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The energy conversion efficiency (η) of silicon heterojunction (SHJ) solar cells is limited by the current losses in the layer stack on the illuminated side. To reduce these losses, hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) was implemented as a window layer in SHJ solar cells. However, the integration of nc-SiOx:H in devices without degradation of fill factor (FF) is still a challenge. To optimize the electron performance of devices, the optoelectronic properties and microstructure of nc-SiOx:H were characterized and analyzed systematically. It was found that the PH3 gas fraction (fPH3) plays a big role on the microstructure, oxygen content, and phosphorus (P) doping efficiency of the films. The highest conductivity, 2.84 × 10–1 S/cm, is obtained at a moderate fPH3 with an optical band gap of 2.26 eV. A ternary model was creatively used to show the variation in the composition of nc-SiOx:H as tuning fPH3. The growth of crystalline phase was accelerated by the P dopants when fPH3 is low, but further increasing fPH3 leads to excessive P inactive dopants, causing a phase transition from nanocrystalline silicon to amorphous silicon in nc-SiOx:H. In this work, the best solar cell with an nc-SiOx:H window layer achieves an FF of 81.4%, a short current density (Jsc) of 39.8 mA/cm2, an open-circuit voltage (Voc) of 731 mV, and an η of 23.7% at the moderate fPH3. A decrease in FF and Jsc is shown with higher fPH3, which is the consequence of the increased front contact resistivity and decreased optical band gap of nc-SiOx:H window layer.

Boosting the contact property of intrinsic hydrogenated amorphous silicon (a-Si:H(i)) film is pivotal to achieving high-efficiency silicon heterojunction (SHJ) solar cells. Here, the microstructure of a-Si:H(i) film is modified with hydrogen dilution ratio using hot wire chemical vapor deposition (HWCVD) for the application into rear-emitter SHJ solar cells. A higher hydrogen content associated with high valence band offset was found to decrease the fill factor FF for low dilution, while high interface defect densities related to epitaxial growth are responsible for the deterioration of both FF and open-circuit voltage VOC for high dilution. In particular, the most compact film prepared at a moderate dilution exhibits the most compact structure with most hydrogen located as isolated hydrogen rather than clustered hydrogen. Finally, high efficiency of SHJ solar cells up to 22.5% was yielded using the optimized a-Si:H(i) layer thanks to a significant enhancement of FF, which is attributed to improved passivation quality and rational band alignment at the a-Si:H(i)/c-Si interface. This work clearly interpreted the correlation between SHJ device parameters and a-Si:H(i)/c-Si interface properties, which might guide the design of a-Si:H passivation layers in pursuit of high-efficiency SHJ solar cells.

Summary Silicon heterojunction (SHJ) solar cells employ nanometer-thin stacks of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) films as carrier-selective contacts. To achieve excellent carrier selectivity, the a-Si:H must be carefully optimized to guarantee an atomically sharp a-Si:H/c-Si interface. In this work, by combining experiments with molecular dynamics and ab initio calculations, we unveil that H atoms bonded to internal-void surfaces in a-Si:H broaden its optical band gap via a filamentary effect near the valence-band maximum. The photovoltaic performance of rear-emitter SHJ solar cells can be significantly improved by tailoring the Si−H bonding state in the front a-Si:H passivation layer, resulting in a power conversion efficiency (PCE) of 23.4% on a 6-in cell. By implementing double antireflection coatings (ARCs) of SiNx and SiOx, the PCE is further improved to 23.9%. More importantly, the ARC devices show prominently improved damp-heat stability without encapsulation in 1,000-h aging at 85°C, 85% relative humidity.

Intrinsic/doped stacked hydrogenated amorphous silicon (a-Si:H) are widely used passivation layers for amorphous/crystalline silicon (a-Si/c-Si) heterojunction solar cells. This work reports that hot wire chemical vapor deposition of doped a-Si:H can significantly modify the property of the underlying intrinsic a-Si:H (a-Si:H(i)) as well as a-Si/c-Si interface passivation, which stems from the in-diffusion of highly reactive atomic hydrogen. Fourier transform infrared spectroscopy, spectroscopic ellipsometry and Raman analyses indicate that the underlying a-Si:H(i) films become more compact and less defected as a result of network reconstruction during doped a-Si:H capping. After this reconstruction, underdense a-Si:H(i) films obtained superior passivation quality than widely used dense layers, despite the inferior quality in the initial state. Effective minority carrier lifetime of c-Si passivated by underdense a-Si:H(i) was 19.9 ms, much higher than 15.2 ms in the case of using dense a-Si:H(i). The porous structure of underdense a-Si:H(i) facilitates hydrogen diffusion towards a-Si/c-Si interface and hence a rapid reduction of interface defect densities occurs, accounting for the better passivation quality. SHJ solar cells (160 μm, 156 × 156 mm2) with industry-compatible process were fabricated, yielding the efficiency up to 23.0% with high Voc values of 741 mV.

