Amaratunga, G

Ore, E

English

Apollo (Cambridge)

 Flexible, Dopant Free a­Si:H Solar Cell  Erenn Ore1, Gehan Amaratunga1   1Department Of Engineering, University Of Cambridge, Cambridge, United Kingdom.   Abstract  —  Dopant free thin film solar cells with the common structure of 𝒁𝒏𝑶:𝑨𝒍 /	𝑴𝒐𝑶𝒙  / 𝒂–𝑺𝒊:𝑯 / 𝑳𝒊𝑭	/ 𝑨𝒍 are fabricated directly on flexible, transparent substrates. All the layers, except the 𝒂–𝑺𝒊:𝑯  layer, are deposited by simple, low temperature physical vapour deposition methods. An open circuit voltage of 𝟎. 𝟕𝟔𝟒	𝑽  and a short circuit current density of 𝟏𝟏. 𝟑𝟏	𝒎𝑨/𝒄𝒎𝟐	 are measured under AM1.5 irradiance. The proof of concept for flexible, dopant free 𝒂–𝑺𝒊:𝑯 solar cell demonstrated here is a step forward towards processing cost-effective, flexible and lightweight thin film solar cells.  Index Terms — amorphous silicon, dopant free, flexible, solar cell. I. INTRODUCTION A typical inorganic thin film solar cell has the core structure of a thin film absorber (A) sandwiched between two wider band gap contact layers (CLs) that facilitate the separate collection of photogenerated electrons and holes from the absorber. CL through which impinging photons enter the absorber first is the window contact layer (WCL). The other CL is the back contact layer (BCL). The core photovoltaic element is abbreviated as WAB, after the initial letter of each layer of the core structure. In order to transport photogenerated electrons and holes to an external circuit, WCL is contacted with a transparent front electrode (TFE), and BCL is commonly contacted with a reflective back electrode (RBE), which also assists recapturing some of the low energy photons that passed through the absorber and BCL without getting absorbed. A thin film WAB solar cell is deposited on a substrate. If the substrate is on the TFE side of the cell, then it has to be transparent so that the most of incoming photons can enter the cell. This thin film WAB cell configuration is referred to as the superstrate structure, Figure 1. If the  substrate is on the RBE side, then it does not have to be transparent. This configuration is referred to as the substrate structure. When intrinsic hydrogenated amorphous silicon (𝑎– 𝑆𝑖: 𝐻) is used as the absorber, the resulting thin film WAB solar cell is here referred to as the 𝑎– 𝑆𝑖: 𝐻 WAB solar cell, which is one of the leading commercial thin film solar cell technologies. For the conventional 𝑎– 𝑆𝑖: 𝐻 WAB solar cell, doped thin film silicon or its alloy (DTF-Si/A) is employed as CL. DTF-Si/A CL is typically processed by a chemical vapour deposition (CVD) method from expensive and dangerous gases that contain the dopant atoms [1][2]. These gasses are phosphine that is used for 𝑛-type doping, and diborane that is used for 𝑝-type doping of thin film silicon or its alloys. In order to prevent dopant contamination of the intrinsic 𝑎– 𝑆𝑖: 𝐻 absorber, which is detrimental to the power conversion efficiency (𝜂) [3], a multi-chamber CVD system is employed for depositing thin film silicon based layers, which are WCL, A and BCL for the conventional 𝑎– 𝑆𝑖: 𝐻  WAB solar cell. The combination of these factors makes the manufacturing process of the conventional 𝑎– 𝑆𝑖: 𝐻  WAB solar cell relatively expensive and complex [3].    Figure 1. A schematic representation of a thin film WAB solar cell with the superstrate structure, deposited on a transparent substrate.  