Gain In Short-circuit Current Density Research Articles

Gradient refractive index-based broadband antireflective coatings and application in silicon solar modules

Gradient refractive index-based broadband antireflective coatings (ARCs) could potentially improve solar power production by reducing the amount of sunlight reflected from the surface of photovoltaic (PV) modules. In this study, refractive index tunable hybrid SiO2 sols with refractive indices from 1.20 to 1.44 were prepared by adjusting different volume ratios and pH values and mixing acid-catalyzed SiO2 with base-catalyzed SiO2. Based on Essential Macleod simulation, five-layer graded refractive index broadband ARCs (FGBA) (n = 1.10–1.44) were prepared using hybrid sols derived from the sol-gel method as the building blocks. The morphological and structural characteristics of each coating layer were systematically investigated with different characterization methods. The average transmittance of the glass substrate coated with FGBA reached 97.57% (380–1800 nm), much higher than that of soda-lime glass (88.49%), and exhibited excellent broadband transmittance enhancement performance. The hardness performance of FGBA reached 5H, and the water contact angle of the surface at annealed 400 °C was 149°. The FGBA exhibited excellent durability performance in thermal cycling tests and abrasion tests. According to the external quantum efficiency test and I-V and P-V curve results, the short-circuit current density gain was 4.83% and the photoelectric conversion efficiency gain was 5.81% for the mini-silicon cell solar modules dipped with FGBA, compared to the undipped modules. This indicates positive prospects for application in outdoor PV plants. Meanwhile, it proves that ARCs are essential components of PV modules and could provide additional solar gains.

Graphene as non conventional transparent conductive electrode in silicon heterojunction solar cells

Chemical Vapor Deposited (CVD) graphene is an attractive candidate as transparent conductive electrode (TCE) for solar cells. Here it is proposed as TCE for silicon heterojunction solar cells (Si-HJSCs) and tested over devices with area up to 4 cm2 realized on n-type c-Si wafers with three different p-type emitters facing the dry-transferred graphene stack: amorphous silicon (a-Si:H), nanocrystalline silicon (nc-Si:H), and nanocrystalline silicon oxide (nc-SiOx:H). A dependence of the cell performance on the material in contact with graphene has been observed with the most promising results in case of nc-Si:H and nc-SiOx emitter. In particular, with respect to reference cells with standard TCE a short circuit current density gain of 1.4 mA/cm2 has been achieved when including an antireflection coating. The present work demonstrates the high quality of dry-transferred CVD-graphene over relatively large area and the feasibility of Si-HJSCs integrating graphene, where the fabrication sequence asks for a transfer process that guarantees a much better contact with the underlying functional layer with respect to more common designs where graphene is transferred onto a support substrate. This approach provides a possible pathway to Si-HJSCs free from conventional plasma-based TCEs and their deleterious effects due to plasma luminescence and ion bombardment, while offering an interesting platform for getting insights in the mechanisms at play at the interfaces with graphene within Si wafer-based devices.

Dual‐Function Light‐Trapping: Selective Plating Mask of SiOx/SiNx Stacks for Silicon Heterojunction Solar Cells

Reducing the production cost of highly efficient silicon heterojunction (SHJ) solar cells is essential for their large‐scale commercialization. Replacing screen printed silver contacts with plated copper is an effective way. However, one needs to deal with the process compatibility without sacrificing the cell performance. In this work, a layer stack of silicon oxide and silicon nitride (SiOx/SiNx) is proposed, serving dual functions of light‐trapping and selective copper plating mask. The SiOx/SiNx stack is prepared by plasma enhanced chemical vapor deposition (PECVD) on a tungsten doped indium oxide (IWO) layer. Optimized SiOx/SiNx/IWO stacks in SHJ solar cells with silver contacts result in excellent anti‐reflection properties and improved external quantum efficiency. A short circuit current density gain of 1.16 mA cm−2 is achieved without the loss in fill factor. This SiOx/SiNx stack is tested as in‐situ deposited selective plating mask for a front‐side copper plating process. An in‐house made light induced plating tool is utilized. A screen printed silver finger is applied as a seed layer for copper plating. The bulk resistivity of Cu/Ag finger reduces to 4.1 E‐6 Ω · cm, which is lower than that of low temperature silver paste (around 6 E‐6 Ω · cm) before copper is plated on it. The dual functions of SiOx/SiNx stack and the process are proven, indicating a good potential for further improving photoelectric properties of SHJ solar cells after well optimization.

