Chen Weijie

Large‐area perovskite films are deposited by a scalable blade coating method on flexible glass substrates at room temperature and in an ambient environment. Additive engineering by ammonium chloride effectively controls the perovskite crystallization and improves film quality. The flexible perovskite module achieves a record efficiency of 15.86% on a large aperture area of 42.9 cm².

Abstract

Perovskite materials are good candidates for flexible photovoltaic applications due to their strong absorption and low‐temperature processing, but efficient flexible perovskite modules have not yet been realized. Here, a record efficiency flexible perovskite solar module is demonstrated by blade coating high‐quality perovskite films on flexible Corning Willow Glass using additive engineering. Ammonium chloride (NH₄Cl) is added into the perovskite precursor solution to retard the nucleation which prevents voids formation at the interface of perovskite and glass. The addition of NH₄Cl also suppresses the formation of PbI₂ and reduces the trap density in the perovskite films. The implementation of NH₄Cl enables the fabrication of single junction flexible perovskite solar devices with an efficiency of 19.72% on small‐area cells and a record aperture efficiency of 15.86% on modules with an area of 42.9 cm². This work provides a simple way to scale up high‐efficiency flexible perovskite modules for various applications.

In this review, the recent developments in understanding the structural evolution of halide perovskites during both formation and degradation using in situ and operando X‐ray scattering techniques are discussed. The benefit of this experimental approach in contrast to ex situ characterization is highlighted and emphasis is laid on the encouraging progress made in terms of upscaling and stability.

Abstract

This review provides an update on the progress in understanding formation and degradation mechanisms in halide perovskites for photovoltaic applications, as supported by in situ and operando X‐ray scattering techniques. The value of these real‐time analyses is particularly high for gaining insights into the structural evolution during crystal formation and decomposition upon exposure to external stress factors. This type of analysis reveals the pathways between starting and end points of a process rather than being limited to comparing states before and after the process. Special attention is put on the successful efforts toward upscaling including deposition techniques that are compatible to roll‐to‐roll processing. These processes are realized using fast annealing procedures. The development of these processes strongly benefited from in situ studies exploring the direct transition from precursor to perovskite without going through observable crystalline intermediate phases. A particular focus of this review is the benefit of using in situ and operando X‐ray scattering techniques to better understand and ultimately improve device stability. The difference between structural stability of thin films and structural stability under device operation is highlighted, convincingly demonstrating the indispensability of operando studies.

Causes of intrinsic and extrinsic instability of perovskite materials and related mechanisms are discussed in terms of their chemical‐bonding nature. Understanding the critical mechanisms rationalizes the chemical approaches to mitigate the degradation in perovskite solar cells.

Abstract

Chemical bonding dictates not only the optoelectronic properties of materials, but also the intrinsic and extrinsic stability of materials. Here, the causes of intrinsic and extrinsic instability of perovskite materials are reviewed considering their correlation with the unique chemical‐bonding nature of perovskite materials. There are a number of key standardized stability tests established by the International Electrotechnical Commission for commercialized photovoltaic modules. Based on these procedures, the possible causes and related mechanisms of the material degradation that can arise during the test procedures are identified, which are discussed in terms of their chemical bonds. Based on the understanding of the critical causes, promising strategies for mitigating the causes to enhance the stability of perovskite solar cells are summarized. The stability of the state‐of‐the‐art perovskite solar cells implies a need for the development of improved stability‐testing protocols to move onto the next stage toward commercialization.

Despite the record efficiency exceeding 25%, the long‐term operational stability of perovskite solar cells is limited by the degradation mechanisms accelerated by the presence of vacancies and defects. In this review, recent engineering strategies ranging from grains to interfaces that mitigate degradation and improve efficiencies are discussed.

