Rear-emitter Silicon Heterojunction Solar Cells Research Articles

Utilization of CaF2/ITO Double‐Layer Anti‐Reflective Coating for Increasing the Efficiency in Rear Emitter SHJ Solar Cells

AbstractA method to minimize optical losses in the front side of rear emitter silicon heterojunction (SHJ) solar cells by applying double‐layer antireflection coating (DLARC) has been examined. The primary aim is to increase the short circuit current density (Jsc) using DLARC that is associated with the cell's absorbance. The optical and electrical properties of single‐layer antireflection coating (SLARC) are studied and compared with calcium fluoride (CaF2)/Indium tin oxide (ITO) DLARC.OPAL 2 is utilized as a simulation tool to increase the optical properties of CaF2/ITO DLARC, and the experimental result verifies its validity. ITO is deposited using sputter system while thermal evaporator is utilized to deposit CaF2 to make DLARC. The SHJ solar cell having DLARC demonstrates the average reflection of 6.55% which is 31.7% less than SLARC, that is 9.59% for the wavelength range of 300 to 1100 nm. The fabricated textured solar cell with CaF2/ITO DLARC shows enhancement in external quantum efficiency from 76.89% to 81.06%. It also indicates an improvement in Jsc to 40.83 mA cm‐2 from 39.91 mA cm–2 and having a conversion efficiency of >21% when compared with that of ITO SLARC.

Functions of oxygen atoms in hydrogenated amorphous silicon oxide layers for rear-emitter silicon heterojunction solar cells

Functions of oxygen atoms in hydrogenated amorphous silicon oxide layers for rear-emitter silicon heterojunction solar cells

Application of rear-emitter silicon heterojunction solar cells with mitigation of the damage on the amorphous silicon by an atomic-layered ZnO

In this study, we evaluated an atomically layered ZnO film that interface damage on a silicon-based solar cell caused by the deterioration of the passivation layer cells due to sputtered plasma. The optical properties of atomically layered ZnO films showed an average value of over 90% from the visible to NIR range, and the reflectance in the solar cell was reduced as a function of the rear reflector. The carrier concentration of ZnO films (8.2 × 1019 cm−3) was better than that of ITO film (4.15 × 1020 cm−3), however, the hall mobility was for ZnO film (16.18 cm2/V-s) was low as compared with that of ITO film (49 cm2/V-s). We proved that ZnO film can reduce interface damage caused by sputtered plasma while maintaining the open-circuit voltage (VOC of 723 mV, FF: 76.4%) by measuring the carrier lifetime and Suns VOC. An efficiency of 22.6% was determined for silicon heterojunction solar cells; the ALD-ZnO film limited damage to the p-type silicon layer and subsequently, contributed toward reduced defect creation, resulting in an enhanced JSC. In addition, we showed that the mitigation of plasma damage through an atomic layer can be beneficial for the efficiency of sensitive optoelectronic devices.

Role of hydrogen in modifying a-Si:H/c-Si interface passivation and band alignment for rear-emitter silicon heterojunction solar cells

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.

Effect on the reduction of the barrier height in rear-emitter silicon heterojunction solar cells using Ar plasma-treated ITO film

In this study, we investigated the effect of plasma treatment on an indium tin oxide (ITO) film under an ambient Ar atmosphere. The sheet resistance of the plasma-treated ITO film at 250 W (37.6 Ω/sq) was higher than that of the as-deposited ITO film (34 Ω/sq). Plasma treatment was found to decrease the ITO grain size to 21.81 nm, in comparison with the as-deposited ITO (25.49 nm), which resulted in a decrease in the Hall mobility. The work function of the Ar-plasma-treated ITO (WFITO=4.17 eV) was lower than that of the as-deposited ITO film (WFITO = 5.13 eV). This lower work function was attributed to vacancies that formed in the indium and oxygen vacancies in the bonding structure. Rear-emitter silicon heterojunction (SHJ) solar cells fabricated using the plasma-treated ITO film exhibited an open circuit voltage (VOC) of 734 mV, compared to SHJ cells fabricated using the as-deposited ITO film, which showed a VOC of 704 mV. The increase in VOC could be explained by the decrease in the work function, which is related to the reduction in the barrier height between the ITO and a-Si:H (n) of the rear-emitter SHJ solar cells. Furthermore, the performance of the plasma-treated ITO film was verified, with the front surface field layers, using an AFORS-HET simulation. The current density (JSC) and VOC increased to 39.44 mA/cm2 and 736.8 mV, respectively, while maintaining a WFITO of 3.8 eV. Meanwhile, the efficiency was 22.9% at VOC = 721.5 mV and JSC = 38.55 mA/cm2 for WFITO = 4.4 eV. However, an overall enhancement of 23.75% in the cell efficiency was achieved owing to the low work function value of the ITO film. Ar plasma treatment can be used in transparent conducting oxide applications to improve cell efficiency by controlling the barrier height.

