Smaller texture improves flexibility of crystalline silicon solar cells

Direct energy conversion from sun light to electricity using crystalline silicon based solar cell modules has been commercially available for several years. Above 20% conversion efficiencies have been realised in laboratory scale since 1990 [1]. The more important issue in the present solar cell fabrication development is to adopt such procedures and equipment that allow practical module efficiencies with less material and processing costs. One way to cut the material costs is to use thinner silicon wafers, which gives more wafers per length of a silicon log.In a high conversion efficiency cell the thickness has to be well below electron diffusion length for collection of the energy from long wavelength part of the solar spectrum. The diffusion length is an indicator of the material quality. Thin crystalline silicon solar cell technology gives the opportunity to choose cheaper, lower quality silicon as a starting material for the cell process without severe reduction in the conversion efficiency.Optimisation of a thin crystalline silicon solar cell design includes efficient light trapping and passivation of the cell surfaces. The proportion of a cell surface area to the cell volume increases as thickness is reduced. This magnifies the relative importance of surface recombination of the charge carriers as compared to the volume recombination in a solar cell. Although the quality demand on the cell material is relaxed, high conversion efficiency thin solar cell technology puts more weight on the quality of the surface passivation.In this paper, the fabrication procedure of thin crystalline silicon solar cell samples used in H.U.T. Electron Physics Laboratory is presented. The sample material is thinned using a 20% hot KOH bath to a nominal 1007 μm thickness. The standard cell sample is a 24 mm × 24 mm square with a p-type substrate, aluminium doped back surface field and phosphorous front diffusion. Aluminium evaporation through a stainless steel mask is adopted as a quick and simple metallisation scheme for both front and back contacts [2].The current-voltage-characteristics together with spectral response data of a typical thin crystalline silicon solar cell is also illustrated for the evaluation of the process. The measurement data is evaluated using the procedures and software developed by the author [2, 3]. The software includes diode modelling of the solar cell current-voltage data and breaking the spectral response to contributions from the emitter, depletion region and the base with numerical estimates on diffusion length, surface recombination velocities and charge carrier lifetimes.

Flexible solar cells have a lot of market potential for application in photovoltaics integrated into buildings and wearable electronics because they are lightweight, shockproof and self-powered. Silicon solar cells have been successfully used in large power plants. However, despite the efforts made for more than 50 years, there has been no notable progress in the development of flexible silicon solar cells because of their rigidity1–4. Here we provide a strategy for fabricating large-scale, foldable silicon wafers and manufacturing flexible solar cells. A textured crystalline silicon wafer always starts to crack at the sharp channels between surface pyramids in the marginal region of the wafer. This fact enabled us to improve the flexibility of silicon wafers by blunting the pyramidal structure in the marginal regions. This edge-blunting technique enables commercial production of large-scale (>240 cm2), high-efficiency (>24%) silicon solar cells that can be rolled similarly to a sheet of paper. The cells retain 100% of their power conversion efficiency after 1,000 side-to-side bending cycles. After being assembled into large (>10,000 cm2) flexible modules, these cells retain 99.62% of their power after thermal cycling between −70 °C and 85 °C for 120 h. Furthermore, they retain 96.03% of their power after 20 min of exposure to air flow when attached to a soft gasbag, which models wind blowing during a violent storm.

Silicon nanowires (SiNWs) show a great potential for energy applications because of the optical confinement effect, which enables the fabrication of highly efficient and thin crystalline silicon (c-Si) solar cells. Since a 10-μm-long SiNW array can absorb sufficient solar light less than 1200 nm, the 10-μm-long SiNW was fabricated on Si wafer to eliminate the influence of the Si wafer. On the other hand, Surface passivation of the SiNWs is a crucial problem that needs to be solved to reduce surface recombination and enable the application of SiNWs to c-Si solar cells. In this study, aluminum oxide (Al2O3) was fabricated by atomic layer deposition for the passivation of dangling bonds. However, owing to a complete covering of the SiNWs with Al2O3, the carriers could not move to the external circuit. Therefore, chemical–mechanical polishing was performed to uniformly remove the oxide from the top of the SiNWs. A heterojunction solar cell with an efficiency of 1.6% was successfully fabricated using amorphous silicon (a-Si). The internal quantum efficiencies (IQE) of the SiNW and c-Si solar cells were discussed. In the wavelength region below 340 nm, the IQE of the SiNW solar cell is higher than that of the c-Si device, which results in an increase of the absorption of the SiNW cells, suggesting that SiNWs are promising for crystalline-silicon thinning.

