Christelle Hinnewinkel
email: christelle.hinnewinkel@univ-lille1.fr

Sylvie Guillerme
email: guillerm@univ-tlse2.fr

Dr. Ajit Menon

Dr Siddartha Krishnan

Nitin Rai
email: nitinrai@atree.org

Eric Maire
email: maire@lmtg.obs-mip.fr

Claude Garcia
email: claude.garcia@cirad.fr

Dr Geography

Sociology

Diversity of rural forests in central Western Ghats (India): views from different stakeholders

Keywords: rural forest, domestic forest, stakeholders' representations, Western Ghats, India

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

meaning of the concept of forest in India. Adding the adjective "rural" will lead to a more nuanced and plural understanding of what forests mean to rural constituencies and what this means to understanding the forest structure and dynamics. This would open the way to debate on how to locate forest in rural system rather than autonomous objects. This will contribute to the discussion on participatory forest management. As Forest Departments are talking about decentralization and involving people, there is a need to understand what types of forests local people use and how they manage it.

With the Indian highest density of trees in rural area and a long history of human-forest interactions, the Western Ghats offers an interesting case study to analyse the diversity of "involving people forests". Today, this 160 000 km² of mountain is considered as one of the 25 biodiversity hotspots of the world (10 000-15 000 species of organisms and among them 40% are endemics)1 by scientists in ecology, and is the homeland of an important population (the population density is higher than 100 hab/km² and reach nearly 1500 hab/km² in Kerala). The combination of those two characteristics generates a debate about the functions and uses of the forests of Western Ghats.

The inhabitants of the Western Ghats, who settled there at different periods in the history, depend on the forest resources and have developed different ways to use the forest resources. In parallel, the implementation of decentralised and participatory forest management by the Forest Departments highlights the question of involving people in forest management. For forest officials "there are many terms for describing forestry activities involving rural community such as social forestry, community forestry, farm forestry, agroforestry, forestry for local community development, upland forestry, rural forestry, etc." (Vyas 2 , 2006). The variety of the terms used shows the multiple attempts of the Forest Departments to handle this question of involving people in forest management and the difficulty to name those forests.

Those two types of local people forest management (the prevalent local people uses and the "involving people forestry" by Forest Department) contribute to the rural economy and in this way constitute a "rural forest" which can take different shapes. The present typology of "rural forests" in the Western Ghats is based on field work in central Western Ghats, discussions with officials from Forest Departments in Tamil Nadu, Kerala and Karnataka and on expert views that emerged from a workshop of academics and practitioners (some work for wildlife protection and others for rural development's NGOs) organized in Coimbatore on 29 th and 30 th of October 2007. From this, emerged the problematic of what a "rural forest" is and what it should be, in other words "what are the local people uses" and what is considered as "a good involving people forest management". The idea of "rural forest" which questions the definition of local people forest management helps to distinguish forests according to their types of management: from forest managed by Forest Departments, to forests used by local people trough co-managed forests and forests managed by local people.

1.

To acknowledge local people forest management Throughout the world there appears a recent trend to acknowledge local people forest management which encompasses a large variety of forest types and practices. Two concepts have been formulated to address this issue: the "domestic forest" [START_REF] Michon | Domestic forests: a new paradigm for integrating local communities'forestry into tropical forest science[END_REF] which names all the forests shaped by local people uses and the "small-scale forestry" [START_REF] Pouchepadass | Non-industrial, smallholder, small-scale and family forestry: what's in a name? Small-scale forest economics[END_REF] which names all the non-industrial practices involved to manage the forests.

A forest used by local people

The concept of "domestic forest" broadly covers local forests, community forests, traditional forests, sacred forests, peasant forests, forest gardens, agroforests, intermediate systems, and all forests characterized by the ethnicity of their managers. "The term domestic forest aims at highlighting the close relationship the domestication process establishes between a specific human group, including its elementary units, i.e., the domestic units, and the forest, transformed and managed to fulfill the needs of that group. Domestic refers to both the process of domestication and the relation to a household or to a group of households" [START_REF] Michon | Domestic forests: a new paradigm for integrating local communities'forestry into tropical forest science[END_REF]. According to the definition "rural forests" are equivalent to domestic forests but this is questionable in the Indian forests context where more than 90% of the forests are owned by the State.

In Western Ghats, as the majority of the forest cover is owned by the State, the diversity of practices which shape the forests varies according to the ownership of the land (Table 1). The main stakeholders directly involved in those practices are the forest officials, the people living in or next to the forest, the environmental NGOs, the social activist NGOs and the forest contractors. Schematically, the roles are divided into two main domains of activities: protection and exploitation which are nowadays related to two different types of property: public and private. The main activities of the Forest Department are on state-owned lands. Since the 1988 National Forest Policy their role has been to protect the forest for ecological purposes. At the opposite, rural people are considered as exploiting the forest resource and these activities are restricted. Their main practices are normally limited to private lands where they manage private forests, plantations and agroforestry systems. But the roles of stakeholders are more integrated than this very simple typology. The Forest Departments used to implement also schemes for afforestation on revenue land (Social forestry). This is not anymore in force now but they implement afforestation schemes on private holdings (distribution of seedlings to farmers through Afforestation programme).

Rural people may also manage sacred groves on private lands as well as on forest lands. Since the 1990's, they have also been involved by the Forest Departments in the management of degraded forest located in public forest lands through Joint Forest Management programmes (JFM). People also gather firewood on all types of land and some Minor Forest Products on private land but also on RF areas given in lease to the company allowed to buy and sell those products, the LAMP societies. Among the rural people, some live inside the Reserved Forests where they grow paddy or millet and sometimes cultivate tea, coffee, and pepper in multicrop system with yam and other tubers.

A forest managed by local people

The concept of "small-scale forestry" focuses on the non-industrial character of the practices that shape the forest. This concept has been formulated in order to name the forestry practices which are acknowledged by the "trend to move away from industrial forestry towards landholder-based forest management and community forestry" (Harisson & al, 2002). Smallscale forestry covers private forestry, family forestry, farm forestry and community forestry which are practiced at a small-scale but there is no reference to firewood collection and minor forest products collection which are important activities for the local people living near the forests of Western Ghats. For instance, the ayurvedic industry, which mainly uses plants grown in forest, generates a trade of 35 milliards of Rupees in India [START_REF] Dejouhanet | Les produits forestiers non ligneux et la gestion de la forêt kéralaise : droit d'usage et droit de contrôle[END_REF] and gives a livelihood for many tribal people.

The confrontation of the concept of "small-scale forestry" with the Western Ghats context highlights the question of the meaning of forest management. Those practices of collection are sometimes not considered as a forestry practices or are considered as mismanagement. In India, many foresters still think that people have to be taught to manage properly the forests but those scientific forestry principles which they teach are in pass to change. With the contribution of the "small-scale forestry" concept, the scientific forestry can integrate the principles of the family, farmers or community forestry. A co-learning process between scientists and local people will give birth to new principles in forestry. This is the condition for a rich and creative local people forest management.

In Western Ghats, the current issue is to constitute a forest managed by local people. All the forests of the Western Ghats are domestic forests, they are shaped by current and deserted practices of local people and they contribute to the livelihood of the rural people even if many practices are considered illegal. The constitution of a vast state-owned forest territory has excluded local people from the making decision process of the management of those Recorded Forests3 . It has restricted or prohibited many of their practices, but nowadays this situation is questioned by the new decentralised and participatory forest approach. Nowadays the Forest Departments work on involving people in forest management, in other words to constitute a forest managed by local people which could be called a "rural forest".

Forest managed by the Forest Department for the requirements of local people

With the National Forest Policy of 1988, industrial forestry on state-owned forests has been deserted. In this context, the trees outside recorded forests have been regularly assessed since 1991 by Forest Survey of India (FSI, 2003). This tree cover outside state-owned forest areas, named Trees Outside Forest (TOF), is acknowledged to contribute significantly to the socioeconomic and ecological status of a country. For the FSI, TOF is of two types: blocks of forest cover bigger than 1 hectare and tree cover 4 . In India, the blocks represent only 3.4 % of the total area and the average tree cover is 12.3 trees/ha of land. Compare to other area in India, TOF is relatively important in Western Ghats: the density of trees is the highest in the country with 21.6 trees/ha and the blocks of forest represent 8.9 % of the area. This TOF is located on two types of land ownership: common lands and private holdings and is the result of different afforestation practices: "rural forestry" and also "farm forestry" or private forestry.

When extension forestry is conducted by Forest Department on common land in order to meet the needs of the rural population, the forest officials call it "rural forestry", i.e. forests regenerated or raised ex-situ where the species involved is seen to contribute to subsistence needs and income and where such regeneration helps meet the forest cover targets prescribed in the national forest policy, which is 33 % for plains and 66 for hills. It encompasses forestry activities on community and village lands, degraded forests, road and railway sides and canal banks for the benefit of rural people. It also includes restoration of derelict areas such as those bearing scars of quarrying, mining, road construction, brick manufacture, lime burning and the like [START_REF] Vyas | Community forestry[END_REF]. For the Forest Departments, those activities are similar to "farm forestry" but the afforestation schemes are implemented on common land and not on private holding as it is in "farm forestry". The management of those plantations is named "rural forestry" because the plantation is supposed to improve the local environment (reduce the erosion, control the water flow) and a part of the benefit of the sale of the timber is allotted to the District in order to maintain roads or water supply system. The benefit is shared with local people but they are not involved in the management of those plantations.

This "rural forestry" comes under the concept of "social forestry" which was first recognized as an important component of forestry for meeting rural needs in the report of the National Commission on Agriculture (NCA) of 1972. During the 1970's and 80's, the objectives of social forestry were discussed and the scope of social forestry was not only the afforestation on common lands but includes also farm forestry, community woodlots and reforestation in degraded lands. In the 1980's, the concept of social forestry was established as forestry "for the people, with the people and by the people". It was a precursor of "involving people" schemes of the Forest Department recommended by the National Forest Policy of 1988.

