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To further increase the efficiency of thin film solar cells, it is critical to enhance the absorption over the full spectral wavelength range in which solar cells generate electricity. In this letter, we present a fullband absorption enhancement method for n-i-p thin film silicon solar cells based on a Quasi-Crystal Structure (QCS) by superimposing Ag random nanotextures on periodically patterned Micro-Cone Substrates. Both light in-coupling and light trapping abilities are significantly improved thanks to the reduction of front-surface reflection originated from the gradually changed refractive index of preserved micro-cone profile after film deposition and the richer guided mode resonances caused by the QCS. An initial efficiency of 10.4% is obtained for QCS-based hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells, which outperforms the planar (efficiency of 7.5%) and randomly nanotextured (efficiency of 8.7%) counter part by 38.7% and 19.5%, respectively. The QCS can also be duplicated for other thin film photovoltaic devices and provides a new approach for creating high-efficiency thin-film solar cells.

To further increase the efficiency of thin film solar cells, it is essential to enhance the absorption over the full spectral wavelength range in which solar cells generate electricity. Here, we present a broadband absorption enhancement method for n-i-p thin film silicon solar cells using Micro-Cone Patterned Substrates (MCPSs). The periodically patterned micro-cone textured profiles are well preserved after the deposition of each thin film layers, thus to fulfill with two critical criteria in solar energy harvesting by enhanced light in-coupling and light trapping. The influence of the aspect ratio (height/period) of the MCPSs on the optical and electrical performance of the hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells is discussed via experiment. We demonstrate that MCPSs based solar cells allow us to achieve an electrical performance (open-circuit voltage and fill factor) comparable to what we obtain on flat and randomly nanotextured reference samples. Thanks to the full-spectral light harvesting enhancement, initial efficiency of 10.1% is obtained for the solar cell based on MCPS with aspect ratio of 0.5, which outperforms the planar (efficiency of 7.5%) and randomly nanotextured (efficiency of 8.7%) counter part by 34.7% and 16.1%, respectively. The micro-cone light harvesting structure can also be duplicated for other thin film photovoltaic devices and provides a new approach for creating high-efficiency thin-film solar cells.

One of the foremost challenges in achieving high-efficiency thin-film silicon solar cells is in devising an efficient light trapping system because of the short optical path length imposed by the inherent thin absorption layers. In this paper, an efficient light trapping system is proposed using a combination of randomly textured surfaces and a one-dimensional photonic crystal (randomly textured photonic crystal; RTPC). The influence of the texture on the optical performance of RTPCs is discussed using the results of an experiment and a finite-difference time-domain simulation. This RTPC back reflector (BR) can provide high reflectivity and strong light scattering, resulting in an increased photocurrent density of the hydrogenated amorphous silicon (a-Si:H) solar cell. As a result, the highly textured RTPC BR yielded an efficiency of 9.6% for a-Si:H solar cell, which is much higher than the efficiency of 7.6% on flat AZO/Ag BR and 9.0% on textured AZO/Ag BR. This RTPC BR provides a new approach for creating high-efficiency, low-cost thin-film silicon solar cells.

We demonstrate the application of metal nanowire (NW) networks as a transparent electrode on hydrogenated amorphous Si (a-Si:H) solar cells. We first systematically investigate the optical performances of the metal NW networks on a-Si:H solar cells in different electrode configurations through numerical simulations to fully understand the mechanisms to guide the experiments. The theoretically optimized configuration is discovered to be metal NWs sandwiched between a 40 nm indium tin oxide (ITO) layer and a 20 nm ITO layer. The overall performances of the solar cells integrated with the metal NW networks are experimentally studied. It has been found the experimentally best performing NW integrated solar cell deviates from the theoretically predicated design due to the performance degradation induced by the fabrication complicity. A 6.7% efficiency enhancement was achieved for the solar cell with metal NW network integrated on top of a 60 nm thick ITO layer compared to the cell with only the ITO layer due to enhanced electrical conductivity by the metal NW network.

