced at a low cost. Furthermore, NiOx can be fabricated via scalable and controlled physical deposition methods such as RF sputtering to facilitate the quest for scalable, solvent-free, vacuum-deposited HaP-based solar cells (PSCs). However, the interface between NiOx and HaPs is still not well-controlled, which leads at times to a lack of stability and Voc losses. Here, we use RF sputtering to fabricate NiOx and then cover it with a NiyN layer without breaking vacuum. The NiyN layer protects NiOx doubly during PSC production. Firstly, the NiyN layer protects NiOx from Ni3+ species being reduced to Ni2+ by Ar plasma, thus maintaining NiOx conductivity. Secondly, it passivates the interface between NiOx and the HaPs, retaining PSC stability over time. This double effect improves PSC efficiency from an average of 16.5% with a 17.4% record cell to a 19% average with a 19.8% record cell and increases the device stability.

Synthesis and application of colloidal CdS quantum dots as interface modification material in perovskite solar cells.

In this study, colloidal CdS quantum dots were synthesized, structurally characterized, and their effect on performance of perovskite solar cells was observed by using them as interface modification agent between TiO2/perovskite. Colloidal CdS quantum dots were synthesized based on two-phase method and characterized by X-ray diffraction and Transmission Electron Microscopy techniques. The average particle size of CdS quantum dots have found to be around 5 nm. Oleic acid was used as capping agent during synthesis to lead solubility in organic solvents. Obtained quantum dots are coated on compact TiO2 layer for surface modification. A decrease was observed when oleic acid capped CdS quantum dots were used at interface, while significant improvement was observed when ligand exchange was carried out by pyridine before perovskite layer. Reference solar cells showed 11.6% efficiency, while pyridine capped CdS modified solar cells' efficiency was 13.2%. Besides the improvement in efficiency, reproducibility of solar cells also was increased by using pyridine capped CdS as interface material.

Impact of the Solvation State of Lead Iodide on Its Two‐Step Conversion to MAPbI3: An In Situ Investigation

AbstractProducing high efficiency solar cells without high‐temperature processing or use of additives still remains a challenge with the two‐step process. Here, the solution processing of MAPbI3 from PbI2 films in N,N‐dimethylformamide (DMF) is investigated. In‐situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) measurements reveal a sol–gel process involving three PbI2‐DMF solvate complexes—disordered (P0) and ordered (P1, P2)—prior to PbI2 formation. When the appropriate solvated state of PbI2 is exposed to MAI (methylammonium Iodide), it can lead to rapid and complete room temperature conversion into MAPbI3 with higher quality films and improved solar cell performance. Complementary in‐situ optical reflectance, absorbance, and quartz crystal microbalance with dissipation (QCM‐D) measurements show that dry PbI2 can take up only one third of the MAI taken up by the solvated‐crystalline P2 phase of PbI2, requiring additional annealing and yet still underperforming. The perovskite solar cells fabricated from the ordered P2 precursor show higher power conversion efficiency (PCE) and reproducibility than devices fabricated from other cases. The average PCE of the solar cells is greatly improved from 13.2(±0.53)% (from annealed PbI2) to 15.7(±0.35)% (from P2) reaching up to 16.2%. This work demonstrates the importance of controlling the solvation of PbI2 as an effective strategy for the growth of high‐quality perovskite films and their application in high efficiency and reproducible solar cells.

Investigation of NiOx-hole transport layers in triple cation perovskite solar cells

Perovskite solar cells with a planar p-i-n device structure offer easy processability at low temperatures, suitable for roll-to-roll fabrication on flexible substrates. Herein we investigate different hole transport layers (solution processed NiOx, sputtered NiOx, PEDOT:PSS) in planar p-i-n perovskite solar cells using the triple cation lead halide perovskite Cs0.08(MA0.17FA0.83)0.92Pb(I0.83Br0.17)3 as absorber layer. Overall, reproducible solar cell performances with power conversion efficiencies up to 12.8% were obtained using solution processed NiOx as hole transport layer in the devices. Compared to that, devices with PEDOT:PSS as hole transport layer yield efficiencies of approx. 8.4%. Further improvement of the fill factor was achieved by the use of an additional zinc oxide nanoparticle layer between the PC60BM film and the Ag electrode.

Thermally Stable Bulk Heterojunction Prepared by Sequential Deposition of Nanostructured Polymer and Fullerene.

A morphologically-stable polymer/fullerene heterojunction has been prepared by minimizing the intermixing between polymer and fullerene via sequential deposition (SqD) of a polymer and a fullerene solution. A low crystalline conjugated polymer of PCPDTBT (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]) has been utilized for the polymer layer and PC71BM (phenyl-C71-butyric-acid-methyl ester) for the fullerene layer, respectively. Firstly, a nanostructured PCPDTBT bottom layer was developed by utilizing various additives to increase the surface area of the polymer film. The PC71BM solution was prepared by dissolving it in the 1,2-dichloroethane (DCE), exhibiting a lower vapor pressure and slower diffusion into the polymer layer. The deposition of the PC71BM solution on the nanostructured PCPDTBT layer forms an inter-digitated bulk heterojunction (ID-BHJ) with minimized intermixing. The organic photovoltaic (OPV) device utilizing the ID-BHJ photoactive layer exhibits a highly reproducible solar cell performance. In spite of restricted intermixing between the PC71BM and the PCPDTBT, the efficiency of ID-BHJ OPVs (3.36%) is comparable to that of OPVs (3.87%) prepared by the conventional method (deposition of a blended solution of polymer:fullerene). The thermal stability of the ID-BHJ is superior to the bulk heterojunction (BHJ) prepared by the conventional method. The ID-BHJ OPV maintains 70% of its initial efficiency after thermal stress application for twelve days at 80 °C, whereas the conventional BHJ OPV maintains only 40% of its initial efficiency.

