Study of optical losses in mechanically stacked dye-sensitized/CdTe tandem solar cells
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Study of optical losses in mechanically stacked dye-sensitized/CdTe tandem solar cells Vincent Barrioz1*, Simon Hodgson1, Peter Holliman2, Arthur Connell2, Giray Kartopu1, Andrew J. Clayton1, Stuart J.C. Irvine1, Shafiul Monir1 and Matthew L. Davies2 1 2 *
Centre for Solar Energy Research, Glynd r University, OpTIC, St Asaph, LL17 0JD, UK. School of Chemistry, Bangor University, Bangor, Gwynedd, LL57 2UW, UK. [email protected] (+44 1745 535 159)
ABSTRACT In a constant effort to capture effectively more of the spectral range from the sun, multi-junction cells are being investigated. In this context, the marriage of thin film and dye-sensitized solar cells (DSC) PV technologies may be able to offer greater efficiency whilst maintaining the benefits of each individual technology. DSC devices offer advantages in the nature of both the metal oxide photo-electrode and dye absorption bands, which can be tuned to vary the optical performance of this part of a tandem device, while CdTe cells absorb the majority of light above their band-gap in only a few microns of thickness. The key challenge is to assess the optical losses with the goal of reaching a net gain in photocurrent and consequently increased conversion efficiency. This study reports on the influence of optical losses from various parts of the stacked tandem structure using UV-VIS spectrometry and EQE measurements. A net gain in photocurrent was achieved from a model developed for the DSC/CdTe mechanically stacked tandem structure. INTRODUCTION In order to increase the photovoltaic (PV) competitiveness and achieve grid parity globally, the research community constantly strives to increase conversion efficiency and, where possible, go beyond the physical limits of single junction cells [1]. In the case of a near optimum direct band gap material (i.e. 1.5 eV), the conversion efficiency limit would be 32 % with a short-circuit current density (Jsc) of 28.9 mA/cm2. A NREL-certified record efficiency of 18.7 %, has recently been reported by First Solar for thin film CdTe solar cells [2]. This follows GE Global Research previous record [3] at 18.3 % which had an impressive 26.95 mA/cm2 over the spectral range of 350 – 850 nm. The exact source from this enhanced blue response is not known, however, it is generally achieved either by thinning the CdS front window layer or by increasing its band gap (Eg) by alloying the CdS (e.g. with Zn [4]). For dye-sensitized solar cells (DSC) conversion efficiencies have reached 12.3 % (Jsc ~ 17.3 mA/cm2 within spectral range of 400 – 700 nm) using Zn-porphyrin dye co-sensitized with a triphenylamine and a cobalt complex redox couple [5]. Exploring alternative routes to higher cell efficiencies is essential. One path to enhance performance is to use multi-junction solar cells in order to minimize thermalization losses. The preference is to use monolithically integrated sub-cells in series as widely reported for III-V concentrated multi-junction solar cells [6] and of increasing attraction for organic/inorganic cells [7,8]. This
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