Cavity-controlled radiative recombination of excitons in thin-film solar cells

Abstract

We study the performance of photovoltaic devices when controlling the exciton radiative recombination time. We demonstrate that when high-quantum-yield fluorescent photovoltaic materials are placed within an optical cavity, the spontaneous emission of the radiative exciton is partially inhibited. The corresponding increase of the exciton lifetime results in an increase of the effective diffusion length and diffusion current. This performance maximizes when the thickness of the cell is comparable to the absorption length. We show that when typical parameter values of thin solar-cell devices are used, the efficiency may improve by as much as three times. © 2009 American Institute of Physics. ͓doi:10.1063/1.3262954͔In many photovoltaic devices, a major limitation to high efficiency stems from a short exciton diffusion length given that excitons recombine quickly before reaching the chargeseparation layer. 1,2 It is assumed possible to reduce some of the recombination rates, except for the radiative one which, in photovoltaics, is generally treated as a material parameter that cannot be changed. However, the radiative rate of recombination is not an immutable property of the radiationmatter interaction, and instead it may be modified by the presence of a cavity or a reflecting surface that alters the electromagnetic environment for the exciton radiation. Indeed, linked to the Purcell effect, 3 it has been shown that the spontaneous emission rate can be inhibited when a radiative system in the excited state is placed near a reflecting surface, 4,5 in a dielectric slab, 6 within a Fabry-Pérot type cavity, 7,8 or photonic crystal. 9,10 Yet, this very compelling phenomenon has found not many interesting real applications. In photovoltaic materials that exhibit a high-quantumyield fluorescence, where radiative recombination is one of the dominant recombination paths, a reduction of the fluorescence rate may result in a larger exciton diffusion length and ultimately a larger short-circuit current or efficiency.In the present letter we analyze in detail this effect on the exciton diffusion current when a thin layer of photovoltaic material is placed between two metallic electrodes to form a Schottky diode-type photovoltaic cell. These two parallel electrodes also produce a nonclosed optical cavity for the exciton radiation. One of the electrodes is a perfect conductor with unit reflectivity while the other is a thin metal electrode 11 with a partial light transmission in order to let the sun photons through but at the same time contribute in forming a cavity for the fluorescence photons. The device configuration that we consider is shown schematically in Fig. 1. The z-axis is perpendicular to the plane of the device, the semitransparent metal electrode has thickness d and the active layer has thickness h. Inside the active region, excitons are generated and transported via diffusion. Charge separation occurs at z = 0 or at the metal electrode with the lower work function.To determine the diffusion current, we use the FengG...

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