Junction formation, current transport and degradation mechanisms in hybrid n-Si/PEDOT:PSS solar cells

Manuela Göbelt^a, Sara Jäckle^a^,^b, Sebastian Schmitt^a^,^b, Gerald Brönstrup^a^,^b, Silke Christiansen^a^,^b, Matthias Mattiza^b

^aMax Planck Institute for the Science of Light - Erlangen, Günther-Scharowsky-Straße, 1, Erlangen, Germany

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Sara Jäckle, 206
Publication date: 5th February 2015

Hybrid inorganic/organic junctions are promising candidates for highly efficient and cost-effective photovoltaic devices. We have studied the hybrid interface consisting of monocrystalline n-type silicon (n-Si) and a highly conductive polymer blend containing the conjugated polymer Poly(3,4-ethylenedioxy-thiophene) (PEDOT) and the polyelectrolyte poly(styrene sulfonate) (PSS). While the light is absorbed in the inorganic silicon substrate the transparent ’metal’-like wide-gap polymer PEDOT:PSS serves as the selective contact. We extract the junction parameters and solar cell characteristics of planar n-Si/PEDOT:PSS photovoltaic cells with small signal current-voltage (dark J-V), capacity-voltage (C-V) measurments as well as their photovoltaic response (illuminated J-V). By varying the doping concentration of the silicon wafer we can monitor the build-in voltage and currents that manifest at this hybrid interface. We are able to show that a strong inversion layer is formed at the surface of the n-type silicon induced by the polymer blend. By a detailed analysis of the differently doped silicon substrates and by applying equations based on common solar cell models for drift-diffusion in pn-junctions and thermionic emission in Schottky junctions to the measured data we can prove that the transport is not governed by majority carriers in the inorganic semiconductor but by minority carrier instead. With this we give a comprehensive explanation of the high open-circuit voltages and promising efficiencies that are observed in n-Si/PEDOT:PSS solar cells. We demonstrate planar n-Si/PEDOT:PSS junctions with open circuit voltages up to 640mV and power conversion efficiencies of 12% [2]. A band diagram of the hybrid junction will be presented. The current in the n-Si/PEDOT:PSS is mainly limited by the charge transfer across the interface and the charge transport in the polymer. By tailoring the morphological structure of PEDOT:PSS the hole transport properties across the n-Si/PEDOT:PSS interface and in the polymer itself are improved, which directly influence the solar cell parameters [1]. We will also examine the stability of the hybrid interface under ambient conditions. While we will unravel the degeneration mechanisms in the n-Si/PEDOT:PSS solar cells, we will also discuss ways to stabilize them. The effects of encapsulation with insulating and conductive transparent metal oxides by low temperature atomic layer deposition on the performance and stability of the n-Si/PEDOT:PSS solar cells are presented. First results of combining this hybrid solar cell concept with silicon nanostructures and the potential of incorporate large arrays of homogenously regular silicon nanowires on multicrystalline thin film silicon on glass [3] will be presented.
Left: Schematic of the fabricated hybrid inorganic/organic n-Si/PEDOT:PSS solar cells. [2] Right: Dependency of the power conversion efficiency (PCE), open-circuit voltage (VOC) and short-circuit current (JSC) of n-Si/PEDOT:PSS solar cells on the doping density (ND) of the silicon substrate. All parameters are extracted under an AM1.5 irradiation spectrum.[2]
[1] Pietsch, Bashouti, Christiansen, J. Phys. Chem. C (2013) 117, 9049−9055 [2] Pietsch, Jäckle, Christiansen, Appl. Phys. A (2014) 115:1109–1113 [3] Schmitt, et al. Nano Letter 12, 4050, 2012

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