Mohamed M. Hilali

Single heterojunction solar cells on exfoliated flexible ∼25 μm thick mono-crystalline silicon substrates

Mono-crystalline silicon single heterojunction solar cells on flexible, ultra-thin (∼25 μm) substrates have been developed based on a kerf-less exfoliation method. Optical and electrical measurements demonstrate maintained structural integrity of these flexible substrates. Among several single heterojunction ∼25 μm thick solar cells fabricated with un-optimized processes, the highest open circuit voltage of 603 mV, short circuit current of 34.4 mA/cm2, and conversion efficiency of 14.9% are achieved separately on three different cells. Preliminary reliability test results that include thermal shock and highly accelerated stress tests are also shown to demonstrate compatibility of this technology for use in photovoltaic modules.

Nanostructured solar cell

Publisher Summary This chapter addresses the nanostructured solar cells that play an important role in enhancing the efficiency of future generations of solar cells, whether they are III–V, II–VI, or hybrid organic–inorganic cells. There is a great deal of potential in multiple approaches for these nanostructures. Nanostructures can also be composed of arrays of individual nanomaterials. Semiconducting quantum dots (QDs) can be combined in a three-dimensional array, often through the use of selfordering. The discrete-like energy levels of the QDs will combine and form bands of allowed energy states in an analogous way in which atomic energy levels combine to produce the energy bands in conventional solids. The role of a nanomaterial or nanostructure in a given photovoltaic solar cell design can vary dramatically. In some cases, the goal may be simply to provide the means to disassociate excitons throughout a bulk material as in the use of colloidal QDs in organic or polymeric solar cells.

Enhanced photocurrent in thin-film amorphous silicon solar cells via shape controlled three-dimensional nanostructures

In this paper, we have explored manufacturable approaches to sub-wavelength controlled three-dimensional (3D) nano-patterns with the goal of significantly enhancing the photocurrent in amorphous silicon solar cells. Here we demonstrate efficiency enhancement of about 50% over typical flat a-Si thin-film solar cells, and report an enhancement of 20% in optical absorption over Asahi textured glass by fabricating sub-wavelength nano-patterned a-Si on glass substrates. External quantum efficiency showed superior results for the 3D nano-patterned thin-film solar cells due to enhancement of broadband optical absorption. The results further indicate that this enhanced light trapping is achieved with minimal parasitic absorption losses in the deposited transparent conductive oxide for the nano-patterned substrate thin-film amorphous silicon solar cell configuration. Optical simulations are in good agreement with experimental results, and also show a significant enhancement in optical absorption, quantum efficiency and photocurrent.

Realization of dual-heterojunction solar cells on ultra-thin ∼25 μm, flexible silicon substrates

Silicon heterojunction (HJ) solar cells with different rear passivation and contact designs were fabricated on ∼25 μm semiconductor-on-metal (SOM) exfoliated substrates. It was found that the performance of these cells is limited by recombination at the rear-surface. Employing the dual-HJ architecture resulted in the improvement of open-circuit voltage (Voc) from 605 mV (single-HJ) to 645 mV with no front side intrinsic amorphous silicon (i-layer) passivation. Addition of un-optimized front side i-layer passivation resulted in further enhancement in Voc to 662 mV. Pathways to achieving further improvement in the performance of HJ solar cells on ultra-thin SOM substrates are discussed.

Light trapping in ultrathin 25 μm exfoliated Si solar cells

The optical absorption in 25-μm-thick, single-crystal Si foils fabricated using a novel exfoliation technique for solar cells is studied and improved in this work. Various light-trapping and optical absorption enhancement schemes implemented show that it is possible to substantially narrow the gap in optical absorption loss between the 25 μm Si foils and industry-standard 180-μm-thick Si wafer solar cells. An improvement of absorption by 58% in the near-infrared (740-1200 nm) range is observed for the 25 μm monocrystalline Si substrates with the use of antireflective coating and texturing. The back reflectance of the metal foil that provides mechanical support to the ultrathin Si semiconductor-on-metal foils is extracted to be ∼51.5%, based on the reflectance matching with the simulated escape reflectance in the sub-bandgap region. The back reflectance is enhanced to ∼58% by incorporating an intermediate silicon nitride layer on the back between the Si and the metal. The incorporation of Al as an improved metal reflector on top of the silicon nitride at the backside of the solar cell results in a 5.8 times enhancement in optical path length as a consequence of the improved effective back reflectance of ∼95%. A thin Si foil solar cell with an unoptimized amorphous Si/crystalline Si heterojunction with intrinsic-thin-layer design with implementation of such light-trapping schemes shows an efficiency of 13.28% with a short-circuit current density (JSC) of 35.97  mA/cm2, which approaches the JSC of industrial wafer-based Si solar cells.