Halide perovskite films processed from solution at low-temperature offer promising opportunities to make flexible solar cells. However, the brittleness of perovskite films is an issue for mechanical stability in flexible devices. Herein, photo-crosslinked [6,6]-phenylC61 -butyric oxetane dendron ester (C-PCBOD) is used to improve the mechanical stability of methylammonium lead iodide (MAPbI3 ) perovskite films. Also, it is demonstrated that C-PCBOD passivates the grain boundaries, which reduces the formation of trap states and enhances the environmental stability of MAPbI3 . Thus, MAPbI3 perovskite solar cells are prepared on solid and flexible substrates with record efficiencies of 20.4% and 18.1%, respectively, which are among the highest ever reported for MAPbI3 on both flexible and solid substrates. The result of this work provides a step improvement toward stable and efficient flexible perovskite solar cells.

For wafers thicker than 200 /spl mu/m, the efficiency of industrial multicrystalline silicon solar cells is independent of wafer thickness. An important efficiency limitation of these thin solar cells is the poor rear surface passivation due to the short minority carrier diffusion length of the aluminium BSF. The most likely reason for the poor quality of the BSF is the poor crystallinity of the BSF due to fast cooling of the cells during metallisation firing. Possible solutions to overcome this limitation are presented.

An 18 μm thin crystalline silicon solar cell was demonstrated, and its best open circuit voltage is 642.3 mV. However, this value is far from the cell’s theoretical upper limit in an ideal case. This paper explores the open circuit voltage losses of the thin silicon solar cell, starting from the ideal case, through first principle calculation and experiments. The open circuit voltage losses come from the introduced recombination due to the non-ideal surface passivation and contacts integration on front and rear surfaces, and edge isolation. This paper presents a roadmap of the open circuit voltage reduction from an ideal case of 767.0 mV to the best measured value of 642.3 mV.

Organic-inorganic hybrid perovskite solar cells have experienced a skyrocketing photoelectric conversion efficiency (PCE) increase in recent years, and become the “rock star” in the field of solar cells. After only 4 years, the certificated PCE of these materials based solid state solar cells have reach the maximum 22.1%, which is comparable to the highest PCE of the commercial silicon-based solar cells and higher than those of the organic solar cells and the dye-sensitized solar cells. Besides the promising efficiency, these materials surpass its competitors with the adaptability to solution- process at low-temperature and in air, and better flexibility, which can lead to lower large-scale production cost and more application potential for wearable electronics. The outstanding device performance of the hybrid perovskite solar cells have attracted intensive research interest, and besides increasing the PCE, lots of efforts have been focused on functional diversification for this solar cells. Compare to the commercial solar cells based on rigid silicon, organic- inorganic hybrid perovskite materials exhibit better flexibility which is beneficial for applications in flexible electronic devices, and due to the mild preparation conditions and the solution processablity, it is easy to prepare a variety of nanostructures. Those advantages have expanded the application scope of the hybrid perovskite materials, especially for fabricating flexible and semi-transparent solar cells which have potential use in building integrated photovoltaic such as solar cell windows and solar cell curtains. Here in this review, we focus on the recent progress of the flexible and semi-transparent solar cells based on organic-inorganic hybrid perovskite materials. In the part of the flexible hybrid perovskite solar cells, two mainstream device structures, namely planar and fiber-shaped devices, are discussed and the device structure and materials selection are analyzed in detail. As for the planar flexible perovskite solar cells, the choosing of the low-temperature processable function materials for the solar cells seems to be one of key factors due to the fact that most flexible planar substrates are vulnerable to the high-temperature treatments. As for the fiber-shaped perovskite solar cells, the choosing of the proper transparent conductive electrode materials is crucial which should have a good balance between the transparency and conductivity, and due to the difficulty of forming high-quality perovskite function layer on the curved surface of a certain wire, the prepare methods have to be upgraded too. In the part of the semi-transparent hybrid perovskite solar cells, two main strategies (reducing film thickness and forming isolated island structure) are discussed and a lot of reported preparation methods are analyzed in detail. As for the former strategy of reducing film thickness, the advantages are the comparatively higher PCE, easier manufacture and better controllability and the disadvantage is the unfavorable brownish color. As for the latter strategy of forming isolated island structure, the advantage is the better full-spectrum transmission and the disadvantages are comparatively more complicated manufacture and worse controllability. The whole paper has presented the development of the flexible and semi-transparent perovskite solar cells so far and gave detailed discussions for the reported works. Based on these analyzations, the organic-inorganic hybrid perovskites will find their special and unique position in the field of flexible and semi-transparent solar cells and may aggressively push the field forward.

AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract We have developed a new light-trapping scheme for a thin-film Si stacked module (Si HYBRID PULS module), where a (a-Si:H/transparent interlayer/microcrystalline Si) thin-film was integrated into a large-area solar cell module. An initial aperture efficiency of 13·1% has been achieved for a 910 × 455 mm Si HYBRID PLUS module, which was independently confirmed by AIST. This is the first report of the independently confirmed efficiency of a large-area thin-film Si module with an interlayer. The 19% increase of short-circuit current of this module was obtained by the introduction of a transparent interlayer that caused internal light-trapping. A mini-module was shown to exhibit a stabilized efficiency of 12%. Outdoor performance of a Si HYBRID (a-Si:H / micro-crystalline Si stacked) solar cell module has been investigated for over 4 years with two different kinds of module (top and bottom cell limited, respectively). The HYBRID modules limited by the top cell have exhibited a more efficient performance than the modules limited by the bottom cell, in natural sunlight at noon. Copyright © 2005 John Wiley & Sons, Ltd. REFERENCES 1 Yamamoto K, Suzuki T, Yoshimi M, Nakajima A. Low temperature fabrication of thin film polycrystalline Si solar cell on the glass substrate and its application to the a-Si:H/Polycrystalline Si tandem solar cell. Proceedings of the 25th IEEE Photovoltaic Energy Conversion, Conference, 1996; 661– 664. 2 Yamamoto K, Yoshimi M, Suzuki T, Tawada Y, Okamoto Y, Nakajima A. Very thin film crystalline silicon solar cells on glass substrate fabricated at low temperature. IEEE Transactions Electron Devices 1999; 46: 2041– 2047. 3 Yamamoto K, Yoshimi M, Tawada Y, Okamoto Y, Nakajima A. Cost effective and high-performance thin film Si solar cell towards the 21st century. Solar Energy Materials and Solar Cells 2001; 66: 117– 125. 4 Meier J, et al. On the way towards high-efficiency thin film silicon solar cells by the ‘micromorph’ concept. Proceedings of the Material, Research Society Symposium, Spring Meeting, San Francisco, 1996; 420: 3– 14. 5 Yamamoto K, et al. Novel HYBRID thin film silicon solar cell and module. Proceedings of the 3rd World Conference on Photovoltaic Solar Energy Conversion, 2003; Paper S20B903. 6 Nakajima A, et al. Improvement on actual output power of thin film silicon HYBRID module. Proceedings of the 3rd World Conference on Photovoltaic Solar Energy Conversion, 2003; Paper 60B1104. Citing Literature Volume13, Issue6September 2005Pages 489-494 ReferencesRelatedInformation

With the continuous development of various new energy sources, flexible solar cells have attracted much attention as a renewable and clean energy source. Compared with rigid solar cells, flexible solar cells have the characteristics of bendability, flexibility, and portability. Choosing the right material and manufacturing process can get twice the result with half the effort in the research of flexible solar cells. This article aims to review the research progress of wearable flexible solar cells, analyze the materials and preparation technologies used in flexible solar cells at this stage, and discuss the influence of some materials and processes on the photoelectric conversion efficiency and flexibility of flexible solar cells. In addition, this paper also discusses the future development prospects and some existing problems of flexible solar cells. With the continuous development of wearable technology, the research of flexible solar cells must be one of the development priorities in the future.

Flexible photovoltaics are covering the way to low-cost electricity. The build-up of organic, inorganic and organic–inorganic solar cells on flexible substrates by printing technologies is to provide lightweight and economic solar modules that can be incorporated in various surfaces. Progress of flexible and lightweight solar cell is interesting for many terrestrial and space applications that require a very high specific power. Thin-film solar cells on polymer films can produce more than 2KW\\Kg specific power. Flexible solar cells are proposed to open up a numerous of possibilities for enabling new applications in consumer electronics and space satellites. Recent research in thin-film electronics has been concentrated on the replacement of the traditional rigid glass plate substrate with plastic or metallic foils. Organic materials bear the potential to develop a long-term technology that is economically viable for large-scale power generation based on environmentally safe materials with unlimited availability. Organic and organic-inorganic photovoltaics (PVs) (third generation solar cells) continue to attract great attention from the PV community, due to their promising features such as low organic–inorganic cost, flexibility and light weight. In this chapter, many of the possible materials for manufacturing of flexible solar cells are discussed.