Thin-film Cu(In,Ga)(Se,S)2 -based solar cell with (Cd,Zn)S buffer layer and Zn1−x Mg x O window layer

(Cd,Zn)S buffer layer and Zn1−xMgxO window layer were investigated to replace the traditional CdS buffer layer and ZnO window layer in Cu(In,Ga)(Se,S)2 (CIGSSe)-based solar cell. (Cd,Zn)S with band-gap energy (Eg) of approximately 2.6 eV was prepared by chemical bath deposition, and Zn1−xMgxO films with different [Mg]/([Mg] + [Zn]) ratios, x, were deposited by radio frequency magnetron co-sputtering of ZnO and MgO. The estimated optical Eg of Zn1−xMgxO films is linearly enhanced from 3.3 eV for pure ZnO (x = 0) to 4.1 eV for Zn0.6Mg0.4O (x = 0.4). The quality of the Zn1−xMgxO films, implied by Urbach energy, is severely deteriorated when x is above 0.211. Moreover, the temperature-dependent current density-voltage characteristics of the CIGSSe solar cells were conducted for the investigation of the heterointerface recombination mechanism. The external quantum efficiency of the CIGSSe solar cell with the (Cd,Zn)S buffer layer/Zn1−xMgxO window layer is improved in the wavelength range of 320–520 nm. Therefore, a gain in short-circuit current density up to about 5.7% was obtained, which is higher conversion efficiency of up to around 5.4% relative as compared with the solar cell with the traditional CdS buffer layer/ZnO window layer. The peak efficiency of 19.6% was demonstrated in CIGSSe solar cell with (Cd,Zn)S buffer layer and Zn1−xMgxO window layer, where x is optimized at 0.211. Copyright © 2017 John Wiley & Sons, Ltd.

Optical confinement in chalcopyrite based solar cells

Potential gains in short-circuit current density related to improvements in optical confinement in chalcopyrite based solar cells are studied and quantified by means of optical simulations. In the first part idealised optical conditions – anti-reflection at front interfaces, high reflection at back contact and light scattering - are introduced by simulating realistic scenarios of Cu(In, Ga)Se2 (CIGS) solar cells with 2000nm thick and 300nm ultra-thin CIGS absorber, including the encapsulation at the front. For anti-reflection effect at front interfaces simulations revealed that in the photovoltaic module structure the most critical reflectance is the reflectance of the front surface of the protecting glass (possible 4.4% gain in short-circuit current density) and not the one at the front transparent conductive oxide contact, as in the case of non-encapsulated solar cell. Introduction of a highly reflective, highly diffusive back reflector is the most crucial point to improve the short-circuit current density of the ultra-thin devices. Potential for 15.8% gain in short-circuit current density related to ideal reflectance and additional 17.4% related to ideal scattering introduced at the back contact was revealed. A concrete example of light management structure was investigated in the second part by employing fully three-dimensional rigorous optical simulations. A semi-ellipsoidal texture was introduced to the substrate of the ultra-thin device. By using ZrN back reflector in simulations the gains in short-circuit current density related to the optimised size of the texture reach 10%, whereas in the case of an ideal highly reflective contact the gain is >22% according to simulations.

Efficiency increase of crystalline silicon solar cells with nanoimprinted rear side gratings for enhanced light trapping

We demonstrate diffractive rear side gratings to enhance the near infrared light trapping and thus the quantum efficiency of wafer based crystalline silicon solar cells. Binary crossed gratings with a period of 1µm, produced via nanoimprint lithography and plasma etching, are electrically decoupled from the solar cell by a thin dielectric passivation layer, creating an electrically flat, but optically rough rear side. We fabricated solar cells with thicknesses of 250, 150 and 100µm and demonstrate a short circuit current density gain due to the grating of 1.2, 1.6 and 1.8mA/cm2 for solar cells with planar front surface. For solar cells with pyramidally textured front surface the grating also leads to a small current density gain in the near infrared of approximately 0.3mA/cm2 according to EQE measurements, leading to the best cell's efficiency of 21.1%. By optical simulations we show the potential of the grating structure and identify losses in the fabricated solar cells.

