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Photoelectric conversion characteristics of a GaAs solar cell with biomimetic antireflective nanostructure are investigated by using an near-field optical beam induced current (NOBIC) technique. The mappings of the nanostructured profile and photocurrent induced by incident light with different wavelengths can be obtained simultaneously. The measured results of NOBIC show the optical conversion efficiency is mostly harvested near the valley of the nanostructures, which serve as light guides for incident light with three different wavelengths. The observation is consistent with the simulation results using a rigorous coupled wave analysis.

Hybrid solar cells (HSCs) based on a mixture of organic and inorganic semiconductor materials attract a lot of attention owing to the combination of desirable properties of both materials. Due to low exciton diffusion length and light harvesting issues, nanostructures are often employed in HSCs. However, most numerical models over-simplify the complicated bulk heterojunction (BHJ) into a homojunction configuration which losses insights to charge transport. Moreover, designing nanostructures to achieve both light harvesting and carrier collection is essential, but rather complicated. In this work, we develop a methodology based on two-dimensional (2D) optical and electrical simulations which are tailored for the hybrid system. The optical simulation employs a finite-difference time-domain (FDTD) technique to calculate the electromagnetic field and obtain the generation rate in the nanostructure. Next, an electrical calculation is based on a 2D self-consistent drift-diffusion and Poisson solver which uses a finite element method (FEM). As a result, our works allows the analysis of optical and electrical properties of nanostructured heterojunction semiconductor materials. The theoretical approach has been validated for P3HT/PCBM BHJ organic solar cells. In this study, hybrid structures based on Si nanowires (NWs)/ poly(3-hexylthiophene)(P3HT) are used as the hypothetical material system, where an interpenetrating network of rectangular channels consisting of SiNWs and P3HT is assumed. The simulation results show that NWs facilitates transmission and absorption of sunlight inside the photoactive layer. We then investigate the device current-voltage characteristics as a function of the recombination rate, barrier heights of anode and cathode, and structural configuration of interfaces. The theoretical approach also allows the optimization of optical and electrical properties of solar cells with embedded nanostructures or nanoparticles.

Silicon nanowire (SiNW) arrays have become a promising structure for photovoltaics in which the surface recombination velocity is an important parameter. In this study, a simple and cost-effective method is presented for producing large-area SiNW arrays with various lengths. We then employ an optical technique based on resonant-coupled photoconductive decay (RCPCD) to provide a contactless measurement for the determination of the surface velocity and bulk lifetime of SiNW arrays. The basic method is to probe the decay of total excess carrier concentration by varying the excitation wavelengths. However, as the initial carrier distribution could be very complicated in SiNWs, we have developed a rigorous couple wave analysis (RCWA) to calculate the absorption of SiNWs in order to obtain the initial distribution of excess carriers. The total excess carrier concentration as a function of time can then be derived by using the simulation results for experimental curve fitting. The model successfully fits the measured data and extracts parameters, which helps to determine both the bulk lifetime and the surface recombination velocity.

Tunnel diodes, also known as Esaki diodes, play an important role in III-V multi junction solar cells. In this work, we theoretically investigated the electrical characteristics of a GaAs tunnel diode and its performance against the illumination conditions such as light intensity, spatial profile, etc. We then developed a methodology to optimize a triple junction InGap/GaAs/Ge solar cell with a GaAs tunnel junction. The conversion efficiency drops with the increase of the concentration ratio is also discussed.

a-Si/a-Si tandem cells have a higher fill factor (FF), and lower light degradation than the single junction due to an intrinsic high electric field. Also, they are thinner and cheaper than a-Si single junction solar cells.[1] This paper presents experiment results of a-Si/a-Si tandem solar cells deposited by a High-Density Plasma Chemical Vapor Deposition (HDPCVD) and uses AMPS-1D to simulate the performance of different material compositions at the recombination junction (RJ), including n-a-Si/p-a-Si, n-a-Si/p-mc-Si, n-mc-Si/p-mc-Si, and n-mc-Si/p-a-SiC.[2,4] In the experiment, although we found that as the thickness of the n-a-Si and p-a-Si layers of the RJ decreased, and the flow rate of PH 3 for the n-a-Si layer of the top cell lowered, the photocurrent (Jsc) and FF increased due to a reduced barrier height at the n-layer that facilitates the recombination and tunneling. The simulation results showed that the n-mc-Si/p-mc-Si composition had the best performance at the tunneling junction, and its highest efficiency could reach 10.4%.

The enhanced photoelectric conversion is demonstrated in nanostructured photovoltaics using colloidal lithography and reactive-ion-etching (RIE) techniques. From the reflectance spectroscopy, trapezoid-cone arrays (TCAs) Si with SiNx passivation layer effectively suppress the Fresnel reflection in the wavelength range from 400 nm to 1000 nm. The power conversion shows the TCAs Si solar cell with 120 nm thickness of SiNx passivation layer achieves 13.736%, which is 8.87% and 2.56% enhancement compared to the conventional KOH-textured photovoltaics and TCAs with 80-nm-thick SiNx, respectively. An optical simulation based on RCWA describes the optimized shape of nanostructure to further reduce reflectance for maximum light absorption.

Light trapping techniques such as textured interfaces and highly reflective back contacts are important to thin-film solar cells. Scattering at rough interfaces inside a solar cell leads to enhanced absorption due to an increased optical path length in the active layers, which is generally characterized by a haze ratio. In this work, we demonstrate the measured haze characteristics of indium tin oxide nanowhiskers deposited on an ITO-coated glass substrate. A theoretical model based on a classical Mie theory is also employed to analyze the scattering effects of nanowhiskers. The calculated haze-ratio of an ITO whisker layer matches the measurement closely.

