The influence of different black silicon (b-Si) front side textures prepared by inductively coupled reactive ion etching (ICP-RIE) on the performance of back-contacted back silicon heterojunction (BCB-SHJ) solar cells is investigated in detail regarding their optical performance, black silicon surface passivation and internal quantum efficiency. Under optimized conditions the effective minority carrier lifetime measured on black silicon surfaces passivated with Al2O3 can be higher than lifetimes measured for the SiO2/SiNx passivation stack used in the reference cells with standard KOH textures. However, to outperform the electrical current of silicon back-contact cells, the black silicon back-contact cell process needs to be optimized with aspect to chemical and thermal stability of the used dielectric layer combination on the cell.
© 2014 Optical Society of America
A successful approach to boost efficiency in solar cells is to increase their spectrally broad band, angular independent light absorption in the absorber layer of the cell by optical light trapping. This is particularly important for the long wavelengths in the range of the band gap of the absorber material where absorption is typically low. In current state of the art silicon solar cells these desired optical properties are achieved by front side modifications such as random pyramids for light trapping and optimized anti-reflection coating ARC. Such front sides need to combine two important features; a very good absorption enhancement in rather thick (> 100μm) silicon wafers, as well as the equally important capability of electrical surface passivation, i.e. the suppression of surface recombination of minority charge carrier, to maintain a high efficiency. The ARC and passivation coatings consist of dielectric layers or layer stacks .
Black silicon (b-Si) technology will in principle also allow in cells based on thin crystalline silicon absorbers with a thickness in the range of 20 to 50μm light absorption close to the Yablonovitch limit . Black silicon surfaces fabricated by e.g. inductively coupled plasma reactive-ion etching (ICP-RIE) can improve the absorption in the wavelength range of 300 – 1100 nm even further by combining superior broadband anti-reflection properties and angular independent light trapping properties in the long wavelength regime . The excellent passivation quality of atomic-layer deposited thin Al2O3 films on ICP-RIE etched b-Si surfaces providing very low surface recombination rates has been demonstrated recently . Currently, significant research is ongoing to integrate black silicon front side textures into solar cells on either front-contacted cells [4–7] or even novel MIS solar cells . However, currently reported efficiencies do not exceed state of the art cell technologies. One of the reasons for this lies in the challenge to properly passivate b-Si surfaces and thus reduce electrical losses by surface recombination in these cells while contacting the cells through the black silicon textures. Therefore, instead of front side contacting, all back side contacted solar cells should be considered. In this study, black silicon surfaces are evaluated in back-contacted back silicon heterojunction PRE-CASH (BCB-SHJ) solar cells . This cell concept enables to place all contacts on the back side, thus, no shadowing effects of a front grid occurs and no contacting through the nanostructured black silicon surface is needed. Because all charge carrier selective contacts are formed on the back side, the front side texturing as well as the front side passivation can be easily modified by only small changes in this rather complex cell process. A schematic scetch of the realized solar cell design is drawn in Fig. 1. Here, the integration of ICP-RIE etched front side structures passivated by thermal ALD deposited Al2O3 films in a PRECASH silicon solar cell is tested in a straightforward way.
Polished boron-doped (1 – 5Ωcm) FZ c-Si wafers with 4″ diameter and thicknesses between 265 μm and 295 μm are used as a base material. Using an SF6 and O2 based ICP-RIE process at temperatures above 20°C  two different surface morphologies were produced on the front side by varying process pressure. While a low process pressure leads to a randomly distributed needle-like surface structure with characteristic heights of around 400 nm and aspect ratios of about 2–3 (labeled texture B, compare Fig. 2(b)), a higher process pressure results in a sponge-like surface structure with characteristic heights of around 200 nm and aspect ratios smaller than one (labeled texture A, compare Fig. 2(a)).
Before depositing about 20 nm Al2O3 films on both sides of the textured wafers an RCA cleaning procedure  was applied. For the Al2O3 deposition by thermal ALD at a temperature of 180°C, Trimethylaluminium (TMA) and H2O were used as precursor materials. The activation of the passivating layers was done by post deposition annealing in a muffle oven (30 min at 385 °C) in ambient atmosphere.