Co-managed forests

While forest blocks represent only 8.9 % of the total area of the Western Ghats, 45 % of the area of this eco-region is under state-owned forests (FSI, 2003). The Forest Departments involved local people in the management of limited part of this vast state-owned forests territory. As a major aim of the National Public Policy of 1988 is to increase the total forest cover of the country, an important part of the activities of the Forest Departments is afforestation on private or on state-owned lands.

Co-management on private land : farm forestry

Afforestation on private lands is whether a private forestry or a scheme of the Forest Departments (National Afforestation Programme, NAP). According to forest officials the plantation of trees on private land with the help of the Forest Department come under the category "involving people forestry" and is named "farm forestry" [START_REF] Vyas | Community forestry[END_REF].

In 2007, the Tamil Nadu Forest Department has launched a vast scheme of farm forestry, the Tree Cultivation in Private Land (TCPL). The local people will manage those plantations and are allowed to sell the timber but they were not consulted to select the species. In Western Ghats, the forest officials have chosen a fast growing species, the silver oak (Grevillea robusta). Seedlings were provided to farmers for planting them as intercrop in their fields (mainly as intercrop with tea). The co-management in this case takes the form of plantations initiated by the Forest Department and managed by the farmers. While the "rural forestry" can contribute to the requirements of all local people, "farm forestry" contributes to meet the requirements of only the landowners. Co-managed forests schemes on public land are more equitable in this sense.

Involving people in forest management of delimited part of the state-owned forests

For most of the forest officials only sacred groves, Reserved Forest leased for Minor Forest Products collection and forests under Joint Forest Management can be considered as rural forests. Even if they implement some schemes to involve people in the management of public forests, this remains on limited areas. For them the forest seems to be irreducibly a separate world which should be protected from the greed of people. The existence among forest officials of a minority in favour of involvement of local communities in conservation is as old in India as the Forest Department itself [START_REF] Guha | Dietrich Brandis et les forêts de l'Inde : la voie délaissée[END_REF]. Since the 1980's, the successful experiences of collective forest management have been carried out in many reserved forests, sometimes on those foresters' initiative [START_REF] Pouchepadass | Introduction[END_REF].

The collection of Minor Forest Products (MFP) or Non-Timber Forest Products (NTFP) is a source of employment, income and subsistence for many agricultural labourers in the Western Ghats. The Forest Departments give lease to collect MFP on reserved forests and the products are sold to specific society allowed to purchase MFP, the LAMPS (LArge MultiPurpose Society). The forestry sector provides 1.6 million of employment in NTFP activities and about 40 to 50 % of State forest revenues in India (Tewari, 1993[START_REF] Sudha | Community forest management and Joint Forest Management : an ecological, economic and institutional assessment in Western Ghats, India, Presented at "Crossing Boundaries[END_REF]. In the Nilgiri-Wayanad at the south fringe of Mudumalai Wildlife Sanctuary, the traditional herbalists among the Paniyan, Mountadan Chetty and other communities collect roots, rhizomes, barks, leaves, flowers, fruits, latex and seeds of medicinal plants growing in the moist deciduous forests of the area. Since 2000, Minor Forest Products 5 collection from protected forests has not been allowed by the Forest Department but as many tribal communities are agricultural labourers, they depend on those products for their livelihood and so still collect them though illegally [START_REF] Ghatak | Bio-diversity and sustainable development in a fringe village in Mudumalai Wildlife Sanctuary Nilgiri Biosphere Reserve -A case study[END_REF].

Recorded forests are not only given in lease by the Forest Departments for collection of forest products, they are also managed with the participation of people. The involving people management forestry is implemented mainly for the management of some sacred groves and of degraded forests under the Joint Forest Management (JFM).

Sacred forests are patches of forest that either belong to a temple or are said to shelter a god or spirit revered by the local inhabitants. The sacred forests are not a space left outside the village life. In Kodagu, the neighbouring villagers do enter into Devarakadus (sacred groves) to harvest forest products they cannot find elsewhere near their houses: firewood, small poles and timber, and minor forest products 6 . In December 2001, the Karnataka State has decided to 5 The main items of minor forest products are Lavangam bark, avaram bark, reeds, shikakal (Acacia officinalis), medicinal plants, honey, bee wax, nellikay (Phyllanthus emblica), gall nuts, wild pepper, wild ginger, wild turmeric, wild mango (Mangifera indica), lemon grass (Cymbopogon citratus), kullirma bark (Machilus macarantha), dammar (Shorea, Canarium, Vateria), camphor. 6 A great variety of minor forest products is collected from the sacred forests. Gums, resins (Canarium strictum), fruits (Mangifera indica, Artocarpus heterophyllus, Garcinia gummi-gutta, Sapindus laurifolius), wild roots, and medicinal plants (Fabaceae climbers) are extracted, and sometimes sold at the city market. Such production can supplement the household revenue. The nutrient-rich topsoil of the forests is sometimes removed and used for plant nurseries [START_REF] Garcia | Les forêts sacrées de Kodaguvaleur écologique, rôle sociale et implications pour la conservation de la biodiversité[END_REF].

implement Joint Forest Management Plan for the Devarakadus. Village committees in collaboration with the Karnataka Forest Department manage the sacred groves while the Forest Department is the owner. A Management Plan is prepared by the Committee and approved by the foresters. The members of the committee are the members of the traditional temple committee completed with several seats reserved for the women and the economically weak part of the population, scheduled castes (SC) and scheduled tribes (ST) [START_REF] Garcia | Les forêts sacrées de Kodaguvaleur écologique, rôle sociale et implications pour la conservation de la biodiversité[END_REF]. This participatory approach for forest management is not new and unique in India.

Several [START_REF] Lele | Joint forest planning and management in the eastern plains region of Karnatakaa rapid assessment[END_REF].

A huge investment in externally-assisted projects and through the allocation of central government funds defines JFM as a massive programme (Rao et al., 2004) and 26 % of the recorded forest areas are under JFM (Sudha et al., 2004). But the implementation of this programme is not easy and requires a major change in the mindsets of forest officers. Foresters find implementation of JFM difficult because they do not have sufficient knowledge and training on social systems and social skills for effective implementation of JFM. Many foresters do share their belief with the foresters of 1950s, that communities are heavily driven by their needs and degrade the forests. Further, foresters are unwilling to lose their authority by leaving behind their regulatory functions in favour of social functions [START_REF] Sood | Implications of Indian foresters' perspectives of Joint Forest Management[END_REF]. This negative attitude seems to be related also to the bureaucratic and hierarchical organizational structure of Forest Department, which provides limited freedom to field level staff. P. [START_REF] Rishi | Joint forest management in India: an attitudinal analysis of stakeholders[END_REF] suggests that continuous training programmes and supervision of senior officers can shaped the attitudes of forest officers. In order to build a mutual understanding between local people and foresters, forests managed by local people could be shown to the foresters.

Forests managed by local people

Local people can develop their own forest management on their own land and in some case on revenue land. While community forestry is considered as rural forest by many practitioners, the forestry on private land is denied to be a rural forest in the name of the principle of equity. But both forestry practices give a framework for the maintenance and creation of local knowledge related to forest and tree management.

Self-initiated community forestry on revenue land

A study of [START_REF] Sudha | Community forest management and Joint Forest Management : an ecological, economic and institutional assessment in Western Ghats, India, Presented at "Crossing Boundaries[END_REF] presents several types of community management systems that exist in Western Ghats. Among them, one community management system exists for about 100 years in Shimoga district in Karnataka, in order to manage forest patches on revenue land. In this area, people collect firewood, leaf litter as cattle bedding and manure in the crop fields but also medicinal plants, mushrooms, spices, bamboo shoots, berries for pickle mainly for self consumption in the surrounding forests. The Karnataka Forest Department is not involved in this self-initiated community forest management; foresters have only provided fund for fencing. A management committee exists and the mode of protection is based on the assignment of guards from a household and a fence has been erected around a portion of the forest. There is no restriction with regard to the collection of leaf litter, fruits, flowers, nuts, seeds, medicinal herbs etc. from the protected patch at any time of the year. However, people are not allowed to cut any green twigs, branches, leaves or bamboo shoots from the patch.

Currently the sharing of costs particularly the human effort for protection is equal, but the cost of human labour for a poor landless and garden owner is surely different. The sharing arrangements are equal for every household has equal but limited access to fuelwood and other NTFPs seems inequitable. First, garden owners have their own large biomass resources and the community forest is only a supplementary source. But for the landless and non-garden owning households with no private biomass resources, community forest is a critical source of biomass and also the cost of alternate sources is very high for the landless and marginal farmers. Secondly, banning of extraction of bamboo will deprive raw materials to the basket weavers and such a regulation may not affect the landed households [START_REF] Sudha | Community forest management and Joint Forest Management : an ecological, economic and institutional assessment in Western Ghats, India, Presented at "Crossing Boundaries[END_REF].

There are not many possibilities to develop rural forest on revenue land due to land scarcity. A field study in Nilgiri-Wayanad (Gudalur taluk in Tamil Nadu) revealed that the poromboke (land owned by the revenue department) represents 14 % of the total area. Even if it is possible to find some lands available, as the forest blocks on private lands occupy less than 8.9 % of the area of the Western Ghats, it is certain that the main area outside recorded forest available to meet the requirements of rural people is the agricultural land.