Because of the high density of tail states in their band gaps, DTF-Si/A CLs absorb a significant portion of the incoming photons [4]. However, the electrons and holes generated by photon absorption in DTF-Si/A CLs do not contribute to 𝜂 of the cell because of the high recombination rates in DTF-Si/A CLs, resulting primarily from doping [5]. Recombination is particularly problematic for the 𝑝-doped DTF-Si/A, which is typically employed as WCL in the superstrate structure [6]. In addition, the high recombination rates present in DTF-Si/A CLs decrease the probability of photogenerated electrons and holes passing through DTF-Si/A CLs towards their designated electrodes.  To circumnavigate the aforementioned disadvantages of using DTF-Si/A, CLs can be made from alternative materials that have higher transparency, and that are safer, simpler and cheaper to deposit. In this investigation, for the flexible, dopant free 𝑎– 𝑆𝑖: 𝐻  WAB solar cell design with the superstrate structure, a layer of non-stoichiometric  molybdenum oxide (𝑀𝑜𝑂J ) [7]-[9]  and a layer of lithium fluoride ( 𝐿𝑖𝐹 ) [7][8][10] are used as WCL and BCL respectively. Instead of the rigid and heavy quartz-based substrates used for the rigid, dopant free 𝑎– 𝑆𝑖: 𝐻 WAB solar cells investigated in our previous works [7][8], a flexible and lightweight transparent substrate is employed for the flexible cell investigated in this work. II.  SOLAR CELL STRUCTURE AND PROCESSING The flexible, dopant free 𝑎– 𝑆𝑖: 𝐻 WAB solar cell investigated here has the structure of 1	𝜇𝑚 	𝑍𝑛𝑂: 𝐴𝑙  (TFE) | 10	𝑛𝑚  𝑀𝑜𝑂J	 (WCL) | 300	𝑛𝑚  𝑎– 𝑆𝑖: 𝐻  (A) | 1.5	𝑛𝑚  𝐿𝑖𝐹 (BCL) | 300	𝑛𝑚  𝐴𝑙	 (RBE), deposited on a flexible and transparent ClearOhmTM (Cambrios) substrate, which is a flat polyethylene terephthalate film covered by a percolated network of silver nanowires. This substrate weighs less per surface area than the quartz-based substrates. Prior to any deposition step, the ClearOhmTM substrate is rinsed by deionised water and dried with compressed nitrogen gas. The 𝑍𝑛𝑂: 𝐴𝑙  TFE is deposited on this substrate by the radio frequency magnetron sputtering method from a 𝑍𝑛𝑂: 𝐴𝑙V𝑂W  ceramic target. This is a physical vapour deposition (PVD) method. The TFE surface is textured by 𝐻𝐶𝑙 etching. This step is significant in achieving high 𝜂, since the 𝑎– 𝑆𝑖: 𝐻  WAB solar cell with the optimally textured interfaces is able to generate considerably more short circuit current density (𝐽Z[) than the cell with smooth interfaces [11].  The intrinsic 𝑎– 𝑆𝑖: 𝐻 absorber of the flexible cell is processed at 200]𝐶  by the plasma enhanced CVD method, which uses only one CVD deposition chamber. The 𝑀𝑜𝑂J WCL, the 𝐿𝑖𝐹  BCL, and the 𝐴𝑙  RBE are all deposited by thermal evaporation at room temperature in the same single chamber PVD system from stoichiometric 𝑀𝑜𝑂W  powder, from 𝐿𝑖𝐹 powder, and from 𝐴𝑙 pellets respectively. The incomplete solar cell precursors are exposed to ambient air atmosphere between the depositions steps, except between the deposition steps of the 𝐿𝑖𝐹 BCL and the 𝐴𝑙 RBE, which are processed sequentially in the same thermal evaporation deposition run without breaking vacuum. A shadow mask used during BCL | RBE deposition defines the planar size of the flexible cell, which is 4𝑥4	𝑚𝑚V. After the TFE deposition step, the resulting solar cell precursor, i.e. the 𝑍𝑛𝑂: 𝐴𝑙 covered ClearOhmTM substrate, is found to have a prominent curvature. Therefore, in order to reduce the level of uneven film coverage during the subsequent deposition steps, solar cell precursors are clamped flat.  