Mitigating voltage losses in photoelectrochemical cell scale-up

Oxygen (O2) and hydrogen (H2) gas bubbles are the desired products from photoelectrochemical water splitting, but they are also a common source of efficiency losses in photoelectrochemical cells (PECs) that are poorly understood and often difficult to quantify. When attached to the surface of a photoelectrode, a gas bubble can induce optical, kinetic, ohmic, and mass-transport losses. The operating conditions and dynamic behavior that underlie these bubble-induced losses are complex and convoluted, but understanding these dynamics would be invaluable for selecting operating conditions and engineering photoelectrode surfaces to minimize those losses. Toward this end, we employ in situ scanning photocurrent microscopy (SPCM) to investigate the local photocurrent losses associated with isolated H2 bubbles attached to the surface of a photoelectrode. For the first time, we are able to quantify and resolve local bubble-induced photocurrent losses on a photoelectrode at the sub-bubble level. As a basis for this work, Si-based metal–insulator–semiconductor (MIS) photocathodes containing an ultrathin, continuous metal layer are studied. SPCM measurements, combined with optical modeling based on Snell’s law, are used to elucidate relationships between local photocurrent losses and bubble size. These investigations reveal increases in optical losses at larger bubble sizes (radius >100 μm) due to the photoelectrode geometry and the total internal reflection of light from the edge regions of bubbles. Finally, we apply the knowledge gained from single-bubble SPCM measurements to model the “sawtooth” current–time profile associated with dozens of bubbles evolving from a photoelectrode surface under photolimited operation and uniform AM1.5 illumination.

Solar water splitting has the potential to significantly contribute to future greenhouse-gas-free fuels. In photoelectrochemical water splitting, the solar absorber is in direct contact with the aqueous electrolyte. This enables a higher level of device integration, but it also creates additional challenges when compared to decoupled photovoltaics-electrolysis approaches. Here, we report on the design of a photoelectrochemical Schlenk cell processing routine that allows photoelectrochemical interface functionalization under controlled conditions. Interface inhomogeneities that accelerate corrosion are hereby reduced, while the photoelectrochemical window layer from the functionalization reduces voltage and light absorption losses for the photoelectrode. Applied to a dual-junction GaInP/GaAs photoelectrode, we demonstrate solar-to-hydrogen efficiencies of up to 18.8% on increased electrode areas and longer unbiased electrode lifetimes compared to similar previous work. This general processing approach should be transferable to other systems where photoelectrochemical functionalization with optical in situ control is desirable.

The large voltage losses usually encountered in organic solar cells significantly limit the power conversion efficiencies (PCEs) of these devices, with the result that the current highest PCE values in single‐junction organic photovoltaic remain smaller than for other solar cell technologies, such as crystalline silicon or perovskite solar cells. In particular, the nonradiative recombinations to the electronic ground state from the lowest‐energy charge‐transfer (CT) states at the donor–acceptor interfaces in the active layer of organic devices, are responsible for a significant part of the voltage losses. Here, to better comprehend the nonradiative voltage loss mechanisms, a fully quantum‐mechanical rate formula is employed within the framework of time‐dependent perturbation theory, combined with density functional theory. The objective is to uncover the specific contributions of intramolecular vibrations to the CT‐state nonradiative recombinations in several model systems, which include small‐molecule and polymer donors as well as fullerene and nonfullerene acceptors.

