Orthogonal Thin Film Photovoltaics on Vertical Nanostructures
© Ahnood et al. 2015
Received: 3 October 2015
Accepted: 1 December 2015
Published: 16 December 2015
Decoupling paths of carrier collection and illumination within photovoltaic devices is one promising approach for improving their efficiency by simultaneously increasing light absorption and carrier collection efficiency. Orthogonal photovoltaic devices are core-shell type structures consisting of thin film photovoltaic stack on vertical nanopillar scaffolds. These types of devices allow charge collection to take place in the radial direction, perpendicular to the path of light in the vertical direction. This approach addresses the inherently high recombination rate of disordered thin films, by allowing semiconductor films with minimal thicknesses to be used in photovoltaic devices, without performance degradation associated with incomplete light absorption. This work considers effects which influence the performance of orthogonal photovoltaic devices. Illumination non-uniformity as light travels across the depth of the pillars, electric field enhancement due to the nanoscale size and shape of the pillars, and series resistance due to the additional surface structure created through the use of pillars are considered. All of these effects influence the operation of orthogonal solar cells and should be considered in the design of vertically nanostructured orthogonal photovoltaics.
Despite the simplicity of the concept of orthogonal solar cells, there are a number of underlying physical mechanisms which need to be accounted when considering the form factor of orthogonal solar cells. These require development of a design framework which is tailored for the orthogonal devices based on the physical effects uniquely present in this class of devices. Earlier works have demonstrated the clear influence of pillar height and diameter on the efficiency of thin film orthogonal solar cells [3, 12]. This paper builds on the earlier works by considering (i) non-uniformity of the illumination across the depth of the device, (ii) electric field enhancement effects at the nanoscales, and (iii) increased series resistance due to the higher device surface area.
The test structures were fabricated in this study consisted of silicon thin film PV cells deposited on vertical nanostructures and on a flat ITO-coated glass substrate as reference samples. Where vertical nanostructures were used, they consisted of either an array of MWCNTs or ZnO nanowires with their growth deposition methods reported in earlier works [6, 13]. The PV cells consisted of p-i-n type structure deposited using plasma-enhanced chemical vapor deposition with their deposition methods reported in earlier works . The thicknesses of active layers used here are p-type amorphous silicon carbide (20 nm), intrinsic a-Si:H (300 nm), n-type nanocrystalline silicon (40 nm). PV cell measurements were performed using Keithley source meter 2400, under dark and various illuminated conditions. Simulations were performed using SPICE based module on a double diode circuit module with series and parallel parasitic resistances (AimSpice software).
Results and Discussions
Figure 2b shows the similar parallel-configured solar cell segments in the case of the orthogonal solar cells. As the light travels through the orthogonal device, a proportion of the illumination is absorbed in each segment, with the material absorption coefficient determining the proportion of light transmitted through to the next segment. This gives rise to variable illumination intensity at each segment of the orthogonal solar cell. Furthermore, the wavelength dependence of the absorption coefficient in many materials leads to a variation in the illumination spectrum reaching each segment. These differences in the illumination intensity and spectrum give rise to a variability in the electrical characteristics from the various segments of the orthogonal solar cell.
The first segment, closest to the point of light entering the orthogonal solar cell, experiences the highest intensity of illumination and therefore the highest VOC and ISC compared with the rest of the segments. Here, the illumination spectrum will be typically 1.5 AM spectrum, with the values of VOC, FF, and JSC of the segment comparable to a planar solar cell with similar thickness. On the other hand, the final segment is exposed to a lower light intensity, thus exhibiting a lower VOC and ISC with a different FF compared with the first segments . Although the variation in the electrical performance between different segments is marginal in short pillars, this variation is exacerbated in longer structures. As shown in Fig. 2c, this variability has a detrimental impact on the overall VOC of orthogonal solar cell. All segments are connected in parallel, which infers the same VOC for across all segments. The comparatively poor electrical performance of the lower segments, due to the weak illumination, leads to a reduction in the overall VOC of the orthogonal device. An orthogonal solar device biased at the open circuit point will only have a small portion of its segments operating at the open circuit condition. The upper segments with a larger VOC will still be generating power, which is then internally consumed by the solar cell itself in the lower segments with lower VOC, as depicted in Fig. 2c, d.
Equation 5 points to the dependence of the VOC to the number of segments and therefore the length of the orthogonal solar cell as well as the illumination spectrum. This is to say that certain orthogonal solar cells are better situated in certain illumination spectrum.