Luminescent borate glass for efficiency enhancement of CdTe solar cells

Rare-earth (RE) doped borate glasses are investigated for their potential as photon down-shifting cover glass for CdTe solar cells. The barium borate base glass is doped with trivalent rare-earth ions such as Sm3+, Eu3+, and Tb3+ showing an intense luminescence in the red (Sm3+, Eu3+) and green (Tb3+) spectral range upon excitation in the ultraviolet and blue. Additionally, the glasses are double-doped with two RE ions for a broad-band absorption. The gain in short-circuit current density of CdTe solar cells with different thicknesses of the CdS buffer layer is calculated. Though the single-doped glasses already reveal a slight increase in short-circuit current density, the double-doped glasses allow for higher efficiency gains since a significant broader spectral range is covered for absorption. For a Tb3+/Eu3+ double-doped glass with a RE doping level of 1at% each, an efficiency increase of 1.32% can be achieved.

Optical properties of down-shifting barium borate glass for CdTe solar cells

CdTe thin film solar cells have a poor response in the ultraviolet and blue spectral range, mainly due to absorption and thermalization losses in the CdS buffer layer. To overcome this efficiency drop in the short wavelength range trivalent rare-earth doped barium borate glass is investigated for its potential as frequency down-shifting cover glass on top of the cell. The glass is doped with either Tb3+ or Eu3+ up to a level of 2.5at.% leading to strong absorption in the ultraviolet/blue spectral range. Tb3+ shows intense emission bands in the green spectral range while Eu3+ emits in the orange/red spectral range. Based on rare-earth absorption and luminescence quantum efficiency the possible gain in short-circuit current density is calculated.

Silicon oxide buffer layer at the p–i interface in amorphous and microcrystalline silicon solar cells

The use of intrinsic silicon oxide as a buffer layer at the p–i interface of thin-film silicon solar cells is shown to provide significant advantages. For microcrystalline silicon solar cells, when associated with highly crystalline i-layers deposited at high rates, all electrical parameters are improved. Larger efficiency gains are achieved with substrates of increased roughness. For cells with an improved i-layer material quality, there is mainly a gain in short-circuit current density. An improvement in carrier collection in the blue region of the spectrum is systematically observed on all the cells. The presence of a silicon oxide buffer layer also promotes the nucleation of the subsequent intrinsic microcrystalline silicon layer. In amorphous silicon solar cells, the silicon oxide buffer layer is proven to act as an efficient barrier to boron cross-contamination, eliminating the need for additional processing steps (e.g. water vapor flush), while providing a wide bandgap material at the interface. The implementation of silicon oxide buffer layers for both types of cells thus provides a decisive improvement, as it allows extremely fast deposition of the full p–i–n stack of layers of the cell in a single-chamber configuration while providing a high-quality substrate-resilient p–i interface.

Comparison of charge distributions in CIGS thin-film solar cells with ZnS/(Zn,Mg)O and CdS/i-ZnO buffers

The commonly used CdS/i-ZnO buffer system in Cu(In,Ga)Se 2 (CIGS) thin-film solar cells was substituted by ZnS/(Zn,Mg)O. ZnS has a higher transmission in the short wavelength range due to the higher bandgap energy E g = 3.7 eV compared to CdS with E g = 2.4 eV. Unfortunately, in our experiments the resulting gain in short-circuit current density j SC as the result of reduced absorption losses in the blue wavelength region is mostly accompanied by a decrease in open-circuit voltage V OC of the devices with ZnS buffer. This contribution discusses possible explanations for the systematically lower open-circuit voltages of the devices with a ZnS buffer layer. The carrier collection properties of the devices with a ZnS buffer were investigated by electron beam induced current measurements in the junction configuration. The maximum of the collection probability for ZnS cells is located in the CIGS bulk and not near the buffer/CIGS interface like for solar cells with CdS buffer. Additionally, we observed a larger space charge width compared to devices with a CdS buffer. This finding concurs with the considerably lower capacitance values and also lower charge densities in ZnS-buffered devices, as determined by capacitance voltage measurements. Based on these findings, the main reason for the lower open-circuit voltages of our ZnS devices is that the charge densities are lower than for the CdS/i-ZnO cells.