After completing the last deposition step, the flexible solar cells are annealed in a conventional oven in ambient air atmosphere for one hour at 120]𝐶, since the post-deposition annealing treatment at this temperature is found to improve 𝜂 of the 𝑎– 𝑆𝑖: 𝐻 WAB solar cell with the 𝑀𝑜𝑂J	WCL [9] and 𝜂 of the 𝑎– 𝑆𝑖: 𝐻 WAB solar cell with the 𝐿𝑖𝐹 BCL [10]. III. SOLAR CELL CHARACTERISATION  The current density and voltage (𝐽𝑉) characteristics of the solar cells are determined by using a PASAN AM1.5 solar simulator system with AM1.5 irradiance. The 𝐽𝑉 characterisation is employed for calculating the values of 𝐽Z[, of the open circuit voltage (𝑉a[), of the series resistance (𝑅Z) and of the shunt resistance (𝑅Zcd).  The 𝐽𝑉 measurements are carried out at room temperature. The cells are also clamped flat during these measurements. IV. RESULTS AND DISCUSSION Despite their curved shapes, the flexible and dopant free 𝑎– 𝑆𝑖: 𝐻 WAB solar cells are physically stable. There is no visible cracking, when they are flexed repetitively. The flexible cells also have good adherence to their substrate with no observed peeling.  The prominent curvature of the cell precursor observed after the 𝑍𝑛𝑂: 𝐴𝑙  TFE deposition can be explained by the difference in the thermal expansion coefficients of the ClearOhmTM substrate and the	𝑍𝑛𝑂: 𝐴𝑙 TFE. As TFE and the substrate cool following the 𝑍𝑛𝑂: 𝐴𝑙  deposition, they are likely to contract at significantly different rates. This would increase the stress at the interface between the 𝑍𝑛𝑂: 𝐴𝑙 TFE and the substrate [12], resulting in the associated cell precursor getting curved. The curvature developed after the 𝑍𝑛𝑂: 𝐴𝑙  deposition is observed to dominate the over-all curvature of the fully processed solar cells.  The increasing stress with increasing film curvature in the 𝑍𝑛𝑂: 𝐴𝑙  TFE is expected to be inversely correlated with the conductivity of TFE [13]. Therefore, the prominent curvature of the flexible solar cell would explain the high 𝑅Z value of 36.6	Ω. 𝑐𝑚. 𝑅Z of a few ohms or more is known to degrade solar cell performance [2]. Alternate clamping and relaxing of the curved cell precursor during different deposition steps, and clamping of the curved solar cells during 𝐽𝑉 characterisation are likely to increase the probability of pinhole formation. The 𝐿𝑖𝐹 BCL is especially susceptible to pinhole formation, as it is significantly thinner than the other layers.  Pinholes cause leakage current, which is associated with low 𝑅Zcd [14]. Hence, pinhole formation is the likely reason for the very low 𝑅Zcd value of 225	Ω. 𝑐𝑚V, which is within the range of the 𝑅Zcd values that are found to degrade solar cell performance [2]. Since fill factor (𝐹𝐹) is highly influenced by both 𝑅Zcd and 𝑅Z [2], their unfavourable values are considered to be the main reasons for the inferior 𝐹𝐹 of the flexible, dopant free 𝑎-𝑆𝑖: 𝐻 WAB cell, compared to 𝐹𝐹 of the rigid, dopant free 𝑎-𝑆𝑖: 𝐻 WAB cell with the same structure that is deposited on an Asahi U substrate in the same deposition systems under the same deposition conditions [15], Figure 2. Asahi U substrate used for this rigid cell is an Asahi type quartz that is coated with a textured, conductive and transparent layer of 𝑆𝑛𝑂V: 𝐹 [16], which functions as TFE for the rigid cell.    