Abstract

Lead halide perovskite solar cells now show excellent efficiencies and encouraging levels of stability. Further improvements in performance require better control of the trap states which are considered to be associated with vacancies and defects at crystallite surfaces. Herein, a reflection on the ways in which these traps can be mitigated is presented by improving the quality of the perovskite layer and interfaces in fully assembled device configurations. In this review, the most recent design strategies reported in the literature, which have been explored to tune grain orientation, to passivate defects, and to improve charge‐carrier lifetimes, are presented. Specifically, the advances made with single‐cation, mixed‐cation and/or mixed‐halide, and 3D/2D bilayer‐based light absorbers are discussed. The interfacial, compositional, and band alignment engineering along with their consequent effects on the open‐circuit voltage, power conversion efficiency, and stability are a particular focus.

A rapid layer‐specific annealing on perovskite active layers enabled by UV LED is developed, and efficiency close to 19% in a simple planar inverted structure of ITO/PEDOT:PSS/MAPbI₃/PC₇₁BM/Al without any device engineering is demonstrated. The results demonstrate that if the UV dosage is well managed, UV light is capable of annealing perovskite into high‐quality film rather than simply damaging it.

Abstract

A rapid layer‐specific annealing on perovskite active layer enabled by ultraviolet (UV) light‐emitting diode (LED) is demonstrated and efficiency close to 19% is achieved in a simple planar inverted structure ITO/PEDOT:PSS/MAPbI₃/PC₇₁BM/Al without any device engineering. These results demonstrate that if the UV dosage is well managed, UV light is capable of annealing perovskite into high‐quality film rather than simply damaging it. Different in principle from other photonic treatment techniques that can heat up and damage underlying films, the UV‐LED‐annealing method enables layer‐specific annealing because LED light source is able to provide a specific UV wavelength for maximum light absorption of target film. Moreover, the layer‐specific photonic treatment allows accurate estimation of the crystallization energy required to form perovskite film at device quality level.

The photochemistry of halide‐related defects affects the optoelectronic properties of lead–halide perovskite semiconductors and their reactivity to external stimuli such as light and environmental molecules.

Abstract

The presence of various types of chemical interactions in metal‐halide perovskite semiconductors gives them a characteristic “soft” fluctuating structure, prone to a wide set of defects. Understanding of the nature of defects and their photochemistry is summarized, which leverages the cooperative action of density functional theory investigations and accurate experimental design. This knowledge is used to describe how defect activity determines the macroscopic properties of the material and related devices. Finally, a discussion of the open questions provides a path towards achieving an educated prediction of device operation, necessary to engineer reliable devices.

The latest breakthroughs in interface and defect engineering as applied to metal halide perovskite solar cells and light‐emitting diodes (LEDs) are reviewed in order to shed light on their necessity and importance in tuning the optoelectronic properties of devices in an attempt to realize the best‐performing solar cells and LEDs.

Abstract

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge‐carrier diffusion lengths, and high defect tolerance. Perovskite‐based solar cells and light‐emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect‐free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.

Although high power conversion efficiency of up to 23.3% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Nonradiative recombination and charge back transfer at interfaces are mainly responsible for conversion loss. Interface engineering is the most important approach toward the theoretical efficiency in PSCs.

Abstract

Organic–inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Obviously, trap‐assisted nonradiative (also called Shockley–Read–Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post‐treatment are explored. The focus herein is on the recombination at perovskite/electron‐transporting material and perovskite/hole‐transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap‐assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Rapid advancement of perovskite solar cells confronts the challenges of reliable measurement, which is important for data analysis and results reproduction. Major measurement methods and the key factors affecting evaluation are summarized. A measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Abstract

Perovskite solar cells (PSCs) have undergone an incredibly fast development and attracted intense attention worldwide owing to their high efficiency and low‐cost fabrication. However, it is challenging to make a reliable measurement of PSCs, which creates great difficulty for researchers to compare and reproduce published results. Herein, the major measurement methods and key factors affecting evaluation of PSCs are summarized, such as hysteresis in current–voltage measurement, calibration of solar simulators for less mismatch in spectra and light intensity, and the area for the calculation of current density and power conversion efficiency. PSCs are also compared with n–i–p or p–i–n structures that exhibit different feedback under the same measurement methods. Finally, a measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Low‐toxicity tin‐based perovskites have excellent optical and electrical properties, and are a good alternative for lead‐based perovskites. However, the performance and stability of tin‐based perovskites are not comparable. The properties of tin‐based perovskite films and the performance of tin‐based perovskite solar cells are reviewed. The current challenges and a future outlook for Sn‐based perovskites are discussed.