Management of trees on private holding: agroforestry and private forestry

In Nilgiri-Wayanad, the cultivated area occupies more than 50 % of the total area, mainly planted under agroforestry systems, from simple plantations of areca nut, coffee or tea to mixed agriculture with tea, coffee, areca nut, silver oak, pepper, yams and other tubers. Tea is the dominant crop. In the south and south-east of the area big plantations (more than 200 ha) are numerous but in between them small owners also grow tea. In the west of the area (near the Kerala border), the small farmers have developed agroforestry systems which are the consequences of complex cultural, legal and economic interstices. The farmers are predominantly Malayalam-speakers which explains the domesticity of forests. Economically the structure of these agroforestry systems i.e. tea and species cultivation interspersed with silver oak, areca nut and jack trees are, like in the neighbouring Kerala, production oriented in order to maximise income opportunities and minimise natural and market risks. Legally agroforestry systems are evidences for cultivation and occupation and moral economic statements reflecting migrant farmer ethic of migration, hard labour and subsistence. Those statements can be employed legally as proof of occupation and thus regularisation and also ideologically in resistance of evictions [START_REF] Krishnan | Deforestation and conflict in the Nilgiri Wynaad: a sociological study of protest, resistance and legal dispute[END_REF]. In a way, the trees planted in those agroforestry systems symbolically represent the implantation of the migrant farmer into the rural system.

For some people, agroforestry systems are not considered as rural forest because it is on private holdings and not accessible to all the community. Only community forestry can come under "rural forest" because the benefit should be shared among the group and access to the resource should be free. However this understanding of rural forests is normatively constrained because equity seems to be a defining trait of the concept. But from a spatial perspective agroforestry systems if located in rural areas can also be called rural forests.

Private owners of agroforestry farms may also be rural actors, albeit elitist.

For some academics in social sciences when agroforestry is practiced on a private holding, it is considered as community forestry because "the community creates the framework of law and custom which gives the holder tenure in the land and trees and creates the mechanisms which protect tenure" [START_REF] Bruce | Community forestryrapid appraisal of tree and land tenure[END_REF]. Even if private landholding system came into force after abolishing customary tenures and has disturbed the indigenous forestry and agriculture practices, it is not realistic to consider rural forest without taking into account tree planting on the agricultural holding. As the household which is making decisions about trees on his holding is involved in a farming system which overflows the holding into the common lands and sometimes into the Reserved Forests, the two systems (management of trees on private land and management of trees on common land) cannot be separated. This is confirmed in Kerala where agroforestry systems present the advantage that it can cover most of the households' needs for food and fuel, depending upon the size of the holding. The household's decisions about trees are made in terms of its overall access to tree products, whether on or off the holding. The kerala forest gardens respond also to the desire to preserve a wooded area around the house. This "domestic forest" is considered by farmers to be more efficient than the "natural forest" whose trees grow slowly and do not provide as much useful products for a family in a limited area. It consists of trees selected according to the needs of farmers or as a "financial reserve". Trees are perceived as "indispensable to man" and the home garden is regarded as a high value multifunctional production system [START_REF] Guillerme | Pratiques agroforestières et stratégies paysannes au Kérala (Inde du Sud), dynamique rurale en contexte de forte pression démographique[END_REF].

As the agroforestry systems are the product of law and custom, the private forestry in India is also the product of socio-political context. The State Governments regulates felling of private trees and transit of forest product in order to protect forests and maintain ecosystem services. Those State Forest Laws that regulate user rights of the land and its product act as major disincentives of the private forestry [START_REF] Sreedharan | Policy initiatives for tree planting in private lands[END_REF]. For instance, the Tamil Nadu Preservation of Private Forests Act of 1949 prevents the destruction of private forest and the Tamil Nadu Private Forest (Assumption of Management) Act of 1961 gives the power to the State to take over a private forest and manage it for a period of time. The public perception that land under tree culture is liable to acquired by Government and also the imperfection of the market discourage private initiatives [START_REF] Sreedharan | Policy initiatives for tree planting in private lands[END_REF]. The economic benefit of agroforestry systems and private forests are not shared among all the rural community but those forests contribute to some ecosystems services and interact with the other land uses and it is difficult to fulfill the criteria of equity. Equity is not warranty in community forestry, as Sudha & al (1998) explain about the Shimoga example and rurality cannot be reduced to equity alone. It is a culture with its own contradictions and knowledge that need to be understood to ensure sustainable forestry.

Forests used by local people

The forests of the Western Ghats are recognised to "sustain many rivers that are the lifelines of peninsular India and thus provide valuable ecosystem services not only in the form of water but also in the form of soil retention, climate regulation, carbon sequestration, and as reservoirs of pollinators and natural predators of pests" 7 . Based on this scientific assessment the National Forest Policy in force stipulates the state-owned forests need to be protected from degradation in order to maintain their ecosystem services. At the same time, the Forest Commission states that "the first charge of forest products must be in favour of the 7 From the Western Ghats Forum neighbouring people. Sustainable management of forests and proper use of forest products can help in alleviating poverty of the forest dwellers. Low productivity is a result of unsustainable management and consequent degradation".

So for many forest officials rural forest should be developed in order to fulfil the needs of rural people and like thus reduce the pressure on the state-owned forests. In order to combine the two priorities of the forest public policy, they have created an essential dichotomy between forests dedicated to people requirements and protected forests which are dedicated to critical wildlife habitat and other ecosystem services. After many years of forest policies excluding local people from the management of forests, many uses of the forests are considered illegal and are pointed by many forest officials as the reason for the forest degradation. At the opposite, social academics question this "dichotomic representation" of the forest and suggest that this excluding policy prevents people from transforming their forest uses into an effective forest management.

Firewood, a major need for local people

There are millions of people in this country who depend upon wood as a source of energy for cooking and to a lesser degree for heat. The rural poor have no access to commercial sources of fuel and have no option but to collect firewood. Under the aegis of the Forest Act, 1927, the Forest Department used to permit the extraction of deadwood for the fuel needs of the people living within and around forests, for their personal bona fide use. About three decades ago, the State Governments started passing orders permitting sale of 'head-loads' of firewood and head-loads were defined as a quantity that could be transported by rail, truck, cart or bicycle.

The Ministry of Environment and Forests estimates that the forests in India can yield 40 million tones of timber on a sustainable basis while the current extraction is perhaps around 500 million tones. Unless alternate sources of fuel, wood or LPG, are made available to the population, particularly those dwelling in and around forest and protected areas, they have no option but to depend on forests and trees available They collect firewood on all wooded areas. FSI estimates that 51 % of the fuelwood is collected from the forest areas and 49 % from the non-forest areas of the country [START_REF] Singh | Firewood from forests -How much worth is it? Indian Forester[END_REF].

In term of uses, there is no a dichotomy in representation of the forests: firewood is collected on whether private land or common land or sacred groves or protected forests. Western Ghats rural communities depend on local vegetation for their biomass needs; they collect them in forests, land under tree crops, groves, plantations, permanent pastures, grazing lands, farms, homestead gardens etc [START_REF] Sudha | Community forest management and Joint Forest Management : an ecological, economic and institutional assessment in Western Ghats, India, Presented at "Crossing Boundaries[END_REF]. Regarding these practices of collection, there are many interactions between forest area and other land uses. Those interactions are historically built and expressed in different ways according to the culture of the local people.

A "non-dichotomic representation" of the forest

The world of the Western Ghats' forests and the agrarian society of coastal plains have been in continuous interaction for many centuries [START_REF] Chandran | On ecological history of the Western Ghats[END_REF]. People living in the Ghats are not irreducibly ethnically distinct groups from people of the plains, and doomed to forest existence for ages. The forest is there, across the generations, as a space of social movement. It clearly has a long history of migration and colonization determined by the effects of wars and disasters, and the people who live there remember that they are often from elsewhere [START_REF] Pouchepadass | Introduction[END_REF]. This is, for example, well illustrated by the history of the Badaga, an agriculturist community constituted in the Nilgiri Hills after several migration phases mainly from Mysore plateau.

The badaga vocabulary to name the forest indicates this interpenetration between forest and agriculture worlds [START_REF] Hinnewinkel | La montagne convoitée -Contribution à l'étude des dynamiques environnementales et sociales dans les Nilgiri[END_REF]. Historically sole meant the sholas that was in between the folds of grasslands. It is this forest type they historically interacted with. The forest is also named by bana and kadu which refer to both forest and agriculture worlds. The first is a forest / a jungle / a grove and also a forest clearing / a forest garden. The second is used both for the wilder side of the forest (jungle / forest / wood / unknown area) and the domestic side of the forest (forest clearing / someone else's field). There is pekadu for wilderness and kaduhola or solehola for temporary fields. Since colonial advent they have settled largely as agriculturists and now as monoculturists. There is no dependence on natural forests now. The word kadu is now sometimes used in reference to tea gardens which is sometimes also called tottam, garden. Agriculture lands are called hola. Sometimes eucalyptus and wattle plantations where they collect firewood from are called sole. For Badaga, it seems that forest is a wild area, unknown, inhabited sometime by spirits or gods, and a cultivated area, and of course a reserve of land for agriculture (tudi is wood with fertile soil sometime crossed by a river). As domestic forests, kadu and bana are shaped by a variety of practices.

Is it possible to consider legally this domestic forest as rural forest? Forest officials have a negative answer and state that the practices related to those domestic forests have disappeared today, due to population growth. For them, if all the forests are legally considered as rural forest, this will lead to the overexploitation of the forest resources. At the opposite, many social academics argue that this situation is generated by the socio-political context and are in favour of an important change in forest public policies.

Conclusion

The idea of "rural forest" helped to categorize the diversity of "involving people forests" and in this way to analyse the conflicting views regarding "involving people forests" which has been a main issue in India forest management since National Forest Policy of 1988. It highlights the confrontation between the normative approach of the forest which is recommended by the State, and the pragmatic approach based on forest uses of local people. While the first approach institutes a major dichotomy between forests to meet the requirements of rural people and protected forests, the second one questions the established order.

