Date Published: February 01, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Qianqian Lin, Zhiping Wang, Henry J. Snaith, Michael B. Johnston, Laura M. Herz.
Perovskite solar cells have shown a meteoric rise of power conversion efficiency and a steady pace of improvements in their stability of operation. Such rapid progress has triggered research into approaches that can boost efficiencies beyond the Shockley–Queisser limit stipulated for a single‐junction cell under normal solar illumination conditions. The tandem solar cell architecture is one concept here that has recently been successfully implemented. However, the approach of solar concentration has not been sufficiently explored so far for perovskite photovoltaics, despite its frequent use in the area of inorganic semiconductor solar cells. Here, the prospects of hybrid perovskites are assessed for use in concentrator solar cells. Solar cell performance parameters are theoretically predicted as a function of solar concentration levels, based on representative assumptions of charge‐carrier recombination and extraction rates in the device. It is demonstrated that perovskite solar cells can fundamentally exhibit appreciably higher energy‐conversion efficiencies under solar concentration, where they are able to exceed the Shockley–Queisser limit and exhibit strongly elevated open‐circuit voltages. It is therefore concluded that sufficient material and device stability under increased illumination levels will be the only significant challenge to perovskite concentrator solar cell applications.
Perovskite solar cells have emerged over the last few years as highly promising photovoltaic applications, with power conversion efficiencies (PCEs) improving at an unprecedented rate. Recent optimized PCE values1 near 22.1% are now comparable with the most efficient thin film analogues, such as cadmium telluride and copper indium gallium selenide (CIGS) cells. Meanwhile, fundamental optoelectronic properties of hybrid perovskites have also been extensively investigated, revealing high charge‐carrier mobility and low recombination rates—key advantages for operational devices.2, 3, 4, 5 However, a few key challenges remain if perovskite solar cells are to move toward commercial manufacture, as for example long‐term stability, current–voltage hysteresis, and up‐scaling issues.6, 7, 8, 9, 10 Some excellent progress has been made on the issue of stability, with recent developments suggesting that perovskite solar cells may now deliver stable output for a few thousand hours under 1 sun (AM 1.5G) illumination.11, 12 Measures to improve stability include the use of mixed‐cation perovskites,13 and two‐dimensional (2D)14 and quasi‐2D perovskites11, 12 that incorporate spacer layers. Furthermore, efficient large‐area perovskite solar cells and modules have been successfully demonstrated,15, 16 indicating incipient success of up‐scaling approaches.
Our evaluations presented in this work are based on basic rate equations used to determine charge‐carrier concentrations inside a device under different operating conditions. A range of different charge‐recombination and ‐extraction rate parameters are used as input to the calculations that reflect what has so far experimentally been determined for these materials. We assume for the purpose of these calculations that the HOIPs are sufficiently stable under high illumination conditions, exhibiting negligible degradation. Given the current rapid progress in device and materials engineering, the stability of HOIPs and related devices may well be improved so that they can ultimately operate under the much harsher concentrator conditions for an extended period. Hot carriers effects, which could potentially improve matters at high charge carrier densities,29, 30 are not included in this work, which is based on the assumption that electron‐ and hole‐transporting layers are optimized to extract carriers at normal lattice temperature. In order to simplify our model, we also excluded series resistance and shunt resistance in this work. Ideally, the effect of parasitic resistances can be minimized by a decrease in device area and an enhancement in the junction quality. In addition, we assume that reflection losses are minimal, which can be achieved through suitable surface coatings. Under these conditions, the only unavoidable efficiency losses are caused by defect‐mediated charge‐carrier recombination and device performance can be calculated based on charge extraction and recombination rate parameters.
In summary, we have demonstrated that perovskite solar cells will fundamentally harvest photons more efficiently at certain regimes of high solar concentration, where they should be able to exceed the Shockley–Queisser limit and reach extraordinarily high open‐circuit voltages close to 1.4 V (i.e., losses limited to near 200 meV). Suppression of trap‐mediated (Shockley–Read–Hall) recombination of charge‐carriers is the key toward achieving this goal, but we argue that the required level is already approached, with charge‐carrier lifetimes in the low‐density regime now often exceeding microseconds. We therefore conclude that material and device stability under increased illumination levels would be the main challenge toward implementation of perovskite concentrator solar cells. Some key issues will need to be addressed in this regard, including the thermal and photostability of HOIPs and interlayers, and the long‐term stability of the devices. Moreover, there is clear room for improvement in the electrical sheet resistance of the TCEs, and the charge‐carrier mobility of the interlayers that are currently typically used. These issues could potentially be addressed by better control over doping and minimization of the effective thicknesses of such auxiliary layers. We note that such obstacles will in any case also have to be addressed for long‐term stable performance of perovskite solar cells under standard conditions. Most PV technologies that have achieved such stable operation in the field (e.g., silicon and GaAs) ultimately also qualified for solar concentrator applications. Therefore, we believe that further development of perovskite solar cells has the potential to ultimately allow their use in concentrating photovoltaics. Finally, the use of perovskite tandem cells under solar concentration will be a highly promising goal, given that the most efficient photovoltaic devices are currently working as a tandem structure under concentrator conditions.
The authors declare no conflict of interest.