The enhanced photo-response of Textured Si and an introduction about the latest study in Textured Si solar cell

Qiu Ying

Supervisor: Lu Ming

Department of Optical Science and Engineering, Fudan University Shanghai 200433, China

Abstract:

A close to unity and all-solar-spectrum photoabsorption by ion-sputtering induced Si nanocone arrays is reported, with the maximal absorbance for l = 350 to 1100 nm higher than 96%, and that for l = 1100 to 2000 nm higher than 92%. Photocurrents of Si with different absorptivities were investigated. Two Al ribbons 5 mm apart were evaporated onto Si surface, and the surface current was measured with a 2V DC voltage across the Al electrodes. The photocurrent was characterized as I₁-I₀, where I₁ is the surface current at the presence of light illumination and I₀ the dark current. Some reviews of latest researches are as follows.

Researchers in Germany present a wet chemical process for nanoscale texturing of Si surfaces, which results in an almost complete suppression of the reflectivity in a broad spectral range, leading to black Si surfaces. The process affects only the topmost 200–300 nm of the material and is independent of the surface orientation and doping. Thus, it can be applied to various structural forms of bulk silicon (single, poly-, or multicrystalline) as well as to thin Si films (amorphous or microcrystalline).

Another job is about an amorphous silicon [a-Si]-solar cell with a nanocone array structure, which was implemented by reactive-ion etching through a polystyrene nanosphere template. The nanostructure could provide the efficient carrier collection. Owing to the better carrier collection efficiency, efficiency of a-Si solar cell was increased from 1.43% to 1.77% by adding the nanocone structure which has 24% enhancement. Further passivation of the a-Si:H surface by hydrogen plasma treatment and an additional 10-nm intrinsic-a-Si:H layer, the efficiency could further increase to 2.2%, which is 54% enhanced as compared to the planar solar cell.

Researchers in Philips Research Laboratories present an entirely new concept that suppresses the reflection of light from a silicon surface over a broad spectral range. A two-dimensional periodic array of subwavelength silicon nanocylinders designed to possess strongly substrate-coupled Mie resonances yields almost zero total reflectance over the entire spectral range from the ultraviolet to the near-infrared. This new antireflection concept relies on the strong forward scattering that occurs when a scattering structure is placed in close proximity to a high-index substrate with a high optical density of states.

Heterojunction solar cells are prepared by depositing ultrathin amorphous Si film on the nano-patterned Si structures in Nanjing University, and the short circuit current density increases to 37.2 mA/cm2 and the power conversion efficiency is obviously improved compared to the reference cell on flat Si substrate.

The latest research shows that both surface recombination and Auger recombination exist in wafer-based nanostructured silicon solar cells. By identifying the regimes of junction doping concentration in which each mechanism dominates, researchers in National Renewable Energy Laboratory were able to design and fabricate an independently confirmed 18.2%-efficient nanostructured ‘black-silicon’ cell that does not need the antireflection coating layer(s) normally required to reach a comparable performance level.

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Applications of ZnO nanorod array for solar cell

   Yang  Qin

Tutor: Jiada Wu

Department of Optical Science and Engineering, Fudan University Shanghai 200433, China

Abstract

The two important directions of application of solar energy, one is changing it into heat energy, another is changing it into electric energy. And its merits are no transmission parts, no noise, no pollution and renewable. It won't be exhausted. That's the key point.

     The world energy consumption is reaching 16 terawatts and is predicted to triple by 2050. The research and development of solar cells has been driven by this quickly increasing energy consumption and further accelerated by increasing concerns about carbon emission from fossil fuel-based energy sources. As a result, photovoltaics have seen more interest in technological development in recent years. Although silicon p-n junction solar cells have been the most used solar cell system because of their advantages in high efficiency and reliability, the high cost of silicon solar cells has limited their applications.

              Compare to conventional solar cells, organic and hybrid solar cells have many advantages, light weight, flexibility and potential low cost due to their compatibility with versatile solution processing method. The choice of inorganic semiconductors starts with CdSe and extends to TiO₂ and ZnO. Among these inorganic semiconductors, ZnO has attracted a lot of attention for being used in HSCs because ZnO is less toxic than mang other II-VI semiconductors and is relatively easy to synthesize in large quantities with low cost techniques. As an n-type semiconductor with a wide band gap, ZnO also has very good optical transmittance, which makes ZnO an excellent candidate as an efficient cathode buffer layer or a transparent electrode in OSCs and HSCs. In ZnO-NR/organic HSC device architecture, devices consist of rigidly connected and vertically aligned ZnO nanorods, which can provide direct pathways for electron transport. And it's easily synthesis on many substrates by simple method.

              In this report I will introduce some applications about ZnO nanoro for solar cells, including some work of our group in the past time, my work at the present and the future plan.

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Lateral photovoltaic effect in organic semiconductor related structures

Wang Chao

Advisor: Prof. Ni Gang

Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education) and Department of Optical Science and Engineering, Fudan University Shanghai 200433, China

Abstract

In the past few decades, organic semiconductors (OSCs) have attracted a lot of interest for the application of optoelectronics devices. Organic light-emitting diodes (OLEDs) have become a promising candidate for the next-generation flat panel displays, and organic solar cells are also challenging the commercial inorganic solar cells due to the intriguing combination of low weight and cost. In comparison with the transverse photovoltaic effect used in the solar cells, lateral photovoltaic effect (LPE) also has a wide range of applications, especially position sensitive detectors (PSDs). Under a nonuniform irradiation on a semiconductor device (such as p-n junction), an additional photovoltage parallel to the plane of the film can be observed. Besides in p-n junctions, the effect has been observed in various structures, such as metal/semiconductor superlattices, metal-oxide-semiconductor (MOS), metal- semiconductor (MS) structures and perovskite heterostructures. Recently, large LPE was also found in nanometer composite films on Si substrates. Till now, only few studies have been reported for LPE in organic semiconductor related devices. Thus, it is interesting to study what will happen if the upper layer is replaced by metal doped organic semiconductor films in the MOS/MS structures. We investigated the lateral photovoltaic effect in Co-Alq3 granular films on Si substrate and Al-Alq₃/SiO₂/Si structure. And observed a large sensitivity and a good linearity in the organic semiconductor related structure, discussed its possible mechanisms, proposed the competition mechanism which is successfully used to explain the specially irreversible of LPV and we found the regulation between the sensitivity and the Al volume fraction in Al-Alq₃ layer, which can be interpreted in conductive network mechanism.

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