The definition of "rural forest" is more or less broad according to the stakeholders' views. Academics in social sciences, who attempt to consider the local practices and indigenous views, argue for a broader definition in order to question the meaning of a protected forest, of sustainable forestry practices, and of the share of resources. For them, forest management can be sustainable only if people needs are fulfilled. At the opposite, for some practitioners, rural forests are limited to the wooded areas in common lands. This view will be followed by many forest officials who could see the potential in rural forest as a mean to reduce the pressure on the state-owned forest that should be protected for its ecological value. In between this two views, joint forest management approach attempts to involve people, but how this new approach can be sustainable if the socio-economical and cultural needs of the local people are ignored? The integration of people knowledge about trees and forest management in the Management Plan of the forest under Joint Forest Management can be a way to reinforce a sustainable forest management instead of the afforestation actually practiced.

In sum up, according to the stakeholder representations, the typology of rural forests in Western Ghats is whether limited to forests managed by a community or includes all comanaged forests or includes all forests to meet the requirements of local people or includes all the domestic forests (Fig 1). If a rural forest is constituted to involve people in forest management, how to delimit/define it and is it sustainable to involve people in forest management without taking into account their needs and acknowledging their practices?

Fig. 1 :

 1 Fig. 1: Different representations of rural forests

Table 1 :

 1 Main forest uses that shape the forests of Western Ghats according to the land ownership

	Land ownership	State-owned lands
	Forest uses	Revenue lands	Recorded forests	Private holdings
	Protection for ecological functions		Reserved Forest
	Religious protection	Burial grounds	Sacred groves
	Afforestation, Restoration	Social forestry	Afforestation programme JFM (VSS, VFC) Farm forestry
	Silviculture	TE and plantations	Private forestry
	Collection of MFP	Areas in RF leased to LAMPS Plantations or agroforestry
	Collection of FW	on all areas with wooded vegetation
	Agriculture practices	Tribal agriculture lands in RF	Agroforestry
	Dark grey: practices in force. Light grey: practices deserted during the last 15 years.
	Italic: name of the schemes implemented by Forest Departments.
	TE: Timber extraction; MFP: Minor Forest Products; FW: Firewood; RF: Reserved Forests; JFM: Joint Forest
	Management; LAMPS: LArge MultiPurpose Society.

  experiences of participatory forest management of recorded forests were experimented in West Bengal in the 1970's but this approach was institutionalized in the National Forest Policy of 1988 and gained momentum with the Central Government Circular Concerning Joint Forest Management No. 6-21/89-P.P. This circular of June 1990 provides the framework for the implementation of Joint Forest management (JFM) programme: the aim is to protect, regenerate and rehabilitate degraded recorded forest, jointly with the Forest Department and the local communities. It is a decentralised approach which links people rights to use the forest resource with people responsibility to sustain the resource. It means the recorded forests cease to be purely state-controlled resource and become defined patched to which specific village communities would have exclusive and assured access

R J Ranjit Daniels on the website of Western Ghats Forum: http://www.westernghatsforum.org

G.P.D. Vyas is a Deputy Conservator of Forest.

In India "recorded forest areas" are "Reserved Forest", "Protected Forest" and "Unclassified Forest" defined by Indian Forest Act.

Tree cover is the average number of trees on cultivable land. It is estimated through analysis of high resolution satellite data(FSI 2003).

Acknowledgments

This work was carried out with the financial support of the « ANR-Agence Nationale de la Recherche -The French National Research Agency » under the « Programme Agriculture et Développement Durable », project « ANR-06-PADD-014, Popular ».

Domestic forests

Forests to meet the requirements of local people


Adrien Stoliaroff

Alicia Lecomte

Oleg Rubel

Stéphane Jobic

Xianghua Zhang

Camille Latouche
email: camille.latouche@univ-nantes.fr

Xavier Rocquefelte
email: xavier.rocquefelte@univ-rennes1.fr

Deciphering the role of key defects in Sb 2 Se 3 , a promising candidate for chalcogenide based solar cells

Keywords: Sb 2 Se 3, DFT, point defects, photovoltaics, chalcogenides, absorber, modelling

Herein we report a thorough investigation on Sb 2 Se 3, a promising absorber material for photovoltaic applications, using state of the art quantum methods to understand the impact of defects on its electrical properties. The results show that despite a rather small

stability domain, Sb 2 Se 3 is easy to synthetize because there is no other possible stable competing binary phase in the Sb/Se system. Our calculations prove that formation of intrinsic n-type defects is unlikely, because Sb vacancies restrain the Fermi level from reaching the CBM vicinity. In contrast, intrinsic p-type semiconductor behavior is expected due to Sb Se antisite defects. Doping is a commonly used technique to impact on the charge carrier concentration as well as the charge carrier nature. In that context, several extrinsic defects were considered, based on tin and copper to enhance the native p-typeness, and halogenides (Cl, Br, I) to induce n-type doping in Sb 2 Se 3 . Our results tend to prove that Sb 2 Se 3 :Cu(p)/Sb 2 Se 3 :I(n) might be a viable homojunction for photovoltaic devices.

I. Introduction

The need for exploration of sustainable clean energies has become a global consensus, due to the depletion of fossil fuels and increasing concerns of environmental protections. [START_REF] Chu | Opportunities and Challenges for a Sustainable Energy Future[END_REF] Nowadays, the most commercially competitive technology among thin-film photovoltaics (PV) is CdTe due to its high device efficiency in relation to its excellent optoelectronic properties, [START_REF] Burst | CdTe Solar Cells with Open-Circuit Voltage Breaking the 1 V Barrier[END_REF][START_REF] Zhao | Monocrystalline CdTe Solar Cells with Open-Circuit Voltage over 1 V and Efficiency of 17%[END_REF] and the relatively low cost of its production. CIGS based photovoltaic cells and modules offer higher performances but suffer from the scarcity of gallium and indium and lesser deposition rate due to the complexity of the absorber composition and its inherent versatility. Sb 2 Se 3 , with earth-abundant constituents, emerged recently as a very promising non-toxic alternative to CdTe and CIGS PV. Indeed, Sb 2 Se 3 -based solar cells received these last few years a strong incentive that led to a continuous and impressive increase in the conversion efficiency, . Actually, this latter shifted successively from 2.6 and 2.7 % in 2014, [START_REF] Zhou | Solution-Processed Antimony Selenide Heterojunction Solar Cells[END_REF][START_REF] Leng | Selenization of Sb 2 Se 3 Absorber Layer: An Efficient Step to Improve Device Performance of CdS/Sb 2 Se 3 Solar Cells[END_REF] to 4.7 % in 2015, [START_REF] Liu | Improving the Performance of Sb 2 Se 3 Thin Film Solar Cells over 4% by Controlled Addition of Oxygen during Film Deposition[END_REF] 5.6, 5.8 and 6.5 % in 2017, [START_REF] Chen | Accelerated Optimization of TiO 2 /Sb 2 Se 3 Thin Film Solar Cells by High-Throughput Combinatorial Approach[END_REF][START_REF] Liu | Enhanced Sb 2 Se 3 Solar Cell Performance through Theory-Guided Defect Control[END_REF][START_REF] Chen | 6.5% Certified Efficiency Sb 2 Se 3 Solar Cells Using PbS Colloidal Quantum Dot Film as Hole-Transporting Layer[END_REF] 7.04 % in 2018, [START_REF] Chen | Efficiency Improvement of Sb 2 Se 3 Solar Cells via Grain Boundary Inversion[END_REF] to reach finally 9.21 % in 2019. [START_REF] Li | 9.2%-Efficient Core-Shell Structured Antimony Selenide Nanorod Array Solar Cells[END_REF] This very fast improvement is very encouraging and suggests that characteristics may still progress in the near future, in particular through a better control of interfaces, and a mastering of charge carrier concentrations. To do so, the understanding of the impact of defects on transport properties is crucial as they play a pivotal role on thin-film solar cell performances, as well as grain boundaries and interfaces, two subjects not discussed herein.

Naturally, antimony selenide (Sb 2 Se 3 ) is a p-type semiconductor with a onedimensional crystal character. [START_REF] Petzelt | Far Infrared Dielectric Dispersion in Sb 2 S 3 , Bi 2 S 3 and Sb 2 Se 3 Single Crystals[END_REF] Mobility of minority carriers (n) is decent for the target application (∼10 cm 2 .V -1 .s -1 ) as well as carrier lifetime (∼60 ns according to transient absorption measurements) [START_REF] Chen | Characterization of Basic Physical Properties of Sb 2 Se 3 and Its Relevance for Photovoltaics[END_REF] . Moreover, Sb 2 Se 3 displays all the requisites to be an excellent PV absorber, i.e. a high absorption coefficient (>10 5 cm -1 ) [START_REF] Zhou | Solution-Processed Antimony Selenide Heterojunction Solar Cells[END_REF] and an optical gap of ca. 1.2 eV, i.e. a value next to the optimal one for a single junction solar cell) [START_REF] Chen | Optical Properties of Amorphous and Polycrystalline Sb 2 Se 3 Thin Films Prepared by Thermal Evaporation[END_REF] .In addition, theoretical calculations [START_REF] Filip | Quasiparticle Band Structures of Stibnite, Antimonselite, Bismuthinite, and Guanajuatite[END_REF][START_REF] Patrick | Structural and Electronic Properties of Semiconductor-Sensitized Solar-Cell Interfaces[END_REF] alluded Sb 2 Se 3 can be regarded as a promising candidate for achieving power conversion efficiency (PCE) as high as 20%. A recent investigation concerning the point defects of Sb 2 Se 3 demonstrated the role of antimony vacancies as a pinner of the Fermi level. In this study, the authors used screened hybrid functional HSE of which they tweaked the share of Hartree-Fock exchange at short range (α parameter). [START_REF] Huang | Complicated and Unconventional Defect Properties of the Quasi-One-Dimensional Photovoltaic Semiconductor Sb 2 Se 3[END_REF] The density functional theory was used together with many body approaches to investigate the impact of both intrinsic and extrinsic A c c e p t e d m a n u s c r i p t defects on the characteristics of Sb 2 Se 3 . Indeed, our simulation focuses on point defects to create p-type and n-type semiconductors with high hole or electrons concentrations.