Electric Field Confinement
In the case of device shown in the Fig. 5a with inner and outer radiuses of 50 and 350 nm, respectively, the structure with a metallic sidewall coating would exhibit an electric field enhancement factor of 3.1 close to the central pillar and a reduction in the electric field by a factor of 0.44 times close to the outer sidewall, relative to planar structure. Consider a p-i-n junction a-Si:H solar cell, which is a discrete heterojunction thin film solar cell dominated by drift process. Electric field across such device is a significant factor in the collection efficiency of photogenerated carriers. In such device, typically any gain due to electric field enhancement close to the pillar electrode is quenched by the reduction in the electric field close to the outershell electrode. The devices overall performance will be limited by the collection efficiency of the carriers close to the outer sidewall. A different outcome can be envisaged in the case of devices dominated by diffusion and dissociation processes, such as bulk heterojunction organic solar cells or single-crystal silicon solar cells. Although the electric field does not play a role in the separation of the photogenerated carries, it nevertheless affects the performance of the device through influencing the contact resistance and charge transfer to the electrodes.
Given the pillar lengths of up to few micrometers, it is feasible to omit the outer metallic sidewall and use a highly doped semiconductor layer on side walls in conjunction with a metallic mesh film at the base of the pillar as charge collection layer. In this configuration, the charge generated in the intrinsic layers are collected through the CNT and the n-type doped layer and transferred onto the bottom of the pillar where metal contacts provide a low impedance path for the collection of the carriers. The difference between the electric field penetration depth in doped semiconductor film compared with that of metal film leads to different electric field distribution within the structure. The electric field penetration depth of metallic films is in the order of few angstroms  and therefore few orders of magnitude thinner than thickness of the metal electrodes used in devices, whereas electric field penetration depth is comparable to a typical doped layer in the range of nanometers . Metal outer shell of nanopillar solar cell acts as the boundary for the electric field, while without the metal outer shell, the electric field extends into the final dope layer, effectively increasing in the r 2 in Eq. (7), and thus mitigates the quenching effect of outer sidewall. The electric field enhancement can be advantageous for the performance of certain type of devices. It has been shown to improve the charge collection efficiency in the organic solar cells [23, 28, 29] and enhance the operating speed and response of photodiodes .
As shown in Fig. 7a, there is a significant drop in the VOC with the increase in series resistance. This reduction is most pronounced for longer pillar. Indeed, the shortest pillar’s VOC response is consistent with the response of planar devices where the VOC is not expected to vary significantly with the increase in series resistance. The variation in the ISC with the series resistance is more pronounced for the longer pillar as depicted in Fig. 7b. Here, high series resistance effectively decouples segments furthest away from the probing point and subsequently results in a sharp drop in the ISC with increase in the series resistance, particularly in taller structures. Once again, this variation is minimal for orthogonal devices with shortest pillar highest, in line with the characteristics of planar devices. As shown in Fig. 7c, series resistance adversely affects the FF of the all orthogonal devices including shorter structures. The combination of these performance indices leads to the variation in the maximum power output with series resistance shown in Fig. 7d. It highlights the fact that the reduction in the PMax and subsequently solar cell efficiency with increase in the series resistance is most pronounced in the case of taller structures. Reduction in pillar height reduces the influences of series resistance on the PMax and the subsequent device efficiency.
Resistivity and segment resistance of a number of different thin film materials used as the charge collection and scaffolding in vertical solar cells
1E + 05
1E + 07
8E + 08
1E + 09
In this work, the effects that influence the performance of orthogonal solar cells are investigated. It is shown that although orthogonal devices have a potential efficiency gain compared to planar devices, a careful design is required in order to achieve their optimum performance efficiency. We discuss the role of optical non-uniformity across the depth of the solar cell. As light travels through the orthogonal solar cell, its intensity and spectrum change. This change in the illumination translates in a variable output characteristics from parallel-connected segments of the solar cell. Lower segments, owing to their lower open circuit voltage due to lower intensity illumination, act as power sink consuming the power generated by the upper segments. This effect is worsened when considering the recombination losses in the device, which are prevalent in thin film solar cells.
Ordered array of nanopillars has widely been demonstrated as a means of enhancing broadband light absorption of materials. Device simulation suggests that in the case of closely packed nanopillar array, the light intensity decays cross the depth of the device . However, in addition to the absorption, scattering plays an important role in the optical properties of nanopillar arrays . A simple optical scattering may result in a more uniform illumination as a function of depth of the pillars. Despite this, the overall non-uniform illumination and the role it plays on the performance of orthogonal solar cells remain an issue to consider. The electric field enhancement due to the nanoscale size shape of the pillars used in orthogonal solar cells is discussed. The design of the pillar structure, in particular the effect of metallic side wall, on the electric field distribution of the device is considered and shown that with a careful design, it is possible to exploit this effect. It is shown that series resistance due to the additional surface structure created through the use of pillars can play an import role in the device performance. All of these effects have the potential for reducing the performance of orthogonal solar cells and should be considered in device designs.
This work has considered the influence of physical effects such as illumination non-uniformity, electric-field confinement, and series resistance separately in order to qualitatively examine the specific contribution of each of these on the operation of orthogonal PV devices. However, the interplay between these physical mechanism means coupled optical/electrical models [34–36] provide a more accurate quantitative description of the device performance.
AA acknowledges support of the Melbourne Materials Institute’s seed grant, University of Melbourne. ZH was supported by Shenzhen Innovation Fund (JCYJ20130329180806949).
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