Abstract

The tremendous interest focused on organic–inorganic halide perovskites since 2012 derives from their unique optical and electrical properties, which make them excellent photovoltaic materials. Pb‐based halide perovskite solar cells, in particular, currently stand at a record efficiency of ≈23%, fulfilling their potential toward commercialization. However, because of the toxicity concerns of Pb‐based perovskite solar cells, their market prospects are hindered. In principle, Pb can be replaced with other less‐toxic, environmentally benign metals. Sn‐based perovskites are thus the far most promising alternative due to their very similar and perhaps even superior semiconductor characteristics. After years of effort invested in Sn‐based halide perovskites, sufficient breakthroughs have finally been achieved that make them the next runners up to the Pb halide perovskites. To help the reader better understand the nature of Sn‐based halide perovskites, their optical and electrical properties are systematically discussed. Recent progress in Sn‐based perovskite solar cells, focusing mainly on film fabrication methods and different device architectures, and highlighting roadblocks to progress and opportunities for future work are reviewed. Finally, a brief overview of mixed Sn/Pb‐based systems with their anomalous yet beneficial optical trends are discussed. The current challenges and a future outlook for Sn‐based perovskites are discussed.

The high‐performing single‐junction organic solar cell blend, PM6:Y6, is examined to obtain an in‐depth understanding of the voltage losses, and charge recombination and extraction dynamics. The devices exhibit remarkable extraction coupled with moderate recombination losses. This behavior can most likely be credited to a beneficial morphology as evidenced by atomically resolved ¹⁹F magic‐angle‐spinning solid‐state NMR analysis.

Abstract

The highly efficient single‐junction bulk‐heterojunction (BHJ) PM6:Y6 system can achieve high open‐circuit voltages (V _OC) while maintaining exceptional fill‐factor (FF) and short‐circuit current (J _SC) values. With a low energetic offset, the blend system is found to exhibit radiative and non‐radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies reveal that the device shows moderate non‐geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques are employed to understand the phase separation, long‐range ordering, as well as donor:acceptor (D:A) inter‐ and intramolecular interactions at an atomic‐level resolution. This is achieved using photo‐conductive atomic force microscopy, grazing‐incidence wide‐angle X‐ray scattering, and solid‐state ¹⁹F magic‐angle‐spinning NMR spectroscopy. The synergy of multifaceted characterization and device physics is used to uncover key insights, for the first time, on the structure–property relationships of this high‐performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing reveals that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high J _SC and FF to be retained despite the low energetic offset.

The in situ reduction of parasitic Sn⁴⁺ to Sn²⁺ by metallic tin powder effectively reduces Sn⁴⁺ content and thereby decreases the trap density of the perovskite films, giving rise to a remarkably long charge carrier lifetime and favorable energy‐level alignment at the interfaces. Consequently, a high power conversion efficiency of 20.7% is achieved for low‐bandgap Pb–Sn‐alloyed perovskite solar cells.