BCB-SHJ PRECASH solar cells were prepared on both plasma textured wafers. The same type of solar cells but with standard front sides were additionally prepared as reference samples. The standard front side consists of random pyramids passivated by a SiO2/SiNx stack. The fabrication process of the solar cells is similar to the one described in . However, as a consequence of the conclusions drawn in  by Haschke et al. the a-Si:H emitter layer now also acts as passivation layer of the area surrounding the rather small cells (1cm2). The etching time for structuring the a-Si:H emitter was adapted to the findings reported in  by Greil et al. to ensure proper removal of the a-Si:H emitter at locations where the absorber contacts are formed at a later stage of the process. Different cells with with varying back surface field (BSF) to emitter contact areas ABSF = 2.5% and 5% on the backside have been realised on the different textures. The front side passivation layer is covered by approximately 80 nm intrinsic a-Si:H to protect it during the processing. This layer is removed at the end of the process as it would strongly decrease the short circuit current density jSC of the solar cells. The a-Si:H(i) protection layer on the SiO2/SiNx stack of the reference sample was removed using an HF/HNO3/H3PO4 based silicon etchant. On the b-Si samples, this etchant could not be used as it would strongly degrade the Al2O3 film passivation. Hence, we used NaOH (0.6 %) to remove the a-Si:H(i) on the b-Si samples. However, the etching process was not optimized and therefore is inhomogeneous, leaving some residuals of a-Si:H(i) on the surface.
To check the cell process stability of the front side passivation the effective minority charge carrier lifetimes τeff of the samples were measured at different stages of the cell process via the quasi-steady-state photo-conduction (QSSPC) method in transient or generalized mode with a Sinton WCT 120 lifetime tester [13, 14]. The illuminated IV curves of the fabricated solar cells were measured using a Wacom WXS-156S - L2, AM1.5GMM dual source (tungsten and halogen lamp) sun simulator with class AAA characteristics and a shadowing mask with an opening as large as the active cell area. To further analyze the influence of the different front side textures on cell level, optical reflection and external quantum efficiency measurements under white bias illumination of 0.1 suns have been conducted.
2. Results and discussion
Due to their different morphologies (see Fig. 2), sample A and B have different optical properties as can be seen in Fig. 3 in which the reflectivity of the prepared solar cells is plotted. In the wavelength range of 300–1100 nm the reflection of sample A (sponge like surface structure) is always above 9 %, thus relatively high. Sample B (needle-like surface with higher aspect ratio) has a much lower reflectivity of below 2% in the same wavelength range. In comparsion with the random pyramidal textured surface coated with an additional anti-reflection coating which acts as a reference in this work, the b-Si surface structure of sample B outperforms the reference structure in the short wavelength range 300–400 nm as well as in the long wavelength range 800–1100 nm. The reference and sample B perform similar in the wavelength range of 400–800 nm, from which a significant portion of the solar photons come from. For a fairer comparison we calculated the maximum achievable photocurrent (MAPC). This ideal value gives the photocurrent under the assumption that every absorbed photon with a photon energy larger than the bandgap of silicon produces an electron-hole pair that contributes to the current. The reference texture gives a MAPC of 44.86 mA × cm−2, the value of texture A is 7.89% lower (41.32 mA × cm−2) and texture B beats the reference structure slightly by 0.56% (45.11 mA × cm−2).
Plasma texturing of c-Si usually increases surface recombination drastically due to the increased surface area and a lower bulk lifetime close to the surface as a consequence of defects introduced during the etching process . However, effective lifetime measurements of the plasma textured samples A and B, both bifacially passivated by 20 nm Al2O3 layers show excellent effective lifetime values τeff, cf. Fig. 4. QSSPC measurements reveal lifetimes above 1.5 ms. The higher effective lifetime of the sample A which has a smaller effective front surface area than sample B indicates that the effective lifetime in these samples is limited by the surface area.
To check the stability of the Al2O3 front side passivation during the cell processing the effective lifetimes of the samples have been measured again after the substitution of the Al2O3 passivation on the back side by an n-doped a-Si:H layer, cf. Fig. 4. A reduction of lifetime during this step can be observed for both samples. This degradation is mainly due to the higher interface recombination at the a-Si:H (n)/c-Si interface on the back side. The a-Si:H(i) protection layer, which stays on the front side during the cell process, keeps the lifetimes at these still sufficient values. Without this protection layer the lifetimes would reduce to values below 100 μs due to detrimental impacts during the cell process. In comparison with a sample textured by random pyramids on both sides and passivated by the SiO2/SiNx reference stack, all Al2O3 passivated samples reveal higher effective lifetimes (see Fig. 4).