The development of flexible solar cells is necessary for achieving market competitiveness through the implementation of low cost solar cells and for applying customized business models, such as Building Integrated Photovoltaics (BIPV). Cu2ZnSn(S,Se)S4 (CZTSSe) thin films have attracted attention as low-cost absorber materials for solar cells; however, further studies are required to develop flexible solar cells from this material and to achieve a high power conversion efficiency. In this work, the carrier separation characteristics of flexible CZTSSe thin-film solar cells on Mo foil substrates were identified. The surface electrical characteristics of the CZTSSe absorber layer indicate that the topography, surface potential, and local current are proportional to one another, in contrast to the characteristics exhibited by CIGS cells. For the CZTSSe absorber layer, an upward band bending-like band structure formed at and near the GBs, and a current path formed along the IGs as holes collected toward the GBs, and electrons collected toward the IGs. In contrast to that in CIGS cells, the carrier separation induced by the formation of the upward band bending-like band structure at the GBs is crucial in CZTSSe thin-film solar cells. Furthermore, the positive potential distribution at the IGs and average potential difference showed a proportional relationship to the local current and PCE. Through the implementation of these carrier separation characteristics, a flexible CZTSSe thin-film solar cell on a Mo foil substrate achieved a PCE of over 7%. Acknowledge: This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) (No. 20173010012980) and by the DGIST R&D Programs of the Ministry of Science, ICT & Future Planning of Korea (19-BD-05).

For the development of high efficiency Cu(In,Ga)Se 2 (CIGS) flexible solar cells on polymer films with efficient and robust deposition process the so-called three-stage evaporation processes were modified. CIGS layers were grown by co-evaporation of constituent elements at low substrate temperature compatible with the stability of the polyimide film. The CIGS layers grown with our standard process are compositionally graded, especially the Ga has a strong grading profile across the layer thickness. Different variations of the standard evaporation process were investigated, especially in view of changing the energy band gap profiles in the absorber layer for achieving further improvements in high efficiency solar cells. Further on, the incorporation of sodium during the CIGS deposition was optimized. Structural and chemical composition properties of CIGS layers were characterized with SEM, XRF, SIMS and the photovoltaic properties were characterized with I–V and quantum efficiency measurements. A flexible CIGS solar cell on polyimide film with a record efficiency of 17.6% has been developed.

In the wet-texturing process, a smaller etch depth of 8 µm was established on each side of the thin silicon wafer. Formation of a very small size (1–3 µm) pyramidal structure was confirmed on a thin c-Si surface through the use of SEM micrographs. For this silicon surface with small pyramidal structures, a surface reflectance of 15.14% was observed. Saw damage removal was employed for the second and the third sets of silicon wafers in a NaOH (8 wt%) solution, and the wafers were then textured in a mixture of NaOH (1.5 wt%), de-ionized water and Isopropyl alcohol (IPA, 6–8 wt%) at 80 °C for 18 min and 24 min, respectively. In this process, the scraped depths were 16 and 26 µm on each side of the second and the third sets of silicon wafers, and the pyramidal sizes were 4–9 µm and 9–12 µm respectively. We found that the reflectances of these sets of wafers were 14.98% and 14.78%, respectively. Our novel approach of ultrasonic cleaning, hot de-ionized water treatment, and texturing for thin c-Si wafers yielded small-sized (1–3 µm) uniform pyramidal structure that were stable and suitable for metalized screen printing. Even though the reflectance was high for the first set of wafers the illuminated current-voltage (LIV) measurements confirmed the wafers’ superiority over the conventional textured c-Si solar cells in terms of the open-circuit voltage and the fill factor. The small-size pyramidal-structured thin single-crystalline silicon solar cells showed a 17.9% efficiency. Since no NaOH solution was used for saw damage removal in the first set of silicon wafers, the production cost for commercial production of large-area single-crystalline silicon solar cells was reduced by nearly 20%.

Enhanced mechanical endurance and power conversion efficiency in flexible Cu2ZnSnS4 solar cells using ZnO/Ag-Ag2O/ZnO film

Flexible solar cells have launched potential applications in diverse areas, such as civil engineering, consumer electronics, electric automobile, and aerospace use. In recent years, perovskite solar cells (PSCs) have been successful with a rocketing power conversion efficiency (PCE) from 3.8% to 25.2% in the last decade. Due to its material flexibility, together with low‐cost and facile fabrication processes, perovskites have certainly been considered as a promising candidate for the next generation of flexible solar cells. So far, the PCE of single‐junction flexible perovskite solar cells (FPSCs) has reached 19.51%, which retains researchers' great enthusiasm for further development and applications of FPSCs. Herein, a brief review of the recent advances in the structural components of FPSCs is presented, including perovskite absorber layers, flexible substrates, transparent bottom electrodes, and charge carrier transport layers. In particular, the focus is on the development of low‐temperature fabrication processes of the aforementioned compositional layer structures. Finally, noticeable achievements and annual milestones are discussed and summarized, and suggestions for further improvement of FPSCs and their ways toward commercialization are presented.