Paul J. Roland

Improving the Performance of Formamidinium and Cesium Lead Triiodide Perovskite Solar Cells using Lead Thiocyanate Additives

Formamidinium lead triiodide (FAPbI3 ) is considered as an alternative to methylammonium lead triiodide (MAPbI3 ) because of its lower band gap and better thermal stability. However, owing to the large size of FA cations, it is difficult to synthesize high-quality FAPbI3 thin films without the formation of an undesirable yellow phase. Smaller sized cations, such as MA and Cs, have been successfully used to suppress the formation of the yellow phase. Whereas FA and MA lead triiodide perovskite solar cells (PVSCs) have achieved power conversion efficiencies (PCEs) higher than 20 %, the PCEs of formamidinium and cesium lead triiodide (FA1-x Csx PbI3 ) PVSCs have been only approximately 16.5 %. Herein, we report our examination of the main factors limiting the PCEs of (FA1-x Csx PbI3 ) PVSCs. We find that one of the main limiting factors could be the small grain sizes (≈120 nm), which leads to relatively short carrier lifetimes. We further find that adding a small amount of lead thiocyanate [Pb(SCN)2 ] to the precursors can enlarge the grain size of (FA1-x Csx PbI3 ) perovskite thin films and significantly increase carrier lifetimes. As a result, we are able to fabricate (FA1-x Csx PbI3 ) PVSCs with significantly improved open-circuit voltages and fill factors and, therefore, enhanced PCEs. With an optimal 0.5 mol % Pb(SCN)2 additive, the average PCE is increased from 16.18±0.50 (13.45±0.78) % to 18.16±0.54 (16.86±0.63) % for planar FA0.8 Cs0.2 PbI3 PVSCs if measured under reverse (forward) voltage scans. The champion cell registers a PCE of 19.57 (18.12) % if measured under a reverse (forward) voltage scan, which is comparable to that of the best-performing MA-containing planar FA-based lead halide PVSCs.

Low-temperature plasma-enhanced atomic layer deposition of tin oxide electron selective layers for highly efficient planar perovskite solar cells

Recent progress has shown that low-temperature processed tin oxide (SnO2) is an excellent electron selective layer (ESL) material for fabricating highly efficient organic–inorganic metal-halide perovskite solar cells with a planar cell structure. Low-temperature processing and a planar cell structure are desirable characteristics for large-scale device manufacturing due to their associated low costs and processing simplicity. Here, we report that plasma-enhanced atomic layer deposition (PEALD) is able to lower the deposition temperature of SnO2 ESLs to below 100 °C and still achieve high device performance. With C60-self-assembled monolayer passivation, our PEALD SnO2 ESLs deposited at ∼100 °C led to average power conversion efficiencies higher than 18% (maximum of 19.03%) and 15% (maximum of 16.80%) under reverse voltage scan for solar cells fabricated on glass and flexible polymer substrates, respectively. Our results thus demonstrate the potential of the low-temperature PEALD process of SnO2 ESLs for large-scale manufacturing of efficient perovskite solar cells.

Wiring-up carbon single wall nanotubes to polycrystalline inorganic semiconductor thin films: low-barrier, copper-free back contact to CdTe solar cells.

We have discovered that films of carbon single wall nanotubes (SWNTs) make excellent back contacts to CdTe devices without any modification to the CdTe surface. Efficiencies of SWNT-contacted devices are slightly higher than otherwise identical devices formed with standard Au/Cu back contacts. The SWNT layer is thermally stable and easily applied with a spray process, and SWNT-contacted devices show no signs of degradation during accelerated life testing.

Post-deposition processing options for high-efficiency sputtered CdS/CdTe solar cells

CdCl2 activation near 400 °C is known to be critically important for obtaining high efficiency CdS/CdTe solar cells. However, this treatment step behaves differently on high-temperature-grown CdTe than on lower-temperature-grown CdTe layers such as those grown by sputtering. On sputtered films, the post-deposition activation produces grain-boundary passivation, sulfur diffusion into CdTe, and substantial grain growth. Nevertheless, we find the CdCl2 process for sputtered films to be characterized by a single activation energy that we interpret as applying to S diffusion into CdTe. We find this activation energy to hold for CdCl2 treatments from 370 to 440 °C. The completed CdS/CdTe solar-cell structures showed somewhat poorer initial performance with activation above 420 °C, but, in this case, the cell efficiency increased after accelerated life testing at 85 °C, open-circuit biasing and one-sun illumination. With an optimized CdCl2 activation process, the use of oxygenated sputtered CdS, and low-iron sod...