W e would like to start off by thanking the authors of the comment, Wetzelaer and Blom, for their very helpful and constructive analysis. They present an interesting alternative view on an important and timely research topic. Before discussing the mechanism suggested in their comment, from the viewpoint of our experimental results, we would like to summarize our findings. In our work, 1 we experimentally showed that • The contact between donor molecules in the active layer of organic solar cells (OSCs) and a molybdenum oxide (MoO 3) hole extraction layer (HEL) causes an increase in nonradiative recombination losses, proportional to the extent of contacts. • We attributed these losses to surface recombination, and we proved that the losses can be suppressed by inserting a thin interfacial fullerene layer at the anode side. • Analyzing various donor−acceptor mixing ratios, with and without a fullerene-modified HEL, we decoupled and quantified the contribution from surface recombi-nation on the total nonradiative losses occurring in these devices. In the best case, we showed an improvement of 150 meV in V OC , as compared to the reference device. This demonstrates that surface recombination is a considerable contributor to nonradiative voltage losses in these solar cells, which are otherwise commonly occurring through charge-transfer states or energetic trap states because of defects in the bulk. We consider this the main result of our work. • Measurements by a modified charge extraction by linearly increasing voltage (CELIV) technique provided evidence that the improvement in V OC could be attributed to an enhanced built-in potential (V bi), reducing the presence of minority charge carriers at the respective electrodes. Although the authors of the comment in general agree with our experimental findings, they argue that the V bi does not play a direct role in suppressing the surface recombination of minority carriers. The introduction of a C 60 interlayer, they argue, renders the MoO 3 contact ohmic. 2 The reduced anodic injection barrier simultaneously increases the V bi , minimizes nonradiative voltage losses upon the extraction of majority carriers (holes), and suppresses minority-carrier (electron) surface recombination, the latter being the result of hole accumulation and associated band bending near the ohmic hole contact. Therefore, the ohmic contact formation suppresses both majority-and minority-carrier surface recombination losses, whereas the built-in voltage per se, they reason, does not play a major role. It is our opinion that the authors of the comment provide a very reasonable alternative explanation for the reduced surface recombination. Injection barriers at the contacts are well-known to be detrimental for the performance of OSC as being a major cause of a reduced V bi and increased surface recombination. Injection barriers have been shown to reduce not only the V OC but also the fill factor (FF), sometimes even leading to s-shaped JV curves. 3−5 It has been suggested that, in the case of very large injection barriers, the V OC is given by V bi. 6,7 Although V bi is determined by the difference in the work functions of the contacts, because of Fermi level pinning and the associated band bending, the built-in potential across the active layer, the effective V bi , typically cannot exceed the effective gap of the bulk-heterojunction blend. In other words, the anode Fermi level pins to the highest occupied molecular orbital (HOMO) of the donor, whereas the cathode pins to the lowest unoccupied molecular orbital (LUMO) of the acceptor. In addition, because of disorder, Fermi level pinning typically occurs to discrete gap or tail states, causing additional band bending, which further limits the effective V bi. 8,9 If there is an injection barrier at one contact, an increase or decrease in this barrier is directly reflected in the V bi. 5

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

In this work, deep convolutional neural networks (CNNs) are used to speed up the calculation of spatial nonuniform device parameters, voltage and fill factor (FF) losses in crystalline Si solar cells. Luminescence images on finished solar cells, specifically as‐measured electroluminescence (EL) and photoluminescence (PL) images, are used as input features to train the CNN models and losses analysis as output is from a finite‐element modeling program called Griddler. From the analysis of a small dataset of 250 commercial grade solar cells, the CNN models are able to predict the spatially nonuniform distribution of contact resistance and defect parameters of the devices under test and exhibit good performance in the inference of voltage and FF losses. As EL and PL images are widely collected in‐line production data, in the result, the possibility of deploying 2D spatial power loss analysis is implied as a fast, in‐line, and nondestructive analytic tool for solar cells manufacturing.

Impact of side-chain engineering on quantum efficiency and voltage losses in organic solar cells

Dark current density-voltage measurements were performed in a temperature range 110–290 K in both forward and reverse bias on a 0.5 cm2 solar cell sample to determine current and voltage losses in a CIGS solar cell. For the first time a JD − V model applicable in a wide temperature range was constructed by adding non-ideality terms to the Shockley diode equation and comparing the experimentally acquired data to the model simulation results. The shunt current was modeled assuming space charge limited current by a uniform distribution of trap levels. A total of only 9 adjustable parameters in the final JD − V model sufficed to simulate all experimental data simultaneously, i.e., no re-estimation had to be performed at different temperature levels. The estimates for the diode ideality factor n = 1.8 and the diode activation energy ΔE = 1.20 eV coincide with values typically obtained for CIGS solar cell dominated by recombination in the absorber. The quality of the CIGS solar cell used in this study was also confirmed by the low back contact activation barrier of 52 meV and the high shunt resistance. We demonstrate that this flexible model can be used to make an initial prediction of the resistive losses and non-idealities in thin film solar cells.

Due to the small dielectric constant of pristine conjugated polymers or small molecules, the photogenerated electron-hole pairs (excitons) are strongly bound because of Coulombic attraction, typically 0.2–0.5 eV (Brédas et al., 1996; Alvarado et al., 1998; and Arkhipov and Bässler, 2004) and are unable to dissociate automatically. The most effective way to solve the issue is to introduce another component possessing electron affinity into the photo-active film. The intermixing of electron donor (D) and electron acceptor (A) materials form the type II heterojunctions, where both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the electron donor are higher than the electron acceptor. In particular, new interfacial electronic states between donor and acceptor are formed, called charge transfer (CT) states, where interesting optoelectronic processes, such as charge separation and recombination, take place (Benson-Smith et al., 2007).

