Small Area Solar Cells Research Articles

Coherent growth of high-Miller-index facets enhances perovskite solar cells.

Obtaining micron-thick perovskite films of high quality is key to realizing efficient and stable positive (p)-intrinsic (i)-negative (n) perovskite solar cells1,2, but it remains a critical challenge. Here, we report an effective method for producing high-quality, micron-thick formamidinium-based perovskite films by forming coherent grain boundaries, where high-Miller-index-oriented grains grow on the low-Miller-index-oriented grains in a stabilized atmosphere. The resulting micron-thick perovskite films, with enhanced grain boundaries and grains, showed stable material properties and outstanding optoelectronic performances. The small-area solar cells achieved efficiencies of 26.1%. The 1-square-centimeter devices and 5 cm × 5 cm minimodules delivered efficiencies of 24.3% and 21.4%, respectively. The devices processed in a stabilized atmosphere presented a high reproducibility across all four seasons. The encapsulated devices exhibited superior long-term stability under both light and thermal stressors in ambient air.

Impurity-healing interface engineering for efficient perovskite submodules.

One issue that always strands the scaling-up development of perovskite photovoltaics is the significant efficiency drop when enlarging the device area, which is caused by the inhomogeneous distribution of defected sites1-3. In the narrow band gap formamidinium lead iodide (FAPbI3), the native impurities of PbI2 and δ-FAPbI3 non-perovskite could induce unfavored non-radiative recombination, as well as inferior charge transport and extraction 4,5.Here, we develop an impurity-healing interface engineering strategy to well address the issue both in small-area solar cell and large-scale submodule. With the introduction of a functional cation, 2-(1-cyclohexenyl)ethyl ammonium, two-dimensional (2D) perovskite with high mobility is rationally constructed on FAPbI3 to horizontally cover the film surface and vertically penetrate to the grain boundaries of 3D perovskites. Such unique configuration not only comprehensively transforms the PbI2 and δ-FAPbI3 impurities into stable 2D perovskite and realize a uniform defect passivation, but also provides interconnecting channels for efficient carrier transport. As a result, the FAPbI3-based small-area (0.085 cm2) solar cells achieve a champion efficiency over 25.86% with a notably high fill factor (FF) of 86.16%. More encouragingly, the fabricated submodules with the aperture area of 715.1 cm2 obtain a certified record efficiency of 22.46% with a good FF of 81.21%, showcasing the feasibility and effectualness of the impurity-healing interface engineering for scaling-up promotion with well-preserved photovoltaic performance.

Extended-bandwidth solar nano-concentrator integrated with graphene-metamaterial for photovoltaic energy harvesting applications

Concentration photovoltaics are used to achieve solar energy focusing on small-area solar cells, thus obtaining a high efficiency at a reasonable cost. An array of gold plasmonic concentrator cells is proposed here, which can nano-focus the solar radiation down to 60 nm spot size. It works over the entire solar energy bandwidth up to 2400 nm wavelength, which is extended beyond the conventional solar cells known maximum operating wavelength of 1800 nm. The concentrator consists of a coaxial waveguide with a tapered section that can plasmonically nano-focus solar radiation, followed by a straight waveguide section to propagate focused light along a graphene metamaterial for optical absorption. The metamaterial is integrated within the 60 nm gap of the straight waveguide section. The metamaterial consists of 10 concentric graphene shell layers, which are cross-connected by 5 horizontal graphene annuli. The concentrator and metamaterial design maximizes the solar optical absorption efficiency within the graphene metamaterial. The graphene optical absorption is enhanced up to 27 times with an almost flat response covering most of the solar bandwidth. The plasmonic absorption within the gold surfaces of the structure is found to be reasonable. The enhanced optical absorption of the weak solar radiation received within the extended band (1800–2400 nm) can reach up to 50%. The concentrator device is polarization insensitive with a 120o field-of-view. The enhanced optical power absorption in graphene metamaterial could be used in photovoltaic energy harvesting applications.