Although the theoretical power conversion efficiency (PCE) of low‐bandgap Pb–Sn‐alloyed perovskite solar cells (PSCs) is higher than that of its conventional pure Pb counterpart, its device performance currently has been severely restricted by the large open‐circuit voltage (V _oc) loss. Herein, it is discovered that the Sn⁴⁺‐induced trap states of the perovskite film can be effectively suppressed by introducing excess Sn powder into the precursor solution (FASnI₃) to reduce the Sn⁴⁺ content. As a result, the average charge carrier lifetime of the perovskite film increases remarkably from 115 to 701 ns due to the suppressed nonradiative recombination, and the energy levels have up‐shifted by about 0.27 eV, rendering a more favorable energy‐level alignment at the interface. Ultimately, the champion PSCs using a low‐bandgap (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite film with Sn⁴⁺ reduction show a high V _oc of 0.843 V corresponding to a V _oc loss as low as 0.397 eV and a high fill factor of 80.34%, leading to an impressive PCE of 20.7%, which is one of the few instances of a PCE over 20% for low‐bandgap mixed Pb–Sn PSCs to date.

Organic amine cation, GA⁺ is intentionally incorporated in MA_0.7FA_0.3PbI₃ perovskite to stiffen the inorganic Pb–I lattice, restrain the formation of iodine vacancies defects, and reduce ion diffusion. Solar cells based on this component engineering and PFN‐Br interfacial strategy demonstrate an enhanced power conversion efficiency value over 21% for SnO₂‐based planar perovskite solar cells and excellent thermal stability.

Tin oxide (SnO₂) offers its advantages in widespread applications that require efficient carrier transport. However, the usages of SnO₂ in organic solar cells are hindered because of dangling bonds on the surface of SnO₂. Herein, PFN‐Br as an interfacial layer to tailor the work function of SnO₂ is adopted, making it an ideal candidate for interfacial material in organic electronics. Meanwhile, such an efficient SnO₂/PFN‐Br electron transport layer (ETL) can also be applied to perovskite devices and achieve competitive efficiency. To eliminate current–voltage hysteresis and improve poor thermodynamic stability of perovskite solar cells (PSCs), 5 mol% of guanidinium iodide (GAI) into the (MA) _x (FA)_1 − x PbI₃ precursor solution is incorporated, enabling the formation of triple‐cation perovskite films with excellent optoelectronic quality and stability. The combination of an SnO₂/PFN‐Br ETL and GAI doping strategy finally delivers power conversion efficiencies over 21% and negligible current–voltage hysteresis in planar PSCs. These improvements arise from the strong hydrogen bonding caused by the incorporation of GA⁺. It can stiffen the inorganic Pb–I lattice of the unit cell and restrain the formation of iodine vacancies defects. Moreover, the strong hydrogen bonding can immobilize iodide ion and thus enhance the thermal stability of the corresponding device.

The efficiency and stability of 2D perovskite solar cells are synergistically improved through metal ion doping. The hole extraction and transport abilities are significantly enhanced by Cu ion doping in the NiO _x layers, while the optoelectronic properties of the BA₂MA₃Pb₄I₁₃ (BA = butylamine; MA = methylammonium) layers are effectively improved with Cs ion doping.

Abstract

2D perovskites hold a great prospective to create highly efficient and stable solar cell devices. In order to explore their full potential, every component of 2D perovskite solar cells (PSCs) has to be carefully designed and engineered. Herein, the metal ion doping strategy is taken to optimize both the hole transport layers (HTLs) and the light absorbing layers of the BA₂MA₃Pb₄I₁₃ (BA = butylamine; MA = methylammonium) based 2D PSC devices. The hole extraction and transport abilities are significantly enhanced by Cu ion doping in the nickel oxide layers, while the optoelectronic properties of the BA₂MA₃Pb₄I₁₃ layers are effectively improved with Cs ion doping. The synergistic incorporations of Cu and Cs ions have boosted the device power conversion efficiency to 13.92%, the highest for 2D PSCs based on inorganic HTLs. In addition, the inorganic nature of the Cu doped nickel oxide film and the high quality of the Cs doped 2D perovskite film also endow the PSC device with extraordinary humidity and thermal stabilities.

A postdeposition treatment is designed to modify the photoactive layer of perovskite solar cell (PSC) by spin‐coating p‐aminobenzoic acid (PABA). The PABA treatment can enhance V _OC, fill factor, and power conversion efficiency. The performance improvement is attributed to the suppression of carrier trap states. PABA post‐treatment provides a promising strategy and potential option for high performance solar cells.

