Photocurrent enhancement of dye solar cells by efficient light management

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Abstract

We studied the possibility of photocurrent enhancement in Dye Solar Cells (DSCs) by means of two strategies of light management, namely the use of a scattering layer (SL) and the efficient coupling of incoming light into the cell. The first leads to a significant enhancement in conversion efficiency, as evident from IV characteristics and Incident Photon-to-Current Efficiency (IPCE) measurements of cells provided with SL. The latter was investigated by using angle resolved Attenuated Total Reflection (ATR) technique that evidenced a strong angular dependence of the photocurrent and further enhanced upon using a coupling prism. Both strategies are found to be based on the increase of the light path inside the active layer.

Introduction

Dye Solar Cells (DSCs) are electrochemical devices developed in 1991 by Graetzel and O’Regan [1], [2]. Since then progress has led to record power conversion efficiency of up to 11.58% [3], [4], [5], [6]. The basic arrangement of a dye solar cell consists of a 1015μm thick layer of titanium dioxide (TiO2), i.e. a large band gap semiconductor, sandwiched between two transparent and conducting glass substrates (Fluorine doped Tin Oxide, FTO). The titania must be mesoporous, in order to achieve a high surface/volume ratio to bond a large number of dye molecules. Dye molecules can absorb incident light and a percentage of photogenerated electrons can be transferred to the photoelectrode. A liquid electrolyte based on the I/ I3 redox couple has a regenerative effect for dye cations formed after the loss of electrons. Finally, the platinum layer on the counterelectrode acts as catalyst in the reduction process between I3 and I, allowing to close the circuit.

DSCs are thought to offer wide margins of efficiency improvement. Usually, many efforts are devoted to find new dyes for enhanced absorption and electron injection or to improve electrolytes and nanoporous titania morphology for a better current conduction. Actually, also light management strategies are being developed regarding the photon spatial optimization. In this frame, we made use of two complementary approaches, a scattering layer (SL) on the backside (Fig. 1) of the titania and a refractive element on the topside (Fig. 2). Both are devoted to increase the light path inside the active layer respectively by means of the enhanced back-scattering reflection and of the incident angle optimization.

Section snippets

Materials and experimental details

Conducting glass substrates (15 Ω/sq) were bought from Pilkington. The titanium dioxide T/SP transparent paste (13 nm particle size), the N719 dye, and the Pt-catalyst used for counterelectrode were purchased from Solaronix. Dyesol provided the two scattering titania pastes: WER2-0 and WER4-0 (150–250 nm and 250–350 nm particle diameter respectively) and the High Stability Electrolyte (HSE). The photoanodes were deposited by the doctor blade technique.

In the frame of a light management strategy

IV and IPCE measurements

IV measurements were carried out with an Agilent Source Meter under a KHS Solar Constant 1200 Sun Simulator at AM1.5 and 100 mW/cm2 illumination conditions. IV characteristics are shown in Fig. 3 and cell parameters reported in Table 1. We observed a 47% relative enhancement of density of photocurrent (Jsc) for cell B compared to cell A. In cell B, the TiO2 particles whose diameter is comparable to the wavelength of visible light cause a scattering of the incident light thus increasing the

Angular measurements

In this section we show the angular dependence of the photocurrent. Measurements are executed by means of an Attenuated Total Reflection technique (ATR, commonly used in surface plasmon resonance characterization), implemented on a previously described setup [13]. A collimated single wavelength light beam, He–Ne laser source at λ=633nm, is used to illuminate the cell, directly or through the input facet of a 45 BK7 glass coupling prism (see Fig. 2) mounted on the glass substrate (nsnBK7=1.515

Acknowledgment

The financial support was obtained by Lazio region grant “Polo Solare Organico Regione Lazio”.

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