Controlling a crystalline seed layer for mirocrystalline silicon oxide window layer in rear emitter silicon heterojunction cells

Remarkable progress has been made in the improvement of rear-emitter silicon heterojunction (RE-SHJ) solar cells with the use of very thin n-type front contact layers. However, further reducing the thickness of the front window layers while maintaining high conductivity for mitigating the parasitic absorption and carrier collection loss has proven challenging. In this study, we implement controlling a seed layer for achieving ultra-thin, high crystalline and conductivity of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiOx:H) front window layer in RE-SHJ solar cells. By using a seed layer, the crystallinity confirmed by Raman and TEM measurements, and the conductivity of the n-µc-SiOx:H front layers are significantly enhanced compared with that without using the seed layer. This leads to a remarkable increase in the open-circuit voltage (Voc) by 6 mV and fill factor (FF) by 4.11% while maintaining a high short-circuit current density (Jsc) in range of 38 mA/cm2. A high cell performance of 21.1% is obtained with the use of an optimised seed layer.

Rear-emitter silicon heterojunction solar cells with atomic layer deposited ZnO:Al serving as an alternative transparent conducting oxide to In2O3:Sn

Here high-efficiency (above 21%) large-area silicon heterojunction solar cells with atomic layer deposited ZnO:Al as front- or back-side transparent conducting oxide are demonstrated. Photoconductance decay measurements indicate that the excellent chemical passivation provided by the a-Si:H(i,p) and a-Si:H(i,n) stacks is preserved upon deposition of ZnO:Al, and that field-effect passivation losses for the a-Si:H(i,p)/ZnO:Al contact can be mitigated by lowering the Al doping level. Use of low Al-doping is enabled by the rear-emitter configuration which, in addition to facilitating the a-Si:H(i,p)/ZnO:Al contact engineering, enables a higher photocurrent due to the decrease in free-carrier absorption in ZnO:Al. The results encourage the use of In-free transparent conducting oxides in silicon heterojunction solar cells, as the replacement of In2O3:Sn without efficiency loss is demonstrated.

Ultra-thin stack of n-type hydrogenated microcrystalline silicon and silicon oxide front contact layer for rear-emitter silicon heterojunction solar cells

We demonstrate the clear advantage of a n-type hydrogenated microcrystalline silicon (n-µc-Si:H) seed layer on the optoelectronic properties and crystallisation behaviour of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiOx:H) front contact layers. The presence of a non-oxidic n-µc-Si:H seed layer can reduce the thickness and refractive index of the n-µc-SiOx:H front layer significantly while maintaining a high degree of crystallisation and excellent conductivity. This leads to increase in short-circuit current density (Jsc) by 2.64% and open-circuit voltage (Voc) by 0.56% in comparison to that of a device without the seed layer. The enhancement in Jsc can be attributed to the reduction in parasitic absorption loss in the extremely thin front layer. In addition, the improvement in Voc can result from enhanced surface passivation of the wafer due to seed layer growth in very high hydrogen plasma environment which can play a role as the hydrogen post-plasma treatment. The low thickness of the n-µc-SiOx:H front layer yields lower internal recombination losses. In conjunction with an optimised n-µc-Si:H seed layer and n-µc-SiOx:H front layer, we obtained a high conversion efficiency value of 21.8% with Voc of 727 mV, Jsc of 39 mA/cm2, and FF of 77% among the fabricated cells in laboratory.