II. Computational details

DFT computations using semi-local PBE functional [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] on a 1x3x1 supercell containing 60 atoms were performed within the projector augmented-wave (PAW) scheme as implemented in the VASP software (Vienna Ab-initio Simulation Package) [START_REF] Kresse | Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set[END_REF][START_REF] Kresse | Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set[END_REF][START_REF] Kresse | From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method[END_REF] , similarly to Liu et al and Savory and Scanlon. [START_REF] Liu | Enhanced Sb 2 Se 3 Solar Cell Performance through Theory-Guided Defect Control[END_REF][START_REF] Savory | The Complex Defect Chemistry of Antimony Selenide[END_REF] The energy cut-off for plane waves was set at 350 eV. The first Brillouin zone was sampled with a 4x4x4 mesh following a Monkhorst-Pack scheme for structural relaxation and 5x5x5 mesh for accurate total energy calculations, respectively. [START_REF] Monkhorst | Special Points for Brillouin-Zone Integrations[END_REF] We compare different methodologies to account for the dispersion forces, including or not Becke-Johnson damping factor (see Table S1). [START_REF] Grimme | Effect of the Damping Function in Dispersion Corrected Density Functional Theory[END_REF] Without any dispersion, PBE catastrophically overestimates the a parameter (+8.0%), highlighting the need for dispersion corrections (only ±1% for b and c). GD3 scheme performs much better with a deviation on cell parameters with respect to experiment of ±2%. GD3-BJ overcorrects dispersion effects and lead to a systematic underestimation of cell parameters. This is particularly visible on the volume of the cell. Therefore, all the data discussed herein are with the PBE + GD3 scheme. [START_REF] Grimme | Effect of the Damping Function in Dispersion Corrected Density Functional Theory[END_REF][START_REF] Grimme | A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu[END_REF] Refined lattice parameters (see Supporting Information (SI)) turn out to be in very good agreement with respect to available experimental data. Indeed, the optimized a, b and c parameters are computed at 12.015 Å, 4.019 Å and 11.465 Å, respectively. They match the experimental ones obtained via X-ray diffraction, i.e. 11.805 Å, 3.988 Å and 11.662 Å, for a, b, and c, respectively (SG: Pnma). [START_REF] Kyono | Selenium Substitution Effect on Crystal Structure of Stibnite (Sb 2 S 3 )[END_REF] 1 2 %. Defect formation enthalpies have been calculated using the supercell approach, as given in equation ( 1).

(1) ∆𝐻 𝐷,𝑞 𝑓𝑜𝑟𝑚 (𝜇 𝐸 𝐹 ) = ( 𝐸 𝐷,𝑞 -𝐸 𝐻 ) + ∑ 𝑖 𝑛 𝑖 𝜇 𝑖 +𝑞 𝐸 𝐹 + 𝐸 𝑐𝑜𝑟𝑟 (where E D,q and E H are the total energies of the faulted and perfect cell respectively, µ i is the chemical potential of the i th chemical species, µ EF is the chemical potential of the electrons (Fermi level) and E corr are the various corrections included to account for the limitations of the supercell model. Band edges positions were corrected based A c c e p t e d m a n u s c r i p t on the results of a GW calculation. An electrostatic potential alignment correction was taken into account. Spurious electrostatic interactions between charged defects and their periodic images were dealt with using Makov-Payne style correction. Moss-Burstein type band-filling effect and Perturbed Host States correction were taken into account as well. In the dilute limit, the variation of volume due to the creation of a defect is negligible so that the calculated defect formation energy reasonably approximates the formation enthalpy. All defect post-treatments were executed with the PyDEF code. [START_REF] Stoliaroff | 2.0: An Easy to Use Post-Treatment Software for Publishable Charts Featuring a Graphical User Interface[END_REF][START_REF] Péan | Presentation of the PyDEF Post-Treatment Python Software to Compute Publishable Charts for Defect Energy Formation[END_REF] The energy state at which the charge of a given defect is changing, i.e. the transition state, was determined by plotting enthalpy vs. Fermi level ( ) 𝜇 𝐸 𝐹 position. It can be expressed as

𝜖 𝑞1,𝑞2 = - ΔH D,𝑞 1 (0) -ΔH D,q 2 (0) 𝑞 1 -𝑞 2 (2)
It should be noted that we have tested the consistency of our results by using a larger supercell, namely 2x3x2 supercell. It leads to no significant changes as evidenced in Table S2.

The defect concentration, of a defect in charge state , can be approximated 𝑛 𝐷,𝑞 (𝐸 𝐹 ) 𝐷 𝑞 using a Boltzmann distribution depending on the position of the Fermi level during synthesis E F , as expressed in:

𝑛 𝐷,𝑞 (𝐸 𝐹 ) ≈ 𝑁 • exp ( -Δ𝐻 𝐷,𝑞 𝑓𝑜𝑟𝑚 (𝐸 𝐹 ) 𝑘 𝐵 𝑇
) [START_REF] Zhao | Monocrystalline CdTe Solar Cells with Open-Circuit Voltage over 1 V and Efficiency of 17%[END_REF] where is the Boltzmann constant, is the number of sites available to the defect 𝑘 𝐵 𝑁 divided by the cell volume and T is the temperature of the synthesis. The entropic contribution to the Gibbs' free energy is considered negligible with respect to the enthalpic terms. The material is globally charge neutral, which sets the value of the Fermi energy so that to compensate the charges of the carriers (holes and electrons) with the ones of the defects. Numerically, this translates as the following equation, whose resolution leads to the determination of the Fermi level:

-𝑛 𝑒 (𝐸 𝐹 ) + 𝑛 ℎ (𝐸 𝐹 ) + ∑ 𝐷 𝑞 𝐷 • 𝑛 𝐷,𝑞 𝐷 (𝐸 𝐹 ) = 0 (4)
A c c e p t e d m a n u s c r i p t

Here, and are the concentrations of free electrons and holes, respectively, 𝑛 𝑒 (𝐸 𝐹 ) 𝑛 ℎ (𝐸 𝐹 ) for a Fermi energy at a given temperature. They are given by the following equations:

𝑛 𝑒 (𝐸 𝐹 ) = ∫ +∞ 𝐸 𝐶 𝑔 𝑒 (𝜇) • 𝑓 𝐹𝐷 (𝜇 -𝐸 𝐹 ) • 𝑑𝜇 (5)
and

𝑛 ℎ (𝐸 𝐹 ) = ∫ 𝐸 𝑉 -∞ 𝑔 ℎ (𝜇) • (1 -𝑓 𝐹𝐷 (𝜇 -𝐸 𝐹 )) • 𝑑𝜇 (6)
where is the Fermi-Dirac function:

𝑓 𝐹𝐷 (𝜇 -𝐸 𝐹 ) 𝑓 𝐹𝐷 (𝜇 -𝐸 𝐹 ) = 1 1 + exp ( 𝜇 -𝐸 𝐹 𝑘 𝐵 𝑇 ) ( 7
)
and and are the density of states of respectively electrons and holes for a 3D 𝑔 𝑒 (𝜇) 𝑔 ℎ (𝜇) solid: 1a). To some respect, if only short Sb-Se interactions are taken into account (strong covalent bonds,  Sb_Se > 0.24 ( Sb_Se = bond valence between Sb and Se atoms) [START_REF] Brese | Bond-Valence Parameters for Solids[END_REF] ), Sb 2 Se 3 can be regarded as 1 / ∞ [Sb 4 Se 6 ] infinite ribbons running along the b-axis of the orthorhombic cell (Figure 1b). If one considers medium Sb-Se distances (medium covalent bonds, 0.14 <  Sb_Se <0.24), these ribbons condense along the c direction to define 2 / ∞ [Sb 2 Se 3 ] infinite layers lying in the (100) planes (Figure 1c). These planes interact altogether via very weak Next, we investigate the electronic structure of the ideal cell, as shown in Figure 2 to determine the direction in which the photogenerated charge carriers flow in order to favor charge extraction in the device. The valence band maximum (VBM) is reached between Γ and Z (0,0,0.5) in reciprocal space, along the c* axis. The bandgap is direct and calculated at 0.77 eV with PBE-GD3 functional. As expected in GGA, the gap is underestimated with respect to the experimental optical value of ~1.2 eV. [START_REF] Zhang | Scalable Low-Band-Gap Sb 2 Se 3 Thin-Film Photocathodes for Efficient Visible-Near-Infrared Solar Hydrogen Evolution[END_REF][START_REF] Vadapoo | Electronic Structure of Antimony Selenide (Sb 2 Se 3 ) from GW Calculations[END_REF] From the analysis of the atomic structure, the highest dispersion is along the Γ-Y segment in reciprocal space, and then along the Γ-Z segment. Clearly, it corresponds to the directions defining the 2 / ∞ [Sb 2 Se 3 ] layers in direct space. The dispersion of the bands is smaller along the Γ-X segment, which corresponds to the direction perpendicular to the 2 / ∞ [Sb 2 Se 3 ] layers, i.e. a direction where Sb-Se bonds are weak. The highest occupied electronic band along this segment does not reach the Fermi level (the band lies a few meV below E F even at the X high-symmetry point). Thus, based on the analysis of the band structure, free holes are counterintuitively more likely to propagate along the c axis, i.e. the in-plane direction perpendicular to the chains. A similar conclusion would prevail for free electrons after examination of the dispersion of the conduction band.