As silicon photovoltaics evolve towards thin-wafer technologies, efficient optical absorption for the near-infrared wavelengths has become particularly challenging. In this work, we present a solution that employs combined micro- and nano-scale surface textures to increase light harvesting in the near-infrared for crystalline silicon photovoltaics, and discuss the associated antireflection and scattering mechanisms. The surface textures are achieved by uniformly depositing a layer of indium-tin-oxide nanowhiskers on micro-grooved silicon substrates using electron-beam evaporation. The nanowhiskers facilitate optical transmission in the near-infrared by functioning as impedance matching layers with effective refractive indices gradually varying from 1 to 1.3. Materials with such unique refractive index characteristics are not readily available in nature. Compared to the reflectance of the conventional silicon solar cell, the combined textures structure provided broadband high absorption, especially in the near infrared region. As a result, the solar cell with combined textures achieves over 90% external quantum efficiencies for a broad wavelength range of 460 to 980 nm, which is crucial to the development of advanced thin-substrate silicon solar cells. Due to the high photocurrent contributed to the performance, the compound textured solar cell increased the 1.1% absolute power conversion efficiency, from 16.1% to 17.2%.

The enhanced photoelectric conversion is demonstrated in nanostructured glass substrates for a-Si thin film solar cell. The nanostructured glass substrates were fabricated using nanosphere lithography and RIE technique. The nanostructure substrates provide antireflective and light scattering characteristics, which enhance the broadband light absorption, especially in near-infrared range. Finally we demonstrate the patterned glass substrates (nipple shape arrays) which are useful in light absorption of a-Si thin film solar cell. Compared to a flat glass substrate cell and Asahi-U glass substrate cell, the power conversion efficiency enhancement achieved 48.4% and 3.1%, the Jsc enhancement achieved 51.6% and 8%.

The GaN/In 0.25 Ga 0.75 N double-heterojunction solar cells with rough and nipple arrays Indium Tin Oxide (ITO) surface texture was fabricated by the inductively coupled plasma reactiveion etching (ICP-RIE) system. The peak of the external quantum efficiency for the samples with rough and nipple arrays ITO surface were each reached 29.17% and 34.25% at 420nm, and the enhancement with nipple arrays achieved highly 40% compared with the flat surface one. The results of the angle dependent current density were shown the outstanding antireflection for wide incident angle from 0° to 60°.

Light trapping in amorphous silicon thin film solar cells has been an intensive study owing to the low absorption coefficient in near-infrared. We demonstrate a frontal pre-patterned substrate (PPS) on amorphous silicon solar cells, utilizing scalable colloidal lithography, to serve both functions of anti-reflection and light trapping effect. We show that a solar cell with front pre-patterned substrate exhibits broadband enhanced external quantum efficiency due to both anti-reflection and light-trapping, with respect to an industrial standard cell using an Asahi U glass substrate which is mostly optimized for light trapping. The power conversion efficiency of the pre-patterned cell is measured 8.38%, which shows 56.34% and 8.83% enhancement compared to the reference cell with a flat substrate and the commercialized Asahi U-type substrate, respectively. Moreover, the angle-resolved absorption spectroscopy shows superior optical coupling to the absorber layer at large angles of incidence (AOIs), which guarantees sufficient light harvesting for the entire day. We also present a design optimization of frontal pre-patterned substrate with broadband antireflective subwavelength structures based on the theoretical calculation using a rigorous coupled wave analysis (RCWA) method. We can find the optimization size of PPS is 500nm bottom width and 450nm height from the simulation enhancement mapping figure.

Thin wafer-based solar cells have the potential to significantly decrease the cost of photovoltaics. Light trapping is particularly critical in such thin-wafer crystalline silicon solar cells in order to increase light absorption and hence cell efficiency. In this article we investigate the indium-tin-oxide nanowhisker on textured silicon surface for enhancing the near-infrared absorbance of silicon photovoltaics. The nanowhiskers facilitate optical transmission in the near-infrared by functioning as impedance matching layers with effective refractive indices gradually varying from 1 to 1.3. Materials with such unique refractive index characteristics are not readily available in nature. As a result, the solar cell with combined textures achieves quantum efficiencies enhancement for a broad wavelength range of 900 to 1100 nm, which is crucial to the development of advanced thin-substrate silicon solar cells.

For thin film silicon solar cells and modules incorporating amorphous (a-Si:H) silicon as absorber materials, light trapping, i.e. increasing the path length of incoming light, plays a decisive role for device performance. Here we demonstrate the novel diffuser superstrate for thin film solar cell. For the optical measurement, the absorption reveals that the diffuser superstrate can effectively enhance the optical path for long wavelength. For the short wavelength, the randomly textured surface provides a superior antireflective effect compared with the flat glass devices. The novel diffuser superstrate provides effective antireflection and light trapping over a broad spectral range and a wide set of incidence angles, which leads to much higher power efficiency than that of the flat film devices. Further, the electric properties, such as the external quantum efficiency and the curve of current density vary with voltage, will be investigated and presented in the full paper.

Radial p-n junction structures are of interest in photovoltaics as they decouple light absorption from minority carrier collection. In this study, we design photovoltaic devices based on silicon nanohole arrays. The nanoholes are fabricated using polystyrene sphere lithography, which is a scalable and cost effective method to fabricate silicon nanostructures in large areas. A post-RIE passivation treatment was applied to prevent seriously surface recombination. Optical reflection characteristics of the optimum nanohole array are theoretically investigated to determine a maximum photocurrent output.