Substrate-type Solar Cells Research Articles

Back-contact barrier analysis to develop flexible and bifacial Cu(In,Ga)Se2 solar cells using transparent conductive In2O3: SnO2 thin films

The presence of a back-contact (BC) barrier disturbs the current density-voltage (J-V) characteristics of solar cells by limiting the majority carrier transport, which is commonly referred as “rollover effect”. In this study, the majority barrier height as high as 131 meV at the Cu(In,Ga)Se2 (CIGS)/Mo-BC interface was calculated using the temperature dependent J-V characteristics. Subsequently, the lift-off process was applied to peel off the substrate-type solar cells from Mo-BC to investigate the origin of the rollover effect. The MoSe2 layer was detected on lift-off Mo samples using energy dispersive X-ray spectroscopy and the grazing-incidence X-ray diffraction measurements. Finally, optically transparent In2O3:SnO2 (ITO) thin films were deposited on the lift-off CIGS samples to fabricate flexible and bifacial solar cells. The solar cell with ITO-BC showed improved fill factor and efficiency without rollover effect, owing to highly Ohmic-contact at CIGS/ITO interface. In this contribution, it has been demonstrated that MoSe2 layer, i.e. with unreferred crystallographic orientation, can be removed by lift-off process to develop bifacial CIGS solar cells on flexible substrates. The bifacial cell with ITO-BC showed efficiencies of 10.1% and 2.8% for frontside and backside illuminations, respectively.

Superstrate-type flexible and bifacial Cu(In,Ga)Se2 thin-film solar cells with In2O3:SnO2 back contact

Bifacial and flexible Cu(In,Ga)Se2 (CIGS) thin-film solar cells with superstrate-type structure were fabricated. For this fabrication, the substrate-type solar cells were first transferred from Mo/Glass substrates to 50-µm-thick flexible fluorinated ethylene propylene (FEP) copolymer films as superstrates using a lift-off process. The 300-nm-thick In2O3:SnO2 (ITO) thin-films were subsequently deposited on the rear sides of the CIGS absorber layers as back contacts (BC) of the finished bifacial devices with the structure of FEP/Epoxy/Al/Ni/Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CdS/CIGS/ITO/Ni/Al. Particular attention was paid to maintain the high optical and electrical transport properties of ITO-BC as well as to improve the Ohmic-like contact behavior at the CIGS/ITO interface to improve cell performances. Under the optimized ITO preparation, the resulting ITO thin film possesses the average optical transparency of ITO (>77%) in the wavelength range of 400–1100 nm, and the resistivity of 6.1 × 10−4 Ωcm with corresponding high carrier mobility of 42.8 cm2/Vs, as well as Ohmic-like characteristics at the CIGS/ITO interface can be realized with shunt resistance as low as 5.5 Ωcm2. Consequently, the flexible and bifacial CIGS thin-film solar cell was fabricated with efficiencies of 9.1% and 2.4% for frontside and backside illuminations, respectively. In this study, highly transparent and conductive ITO thin films have been successfully used as BC in flexible and bifacial CIGS solar cells.

Improvement of In2S3/ZnCuInS2 interfaces for wide‐gap solar cells

AbstractWe have fabricated superstrate In2S3/ZnCuInS2 (Zn2xCu1‐xIn1‐xS2, ZCIS) solar cells as the top cell of multi‐junction solar cells. The ZCIS and In2S3 thin films were deposited by spray pyrolysis deposition (SPD) and an efficiency of 4.4% was achieved with bandgap of 1.8 eV. However, in the structure of superstrate type solar cells, interdiffusion have been caused between the buffer layer and the ZCIS layer. In order to suppress the interdiffusion, we have fabricated In2S3/ZCIS substrate type solar cells. The thickness of the interdiffusion region decreased from 70 nm of the superstrate cell to 15 nm of the substrate cell. As a result, the substrate cell with bandgap of 1.8 eV showed VOC of 0.71 V and FF of 0.55 higher than those of the superstrate cells. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High efficiency multi-junction thin film silicon cells incorporating nanocrystalline silicon

Significant advances have been made in the recent years to improve the efficiency of single- and multi-junction solar cells incorporating nanocrystalline silicon (nc-Si:H). The improvements have resulted from the development of high quality intrinsic material, novel doped layers, superior back reflector and appropriate device design. Stable active-area efficiency of 13.6% for small-area cells and aperture-area efficiency of 11.85% for large-area devices have been achieved. In this paper, we discuss the key activities that led to these high efficiencies with emphasis on substrate-type solar cells. Manufacturing issues and future research directions to improve efficiency further are also discussed.

Optical properties of silicon-based thin-film solar cells in substrate and superstrate configuration

Abstract The optical properties of microcrystalline silicon substrate-type solar cells with a front ZnO thickness of 500–1000 nm as required for monolithic series connection in PV-modules are investigated. The surface texture of the front ZnO was varied to study the possibility to reduce reflection losses and to improve light trapping. Before encapsulation, certain textures of the front ZnO exhibit improved light coupling for substrate solar cells. For substrate solar cells encapsulated with EVA and a glass cover, however, additional texture of the front ZnO has hardly any effect. Encapsulation also removes the antireflection conditions in case that the front ZnO was originally designed as ARC. So far, the quantum efficiency of encapsulated substrate solar cells does not show the high level as observed in superstrate solar cells possibly due to parasitic absorption in the front and back contact and the not optimized texture of the reflector (substrate).