(8) 𝑔 𝑒,ℎ (𝜇) = 1 4𝜋 2( 2𝑚 * 𝑒,ℎ ℏ 2 ) 3/2

𝜇

III. Results and discussions

A. Perfect Cell

The projection of the density of states (DOS) on the atomic orbitals show that the valence band mainly comes from the selenium orbitals whereas the conduction band has a mixed antimony and chalcogenide character. It is important to reproduce accurately the bandgap for the following point defect study. To overcome the bandgap problem of GGA, a GW0 calculation was performed on top of the PBE-GD3 geometry, yielding a gap value of 1.24 eV, in excellent agreement with the experimental gap. We have used this value to correct the bandgap for all faulted cell calculations (vide infra).

In thin films, one usually observes Sb 2 Se 3 rods growing preferentially along the chain direction. [START_REF] Li | Improvement in Sb 2 Se 3 Solar Cell Efficiency through Band Alignment Engineering at the Buffer/Absorber Interface[END_REF][START_REF] Tao | Investigation of Electronic Transport Mechanisms in Sb 2 Se 3 Thin-Film Solar Cells[END_REF] This encourages us to assess the propensity of this anisotropic material to absorb light along the different spatial directions. In order to do so, the imaginary part of the dielectric function is presented in Figure 3. The absorption of incoming light will generate free electron-hole pairs harvested in the form of an electrical current. Fast and efficient design of devices requires the optimization of the electrical conductivity of the material. In order to rationalize the fair p-type conduction of undoped Sb 2 Se 3 , [START_REF] Chen | Characterization of Basic Physical Properties of Sb 2 Se 3 and Its Relevance for Photovoltaics[END_REF] the intrinsic point defects of the material, namely vacancies, self-interstitials and antisites were investigated.

B. Intrinsic defects to determine the origin of the electronic properties

In that context, we first need to set the values of the chemical potentials to properly account for the experimental synthesis conditions. As aforementioned, Sb 2 Se 3 is the only binary compound of the Sb-Se system, so that the limits of its stability domain are set by the reference phases of each element i.e. elemental Se and Sb in their hexagonal and rhombohedral forms, respectively, as shown in Figure 4. Two limit cases have to be envisioned, i.e. the Se-rich/Sb-poor conditions at the frontier with the domain of the selenium (where the potential of Sb is fixed by rhombohedral antimony) and the Sepoor/Sb-rich ones on the Sb side (where the potential of Se is fixed by hexagonal selenium). The formation enthalpy of all five possible vacancies in Sb 2 Se 3 (V Sb1 , V Sb2 , V Se1 , V Se2 , V Se3 ) are summarized in Figure 5. The transition levels are in total agreement with computations performed using hybrid functional HSE06-D3 performed by Savory and Scanlon which fully validates our methodology. [START_REF] Savory | The Complex Defect Chemistry of Antimony Selenide[END_REF] Huang et al also showed that the transition level positions are not affected by going from HSE to PBE. [START_REF] Huang | Complicated and Unconventional Defect Properties of the Quasi-One-Dimensional Photovoltaic Semiconductor Sb 2 Se 3[END_REF] Here, both antimony vacancies V Sb1 and V Sb2 present an acceptor character with a transition level 0/-1 0.15 eV above the VBM. In both synthesis conditions, they possess a quite high enthalpy of formation which suggests that their concentration in the material will not be important (vide infra). Moreover, and most importantly, the selenium vacancies are lower than Sb ones in enthalpy at low , especially for Se-poor/Sb-rich synthesis conditions, and are 𝜇 𝐸 𝐹 (very deep) donor defects which will capture the holes produces by the formation of V Sb ("hole killer" defects). Thus, the observed p-type conductivity cannot result from V Sb . The formation enthalpy of V Sb becomes negative as the Fermi level approaches the CBM. If the Fermi level enters this region, these vacancies would be so easy to form that they would drive the ongoing synthesis off-stoichiometry to finally lead to another phase (according to the stability domain, demixing into elemental Sb and Se). Thus, the zero enthalpy crossing of V Sb marks the limit of a forbidden domain for the Fermi level (n-type dopability limit). In other words, cannot explore the region close to the CBM, so even doping 𝜇 𝐸 𝐹 cannot lead to n-type conductivity in such conditions. The dopability limit is deep in the gap for the Se-rich/Sb-poor atmosphere (0.25 eV below the CBM) and tolerable for the Se-poor/Sb-rich limit case (0.09 eV below the CBM), so the excess in Selenium is detrimental to n-type doping.

Then, we investigate the formation of self-interstitials. To locate the possible interstitial sites, we use the plots of isosurfaces of the charge density as drawn in Figure S1. Both Se i and Sb i defects are deep defects and will therefore barely affect the electronic conductivity of the material. The formation enthalpy of the selenium interstitial is lower than that of the antimony one, and inferior to 2 eV. Additionally, steric and charge effects may also play a role, the anion being smaller than the cation.

None of the defects investigated so far give a satisfactory explanation for the natural ptype conductivity of the undoped material. Antimony has the peculiar property to be able to show both cationic, Sb 3+ (as in Sb 2 Se 3 ), and anionic, Sb 3-(as in AlSb), characters, by, respectively, emptying and filling the 5p electronic orbital. antisite, Sb atom is bonded to Sb 1 and the associated defect has a shallower transition level (0.23 eV). Its formation enthalpy is lower than that of the hole killers V Se . It is thus the shallowest intrinsic acceptor, even though the position of its transition level is not in the optimal region situated under 0.1 eV (a few k B T) above the VBM. This clearly demonstrates that the intrinsic p-type conductivity of Sb 2 Se 3 is likely due to the formation of Sb Se3 antisites. This result is fully consistent with previous literature performed with the HSE06 hybrid functional, [START_REF] Savory | The Complex Defect Chemistry of Antimony Selenide[END_REF] which fully validates the use of PBE-D3 to describe the defect physics in this material.

Now that we have determined the formation enthalpies and electronic properties of a wide range of intrinsic defects, we can estimate their concentration at a given synthesis temperature. Indeed, the material is globally charge neutral, so that the electrostatic charges of the different defects and charge carriers must sum up to zero. This can be written as equation 4. n e is the concentration of electrons, n h of holes, derived from the density of states, and the concentration of defect. As the concentrations are functions 𝑛 𝐷,𝑞 𝐷 of the Fermi level, solving it through a standard root-finding algorithm method yields the Fermi level during crystal growth at a given growth temperature (T growth ). The obtained defect concentrations are shown in Figure 6. For a material synthesized at 550 K, which corresponds to a realistic temperature in a physical vapor deposition of a Sb 2 Se 3 thin film, the most prevalent defect in Se-rich/Sb-poor conditions is the Se Sb1 antisite with a concentration of ~6 10 13 cm -3 (Fermi level at 0.57 eV for T = 550 K). This defect exhibits only one transition level deep in the gap and is thus of weak interest with respect to the electrical properties. The second most important defect is V Sb1 up to T growth ~790 K and Se Sb2 for higher synthesis temperatures. Regardless of the growth temperature, such material is expected not to display electrical properties sufficient for device application. In Se-poor/Sb-rich conditions for the same growth temperature of 550 K, the major defects are Sb Se2 and Sb Se3 , exhibiting a concentration of ~10 14 cm -3 . Sb Se3 is responsible for the p-type character of the material. Such concentrations are similar to those met in silicon, the base material for the electronic industry and therefore used in photovoltaics. [START_REF] Fahey | Point Defects and Dopant Diffusion in Silicon[END_REF] However, the transition level is a bit far from the VBM, possibly explaining the not so good observed electrical properties. Computed positive charge carrier (hole) concentrations (n h ) at room temperature are of 5.5 10 13 cm -3 in Se-rich/Sbpoor atmosphere and 7.2 10 14 cm -3 in Se-poor/Sb-rich.

A c c e p t e d m a n u s c r i p t

Due to its fast cooling just after formation, the stoichiometry of the material at high temperature l (e.g. ca. 550 K) can be considered as quenched at room temperature. Thus, all the defects created during the high temperature process are frozen when the system cools down from growth to (device) operating temperature. Solving the charge neutrality equation at fixed defect concentrations then yields the Fermi level at operating temperature. The result is shown in Figure 7

C. Sn-doping

The first dopant considered is tin, an earth-abundant and low-cost element, that was recently tried out experimentally. [START_REF] Chen | Enhanced Electrical Conductivity and Photoconductive Properties of Sn-Doped Sb 2 Se 3 Crystals[END_REF] Two types of defects must be considered, cationic substitution and interstitial. Substituting an antimony atom (Z=51) with a tin atom (Z=50) is expected to form an electronic hole, thus leading to p-type doping. At the opposite, a tin interstitial is expected to be a donor defect as it can release an electron by switching from neutral to positive states of charge. The chemical potential of the dopant is usually taken at the reference thermodynamical value. As we recently discussed, [START_REF] Stoliaroff | New Insights into the Determination of Maximum Chemical Potentials to Account for Alkali Doping in β-In 2 S 3 by Ab Initio Calculations[END_REF] this corresponds to synthesis conditions at the limit of formation of the reference thermodynamic compound, tin metal in tetragonal (β) form in this case. However, this does not reflect the synthesis conditions used to incorporate a dopant, i.e. a dilute point defect in the Sb 2 Se 3 host lattice.

Choosing the standard chemical potential value for the doping chemical species will strongly favor dopant-related defects (here tin interstitials and substitutions), so much that they will exhibit a negative defect formation enthalpy, thus driving the formation of another phase.

