Enhancing the performance of silicon solar cells via the application of passive luminescence conversion layers

Abstract

This paper discusses ways of reducing the two major losses encountered in a single-junction photovoltaic (PV) device—that of sub-band gap transmission and lattice thermalisation losses—via the application of passive luminescent devices called up- and down-converters, respectively. Down-conversion (DC) results in the generation of more than one lower energy photon being generated per incident high-energy photon, while up-conversion (UC) generates one photon with energy for every two or more sub-band gap photons absorbed. A related process of wavelength down-shifting (DS) is similar to DC except that the external quantum efficiency of the DS process is less than unity.

Introduction

Photovoltaic (PV) devices fabricated from silicon (Si) wafers dominate the marketplace and comprised 94% of all module shipments in 2004 [1]. In addition, the dominance of bulk Si over other thin-film technologies has been increasing year by year. While it is acknowledged that thin-film and more advanced “third generation” solar cells [2] undoubtedly have an important role to play in the future, manufacturers of Si solar cells are presenting the more recent generations of PV with a rapidly moving target.

For any single-junction solar cell, the band gap energy (E_g) of the semiconductor from which the PV device is fabricated establishes a fundamental upper limit for its conversion efficiency. The two major loss mechanisms that need to be overcome to significantly enhance device efficiencies are lattice thermalisation and transparency to sub-band gap photons. For the case of lattice thermalisation, a photon with high energy creates an electron–hole (e–h) pair, however, the photoexcited pair quickly loses energy in excess of the band gap and the extra energy is lost as heat within the device. This is illustrated by process

for a typical p–n junction band diagram in Fig. 1, while the transparency of the semiconductor to sub-band gap photons is denoted as process

in Fig. 1. A further loss mechanism is the recombination of photoexcited e–h pairs (see

in Fig. 1), however, the impact of this parameter can be minimised by maintaining high minority carrier lifetimes in the semiconducting material and does not contribute greatly to the theoretical efficiency limit. Using the principle of detailed balance between incident and escaping photons and extracted electrons, Shockley and Queisser demonstrated that the one-Sun efficiency limit for a single-material cell is around 31% with an optimal band gap of 1.3 eV [3]. This assumes that the only unavoidable losses from the device are the emission of photons produced by radiative recombination, and includes voltage drops across the contacts and junction, denoted by processes

and

in Fig. 1. With a slightly non-optimal band gap, the Shockley–Queisser one-Sun efficiency limit of a silicon ($E_{\mathrm{g}}=1.12\mathrm{eV}$) device is further reduced to 30% [3].

The only existing PV device that addresses lattice thermalisation and sub-band gap losses is the tandem solar cell, a series-connect stack of two–five junctions made from semiconducting materials with a decreasing band gap. Green has described the theoretical upper efficiency limits of tandem devices, being able to achieve efficiency limits ranging from 31% to 68.2% for one and an infinite number of band gaps, respectively, using unconcentrated sunlight [2]. When operating under the maximum theoretical solar concentration of around 46,000 times, these values increase to 40.8% and 86.8%, respectively [2].

The best lab-scale Si solar cells with a one-Sun efficiency of $\eta =24.7\%$ have already achieved results that are very close to their theoretical limit. Motivated by the predicted dominance of Si PV technologies for the next decades, research has been performed on the application of luminescence up-conversion (UC) [4], [5], [6] and down-conversion (DC) [7] to single-junction PV devices in an effort to bridge the gap between the realisable efficiencies on Si devices and the ultra-high efficiencies expected of third generation PV.

There are both benefits and challenges in successfully applying UC and DC to PV technologies. Both UC and DC components are passive, optical devices with carrier collection still performed via the single PV junction. Therefore, they have a distinct advantage over tandem solar cells, where the photocurrents generated in upper and lower cells must match throughout the day in order to avoid significant mismatch losses. In addition, the application of UC and DC layers, at least initially, to PV does not require modification of the existing solar cell as the layers are passive and purely optical in operation.