One of the foremost challenges in designing thin-film silicon solar cells (TFSC) is devising efficient light-trapping schemes due to the short optical path length imposed by the thin absorber thickness. The strategy relies on a combination of a high-performance back reflector and an optimized texture surface, which are commonly used to reflect and scatter light effectively within the absorption layer, respectively. In this paper, highly promising light-trapping structures based on a photonic crystal (PC) for TFSCs were investigated via simulation and experiment. Firstly, a highly-reflective one-dimensional photonic crystal (1D-PC) was designed and fabricated. Then, two types of 1D-PC-based back reflectors (BRs) were proposed: Flat 1D-PC with random-textured aluminum-doped zinc oxide (AZO) or random-textured 1D-PC with AZO. These two newly-designed BRs demonstrated not only high reflectivity and sufficient conductivity, but also a strong light scattering property, which made them efficient candidates as the electrical contact and back reflector since the intrinsic losses due to the surface plasmon modes of the rough metal BRs can be avoided. Secondly, conical two-dimensional photonic crystal (2D-PC)-based BRs were investigated and optimized for amorphous a-SiGe:H solar cells. The maximal absorption value can be obtained with an aspect ratio of 1/2 and a period of 0.75 µm. To improve the full-spectral optical properties of solar cells, a periodically-modulated PC back reflector was proposed and experimentally demonstrated in the a-SiGe:H solar cell. This periodically-modulated PC back reflector, also called the quasi-crystal structure (QCS), consists of a large periodic conical PC and a randomly-textured Ag layer with a feature size of 500–1000 nm. The large periodic conical PC enables conformal growth of the layer, while the small feature size of Ag can further enhance the light scattering. In summary, a comprehensive study of the design, simulation and fabrication of 1D-PC- and 2D-PC-based back reflectors for TFSCs was carried out. Total absorption and device performance enhancement were achieved with the novel PC light-trapping systems because of their high reflectivity or high scattering property. Further research is necessary to illuminate the optimal structure design of PC-based back reflectors and high solar cell efficiency.

Ga0.51In0.49P/In0.01Ga0.99As/Ge triple-junction solar cells for space applications were grown on 4 inch Ge substrates by metal organic chemical vapor deposition methods. The triple-junction solar cells were obtained by optimizing the subcell structure, showing a high open-circuit voltage of 2.77 V and a high conversion efficiency of 31% with 30.15 cm2 area under the AM0 spectrum at 25 °C. In addition, the In0.01Ga0.99As middle subcell structure was focused by optimizing in order to improve the anti radiation ability of triple-junction solar cells, and the remaining factor of conversion efficiency for middle subcell structure was enhanced from 84% to 92%. Finally, the remaining factor of external quantum efficiency for triple-junction solar cells was increased from 80% to 85.5%.

Controllable light utilization in silicon-based-thin-film solar cells with a DBR structure have been simulated. A experimental model cells constructed by a-Si pin/ZnO (~70 nm)/P+muc-Si(20 nm) were verified. The ratio of Isc increment can be reached to 14.1%.

The key factor for high efficiency of tandem Si-based thin film solar cells is to optimize the whole configuration for improving its utilization of solar energy besides the active layers of device-quality level. Several aspects reached in our lab are discussed in this paper. A double structure of P + window layer was proposed and studied. An interlayer between top-cell and bottom-cell in the tandem structure for forming a micro-cavity in top-cell was simulated by the software programmed by ourselves. Finally, the results of two types of tandem solar cells, namely, a-Si/a-Si and a-Si/μc-Si with the light trapping back-reflection structure are reported.

An efficient light trapping scheme named as textured conductive photonic crystal (TCPC) has been proposed and then applied as a back-reflector (BR) in n-i-p hydrogenated amorphous silicon (a-Si:H) solar cell. This TCPC BR combined a flat one-dimensional photonic crystal and a randomly textured surface of chemically etched ZnO:Al. Total efficiency enhancement was obtained thanks to the sufficient conductivity, high reflectivity and strong light scattering of the TCPC BR. Unwanted intrinsic losses of surface plasmon modes are avoided. An initial efficiency of 9.66% for a-Si:H solar cell was obtained with short-circuit current density of 14.74 mA/cm2, fill factor of 70.3%, and open-circuit voltage of 0.932 V.

For thin-film silicon solar cells (TFSC), a one-dimensional photonic crystal (1D PC) is a good back reflector (BR) because it increases the total internal reflection at the back surface. We used the plane-wave expansion method and the finite difference time domain (FDTD) algorithm to simulate and analyze the photonic bandgap (PBG), the reflection and the absorption properties of a 1D PC and to further explore the optimal 1D PC design for use in hydrogenated amorphous silicon (a-Si:H) solar cells. With identified refractive index contrast and period thickness, we found that the PBG and the reflection of a 1D PC are strongly influenced by the contrast in bilayer thickness. Additionally, light coupled to the top three periods of the 1D PC and was absorbed if one of the bilayers was absorptive. By decreasing the thickness contrast of the absorptive layer relative to the non-absorptive layer, an average reflectivity of 96.7% was achieved for a 1D PC alternatively stacked with a-Si:H and SiO2 in five periods. T...