Four-terminal perovskite/silicon series solar cells with 28% efficiency achieved by suppressing edge recombination

Although the commercial application of solar cells pursues scalable and large-area devices, small-area solar cells on a scale of several centimeters possess many advantages such as low fabrication cost and facile high-throughput screening in the research laboratory. Most emerging photovoltaic technology starts from the studying of small-area devices. Recently, perovskite/silicon tandem solar cells have aroused extensive research interest because they can break through the radiative efficiency limit of single-junction solar cells. However, when commercial large-area silicon cells are cut into small pieces with a few squared centimeters in area for laboratory use, there occurs a significant efficiency loss, limiting the performance of tandem cells. Herein, to eliminate the thermal damage caused by the traditional laser cutting method and also reduce the non-radiative recombination of heterojunction silicon cells after being cut, a cold-manufacturing method of grinding wheel dicing is used to cut heterojunction silicon cells. This method is realized by high-speed mechanical grinding accompanied by liquid washing, which avoids damaging the edge of solar cell caused by heat. Compared with the device cut by laser, the heterojunction silicon cells cut by the cold-manufacturing method exhibit less cross-sectional damage. The measurements by scanning electron microscopy (SEM) and three-dimensional optical profilometer reveal that the morphology of the device edge is smoother than the counterpart cut by laser. Device physics measurements including electrochemical impedance spectrum(EIS), dark current-voltage curves, transient photovoltage (TPV), transient photocurrent (TPC), and the dependence of short-circuit current density and open-circuit voltage on light intensity reveal that the cold-manufacturing method can significantly prevent the heterojunction silicon cells from non-radiatively recombining after being cut. These results indicate that the edge-recombination of the silicon solar cells cut by grinding wheels is reduced compared with that cut by laser. As a result, statistical analysis of the device performance reveals that both the open-circuit voltage and fill factor of the device are improved, and the average photoelectric conversion efficiency increases by an absolute efficiency of ~1%. Stacking the obtained silicon cells with the normal transparent perovskite solar cells, the obtained four-terminal perovskite/silicon tandem solar cells deliver an efficiency of over 28%. This work emphasizes the importance of reducing efficiency loss during manufacturing the heterojunction silicon solar cell in fabricating high-performance silicon-based tandem solar cells.

Engineering high-emissive silicon-doped carbon nanodots towards efficient large-area luminescent solar concentrators

Luminescent solar concentrators (LSCs) are devices that can collect sunlight from a large area, concentrating it at the borders of the slab, to achieve efficient photovoltaic conversion when small area solar cells are placed at their edges, realizing building-integrated photovoltaics. Efficient luminophores in terms of high luminescence quantum yield are needed to obtain high-performance LSCs. A key point is the ability to engineer the Stokes shift (i.e. the difference between the maximum of the absorption and emission spectra), to minimize reabsorption processes. In this work, we report novel silicon-doped carbon nanodots (Si-CDs) with an ultrahigh quantum yield (QY) up to 92.3% by a simple hydrothermal method. Thin-film structured LSCs (5 × 5 × 0.2 cm3) with different concentrations of Si-CDs are prepared by dispersing the Si-CDs into polyvinyl pyrrolidone (PVP) matrix, and the optimal power conversion efficiency (PCE) of LSCs can be as high as 4.36%, which is nearly 2.5 times higher than that prepared with silicon-undoped CDs. This Si-CDs/PVP film LSC has a high QY of 80.5%. A large-area LSC (15 × 15 cm2) is also successfully fabricated, which possesses a PCE of 2.06% under natural sunlight irradiation (35 mW·cm−2), one of the best reported values for similar size LSCs. The efficient Si-CDs as luminescent substances for high-efficiency large-area LSCs will further give an impetus to the practical exploitation of LSCs.

Design and optimization of transparent scattering solar concentrator based on SiO<sub>2</sub> aerogel

Scattering solar concentrators (SSCs), an important component of transparent/translucent photovoltaic devices, can concentrate large-area sunlight on small-area solar cells while allowing some sunlight to pass through the devices. However, owing to the lack of suitable scattering materials, there have been few reports on SSCs in recent years. In this study, we fabricated SiO<sub>2</sub> aerogel-based SSCs and tested their performances. The photoelectric performance was found to be moderate. Additionally, the results demonstrated excellent transmittance and color rendering index, which meet the lighting requirements of the windows. A Monte Carlo ray tracing program was developed to simulate an SSC and analyze the fate of all photons. We also analyzed the multiple scattering mechanism in SSCs that damages the photoelectric efficiency of a device via theoretical simulation. Finally, we proposed an anisotropic scattering device that can increase the primary scattering and suppress multiple scattering, resulting in excellent photoelectric efficiency.

Laser Processing Optimization for Large-Area Perovskite Solar Modules

The industrial exploitation of perovskite solar cell technology is still hampered by the lack of repeatable and high-throughput fabrication processes for large-area modules. The joint efforts of the scientific community allowed to demonstrate high-performing small area solar cells; however, retaining such results over large area modules is not trivial. Indeed, the development of deposition methods over large substrates is required together with additional laser processes for the realization of the monolithically integrated cells and their interconnections. In this work, we develop an efficient perovskite solar module based on 2D material engineered structure by optimizing the laser ablation steps (namely P1, P2, P3) required for shaping the module layout in series connected sub-cells. We investigate the impact of the P2 and P3 laser processes, carried out by employing a UV pulsed laser (pulse width = 10 ns; λ = 355 nm), over the final module performance. In particular, a P2 process for removing 2D material-based cell stack from interconnection area among adjacent cells is optimized. Moreover, the impact of the P3 process used to isolate adjacent sub-cells after gold realization over the module performance once laminated in panel configuration is elucidated. The developed fabrication process ensures high-performance repeatability over a large module number by demonstrating the use of laser processing in industrial production.