Silicon heterojunction (SHJ) solar cell that combines traditional pure H 2 plasma treatments has been frequently reported in the literature. However, this method requires an individual gas blending step between a-Si:H film deposition and post H 2 plasma treatment to stabilize the gas environment in the PECVD chamber. Here, we report the introduction of residual SiH 4 molecules in H 2 plasma to treat SHJ solar cell devices. In contrast to the traditional H 2 plasma treatments, it requires no time interval between the a-Si:H film deposition and H 2 plasma treatment, i.e., we merely closed the SiH 4 inlet after the a-Si:H deposition. In the meantime, all other PECVD parameters were kept unchanged. Taking advantage of the decreasing SiH 4 density during the H 2 plasma process, a dense silicon layer was grown onto the top layer of the as-deposited a-Si:H film, which inhibited free H atoms effusing out of the low-mass-density a-Si:H network. The better a-Si:H/c-Si interface passivation results in improvements to both the short-circuit current density ( J sc ) and open-circuit voltage ( V oc ) of the SHJ solar cell in comparison to the counterpart cell treated by the traditional pure H 2 plasma. For instance, when the n + a-Si:H window layer is as thin as ~ 1.8 nm, the power-conversion efficiency skyrockets from 2.35% treated by the traditional pure H 2 plasma to 16.09% treated by the H 2 plasma containing residual SiH 4 molecules. For a thicker n + a-Si:H window layer of ~ 4.3 nm, the efficiency is also enhanced from 20.66% to 22.74%. This finding paves the way for a more efficient H 2 plasma treatment in pursuit of an outstanding SHJ solar cell.

Titanium and tungsten co-doped indium oxide (IWTO) films are deposited via low-damage reactive plasma deposition technique at room temperature (RT) and heating temperature (HT). The effects of oxygen flow rates (FO2) on the structural, optical and electrical properties of the IWTO films are investigated after air annealing. X-ray diffraction and scanning electron microscopy results show that the IWTO films are polycrystalline structure exhibiting a preferred (222) crystal plane orientation. As FO2 increases from 10 sccm to 80 sccm, carrier concentration decreases by an order of magnitude for both RT and HT condition, while carrier mobility first increases and then decreases. It is noteworthy that films fabricated at HT show highest mobility (92.1 cm2 V−1 s−1) and lowest resistivity (2.13 × 10−4 Ω cm) due to better crystallinity and hence decreased grain boundary scattering. In addition, the average transmittance of the IWTO films exceeds 89.5% in the wavelength range of 400–1200 nm. Finally, silicon heterojunction solar cells with a high efficiency of 23.8% (with open-circuit voltage (Voc) of 0.746 V, short-circuit current density (Jsc) of 38.7 mA cm−2 and fill factor (FF) of 82.9%) is achieved with the application of the high-performance IWTO film.

Decreasing the contact resistance between hydrogenated amorphous silicon (a-Si:H) and transparent conductive oxide film (TCO) is beneficial for achieving high efficiency silicon heterojunction (SHJ) solar cells. This study reports the implementation of trimethyl boron B(CH3)3 (TMB) doped p-type a-Si:H (a-Si:H(p)) film as hole transport layer contacting with indium-free aluminum doped zinc oxide (AZO) in SHJ solar cells. The influence of doping concentration on the nanostructure of a-Si:H(p), TCO/a-Si:H(p) contact resistivity as well as the resultant cell performance was systematically investigated. It was found that excessive TMB doping results in more carbon and voids inside the films and reduces the doping efficiency, lowering the conductivity and increasing the contact resistivity. a-Si:H(p) film with low defect density and high doping level was obtained at a moderate doping concentration, which facilitates tunneling transport for holes to overcome the high energy barrier at the a-Si:H(p)/AZO interface and results in a low contact resistivity down to 0.14 Ωcm2. The optimized low-resistivity a-Si:H(p)/AZO contact enables a fill factor above 81% and efficiency of 23.6% for M2 SHJ solar cells, which is comparable with 23.7%-efficient cells using traditional tin doped indium oxide (ITO). To our knowledge, this is the highest efficiency for AZO-implemented SHJ cells without double anti-reflection layer and silver back reflector. This work provides design principles on how to achieve high-efficiency SHJ cells with low resistive loss at the hole contact side via doping engineering.