High reproducibility of perovskite solar cells via a complete spin-coating sequential solution deposition process

The controllability of conditions and parameters, in a certain procedure that adopted to prepare perovskite absorption layer, play a crucial role in morphology and crystallization of the resulting films, which is of great impact on the reproducibility of solar cells. Here we propose a complete spin-coating sequential solution deposition (spinning-SSD) process, in which a high concentration CH3NH3I precursor was spin-coated on PbI2-coated substrate insteading of immersing procedure; the pristine isopropanol was spin-coated to ameliorate the surface morphology and improve the quality of the resulting CH3NH3PbI3 films both before and after the spin-coating of CH3NH3I solution. Utilizing this spinning-SSD process, we fabricated meso-structured perovskite solar cells, and these devices exhibited decent performance and high reproducibility, achieving an average power conversion efficiency of 11.3% with the standard deviation as low as ±0.52. This optimized approach is expected to be used for fabricating high-performance perovskites solar cells with a full printable process.

Mixed perovskite based on methyl-ammonium and polymeric-ammonium for stable and reproducible solar cells.

We have described the compositional engineering of mixed perovskite based on methyl- and polymeric-ammonium. The resulting perovskite films containing moisture-resistant 2D materials exhibit a maximum power conversion efficiency of over 15% with high reproducibility and good stability, offering greater tunability at the molecular level for material optimization.

Improved morphology control using a modified two-step method for efficient perovskite solar cells.

A two-step wet chemical synthesis method for methylammonium lead(II) triiodide (CH3NH3PbI3) perovskite is further developed for the preparation of highly reproducible solar cells, with the following structure: fluorine-doped tin oxide (FTO)/TiO2 (compact)/TiO2 (mesoporous)/CH3NH3PbI3/spiro-OMeTAD/Ag. The morphology of the perovskite layer could be controlled by careful variation of the processing conditions. Specifically, by modifying the drying process and inclusion of a dichloromethane treatment, more uniform films could be prepared, with longer emission lifetime in the perovskite material and longer electron lifetime in solar cell devices, as well as faster electron transport and enhanced charge collection at the selective contacts. Solar cell efficiencies up to 13.5% were obtained.

In-situ optical emission spectroscopy for a better control of hybrid sputtering/evaporation process for the deposition of Cu(In,Ga)Se2 layers

In this work, we have developed a hybrid one-step co-sputtering/evaporation Cu(In,Ga)Se2 (CIGS) process, where Cu, In and Ga are sputtered simultaneously with the thermal evaporation of selenium, thus avoiding the H2Se use. An appropriate control of the selenium flux is very important to prevent the target poisoning and hence some material flux variations. Indeed, the control of the CIGS composition must be rigorous to ensure reproducible solar cell properties. In this regard, a study of the correlations between plasma species and thin film composition, structure and morphology has been performed by varying power values and Se evaporation temperature in the 170 to 230°C range. We started by studying the plasma with a powerful technique: optical emission spectroscopy, following light emissions from different plasma species: sputtered Cu, Ga, In but also evaporated Se. Hence, we determined the Se flow threshold avoiding target poisoning and the main parameter controlling the CIGS composition. Obviously, we also focused our interest on the material. We measured film composition and thickness of the samples with X-ray fluorescence and by energy dispersive X-ray. Different phases formed during the process were identified by Raman spectroscopy and X-ray diffraction. The optoelectronic cell properties showed promising efficiency of 10.3% for an absorber with composition ratios of [Cu]/([In]+[Ga])=1.02 and [Ga]/([In]+[Ga])=0.44. Finally, this work shows that we are able to control this hybrid process and thus the structure and composition of CIGS thin film for industrial transfer in the photovoltaic field.

Bifacial configurations for CdTe solar cells

We present a different back contact for CdTe solar cell by the application of only a transparent conducting oxide (TCO), typically ITO, as a back electrical contact on all-PVD CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same time allows to realize bifacial CdTe solar cells, which can be illuminated from either or both sides. Also devices with thin CdTe layers (from ∼2 μm down to 1 μm) have been prepared to improve the conversion efficiency on the back side illumination, which is limited by the collection of carriers far away from the junction and to reduce the amount of material in the CdTe device. Reproducible solar cells exceeding 10% efficiency on the front side illumination and exceeding 3% on the back side illumination are reported.

CELLO: an advanced LBIC measurement technique for solar cell local characterization

A solar cell is a large area device, and thus its global IV-characteristic and efficiency depend strongly upon its local properties. Local defects, such as a locally reduced diffusion length, a strong local shunt resistance or a high local series resistance will adversely influence the cell’s global properties. Experimental techniques suitable for mapping the spatial distribution of such local parameters can provide valuable information, and thus help to improve the technology for the production of efficient and reproducible solar cells. The LBIC (Light Beam Induced Current) technique [1] allows calculation of the local diffusion length of the solar cell material from local photocurrent data obtained under short-circuit conditions. As well as allowing LBIC measurements, the PVScan 5000 mapping analyzer by NREL [2] can be used to map defects and grain boundaries using reflectivity data and special surface etching. Localized shunts can be mapped by CCD cameras [3,4] or nematic liquid crystals [5]. The electron-beam-induced current (EBIC) is an alternative technique for the investigation of defects in solar cells [6]. Destructive techniques, MASC [7] for local IV-characterisation and RAMP [8] using a scanning (by scratching) tungsten electrode, have also recently been developed.