Exfoliated sub-10μm thin germanium for cost-effective germanium based photovoltaic applications

Germanium based photovoltaic cells have found applications in both space/satellite and terrestrial systems such as high-efficiency multi-junction concentrator solar cells. In these applications, the main emphasis is on increasing the cell efficiency, as opposed to cost reduction. Recently, there has been more widespread application of Ge in other areas such as low light conditions and thermophotovoltaic (TPV) systems where cost-effectiveness is important. In order for this to be commercially viable, a low-cost solution for developing Ge cells would be essential. To this end, our approach is to minimize the amount of Ge material used. In this paper, we demonstrate for the first time a novel exfoliation technology capable of producing sub-10μm thin flexible Ge foils for cost-effective Ge based photovoltaic applications. First, computer simulation was used to study the influence of substrate thickness on the open circuit voltage and efficiency of Ge single-homojunction cells. Then, our novel exfoliation technology was applied to Ge cells to realize thin flexible foils. Our preliminary experimental results show an increase in open-circuit voltage from 150 mV (non-exfoliated cell) to 270 mV (exfoliated cell), in agreement with our simulation results.

Nanostructured Silicon-Based Photovoltaic Cells

In recent years significant research has been conducted on the materials, design, and device physics of nanostructured solar cells to obtain enhanced performance. While there are several promising results, practical deployment of these nanostructured cells is quite limited due to the need for (1) cost-effective, scalable fabrication techniques; (2) readily available raw materials; and (3) cells that can reliably perform as installed with minimal performance degradation. This book chapter focuses on nanostructured silicon solar cells. Silicon has been chosen in this chapter as it is an abundantly available raw material; silicon cells are well understood due to broad deployment of conventional monocrystalline, multi-crystalline, and amorphous silicon solar cells; and silicon cells also are by far the dominant one in terms of production scale in the photovoltaic industry.

Amorphous/crystalline silicon heterojunction solar cells via Remote plasma chemical vapor deposition: Influence of hydrogen dilution, RF power, and sample Z-height position

Single heterojunction hydrogenated amorphous silicon/monocrystalline silicon (a-Si:H/c-Si) cells of varying hydrogen dilution ratios, RF power, and sample position with respect to the plasma discharge were fabricated using remote plasma enhanced chemical vapor deposition (RPCVD). Only p-type doped a-Si:H is investigated in this paper, without intrinsic a-Si:H (i-layer) passivation. It was found that a hydrogen dilution ratio, R ≤ 2 is necessary to minimize the degradation of the open-circuit voltage (VOC). Furthermore, a comparison of the RF power on device performance shows enhancements in cell performance, including approximately ~20 mV improvement in VOC with 30 W vs. 60 W, which may be attributed to a reduction in ion-bombardment damage. Finally, it was found that a higher sample z-height position (away from plasma glow) yields improvements in JSC and VOC of 1.6 mA/cm2 and 9 mV, respectively at a low RF power of 7 W. These results suggest that the RPCVD is a potential technology to deliver low plasma damage and improved passivation for Si heterojunction solar cell.

Improved cleaning process for post-texture surface contamination removal for single heterojunction solar cells on ∼25µm thick exfoliated and flexible mono-crystalline silicon substrates

An improved cleaning process is developed to remove surface contamination before depositing amorphous silicon on a textured surfaces of ~25μm thick exfoliated and flexible monocrystalline substrates. Auger electron spectroscopy is used to monitor the change in atomic percentage on the surface to reflect the effect on contamination removal due to different cleaning processes. Quantitative analysis of the spectra shows significant reduction (~0.9% atomic) in key contaminant i.e. potassium with the newest cleaning procedure. Electrical data is collected on completed single heterojunction cells on exfoliated substrates to compare between the old process and the new improved process for cleaning the textured surface. An overall enhancement of 22mV and an absolute increase of 1.5% in conversion efficiency are observed with the new and improved cleaning procedure.

Exfoliated thin, flexible monocrystalline germanium heterojunction solar cells

A thin, flexible monocrystalline germanium (c-Ge) heterojunction solar cell has been developed based on a cost-effective kerfless exfoliation process and remote plasma-enhanced chemical vapor deposition (RPCVD) of hydrogenated amorphous silicon (a-Si:H). The performance of the exfoliated 50μm thick and bulk 500μm Ge heterojunction cells are compared in this paper. A superior conversion efficiency of 5.28% was achieved with the 50μm exfoliated Ge cell versus 1.78% for the bulk Ge cell, in agreement with simulation results. A record fill factor of 58.1% for an a-Si:H/c-Ge heterojunction cell is obtained with the exfoliated cell. Moreover, the conversion efficiency achieved with the 50μm exfoliated cell (without intrinsic a-Si:H passivation) is comparable to the best reported in literature with bulk Ge heterojunction cells and intrinsic a-S:H passivation.