Analysis and characterization of iron pyrite nanocrystals and nanocrystalline thin films derived from bromide anion synthesis

We use a solution-based hot injection method to synthesize stable, phase pure and highly crystalline cubic iron pyrite (FeS2) nanocrystals, with size varying from ∼70 to 150 nm. We use iron(II) bromide as an iron precursor, elemental sulfur as the sulfur source, trioctylphosphine oxide (TOPO) and 1,2-hexanediol as capping ligands, and oleylamine (OLA) as a non-coordinating solvent during the synthesis. We report on the influence of hydrazine treatment, and of thermal sintering, on the morphological, electronic, optical, and surface chemical properties of FeS2 films. Four point probe and Hall measurements indicate that these iron pyrite films are highly conductive. Although they are unsuitable as an effective photovoltaic light-absorbing layer, they offer clear potential as a conducting contact layer in photovoltaic and other optoelectronic devices.

Enhanced Grain Size, Photoluminescence, and Photoconversion Efficiency with Cadmium Addition during the Two-Step Growth of CH3NH3PbI3

Control over grain size and crystallinity is important for preparation of methylammonium lead iodide (MAPbI3) solar cells. We explore the effects of using small concentrations of Cd2+ and unusually high concentrations of methylammonium iodide during the growth of MAPbI3 in the two-step solution process. In addition to improved crystallinity and an enhancement in the size of the grains, time-resolved photoluminescence measurements indicated a dramatic increase in the carrier lifetime. As a result, devices constructed with the Cd-modified perovskites showed nearly a factor of 2 improvement in the power conversion efficiency (PCE) relative to similar devices prepared without Cd addition. The grains also showed a higher degree of orientation in the ⟨110⟩ direction, indicating a change in the growth mechanism, and the films were compact and smooth. We propose a Cd-modified film growth mechanism that invokes a critical role for low-dimensional Cd perovskites to explain the experimental observations.

Effects of electric field on thermal and tunneling carrier escape in InAs/GaAs quantum dot solar cells

The effects of electric field on carrier escape in InAs/GaAs quantum dots embedded in a p-i-n solar cell structures have been studied by quantum efficiency. Via band structure simulation, effective barrier height of carriers inside QDs is reduced with increasing local electric field, so tunneling and thermal escape are enhanced. At 300K, when electric field intensity is below 40kV/cm, thermal escape is dominant in all confined states in QDs; when electric field intensity is above 40kV/cm, tunneling is dominant in shallow confined states and thermal escape is dominant in the ground state of QDs.

Effect of electric field on carrier escape mechanisms in quantum dot intermediate band solar cells

Carrier escape and recombination from quantum dot (QD) states reduce the probability of two-step photon absorption (TSPA) by decreasing the available carrier population in the intermediate band (IB). In order to optimize the second photon absorption for future designs of quantum dot embedded intermediate band solar cells, the presented study combined the results of simulations and experiments to quantify the effect of electric field on the barrier height and the carrier escape from the QDs in InAs/GaAs quantum dot solar cells with five-layer QD superlattices. The electric field dependent effective barrier heights for ground state electrons were calculated using eight band k·p theory at short circuit conditions. With an increase in electric field surrounding the QDs from 5 kV/cm to 50 kV/cm, the effective barrier height of the ground state electrons was reduced from 147 meV to 136 meV, respectively. Thus, the increasing electric field not only exponentially enhances the ground state electron tunneling rate...

Influence of interparticle electronic coupling on the temperature and size dependent optical properties of lead sulfide quantum dot thin films

We report on the quantum dot(QD) size, temperature, and inter-dot coupling dependence on the optical absorption and emission for PbSQDthin films. Inter-dot coupling is induced by ligand exchange from oleic acid to 1,2-ethanedithiol, and the expected band gap red-shift observed for coupled QDthin films is accompanied by a modification to the temperature-dependence of the band gapenergy. The amplitude and temperature dependence of the photoluminescence(PL) Stokes shift support recombination via a mid-gap state and also indicate that the application of band gap-specific models to fit the temperature dependence PL peak energy is inadequate. Electronically coupled QDthin films show PL quenching with decreasing temperature, following a Boltzmann model which is consistent with thermally activated carrier transport. Enhancing the inter-dot coupling results in the dynamic PL decay signal changing from single- to bi-exponential behavior, reveals a size-dependent transport activation energy, and yields a negative temperature dependent band gapenergy for the smallest QD diameters.

Enhancing the efficiency of CdTe solar cells using a nanocrystalline iron pyrite film as an interface layer

We use thin film nanocrystalline (NC) iron pyrite as an interface layer at the back contact of CdS/CdTe solar cells. In both spattered and CSS deposited CdTe devices, improvements in Voc and FF were obtained after the inclusion of the NC FeS2 layer. Repeated tests show that Voc increases by >30 mV and FF increases by ~3.5% for a standard CdTe device having efficiency of ~13%. The devices tested at STC show a relative increase in the power conversion efficiency in the range of 5% to 9%.