Abstract

Various approaches of interface engineering are shown to be effective in improving the device performance of organic–inorganic hybrid perovskite solar cells (PSCs). The modification of the photoactive layer of PSC, CH₃NH₃PbI₃ (MAPbI₃), by spin‐coating a layer of p‐aminobenzoic acid (PABA), which can significantly enhance the open‐circuit voltage (V _OC), the fill factor (FF), and the power conversion efficiency (PCE) of PSCs, is herein reported. The champion device shows a short‐circuit current (J _SC) of 22.83 mA cm⁻², V _OC of 1.167 V, FF of 0.768, and PCE of 20.47%. The improvement in photovoltaic performance is attributed to the suppression of carrier trap states and the improvement in the morphologies of perovskite films. This work demonstrates a simple and effective protocol to enhance the device performance, and provides an insight into the influence of PABA post‐treatment on the charge carrier dynamics.

A compact TiO₂/WO₃ bilayer film is fabricated as electron transport layer (ETL) in perovskite solar cells. Compared to the single WO₃ layer, the bilayer efficiently covers the fluorine‐doped tin oxide (FTO), avoids the direct contact between perovskite and FTO, decreases the risk of recombination. Finally, the bilayer ETL based device achieves a superior power conversion efficiency of 20.14%.

Abstract

It is crucial to retard the carrier recombination and minimize the energy loss at the transparent electrode/electron transport layer (ETL)/perovskite absorber interfaces to improve the performance of the perovskite solar cells (PSCs). Here, a bilayered TiO₂/WO₃ film is designed as ETL by combining atomic layer deposition (ALD) technology and spin‐coating process. The ALD‐TiO₂ underlayer fills the fluorine‐doped tin oxide (FTO) valleys and makes the surface smoother, which effectively avoids the shunt pathways between perovskite layer and FTO substrate and thereby suppresses electron–hole recombination at the interface. Moreover, the presence of hydrophilic TiO₂ underlayer is helpful in forming a uniform and compact WO₃ layer which is beneficial for extracting electron from perovskite to ETL. Meanwhile, the lower valance band minimum level of TiO₂ relative to WO₃ can efficiently enhance the hole‐blocking ability. By employing the optimized TiO₂ (7 nm)/WO₃ bilayer as ETL, the resulting cell exhibits an obviously enhanced power conversion efficiency of up to 20.14%, which is much better than the single WO₃ or TiO₂ ETL based device. This work is expected to provide a viable path to design ultrathin and compact ETL for efficient PSCs.

Among several solution process photovoltaics, perovskite solar cells are evolving at an impressive pace, emerging as the most promising next‐generation photovoltaic devices. Herein, the recent developments in laser technology applicable to perovskite‐based solar devices, technological and process aspects, and an outlook on future applications are reported.

In the last decade, hybrid organic–inorganic perovskite‐based solar cells (PSCs) have shown an impressive rate of growth in performance, reaching power conversion efficiencies (PCEs) comparable with the ones exhibited by crystalline silicon devices. Recently, perovskite‐based solar modules (PSMs) have been developed, showing a similar pace in the progress of the reported PCE. Nevertheless, scaling up the dimensions of devices is not a trivial process. To this effect, different deposition and manufacturing techniques have to be implemented. Laser apparatuses have been demonstrated to be fundamental in the production of PSMs, due to the extreme precision needed for manufacturing processes. Herein, an overview of the recent progresses in the application of laser systems in the production of perovskite‐based solar devices is provided. In particular, lasers are used in small‐area PSCs to realize pulsed laser deposition procedures for the realization of perovskite layers and novel electrodes. In the field of PSMs, lasers have boosted the exploitation of substrates, minimizing the dimension of interconnection areas between the cells that form a module and providing the necessary accuracy, repeatability, and level of automation needed for the future industrialization of perovskite‐based solar technology.

