Improving the efficiency of rear emitter silicon solar cell using an optimized n-type silicon oxide front surface field layer.

Optical and electrical characteristics of n-type nano-crystalline-silicon oxide (n-µc-SiO:H) materials can be varied to optimize and improve the performance of a solar cell. In silicon heretojunction (SHJ) solar cells, it can be used to improve carrier selectivity and optical transmission at the front side, both of which are vitally important in device operation. For this purpose, the n-µc-SiO:H was investigated as the front surface field (FSF) layer. During film deposition, an increased CO2 flow rate from 0 to 6 sccm resulted in changes of crystalline volume fractions from 57 to 28%, optical band-gaps from 1.98 to 2.21 eV, dark conductivities from 7.29 to 1.1 × 10-5 S/cm, and activation energies from 0.019 to 0.29 eV, respectively. In device applications, a minimum optical reflection was estimated for the FSF layer that was fabricated with 4 sccm CO2 (FSF-4), and therefore obtained the highest external quantum efficiency, although short circuit current density (Jsc) was 38.83 mA/cm2 and power conversion efficiency (PCE) was 21.64%. However, the highest PCE of 22.34% with Jsc = 38.71 mA/cm2 was observed with the FSF prepared with 2 sccm CO2 (FSF-2), as the combined opto-electronic properties of FSF-2 were better than those of the FSF-4.

Effective Light Trapping in Thin Film Silicon Solar Cells with Nano- and Microscale Structures on Glass Substrate.

For thin film silicon-based solar cells, effective light trapping at a broad range of wavelengths (400-1100 nm) is necessary. Normally, etching is only carried out with TCOs, such as SnO2:F and impurity doped ZnO, to form nano-sized craters in the surface morphology to confer a light trapping effect. However, in this study, prior to ZnO:Al etching, periodic structures on the glass substrates were made by photolithography and wet etching to increase the light scattering and internal reflection. The use of periodic structures on the glass substrate resulted in higher haze ratios in the range from 550 nm to 1100 nm, which is the optical absorption wavelength region for thin film silicon solar cells, than obtained by simple ZnO:Al etching. The periodically textured glass with micro-sized structures compensates for the low haze ratio at the middle and long wavelengths of wet etched ZnO:Al. ZnO:Al was deposited on the periodically textured glass, after which the ZnO:Al surface was also etched randomly using a mixed acid solution to form nano-sized craters. The thin film silicon solar cells with 350-nm-thick amorphous silicon absorber layer deposited on the periodic structured glass and etched ZnO:Al generated up to 10.68% more photocurrent, with 11.2% increase of the conversion efficiency compared to the cell deposited on flat glass and etched ZnO:Al.

Boron Doped Nanocrystalline Film with Improved Work Function as a Buffer Layer in Thin Film Silicon Solar Cells.

We investigated thin film silicon solar cells with boron doped hydrogenated nanocrystalline silicon/ hydrogenated amorphous silicon oxide [p-type nc-Si:H/a-SiOx:H] layer. First, we researched the bandgap engineering of diborane (B2H6) doped wide bandgap hydrogenated nanocryslline silicon (p-type nc-Si:H) films, which have excellent electrical properties of high dark conductivity, and low activation energy. The films prepared with lower doping ratio and higher hydrogen dilution ratio had higher optical gap (Eg), with higher dark conductivity (σ(d)), and lower activation energy (Ea). We controlled Eg from 2.10 eV to 1.75 eV, with σ(d) from 1.1 S/cm to 7.59 x 10(-3) S/cm, and Ea from 0.040 eV to 0.128 eV. Next, we focused on the fabrication of thin film silicon solar cells. By inserting p-type nc-Si:H film into the thin film silicon solar cells, we achieved a remarkable increase in the built-in potential from 0.803 eV to 0.901 eV. By forming p-type nc-Si:H film between SnO2:F/ZnO:Al (30 nm) and p-type a-SiOx:H layer, the solar cell properties of open circuit voltage (Voc), short circuit current density (Jsc), and efficiency (η) were improved by 3.7%, 9.2%, and 9.8%, respectively.