Study and development of rear‐emitter Si heterojunction solar cells and application of direct copper metallization

AbstractWe have presented a comprehensive study on the development of rear emitter silicon heterojunction (SHJ) solar cells, which shows more freedom for device optimization than the standard front emitter SHJ counterparts. The optimization of the p‐type hydrogenated amorphous silicon (a‐Si:H(p)) layer is liberated from the parasitic absorption issue, while the n‐type hydrogenated amorphous silicon (a‐Si:H(n)) layer is optimized considering the tradeoff between the light absorption loss and field‐effect passivation. The front transparent conductive oxide (TCO) layer can be designed stressing on its optical properties because the lateral transport of the majority carriers at the front side of the cell is supported by the Si substrate, while the rear TCO layer is free to be tuned for suppressing the TCO/a‐Si:H(p) contact resistance. A rear emitter SHJ solar cell (225.47 cm2) fabricated by industry compatible process has been achieved with an efficiency of over 22%. We have further demonstrated the replacement of screen‐printed silver metallization with a low cost direct copper metallization in the rear emitter SHJ solar cells. We report an exciting 22.06% cell efficiency which is comparable to that of the screen‐printed counterpart.

Comparative study on front emitter and rear emitter n-type silicon heterojunction solar cells: The role of folded electrical fields

Here, we presented a comparative investigation on the front and rear emitter n-type silicon heterojunction (SHJ) solar cells. The cell performances showed that the rear emitter SHJ solar cell featured higher open-circuit voltage (Voc), fill factor (FF) and conversion efficiency (Eff.), but suffering from a lower short-circuit current density (Jsc). The poor Jsc agreed well with the external quantum efficiency (EQE) results, that rear emitter SHJ had a inferior EQE than the front emitter ones, although the thinner a-Si:H(n+) layer had a better optical transmittance than that of a-Si:H(p) layer. Then, by adjusting the thickness of two sides intrinsic hydrogenated amorphous silicon (a-Si:H(i)) layers in the rear emitter SHJ solar cells, the Eff.s were further improved. It was believed that the folded electrical fields formed by ITO, doped a-Si:H layers, and n-type crystalline silicon (c-Si(n)) substrate played a great role on the cell performances. Consequently, by combining the optimized thickness of doped and intrinsic a-Si:H layers, for the rear emitter SHJ solar cells, a full-area certified Eff. of 21.71% on large area of 238.9 cm2 substrate was obtained, with a Voc of 0.731 V, Jsc of 37.46 mA/cm2, and FF of 0.793. Note that, the 21.71% full-area conversion efficiency relates to an aperture-area conversion efficiency of 22.57%.

>21% Efficient Silicon Heterojunction Solar Cells on n- and p-Type Wafers Compared

The properties and high-efficiency potential of front- and rear-emitter silicon heterojunction solar cells on n- and p-type wafers were experimentally investigated. In the low-carrier-injection range, cells on p-type wafers suffer from reduced minority carrier lifetime, mainly due to the asymmetry in interface defect capture cross sections. This leads to slightly lower fill factors than for n-type cells. By using high-quality passivation layers, however, these losses can be minimized. High open-circuit voltages (Vocs) were obtained on both types of float zone (FZ) wafers: up to 735 mV on n-type and 726 mV on p-type. The best Voc measured on Czochralski (CZ) p-type wafers was only 692 mV, whereas it reached 732 mV on CZ n-type. The highest aperture-area certified efficiencies obtained on 4 cm2 cells were 22.14% (Voc = 727 mV , FF = 78.4%) and 21.38% (Voc = 722 mV, FF = 77.1%) on n- and p-type FZ wafers, respectively, proving that heterojunction schemes can perform almost as well on high-quality p-type as on n-type wafers. To our knowledge, this is the highest efficiency ever reported for a full silicon heterojunction solar cell on a p-type wafer, and the highest Voc on any p-type crystalline silicon device with reasonable FF.