The power conversion efficiencies of all produced plasma textured cells are mainly limited by a low shunt resistance. The origin of this problem is under further investigation. In the following discussion we focus on samples of each texture type, random pyramid texture and both plasma textures, that show the highest shunt resistance. In figure 5 the illuminated IV curve of a cell with texture A (sponge like plasma texture) and of a reference cell with the same emitter to BSF area ratio of 2.5% is plotted. The curves are measured at slightly different temperatures. Besides the aforementioned lower shunt resistance in comparison to the reference, this sample reveals a lower short circuit current than the reference cell of 35mA × cm−2, which constitutes another limiting factor for efficiency. This is a direct result of the higher reflection of texture A relative to the random pyramid structure, since the IQE of this cell is very similar to the one of the reference for a broad wavelength range (Fig. 3). This indicates that the front side recombination velocities in both cells (texture A cell and reference cell) are similar. The illuminated IV measurement of the best cell with plasma texture B and an emitter to BSF area ratio of 5% shows a significantly lower IQE compared to the others, see Fig. 3. The current limitation in this cell is caused by a high front side recombination velocity and by parasitic shading of a-Si:H(i) residuals of the protection layer. Both effects lead to a loss in short circuit current, overcompensating the optical enhancement gained by the low reflection of this texture. This is seen as well in the illuminated IV curve plotted in Fig. 6.
For an optimised production process, several issues have to be taken into account. First, the front side Al2O3 passivation needs to be activated during the production process and maintained throughout. A simplest idea would be apply the front side texture and Al2O3 passivation after the back side was done. Thereby the main difficulty is to thermal activated the frontside activation without damaging the back side passivation. One way to achieve this is to optimise the used a-Si:H PECVD layers for better temperature stability. Possibly fast rapid thermal annealing at tempretures above 300°C is a methode to activate the Al2O3 passivation without decreasing the passivation quality of the a-Si:H layers. Another maybe less challenging way could be to protect the Al2O3 by an additional layer such as e.g. TiO2 which should be more etch-resistant than Al2O3. A more chemically stable front side would be beneficial for the stripping of the used front side protecting layer to avoid shading residuals of this layer on the front. Second, a point-contacting scheme is beneficial when large emitter area fractions are needed due to poor diffusion lengths and low contact resistivities are easily achievable. Both, generally does not hold true when using a-Si:H as heterojunction contacts. Normally, high quality wafers with high lifetimes are used to benefit from the excellent surface passivation of a-Si:H and the realisation of a low-ohmic majority contact using a-Si:H layers is difficult . To this end, as suggested by numerical simulations by Chen et al. , a larger area fraction for the majoritiy contact should be used in an optimised cell design. Third, the passivation of the area surrounding the solar cell should be realised by the the a-Si:H layer stack used for the BSF or by a dielectric to avoid issues raised by a non-illuminated diode in parallel to the solar cell.
We prepared back contacted silicon heterojunction PRECASH solar cells with a black silicon texture at the front side passivated with ALD-deposited Al2O3. Compared to a reference standard front side texture of random pyramids and a SiN/SiO2 passivation, the b-Si texture results in an broad band very low reflectively throughout the whole spectral range of wavelengths. The effective minority charge carrier lifetimes measured on plasma textured samples passivated with Al2O3 are higher compared to the lifetimes measured for the passivation stack used in the reference cells. However, to really tap the full potential of the b-Si surface in solar cell devices the production process of the cells needs further adaptation such as an optimised protection of the Al2O3 front side passivation during cell processing.
The authors gratefully acknowledge the support of E. Conrad, K. Jacob, and M. Mews in solar cell preparation. T. Hänel and K. Mack are acknowledged for support in measuring. Furthermore, we like to thank R. Ferré and R. Gogolin from ISFH for providing the front side of the reference. The German Federal Ministry of Education and Research (BMBF) is acknowledged for funding within the research college STRUKTURSOLAR and the project grant PHIOBE.
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