Crystalline Fraction and Doping Concentration Effect on Heterojunction Solar Cells n-Doped µc-Si:H Back Surface Field Layer.

The back surface field (BSF) plays a vital role for high efficiency in the Heterojunction Intrinsic Thin (HIT) film solar cell. This paper investigated the effect of crystalline volume fraction (Xc) and 1% hydrogen diluted phosphine (PH3) gas doping concentration of the n-type µc-Si:H back surface file (BSF) layer. Initially, the thickness of the n-type µc-Si:H BSF layer was optimized. With increase in Xc from 6% to 59%, the open circuit voltage (Voc) increased from 573 mV to 696 mV, and the fill factor (FF) also increased from 59% to 71%. In the long wavelengths region (≥ 950 nm), the QE of the solar cells decreased over the optimized Xc of the n-doped micro BSF layer, due to the defects of a film. In the second part of this paper, the effect of high conductivity n-type µc-Si:H BSF layer with optimized thickness on the performance of HIT solar cells was investigated, by doping gas ratio variation. Even though Xc decreased, conductivity was increased, with increasing PH3 doping concentration. Under the optimized condition, a n-µc-Si:H BSF layer has a dark conductivity of 2.59 S/cm, activation energy of 0.0519 eV, and X, of 52%. The conversion efficiency of 18.9% was achieved with a Voc of 706 mV, fill factor of 72%, and short circuit current density of 37.1 mW·cm(-2).

Correlation Between the Raman Crystallinity of p-Type Micro-Crystalline Silicon Layer and Open Circuit Voltage of n-i-p Solar Cells.

This article mainly discusses the difference between p-i-n and n-i-p type solar cells. Their structural difference has an effect on cell performance, such as open circuit voltage and fill factor. Although the deposition conditions are the same for both p-i-n and n-i-p cases, the substrate layers for depositing p-type microcrystalline silicon layers differ. In n-i-p cells, the substrate layer is p-type amorphous silicon oxide layer; whereas, in p-i-n cells, the substrate layer is ZnO:Al. The interfacial change leads to a 12% difference in the crystallinity of the p-type microcrystalline silicon layers. When the p-type microcrystalline silicon layer's crystallinity was not sufficient to activate an internal electric field, the open circuit voltage and fill factor decreased 0.075 V and 7.36%, respectively. We analyzed this problem by comparing the Raman spectra, electrical conductivity, activation energy and solar cell performance. By adjusting the thickness of the p-type microcrystalline silicon layer, we increased the open circuit voltage of the n-i-p cell from 0.835 to 0.91 V.

Triple-junction hybrid tandem solar cells with amorphous silicon and polymer-fullerene blends.

Organic-inorganic hybrid tandem solar cells attract a considerable amount of attention due to their potential for realizing high efficiency photovoltaic devices at a low cost. Here, highly efficient triple-junction (TJ) hybrid tandem solar cells consisting of a double-junction (DJ) amorphous silicon (a-Si) cell and an organic photovoltaic (OPV) rear cell were developed. In order to design the TJ device in a logical manner, a simulation was carried out based on optical absorption and internal quantum efficiency. In the TJ architecture, the high-energy photons were utilized in a more efficient way than in the previously reported a-Si/OPV DJ devices, leading to a significant improvement in the overall efficiency by means of a voltage gain. The interface engineering such as tin-doped In2O3 deposition as an interlayer and its UV-ozone treatment resulted in the further improvement in the performance of the TJ solar cells. As a result, a power conversion efficiency of 7.81% was achieved with an open-circuit voltage of 2.35 V. The wavelength-resolved absorption profile provides deeper insight into the detailed optical response of the TJ hybrid solar cells.

Investigation of structural disorder using electron temperature in VHF-PECVD on hydrogenated amorphous silicon films for thin film solar cell applications.