Research on compositional engineering can realize power conversion efficiency (PCE) over 25%. Interfacial engineering along with optimal perovskite solar cell device structure is expected to lead to stable and theoretical PCE over 30%.

Abstract

Discovery of the 9.7% efficiency, 500 h stable solid‐state perovskite solar cell (PSC) in 2012 triggered off a wave of perovskite photovoltaics. As a result, a certified power conversion efficiency (PCE) of 25.2% was recorded in 2019. Publications on PSCs have increased exponentially since 2012 and the total number of publications reached over 13 200 as of August 2019. PCE has improved by developing device structures from mesoscopic sensitization to planar p‐i‐n (or n‐i‐p) junction and by changing composition from MAPbI₃ to FAPbI₃‐based mixed cations and/or mixed anion perovskites. Long‐term stability has been significantly improved by interfacial engineering with hydrophobic materials or the 2D/3D concept. Although small area cells exhibit superb efficiency, scale‐up technology is required toward commercialization. In this review, research direction toward large‐area, stable, high efficiency PSCs is emphasized. For large‐area perovskite coating, a precursor solution is equally important as coating methods. Precursor engineering and formulation of the precursor solution are described. For hysteresis‐less, stable, and higher efficiency PSCs, interfacial engineering is one of the best ways as defects can be effectively passivated and thereby nonradiative recombination is efficiently reduced. Methodologies are introduced to minimize interfacial and grain boundary recombination.

Recent progress in first‐principles simulations of carrier recombination in halide perovskites is reviewed. Misunderstandings relating to the impact of the Rashba effect on radiative recombination are clarified. The origin of exceptionally strong Auger recombination and avenues for improved materials design are discussed. Critical analysis of the recombination mechanisms reveals fruitful directions for improving the performance of halide perovskites.

Abstract

In recent years, there have been remarkable developments in halide perovskites, which are used in highly efficient optoelectronic devices and exhibit intriguing materials physics. Detailed knowledge of carrier recombination mechanisms is essential for understanding their excellent performance and to further increase their efficiency. Obtaining such knowledge is challenging however, and different studies have reached divergent conclusions in some cases. This progress report outlines the critical developments in understanding the carrier recombination mechanisms in halide perovskites from a computational perspective. The primary focus is radiative and Auger recombination, since they have not been systematically assessed and discussed before, and a number of important issues have been actively debated. This comprehensive discussion of the carrier recombination mechanisms is aimed at establishing physically justified insights that can form the basis for better materials and devices design.

Publication date: Available online 26 November 2019

Source: Nano Energy

Author(s): Chong Chen, Fumin Li, Liangxin Zhu, Zhitao Shen, Yujuan Weng, Qiang Lou, Furui Tan, Gentian Yue, Qingsong Huang, Mingtai Wang

Graphical abstract

Device structure and charge separation diagram.

A method is presented to spatially resolve the space charge region in organic photovoltaics (OPVs) by measuring white‐light bias external quantum efficiency and optical modeling. A method is demonstrated for a model polymer/fullerene blend with imbalanced carrier mobilities. Furthermore, numerical and analytical means are derived to discuss the general thickness limits for OPVs with imbalanced transport.

Space charge effects can significantly degrade charge collection in organic photovoltaics (OPVs), especially in thick‐film devices. The two main causes of space charge are doping and imbalanced transport. Although these are completely different phenomena, they lead to the same voltage dependence of the photocurrent, making them difficult to distinguish. Herein, a method is introduced on how the build‐up of space charge due to imbalanced transport can be monitored in a real operating organic solar cell. The method is based on the reconstruction of quantum efficiency spectra and requires only optical input parameters that are straightforward to measure. This makes it suitable for the screening of new OPV materials. Furthermore, numerical and analytical means are derived to predict the impact of imbalanced transport on charge collection. It is shown that when charge recombination is sufficiently reduced, balanced transport is not a necessary condition for efficient thick‐film OPVs.