Organic solar cells (OSCs) are a next-generation photovoltaic technology that convert solar energy to electrical energy. They have attracted great attention due to their advantages of low cost, ease of synthesis, light weight, mechanical flexibility, and roll-to-roll processability. In the past decades, owing to the development of the materials, device optimization and the understanding of the working mechanism, the power conversion efficiency (PCE) has been boosted to ~19%. However, the efficiency of the OSCs is still not comparable to the conventional inorganic solar cells and emerging perovskite solar cells due to the large open-circuit voltage loss (Vloss). In addition, it is also important to obtain efficient charge generation while reducing the Vloss. Thus, understanding the loss mechanisms in the OSCs is significant for achieving further improvement.

Nonradiative losses to the open-circuit voltage are a primary factor in limiting the power-conversion efficiency of organic photovoltaic devices. The dominate nonradiative loss in the bulk is intrinsic to the active layer and can be determined from the quasi-Fermi-level splitting (QFLS) and the radiative thermodynamic limit of the photovoltage. Quantification of the QFLS in thin-film devices with low mobility is challenging due to the excitonic nature of photoexcitation and additional sources of nonradiative loss associated with the device structure. This work outlines an experimental approach based on electromodulated photoluminescence, which can be used to directly measure the intrinsic nonradiative loss to the open-circuit voltage, thereby quantifying the QFLS. Drift-diffusion simulations are carried out to show that this method accurately predicts the QFLS in the bulk of the device regardless of device-related nonradiative losses. State-of-the-art PM6:Y6-based organic solar cells are used as a model to test the experimental approach and the QFLS is quantified and shown to be independent of device architecture. This work provides a method to quantify the QFLS of organic solar cells under operational conditions, fully characterizing the different contributions to the nonradiative losses of the open-circuit voltage. The reported method will be useful not only in characterizing and understanding losses in organic solar cells but also in other device platforms such as light-emitting diodes and photodetectors.

Effects of Aluminum in Metallization Paste on the Electrical Losses in Bifacial N-type Crystalline Silicon Solar Cells

Multiple-cation lead mixed-halide perovskites (MLMPs) with tunable band gaps have been demonstrated as ideal candidates to achieve perovskite solar cells with high efficiencies. It is well-known that a large open-circuit voltage (VOC) loss caused by nonradiative recombination still limits the approach to the Shockley-Queisser limit. However, there are few comprehensive contributions regarding the origin and pathway of nonradiative recombination in n-i-p structured MLMPs. Here, we compare the performance of MLMPs containing different halides and analyze the energy loss and interface trap-assisted nonradiative recombination characterizations. It is found that Br-containing devices with a lower interface trap density of 3.2 × 1013 cm-2 obtain a high VOC of 1.12 V, a small energy loss of 0.02 eV, radiative recombination current density of 8.05 × 10-21 A m-2, and total recombination current density of 22.16 mA cm-2. This work provides an opportunity to understand the device physics and reveals the nature of nonradiative recombination based on experiment and simulation.

We investigate power losses in back-contact back-junction monocrystalline thin-film silicon solar cells fabricated using the porous silicon layer transfer process. Our loss analysis combines two-dimensional finite element modeling and resistance network simulations. The input parameters of the finite element modeling are determined experimentally by measuring saturation current densities and sheet resistances on test samples prepared identically to the solar cells. Characteristic solar cell parameters such as short circuit current, open circuit voltage, fill factor, and efficiency of measured and network simulated current voltage characteristics investigated in this study match within an uncertainty of 5%. Free energy loss analysis serves as comparison of all losses in units of power per area at the maximum power point. The largest loss is bulk recombination due to a carrier lifetime of 2 μs in the epitaxial Si layer. Further significant losses result from recombination at the base contacts characterized by a diode saturation current density of 50 000 fA cm−2 as well as resistive losses due to lateral majority carrier current flows within the solar cell base and contact resistance losses.

An 18 μm thin crystalline silicon solar cell was demonstrated, and its best open circuit voltage is 642.3 mV. However, this value is far from the cell’s theoretical upper limit in an ideal case. This paper explores the open circuit voltage losses of the thin silicon solar cell, starting from the ideal case, through first principle calculation and experiments. The open circuit voltage losses come from the introduced recombination due to the non-ideal surface passivation and contacts integration on front and rear surfaces, and edge isolation. This paper presents a roadmap of the open circuit voltage reduction from an ideal case of 767.0 mV to the best measured value of 642.3 mV.