Defining the chemical potential of the extrinsic dopant Sn, thus, requires taking into account all Sn-based phases, participating to the ternary Sn-Sb-Se system, i.e. β-Sn, SnSe 2 and SnSe. We can then define the limits of the chemical potential of tin, µ Sn , with β-Sn for the upper limit and for the lower limit, SnSe 2 and SnSe in Se-rich and Se-poor atmospheres, respectively (Table 1). Figure 8 provides the evolution of the formation energy with respect to the Fermi level position in the bandgap for the defects involving Sn. For Sn Sb1 , the curve resulting from the use of the upper limit (β-Sn as reference) is shown (dashed red line). It evidences a shift of 1.14 and 0.55 eV towards the lower energies for Se-rich and Se-poor conditions, respectively. In summary, by considering S2. As the shape of the defect concentration vs.

growth temperature curve has been discussed previously in Figure 6, we now only consider one representative growth temperature of 800 K. This temperature is close to the reported synthesis temperature (550 °C) by Huang and coworkers. [START_REF] Zhou | Thin-Film Sb 2 Se 3 Photovoltaics with Oriented One-Dimensional Ribbons and Benign Grain Boundaries[END_REF] The resulting concentrations are shown in Figure 9. The main defect is the extrinsic substitution Sn Sb as mentioned beforehand. The selenium vacancies in Se-poor conditions and Se Sb antisites in Se-rich conditions are also favored. They may act as recombination centers detrimental to the device and might hinder the gain in hole concentration.

In conclusion, the incorporation of tin will generate some additional holes in the material and thus will improve the p-type conductivity. However, due to the not optimal position of the transition level and the non-negligible formation of deep defects, the effect might not be as important as desired. Most importantly, the amount of tin which can be The oxidation state of copper is expected to be +I in this material. Thus, Cu i can be charged +1/0. This is a donor defect. It has a positive and not too high formation enthalpy (~1eV), however the transition level +1/0 is 0.37 eV below the CBM. Indeed, the level is unambiguously too deep to induce any n-type doping of the bulk material at room temperature, contrary to what was expected experimentally. [START_REF] Chen | Efficiency Improvement of Sb 2 Se 3 Solar Cells via Grain Boundary Inversion[END_REF] The n-type doping after CuCl 2 chemical bath proposed in this reference must be due to the Cl anion instead (vide A c c e p t e d m a n u s c r i p t infra). Next, we investigate the possible cationic substitutions in both Sb crystallographic sites. In Sb 2 Se 3 , the oxidation state of antimony is +III. Thus, the charge state of Cu Sb varies between 0 and -2. This is an acceptor defect. In both synthesis conditions, the formation enthalpy of Cu Sb is quite low (<1.7 eV). The formation enthalpy of Cu Sb1 is 0.39 eV lower than that of Cu Sb2 . The 0/-1 acceptor transition level is 0.25 eV above the VBM. One should recall that electronically active levels should be within a few k B T from the band extrema to be thermally activated, so this level is active but a bit too deep. This is compensated by a low formation enthalpy of the defect which guarantees a high defect concentration. In short, each individual Cu Sb defect has a fair but not so high probability of generating a charge carrier (hole), but this is compensated by the important number of Cu Sb defects in the doped material. Then, we calculate the defect concentrations for all copper-related defects along with the intrinsic defects of the material as presented in Figure 11. In Sepoor/Sb-rich synthesis conditions, the concentration of the (very poor) donor Cu i is compensated by the formation of intrinsic antisite acceptors Sb Se in comparable amount.

Due to this competition, the Fermi level at room temperature (T= 300 K) lies 0.22 eV above the VBM, resulting in a slight p-type improvement. In Se-rich/Sb-poor atmosphere, the situation is similar with the competition between the extrinsic acceptor Cu Sb1 and extrinsic donor Cu i , resulting in the Fermi level at room temperature pinned 0.13 eV above the VBM. These simulations tend to prove the stabilization of both Cu i and Cu Sb defects is possible, allowing a modest improvement of the p-type conductivity, especially in Serich/Sb-poor synthesis conditions.

A c c e p t e d m a n u s c r i p t The chemical potential is set by the limit of formation of SbCl 3 in both Se-poor and Se-rich limit conditions. The value is roughly 1 eV lower than the reference value, thus the amount of dopant which can be inserted is expected to be low. The formation enthalpy of chlorine interstitial is higher than that of the substitutions.

The anionic substitutions are donor defects with transition levels located ~0.2 eV under the CBM for Cl Se1 and Cl Se2 and ~0.05 eV for Cl Se3 .

The latter is thus a very good donor defect. It exhibits a formation enthalpy of 1.25 eV so is expected to be quite abundant in the material. One should mention that the Cl i acceptor defect crosses the lines of the donors (Cl Se1-3 ) and, consequently, may prevent a good conductivity and potentially acts as electron killer defect.

A c c e p t e d m a n u s c r i p t

The cases of iodine and bromine are more promising than chlorine. In the case of bromine, the ternary phase SbSeBr limits the possible range of values for the chemical potential of Br. This time, the three Br Se substitutions behave the same with a -1/0 transition level around 0.06 eV under the CBM. Consequently, all of them form very good donor defects.

The interstitial is much higher in enthalpy and does not cross the lines of the substitutions in the band gap, although the enthalpy remains close to the interstitial near the CBM.

Concerning I doping, the halogen chemical potential is set by SbSeI. The substitutions exhibit a -1/0 transition level roughly 0.10 eV below the CBM as for Br. In this case, the insertion of iodine in interstitial site is the most energy consuming defect among the three halogens and never crosses the lines formed by the formation enthalpies of I Se1-3 .

Consequently, this dopant seems to be the most favorable one.

In short, all three halogens considered are expected to significantly improve the electronic conductivity of the Sb 2 Se 3 host. Our results provides a reasonable explanation for the n-type doping induced by CuCl 2 chemical bath treatment proposed in a previous work. [START_REF] Chen | Efficiency Improvement of Sb 2 Se 3 Solar Cells via Grain Boundary Inversion[END_REF] In terms of chemical potential, iodine has the highest allowed value of the three elements (the lowest in absolute value). This means that more iodine can be incorporated in the structure without destabilizing it as compared to bromine or chlorine. Based on this observation, only defect concentrations of defects involving iodine are presented in Figure 13. (see figures S6 and S7 for chlorine and bromine).

A c c e p t e d m a n u s c r i p t without phase separation, leading to a strong decrease of electric resistivity (eight orders of magnitude). Indeed, the obtained materials are stable n-type semiconductors. 38

IV. Conclusion

We conducted a thorough DFT investigation on the intrinsic defects of Sb 2 Se 3 and their potential ability to induce p-type and n-type doping of the material. According to our calculations, V Se act as hole killer defects, deteriorating the p-type conductivity of the material. V Sb are too high in enthalpy to be responsible for the p-type and set the n-type dopability limit. Thus, an excess in Selenium during synthesis would be detrimental to ntype doping. Thanks to the ability of Sb to act both as anion and cation, antisites in this material are fairly achievable defects. In particular, Sb Se on the Se site bonded to the hexacoordinated antimony (Se 3 ) exhibit a fair shallow transition level (0.23 eV) and a low formation enthalpy and is the best candidate to explain the (poor) intrinsic p-type conductivity of Sb 2 Se 3 . The study of tin and copper to achieve extrinsic p-type doping and halogens substitution for n-type doping of the absorber show that tin does not much improve the conductivity, while copper and the halogens provide good conductivities. Our results demonstrate that among the three investigated halogens, iodine seems the most promising dopant for the Sb 2 Se 3 matrix. Consequently, Sb 2 Se 3 :Cu(p)/Sb 2 Se 3 :I(n) appears as a viable homojunction for photovoltaic devices.

V. Supporting Information

Table S1 Comparison of cell parameters computed with different dispersion corrections with respect to experimental structure; Table S2 Comparison of defect formation enthalpy for two supercells; Table S3 Calculated Fermi levels and free carrier

VI. Conflicts of interest

There are no conflicts to declare.

Sb 2

 2 Se 3 crystallizes in the stibnite structure type (Pnma, SG n° 62). Two and three types of antimony and selenium sites are crystallographically distinguishable, respectively (all of them are located at a 4c Wyckoff position). [Sb(1)Se 6 ] and [Sb(2)Se 7 ] polyhedra can be viewed as strongly distorted octahedra (three short (d Sb-Se < 3.1 Å) and three medium (3.1 Å < d Sb-Se < 3.3 Å) Sb-Se distances), and bicapped square pyramids (five short (d Sb-Se < 3.1 Å) and two long (d Sb-Se > 3.3 Å) Sb-Se distances) (Figure

  s c r i p t interactions (3.3 Å < d Sb-Se < 3.6 Å, 0.06 <  Sb_Se <0.14) along the a-axis to generate a tridimensional array (Figure1d). Let us notice that Sb 2 Se 3 is sometime also described in the Pbnm unconventional space group which may be a severe source of misunderstanding in discussion of physical properties in the direct and reciprocal spaces.[START_REF] Li | 9.2%-Efficient Core-Shell Structured Antimony Selenide Nanorod Array Solar Cells[END_REF]

Figure 1 .

 1 Figure 1. Sb 2 Se 3 crystallographic structure (SG: Pnma) (a) [Sb(1)Se 6 ] and [Sb(2)Se 7 ] polyhedra, bond are colored according to their length (< 3.1 Å: black, < 3.3 Å: orange, < 3.5 Å: grey) 26 (b); infinite 1 / ∞ [Sb 4 Se 6 ] ribbons running along the b-axis (c) condensation of 1 / ∞ [Sb 4 Se 6 ] ribbons along the c-axis to define 2 / ∞ [Sb 2 Se 3 ] layers running in the (100) planes when considering Sb-Se bond lengths comprised between 3.1 and 3.3 Å (d);

A

  c c e p t e d m a n u s c r i p t condensation of 2 / ∞ [Sb 2 Se 3 ] layers along the a-axis to define a tridimensional array. The conventional cell borders are drawn in black.

