Photovoltaic & Optoelectronic Device Group

Abstract:

Photovoltaic cells are generally optimised for operation at 25 °C under the AM1.5 solar spectrum. This is useful for comparing different technologies, but ultimately energy yield (the energy output across a year of operation) is what we aim to maximise. This is generally estimated using the power conversion efficiency (PCE) of a cell at 25 °C and its temperature coefficient (how PCE changes with temperature). For monolithic tandem cells, this method is less accurate as current matching between each sub cell is required for maximum performance. To understand how to optimise tandems for energy yield, we fabricated perovskite/Si (30.8% PCE) and all-perovskite (25.4% PCE) tandems and tested them over a range of temperatures. Opposing temperature dependence with bandgap - perovskite bandgaps tend to widen with increasing temperature, whereas Si narrows - results in a worse temperature coefficient for perovskite/Si (-0.23%/°C) compared to all-perovskite tandems (-0.17%/°C). Increasing the Cs concentration at the perovskite A-site reduces the bandgap widening effect, resulting in an improved temperature coefficient (-0.19%/°C). Surprisingly, when estimating energy yields using real-world data, this improved temperature coefficient does not translate to increased yields. Instead, we find that the opposing temperature/bandgap correlation of perovskite/Si tandems is an advantage as the solar spectrum is blue-shifted compared to AM1.5 when cells are warmest and operate at their peak power output. Thus, we expect an increase in energy yield of up to 3% for optimised perovskite/Si tandems compared to other tandem technologies with the same PCE. Significantly, this equates to a full PCE point increase under standard conditions for optimised (> 30%) perovskite/Si tandems.

Abstract:

Despite significant advances in wide band gap perovskite solar cells over the past several years, non-radiative recombination at the perovskite-electron transport interface continues to be a critical obstacle limiting device performance. This study presents a solution utilizing a thin interlayer of blended fullerenes [6], [6]-phenyl C61 butyric acid methyl ester (PCBM) and indene-C60 bis-adduct (ICBA) between the 1.77 eV perovskite and evaporated C60. Optimizing the blend to a trace 2% by mass PCBM in ICBA results in a hybrid electron transport layer (ETL) with improved energetic alignment, stronger molecular ordering, an order of magnitude higher electron mobility compared to neat PCBM or ICBA. When combined with surface passivation, this approach resulted in devices with 19.5% steady state efficiency, a fill factor of 0.85 and open-circuit voltage (VOC) of 1.33 V, which is within 10% of the radiative limit of VOC for this bandgap. Here we highlight the complex nonlinear behavior with fullerene mixing, and how controlling the energetics and crystallinity as well as the electronic properties of these materials is vital in mitigating nonradiative recombination and achieving high performing wide band gap perovskite solar cells.

Abstract:

Transparent conductive films are key components of many optoelectronic devices but are often made from either scarce or brittle materials like indium tin oxide. Carbon nanotube-polymer films offer an abundant and flexible alternative. Here, we report how the dimensions of the carbon nanotube raw material affect their thin film performance and thickness yield when processed with the polymer ethylene-vinyl acetate. We perform chemical doping with several halogenated metals and find the electron affinity of the metal to be a good indicator of p-doping effectiveness. We identify CuCl2 as low-cost alternative to the established gold chloride dopants. Optimising the dopant deposition method allows us to reduce the effect of doping on the optical transmittance. Percolation analysis of our films demonstrates that optimized single-walled carbon nanotube-ethylene-vinyl acetate films show no sign of percolation effects down to thicknesses of 5 nm. Finally, we produce transparent touch-sensitive devices. Comparing several of these devices, we find a linear relationship between the sheet resistance and the on/off ratio of the touch sensing that can be used to determine a threshold film thickness. Using doped carbon nanotube-ethylene-vinyl acetate films increases the on/off ratio and allows us to fabricate touch-sensitive devices with an on/off ratio of 10 at 95% optical transmittance. This clearly demonstrates the potential of these films for transparent touch-sensitive applications.

Abstract:

Tandem photovoltaics hold great potential to surpass the efficiency limit of single-junction solar cells. Detrimental structural defects and chemical reactions at buried interfaces of subcells considerably impede the performance of integrated tandems. Here, we devise a mercapto-functionalized mesoporous silica layer as a superstructure at the buried interface to modulate the crystallisation, eliminate nanovoids, passivate defects, and suppress the oxidation of Sn(II) in the tin–lead perovskite films, contributing substantially to reduce charge carrier losses and improve stability in positive-intrinsic-negative structured devices. Consequently, the tin–lead perovskite single-junction cells show efficiency values of up to 23.7% with the best open-circuit voltage of 0.89 V. With the enhanced subcells, our double-junction tandems show efficiency values of 29.6% (certified 29.5% and steady-state 28.7%) and 24.7% on solar cells and 11.3 cm2 mini-modules, respectively. Encapsulated tandems maintain 90% of initial efficiency after 445 h of maximum power point tracking under simulated 1-sun illumination.