Figure 2.  The 𝐽𝑉 curves of the best performing flexible, dopant free 𝑎-𝑆𝑖: 𝐻 WAB cell (red), and of the rigid, dopant free 𝑎-𝑆𝑖: 𝐻 WAB cell (blue) [15]. Both cells have identical layers expect their TFEs and substrates.  By optimising the 𝑍𝑛𝑂: 𝐴𝑙 deposition on the ClearOhmTM substrate, the 𝑍𝑛𝑂: 𝐴𝑙  film stress can be reduced [13], resulting in TFE that has better conductivity and reduced curvature. Therefore, it should be possible to improve the parasitic resistance values of 𝑅Zcd and 𝑅Z, so that a better 𝐹𝐹 can be achieved for the flexible cell. The best performing flexible, dopant free 𝑎– 𝑆𝑖: 𝐻  WAB cell has 𝑉a[  of 0.764	𝑉  and 𝐽Z[  of 11.31	𝑚𝐴/𝑐𝑚V . The low transparency of the annealed ClearOhmTM substrate is one of the main reasons for the relatively low 𝐽Z[ , Figure 2. Furthermore, due to the imperfect mechanical clamping of the curved cells during 𝐽𝑉 characterisation, the active cell area for impinging photons is noticeably smaller than 4𝑥4	𝑚𝑚V , which is used for 𝐽Z[ calculations. Consequently, the 𝐽Z[ value calculated from the 𝐽𝑉  characterisation data underestimates the value of 𝐽Z[ for the flexible cells.  It is clear that 𝐽Z[  can be improved by using a flexible substrate that retains higher transparency than the ClearOhmTM film, after cell processing is completed.  𝐽Z[ can also be improved by using the substrate cell configuration, where the solar cell is deposited in reverse order on a flexible substrate, which does not need to be transparent. This configuration should increase the amount of photons reaching the absorber. One of the main reasons for the relatively low 𝑉a[ value of the 𝑎– 𝑆𝑖: 𝐻 WAB solar cell with 𝑀𝑜𝑂J	WCL is attributed to air exposure of the cell precursor between the deposition steps of 𝑀𝑜𝑂J	WCL and the 𝑎– 𝑆𝑖: 𝐻 absorber [9]. Thus, depositing the 𝑀𝑜𝑂J	and 𝑎– 𝑆𝑖: 𝐻  layers sequentially without breaking vacuum is expected to improve the 𝑉a[ . In addition, 𝑉a[  is known to increase with increasing 𝑅Zcd  [14]. Therefore, increasing the 𝑅Zcd value is expected to further improve 𝑉a[.  V. CONCLUSION The proof of concept for the flexible, dopant free 𝑎– 𝑆𝑖: 𝐻 WAB solar cell is demonstrated. The processing of this cell eliminates the use of dangerous dopant gases and the need for multi-chamber CVD reactor. Combination of these factors should make the 𝑎– 𝑆𝑖: 𝐻  WAB solar cell manufacturing process safer, simpler and cheaper. Furthermore, the use of lightweight and flexible substrate increases the application range for the dopant free 𝑎– 𝑆𝑖: 𝐻 WAB solar cells.  Since this is only a proof of concept device, the power conversion efficiency parameters of 𝑉a[, 𝐽Z[, and 𝐹𝐹 have the potential to be improved by deposition optimisation; especially regarding the 𝑍𝑛𝑂: 𝐴𝑙 and the 𝑀𝑜𝑂J	layers; and by employing alternative flexible and transparent substrates; or by employing flexible and opaque substrates in the substrate solar cell configuration. Note that 𝑀𝑜𝑂J  can be replaced by other dopant free materials that have the suitable properties to be used as WCL, such as 𝑉V𝑂J  [17], 𝑃𝐸𝐷𝑂𝑇: 𝑃𝑆𝑆  and 𝐶𝑢V𝑂J  [7]. Similalry, 𝐿𝑖𝐹 can be replaced by other dopant free materials that have the suitable properties to be used as BCL, such as 𝑀𝑔𝑂 [15].  