Perovskite modules with 99% geometrical fill factor using point contact interconnections design

AbstractThin‐film photovoltaic technology, based on hybrid metal halide perovskites, has achieved 25.2% and 16.1% certified power conversion efficiencies for solar cell and solar module devices, respectively. Still, the gap between power conversion efficiency of small area solar cells and large area solar modules is greater than for any other photovoltaic technology. Analysis of loss mechanisms in n‐i‐p solution processed devices defined layer inhomogeneity loss and inactive area loss as the two most prominent loss mechanisms in upscaling. In this study, we focus on minimizing inactive area loss. We analyze the point contact interconnections design and demonstrate it on perovskite thin‐film solar modules to achieve a geometrical fill factor of up to 99%. Numerical and analytical simulations are utilized to optimize interconnections and solar module design and balance inactive area loss, series resistance loss, and contact resistance loss.

Regional measurements to analyze large-area luminescent solar concentrators

Abstract Experimentally analyzing the photovoltaic (PV) performance and the photon transport mechanism of large-area luminescent solar concentrators (LSCs) (>200 cm2) has proven difficult because common solar simulators and integrating spheres have limited measuring capacity. This report introduces an approach of regional measurements to address these issues using common laboratory instruments. In this approach, the LSC is configured to have different surface and edge regions. The surface regions are sequentially illuminated by a low-cost solar simulator, and the edge regions sequentially measured by a small-area solar cell or a general photoluminescence (PL) spectrometer. The methodology is validated through the comparison with the conventional method and Monte Carlo ray-tracing simulation in the study of the PV performance of a series of R305-based LSCs. Experimental results from the regional measurements reveal important the photon transport mechanism of the LSCs. The results show that severe photon transport loss exists within a surface distance (dsurf) of 5 inches (12.7 cm), and photons with the longest average transport distance ( max) experience number of absorption events (Nabs) for an average of 11.3 times. The approach of regional measurements can effectively measure the PV performance and investigate the photon transport mechanism of large-area LSCs.

Design of novel thiazolothiazole-containing conjugated polymers for organic solar cells and modules

One of the major challenges in the field of organic photovoltaics is associated with high-throughput manufacturing of efficient and stable organic solar cells. Practical realization of technologies for production of large-area organic solar cells requires the development of novel materials with a defined combination of properties ensuring sufficient reliability and scalability of the process in addition to good efficiency and operation stability of the devices. In this work, we designed two novel polymers comprising thiazolothiazole units and investigated their performance as absorber materials for organic solar cells and modules. Optimized small-area solar cells based on P1/[70]PCBM ([6,6]-phenyl-C71-butyric acid methyl ester) blends exhibited promising power conversion efficiency (PCE) of 7.5%, while larger area modules fabricated using slot die coating showed encouraging PCE of 4.2%. Additionally, the fabricated devices showed promising outdoor stability maintaining 60–70% of the initial efficiency after 20 sun days being exposed to natural sunlight at the Negev desert. The obtained results feature the designed polymer P1 as a promising absorber material for a large-scale production of organic solar cells under ambient conditions.

Development of Printable Organic Solar Cells in a Large Area

Solar cells based on the photovoltaic effect are an effective method to convert solar energy into electricity. It has become attractive to find solar cell technologies which exhibit lower manufacturing costs by utilizing inexpensive and abundant materials. Some additional advantages conveyed by organic solar cells are that they are lightweight and flexible and allow for scalable material production and cost effective roll-to-roll coating and printing techniques. Efficiency of >10% was disclosed in small area solar cells with various device architectures, which indicated the rapid progress over the last several years and the great potential of organic solar cells as an alternative source of energy. In order for organic solar cells to fully mature from research and development into cost effective products, solar cell technology at a small size needs to be realized at a large scale. And for this, it is highly important to develop new conjugated polymers and understand the fundamental working mechanism of the materials. In the presentation, we developed new conjugated polymers for large-area solar cell technologies and studied the effects of conjugated polymers on solution processing and photovoltaic performance in large area organic solar cells.