Electrode distances and gas flow ratios are important parameters for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma-enhanced chemical-vapor deposition (VHF-PECVD) system. In this work, we investigated the relationship between the electrode distances and gas flow ratios on the properties of i-type a-Si:H films. The electrical, chemical and structural properties are improved with decreasing electrode distances (20-40 mm) at a hydrogen ratio [R (H2/SiH4) = 4], due to the low electron temperature and heating effect. A low electron temperature generates silane-related-reactive species (SiH3) and decreases structural disorder resulting in high quality i-type a-Si:H films. The electrical, chemical and structural properties of the a-Si:H films are confirmed using Al coplanar electrodes, FTIR, Raman spectroscopy, and spectroscopy ellipsometry (SE). When a solar cell is fabricated using the a-Si:H film, J(sc) of 13.2-14.8 mA/cm2, photoconductivity of 1.5 x 10(-5)-8.6 x 10(-6) S/cm, Si--H2 content of 0-1.24 at.%, and hydrogen content of about 10 at.% are obtained. These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process.

Influence of n-doped µc-Si:H back surface field layer with micro growth in crystalline-amorphous silicon heterojunction solar cells.

The back surface field (BSF) plays an important role for the efficiency of the heterojunction intrinsic thin-film (HIT) solar cell. In this paper, the effect of thickness variation in n-type micro crystalline BSF layer was investigated by Raman and spectroscopy ellipsometry. As we increase the crystalline volume fraction (X(c)) from 6% to 59%, the open circuit voltage (V(oc)) increases from 573 to 696 mV with increase in fill factor from 59% to 71%. However, we observed that V(oc) and FF are decreased over 59% X(c) of n-type µc-Si:H BSF layer. It seems that higher X(c) micro layer include lots of defects. The quantum efficiency (QE) measurements were demonstrated on optimized thickness of n-doped micro BSF layer. In the long wavelengths region, the QE slightly increases with increasing the n-type µc-Si:H BSF layer thickness from 10 to 40 nm because of BSF effect, whereas the QE decreases when n-type µc-Si:H BSF layer thickness increases from 40 to 120 nm due to defects in the layer. The performance of heterojunction solar cell device was improved with the optimized thickness on n-doped micro BSF layer the best photo voltage parameters of the device were found to be V(oc) of 696 mV, short-circuit current density of 36.09 mA/cm2 and efficiency of 18.06% at n-doped micro BSF layer thickness of 40 nm.

A novel method to make boron-doped microcrystalline silicon thin films with optimal crystalline volume fraction for thin films solar cell applications.

Highly conducting boron-doped microcrystalline silicon (p-type µc-Si:H) thin films have been prepared by radio frequency plasma-enhanced chemical-vapor deposition (RF-PECVD). In this work, the effects of hydrogen dilution, doping ratio, plasma power, deposition pressure and substrate temperature on the growth and the properties of boron-doped microcrystalline silicon (p-type µc-Si:H) thin films are investigated. The electrical, chemical and structural properties are improved with increasing crystallite, which depends on the plasma conditions. For various plasma parameters, the crystalline volume fraction (X(c)), dark conductivity (σ(d)), activation energy (E(a)), hydrogen content (C(H)), surface roughness (S(r)), and micro void fraction (R*) were measured, and they were 0-72%, 4.17-10(-4) S/cm-1.1 S/cm, 0.041-0.113 eV, 3.8-11.5 at.%, 3.2 nm-12.2 nm, and 0.47-0.80, respectively. The film with R* of 0.47 and C(H) of about 5 at.% belonged to a region of low disorder, and acted as a good passivation layer.

Effects of target angle on the properties of aluminum doped zinc oxide films prepared by DC magnetron sputtering for thin film solar cell applications.

An aluminum doped zinc oxide (AZO) films for front contacts of thin film solar cells, in this work, were prepared by DC magnetron sputtering with different target angles. Effects of target angles on the structural and electro-optical properties of AZO films were investigated. Also, to clarify the light trapping of textured AZO film, amorphous silicon thin film solar cells were fabricated on the textured AZO/glass substrate and the performance of solar cells were studied. The surface became more irregular with increasing the target angle due to larger grains. The self-surface textured morphology, which is a favorable property as front layer of solar cell, exhibited at target angle of 72.5 degrees. We obtained the films with various opto-electronic properties by controlling target angle from 32.5 degrees to 72.5 degrees. The spectral haze increased substantially with the target angle, whereas the electrical resistivity was increased. The conversion efficiency of amorphous silicon solar cells with textured AZO film as a front electrode was improved by the increase of short-circuit current density and fill factor, compared to cell with relatively flat AZO films.