Also note that the flexible, light-weight, and dopant free thin film WAB solar cell concept demonstrated here can be applied to the other thin film solar cells, including multiple junction thin film silicon solar cells, such as the micromorph solar cell.  We thank Prof Miro Zeman of Delft University of Technology, Delft, Netherlands for giving us access to his laboratory for this work.  REFERENCES [1]  V. M. Fthenakis, P. D. Moskowitz, “Characterization and control of phosphine hazards in photovoltaic cell manufacture,” Solar Cells, vol. 22, pp. 303-317, 1987. [2] T. Markvart, L. Castafier, Practical Handbook of Photovoltaics: Fundamentals and Applications. Oxford, UK: Elsevier, 2003. [3]  A. Shah, J. Meier, A. Buechel, U. Kroll, J. Steinhauser, F. Meillaud, H. Schade, D. Domine, “Towards very low-cost mass production of thin-film silicon photovoltaic (PV) solar modules on glass,” Thin Solid Films, vol. 502, pp. 292-299, 2006. [4] J. Poortmans, V. Arkhipov, Thin Film Solar Cells: Fabrication, Characterization and Applications. Chichester, UK: John Wiley & Sons, 2006. [5] R. A. Street, Hydrogenated Amorphous Silicon. Cambridge, UK: Cambridge University Press, 1991.  [6] H. Okamoto, Y. Nitta, T. Yamaguchi, Y. Hamakawa, “Device physics and design of a-Si ITO / p-i-n heteroface solar cells,” Solar Energy Materials, vol. 2, pp. 313-325, 1980.  [7] E. Ore, G. Amaratunga, “Thin heterojunction a-Si:H photovoltaic cell design with no doped a-Si:H layers”. In 40th IEEE Photovoltaic Specialist Conference (PVSC), 2014, pp. 3087-3089. [8]  E. Ore, G. Amaratunga, "Cylindrical ultra-thin a-Si:H photovoltaic cell with no doped layers," MRS Advances, vol. 2, issue 15, pp. 825-833, 2017. [9] E. Ore, J. Melskens, A. Smets, M. Zeman, G. Amaratunga, “MoOx hole collection layer for a-Si:H based photovoltaic cells,” MRS Advances, vol. 1, issue 14, pp. 977-983, 2016. [10] E. Ore, J. Melskens, A. Smets, M. Zeman, G. Amaratunga, “Ultra-thin LiF layer as the electron collector for a-Si:H based photovoltaic cell,” MRS Advances, vol. 2, issue 15, pp. 863-867, 2017. [11] A. Čampa, O. Isabella, R. van Erven, P. Peeters, H. Borg, J. Krč, M. Topič, M. Zeman, “Optimal design of periodic surface texture for thin-film a-Si:H solar cells,” Progress in Photovoltaics: Research and Applications, vol. 18, pp. 160-167, 2010. [12] A. Bishop, Vacuum Deposition Onto Webs, Films, And Foils. Norwich, New York, USA: William Andrew Publishing, 2007.  [13] J. S. Fernandez, F. B. Naranjo, “Optimization of aluminum-doped zinc oxide films deposited at low temperature by radio-frequency sputtering on flexible substrates for solar cell applications,” Solar Energy Materials & Solar Cells, vol. 94 pp. 157-163, 2010. [14] R. D. Rugescu, Solar Energy. Olajnica, Vukovar, Croatia: Intech, 2010. [15] This work is carried out by the authors of this paper and it is not published yet.  [16] K. Sato, Y. Gotoh, Y. Wakayama, Y. Hayashi, K. Adachi, H. Nishimura, “Highly textured SnO2:F TCO films for a-Si solar cells”, Rep. Res. Lab. Asahi Glass Co. Ltd, vol. 42, pp. 129-137, 1992. [17] E. Ore, G. Amaratunga, S. De Wolf, “HIT solar cell with V2Ox window layer,” MRS Advances, vol. 2, issue 53, pp. 3147-3156, 2017.       

Flexible, Dopant Free a-Si:H Solar Cell

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Flexible, Dopant Free a-Si:H Solar Cell

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