Surfactant-controlled ink drying enables high-speed deposition of perovskite films for efficient photovoltaic modules

Novel photovoltaic technologies such as perovskites hold the promise of a reduced levelized cost of electricity, but the low-cost potential depends on the ability to scale-up solution-based deposition. So far, complex fluid dynamics have limited the solution deposition of uniform pinhole-free organic–inorganic perovskite thin films over large areas. Here, we show that very small amounts (tens of parts per million) of surfactants (for example, l-α-Phosphatidylcholine) dramatically alter the fluid drying dynamics and increase the adhesion of the perovskite ink to the underlying non-wetting charge transport layer. The additives enable blading of smooth perovskite films at a coating rate of 180 m h–1 with root-mean-square roughness of 14.5 nm over 1 cm. The surfactants also passivate charge traps, resulting in efficiencies over 20% for small-area solar cells. Fast blading in air of perovskite films results in stabilized module efficiencies of 15.3% and 14.6% measured at aperture areas of 33.0 cm2 and 57.2 cm2, respectively. Scaling up perovskite film deposition necessitates controlling the film formation dynamics. Here, Deng et al. use amphoteric choline surfactants to blade-coat well-passivated films, reaching module efficiencies of ~15% for aperture areas up to 57 cm2.

Effect of oxide based graded buffer and bottom n-layer on the performance of the single junction amorphous silicon solar cells

We have developed oxide based window layer, graded buffer layer and n-layer by radio frequency plasma enhanced chemical vapour deposition (rf-PECVD) method for use in the fabrication of single junction amorphous silicon (a-Si) small area (1 cm2) solar cell. From our earlier study we had observed that higher stabilized efficiency can be obtained using oxide based window and buffer layer. In this study, we have replaced conventional buffer layer by graded buffer (graded with four different band gaps) and conventional n-a-Si:H by wide band gap n-a-SiO:H to improve p/i interface and current density further of a single junction solar cell. The use of these two materials in a single junction a-Si structure enhances the open circuit voltage (Voc) and short circuit current of the cell and the conversion efficiency as well. Graded buffer layer can minimize the p/i interface defects to a large extent causing an improvement of Voc. The bandgap of n-a-SiO:H film is much higher and optical absorption is less than that of n-a-Si:H film having the dark conductivity almost same. Due to low absorption and wide optical gap of n-oxide layer allows more fraction of light enter into the absorber layer after reflection from back reflector resulting in increase in short circuit current. 9.53% of initial efficiency is achieved by the use of graded buffer layer and wide band gap n-layer in single junction small area (1 cm2) solar cell.

Modeling of Edge Losses in Al-BSF Silicon Solar Cells

Specific photovoltaic (PV) applications can require the use of limited-area solar cells. In that case, the edge of the device can have a considerable influence on the conversion efficiency. In this study, limited-area solar cells were fabricated by laser scribing and subsequent mechanical cleavage of large aluminum back surface field solar cells. First, laser parameters were optimized in order to limit the loss in conversion efficiency. It was shown that laser scribing must be performed on the rear side of the device in order to avoid the formation of shunts. The variation in the laser conditions could not improve the cell efficiency, as the discontinuity of the crystal lattice has a prominent impact compared with defects induced by laser scribing and cleaving. The fabrication of various cell geometries confirmed that the reduction of edge recombination was possible by simply limiting the cell periphery over its area. Specific characterization of the fabricated devices was carried out in order to understand the influence of the cell area on the performance. Using the two-diode model, two methods were used to extract the saturation current densities induced by metallization-related recombination ( $J_{{\rm{0m}}}$ ) and edge recombination ( $J_{{\rm{0e-dr}}}$ ). From this model, it is thus possible to predict the open-circuit voltage of any cell format. This study is particularly relevant for specific PV applications requiring the use of small-area solar cells and can be applied to other silicon cell technologies.

ITO-free large-area flexible organic solar cells with an embedded metal grid

We report on ITO-free large-area flexible organic solar cells with embedded thick metal grids. Embedded thick metal grids on flexible substrates allow uniform spin coating of thin layers of PEDOT:PSS (PH1000) semitransparent electrodes and organic semiconductor layers. A transparent SU8 photo-patternable polymer is used as a flexible substrate. Electroplated copper metallic grids, up to 15μm thick, defined by photolithography, produce a shadow area of 5.5% of the active area. Inverted solar cells with an active area of 9.3±0.2cm2, exhibited a fill factor of 0.53±0.06, and an open-circuit voltage of 808±5mV and a short-circuit current density of 5.5±0.5mA/cm2, yielding a power conversion efficiency of 2.4±0.4% under 100mW/cm2 air mass 1.5G illumination. The fill-factor and open circuit voltage values of large-area solar cells are comparable to values reported on small-area solar cells.