Inserted layer of AZO thin film with high work function between transparent conductive oxide and p-layer and its solar cell application.

We report aluminum doped zinc oxide (AZO) films with high work function as an insertion layer between transparent conducting oxides (TCO) and hydrogenated amorphous silicon carbide (a-SiC:H) layer to improve open circuit voltage (V(oc)) and fill factor (FF) for thin film solar cells. Amorphous silicon (a-Si:H) solar cells exhibit poor fill factors due to a Schottky barrier at the interface between a-SiC:H window and TCO. The interface engineering is carried out by inserting an AZO layer with high work function (4.95 eV at O2 = 2 sccm). As a result, V(oc) and FF improved significantly. FF as high as 63.35% is obtained.

Reduction of tail state on boron doped hydrogenated amorphous silicon oxide films prepared at high hydrogen dilution.

In this report, we have investigated on the defect state of diborane (B2H6) doped wide bandgap hydrogenated amorphous silicon oxide (p-type a-SiO:H) films prepared using silane (SiH4), hydrogen (H2) and nitrous oxide (N2O) in a radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) system with different hydrogen dilutions. The films prepared with higher hydrogen dilution show lower Urbach energy (Eu), lower microstructure (R*), lower short and medium range disorder (omegaTO, Gamma(TO), I(TA)/I(TO), I(LA)/I(TO)), higher dark conductivity (sigma d) and higher refractive index (n) with high optical gap (Eg). Eu decreases from 248 meV to 153 meV, and R* decreases from 0.46 to 0.26, Raman peak omegaTO-TO mode position shifts from 480.24 to 483.28, GammaTO-full width half maximum of omegaTO decreases from 78.16 to 63.87, I(TA)/I(TO)-the ratio of integrated area of TA and TO mode decreases from 0.624 to 0.474, I(LA)/I(TO)-the ratio of integrated area of LA and TO mode deceases from 0.272 to 0.151, sigma d increases from 4.6 x 10(-7) S/cm to 1.1 x 10(-6) S/cm, n increases from 3.70 to 3.86. Reduced Nd, Eu and R* at wide Eg indicates that the films are more useful for solar cell window layer. Applying this layer to a single junction solar cell shows open circuit voltage (Voc) = 0.80 V, short circuit current density (Jsc) = 16.3 mA/cm2, fill factor (FF) = 72%, efficiency (eta) = 9.4%.

Two-step texture process for high-efficiency crystalline silicon solar cell applications.

Anisotropic etching of single-crystalline solar cells is used to increase the light absorption and surface area, which can improve the conversion efficiency. However, the conventional anisotropic etching process is limited for increasing surface area. For high-efficiency solar cells, unique surface structures are necessary. We present a new two-step texture process that involves combining dry etching and wet etching to produce a high-aspect-ratio surface structure for high-efficiency solar cells. Using this process, we achieved pillar-type surface structure with 1:1.9 aspect ratios in reactive ion etching (RIE), and the aspect ratio was increased further to 1:2.6 by the anisotropic wet etching process. The reflectance of the c-Si wafer was reduced from 24% to 12% by this two-step texturing process. This new technique can be used to increase the aspect ratio and surface area for high-efficiency c-Si solar cells.

Effect of carrier injection stress in thin film solar cells by impedance spectroscopy.

In amorphous silicon solar cells, degradation is directly related to V(oc), FF and cell performance. The dependence of the stability of thin film amorphous silicon solar cells is studied in terms of the volume fraction of B2H6 in the p-layer. When the volume fraction of B2H6 is increased by an order of magnitude, the doping-induced defects tend to increase quite rapidly. Low-doped p-type a-SiO(x) layers had better initial properties but rapidly degraded. Heavily doped p-type a-SiO(x) layers had lower initial properties but displayed better stability. The improvement in stability is explained in conjunction with the capacitance and resistance values of impedance spectroscopy. When the B2H6 gas flow rate is increased, the cell is degraded showing a capacitance decay decrease from 51.75% to less than 18.18%. In addition, the increase in the resistance decreased from 90.90% to 11.73%.