A

  photogenerated charge carriers. However, this may require a strong deposition process tuning to overcome the natural orientation along the b axis.

Figure 2 .

 2 Figure 2.Electronic band structure (a) and densities of states (b) of Sb 2 Se 3 calculated with PBE-GD3. G (0,0,0), X (0.5,0,0), Y (0,0.5,0), Z (0,0,0.5).

  A c c e p t e d m a n u s c r i p t

Figure 3 .

 3 Figure 3. Imaginary part of the dielectric function of Sb 2 Se 3 calculated with PBE functional.

  A c c e p t e d m a n u s c r i p t

Figure 4 .

 4 Figure 4. Stability domain of Sb 2 Se 3 vs µ Sb (bottom axis) and µ Se (top axis). The deviation of chemical potential from the reference most thermodynamically stable phase is indicated in brackets.

Figure 5 .

 5 Figure 5. Formation enthalpies of the vacancies vs. Fermi level in Se-rich (top) and Se-poor (bottom) conditions. Each line corresponds to the charge (encapsulated) exhibiting the lowest formation enthalpy. Points mark transition levels. Brackets highlight the dopability limit.

  A c c e p t e d m a n u s c r i p t Thus, the formation of an antisite, i.e. the substitution of Se by Sb and vice versa, is deemed possible. Regardless of the synthesis conditions, the enthalpies are under 2 eV which supports our claim that antisites can be abundant in this material. Se Sb antisites are very deep donor defects ( ~0.70 eV under CBM) which will not affect the electronic 𝜇 𝐸 𝐹 properties. Sb Se antisites are acceptor defects with a low formation enthalpy (<2 eV). Sb Se1 and Sb Se2 antisites exhibit almost the same behavior with a transition level 0/-1 located 0.33 and 0.40 eV, respectively, above the VBM. They both occupy a corner of the square based pyramid of Sb 2 but the Sb 2 -Sb Se1 and Sb 2 -Sb Se2 bond lengths differ. For the Sb Se3

Figure 6 .

 6 Figure 6. Intrinsic defect concentrations in Sb 2 Se 3 with respect to synthesis conditions.

Page 14 of 30 A

 30 c c e p t e d m a n u s c r i p t

2 ( 3 4

 23 for the two synthesis atmospheres and a growth temperature of 550 K. The behavior is typical of a p-type semiconductor: at low temperature the Fermi level is pinned close to the VBM and when the operating temperature increases, the Fermi level moves towards the middle of the bandgap. At very high temperatures (see SI), the simulation would follow the theoretical behavior 𝐸 𝐹 = 𝐸 𝑔 FigureS5) (these high temperatures are of course not reachable due to the + the material, but our calculations prove that our model matches semiconductor physics).

Figure 7 .

 7 Figure 7. Simulated Fermi level as a function of operating temperature for a model synthesis temperature of 550 K in different growth conditions (Se-rich/Sb-poor solid line and Se-poor/Sb-rich dashed line).

A c c e p t e d m a n u s c r i p t

SnSe 2 and

 2 SnSe phases, positive defect formation enthalpies are obtained, which means that conditions suited for the doping of the Sb 2 Se 3 host by tin are reachable. The depth of this maximum chemical potential value for tin to avoid the formation of an undesired phase gives us information about the amount of tin that should be included in order not to destabilize the material. Here, chemical potential values are quite deep (lower than -4 eV), which means that the Sn/Sb ratio should be low in order not to destabilize the matrix. The 0/-1 transition level is located ~0.24 eV above the VBM. It is roughly at the same position in the bandgap as the Sb Se3 antisite responsible for the native (poor) p-type conductivity of the material. Extrinsic defect concentrations can be deduced from equation 4. Of course, intrinsic defects are still present and must also all be taken into account. Due to the global charge balance, the addition of the external dopant affects the concentrations of the intrinsic defects as well. Additionally, considering the material quenched at growth temperature (800 K), freezing the lattice defects, the Fermi level at room temperature can be deduced from equation 4 with fixed concentrations obtained at growth temperature. All Fermi level values are summarized in Table

A c c e p t e d m a n u s c r i p t introduced inside the Sb 2

 2 Se 3 host without destabilizing the structure is small, SnSe 2 and SnSe phases being highly thermodynamically stable in Se-rich/Sb-poor and Se-poor/Sbrich, respectively. For the rest of the study, we will always take into account the phases in competition with Sb 2 Se 3 in the ternary phase diagram to determine the chemical potential of the dopant. It has been indeed experimentally demonstrated that the substitution of Sb by Sn can improve the electrical conductivity of Sb 2 Se 3 .[START_REF] Chen | Enhanced Electrical Conductivity and Photoconductive Properties of Sn-Doped Sb 2 Se 3 Crystals[END_REF]

A c c e p t e d m a n u s c r i p tFigure 8 .

 8 Figure 8. Sn-extrinsic point defect formation enthalpy vs. Fermi level in Se-rich (top) and Se-poor (bottom) conditions. Each line corresponds to the charge (encapsulated) exhibiting the lowest formation enthalpy. Points mark transition levels. The dashed red line corresponds to the formation enthalpy of Sn Sb1 calculated using the potential for tin in its elemental form to illustrate how this choice favors the decomposition of the host (negative defect formation enthalpy).

A c c e p t e d m a n u s c r i p tFigure 9

 9 Figure 9 Estimated defect concentrations for tin doping at T growth = 800K in Se-poor (top) and Se-rich (bottom) conditions, together with intrinsic defects.

Figure 10 .

 10 Figure 10. Cu-extrinsic point defect formation enthalpy vs. Fermi level in Se-rich (top) and Se-poor (bottom) conditions. Each line corresponds to the charge (encapsulated) exhibiting the lowest formation enthalpy. Points mark transition levels.

A c c e p t e d m a n u s c r i p tFigure 11 .

 11 Figure 11. Estimated defect concentrations for Cu-doping at T growth = 800 K in Se-poor (top) and Se-rich (bottom) conditions, together with intrinsic defects.

A c c e p t e d m a n u s c r i p tFigure 12

 12 Figure 12 Cl/Br/I-extrinsic point defect formation enthalpy vs. Fermi level. Each line corresponds to the charge (encapsulated) exhibiting the lowest formation enthalpy. Points mark transition levels.

Figure 13 .

 13 Figure 13. Estimated defect concentrations for iodine doping doping at T growth = 800K in Se-poor (top) and Se-rich (bottom) conditions, together with intrinsic defects. For I as dopant, the Fermi level, both during crystal growth and during device use at room temperature, is pushed up towards the conduction band, undoubtedly the manifestation of n-type doping. Nevertheless, antimony-related p-type defects, i.e. Sb Se antisite in Se-poor atmosphere and V Sb in Se-rich atmosphere, still play an important role, being the second most concentrated defects after the donor I Se2 substitution. Experiments have been performed with iodine doping of Sb 2 Se 3 and it has

  concentrations at T growth = 800K and T room = 300K; Figure S1 Charge density isosurfaces (yellow) in ideal Sb 2 Se 3 conventional cell; Figure S2 Formation enthalpies of the vacancies vs. Fermi level; Figure S3 Formation enthalpies of Sb and Se interstitials vs. Fermi level; Figure S4 Formation enthalpies of antisites in Sb 2 Se 3 vs. Fermi level; Figure S5 Simulated Fermi level as a function of operating temperature for a model synthesis temperature of 550 K in Se-rich/Sb-poor synthesis conditions; Figure S6 Estimated defect concentrations for chlorine doping at T growth = 800K; Figure S7 Estimated defect concentrations for bromine doping at T growth = 800K.

A c c e p t e d m a n u s c r i p t

A c c e p t e d m a n u s c r i p t A c c e p t e d m a n u s c r i p t

Table 1 .

 1 Comparison between reference chemical potential and maximum chemical potential value for Sb 2 Se 3 doping obtained by considering the stability domain of the Sb 2 Se 3 host in the Sb-Se-X ternary system.

	Dopant	µ 0 (eV)	Phase	𝜇 𝑆𝑒 -𝑅𝑖𝑐ℎ 𝑚𝑎𝑥	(𝑒𝑉)	phase	𝜇 𝑆𝑒 -𝑃𝑜𝑜𝑟 𝑚𝑎𝑥	(𝑒𝑉) phase
	Sn	-3.98	β Sn (I4 1 /amd)	-5.12	SnSe 2	-4.53	SeSn
	Cu	-4.24	fcc Cu (F m-3 m)	-4.50	Cu 3 SbSe 4 -4.21	CuSbSe 2
	Cl	-1.79	Cl 2(g)	-2.87	SbCl 3	-3.10	SbCl 3
	Br	--*	Br (l)	-2.48	SbSeBr	-2.72	SbSeBr
	I	-1.78	I 2(s) (C m c a)	-2.06	SbSeI	-2.30	SbSeI

* The reference thermodynamic state for Br is liquid bromine which was not calculated herein.

Page 18 of 30

VII. Acknowledgements

The first two authors equally contributed to this work. A.L. is indebted to Region Bretagne and Direction Générale de l'Armement for financial support during her PhD. A. S. thanks the CNRS and Region des Pays de Loire for funding. CCIPL (Centre de Calculs Intensifs des Pays de Loire) is aknowledged for computational resources. A.S. and C.L. thank access to the HPC resources of TGCC under allocation 2019-A0070911052 made by GENCI. A.L. and X.R. thank access to the HPC resources of [TGCC/CINES/IDRIS] under allocation 2017-A0010907682 made by GENCI.

VIII. Notes and references