To improve the doping efficiency of boron (B)-doped hydrogenated amorphous silicon [a-Si:H(p)] films, a three-step post-B-doping method was developed. This post-treatment method presents the potential to enhance not only the B content but also the hydrogen content in a-Si:H(p) films by increasing the number of treatment times. Based on secondary ion mass spectroscopy and dark conductivity measurements, the B concentration and efficiency of B-doping in a-Si:H(p) films were effectively improved by the three-step B-doping treatment. Furthermore, it was demonstrated that the atomic hydrogen generated during the B-doping process could diffuse into the a-Si:H(p) film and the underlying a-Si:H(i) layers, which is beneficial for suppressing the carrier recombination in the a-Si:H(p/i) passivation layers. There was an absolute increase of 600 μs in the effective minority carrier lifetime in the standard a-Si:H(n)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(p) structure by the 3× three-step treatment on the emitter side. Consequently, enhancements in both the open circuit voltage and the fill factor were observed, resulting in a 0.28% absolute gain (approximately) in the conversion efficiency of silicon heterojunction cells.

In this study, we performed post hydrogen treatment separately after the deposition of i/n a-Si:H window layers. The treatment was carried out in Cat-CVD chamber by inletting hydrogen gas and driving the dissociated atomic hydrogen into the layers by thermal irradiation. Owing to the increased Si-H bonds in the films, the defect states reduced and the optical bandgaps widened. This process was applied to silicon heterojunction solar cell fabrication, the cell efficiencies were improved apparently from 22.15% to 22.76% ($J _{\mathbf {sc}}$textbf from 38.0 to 38.2 mA/cm$^{\mathbf {2}}$, $V _{\mathbf {oc}}$ from 728.4 mV to 735.3 mV, $FF$ from 80.0% to ${81.1{\%}). }$

Correlation between surface passivation of crystalline silicon and the microstructure of silicon passivation layer has been investigated. Based on the optical constants determined by Spectroscopic Ellipsometry measurements, effective medium approximations has been employed to simulate the microstructure of silicon thin film with the mixed phase, including amorphous structure and spherical or ellipsoidal crystallites. A key factorf c , describes the volume fraction of nanocrystallites in the amorphous network, has a significant role to improve the fill factor of silicon heterojunction solar cells. Moreover, the cell performance can be improved by the increasing material density of intrinsic silicon interface layer.

The microstructure evolution of hydrogenated amorphous silicon (a-Si:H) films, deposited by plasma enhanced (PE) and catalytic (CAT) chemical vapor deposition, have been investigated with the increasing thickness. It has been found that crystallites prone to present in the initial growth of a-Si:H deposited by CAT method, whose film has a more compact structure and stronger bonding energy than the film deposited by PE method. At a premise of same conversion efficiency, open circuit voltage and fill factor of the silicon heterojunction solar cells fabricated by CAT are higher than that of the cell fabricated by PE.

A comprehensive study of the microstructures and properties of n-type hydrogenated amorphous silicon (n-a-Si:H) films, deposited by catalytic chemical vapor deposition, for the window layers of silicon heterojunction (SHJ) solar cells is presented. With increasing hydrogen-to-silane dilution ratio (RH), the deposited films first become dense, after which they loosen. With further increases in RH, the films tend to crystallize with native post-oxidization. The doping efficiencies of phosphorus in the various n-a-Si:H films are similar, but the upper surface doping levels of the films are affected by RH. The post-oxidized n-a-Si:H film is more transparent at short wavelengths than a dense film deposited at low RH, exhibiting an external quantum efficiency gain of 20% at 300 nm. Finally, a higher efficiency and short-circuit current density (Jsc) are obtained with the post-oxidized n-type a-Si:H window layer; a Jsc gain of 0.25 mA/cm2 and an efficiency increase of 0.36% were achieved for the optimized SHJ solar cell. At the device level, a dense intrinsic a-Si-H passivated layer is beneficial for suppressing fill-factor (FF) deterioration. The natively post-oxidized n-a-Si:H window layer is a potential choice for improving Jsc by apparently enhancing light absorption in crystalline silicon at short wavelengths.