PassDop rear side passivation based on Al2O3/a-SiCx:B stacks for p-type PERL solar cells

In this study we present a new approach for p-type PERL solar cells based on an Al2O3 passivation in combination with boron doped amorphous SiCx. During a laser diffusion process, the rear side passivation is locally opened and simultaneously dopants are driven from the stack into the silicon to create a local back surface field resulting in a sheet resistance in the range of 15Ω/sq. We show that the main contribution to the dopants in the local back surface field originates from the a-SiCx:B layer, while aluminum is mainly present near the surface. This p-type PassDop stack is compatible with a firing step, reaching surface recombination velocities lower than 3cm/s. With this stack, small area solar cells on p-type float-zone silicon were processed, achieving energy conversion efficiencies up to 21.4% and fill factors of up to 82.5% with PVD contacts in a proof-of-concept batch.

Firing-stable PassDop passivation for screen printed n-type PERL solar cells based on a-SiNx:P

Rear side passivation and local back surface field formation are two of the main technological challenges for n-type PERL silicon solar cells. A promising approach is the PassDop process. This process combines a phosphorous doped passivation layer deposited on the rear side with a subsequent laser process to create both a local contact opening as well as a local back surface field. In this paper we introduce a new layer system based on doped amorphous silicon nitride (the fPassDop process) which is able to passivate the n-type surface after a firing step as typically used for screen printed contacts. After the firing step, an effective recombination velocity <5cm/s can be reached with this layer. The measured sheet resistance is in the range of 60Ω/sq after the laser process. In a first test the fPassDop process is applied to small area solar cells achieving a conversion efficiency of 21.3% (675mV Voc). Additionally, we fabricated large area n-type solar cells with screen printed front side contacts achieving 20.1% efficiency and Voc of 668mV.

Analysis of electroluminescence images in small-area circular CdTe solar cells

The electroluminescence (EL) imaging process of small area solar cells is investigated in detail to expose optical and electrical effects that influence image acquisition and corrupt the acquired image. An approach to correct the measured EL images and to extract the exact EL radiation as emitted from the photovoltaic device is presented. EL images of circular cadmium telluride (CdTe) solar cells are obtained under different conditions. The power-law relationship between forward injection current and EL emission and a negative temperature coefficient of EL radiation are observed. The distributed Simulation Program with Integrated Circuit Emphasis (SPICE®) model of the circular CdTe solar cell is used to simulate the dark J-V curve and current distribution under the conditions used during EL measurements. Simulation results are presented as circularly averaged EL intensity profiles, which clearly show that the ratio between resistive parameters determines the current distribution in thin-film solar cells. The exact resistance values for front and back contact layers and for CdTe bulk layer are determined at different temperatures, and a negative temperature coefficient for the CdTe bulk resistance is observed.

CuInSe2-Based Thin-Film Photovoltaic Technology in the Gigawatt Production Era

The objective of this paper is to review current status and future prospect on CuInSe2 (CIS)-based thin-film photovoltaic (PV) technology. In CIS-based thin-film PV technology, total-area cell efficiency in a small-area (i.e., smaller than 1 cm2) solar cell with top grids has been over 20%, while aperture-area efficiency in a large-area (i.e., larger than 800 cm2 as definition) monolithic module is approaching to an 18% milestone. However, most of the companies with CIS-based thin-film PV technology still stay at a production research stage, except Solar Frontier K.K. In July, 2011, Solar Frontier has joined the gigawatt (GW) group by starting up their third facility with a 0.9-GW/year production capacity. They are keeping the closest position to pass a 16% module-efficiency border by transferring the developed technologies in the R&D and accelerating the preparation for the future based on the concept of a product life-cycle management.

CuInSe2-Based Thin-Film Photovoltaic Technology in the Gigawatt Production Era

The objective of this paper is to review current status and future prospect on CuInSe2 (CIS)-based thin-film photovoltaic (PV) technology. In CIS-based thin-film PV technology, total-area cell efficiency in a small-area (i.e., smaller than 1 cm2) solar cell with top grids has been over 20%, while aperture-area efficiency in a large-area (i.e., larger than 800 cm2 as definition) monolithic module is approaching to an 18% milestone. However, most of the companies with CIS-based thin-film PV technology still stay at a production research stage, except Solar Frontier K.K. In July, 2011, Solar Frontier has joined the gigawatt (GW) group by starting up their third facility with a 0.9-GW/year production capacity. They are keeping the closest position to pass a 16% module-efficiency border by transferring the developed technologies in the R&D and accelerating the preparation for the future based on the concept of a product life-cycle management.